TEMPERATURE AND WATER INDUCED SOFTENING BEHAVIOUR OF WOOD FIBER BASED MATERIALS

Lennart Salmen Department of Paper Technology The Royal Institute of Technology S-100 44 STOCKHOLM 70 SWEDEN

Library I

TEMPERATURE AND WATER INDUCED SOFTENING BEHAVIOUR OF WOOD FIBER

BASED MATERIALS

Lennart Salmen

Department of Paper Technology The Royal Institute of Technology S-100 hk Stockholm 70 Sweden

Swedish Forest Products Research Laboratory Paper Technology Department Box 560A S-11A 86 Stockholm Sweden

Akademisk avhandling som med till stand av Kungl. Tekniska Hogskolan i

Stockholm framlagges till offentlig granskning for avlaggande av teknisk

doktorsexamen, fredagen den 26 februari 1982, kl . 10.00 i Kollegiesalen,

Administrationsbyggnaden, Valhallavagen 79, Kungl. Tekniska Hogskolan,

Stockholm. Avhandlingen forsvaras pa svenska.

Gardens Point A22810463B Stockholm 1982 Temperature and water induced softening behaviour of wood fiber based materials

by

Smmrn - ,

TEMPERATURE AND WATER INDUCED SOFTENING BEHAVIOUR OF WOOD FIBER BASED

MATERIALS

by Lennart Salmen

ABSTRACT

The composite structural material comprising paper and wood is analysed

in terms of thermal softening and hygroplasticization with regard to its

constituent polymers. In particular, an attempt is made to relate the

softening occurring under dry conditions to that occurring under moist

conditions.

It is proposed that the semi-crystal 1ine cellulose exhibits a broad

transition region and thus displays a gradual softening at increasing

moisture contents. The crystallites restrict the motion of the tie

molecules between the crystallites in the microfibrils and thus shift

the transition in these regions to higher temperatures or higher moisture

contents. For the amorphous carbohydrates the plasticizing effect of

water is estimated from a free volume related theory of Kaelble.

A mechanical model of the wood fiber is suggested where the cellulose

microfibrils act as the reinforcements in a matrix of hemicelluloses.

The fiber wall is built up as a laminate. Based on this model the

macroscopic response to softening of the individual components is calcu­

lated. It is suggested that the softening effect due to water immersion

results from a softening of the disordered zones between the cellulose

crystallites in the microfibril. Under these conditions the fiber is

best represented by a discontinuous reinforced system in which only the

cellulose crystals act as reinforcing elements.

By an immersion technique in hot silicone oil it has been possible to

study the properties of dry paper and wood at temperatures up to 250°C

where fast chemical reactions otherwise interfere. For most papers the

main softening occurs at 230 C.

It is demonstrated that native or only slightly modified lignin under

dry conditions softens at 205 C or thereabove. The differences in the

softening behaviour of wood along and across the grain are shown to be

predictable from the structural arrangement of the components within the

wood. Under both dry and water-immersed conditions an apparent acti­

vation energy has been obtained for the glass transition of lignin.

The softening of paper in the RH-range of 0 % to 95 % has been measured

over the temperature range of -25 C to 65o C. By a temperature shift of

the curves at different moisture contents a master curve is constructed.

The shift terms follow the ones predicted from free volume theories. It

is concluded that the changes in elastic properties of paper with in­

creasing moisture content are determined by a softening of the amorphous

carbohydrates. The presence of dried in stresses is shown to be un­

affected by the softening in this RH range studied. It is proposed that

the dried in stresses are mainly due to restraints imposed in the dis­

ordered regions of the cellulose.

Key words: cellulose

f'ibers

glass transit-ion

hemieellulose

lignin

moistuve

paper

wood

TEMPERATURE AND WATER INDUCED SOFTENING BEHAVIOUR OF WOOD FIBER BASED

MATERIALS

by Lennart Salmen

TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2. SOFTENING OF WOOD POLYMERS 3

2.1 Thermoplasticizatlon 3

2.2 Hygroplasticization 8

3. MODEL FOR THE SOFTENING BEHAVIOUR OF WOOD FIBERS 16

3.1 The microfibrillar structure 16

3.2 Mechanical composite models of the cell wall 17

3.3 The effects of hygroplasticization on single fibers 20

4.. VISCOELASTIC PROPERTIES OF WOOD UNDER WATER-SOAKED

CONDITIONS 26

5. SOFTENING OF PAPER

5.1 Experimental method

5.2 Thermal softening

5.3 Influence of moisture

6. ADDITIONAL REMARKS

7. ACKNOWLEDGEMENTS

8. REFERENCES

30

30

30

37

47

1. INTRODUCTION

This thesis describes an attempt to analyse the softening behaviour

observed for the individual wood polymers and for wood fiber based

materials, and from this to present a unified picture of the softening

behaviour of the composite systems of wood fibers, wood and paper. In

particular a comparison is made between softening under dry conditions

and softening at different moisture contents. The softening of these

systems as a result of changes in temperature or moisture content is

essential in most applications of these products. The softening be­

haviour of the individual wood polymers, cellulose, hemicelluloses and

lignin is however still inaccurately known. Knowledge of the inter­

action of these components in the composite structure of wood fibers is

a 1 so very 1imited.

In presenting this analysis, the systems are discussed in order of

increasing complexity. Thus the softening temperatures reported for the

individual dry components are first examined. A discussion of the

plasticizing effect of water on the carbohydrates is then presented, and

the role of the microfibril structure is discussed in terms of the

softening temperature of cellulose. From these considerations, a

micromechanical model of the cell wall of tracheids is proposed where

the softening effects of the individual wood components can be simulated.

Measurements of the softening effects in paper and wood resulting from

changes in moisture and temperature are then presented. The effects on

the moduli of these composite materials are discussed in relation to the

softening temperatures of the individual wood components.

The following papers, referred to subsequently by Roman numerals,

provide the basis for this thesis:

I "The influence of water on the glass transition temperature of

cellulose."

Lennart Salmen and Ernst Back.

Tappi 60 (1977), 12:137.

II "The cell wall as a composite structure."

Lennart Salmen in "Paper Structure and Properties", Ed. Bristow,

J.A., Marcel Dekker Inc. To be published.

III "The fundamentals of energy consumption during viscoelastic and

plastic deformation of wood".

Lennart Salmen and Christer Fellers.

Based on a paper given at the Int. mech. pulping conf., Oslo June

1981. To be published.

IV "Simple stress-strain measurements on dry papers from -25 C to

250°C."

Lennart Salmen and Ernst Back.

Svensk Papperstidning 8>0 (1977), 6:178.

V "Effect of temperature on stress-strain properties of dry papers."

Lennart Salmen and Ernst Back.

Svensk Papperst idning 81 (1978), 10:341 .

VI "Thermal softening of the components of paper: Its effect on

mechanical properties."

Lennart Salmen.

Pulp & Paper Canada, Trans Tech Sec £ (1979), 3:TR kS.

VII "Moisture-dependent thermal softening of paper evaluated by its

elastic modulus."

Lennart Salmen and Ernst Back.

Tappi 63 (1980), 6:117.

3.

2. SOFTENING OF WOOD POLYMERS

The structural rigidity of wood fibers and fiber products is greatly

influenced by the stiffness of their main polymeric components: cellu­

lose, the hemicelluloses and lignin. As these polymers are to various

extents hygroscopic, the influence of the plasticizing action of water

is also important. Although the softening of cellulose, hemicellulose

and lignin under both dry and moist conditions has been studied indivi­

dually, their respective glass transition temperatures (Tg's) are still

inaccurately known. The selection of softening temperatures and/or

critical levels of plasticizers in wood polymers therefore requires a

discussion of the various published data. It is essential to take these

variables, temperature and water content, into account when predicting

the mechanical behaviour of cellulosic materials as they are responsible

for the major changes in the properties of the matrix in these structu­

res.

2.1 Thermoplasticization

Polymers generally exhibit several more or less distinctive second order

transitions due to the onset of movements of different segments or side

groups of the polymer chain. Apart from the glass transition, these

other secondary transitions are of minor importance for most of the

mechanical properties of a polymer, except maybe for the impact strength

(1). It should be noticed that a second order transition temperature is

dependent on the frequency used for the measurements (1). The glass

transition may be characterized by an apparent activation energy of 40 kJ/mol

or greater (2). For most polymers a value between 100 and 500 kJ/mol

is found in the frequency range of 1 to 10 Hz (2,3).

For semicrystal1ine polymers the presence of crystalline regions within

the polymer interferes with the transition process in the amorphous

regions. Obviously the glass transition will diminish in significance

as the crystallinity increases and may at high crystal1inities totally

vanish due to too small volumes of amorphous material (4). Crystallites

will broaden the transition region (1,5). For many polymers, an increase

4.

in Tg is noticed with increasing crystallinity as has for instance been

observed in poly(ethylene oxide) (6). An explanation of this effect has

been sought through the use of network theory for crosslinked polymers

or alternatively by considering the crystallites as reactive fillers,

thus attributing the effect to a shortening of the amorphous chain

segments between crystallites (1,7). Many other possible explanations

exist (1).

Fig. 1

The origin of the two glass

transitions Tg (L) and Tg (U)

according to Boyer (7).

For semicrystal1ine polymers, the concept of a double glass transition

has been introduced by Boyer (7). The lower T (L) is attributed to

amorphous material completely free from restraints caused by the crys­

tallites, i.e. dangling cilia (loose ends), while the upper T (u)

arises from the amorphous material under restraint by the crystallites,

i.e. tie molecules or loose loops, see fig. 1. This situation is

perhaps easier to visualize in the well-known fringed micellar model

than in the folded chain model in fig. 1. The T (l_) would thus be

unaffected by the crystal 1inity whereas the T (u) would shift to higher

temperatures the higher the crystal 1inity. Especially for polymers such

as polyvinylfluoride, polyvtny1idenefluoride and poly(ethylene oxide)

can the observed transitions be explained in this way (7,8,9).

Cellulose

Cellulose, or poly -glucopyranose, is the predominant structural

constituent in wood fibers. It is a linear polymer with cellobiose as

repeat unit. The degree of polymerisation (DP) of cellulose in native

plants is of the order of several thousands, which is reduced to around

1000 in pulps and even lower in regenerated forms. The presence in some

cases of linked glucans of lower DP values is thought to arise from

5

degraded cellulose. Cellulose occurs in nature solely in a semi-crystal­

line state. The crystallites are known to exist in at least four poly­

morphic forms. Native cellulose is found in the cellulose I form,

whereas regenerated and mercerized cellulose usually crystallize in the

eellulose I I form.

The transitions occurring in cellulose have been thoroughly reviewed by

Klason and Kubat (10), by Kaimin et al. (11) and by Jakobson and Erinsz

(12). A well defined transition exists at -70 C, 1.0 Hz, with an activa­

tion energy of 50 kj/mol. This transition is probably due to motion of

the C6-methylol group (13). Other transitions have been reported in the

regions of -30°C, 20°C and 120°C, the latter considered by Kaimin et al.

(11) to be due to movements of chain segments in the amorphous cellulose.

However, interaction between residual amounts of water and the cellulose

(10) or, at 120 C, evaporation of small amounts of water (10), may be

alternative explanations.

The glass transition of dry cellulose is claimed by most authors to

occur around 2300 C. Due to both auto-crosslinking and degradation at

these high temperatures (14), normal methods of studying the thermal

properties of polymers have been difficult to employ and the ones used

do not always make a clear distinction from first order transitions.

Various reported values are given in table I. For the methods here

given the frequency dependence may explain a difference between measure­

ments of some ten degrees.

Table I. Reported transition temperatures for cellulose in the range of 150° to 300°C.

T, °C

237-253 (x)

175,230 (x)

160 (x) , 230

145-196, 200-

230 (x)

(x)

-236 (x)

Method of determinati

Therma1 compressibi1ity

Sonic pulse velocity

Torsion pendulum

Forced vibration

Thermal expansion

ion Frequency

5 kHz

110 Hz

- 1 Hz

Ref.

(15)

(16)

(17)

(18)

(19)

(x) claimed by the authors to be the glass transition.

6.

In a thorough analysis of various cellulose samples, Yano et al. (18)

have suggested that transitions at 200O C for amorphous cellulose and at

236°C for cellophane are associated with the glass transition. The

activation energies for these transitions in the frequency range of 3-5

to 110 Hz were 206 kJ/mol for amorphous cellulose and 196 kJ/mol for

cellophane. On the other hand Zeronian and Menefee (17), from torsion

pendulum measurements on ramie fibers and mercerized ramie, tentatively

claim that the Tg occurs at 160° or as a double Tg with a Tg (L) at 160O C

and a Tg (U) at 230O C. However, data obtained by differential scanning

calorimetry and infrared spectroscopy by Hatakeyama et al. (20) indicate

the lower of these transitions to be due to a recrysta11ization of

amorphous regions of cellulose on heating. It is also recognized that

for cellulose acetates the Tg decreases with increasing acetylation,

being 195°C for diacetate and 175O C for triacetate (21). Thus an even

higher value would seem reasonable for the Tg of cellulose, i.e. at

about 230O C.

Hemicellulose

Hemicellulose is a collective name for various polysaccharides, except

the poly -glucopyranose, present in plant cell walls. They are

generally made up of more than one saccharide. In softwood the major

hemicellulose is 0-acetylgalactoglucomannan with lesser amounts of

arabino-4-0-methylglucurono xylan while in hardwoods 0-acetyl-4-0-

methylglucurono xylan dominates with lesser amounts of glucomannan

present. Extracted hemicelluloses have DP-values ranging between 150

and 200. Hemicelluloses are probably amorphous in their naturally

occurring state although they may crystallize when isolated (22).

For isolated dry hemicellulose preparations, the glass transition has

been observed over a rather broad range of temperatures, from 150 to

220°C, table II.

This variation in Tg is due partly to differences in chemical composi­

tion, for instance the existence of flexible side groups which lower the

transition temperatures due to a reduction of the molecular packing

efficiency. Data of Alfthan et al. (25) on various oligosaccharides

indicate that acetylation may cause a reduction in T of hemicel1uloses

of about 10 to 30°C.

7.

Table II. Glass t r a n s i t i o n temperatures of dry hemicelluloses.

Hemicellulose Tg, °C Ref.

Glucomannan 181 (15)

0-acetyl-4-0-methylglucurono xylan 148 (23)

4-0-methylglucurono xylan 217 (15)

arabino-4-O-methylglucurono xylan 180 {2k)

Lignin

Lignin is an aromatic polymer built up of three primary precursor monomers,

basically phenylpropanoid units. The lignin forms a three dimensional

network with crosslinks of several types. The structure differs between

softwoods and hardwoods and also between the middle lamella lignin and

that present in the cell wall (26).

The lignin extracted from wood and wood fibers differs in various respects

from the native lignin within the wood. It is thus not surprising that

the Tg's observed for lignin preparations vary as much as from 124O C to

193OC, table III. High temperatures also tend to change the samples in

various ways and it is very often found that the measurements exhibit a

great deal of hysteresis. The Tg of the various lignins also depends on

the molecular weight, as noticed by Goring (15) and Hatakeyama et al.

(27). Particularly for samples of high molecular weight, the Tg increases

substantially (15). With increasing degree of crosslinking, the Tg of a

polymer is also substantially increased as is for instance apparent in

crosslinked polystyrene (30). Thus native lignin may have a Tg at a

higher temperature than any of the lignin preparations.

Table III. Glass transit-Con temperatures of dry l-Cgn-Cns.

Type of lignin Tg , OC Ref.

Thiolignin 124 (27)

Dioxane 1ignin 138 (28)

Thiolignin 174 (28)

Bjorkman lignin 150 (29)

Periodate lignin 193 (15)

Enzyme 1ignin 192 (15)

8.

2.2 Hygroplasticization

Hygroplasticization, i.e. the action of water as a plasticizer, is of

particular interest in the case of carbohydrates. The interaction of

water with cellulose is a complex phenomenon, where the different stages

of water sorption range from tightly bonded water molecules to free

water primarily acting as a swelling medium. There is a strong inter­

action between the water molecules first adsorbed and the hydroxyl

groups in cellulose through hydrogen bonding. Hysteresis is observed in

the sorption isotherm for water in cellulose as for most other water-

polymer systems (31). However, Higgins has shown that the elastic

modulus is related only to the moisture content irrespective of whether

it is reached by absorption or desorption (32).

General plasticization relations

A plasticizer is in general a monomeric molecule added to a polymer to soften

it, i.e. to lower its glass transition temperature and thereby its stiffness

at a given temperature. Most plasticizers used have a low Tg in the

range of -150 to -50O C. Many equations of both empirical and theoretical

natures have been put forward to describe the lowerinq of the Tg of the

mixture by the addition of a plasticizer.

Relations applicable up to large amounts of added plasticizer have been

given by Kelley and Bueche (33) who related Tg to free volume, by

Couchman and Karasz (34) considering volume or entropy continuity con­

ditions, by DiMarzio and Gibbs (35) using a statistical mechanical

interpretation of composition effects on Tg , by Kaelble (36) relating T

to the cohesive energy and lattice coordinate numbers and by Nose (37)

assuming a hole theory.

The equations of Kelley and Bueche, of Kaelble and of Couchman and

Karasz are mathematically very similar and under certain conditions

identical (34) whereas the equation of Nose contains similar parameters

but in a different form. The DiMarzio-Gibbs method does not appear to

provide an explicit expression for T in terms of composition. The

merits of the different equations have not yet been established and

9.

for practical purposes the choice could merely be based on the avail­

ability of values for the various parameters in the respective equations.

To calculate the effects of water on the T 's of the carbohydrates the

equation of Kaelble (36) has been chosen since the parameters there

required can be estimated with reasonable accuracy.

The Kaelble relation is given by

[1]

where Xp and Xn are mole fractions of monomer units of the polymer P

and diluent, i.e. plasticizer, D respectively, and hp and hn are corre­

sponding parameters given by the relationship

where Z is the lattice coordination number in the glass state, 9

the change in specific heat capacity at Tg at constant volume and

R is the gas constant. The parameter h may also be calculated from

and U is the molar cohesive energy, is the solubility parameter and

is the molar volume.

The more generally used Kel1ey-Bueche equation (33) gives exactly the

same estimate of Tg as the Kaelble equation [1] if

[2] h =

[1]

i s

10.

Tg = K [6]

where K = 0.11 for polymers and 0.082 for diluents.

The plasticizing effect of water on various polymers has recently been

recognized for its detrimental effect on the mechanical properties of

epoxy resins and also for its importance for biological materials. For

the small amounts sorbed in epoxy resins the plasticizing action of

water has been shown to follow the polymer-plasticizer relations (39).

For polymers sorbing large amounts of water such as poly(2-hydroxy-

ethylmethacrylate), PHEMA, these relations also apply, as measured by

Sung et al. by differential scanning calorimetry (DSC) up to moisture

contents of 50 % by weight (40). For the PHEMA-water system the water

first sorbed leads to a volume contraction (41). Volume contraction

also occurs when polymer and diluent are mixed in the case of other

water absorbing polymers such as natural fibers and polyamides (31).

For many polymers a limiting value is reached after which additional

water does not further decrease the Tg . Examples of such are poly(6-

aminohexanoic acid), poly(hexamethylene adipamide), poly-N-viny1pyrro-

lidone (42), polyhydroxymethy1ene (43) and thiolignin (28). It has

been suggested (42) that the decrease in Tg due to addition of water is

related to a replacement of intermolecular hydrogen bonds in accessible

regions of the polymer. The limiting value of Tg is reached at a point

corresponding to the amount of water required for complete interaction

with all such bond sites. Of course, this theory does not explain the

behaviour of polymers which show no limiting value of T with increasing

water content. The softening effect of water on poly(hexamethylene

adipamide) has also been shown to follow the general relations at

moisture contents below the limiting amount (Paper I).

In semicrystal1ine polymers, the relations describing the effect of a

plasticizer upon Tg differ from those for amorphous polymers. In many

cases the plasticizer cannot penetrate the crystallites although ad-

sorption on the surfaces of the crystallites may occur (31). Ellis et

al. (44) have shown that crosslinks in the polymer cause the depression

of Tg by a diluent to be increased, and they postulate that the same

phenomenon may occur in semicrystal1ine systems.

11.

Carbohydrates

Data on the influence of moisture content on the glass transition

temperature of cellulose are conflicting. The glass transition has been

claimed to occur at room temperature from moisture content values

below 6 % to above 50 %, while still other investigations have detected

no transition at all.

In a semicrystal1ine polymer such as cellulose, the amorphous part may

exist in different regions, from regions undisturbed by the crystallites

to regions where the molecules have a very restricted mobility due to

the linkage to the crystallites. It seems reasonable to assume that the

T will be shifted to higher temperatures due to these restraints.

Therefore a broadening of T on the high temperature side will result

(45).

Similarly, it is proposed that cellulose may exhibit a gradual softening

at increasing moisture contents as a consequence of the restraints

imposed by the cellulose crystallites.

For completely amorphous cellulose the behaviour is expected to be

similar to that of the amorphous hemicelluloses. For both these carbo-

hydrates the plasticizing effect of water may be estimated from general

polymer-plasticizing equations. Here the equation of Kaelble has been

applied (Paper I) .

In order to estimate the constant hp in the Kaelble equation for cellulose,

reported measurements of the plasticizing action of various diluents

have been inserted in the equation. Using measurements by Kargin et al.

(kS) for the plasticization of amorphous cellulose with triethylphenyl-

ammonium hydroxide for the transition that in dry cellulose occurs at

, a value of 151 for hp of cellulose has been calculated (Paper I).

This value is consistent with the value calculated from data for the

effect of water on the Tg of glucose by Luyet and Rasmussen (47) con-

sidering that the monomer and polymer in general have a similar molar

cohesive energy, and applying equation [3].

Since their structure is chemically similar to that of cellulose, the

constant hp for the hemicelluloses was taken to be the same as for

cellulose.

12.

The data used to calculate the water plasticizing effect are given in

table IV.

The parameters hD and hp for the cellulose-water system can be compared

with those applicable in the Kelley-Bueche relation. Using equation

Table IV. Data for the aalculation of plasticization effects according to the Kaelble equation (36).

Water

Triethylphenyl -ammonium hydroxide

Cellulose

Xylan

Glucose

136

120

493

463, 493

273

71

280

151

151

263

Calculations for xylan of the dependence of Tg on moisture content, based

on two values of T , 190°C and 220°C, are given in fig. 2. 9p

10 20 30

Moisture content, %

Fig. 2

Influence of water on the glass transition of hemi-celluloses as calculated using the approach of Kaelble (36) for two values of the T of xylan; 190° and g 220 C. Measurements on various hemicelluloses are included where the triangles refer to those by Goring (15), the circles to those by Hakkinen and Toroi (49) and the squares to those by Takamura (29). For comparison the calculated relationship for a completely amorphous cellulose is also included.

is estimated to be 2.73. This is close to the

value 2.82 estimated for the corresponding entity

[ 6 ] ,

48 .1 18

108

13

Cousins (48) has demonstrated that for isolated hemicel1uloses, xylan

and glucomannan from Pinus radiata, the mechanical properties show a

typical glass-rubber behaviour with increasing moisture uptake indicat­

ing a transition at about 30 % moisture content at 20o C, which is in

close agreement with the calculated curves in fig. 2. For intermediate

moisture contents, data of Goring (15), Hakkinen and Toroi (49) and

Takamura (29) on the rate of compression of pulverized samples of vari­

ous hemicelluloses are included in fig. 2 and essentially confirm the

calculated behaviour given for xylan.

Due to its chemical similarity, amorphous cellulose can be expected to

behave in a similar way to the hemicelluloses, as indicated by the

calculations shown in fig. 2 with of cellulose taken as 220o C. For

the semicrystalline cellulose the fact that the cellulose I crystals do

not absorb water (50), has to be considered. Therefore, for a given

moisture content in the amorphous regions the moisture content based on

the total weight of the sample will be a function of the degree of

crystalliniry. Fig. 3 shows the softening temperature calculated from

the Kaelble equation as a function of the moisture content based on the

total weight and different degrees of cellulose crystalliniry (Paper I).

Fig. 3

Influence of water on the glass transition temperature of cellulose calculated using the approach of Kaelble (36). The filled symbols refer to experimental NMR data by Ogiwara et at. (51) for cotton and various treated dissolving pulps and the open symbols refer to torsion pendulum measurements by Tokita (52) on viscose rayon.

10 20 30 MOISTURE CONTENT, %

Some data said to show the plasticized softening temperature of cellulose

due to water, based on NMR-measurements (51) and torsion pendulum measure-

ments (52), are included in the figure. Tokita found a small loss

maximum varying with moisture content between 37o C at 16 % moisture

content and 72°C at 9 % moisture content when measuring the torsional

modulus of viscose rayon (52). From measurements at three different

frequencies, 0.091, 144 and 520 Hz, he estimated the activation energy

to be about 795 kJ/mol. The observed loss maxima are however very

vague. Tsuge and Wada, studying the dielectric dispersion on cellophane

between 2 and 17 % moisture content at 35 C, also interpret the data as

a transition in this moisture content region (53).

The water contents at these different transition temperatures are in the

range associated with specific hydration water or bound water, as opposed

to free water (54). Westman (55) for instance related the amount of

bound water in cellulosic gels (67 % crystal1inity Crl, X-ray) to a

local minimum in the specific volume curve for cellulose-water mixtures,

found at 11.6 % by weight of water at 20°C. It may thus be that the

above-mentioned transitions are associated with the onset of the presence

of free water.

At higher moisture contents than those discussed above, creep data for

cellulosic gels by Westman (56) and measurements of the dynamic modulus

during drying of paper by Htun (57) indicate a significant softening

effect occurring above 1 gram water per gram amorphous cellulose at

20 C. Htun estimated the activation energy for the relaxation process

to be about 56 kJ/mole from a time-temperature shift below the transition

temperature. Considering the specific levels of the modulus and its

change at this transition as well as the calculated activation energy,

this softening may be associated with a glass transition of cellulose

although this is not explicitly stated by these authors. This transition

indicated by Westman (56) and Htun (57) occurs at a much higher moisture

content than that predicted for the amorphous materials. This can be

understood to be due to the effect of crystallite restraints on the

segment mobility which shift the transition to longer times, which is

equivalent to a shift to higher temperatures or higher moisture con­

tents.

14

15

Thus it is probable that the softening in cellulose occurs over a broad

range of moisture contents. At room temperature the range spans from a

lower limit at about 80 % RH to an upper limit reached when the material

is immersed in water. This type of behaviour is supported by dynamic

mechanical studies on cellulose fibers where it is shown that the mecha­

nical damping steadily increases from about 80 % RH (58).

Several studies have been made on the influence of different softeners

on lignin. Effective plasticizers, such as dimethyl phthalate, may

reduce the to about 60°C at 20 %

plasticizer content (28). For such plasticizers the general plastici-

zing relations describing their effects on T have been shown to apply.

For water, however, the solubility is very limited. Thus due to the low

degree of interaction between this plasticizer and the polymer, rela-

tions based on free volume concepts are difficult to envisage. A small

amount of water has, however, been shown drastically to reduce the Tg

(15,28,29) for example for a thiolignin from 174°c down to 115°C at 5 %

moisture content (28). Further addition of water shows comparatively

little effect. Nakamura et al. (59) have by studies on a model lignin

sample of poly(4 hydroxystyrene), which was compared with poly(4-acetoxy-

styrene) hydrolysed to different degrees, shown that the rapid decrease

of T due to addition of water is proportional to the amount of hydroxyl

groups in the sample.

The limiting moist glass transition temperature of lignin can be lowered

considerably by sulphonation, the extent of this reduction being pro-

portional to the degree of sulphonation (60) . Yeo and Eisenberg have

recognized that for a polyelectrolyte the maximum loss coefficient in-

creases and the rubbery modulus decreases with increasing plasticizer

content (61). The ionizable groups in the lignin should thus play an

important role with regard to the softening mechanism.

Lignin

16.

3- MODEL FOR THE SOFTENING BEHAVIOUR OF WOOD FIBERS

3.1 The microfibrillar structure

In its naturally occurring state, cellulose exists in the form of micro-

fibrils, which are regularly ordered within the cell wall of plants.

These microfibrils have a diameter of about 20-45 (62) with an extreme

length which may even extend through the entire fiber. In native

cellulose the chains in the crystallites of the cellulose (cellulose I)

have been shown to exist in a parallel extended configuration (63) with

the chain axis parallel to the axis of the microfibril. Several pub-

lished investigations (see 64) tend to favour the idea that the cellu-

lose chains in the microfibrils a re disturbed from their parallel order

in certain regularly occurring regions. Thus many models, originating

with the fringed micellar theory, have been proposed for microfibrils

with recurring disordered regions. In pulp fibers the crystallite

length has been found to be about 800 (65). Stöckmann (66) in con-

sidering the growth process of the cell wall suggests that in wood the

only disturbances existing a re lattice defects of the cellulose crystal.

These defects are converted to more or less amorphous regions in the

processes of pulping and beating (66). These disordered zones between

the crystallites are accessible to water (67). It is therefore reason­

able to assume that these undergo thermal softening as well as hygroplasti-

cizat ion.

The disordered regions consist mainly of tie molecules between the

cellulose crystallites. This means a restricted chain mobility in these

regions, which shifts the Tg to higher temperatures or higher moisture

contents. Loose chain ends of sufficient length extending from crystal-

lites could exhibit a Tg more comparable with that of amorphous carbo-

hydrates. The drastic differences between the mobility of the segments

belonging to the tie molecules and that of the free chain ends may then

be reflected as a broad softening range for cellulose. The data for moist

cellulose seem, as previously discussed, to indicate such a broadening

of the transition region.

The model of the microfibrillar structure here visualized in order to

describe mathematically the softening behaviour of cellulosic fibers is

Stockmann

17.

shown schematically in fig. 4. The microfibril is considered to have

disordered regions regularly spaced along its length with a surrounding

matrix of hemicelluloses and loose cellulose chain ends. In the follow-

ing discussion of the model the matrix material is simply termed hemi-

celluloses. This matrix of hemicelluloses is here considered to exhibit

a softening temperature comparable with that of amorphous carbohydrates

implying a softening at room temperature at about 80 % RH. The dis-

ordered regions of the microfibrils represent restrained cellulose

chains which should display a softening at considerably higher humidities

and complete softening may not be achieved until the fiber is immersed

in water. In the model, the effect of this softening may be taken into

account by considering these zones to become a part of the matrix

material when the fiber is immersed in water. Thus, under wet conditions,

the microfibrilar structure can be viewed as a discontinuous reinforced

composite with the cellulose crystals as the reinforcements in a matrix

of hemicelluloses (Paper II).

Cellulose microfibril

Crystalline

Soft

Glass

Hemlcelluloses and cellulose chain ends

Fig. 4 Model representation of the microfibril with its surrounding matrix in wood pulp fibers under different environmental conditions. The humid conditions refer to an RH above 80 % while the wet state is reached when the fibers are immersed in water.

3.2 Mechanical composite models of the cell wall

The properties of wood and wood fibers have been described in terms of

various models of the structural organization of their components (68-

72). In idealizing the helical winding of microfibrils in the cell

DRY HUMID WET

18.

wall, Muench (68) considered the cell wall to be composed of several

sets of helical springs embedded in a matrix of non-crystalline material.

Other approaches of e.g. Mark (71) and Schniewind (72) have been based

on the concept of a layered structure. The cell wall model here adopted

is based on the concept of a laminated structure, thus recognizing the

different layers in the tracheid cell wall. Each cell wall layer is

assumed to consist of cellulose microfibrils embedded in a matrix of

hemicelluloses. The lignin present is considered to exist in separate

isotropic lamellae located in the middle of each cell wall layer, as is

schematically shown in fig. 5- (Paper II)

Fig. 5 Model representation of the cell wall layers in a traeheid.

This model involves a certain degree of simplification. Kerr and Goring

(73) have for instance concluded on the basis of electron microscope

studies that the cell wall has an interrupted lamella structure where

the dimension of a given lignin or carbohydrate entity is greater in the

tangential direction of the fiber wall than in the radial direction.

They also concluded that the hemicelluloses not only exist as a matrix

around the microfibrils but must also be present in the lignin-con-

taining entities. The presence of covalent bonds between lignin and

hemicelluloses (74) indicates a close association between these com-

ponents. Yet there is still no evidence that these polymers are miscible

in the sense that they mechanically react as a homogeneous component

i.e. exhibiting a single glass transition temperature (Paper VI). It is

therefore here considered that the cellulose, hemicelluloses and lignin

components exist in a heterogeneous composite and exhibit separate glass

transitions.

19.

As previously stated, the softening of the disordered regions of the

cellulose microfibrils is here visualized as being due to the fact that

the composite changes from being a continuous reinforced structure to a

discontinuous reinforced one.

Estimates of softening effects in wood and wood fibers based on a

laminate concept have previously been made by Cave (75), who calculated

the influence of humidity on the longitudinal modulus of wood, and by

Mark (76), who estimated the influence of structural factors when the

matrix softened but only for a single fiber wall. In these estimates

the reinforcing microfibrils have been considered as continuous and not

influenced by the environment.

Micromechanica1 relations for discontinuous composites

The elastic behaviour of a discontinuous composite system depends not

only on the properties of the two components and the volume fraction of

the reinforcing material, but also on their size, shape and orientation

and on the state of adhesion between the reinforcement and matrix.

Several approaches have been made to predict the stiffness of such

systems, as reviewed by Ashton, Halpin and Petit (77).

A convenient equation to calculate moduli of widely different morpho­

logical systems has been developed by Halpin and Tsai (77,78) given as

where

n - (Ef/Em - 1)/(Ef/Em + ) [8]

E is the modulus of the composite, Ef, that of the reinforcement and Em

that of the matrix, Vf is the volume fraction of the reinforcement and

is a shape factor of the reinforcing elements. For the modulus along

the axis of the reinforcements = 2 , where is the length to

diameter ratio, i.e. the aspect ratio. This equation may be applied to

20.

systems of spherical inclusions, where = 1 as well as to continuous

fibrous reinforcemnts where

Halpin and Kardos (79) have applied this equation to semicrystal1ine

polymers where they related the difference in stiffness between crystalline

polyolefins and natural rubber to the crystal morphology via the shape

factor. For these systems, the measured shape factor gave a reasonable

estimate of the elastic modulus of the polymer. On the other hand,

Porter et al. (80) have found the aspect ratio of crystallites in

ultraoriented semi-crystalline polyethylene calculated from the Halpin-

Tsai equation to be greater than the measured value.

For the composite system of the cell wall, however, this equation fulfils

the present aim of allowing the cellulose reinforcement in the fiber

wall to be considered both as a continuous microfibril and as discrete

crystals. This model then makes it possible to calculate the fiber

stiffness both under dry conditions and when immersed in water, going

from a continuous reinforced system to a discontinuous one.

3.3 The effects of hygroplasticization on single fibers

The cell wall model here proposed provides a means of estimating the

effect of softening of the individual wood components on the properties

of single fibers. In particular, an analysis is made of the differences

between the hygroplasticization occurring at about 80 % RH, thought to

be mainly a consequence of hemicellulose softening, and that occurring

when the fibers are immersed in water.

As previously discussed, the disordered regions of the microfibril are

assumed to be plasticized at water contents reached only when the mate-

rial is immersed in water. Under dry or humid conditions, where these

disordered regions are considered to be unaffected by water, a value of

50,000 for the reinforcing shape factor has been chosen as adequate

to simulate the "infinitely" long reinforcement. When these disordered

regions soften, a shape factor of about 25 would be applicable to most

fibers if the crystallites alone were considered as reinforcements. Due

to the restrictions imposed by the crystallites the interconnecting

disordered chains may in reality not soften to the same extent as the

21 .

hemicelluloses, thus implying longer reinforcing elements. Since the

aim of the model here suggested is simply to simulate the effect of

softening of the cellulose microfibrils, a value of 500 for the shape

factor is here taken as representative of all the different cellulose

containing fiber types. (Paper II)

The calculated effects of this change in shape factor, representing the

softening of the disordered regions, on the elastic moduli of a kraft

pulp fiber with an S2 fibril angle of 20o are compared in fig. 6 with

the effects of a softening of the hemicelluloses alone. The relative

rigidities are given, where the values are the rigidities corre­

sponding to = 50,000 with the hemicelluloses assumed to be in the

glassy state. The curves show the behaviour when the hemicelluloses are

assumed to be soft. The intercept on the ordinate thus represents the

loss of rigidity due to the softening of the hemicel1uloses. (Paper II)

50000 10000 5000 1000 500

Shape factor, 1/d

Fig. 6 The calculated influence of the shape factor for the reinforcing cellulose crystals on the relative rigidities of a kraftt pulp fiber with an S2 fibril angle of 20°. E refers to the rigidities corresponding to = 50,000 with ihe hemicelluloses assumed to be in the glassy state. The curves are given for hemicelluloses in the soft state. refers to the longitudinal fiber modulus, Ey to the transverse fiber modulus and torsion to the torsional stiffness of the fiber. (Paper II)

These calculations predict that the relative rigidities should be more

sensitive to the shape factor of the reinforcements than to the softening

of the matrix component, here hemicellulose. The influence of the

22.

reinforcements diminishes in importance in the sequence: longitudinal,

transverse and torsional stiffness. The magnitude of these effects is

surprisingly independent of fibrillar angle up to about 30 (fig. 12

Paper II).

Comparison with experimental data

Data on the longitudinal modulus of dichlorite delignified tracheids

presented by Kersavaga (81) show that up to a moisture content of 18 %

corresponding to 83 % RH the loss in relative rigidity is only about

11 %. This is in close agreement with the reduction of about 15 % here

calculated for the softening of the hemicelluloses alone. When these

fibers are immersed in water the modulus drops still further to a total

reduction of about 50 % (81). Thus it seems likely that no appreciable

softening of the disordered zones of the microfibrils takes place in the

range of moisture contents up to a level equivalent to 85 % RH.

The assumption that the response of a fiber to moisture in the range of

different relative humidities reflects the softening of the hemicellu-

loses may be tested in greater detail using torsional stiffness data on

a kraft pulp fiber of Kolseth et al. (58). Utilizing data presented by

Cousins (48) for the modulus of hemicelluloses as a function of moisture

content the corresponding change in relative torsional rigidity from 5 %

moisture content (25 % RH) has here been calculated as shown in fig. 7.

The cross-sectional swelling of the fiber is not included in these

calculations. This swelling may cause the fiber to appear stiffer due

to an increase in the moment of inertia. The calculated relative torsional

rigidities should therefore represent a lower limit compared with experi­

mental values.

It may be demonstrated that the correction for swelling applicable to

the calculated values of the torsional rigidity is proportional to the

swelling which is approximately given by the increase in moisture con­

tent (Paper II). The corrections to the data given in fig. 7 are thus

comparatively small and the quantitative agreement up to 15 % moisture

content (90 % RH) should still be considered quite acceptable.

23.

Fig. 7 The influence of moisture content on the relative torsional stiffness of a kraft fiber. The points refer to measurements on a kraft pulp fiber by Kolseth et al. (58). The line shows the calculated changes assuming hemicellulose softening according to data of Cousins (48). (Paper II)

It should be recognized that the predicted values only take account of

matrix softening occurring in the humidity region between 25-90 % RH.

At higher humidities a gradual softening of the disordered. regions In

the microfibrils may reduce the relative rigidity appreciably. The

effect of this softening can only be accounted for in the model when

these disordered regions soften fully on immersion in water. Therefore

it is to be expected that in the RH region above 90 % that will be poor

agreement between the experimental values of the relative rigidity and

the predicted values based only on matrix softening. A comparison with

experimental data should therefore be limited to moisture contents

between 25 and 90 % RH.

In table V relative torsional rigidities, comparing the conditions at

90 % RH with those at 25 % RH, are given for a number of different

tracheids as measured by Kolseth and Ehrnrooth (82). The calculated

values are in the same range as the measured ones and show in some cases

a lower value, which is to be expected since the changes in cross-

sectional area due to swelling have been ignored. As seen in fig. 8

neither the experimental nor the predicted data show any great variation

with the chemical composition of the fiber. The torsional relative

rigidity has been calculated for a composition of 40 % carbohydrate

crystal 1inity, i.e. 40 % reinforcing material and 60 % matrix material,

in the lower line and for 80 % carbohydrate crystal linity in the upper

line. The shaded area thus covers the normal range of carbohydrate

crystal linity in wood pulp fibers. The measured values of the relative

24.

torsional stiffness of the fibers in this figure have been corrected

assuming a swelling of 10 % (Paper II). For ramie fibers with their low

content of matrix material, the decrease in relative torsional stiffness

is also similar to the range here predicted (58,83).

Apparently the model here given agrees with experimental results with

respect to

1. The qualitative change in relative torsional rigidity in the hu-

midity region between 25-90 %.

2. The insensitivity of the change in relative modulus to fiber compo-

sition.

3. The magnitude of the changes in relative modulus between 25-90

% RH.

It may therefore be concluded that the assumption of matrix softening

gives a good description of the changes in fiber rigidity in this humi­

dity region.

When the fibers a re immersed in water, the measured relative tensile

rigidity as given by Kolseth and Ehrnrooth (82), comparing the wet

condition with that at 50 % RH (table V ) , is reduced much more than a

softening of the hemicel1uloses alone would account for, as is apparent

from the calculated effect given in fig. 6. This fact can only properly

be accounted for in the model by assuming a change in the shape factor

as a reflection of the softening of the disordered regions. The

calculated relative tensile rigidities given in table V are based on a

reduction to = 500. As the cellulose microfibrils essentially

determine the extensional stiffness of the tracheids, it is evident that

a softening of the disordered regions will play an important role with

regard to the hygroelastic behaviour of pulp fibers. It is however

notable that this type of softening does not have to be included in the

model, until the fibers are wetted, to obtain agreement between experi-

mental and calculated values of the relative rigidity.

25.

Relative rigidities due to different changes in the environment for a number of various softwood tracheids. Measurements according to Kolseth and Ehrnrooth (82). Calculations according to the micromechanical model here given. ("Payer II)

Fiber

TMP

d ichlori te 1ignified

" "

sulphate

bleached sulphate

bleached sulphate

holocellul

high yield sulphate

high yield sulphate

de-TEMP

ose

Relative

Cellu­lose

44

45

47

54

74

78

80

77

60

72

compos

Hemi-cellu-loses

30

30

31

36

18

22

20

21

19

20

tion (%)

Lignin

26

25

22

10

8

0

0

2

21

8

Relat ive rig

Torsion 90%/25% RH meas. calc.

0.52

0.52

0.49

0.56

0.47

0.48

0.55

0.52

0.55

0.52

0.52

0.49

0.51

0.50

0.50

0.50

0.53

0.50

dities

Tension wet/50% meas.

0.4

0.2

0.2

0.2

0.4

0.3

E/Eo RH calc.

0.25

0.23

0.23

0.25

0.29

0.25

10 20

Lignin content, %

Fig. 8 The relative torsional rigidity for fibers as a function of lignin content. The calculated lines represent the range of carbohydrate crystallinities from 40 %, the lower line to 80 %, the upper line. Measured data indicated by points are taken from table V3 but in this case the data are corrected for an assumed swelling of 10 %.

Table V.

26.

4. VISCOELASTIC PROPERTIES OF WOOD UNDER WATER-SOAKED CONDITIONS

When wood fibers are immersed in water, the amorphous carbohydrates will,

according to the previous discussion, be in a soft stage at room tempe­

rature. As various lignin preparations have their softening limit in

water at about 100°C (15,28), it is likely that native lignin is still

in its glassy state at room temperature. Lignin may then have a more

pronounced influence on the temperature dependence of the elastic modulus

of water-impregnated wood. This has a special interest in connection

with the defibration of wood.

In this work, the viscoelastic properties of wet wood samples of Norwegian

spruce (Picea abies) have been studied with dynamic mechanical measure­

ments between 20° and 140°C (Paper III). The absolute value of the

complex modulus along and across the grain at 10.0 Hz is given in fig. 9.

For samples that have not been steam-treated before the tests there is

an irreversible softening during the first rise in temperature which is

also manifested in an increase in the mechanical loss coefficient.

Fig. 9

The dynamic elastic modulus for water-soaked wood samples of Norwegian spruce, Picea abies, along and across the grain. The modulus is given as the absolute value of the complex modulus, i.e. \E*\. (Paper III)

0 50 100 150

27.

The maxima noticed in the loss coefficient tan & over the temperature

span as shown in fig. 10 have, by analogy with the known softening limit

of isolated lignin samples (28) and of the lignin in NSSC-paper (84),

been attributed to the wet glass transition of native lignin. The

correlation with softening in the lignin is also supported by the

structural features of the wood, indicating a much greater sensitivity

to the lignin properties across than along the grain (Paper II).

Fig. 10

Mechanical loss coefficient as a function of temper-ature for water-soaked wood samples along and across the grain. The measurements refer to stearm treated wood. (Paper III)

0 20 50 100 150 Temperature, °C

To facilitate a comparison of the transition region along and across the

grain, a normalization procedure has to be used which is independent of

the geometry of the sample and of the structural arrangement of the con-

stituents. A suitable normalization of the loss coefficient can be achieved

by setting its maximum equal to 1. Thus the normalized loss factor

is given by

[9]

where tan is the loss coefficient at the peak of the dispersion (85).

In fig. 11 these normalized loss coefficients (tan 6) are compared for

the two directions in wood. The rather close correlation between the

shapes of the curves indicates that the viscoelastic properties of the

28.

wood at temperatures around 100 C are dominated by the lignin irrespec-

tive of grain direction. The discrepancy at lower temperatures may be

due to the tail end of a transition below the measured temperature

range. This softening has a larger influence on the cross direction

properties.

Temperature, °C

A small difference in the softening temperatures along and across the

grain may be noticed, but it is too small to be given any significance.

Judging from the loss modulus, the differences are greater but the

shapes of these curves are severely affected by the additional loss due

to the existance of a broad transition region in the lower end of the

measured temperature range (Paper III). Yang et al. (85) have noticed

no shift in the softening maximum of the epoxy or of the dispersion

curve for laminates of fiber-reinforced epoxy resin with different

stacking sequences, despite the different stress states.

Analysis of the activation energy

Measurements at different frequencies, between 0.2 Hz and 20 Hz, show a

shift in the tan 6 maximum towards lower temperatures at lower frequen-

cies, analogous to the behaviour of second order transitions in poly-

mers. The frequency dependence for a glass transition can be approxi-

mated by an Arrhenius-type equation given by

[10]

where f = the frequency

fo = pre-exponential factor

Fig. 11

Relative mechanical loss coefficient versus temperature for water-soaked wood samples along and across the grain.

29.

= apparent activation energy in J/mol

R = gas constant (8.3143 J/mol K)

T = temperature in K

Thus over a limited frequency range an apparent activation energy may be

calculated for the glass transition. The measured data for wood samples

across the grain are given in an Arrhenius plot in fig. 12. The apparent

activation energy thus calculated from our measurements on wet wood

across the grain is 395 kJ/mol. Data of Becker et al. (86) on wet wood

samples of spruce measured in torsion in the radial direction are also

included in fig. 12. These data agree reasonably well with the relation

found between frequency and inverted temperature although a slight shift

to higher temperatures at the highest frequency is noted. A significant

deviation from the Arrhenius equation at high frequencies is general for

a glass transition. The apparent activation energy for a glass

transition normally has a value of about 40 kJ/mol and upwards with a

general trend towards an increasing with increasing T (2,3).

Here a comparison may be made with polystyrene with a T of 100°C and a

2.0 2.2 2.4 2.6 2.8

1/T x 10-3 ( K-1)

Fig. 12

Arrhenius plot, log frequency versus reciprocal absolute temperature, for the major transitions in dry and wet lignin-containing samples. Data represented by rings are taken from measurements of Becker et al. (86). Data for the dry lignin are discussed in chapter 5. 2.

of 423 kJ/mol (87) and polycarbonate with a T of 150°C and a of

481 kJ/mol (88). Thus the value calculated for the activation energy of

the measured transition in wet wood is well with in the range expected

for a giass transition.

30.

5. SOFTENING OF PAPER

5.1 Exferimental method

The measurement of the softening of dry wood fiber products is accompanied

by considerable problems due to the rapid degradation and auto-crosslinking

reactions which take place at temperatures in the vicinity of these

transitions, i.e. above 200 C (14). Measurements of the temperature-

dependence at lower temperatures when the product contains water are

also troublesome due to the changes in moisture content which occur as

a result of the interrelation between absorbancy, relative humidity and

temperature.

To overcome these problems a method has been developed in which a strip

of the material is immersed in a thermostated silicone oil. As a result

of the rapid heating rates thus achieved it is possible to determine the

mechanical properties of the dry material at high temperatures before

any reactions noticeably alter its properties. (Paper IV) It is possible

that the inert silicone oil also decreases the rates of the reactions

taking place. Due to the very low solubility of water in the silicone

oil, the moisture content of immersed strips containing moisture does

not change if the temperature used is not too high, i.e. below about

50°C (Paper VII).

5.2 Thermal softening

The elastic modulus of dry papers, as measured by the immersion technique

in silicone oil, decreases fairly linearly with temperature on a logarithmic

scale up to about 200 C, after which it drops significantly, as seen in

fig. 13. This softening taking place between 200 and 250 C seems to be

greater for papers containing more amorphous material i.e. the papers of

NSSC and thermomechanical pulp (Paper V ) . A softening in this region

has also previously been noticed for various wood fiber preparations (15,89).

Measurements on an NSSC-paper are shown in more detail in fig. 14, in

which two regions of softening are discerned, one at the other at

for a testing rate of 1.7.10 /s. The softening temperatures

31.

indicated have been taken as the temperatures corresponding to maxima in

the slope of lines fitted to these measurements. These slopes have been

obtained by a linear regression calculation applied to the measurements

over a temperature span of 30°C, successively repeated for every 2o C. For

all other papers measured, except for those based on NSSC-pulp, no

apparent decrease in the modulus was obtained until about 220o C.

0

-0.5

-1.0

-1.5

In E/E0

100 200 oC temperature

Fig. 13

The natural logarithm of the relative elastic modulus E/Eo versus temperature for dry papers. Eo refer to the modulus at 20o C. (Paper V)

The influence of iignin softening

In order to determine the origin of the different softening for the

NSSC-papers, the same NSSC-pulp was in one case selectively extracted

with alkali to remove hemicel luloses and in another case de'ignified

with dichlorite. Rate studies and the addition of plasticizer were also

used to characterize the transitions.

Fig. 14

The natural logarithm of the elastic modulus In E versus temperature for a dry NSSC fluting in the machine direction. (Paper VI)

32.

The influence of extraction procedures on an NSSC-pulp are shown in

fig. 15. After a dichlorite delignification the softening at 205oC

disappears, whereas a hemicellulose extraction leaves this softening

unaltered. The influence of the extraction procedures on the other

parts of the curve is marginal for either of the treatments. It thus

seems reasonable to conclude that the softening at 205oC is due to a

transition taking place in the lignin phase. The temperature here

indicated also seems reasonable for a T of a lignin that has only been

mildly changed from its native structure. On a birch veneer, a distinct

transition detected across the grain at 205 C was also ascribed to the

softening of native lignin (fig. 4, Paper VI).

The presence of lignin in the different papers made of NSSC-pulp and in

the corresponding extracted pulps also shows up as a large increase in

stretch at rupture above the softening temperature of the lignin (fig. 5,

Paper VI). This may be ascribed to the effects of plastic flow of the

1ignin.

Fig. I5

In E versus temperature for NSSC-fluting medium in the machine direction (MD) and cross direction (CD) and for random sheets of hemicellulose-extracted and delignified NSSC-pulps. In order to separate the curves, different arbitrary constants have been added to the different In E-curves. (Paper VI)

0 100 200 TEMPERATURE °C

33.

This softening of lignin noticed in the NSSC-paper under dry conditions

has been further analysed by measuring the elastic modulus at different

rates of deformation as seen in fig. 16 (Paper IV). This rate is compa-

rable with the role of frequency in dynamic measurements.

Temperature,°C

Fig. 16 Modulus of elasticity versus temperature at different strain rates for a dry fluting of 112 g/m2 in the machine direction. (Paper IV)

Although precise softening temperatures are here difficult to assign,

these have been taken as the temperatures at the maxima in the slopes of

the modulus-temperature curves. These data are also incorporated in the

Arrhenius plot in fig. 12. An arbitrary constant has been added to the

straining rates so that they fit into the same diagram as the data for

wet wood. The apparent activation energy thus calculated equals 456

kJ/mol. This is in reasonable agreement with the value obtained for the

native lignin in wood under wet conditions. It has previously been noticed

that the activation energy for the glass transition decreases when a

material contains water (90).

The lignin softening may be further studied by adding a plasticizer.

This decreases the T and allows the effect of softening to be detected

at a temperature at which it is easier to operate. Here ethylene glycol

has been chosen as a suitable plasticizer, being a rather good solvent

for the wood polymers and having a high boiling point (198°C). The

ethylene glycol has a solubility parameter of 33.4 • 103 (J/m3 )1/2 and

is one of the diluents which come closest to the value of for water

48 • 103 (J/m3)1/2 (91). Both these diluents have high hydrogen bonding

potential. Measurements on an NSSC-paper show that the transition

region is successively lowered by increasing amounts of ethylene glycol,

fig. 17.

34

0 50 100 150 200 250 Temperature, C

Fig. 17 Specific elastic modulus for an NSSC paper containing different amounts of ethylene glycol as a function of temperature.

Increasing the ethylene glycol content further indicates a softening

limit of about 110°C (84). The samples were prepared by absorption of

ethylene glycol vapor at low pressure and 60 C for 30 days. The existence

of residual amounts of water was measured by Karl-Fischer titration.

The elastic modulus was measured for series of samples with different

amounts of ethylene glycol, with a water content of less than 0.5 %,

according to the previously described method of heating in silicone oil.

Although the softening of hemicelluloses and cellulose may also be

affected by the ethylene glycol, the transitions observed for the

plasticized samples have been shown by comparison with corresponding

measurements on delignified NSSC samples to relate almost completely to

the lignin softening (84). Sadoh (92) has recently shown that the

softening temperature for wood immersed in either ethylene glycol or

water occurs at about the same temperature of 80o C at 0.02 Hz, this

being the limiting softening temperature of lignin in these diluents.

35.

In the case of papers made from other high yield pulps such as thermo-

mechanical pulps, no specific softening temperature of lignin could be

detected in the modulus-temperature curve, as seen in fig. 18 where the

effects of delignification and hemicellulose extraction are compared

(Paper VI). Baldwin and Goring (89) in measuring the thermal com-

pressibility of wood also noticed a single softening in a range similar

to that here measured for the samples of TMP. A large increase in the

breaking stretch, observed in papers based on NSSC-pulps as a consequence

of lignin softening, is however also noticed for the lignin-containing

TMP-papers at about the same temperature, indicating that lignin softening

occurs in these papers above 205o C (Paper VI).

The resulting change in modulus due to this lignin softening may be

concealed in these papers by the subsequent softening at 230o C. It thus

seems probable that the sulphonation of the lignin, as in the NSSC-

pulps, may either change the softening behaviour of the dry lignin or

due to a relocation alter its way of interacting in the stress-transferring

mechanisms of these papers and thus make its softening detectable.

Fig. 18

In E versus temperature for sheets of thermomechanical pulp of aspen and for the same pulp hemicellulose-ex tracted , and for sheets of TMP of spruce and for the same pulp delignified. In order to separate the curves, different arbitrary constants have been added to the different In E-curves. (Paper VI)

0 100 200 TEMPERATURE °C

36.

The influence of carbohydrate softening

In all the temperature spectra of the modulus obtained for different

papers no indication of any specific hemicellulose transition has been

discerned. However, de Ruvo and Bredhe noticed a gradual softening

above 180°C for a bleached sulphite fiber (93) which they attributed to

a gradual softening of carbohydrates. For a paper of bleached kraft

pulp, Young also noticed a softening maximum at 160o C which was particularly

apparent for a low basis weight sample (94). Thus, it may be that the

higher slopes noticed in the region of 160o to 180o C for the measurements

on high yield pulps indicate the presence of a small transition. This

softening may be ascribed to a softening of some of the hemicelluloses

as has also been suggested by other authors (95).

The hemicellulose extraction appears also to have little effect on the

main softening noticeable in most papers around 230o C, (Paper VI).

However, as the two hemicellulose extractions done on an NSSC-pulp and a

TMP-pulp still left the samples with a hemicellulose content of 8 and

14 % respectively, the effect may be expected to be small. The fiber

model presented in chapter 3 predicts a rather low sensitivity of the

relative modulus to the amount of hemicellulose as is evident in fig. 8.

The softening at 230o C has nevertheless in many previous observations

(15,16), been attributed to the cellulose.

Due to interactions in this composite system, it is not unlikely that

the transitions may have been broadened, as may for instance occur, with

grafted polymers on cellulose (96). The loss in relative rigidity as a

function of temperature up to 170o C is however unaffected by hemicellulose

or lignin removal, thus indicating that no transitions due to these

materials take place in this temperature range (Paper VI). It is instead

suggested that this rigidity loss reflects the changes in modulus of the

microfibrils. It has been indicated that the rigidity loss measured up

to 170o C depends on the cellulose crystal1inity of the sample (Paper

VI). This supports the model here described of cellulose microfibrils

acting as reinforcements in the cell wall structure and therefore being

responsible for the elastic properties as long as no transitional changes

occur in the other components.

37.

It thus seems likely that in the dry state all the components, namely

lignin, hemicellulose and cellulose, soften in the same temperature

range, so that the softening regions overlap to such a degree that they

cannot be resolved. Only in special cases, as in the cross direction of

veneer, or in paper of high-yield sulphite pulp, where lignin is modi-

fied, can an individual softening temperature be discerned.

5.3 Influence of moisture

In view of the present knowledge of the effect of water as a softener

for carbohydrates, a thorough analysis of the influence of temperature

and moisture on the modulus of a kraft paper has been performed to

elucidate the dependency for paper. The tensile properties have been

measured for a kraft sack paper over a temperature range from -25o C to

+65o C in the humidity range of 0-20 % moisture content by the immersion

technique in silicone oil (Paper VII). The properties are illustrated

by the failure envelopes of fig. 19. It is apparent that the stress-

strain curves of the paper change from a rather brittle type of appear-

ance to a more ductile one as the moisture content increases.

Fig. 19

Failure envelopes for a kraft sack paper in the machine direction at temperatures from -25 to 65o C and moisture contents of 0,, 5, 10, 15 and 20 %. Strain rate = 0.83 % /s. Dry basis weight 105 g/m2 . (Paper VII)

0 1 2 3 4 ELONGATION %

38.

Drying stresses

In the making of paper, the sheet is often subjected to drying stresses,

which give rise to internal stresses in the final paper and thus increase

its modulus (57). The dried-in stresses and the subsequent release of

these upon wetting are often claimed to be responsible for the decrease

in the modulus with increasing humidity. In fig. 20 the effects of

increasing moisture content are compared for two papers whose only

difference is that one is dried completely restrained in drying frames

(57) while the other is dried as free from restraints as possible. The

large difference in modulus, over 3 times for these papers clearly shows

the effects of larger dried-in stresses in the samples dried under re-

straint than in those freely dried (57). A comparison of the relative

effects of moisture on the elasticity shows however no difference

between the two drying modes. Thus the internal stresses built in

during drying in the samples dried under restraint are unaffected by the

plasticization due to water in the region of moisture contents here

studied.

It is here suggested that the dried-in stresses are mainly due to

restraints imposed in the disordered parts of the cellulose. In accor­

dance with the earlier proposed softening relations these disordered

zones will soften at higher moisture contents than those reached in

these measurements. This hypothesis may also be supported by the work

of Htun (57) who noticed that the drying stress of paper, which is pro­

portional to the elastic modulus, increases rapidly during drying in the

range of moisture contents between 55 and 40 % at a drying temperature

of 20o C. Thus the internal stresses in the paper build up during this

stage of the drying process, due to the glass-rubber transition in the

disordered regions.

The influence of moisture on the elastic modulus in the RH-range (0-90 %

RH) is thus probably related to changes in the amorphous material sur-

rounding the fibrils, mainly the hemicelluloses.

39.

Fig. 20 Specific modulus of elasticity and the relative elastic modulus E/E versus moisture content for a kraft paper in MD dried to two different stress levels. E refers to the modulus under dry conditions.

The equivalence of moisture and temperature

The influence of temperature at different moisture contents on the

specific elastic modulus is given in fig. 21 for a kraft sack paper

of 105 g/m2 (jf. fig. 19). This paper has a relative composition of 7 %

lignin, 16 % hemicellulose and 77 % cellulose (Paper V ) . At all moisture

contents, the modulus decreases with increasing temperature but a

significant softening region which shifts towards lower temperatures

with increasing moisture content can be discerned.

In many cases the viscoelastic properties of amorphous polymers show a

time-temperature equivalence. By analogy with this well known time-

temperature equivalence, expressed for instance by the WLF-equation, a

similar time-plasticizer concentration equivalence (97), or a tempera-

ture plasticizer concentration equivalence also exists. Here such a

relation may be applied to the measurements on the kraft paper. Ob-

viously paper does not represent the type of homogeneous amorphous polymer

to which the classical superposition principles have been applied. The

use of time-temperature and time-plasticizer concentration superposition

on semi-crystal line systems has been questioned and it has only been

applied to a limited number of such polymers (1,98-100). On cellulosic

systems, the principles have been applied to the g-transition at -70 C (101)

and to the main transition under wet conditions measured by Htun (57).

Fig. 21 Specific modulus of elasticity for a kraft sack paper vs. temperature at different moisture contents (% of moist paper). (Taper VII)

To account for the swelling effects on the elastic properties, a vertical

shift is usually applied in the type of diagram shown in fig. 21. Most

simply this is accounted for by the assumption that the modulus is pro-

portional to the polymer content per unit volume of the system, giving

the relation

[11]

where v is the volume fraction of the polymer and v0 the reference volume

fraction of polymer (102). Here this correction is already accounted for

as the modulus is given in terms of the specific elastic modulus, related

to the density at a reference condition of 20°C, 9 % moisture content.

In amorphous polymers, temperature corrections are made using the expression

[12]

40.

4 1 .

where r is the density and T the temperature in K (97). This correction

factor is derived from the kinetic theory of rubber elasticity. Its

application to semicrystal1ine polymers can be questioned and it is used

rather arbitrarily in such systems (98,99). Here the density factor is

already accounted for in the data of fig. 21 as discussed above, while a

temperature correction has been applied with respect to a reference

temperature taken as 200 C. This correction gives an improved agreement

between the shifted curves of the kraft paper.

In fig. 22 the individual moduli curves given in fig. 21 for the kraft

paper at different moisture contents have been corrected to 20°C and

have then been shifted horizontally along the temperature axis to form a

master curve, the curve at 9.0 % moisture content being taken as the

reference curve.

It can be seen that the curves fit fairly smoothly into the constructed

master curve in the region of high temperature, i.e. above the transit­

ion. Below the transition, superposition does not apply.

-50 0 50 100 150

Temperature °C

Fig. 22 Master curve of the reduced elastic modulus for a kraft sack paper. The master curve is constructed with a reference of 9.0 % moisture content.

kl.

Fig. 23 The shift term rT corresponding to the temperature shift made in fig. 22 as a function of moisture content for each shifted curve -solid tine. The broken line is calculated from a free volume theory.

In fig. 23 the shift term rT used for the construction of the master

curve is given as a function of the corresponding moisture content.

Evidently there is a deviation from the general trend below 6.5 %

moisture content indicating that a different mechanism here applies.

This shift term may be compared with the shift predicted from the in-

crease in free volume with the increasing amount of plasticizer by

either the Kelley and Bueche equation or the Kaelble equation [1] as

described in chapter 2. Thus, using the value 2.82 for raD/rap as

derived earlier and a crystallinity of 55 % for this kraft sack paper,

estimated from the respective areas of the amorphous and crystalline

regions in X-ray diffractograms, this shift rT has been calculated with

reference to 9 % moisture content based on the total mass. The calcu-

lated shift agrees closely with the experimental shift as seen in fig.

23, indicating that the softening is due to a second order transition

behaviour. Below 6.5 % moisture content, the measured and calculated

shifts do not agree due to the fact that these measurements are too far

below the transition here studied. Riemen and Kurath also noticed a

softening maximum for a paper of bleached sulphite pulp at about 10 %

moisture content at room temperature which they attributed to the plas-

ticizing effect of water (103).

The transition here observed may, as previously discussed, be due to the

plasticizing effect of water on the softening of the matrix material

surrounding the microfibrils, i.e. mainly the hemicelluloses.

43.

Effects of crystallinity

The measurements indicate that the change in elastic properties of paper

with changing moisture content is determined by a softening of the

carbohydrate material making up the matrix material around the micro-

fibrils. The effect on papers of various composition is illustrated in

fig. 24 for a cotton 1 inter, a kraft sack paper and an NSSC-fluting

(Paper VII).

Fig. 24

Relative moduli E/E0 for a paper of cotton linters, a kraft paper and an NSSC fluting medium vs. moisture content as a percentage of moist paper. E0 represents the corresponding dry modulus. Temperature: 20°C. (Paper VII)

The differences between these papers are due mainly to the differences

in the crystallinity of the carbohydrates i.e. of the amount of material

that does not absorb water. As water does not enter the crystalline

cellulose I structure (50) these measurements may instead be given as a

function of the moisture content calculated with respect to amorphous

carbohydrates only. The crystallinities of the carbohydrates for these

papers have been estimated from X-ray diffractograms to be 68 % for the

cotton linters, 55 % for the kraft sack and 49 % for the NSSC-fluting,

based on the areas of the crystalline and amorphous regions. The degree

of crystallinity for these cellulosic samples was calculated according

to the concept of Jayme and Knolle (104) as the integrated intensity of

the crystalline peaks divided by the integrated intensity for both the

crystalline and amorphous regions. The intensity of the radiation

diffracted by amorphous hemicellulose has its maximum at the same angle

as does that of the radiation diffracted by the amorphous parts of the

cellulose (104). Thus the crystallinity values calculated are based not

only on the cellulose but also on the hemicelluloses present. The

sorptive capacity of lignin has for various preparations been found to

Salmen-4 be from 30 % to above 50 % of the sorptive capacity of hemicelluloses

(29,48,105,106). For the calculations it is here assumed that the lignin

present absorbs half as much water per unit weight as the amorphous

carbohydrates. With these assumptions the moisture contents in the

different papers are converted to relate only to the amorphous carbo-

hydrates.

As seen in fig. 25, there is fairly good agreement between these samples

considering the uncertainties in the crystal linity values of the carbo-

hydrates and how they affect the morphological structure of the fibers,

and also in the effects of the various amounts of lignin present. In

the amorphous material, the moisture is probably absorbed to the same

extent in the different fibers at a given RH. On a relative humidity

scale the softening will then occur at the same relative humidity, since

the difference on the normal moisture content scale is only an apparent

difference due to the fact that the papers contain different amounts of

material that absorb water.

Fig. 25

Relative rigidity E/E0 as a function of moisture content in grams of water per grams of water-containing amorphous carbohydrates. Temperature: 20 C.

The softening behaviour depicted above indicates that the relative

amount of amorphous carbohydrates has only a minor effect on the manner

in which the relative rigidity changes with changing moisture content.

Similar findings were also obtained for the fiber properties as shown

for the relative torsional rigidity in fig. 8.

44.

45.

6. ADDITIONAL REMARKS

The analysis here made of the different softenings observed in materials

based on wood fibers, e.g. wood, wood pulp fibers and paper has shown

that these softenings can be related to the softening points of the

individual wood polymers, i.e. cellulose, hemicelluloses and lignin.

In the case of wood and wood pulp fibers the degree of softening can be

fairly well predicted from structural considerations. In the case of

paper, the degree of softening depends both on the properties of the

individual fibers and on the interaction between them, i.e. the complex

structure of paper must be taken into account.

Paper may in many instances be considered as a network of its constituent

fibers. It is not, however, easy to visualize the relationship between

the fiber and the complex structure of paper. Theoretical estimates of

this relationship were initiated by Cox (107), who considered paper as a

two-dimensional network of randomly oriented, infinitely long, straight,

linearly elastic fibers. Cox obtained a modulus for this paper of one

third of the fiber modulus. To take into account the structure of the

network, a correction factor has been introduced which differs somewhat

between the theory of Cox and a number of similar theories that have

subsequently been presented (108).

Recently Page et al. (109) have been able to show experimentally that

for a well-bonded sheet made of straight fibers the modulus is indeed

given by the Cox theory, i.e. 1/3 of the longitudinal fiber modulus.

For such papers the theory predicts that the relation between the paper

and its constituent fiber should be constant irrespective of the surround-

ing environment. In the studies in chapter 5.3 of the influence of

moisture content (fig. 24), however, the papers studied showed a larger

decrease in the relative elastic modulus than that expected from a

measurement of the decrease in longitudinal fiber modulus. The calcu-

lations given in chapter 3 for the fiber modulus show that the relative

fiber modulus decreases by only about 10 to 20 % of its value with a

change in moisture content in the RH range of 0 to 95 % RH, fig. 6.

Measurements by Kersavage (81) on single fibers has shown a similar

decrease in the relative longitudinal modulus in this range of relative

46.

humidities. Thus the actual changes of about 60 % measured for papers

do not correspond with the results predicted from single fibers using

the simple Cox theory. However, for the less well bonded sheets in the

normal density range, a more realistic theory should also account for

shear stresses as well as properties in the transverse fiber direction.

In their equation for the paper modulus, Page et al. have taken the

shear modulus into account (109). More sophisticated theories for low

density papers have recently been developed (110) which may here be more

applicable. There is, nevertheless, still a lack of experimental data

relating the moisture dependence of the paper to that of the fiber.

47.

7. ACKNOWLEDGEMENTS

I wish to express my sincere gratitude to Dr Alf de Ruvo, Head of the

Paper Technology Department at the STFI for continuous support, inspi-

ration and guidance throughout the preparation of this thesis.

! am indebted to Professor Ernst Back, co-author of four of the papers,

for initiating this work and for his support and constructive criticism

throughout the work. I also wish to express my gratitude to Mr Petter

Kolseth, Dr Mikael Rigdal, Dr Lennart Westman, Dr Wyn Brown of Uppsala

University and Dr Christer Fellers for their constructive criticism and

fruitful discussions during the preparation of this thesis. Thanks are

also due to Professor Bo Norman, Dr Leif Carlsson and Dr Myat Htun for

their valuable criticism and friendly support, as well as to Dr Richard

Mark, ESPRI, Syracuse, USA, for valuable comments on the laminate cell

wall model given in Paper II.

Special thanks are expressed to Mr Sune Karlsson, Mr Jan-Erik Wiken and

Ms Senada Angelova for their skilful experimental assistance, and to

Mr Stig Almgren, Mr Christer Brostam, Mr Sune Holm and Mr Arne Johansson

for their skilful electrical and mechanical design of instruments used.

! also wish to extend my thanks to Ms Christina Benckert and Ms Inger

Lindegren for their patient typing of the manuscript, to Ms Gunilla de

Ruvo for drawing the figures and to Mr Anthony Bristow for the linguistic

revision of the manuscript.

Financial support from "Stiftelsen Cellulosa- och Pappersforskning" is

gratefully acknowledged.

Finally, I wish to express my appreciation and thanks to my wife Kristina

and my son Christofer for their patience and understanding during the

preparation of this thesis.

48.

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95. Htun, M. and de Ruvo, A. Cellulose Chem. Technol. (in press).

96. Ehrnrooth, E., Kolseth, P. and de Ruvo, A. In "Fibre-Water Interactions in Paper-making". Technical Division, B.P. & B.I.F. (1978), 715.

97. Halpin, J.

In "Composite Materials Workshop", Ed. Tsai, S.W., Halpin, J.C. and Pagano, N.J., Technomic Publishing Co., Inc., Stamford, Conn. 1967.

98. Onogi, S., Sasaguri, K., Adachi, T. and Ogihara, S. J. Polym. Sci. 58(1962),1.

99. Kunugi, T., Isobe, Y., Kimura, K., Asanuma, Y. and Hashimoto, M. J. Polym. Sci. 24(1979),923.

100. Quistwater, J.M.R. and Dunel1, B.A. J. Appl. Polym. Sci. 1(1959), no. 3:267.

101. Haughton, P.M. and Sellen, D.B. J. Phys. D:Appl. Phys. 6(1973),1998.

102. Janacek, J. and Ferry, J.D. J. Polym. Sci. A2 7(1969),1681.

103. Riemen, W.P. and Kurath, S.F. Tappi 47(1964), no. 10:629.

104. Jayme, G. and Knolle, H.

Das Papier 18(1964), no. 6:249.

105. Cousins, W.J. Wood Sci. Techn. 10(1976),9.

106. Christensen, G.N. and Kelsey, K.E. Autralian J. Appl. Sci. 9(1958), no. 3:265.

107. Cox, H.L. Brit. J. Appl. Phys. 3(1952), no. 3:72.

108. Algar, W.H. In "Consolidation of the Paper Web" Ed. Bolam, F. Techn. Sec. BP & BMA 1966, 814.

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Paper I

Keywords Cellulose Glass transition temperature Moisture Plasticizing Thermal properties

Abstract The approach of Kaelbe for calculating the effect of plasticizers on the glass transition temperature of the system polymer- plasticizer is discussed and com-pared with reported experimental data. By using these equations of Kaelbe, one can calculate the softening effect of water on cellulose. The results are shown to be in reasonable agreement with available data for the glass transition tempera-ture of wet and plasticized cellulose. The glass transition temperature for cel-lulose containing variable amounts of water is calculated for cellulose material of different degree of crystallinity. Also the softening effect of water in cellulose systems containing hemicellulose and lignin is discussed.

The influence of water on the glass transition temperature of cellulose

N. L. Salmen and E. L. Back

The glass transition temperature is the temperature at which an amorphous polymer changes from a hard glassy form into a rubber-like plastic form or to a viscous fluid. At this glass transition, which is a secondary transition, the temperature derivatives of both physi­cal and mechanical properties of the polymer change. The glass transition temperature is related to the onset of a certain degree of movement in the main chain and is explained by many theories as being related to the free volume of the polymer, i.e., the volume not occupied by molecules.

It is assumed that above the glass transition temperature the free volume is so large that a significant chain mo-tion is possible and exists, for example, a rotation of segments. From this it is apparent that a polymer, especially one with side groups, can exhibit several secondary transitions, as for example when the free volume becomes suffi-cient for the rotation of a single side group. The secondary transitions of cel-lulose are estimated to be at 220°C for main chain movement, that is, the true

N. L. Salmen and E. L. Back, The Swedish Forest Products Research Laboratory, Box 5604, S-11486, Stockholm, Sweden.

glass transition (1-5), and at 0°C for movement in the gluco-pyranose ring (6, 7).

EFFECT OF PLASTICIZER

Plasticizers significantly affect the properties of a polymer and thus also the glass transition temperature. These effects are of great interest in the plas-tics industry and also in papermaking processes. They also influence the mois-ture dependence of paper properties. Several theories have been proposed to describe and predict the reduction in the glass transition temperature for a polymer with added diluent, i.e., plas-ticizer. The most well-known ones are that of Kelley and Bueche (8), using the temperature dependence of the frac-tional free volume and the statistical mechanical theory of Gibbs and Di Mar-zio (9).

A recent relation based on the lattice theory has been introduced by Kaelbe (10). Here the glass transition tempera-ture is related to the cohesive energy of the molecules, which is one of the fac-tors which determines the glass transi-tion temperature according to Boyer (11). On the other hand, Kaelbe's equa-tion does not consider the stiffness of the

polymer chains, which is also important for the glass transition temperature. Kaelbe's approach has an important advantage. It makes use of some well-known characteristics for polymers which facilitates its application:

The molar cohesive energy U can be considered to be independent of temper-ature and can be calculated from the cohesive energy density d2 and the molar volume v according to:

(2)

If the cohesive energy density, d2, is not known, the cohesive energy U can be estimated according to Hayes' empir-

Reprinted from Tappi, The Journal of the Technical Association of the Pulp and Paper Industry, Vol. 60, No. 12, December 1977. Copyright, 1977 by Tappi and reprinted by permission of the copyright owner

Tg =

where

= molar cohesive energy at the glass transition temperature Tg

= difference between specific heats at constant volume above and below Tg

= lattice coordination number for the rubber state = lattice coordination number for the glass state

Vg

Z L

Z G

h

ical relation (12) where the contribution of the structural groups to the cohesive energy are summed.

According to Kaelbe, if it is assumed that Hayes' summation of the molar cohesive energies is valid for different molecules in a mixture, Eq. 1 can be rearranged to:

MOLE FRACTION OF PLASTICIZER

Fig. 1. The glass transition temperature for two polymer- plasticizer systems. The lines are calculated according to Kaelbe's ap­proach. Symbols give experimental values according to Jenckel and Heusch {13), Kelley and Bueche (8), Adachi et al. (14), and Woodward et al. (15).

XD = mole fraction for plasticizer

hD - constant in Eq. 1 for plasticizer

hp = constant in Eq. 1 for polymer

Equation 3 is mathematically similar to the volume-related equation pro-posed by Kelley and Bueche. The equa-tion proposed by Kelley and Bueche should, therefore, have the same valid-ity as that of Kaelbe which is used here, except that the latter contains constants which are more easily acces-sible.

Here, Eq. 3 has been applied to some systems reported in the literature, using data listed in Table I.

Figure 1 shows glass transition tem-peratures for a number of plasticized systems. The lines are those derived ac-cording to the Kaelbe approach. The figure includes data for systems of (a) polystyrene plasticized with ethyl-acetate given by Jenckel and Heusch (13), (b) polymethylmethacrylate, PMMA, plasticized with diethylphtha-late given by Kelley and Bueche (8), and (c) polyvinylacetate, PVAc, plasticized with toluene given by Adachi etal. (14).

For all these plasticized amorphous polymers, Kaelbe's approach shows good agreement with the experimental data.

A glass transition temperature is somewhat dependent on the time scale of the experiment as well as on the method used. Consequently, the glass transition of the pure components has been used here as reported in the sys-tems described, without any attempt to correct for this effect.

Figure 1 also includes data for a par-tially crystalline polyhexamethylene adipamide, nylon 6-6, plasticized with water, as reported by Woodward (75). For the present calculations the glass transition temperature of water has been set to 136°K according to Rasmus-sen and MacKenzie (16). The crystallin-ity of nylon 6-6 has been calculated to be

Table I. Data fo r the Calculation of the Properties of Plasticized Systems According to Kaelbe's Approach

Material

Water Ethylacetate Toluene Diethylphthalate Triethylphenyl-

ammonium hydroxide

PMMA Polystyrene PVAc Nylon 6-6 Cellulose

Transi-tion

tempera-ture

(Tg), K

1 3 6 ( 1 6 ) 1 1 8 ( 2 8 ) 1 3 2 ( 7 4 ) 1 8 2 ( 2 9 )

1 2 0 3 7 7 ( 8 ) 3 5 9 ( 1 3 ) 3 1 2 ( 7 4 ) 3 7 0 ( 7 5 ) 4 9 3 ( 7 ) 2 7 3 ( 6 )

Cohesive energy dens i ty

(d2), kJ/m3 . 10-3

2254 ( 7 0 ) 347 ( 7 0 ) 3 3 2 ( 7 0 ) 4 2 3 ( 7 0 )

5 3 5 ( 2 7 ) 4 0 2 ( 2 7 ) 5 3 5 (27) 7 7 4 ( 1 0 )

Molar vo lume (v),

m3 /mole • 106

1 8 ( 7 0 ) 9 9 ( 7 0 )

1 0 7 ( 7 0 ) 1 9 7 ( 7 0 )

8 5 9 9 7 3

2 0 9

F r o m Eq. 1 (h),

J/mole° K • 1 0 - ' 2

2.98 2 .91 3 .12 4 .58

. 1 . 7 2 1.28 1.11 1.38 4 .70 6 .3 3

1 0 . 8 5

43% using the actual density and the densitites of a completely amorphous and a completely crystalline nylon 6-6 given by Miller (17). The plasticizing effect must be calculated only on the amorphous part of the polymer. As is evident in Fig. 1, the Eq. 3 also de-scribes this system sufficiently well.

The application of Kaelbe's approach makes use of some assumptions that are questionable or at least not established for very polar molecules. One is that the cohesive energies are also measures of the interaction of polar molecules where specific interactions, such as hydrogen bonding, are possible and where polar interactions can depend on steric fac-tors. It is further assumed that the molar cohesive energy, U, is indepen-dent of temperature and can be calcu-lated for polar molecules. Despite these shortcomings, the comparison in Fig. 1 indicates that Eq. 3 can be useful to estimate or predict the glass transition temperatures of plasticized polymer systems.

WATER AS PLASTICIZER

The plasticizing effect of water on cel-lulosic material is of great importance. Water is an effective softening agent for cellulose and is present in almost every process where cellulose and paper are produced, converted, and finished. This softening is of special interest, for example, for the press-drying of hard fiber building board, where it gives the stiff fiber bundles sufficient flexibility to create a large bonding area. Another example is the corrugating of fluting, where water makes the paper moldable around the roll profile in the corrugator nip.

The effect of water on the glass transi-tion temperature of cellulose has, how-ever, been studied very little. Con-sequently, this effect has been calcu-lated here using Eq. 3 and data in Table I, the glass transition temperature of pure amorphous water and cellulose being taken as 136°K and 493°K respec-tively, as reported in the literature.

The transition in cellulose at 273°K, attributed to pyranose ring movement, has been shown by Kaimins and Ioelo-vich (6) to be affected by water in the same manner as a glass transition. This transition can be assumed to depend on the intermolecular forces of the cellulose chain, i.e., the molar cohesive energy, U, of cellulose. Using the data of Kaimins (6), the molar cohesive energy of cellulose thus can be calculated according to Eqs. 1 and 3 to be 296 kJ/mole. Naturally, the molecular rearrangement in the lat-tice is different at the two transitions of cellulose at 273°K and 493°K but, know-ing the molar cohesive energy of cel-lulose, the constant hp for cellulose at the glass transition at 493°K can be cal-culated using Eq. 1 to be 633 J/mole°K. So far this seems to be the most easily

Vol. 60, No. 12 December 1977 I Tappi

where T g = glass transit ion temperature for

p polymer

138

accessible way of calculating the cohe-sive energy of cellulose. Using this value of the constant hp for cellulose at 493°K, results of Kaelbe's approach can now be compared with the experimental data of Kargin et al. (1) for the plasticized system cellulose-triethylphenylammo-nium hydroxide. The glass transition temperature for triethylphenylammo-nium hydroxide was estimated to be 120°K by extrapolating both the molar and the weight fraction data of the sys-tem mentioned above. Its molar cohesive energy was calculated to be 141 kj/mole according to Hayes (12). From Fig. 2 it appears that Eq. 3 is in good agree-ment with the experimental data.

Using the previously mentioned val-ues of the cohesive energies and glass transition temperatures, the effect of water on the glass transition tempera ture for amorphous cellulose is also cal-culated according to Eq. 3 and included in Fig. 2. Here the calculation is given with the water content as the molar fraction of the amorphous portion only of the cellulose, as it is in this amorph-ous portion that the water is absorbed. Also, it is assumed that the crystalline regions and their arrangement, such as in micelles distributed in the amorph-ous phase, have no effect on the glass transition temperature of the amorph-ous phase.

For a given moisture content in the amorphous portion, the moisture con-tent of the total material will naturally depend on the degree of crystallinity of the specific cellulose material con-cerned. The crystallinity can, for exam-

ple, vary for different cellulose fiber materials from 50 to 85%, and for vis-cose cellulose from 25 to 40%, according to Casey (18). In Fig. 3, the data from Fig. 2 are recalculated and presented for cellulose of 35, 55 and 80% crystallinity. In this way, the softening effect of water on cellulose is quantified. In this and the following calculations the water content is given as a percentage of the total weight, i.e., as g of water per g of cellulose plus water. One objection to these calculations is that the first 1% of water is known to be bonded strongly to the cellulose, i.e., it is packed very closely. This could have a minor effect on accuracy of the calculations.

Some measurements have been pub-lished by Naimark et al. for the glass tran-sition temperatures of cellulose materials saturated with water (19), which to some degree verify the relation. The glass transition temperature in systems saturated with water is stated to be about -25°C for viscose and about -45°C for cotton cellulose. The water content, excluding capillary water can be estimated according to Stamm to be between 27.5 and 29.5% in saturated viscose and about 14.5% in saturated cotton cellulose (20). These data have been plotted in Fig. 3, where they fit fairly well with the calculated lines.

The large plasticizing effect of water is also indicated by stiffness measure-ments on fibers by Bryant and Walter (21), who estimated the glass transition temperature for both saturated cotton and saturated viscose to be below 0°C.

NMR data by Ogiwara et al. for the glass transition temperatures of various cellulose materials softened by water have also been published (22). The cellu-

lose is treated in different ways to give different degrees of crystallinity. Figure 4 gives published data for cotton and for dissolving pulp of different crystallini-ties. It is evident that the experimental curves are very similar to those calcu-lated according to Kaelbe's approach.

EFFECT ON PAPER Naturally, cellulosic material is often

present in a fibrous structure in an in-homogeneous mixture together with hemicellulose and lignin. When exist-ing in a homogeneous mixture, cellulose and hemicellulose chains can, because of their similar chemical structures, form a system with a joint glass transi-tion temperature, somewhere between that of the two components, both in the dry state and with respect to the effect of water. The transition in the pyranose ring for holocellulose has been shown by Kaimins and Ioelovich to occur between the transition temperatures for xylan and cellulose (7). Similarities with amorphous cellulose are also noticed in the effect of water on the glass transi-tions of various modified hemicelluloses reported by Goring (3) and Takamura (23).

Native lignin is considerably more hydrophobic than cellulose. In fibers it exists partly separated from the hy-drophilic cellulosic components. In some pulping processes, the lignin becomes more hydrophilic by chemical reaction. Therefore, its glass transition tempera-ture could be thought to be lowered by water, as has been shown by Goring to be the case for periodate lignin and dioxane lignin (3). The lignin in the structure will interact with the cel-lulosic compound and make the soften-ing of cellulose in the fiber or in the wood material less distinct. For exam-

MOLE FRACTION OF PLASTICIZER

Fig. 2. The glass transition temperature for amorphous cellulose with two plasticizers. The lines are calculated according to Kaelbe's approach. Symbols give experimental values according to Kargin er a/. (1).

Tappi I December 1977 Vol. 60, No. 12

Fig. 3. The glass transition temperature for the cellulose-water system with cellulose of different degrees of crystallinity. The lines are calculated according to Kaelbe's ap-proach. Water content is given as percent-age of the total weight. The lower line crossing the Tg lines of different crystal-Unities indicates completely swollen cellu-lose where Tg lines end.

WATER CONTENT IN CELLULOSE, %

Fig. 4. The glass transition temperature for the cellulose-water system with cellulose of different degrees of crystallinity. Symbols give experimental values for cellulose treated to give different degrees of crystallinity according to Ogiwaraet al. (22). The broken lines are calculated theoretically according to Kaelbe as in Fig. 3.

139

ple, the softening temperature of wood saturated with water has been reported by Becker and Noack (24) and Hoglund et al. (25) to be between 80°C and 90°C in agreement with the glass transition temperatures for saturated modified lignins measured by Goring (3) and Takamura (23).

For many paper materials, the prop-erties in question are expected to be re-lated mainly to the cellulosic compo-nents. The plasticizing effect of water has been measured on printing paper at 30°C by Tsuge and Wada (26) in studies of dielectric dispersion and creep char-acteristics, showing the glass transition to occur at a moisture content of 3%. For cellophane with its lower crystallinity the transition was found to occur at a moisture content of 6% at 35°C. Since these transitions were defined as the beginning of an anomaly, they may in-dicate somewhat too low a moisture content.

The crystallinity, as well as the mor-phological structure of fibers consisting of fibrils with unevenly distributed chemical components, can influence the resulting softening at the glass transi-tion. This means that the softening can be more difficult to evaluate by mechan-ical methods. Perhaps this crystallinity of cellulose could explain the difficulties experienced by Goring (3) and Takam-ura (23), using the same method, in

measuring the plasticizing effect of water on cellulose samples.

With these restrictions in mind it can be concluded that at the present time Eq. 3 of Kaelbe and Fig. 3 can serve to summarize the softening effect of water on cellulosic materials, excluding wood, and can serve to stimulate further ex-perimental work.

LITERATURE CITED 1. Kargin, V. A., Kozlov, P. V., and Wang,

Nai-Ch'ang., Doklady Akad. Nauk. SSSR 130(2): 356 (1960).

2. Alftan, E., de Ruvo, A., and Brown, W., Polymer 14(7): 329 (1973).

3. Goring, D. A. L.,Pulp Paper Mag. Can. 64(12): T-517 (1963).

4. Back, E. L. and Didriksson, E. I. E., Svensk Papperstid. 72(27): 687 (1969).

5. Naimark, N. I., and Fomenko, B. A., Vysokomol. Soyed. B 13(1): 45 (1971).

6. Kaimins, I. F., and Ioelovich, M. Ya., Vysokomol. Soyed. B 15(70): 764 (1973).

7. Kaimins, I. F., and Ioelovich, M. Ya., Khim.Drev.,(2): 10(1974).

8. Kelley, F. N., and Bueche, F., J. Poly-mer Sei. 50(154): 549 (1961).

9. DiMarzio, E. A., and Gibbs, J. H., J. Polymer Sci., Part A (4): 1417 (1963).

10. Kaelbe, D. H„ "Physical Chemistry of Adhesion," Wiley-Interscience, New York, 1971.

11. Boyer, R. F., Rubber Chem. Technol 36(5): 1303 (1963).

12. Hayes, R. A., J. Appl. Poly. Sci. 5(15): 318(1961).

13. Jenckel, E., and Heusch, R., Kolloid Zeitschrift 130 (2): 89 (1953).

14. Adachi, K., Hattori, M., and Ishida, Y., J. Polymer Sci. Polym. Phys. Ed. 15(4): 693 (1977).

15. Woodward, A. E., Crissman, J. M., and Sauer, J. A., J. Polymer Sci. 44(143): 23 (1960).

16. Rasmussen, D. H., and MacKenzie, A. P., J. Phys. Chem. 75(7): 967 (1971).

17. Miller, R. L., In "Polymer Handbook," Interscience, New York, 1966, Section HI, pp. 1-60.

18. Casey, J. P., "Pulp and Paper," 2nd ed., Interscience, New York, 1960, Vol 1, p. 19.

19. Naimark, N. I., Fomenko, B. A., and Ingnateva, E. V., Vysokomol. Soyed. B 17(5): 355 (1975).

20. Stamm, A. J., "Wood and Cellulose Sci-ence," Ronald Press Co., New York, 1965.

21. Bryant, G. M., and Walter, A. T., Text. Res. J. 29(3): 211 (1959).

22. Ogiwara, Y., Kubota, H., Hayashi, S., and Mitomo, N., J. Appl. Polymer Sci. 14(2): 303 (1970).

23. Takamura, N., J. Japan Wood Res.Soc. 14(4): 75 (1968).

24. Becker, H., and Noack, D., Wood Sci. Technol 2(1): 213 (1968).

25. Hoglund, H., Sohlin, U., and Tistad, G, Tappi 59 (6): 144 (1976).

26. Tsuge, K. and Wada, Y., J. Phys. Soc. Japan 17 (1): 156 (1962).

27. Hansen, C. M., Ph. D. thesis, Danmarks Tekniska Hogskola, Kopenhamm, 1967.

28. Lesikar, A. V., J. Phvs. Chem. 80(9): 1005 (1976).

29. Johari, G. P., and Goldstein, M., J. Chem Phys. 55(9): 4245 (1971).

Received for review April 11, 1977. Accepted Aug. 3, 1977.

Correction:

Reference 10 as well as the name of this author appearing in the article should read "Kaelble, D.H.". The authors apologize for these typographical errors.

Paper II

1.

THE CELL WALL AS A COMPOSITE STRUCTURE

By Lennart Salmen

To be published in "Paper Structure and Properties"

Ed. J. Anthony Bristow, Marcel Dekker Inc., New York

Introduction

In a composite material the structural arrangement of the components has

a strong influence on the mechanical properties. Commercial reinforced

plastics, for instance, are designed to utilize co-operative effects of

the components in order to make a product which is superior to each of

the constituent materials. In the case of wood, nature has evolved a

construction for the composite cell wall of a wood fibre not only to

meet mechanical demands but also to promote water transport etc. The

layered structure of the cell wall contains fiber layers at different

angles and intermediate layers of lignin and this provides resistance to

both tensile and compression forces in both the fiber direction and the

perpendicular direction.

To predict the properties of a composite it is essential to arrive at an

understanding of the structure of the material and also of the mechanical

properties of the individual components under different conditions. In

view of the complexity of the fiber system, a high degree of simplifi-

cation is necessary. Previously the orthotropic elasticity theory of

composite materials has been used by e.g. Mark (l), Cave (2) and Schniewind

(3) to show the effects of crystal 1 inity, microfibrillar orientation and

mechanical properties of the matrix polymers. They have all regarded

the cellulose microfibril as a continuous reinforcing element. However,

irregular zones are known to exist along the length of the microfibril

{U) and the consequence of this for the mechanical properties of paper

has also been discussed (5, 6). In this study the mechanical consequences

of a softening of these zones are demonstrated. This softening may be

anticipated to occur under wet conditions and the system is then treated

2.

as a composite with d i s c o n t i n u o u s reinforcing elements. The difference

between the dry and the wet state is then a shift from continuous to

discontinuous reinforcing elements. With changes in the surrounding

temperature and humidity, the matrix wood polymers change from a glassy

to a rubbery state and this also influences the mechanical properties of

the fiber. The development of analytical theories for laminated compo-

site materials has provided tools which facilitate a rational analysis

of the mechanical properties of wood fibers. Here an attempt is made to

extend the model put forward by Mark (l), especially with regard to the

calculation of the effect of the softening of the wood polymers when the

consequences of different modes of reinforcement are incorporated.

Cel1 wal1 structure

The cell wall of tracheids has a structure that resembles man-made

fiber-reinforced composites. The basic reinforcing element is the

cellulose microfibril which is surrounded by a stress-transferring

matrix of amorphous wood polymers. The cell wall consists of several

layers, which are mainly distinguished by differences in the orientation

of the cellulose microfibrils (7), Fig. 1. The outermost of these

layers is denoted the primary wall. This primary wall, with a thickness

of 0.06 um, consists of a loose aggregation of fibrils randomly arranged

on the outer surface and oriented more or less transversely to the fiber

axis on the inner surface. Next comes the secondary wall with its S1,

S2 and S3 layers with parallel fibrils arranged at specific angles in

the different layers. The S1 layer has a crossed fibrillar texture at a

large angle to the fiber axis and a thickness of 0.1-0.2 ym. In the S2

layer, with a thickness of 1-5 ym, the fibrils are wound around the axis

in a steep helix at an angle usually between 10 and 30 degrees to the

fiber axis. In the S3 layer, the fibrils are arranged parallel to each

other, again at a large angle to the fiber axis. This layer has a

thickness of 0.1 um. Of these layers, the S2 is by far the most domi-

nant, making up about 70 to 80 % of the cell wall, so that the mechanical

properties of the fiber are largely influenced by the properties of this

layer.

cell wall layers

The fibrils in each layer are mainly regarded as being built up of

microfibrils with a diameter of 20-45 Å (8) consisting of crystalline

cellulose with the polymer chain parallel to the axis of the micro-

fibril. Several authors favour the idea that the microfibrils are

disturbed from their parallel order in certain regions and many models

of crystalline microfibrils with order defects have been proposed.

Fig. 2 shows the concepts of Hess et al (9) in which the irregularities

in the structure occur at regular intervals along the length of the

microfibril. In reality these disordered zones may occur more randomly.

The irregular zones have been reported to occur at intervals of 100 to

800 Å (10). Stockmann has suggested that these disordered regions in

wood consist only of crystal defects which in the pulping and bleaching

processes are converted to more or less amorphous regions (10). The

extent of these zones increases with increasing severity of the pulping

process (5, 10, 11). The disordered zones between the crystallites are

not purely amorphous but have a less ordered structure than the pure

crystallites. It has been demonstrated that these zones are accessible

to water (12) and it is therefore reasonable to assume that they can be

plasticized by water and increasing temperature.

4.

microfibrillar structure

The cellulose crystal is anisotropic in the plane transverse to the

chain axis but any occurrence of a preferential orientation is here

disregarded. Electron micrographs of the microfibril arrangements in

plant cell walls show that the microfibrils are not strictly arranged

solely in the plane of the cell wall. However, the frequency of devi-

ation from the plane is small and probably unimportant (2).

The structural arrangement of the amorphous wood polymers, hemicellulose

and lignin, is still the subject of debate. At least a portion of the

hemicellulose appears to be associated with the cellulose (13, 14).

There are indications that the hemicelluloses show some degree of pre-

ferred orientation associated with the cellulose alignment (15, 16)

so that the hemicellulose may not behave as an isotropic material.

Lignin is a crosslinked polymer based on the phenylpropanoid unit. It is

probably isotropic in nature (17). Stone et al (18) suggested that the

major part of the lignin is arranged in tangentially concentric layers

in the cell wall, a conclusion derived from a selective delignificat ion

of the fiber which only changed the dimensions of the wall thickness

without shrinkage of the whole fiber. On the other hand, Kerr and

Goring (13), on the basis of electron microscope studies, tend instead

5.

to favour an interrupted lamella structure where the dimension of a

given lignin or carbohydrate entity is greater in the tangential direc-

tion of the fiber wall than in the radial direction.

Softening behaviour of wood polymers

The properties of the amorphous wood polymers are dependent on the

environmental conditions. The polymers change from glassy to rubbery

materials with increase in the surrounding temperature and humidity. It

is well established that lignin and hemicellulose are essentially thermo-

plastic polymers (19, 20), and that isolated lignin and hemicellulose

under dry conditions become rubbery in the temperature interval between

180-220 C. In water, the transition temperature for native lignin is

lowered to 80-90 C whereas hemicellulose is softened at room temperature.

When the cellulose is immersed in water, as in the case of pulp, the

water is assumed to penetrate the irregular zones of the cellulose

microfibrils which then soften and drastically reduce the elastic modulus.

This view is supported by the large decrease in modulus observed for

cellulosic samples immersed in water as encountered by Htun (21) in

drying experiments on paper and by Westman (22) for creep properties of

cellulosic gels.

Micromechanical cell wall model

Wood consists of many different cell types. However, in softwood, the

tracheids are by far the dominant cells making up nearly 98 per cent by

weight of the wood (7). Thus a model restricted to the properties of

tracheids could be expected to describe softwood or fibers from softwood

adequately.

The laminate model of the tracheid wall here adopted depicts each cell

wall layer as consisting of cellulose microfibrils embedded at a certain

angle in a matrix of amorphous hemicellulose. Under equilibrium condi-

tions, with a certain relative humidity, the reinforcing cellulose

material is considered to span over several crystalline and irregular

zones thus making the layer a continuous fiber composite. Under wet

6.

conditions, the irregular zones may be softened. This may be accounted

for by considering these zones to be part of the matrix material. Thus

under wet conditions the reinforcing component is solely the cellulose

crystallites and the layer is a discontinuous fiber composite. The

lignin present in the fiber wall is considered in a manner similar to

the model of Cave (23) as existing in separate isotropic lignin lamellae

alternating with the cellulose-containing lamellae. Here these lignin

lamellae are grouped together to a single layer located in the middle of

each of the fiber layers.

Unless otherwise stated, the fiber is here considered to be built up of

an S2 layer with a fibril angle of 20 while all the other layers are

given a fibril angle of 70 . The thickness of the respective layers are

taken as 16 % for P plus S1, 76 % for S2 and 8 % for S3.

The laminate model structure here adopted for wood and wood fibers is

shown in Fig. 3. The structure of wood may be regarded as consisting of

fibers with a square cross-section forming a structural element, the

common wall, which consists of two fiber walls with a middle lamella in

between, Fig. 3a. The fibril angles in the one fiber wall will then be

opposite to those in the fiber wall on the other side of the middle

lamella. Thus our structural element consists of an anti-symmetric

laminate adequate to describe stresses in the fiber direction. In the

transverse direction, however, the interaction of the cell wall corners

has to be considered making the calculations more complicated. The

model here used is given in Appendix 2.

wood cell wall element fiber wall element

a) b)

Fig. 3

a) Wood cell wall element cons i s t ing of a tracheid wall, a middle lamella (ML) and a tracheid wall. Thus the element consist of the layers S3, S2, S1, P, ML, P, S1, S2, S3.

b) Fiber wall element of tracheid. The element consists of the layers P, S1, S2, S3, S3, S2, S1, P.

In the case of fibers making up a sheet of paper the single fiber must

be considered. Well-beaten fibers or fibers of low yield are collapsed

in the paper sheet, i.e. the square fiber has been flattened out so that

its inner surfaces contact each other. In this case the angle of the

fibrils in a layer in the front fiber wall is opposite to that in the

back fiber wall, fig. 3b. Thus the fibers in the paper sheet can also

be viewed as an anti-symmetric laminate.

The material constants for the components of the cell wall needed to

calculate the fiber properties are still not established in detail.

However both experimental results and theoretical considerations have

given values that are sufficiently reliable for the scope of this model.

The data used are summarized in Table 1. No data are available for a

softened lignin. Due to the high degree of crosslinking in native

7.

8.

lignin, it can be assumed that the elastic modulus of soft lignin will

not be as low as that of normal uncrosslinked polymers (27). Here the

modulus has been assumed to be only slightly less than two decades lower

than that of the stiff sample. For hemicellulose, transverse isotropy

is assumed. The ratios between the moduli are in accordance with the

suggestions of Cave (23).

Table 1 Data for mechanical properties of cell wall components.

below Tg above Tg

cellulose

Ex (N/m2) 13.4.1010 (Mark (24))

E (N/m2) 2.72 1010 (Mark (24))

G (N/m2) 0.44 1010 (Mark (24))

v 0.1 (Mark (24))

50,000 (estimated) 500 (estimated)

hemicellulose

Ex (N/m2) 8.109 (Cousins (25)) 2.107 (Cousins(25))

E (N/m2) 4.109 1.107

G (N/m2) 2.109 ' (estimated) 0.5.107 (estimated)

vx 0.2 0.2

1i gni n (i sotropic)

E (N/m2) 4-109 (Cousins (26)) 6-107 1

G (N/m2) 1.5*109[(estimated) 2.25*107/(estimated)

v 0.33 I 0.33 i

E = Young's modulus G = shear modulus v = poisson's ratio

= reinforcement shape factor; crystallite length divided by thickness.

Micromechanical lamination theory

The fundamental steps in the cell wall model calculations involve

analytical investigations on two levels of abstraction known as micro-

mechanics and macromechanics. Micromechanics recognizes the true

9.

identity of the composite. It enables the mechanical properties of

unidirectional composites to be calculated from those of the constituent

materials. In macromechanics, the individual plies oriented at differ­

ent angles are combined to form a laminate in order to obtain the fiber

properties. These calculations then allow the macroscopic moduli to be

related to those of the individual components of the composite. Several

theories for predicting the mechanical properties of filled composites

have been developed ranging from empirical to exact methods based on

elasticity theory, as reviewed by for instance Chow (28) and Ashton,

Halpin and Petit (29). For the micromechanical considerations, the

equations of Halpin & Tsai (29) which account for the shape of the

reinforcing material have here been chosen. Although these are simple

empirical expressions approximating to formal elasticity theory, they

have been shown to be reasonably accurate for values of Ef/Em up to

1000, as long as the fraction of reinforcing material does not approach

unity. The equations for the modulus of elasticity E and for the

poisson ratio in the x direction are:

10.

limits. For pulp fibers under wet conditions, the shape factor is

considered to be reduced as a consequence of the softening of disordered

regions. If only the crystallites were considered as reinforcements

this would imply a shape factor of about 25 for most fibers. Due to the

restrictions imposed by the crystallites, the interconnecting disordered

chains may in reality not soften to the same extent as the amorphous

hemicelluloses, thus implying longer reinforcing elements. It should be

stated that the aim of the model here suggested is simply to simulate

the effect of softening of the cellulose microfibrils. Thus, a value of

In this case, the reinforcing material is thus characterized by the

factor (a/b) where a is the dimension in the axis of stress and b is the

dimension perpendicular to this axis.

The microfibril is assumed to have a circular cross-section which gives

a value of a/b equal to unity.

The important implicit assumptions in these equations include the assump­

tions that both filler and matrix are homogeneous and linearly elastic,

that filler and matrix are free of voids, that there is perfect contact

between filler and matrix at the interface and that the filler is per-

fectly dispersed.

The micromechanical calculations give the engineering moduli of the

individual plies in the axis of the reinforcement and transverse to it.

By the application of lamination theory (see Appendix l) these moduli

are converted to moduli which are dependent on the angle of the rein­

forcement. The engineering moduli of the total structure of the fiber

wall with its individual plies can then be calculated from a combination

of the off-axis moduli of all the plies.

11.

Depending on the boundary conditions, the relations between forces,

moments, strains and curvatures can be obtained which permit an easy

derivation of the engineering moduli.

In the case of a fiber which is considered to be an an t i -symmetr ic

laminate, the interactions between odd and even functions yield the

following relations between strain, forces and curvatures according to

equations 21 and 25 of Appendix 1.

The mounting is considered to be firm so that the twisting K6, is equal

to zero. The transverse force N2 is also zero giving the engineering

modulus:

E1 = N1/E1h = 1/A*11 h [5]

where h is the thickness of the laminate.

In torsion the clamps are considered to be balanced so that the forces

N1 and N2 are both equal to zero. From the partially inverted relations

given in equations 21 and 25 in Appendix 1, the torsional stiffness

T = M6/M6 is then given as:

When fibers and wood samples are tested in the longitudinal direction,

the strain is then given by:

[ 4 ]

[6]]

0 10 20 30 40 50 60 70 80 90 S2 Fibril angle

Fig. 4

The calculated elastic modulus as a function of S2 fibril angle for a fiber when the fiber is (a) free to twist and (b) prevented from twisting.

The influence of boundary conditions in our case is illustrated in

figure 4 for the fibre tensile modulus, the case where twisting is

prevented being compared with that when twisting can occur freely. This

behaviour is derived from the partially inverted and fully inverted

forms of the relations, according to equations 21 and 23 in Appendix 1.

Classical lamination theory assumes the laminate to be thin and that the

normal to the middle plane remains straight and of constant length

during deformation. Wood fibers have, of course, a finite width and a

rather substantial thickness and are thus far from ideal. However the

ten-fold ratio of width to thickness makes the assumptions not too

crude.

Comparison with experimental data - effect of fibril angle

The validity of the model presented may be judged by comparing the

calculations with measurements of the dependence of modulus on the

fibrillar angle of the S2-layer of single fibers and wood.

12.

13.

For single fibers, extensive measurements have been carried out by Page

et al (30) on black spruce fibers prepared from a holocellulose pulp and

from a kraft pulp at 45 % yield. Their data are shown in Fig. 5 together

with calculations based on the present model assuming that these fibers

contain no lignin and that the hemicellulose content is 35 %. Page

points out that many of the fibers measured have been damaged during the

process of isolation. These fibers contain crimps, microcompressions,

dislocations etc. and show a lower modulus within the span at a certain

fibrillar angle. Thus it may be expected that the highest value ob-

tained for the elastic modulus of the fibers should represent the un-

damaged fibers. Our calculations assume that the lumen is collapsed and

that the opposing fiber walls are bonded together whereas Page et al

(30) assume that the cell wall corners transmit the shear stresses

between the walls and thus restrict shear deformation. Their boundary

conditions are essentially equivalent to a restriction across the lumen.

However these considerations do not automatically lead to the conclusion

that the shear strain is zero as stated by Page, but merely to the

conclusion that this value depends on the testing condition i.e. whether

the clamping restricts this deformation or not. Of more principal

concern is the fact that resulting curvatures and acting moments which

will interact according to the coupling matrix [B] in equations 13 and

14 of Appendix 1 are neglected. These can only be disregarded for a

symmetric laminate where all the elements in this matrix are equal to

zero. The calculated curve in fig. 5 is nevertheless in good agreement

with a curve calculated by Page et al (30), but in the present case the

curve is predicted from moduli of the individual components and does not

merely represent a fit to the experimental data.

The elastic modulus of pulp fibers as a function of the S2 mean fibril angle. The points refer to single fiber measurements by Page et al. (30). The curve is calculated assuming the fibers to be lignin-free and to contain 35 % hemicellulose.

For wood the variation of the modulus with the fibril angle of the S2-

layer of softwood is shown in Fig. 6 with data taken from Cave (2). The

curve included in the figure is calculated on the basis of the assump-

tion that the wood contained 28 % lignin and 31 % hemicellulose. The

agreement between the present theory and the experimental results is

good with respect both to the magnitude of the modulus and to its depen-

dence on fibril angle. Some of the higher values at low fibril angle

may be due to the fact that these fibers have a higher proportion of the

S2-layer.

Fig 5.

15.

Fig. 6

The elastic modulus of wood along the grain as a function of the S2 mean fibril angle. The points refer to measurements on Pinus radiata by Cave (2). The curve is calculated assuming the wood to contain 28 % lignin and 31 % hemicellulose.

0 10o 20o 30° 40° 50°

Mean fibril angle

These calculations lead to similar conclusions as those of Cave (2) but

are significantly higher than the predictions of Mark (1). In calcu­

lating the longitudinal modulus of wood, Mark (1) assumed that it could

be obtained from a laminate of one single cell wall. As has been pointed

out by Schniewind and Barrett (31) and has also been recognized by Mark

and Gillis (32) the theory in this form takes no account of the stiffening

influence of neighbouring cells in preventing torsion under axial tension.

This obviously leads to values of the longitudinal modulus that are too

Comparison with experimental data - softening effects

At room temperature an increase in the moisture content of a fiber is

assumed first to soften the hemicelluloses, whereas the disordered

zones in the microfibrils only become soft when high moisture contents

are reached upon water immersion.

Fig. 7 The influence of shape factor l/d for the reinforcing cellulose crystals on relative rigidities of a kraft fiber, Eo is the rigidity correspon­ding to l/d = 50,000 with hemicelluloses assumed to be glassy. The curves show the behaviour when -the hemicelluloses are assumed to be soft. E refers to the longitudinal fiber modulus, Ey to the transverse fiber modulus and torsion to the torsional stiffness of the fiber.

The softening of the irregular zones of the cellulose microfibrils is

accounted for in the model by a variation in the shape factor for the

reinforcements. Thus under wet conditions the reinforcing microfibrils

= 50,000) are considered to be chopped into smaller fragments of

cellulose crystallites which remain as the reinforcements

The calculated effects of this change in on the elastic moduli of a

pulp fiber are compared in Fig. 7 with the effects of a softening of

only the hemicelluloses. The relative rigidities are here given for a

fiber representing a kraft cook. The E -values are the rigidities

corresponding to = 50,000 with the hemicelluloses assumed to be in

the glassy state. The curves show the behaviour when the hemicelluloses

are assumed to be soft. The intercepts on the ordinate thus represent

the loss of rigidity due to the softening of the hemicelluloses.

The calculations indicate that the relative rigidity is more sensitive

to the change in the shape factor of the reinforcements than to the

softening of the hemicellulose matrix. The change in the shape factor

also has a more pronounced effect on the longitudinal stiffness than on

the transverse or torsional stiffnesses. This is commented on further

in the following pages.

Thermal_softenimg_of dry_wood

Measurements of the elastic modulus of rotary cut birch veneer under dry

conditions (33) show a large softening across the grain, Fig. 8a. The

transition at 205o C has been attributed to softening of the lignin

component (33). Along the grain no softening regions can be discerned.

The effects of softening of lignin on the moduli of dry wood have been

calculated according to the laminate model, Fig. 8b. Apparently the

major experimental results are confirmed, i.e. regardless of whether the

hemicelluloses are stiff or softened, lignin softening causes a large

drop in modulus in the transverse direction but hardly any noticeable

change in the fiber direction.

Fig. 8 Thermal softening of dry wood. a) (Upper figure). The relative modulus of dry birch as a function of temperature according to data by Salmen (33). In this figure, the softening degree calculated in b) across the grain for the case of soft hemicelluloses is indicated.

b) (Lower figure). The relative modulus of dry wood calculated as a function of the lignin modulus for the two cases of soft and stiff hemi­celluloses.

17.

18.

For the case of wood across the grain, a comparison of the calculated

changes of the elastic modulus with the measured loss of rigidity,

in the lignin softening region around 205o C, indicates that most of the

hemicelluloses are probably already softened at this temperature. The

calculated relationship across the grain for the case of soft hemi­

celluloses is indicated in fig. 8a as a softening range. The measured

loss in modulus for the sample across the grain prior to 205oC is probably

due to the softening of the native hemicelluloses, which mostly occurs

between 150 and 200oC (33). In wood, this hemicellulose softening

probably occurs over a rather broad temperature range due to inter-

actions in the complex structure. An alternative explanation, consider-

ing the hemicelluloses to be glassy, would be that the highly cross-

linked lignin does not soften to the extent as assumed in the given

calculations. However, this would imply a change in the elastic modulus

of the lignin over the softening temperature of only one decade, which

seems to be too low a value.

Influence of moisture content on wood

At room temperature, an increase in the relative humidity is assumed

mainly to result in a softening of the hemicelluloses. Measurements by

Cousins on two different hemicelluloses (25) have shown that a transi­

tion takes place at about 80 % relative humidity with a change of modulus

from 8 . 1 0 9 N/m2 under completely dry conditions to 1 • 107 N/m2 at

about 95 % R.H. The resulting calculated softening in wood is presented

in Fig. 9 together with data by Carrington on spruce (34). The calcu­

lated losses in relative rigidity across the grain are greater than

those measured. This may be ascribed to the inadequacy of the model

to describe properly the stress transfer in wood. On the other hand,

the discrepancy may be explained by several molecular features as for

example the presence of crystalline hemicelluloses as proposed by Page

(35). The presence of bonds between the hemicelluloses and lignin may

also restrict the motion of the hemicellulose chains and thus increase

its modulus. During pulping these bonds are suggested to be broken.

This hypothesis is based on the lowering of the modulus when wood is

steam treated (36). Along the grain, both Carringtons measurements and

19.

the present calculations indicates that the sensitivity to the moist

hemicellulose modulus is low.

Fig. 9

Softening of wood due to an increase in relative humidity.

Data of the relative modulus of spruce as a function of relative humidity according to Carrington (34). The lines show the calculated changes assuming hemicellulose softening according to data of Cousins (25).

A further increase in the moisture content, as when the wood samples are

immersed in water or even pressure-impregnated, has no effect on the

moduli of wood (37, 38). Thus it is not necessary to include a transi-

tion from continuous to discontinuous elements to describe the influence

of water immersion.

Two reasons for this behaviour are here considered. In wood the dis-

ordered regions may, as Stockmann has suggested (10), consist only of

defects in the lattice structure of the crystallites. Such irregulari-

ties are not susceptible to softening and thus will not reduce the

length of the reinforcing elements upon water immersion. Another possible

effect is that swelling restrictions hinder water uptake in the dis-

ordered regions so that the moisture contents needed for a softening to

take place are not reached.

Thermal_softening of_wet_wood

For wood under wet conditions, only the hemicelluloses are considered to

be soft at room temperature. As discussed in the previous section the

cellulose microfibrils in wood are unaffected by water. Measurements of

the temperature dependence of the elastic modulus of wet samples of

spruce wood between 20 and 160°C show a substancial softening as seen in

Fig. 10. The transition which may be attributed to the softening of

moist lignin, occurs at 95°C. The calculated effect of lignin softening

on the modulus across the grain is indicated in the figure as a softening

range around 95°C. The shape factor of the reinforcements is here

considered to be 50,000. The gradual loss in rigidity observed as a

function of temperature below the lignin softening range is probably due

to an increasing softness of the hemicel1uloses. Along the grain the

sensitivity to the calculated effect of lignin softening is low. The

experimental data do not show any dramatic changes to contradict this

prediction.

Fig. 10

Thermal softening of wet wood.

Measurements of the relative modulus for wet spruce wood along and across the grain are given as a function of temperature. The calculated decrease in the relative modulus across the grain as a result of lignin softening is indicated.

20.

Influence of moisture content on fibers

The response of a single fiber to moisture over a range of different

humidities is mainly considered to be due to softening of the hemi-

celluloses. Utilizing the results obtained by Cousins (25) for the

modulus of hemicelluloses as a function of moisture content, the corre-

sponding change in a fiber may be calculated as in Fig. 11a for a kraft

fiber in torsion. The deviation between calculated and experimental

results obtained by Kolseth et al (39) is mainly found below 5 % moisture

content, so that if the condition at 5 % moisture content is taken as

starting point there is good agreement as indicated in Fig. 11b.

Fig. 11

The influence of moisture content on the relative torsional stiffness of a kraft fiber. Measurements according to Kolseth et al. (39). The lines show the calculated changes assuming hemicellulose softening according to data of Cousins (25).

a) (Upper figure.) Comparison with 0 % moisture content as starting point.

b) (Lower figure.) Comparison with 5 % moisture content as starting point.

2 1 .

22.

In these calculations the effect of the swelling of the fiber is not

taken into account. According to the estimations given in Appendix 3

the fiber swelling will imply a correction of the calculated torsional

rigidities amounting to an increase in proportion to the increase in

moisture content. Still it is evident that most of the softening effect

in this range of moisture contents can be accounted for by the softening

of the hemicelluloses.

These data do not exclude softening effects on the reinforcing micro­

fibrils, since the torsional stiffness is somewhat less sensitive to the

stiffness of these components as is evident in fig. 7. Data of Kersavaga

(40) on the longitudinal modulus of delignified tracheids show however a

loss in relative rigidity of only about 11 % up to a moisture content of

18 %. Since the estimated loss in relative rigidity due to hemicellu-

lose softening is about 15 % (figure 7 ) , the microfibrils are considered

to be unaffected by changes of moisture content in this range.

A further comparison between the model and fiber measurements is given

in Table 2. The calculated relative torsional rigidity for fibers of

different origins is about the same when variations in relative humidity

are considered. The measured relative torsional rigidity is also almost

the same for the different fibers. The scatter in data for fibers of

similar composition is however rather large.

When the fibers are immersed in water the measured relative tensile

rigidity is reduced much more than only the softening of hemicelluloses

would account for. Data by Kolseth and Ehrnrooth (41) given in Table 2

show a reduction of about 70 % while the calculated loss due to hemi-

cellulose softening is only 15 % as seen in fig. 7. The large softening

effect measured can only properly be accounted for by a change of the

shape factor t/d as a reflection of the softening of the disordered

regions. The calculated values here are based on a reduction to Z/d =

500.

It is evident that it is the cellulose microfibrils that essentially

determine the stiffness of the thracheids. Softening of the disordered

regions will therefore play an important role with regard to the hygro-

elastic behaviour of pulp fibers.

23.

Table 2 Relative change in modulus for various fibers due to different changes in the environment. Measurements of Kolseth and Ehrnrooth (41). The measured relative torsional rigidities are uncorrected for the effects of swelling.

relative composition relative rigidity (%) torsion tension

90% RH/25% RH wet/50 % RH fiber cellu- hemi- lignin mea- calcu- mea- calcu-

lose cellu- sured lated sured lated lose

TMP 44.2 29.5 26.3 0.4 0.25

dichlorite deligni-

fied TMP 45.2 30.1 24.7 0.52 0.52

46.5 31.0 22.5 0.52 0.52 0.2 0.25

54.1 36.0 9.9 0.49 0.49 0.2 0.23

sulphate 74.0 17.8 8.2 0.56 0.51

bleached sulphate 78.3 21.4 0.3 0.47 0.50

bleached sulphate 80.4 19.4 0.2 0.48 0.50 0.2 0.25

holocellulose 77.3 21.2 1.5 0.55 0.50 0.4 0.29

high yield sulphate 59.5 19.1 21.4 0.52 0.53

high yield sulphate 71.7 20.3 8.0 0.55 0.50 0.3 0.25

Influence of moisture content on paper

The properties of paper reflect to some degree the properties of its

individual building elements, i.e. the fibers. The derivation of paper

properties from the fiber properties is, however, not well established.

Measurements by de Ruvo et al (42) have shown that the relative changes

in torsional rigidity of paper due to moisture and temperature changes

are similar to the changes in relative torsional stiffness of the fibers.

The calculated effects of softening of the hemicelluloses on the tensile

and shear modulus of single fibers are shown as a function of fibrillar

angle in Figure 12. This softening represents a change in relative

humidity from 0 to 95 % R.H. It is interesting to note that the effects

of fibril angle in both torsion and tension are marginal in the interval

of the most common fibril angles, 10 to 30 (7). Thus the relative

change in modulus for a certain change in conditions can justifiably be

determined by testing only a few fibers. These levels of fiber softenings

are given here as a reference for the comparison with the corresponding

softening for paper.

24.

The calculated effect of the softening of hemicelluloses on the relative modulus in tension and torsion of fibers of different fibril angles. The calculated effect on paper simulated by a quasi-isotropic model is shown by the broken line.

For an isotropic paper the random array of fibers and thus also of

microfibrils may be simulated by a quasi-isotropic model in which the

paper is seen as a multilayered material where the direction of the

reinforcing elements varies in different layers. It can be demonstrated

that a minimum of three different ply orientations (0 , 45 , 90 ) is

needed to obtain isotropic stiffness properties (43). The merit of this

quasi-isotropic model is that it enables the elastic properties to be

quantified in terms of the properties of the constituents of the fiber.

The calculated response to hemicellulose softening for a random, quasi-

isotropic paper is also included in fig. 12. It should be recognised

that the quasi-isotropic model represents a completely homogeneous sheet

containing no voids and that the presence of such would substantially

reduce the absolute value of the modulus of the paper sheet.

A change in humidity from 0 to 90 % RH lowers the relative elastic

modulus of paper to about 0.50 (44), which is considerably greater than

the value predicted for paper by the laminate theory, as is evident in

Figure 12. Thus the network structure of normal paper must introduce

other factors which affect the humidity dependency. For well-bonded

sheets, Page suggests (45) that the elastic modulus of the paper is

directly proportional to the longitudinal fiber modulus. However, as

seen in Figure 12, the relative tensile fiber modulus for a softening of

the hemicelluloses is only comparable with the levels of relative rigi-

dities measured on papers for fibers of extreme fibril angles; 35 and

25.

above. Both the model of Page (45), giving the paper modulus as 1/3 of

the fiber modulus, and the laminate model represent a well-bonded high-

density sheet, whereas a normal paper of lower density exhibits quite a

different stress state which might lead to a larger reduction in the

relative rigidity over the RH-range. For paper, none of these proposed

models can thus account for the loss in stiffness due to moisture.

Several other theories exist where the structural arrangement of the

fibers in the network of paper is considered. All of these contain,

however, structural quantities which it is not yet possible to measure.

Final remarks

The model presented gives a rational way to estimate both qualitatively

and quantitatively the moduli of fibers and wood under different environ­

mental conditions. The laminate model used makes it possible to account

for the coupling between tension and twisting of the layered structure

that represents a fiber, a factor hitherto neglected in estimations of

fiber moduli .

It is clear that the wet end properties are particularly sensitive to

the reinforcing cellulose crystals. The effects of these can appro­

priately be simulated by assuming a change in the dimensions of these

reinforcing elements. Thus under wet conditions the composite structure

of a pulp fiber is best represented by a discontinuous reinforced system.

The knowledge which the model provides of how the fiber structure

responds to softening of the lignin, hemicellulose and cellulose makes

it a useful tool in estimating the merits of various methods of improve­

ment of the fiber properties.

Acknowledgements

The author wishes to thank Dr Alf de Ruvo, Dr Christer Fellers and Mr

Petter Kolseth for valuable advice and encouragement during this work.

The linguistic revision performed by Mr Anthony Bristow is also grate­

fully acknowledged.

26.

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27-

17. Goring, D.A.I.

In: "Lignins, Occurrence, Formation, Structure and Reactions" (Ed. by K.V. Sarkanen and C.H. Ludwig). Wiley Inter-Science, New York 1971.

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2k. Mark, R.E. "Cell Wall Mechanics of Tracheids". Yale University Press 1967.

25. Cousins, W.J. Wood Sci. Techn. 12(1978) 161.

26. Cousins, W.J. Wood Sci. Techn. 10(1976) 9.

27. Nielsen, L.E. "Mechanical properties of polymers and composites". Marcel Dekker, Inc., New York 1974.

28. Chow, T.S. J. Material Sci. 15 (1980) 1873.

29. Ashton, J.E., Halpin, J.C. and Petit, P.H.

In: "Primer on Composite Materials: Analysis". Technomic, Stamford Conn. 1969. Chapter 5.

30. Page, D.H., El-Hosseiny, F., Winkler, K. and Lancaster, A.P.S. Tappi 60(1977) no 4:114.

31. Schniewind, A.P. and Barrett, J.D. Wood and Fiber 1 (1969) no 3:205.

32. Mark, R.E. and Gillis, P.P. Wood and Fiber 2.(1970) no 2:79.

28.

33. Salmen, L. Pulp Pap. Can., Trans. Tech. Sec. 5 (1979) no 3:TR 45 .

34. Carrington, H. Aeron. J. 26(1922) 462.

35. Page, D.H. Wood Fiber 2(1976) no 4:246.

36. Baldwin, S.H. and Goring, D.A.I. Svensk Papperstidning. 71(1968) no 18:646

37. Kollman, F. and Krech, H. Holz als Roh- und Werkstoff 18(1960) no 2:41.

38. Suzuki, M. J. Japan Wood Res. Soc. 26 (1980) no 5:299.

39. Kolseth, P., de Ruvo, A. and Tulonen, J. To be published.

40. Kersavage, P.C. Wood and Fiber 5 (1973) no 2:105.

41. Kolseth, P. and Ehrnrooth, E. In: "Paper structure and properties" Ed. Bristow, J.A., Marcel Dekker Inc. To be published.

42. de Ruvo, A., Lundberg, R., Martin-Lof, S. and Soremark, C.

In: "The fundamental Properties of paper related to its uses' BP & BIF 1976 p. 785.

43. Halpin, J.C. and Pagano, N.J. J. Comp. Mater. 13 (1969) 720.

44. Salmen, N.L. and Back, E.L. Tappi 63(1980) no 6:117.

45. Page, D.H., Seth, R,S. and de Grace, J.H.

Tappi 62(1979) no 9:99.

46. Jones, R.M.

"Mechanics of composite materials". McGraw-Hill, New York, 1975.

47. Tsai, S.W. and Hahn, H.T. "Introduction to composite materials". Technomic, Westpoint, Conn. 1980.

48. Agarwal, B.D. and Broutman, L.J. "Analysis and performance of fiber composites". John Wiley & Sons, New York, 1980.

49. Meredith, R. J. Text. Inst. 48(1957) no 6:T163.

APPENDIX 1

Stress-strain relations for laminated orthotropic materials

For a three-dimensional stress state in an orthotropic material the

following strain-stress relations exist (46) :

where the [S] matrix is the compliance matrix. For a sheet-like material

or a ply in a laminate it is common to assume a plane stress state, i.e.

where the third direction denotes the thickness direction. It is then

evident from the compliance matrix that Y23 = 0, Y31 = 0 and that e3

becomes a dependent component so that equation (l) reduces to

For the plane stress state several systems of notation are used. Following

Tsai and Hahn (47) who used the notation x and y for the principal

directions of a ply and s for shear we obtain.

(3)

29.

30.

giving

(7)

The compliance components Sij are related to the engineering constants

by:

where Ex , Ey and Gxy respectively are the elastic moduli in the principal

material directions and the in-plane shear modulus and vx and vy are

the poisson ratios for stresses in the principal directions.

The strain-stress relations in equation (4) can be inverted to obtain

the stress-strain relations

The inversion [Q] = [S -1 ] is here given by

31.

The stiffness components Qij are related to the engineering constants by

If the principal axes of the sheet or ply are not aligned with the

reference coordinate axes, the stress-strain relations have to be trans­

formed to the new coordinate system. For an orientation of the principal

axes of an angle 9 counter-clockwise to the coordinate axes the trans­

formations are given below. Thus the positive angle 9 is defined from

off-axis to on-axis.

The stress-strain relations in off-axis coordinates may then be given by

where the elements are given by

32 .

For a laminate it is assumed that all layers are in a state of plane

stress, that all layers are perfectly bonded together and that the

strain components are linear functions of the thickness coordinate.

With these assumptions and using the stress-strain relation for each

ply, the relationships between the external forces and moments acting on

the laminate and the midplane strains and curvatures can be established.

The general matrix expressions of these equations are

where [N1, N2, N6] and [M1, M2, M6] are the resultant forces and moments

per unit width respectively, and [e1, e2, e6] and [K1, K2, K6] are the

midplane strains and curvatures respectively. Because of symmetry

conditions such as

it follows that

Aij =Aji, Bij = Bji and Dij = Dji

Qij=Qji

33.

The e lements A i j , B i j and D i j a re

(15)

(16)

(20)

(17)

where (Qij)k is the so-called reduced stiffness matrix for ply k and is

calculated from Equation (12). The ply coordinates zk are defined as

(18)

(19)

where hk is the thickness of each ply

The relations 13 and 14 may be given in a more compressed form as

By matrix operations these relations can be partially inverted to

(21)

where

[A*] = [A-1]

[B*] = -[A-1][B]

[C*] = [B][A-1] = -[B*]T

[D*] = [D] - [B][A-1][B]

(22)

34.

The fully inverted form is given by

(23)

[A '] = [A*] - [B*][D*-1][C*]

[B ' ] = [ B * ] [ D * - 1 ]

[ C 1 ] = - [ D * - 1 ] [ C * ]

[ D ' ] = [ D * - 1 ]

(24)

It may be recognized that for a symmetrical laminate, i.e. a laminate

where each ply above the geometric midplane with a certain orientation q

is counterbalanced by an identical ply with the same orientation at the

same distance below the midplane, the coupling matrix B equals zero.

For an antisymmetrical laminate, i.e. a laminate where each ply above

the geometric midplane with a certain orientation q is counterbalanced

by an identical ply with the orientation -q at the same distance below

the midplane, the following components of modulus are equal to zero

because of interaction between odd and even functions.

B11, B22, B12, B66, D16, D26, A16 and A26

giving the following relations

(25)

For further reading the works of Jones (46), Tsai and Hahn (47) , Ashton,

Halpin and Petit (29) and Agarwal and Broutman (48) are recommended.

where

35..

APPENDIX 2

Mechanical model of softwood

The geometrical arrangement of cells within softwood is complicated and

this arrangement has to be taken into consideration in any attempt to

explain the macroscopic properties of wood.

In the longitudinal direction the Young's modulus is determined solely

by the properties of the multilayered cell wall depicted in figure 3a,

since all the components occur in a parallel coupling and no twisting of

the wood is considered to be possible. Thus all the isotropic lignin

component can be considered to be collected into a single layer in

parallel with the multilayered cell wall.

The transverse properties however are affected by the coupling of the

different structural elements. As shown in fig. A, the load is considered

to be transmitted through a region representing the common cell wall in

the direction of stress. The dimensions of the common cell wall have

been calculated from the density of the wood, assuming square shaped

cross sections of the fibers and a density of the cell wall of 1.5 g/cm3 .

The stress-transmitting zone can be described by a coupling in series of

the different structural elements; the lignin middle lamella, the corner

section of the cell wall and a transverse cell wall element as shown in

fig. B. The lignin middle lamella is considered to be composed of 20 %

of the lignin present in the cell wall.

The cell wall corner element in particular gives rise to complicated

considerations since it is not symmetrical in the direction of stress.

It consists of a parallel coupling of the real cell wall corner and a

middle lamella.

The real cell wall corner is divided along a diagonal so that the lower

portion in fig. C consists of transverse cell wall layers while the

upper portion consists of cell wall layers perpendicular to the direc-

tion of stress, i.e. all the reinforcing microfibrils and lignin lamellae

are considered to be perpendicular to this stress direction.

36.

For this corner a small element d£ is considered in the vertical direc­

tion; this is made up of a coupling in series of portions of the above

transverse and perpendicular layers.

The cell wall corner is made up of a coupling in parallel of elements d£

over the width k of the corner.

For a coupling in series of two materials the elastic modulus is given

by:

where E is the elastic modulus

V is the volume fraction

For a coupling in parallel the elastic modulus is given by:

E = E1 V1 + E2 V2 (2.2)

The total modulus can thus be obtained by integration of the expression:

( 2 .1 )

(2.3)

where E1 is the modulus of the transverse cell wall

E2 is the modulus of the perpendicular cell wall

(2.4)

If instead a small element di is considered in the horizontal direction,

and the integration is carried out over the vertical distance I, the

following relation exists:

(2.5)

(2.6)

(2.7)

This equation does not give the same value as equation (2.4) since there

is a different emphasis on the parallel and serial couplings.

As a compromise the average value of these two estimates has been taken

as representative of the corner element.

STRESS TRANSMITTING ELEMENTS

Fig. A Stress transmitting zones in wood across the grain.

Lignin middle lamella

Corner section

Transverse cell wall element

Fig. B The division of the stress trans- Fig. C Cell wall corner mitting zone into structural elements. element.

37.

38.

APPENDIX 3

Estimate of the correction of the fiber torsional rigidity due to swelling

The torsional rigidity T of a fiber may be given by (49)

T = constant G • A2 (3.1)

where A is the fiber cross-section area and G its shear modulus. The

shear modulus G may simply be estimated by

G = Gf Vf + 6m(1-Vf) (3.2)

where Gf and Gm are shear moduli of the reinforcements and the matrix

respectively and Vf is the volume fraction of the reinforcements.

Consider a fiber swelling amounting to X • V where V is the original

fiber volume. As the fiber length can be considered unaffected by the

swelling the swollen area is given by X • A. As the swelling only takes

place in the matrix material this implies that the volume fraction of

reinforcements in the swollen state amounts to V f/Z. The torsional

rigidity T, calculated on the unswelled cross-section, may then be

compared with the torsional rigidity T' where the swelling has been

taken into account by

as Gf » G and m

if Vf is not approaching zero then

(3.3)

Thus the correction of the calculated torsional rigidity due to fiber

swelling is proportional to the increase in cross-section area, i.e.

increase in fiber volume approximately given by the increase in moisture

content.

Paper III

1

THE FUNDAMENTALS OF ENERGY CONSUMPTION DURING VISCOELASTIC

AND PLASTIC DEFORMATION OF WOOD

N.L. Salmen and C. Fellers

Swedish Forest Products Research Laboratory

Stockholm, Sweden

Synopsis

The fundamental aspects of energy consumption in a viscoelastic material are discussed in relation to the defibration of wood for the manufacture of mechanical pulp. The differences between viscoelastic and plastic deformations and the importance of the latter for making the fibers suitable for papermaking are clari­fied. It is proposed that the efficiency of the defi­bration process be characterized by the structural changes within the wood as reflected by changes in the elastic modulus. Measurements indicate that the fib­rillation process has its optimum efficiency at a higher temperature than does the fiber separation, i.e. at a higher temperature than the optimum temperature for mechanical energy absorption by the wood. Thus fiber separation and f1exibi1isation should perhaps be carried out at different temperatures for optimum efficiency.

Introduction

The mechanical refining processes for producing woodpulp for paper

consume large amounts of energy. The increasing awareness during the

last decade of the limitations in our energy yielding resources and of

their increasing value has made it increasingly important to reduce

the energy consumption in such processes.

In the refining and grinding processes it is generally considered that

only a small portion of the energy put into the system is actually

utilized in the separation of fibers (1-3). In addition, it is con­

sidered that if the fibers produced are to be suitable for papermaking

a certain amount of mechanical flexing of the fibers, creating an "in­

ternal fibrillation", is needed (4), This is accomplished by the

2

cyclic deformations to which the wood or chips are subjected during the

fiber-separation process. These cyclic deformations range from small

viscoelastic to large plastic deformations. The viscoelastic deforma­

tions, by definition, only produce heat in the system, while it is the

plastic deformations which are essential to make the fibers flexible.

These cyclic deformations of the wood consume large amounts of energy,

and it is thus essential to gain more knowledge about how the energy

input is utilized, if any substantial saving in energy consumption is to

be possible.

The aim of this paper is to report an investigation of the nature of

energy consumption by of wood during cyclic loading, both in the visco­

elastic and plastic regions of deformation. It is thus hoped to obtain

tools for a better optimization of the defibration process.

Energy consumption in refining processes

During refining and grinding, wood is subjected to an undefined deformation

process mainly in compression and shear which ultimately separates the

fibers from each other. The energy needed for the breakdown of wood

into fibers is in all estimations found to be relatively small. Van der

Akker (1), considering the strain energy required to create voids,

estimated an energy of 0.01 kWh/ton for fiber separation, whereas Lamb

(2), measuring the energy for surface formation, found a value equal to

35 kWh/ton, which is similar to the value found by Atack et al (3). The

amount of energy required for fibrillation has been stated by van der

Akker (1) to be about 0.4 kWh/ton, while Atalla and Wahren (5) estimated

the energy to fibril late the fibers completely down to the level of

elementary fibrils to be 23 kWh/ton, judging from the energy absorbed in

the generation of new surfaces. From the value of the specific area of

groundwood created, Lamb (2) estimated the refining energy to be 140 kWh/ton.

Nissan (6), instead, considered the total amount of energy transferred

to the wood and fibers as they pass through a refiner if all the impacts

led to delamination and found this to be about 300 kWh/ton. These

levels of energy consumption should be compared with those actually

required for the commercial production of, for example, a newsprint

furnish which is about 1500 kWh/ton for groundwood and 2000 kWh/ton for

TMP.

3

The internal fibrillation and flexibi1isation is of great importance in

order to be able to make strong paper. Essentially the bonding is

enhanced by the greater ability of the fibers to conform to each other.

The optimum conditions for the mechanical action in defibration have

been estimated from small scale viscoelastic measurements by Hoglund et

al (7) and subsequently used by others (8,9). Although such measure­

ments are indicative of the softening of lignin, and may be important in

establishing the site of the breaking zone between fibers, there Is no

evident correlation between them and the fibrillation action. Further­

more, when the wood is plastically deformed in the pulp manufacturing

process, its properties change so that it may behave quite differently

from the original sample. Transformations from viscoelastic to plastic

deformations are thus very complicated, if they are at all possible.

Obviously the kneading action that fibrillates the fibers is essential

in order to make the fibers suitable for papermaking. However, know­

ledge regarding the amount of energy that is needed to achieve such

plastic deformations is still very limited. Before this knowledge is

obtained, all estimates of the efficiency of refining must be considered

to be of limited value.

Fundamentals of energy consumption

When a viscoelastic material like wood is subjected to a stressing -

destressing cycle, even at very small deformations, the material shows

a hysteresis loop in the stress - strain diagram, schematically shown in

fig. 1. The area within this loop represents energy taken up by the

material which is lost as internal frictional heat.

For linear viscoelastic deformations when both stress and strain are

sinusoidal functions of time, the energy consumed can be calculated from

the following relations:

giving

(1)

(2)

4.

(3)

strain as a function of time

= stress as a function of time

= energy adsorbed per cycle

= strain amplitude

stress amplitude

= angular frequency

= time

= phase angle

= mean strain

= mean stress

When the material is cyclically stressed to larger deformations it is

plastically deformed. In this case there is always a viscoelastic

component present, so that it is impossible to relate any area in a

cyclic test to plastic energy consumed. During the cyclic fatigue the

propertiesof the material continuously change. However the energy

needed per cycle for the creation of new surfaces is extremely small in

comparison with the viscoelastic energy absorbed, and it is in practice

impossible to quantify. Apparently, the minimum amount of energy to

cause a cohesive failure would be used if the material were immediately

stressed to the breaking point. However, as previously stated two types

of structural break-down are essential in mechanical pulping; the one to

create fibers with suitable fracture surfaces, the other to give the

fibers an internal fibrillation. The creation of micro-cracks distri­

buted within the fiber may well require a cyclic fatigue process which

by its nature consumes a lot of energy only as heat losses. Other

energy losses may also be envisaged, such as friction and idling losses

of transporting water, which should also be minimized. By experimental

studies and a theoretical model, Marton et al (10) estimated that only

about 20-55 % of the energy is consumed for the defibration and fiber

treatment. In the following, however, only the energy consumption due

to the deformation of wood is discussed.

where

5

Efficiency concepts

The efficiency of the refining process should be related to the energy

absorbed by the wood, as this represents work done in this operation.

In the viscoelastic range this energy is related to the viscous stress

a sin d, which represents the viscosity response where the mechanical

energy is lost as internal frictional heat. This energy is given by

(4)

where the loss modulus

This equation can be related to the efficiency concept E1 • A, given by

Hoglund et al (7), where A = P tan d, the internal friction, and E1 is

the storage modulus. This relation originates from a consideration of

refining treatment at a given degree of deformation, e , assuming that

the viscoelastic energy absorption is a measure of useful treatment.

The relation may be derived from equation 3 by substituting

(5)

However, relations such as this are of uncertain value since the visco­

elastic energy consumption is in no evident way related to the structu­

ral breakdown which is the essential process. We suggest instead that

the efficiency concept should be related to the fatigue process.

It may be anticipated that changes in the elastic modulus of the wood

reflect structural changes which are important for the creation of

flexible fibers. We thus propose that the efficiency of the refining

process be measured by the change in elastic modulus per unit energy

consumed in the process; either the integrated relative change in modulus

per total integrated energy, Q1 , or the differential change in modulus

per energy increment, Q2

6

where

Q = efficiency of the structural breakdown process

rE/Eo = relative change in modulus per cycle

n = number of cycles

At this stage it is not at all self-evident which is the most efficient

loading mode to cause internal fibrillation, nor is it clear in which

direction the modulus should be measured to reflect such changes in the

fibers. In this study the change in elastic modulus is measured in the

same loading mode as the cyclic fatigue process is carried out.

Experimental

The wood used has been heartwood of Norwegian spruce, Picea abies.

Specimens have been cut out for tests both along and across the grain.

The wood pieces were shaped with necks with a cross section of 7 x 7 mm

for tests along the grain and 15 x 50 mm across the grain. The test

pieces were carefully saturated with water before testing. The dry wood

samples were placed in an autoclave and evacuated for 3 h, after which

water was sucked in and a pressure of 0-5 MPa was applied for 4 h.

Unless otherwise stated the specimens were steam-treated for 1 h at

135°C prior to testing.

The mechanical measurements were carried out in an autoclave built up

around a servohydraulic equipment of the Material Testing System (MTS).

The deformation of the wood was measured with an extensometer attached

to the sample. Below 100 C the samples were sprayed with heated water

while above saturated steam was used. The temperature was measured with

(6)

(7)

7

a small thermocouple placed inside the sample. Viscoelastic measurements

were performed with a mean stress of zero while fatigue tests were done

in compression. Stress and strain signals were sampled 23 to 25 times

per loop and evaluated by a computer.

The energy consumption AW was calculated by integrating pairs of stress

and strain values from 10 consecutive loops, to obtain the mean area D

of a single hysteresis loop.

Tan 6 was calculated using this area, D, and the elastic strain energy U

according to the following equations:

(9)

In this presentation the elastic modulus referred to as E is given by

the absolute value of the complex modulus |E*| where

(10)

Due to the small value of tan 6 for wood |E*| is essentially equal to

the storage modulus E1. The modulus is based on the macroscopic

dimensions of the wood, thus including its void volume.

Viscoelastic properties

The elastic properties of wood have been measured under water-soaked

conditions between 20°C and 140°C, as shown in Figure 2. Here samples

have been tested without prior steam treatment. The tests were carried

out at a frequency of 10.0 Hz. The modulus along the grain is naturally

much higher than that across the grain by a factor of about 20. These

samples are subjected to an irreversible softening during the first rise

In temperature. This has also been noticed by Hoglund et al (7) and

similar effects by Stone (11) and Lagergren et al (12). After a steam-

(8)

8

treatment, if it is sufficiently lengthy, the modulus reaches an equi­

librium value, also noticed by Hoglund (7). Successive runs up and down

in temperature then have no effect on the temperature dependence of the

modulus. This irreversible softening may be due to hydrolytic bond

breakage between the wood components (13), especially of lignin-hemi-

cellulose bonds. This is also manifested in the increase of the mecha­

nical loss coefficient, tan 6, due to the steam-treatment, as exempli­

fied for samples across the grain in Figure 3. This maximum in internal

friction is related to the glass transition of the wet lignin, previously

stated by Atack and Heitner (14). Measurements at different frequencies

between 0.2 and 20 Hz show a shift of the loss maximum towards higher

temperatures of approximately 8,5 C per decade of frequency, from which

an apparent activation energy of 395 kJ/mol for this transition can be

calculated. This value indicates that the transition is indeed a glass

transition i.e. the T of wet lignin. Along the grain the reinforcing

microfibrils of cellulose diminish the role of lignin as a load-bearing

component and thus make the energy losses in the lignin lower. In the

perpendicular direction the stress is transferred through the middle

lamella making the energy losses in the lignin much more pronounced.

Plastic deformation

Compression and shear are presumably the dominating modes of loading in

the defibration processes occurring in a refiner or a grinder. Most of

the refining probably takes place across the grain of the wood or chips.

Here we have chosen to study compression fatigue across the grain as

being the most easily attainable.

The energy consumption in the linear viscoelastic range is, as previous­

ly stated, proportional to the loss modulus. This is given in Figure 5

as a function of temperature for samples along and across the grain.

The maxima occur at a somewhat lower temperature than the corresponding

maxima in the mechanical loss coefficient tan d. Usually the maximum in

tan 6 is used as a measure of the loss process as, in general and espe­

cially in this case, the loss modulus is so low that the scatter is

substantial. Across the grain, the loss modulus has its maximum at

about 80 C which, according to the efficiency relation (equation 5) pro­

posed by Hoglund et al (7), should correspond to the most efficient

defibration temperature at this frequency of treatment of 10.0 Hz.

9

Also, when the deformation is increased beyond the linear viscoelastic

region, the energy consumption per cycle is greater at 80°C than at

higher temperatures as seen in Figure 6 for samples across the grain.

Here the deformation of the sample has been successively increased after

about 200-500 cycles at each amplitude, thus some influence of a pro­

gressive fatigue of the wood may be present.

The increasing deformation affects the mechanical loss coefficient as

seen in Figure 7 where it is given as a function of stress and strain

amplitude for the case of cyclic compression across the grain. At low

stresses in the viscoelastic region tan 6 is almost unaffected by the

stress amplitude whereas at higher stresses the internal friction pro­

gressively increases partly due to a cyclic plastic deformation. When

the amplitude is again decreased it is revealed that a large portion of

the non-linear part of the curve is reversible which must be attributed

to a non-linear viscoelastic deformation. Thus these results support

the earlier statement that it is almost impossible to determine the

amount of plastic energy put into the system; the energy absorption can

only be inferred from the changes introduced in the material. For the

wood samples tested the permanent changes in the mechanical loss coeffi­

cient occurring at 100 and 135 C may either be due to structural changes

in the lignin or to a structural breakdown in the microfibril structure

making the lignin a more important load-bearing part. At 80 C the

deformation studied gives rise to no change at all in the loss coeffi­

cient tan 6. The elastic modulus also remains unaffected. Thus the

properties of the wood are not changed and the energy consumed during

the deformations shown in Figure 6 has only produced heat. The relating

of the maximum in the energy absorption to the efficiency in defibration

as proposed by earlier workers must therefore be viewed sceptically.

Cyclic deformations of sufficient amplitude will ultimately fatigue a

material to the point of cohesive failure. Under these conditions the

test sample will continuously change its properties, which may be ob­

served as a change in the elastic modulus. This effect is shown for

wood in Figure 8 for four different compression loads for tests across

the grain at 100°C. Apparently the larger the amplitude the less energy

has to be put in to reach a certain breakdown of the structure. If the

level of deformation is too low, here corresponding to a compressive

10

stress of 170 kPa, the treatment evidently has no effect although energy

is still consumed. The end point here represents 100 000 cycles. It is

thus evident that large amounts of energy that only give rise to heat

can easily be supplied to the system. In Figure 9 where the efficiency

Q1 of the process according to equation 6 is plotted against energy ab­

sorbed it is evident that the process rapidly becomes less efficient as

the number of cycles increases. Similar curves are obtained for the

differential efficiency Q2. Thus only the first few deformations are

really effective and each subsequent cycle contributes very little to

the structural breakdown. However, it may well be that the structural

changes if the wood is subjected to an impact type of deformation are

different from those achieved by small scale deformations so that a

fatigue process is actually necessary. Merely from the standpoint of a

structural breakdown, the higher the stress amplitude the more efficient

the process.

To compare samples tested at different temperatures a good reference is

to compare at equal energy consumption per cycle. In Figure 10, fatigue

tests at the two temperatures of 80 and 100 C have been compared at an

energy consumption per cycle of 3700 J/m3. From the relative change in

modulus the mechanical treatment is judged to be more effective at

100 C. These results then indicate that higher temperatures should be

used than those indicated by previous viscoelastic estimates, which at

this frequency of 10.0 Hz indicate 80 C as being optimal. However,

until the structural changes at the different temperatures have been

classified care should be taken since changes can occur in both the

middle lamella lignin and the fiber phase, whereas the changes in the

fiber phase will be the most valuable. One should also bear in mind the

importance of the lignin softening temperature as a factor determining

the fracture surface between fibers.

Final remarks

Two processes, the one of creating fibers with suitable fracture surfaces

and the other of giving the fibers an internal fibrillation, have to be

optimized in order to improve the efficiency of the defibration process.

The maximum in internal friction of wet wood around 95°C, 10.0 Hz,

reflects the softening of the lignin component. This softening maximum

11

has some importance for the fracture surface between fibers (15). Thus

to produce long intact fibers with good bonding areas the lignin has to

soften somewhat but not to the extent that the separation takes place in

the middle lamella. Microscopic examination of fracture surfaces indicates

that at 100 C the separation takes place in the middle lamella whereas

at 80 C the fibers have been broken along the cell wall.

The measurements in this investigation have been focused on the internal

fibrillation, i.e. the structural breakdown of wood in fatigue. It is

shown that these structural changes, due to cyclic compression deformations,

are introduced with lesser amounts of energy consumed at 100 C than at

80 C. Due to the fact that the energy consumed in this process is a

large part of the total energy consumption, attention has to be focused

on its optimisation. The structural breakdown process is thus more

efficiently performed at higher temperatures than the optimum temperature

for fiber separation. It therefore seems probable that the fiber sepa­

ration and the flexibi1ization should be carried out at different tempe­

ratures in order to increase the efficiency of the refining processes.

Acknowledgements

The authors would like to extend their thanks to Mr Sune Karlsson for

his excellent contribution to computer programs as well as to the running

of the tests. Thanks are also due to Mr Petter Kolseth for valuable

comments on the manuscript as well as to Mr Anthony Bristow for linguis­

tic revision. Financial support from "The National Swedish Board for

Technical Development" is gratefully acknowledged.

Literature cited

1. Van der Akker, J.A., "Energy considerations in the beating of pulp"

In "Fundamentals of Papermaking fibers". Ed. Bolam, F. Techn. S e c ,

British Paper and Board Makers Ass. 1958, p. 435.

2. Lamb, G.E.R., "Energy consumption in mechanical pulping", Tappi

45 (1962) No. 5, 364.

12

3. Atack, D., May, D., Morris, E.L. and Sproule, R.N., "The energy of

tensile and cleavage fracture of black spruce", Tappi 44 (1961)

No. 8, 555.

4. Stone, J.E., Scallan, A.M. and Abrahamson, B., "Influence of

beating on cell wall swelling and internal fibrillation", Svensk

Papperstidn. 71 (1968) No. 19, 687.

5. Atalla, R.H. and Wahren, D., "On the energy requirement in refining"

Tappi 63 (1980) No. 6, 121.

6. Nissan, A.H., Lectures on fiber science in Paper. Joint Textbook

Committee of the Paper Industry, Tappi, Atlanta, 1977.

7. Hoglund, H., Sohlin, U. and Tistad, G., "Physical properties of

wood in relation to chip refining", Tappi 59 (1976) No. 6, 144.

8. Vikstrom, B. and Hammar, L.-A., "Defibration in chemimechanical

pulping", Svensk Papperstidn. 82 (1979) No. 6, 171.

9. Becker, H., Hoglund, H. and Tistad, G., "Frequency and temperature

in chip refining", Paperi ja Puu 59. (1977) No 3, 123.

10. Marton, R., Tsujimoto, N. and Eskelinen, E., "Energy consumption

in thermomechanical pulping". Paper given at the Symp of Fundamental

Concepts of Refining, Appleton. Sept 16-18, 1980.

11. Stone, J.E., "The rheology of cooked wood. II Effect of temperature".,

Tappi 38. (1955) No. 8, 452.

12. Lagergren, S., Rydholm, S. and Stockman, L., "Studies on the

interfibre bonds of wood. Part I: Tensile strength of wood after

heating, swelling and delignification"., Svensk Papperstidn. 60

(1957) No. 17, 632.

13. Baldwin, S.H. and Goring, D.A.I., "The thermoplastic and adhesive

behaviour of thermomechanical pulps from steamed wood", Svensk

Papperstidn. 71, (1968) No. 18, 646.

13

14. Atack, D. and Heitner, C, "Dynamic mechanical properties of

sulphonated eastern black spruce", Pulp Paper Can., Trans. Techn.

Sec. 5 (1979) No. 4, TR 99.

15. Koran, Z. , "Energy consumption in mechanical fiber separation".,

Paper presented at the CPPA annual meeting 1980 in Montreal, Canada.

1981-12-01

NLS/ili/CB

14

Figure 1. The hysteresis loop; relation between stress and strain in a dynamic mechanical test with sinusoidally varying strain.

0 50 100 150 Temperature, °C

Figure 2 The dynamic elastic modulus for wood samples along and across the grain. For these samples that have not been steam treated before the test an irreversible softening is seen during the first rise in temperature.

0 20 50 100 150 Temperature, °C

Figure 3 Mechanical loss coefficient as a function of temperature for

samples across the grain that have not been steam treated

before the test.

Figure 4 Mechanical loss coefficient as a function of temperature for

steam-treated samples along the grain.

15

Figure 5. The loss modulus

E" for wood samples along

and across the grain.

0 50 100 150 Temperature, °C

Figure 6 Energy consumption per cycle as a function of strain amplitude

at various temperatures for tests in compression across the

grain.

16

17

(b)

Figure 7 Mechanical loss coefficient as a function of (a) stress and (b) strain amplitude for test on samples in compression across the grain at various temperatures. The stress is successively increased after about 200-500 cycles at each level and finally reduced to the initial stress level.

Figure 8 The permanent change of the elastic modulus as a function of

energy absorbed for fatigue tests in compression across the

grain at 100°C. Parameter is the stress range 2 oo.

Figure 9 Efficiency of the structural break-down process as a function

of energy absorbed for tests in compression across the grain

at 100 C, Parameter is the stress range 2s0 .

18

Figure 10 The permanent change of the elastic modulus as a function of

energy absorbed. The two temperatures 100°C and 80°C are

compared at the same energy consumption per cycle.

19

Paper IV

Simple stress-strain measurements on dry papers from —25°C to 250°C N. LENNART S A L M E N and ERNST L. BACK, Swedish Forest Products Research Laboratory, Stockholm

K E Y W O R D S : Tensile strength, Elastic strength, Stretch, Thermal properties, Glass transition temperature, Paper

S U M M A R Y : A method has been developed for determina­tion of the stress-strain properties of dry paper at high tem­peratures. Such determinations can give useful information with respect to the mouldability of fluting in the corrugating process.

In this method the strength characteristics are measured within a few seconds in an environment of a pre-thermostated oil which does not affect the properties of the paper. The tensile strength and the modulus of elasticity appears to de­crease about linearly from —25°C up to 170°C while the stretch at rupture increases slightly. Above 200°C for a fluting the modulus of elasticity decreases rapidly while the stretch increases considerably due to softening of the mate-rial.

The method can easily be used to estimate the glass transi­tion for paper qualities under dry conditions.

• En metod har utvecklats for bestamning av spiinnings-tojningsegenskaper for torra papper vid hoga temperaturer , dvs upp till 250°C. Bestamningarna kan ge informationer om bl a formbarheten hos fluting i korrugeringsprocesscn.

Med denna metod mats papperets styrkeegenskaper inom nagra fa sekunder i en omgivning av en inert fortermosta-terad olja, som inte paverkar papperet. Brottlasten och elasti-citetsmodulen har befunnits minska linjart med temperaturen mellan —25° och 170°C, medan brottojningen okar nagot. Vid temperaturer over 200°C minskar elasticitctsmodulen snabbt, medan brottojningen exempelvis hos fluting okar be-tydligt, vilket tydcr pa mjukgoring av materialet.

Denna metod kan anvandas for att pa enkelt satt uppskatta glastemperaturen for papperskvaliteter i vattenfritt tillstand.

• Eine Methode zur Messung von Spannungs-Dehnungs-Eigenschaften fur trockene Papicre bei hohen Temperaturen bis auf 250°C ist entwickelt worden. Diese Messungen geben verwendbare Information iiber z.B. die Formbarkeit von Fluting-Papier.

In dieser Methode werden die Festigkeitscigenschaftcn in einigen Sekunden in einer Umgebung von vorerwarmten inerten Ol registriert, welche die Eigenschaften des Papiers nicht beeinflusst.

Der Bruchwiderstand und das Elastizitatsmodul hat eine lineare Abnahme zwischen —25°C und 170°C, wahrend die Bruchdehnung eine kleine Steigerung hat. Uber einer Tem-peratur von 200°C fiillt der Elastizitatsmodul sehr schnell ab, wahrend die Bruchdehnung bei Fluting-Papieren ansteigt, was einem Erweichen des Materiales entspricht.

Mit dieser Methode ist es moglich, die Glastemperatur von trockenen Papieren einfach zu ermitteln.

ADDRESS OF T H E A U T H O R S : STFI , Box 5604, S-114 86 Stockholm, Sweden.

In the corrugating nip, the fluting is softened under the influence of moisture and heat at temperatures around 150°C reached in the fluting. This makes it possible to form the desired corrugating profile. Absence of moist­ure will increase the softening temperature for the ligno-cellulosic material (1). One way of estimating the mould-ability of a paper is to study its modulus of elasticity versus temperature. Therefore a method has been devel­oped which enables stress-strain measurements on dry paper up to 250°C. In this method a dry paper strip is quickly heated by a pre-thermostated oil and tested with­in 10 seconds.

At high temperatures there will occur some auto-cross-

linking of the cellulosic material (2). The rate of cross-linking increases exponentially with temperature. Below 250°C the reaction velocity is slow enough to permit testing periods less than 10 seconds without any signi­ficant influence on the dry stress-strain properties of the paper (3). The short testing period in the method present­ed thus minimizes the influence of auto-crosslinking as well as the influence of degradation of lignocellulosic material. It is made possible due to the good heat capac­ity of the oil and the rapid heat transport from the penetrating liquid to the solid paper.

Previously the modulus of elasticity has been measur­ed at high temperatures with indirect methods, such as a torsional pendulum (4) and sonic pulse technique (5), but no direct measurements have been published for tem­peratures up to 250°C.

Apparatus

The equipment consists of a testing chamber connected to a temperature regulating oil bath. Fig. 1 shows the testing chamber which contains:

• two clamps for tensile tests of paper permitting a test­ing span of 100 mm. The upper clamp is movable

• a contact regulating thermometer • an additional heater • a control thermometer • outlets and inlets for oil and air.

The paper strip is inserted between the clamps in the chamber through a hermetically closeable door at the front of the chamber. The chamber space is only 1.5 litres which makes the filling time short. The contact surfaces of the paper clamps are either blasted or ground with cemented emery-cloth to prevent slipping. To fill up, circulate and empty the oil in the chamber, a circu-

Reprint from S v e r s k p a p p e r S t i d n i n g no. 6-1977 80 (1977) 178—183

lating pump is connected to the thermostatic bath. The oil must stand temperatures up to 250°C without degrad­ing and its viscosity must allow pumping. It must not in any way affect the paper. The vapour pressure of the oil sets a limit to the temperature possible to apply because of endurable working conditions. Therefore, the ventila­tion around the apparatus must be efficient. It is suitable to use oils of different viscosities to cover the total tem­perature range up to 250°C. Silicone oil DC 200 (6) of the viscosity 20 mm2/s has been used for the tempera­ture range —25° to +50°C, while the viscosity 50 mm2/s has been used to cover the range 20° to 170°C and the viscosity 350 mmVs from 150° to 250°C. However, if this oil is used frequently above 200°C, it has to be ex­changed after a period of time because of degradation. To avoid this, a high temperature stable silicone oil DC 710(6) can be used.

Experimental procedure In order to improve the reproducibility, commercial pa­pers to be tested were cycled twice between 45% and 90% relative humidity to remove most or part of the drying stresses. This was checked by measuring the hygro-instability (7). Handmade paper sheets were dried stress free between two blotting-papers. From these sheets strips were cut 150 mm long and 15 mm wide and dried at a temperature of 20°C over P205 according to the following two steps:

1. Appropriate number of samples two days in a desic­cator.

2. Two days in a hermetically closed test tube above P2O5.-, one sample per tube.

No remaining moisture was detectable after this pro­cedure according to drying tests (8) or Karl Fischer titra­tion (9). After 30 seconds exposure in air of 15% r.h., 20°C the samples still showed a moisture content less than 0.5%. During the experiment the oil bath and the testing chamber were thermostated with the oil circulat­ing. If temperatures less than 150°C were to be used, the oil was dried with anhydrous calcium sulphate, always present in the oil in small bags. The remaining moisture content in the oil could easily be checked by Karl Fischer titration (10). Care was taken to arrange for a relative humidity of the surrounding air as low as possible. The equipment thus was set up in a room of 15% relative humidity and 20° C.

The paper strip to be tested was quickly transferred from the test tube to the clamps and the chamber im­mediately filled with oil. The total time elapsed until the oil reached the paper was less than 30 seconds, normally 20 seconds, so that any water absorption would be very small. In an additional 10 seconds, normally in 5 seconds, the sample is totally submerged and the straining opera­tion completed. The time for the sample to reach tem­perature equilibrium at 250°C has been measured to about 2 seconds. The stress-strain testing was performed at a constant strain rate in an universal tensile tester, type Alwetron TCT 20 (11).

Reliability and reproducibility of the method

The silicon oil used has been tested for its influence on paper properties. For three different dry papers, a kraft sack paper of 114 g/nr, a fluting A of 112 g/nf and a fluting B of 112 g/m8, the dry stress-strain characteris­tics in air and oil have been compared at 20°, 45° and 90°C. Fig. 2 and table 1 show that no significant dif­ference can be detected between testing conditions in oil

Table 1. Stress-strain characteristics and their 95% con­fidence intervals based on ten strips, for dry papers in oil respectively air at 20°, 45° and 90°C. Strain rate 1.7 • W-3/s(0.l7%/s).

Fig. 2. Stress-strain diagrams for fluting A of 112 g/m2 in the machine direction surrounded by oil and air respectively at different temperatures. Strain rate 1.7 • 10 Vs (0.17%/s). Mean curves based on six strips.

179

Table 2. Stress-strain characteristics and their 95% con­fidence interval based on six strips, for dry paper a flut­ing B of 112 g/m' in MD in oil at different wetting periods (upper part of table) and at different heating periods (lower part of table). Strain rate 1.7 • 10~s/s (0.17%/s).

Tests have also been made in silicon oil, both after 10 seconds and 5 minutes of thermostating at 45° and 150°C with identical results, which is obvious from the upper part of table 2.

From the time the paper strip is removed from the drying test tube until it is totally wetted by the oil, it has the possibility of absorbing moisture during a short period of time, usually 20 seconds, in air of 15% r.h., 20°C. This period, however, is so short that the resulting moisture content in the papers becomes less than 0.5%. The papers are also further dried in the oil, because a drying agent is added if the temperature used is less than 150°C. Thus the negligible amount of water present could not appreciably affect the stress-strain properties which may be accepted as valid for dry paper (12).

At high temperatures some degree of auto-crosslinking of cellulose and hemicellulose always occurs, resulting in an increasing modulus of elasticity and, to a lesser de­gree, an increasing tensile strength while the stretch at rupture will decrease (13).

The influence of auto-crosslinking was determined by a comparison performed at 190°C after 5 seconds and 60 seconds, and at 250°C after 5 seconds and 15 seconds of thermostating in the oil. The lower part of table 2 shows no significant difference on the dry stress-strain proper­ties for these different heating periods. Therefore, in this method the effect of crosslinking as well as the effect of any degradation of the ligno-cellulosic material can be neglected when the total testing period is maximum 10 seconds in the oil for temperatures up to 250°C.

The standard deviation expressed as the coefficient of variation of the tensile strength, of the stretch at rupture and of the modulus of elasticity is rather constant over the whole temperature range from —25° to 250°C, as shown in fig. 3. The coefficient of variation is also of the same magnitude as that found in normal stress-strain testing of paper (3). Obviously, the coefficient of varia-

-50 0 100 200 °C temperature

Fig. 3. Coefficient of variation versus temperature for tensile strength, triangels, stretch at rupture, squares and modulus of elasticity, rings, of fluting B of 112 g/m2 based on six strips per temperature. Strain rate 1.7 • 10-Vs (0.17%/s).

tion is somewhat larger for the stretch at rupture than for the other two parameters.

The dependence of the stress-strain curve on temperature The influence of temperature on the stress-strain charac­teristics of dry paper was investigated between —25 °C and +250°C. The following results refer to the machine direction, MD, and the cross direction, CD, of one quali­ty of fluting of 112 g/m2. As it could be expected the stress-strain curves were greatly influenced by the tem­perature. This is illustrated in fig. 4. The curves can be

Fig. 4. Stress-strain diagrams of a dry fluting of 112 g/m2 in the machine direction at different temperatures. Strain rate 1.7 • 10-3 /s (0.17%/s). Mean curves based on six strips at each temperature.

180

Fig. 5. Tensile strength, stretch at rupture and modulus of elasticity versus temperature for a dry fluting of 112 g/m2. In the machine direction, one strip for each marking and in the cross direction, a mean curve based on one strip for each 2°C. Strain rate 1.7 • 10Vs (0.17%/s).

connected at their points of rupture to a failure envelope (14), which might be a suitable way of comparing dif­ferent papers when subjected to stress at different tem­peratures.

The tensile strength and modulus of elasticity will decrease when the temperature is increased. Anderson and Berkyto (12) have earlier determined the stress-strain properties of dry paper in nitrogen atmosphere between —50 and + 150°C. In their study, they present­ed a linear decrease of the tensile strength and of the modulus of elasticity respectively versus the temperature. This is in line with the results of our measurements, shown in fig. 5, where an approximate linearity up to 170°C is indicated although deviations can be noticed. The stretch at rupture increases sligthly in MD and doubles in CD up to 200°C. The slow decrease with tem-perature of the clastic modulus is something that nor-mally occurs for glassy polymers (15).

Above 200°C there is a marked decrease of the mod-ulus of elasticity in MD, which according to theories applicable to polymers resembles the effect of a glass transition, i.e. a softening of the glassy polymer (15). This decrease is smaller than that of amorphous poly-mers, which is understandable since the chrystalline cel-lulose in the paper does not transform at this tempera-ture. The softening is also seen in the stretch at rupture which increases considerably above 220°C both in MD and CD, while the tensile strength decreases slightly more at temperatures above than below 200°C .

In normal stress-strain testing of paper, the tensile strength increases with an increase of the strain rate. The modulus of elasticity sometimes increases as well, depending on paper and the span used, while the stretch at rupture is rather unaffected (16, 17).

These relations have been found similar over the whole temperature range. Fig. 6 shows this for the elastic modulus.

Above the glass transition, however, the stretch at rupture increases, with increasing strain rate. This effect

Fig. 6. Modulus of elasticity versus temperature at different strain rates for a dry fluting of 112 g/m3 in the machine direction based on six strips for each temperature interval of 100C.

could be due to the auto-crosslinking reactions occurring during the longer testing times. For the modulus of elasticity, which is always determined at testing times less than 10 seconds, these reactions will have no effect.

According to the results obtained the method present­ed can be used for the estimation of the glass transition of dry paper. Such direct stress-strain measurements are also useful for the evaluation of the strength and stretch of paper at high temperatures, where such knowledge is valuable. Results for different paper qualities will be discussed in a following paper.

Determination of the glass transition

The glass transition temperature is of interest since at this temperature a paper softens more significantly and therefore, for example, becomes more mouldable. Many polymer properties change their temperature derivative at the glass transition temperature. In this investigation the glass transition is defined as the inflection point of the rapidly descending S-formed curve of the modulus of elasticity versus temperature (18).

For the estimate of the glass transition of papers, three testing conditions have to be considered, the strain rate, the fiber orientation in the sheet and the distribution over the chosen temperature Interval of the number of strips to be evaluated. It has been mentioned earlier, that an increasing strain rate increases the modulus of elastic­ity and that the change in the modulus of elasticity around the glass transition becomes more distinct, as shown in fig. 6. In the machine direction of a paper, the change in the modulus of elasticity with temperature is greater than in the cross direction (5). Fig. 7 shows that it would be of advantage to use the machine direc­tion for evaluating the glass transition. This might be due

150 200 250 °C

temperature

Fig. 7. Modulus of elasticity versus temperature for different testing series of a dry fluting of 112 g/m2. Strain rate 1.7 • 10-3/s (0.17%/s).

182

0 100 200 °C temperature

Fig. 8. Temperature derivative of modulus of elasticity versus temperature for a dry fluting of 112 g/m- in the machine di­rection based on one strip for each 2°C. Strain rate 1.7 • 10-;7s (0.17%/s).

to the higher elastic modulus in the machine direction partly caused by dried in stresses, which presumably are released to a larger extent above the glass transition of the paper.

To determine a suitable distribution of tests over the temperature interval, two series were compared, one with one paper strip for each 2°C, and one with six pa­per strips every 10°C. For the last series, as stated above, the standard deviation of the measurements was similar to that of normal tensile tests of paper i.e. a coefficient of variation of 4 to 6%. This means that the absolute value of the standard deviation decreases with the tem­perature determinations of the modulus of elasticity and the tensile strength but increases with the temperature as far as stretch at rupture is concerned. Fig. 7 indicates that the glass transition can be more easily estimated if the test strips are distributed one by one with small tem­perature intervals. This is clearly shown in fig. 8, where the temperature derivative of the modulus of elasticity above 200°C shows a peak at the glass transition. This derivate is calculated successively from a linear regres­sion of the modulus of elasticity versus temperature, over 20 following tests, with use of a dator.

The stretch at rupture, presented in fig. 5 versus tem­perature increases rapidly above 220°C both in MD and CD, also indicating the glass transition. The temperature derivative of the stretch at rupture, calculated as above, has a large peak at the same temperature as the modulus of elasticity and also nearly the same curvature, although the correlation at low temperatures is less pronounced. These both temperature derivative curves indicate two secondary transitions previously reported. These transi­tions are, however, according to Klason and Kubat (19), induced by small amounts of residual water. From all these results for the 112 g/m2 fluting B the glass transi­tion is estimated to 220° C.

Accordingly, the best way to estimate the glass transi­tion is from stress-strain measurements in the machine direction at a high straining rate and with one strip each at small temperature intervals, evaluating both the modulus of elasticity and the stretch at rupture.

References 1. Goring, D. A. L.: Thermal softening of lignin, hemicel-

lulose and cellulose. Pulp Paper Mag. Can. 64 (1963) 12, T-517.

2. Back, E. L.: Thermal Auto-Crosslinkine in Cellulose Material. Pulp Paper Mag. Can 68 (1967) 4, T-165.

3. Stenberg, E., Back. E. I..: Vatforstyvning genom varme-bchandling av lopande pappcrsbana. Del 1. Inverkan pa euenskaper hos kraftliner i vatt och torrt tillstand. Medd. Svenska Triiforskningsinst. B.164 (1973).

4. de Ruvo, A. et al.: The influence of temperature and humidity on the elastic and expansional properties of paper and the constituent fibre. Medd. Svenska Triiforsk­ningsinst. 13:183 (1973).

5. Back, E. L.. Didriksson, E. 1. E.: Four secondary and the glass transition temperatures of cellulose, evaluated by sonic pulse technique. Svensk Papperstidn. 72 (1969) 687.

6. Dow Corning 200 fluid. Dow Corning Internat ional Ltd., Brussels, Belgium.

7. SCAN-P 28:69. Dimensions-hygroinstabilitet hos papper och papp. Svensk Papperstidn. 72 (1969) 411.

8. SCAN-P 4:63. Fukt i papper och papp. Svensk Pappers­tidn. 66 (1963) 6.

9. Mitchell. J. Jr.: Determination of moisture in native and processed cellulose. Ind. Eng. Chem., Anal. Ed. 12 (1940) 390.

10. Almy. E. G.. Griffin. W. C, Wilcox, C. S.: Ind. Eng. Chem. Anal . Ed. 12 (1940) 392.

11. Alwetron, T C T 20. Lorentzen & Wett re , Stockholm, Sweden.

12. Andersson, O., Berkyto, E.: Some factors affecting the stress-strain characteristics of paper. Svensk Papperstidn. 54(1951) 437.

13. Stenberg, E., Back, E. L.: Vatforstyvning genom varme-behandling av lopande pappersbana. Del 2. Egcnskaper hos behandlad fluting. Medd. Svenska Traforskningsinst. B.191 (1973).

14. Smith, J. P.: J. Polymer Sci. Al (1963) 3597. 15. Meares, P.: Polymers structure and bulk properties. Van

Nostrand Co. Ltd., London (1967). 16. Malmberg, B.: Remslangd och tojningshastighet vid

spannings-tojnincs-matning pa papper. Svensk Pappers­tidn. 67 (1964) 690.

17. Gdttsching, L.: Das Festigkeitsverhalten von Papier unter statischer und dvnamischer Beanspruchung. Papper och Tra 9 (1970) 535."

18. Mercier, J. P.: Glass transition temperature . J. of Paint Techn. 43 (1971) 561.

19. Klason. C, Kuhat, J.: Thermal transitions in cellulose. Svensk Papperstidn. 79 (1976) 494.

(Manuscript received September 30. 1976)

Paper V

Effect of temperature on stress-strain properties of dry papers N. LENNART SALMEN and ERNST L. BACK, Swedish Forest Products Research Laboratory, Stockholm

KEYWORDS: High temperature tests, Low temperature tests, Paper grades, Rupture work, Stress strain properties, Tensile strength.

SUMMARY: The influence of temperature on the tensile stress-strain properties of dry papers has been evaluated in the range of —25° to +250°C. The tensile strength appears to decrease linearly with temperature up to 200°C and for all papers investigated with approximately 0.3% per °C. A linear decrease with temperature has also been found for the loga­rithm of the modulus of elasticity up to 200°C, with for all papers investigated about equal slope of 0.0031 per °C in accordance with a theory of Nissan.

The thermal softening of the dry papers can be noticed in the modulus of elasticity above 220°C especially for the pa­pers of higher pulping yield. For these papers there is also a pronounced increase in the stretch at rupture, accompanying this softening.

The most characteristic difference between papers of dif­ferent chemical composition is in the failure envelope, i.e. the line connecting the points of rupture of the stress-strain diagrams over the temperature range.

• Tempcraturens inverkan pa spannings-tojningsegenskaper-na hos hclt torra papper har studerats i temperaturintervallet fran —25° till +250°C. Harvid har dragstyrkan befunnits minska linjart med temperaturen upp till 200°C med ungefar 0,3 % per °C for alia papper. Upp till 200°C minskar aven logaritmen for elasticitetsmodulen linjart med temperaturen och med nastan samma lutning for alia har undersokta pap­per i cnlighet med en teori av Nissan. Over 220°C framgar av elasticitetsmodulen den termiska mjukningen av de torra papperen, speciellt de av hogt utbyte. For dessa papper note-ras ocksa samtidigt en betydande okning i brottojning, vilken atfoljer mjukningen.

Den mest karakteristiska skillnaden mellan papper av olika kemisk sammansattning ligger i den s k granskurvan for brott, dvs den linje som over hela tcmperaturomradet sam-manbinder brottpunkten i belastnings-tojningsdiagrammen.

• Dcr Einfluss dcr Tempcratur auf die Spannungs-Deh-nungs-eigenschaften von trockenen Papieren ist fur das Tem-peraturintervall von —25° bis +250°C untersucht worden. Es zeigt sich, dass die Bruchspannung mit der Temperatur bis 200°C linearen abnimmt mit ungefahr 0.3% per °C fiir alle Papicre. Bis 200°C fallt auch der Logarithmus des Elastizi-tatsmoduls fiir die Papiere gleichartig lineare ab.

Uber 220°C zeigt der Elastizitatsmodul der trockenen Pa­piere von eine Erweichung des Materiales. Diese ist merk-bar speziell fur Papiere von hoher Ausbeute. Solche Papiere bekommen auch eine bedeutende Steigerung von der Bruch-dehnung durch die Erweichung.

Ein karaktaristischer Unterschied zwischen Papieren verschicdener chemischer Zusammensetzung ist die umfas-sende Bruchgrenzkurve, dass heisst die Linie die die Bruch-stelle der Spannungs-Dehnungs-kurven uber das untersuchte Temperaturbereich zusammenbindet.

ADDRESS OF THE AUTHORS: STFI, Box 5604, S-114 86 Stockholm, Sweden.

Due to experimental difficulties the influence of the temperature on physical properties of paper has not been adequately studied even though the temperature is important in many operations such as the corrugating of fluting, the filling of paper sacks with hot cement, storage of frost packings and maintaining paper strength in the drying section.

In this report the stress-strain characteristics of dry papers are given for the interval from — 25°C to

Fig. 1. Stress-strain diagrams of a dry NSSC fluting of 112 g/nr in the machine direction, MD, and the cross direction. CD. Strain rate 1.7 • 10-Vs (0.17%/s). Mean curves based on six strips at each temperature.

+ 250°C. The method used is based on direct heating or cooling of the dry paper in an inert pre-thermostatcd silicon oil. The period of heating is so short that the influence of cross-linking or degradation reactions even at 250°C is negligible (1).

Influence on stress-strain curves, "failure envelopes"

The temperature exerts a considerable influence on the mechanical properties of paper as well as on other ma­terials. This is illustrated in fig. 1, where the tensile properties of a NSSC fluting is shown both in the machine and in the cross direction. It is evident that the tensile strength and the modulus of elasticity are greatly reduced in both directions when the temperature is in­creased from —25 °C to +250°C.

The influence of temperature is better visualized if the points at rupture are connected to form a failure en­velope (2). This is illustrated in fig. 2 for different pa­pers: kraft sack paper, NSSC fluting, paper made from thermomechanical pulp, and machine-glazed sulphite paper. The yields and chemical compositions of the pulps from which the different papers were made are given in table 1. Apparently there are significant differences between these papers, especially in the values of the stretch at rupture as a function of the temperature. The two papers of the highest pulping yield and with a high lignin content show a marked increase in the stretch at rupture above 220°C, while the modulus of elasticity decreases more rapidly above than below this tempera-

Reprint from S v e n s k papperstidning no. 10-1978 81 (1978) 341—346

Fig. 2. Failure envelopes of dry papers: a kraft sack paper of 114 g/ma, a NSSC fluting of 112 g/m2, a machine-glazed, MG, sulphite paper of 78 g/nr and hand sheets of spruce thermo-mechanical pulp, TMP, of 150 g/m2. The papers are tested in the machine direction at a strain rate of 1.7 • 103/s (0.17%/s) and for the TMP-paper at 5.0 • 10-3/s (0.50%/s).

ture. This should be indicative of the softening of amorphous components in these papers. The papers of low pulping yield have too high a content of crystalline material for such pronounced softening of amorphous polymer material to appear.

Table 1. Pulping yield and relative composition of the papers investigated.

The effects of lignin and amorphous carbohydrates in this respect will be treated in some detail in a following paper.

Thus, the effect of temperature on the mechanical properties of papers seems mainly to be a function of the composition of the paper. This may again be illus­trated in fig. 3 both by comparing stress-strain curves for two papers similar in chemical composition, a kraft sack paper and a kraft liner in the machine direction and by comparing stress-strain curves for NSSC fluting in the machine and cross directions respectively, in both cases with temperature as parameter. The temperature dependence is similar. For the kraft papers the failure

Fig. 3. Comparison of failure envelopes of dry papers of similar chemical composition: a kraft liner of 110 g/m3

compared with a kraft sack paper of 114 g/m2 both tested in the machine direction, a NSSC fluting of 112 g/m' tested in the machine and in the cross direction, MD and CD respec­tively.

342

Cotton linters

Kraft sack paper (Pinus silvestris)

Kraft liner (Pinus silvestris)

Machine-glazed sulphite paper (Picea abies)

Fluting (mainly Be tula verrucosa)

Thermomechanical pulp, spruce (Picea abies)

Thermomechanical pulp,

(Populus tremuloides)

46

48

~ 5 2

80

~ 9 5

~ 9 4

100

77

76

74

48

46

50

16

16

17

29

25

27

7

8

9

23

29

23

Paper Pulping Composition % Yield Cellulose Hemicel- Lignin % lulose

Fig. 4. Tensile index versus temperature for dry papers: isotropic hand sheets of cotton linters of 100 g/m2 and of a spruce thermomechanical pulp, TMP, of 150 g/m2, a kraft liner of 110 g/m2 and a machine-glazed, MG, sulphite paper of 78 g/nr. The kraft liner and the sulphite paper are tested in the machine direction.

envelopes are about parallel, with the kraft sack paper showing highest stretch at rupture. The failure envelope for paper made from cotton linters displays about the same temperature dependence as that of papers made from low yield pulps.

Effect on tensile strength

The tensile strength of dry papers decreases monoto­nously as the temperature increases from —25°C to 250°C. In the glassy state, i.e. for the dry papers below 200°C, a linear relation is found between tensile strength and temperature for all papers investigated in this work. This is illustrated in jig. 4 for papers made from cotton linters and from thermomechanical pulp, for kraft liner and a machine-glazed sulphite paper.

A linear relationship between tensile strength and temperature for dry papers has previously been reported

for a few papers in the interval from —50°C to +150°C (3).

Theoretically, fracture under load in polymer solids can be described as a thermally activated process (4), also found experimentally for creep measurements on board (5) and papers (6). Thus the tensile strength should be inversely proportional to the temperature, in agreement with the results of fig. 4.

The relative decrease in strength based on the value at 20°C and given in table 2 is nearly the same for all papers investigated, varying from 0.25 to 0.35% per °C. Above 200°C the tensile strength for the papers listed which still can be subjected to a high stress deviates from the linear relationship and the rate of reduction increases. This is perhaps an effect of chain or fiber pull out or a combined effect of such occurrence which has a tendency to take place near or above the softening temperature of the amorphous bonding material (7).

Effect on tensile energy absorption

The influence of temperature on the tensile energy absorption (TEA) of dry papers cannot easily be genera­lized. Table 3 shows that the TEA decreases with the

Table 3. Tensile energy absorption of dry papers at dif­ferent temperatures.

Cotton linters Kraft sack paper, MD Kraft liner, MD Machine-glazed sulphite paper, MD Fluting, MD Fluting, CD Thermomechanical pulp, spruce Thermomechanical pulp,

Average coefficient of variation 10%.

Table 2. Tensile properties of dry papers and their temperature dependence.

1.7 24.0 1.20 1.7 87.6 5.0 86.0 1.7 92.3 1.7 76.2 1.7 31.6 5.0 43.6 5.0 29.5

4.95 5.13

10.53 4.56 1.80 1.99 1.54

3.50 2.12 1.77 0.93 1.08 1.28 2.07 2.16

0.32 0.29 0.26 0.35 0.25 0.25 0.27 0.33

2.9 3.1 2.8 2.5 3.2 3.5 3.2 3.7

Average for papers tested: 0.29 3.1

Cotton linters Kraft sack paper, MD Kraft liner, MD Machine-glazed sulphite paper, MD Fluting, MD Fluting, CD Thermomechanical pulp, spruce Thermomechanical pulp, aspen

65 117 98

39 50 28

90

59

— 110 89

24 49 35

99

52

30 82 76

14 48 35

94

40

30 54 54

7 40 33

79

34

30 31 23

3 30 28

90

35

Strain Dry properties at 20° C Relative decrease d(1nE)/dT rate in strength, based ( — l 0 - 3 / C ) (10 -3/s) Tensile Modulus of Stretch at on strength at 20°C

index elasticity rupture (%/ oC) (Nm/g) (GPa) (%)

temperature for papers produced from low yield pulps, but there is a difference between the papers. High yield pulps appear to imply a nearly constant TEA, even though for some of these papers a small maximum in the lower temperature range can be noticed. For the papers made from thermomechanical pulps the TEA even increases above 200°C.

The effect of temperature on the TEA has previously been described for kraft liner with constant moisture content by Benson (8). He reported a relatively large decrease in TEA from 15.5°C to 48.9°C in the RH range of 50 to 65%. On the other hand, Wink (9) has found that under almost identical conditions the TEA has a small maximum in this temperature range, based on the average change in TEA for five different papers: kraft, rag, sulphite and two newsprints.

Effect on stretch at rupture

The temperature, as well as the moisture content (3), has a greater effect on the stretch at rupture in the cross direction. CD, than in the machine direction, MD. The pronounced difference of the effect of temperature in the different paper directions is illustrated in fig. 5 showing the results obtained for fluting in MD and CD. At —25°C the stretch at rupture in these directions is equal.

Above 200°C papers made from high yield pulps show a pronounced increase in the values of the stretch at rupture, not noticeable when low yield pulps are used, and probably due to the softening of amorphous com­ponents. This is exemplified in fig. 5 referring to a paper made from thermomechanical pulp. The stretch at rup­ture for papers made from pulps of a low yield decreases monotonously for an increasing temperature from a maximum in the range of 50° to 120°C, at least in the machine direction. This is shown for a kraft liner in the

same figure. Since the stretch at rupture is determined by the tensile strength, and the modulus of elasticity obtained and also by the plastic properties, which all vary with temperature, a general rule for the effect of temperature on the stretch at rupture cannot be estab­lished on the tested material.

Effect on the modulus of elasticity

In the glassy state the modulus of elasticity for most polymers will slightly decrease with increasing tempera­ture. The linear relationship in the Hookean part of the stress-strain curve is in the glassy state explained as depending on an instantaneous shift of the segments of the polymer from their potential energy minima, when stress is applied to the polymer. As this is an energy ac­tivation process it explains the reduction in the modulus when the temperature is increased (10).

For a few dry papers Andersson and Berkyto have reported a linear reduction in the modulus of elasticity versus temperature between —50°C and 150°C (3).

In the data presented here the temperature depen­dence of the modulus of elasticity can be said to be fairly linear up to 200°C for all the papers tested, al­though the lines are slightly concave upwards. Above 200°C indications of softening can be seen. This is illustrated in fig. 6 where four different papers, a paper made from cotton linters, a kraft liner, a NSSC fluting and a machine-glazed sulphite paper are compared. However, in the region below 200°C a nearly linear correlation is obtained if the natural logarithm of the

Fig. 5. Stretch at rupture versus temperature for dry papers: a NSSC fluting of 112 g/m2 tested in the machine direction, MD, and the cross direction, CD, a kraft liner of 110 g/m2

tested in MD and isotropic hand sheets of a spruce thermo­mechanical pulp, TMP, of 150 g/m2.

Fig. 6. Modulus of elasticity versus temperature for dry pa­pers: isotropic hand sheets of cotton linters of 100 g/m2, a kraft liner of 110 g/m3, a fluting of 112 g/m2 and a machine glazed, MG, sulphite paper of 78 g/m2. The last three papers are tested in the machine direction.

modulus of elasticity, In E, is plotted against the temper­ature. This is in agreement with the theoretical predic­tions by Nissan derived from the stress-strain depen­dence for a hydrogen-bonded solid (11). He estimated the relative change in the modulus of elasticity d (In E)/ dT for cellulosic material to be between 2.1 • l0-3 and 6.3 • 10-3 considering physical properties of cellulose and its derivatives. The results obtained on papers investigated here agree well with these predictions giving on the average d (In E)/dT = 3.1 • 10-3 as illustrated in table 2 and fig. 7.

Roughly, the relative change in the modulus of elasti­city dE/dT for dry papers could be estimated to be be­tween 0.20 and 0.30% per °C which is of the same magnitude as estimated for the shear modulus of some dry papers by de Ruvo et al. (12) and for the tensile modulus by Andersson and Berkyto (3).

The softening of dry papers, secondary transitions

It was mentioned before that the softening of dry papers occurring above 220°C is reflected in an increased decline in the modulus of elasticity with the tempera­ture. For papers with a large content of amorphous material an increase in the value of the stretch at rup­ture, (i.e. in the breaking elongation), is also noticeable in the range where the modulus of elasticity falls off rapidly, i.e. in the range above 220oC.

In fig. 8 the natural logarithm of the modulus of elasticity in the softening area is given for a few dry papers, a kraft sack paper, a machine-glazed sulphite paper, a NSSC fluting and a paper made from thermo-mechanical pulp. For all these papers the modulus of elasticity falls off noticeably above 220°C. For the fluting another pronounced decrease in modulus is also seen, starting at 200°C.

The softening between 220° and 240 °C is in agreement with the previously reported evaluations of a glass transition temperature of cellulose at about 230° C (13,

Fig. 8. The natural logarithm of the modulus of elasticity versus temperature between 150° and 250°C for dry papers: a machine-glazed, MG, sulphite paper of 78 g/m2, a NSSC fluting of 112 g/m2, a kraft sack paper of 114 g/m2 and hand sheets of spruce thermomechanical pulp, TMP, of 150 g/m3. The first three papers are tested in the machine direction at a strain rate of 1.7 • 10-3/s (0.17%/s) and the hand sheets of thermomechanical pulp at a strain rate of 5.0 • 10-3/s (0.50%/s).

14, 15). A softening around 205°C for papers from pulps of a low yield has been indicated in some earlier work (15). The softening measured for the fluting will be discussed in a following paper.

The measurements reported herein do not definitely indicate other secondary transitions. According to a proposal by Klason and Kubat (16), such transitions previously reported at room temperature and 100°C are induced by small amounts of residual water.

Experimental The tensile tests were performed in a universal tensile tester, type Alwetron TCT 20 (17). The paper strips were clamped in and then heated to the measuring tern perature directly by an inert pre-thermostated silicon oil within a few seconds, as described (1). The papers were dried for several days over P 2 0 5 at 20°C before testing, resulting in almost completely dry papers, that is a moisture content less than 0.5%.

The stress-strain testing was performed at constant strain rate selected for different papers as 1.7 • 10-3 or 5.0 • l0-3 per second as listed in table 2 in order to com­plete the testing within 10 seconds. Thus auto-crosslink-ing at high temperatures could be avoided and an ac­curate evaluation of the stress-strain diagram was pos­sible. The difference in the strain rates used has no significant influence on the relative properties of the papers or their softening temperatures (1).

345

The testing span was 100 mm and the width of the paper strips 15 mm. To improve reproducibility, part of the dried-in stresses were removed by cycling the com­mercial papers twice between 45% and 90% relative humidity. Hand sheets were made isotropic and dried mainly stress-free between two blotting-papers. The stress-strain properties were tested over the temperature range with approximately one strip at each 2°C. The modulus of elasticity is here based on the dry thickness of the paper at 20°C.

The chemical composition of the papers were analysed and calculated according to a procedure outlined by Aurell and Harder (18, 19) .

Acknowledgements

The authors wish to express their thanks to Mrs. Senada Angelova for a very good and patient experimental assistance.

References 1. Salmen, N. L., Back, E. L.: Simple stress-strain measure­

ments on dry papers from —25°C to 250°C. Svensk Papperstidn. 80(1977) 178.

2. Smith, J. P.: Ultimate tensile properties of elastomers. I. Characterisation bv a time and temperature independent failure envelope. J. Polymer Sci. A 1 (1963) 3597.

3. Andersson, O., Berkyto, E.: Some factors affecting the stress-strain characteristics of paper. Svensk Papperstidn. 54(1951)437.

4. Zhurkov, S. N.: Kinetic concept of the strength of solids. Intern. J. Fracture Mechanics. 1 (1965) 311.

5. de Ruvo, A., Lundberg, R.: Livslangd for kartong under konstant belastning och varierande klimatbctingelser. Medd. Svenska Traforskningsinst. B: 131 (1972).

6. Guthrie, J. L., Fulmer, G. E.: Characterization of satur­ated cellulosic webs by the creep failure method. Tappi 52(1971)2181.

7. Llovd, B. A., Devries, K. L., Williams, M. L.: Fracture behavior in nylon 6 fibers. J. Polymer Sci. A-2 (1972) 1415.

8. Benson, R. E.: Effects of relative humidity and tempera­ture on tensile stress-strain properties of kraft linerboard. Tappi 54 (1971) 699.

9. Wink, W. A.: The effect of relative humidity and tem­perature on paper properties. Tappi 44 (1961): 6, 171A.

10. Meares, P.: Polymers structure and bulk properties. Van Nostrand Co. Ltd., London (1967).

11. Nissan, A. H.: The rheological behaviour of hydrogen-bonded solids. Trans. Faraday Soc. S3 (1957) 710.

12. de Ruvo, A., Lundberg, R., Martin-Lof, S., Soremark, C: Influence of temperature and humidity on the elastic and expansional properties of paper and the constituent fibre. Paper presented at the British Paper and Board Makers Association Cambridge symposium 1973. Tech­nical Section 785 (1976).

13. Goring, D. A. L.: Thermal softening of lignin, hemicel-lulose and cellulose. Pulp Paper Mag. Can. 64 (1963) 12, T-517.

14. Naimark, N. J., Fomenko, B. A.: Glass transition tem­perature of cellulose. Vysokomol. Soyed. B 13 (1971) 45.

15. Back, E. L., Didriksson, 'E. I. E.: Four secondary and the glass transition temperatures of cellulose evaluated by sonic pulse technique. Svensk Papperstidn. 72 (1969) 687.

16. Klason, C, Kubdt, J.: Thermal transitions in cellulose. Svensk Papperstidn. 79 (1976) 494.

17. Alwetron, TCT 20. Lorentzen & Wettre, Stockholm, Sweden .

18. Aurell, R.: Kraft pulping of birch. Part 1. The changes in the composition of the wood residue during the cook­ing process. Svensk Papperstidn. 67 (1964) 43.

19. Aurell, R., Hartler, N.: Kraft pulping of pine. Part 1. The changes in the composition of the wood residue during the cooking process. Svensk Papperstidn. 6S (1965) 59.

(Manuscript received A ugust 19, 1977)

346

Paper VI

Salmdi - 10

Transactions of the Technical Section

Volume 5, No. 3 September, 1979

Thermal Softening of the Components of Paper: Its Effect on Mechanical Properties by N. LENNART SALMEN

In order to investigate the fluence of softening and interactions between the chemical component: of paper, the temperature dependence of the modulus of elasticity for various dry papers was studied up to 250° C. It is suggested that cellulose, hemicel-lulose and lignin act as separate com­ponents in the composite material of paper with glass transition tempera­tures of 230°C for cellulose, 205°C for lignin and 165°C to 175°C for the hemicelluloses. The cellulose com­ponent is interpreted to be the main stress-transferring element in paper.

N. Lennart Salmen, Swedish Forest Products Research Lab. Stockholm, Sweden: now with PPR1C, 570 St. John's Blvd., Pte. Claire, Que. H9R 3J9

Since the softening at the glass transition temperature is of great im­portance for the mechanical properties of polymers, it has lately also been studied in cellulose containing mate­rials. Most authors have studied the isolated components, or some modifi­cations of these, in order to under­stand the behaviour of wood and pulp. Glass transition temperatures for the individual components have thus been reported, for cellulose 220-250°C (1,2,3,4), for some modified hemicel-luloses 165-215°C (1,2,4,5) and for some modified lignins 135-235 C somewhat dependent on the molecular weight( 1,4,5,6).

However, in a composite ma­terial such as paper, the softening of the individual components may be greatly influenced by structural factors. This paper reports measure­ments of the temperature dependence of the properties of dry wood and paper under tensile stress, carried out in order to understand the interaction of their components. The results are discussed in relation to the material as a composite of cellulose, amorphous

and crystalline, hemicellulose and lignin. The softening of the materials has been evaluated by analysis of the modulus of elasticity and of the flow properties from tensile measurements on dry strips of the material at dif­ferent temperatures from 20° to 250°C.

Previously, the softening in the dry wood or paper material has not been studied much. Measurements by Back & Didriksson(7) on dry paper of cotton linters and bleached kraft, re­veal a glass transition temperature of 230°C with some minor indications of softening at lower temperatures. Vari­ous measurements by Goring(l) on powder of spruce sulphite pulp and kraft pulp, by Baldwin and Goring(8) on powder of untreated and steam-cooked aspen poplar, white birch and black spruce chips, exhibit a softening only when dry at 230°C to 250°C, while Kawakami et al (9) report a softening of powder of makanba wood at 260 C. Measurements on single dry bleached sulphite fibres in torsion by de Ruvo and Brehde(lO), showed a slight increase in the logarithmic de-

crement at higher temperatures, which they attribute to a gradual softening of amorphous carbohydrates above 180°C. The transitions at room tem­perature and 100°C previously re­ported in the literature for paper are probably, according to Klason and Kubat(ll), induced by small amounts of residual water.

BACKGROUND - GLASS TRANSITION IN MIXED POLYMER SYSTEMS

The glass transition temperature is the temperature at which an amorphous polymer changes from a hard glassy form into a rub­bery form, which results in a large softening and is therefore of great technical importance. This glass transition of a polymer is influenced by many structural factors. Among the most important ones are the average molecular weight, the crystallinity and the microscopic arrangement in a com­posite material.

For polymers in general, it has empirically been established that the glass transition temperature, Tg, in­creases with average molecular weight, Mn, up to a temperature, Tg at infinite molecular weight:

Tg = Tg° - K/Mn

where K is a characteristic constant for each polymer(12).

The behaviour is explained by Nielsen(12) as being due to free chain ends which introduce extra free volume by disrupting the packing of the molecules. This relation has been verified for cellulose by Alfthan et al on oligosaccharides of different chain lengths(2). For amorphous cellulose, Alfthan et al calculated the limiting glass transition temperature, Tg°, to be 217°C. The relation found also shows that a reduction in the degree of polymerisation for cellulose to 100 glucopyranose units reduces the glass transition temperature by less than five degrees.(2)

The crystallinity of the polymer normally affects the glass transition temperature only slightly. An increase in the glass transition temperature with increasing crystallinity, which has sometimes been noticed, has been at­tributed by Nielsen(12) to a shorten­ing of the amorphous chain segments between crystallites. For cellulose, the difference between the calculated 217° C for amorphous cellulose(2) and the measured 230-250°C for partly crystalline cellulose (1,3,4) might partly be related to the effect of crystallites.

However, the crystallinity has a pronounced effect on the extent of

softening. Softening takes place only in the amorphous regions of the polymer and therefore, above the glass transition temperature, crystallites act as rigid fillers in a soft amorphous matrix. The extent of softening at the glass transition temperature is there­fore rapidly reduced as the degree of crystallinity increases(12), which is very obvious for cellulose.

In a multicomponent material, the structure of the composite greatly influences the extent of softening and also the glass transition temperature. In a homogeneous system, like most copolymers, only one glass transition temperature is observed with its posi­tion dependent on the volume frac­tions of the components.

In a heterogeneous system, like most polyblends, however, two glass transitions will be detectable, each representing the transition of the individual components. For a hetero­geneous system, also the arrangement of the two phases influences the mechanical behaviour. In pure tension, the two extremes could be considered to be either with the components in parallel strings or in series respectively. With strings in parallel the more rigid one becomes the stress-transferring unit and essentially determines the elastic modulus, while with strings arranged in series the elastic modulus is most dependent on the softer phase.(12)

RESULTS

Softening temperatures of composites of cellulose, hemicellulose and lignin

The method used here employs conditioning of dry papers in the temperature range of -25° to 250°C

- 2 -

within a few seconds. This is very important to prevent interfering degradation and chemical reactions at high temperatures. Measurements of the modulus of elasticity of several dry papers have shown a softening around 230°C, attributed to the glass transi­tion of cellulose(13). Figure 1 /nE, here being plotted against tempera­ture, shows however, in the case of birch NSSC-fluting medium, a pro­nounced softening at 205°C together with the one at 230°C. Here there is also a slight indication of softening at about 170°C.

In order to study the origin of the softening, the same NSSC-pulp was, in one case, hemicellulose-extracted with alkali and, in another case, delignified with sodium di-chlorite (Table IV). The effects of these chemical treatments, on the In E versus temperature relationship for laboratory sheets of these pulps are illustrated in Figure 2. It is evident that the softening at 205°C disappears on delignification whereas neither of the chemical treatments has any great influence on the softening at 230°C. Although less pronounced in the sheets of hemicellulose-extracted NSSC-pulp, there still is an indication of softening at about 170°C in all these papers.

In NSSC-fluting medium in tor­sion, Htun and de Ruvo(14) have also found indications of softening at about the same temperatures as those given here.

Measurements on laboratory sheets of aspen thermomechanical pulp, and of this pulp hemicellulose-extracted with alkali, show that an indication of softening at about 175°C disappears when hemicellulose is

Fig. 1. The natural logarithm of the modulus of elasticity. In E, versus temperature for a dry NSSC-fluting medium in the machine direction.

Fig. 2. In E versus temperature for NSSC-fluting medium in the machine direction (MD) and cross direction (CD) and for sheets of hemicellulose-extracted and delignified NSSC-pulps. In order to separate the curves, different constants have been added to the different In E-curves.

removed, Figure 3. When a spruce thermomechanical pulp is delignified with sodium dichlorite no influence on the temperature-dependence of the logarithm of the elastic modulus for laboratory sheets was observed, as is evident in the lower part of Figure 3. Here again, in both these papers, a minor transition was noticeable at 175°C.

Measurements on birch rotary cut veneer, with a thickness of 0,7 mm crosswise and in the fibre direction, reveal a sharp transition in the modulus of elasticity at 205°C, when tested crosswise, whereas in the fibre direction the most pronounced lowering of the modulus

Fig. 3. In E versus temperature for sheets of thermomechanical pulp, TMP, of aspen and for the same pulp hemicellulose-extracted, for sheets of TMP of spruce and for the same pulp delignified. In order to separate the curves, different constants have been added to the different In E-curves.

takes place above 240 C, as shown in Figure 4. Simplifying the structure of wood we here regard it as a composite material of cellulosic fibres in a matrix of lignin. Since the cellulosic fibres can be regarded as the stiffer component in both the fibre direction and the transverse direc­tion(15), the elastic modulus when tested with the components parallel will be more dependent on the cel­lulose. However, when tested crosswise with the components in series, the lignin will have the greater influence. Thus the softening in the veneer cross­wise at 205°C could be attributed to the glass transition of lignin and the softening in the fibre direction above 240°C to cellulose.

From all the above measure­ments the conclusion can be drawn that in the dry state the amorphous cellulose in papers or wood materials softens at 230°C, and the lignin at 205°C. The indication of a small transition at 165°C to 175°C is pos­sibly due to the hemicellulose present.

Influence on failure envelope

The influence of the softening of different components in paper on the mechanical properties can be sum­marized in the form of stress-strain failure envelopes(16). In Figure 5 such failure envelopes are shown for birch NSSC-fluting medium and for sheets made from the corresponding pulp, hemicellulose-extracted with alkali or delignified with sodium dichlorite. The extraction of hemicellulose has no major influence on the form of the failure envelope. Both the NSSC-fluting and the corresponding hemicel­lulose-extracted paper show a marked

Fig. 5. Failure envelopes of NSSC fluting medium in MO of paper of hemicellulose-extracted NSSC pulp and of delignified NSSC-pulp. For the two latter papers isotropic properties are shown.

- 3 -

increase in the stretch at rupture above 200°C accompanying the softening of the papers, a behaviour typical for high lignin pulps(13). After delignifica-tion, this effect disappears and instead a decrease of stretch with temperature occurs. This behaviour was found typical for papers containing only small amounts of, or no lignin(13).

The birch veneer tested in the cross direct ion also shows a pronounced increase in the stretch at rupture accompanying the softening of the lignin at 205°C, as shown in Figure 6. In the fibre direction, the veneer cracked longitudinally and thus no true breaking points could be established.

For the aspen thermomechanical pulp no change in the appearance of the failure envelope was observed after the hemicellulose-extraction. Delignifi-cation of the spruce thermomechanical pulp diminished the increase in elonga­tion above the softening of the paper and also lead to a decrease in the stretch at rupture between 220°C and 250°C.

Influence on the elastic modulus below softening

Neither delignification nor extraction of hemicellulose had any significant effect on the temperature dependence of the elastic modulus, d (In E)/dT, below 170°C, as seen in

Table I. This indicates that the stress-transferring elements in paper is mainly the cellulose component, also suggested by Ehrnroot et a/(17). The contributions to stress-transfer from hemicellulose and sulphite lignin are so small, the softening of these amor­phous components in these tests of paper is scarcely detectable.

The difference in d(/n E)/dT noticed between machine and cross directions for NSSC-fluting medium could perhaps be due to the orienta­tion.

Reliable values of crystallinity could be determined by infTa-red spectra of samples containing only small amounts of lignin. The crystallinity of the cellulose was calculated via the chemical composition and the correla­tion between infra-red ratio and crystallinity according to moisture regain(19).

According to a theory of Nissan(18), relating the elastic proper­ties in paper to the cellulose com­ponent, the temperature dependence of the modulus of elasticity in the glassy state will depend on the degree of crystallinity of the cellulose. Thus, for to ta l ly crystalline cellulose, d(7n E)/dT was reported to be -2.1 • 10*3/°C, while for celluloid with a lower crystallinity a value of -3.8 • 10-3/°C has been estimated(18).

Measurements on papers of low lignin content, MG-sulphite, liner, kraft sack and cotton linters previously reported(13), as well as on sheets of delignified NSSC-pulp and TMP spruce seem to support this theory, as seen in Table II where d (In E)/dT and crystallinity are given for these papers.

DISCUSSION

The measurements of the modulus of elasticity for various chemically treated papers show that neither delignification nor extraction of hemicellulose has any effect on the temperature of softening at 230°C. With delignification of NSSC-pulp, one t ransi t ion at 205° disappears. Table 111. The indications of softening around 170°C in all the measurements, except those for hemicellulose-extracted TMP points to the real exis­tence of this transition. However, pre­vious measurements by the author(13) on papers of comparatively low hemi­cellulose content, cotton linters, kraft sack, kraft liner and MG-sulphite paper show no significant indications of such a softening at 170°C.

Alfthan et al have by extrapola­tion from oligosaccharides found the limiting glass transition temperature for xylodextrin to be 200°C(2). Side

- 4 -

Fig. 6. Failure envelope of birch veneer in the cross direction.

TABLE I

The Decrease In the Logarithm of the Modulus of Elasticity with Temperature for Vartow Papers.

d (/n E) /dt for T < 170°C Patter ( x l O ' )

NSSC fluting medium MD -2.77/°C

NSSC fluting medium CD - 3.35/° C

non-oriented papers:

Hemicellulose-extracted

NSSC-pulp - 3.257° C

delignified NSSC-pulp -3.32/°C

TMP, aspen -4.29/°C

hemicellulose-extracted

TMP-aspen -4.43/°C

TMP, spruce -3.23/°C

delignified TMP, spruce 3.35/oC

groups that reduce the efficiency of the molecular packing, as in the glucu-ronoxylans, also reduce the glass tran­sition temperature. Thus it is probable that the glass transition temperature of native hemicelluloses is less than 200°C.

The above indicates that the glass transition temperature for the components in paper are about 230°C for cellulose, 205°C for native or slightly modified lignin, and 165 C to 175oC for native hemicelluloses.

The fact that individual transi­tions are observed for the paper com­ponents as well as that changes in the chemical composition of the papers do not alter the positions of the glass, transition temperatures indicate that cellulose, lignin and hemicellulose act as separate components in a hetero­geneous composite. Paper fibres may thus, perhaps be regarded as cellulosic microfibrils interrupted by spaces of hemicellulose and lignin. However, it cannot be excluded that the structural arrangement within the fibre and the paper may affect these results or that some of the hemicelluloses may have a glass transition temperature of about the same as that for cellulose.

For the papers studied and also for previous measurements on papers of low yield pulp(13) softening of lignin is only detectable in NSSC-fluting medium which indicates that the more hydrophilic sulphite lignin interacts more strongly in the stress-transferring mechanism. The different behaviour of sulphite lignin compared to kraft lignin has also been indicated by Hartler and Mohlin in their bond strength studies(20).

In the plastic region, the soften­ing of lignin has a marked effect in increasing the extensibility, which is clearly demonstrated by the failure envelopes of different pulps. In papers containing only small amounts of lignin, the plastic region at high tem­peratures is rather small. The fact that the softening of lignin makes the paper more extensible can be interpreted as being due to the achievement of a better stress-distribution within and between the fibres.

EXPERIMENTAL

Before testing, the paper strips were dried for several days over P2O5 at 20°C, resulting in almost complete­ly dry strips, that is, a moisture content less than 0.5%. The tensile tests were performed in a tensile tester type Alwetron TCT 20(21). The test­ing span was 100 mm and the width of the paper strips 15 mm. The paper strips were clamped and then heated to the measuring temperature directly

TABLE IIl

Paper

NSSC fluting medium MO NSSC fluting medium CD hemicellulose-extracted NSSC pulp delignified NSSC pulp TMP, aspen hemicellulose-extracted TMP, aspen TMP, spruce delignified TMP, spruce

1

170 162 167 168 177

175 175

170

Transition temperatures inflection point

II

205 204 202

205

°C

III

230 232 230 232 235 230 232 230

230

by an inert prethermostated silicon oil within a few seconds, as described(22).

The stress-strain tests were per­formed at a constant strain rate selected for different papers as 1.7 • 10-3 or 8.3 • 10-3 per second in or­der to complete the test within 10 seconds. Thus auto-cross linking at high temperatures could be avoided as previously tested(22), and an accurate evaluation of the stress-strain diagram was made possible. The difference in the strain rates used has no significant influence on the relative properties of the papers or their softening tempera-tures(22). The stress-strain properties were tested over the temperature range with one strip at approximately each 2°C intervals. The initial modulus of elasticity is here based on the dry thickness of the paper at 20 C, measured at a pressure of 100 kPa.

The transition temperatures were evaluated as maxima in the derivative curve of the logarithm of the modulus of elasticity against temperature.

To improve reproducibility, part of the dried-in stresses were removed by cycling the commercial papers twice between 45% and 90% relative humidity. Isotropic hand sheets were dried mainly stress-free between two blotting-papers. The delignification of pulps was performed by a treatment in

- 5 -

sodium dichlorite, following a conven­tional technique recommended in the literature. The extraction of hemicel­lulose was performed by treatment in alkali according to a procedure given by Spiegelberg(23).

The chemical compositions of the papers were analysed and cal­culated according to a procedure out­lined by Aurell and Hartler(24,25) Table IV.

The chemical treatments caused no mercerization, tested by the x-ray diffraction spectra of disintegrated samples according to Jayme(26). The degree of polymerisation was checked by measurement of the CED intrisic viscosi ty(27) . The hemicellulose extraction of the NSSC-fluting had no major effect on the degree of sul-phonation, measured as total sulphur to be about 1% on the lignin.

ACKNOWLEDGEMENT

The author wishes to thank Dr. Ernst L. Back for valuable discussions concerning this work and Mr. Jan-Erik Wiken for skillfull technical assistance.

REFERENCES 1. GORING, D.A.I., Thermal soften­

ing of lignin, hemicellulose and cellulose. Pulp Paper Mag. Can. 64 (1963) No. 12, T-517

TABLE IV

Relative Compositions of Papers Investigated

Paper

NSSC-fluting medium (mainly Betula verrucosa) hemicellulose-extracted NSSC-pulp delignified NSSC-pulp

thermomechanical pulp, aspen (Populus tremuloides) hemicellulose-extracted TMP, aspen

thermomechanical pulp, spruce Picea abies) delignified TMP, spruce

Composition (%}

Cellulose Hemicellulose Lignin

48 67 60

50 63

46 73

29 23 8 25

39 1

27 23 14 23

25 29 25 2

2. ALFTHAN, E., DE RUVO, A., BROWN, W., Glass transition tem­peratures of oligosaccharides. Polymer 14 (1973) No. 7, 329

3. NAIMARK, N.I., FOMENKO, B.A., The glass transition of cel­lulose. Vysokomol, Soyed. B13 (1971)No. 1,45

4. TAKAMURA, N., Softening of fibre components in hot pressing of fibre mat. J. Japan Wood Res. Soc. 14 (1968) No. 4, 75

5. STONE, J.E., SCALLAN, A.M., The influence of heat on solvent-exchange-dried wood compo­nents. Pulp Paper Mag. Can. 66 (1965)T-440

6. HATAKEYAMA, H., NAKANO, J., Nuclear Magnetic Resonance Studies on Lignin in Solid State. Tappi 53 (1970) No. 3,472

7. BACK, E.L., DIDRIKSSON, E.I.E., Four secondary and the glass transition temperatures of cellulose, evaluated by sonic pulse technique. Svensk Papperstid. 72 (1969) No. 21, 687

8. BALDWIN, S.H., GORING, D.A.I., The thermoplastic and adhesive behaviour of thermo-mechanical pulps from steamed wood. Svensk Papperstid. 71 (1968) No. 18, 646

9. KAWAKAMI,H.,SHIRAISHI,N., YOKOTA, T., Thermal softening of wood and wood-polymer com­posites Mokuzai Gakkaishi 23 (1977)143

10. De RUVO, A., BREHDE, L., The influence of temperature and humidity on the mechanical behaviour of single pulp fibres grafted with polyacrylamide. Cel­lulose Chem. Technol. 1 (1973) No. 2, 191

11. KLASON, C, KUBAT, J., Ther­mal transitions in cellulose Svensk Papperstid. 79 (1976) No. 15, 494

12. NIELSEN, L.E., "Mechanical Properties of Polymers and Com­posites", Marcel Dekker Inc., New York 1974

13. SALMEN, N.L., BACK, EX., Ef­fect of temperature on stress-strain properties of dry papers. Svensk Papperstid. 81 (1978) No. 10,341

14. HTUN, M., De RUVO, A., Ther­mal treatment of lignin - contain­ing handsheets. (In press) Cel­lulose Chem. Technol.

15. MARK, R.E., Mechanical be­haviour of the molecular com­ponents of fibers in "Theory and Design of Wood and Fiber Com­posite Materials", Ed. Jayne, B.A. Syracuse University Press, Syra­cuse, 1972

16. SMITH, J.P., Ultimate tensile pro­perties of elastomers. I Characteri­sation by a time and temperature

independent failure envelope. J. Polymer Sci. Al (1963) 3597

17. EHRNROOTH,E.,KOLSETH,P., De RUVO, A., The influence of matrix composition and softening on the mechanical behaviour of cellulosic fibers. "Fibre-Water In­teractions in Paper-making". Tech­nical Division, B.P., & B.I.F. (1978)715

18. NISSAN, A.H., "Lectures on fibre science in paper", Pulp and Paper Technology Series No. 4, Joint Textbook Committee of the Paper Industry, 1977

19. NELSON, M.L., O'CONNER, R.T., Relation of certain infrared bands to cellulose crystallinity and crystal lattice type. J. Appl. Pol. Sci. 8 (1964) 1325

20. HARTLER, N. MOHLIN, U-B., Cellulose fibre bonding. Part 2. Influence of pulping on interfibre bond strength. Svensk Papperstid. 78 (1975) No. 8, 295

21. Alwetron TCT 20. Lorentzen & Wettre, Stockholm, Sweden

22. SALMEN, N.L., BACK, E.L., Simple stress-strain measurements on dry papers from -25°C to 250°C. Svensk Papperstid. 80 (1977) No. 6, 178

23. SPIEGELBERG, H.L., The effect of hemicelluloses on the mechani­cal properties of individual pulp fibers. Tappi 49 (1966) No. 9, 388

24. AURELL, R, Kraft pulping of birch. Part 1. The changes in the composition of the wood residue during the cooking process. Svensk Papperstid. 67 (1964) No. 2,43

25. AURELL, R. HARTLER, N., Kraft pulping of pine. Part 1. The changes in the composition of the wood residue during the cooking process. Svensk Papperstid. 68 (1965), No. 3,59

26. JAYME,G., KNOLLE, H., Beitrag zur empirischen rontgenogra-phischer Bestimmung des Kristal-l ini tatsgrades cellulosehaltiger Stoffs. Das Papier 18 (1964) No. 6, 249

27. SCAN-C 15:62, Viscocity of cel­lulose in Cupriethylenediamine solution (CED), Svensk Pap­perstid. 65 (1962) No. 22, 921

REFERENCE: SALMEN, N.L. Thermal softening of the components of paper and its effect on mechanical properties. Transactions of the Technical Section, Vol. 5(3) TR 45-50 September 1979. Paper presented at the 65th Annual Meeting of the Technical Section, Canadian Pulp & Paper Association, Montreal, Quebec, January 29 - February 2, 1979. Not to be reproduced without permission from this organization. Manuscript received November 10, 1978; approved by the Review Panel, May 16, 1979.

ABSTRACT: In order to investigate the influence of softening and interactions between the chemical components of paper, the temperature dependence of the modulus of elasticity for various dry papers was studied up to 250°C. It is suggested that cellulose, hemtcellulosa and lignin act as separate components in the composite material of paper with glass transition temperatures of 230°C for cellulose, 205° C for lignin and 165°C to 175° C for the hemicelluloses. The cellulose component is interpreted to be the main stress-transferring element in paper.

RESUME: En vue d'examiner I'influence de I'amollissement et des interactions entre les composants chimiquet du papier, nous avont etudie, jusqu'a 250 C, la dependence du module d'elacticite de divert papiers tecs au regard de la temperature. II semble que la cellulose, I'hemiceilutose et la ligntne agissent comme composants separes au sein de la substance heterogene du papier, atom que lea temperatures de transition sur le verre sont de 230° C pour la cellulose, 206° C pour ta lignine et de 166°C a 175°C pour I'hemicellulose. Tout porta a croire que le compoeant de cellulose est le principal agent de transfert de la tension dens le papier.

KEYWORDS: HEAT TREATMENT, SOFTENING, PAPER, TEMPERATURE, ELASTIC STRENGTH. CELLULOSE, HEMICELLULOSES. LIGNINS, GLASS TRANSITION. TEMPERATURE, MECHANICAL PROPERTIES, TENSILE STRESS, CORRUGATING MEDIUM. VENEERS, NSSC PULPS, THERMOMECHANICAL PULPS, BETULA, POPULUS, CHEMICAL TREATMENT, DELlGNIFlCATION.

Reprinted from TRANSACTIONS, Vol. 5, No. 3, TR45-TR50, September, 1979.

Paper VII

Moisture-dependent thermal softening of paper, evaluated by its elastic modulus ABSTRACT To study the combined influence of moisture and temperature on paper strength properties, especially on the elastic modulus, tensile properties have been measured in the temperature range -25°C to 65° C and at moisture contents from zero to 20%, corresponding to a range of 0% to 95% relative humidity. Detailed data are presented for the specific modulus of elasticity in this moisture and temperature range for a kraft sack paper and at 20°C for other papers. The temperature derivative of the elastic modulus as well as its moisture derivative show distinct transition regions in which the modulus falls off rapidly. This region is interpreted as being the glass transition for the cellulose-hemicellulose water sytem. It is shown that water acts as a softener for paper, influencing the glass transition of the cellulosic components to an extent predicted earlier using the approach of Kaelble. The effects of paper crystallinity are also discussed in accordance with this view.

KEYWORDS Cellulose Crystallinity Elastic strength Kraft papers Moisture Transition temperature

N. Lennart Salmen and Ernst L. Back Swedish Forest Products Research Laboratory, Box 5604, S 114 86 S tockho lm, Sweden

It is well known that the elastic modulus and most mechanical properties of paper depend on the moisture content. The moisture dependence of the elastic modulus has been related by Nissan (1) and by Higgins (2) to the breaking of hydrogen bonds, with equations pro­posed for describing this process. For polymers, similar effects of a softener result from lowering the glass transition temperature, Tg.

When the tensile properties of paper are considered over a reasonable tem­perature and humidity range, as sum­marized in the form of tensile failure envelopes in Fig. 1, it is evident that the load-elongation curves for a dry paper indicate a relatively brittle material, whereas for a moist paper they indicate a relatively soft material. Such differ­ences can be expected if the material passes its glass transition point while going from the dry to the moist state. Accordingly, the tempera ture and moisture dependence of the elastic modulus of paper will be discussed in terms of the variation of the glass transition temperature of the carbo­hydrates, i.e., of amorphous cellulose and hemicellulose, with water as a softener.

A glass transition is characterized by the onset of motion of larger chain segments, which implies a breaking of intermolecular forces. In cellulose, these are predominantly hydrogen bonds. Thus an interrelation between the glass transition and a proriounced increase of the hydrogen bond mobility should exist.

Water as a softener The glass transition temperatures of the amorphous components of paper have been measured (3). For dry cellu­lose, the glass transition temperature is as high as 230°C (3-6), while for hemicellulose it has been measured between 165°C and 225°C (3, 7) de­pending on its composition and modi­fication. In mechanical testing, a glass transition shows up as a large reduction in the elastic and shear moduli. These moduli for a completely amorphous polymer may fall off two or three dec­ades within a small temperature range (8), while for a partially crystalline polymer such as nylon, polyoxymeth-ylene, or cellulose (4), they fall off much less.

From tensile tests, the glass transition

temperature can be estimated as the maximum derivative of the elastic modulus with respect to temperature. However, the temperature evaluated to correspond to the glass transition is somewhat dependent on the time scale of the experiment.

ELONGATION. %

1. Failure envelopes for a kraft sack paper in the machine direction at temperatures from -25 to 65°C and moisture contents of 0, 5, 1O, 15, and 20%, based on moist paper. Strain rate = 0.83%/s.

Reprinted from Tappi, The Journal of the Technical Association of the Pulp and Paper Industry. Vol. 63, No. 6, June 1980, Copyright, 1980 by TAPPI, and reprinted by permission of the copyright owner.

Softening agents are molecules of low molecular weight and of low glass

TEMPERATURE.°C 2. Specilic modulus of elasticity for a kraft sack paper vs. temperature at different mois­ture contents (% of moist paper).

transition temperature which interact with the intermolecular bonds of the polymer and thus shift its glass transi­tion to a lower temperature. For cellu­lose, the softening agent must be able to interact with the hydrogen bonds, e.g., water. For other hydrogen-bonded polymers, such as nylon 6-6, the effect of water as a softener in reducing the glass transition temperature and there­by influencing the mechanical proper­ties has been clearly demonstrated by Quistwater and Dunell (9).

It has been suggested on the basis of NMR measurements by Ogiwara et al. (10) and dielectric dispersion measure­ments by Tsuge and Wada (11) that in the presence of sufficient water the glass transition point of cellulose is reduced to room temperature or below. We have also estimated this reduction of the glass transition temperature of cellulose by water (12) using the ap­proach of Kaelble (13). Calculations on some hemicelluloses show a similar dependence on water.

Since water is believed to act as a softener for the cellulosic components, it is assumed that their glass transition point in the wet or moist paper can be determined from the relation between its elastic modulus and temperature at different moisture contents.

In Fig. 2 the specif ic elastic modulus-modulus related to the basis weight (see Experimental)—of a kraft paper is given as a function of temperature at different moisture contents. A softening region, which shifts towards lower tem­peratures with increasing moisture content, can be discerned. The softening is not very pronounced, which might be expected because of the relatively high

MOISTURE CONTENT, %

3. Upper diagram: Specific modulus of elas­ticity for a kraft sack paper vs. moisture content in % of moist paper at -25°C to 65°C; average of 10 samples; moisture con­tent achieved by absorption; dotted line at 20oC shows results with moisture content achieved by desorption. Lower diagram: The derivative of the specific modulus of elasticity with respect to moisture content, dE/dM, and with respect to temperature, dE/dT, vs. moisture content at various temperatures.

118

MOISTURE CONTENT, %

4. Glass transition temperature for cellulose of different degrees of crystallinity vs. mois­ture content in % of moist material. Solid lines are those previously calculated (12) with the absorption restriction of the cellu­lose-water phase indicated by the thin line. The circles are estimated from the derivative of the specific modulus with respect to moisture, and the triangles with respect to temperature for the kraft sack paper studied (crystallinity = approx. 70%).

degree of cellulose crystallinity in paper. The softening is more evident when the modulus is plotted against moisture content as in the upper part of Fig. 3.

As shown by Higgins (14), the modulus is only dependent on the moisture con­tent, regardless of whether it is reached by absorption or desorption, as indicated here by the 20°C curve in the upper part of Fig. 3. Thus, it is not a function of relative humidity but only of the amount of softener present, i.e., water. However, longer periods at high humidi­ties, i.e., above the softening point, will result in recrystallization, as shown by Kimura et al. (15), thus altering the above relationship.

To define the softening temperature, the temperature derivative as well as the moisture content derivative of the tensile modulus are plotted against moisture content in the lower part of Fig. 3. The peaks of the two derivatives coincide well and are shifted towards a higher moisture content as the tem­perature decreases. The broadening of the peak, i.e., of the transition region of the elastic modulus at lower tempera­tures (higher moisture contents), as seen in the lower part of Fig. 3, is also typical of the effect of softeners on the glass transition of polymers (8).

The measured softening peaks for the kraft sack paper of Fig. 3 are plotted in Fig. 4 together with the previously calculated (12) effect of water on the glass transition temperature of cellulose using the approach of Kaelble (13). The degree of crystallinity for the kraft paper examined has been esti­mated by X-ray measurements to be about 70% of total carbohydrate mate­rial. The results best fit to a curve of 63% crystallinity, which is a reasonable agreement. In the X-ray spectra, semi-ordered regions contribute to the crys­talline portion. Since these regions may participate in water absorption, the crystallinity measured by X-rays is somewhat too high with regard to cal­culations of effects of a softener.

In a temperature-humidity interval similar to that studied here, Tokita (76'), in measuring torsional modulus of viscose rayon, found a transition with an activation energy suggesting a major transition, i.e., the glass transition of cellulose. Also, measurements by Cousins (17) of the elastic modulus of hemicelluloses at 20°C over the entire humidity range indicate a similar be­havior as found here for paper with a degree of softening corresponding to our calculations for totally amorphous carbohydrates. However, where cellu­lose is the major carbohydrate com­ponent, as for dry papers, the authors (4) have suggested the cellulose compo­nent to be the main stress-transferring element in tension.

The measurements presented here indicate that the glass transition of the

Vol. 63, No. 6 June 1980 / Tappi

cellulosic components of paper might be reduced by water and that this is the most probable explanation for the action of water on its mechanical properties.

Paper crystallinity Since water is only absorbed in the amorphous regions, the glass transition and thus the effects on the elastic mod­ulus will occur at different overall moisture contents for papers of different degrees of crystallinity, as indicated in Fig. 4.

Additional verification is given in Fig. 5, where the relative moduli E/Eo (Eo representing the corresponding dry modulus) are compared for a paper of cotton linters. a kraft paper, and a neutral sulfite semichemical (NSSC) fluting medium, with estimated crys-tallinities by the X-ray method of about 85, 70, and 65%, respectively. The de­rivative with respect to moisture content of these curves gives softening maxima of 7, 9.5, and 11% moisture content, respectively, for these papers. These results are then plotted in Fig. 4 as well and show reasonable agreement with the calculated effects of cellulose crys­tallinity, although the X-ray measure­ments show somewhat higher values, as discussed above.

In a recent study (4) of the tensile properties of completely dry papers, we suggested that the crystalline frac­tion of the cellulose contributes to the elastic modulus. The same is indicated here by data for different papers, given in Fig. 5. If only theamorphous fraction of the paper structure had contributed to the modulus, the moisture dependence in the dry region, i.e., in the glassy state, would have varied noticeably, with a greater slope for the more crys­talline paper.

At equal relative humidity, papers of low crystallinity will absorb more water than those of high crystallinity, as ex­emplified in Fig. 6 for cotton linters paper and the kraft paper. This fact has long been used for estimating crys­tallinity of cellulosic materials (18, 19) where water is considered nearly exclusively absorbed by the amorphous phase. Since only this amorphous phase exhibits a glass transition, the transition for different papers will occur at a single relative humidity when condi­tioned in the same way. This is illus­trated in Fig. 6 as well.

The above relations will, however, be somewhat dependent on lignin content because of low water sorption in this phase. In this context, hemicellulose is considered as part of the amorphous cellulose phase. Also, there might be some influence from the inhomogeneity of the fibrous structure.

These measurements refer to the role of water as a cellulose plasticizer, thereby affecting the elastic modulus

of paper in a predictable way. The effects on the other tensile properties are as expected according to this inter­pretation. However, it is noteworthy that in the vicinity of a transition region linear relationships seldom occur.

Experimental Materials The papers studied were:

• A commercial kraft sack paper of 46% yield of Pinus silvestris and 105 g/m2 dry basis weight containing no wet-strength additives and partly dried as an air-borne web, density 577 kg/m3.

• An NSSC fluting medium, based mainly on Betula verrucosa of 116 g/m2, density 542 kg/m3.

• Handmade sheets of cotton linters of 100 g/m2, density 460 kg/m3.

The commercial papers were tested in the machine direction. Before testing, samples were cycled twice between 45% and 90% R.H. for 24 hr each to release part of the dried-in stresses.

Strength testing

Stress-strain tests were performed in a tensile tester, type Alwetron TCT 20 (20), at a strain rate of 0.83%/s. The test span was 100 mm and the strip width 15 mm. The results were analyzed by a computer to print out directly the tensile parameters presented here. The modu­lus is given as specific elastic modulus (elastic modulus divided by density) which is adequate to use for an in-homogeneous material such as paper. This was achieved by dividing the tensile stiffness by the dry basis weight. Dry tensile properties were tested after drying the paper strips over P205 (21). Different moisture contents were achieved after conditioning from the drv state for 2 days at different relative humidities in the 25% to 95% R.H. range. The moisture content is given as percentage of total weight. The test temperature in the range of -25°C to 65°C was achieved by rapid heating or cooling of the strips in thermostated inert silicone oil (21) or conditioned in air at 20°C.

Measurements refer either to 6-10 strips conditioned at a specific relative humidity and tested at agiven tempera­ture, or to a number of such strips tested at variable temperatures in the range -25 to 65°C with approximately one strip for every degree.

Measurements in inert silicone oil were compared for the kraft paper at 20°C and 46°C with measurements over the same moisture range in air from 0% to 20% moisture content, as exemplified in Fig. 7. The modulus of elasticity was found to be the same, while the strength and stretch were slightly reduced in the silicone oil as shown by Robertson (22).

Moisture content was analyzed both by drying at 105°C (23) and by Karl Fischer titration (24) before and after heating or cooling in silicone oil. Up to 46°C, no moisture escape with time

MOISTURE CONTENT. %

5. Relative moduli E/E0 for a paper of cotton linters, a kraft paper and a NSSC fluting medium vs. moisture content in % of moist paper. E0 represents the corresponding dry modulus. Crystallinities estimated by X-ray method: 85, 70, and 65%, respectively.

RELATIVE HUMIDITY %

6. Moisture content for kraft sack compared with cotton linters vs. % relative humidity. The paper moisture contents are achieved by absorption. T9 is the measured softening maxima for these papers.

ELONGATION, % 7. Load-elongation curves for a kraft sack paper at 20oC measured in air and in silicone oil at different moisture contents. Strain rate =0.83%/s.

119

could be noticed. At 65°C, where mois­ture was observed to escape, measure­ments were made after 12, 20, 30, and 60 s of heating in oil, to estimate initial properties. Control measurements in air at 50°C and 60% R.H. and at 65°C and 65% R.H. showed good agreement with the predicted properties.

The crystallinity of the papers was measured by X-ray diffraction spectra of disintegrated samples, according to a procedure of Jayme (25), and calculated from the intensities of the crystalline and amorphous peaks.

Literature cited 1. Nissan, A.H., Macromolecules 9(5): 840

(1976). 2. Higgins, H.G., and Balodis, V., In "Frac ­

t u r e , " (C.J. Osborn, ed.), Univers i ty of Melbourne, Melbourne, 1965.

3. Goring, D.A.I., Pulp Paper Mag. Can. 64(12): T517 (1963).

4. Sa lmen, N.L. , CPPA Trans. Tech. Sec.

5(3): T R 4 5 (1979). 5 . N a i m a r k , N . I . , a n d F o m e n k o , B . A.,

Vysokomol Soyed. B 13(1): 45 (1971). 6 . Back , E .L . . a n d Dikr iksson , E . I . E . ,

Svensk Papperstid 72(21): 687 (1969). 7. Gil lham, J.K., A.I.Ch.E.J. 20(6): 1066

(1974). 8. Nielsen, L.E. , "Mechanical Proper t ies

of Polymers and Composites," Marcel Dekker Inc.. New York, 1974.

9. Quis twater . J.M.R., and Dunell , B.A., J. Appl Polymer Sci. 1(3): 267 (1959).

10. Ogiwara , Y., Kubota , H., Hayashi , S., and Mitomo, N. , J. Appl. Polymer Sci 14(2): 303 (1970).

11. Tsuge , K., and Wada , Y., J. Phys. Soc. Japan 17(1): 156 (1962).

12. Sa lmen, N.L. , and Back, E.L., Tappi 60(12): 137 (1977).

13. Kaelble, D.H., "Physical Chemis t ry of Adhes ion , " Wi ley - In t e r sc i ence , New York, 1971.

14. Higgins , H.G., Appita 12(1): 1 (1958). 15. K imura , M., H a t ak ey ama , T., and Na-

kano, J., J. Appl. Polymer Sci. 18(10): 3069 (1974).

16. Toki ta ,N. , J. Polymer Sci. 20:515(1956).

17. Cousins, W.J., Wood Sci Technol. 12(3): 161 (1978).

18. Jeffries, R., J. Appl. Polymer Sci. 8(3): 1213 (1964).

19. Valentine, L., Chem. Ind. 43:1279 (1956). 20. Alwetron TCT 20: Lorentzen & Wet t re ,

Stockholm, Sweden. 21 . Sa lmen , N.L. , and Back, E.L. , Svensk

Papperstid. 80(6): 178 (1977). 22. Robertson,A.A., Tappi53(7): 1331(1970). 23. SCAN-P 4:63; Svensk Papperstid. 66(1):

8 (1963). 24. Mitchel l , J., J r . , Ing. Eng. Chem., Anal

Ed. 12(7): 390 (1940). 25. J a y m e , G., and Knolle, H., Das Papier

18(6): 249 (1964).

The skillful technical assistance of Jan-Erik Wiken is gratefully acknowledged.

Received for review March 9, 1979. Accepted Dec. 3,1979. Based on a paper presented at the International Paper Physics Conference, jointly sponsored by CPPA, TAPPI. and the Fundamental Research Committee of the BPBMA. held in Harrison, B.C.. Canada, Sept 17-19,1979.

Vol. 63, No. 6 June 1980 / Tappi