Dissolution of cellulose for textile fibre applications513750/... · 2012-08-27 · textile fibre applications Martin Kihlman Karlstad University Studies | 2012:16 Chemical Engineering

Dissolution of cellulose for textile fibre applications

Martin Kihlman

Karlstad University Studies | 2012:16

Chemical Engineering

Faculty of Technology and Science



Martin Kihlman

Distribution:Karlstad University Faculty of Technology and ScienceChemical EngineeringSE-651 88 Karlstad, Sweden+46 54 700 10 00

© The author

ISBN 978-91-7063-420-3

Print: Universitetstryckeriet, Karlstad 2012

ISSN 1403-8099


LICENTIATE THESIS

Martin Kihlman


WWW.KAU.SE

i

Abstract

This thesis forms part of a project with the objective of developing and

implementing a novel, wood-based, process for the industrial production of

cellulose textile fibres. This new process should not only be cost effective but

also have far less environmental impact then current processes. Natural and

man-made fibres are usually plagued with problems (e.g. economic and

environmental) and are unsuitable in meeting growing demands. The focus of

this thesis was therefore to investigate the dissolution of cellulose derived from

various pulps in novel aqueous solvent systems.

It was shown that cellulose could be dissolved in a NaOH/H2O solvent

at low temperatures (< 0°C) and that such an alkaline solvent can be improved

regarding the solubility, stability and rheological properties of the cellulose

dopes formed if different additives (salts or amphiphilic molecules) are used.

The effect of different kinds of pretreatment (individually and combined) and

the influence of pulp properties on cellulose accessibility and dissolution were

also evaluated. These pretreatments affected, as expected, some characteristic

properties of the pulps mainly by reducing the DP but also, for example,

changing the composition of the carbohydrates. Not only did the pretreatment

affect the solubility it also increased the stability of the cellulose dopes,

resembling the effect of chemical additives to the NaOH system. According to

multivariate data analysis it was established that, of the pulp properties

analyzed, only the composition of carbohydrates and the DP had a significant

influence on the solubility of the pulps used in this study. Finally, it was

emphasized that the dissolution of cellulose pulps seemed to be controlled by a

very complex interaction between both kinetic and thermodynamic parameters.

Keywords: alkaline aqueous solvent systems; cellulose; dissolution; dissolving

pulp; kraft pulp; pretreatment; pulp characterization; sulfite pulp; textile fibres

ii

Sammanfattning

Denna avhandling var en del av ett projekt där främsta målet var att utveckla

och implementera en ny industriell process för framställning av textilfiber

baserat på cellulosa från ved. Den framtagna processen skulle vara mer

ekonomisk än dagens processer samt ledande vad avser låg miljöpåverkan. De

natur- och konstfibrer som finns tillgängliga idag plågas med allvarliga

kostnads-, miljö- och processproblem och de anses därför inte som lämpliga

som framtidens textilfiber. Avhandlingens syfte var därför att förbättra

tillverkningsprocessen av textilfiber genom att undersöka upplösning av

cellulosa, från olika typer av vedbaserade massor, i nyutvecklade vattenbaserade

lösningsmedel.

Det visade sig att cellulosa kunde lösas upp i ett NaOH/H2O baserat

lösningsmedel vid låga temperaturer (< 0°C) och att lösningsmedlet kunde

förbättras ytterligare genom att tillsätta olika typer av additiv (salter eller amfifila

föreningar) med avseende på upplösning, stabilitet och reologiska egenskaper

av cellulosalösningen. Dessutom var effekten av förbehandlingarna (enstaka

eller multipla) samt inverkan av massaegenskaperna på cellulosans tillgänglighet

och upplösning undersökt. Som förväntat påverkade de olika förbehandlingarna

egenskaperna hos de olika massorna. Mestadels genom att reducera DP men

även, som ett exempel, ändra kolhydratsammansättningen. Förbehandlingarna

påverkade inte bara upplösningen av cellulosan utan även stabiliteten av

cellulosalösningen. Av de olika massaegenskaperna analyserade i den här

avhandlingen förefölls det, enligt den multivariata dataanalysen, att de enda

egenskaperna som hade en signifikant inverkan på upplösningen för de olika

massorna var DP och kolhydratsammansättningen. Det bör påpekas att

upplösning av cellulosa i vattenbaserade system verkar vara en väldigt komplex

interaktion mellan både kinetiska och termodynamiska parametrar.

Nyckelord: alkaliskt vattenbaserade system; cellulosa; dissolvingmassa;

förbehandling; karakterisering av massa; sulfatmassa; sulfitmassa; textilfibrer;

upplösning

iii

List of papers

The following publications, referred to as Papers I, II and III, comprise the

foundation of this licentiate thesis.

I. Kihlman, M., Wallberg, O., Stigsson, L. & Germgård, U. (2011).

Dissolution of dissolving pulp in alkaline solvents after steam explosion

pretreatments. Holzforschung, 65 (4), 613-617.

II. Kihlman, M., Aldaeus, F., Chedid, F. & Germgård, U. (2012). Effect of

various pulp properties on the solubility of cellulose in sodium

hydroxide solutions. Holzforschung, DOI: 10.1515/hf-2011-0220.

III. Kihlman, M., Medronho, B., Romano, A., Germgård, U. & Lindman, B.

(2012). Kinetic and Thermodynamic Effects on Cellulose Fibre

Dissolution in an Alkali Based Solvent: Influence of Additives and

Pretreatments. Submitted to Holzforschung.

Results relating to this thesis have been reported at the following conferences:

1. Kihlman, M., Medronho, B., Trey, S., Stigsson, L. & Germgård, U.

CelluNova – A future textile fibre concept based on a sustainable raw

material. Poster presentation. International Workshop on Wood Biorefinery

and Tree Biotechnology, Örnsköldsvik, Sweden, 21-23/6-2010. Published in

proceedings pp. 77.



material. Poster presentation. 11th European Workshop on Lignocellulosics

and Pulp, Hamburg, Germany, 16-19/8-2010. Published in proceedings pp.

307-308.



material. Oral presentation. FPIRC & PAPSAT Summer Conference,

Karlstad, Sweden, 24-26/8-2010. Published in the proceedings of the

meeting.

iv

4. Kihlman, M., Stigsson, L., Westin, M. & Germgård, U. Regenerated

cellulosic fibres for textile applications. Oral presentation. IAWS - Novel

materials from wood or cellulose, Stockholm, Sweden, 31/8-2/9-2011.

Published in proceedings pp. 24-25.

5. Kihlman, M., Stigsson, L., Westin, M. & Germgård, U. Regenerated

cellulosic fibres for textile applications. Poster presentation. 1th Avancell

Conference, Göteborg, Sweden, 18-19/10-2011. Published in the

proceedings of the meeting.

6. Kihlman, M., Wawro, D., Perzon, E., Hedlund, A. & Germgård, U.

Preparation of cellulosic fibers in semi-pilot scale from NaOH-solutions.

Poster presentation. 243rd ACS National Meeting, San Diego, CA, USA, 25-

29/3-2012.

The author’s contributions to the papers

Paper I The experimental work and writing were performed by Martin

Kihlman with the exception of the steam explosion pretreatment

and the section describing the performance, both of which were

carried out by the co-author, Ola Wallberg.

Paper II Martin Kihlman carried out all of the experimental work apart

from the SEC, carbohydrate and WRV analysis and the

multivariate evaluation, which were performed by the co-authors

at Innventia. Martin is also the main author, with the exception of

the section evaluating the results from the multivariate analysis of

the pulp properties.

Paper III The work was divided equally between Martin Kihlman and Bruno

Medronho.

v

Table of contents

Abstract ...........................................................................................................................i

Sammanfattning ........................................................................................................... ii

List of papers ............................................................................................................... iii

The author’s contributions to the papers ............................................................ iv

Table of contents ......................................................................................................... v

List of Figures ......................................................................................................... vi

List of Tables ......................................................................................................... vii

Abbreviations ............................................................................................................ viii

1. Introduction ................................................................................................... 1

1.1. Wood .............................................................................................................. 2

1.2. Dissolving pulp ............................................................................................. 3

1.3. Cellulose ......................................................................................................... 4

1.3.1. Molecular structure .................................................................................... 4

1.3.2. Supramolecular structure ............................................................................ 6

1.4. Solvent systems ............................................................................................. 9

1.5. Dissolution of cellulose in NaOH systems ............................................. 10

1.6. Pretreatments .............................................................................................. 13

1.7. Regenerated cellulose fibres ...................................................................... 13

1.8. Objective ...................................................................................................... 14

2. Materials and Methods .......................................................................... 15

2.1. Materials ....................................................................................................... 15

2.1.1. Pulps ....................................................................................................... 15

2.1.2. Solvent systems......................................................................................... 15

2.2. Methods ....................................................................................................... 16

2.2.1. Pretreatments ........................................................................................... 16

2.2.1.1. Steam explosion ................................................................................... 16

2.2.1.2. Hydrothermal ...................................................................................... 17

vi

2.2.1.3. Ethanol-acid ....................................................................................... 17

2.2.2. Preparation of solutions ............................................................................ 18

2.2.3. Analysis .................................................................................................. 18

2.2.3.1. Pulp characterization ............................................................................. 18

2.2.3.2. Characterization of the solutions ............................................................... 19

3. Results and Discussion ......................................................................... 22

3.1. Effects of the pretreatments ..................................................................... 22

3.2. The influence of pulp properties on solubility ....................................... 24

3.2.1. Multivariate data analysis ........................................................................ 27

3.3. Aqueous solvent systems ........................................................................... 29

3.3.1. Effects of ZnO additions .......................................................................... 32

3.4. Pulp consistency ......................................................................................... 35

3.4.1. Effects of EtOH/HCl treatment ............................................................. 37

4. Conclusions .................................................................................................. 41

5. Future studies ............................................................................................. 42

6. Acknowledgements .................................................................................. 43

7. References ..................................................................................................... 44

List of Figures

Figure 1. The structure of a wood fibre cell wall ................................................... 2

Figure 2. The molecular structure of cellulose ....................................................... 5

Figure 3. The intra- and intermolecular bonds within cellulose molecules. ....... 6

Figure 4. Interconversion of the cellulose polymorphs ........................................ 7

Figure 5. A schematic representation of the fringed fibril model. ....................... 7

Figure 6. The supramolecular structure of cellulose .............................................. 8

Figure 7. A classification of cellulose solvent systems .......................................... 9

Figure 8. Possible interactions between Na+ ions and cellulose molecules ..... 10

Figure 9. Schematic representation of the mercerization process ..................... 11

Figure 10. Phase diagram of the NaOH/H2O/cellulose system ....................... 11

Figure 11. Schematic illustration of the STEX equipment ................................. 17

Figure 12. Examples of optical micrographs with different solubility ranking 20

Figure 13. The DPη2 obtained after STEX pretreatment. ................................... 22

Figure 14. Molecular mass distributions determined by SEC ............................ 23

vii

Figure 15. The influence of DPη2 on solubility in NaOH/urea/thiourea. ....... 26

Figure 16. The influence of DPη2 and the content of hemicelluloses on

solubility in NaOH/urea/thiourea. .................................................. 26

Figure 17. The influence of the width and length of the fibres on solubility in

NaOH/urea/thiourea.. ......................................................................... 27

Figure 18. Loadings bi plot...................................................................................... 28

Figure 19. Variable importance plot....................................................................... 29

Figure 20. The solubility of STEX D (339) pulp ................................................. 30

Figure 21. Solubility of the HT B V67 (310) pulp ............................................... 30

Figure 22. The solubility of STEX D (160) in the NaOH/urea/thiourea

system under various conditions ....................................................... 31

Figure 23. Optical micrographs of 6% HT B V67 (263) dissolved in NaOH

(8.5%) and NaOH/ZnO (8.5:0.8%). ................................................. 32

Figure 24. G´ and G´´ as a function of time. ....................................................... 34

Figure 25. Shear viscosity of 6% HT B V67 (263) pulp dissolved in the NaOH

(8.5%) and the NaOH/ZnO (8.5:0.8%) systems ............................. 34

Figure 26. Shear viscosities of HT B V67 (263) ................................................... 35

Figure 27. Solubility of STEX D (160) pulp in the NaOH/urea/thiourea

system as a function of pulp consistency. ........................................ 36

Figure 28. Solubility of cellulose versus pulp consistency using high efficiency

and standard efficiency mixers. ........................................................... 37

Figure 29. G´ and G´´ as a function of time ........................................................ 38

Figure 30. Optical micrographs of (a) 6% EA/HT B V67 (234) pulp, along

with (b) 4%, (c) 6% and (d) 9% EA B V67 (393) pulps dissolved in

NaOH/ZnO (8.5:0.8%). ..................................................................... 40

List of Tables

Table 1. Properties used to describe dissolving pulp ............................................. 3

Table 2. Solvent systems utilized for dissolution of cellulose. ........................... 10

Table 3. The pulps used in this study. ................................................................... 15

Table 4. Pulp characterization ................................................................................. 25

Table 5. Pulp characterization ................................................................................. 25

viii

Abbreviations

AGU anhydroglucopyranose unit

DP degree of polymerization

DPw degree of polymerization; calculated from the measured

weight-average molecular mass

DPη degree of polymerization; calculated through its correlation

to the pulp’s intrinsic viscosity

G´ storage modulus

G´´ loss modulus

HPAEC-PAD high performance anion exchange chromatography - pulsed

amperometric detector

HT hydrothermal

LODP leveling-off degree of polymerization

MCC microcrystalline cellulose

mc% moisture content

ML middle lamella

Mn number-average molecular mass

Mw weight-average molecular mass

NaOH sodium hydroxide

OH hydroxyl groups

P primary wall

PC principal component

PDI polydispersity index

PEG polyethylene glycol

SEC size-exclusion chromatography

S1/S2 secondary wall

STEX steam explosion

T tertiary wall

W lumen

WRV water retention value

wt% weight percent

ZnO zinc oxide

1

1. Introduction

Cellulose derived from wood is a highly interesting polymer for producing bio-

based products with respect to sustainable development that currently

characterizes almost all industries. A crucial problem is, however, its low

solubility in common solvents. The textile and apparel industries have utilized

wood cellulose for over a century already. Not only the raw material but also

the whole process are, however, characterized today by a sustainable and

ecological vision, which is why different research groups are currently working

on the development of novel, environmentally friendly, solvents for the

dissolution of cellulose.

Novel approaches for the dissolution of wood cellulose have been

thoroughly investigated in this thesis. The main focuses were dissolution in

alkaline aqueous solvent systems and the examination of various methods

suitable for pretreating the cellulose.

2

1.1. Wood

Wood is a heterogeneous organic composite of elongated fibres. Due to the

anisotropy of the fibre directions, both longitudinal and tangential, wood may

have varying properties and functionalities. The cell wall of each fibre is

composed mainly of cellulose, hemicellulose and lignin; depending on the

species, the quantities of those three molecular components vary from 40–50%,

15–30% and 20–30%, respectively. The structure of the cell wall consists of

several layers. The lumen (W) is the central cavity and is surrounded by the

tertiary wall (T), the winding (S1) and the main body (S2) of the secondary walls,

the primary wall (P) and, finally, the middle lamella (ML), which holds the fibres

together, as illustrated in Figure 1. The tertiary wall consists mainly of lignin

while the primary wall is composed, more or less, of hemicellulose and

cellulose. A large fraction of the cellulose content is distributed in the winding

and main body walls whereas the middle lamella consists of pectin’s and lignin.

Figure 1. The structure of a wood fibre cell wall (Krässig 1993).

The density of the wood and the shape of the fibres correlate to the mechanical

properties of the wood, which varies depending on the species. Trees bearing

needles, e.g. spruce or pine, are usually referred as softwoods whilst those with

leaves, e.g. birch and eucalyptus, are known as hardwoods. The wood fibres in

these two types of trees vary: softwood has long fibres and hardwood has

shorter, and thus more densely packed, fibres. The longer fibres of softwood

make it stronger than hardwood.

3

1.2. Dissolving pulp

Pulp consisting of a high purity of cellulose (9098%) and low amounts of

hemicellulose, lignin and extractives is often referred as dissolving pulp or

cellulose pulp. Compared to a regular paper pulp, dissolving pulp is

characterized by properties such as a low pulp yield (approx. 35%), a high -

cellulose content, narrow molecular weight distributions, low ash content, etc.

(Christoffersson 2005). Table 1 shows some of the properties of a typical

dissolving pulp.

Table 1. Properties used to describe dissolving pulp

(Christoffersson 2005).

Properties Unit

Viscosity ml/g

Yield %

R18 (alkali resistance in 18% NaOH) %

R10 (alkali resistance in 10% NaOH) %

DPw DPn

Polydispersity index (DPw/DPn)

Ash %

Kappa Number

Brightness %ISO

Dissolving pulp is manufactured using two main processes: the acid sulfite and

the prehydrolyzed-kraft. Prehydrolyzed-kraft cook, as the name reveals, is a

two-stage process in which the prehydrolysis step reduces the content of

hemicellulose and the following kraft cook reduces the content of lignin (Saka

& Matsumura 2004). Although other processes have also been suggested as

alternatives to manufacturing of dissolving pulp, including prehydrolyzed-

soda/anthraquinone cook (Reguant et al. 1997), different types of organosolvs

(Sixta et al. 2004) and prehydrolyzed-alkaline/sulfite cook (Kordsachia et al.

2004), none of them are currently used on an industrial scale.

Dissolving pulp is well suited as raw material for cellulose products such

as staple/filament fibres and various derivatives of ethers and esters. The main

end-use markets for dissolving pulp are viscose fibres (rayon), acetate tow and

fibres (filters), microcrystalline cellulose, MCC, (pharmaceutical and food

additives) and nitrocellulose (paints and explosives).

4

1.3. Cellulose

Cellulose is an organic polymer that exists in the cell wall of all green plants

with the purpose of ensuring that their structure is mechanically stable. It is

thereby the most abundant organic compound on Earth. The present volume

of cellulose is believed to be around 700 billion tons, with a high annual rate of

growth. It is a polysaccharide based on the monomer anhydroglucopyranose

unit (AGU) with the molecular formula C6H10O5 (Klemm et al. 2005;

O'Sullivan 1997).

Cellulose is an almost inexhaustible resource; it is used either in its native

or derivate form in a wide range of industries of which pharmaceuticals and

apparel are two of the largest. The fact that the cellulose molecule is a

renewable, biodegradable and environment-friendly biomaterial has resulted in

other industries showing an increased interest in its use. The fuel and energy

sectors have more recently begun to utilize cellulose: moreover other wood

components, such as hemicellulose and lignin, have also become an interesting

source of raw material for the production of bioethanol and biodiesel.

1.3.1. Molecular structure

The AGU monomers, which compose the cellulose molecule, are linked

together by β-(1→4)-glycosidic bonds that are formed between the carbon

atom C1 and the C4 of an adjacent unit, as illustrated in Figure 2. Every second

AGU is rotated 180° to adjust the preferred bond angles of the β-(1→4)-

glycosidic bond. A dimer of AGUs (cellobiose) can therefore be regarded as

being the basic unit of the cellulose molecule. This also implies that the

cellulose molecule could be considered as being a linear, unbranched, polymer

with repeating units (Krässig 1993).

Each AGU in the cellulose chain, as derived from X-ray diffractions and

NMR measurements, is assumed to have a 4C1 chair conformation (Klemm et

al. 2001b). Should such be the case, the hydroxyl groups (OH) and the

hydrogen atoms are positioned at the equatorial and axial plane, respectively.

Cellulose molecules therefore have an intrinsically structural anisotropy, with

sides of striking difference in polarity. Thus, cellulose has to be regarded as an

amphiphilic molecule the stability of which is governed mainly by both

hydrophobic and hydrophilic interactions (Yamane et al. 2006; Lindman et al.

2010).

5

Figure 2. The molecular structure of cellulose (adapted from Klemm et al. 2001b).

The chain length of the cellulose molecule is expressed as the constituent of

AGUs, which is commonly known as the degree of polymerization (DP) and

varies depending on its origin. Native cellulose, isolated from cotton, wood,

etc., has a DP ≥ 1000 while regenerated cellulose fibres and powder have a DP

of 250-600 and ≤ 200, respectively. The DP is always an average value since the

cellulose substrate is a polydisperse mixture of polymers with varying chain

lengths. The correlation between the DP and the weight-average molecular

mass (Mw) of the cellulose molecule is expressed in Equation 1.

DP = Mw/162 Eq. 1

Each AGU possesses a hydroxyl group on the carbon atoms C2, C3 and C6. It is

these that provide the opportunity of modifying/derivatizing the cellulose, in

different manners, with reactions that are commonly used in primary and

secondary alcohols, e.g. esterification and etherification. The hydroxyl groups

on each ends of the molecule also give the molecule a chemical polarity (Egal

2006). The reducing end group (on C1) has an AGU with a free carbon atom

whilst the non-reducing end group (on C4) has an AGU that is involved in the

glycosidic linkage, see Figure 2.

Various analyses have proved the existence of intramolecular hydrogen

bonds between the hydroxyl groups of C3, on one AGU, to the O5 of the

adjacent AGU, and between C2 and the C6 of the following AGU (Blackwell et

al. 1977), as illustrated in Figure 3. Both Krässig (1993) and Klemm et al.

(2001b) claim that these hydrogen bonds are of great relevance to the stiffness

and rigidity of the single chained cellulose molecule.

6

Figure 3. The intra- and intermolecular bonds within cellulose molecules (University of

Cambridge 2011).

1.3.2. Supramolecular structure

The native cellulose molecule has not only the ability of generating

intramolecular hydrogen bonds (i.e. within a molecule) but also intermolecular

bonds (i.e. between molecules) from C6 in one chain to C3 on the neighbouring

molecule; both kinds of hydrogen bonds are shown in Figure 3. The cellulose

molecules have the tendency to aggregate into highly well-defined order regions

due to their chemical structure and ability to generate intra- and intermolecular

hydrogen bonds, hydrophobic interactions and attractive van der Waals forces.

These are normally known as “crystalline regions” and can be arranged in

different polymorphs named cellulose I (Iα and Iβ), II, III (IIII and IIIII) and IV

(IVI and IVII) (O'Sullivan 1997; Egal 2006). The native cellulose, cellulose I, has

a parallel crystalline structure composed of the phases Iα and Iβ where the latter

is thermodynamically more stable than the former. The composition of Iα and Iβ

varies with the origin of the raw material: cellulose extracted from higher plants,

(e.g. wood) consists mainly of Iβ whereas that from more primitive organisms,

(e.g. bacteria) is enriched with Iα. Cellulose I can be converted into cellulose II

either by regeneration or mercerization. This polymorph, in contrast to

cellulose I, has an antiparallel crystalline structure (Kolpak & Blackwell 1976).

Moreover, cellulose II appears to have a more complex network of hydrogen

bonds (both intra- and intermolecular) that apparently results in a more dense

and stable structure. The two final polymorphs, III and IV, can be obtained

7

when treating cellulose I and II with ammonia and cellulose III with heat,

respectively, as shown in Figure 4.

Figure 4. Interconversion of the cellulose polymorphs (O'Sullivan 1997).

The crystalline order is not uniform throughout the whole structure and, in

addition, there are less ordered regions called “amorphous regions”. Different

models describing the construction of the cellulose molecule (i.e. the crystalline

and amorphous regions) have been proposed throughout the years. A two-

phase model known as the “fringed fibril” (or “fibrillar”) model is the one

generally accepted today and it does, most likely, describe the heterogeneous

accessibility of the cellulose (Krässig 1993). Figure 5 illustrates this model,

where the squares represent the crystalline regions and the strains the

amorphous.

Figure 5. A schematic representation of the fringed fibril model. The squares illustrate the

crystalline regions while the strains illustrate the amorphous areas (Klemm et al. 2005).

8

The cellulose fibre is composed of elementary crystallites, which are the

smallest morphological units. These entities (crystalline regions) are linked

together through less ordered regions (amorphous regions), forming “micro-

fibrils” (also known as “elementary fibrils”) with a cross-section of around 3-20

nm. Through van der Waals forces, hydrogen bonds and hydrophobic

interactions these fibrils can aggregate into larger fibrils called “macro-fibrils”

with diameters of a few tens of nanometer up to the micrometer range (Klemm

et al. 2001b). This bundling continues to the final fibre structure shown in the

schematic illustration in Figure 6.

Figure 6. The supramolecular structure of cellulose: cellulose fibres, macro-fibril, micro-fibril

and cellulose molecules (Egal 2006).

9

1.4. Solvent systems

Cellulose is not a thermoplastic polymer and degrades before its melted, thus

melting is not an option. Dissolution is a process step that is therefore

necessary in order to transform the solid cellulose into a liquid phase. However,

due to its well defined structure, cellulose is barely accessible to any organic and

inorganic solvents (Krässig 1993; Heinze & Koschella 2005). The solvent

systems available can be classified into two basic groups, derivatizing and non-

derivatizing solvents, the latter of which can be divided into two subgroups,

namely aqueous and non-aqueous media, see Figure 7. Derivatizing solvents, as

the name reveals, modify the cellulose prior to dissolution. The viscose process

utilizes carbon disulfide prior to the dissolution step to form a soluble cellulose

derivatized intermediate. These solvents react with one, or several, of the three

reactive hydroxyl groups in the cellulose, thus making it more soluble in

common solvents. The non-derivatizing solvents, on the other hand, do not

modify the cellulose prior to dissolution but dissolve it instead, by disturbing

the forces keeping it together without any chemical modification.

Figure 7. A classification of cellulose solvent systems (adapted from Heinze & Koschella

2005).

Various types of solvents are available. Table 2 summarizes some of the

solvents utilized either in industrial processes or small laboratory-scale research.

The main focus in this thesis was placed on the aqueous alkaline solvents

since they can be considered as being inexpensive, harmless and environment-

friendly. The NaOH based solvents seemed the most suitable since the

objective of this project was to develop a more economically viable and

environment-friendly process for the production of textile fibres (when

compared to e.g. viscose and cotton).

10

Table 2. Solvent systems utilized for dissolution of cellulose.

Solvent

Classification

Utilized Non-derivatized Derivatized

Non-aqueous Aqueous

LiCl/DMAc x - - Size exclusion chromatography

CS2+NaOH - - x Viscose process

NMMO - x

Lyocell process

DMSO/TBAF x x - Research

Ionic liquids x x - Research

NaOH+additive(s) - x - Research

1.5. Dissolution of cellulose in NaOH systems

The interaction between cellulose and aqueous alkali solutions has been a very

important subject of research for more than a century. It began over 160 years

ago when John Mercer noticed that cotton fibres swelled when immersed in

strong NaOH solutions (Klemm et al. 2001a). He observed certain

morphological changes: that the fibre shrank in both dimensions, thus

becoming denser. The phenomenon came to be known as “mercerization”,

after the observer. Today it is well known that the morphological structure and

the properties of the native cellulose changes when it comes into contact with

strong alkali solutions. The amorphous regions between the fibrils are the

starting point for swelling. The alkali ions penetrate further into the more

crystalline regions of the cellulose. This generates an important intermediate

called “alkali cellulose” which, compared to the native cellulose, has a more

open and reactive structure. Figure 8 illustrates various possible interactions

between the Na+ ions and cellulose molecules.

Figure 8. Possible interactions between Na+ ions and cellulose molecules (adapted from Fink

et al. 1995).

Furthermore, through mercerization and washing, native cellulose can change

its polymorph structure from cellulose I to cellulose II, as illustrated

11

schematically in Figure 9 (adapted from Okano & Sarko (1985)). The alkali

cellulose has an antiparallel conformation resembling the cellulose II structure

which is, in terms of energy and entropy, more favorable than the cellulose I.

Thus, during mercerization, the crystalline regions of cellulose I gradually

transform into alkali cellulose; it finally assumes, when washed, a cellulose II

antiparallel conformation (Okano & Sarko 1984; Okano & Sarko 1985; Egal

2006).

Figure 9. Schematic representation of the mercerization process (according to Okano &

Sarko 1985).

Despite the fact that strong alkali solutions mercerize cellulose, Sobue et al.

(1939) discovered that, under certain conditions, NaOH/H2O solutions partly

dissolve cellulose. They realized that by changing the conditions, i.e. the

temperature and NaOH concentration, different types of alkali celluloses could

be obtained: this is represented in their phase diagram in Figure 10. In the

region denoted Q (temperature: -10−4°C, [NaOH]: 6−10%) the NaOH/H2O

solution acts as a direct solvent for cellulose.

Figure 10. Phase diagram of the NaOH/H2O/cellulose system (Sobue et al. 1939).

12

Cellulose dissolution in alkaline solvents has been studied widely since the

middle of the 1980s (Kamide & Okajima 1987; Yamashiki et al. 1988; Kamide

et al. 1992; Isogai & Atalla 1995; Isogai & Atalla 1998; Zhang et al. 2002; Egal

2006; Kihlman et al. 2011; Kihlman et al. 2012). Isogai & Atalla (1998) studied

the dissolution of cellulose of different origins and observed that only low

molecular weight cellulose could be dissolved fully. In addition to the reduced

ability of alkaline solvents to dissolve cellulose, the stability of

NaOH/H2O/cellulose solutions during storage is also a disadvantage, since

they have a strong tendency to gel with increasing time and temperature (Roy et

al. 2003). One way of enhancing both stability and dissolution has been to use

additives in the cellulose-NaOH-water systems. Examples of these systems are

provided by Zhang et al. whose work during the last decade has provided some

new solvent systems, including NaOH/urea (Zhou & Zhang 2000) and

NaOH/thiourea (Zhang et al. 2002). Other NaOH-based solvent systems, e.g.

NaOH/polyethylene glycol, PEG, (Yan & Gao 2008) and NaOH/ZnO

(Wawro et al. 2009) were also developed. The role of the additives, however, is

not fully understood. Some authors claim that the NaOH and the additive(s)

interact and react with the cellulose, thereby improving solubility (Zhou et al.

2004; Lue et al. 2007); others suggest that only the NaOH hydrates react with

the cellulose, while the additive-hydrates prevent re-aggregation of the cellulose

molecules and consequent gelation (Cai & Zhang 2005). Recently, the role of

sodium zincate, Zn(OH)42−, (a possible reaction product of ZnO with NaOH)

was analyzed by Yang et al. (2011), who claimed that Zn(OH)42− can form

stronger hydrogen bonds with cellulose than hydrated NaOH. Additionally, and

somewhat contradictory, the authors also consider that Zn(OH)42− plays an

important role in breaking the intermolecular hydrogen bonds of cellulose,

leading to an enhancement of the dissolution ability. This, in fact, reflects the

rather general and accepted understanding of the insolubility of cellulose in

water that takes the fact that cellulose can form strong intra and intermolecular

hydrogen bonds for granted. Thus, the suggested role of compounds such as

ZnO, urea, thiourea, etc. is that they can “break” these hydrogen bonds. As far

as a simple hydrogen–bonding mechanism is concerned, it is clear that all

hydrogen-bond interactions involved (cellulose-cellulose, cellulose-water and

water-water) are comparable in magnitude: ca. 20 kJ/mol (Åstrand et al. 1995;

Canuto et al. 2004). This makes it unclear as to why the Zn(OH)42− anion, or

ions derived from other additives, should form stronger hydrogen bonds with

cellulose than with water and, at the same time, be capable of selectively

breaking cellulose intermolecular hydrogen bonds.

13

1.6. Pretreatments

Aqueous NaOH solutions, as mentioned above, dissolve low molecular weight

cellulose (Sobue et al. 1939; Isogai & Atalla 1998). Cellulose can be pretreated

prior to the dissolution step if the intention is to dissolve higher molecular

weight cellulose or higher cellulose contents. Several types of pretreatments

have been developed and can be classified into physical, chemical and

biological. The idea behind pretreatment, also known as “activation”, is to

increase the accessibility of the solvent to the whole cellulose structure by

disrupting the aggregated cellulose molecules and decreasing the highly ordered

(crystalline) regions, thus enhancing the swelling and dissolution ability of the

solvent. However, besides improving dissolution, these activation procedures

are typically accompanied by degradations of the cellulose polymer, leading to

lower DP values (Le Moigne 2008).

Kamide & Okajima (1987); Yamashiki et al. (1990a; 1990b; 1990c); Wang

et al. (2008); Wawro et al. (2009); Ibarra et al. (2010); Le Moigne et al. (2010)

and Trygg & Fardim (2011) all mention pretreatment methods that increase the

accessibility of cellulose. The use of, for example, steam explosion (STEX),

hydrothermal (HT), enzymatic and hydrolysis pretreatment methods allows

aqueous NaOH solvents to dissolve greater amounts of cellulose and/or

cellulose with a higher DP.

1.7. Regenerated cellulose fibres

Regenerated cellulose fibres are produced from raw materials that form fibres

naturally, such as dissolving pulp and cotton linters. The first commercial

regenerated cellulose fibre was the viscose fibre, also known as “rayon”. In the

year 1892 C.F. Cross and E.J. Bevan patented the process called “viscose”

following their discovery that cellulose, after being derivatized with carbon

disulfide, CS2, could be dissolved in diluted sodium hydroxide (Woodings

2001). Today there are several processes for manufacturing regenerated

cellulose fibres, e.g. using the Lyocell process, acetate and cuprammonium.

In general terms, the way in which cellulose fibres are produced is similar

in all of the processes. The cellulose raw material is, in its native or derivatized

form, dissolved in a solvent system. The dope is then coagulated and

regenerated into fibres. A process called “spinning” (e.g. wet spinning), is used

in all processes to regenerate the dissolved cellulose into the final fibre form.

The spin dope (i.e. cellulose solution) is forced to pass through the small holes

of a spinnerette into a coagulation medium in order to form a continuous

14

thread composed of several filaments. The coagulation medium used acts as an

anti-solvent system, and therefore solidifies and precipitates the cellulose

(Vehviläinen et al. 2008).

According to the Oerlikon Textile GmbH & Co. (2010) report “The Fiber

Year 2009/10” the global production of fibre in the year 2009 exceeded 70

million tons of which synthetic, fossil-based, man-made fibres comprised the

majority, i.e. 40.3 million tons. Only a small fraction of the total fibre

production, a mere 3.8 million tons, was attributed to cellulose fibres, of which

viscose staple fibres accounted for 2.7 million tons. The increase in the amount

of cellulose fibres was nevertheless 7.7% and is predicted to rise even further

since the demand for fibres, and the population of the world, is continuously

increasing.

1.8. Objective

This thesis forms part of a project where the first objective was to develop and

implement sustainable technology for the industrial production of cellulose

textile fibres from wood that has a lesser impact on the environment than

existing processes. The current situation regarding natural and man-made fibres

is that not only are they plagued with problems that include the utilization of

pesticides and toxic chemicals, a huge water consumption and a lack of suitable

agricultural land, they are also unable to meet the increasing demands. The ever

increasing price of crude oil and the intensified attention being paid to

environmental issues mean that the need for new, economically viable and

environment-friendly textile fibres will increase drastically in the near future.

The second objective was to aid the Scandinavian forest product industry

in converting pulp mills from producing paper pulp to staple fibre for textile

applications. There is a pronounced need for establishing novel, efficient and

economical systems for cellulose dissolution and regeneration: these should be

based preferably on alkaline aqueous systems to facilitate integration within a

kraft pulp mill.

The contribution to the project by the author was to develop novel

approaches for the dissolution of cellulose, not only investigating various

aqueous solvent systems but also different pretreatment methods. The author

was also involved in the coagulation and regeneration processes of the cellulose.

15

2. Materials and Methods

2.1. Materials

2.1.1. Pulps

Several types of pulps were used in this study, namely one paper grade pulp,

two dissolving pulps and five pretreated pulps, as presented in Table 3. The

pulp denoted D was pretreated using the steam explosion method at various

conditions and the resulting pulps (STEX D) acquired different properties, e.g.

DP. Each pulp will hereafter be denoted by the letter shown in Table 3,

followed by the DP value in brackets, e.g. HT B V67 (310).

Table 3. The pulps used in this study.

Pulp Pretreatment Denoted as Supplier DPη2

Sulfate birch paper pulp - S Södra, Sweden 1097

Sulfite spruce dissolving pulp

- D Domsjö Fabriker, Sweden

745

Prehydrolyzed kraft Southern pine dissolving pulp

- B V67 Buckeye Tech. Inc., USA

637

Sulfate birch paper pulp Steam explosion (STEX)

STEX S Södra, Sweden 202

Sulfite spruce dissolving pulp

Steam explosion (STEX)

STEX D Domsjö Fabriker, Sweden

160 − 691


Hydrothermal (HT) HT B V67 Buckeye Tech. Inc., USA

310, 263


Ethanol-acid (EA) EA B V67 Buckeye Tech. Inc., USA

393


EA + HT EA/HT B V67

Buckeye Tech. Inc., USA

234

2.1.2. Solvent systems

The pulps were dissolved in aqueous alkaline solvent systems, with or without

additives. A series of different solvents were used: NaOH (8.5%),

NaOH/urea/thiourea (8:8:6.5%), NaOH/ZnO (9:1%; 9:0.5% and 8.5:0.8%),

NaOH/PEG (9:1%) and NaOH/ZnO/Surf A (8.5:0.8:0.6%). The solvent

systems were always freshly prepared with distilled water and the

concentrations were calculated as weight percent (wt%) of the total solution.

The alkali used in the solvents was NaOH received in the form of pellets

from Merck (Darmstadt, Germany). The polyethylene glycol (PEG 2000) was

16

also obtained from the same sourced but as a powder. Urea, thiourea and zinc

oxide (ZnO) were obtained from Sigma-Aldrich (St. Louis, MO, USA) all in

powder form. All of the chemicals, with the exception of a zwitterionic

surfactant solution which, due to protection and patenting reasons, is denoted

here as “surf A”, were of analytical grade and used as received.

2.2. Methods

2.2.1. Pretreatments

2.2.1.1. Steam explosion

Figure 11 is a schematic illustration of the STEX equipment (Bösch et al. 2010).

The reactor was supplied with high pressure steam from an electrical boiler. A

valve was used to reduce the steam pressure to the level desired, corresponding

to a certain temperature (190–226°C). The reactor was loaded with the pulp,

which had a 50% moisture content (mc%), and the inlet valve was closed. The

reactor was then heated rapidly by an injection of high pressure steam to reach

the temperature set-point. The temperature in the reactor was then maintained

by the control valve. At the end of the reaction time, the reactor outlet valve

was opened; the overpressure in the reactor allowed the material to be

evacuated into the flash vessel, where excess steam was flashed from the slurry.

The control system comprised a PM851K01 controller and an industrial

IT 800xA 5.0 (ABB, Västerås, Sweden). Three thermocouples measured the

temperature in the chamber (TR), in the space between the chamber wall and

the mesh (TS), as well as in the jacket (TJ). The pressure in the reactor was

monitored by the pressure sensor PR. The heating cycle was initiated by heating

the jacket, followed by heating of the chamber. After attaining the temperature

desired in the chamber, the steam inlet valve was closed and steam was

introduced through a PID-regulated control valve to maintain a constant

temperature.

The S pulp was treated with 0.2% H2SO4 at 226°C for 15 min (generating

STEX S), whereas the D pulp was treated at various temperatures (190–226°C)

and times (1–15 min) but without the H2SO4, to generate different STEX D

pulps.

17

Figure 11. Schematic illustration of the STEX equipment (Bösch et al. 2010).

2.2.1.2. Hydrothermal

The hydrothermal treatment was performed according to the procedure

developed by Struszczyk et al. (2009), which involves the pulp being exposed to

a mixture of water and a small addition (< 0.1%) of ascorbic acid during mixing

for 40 min at 80 rpm at 170°C.

2.2.1.3. Ethanol-acid

The EtOH/HCl pretreatment procedure was carried out as described elsewhere

(Trygg & Fardim 2011). A volume of 400 cm3 of ethanol was pre-heated in a

water bath to 60ºC and, after thermal equilibration, 16 cm3 of hydrochloric acid

(37 wt%) were added. Then, 16g of cellulose pulp were added whilst being

stirred to ensure an even cellulose contact and distribution in the ethanol-acidic

medium. The reaction was quenched after 30 min by adding several liters of

cold distilled water. The treated pulp was then filtered and washed extensively

with distilled water until the filtrate attained a neutral pH.

18

2.2.2. Preparation of solutions

The freshly prepared solvent systems were pre-cooled either to -10°C (Paper I)

or -4°C (Paper II-III) and the cellulose to around 0°C. When the cellulose was

added, the temperature of the mixture rose to approx. -1°C and, finally, the

temperature rose during mixing to around +4°C. The solutions were stirred for

10 to 20 min at approx. 500 rpm depending on the pulp used: each solution was

prepared in duplicates. Each solution had a total weight of 100g.

2.2.3. Analysis

2.2.3.1. Pulp characterization

A FiberTester analysis was performed to determine the fibre dimensions as

outlined in the ISO 16065-2 standard.

The water retention value (WRV) of the different pulps was determined

according to the SCAN-C 62:00 standard.

The carbohydrate compositions were determined according to the SCAN-CM

71:09 standard, using high performance anion exchange chromatography with a

pulsed amperometric detector (HPAEC-PAD) to determine the

monosaccharides. The content of lignin was determined according to TAPPI T

222 and TAPPI UM 250 standards, with samples being analyzed in duplicate.

The content of cellulose was approximated as being equal to the glucose

content relative to the total amount of carbohydrates and lignin; the content of

hemicelluloses was assumed to be equal to the sum of arabinose, galactose,

xylose and mannose relative to the total amount of carbohydrates and lignin;

the lignin content was assumed equal to the sum of acid-insoluble and acid-

soluble lignin relative to the total amount of carbohydrates and lignin.

The DP was calculated both through its correlation to the pulp´s intrinsic

viscosity [η] as measured according to the ISO 5351 standard by employing

Equation 2 (Immergut et al. 1953) and Equation 3 (Evans & Wallis 1989)

below.

DPη0.905= 0.75*[η] Eq. 2

DPη0.90= 1.65*[η] Eq. 3

19

and from the measured weight-average molecular mass (Mw) using Equation 1.

DP calculated from Equation 1 is denoted here as DPw while DPs calculated

from Equations 2 and 3 are denoted as DPη2 and DPη3, respectively.

The molecular masses were determined relative to the polystyrene

standards using size-exclusion chromatography (SEC) with tetrahydro furan as

the eluent, three PLgel 10 µm MIXED-B LS 300 x 7.5 mm and an PLgel 10 µm

Guard 50 x 7.5 mm column, a reflective index detector and, finally, the software

Cirrus v 3.1 (Polymer Laboratories, Amherst, MA, USA). Prior to SEC analysis,

the samples were tricarbanilated by heterogeneous synthesis with phenyl

isocyanate (Evans et al. 1989; Drechsler et al. 2000). The conversion to

cellulose tricarbanilate included activation in pyridine, reaction with the phenyl

isocyanate at 80°C for 48 h, quenching by cooling and the addition of

methanol, precipitation of cellulose tricarbanilate with 100 ml methanol and

625 µl acetic acid and washing with deionized water and methanol, before being

dried.

A multivariate evaluation of the pulp properties for the various pulps using the

solubility rank as the response was performed using the software SIMCA-P+

12.0.1 (Umetrics AB, Umeå, Sweden) (Strunk et al. 2011). Despite the response

factor not being truly continuous, the results may give good indications of

interesting correlations.

2.2.3.2. Characterization of the solutions

The solubility test was performed as described by Jin et al. (2007). The solutions

were centrifuged for 1.5 h at 4500 rpm at 3°C in order to separate the

undissolved cellulose residuals from the dissolved cellulose. The undissolved

material was then washed using vacuum filtration: 20 times with water and 3

times with acetone. Finally, the washed material was dried at 105°C for 3 h. The

dissolved cellulose was regenerated with 2 M H2SO4 before being subjected to

the same washing and drying procedure as the undissolved material.

The solubility (Sa) was calculated according to Equation 4 (Jin et al. 2007):

Sa = w1/(w1+w2)*100% Eq. 4

where w1 and w2 are the amounts of dissolved cellulose and undissolved

materials, respectively. This method was reported in Paper I and used to

quantify the amount of dissolved cellulose.

20

In Paper II a ranking system for the solubility of the pulp fibres was

introduced whereby the solubility was estimated by allocating it a number

between one and five, with 1 = very poor, 2 = poor, 3 = intermediate, 4 =

good and 5 = very good dissolution. This solubility ranking was based on a

microscopic determination of the residual amount of fibres present after the

dissolution stage and is thus a subjective assessment method. Figure 12 shows

examples of two solutions: the image on the left has been allocated a solubility

rank of 1 and the one on the right a rank of 3. A quantitative analysis of the

weight of the solid material after centrifugation at 5000 rpm for one hour was

also made in order to reinforce the microscopic results.

Figure 12. Examples of optical micrographs with different solubility rankings. Left: a

cellulose solution ranked as 1, with 2% S (1097) pulp in the NaOH/urea/thiourea (8:8:6.5%)

system. Right: a cellulose solution ranked as 3, with 3% HT B V67 (310) pulp in the

NaOH/ZnO (9:0.5%) system.

An Olympus BX51 microscope (Hamburg, Germany) equipped with a

ColorView III Soft imaging system (Münster, Germany), was used to evaluate

the quality of the dissolution in terms of transparency, birefringence and

fraction of undissolved fragments. A small aliquot of the cellulose solution was

transferred to a microscopic glass slide, covered with a cover glass and viewed

in the microscope between crossed polarizers. A magnification of 200x was

used in each trial.

The rheology experiments were performed on a Physica UDS 200 rheometer

(Ostfildern, Germany) using a cone-and-plate measuring geometry (1°, 50 mm

diameter). The instrument was equipped with an automatic gap setting. A

temperature control unit ensured a temperature variation in the sample

chamber not larger than 0.1°C from the set value of 5°C. The shear viscosity

( ) of the samples was determined in nonlinear rotational measurements. The

small-amplitude dynamic tests, on the other hand, provided information of the

21

linear viscoelastic behavior of the materials through the determination of the

complex shear modulus, G*(ω), as illustrated by Equation 5 (Larson 1998):

G*(ω) = G´(ω) + i G´´(ω) Eq. 5

where G´(ω) is a measure of the reversible elastic energy and G´´(ω) represents

the irreversible viscous dissipation of the mechanical energy as a function of

frequency, ω. The gelation kinetics were monitored by recording the evolution

of G´ and G´´ over a fixed period, at a constant ω = 1 Hz and constant shear

stress of 1 Pa. The shear stress used was found to be within the linear

viscoelastic regime.

22

3. Results and Discussion

3.1. Effects of the pretreatments

Cellulose with a low molecular mass can be dissolved in aqueous NaOH (Sobue

et al. 1939; Isogai & Atalla 1998), as shown above. Pretreatment of the cellulose

prior to the dissolution step is, however, necessary for greater cellulose contents

and molecular masses.

Three different pretreatment methods were utilized in this study, namely

steam explosion, hydrothermal and EtOH/HCl (Paper I-III). There is little

doubt that they all altered the morphology of the pulp fibres (see DPη2 in Table

3). Figure 13 shows how the DP of the D (745) pulp was affected by the steam

explosion pretreatment at various conditions (Paper I). As seen in the figure,

harsher conditions reduced the DP quite substantially: from 745 to 160. The

same figure also reveals that the leveling-off degree of polymerization (LODP)

seemed to be reached when the cellulose was treated at 226°C (▲) for more

than 10 minutes. According to Battista et al. (1956) and Håkansson (2005) the

LODP correlates to both the average length of the cellulose elementary

crystallites and the fibre fragments obtained after the treatment.

Figure 13. The DPη2 obtained after STEX pretreatment.

x = non-treated, ● = 190°C, ■ = 210°C and ▲ = 226°C

0

100

200

300

400

500

600

700

800

0 5 10 15 20

DPη

2

Time (min)

23

The distribution of the molecular masses of the untreated, and their

corresponding pretreated, pulps were investigated and are represented in

Figures 14a and b, respectively. The curves of the pretreated pulps (Figure 14b)

have been displaced to the left, due to a reduced molecular mass. It can also be

noted that the polydispersity indexes (PDI) of the two dissolving pulps were

reduced whilst it increased for the paper pulps. PDI is a value calculated from

the correlation between weight-average molecular mass (Mw) and number-

average molecular mass (Mn), see Equation 6, and can be estimated from the

width of the curve. A narrow curve implies a low PDI, where the distribution

of the molecular mass of the polymer is small. A wider curve, conversely,

indicates polymers with a more heterogeneous molecular mass distribution (i.e.

a higher PDI).

DPI = Mw/Mn Eq. 6

Figure 14. Molecular mass distributions determined by SEC of (a) untreated pulps and (b)

pretreated pulps. Pulps: solid line = S (1097)/STEX S (202), dashed line = D (745)/STEX D

(160), dotted line = B V67 (637)/HT B V67 (310).

The DP and PDI are believed to influence both the solubility of the cellulose

and the strength of the final textile fibre. The DP normally has a linear

correlation to the tensile strength of a regenerated cellulose fibre, i.e. the higher

24

the DP the higher the tensile strength. However, there are different opinions

regarding the relationship in which the DP no longer influences the tensile

strength of the fibre (Coley 1953). The tensile strength reaches a plateau at a

certain DP: according to Coley (1953), this is around 350 for rayon fibres. An

increased DP, however, has a negative impact on solubility. When a polymer

has a low DP, its solubility is normally high. One reason for this is that the

number of molecules, and thus the entropic term, becomes greater. The smaller

the molecular mass the stronger the entropic driving force for dissolution.

Therefore, a reduction in DP is a necessary compromise to be able to process

the cellulose into regenerated cellulose fibres even though it may mean a

reduced tensile strength of the final fibres produced.

A large PDI (wider curve) of the cellulose can have a negative impact on

the quality of the cellulose solution. The fraction of cellulose of low molecular

mass probably dissolves easily (faster) while the higher molecular mass fraction

may not dissolve and be present as insoluble fragments. Obviously, the solution

is heterogeneous, which will cause extra difficulties in further processing

(regenerated cellulose fibres). Additionally, it is expected that a larger PDI will

influence the tensile strength of the spun fibres, although no systematic work

has been carried out on the subject.

3.2. The influence of pulp properties on solubility

Pulps were characterized to investigate how the various properties influenced

their solubility in aqueous solvent systems (Paper II). The properties

characterized were: fibre dimensions (length, width and coarseness), average

degree of polymerization, WRV and the content of carbohydrates. The results

were compiled and are summarized in Tables 4 and 5. The common

denominator for the three untreated pulps was the fact that all of their

properties changed after the pretreatments; some more than others. The

outstanding characteristic of the paper grade pulps (i.e. S and STEX S) was that

they both had a higher content of hemicelluloses (> 9%) when compared to the

dissolving pulps and their corresponding pretreated pulps, which consisted of

almost pure cellulose (ca. 95–98%).

25

Table 4. Pulp characterization: dimensions of the fibres and WRV.

Dimensions of the fibres

Pulp Length (mm)

Width (μm)

Coarseness (μg/m)

WRV (gH2O/gFibre)

S (1097) 0.93 20.6 85 1.07

D (745) 1.72 25.5 134 0.72

B V67 (637) 2.66 32.1 182 0.69

STEX S (202) 0.86 20.0 91 1.56

STEX D (160) 0.95 24.0 103 1.34

HT B V67 (310) 2.45 30.3 199 0.66

Table 5. Pulp characterization: contents of carbohydrate and lignin, average molecular mass

(DPη and DPw) and PDI.

All of the pulps characterized were dissolved in the NaOH/urea/thiourea

(8:8:6.5%) system. The solutions were evaluated according to the ranking of

solubility mentioned previously.

In Figure 15 the solubility of six pulps in the NaOH/urea/thiourea

system are expressed as a function of DPη2. The solubility increased as the DP

decreased, as expected. However, the pulp ranked with the highest solubility,

HT B V67 (310), did not present the lowest DP value. Thus, despite DP being

one of the most important parameters controlling dissolution, this indicates that

there are also other factors that influence the solubility of pulp fibres.

Pulp Cellulose

(%)

Hemi- celluloses

(%)

Lignin (%)

DPη2 DPη3 DPw PDI

S (1097) 76.8 21.9 1.3 1097 2738 2499 1.8

D (745) 96.1 2.6 1.3 745 1856 3902 6.7

B V67 (637) 95.9 3.5 0.6 637 1586 2183 5.0

STEX S (202) 85.4 9.2 5.4 202 501 1539 14.7

STEX D (160) 98.0 1.2 0.8 160 394 373 2.7

HT B V67 (310) 96.0 3.1 0.9 310 770 1040 4.7

26

Figure 15. The influence of DPη2 on solubility in NaOH/urea/thiourea.

Pulps: ○ = S (1097), □ = D (745), ∆ = B V67 (637), ● = STEX S (202), ■ = STEX D (160)

and ▲= HT B V67 (310). Solubility: 1 = very poor, 2 = poor, 3 = intermediate, 4 = good and

5 = very good dissolution.

Figure 16 illustrates how the DP and the content of hemicellulose of each pulp

affect solubility. Both of them showed clear trends: a low content of

hemicellulose (≤ 5%) and, as discussed above, a low DP seem advantageous for

the solubility of both the untreated (unfilled dots) and the pretreated (filled

dots) pulps. This result was in agreement with Le Moigne & Navard (2010),

who stated that “A higher content of hemicellulose leads to an increased

embedding of the cellulose microfibrils in the hemicellulose matrix, thus

making it harder to dissolve”. A low content of hemicellulose is an important

characteristic of dissolving pulps and, most likely, one of the reasons why such

pulps are suitable, and normally used, as the raw material for producing

regenerated cellulose fibres.

Figure 16. The influence of DPη2 and the content of hemicelluloses on solubility in

NaOH/urea/thiourea. Pulps: ○ = S (1097), □ = D (745), ∆ = B V67 (637), ● = STEX S

(202), ■ = STEX D (160) and ▲= HT B V67 (310). The numbers next to the symbols

represent their solubility ranking, where: 1 = very poor, 2 = poor, 3 = intermediate, 4 = good

and 5 = very good dissolution.

1

2

3

4

5

0 500 1000

So

lub

ilit

y

DPη2

0

5

10

15

20

25

0 500 1000

Hem

ice

llu

los

e (

%)

DPη2

2

1

2 5 4

3

27

Not only were the properties on or below the microscopic level of the pulps

characterized but also those on the macroscopic level (i.e. dimensions of the

fibres). Figure 17 indicates that the length and the width of the fibres were

correlated linearly for the pulps investigated, i.e. the degradation and

defibrillation of the fibres occurred simultaneously. Since the samples were

distributed randomly along the curve with respect to the solubility ranking,

neither of the two parameters visualized in Figure 17 seemed to exert a

significant influence on the solubility of the pulp fibres. This is in agreement

with the commonly held view that the solubility of cellulose is not determined

by macroscopic properties (e.g. length of the fibre) but rather by properties

prevailing at the micro and nano levels, i.e. in regions close to the molecular

and supramolecular architecture of the cellulose, in the cell wall within the

fibrils and the fibril aggregates.

Figure 17. The influence of the width and length of the fibres on solubility in

NaOH/urea/thiourea. Pulps: ○ = S (1097), □ = D (745), ∆ = B V67 (637), ● = STEX S

(202), ■ = STEX D (160) and ▲= HT B V67 (310). The numbers next to the symbols

represent their solubility ranking, where: 1 = very poor, 2 = poor, 3 = intermediate, 4 = good

and 5 = very good dissolution.

3.2.1. Multivariate data analysis

As the solubility ranking is not only qualitative, a multivariate evaluation of the

pulp properties was also performed. Such an evaluation may provide

indications of which properties have a positive influence on solubility. The

properties were, through the multivariate evaluation, quantified tentatively by a

principal component (PC) analysis; two components were found to explain

almost 80% of the variation, of which PC 1 accounted for the majority. This is

expressed in Figure 18. The way to interpret such a plot, in a simplified way, is

0

5

10

15

20

25

30

35

0 1 2 3

Wid

th (μ

m)

Length (mm)

2

3 1

2

5

4

28

to compare the location of the response (solubility) with the location of the

factors, i.e. the pulps and the corresponding properties. For instance, if the

location of the factor “hemicellulose” is compared to the location of the

property “solubility”, they are to be found on opposite sides of the plot in

Figure 18. Thus, they are inversely correlated; i.e. a low content of

hemicellulose should facilitate the solubility of the fibres. The results shown in

the figure are consistent with the observations above, with high levels of

solubility correlating mainly to low DP and low contents of hemicellulose.

These results are also in good agreement with the results obtained by

Christoffersson (2005) and Strunk et al. (2011), who used the pulp reactivity as

their response instead of the solubility ranking. Furthermore, the model

predicts that STEX D (160) and HT B V67 (310) have the highest and most

similar solubility. This indicates that other properties than those included in this

study affect the solubility of cellulose fibres in alkaline solvent systems.

Figure 18. Loadings bi plot of the principal components 1 and 2 in the partial least-squares

model. The properties measured (FL = fibre length, FW = fibre width, COA = fibre

coarseness, WRV = water retention value, DPη3 = degree of polymerization as measured by

viscometry, DPw = degree of polymerization as measured by SEC, PDI = polydispersity index,

CEL = cellulose content, HEM = hemicelluloses content and LIG = lignin content) are

plotted as circles, the response parameter (SOL = solubility ranking) as a filled dot and the

pulps as crosses.

The importance of each variable was also visualized in a variable importance

plot based on the principal component analysis, as displayed in Figure 19. The

results once again show that the DP and composition of the carbohydrates (i.e.

FL

FW

COA

WRV

CEL

HEM

LIG

DPn3

PDI

DPw

SOL

S

D B V67

STEX S

STEX D

HT B V67

-1

0

1

-1 0 1

PC

2

PC 1

29

cellulose and hemicelluloses) are the major and sole significant parameters to be

considered (the error bar does not cross zero).

Figure 19. Variable importance plot for the partial least-squares model. DPη3 = degree of

polymerization as measured by viscometry, DPw = degree of polymerization as measured by

SEC, HEM = hemicelluloses content, CEL = cellulose content, COA = fibre coarseness, FW

= fibre width, FL = fibre length, WRV = water retention value, PDI = polydispersity index

and LIG = lignin content.

The conclusions that can be drawn from these analyses are the following: 1) the

dissolution of pulps seems to be controlled by a very complex interaction

between various parameters, 2) the solubility of the cellulose molecules are

most certainly determined by properties prevailing beneath the fibre

dimensions, 3) properties other than those investigated here also affect

solubility and, finally, 4) an “ideal” pulp should have a low DP and a low

content of hemicelluloses if it is to be fully soluble in alkaline solvent systems.

3.3. Aqueous solvent systems

A variety of alkaline aqueous solvent systems were investigated in this study

(Paper I-III), see section “Materials and Methods - Solvent systems”. In Paper I the

ability of three different solvent systems to dissolve STEX D (339) was

compared (Figure 20). As shown in Figure 20 all three solvents had a low

solubility capacity for the STEX-pretreated dissolving pulp, with the

NaOH/urea/thiourea system being the most efficient. Figure 21 illustrates that

the NaOH/urea/thiourea system was not only more suitable for dissolving the

STEX-pretreated pulp but also for the HT B V67 (310) pulp (Paper II). These

results are in agreement with the data obtained by Zhang et al. (2010).

-1

0

1

2

3

DPn3 DPw HEM CEL COA FW FL WRV PDI LIG

Va

ria

ble

im

po

rta

nce

30

Figure 20. The solubility of STEX D (339) pulp at 5% consistency obtained for three

different solvent systems. Solvents: NaOH/urea/thiourea (8:8:6.5%), NaOH/ZnO (9:1%)

and NaOH/PEG (9:1%).

Figure 21. Solubility of the HT B V67 (310) pulp in the NaOH/urea/thiourea (8:8:6.5%) and

NaOH/ZnO (9:0.5%) systems. ● = NaOH/urea/thiourea and ■ = NaOH/ZnO. Solubility:

1= very poor, 2 = poor, 3 = intermediate, 4 = good and 5 = very good dissolution.

A number of researchers have reported the beneficial effect of using pre-cooled

NaOH-based solvent systems (Cai & Zhang 2005; Lue et al. 2007; Zhang et al.

2010). The overall process of dissolution of cellulose is exothermic, according

to Qi et al. (2008), and thus favored by lower temperatures. They illustrated the

correlation between solubility (Sa) and temperature as an Arrhenius equation

(Equation 7), and observed that when the temperature of the solvent was

lowered, the solubility of the cellulose increased.

ln Sa = ln A-(Ea/RT) Eq. 7

1

2

3

4

5

2 3 4 5

So

lub

ilit

y

Pulp consistency (%)

31

In Paper I the effect of a pre-cooled solvent system was investigated. Figure 22

displays the solubility of the STEX D (160) pulp in the NaOH/urea/thiourea

system under various dissolution conditions, and shows that a decrease in

temperature clearly increases solubility. The results obtained indicate that the

dissolution of cellulose is kinetically controlled. Prolonging the storage time

allowed the solubility to be increased from 82 to 94%. Thus, a more efficient

mixer should provide a similar effect as prolonging the storage time, since this

will clearly increase the diffusion rate of the solvent into the walls of the fibres.

The subject of kinetics will be discussed shortly.

Figure 22. The solubility of STEX D (160) in the NaOH/urea/thiourea system under various

conditions: time (for how long the solutions were stored at 3°C), temperature of the solvent

and concentration of cellulose in the dope.

Despite the fact that the NaOH/urea/thiourea system seemed to be the most

favorable alkaline aqueous solvent system for the dissolution of cellulose, other

factors contributed to this track nevertheless being abandoned. One idea of this

project was to integrate the novel textile fibre manufacturing process into an

existing pulp mill, thus allowing textile fibres to be produced on the same site.

However, during the past few decades, the authorities in the Nordic countries

have focused their environmental protection legislation concerning effluent

water on emissions of nitrogen compounds. Utilization of urea in a pulp mill

leads, sooner or later, to the emission of compounds that contain nitrogen, so

the use of urea in a textile fibre plant is therefore not a viable option. Moreover,

the addition of urea to a pulp mill utilizing a chemical recovery system will

result in the formation of NOx gases, which have a negative impact on the

environment either in the form of acid rain or acting as a fertilizer in the

receiving water and soil. The increased use of compounds such as urea in the

60

65

70

75

80

85

90

95

100

16 h 16 h 16 h 60 h

23°C -10°C -10°C -10°C

5% 5% 1% 5%

So

lub

ilit

y (

%)

32

pulp and paper industry is thereby highly restricted. Alternative NaOH/H2O

systems, with or without additives, are therefore those investigated henceforth.

3.3.1. Effects of ZnO additions

Visualizing the cellulose solutions in a polarized optical microscope can provide

indications of how the dissolution process of the pulp fibres proceeds. Figure

23 (Paper III) shows two images from the optical microscope assessment of

solutions with 6% HT B V67 (263) dissolved in NaOH (8.5%), Figure 23a, and

NaOH/ZnO (8.5:0.8%), Figure 23b, respectively. As can be seen, more

undissolved materials were observed in the solution with only NaOH than in

that with NaOH/ZnO, indicating that the former is a less favorable solvent

system for dissolution of pulp fibres. Figure 23a shows that there is a high

density of swollen undissolved fibres and discs still present in the solution when

ZnO is not used. However, only an insignificant fraction of discs and small

fragments could be seen after the addition of just a small amount of ZnO,

indicating the high dissolution power of the solvent (Figure 23b). Although, the

role of additives to dissolution of cellulose is not fully understood, as

mentioned above, fundamental physicochemical aspects of polymers have been

reviewed recently, with particular focus being placed on cellulose (Lindman et

al. 2010; Medronho et al. 2012). From their analyses, it becomes clear that

polyelectrolytes are much more soluble than nonionic polymers due to the

entropy of the small counterions. Therefore, the suggestion made here is that

the Zn(OH)42− anion is simply charging up the cellulose by associating with it

and, as a consequence of the formation of a cellulose-zincate charged complex,

providing it with enhanced solubility.

Figure 23. Optical micrographs of 6% HT B V67 (263) dissolved in (a) NaOH (8.5%) and (b)

NaOH/ZnO (8.5:0.8%). The bar represents 200 μm.

33

A small addition of ZnO not only increases the dissolution power of the

solvent but also prevents gelation: i.e. it increases the stability of the solutions

and decreases the shear viscosity, as can be seen in Figures 24 and 25,

respectively. Typically, cellulose dopes are unstable and the self-association of

cellulose chains results in gelation of the system. It is expected that the

electrostatic repulsion between the charged polymer backbones (due to the

hypothesis of the cellulose-zincate charged complexes) contribute to the

stability of the solution, preventing re-association of the cellulose. The stability

of the cellulose dope can be seen in Figure 24 where fresh samples (with and

without ZnO) were allowed to rest at +5ºC while the viscoelastic parameters

G´ (elastic contribution) and G´´ (viscous contribution) were measured over

time. The sample with ZnO has a more pronounced liquid-like behavior; the

G´ and G´´ moduli are far apart (grey curves) indicating that gelation is not

occurring. The cellulose dope without ZnO, on the other hand, shows a higher

storage modulus (G´); after 1 hour, both moduli almost crossed, indicating that

gelation occurs much faster than in the case of the cellulose dope with ZnO.

This phenomenon, i.e. the prevention of gelation by the addition of ZnO, has

also been observed with other additives, such as surf A. Since the cellulose

molecule may be regarded as an amphiphilic polymer it is believed that

amphiphilic co-solutes, e.g. surf A, facilitate not only the aqueous dissolution of

cellulose (by reducing viscosity and increasing the amount of cellulose in the

dope) but also increase the stability of the solution (by decreasing the

viscoelastic properties and kinetics of gelation), as these compounds would

weaken the hydrophobic interactions responsible for aggregation rather than

break hydrogen bonds.

34

Figure 24. G´ (filled squares) and G´´ (empty circles) as a function of time. Gelation of 6%

HT B V67 (263) pulp dissolved in the NaOH (8.5%) (black symbols) and the NaOH/ZnO

(8.5:0.8%) systems (grey symbols).

Figure 25. Shear viscosity of 6% HT B V67 (263) pulp dissolved in the NaOH (8.5%) (filled

squares) and the NaOH/ZnO (8.5:0.8%) systems (empty squares).

The influence of ZnO and surf A when added to the EA/HT B V67 (234)

solutions was also studied. The shear viscosities of different samples are

compared in Figure 26, where it can be seen that the addition of ZnO to the

EA/HT B V67 (234) system (filled black triangles) decreased the viscosity

considerably. A slightly better rheological performance was also observed when

the surf A was included in the latter system; see the empty diamonds in Figure

35

26. This could be of importance because it opens up the possibility of being

able to dissolve greater amounts of cellulose whilst maintaining the viscosity.

Figure 26. Shear viscosities of HT B V67 (263) without ZnO (filled black circles), EA/HT B

V67 (234) without ZnO (empty grey circles), EA/HT B V67 (234) with ZnO (filled black

triangles) and EA/HT B V67 (234) with ZnO and surf A (empty diamonds). All samples were

prepared in a NaOH (8.5%) solution having a cellulose content of 6%. The additives used

were 0.8% ZnO and 0.6% surf A.

3.4. Pulp consistency

One important, and critical, factor of textile fibres is its tensile strength (both

dry and wet strength) which could be affected by several properties, such as the

DP of the cellulose, its concentration (i.e. consistency) and the spinning

conditions (Vehviläinen et al. 2008). One drawback of the alkaline aqueous

solvent systems utilized in this project was the low dissolution ability of

cellulose. In a commercial textile fibre process it is advantageous to use as high

pulp consistency as possible, as this reduces the amount of both energy and

chemicals required. It has been reported in the literature that NaOH-based

solvents could dissolve cellulose with a concentration of up to around 6-7%.

Other processes, e.g. the viscose and the Lyocell process, on the other hand,

utilize around 8-10% and 10-15% cellulose, respectively (Rosenau et al. 2001).

Therefore, one aim throughout the entire project was to increase the

concentration of cellulose in the spin dopes.

Figure 27 illustrates how the solubility of STEX D (160) pulp in a

NaOH/urea/thiourea system is affected by its consistency (Paper I): as

expected, the solubility decreased as the concentration increased. Not even at

36

the lowest pulp consistency of 0.5% was it possible to have a fully dissolved

solution: insoluble remains were always present.

Figure 27. Solubility of STEX D (160) pulp in the NaOH/urea/thiourea system as a function

of pulp consistency.

One of the reasons why various pulps, pretreatments, conditions were

compared in this project was the interest in increasing the concentration of

cellulose in the spin dopes. The kinetics of dissolution was also a parameter that

was evaluated in this study. It is known that the kinetic barrier for dissolution is

much more important for macromolecules than for low molecular polymers, as

mixing is easier and molecular diffusion faster (Lindman et al. 2010). In Figure

28, taken from Paper II, the solubility of the HT B V67 (310) pulp in the

NaOH/ZnO (9:0.5%) system utilizing two types of mixers can be seen. As the

figure indicates, more efficient mixing facilitates the solvent-cellulose contact;

this enhances solubility and permits a higher pulp consistency to be used. This

result was consistent with the results obtained by Zhang et al. (2010), who claim

that the solubility of cellulose increases with an increase in the stirring rate.

Mass transfer effects therefore play an important role in the dissolution process

of cellulose molecules.

76

79

82

85

88

91

94

97

100

0 1 2 3 4 5

So

lub

ilit

y (

%)


37

Figure 28. Solubility of cellulose versus pulp consistency using high efficiency (▲) and

standard efficiency () mixers. The HT B V67 (310) pulp and the NaOH/ZnO (9:0.5%)

system were used. Solubility: 1= very poor, 2 = poor, 3 = intermediate, 4 = good and 5 = very

good dissolution.

3.4.1. Effects of EtOH/HCl treatment

It is often stated that the dissolution of cellulose is dependent on the conditions

used, such as temperature and time, and the properties of the raw material (e.g.

DP). This gives a clear indication that kinetic, rather than thermodynamic,

control is decisive. Given this, an additional pretreatment method to the already

pretreated pulp was investigated to examine whether or not it could improve

the degree of solubility further. The hydrothermally treated Buckeye dissolving

pulp (HT B V67) was therefore treated further with EtOH/HCl, resulting in

the EA/HT B V67 (234) pulp (Paper III). In Figure 29, taken from Paper III,

6% HT B V67 (263) and 6% EA/HT B V67 (234) pulps were dissolved in the

NaOH (8.5%) system. Compared to the HT B V67 (263) solution both of the

viscoelastic parameters (i.e. G´ and G´´) for the EA/HT B V67 (234) solution

were very low, and no crossover was observed after 1 h (black curves). The HT

B V67 (263) sample, on the other hand, almost gelled after 1 h (grey curves).

This shows that the additional EtOH/HCl treatment increases both the

solubility and stability of the solution efficiently, resembling the effect caused

by the addition of ZnO. Thus, it seems that subjecting the cellulose to a series

of pretreatments could replace the need for additives to the NaOH system.

1

2

3

4

5

2 3 4 5 6

So

lub

ilit

y


Improved mixing efficiency

38

Figure 29. G´ (filled squares) and G´´ (empty circles) as a function of time. Gelation of 6%

HT B V67 (263) (grey symbols) and EA/HT B V67 (234) (black symbols) pulps dissolved in

the NaOH (8.5%) system.

The EtOH/HCl treatment has two main effects: it reduces the DP and

ruptures the primary cell wall (Trygg & Fardim 2011). The former has a direct

consequence on the translational entropy of mixing of the system. In other

words, the lower the molecular weight the stronger the entropic driving force

for dissolution. The latter, and principle, effect is a weakening of the cell wall

structure, allowing the solvent to penetrate throughout the fibre and thus

dissolve it. Typically, when a polymer sample is placed in a solvent, the solvent

molecules make contact relatively quickly with the polymer and penetrate into

its surface. This often results in the outermost surface taking on a gel-like

consistency which, in turn, then retards dissolution. The EtOH/HCl treatment

impairs the first barrier to fibre dissolution, thereby allowing the solvent

molecules to permeate and diffuse easily and faster through the internal parts of

the cellulose fibres. This clearly increases the kinetics of dissolution.

Although the EA/HT B V67 (234) pulp was dissolved in the NaOH

(8.5%) system its solubility was investigated further in the NaOH/ZnO

(8.5:0.8%) system. The pulp consistency of 6% was adopted and optical

microscopy was used to evaluate the solubility, as can be seen in Figure 30a.

Hardly any fragments were visible at all indicating that, under the conditions

used, the NaOH/ZnO (8.5:0.8%) system appeared to be a good solvent for

dissolving the EA/HT B V67 (234) pulp.

39

The next step was to use the EtOH/HCl mixture directly on the B V67 (637)

pulp without the HT treatment (Paper III). This untreated Buckeye dissolving

pulp had a reasonably high DP (637) and, without activation, it was insoluble in

NaOH solvents. After treatment with EtOH/HCl the resulting material, EA B

V67 (393), had a DP of 393 as measured by its intrinsic viscosity (see Table 3).

The EA B V67 (393) pulp that was generated was then dissolved in the

NaOH/ZnO (8.5:0.8%) system following the standard procedure used in

Paper II. In Figure 30 it can be seen how the concentration of EA B V67 (393)

influences its dissolution. With a 4% pulp consistency, full dissolution was

observed after 20 min mixing (Figure 30b); no undissolved material was noted

in the optical micrograph. When the consistency was increased further to 6% a

very viscous, but transparent, solution was obtained (Figure 30c). In this case,

only a residual fraction of undissolved fragments could be observed but this

was easily extracted by filtration. Finally, 9% EA B V67 (393) pulp was used

and, despite the micrograph showing a low fraction of undissolved material and

fibres, the sample gelled quite quickly (< 20 min) during the vigorous mixing

procedure (Figure 30d). In any case, it was emphasized that, at the very least,

6% EA B V67 (393) pulp, with an average DP of 393, could be dissolved in an

alkaline aqueous solvent system such as NaOH/ZnO (8.5:0.8%). This

demonstrates a considerable improvement when compared with most of the

previous studies undertaken using this solvent system.

40

Figure 30. Optical micrographs of (a) 6% EA/HT B V67 (234) pulp, along with (b) 4%, (c)

6% and (d) 9% EA B V67 (393) pulps dissolved in NaOH/ZnO (8.5:0.8%).

41

4. Conclusions

Some of the parameters that affect the dissolution of cellulose in different

environmentally friendly solvents have been compared in this thesis. Those

studied were: pulp properties, pretreatments, additives, kinetic and

thermodynamic aspects (hydrophobic interactions, hydrogen bonds and the

effects of charges).

The dissolution of cellulose seemed to be controlled by a very complex

interaction between various parameters. It was emphasized that the solubility of

the cellulose molecules was almost certainly determined by properties prevailing

in regions close to the molecular and supramolecular architecture, and not by

macroscopic properties. Furthermore, it was established that, of the pulp

properties investigated, only the composition of the carbohydrates and the DP

had a significant influence on the solubility of the pulps used. It should also be

noted that properties other than those investigated, e.g. crystallinity, may be of

importance to the solubility of cellulose.

It was further confirmed that a more efficient mixing process facilitated

the solvent-cellulose contact and thus solubility. Kinetic control of dissolution

is, as is widely known, much more important for macromolecules than for low

molecular mass solutes since mixing is easier and molecular diffusion faster, in

the latter. It was also concluded that the pretreatments and the various additives

to the NaOH system played significant roles in dissolution. The addition of

ZnO and amphiphilic molecules, such as the “surf A”, was observed to

improve the kinetics of dissolution and stability of the cellulose dopes.

Moreover, the pretreatments studied efficiently increased both the solubility

and stability of the cellulose dopes, resembling the effect of additives to the

NaOH system.

42

5. Future studies

Knowledge of what occurs on a molecular level during the dissolution of

cellulose is very limited and it would therefore be of value to study this

phenomenon further. It would also be interesting to continue the investigation

of the role played by additives in dissolution: do they interact with the cellulose

and, if so, how? The behavior of cellulose in solutions and the re-aggregation

(gelation) phenomenon are other important, and intriguing, subjects that

deserves further attention.

This study has confirmed that mass transfer effects are important in the

dissolution process. Future studies involving the effects of higher shear stress

mixers, e.g. MC mixers, should therefore be undertaken.

Future work would also involve scale-up procedures, such as optimizing

the conditions of the various sub-steps of the production of cellulose-based

textile fibres (pretreatment, dissolution and spinning) and investigating the

robustness of the whole process.

43

6. Acknowledgements

Firstly, I wish to express my sincere gratitude to my head supervisor, Prof. Ulf

Germgård, for all his support and commitment to this licentiate thesis. Thank

you, Ulf, for all of the encouraging and fruitful discussions we had.

I would also like to acknowledge my two assistant supervisors, Dr. Mats Westin

(Deputy Centre manager, SP Trä) and Lars Stigsson (CEO, KIRAM AB) for

their expert knowledge, competence and constructive feedback. Lars: a special

thanks for allowing me to be a part of this project.

The CelluNova project, a part of the EcoBuild Excellence Centre, SSF,

Vinnova and KK-stiftelsen are acknowledged for their financial support.

My gratitude goes to the participants of the CelluNova project for keeping the

project running and extremely interesting throughout all our meetings and

collaborations, as well as for assisting me with the necessary analyses and

materials. You all are thanked for taking an interest in this project.

I am most thankful to all of my co-authors for being part of this thesis,

especially Dr. Bruno Medronho. Bruno: you are thanked not only for your

skillful knowledge in this field and your help in my research through

collaboration and inputs, but mostly for the new friendship that we established.

To all my friends and colleagues at Karlstad University whom I met during my

time as a Ph.D. student: I am sincerely grateful for all your help, the many

fertile discussions that we had and, not least, for bringing joy to my work every

day.

Finally, I wish to express my gratitude to my lovely Anna and family for all your

love and support. I am truly honored having all of you by my side.

44

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