Pine-Polyethylene (Wood -Polymer) Composites: Synthesis

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Pine-Polyethylene (Wood -Polymer) Composites:Synthesis and Mechanical BehaviorCharacterization.Parviz S. RaziLouisiana State University and Agricultural & Mechanical College

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PINE-POLYETHYLENE (YVOOD-POLYMER) COMPOSITES: SYNTHESIS AND MECHANICAL BEHAVIOR

CHARACTERIZATION

A Dissertation

Submitted to Graduate Faculty o f the Louisiana State University and

Agricultural and Mechanical College In partial fulfillment of the

Requirements for the degree o f Doctor of Philosophy

in

The Interdepartmental Program in Engineering Science

byParviz S Razi

B. S., College of Science and Technology, Tehran, Iran. 1972 MBA. Louisiana State University, Baton Rouge, 1980 M. S., Louisiana State University, Baton Rouge, 1983

August. 1999


UMI Number: 9945734

UMI Microform 9945734 Copyright 1999, by UMI Company. All rights reserved.

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ACKNOWLEDGEMENT

The author wishes to express sincere appreciation to Dr. Aravamudhan Raman for his

continuous advice and encouragement during the preparation and advancement o f this

project. In addition, special thanks are expressed to Dr. Portier, Institute for

Environmental Studies, for his support o f this research and to Dr. Collier, Chemical

Engineering Department, for the use o f the extruder and other facilities. Appreciation is

also extended to Dr. Pang, Dr. McCarley, Dr. Schulz and Dr. Joshi for their valuable

advice. Special thanks to my very patient wife, Manijeh, and to my two sons, Sam and

Keyan, for their patience and loyal support.

ii


TABLE OF CONTENTS

ACKNOWLEDGMENT.........................................................................................................ii

LIST OF TABLES.................................................................................................................. v

LIST OF FIGURES................................................................................................................ vi

LIST OF NOMENCLATURE.............................................................................................. ix

ABSTRACT.............................................................................................................................x

CHAPTER IINTRODUCTION....................................................................................................... 1

1.1. Mechanical Properties o f Wood-Polymer Composite.................... 21.2. Effect o f Coupling Agents................................................................41.3. Fracture and Fracture Toughness......................................................6

1.3.1. K[C...........................................................................................6

1.3.2. Crack Opening Displacement (COD)..................................71.3.3. The Crack Extension Force, G ............................................8

1.3.4. J Integral................................................................................ 91.3.5. R Curve................................................................................12

1.4. Objectives..........................................................................................21

CHAPTER 2EXPERIMENTAL PROCEDURES...................................................................... 22

2 .1 . Common Characterization Techniques Used................................ 222.1.1. Scanning Electron Microscopy......................................... 222.1.2 Tensile Testing...................................................................222.1.3. Instrumented Impact Testing............................................. 22

2.2. Experiments Pertaining to Mechanical Properties........................232.2.1 Preparation o f Wood-Polymer Composites Samples 23

2.3 Experiments Pertaining to Effect of Coupling Agentsand Surface Modification................................................................242.3.1. Preparation o f Wood-Polymer Duplex Samples............ 25

2.4 Experiments Pertaining to Impact Fracture Properties................. 262.4.1. Preparation o f Composites Samples................................. 26

2.5. Experiments Pertaining to Fracture Resistance............................ 27

CHAPTER 3RESULTS AND DISCUSSION: STATIC MECHANICALPROPERTIES OF PINE CHIP - HDPE COMPOSITES INTENSION AND BENDING................................................................................... 36

3.1. Tensile Mechanical Properties.......................................................363.2. 3-Point Bending Testing................................................................. 363.3. Nail Removal Testing......................................................................37

iii


CHAPTER 4RESULTS AND DISCUSSION: INTERFACE MODIFICATIONAND INTERFACE BONDING IN PINE WOOD - HDPE COMPOSITES 48

4 .1. The Influence o f Pre-treatment in NaOH or H2SO4 on theBond Strength Between Wood and Polymer..............................48

4.2. The Influence o f Coupling Agents on the Bond StrengthBetween Wood and Polymer........................................................ 49

4.3. The Influence o f Surface Morphology o f Woodstockand he Bond Strength Between Wood and Polymer.................. 50

CHAPTER 5RESULTS AND DISCUSSION: IMPACT FRACTUREPROPERTIES OF PINE CHIP - HDPE COMPOSITES....................................58

5.1. Instrumented Impact Testing.......................................................... 585.2 Experimental Results.......................................................................595.3 Energy to Cause Failure..................................................................645.4 Mechanism for Energy Absorption and Fracture

Morphology.................................................................................... 6 6

CHAPTER 6RESULTS AND DISCUSSION: FRACTURE RESISTANCEAND FRACTURE TOUGHNESS OF PINE CHIP - HDPE COMPOSITES...73

6 .1. Kic Determination Using ASTM Standard Procedure..................736.2. JIc Determination Using ASTM Standard Procedure..................736.3. R Curve Determination................................................................... 746.4. Analysis o f Data.............................................................................. 746.5. The critical Stress Intensity Factor for Particulate Filled

Composites..................................................................................... 78

CHAPTER 7CONCLUSIONS AND SUMMARY .................................................................. 87

CHAPTER 8

SUGGESTIONS FOR FUTURE WORK............................................................. 928 .1. Evaluation o f Krif,com and K[C for Composites with 60

vol. % Pine Wood Chips................................................................928.2. Modeling o f Crack Growth and Delamination of the Wood

Chip - polymer Interface................................................................928.3. Fatigue and Creep Characterization...............................................928.4. Effect o f Environmental Variables................................................ 938.5. Damping Characteristics................................................................. 93

BIBLIOGRAPHY.................................................................................................................94

VITA...................................................................................................................................... 99


LIST OF TABLES

Table 1 Mechanical properties o f southern pine (\vood)-polvethylene (polymer)composites with various wood chip sizes and concentrations...................40

Table 2. Summary of different wood surface treatments and experimentalcoupling strengths between woodstock and polymer in southern pine- high density polyethylene coupled samples................................................52

Table 3. Drop weight impact fracture properties of southern pine (wood) -polyethylene ( polymer) composites with various wood chip sizes and concentrations................................................................................................ 61

Table 4. The test result summary' for the K[C tests (as per ASTM E399-83) for pine-HDPE composite sample, (medium size wood chips in 50%

volume concentration)...................................................................................81

Table 5 The test result summary for the Ji0 tests (as per ASTM E813-89) for pine-HDPE composite sample, (medium size wood chips in 50% volume concentration)...................................................................................81

Table 6 Comparison of the Knr.com and K[C.com obtained using ASTM E561-76Tand Kq obtained using ASTM E399-83.......................................................81

V


LIST OF FIGURES

Figure 1 . Crack opening displacement.............................................................................. 7

Figure 2. (a) Elastic body containing a crack o f length 2a under load P: (b) load Pversus displacement. e, diagram........................................................................8

Figure 3. Load-displacement and compliance-crack length curves............................... 10

Figure 4. Definition of the J integral.................................................................................10

Figure 5. Physical interpolation of the./ integral. The./ integral represents thedifference in potential energy (shaded area) of identical bodies containing cracks of length a and a - da ........................................................ 1 1

Figure 6 . R curve for (a) brittle and (b) ductile material..............................................13

Figure 7. Definition of K in terms of crack parameters.................................................. 14

Figure 8 . Definition of J for a physically deforming material.........................................15

Figure 9. Steps in the crack growth process..................................................................... 16

Figure 10. A J-R curve........................................................................................................ 17

Figure 11 The morphology of pine (wood) chips used to produce WPCs; (a) small,(b) medium and (c) large...............................................................................29

Figure 12. Schematic diagram of an adapter used for the production of compositesamples of pine (wood)-PE............................................................................31

Figure 13. The resultant composite sample used for tensile testing............................. 32

Figure 14. Schematic diagram of (a) the drop weight impact tester (b) condition at the instant of impact, (c) specimen holder and (d) specimen to be fitted between the holder plates in (c)..................................................................... 33

Figure 15. The dimensions of compact tension specimens used for K[C and J[Cdetermination................................................................................................... 34

Figure 16. The dimensions of compact tension specimens used for R curvedetermination..................................................................................................35

Figure 17. Effects o f pine (wood) chip size and concentration on UTS of pine PEcomposites...................................................................................................... 41

vi


Figure 18.

Figure 19-1.

Figure 19-2.

Figure 19-3.

Figure 20.

Figure 21.

Figure 22.

Figure 23.

Figure 24.

Figure 25.

Figure 26.

Figure 27.

Figure 28.

Figure 29.

Figure 30.

Effects o f pine (wood) chip size and concentration on break strain ofpine PE composites........................................................................................ 42

Effects o f pine (wood) chip size and concentration on peak load of pine PE composites.................................................................................................43

Effects o f pine (wood) chip size and concentration on MOR of pine PE composites...................................................................................................... 44

Effects o f pine (wood) chip size and concentration on stiffness o f pine PE composites................................................................................................45

The effects o f pine (wood) chip size and concentration on nail pull-out force...................................................................................................................46

The morphology o f the fracture surface of a stretched and broken sample............................................................................................................... 47

The surface morphology of untreated pine (wood) sample.........................53

The surface morphology of pine samples treated in the solutions of (a) NaOH and (b) FTSO-t...................................................................................... 54

The surface morphology of pine (wood) samples treated with NaOH and then with (a) CA1, (b) CA2, and (c) CA3.....................................................56

The surface morphology of pine (wood) samples treated with H;SO.i and then with (a) CA1, (b) CA2, and (c) CA3.....................................................57

Example o f load vs. deflection and energy vs. deflection curves in an instrumented drop weight impact test for a sample with large pine (wood) chips and 60% concentration............................................................ 58

Example o f load vs. time and energy vs. time curves in an instrumented drop weight impact test fora sample with large pine (wood) chips and 60% concentration.......................................................................................... 59

Effect o f pine (wood) chip size and concentration on Maximum load sustained...........................................................................................................62

SEM fractograph of a WPC specimen with large pine (wood) chips and 60% volume concentration. One large pine (wood) chip with a layer o f peeled polymer hanging on it (top) is seen....................................................63

Effect o f pine (wood) chip size and concentration on deflection at maximum load sustained................................................................................6 8

vii


Figure 3 1. Effect of pine (wood) chip size and concentration on energy to maximumload sustained.................................................................................................. 69

Figure 32. Effect of pine (wood) chip size and concentration on total energy tofailure by full tup penetration.........................................................................70

Figure 33-1. The morphology o f fractured surfaces o f samples broken in drop weightimpact test: specimen with 60% volume concentration of small pine (wood) chips (a) top surface (b) bottom surface..........................................71

Figure 33-2. The morphology o f fractured surfaces o f samples broken in drop weightimpact test: specimen with 60% volume concentration of large pine (wood) chips (a) top surface, and (b) bottom surface................................. 72

Figure 34. Load vs. crack opening displacement curve for the pine-polyethylenecomposite sample, (medium size pine (wood) chips in 50% volume concentration)..................................................................................................82

Figure 35. True Stress and engineering stress vs. a crack length plot for pine-polvethvlene composite sample, (medium size pine (wood) chips in 50% volume concentration).....................................................................................82

Figure 36. Engineering FQ vs. crack length plot for the pine-polyethvlene compositesample, (medium size pine (wood) chips in 50% volume concentration)..83

Figure 37. True FQ vs. crack length plot for the pine-polyethylene composite sample.(medium size pine (wood) chips in 50% volume concentration)............... 83

Figure 38. Slope of FQtnie vs. crack length plot for the pine-polvethylene compositesample, (medium size pine (wood) chips in 50% volume concentration)..84

Figure 39. Resistance curve. KJ'[,-ng/E vs. crack length plot for the pine-polyethylenecomposite sample, (medium size pine (wood) chips in 50% volume concentration)...................................................................................................85

Figure 40. Slope of R curve vs. crack length plot for the pine-polyethylenecomposite sample, (medium size pine (wood) chips in 50% volume concentration)...................................................................................................8 6

viii


LIST OF NOMENCLATURE

PE Polyethylene

HDPE H igh Density PE

PP Polypropylene

WPC Wood-Polvmer Composite

WPEC Wood-Polvethvlene Composite

CA Coupling Agent

UTS Ultimate Tensile Strength

MOR Modulus of Rupture

LEFM Linear Elastic Fracture Mechanics

EPFM Elasric-PIasdc Fracture Mechanics

ESEM Environmental Scanning Electron Microscope

CTOD Crack Tip Opening Displacement

FEM Finite Element Method

K,c Critical Stress Intensity Factor

COD Crack Opening Displacement

Knie Stress Intensity Factor Calculated Using Unbroken Ligament Dimension Ahead of the Crack Tip (i.e., W-a) Instead of ’\V \


ABSTRACT

Processing o f recycled wood chips, recently produced after extraction o f creosote from

telephone posts and railroad crossties, and combining them with a recycled polymer for the

synthesis o f novel composite materials would initiate a new trend toward preservation o f natural

resources. The challenge is taken in this work to produce and study the properties o f such

materials for further advancement o f science.

In the present study, addition o f pine wood chips to HDPE reduced the tensile strength

and break strain in tensile loading. Smaller wood chips generally resulted in smaller reductions.

Peak load, modulus o f rupture and stiffness were slightly higher at 40% pine wood chip

concentration in the composite than the polymer alone: they later decreased with increasing

concentration o f wood chips. Optimal mechanical properties in these composites were

produced by pine wood chips smaller than 0.125 inch in size, at around 40 vol. % in

concentration. Pre-treatment od wood chips in a suitable solurion o f NaOH caused an increase

in the coupling strength between protruding wood fibers and polymer. Such a treatment

followed by a second one with vinyltrimethoxysilane was found to be the best for obtaining

maximum bonding strength. Impact testing o f the prepared samples showed that more fracture

resistant wood-polymer composites were those with larger wood chips at the higher concentration

range o f 50 to 60 vol. %. In contrast composites with 60% fine wood chips would fail easily at

energy' levels far less than those required to break the polymer alone.

Variation o f (K<.ng)2/E vs. crack length, the R curve, indicates three regions. The point

o f transition from region I (elastic) to region II is considered as a critical point o f fracture

process initiation, Ki,t.com. The transition from the state o f stable crack growth, region II. to

region III is considered to be at the point o f instability. Likewise, the point o f inflection on the

plot o f K,rae vs. crack length, corresponding to a large change in slope, indicates also the point o f

instability. Kcng corresponding to this critical point o f inflection is proposed to be K[c, the real

fracture toughness o f these composites.

x


CHAPTER 1

INTRODUCTION

Wood and discarded plastics, which present challenges for solid waste disposal,

have long been recognized as potential recyclable materials. Recently, recycled wood

chips were produced by the process o f extraction of creosote from telephone posts and

railroad crossties. Wood chips produced by this process provides us with an abundant

source of cellulosic waste material. Consideration has been given to making use of

these wood chips by combining them with polymer for the synthesis o f novel composite

materials.

Wood-polymer composites (WPCs) have elicited considerable attention due to

their improved properties. These properties range from increased strength to

dimensional stability and to resistance to biodeterioration, Katsurada et al., (1983).

Additionally, the development o f novel WPC materials provides the opportunity for the

utilization of abundant cellulosic wastes and recycled plastics. Traditionally, WPCs are

obtained by impregnating wood with liquid monomers, and then polymerizing those

monomers in situ, using catalvsis-heat or radiation processes. Because of the materials

involved and the production process, the properties o f WPCs should largely depend on

the nature and properties of wood, polymer and the polymerization process involved,

Lauke et al. (1985) and Okajimaet al. (1970).

Above facts have attracted much research toward the WPCs since 1960. Their

physical and mechanical properties, water and humidity absorption, as well as

dimensional stability have been profoundly investigated by varying the factors

mentioned before. It has been found that WPC exhibited improved strength properties,


dimensional stability and resistance to biodeterioration. Manrich et al. (1989) and the

extent of these improvement in properties indeed was found to be directly related to the

nature of wood, polymer and the processing.

Till now, WPCs used in research and industry were produced from sawdust and

fine wood particles obtained as by-products in wood processing. Prior studies have

focused on moldable viscose composites, Sain et al. (1994). As a result, in some

processes the mixtures o f polymer and wood chips were kneaded, Otaigbe (1991), after

mixing for further reduction in particle size. However, the composites made of coarse

wood chips and polymer matrix have not been developed and their properties not

explored. Above facts can have a profound effect on the industry today and on the use

of wood-polymer composites. A better understanding is needed for the development of

more usable materials made of polymer and wood.

1.1. Mechanical Properties of Wood-Polymer Composites

There is a growing body of literature covering a wide variety of subjects in the

polymer-wood composite area. Wood-polymer composites are obtained by

impregnating wood with liquid monomers, and then polymerizing those monomers in

situ, using catalysis-heat or radiation processes. The mechanical properties of WPC are

expected to depend on properties of the wood, polymer, the polymerization process and

the interface bonding, Otaigbe et al. (1985).

WPCs have been the subject of much research recently. Their physical and

mechanical properties, water and humidity absorption, as well as dimensional stability

and biodeterioration, have been investigated by varying several factors. Due to the

profound effect of the interface between wood and polymer on the mechanical


properties, the bonding between wood and polymer, and the influences of different

coupling agents on the properties of the interfaces have also been the topic o f many

research efforts, Kenega et al. (1962).

The study of melt rheology of composites o f polypropylene filled with wood

flour, at filler concentrations of 3-20 wt%, has indicated an increase of melt viscosity

and a decrease o f polymer’s elasticity due to addition of filler and increase of its

concentration, respectively, Maiti et al. (1989). In an update review paper by Wright

and Mathias (1993) new tests and techniques developed for wood-polymer composites

were described. Processability and viscoelasticity properties of composites of wood and

polystyrene were studied by Hon, et al. (1993), where the addition of benzyl chloride as

plastisizer in the polyblend was investigated. This study also revealed the existence of a

single damping peak for the composite regardless o f the blending ratio. Other studies

have concentrated on the thermoplasticization of wood, which can be achieved by (a)

etherification, (b) esterification, and (c) grafting reaction, Hon et al. (1989)]. Khan et

al. (1992:45) studied the effect of combination of monomers in radiation induced wood-

plastic composite and later studied the effect of other additives on these composites,

Khan et al. (1992:49). Using balsa wood and copolymer containing ethyl a-

(hydroxymethyl) acrylate (EHMA)-styrene mixtures, Wright et al. (1993) developed

lightweight composites with improved dimensional stability and good mechanical

properties o f balsa wood. Other researchers studied the thermal properties, Murthy et

al. (1982), o f tropical wood - polymer composites using oxygen index,

thermogravimetry, differential scanning calorimetry, and elemental analysis. This

group also analyzed the dynamic mechanical properties of their composites, Yap et al.


(1991). Khan et al (1993)] also studied the physio-mechanical properties o f vvood-

plastics composites. Infrared spectroscopy (IR) is widely used to elucidate chemical

structures, enabling functional groups and linkages to be identified. Khan, et al.

(1991:45) studied and observed the graft polymerization o f wood components and

acrylonitrile and styrene. However, highly improved spectra o f wood - polymer

composites have been obtained and studied using Fourier Transformed Infrared (FTIR)

spectrophotometers, compared to the conventional dispersive IR instruments.

1.2. Effect of Coupling Agents

Mechanical properties o f composites are a direct function of the interface

bonding between the strengthening fiber or particulates and the matrix. Composites o f

wood and polymers are not exceptions. Recently, through an investigation on the

mechanical behavior o f wood-polyethylene WPECs, it was observed that fracture of the

WPECs was caused by delamination of wood chips from polymer matrix, Razi et al.

(1997). Since the interface between the wood and the polymer matrix is often the

shortfall of WPCs, the bonding strength on the interface plays a key role in determining

the mechanical properties of the WPCs, Ebewele et al. (1991). Studies have shown that

the tensile properties o f a WPC can be improved by the addition of a compatibilizer or a

coupling agent to enhance the bonding force at the interface, or by the application of a

thin polymer layer on the outer surface of the composite material in order to decrease or

eliminate possible sites for the initiation of cracks on the surface, Maldas et al. (1989).

Coupling agents are hybrid organic-inorganic compounds that bridge the

interface between the wood fiber and polymeric matrix. An adhesion promoter, based

on maleic anhydride modified polypropylene, was found to behave as a true coupling

4


agent, i.e., impact strength and elongation value increased significantly, while the

elastic modulus remained unchanged, Dalvag et al. (1985). Isocyanates also were used

to improve the mechanical properties of the composites based on several polyolefins

(PP, LDPE, HDPE) and wood pulp fibers, Coran et al. (1982). Zadorecki, et al. (1986)

found that some coupling agents (trichloro-s-triazine and di-methylolmelamine)

produced covalent bonds between cellulosic materials and polymer matrix leading to

the modified performance and reduced sensitivity to water, Bataille et al. (1990). This

idea has been proved by the research of Maldas, et al. (1989), in which phthalic

anhydride was selected as a coupling agent for wood fiber-polystyrene composites and

it was found that the benzene ring of phthalic anhydride can interact with the benzene

ring of polystyrene and an anhydride group can link to the -OH group of cellulose.

Another strategy to enhance the interfacial adhesion in WPCs is through surface

modification of the wood. Corona discharge treatment on polymer has been shown to

improve the adhesion in polyethylene/cellulose composites, Dong et al. (1993).

Compatibility between wood pulp fibers and thermoplastics may be improved by the

surface grafting of a polymeric segment having a solubility parameter similar to the host

polymeric matrix, Kokta et al. (1983). Recently, 0 2-, Ar- and NH3-pIasma treatment

was found to be able to improve the wettability and adhesion of silicone rubber, Lai et

al. (1996).

Traditionally, the effect of coupling agents on the bonding behavior in

composites is evaluated by testing the mechanical properties of the composites. An

enhancement o f the properties indicates the positive effect o f the coupling agent and the

surface treatment. This method, however, does not reveal the influence of other

5


variables and could lead to erroneous deductions. There is a need for a more direct

measurement of bonding strength and characterization of bonding in these composites.

1.3. Fracture and Fracture Toughness

One o f the important properties for failure safe design is the fracture toughness

of composite material. Traditionally linear elastic fracture mechanics (LEFM)

parameters and elastic - plastic fracture mechanics (EPFM) have been used to

characterize the fracture resistance o f composites.

In order to facilitate the following discussion, a brief description of a few

important fracture parameters are described.

1.3.1. Kic:

Kjc is the critical stress intensity factor under conditions o f plane strain, which

is characterized by small-scale plasticity at the crack tip. The material is fully

constrained in the thickness direction. When determined under this condition, K[C will

be a material constant. Thus, when one needs to characterize materials by their

tenacity, in the same way that one characterizes materials by their ultimate tensile

strength or tensile yield strength, only valid Kjc data should be considered.

Kc is the critical stress intensity factor under conditions o f plane stress, which is

characterized by large plasticity at the crack tip. In this case, the through-thickness

constraint is negligible. Kc value can be up to two times greater than the Kic values for

the same material. In general, Krc depends on temperature T, strain rate, and on

metallurgical variables.


1.3.2. Crack Opening Displacement (COD):

The development o f a plastic zone at the crack tip results in a displacement of

the faces without crack extension. The relative displacement of opposite crack edges is

called the crack opening displacement, Figure 1. It is suggested that when this

displacement at the crack tip reaches a critical value 5C, fracture ensues.

Figure 1. Crack opening displacement

One important point about COD is that theoretically, Sc can be computed for

both elastic and plastic materials. It allows one to treat fracture under plastic condition.

The only drawback is that we cannot define a single critical COD value for a given


material in a manner equivalent to that o f Kic, as the COD value will be affected by the

geometry o f the test specimen.

1.3.3. The C rack Extension Force, G: The elastic strain energy release rate with

respect to the crack area or the crack extension force G is a parameter that can be used

to characterize the fracture condition in a manner similar to the stress intensity factor K.

G can be interpreted as a generalized force.

P

P

i I. JX dP

Ir 't

IIIX^ — de

----------►

Figure 2. (a) Elastic body containing a crack of length 2a under load P; (b) load Pversus displacement, e, diagram

If we consider an elastic body o f uniform thickness B containing a through

thickness crack o f length 2a and let the body to be loaded as shown in Figure 2, with

increasing load P, the displacement e o f the loading point increases, as shown. The

potential energy stored in the body is

Ui= V-l P e before crack extension and

U2 - Vi (P - S P ) ( e + 8 e ) after the crack extension.

8


The change in the potential energy, U2 -Ui, is given by the difference in the

areas o f the two crosshatched regions I in the Figure. The crack extension force, G, per

unit length is

G = lim^_x) S U SA where SA = B Sa

It is convenient to evaluate G in term of compliance c o f the material defined

as

e = c P

Ignoring the higher-order product terms, and by applying further manipulations,

we can write:

G = VziP2 B )(8c 5a)

From this relation we see that G is independent o f the rigidity of the

surrounding structure or the test machine. It depends only on the change in compliance

of the cracked member due to crack extension. Thus, to obtain G for a material, all we

need to do is to determine the compliance of the specimen as a function of crack length

and measure the gradient o f the resultant curve, dc da, at the appropriate initial crack

length, Figure 3.

1.3.4. /Integral:

The J integral is defined as a line integral, independent of path, along a curve T

surrounding the crack tip. Mathematically, it is shown that

•/ = I r ( W d y - T (5u / dx ) ds)

where W is the strain energy density function, T is the traction vector perpendicular to

the surface T, u is the displacement in the x direction and ds is an arc along T, Figure 4.

9


Disphicement,

(dafdc)O

aCrack Length, a

Figure 3. Load-displacement and compliance-crack length curves, Paris et al.(1979).

y

>- X

PlasticZone

Figure 4. Definition of the J integral, Rice and Rosengren (1968).

From a physical point o f view, the J integral represents the difference in the

potential energy of identical bodies containing cracks of length a and a + da, that is,

J = - ( U B ) (d U/ d a )

where U is the potential energy, a the crack length, and B the plate thickness, Figure 5.

>o


a+da

A

Figure 5. Physical interpolation o f the J integral. The J integral represents the difference in potential energy (shaded area) o f identical bodies containing cracks o f length a and a + da

U is equal to area under the load versus displacement curve. Figure shows this

interpretation, where the shaded area is dU = J B da. J measures the critical energy

associated with the initiation o f crack growth accompanied by substantial plastic

deformation.

The path independence o f the J integral, together with this interpretation in

terms of energy, makes it a powerful analytical tool. The J integral is path independent

in the case of material behaving elastically, linear or non-linear. When there occurs

extensive plastic deformation, the current practice is to assume that the plastic yielding

can be described by deformation theory of plasticity. According to this theory, stresses

and strains are functions only o f the point of measurement and not of the path taken to

get to that point. As in the case o f slow, stable crack growth, there will be relaxation of


stresses at the crack tip, so there will be a violation o f this postulate. Thus, the use o f J

integral should be limited to the initiation of crack propagation, by stable or unstable

processes.

1.3.5. R Curve:

The R curve characterizes the resistance of a material to fracture during slow

and stable propagation o f a crack. An R curve graphically represents this resistance to

crack propagation o f a material as a function of a crack growth. With increasing load in

a cracked structure, the crack extension force G at the crack tip also increases, Figure 6 .

However, the material at the crack tip presents a resistance R to the crack

growth. The failure will occur when the rate of change o f the crack extension force

(5G/3a) equals the rate o f change o f this resistance to crack growth in the material

(5R/da). The resistance o f the material to crack growth, R, increases with an increase in

plastic zone size. The plastic zone size increases non-linearly with a; thus, R will also

be expected to increase non-linearly with a. G increases linearly with a. Figure 6

above shows the instability criterion: the point of tangency between the curves G

versus a and R versus a. Figure also shows the R curve for a brittle material and the

R curve for a ductile material. Crack extension occurs for G > R. Considering the G

line for a stress <x’; the crack in this material, at this stress cF, will grow only from a0

to a ' since G > R for a <a '. G < R for a > a ’ and the crack does not extend beyond a

As the load is increased, the G-line position changes, as indicated in the Figure. When

G becomes tangent to R, unstable fracture ensues. The R curve for a brittle material is a

“square” curve and the crack does not extend at all until the contact is reached, at which

point G = GC and the unstable fracture follows.

12


Gc

a

&o

aa — ►

Figure 6 . R curve for (a) brittle and (b) ductile material, Clark (1969).

At this point a comparison of these methods is offered. The most important

consideration in the application of the LEFM’s K analysis is that the test material be

essentially linear elastic. This application uses the principle of a unique linear elastic

crack tip field. This field, which has a unique stress and strain distribution, is

characterized by a single parameter, K, the crack tip stress intensity factor, which

determines the magnitude o f the field, Figure 7. This uniqueness provides a method for

directly correlating the test results, which measure fracture properties with the fracture

behavior of the structural components.

13


2 - "/ cos—sin —cos —V2 «r 2 2 2

Q\3 = 2̂3 = 0^33 = 0 (plane stress)^33 = K0 „ +0 = ) (plane strain) Then: K is the Intensity of the

Elastic Field Surrounding the Crack Tip

if w < r « Planner Dimension1 k 2w = j

6 k a Q

K = a cjjj V (7t . a) where a = crack lengtha = specimen geometry or compliance

Figure 7. Definition o f K in terms of crack parameters, Begley et al. (1972)..

If large-scale plastic stresses and strains are encountered, the K parameter is no

longer a good characterization o f the crack tip field. This limitation restricts the use o f

LEFM and its fracture toughness characterization to higher strength and lower

toughness material, ASTM E399-83, ASTM (1995), and a method for extending these

principles to include larger scale plasticity in materials should also be considered.

Hutchinson (1968) and Rice and Rosengren (1968)] developed the plastic crack

tip field stress-strain analysis. They showed the existence o f a unique stress and strain

distribution with a single characterizing parameter, J, to describe the magnitude of these

stresses and strains, Figure 8 .

ElasticZone


if D<r«PIanar Dimensions, then: J is the Intensity o f the Plastic Field Surrounding the Crack Tip for Power Law Hardening Material

e£'0

Figure 8. Definition of J for a physically deforming material, Rice (1968).

The parameter J came from the path independent J integral developed by Rice

(1968); it could be used to characterize the fracture toughness and sub-critical crack

growth for cases o f large scale plasticity in a way that is analogous to the use o f K for

linear elastic cases.

The development of the crack tip field equations for large scale plasticity

effectively has extended the capability o f fracture mechanics from the linear elastic

regime into the elastic plastic regime where J now has replaced K as the parameter to

characterize fracture type behavior in more ductile and plastic material. In an analogy

to the linear elastic toughness Kic, the elastic-plastic fracture toughness is labeled J[C,

Begley et al. (1972). A complete description o f the ductile fracture process includes

four steps from a sharp crack tip starting point: the crack tip blunting during initial

stress, the initiation o f stable crack growth, the continued stable crack growth, and the


ductile instability, as shown in Figure 9. One of these steps, the initiation o f the tearing

crack from the blunted crack tip, is taken as the point to specify Jrc.

Start Sharp Crack Tip(Fatigue Precrack

Step 1: Crack Tip Blunting ^

Step 2- Initiation of Stable Crack ---------------------------Growth '

S t e p 3 . Continued Stable Crack Growth

Step 4: Ductile Instability v

Figure 9. Steps in the crack growth process

The entire process o f ductile cracking is better described by the crack growth

resistance curve, R curve, where a driving force is plotted as a function of crack

extension, Figure 10. For the ductile fracture case J can be plotted against physical

crack extension, Landes et al. (1977).

The R curve could then be used to describe the initiation o f the ductile cracking,

Jic and the process o f stable crack advance. The method for determination of Jic has

been standardized in ASTM E 813-89.

16


Step 4: Ductile InstabilityStep 3: Stable Crack Growth

a

Step 2: Initiation of Stable Crack Growth

M

Step 1: Crack Tip Blunting

&O

Start: Sharp Crack

Crack Extension

Figure 10. A J-R curve, Clarke (1979).

Although the EPFM methodology extends the capability o f the fracture

characterization well beyond elastic regime, there are also limitations to consider, Paris

et al. (1972). The concept of a unique stress and strain field in plastic zone requires that

the field should not be disturbed by structural or specimen boundaries. A specific

requirement is that specimen dimensions be greater than M J/ao, where <To is the flow

stress and M is a constant developed to satisfy the stress field requirement, Clarke et al.

(1979).


Hutchinson and Paris (1979) showed that the concept o f a J field is valid for the

growing crack, if certain conditions can be maintained. These were quantified by Shih

and labeled “conditions for J-controlled crack growth.”, Shih et al. (1980). They

include M = 25 and

© = (b/JXdJ/da) > 5

Aa/b < 0.1

where b is remaining uncracked ligament length.

In developing an R curve for stability analysis, very often an extensive amount

of crack growth is needed to establish an instability point. The restriction in above

equations limits the amount o f the crack extension so that many times this intersection

cannot be reached. Specimen sizes are often limited by material availability and there

may be a need to exceed these limits. However, R curves tend to be geometry-

dependent when crack growth exceeds the limits. A geometry independent R curve for

greater amounts o f crack growth has been developed by Ernst (1983) in which he

suggested a modified J parameter, Jm, which can be used to characterize the R curve

behavior. Experimental results showed that geometry independence was maintained in

the R curve behavior for crack growth well in excess o f the limits o f the above equation.

As a result o f all o f the above developments, there is a much extensive list o f

parameters available to characterize fracture toughness of composite materials. Among

the different fracture parameters, critical stress intensity factor (Kic), critical J-integral

(Jic), critical crack tip opening displacement (CTODc), fracture energy (Gf) and the R-

curve analysis have been used mainly by researchers to describe the fracture behavior of

particulate-filled composites. The resistance curve methods are used to characterize the

18


resistance to fracture o f slow stable crack extension in the material and provides a

toughness record as the crack is driven stablv into the fracture process zone, caused by

an increase in the applied loads.

Fracture study toward the understanding of the near tip behavior of material

ahead of cracks in particulate composites consisting o f semi-rigid polyhedral particles

in a soft matrix has shown a strong surface influence on the lowest eigenvalue at the

free surface. Smith et al. (1990).

The particulate-filled polymer-concrete composites have been studied for their

fracture characteristics. Cook and Crookham (1978) observed that K[C and stiffness

decreased in polvmer-portland cement concrete (PPCC) matrix composites, regardless

of polymer type, mix and treatment. Polymer impregnation, on the other hand,

increased the fracture toughness and failure strain. Polymer impregnated concrete was

found to be notch sensitive with BQc having a limiting value between the notch-to-depth

ratio of 0.35 and 0.42, beyond which Ktc generally decreased. Alezksa and Beaumont

[1975] have observed that the work to fracture was enhanced in the concrete system

when impregnated with an acrylic polymer.

The strain energy release rate (fracture energy) for polyamine-modified urea-

formaldehyde-polymer-bonded wood joints at crack initiation (G[C) and arrest (Gca)

were calculated from the observed loads at crack initiation and arrest. High fracture

energy means that a large amount of energy is required to initiate a crack, and this

suggests the existence o f a good joint. Alezksa and Beaumont [1975], Microcracking

such as debonding between fillers and matrix in composite material is an important

form of damage. Zhou and Lu (1991) studied the damage process induced by

19


microcracks, its nucleation and extension process in particle-filled composite materials.

They derived a damage variable D from an analysis of the microcrack system, which

was a function o f microcrack size and density.

Zdenek et al. (1990) developed a random particle model for fracture of brittle

particle-filled composite materials. In this model the particles were assumed to be

elastic and have only axial interactions, as in a truss. The idea of particle simulation

was originally proposed by Cundall (1971) and Serrano and Rodrigues (1973). These

works dealt with rigid particles that interact by friction and simulate the behavior of

granular solids such as sand. An extension of this method was introduced by

Zubelewicz and Bazant (1987). In their model they neglected the shear and bending

interaction o f neighboring particles throughout their contact layers. It was verified that

random truss yielded a realistic strain-softening curve, but the study did not include the

fracture mechanics, nor the size effect aspects.

Polymer - matrix composites are normally not brittle and fracture parameters

determined using LEFM and initial notch depth methods do not represent the fracture

properties of these systems. Due to non-linear slow crack growth prior to peak stress

intensity in polymer composite materials, models with two or more parameters may be

more suitable to explain the fracture process. In order to establish the fracture criteria

and fracture toughness model for polymer composites made with polyethylene and pine

(wood) particles, it needs to be shown that tests on specimens with significantly

different dimensions and notch depth produce nearly similar results within experimental

errors.


1.4. Objectives

Previous studies have shown that WPCs exhibit improved strength, dimensional

stability, and resistance to biodeterioration. The effect of wood particle size and its

concentration on the properties and the effect o f coupling agents on the interface

bonding, as well as fracture mechanism and fracture toughness of the WPCs have not

been thoroughly studied. Thus, the objectives of the present research are:

(i) to investigate and increase our understanding of the mechanical properties of

WPCs:

(il) to investigate the effects of coupling agents on the interface bonding in

WPCs and the development o f a direct method for wood-polvmer bonding

force measurement:

Cui) to study their impact fracture properties, and

(iv) to define fracture resistance behavior and fracture toughness of these

particulate-filled polvmer-matrix composites.

21


CHAPTER 2

EXPERIMENTAL PROCEDURES

2.1. Common Characterization Techniques Used

In order to determine the mechanical properties and surface characteristics of

pine-HDPE composites, following characterization techniques were utilized.:

2.1.1. Scanning Electron Microscopy

A scanning electron microscope (SEM) and an environmental scanning electron

microscope (ESEM) were utilized to observe the fracture surfaces sample at room

temperature in a low vacuum o f about 5 mm Hg. Additionally an SEM was used to

observe the surface morphologies o f untreated and treated wood pieces and also the

fractured interface on polymer after the bending, tensile, impact and resistance tests.

Fracture surfaces were also viewed with a low power stereoscope to study the features of

fracture under different modes o f fracture.

2.1.2. Tensile Testing

A universal hydraulic tensile test unit MTS 810 was used for the tensile testing.

This unit was used for the mechanical property characterization, surface bonding force

and fracture toughness determination.

2.1.3. Instrumented Impact Testing

Impact fracture properties o f the WPCs were determined using an instrumented

Dynatup Impact Tester, Model 8225 . The instrumented impact testing was carried out to

determine damage resistance, maximum load sustained under impact, deflection to

maximum load, energy to maximum impact load, and total energy to fracture under

multiaxial high speed deformation.


2.2. Experiments Pertaining To M echanical Properties

Mechanical properties of pine (\vood)-polyethvlene (polymer) were obtained

using specimen prepared using materials and preparation techniques as described below.

Polymer: High-density polyethylene (HDPE) 'EA60-007' in powder form, provided by

PAXON Chemical Company, Baton Rouge Louisiana, was selected in order to obtain

good homogeneity and uniformity o f the composites. Pelletized high density

polyethylene provided by Exxon Chemical Company was also selected as the polymer

base for the composites used to study the effect of coupling agents and surface

modification.

Wood: Southern pine (genus and species) was chosen in this study because it is widely

used in the production of creosote-preserved telephone post. Small wood chips with

various sizes and shapes were prepared using a commercial chipper. First, wood strips of

size 0.5”x l”x8” were cut from 2”xl0”x8" boards along the grain direction and then fed

into the chipper from which wood chips o f random sizes were produced. Then the wood

chips were soaked in 1 N sodium hydroxide (NaOH) solution for 24 hours, drained and

dried in an oven at 70° C for 48 hours.

2.2.1. Preparation of Wood-Poly mer Composites Samples

Three stage sifters with mesh sizes o f 0.5” , 0.25” , and 0.125” were used to sort

out the chips and discard the wood chips that could not pass through the sieve with the

largest mesh. Samples entrapped between the sieves of 0.5” and 0.25” mesh size were

designated as "large”, between the sieves o f 0.25” and 0.125” mesh size as "medium",

and those passing through the sieve with 0.125” opening as "‘small” . Figure 12 (a), (b)


and (c) show the morphology o f the "small’’, "medium” and "large” wood chips,

respectively.

Rectangular WPC samples were prepared in a 5” x 13” x 0.75” steel mold with

removable lid. The mold was filled with pre-mixed wood and polymer and then heated in

a constant temperature furnace. The mold was then heated in a constant temperature

furnace at 155 °C for 3 hours. After the melting of the resin a pressure of 500 psi was

applied for 3 minutes, and the block was subsequently cooled to room temperature.

Resultant rectangular samples were hvdraulically pressed out of the molds and were later

cut into specific dimensions for testing.

The mechanical properties of the WPCs were determined by tensile testing, 3-

point bending and nail removal testing on an MTS tensile tester. All tests were

performed following guidelines specified in relevant ASTM standards. The samples for

tensile and bend tests were cut in compliance with ASTM D 1037-89 (for wood-base

fiber and particle panel materials) specification [ASTM 1989]. The strain rate in tensile

testing and the rate of head movement in bend testing were 0.2 inch/min. Prior to the

mechanical tests, conditioning treatment was undertaken on all the samples at 23±1°C

and 50% humidity for at least 12 hours. Four identical samples were tested in every

experiment and consistent results were averaged and reported.

2.3. Experiments Pertaining to Effect of Coupling Agents and SurfaceModification

Three different coupling agents (CA), provided by SIGMA-ALDR1CH Scientific,

were chosen. Following is a list of their names and their molecular formulas:


OCHj

CA1: vinvltrimethoxvsilane: CH,0— Si—CH=CH2.

OCH3

O OCH,: : i

CA2: 3-(trim ethoxysilyl)propyi methacrylate: H2C=C—C—OCH2 CH2 CHt—Si—OCH3,! 1

CH3 OCHj

OCHj

CAS: 3-glycidoxypropvltrim ethoxysilane: CH,0— Si—CH2 CH: CH2OCH3.

OCH,

2.3.1. Preparation of Wood-Polymer Duplex Samples

The southern pine wood board was cut into rectangular plates of 2.5 cm x 2.5 cm x

15 cm and turned by lathe machine into round stocks of 2 cm dowels and 15 cm length.

ASTM 400 grit sand paper was used to polish the end sides of the woodstocks.

Two kinds of surface treatment including treating in 1 N sodium hydroxide

(NaOH) or 1 N sulfuric acid (H2SO4 ) solution and/or in coupling agents were provided on

polished samples. In the former treatment, samples were soaked in the solution (NaOH

or H:SO.t) for 24 hours, drained and then kept in an oven at 103° C for 24 hours to dry.

In the latter treatment, the samples were soaked in the coupling agent for 3 hours and

then dried in the oven at 103° C for 24 hours. All studied samples with different

treatments are listed in Table 1.

To produce wood-polymer interface for the measurement of bonding force

between the wood and the polymer, a Berstorff twin screw extruder model ZE25X28D

was used. An adapter, as shown in Figure 13, was machined such that its one end fitted

the entrance of the extruder and a woodstock could be inserted into it through the other

2 5


end. During operation, the output temperature o f the extruder was kept at 215° C and the

pressure on discharge at the contact point was maintained between 0.7 and 0.8 MPa. The

melt polymer as well as the woodstock were pushed out through the end o f the adapter

where the woodstock was inserted. After the polymer was pushed through 2/3 length of

the adapter, the adapter was removed from the extruder and cooled down in a water bath

without wetting the sample inside. Then, the composite sample was pressed out using a

hydraulic jack from the side of polymer. The resultant duplex sample with a single

wood-polymer interface is shown in Figure 14. Two holes were drilled one on each side

o f the duplex sample in order to pin the sample for the tensile testing. The strain rate

used in tension was 0.2 in/min. and the stress at the point of fracture was considered as a

measure of the coupling force between the woodstock and the PE. Four identical samples

for each treatment were tested to obtain consistency and the average value of the

corresponding bonding force. An SEM was used to observe the surface morphologies of

untreated and treated wood pieces and also the fractured interface on polymer after the

tensile test.

2.4. Experiments Pertaining to Impact Fracture Properties

Vinyltrimethoxysilane, 98%, (CA1), was chosen as the working coupling agent

for this experiment due to its good performance.

2.4.1. Preparation of Composites Samples

Samples were prepared following the procedure described in the section 2.1.

However samples for these experiments, once chosen from among the blocks with most

uniform distribution o f wood chips, were sorted out and cut into 4” x 4” x 0.37” plates for

testing.

2b


Impact fracture properties of the WPCs were determined using an instrumented

Dynatup Impact Tester, Model 8225 . The instrumented impact testing was carried out to

determine damage resistance, maximum load sustained under impact, deflection to

maximum load, energy to maximum impact load, and total energy to fracture under multi-

axial high speed deformation. Test specifications given in ASTM 3763-86 were followed,

which require an unsupported region beneath the specimen that is six times the tup diameter

in order for the failure mode to be multidimensional. The configuration (Figure 15) also

allows the falling tup to completely penetrate the specimen, relinquishing less than 33% of

its energy in the process to fracture of the specimen. A total cross-head hammer weight of

7.463 lb. was used in this experiment. The original rest position of this weight from the

point of impact on the specimen determines the velocity of the tup and the initial impact

energy of the test according to the kinetic energy equation v2 = 2gh, where v is the velocity,

g is the gravitational acceleration, and h is the free fall distance to the point of impact

During the fracture test, the instruments crosshead was raised to a maximum height of 48

inches to allow for the complete penetration of the tup through the specimens. Four

identical samples were tested in every experiment and nearlv-consistent results were

averaged. The scatter in the results obtained from different specimens for all properties

ranged between 2.3% and 4.6%.

2.5. Experiments Pertaining to Fracture Resistance

Rectangular WPC samples were prepared in a 5” x 13” x 5” steel mold with

removable lid. NaOH-treated medium size wood chips were first soaked in the coupling

agent and dried in a furnace at 70°C for 24 hours. They were then weighed and mixed with

polymer resin in suitable proportion to yield 50 vol.% o f wood chips in the composite and

27


loaded in the mold. The mold was then heated in a constant temperature furnace at 155°C

for 3 hours. After the melting of the resin a pressure of 500 psi was applied for 3 minutes,

and the block was subsequently cooled to room temperature. Resultant rectangular samples

were hvdraulically pressed out o f the molds and cleared. Compact tension specimen

samples of size 2.5” x 2.4” x I” were cut from the block and the ones with the most uniform

distribution of wood chips were sorted out to be prepared as per ASTM E 399 83, [ASTM

1995]. For R-curve determination compact tension specimens of size* 4.5” x 6” x 1”

dimension were cut from the main block and the ones with the most uniform distribution of

wood chips were sorted out to be prepared for testing as per ASTM E 561-76 [ASTM 1995],

Straight-through notches were cut through the samples with crack plane orientation of S-T

as given in ASTM E 399-83. Final sample geometry for K[C and J[C determination is shown

in Figure 15 and for R-curve determination in Figure 16. The initial crack extensions for R

curve samples were produced by the edge of a razorblade to 0.213-inch depth. Due to the

randomized nature of the method used to prepare samples, as described earlier, wood

particles are assumed to be randomly distributed in the matrix with no preferred orientation.

Initial fatigue cycling was used as prescribed in the standards in order to initiate and locate

the pre-crack at the root of the notch.

’ Note that the dimensions o f specimens used in this study do not conform to the recommendations given in the standard.

28


(b)

Figure 11 The morphology o f pine (wood) chips used to produce WPCs; (a) small, (b) medium and (c) large

29


(C)

Figure 1

30


j .U cm

h H

u > oHO cm [D

0.6 cn- êei r:ia re- oc-d

16.0 cn!.0 cn

i.0cn iO tubing for* wood dowels

ro copper tubing For support

Figure 12. Schematic diagram of an adapter used for the production of composite samples of pine (vvood)-PE


Figure 13. The resultant composite sample used for tensile testing

32


L

~ ^ 3 ~

tt

J

(a) (b)

(d)

(c)

Figure 14. Schematic diagram o f (a) the drop weight impact tester (b) condition at the instant of impact, (c) specimen holder and (d) specimen to be fitted between the holder plates in (c)


Figure 15. The dimensions o f compact tension specimens used for KIc and J[c determination

34


c

Figure 16. The dimensions of compact tension specimens used for R curve determination

35


CHAPTER3

RESULTS AND DISCUSSION:STATIC MECHANICAL PROPERTIES OF PINE CHIP - HDPE COMPOSITES

IN TENSION AND BENDING

3.1. Tensile Mechanical Properties

Tensile mechanical properties o f the WPCs were studied initially as a part o f

this investigation. The average tensile properties obtained are listed in Table 1. Figure

17 and 18 show the experimental results o f ultimate tensile strength (UTS) and break

strain. Regardless of wood chip size in the composites, there was a sharp decrease in

UTS of the composites in the range o f 0 to 40% wood chip concentration and no

significant further change in this property in the range o f 40% to 60% wood chip

concentration. Also, a smaller decrease in UTS was found in the composites with

smaller wood chips. The break strain changed in a similar manner for all composites. It

showed significant reduction in the range of 0 to 40% chip concentration and no further

change thereafter. It can be concluded from these results that the introduction of wood

chips to polymer significantly deteriorated the tensile properties. The experimental

results suggest that tensile applications of composites with more than 40% wood chips

concentration should be limited.

3.2. 3-Point Bending Testing

The peak load, modulus o f rupture (MOR) and stiffness o f WPCs were

determined by 3-point bend testing, as shown in Figure 19-1, 19-2 and 19-3. In the

concentration range between 0 and 40%, there were trivial changes in peak load and

MOR, but stiffness increased significantly. Between 40% and 50% wood chip

concentration, a slight decrease in peak load and MOR was found for the composites

36


with small and medium wood chips. But, there was a serious decrease in these

parameters for the composite with large wood chips. In the same concentration range,

stiffness was found to increase slightly for the composite with small wood chips,

decrease slightly with medium wood chips and decrease appreciably with large wood

chips. Afterwards, these three parameters decrease slightly for the composites with

small and medium wood chips, but no further change was found for large wood chips.

3.3. Nail Removal Testing

Figure 20 shows the experimental results of nail-removal testing. The nail-

removal characteristic is very important if the present composite is to be used as a

construction material. It is found that in the concentration range between 0 and 50% the

nail pull-out force is increased by the increasing wood chips concentration (except for a

small decrease for the composite with large wood chips in the concentration range

between 40% and 50%). The highest increase in the pull-out-force occurs in the small

wood chip composite with around 50% wood chip concentration. In the range of 50%

to 60% wood chip concentration, the experimental results show a decrease in the nail

pull-out force for all composites.

Thus, the mechanical properties o f the WPCs are found to vary with

concentration and size o f the wood chips in the composites. In these composites, wood

chips are found encapsulated in a matrix o f flexible polymer and aligned parallel to the

surface by the compressive force applied to the composite after melting. As a result,

there is no binding between two adjacent wood chips and the thickness o f the polymer

ligament between two adjacent wood chips would depend on the wood chip

concentration in the mixture. In the process o f external tension, the difference in tensile

37


deformation between wood chips and polymer matrix would result in the separation or

cracking along the interface. The initiation and propagation of cracks cause the

degradation o f tensile strength and elongation.

The morphology o f fracture surface of a stretched and broken sample is shown

in Figure 21. it could be inferred from the features observed that fracture o f the tensed

sample occurred by the initiation of cracks due to debonding at the interface and then

propagation of the cracks along the interfaces. Based on this fact, it can be inferred that

the tensile properties o f the composite largely depend on the bonding forces between

the wood and polymer, which have been deduced, from the present results, to be lower

than the tensile strengths of individual wood and polymer respectively (otherwise the

latter would have broken first). For the composite with larger wood chips in the same

concentration range, more severe degradation in tensile properties was found (Figure

18-1) due to larger deformation mismatch on the wood-polvmer interface and larger

continuous interface area over which the crack can propagate.

The reason for the bonding strength on interface to be lower than the strengths

of individual polymer and wood is more than likely the presence of residual stress on

the interfaces and the weak bonding between the wood and polymer. The composite

with fine wood chips is expected to have lower residual stress on the interface and

possess higher UTS, compared with the ones with medium and large wood chips. This

has been confirmed by experimental results indicating that for WPC with smaller chips,

there is a smaller decreasing rate of UTS, Figure 17. It has been suggested that the

tensile properties of WPC can be improved by the addition of a compatibilizer or a

coupling agent to enhance the bonding force between the wood chips and the polymer

38


matrix, or by the development of a thin polymer layer on the surface o f the composite

material in order to decrease or eliminate possible sites for the initiation of cracks on the

surface, Maldas et ai. (1990).

In the compression condition of 3-point-bending, wood chips in the composites

tend to compress against each other as well as against the polymer matrix. Because of

the bending strain, the polymer matrix will not fracture before a wood chip starts sliding

with respect to adjacent chips. The wood chips in polymer produce extra internal

frictional forces that restrict deformation. Large external compressive forces are

required to initiate sliding. As a result, the encapsulated wood chips in polymer matrix

act as obstacles to compressive deformation o f bulk composite material, w hich explains

the enhanced properties during the compression of the composite with low wood chip

concentration. However, the compressive properties o f composites would deteriorate in

the high concentration range since excessive amounts of wood chips, particularly with

large size, would cause brittleness of the bulk composite.

As shown in Figure 20, smallest wood chips result in largest improvement in

nail pull-out-force of the polymeric composite. It is found from the experimental results

that the addition of wood chips in polymer matrix produces improvement in MOR due

to the extra obstacle to plastic deformation by the wood chips. Also, it has been seen

that the composites with small wood chips obtain largest improvement in MOR. When

a nail is forced into the composite, it produces a compressive strain field in its vicinity.

The matrix with higher MOR will provide larger vertical holding force onto the nail,

which is equivalent to producing a higher friction force on the nail during the pull-out

process. As a result, the nail pull-out-force of the composites with wood chip

39


concentration less than a certain value (50% in this research) increases with increasing

wood chip concentration, compared with that o f plain polymer base. Also, small wood

chips create high modification in the nail pull-out force. However, high concentration

(>50% in this research ) o f wood chip in composite appears to make the structure more

brittle, as stated above, resulting in the decreased nail pull-out force.

Considering the overall correlation between chip size, its concentration and the

mechanical properties of WPCs. the suggested parameters to produce the composite

should be wood chip size of <0.125'" and wood chip concentration of around 40%.

Specific surface treatments for improving the bonding and interfacial properties are

addressed in the next section.

Table 1Mechanical properties o f southern pine (wood)-polyethvlene (polymer) composites with

various wood chip sizes and concentrations

Wood Chip

Size/Concentration

UTS

(psi)

±5.2%

% Strain at

Break

±6.1%

Peak Load

(lb)

±5.5

MOR

(psi)

±~.5%

Stiffness ij

psi ( 10J) j

±6.1% |

i

Nail Pull

out Force

(lb)

-5.4%

0% 2918 3.9 80 3626 165 iI

28.1

Small 40% 1933 1.07 81.5 40243,5 i

30.8

<0.125 in 50% 1578 1.03 77.2 3742 323 | 31.9

60% 1000 1.05 71 2950 280 j 28.9

Medium 40% 1330 1.13 76.6 3386 304 !1 29 2

>0.125 and 50% 1206 0.72 69.2 2958 291 j 29.5

<0.25 in 60% 921 1.07 62.3 2397 283 | 28.8

Large 40% 1151 1.07 73.4 3419 269 | 29.9

>0.25 and 50% 941 1.03 51.8 2066 245 | 29.4

<0.5 in 60% 750 0.70 51.1 1776 239 j 22.3


Specimens with small wood chips

350C

3000 --------------------------------------------------------------------------------

2500 • ’ * — --------------------------------------------------

2000 -----------------

1500 -----------------------------------------1000 i---------------------------------------------500 ------------------------------------------------------------------------------------

20 30 40 50 60 70

Wood Concentration

Specimens with medium wood chips

2500 ,

3000 £ --------------------------------------------------------------------------------------------------

2500 j------------ ^ -----=----------------------------------------------------------------------------

"3* a.<nH-

W ood Chip Concentration

200Q 4—

1500 t - ~X------1000 I -

500 t -

o i-

t r r

500

0

Specimens with large wood chips

-XT

20 30 40 50

wood Chip Concentraoon

Figure 17. Effect o f pine (wood) chip size and concentration on UTS o f Pine-HDPE composites.

41


Specim ens with sm all w ood ch ips

Figure 18.

m 4 F—<x>

® 3(9

•£ 2m *

0 --------------------------------------------------------------------------------0% 10% 20% 30% 40% 50% 60% 70%

Wood Chip C oncentration

(Q<Uk.meo•f 2<u

5 -

4 C—3 ------

V)

S pecim ens with m edium w ood chips

Q i--------------------------------------------------------------------------------0% 10% 20% 30% 40% 50% 60% 70%

Wood C oncentration

<n 4 F— « L - - .

5 3

■§ 2 (03? '

0

Specim ens with large w ood chips

0% 10% 20% 30% 40% 50% 60% 70%Wood Chip C oncentration

Effect of pine (wood) chip size and concentration on break strain of Pine- HDPE composites.

42


Figure 19-1


90

a 80 £-1 70 -

60 f-<s oQ- 50

40

t -

0% 10% 20% 30% 40% 50%Wood Chip Concentration

60% 70%

90

15 7001 60 aoo- 50

40


§ : [ " i .

0% 20% 40% 60% 80%Wood Chips Concentration


90

1 80H 70 - o^ 60(30)Q- 50

400% 20% 40% 60%

Wood Chip Concentration80%

. Effects o f pine (wood) chip size and concentration on peak load of pine- HDPE composites

43



5000

4000

J. 3000

O 2000 stooo

00% 10% 20% 30% 40% 50%

Wood Chip Concentration60% 70%

Spemens with medium wood chips

5000 ,

4000 £-----------------------------------------------------

\ 3000 i -------------------------- ---------------------------a. ! * * 3O 2000 f --------------------------------------------------------- -s ‘

1000 \---------------------------------------------------------

0% 20% 40% 60% 80%Wood Chip Concentration


5000

4000

q. 3000

O 2000s

1000

0

- I

0% 20% 40% 60%Wood Chip Concentration

80%

Figure 19-2. Effects of pine (wood) chip size and concentration on MOR of pine- HDPE composites

44


Specimens with samll wood chips

— 390 n0 340 —

•S 290 - &« 240 —VI0)1 190 ~33 .140 —


_ 340C l

° 290X«A& 240(A(A

£ 1903=


140 —

7 ^ 4 - - I -



_ 340c l<

Z 290 -------------------------------------------------------------------------

% 240 --------------------- ------------------ — I — J ----------------(A - *(A<H 1 9 0 -------------------------------------------------------------------------

i e * '» 140 -------------------------------------------------------------------------


Figure 19-3. Effects of pine (wood) chip size and concentration on stiffness of pine- HDPE composites in bending

45


S pecim ens w ith sm all wood chips

36_ 34 ---------------§ 32 -5 30 =- o3O. 26"5 24 -z 22

200% 10% 20% 30% 40% 50% 60% 70%

W ood Chip C oncentration

Specim ens w ith m edium w ood chipst

32

S’ 30¥ 28= 26 3Q. 24I 22

200% 10% 20% 30% 40% 50% 60% 70%

Wood Chip Concentration

S pecim ens w ith large w ood chips

1 - • i

1 1 -

10% 20% 30% 40% 50% 60% 70%

W ood Chip C oncentration

Figure 20. Effects o f pine (wood) chip size and concentration on nail removal force of pine-HDPE composites

46

<oz

3432302826242220

0%


Figure 21. The morphology o f the fracture surface of a stretched and broken specimen.

47


CHAPTER 4

RESULTS AND DISCUSSION:INTERFACE MODIFICATION AND INTERFACE BONDING IN PINE WOOD

- HDPE COMPOSITES

4.1 The Influence of Pre-treatment in NaOH or H2SO4 on the Bond StrengthBetween Wood and Polymer

The experimental data for bond strength under each treatment condition is

shown in Table 2. A significant increase in coupling strength between the woodstock

and polymer is found for the sample in which the woodstock was treated in the solution

o f NaOH, comparing sample 5 with sample 1, and samples 7, 8 , 9 with samples 2, 3, 4,

respectively. The treatment with the solution of H2SO4 shows detrimental effect on the

bonding strength, comparing sample 6 with sample 1 and samples 1 0 , 1 1 , 1 2 with

samples 2, 3, 4. Sample 7 with NaOH first treatment and using CA 1 next shows the

highest value of coupling strength, w'hile H2SO4 solution treated sample with CA2

further treatment show's the lowest one in the value.

Pre-treatment in the solutions of NaOH and H2SO4 can also change the surface

morphology of woodstock. The surface morphology of untreated wood sample is

shown in Figure 22 w'hich exhibits the cellulose property w'ith tiny fibers protruding out

of the wood matrix. The treatment of woodstock in solution of NaOH produces

straight, well aligned and unidirectional wood fibers on the original wood surface,

Figures 23 (a). How'ever, the treatment in solution of H2SO4 produces curved,

unaligned and scattered wood fibers, Figure 23 (b). In addition to interfacial acid/base

interaction force, the propagated wood fibers penetrating into melt PE during extruding

process can produce extra mechanical bonding after polymerization. The improvement

in bond strength by the solution o f NaOH and the decline by the solution of H2SO4 have

4S


been verified by the experimental results. Because of acid/base interaction (even

though not dominant) and, more importantly, mechanical bonding through wood fibers,

the pre-treatment o f wood in the solution o f basicity can be a consideration of merit to

enhance the bond strength between the wood and PE. This fact has been supported by

comparing experimental data of samples 7, 8 and 9 with those of samples 2, 3 and 4.

On the other hand, it is concluded that pretreatment in the acidic solution should be

avoided due to its detrimental effect, comparing experimental results of samples 10, 11

and 12 with those o f samples 2, 3 and 4.

4.2 The Influence of Coupling Agents on the Bond Strength Between Wood andPolymer

From Table 2, it is found that CA1 has significant influence on the bond

strength, while the influence of CA2 and CA3 on bond strength is trivial.

Usually, coupling agents which are expected to produce some functional groups

on the wood surface can be used as surface modification materials. The chemical

interactions among the components on the interface rely on the chemical bonding

interactions of functional groups in the polymer with complementary functional groups

produced on the treated wood surface. It is known that processing of materials with

coupling agents which increases surface functional groups and surface roughness or

eliminate surface defects on the wood can improve interfacial adherence, Maiti et al.

(1989).

In brief, the coupling agents modify the coupling strength between wood and

polymer by increasing the adherence and bonding between these two materials.


4.3 The Influence of Surface Morphology of W oodstock on the Bond StrengthBetween Wood and Polymer

It has been found that the protruding wood fibers are beneficial for the

improvement of bond strength between woodstock and PE due to the formation of

mechanical bonding. The treatment in the solution o f NaOH produces sharp, well

aligned and unidirectional wood fibers on the surface o f woodstocks which increases

the bonding strength between woodstock and polymer, comparing sample 5 with sample

1. Under this condition, further improvement can be created by treating with coupling

agents, comparing samples 7, 8 and 9 with sample 5. The surface morphologies o f

samples 7, 8 and 9 are given in Figure 24 (a), (b) and (c). It is found that the common

feature of these three surfaces is well grown and oriented wood fibers. Coupling agents

may not have dominant influence on the surface morphology, but enhance the bonding

strength by possible chemical reactions between wood and polymer, as mentioned

above.

In contrast, the treatment in the solution of H2 SO4 produces curved, unaligned

and scattered wood fibers which decreases the bonding strength between wood and

polymer, comparing sample 6 with sample I. Further treatment in CA1 restores the

bonding strength back to the value of untreated sample, comparing sample 1 0 with

sample 6 , but treatments in CA2 and CA3 weaken the bonding strength, comparing

samples 11 and 12 with sample 6 , due to the special interactions between CA2, CA3

and polymer, as discussed above. Figure 25 (a), (b) and (c) show the surface

morphologies of samples 10, 11 and 12. It is found that a relatively thick and flat layer

on the surface of wood is formed after treatments in H 2 S O 4 and coupling agents CA1

and CA2, which may be attributed to the reaction between H2SO4 and the coupling

50


agents. The formed layer covers the surface of woodstock completely in sample 11

(Figure 25 (b)) and prevents penetration of wood fibers into polymer. Thus, the

treatment in the solution of H2SO4 worsens the coupling strength between wood and

polymer, especially in sample 1 1 .

The typical fracture interface on PE in sample 1 includes distributed tiny pores.

It is found through careful observation that these pores are produced due to wood fiber

pulling-out from polymer matrix during the tensile process. The corresponding low

bonding strength tested on sample 1 is more than likely attributed to the weak adherence

between wood and polymer.

Comparing with that o f sample 1, the fracture surface of sample 7 shows the

broken wood fibers embedded in PE matrix. The consequential improvement in

bonding strength indicates better adherence between wood and polymer caused by pre

treatment (solution of NaOH) and application o f coupling agent (CA1). This

observation reveals that good adherence of penetrating wood fibers into PE matrix

strengthens the mechanical bonding between wood and polymer. As a result, the

coupling strength between the woodstock and polymer increases significantly, as

verified by experimental data.

51


Reproduced

with perm

ission of the

copyright ow

ner. Further

reproduction prohibited

without

permission.

Table 2Summary of different wood surface treatments and experimental coupling strengths between woodstock and polymer in southern pine-high density polyethylene coupled

samples

Sample No. NaOH H2S04 CA1 CA2 CA3 Bonding Strength*

(MPa)

1 1.36

2 Y 2.06

3 Y 1.50

4 Y 1.37

5 Y 2.51

6 Y 1.05

7 Y Y 3.28

8 Y Y 2.75

9 Y Y 2.61

10 Y Y 1.36

11 Y Y 0,593

12 Y Y 0.828

Y: corresponding treatment

*:fracture stress at the wood-polymer interface

Figure 22. The surface morphology o f untreated pine (wood)) sample.

53


(b)

Figure 23. The surface morphology of pine samples treated in the solutions of (a) NaOH and (b) H2S 0 4

54


(a)

(b)


(c)

Figure 24. The surface morphology of pine (wood) samples treated with NaOH and then with (a) CA1, (b) CA2, and (c) CAS

I j MJ HD 4 8nn S 009 0 3 P 06001

(a)

56


fc)

Figure 25. The surface morphology o f pine (wood) samples treated with H2 SO4 and then with (a) CA1, (b) CA2, and (c) CA3

5 7


CHAPTER 5

RESULTS AND DISCUSSION: IMPACT FRACTURE PROPERTIES O F PINE CHIP - HDPE COMPOSITES

5.1. Instrumented Impact Testing

Figure 26 depicts typical curves obtained for load vs. deflection and energy vs.

deflection for a pine chip-HDPE composite specimen subjected to tup velocity and impact

energy of 14.22 ft/s (4.33 M/S) and 23.45 ft-Ib.(31.77 J), respectively. The highest point

on the curve, designated the maximum load, corresponds to the onset of material damage

or complete failure. The x-coordinate of this point corresponds to the deflection to

maximum load, which is the distance the impactor travels from the point of impact on the

specimen to the point of maximum load. A more damage resistant material will have a

higher peak; in other words, the value of the maximum load is a function o f the damage

resistance of a material. The energy value corresponding to its deflection point is the

energy to maximum load and represents the amount of energy that is absorbed by material

to be deformed prior to damage initiation.

j2L o a d -r

.Energy

I o k O0.40 °0 04 0 16 0.20

Ch 1 aetlection finch iQ.2i 0.32 0.360.00

Figure 26. Load vs. deflection and energy vs. deflection curves for specimen with small pine (wood) chips and 60% concentration.


Figure 27 shows sample plots of load vs. time and energy vs. time during the test

for wood-polvmer composites. The highest point on the load-time plot also represents the

maximum load. The energy value corresponding to the same x-coordinate as the

maximum load on the energv-time plot is the energy to maximum load, which is the

energy absorbed by the specimen up to the point o f maximum load. The values

corresponding to the maxima and their locations can be compared for different composites

to ascertain their fracture resistances. Critical values corresponding to the peak in these

plots were obtained from impact tests conducted on different WPC samples. The average

impact fracture properties obtained are listed in Table 5 for WPC samples with various

sizes and volume fractions o f wood chips.

Load

P

OQ0.00 0 25 0.50 2.00 2.25

Figure 27. Load vs. time and energy vs. time curves for a specimen with small pine (wood) chips and 60% concentration.

Results indicate that low velocity impact properties of the wood-polymer

composites vary with the concentration and size of the wood chips in the composites.

5.2. Experimental Results

Figure 28 shows plots of comparative values for the maximum load in the drop

weight test for the pine-HDPE WP composites. The experimental results indicated that

regardless of wood chip size and its concentration in the composites, there was an increase

59


initially in the maximum load sustained by all composites with increase in wood content.

This is in agreement with the resistance to sliding offered by the wood chips. Composites

containing small wood chips showed consistent increase in the maximum load by about

40% up to 50% concentration and then the value dropped sharply to a level 25% below that

of the polyethylene matrix itself. Composites made of medium wood chips exhibited an

increase in maximum load by about 50% up to 40% concentration, beyond which there

was a small decrease by about 10%. Composites with large wood chips, on the contrary,

showed a steady increase in the maximum load with increase in wood fraction. The

highest value for the maximum load was exhibited by samples made o f large wood chips

with 60% concentration. These samples sustained a maximum load slightly more than

twice that of the polyethylene itself, improving it by about 130%

In wood-poiymer composites, wood chips are individually encapsulated in a matrix

of flexible polymer and are mostly aligned parallel to the surface by the compressive force

applied to the composite after melting o f the polymer. The average thickness of the

polymer layer between two adjacent wood chips would depend on the wood chip size and

concentration in the mixture. Higher concentration of wood chips in composite would

generally result in a thinner polymer layer between wood particles. With the advent of

external forces, separation or cracking along the generally weak polymer-wood interface

would result. The initiation and propagation of resultant cracks due to delamination

contributes to a weakening of the composite. Crack propagation would be facilitated if the

particles are aligned preferentially.

The morphology of a typical fracture surface of an impact failure sample is shown

in Figure 29. It can be inferred from the features observed that the fracture of the impact

60


sample occurred by the initiation of cracks due to debonding at the interface and

propagation of resultant cracks along the weak interface. This observation implies that the

impact properties of the composite partially depend on the bonding forces between the

wood and polymer.

Table 3Drop weight impact fracture properties o f southern pine (wood l-polyethvlene

(polymer) composites with various wood chip sizes and concentrations.

Average' A verage A verage A verage

Wood Chip Max. Load Deflection a t Max. Energy to Max. Total Ener

Size/Concentration Load Load

Vol% (lb.) (inch) (Jt-lh.) (ft-lb.)

-5.4% - 3.9% =3.5% =4.3%

0% 278 0.290 4.1 9.8

Small 40% 348 0.255 4.6 7.3

<0.125 in 50% 395 0.200 4.2 10.9

60% 210 0.160 1.7 3.9

Medium 40% 610 0.250 4.5 10.8

>0.125 and 50% 378 0.220 4.9 9.4

<0.25 in 60% 376 0.145 3.0 7.1

Large 40% 441 0.270 4.1 9.7

>0.25 and 50% 530 0.195 4 2 7.6

<0.5 in 60% 649 0.190 5.0 21.4

* The values given are the average o f consistent data in four tests for each case. The maximum

deviation (scatter in results) is indicated for each property. The exact deviation for each value

is shown by bars in the corresponding plot in Figures 5. 7. 8, and 9 (Note: I ft.lb = 1.356 J)

61



| too ------------------------------------------------------------------tm0)< 0 -----------------------------------------------------------------------------

0% 10% 20% 30% 40% 50% 60% 70%Wood Chip Concentration


■O 500 - ô Xe 400 :---------------E _ 300X AE 200

I* 100WO< o



■g 800 ••0 700 --------------------------------------------------------E 600 ---------------------------------------------------------------1 _ 500 ~ rX £ 400 ................................ — ............ ...............E 300 c . ~ ' --------------------------------------------Si 200 -------------------------------------------------------------------| 100 -----------------------------------< 0 --------------------------------------------------------


Figure 28. Effect o f pine (wood) chip size and concentration on maximum load sustained under impact

62


Figure 29. SEM ffactograph of a WPC specimen with large wood chips and 60% volume concentration. One large wood chip with a layer of peeled polymer hanging on it (top) is seen.

Results obtained from impact fracture tests indicate that for the same concentration

o f wood chips, fewer larger particles would offer better resistance to compressive

deformation than large number o f fine particles in the particularly for concentrations

beyond 50%. The impact properties o f composites improve in the higher wood chip

concentration range of larger particles, since perhaps larger amounts o f wood chips would

offer more obstacles for compressive deformation, crack growth and fracture. On the

contrary, finer particles will not be able to resist the deformation o f the matrix, and though

their introduction into the composite strengthens it initially at lower concentration, easy

crack propagation seems to result at higher concentrations (at ~ 60%) contributing to a

63


reduction in the maximum load sustained, to a level lower than that o f the matrix polymer

itself.

The deflection at maximum load. Figure 30, shows a consistent decrease of

deflection at maximum load for all composites as wood chip concentration is increased.

The deflection at maximum load starts to level off for samples with large wood chips at

concentrations in excess of 50%. From this general result it can be deduced that impact

fracture would initiate in general more easily with less deflection when the wood fraction

is increased in this kind of WPCs. Only in the case of composites with large size wood

chips does reduced deflection appear to be an indication of an increase in rigidity, since the

maximum load sustained (damage resistance) keeps on increasing with increasing wood

fraction.

53 . Energy to Cause Failure

Figures 31 and 32 show the results for the energy to maximum load and total

energy to failure under the given set of experimental conditions. The total energy to

failure represents the energy that the material would absorb until full penetration of the

impactor. These parameters are determined from measuring the relevant areas under the

load-deflection curve by the data analysis program in the computer attached to the impact

tester. Evaluation of this parameter begins at the initial contact between the impactor and

the sample and continues beyond until complete penetration by the tup is achieved. Figure

31 indicates for small and medium chips that the energy to maximum load remains nearly

the same up to 50% wood chips addition to polymer for all the composites studied.

Addition of more small or medium wood chips leads to a deterioration of this value and

there is a recognizable drop in the range of concentration 50 to 60%. Addition of large

64


wood chips, on the contrary, showed a noticeable rise in the energy in the range of 50% to

60%. This is attributed to the increase in maximum load at nearly constant deflection in

this range of composition.

The total energy absorbed by the wood-polvmer composites for total penetration

by the tup indicates likewise little or no improvement relative to plain polymer up to 50%

wood concentration (Figure 32). The addition o f wood chip particles of all sizes in the

composites seems not to affect the total energy significantly up to 50% concentration of

wood chips, but a rather large drop by about 60% is seen for the composites with small

wood particles in the 50 to 60% concentration range. Only the composites with the large

wood chips show an increase (nearly double) in the concentration range of 50% to 60%.

Considering the larger external force required for the slipping of the wood chips in the

matrix at 60% concentration (Fig 28) and the fact that inclusion of larger wood chips

would present larger contact surfaces with larger friction energy to overcome, one can

explain the increase of energy to maximum load and the total energy absorbed in these

WPC composites with large wood chips at higher concentrations in the 50 to 60% range.

Further work is needed to ascertain whether any other factors influenced the drastic

increase in the impact fracture energy encountered in the composites containing large

wood chips and paradoxically the large reduction in fracture energy in composites with the

small wood chips in the 50 to 60% concentration range. Nevertheless, it is clearly

demonstrated that whereas the WPCs containing 60% small wood chips would fail more

easily than the matrix polymer itself, those containing 60% large wood chips would resist

impact fracture far more efficiently (at least four times better than the analogous


composites with small wood chips) and those containing medium size wood chips would

show minimal effect.

5.4. Mechanism for Energy Absorption and Fracture Morphology

Earlier works involving the evaluation of tensile properties of WPC composites

indicated that the addition of 60% large wood chips to polymer matrix deteriorated the

tensile properties of these composites, Razi et al. (1997). In contrast, as we have shown

here, the impact properties of these composites seem to be improved drastically for

samples with 60% concentration o f large wood chips. The initial failure mechanism

during the tensile failure was due to the debonding at the wood chip - polymer interfaces

and consequent crack initiation. Once initiated, the crack grows more easily over the

entire interface of wood chip and polymer as the specimen is loaded in tension. The only

material resisting the complete separation into two pieces, the polymer matrix at the cross

section, experiences higher level of stress due to the reduced effective area, and, as a result,

fails easily. During the impact test, however, wood chips would start sliding in the soft

polymer matrix more easily and concurrently debonding would occur due to the difference

in strain rates of wood and polymer. Smaller wood chips can be expected to slide more

easily, though in this process they may enable easy crack propagation through

reorientation and alignment o f the delaminated interfaces. In samples with 60%

concentration o f large wood chips, however, there seems to be a greater resistance for

sliding and when sliding is initiated there would also be a possibility of chip-to-chip

interference. This interference of large wood chips would cause increased resistance to

deformation and necessitate higher level of stress and energy to cause fracture. Such


interference does not seem to occur in composites with small wood chips and these appear

to sustain easy crack extension in a continuous manner.

Figures 33-1 and 33-2 show the fracture features at the top and the bottom faces of

tup-penetrated samples containing 60% small and large size wood particles. The

photographs in figure 33-1 corresponding to the specimen containing small wood chips

show a perfectly circular penetration hole both at the top and bottom faces. The surface of

the hole in the penetrated region is also rather smooth and clear-cut These features are

indicative of least resistance to fracture and ability of the material to be punched rather

smoothly. On the contrary, in the case o f an analogous specimen with large wood chips

the hole is somewhat elongated and oval-shaped, and its surface in the penetration zone is

also rugged and non-uniform. Individual wood chips can also be seen in the photograph of

the hole taken at the bottom surface of the specimen, Figure 33-2. Considering that the

specimen plate bends downward when the hit is made and because of tensile stressing

cracks that would formed at the bottom surface, the features observed are indicative of

increased resistance offered for the propagation of the crack and a more tear-type hole

being formed. These observations strengthen the discussion given earlier that the coarse

chips at 60% concentration would enable an increase in the resistance to fracture, whereas

analogous composites with 60% fine chips would break very easily.


Effect of Large Wood Chip C oncentration on A verage Deflection a t Maximum Load

~ 0 .4 -eo —= .££ V 0 .3 c rU <sQl o ? J 0 2 - a E§> E 0.1 — 2 x 0) (B

o -0% 10% 20% 30% 40% 50% 60% 70%


Effect of M edium Wood Chip C oncentration on Average Deflection a t Maximum Load

0.4c *o.2 g 0.3U _1 .2c E 0 2« 3° EI » °'1 £ 0

• I - ___

0% 10% 20% 30% 40% 50% 60% 70%


Effect of Sm all Wood Chip C oncentration on Average Deflection at Maximum Load

0.4

•2 <o 0.3 u o 0>« | 0.2 » 1S’ ra 0.12 s 4>< 0

0% 10% 20% 30% 40% 50% 60% 70%


Figure 30. Effect of pine (wood) chip size and concentration on deflection at maximum load sustained

68



c

4

3

2

1

010% 20% 40% 70%0% 30% 50% 60%



6n o — 5

4

3OS 3

2P 3

1

020% 30% 40% 70% i10% 50% 60%0%


Specim es with large wood chips

_ 6 3 ? 52* -a 40) (ISc oin _i -j

§ O ,2 e 1o> 2 1 -

< « 10 ■

0% 10% 20% 30% 40% 50% 60% 70%


Figure 31. Effect of pine (wood) chip size and concentration on energy to maximum load sustained

69


Specimens with sm all wood chips

g> 10 -o • c £ a 114 3 8 __ ^

I -

<10%0% 20% 40% 50% 70%30% 60%



2 12>»S’<u ~ c £

— £12 1 o>m5v><

10 -r.

£ 4

.....................-I.

0% 10% 20% 30% 40% 50%Wood Chip Concentration

60% 70%


25 r

= £ MJ - T- £

10 -e-

<40% 50% 70%0% 10% 60%20% 30%


Figure 32. Effect of pine (wood) chip size and concentration on total energy to failure by tup penetration

70


(b)

Figure 33-1. The morphology o f fractured surfaces of samples broken in drop weight impact test: specimen with 60% volume concentration of small wood chips (a) top surface (b) bottom surface

71


(b)

Figure 33-2. The morphology o f fractured surfaces of samples broken in drop weight impact test: specimen with 60% volume concentration o f large wood chips (a) top surface, and (b) bottom surface.

72


CHAPTER 6

RESULTS AND DISCUSSION:FRACTURE RESISTANCE AND FRACTURE TOUGHNESS OF PINE CHIP -

HDPE COMPOSITES

Standard procedures described in various ASTM methods were used to

characterize the fracture properties of wood-polymer samples in compact tension type

specimens. The applicability of these procedures to analyze the fracture behavior o f the

wood chip reinforced polvmer-matrix composites is probed into in this chapter. An effort

is made to define the fracture toughness Krc for such composites.

6.1. Kic Determination Using ASTM Standard Procedure

The fiacture toughness, Kic, was determined following the methodology outlined in

ASTM E 399-83, ASTM (1995). The compact tension specimen, C(T)(S-T), used, its size

and configuration are shown in Figure 16. The pre-crack extension o f the notch tip

produced by fatigue was 3.6 mm. Pretest compliance was 0.0056562 mm2/N. Ramp rate

was chosen to be 40 N/sec. The summary o f test results is given in Table 4. As

recommended by the standard, displacement measurements on the load line were carried

out by a double-cantilever clip-in displacement gage. Each experiment was repeated at

least three times with three different samples and the results obtained were averaged. The

average results were then reported.

6.2. Jic Determination Using ASTM Standard Procedure

The Jic measure of fracture toughness determination followed the methodology

outlined in ASTM E 813-89, ASTM (1995). Similar compact tension specimens, C(TXS-

T,), as used in Krc determination described above, were also used in these evaluations. The

summary of test results for Jic determination is given in Table 5. Displacement

73


measurements on the load line were again obtained by a double-cantilever clip-in

displacement gage

6 J . R curve Determination

The resistance R-curve was obtained by applying uniaxial tensile loading on the

compact tension specimens. Slightly larger samples were prepared for these tests due to

the fact that the results were being analyzed to the fullest extent o f material resistance.

Larger samples provided more material and longer test time to study the reaction of

samples to the test variables. For these series of tests, the methodology outlined in ASTM

E 561-76T, ASTM (1995), was followed. Figure 17 shows the size and configuration of

the compact tension specimens, C(T)(S-T), used for R curve determination. The cross

head movement speed was set at 0.1-inch per minute. The crack length was measured

directly by a walking-microscope set up to move parallel to the movement of the crack tip.

Load values were taken corresponding to different values o f preset crack extension. The R

curve was developed subsequently as a plot of K7E vs. crack length a.

6.4. Analysis of Data

Fracture properties of the WPCs were determined using a 810 MTS universal

servo-hydraulic test system in the tensile mode. The K[C and J[C experiments were

performed and controlled by the Teststar software added to the MTS machine.

Using the computerized standard test procedure and the software supplied by

MTS, fracture toughness, K[c. and J[C, were determined and the results are reported in

Tables 4 and 5 as already mentioned. Since these standard tests were designed

originally for metals with limited ductility and since R*; > 0.1, the test results obtained


were characterized as not valid. This was expected for the wood - polymer-matrix

composites due to the step-wise crack growth in these materials.

The plot o f load applied vs. crack front displacement is shown in Figure 34. The

graph indicates an initial rise in the load, which represents the elastic deformation of the

materials. Beyond the maximum load point, where the plastic deformation is expected

to start in the polymer, the load decreases smoothly to the failure point. Figure 35

represents plots of stress-equivalents, what can be designated loosely as uniform

engineering stress and true stress values calculated using B. W in the former case and

B.fW-a) in the later case for the area carrying the load vs. crack extension. It should be

noted that this designation is purely arbitrary and simple for it is well known that the

stress ahead of the crack tip is not uniform and is a function o f the radial distance away

from the crack tip, Rice (1968), [see also Figures 7 and 8].

Engineering stress curve shows a steady decrease past the maximum stress and

continues this trend to failure. The graph for true stress, on the contrary, continues to

increase past the elastic zone to a maximum stress corresponding to a crack length of

about 2 inches. The true stress decreases continuously beyond this point up to complete

failure. This can be interpreted to mean that as the crack grows, wood chips in the

composite are offering resistance initially to crack growth. Under this condition fast

crack growth and complete rupture are not realized until crack grows to a level of

instability initiating faster fracture. In order to determine the point o f instability and the

resultant faster growth, the true stress was felt to be a better indicator. The plot in Fig

35 indicates the fact that the true stress keeps on increasing beyond the initial fracture

initiation point and after reaching a peak level begins to decrease. This point where the

75


true stress begins to decrease can be construed as the point of instability. To determine

the critical points corresponding to the initial crack growth initiation point and the

critical point o f instability for onset of fast fracture, the engineering stress intensity

factor, (obtained by using instantaneous load and initial sample dimensions B and

vt> throughout in the formula for K[ in the ASTM E 561-76T standard), and the true

stress intensity factor * obtained by the use o f similar formula using the remnant

segment, W-a, instead o f W, were plotted against crack length, Figures 36 and 37.

Whereas K^g increases somewhat in a parabolic manner past the elastic limit to reach

peak value of around 1800 psiVin, variation o f Ktrue with crack tip displacement shows

very interesting trend. Contrary to Keng, Kuue reaches very high values of the order of

million psiVin. It increases in a slow manner initially and then, after some growth, it

increases rather rapidly reaching values in the order o f several million psiVin. There is

an inflection in the curve past the crack length of -2 in. The variation of Ktxue vs. crack

extension thus indicates that the point where the Ktrue begins to rise rather sharply could

be considered as the point o f onset of instability. This point is around 2.2 in. of crack

length, in the present case. Corresponding to the point o f intersection of tangents to the

sloping points o f the curve with low and high slopes, respectively.

Further analysis was carried out by determining the slope of Kê vs. crack tip

displacement plot at various locations and a plot was made of dKtme/da vs. crack length,

a, Figure 38. This plot is similar to Ktmc vs. crack length plot, Figure 37. Two different

speeds of crack growth are indicated by the different slopes in the plots. At the point

where the low slope corresponding to the low values begins to change to high values

* Note that the designation Keng and Ktrue are used loosely here to denote K values calculated using

76


corresponding to high slope, at the point of inflection, the critical point (c) of instability

can be located. This can be taken as the point where again the lines corresponding to

the two different slopes intersect, i.e. at about 2.2 in. o f crack length. The Keng value

corresponding to this point o f inflection can then be taken to represent the true fracture

toughness K[c for the composite material, Krc.com-

In essence, it is found that in these composite materials the crack is propagating

in a sub-critical mode up to the point where the proposed instability is reached. The

crack growth up to this point is either under steady resistance offered to crack growth

by the strengthening particles (wood chip) or under progressively decreasing resistance

until the point o f instability is reached. The analysis o f resistance to crack growth will

define the exact characteristics o f the crack propagation in sub-critical mode prior to

and after the point o f instability is reached. This behavior is analogous to crack

propagation in stress corrosion cracking and in fatigue loading. In those cases the initial

critical stress intensity factor at which crack propagation would be initiated is defined as

Krscc and K[fat respectively. In the case of composites, an equivalent point

corresponding to an initial critical stress intensity factor value can be designated as

stress intensity factor for crack growth (fracture) initiation, Kiif.com. This value would

correspond to the normal K[C value derived by following the procedure given in ASTM

E 399-83, ASTM (1995). That is, by giving 5% offset to the initial slope to the elastic

part o f the curve and drawing a straight line, the intersection point of the offset line with

the plot in the crack growth region will define the K[if.cotn in the case of composites.

constant W all the time for the former and the non-cracked ligament o f dimension w-a instead o f w for the latter in the formula for stress intensity factor.

77


6.5. The Critical Stress Intensity Factor for Particulate Filled Composites

Based on the above discussion the K[c that is determined following the ASTM E

399-83 and designated to be Kq is actually the Knf.com as defined here. The variation of

Kime and/or of dKtnie / da vs. crack length, a , is more appropriate to find the point of

instability and the fracture toughness Kjc for composites.

From the data collected in the low slope region up to the point o f inflection and

beyond, the resistance for crack growth can be studied using a method analogous to that

given in ASTM standard 561-76T, ASTM (1995). The analysis in this curve is based

on K2/E. Variation of this value against crack length is given as the fracture resistance

of material. The use of engineering and true K values obtained in this curve shows also

interesting results.

Figure 39 shows the R curve obtained following a procedure similar to the one

given in the ASTM standard. The standard has K^r values for K in K2/E , Hetrzberg

(1996). Various (Keng)2/E values corresponding to the different crack lengths were

plotted against respective crack lengths. The plot indicates that as the load increases

initially, i.e. in the elastic region, (Keng)_/E increases sharply. Subsequent increase with

increasing crack length in the sub-critical crack growth region occurs rather slowly.

The plot further indicates that when the point o f instability is reached, (Keng)2/E seems

to have reached a value very close to the highest attainable in that region. Thus the

resistance curve plot may be interpreted to have three different slopes corresponding to

three different regions. Region I is where the crack is growing barely amidst a lot of

resistance, when the slope is high. After the point of initial fracture (crack growth)

initiation in sub-critical mode the slope seems to be reduced to much lower level in

78


region II o f sub-critical growth. At the point o f instability, the slope changes again and

approaches zero, the value of region III. The fracture occurs first with near zero

resistance in region III.

The slope o f the plot K2/E vs. crack length, a , that is d(K2/E )id(a) vs. crack

length, a , is shown in the Fig 40. Representing K by the simple equation:

K = Y . a . y f MK 2 = Y2 .cr2 .rr.a (1)

K 2 <7 = i ' .cr.— -X .aE E

Since cr (2)

£then:

K 2 , = Y c r .s .K .a (3)E

Where cr in equation (1) is applied stress, e is strain, a is crack length, BC is stress

intensity factor, E is modulus of elasticity and Y is the (compliance) calibration factor.

The or in the final equation (3) can be considered as the resistance stress corresponding

to the force o f resistance offered by the material for crack propagation.

The variation of slope of K2/E vs. a, i.e. the resistance criterion is clearly

indicated in Figure 40. Normalizing the data in the Y-axis to vary between 1 and 0 (of

slope), i.e. 100 to 0% o f resistance to fracture, in the initial elastic region I, the

resistance to crack growth can be considered to be 100%, for the crack is not growing.

This resistance is maintained for a short while and there is a rather large sharp decrease

79


in the value to about 30%. Where the drop occurs, it signifies the initiation of sub-

critical crack growth due to a considerable drop in resistance to crack growth. Further

crack growth in region II the sub-critical crack extension region, occurs under

continuously decreasing resistance, until the latter reaches the value close to zero when

the instability sets in. The latter critical point corresponds to the total loss of resistance

and easy crack propagation beyond that point under zero resistance. The K^g value of

the initial point corresponding to the large drop in resistance would correspond to the

value of Kuf.com, and that o f the point where resistance reaches close to zero would

correspond to the value of k[c.COm- Thus the resistance curve indicates the two critical

points very clearly. The values of Kur.com and Kfe.com obtained from the crack growth

extension study are given in Table 6. Comparing these values to those given in Table 4,

it can be found that the values of KufCOm given in Table 6 are close to the K(c or Kq

obtained by using methodology given by the standard. Thus the standard procedure for

determining K[C according to ASTM E 399-83 enables the determination of the initial

critical stress intensity factor for fracture initiation in sub-critical mode and not the

fracture toughness o f the composite. The latter is obtained from the fracture resistance

curve from the point o f instability, wherefrom further crack propagation should be

about zero resistance. The point o f instability is more clearly defined in the Ktmc vs. a

and/or dK^Jda vs. a plots at the point of inflection, as already outlined. Thus the FQ-ng

value corresponding to the point of inflection gives K[c more accurately.

The results o f the study gives the following data for the 50 vol.% medium size

pine wood chip polyethylene composite samples:


fC[lt-= 1209 psiVin (42.00 N/mmA1.5), Kfc = 1766 psiVin (61.36 N/mtnA1.5), and

Drop in resistance upon sub-critical crack growth initiation ~ 75%

The fracture surface analysis using SEM essentially indicated a brittle type

fracture process. This was evident from the flat nature of the fracture surface. Particles

protruding from the surface were indicative of delamination at the interface and the

crack moving around the particles.

Table 4:The test result summary for the K[C tests (as per ASTM E399-83) for pine-HDPE

composite sample (medium size wood chips in 50% volume concentration)

Kic Test ResultP5 1329.02 NPQ 1329.02 NPmax 961.5 NKQ 39.0274 N/mmA 1.5RA2 0.999879 Correlation CoefficientRsc 0.335604 Strength RatioKQ * KIc Not valid according to

ASTM E399-83

Table 5:The test result summary for the Jic tests (as per ASTM E813-89) for pine-HDPE

composite sample, (medium size wood chips in 50% volume concentration)

___________________ Jic Test Result ______________________JQ 2.78789 N-mm/mmA2Jic ^ JQ____________ ___________________ Not valid according to ASTM 813-89

Table 6:Comparison of the Kiif.COm K[cxom obtained using ASTM E561-76T and Kq obtained

using ASTM E399-83.

ASTM E399-83 ASTM E561-76TK o * K , c Knfcom K[c.com

1120.5 psiVin (39.02 N/mnr’i .5 ) 1209 psiVin (42.00 N/mmA1.5) 1766 psiVin (61.36 N/mmA1.5)

81


400

300

200

100

0 0.2 0.4 0.6 0.8 1 1.2 |

Crack Opeming Displacement (in)

Figure 34. Load vs. crack opening displacement curve for the pine-polyethylene composite sample, (medium size pine (wood) chips in 50% volume concentration)

</3c .

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Figure 35. True Stress and engineering stress vs. a crack length plot for pine-polyethylene composite sample, (medium size pine (wood) chips in 50% volume concentration)

82


200018001600140012001000800600400200

02.50 0.5 I 1.5 2 3

Crack Length (in)

Figure 36. Engineering Ki vs. crack length plot for the pine-polyethylene composite sample, (medium size pine (wood) chips in 50% volume concentration)

5000 4500 4000 3500 3000

to' 2500 \ 2000 'Z 15005 1000* 500

2.51 1.5

Crack Length (in)

0.5

Figure 37. True Kt vs. crack length plot for the pine-polyethylene composite sample, (medium size pine (wood) chips in 50% volume concentration)


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Crack Length (in)Figure 39. Resistance curve, K2icng/E vs. crack length plot for the pine-polyethylene

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CHAPTER 7

CONCLUSIONS AND SUMMARY

Based on the results o f the present study the following conclusions are drawn:

1. Addition of wood chips to polymer reduces the UTS and break strain in tensile

loading o f the resultant composites. Smaller wood chips generally result in smaller

reductions.

2. The peak load, MOR and stiffness in bending tests o f the pine-PE composites have

been found to have increased at 40% wood chip concentration; they then decrease

with additional concentration of wood chips. The pace of change in mechanical

properties of composites depends on the wood chip size, the smallest chip size

showing largest increases.

3. In tensile testing, fracture of samples is caused by delamination of wood chips from

polymer matrix. The debonding at the interface between wood chips and polymer

matrix may have been caused by residual stress left at the interface after

compression o f the composite during preparation of the specimens.

4. Nail pull-out-force is affected by chip size and concentration. It increases with

wood concentration up to 50% and then decreases. Smaller chip size results in

larger nail pull-out-force.

5. According to the experimental results, the required wood chip size and

concentration for optimal mechanical properties are size smaller than 0.125” and

concentyration around 40vol.%, respectively.

6. Pre-treatment in the solution of NaOH is beneficial to increase the coupling strength

between negatively charged (due to OIT) wood and positively charged (due to H*)

87


PE due perhaps to interaction o f acidity/basicity, but predominantly due to the

modification of surface morphology.

7. Protruding wood fibers after treatment are found to be able to form mechanical

bonding at the interface between wood and polymer.

8. Treatment of wood first with NaOH and after drying with vinyltrimethoxysilane is

the best treatment for obtaining maximum bonding strength at the interface and

mechanical properties resultant therefrom. Treatment in H2SO4 increases acidity of

wood and also scatters the fibers on wood surface, weakens the bond, and hence,

should be avoided.

9. Wood chips in the composite act as obstacles for impact-induced compressive-type

deformation.

10. The buried wood chips would rub against each other and produce internal friction

forces that would restrict their sliding in the matrix and, as a result, resist cause damage

under compressive-type impacts

11. Higher concentration of large size wood chips yields higher maximum load, higher

energy to maximum load and total energy absorbed prior to complete failure compared

to analogous samples with medium or small size chips. More rigid wood-polymer

composites are those with larger wood chips at the higher concentration ranges of 50 to

60 vol.% . On the contrary, pine-HDPE WPCs with 60% fine wood chips are the

weakest under impact-type loading and would fail easily at energy levels far less than

those required to break the polymer alone.

12. Compared to which increases in somewhat parabolic manner with respect to

crack growth the corresponding to the true stress sustained by the un-cracked

88


segment increases slowly initially, but rather fast after the crack has grown

considerably.

13. Variation of K ^ / E vs. crack length, the R curve, indicates three regions of

different slopes-region I where the slope is high, region II where the slope is

medium, and region III where the slope is 0.

14. The slopes at various points on the R curve plotted against a, that is d(K2eng/E)/da

vs. a plot yields the true fracture resistance plot. From the initial high value o f the

region I there is a sharp drop at the transition from region I to region II. Slopes at

points on the curve in region II steadily drop and reach a value very close to zero

corresponding to near loss of all resistance and the onset o f crack growth in region

III. Region III crack propagation can be construed to be under near zero resistance.

15. The point o f transition from region I to region II corresponding to a large drop in

fracture resistance is considered as a critical point o f fracture process initiation in

the sub-critical mode. In region II, the crack propagates in a sub-critical manner

16. The transition from region II to region III, corresponding to the onset o f near zero

resistance, can be considered to be the point of instability.

17. The first critical point at low K[ corresponding to initiation of fracture in the sub-

critical mode, is designated as K[if,COm. The 2nd corresponding to the point of

instability is considered as Kjc, the fracture toughness of the composite

18. The standard procedure given in ASTM E 399-83 to determine the fracture

toughness Kic or Kq yields Kn̂ m- K[C can be derived from the resistance curve

corresponding to the point of onset of zero resistance.


19. The variation o f with crack length is a true indicator of the onset of crack

instability. The point o f inflection corresponding to a large change in slope

indicates the point of instability. The engineering stress intensity

corresponding to this critical point o f inflection would be K[c, the fracture toughness

o f the composite.

The salient results obtained can be summarized as follows:

Composite plates of wood chips and high-density polyethylene (HDPE) matrix were

prepared for study in a 5” x 13” x 0.75” steel mold with removable lid by first filling it with

pre-mixed wood and polymer and then heating the mold in a furnace. Resultant

rectangular samples were later cut into specific dimensions for testing. This procedure

resulted in polymer-wood com posite samples with w ood chips aligned somewhat parallel

to the surface by the compressive force applied to the com posite during solidification and

sample removal, especially in samples with low wood chip concentration.

Mechanical properties o f these samples were characterized later. It was found

that the addition o f wood chips to polymer reduces the UTS and break strain in tensile

loading of the resultant composites and smaller wood chips generally resulted in smaller

reductions. In tensile testing, fracture o f samples is caused by delamination of wood

chips from polymer matrix. Nail pull-out-force was found to increase with wood chip

concentration up to 50% and then it decreased with 60% wood chip concentration.

Smaller chip size results in larger nail pull-out-force.

Optimal mechanical properties are attained in composites with wood chips of

size smaller than 0.125” and in concentration around 40 vol. %.


Treatment of wood first with NaOH and after drying with vinyltrimethoxysilane,

is the best treatment for obtaining maximum bonding strength at the interface and

resultant mechanical properties

Under impact type loading, more rigid wood-polymer composites are those with

larger wood chips at the higher concentration ranges o f 50 to 60 vol.%. On the contrary,

pine-HDPE WPCs with 60% fine wood chips are the weakest and would fail easily at

energy levels far less than those required to break the polymer alone.

A method to obtain stress intensity fracture toughness of polymer-matrix

composites is not well defined in the literature. Since this class of composite material

can propagate a crack slowly and sustain reduced loads even after the crack has

advanced considerably, the standard procedure prescribed in ASTM E 399-83 is no

longer applicable. Evaluation of a resistance curve following the procedure given in

ASTM 651-76T indicates that the fracture toughness Korean be correlated to a point of

onset of near zero resistance to fracture. Analysis in this work indicates that this point

approximately coincides with the point o f inflection in the K ^ e vs. crack length, a,.

curve. It is found from further analysis arrived at in this study that the procedure given

in ASTM E 399-83 can be used to determine the stress intensity for fracture initiation

designated as KnriCom, whereas the fQcng value corresponding to the point o f inflection in

the Kitruc vs. crack length, «, curve would closely derive the Kic value of the composite.


CHAPTER 8

SUGGESTIONS FOR FUTURE WORK

In this work the mechanical properties, bonding strength, impact fracture and

static fracture behavior o f pine-HDPE composites were studied. Conditions to obtain

optimum properties have been discussed. However, there are other areas where future

work can be concentrated to better understand the behavior o f these composites in order

to utilize them.

8.1. Evaluation o f Kijfcom and K[C for composites with 60 vol. % pine wood chips

In view of the fact that the composites with 60 vol. % large and small pine wood chips show

interesting impact properties, their Knf.com and K[C should be determined further using the

procedure adopted in this work. The differences in characteristics between slow and fast

crack growth processes could be better understood through such evaluations and

comparisons.

8.2 Modeling of Crack Growth and Delamination o f the Wood Chip-PolymerInterface

Modeling of fracture and crack growth in polymer (viscoplastic)-matrix

composite materials and understanding the effect o f interface strength and brittle

particles imbedded in them can positively help the potential utilization of these materials.

Such a study could help design material with better-engineered fracture properties. Pine

chip- HDPE composites fall into this category o f materials and such a model could

further lead to methods to improve the fracture resistance of these materials. A clearer

understanding of the delamination process in polymer-matrix composites would enable

development of methods and processes that would eliminate the delamination or at least

92


delay its onset to higher stress levels. Nucleation and growth o f voids in the polymer

matrix and at the wood-polvmer interface should be studied more carefully.

8.3. Fatigue and Creep Characterization.

During the mechanical characterization of pine-HDPE composites, the need for

better understanding the fatigue properties of these composites developed. Likewise

stress relaxation under constant and constrained strain-type loading and creep strain

development under constant loading should be studied. These properties would be o f

interest toward structural application o f these composites.

8.4. Effect of Environmental Variables.

Wood-polymer composites have been the subject o f much research during last

decade. Their water and humidity absorption as well as dimensional stability and

biodeterioration have been investigated. These studies have been concentrated mainly on

composites developed by liquid monomers impregnation into wood and their subsequent

in situ polymerization. It is o f interest to find the effect o f varying environmental

moisture and temperature on these materials with different sized wood chips and

correlate them to applications in the industry.

8.5. Damping Characteristics

One of the important properties of polymers and hence of polymer-based

composites is their vibration damping and mechanical energy absorption characteristic.

This property needs to be studied in pine-chip - PE composites as well. It is possible that

on account of this property they can be utilized in panels and sidings of structures that

would be subject to oscillation and vibration. They could find similar application in

acoustic damping devices and structures as well.

93


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VITA

Parviz Sharif Razi was bom in 1951. He graduated from College o f Science

and Technology o f Tehran, Iran, earning his bachelor o f science degree in Mechanical

Engineering (ME) in 1972. He served two years of military service in the Iranian

Navy as an officer and then joined the Navy's engineering department where he

practiced his engineering skills. He started his graduate studies in 1977 at Louisiana

State University where he received his master of business administration (1980) and

master o f science in mechanical engineering (1983) degrees earlier. He worked as a

project engineer/designer and project manager during his professional career. He is

currently a candidate for the degree o f Doctor of Philosophy in Engineering Science

from the Louisiana State University.

99


DOCTORAL EXAMINATION AND DISSERTATION REPORT

Candidate: Parviz S. Razi

Major Field: Engineering Science

Ti-tlfi of Dissertation: Pine—Polyethylene (Wood—Polymer) Composites:Synthesis and Mechanical Behavior Characterization

Approved:

iduate School

EXAMINING COMMITTEE:

X

Date of Examination:

March 25, 1999


Documents

Pine-Polyethylene (Wood -Polymer) Composites: Synthesis