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A Study of Fibre-Matrix Interactions in Biodegradable Kraft Pulp Fibre-Reinforced Polylactic Acid Composites
by
Mandana Fazl
A thesis submitted in conformity with the requirements for the degree of Masters of Applied Science
Department of Chemical Engineering and Applied Chemistry University of Toronto
© Copyright by Mandana Fazl 2012
ii
A Study of Fibre-Matrix Interactions in Biodegradable Kraft Pulp
Fibre-Reinforced Polylactic Acid Composites
Mandana Fazl
Masters of Applied Science
Department of Chemical Engineering and Applied Chemistry
University of Toronto
2012
Abstract
As the plastics sector moves towards sustainable growth and development, natural fibres start to play an important
role as constituents in composite materials in several industries including automotives. However, drawbacks such as
fibre-matrix incompatibility and poor fibre dispersion still exist. In this thesis, Kraft pulp fibre (KF)-Polylactic Acid
(PLA) composites were prepared using thermal compounding and aqueous blending to study fibre-matrix
interactions. Fibre surfaces were also modified to improve fibre dispersion and water absorption properties. A
biorefinery lignin was added to PLA and high density polyethylene (HDPE) as a biofiller and potential interface
modifier. Aqueous blended composites showed better mechanical and dynamic mechanical performance than the
thermally compounded materials. The fibre surface modification improved dispersion and material properties at
higher fibre content. Furthermore, the addition of lignin to polymers resulted in improved mechanical properties in
both PLA and HDPE; however, lignin failed to improve interface bonding between KF and PLA.
iii
Acknowledgments
I am indebted to a number of people for their support and contribution to my work in the last two years,
and wish to extend my gratitude to them here.
My supervisor, Dr. Ning Yan, has imparted great knowledge, guidance and direction throughout my
graduate academic career. Not only did she help me gain insight in the field of biocomposites, but also
taught me important life lessons in the process, and I will always regard her as a great mentor. I am also
grateful for the support of all my colleagues at the Advanced Forest Bio-Materials Laboratory, especially
Jieming Chen for her guidance and feedback on the work on fibre treatment. I would like to extend my
gratitude to the Department of Chemical Engineering and Applied Chemistry and the Faculty of Forestry
for their financial support and to the faculty and staff for their academic and technical support.
Furthermore, I would like to thank Mascoma Canada for their partnership and in-kind contributions and
NSERC for their financial support of the Lignin project.
To my wonderful parents, Massoud Fazl and Nahid Rakizadeh and my precious sisters and their partners
Maryam & Julian and Mahrokh & Ravi, I am eternally grateful to your unconditional love, unwavering
support, and constant encouragement. To my fiancé, Daniel Liao, who never stopped having faith in my
abilities and inspires me to no end, I thank you from the bottom of my heart for being my pillar. I also
extend my deep gratitude to my amazing friends (the strongest women I know) for keeping me sane and
grounded.
Lastly, I would like to dedicate my work to my dear grandfather, who even in his absence, continues to be
my shining light.
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Table of Contents
Table of Contents ........................................................................................................................... iv
List of Tables ................................................................................................................................ vii
List of Figures ................................................................................................................................ ix
List of Appendices ........................................................................................................................ xii
1 INTRODUCTION...................................................................................................................... 1
1.1 Background ......................................................................................................................... 1
2 LITERATURE REVIEW ........................................................................................................... 3
2.1 Composite Theory ............................................................................................................... 3
2.1.1 Reinforcing Mechanisms ........................................................................................ 3
2.1.2 Fibre Matrix Interactions ........................................................................................ 7
2.1.3 Composite Failure Modes ....................................................................................... 8
2.1.4 Prediction of Mechanical Properties ....................................................................... 8
2.2 Biocomposites ................................................................................................................... 11
2.2.1 Cellulose Fibre ...................................................................................................... 12
2.2.2 Lignin .................................................................................................................... 14
2.2.3 Polylactic Acid ...................................................................................................... 16
2.3 PLA-Natural Fibre Composites ........................................................................................ 19
2.3.1 Past and Current Research .................................................................................... 19
2.3.2 Challenges ............................................................................................................. 21
2.4 Research Objectives .......................................................................................................... 23
3 MATERIALS AND METHODS ............................................................................................. 24
3.1 Raw Materials ................................................................................................................... 24
3.1.1 Natural Fillers and Reinforcement ........................................................................ 24
3.1.2 Matrix .................................................................................................................... 24
v
3.2 Raw Material Processing .................................................................................................. 25
3.2.1 Fibre Preparation ................................................................................................... 25
3.2.2 Lignin Processing .................................................................................................. 26
3.3 Composite Processing ....................................................................................................... 26
3.3.1 Aqueous processing .............................................................................................. 26
3.3.2 Thermal compounding .......................................................................................... 27
3.3.3 Compression Moulding ......................................................................................... 28
3.4 Characterization ................................................................................................................ 29
3.4.1 Water uptake ......................................................................................................... 29
3.4.2 Mechanical testing ................................................................................................ 29
3.4.3 Dynamic mechanical analysis ............................................................................... 30
3.4.4 Scanning electron microscopy .............................................................................. 30
3.4.5 Differential Scanning Calorimetry ........................................................................ 31
4 RESULTS AND DISCUSSIONS ............................................................................................ 32
4.1 Kraft pulp fibre in emulsion type and extrusion grade PLA ............................................. 32
4.1.1 Mechanical properties ........................................................................................... 33
4.1.2 Fracture surface morphology (SEM) .................................................................... 37
4.1.3 Dynamic mechanical properties (DMA) ............................................................... 38
4.1.4 Water absorption properties .................................................................................. 40
4.1.5 Melting and crystallization behavior .................................................................... 42
4.1.6 Summary ............................................................................................................... 44
4.2 Modified Kraft Pulp Fibre in Emulsion-type PLA ........................................................... 45
4.2.1 Mechanical properties ........................................................................................... 46
4.2.2 Fracture surface morphology ................................................................................ 49
4.2.3 Dynamic mechanical properties ............................................................................ 49
4.2.4 Water absorption properties .................................................................................. 51
vi
4.2.5 Summary ............................................................................................................... 53
4.3 Lignin as a filler in HDPE and PLA ................................................................................. 54
4.3.1 Lignin as biofiller in HDPE .................................................................................. 54
4.3.2 Lignin as a compatibilizer in PLA-KF composites ............................................... 56
4.3.3 Summary ............................................................................................................... 61
5 Conclusion and Recommendations .......................................................................................... 62
6 References ................................................................................................................................ 64
7 Appendices ............................................................................................................................... 72
vii
List of Tables
Table 1: Possible fibre-matrix interactions in composite materials ................................................ 7
Table 2: Commodity bioplastics (Endres and Siebert-Raths, 2011) ............................................. 11
Table 3: Dimensions of various Kraft pulp fibres (Hutten, 2007) ................................................ 13
Table 4: PLA material properties (NatureWorks LLC) ................................................................ 18
Table 5: Summary of mechanical properties of PLA-natural fibre composites ........................... 20
Table 6: Lignin by-product specifications (Courtesy of Macoma Canada Inc.) ........................... 24
Table 7: Landy PL series product specifications (Miyoshi Oil and Fat Co) ................................ 25
Table 8: Composites prepared using aqueous processing ............................................................. 27
Table 9: Composites prepared using thermal compounding ......................................................... 28
Table 10: Compression moulding process conditions .................................................................. 28
Table 11: Summary of tensile properties for ePLA and pPLA composites, shown with standard
deviations (samples sizes given in Appendix B of this thesis) ..................................................... 33
Table 12: DMA results for ePLA and pPLA composite systems ................................................. 40
Table 13: Water absorption results for ePLA and pPLA composites over a 24-hour period, shown
with standard deviations (samples size, N = 5)............................................................................. 41
Table 14: Melt and crystallization data for pPLA-KF composites, shown with standard deviations
(sample size N = 4) ....................................................................................................................... 43
Table 15: DMA results for ePLA-KFC composites ..................................................................... 50
Table 16: Equilibrium water absorption for ePLA-KF and ePLA-KFC composites with standard
deviations (Sample sizes given in Appendix D) ........................................................................... 52
viii
Table 17: Influence of particle size on mechanical properties with standard deviations (sample
size, N = 12) .................................................................................................................................. 55
Table 18: Dynamic mechanical properties of pPLA-KF-USL composites compared to neat pPLA
and pPLA-KF composites ............................................................................................................. 60
Table 19: Elimination of tensile strength ouliers in ePLA-KF series ........................................... 73
Table 20: ePLA-KFC tensile test raw data and ANOVA ............................................................. 75
Table 21: ePLA-KFC comparison of treatments using LSD method ........................................... 75
Table 22: ePLA-KF tensile test raw data and ANOVA ................................................................ 76
Table 23: ePLA-KF comparison of treatments using LSD method .............................................. 76
Table 24: pPLA-KF tensile test raw data and ANOVA ............................................................... 77
Table 25: pPLA-KF comparison of treatments using LSD method ............................................ 77
Table 26: pPLA-KF-USL tensile test raw data and ANOVA ....................................................... 78
Table 27: pPLA-KF-USL comparison of treatments using LSD method ..................................... 78
Table 28: HDPE-USL tensile test raw data and ANOVA ............................................................ 79
Table 29: HDPE-USL comparison of treatments using LSD method .......................................... 79
Table 30: Density raw data of all composites and ANOVA ......................................................... 80
Table 31: Density comparison of treatments using LSD method ................................................. 80
ix
List of Figures
Figure 1: Classification of composite systems ................................................................................ 4
Figure 2: Fibre reinforced composites (a) Continuous unidirectional fibre; (b) discontinous
unidirectional short fibre; (c) discontinous randomly aligned short fibre ...................................... 5
Figure 3: Stress distribute along a short fibre-reinforced composite (Callister & Rethwisch, 2011)
......................................................................................................................................................... 5
Figure 4: Stress-position profiles of an FRC with an applied force equal to the fibre tensile
strength when the length of embedded fibre is (a) equal to the critical length Lc; (b) longer than
Lc; and (c) shorter than Lc (Callister and Rethwisch, 2011) .......................................................... 6
Figure 5: Failure modes for fibre-reinforced composites ............................................................... 8
Figure 6: Cellulose fibre hierarchy (plant cell diagram courtesy of The National Academy of
Science) ......................................................................................................................................... 12
Figure 7: Lignin is thought to be the product of enzymatic dehydrogenation polymerization of
three phenylpropanoid monomers (L-R): coniferyl, sinapyl and p-coumaryl alcohols (Lebo et al.,
2001) ............................................................................................................................................. 15
Figure 8: Polymerization mechanism for PLA (Henton et al., 2005) ........................................... 17
Figure 9: Schematic of typical DMA storage modulus and loss tangent curves .......................... 30
Figure 10: (a) ePLA-15KF and (b) pPLA-15KF after compression moulding ............................ 32
Figure 11: Normalized tensile modulus (a) tensile strength (b) and elongation at break (c) for
Kraft pulp fibre-reinforced ePLA and pPLA composites. Error bars represent standard deviation.
....................................................................................................................................................... 35
Figure 12: Comparison of theoretical normalized composite elastic modulus (calculated using
HT model) and experimental tensile moduli for Kraft pulp fibre-reinforced ePLA and pPLA
composites. Ec,random is the theoretical composite elastic moduls with randomly aligned fibres, EL
x
and ET are the longitudinal and transverse moduli, respectively. Error bars represent standard
deviation. ....................................................................................................................................... 36
Figure 13: Comparison of theoretical composite tensile strength (HT model) to experimental
values at corresponding elongation for each composite system. Error bars represent standard
deviation. ....................................................................................................................................... 36
Figure 14: Fracture surface of tensile bars for (a) ePLA-15KF and (b) pPLA-15KF .................. 38
Figure 15: Storage modulus and loss tangent curves for neat ePLA and pPLA films, obtained
through DMA. ............................................................................................................................... 38
Figure 16: Work lost during unloading in imperfectly elastic materials ...................................... 39
Figure 17: Water uptake of neat ePLA and pPLA over a 24 hour interval .................................. 40
Figure 18: Normalized water uptake of ePLA-KF and pPLA-KF composites after 24 hours. Error
bars represent standard deviation. ................................................................................................. 42
Figure 19: DSC thermograph of neat pPLA and ePLA ................................................................ 43
Figure 20: Glass transition temperatures of ePLA-KF and pPLA-KF composites ...................... 43
Figure 21: Composite films after compression moulding (a) ePLA-5KF and (b) ePLA-5KFC .. 46
Figure 22: Tensile properties of ePLA-KF and ePLA-KFC composites (a) Tensile modulus; (b)
tensile strength and (c) elongation at break. Error bars represent standard deviation .................. 47
Figure 23: Theoretical elastic modulus of ePLA-KFC composites (HT model) compared to
experimental values ...................................................................................................................... 48
Figure 24: Comparison of theoretical and experimental tensile strength of ePLA-KFC
composites. Error bars represent standard deviation. ................................................................... 48
Figure 25: Fracture surface of an ePLA-15KFC tensile bar specimen ......................................... 49
Figure 26: Storage modulus and loss tangent curves of ePLA-20KF and ePLA-20KFC
composites ..................................................................................................................................... 51
xi
Figure 27: Water absorption curves of ePLA-KF and ePLA-KFC composites monitored over 2
hours .............................................................................................................................................. 52
Figure 28: Tensile properties of HDPE-USL composites (a) tensile modulus; (b) tensile strength.
Error bars represent standard deviation. Sample size is given in Appendix B. ............................ 56
Figure 29: pPLA-KF-USL tensile properties (a) tensile modulus; (b) tensile strength; (c)
elongation at break. Error bars represent standard deviation. Sample sizes are given in Appendix
B. ................................................................................................................................................... 58
Figure 30: Transition region temperatures of neat pPLA, pPLA-KF and pPLA-KF-USL
composites. Error bars represent standard deviation. ................................................................... 60
xii
List of Appendices
Appendix A : Calculation of Tensile Modulus ........................................................... 72
Appendix B : Statistic Analysis .................................................................................. 73
Appendix C : DMA Curves ........................................................................................ 81
C-1: ePLA-KF vs pPLA-KF .................................................................................................... 81
C-2: ePLA-KF vs ePLA-KFC ................................................................................................. 82
Appendix D : Water Absorption Data ......................................................................... 83
D-1: ePLA-KF water absorption data ...................................................................................... 83
D-2: pPLA-KF water absorption data ...................................................................................... 84
D-3: ePLA-KFC water absorption data ................................................................................... 85
1
1 INTRODUCTION
1.1 Background
Research and development in the field of biodegradable composites has been on the rise in the recent
history of material science, with applications being explored both in academia and industry worldwide.
There are many incentives for the use of biodegradable composites in place of conventional petroleum-
derived materials including reduction of environmental impact, offsetting petroleum consumption and
improving waste management associated with plastic products. In the past three decades, special attention
has been paid to the use of biopolymers derived from annually renewable crops strengthened with natural
fibres.
Currently, a major class of bioplastics includes hydrolysable polyesters such as polylactic acid (PLA),
polyglycolic acid (PGA) and polycaroplactone (PCL) (Uhrich et al., 1999). Although these polymers
cannot compete economically with their petroleum counterparts, their use in products with short lifetimes
continues to grow as consumer demand for eco-friendly products increases. PLA has been at the forefront
of this growth, as it is a commercially available commodity plastic in industry today. Though its use was
limited to biomedical applications in the early stages of research (Kulkarni et al., 1966; Cutright et al.,
1971; Zhou and Chang, 1988; Majola et al., 1992) , the commercialization of PLA production by Cargill
Dow in 2002 made it possible for the polymer to be used in other industries as a commodity bioplastic
(Gruber, 2008). PLA has excellent material properties including tensile modulus of 3.5 GPa and tensile
strength of 53 MPa (NatureWorks LLC), which either match or exceed those of widely used commodity
plastics such as polyethylene and polypropylene. Furthermore, PLA can be easily degraded to its
monomer units through its hydrolysable ester bonds and subsequently composted. Conventional methods
currently used in industry, including compression moulding, extrusion, injection moulding, solvent
welding, and film casting can be used to process PLA products, as well as PLA-based composite
materials (Nishikawa et al., 2009; Huda et al., 2006; Myllytie et al., 2010; Bledzki et al., 2009). While the
2
use of petroleum-displacing biopolymers like PLA can reduce CO2 emissions and waste accumulation,
these materials must also be used in a way that is economically sustainable, with long-term use
applications such as automotive parts and building materials. One way to achieve this is through natural
fibre reinforcement, which not only reduces costs but also improves product performance.
The reinforcing capability of natural fibres has been known for centuries even before the mechanics of it
was resolved, as is evident by primitive building materials such as straw-reinforced mud bricks. Natural
fibres hold many advantages over synthetic fibres, as they are widely abundant, less costly, and non-
abrasive. In current years, automotive industries in Europe and North America have been manufacturing
vehicles with partially biodegradable parts, made from fibres such as kenaf, jute, flax and hemp and
petroleum thermoplastics such as polypropylene and polyethylene (Thomas & Pothan, 2009). Cellulose
fibre from pulp has also been studied and incorporated into composites with various applications (Hsieh,
2004; Samir et al., 2005).
In addition to natural fibres, a material that has been garnering much attention in the biomaterials field is
lignin. The potential applications of lignin are demonstrated through a number of publications, with
successful uses as dispersants and flocculants, binding agents, viscosity controlling agents, adhesives and
complexing agents (Lebo et al., 2001). The major source of lignin has traditionally been the pulp and
paper industry, where kraft and sulfate lignins (also called industrial lignin) are major by-products.
However, the emergence of biorefineries for production of biofuels and sugar products has created a new
source of lignin product with different properties than industrial lignins. Applications of biorefinery
lignin include its use as a matrix or filler in bio-composites and a component of polymer blends
(Sevastyanova et al., 2010; Pucciariello et al., 2007; Chen et al., 2009).
There are several challenges associated with the use of natural fibre, filler and biopolymers to develop
biodegradable composites. The major issues currently under study are material incompatibility, poor
fibre-matrix adhesion, and moisture sensitivity. The purpose of this study was to develop biodegradable
natural fibre-reinforced composites from Kraft pulp fibre and polylactic acid with a focus on improving
3
fibre-matrix interactions and overall composite physical properties. Different composite processing
techniques and a fibre modification method were used to achieve this.
2 LITERATURE REVIEW
2.1 Composite Theory
When developing composite materials, scientists attempt to achieve or improve ideal properties that
homogeneous materials fail to demonstrate on their own. Some of these properties may include specific
strength, anti-corrosion characteristics, environmental compatibility, thermal properties, surface
properties, anti-decay capabilities, and flame retardancy. By combining two or more materials with
known desired characteristics, a composite with the best combined properties can be achieved. At the
same time, the cost to performance ratio must be favorable for a composite material to be feasible for use
in industry. In order to achieve this, several factors must be considered in developing composites, namely
the type of composite system, choice of materials, and compatibility between phases as well as processing
methods. This section provided an overview of composite mechanics theory, processing methods, as well
as relevant applications in past and current literature.
2.1.1 Reinforcing Mechanisms
Composite materials can be classified under three major categories based on the nature of the
reinforcement, as is shown in Figure 1. Particle reinforced composites consist of a soft matrix phase and a
more brittle particulate phase that is evenly dispersed in the matrix. The reinforcing mechanism can either
be dispersion-strengthening where very small particles block the movement of the matrix phase, such as
thoria-dispersed nickel (Askeland, 1988), or true particulate reinforcement where large particles impart
unique properties to the matrix phase, such as rubber reinforced with carbon black to induce wear
resistance (Chung, 2010). Structural composites include laminates, in which a continuous ply of each
4
phase is stacked in various orientations and cured together, and sandwiches which contain three distinct
layers, namely two thin facings and a thick core. Mallite is an example of a sandwich panel used in
automotives in which aluminum facings are bonded to end-grain balsa wood (Kulshreshtha & Vasile,
2002).
Figure 1: Classification of composite systems
Fibre-reinforced composites (FRCs) consist of a brittle and strong fibrous reinforcing phase, surrounded
by a more ductile matrix phase. Addition of fibres to a matrix phase improves strength, stiffness and
fatigue resistance, as most of the force applied to the composite is carried by the fibre phase. FRCs can be
furthered classified based on the nature of fibre reinforcement, as shown in Figure 2. Unidirectional
continuous fibre reinforced composites have anisotropic properties, and are designed to give the best
performance when the direction of applied load is parallel to the orientation of the fibres. Discontinuous
short fibre composites may contain unidirectional or randomly aligned fibres, and demonstrate less load
carrying ability than continuous fibre composites. The applications of fibre-reinforced composites are
numerous, ranging from centuries-old straw-reinforced mud bricks, to fiberglass reinforcement in
aerospace composite materials.
5
Figure 2: Fibre reinforced composites (a) Continuous unidirectional fibre; (b) discontinous
unidirectional short fibre; (c) discontinous randomly aligned short fibre
The mechanical properties of FRCs are dictated by a number of parameters, including the individual
properties of the matrix and fibre, proportions of each phase and the strength of interfacial bonding
between the two phases. Ultimately, a composite material should display improved mechanical properties
compared to the neat polymer. Improvement may manifest itself as an increase strength, in which case the
interfacial bond between the matrix and fibre is strong enough to induce stress transfer from the softer
phase into the more brittle phase, resulting in a composite that is stronger than the neat polymer. Figure 3
demonstrates the stress distribution along a short fibre-reinforced composite in which the interfacial bond
is strong enough to keep the fibre embedded in the matrix as a force is applied, and stress transfer
between the matrix and fibre is effectively achieved.
Figure 3: Stress distribute along a short fibre-reinforced composite (Callister & Rethwisch, 2011)
In order to maximize the stress transfer, a critical length of fibre, Lc, must be embedded in the matrix. This
critical length depends on the fibre diameter, d, and tensile strength, σf. At this critical length, the applied
load is countered by the interfacial bond strength, τb, as is shown in equation 1, and the maximum fibre
6
strength is achieved at the midpoint of the fibre length. Fibre lengths longer than Lc provide the best stress
transfer, while any length less than Lc fails to achieve the maximum fibre strength. The stress-position
profiles are demonstrated in Figure 4. Another desirable property may be fracture resistance, which can
be achieved when the interfacial bond is low enough to allow toughening mechanisms such as debonding
and fibre pull-out to occur when a force is applied (Matthews & Rawlings, 1994).
(Equation 1)
Figure 4: Stress-position profiles of an FRC with an applied force equal to the fibre tensile
strength when the length of embedded fibre is (a) equal to the critical length Lc; (b) longer
than Lc; and (c) shorter than Lc (Callister and Rethwisch, 2011)
7
2.1.2 Fibre Matrix Interactions
Interactions between fibre and matrix surfaces can be achieved through a number of adhesive
mechanisms, including electrostatic interactions, chemical bonding, adsorption and wetting, diffusion,
and mechanical interlocking (Hull & Clyne, 1996). The mechanisms are briefly described in Table 1.
Table 1: Possible fibre-matrix interactions in composite materials
Adhesion
Mechanism Description Application to FRCs
Electrostatic
interaction
In cases where the constituents carry electrical
charges opposite to one another, attractive
forces may be sufficient to create an adhesive
bond. However, this interaction can easily be
reversed in the presence of polar solvents and is
not a major source of adhesion in composite
materials.
If the matrix phase carries a net
electrical charge, adhesion can be
promoted by modifying the fibre
surface with oppositely charged
coupling agents, such as ionic
functional silanes (Hull & Clyne,
1996).
Diffusion
Depending on the nature and compatibility of
constituent surfaces, various diffusion
processes can occur at the interface, where
molecules of the two materials diffuse into one
another. This diffusion zone is ideal because it
creates a gradual change in material, thereby
reducing stress concentration at the interface.
Solvent welding is one method of
creating an effective diffusion zone
between the fibre and matrix
phases.
Mechanical
Interlocking
The extent of physical interaction between two
phases dictates the ease with which adhesive
failure can occur at the interface. When
effective mechanical interlocking is achieved,
such as a key and lock interaction, crack
propagation becomes more energy intensive
due to a tortuous crack path at the interface.
Various fibre modifications,
including alkali treatment, can
increase fibre surface roughness
and therefore fibre-matrix
mechanical interlocking (Valdez-
Gonzalez et al., 1999).
Adsorption
and wetting
In order for effective contact to be achieved at
the interface, the solid phase must be wettable
by the liquid phase. Generally, this can be
achieved when the surface energy of the solid
phase exceeds that of the liquid.
Fibre wettability can be modified
through several methods, including
removal of surface impurities or
coating with material of desirable
surface energy.
Chemical
reactions
Interfacial chemical reactions can lead to
various intermolecular bonds, including
covalent, lewis acid/base and donor-receptor
type interactions. These reactions may occur
naturally between chemically reactive
constituents, or induced through processing
techniques.
Chemical bonds between fibre and
matrix surfaces can be achieved
with the use of cross-linking
agents, or heat treatments (Lee,
1992).
8
2.1.3 Composite Failure Modes
Fibre-reinforced composites are susceptible to failure through various mechanisms, including fracture
failure, tensile delamination and shear delamination, demonstrated in Figure 5 (Soboyejo, 2002).
Delamination failures are associated with cracking and debonding at the fibre-matrix inteface, that can
occur as a tensile (5b) or shear force (5c) is applied to the composite. A major failure mechanism in short
fibre composites under uniaxial load has been shown to be fracture failure (Figure 5a) arising from
interfacial debonding and fibre pull-out (Wambua et al., 2003; Gassan, 2001; Ray et al., 2001). Fibre
fracture and matrix failure can also contribute to fracture failure in these composites.
Figure 5: Failure modes for fibre-reinforced composites
2.1.4 Prediction of Mechanical Properties
Rule of Mixtures
Material properties for continuous aligned fibre composites can be predicted using Rules of Mixtures
(ROM), wherein properties are calculated based on the volume weighted average of each composite
constituent. ROM can be used to determine the upper and lower limits of elastic modulus of composites.
The upper limit is calculated as the longitudinal elastic modulus, where a load is applied parallel to fibre
alignment. The derivation is possible by assuming perfect bonding between the fibre and matrix, resulting
in an iso-strain condition under uniaxial load parallel to fibre direction. Furthermore, the lower limit is
calculated as the transverse elastic modulus, where the applied load is perpendicular to fibre alignment. In
this scenario, the strain on the matrix and fibre is no longer the same, and the derivation is made using an
iso-stress condition. The longitudinal elastic modulus, EL, and transverse elastic modulus, ET for
9
composites are given in Equations 2 and 3, respectively. Em and Ef denote the matrix and fibre elastic
moduli, respectively, and vm and vf are the matrix and fibre volume fractions, respectively.
(Equation 2)
(Equation 3)
ROM can be a useful prediction tool for continuous fibre composites and aid in material selection;
however, a more complex derivation is required for short fibre composites.
Shear-lag Theory
The Shear-lag theory was first used by Cox in 1952 to analyze stress transfer in short fibre-reinforced
composites. This theory assumes that stress transfer from the matrix to the fibre occurs through shear
stresses, and uses a force balance at the interface to derive relationships for fibre tensile stress, σf, and
interfacial shear stress, τi*, given in Equations 4 and 5, respectively. This theory assumes that fibres all
have the same length, with aspect ratio s, and are aligned in the direction of applied load.
Equation 4
Equation 5
Equation 6
Furthermore, the composite tensile stress, σc,ROA, can be predicted using the Rule of Averages, by
equating an applied force to the sum of the volume-average stresses of the fibre (derived above) and the
matrix, as is shown in Equation 7. It is assumed that uniform strain, ε, is experienced by the matrix and
10
composite; therefore the average matrix stress is given by σm = Emε. The tensile modulus of short fibre
composites is given in Equation 8.
Equation 7
Equation 8
Random Orientation – Halpin-Tsai Model
Theoretically, a composite gives the best performance when all fibres are aligned in the direction of the
applied force. However, fabricating such composites may not always be cost-effective and practical.
When predicting the mechanical properties of such composites, it is important to take the impact of
random fibre orientation into account. The Halpin-Tsai model (HT) calculates the upper limit (EL) and
lower limit (ET) elastic modulus of a short-fibre composites, given in Equations 9 and 10, respectively
(Sperling, 2005). This model makes use of a geometrical factor, ξ, which describes the aspect ratio of the
short fibres; L and d denote the fibre length and diameter, respectively. The fibre dimensions are
determined experimentally through fibre quality analysis.
where, Equation 9
where, Equation 10
Equation 11
In the case of randomly oriented short fibres, the elastic modulus can be estimated as a value
between the upper and lower limit, as calculated by the HT model (Zadorecki, 1986;). The random
orientation composite modulus is given in Equation 12.
Equation 12
11
2.2 Biocomposites
Although most composite theories have been developed based on synthetic materials, they hold true for
biocomposites as well. In recent decades, interest in replacing synthetic fibres with natural reinforcing
agents has been on the rise and numerous natural materials have been used as fillers and reinforcing
materials in order to reduce costs and environmental impacts. Natural fillers can be derived from plants
(fibrous materials consisting mostly of cellulose), animals (polypeptides and polysaccharides), and
minerals (asbestos, mica).
The global production capacity of biopolymers stands at 1.4 million tons/year (March 2011) and is
expected to grow to 2.8 million tons/year by the year 2015 (Endres and Siebert-Raths, 2011). As
biopolymers become more accessible, their use in composite materials becomes both technically and
economically feasible. A number of commercially available biopolymers are listed in Table 2.
Table 2: Commodity bioplastics (Endres and Siebert-Raths, 2011)
Biopolymer Global Capacity (% of 1.4 million tons/year)
Biodegradable starch blends 26.9%
Polylactic acid (PLA) 16.5%
Bio-polyethylene 14.2
Polyvinyl alcohol (PVAL) 10.4%
Biodegradable polyesters 8.7%
Bio-polyvinyl chloride (PVC) 8.5%
Polyhydroxyalkanoate (PHA) 7.3%
Regenerated cellulose 2.6%
12
The focus of this thesis is on polylactic acid reinforced with lignocellulosic materials, namely cellulose
fibre from hardwood Kraft pulp, and biorefinery lignin. Cellulose and lignin are the most abundant
naturally occurring and renewable organic materials on the planet, and both are important structural
components of plant cell walls. This section provides an overview of the characteristics of these raw
materials and their suitability as constituents in biodegradable composites.
2.2.1 Cellulose Fibre
Cellulose is a natural polymer comprised of repeating d-glucose units linked by glycosidic bonds. Surface
functionalities of cellulose include one primary and two secondary hydroxyl groups, which induce
crystallinity and allow chains to be arranged in closely packed hierarchical arrangements of microfibrils,
fibrils, and fibres, as shown in Figure 6. Crystallinity in cellulose fibres can be as high as 75% (Hon,
2001), a characteristic that imparts strength and stiffness to the fibre.
Figure 6: Cellulose fibre hierarchy (plant cell diagram courtesy of The National Academy of
Science)
13
There are a number of major commodity cellulose fibres available in the global market, each with specific
characteristics and applications. Cotton fibres have a long history in Pulp and Paper and textile industries
and have the highest cellulose content compared to all other fibre sources (Hsieh, 2004). In today’s
market, however, wood is the major source for industrial cellulose and its isolation from the complex
network of hemicelluloses and lignin within the plant cell wall requires pulping and chemical purification
(French et al., 2003).
Traditionally pulp fibre has been used in conventional paper products such as newsprint, tissue and
paperboard. However, the rapid growth of digital media has resulted in declining demand for such
products, and emergence of competitive markets steers traditional pulp production in places like Canada
away from conventional products and more towards innovative applications. Furthermore, increasing
demand for renewable materials in the face of petroleum shortages and unsustainable waste management
creates an opportunity for cellulose fibre to act as a high value constituent for biomaterials. Pulp fibre
displays many desirable material properties, such as high strength, high aspect ratio, and non-abrasiveness
that make it an attractive candidate as a reinforcing agent in composite materials. The aspect ratio of pulp
fibre depends on the tree species, some of which are presented in Table 3.
Table 3: Dimensions of various Kraft pulp fibres (Hutten, 2007)
Species Length (mm) Diameter (μm) Aspect Ratio
Eucalyptus (HW) 1.0 13 77
Birch (HW) 1.9 28 68
Aspen (HW) 1.1 18 61
Beech (HW) 1.2 18 67
Redwood (SW) 6.1 58 105
Douglas Fir (SW) 3.8 40 95
West. Red Cedar (SW) 3.5 35 100
Slash Pine (SW) 4.6 40 115
Loblolly Pine (SW) 3.5 40 88
14
Several studies have shown that the addition of wood pulp fibre, such as Columbus Pine (Ludvik et al.,
2007), Northern Black spruce (Ayan Chakrobaty, 2006), and hardwood maple (Li and Matuana, 2001), to
petroleum derived and renewable polymers results in an improvement of composite properties, including
tensile properties and thermal stability.
2.2.2 Lignin
Lignin is the second most abundant biopolymer after cellulose, and makes up approximately 30% of
organic carbon in the biosphere. The function of lignin in nature is to impart structural integrity to plant
cell walls, preventing water permeation out of cells in the xylem, and protecting plant cells against
pathogens by providing an impermeable barrier (Boerjan et al., 2003). Lignin is one of the three main
components of trees (including cellulose and hemicelluloses) and makes up 24-33% in softwood and 19-
28% in hardwood tree species.
A major source of lignin is the pulp and paper industry, from which 8x105 tons of lignin is produced
every year (Lebo et al., 2001). The types of industrial lignin that are currently available are classified
based on the method by which the lignin was isolated from woody mass. The major classes of industrial
lignin include lignosulfonates derived from sulfite pulping, and kraft lignin isolated from kraft pulping.
Other classes of lignin include isolation by organosolv, acid hydrolysis and enzymatic hydrolysis. The
latter is a process used in biorefinery industries, namely from cellulosic ethanol production. This type of
lignin has not been as extensively studied as the pulp and paper lignins, and is gaining new interest among
material scientists.
2.2.2.1 Structure and Characteristics
Lignin is a three dimensional amorphous polyphenolic compound, made up of three hydroxycinnamyl
alcohols with no regularly repeating units. The three monomers or monolignols are coniferyl, sinapyl and
p-coumaryl alcohols (Figure 7).
15
Figure 7: Lignin is thought to be the product of enzymatic dehydrogenation polymerization of three
phenylpropanoid monomers (L-R): coniferyl, sinapyl and p-coumaryl alcohols (Lebo et al., 2001)
Isolation of lignin in pulping methods requires that it first be made soluble by sulfonation at the side chain
of the phenylpropane unit, then purified through precipitation or using methods like ultrafiltration. The
isolated lignin assumes a different structure and properties depending on the method of isolation. There is
great variability in the recorded molecular weight of lignin, ranging from 3,000 to 20,000 g/mol, while
polydispersities range from 2-12 (Saake et al., 2007).
Lignin isolated from biorefinery processes in which woody mass is delignified through steam explosion
(SE) and enzymatic hydrolysis (EH). In this process, woody mass at a 50% moisture point is heated and
pressurized, and subsequently goes through a rapid decompression to activate wood fibre. Following
decompression, activated fibre then goes through enzymatic hydrolysis to separate the cellulose and
hemicelluloses from the lignin. After hydrolysis, a solid-liquid separation unit isolates the hydrolyzed
sugars in the liquid stream and the lignin in the solid stream. The resulting SE-EH lignin is highly
condensed and relatively unreactive compared to kraft and sulfate lignins due to cleavage of β-O-4
linkages and C5 condensation reactions (Li et al., 2007). It also contains impurities including residual
sugars (monomer, oligomer) and spent enzymes from EH.
2.2.2.2 The case for lignin bio-filler
With the rapid increasing demand on energy and fuel globally and increasing reliance on fossil fuel, the
need to be able to produce energy and chemicals from renewable forest biomass becomes ever more
16
important. The utilization of significant amount of lignin rich residue during the cellulosic ethanol
production is a major consideration for the bio-ethanol producers. Successful transformation of lignin
residue to value added bio-based products will help biorefineries develop more economical viable
process, and at the same time provides an opportunity for these industries to be leaders in implementing
innovative and environmentally friendly initiatives in the energy sector.
2.2.3 Polylactic Acid
2.2.3.1 PLA Chemistry
Lactic acid (2-hydroxypropanoic acid) is the building block of PLA and it has been produced and used
commercially since the 19th century, the first instance of use being in the food industry as a buffering
agent (Hartmann, 1998). Though it can be chemically synthesized, lactic acid is most commonly
manufactured through fermentation of biomass feedstock, including corn, sugar cane and most recently
agricultural wastes (Garde et al., 2000; Sodergaard 2000). The nature of the microorganisms used in the
fermentation process dictates the stereochemistry of the resulting products and the ratio of D(R) and L(S)
lactic acid in the fermentation broth (Degée & Dubois, 2004).
Polymerization of lactic acid into linear aliphatic polyester chains can be carried out by two methods:
polycondensation of lactic acid monomers or ring opening polymerization of lactides (Figure 8). The
former produces low molecular weight polymers (Mn’ of up to 6500), while the latter yields products
with higher molecular weight by using lactides oligomers (condensation intermediates) in a catalytic ring
opening polymerization reaction. This method was used by Cargill Dow LLC to develop a proprietary
continuous process that simultaneously produces lactides and PLA in a melt, making the commercial
production of PLA both technically and economically feasible (Gruber, 2008). Since lactic acid has two
optically active configurations, PLA properties can vary depending on the composition of the D and L
isomers. The isotactic homopolymers poly(L-lactide) and poly(D-lactide) can be highly ordered and
crystalline, because they are synthesized from lactides monomers with the same optical configuration,
17
whereas a mixture of the enantiomers can form racemic stereocomplexes with amorphous regions
(Henton et al., 2005).
Figure 8: Polymerization mechanism for PLA (Henton et al., 2005)
The key feature of PLA that makes it an ideal biodegradable plastic is the presence of hydrolysable ester
bonds in the polymer back bone that allow the polymer chain to break down into low molecular weight
oligomers (Lunt et al., 1998) degraded into CO2 and water by microorganisms (Tokiwa and Calabia,
2006). Hydrolytic degradation can be assisted by the presence of amorphous regions, which can be
controlled through isomer composition in the polymerization feedstock. Commonly used grades of PLA,
including PLA used in this thesis, are semicrystalline polymers, synthesized using L-lactide as the major
constituent, with small amounts of D-lactide and meso-lactide.
18
2.2.3.2 Material properties and Processing
Polylactic acid (PLA) is the first commercially available bioplastic that is derived from annually
renewable crops or biomass, and is produced using low energy processes compared to the petroleum
derived counterparts (Henton et al., 2005). Conventional processing methods currently used in industry,
including compression moulding, extrusion, injection moulding, solvent welding, and film casting can be
used to produce PLA products, as well as PLA-based composite materials (Huda et al., 2009; Myllytie et
al., 2010; Bledzki et al., 2009). Table 4 summarizes material properties of PLA relevant to composite
processing.
Table 4: PLA material properties (NatureWorks LLC)
Property Extrusion / Thermoforming Injection
Moulding Film & Sheets
Specific Gravity 1.24 1.24 1.24
MFR (g/10min) 6 14-80 6-10
Relative Viscosity 4.0 2.5-3.3 3.4-4.0
Peak Melt Temp (°C) 145-160 145-170 145-170
Glass Transition Temp (°C) 55-60 55-60 55-60
Tensile Yield Strength (MPa) 60 62 -
Tensile Strength at break (MPa) 53 54 -
Tensile Modulus (GPa) 3.6 3.7 -
Tensile Elongation (%) 6 3.5 -
PLA is also available as an emulsion-type product that is marketed for use as thermal adhesives or
waterproof coatings (Landy PL Series, Miyoshi Oil & Fat Co.). However, some studies have used the
emulsion-type PLA as the matrix phase in natural fibre-reinforced composites (Nishikawa et al., 2009;
Sujito et al., 2011). Suitable processing methods for producing these composites include prepreg and
compression moulding.
19
2.3 PLA-Natural Fibre Composites
2.3.1 Past and Current Research
In the past two decades, several research groups have carried out studies on various aspects of natural
fibre-reinforced polylactic acid composites, including the Oksman group at Lulea University of
Technology, the Kunioka group at the National Institute of Advanced Industrial Science and Tehcnology,
the Sain group at the University of Toronto, and the Mohanty group at the University of Guelph.
Furthermore, several patents now exist that outline processing methods or a specific combination of
materials to produce biodegradable fibre reinforced PLA composites and products. These include the
patent by Roberts et al. (2010), which discloses environmentally friendly products for packaging and
agricultural applications made from PLA and Kenaf fibre, as well as the patent by Gamstedt et al. (2010),
which outlines a multi-step manufacturing process for producing lignocellulosic fibre reinforced
thermoplastic composites with reduced mechanosorptive creep. While patents are not as readily
accessible as scientific publications, they can provide insight on the applications and commercial viability
of natural fibre-reinforced PLA composites.
In academia, PLA-fibre composite research topics include studies on mechanical and thermal properties,
biodegradability assays, and development of novel composite processing techniques. Natural fibres used
in these studies include flax, kenaf and cellulose fibre, as well as agricultural wastes. The improvement of
mechanical properties of these composites has been an important topic for many researchers. Many
studies have been carried out to determine the impact of fibre type and processing methods on the tensile
strength, modulus and elongation at break of natural fibre-PLA composites. The following table
summarizes some these results.
20
Table 5: Summary of mechanical properties of PLA-natural fibre composites
Source Fibre Type Processing
Method
Tensile
Strength
Tensile
Modulus
Tensile
Elongation
Bodros et
al., 2006 Flax, 30% by weight Film stacking
Improved by
65%,
Improved
by 186%
Reduced by
38%
Oksman et
al., 2003 Flax, 40% by weight
Extrusion and
compression
moulding
No
improvement
Improved
by 110%
Reduced by
55%
Garcia et
al., 2007
Rice Husks, 30 % by
weight Extrusion and
injection
moulding
Reduced by
42%
Improved
by 40% N/A
Kenaf, 30% by weight Reduced by
10%
Improved
by 72%
Mathew et
al., 2006
Microcrystalline
cellulose (MCC), 25
wt% Extrusion and
injection
moulding
Reduced by
27%
Improved
by 39%
Reduced by
29%
Wood Pulp, 25 wt% Reduced by
9%
Improved
by 67%
Reduced by
21%
Wood Flour, 25 wt% Reduced by
9%
Improved
by 75%
Reduced by
29%
Huda et al.,
2005
Wood fibre (Maple
wood flour), 30 wt%
Extrusion and
injection
moulding
No
improvement
Improved
by 96% N/A
Ludvik et
al., 2007
SW Kraft pulp, 11
wt%
Reduced by
29%
Improved
by 38%
Reduced by
43%
Thunwall et
al., 2008
Defibrillated SW
sulfate pulp, 20 vol%
Injection
moulding
Improved by
8%
Improved
by 96%
Reduced by
79%
The Rule of Mixtures dictates that a composite material reinforced with fibres of greater elastic modulus
than the matrix will demonstrate a tensile modulus that is greater than that of the neat polymer. This was a
common observation in all studies where an improvement in tensile modulus was observed at all fibre
loadings. The greatest improvement was seen in the study by Bodros et al., where an improvement of
186% was achieved in PLA-Flax composites prepared using a film-stacking method. The improvement
21
here arises not only from the addition of fibre, but also the processing method, which allows for fibre
alignment and increased reinforcing capability.
The addition of fibre does not have a significant improvement tensile strength of these composites,
however, and in fact reduces the strength in composites with fibres from wood pulp and flour. This
behavior could attributed to poor fibre-matrix interactions and has been a challenge in the field of fibre-
reinforcement using natural materials. The study by Bodros et al. demonstrates one of few cases where an
improvement of tensile strength was achieved by means of film stacking, without any compatibilizers.
Film stacking, however, may not be economically favorable when compared to more common processing
methods such as extrusion and injection moulding of short-fibre composites.
Several studies have used a compatibilizer to improve the fibre-matrix interactions and in turn strength of
the composite. These works include the addition of cellulose esters to wood/PLA composites by Takatani
et al. (2007) to improve the mechanical properties, the use of MFC as a means to prevent crack growth in
PLA/Bamboo/MFC hybrid composites by Okubo et al. (2009), and the application of a novel carding
process combined with treatment of kenaf fibre with a silane coupling agent by Lee et al. (2009) to reduce
water swelling and improve the heat deflection properties.
2.3.2 Challenges
The properties of natural fibres vary not only between species but also depend strongly on the cultivation,
isolation and processing methods. In 2008, Ochi investigated the impact of Kenaf cultivation environment
as well as plant part usage on mechanical properties of Kenaf fibre-PLA composites. The test conditions
included the cultivation of Kenaf fibres at different temperatures (22°C and 30°C), as well as isolating
fibres from different parts of the Kenaf plant (based on length from the root). The most improvements to
tensile strength and modulus were seen in composites with fibres cultivated at higher temperatures, as
well as fibres isolated closest to the root of the plant. This study identifies a major challenge in using
natural fibre reinforcement, and demonstrates how different fibre handling and processing methods
22
directly impact composite properties. The use of regulated and standard processing methods, such as
chemical or mechanical pulping methods, can be one way to address this issue. The use of pulp fibre as a
high-value constituent in biocomposites can also be an opportunity for countries like Canada to steer
away from the conventional pulp and paper sector, and enter new forest product markets. In order to do so
successfully while maintaining high quality and performance standards, a number of technical challenges
associated with the use of pulp fibres must be overcome.
The factors that must be considered to achieve desirable mechanical properties in fibre-reinforced
composites include compatible surface chemistry of the fibre and matrix phases, corresponding surface
energies, as well as ideal interface. In the case of pulp fibre-reinforced composites, incompatibilities exist
due to the hydrophilic nature of fibre surface and the hydrophobic nature of the polymer. This leads to
insufficient bonding adhesion at the interface, as well as poor fibre dispersion, in turn resulting in non-
uniform material properties in the composites. Methods such as fibre surface modification or high shear
processing have been used to improve the fibre dispersion (Islam et al., 2010; O’Reilly & Cavaille, 1997);
however, these methods must be cost effective and most importantly, must not have any negative impact
on the fibre surface available for stress transfer. Finally, the problem of moisture sensitivity of the fibre
must be addressed, as water and moisture absorption in composites can result to failure and subpar
performance compared to composites reinforced with synthetic fibre.
23
2.4 Research Objectives
This thesis focused on the use of Kraft pulp fibre, PLA, and a biorefinery lignin by-product to develop
biodegradable composites. The specific objectives for this thesis were set out as follows:
Improve mechanical properties of Kraft pulp fibre-reinforced PLA composites by using an
emulsion-type polymer and aqueous processing
Improve Kraft pulp fibre dispersion in PLA and moisture sensitivity through a fibre surface
modification with organically modified nano-clay
Determine the viability of a biorefinery lignocellulosic by-product as a biofiller in high density
polyethylene (HDPE) and PLA
Determine the efficacy of the lignocellulosic by-product as a compatibilizer in Kraft pulp-
fibre/PLA composites
24
3 MATERIALS AND METHODS
3.1 Raw Materials
3.1.1 Natural Fillers and Reinforcement
Bleached Kraft pulp fibre (KF) containing >99% cellulose and an average aspect ratio of 37 was used in
this experiment (Domtar Company, Canada). Organically modified montmorrillonite nanoclays were
provided by Southern Clay Products, Inc (USA). Cloisite® 20A and 93A both are clays modified by
quaternary ammonium salts containing dehydrogenated tallow.
A lignin-rich by-product from a cellulosic ethanol conversion process was provided by Mascoma Canada
(Mississauga, Canada). The product specifications are provided in Table 6. The lignin was received as a
water-washed filter cake with varying moisture content, and stored at 4°C.
Table 6: Lignin by-product specifications (Courtesy of Macoma Canada Inc.)
Lignin content
(dry wt %)
Moisture content
(% theoretical)
Major impurities Source
50.8 47.9 Glucose, cellulose,
cellobiose, xylose
Poplar, Pine
3.1.2 Matrix
Emulsion type polylactic acid (ePLA) was provided by Myoshi Oil and Fat Co. Ltd (Japan). The Landy
PL series products are weakly anionic and consist of 40% polymer solids dispersed in solution. Product
specifications are given in Table 7. Extrusion grade PLA in pellet form (pPLA) was provided by Jamplast
Inc (IngeoTM
Biopolymer 2003D, U.S.A.), with a melt temperature of 185°C and specific gravity of 1.24.
Extrusion grade high density polyethylene with a melt index of 31.5g/10 minutes and density of
0.951g/cm3 was also used in this study.
25
Table 7: Landy PL series product specifications (Miyoshi Oil and Fat Co)
Product
name
Average particle size
(μm) Viscosity
(mPa.s)
Minimum film forming temp
(°C)
PL-1000 5 1000 160
PL-2000 2 1000 90
PL-3000 1 2500 20
3.2 Raw Material Processing
3.2.1 Fibre Preparation
TAPPI method T205 was used to produce pulp from Kraft fibre sheets, and water was removed using a
two-stage solvent exchange method with acetone and toluene. Fibres were dried for 48 hours at room
temperature, followed by oven-drying at 70°C for 24 hours.
A fibre surface modification method developed by Chen and Yan (2012) was used to adsorb organically
modified nano clay onto fibre surfaces. Clay content of treated fibres was determined by analyzing ash
content after combustion at 800°C for two hours. Untreated Kraft pulp fibre was used as control. Clay-
treated Kraft fibres will herein be referred to as KFC.
26
3.2.2 Lignin Processing
Lignin filter cakes were dried at room temperature, then delumped using a mortar and pestle, followed
by oven drying for 24 hours at 70°C. The lignin powder contained particles of varying size and required
further processing. Standard U.S. size mesh sieves were used to determine the particle size distribution.
A known mass of lignin was put through three successive mesh sizes to determine particle size distribute
in the following ranges:
Particle size > 35 Mesh: diameter > 500 μm
35 mesh < Particle size < 45 Mesh: 350-500 μm
45 mesh< Particle size < 60 Mesh: 250-350 μm
Particle size < 60 Mesh: <250 μm
Any particles greater than 500 microns were discarded as oversized particles. The resulting course
powder was then used as control and referred to as unscreened lignin (USL). The finest powder obtained
from successive screening with particle sizes less than 250 microns, referred to as fine screened lignin
(FSL), was used to determine the influence of particle size distribution on composite properties.
3.3 Composite Processing
3.3.1 Aqueous processing
A lab scale emulsifier was used to disperse Kraft pulp fibre in 500mL of distilled water. Emulsion type
PLA (ePLA) diluted in 100mL of water was then added to the solution and mixed for 10 minutes to allow
sufficient contact between the PLA particles and fibre in solution. The aqueous mixture was cast in
shallow pans and oven-dried for 24 hours. Table 8 lists the composites produced using this method.
27
Table 8: Composites prepared using aqueous processing
Composite system Sample ID ePLA
(wt %)
Kraft pulp
fibre (wt%)
Clay-treated Kraft
pulp fibre (wt%)
Emulsion type PLA ePLA 100 - -
Emulsion type PLA with Kraft
pulp fibre
ePLA-5KF 95 5 -
ePLA-15KF 85 15 -
ePLA-20KF 80 20 -
Emulsion type PLA with Kraft
pulp fibre treated with organic
nano-clay
ePLA-5KFC 95 - 5
ePLA-15KFC 85 - 15
ePLA-20KFC 80 - 20
3.3.2 Thermal compounding
A Brabender Plasti-Corder twin screw mixer was used to compound extrusion grade pPLA and HDPE
with their respective fillers. The polymer was melted at the specified melt temperature (HDPE at 150°C,
pPLA at 185°C) and allowed to mix for 5 minutes at 20 rpm. Upon stabilization of the apparatus
temperature and torque, the filler was added gradually into the chamber to be mixed with the polymer
melt for 5 minutes at 80 rpm. Compounded samples were cooled at room temperature. Composites
prepared using thermal compounding are listed in Table 9.
28
Table 9: Composites prepared using thermal compounding
Composite system Sample ID pPLA
(wt %)
HDPE
(wt
%)
Kraft pulp
fibre (wt %)
Lignin 1
(wt %)
Extrusion grade
PLA pellets pPLA 100 - - -
Extrusion grade
PLA with Kraft
pulp fibre
pPLA-5KF 95 - 5 -
pPLA-15KF 85 - 15 -
pPLA-20KF 80 - 20 -
High density
polyethylene HDPE - 100 - -
HDPE with
unscreened Lignin
1 (USL)
HDPE-10USL - 90 - 10
HDPE-20USL - 80 - 20
HDPE-30USL - 70 - 30
HDPE-40USL - 60 - 40
HDPE with fine-
screened Lignin 1
(FSL)
HDPE-10FSL - 90 - 10
3.3.3 Compression Moulding
Both aqueous and thermally compounded materials were pressed into thin films of thickness 0.3 to 0.5
mm thickness using a Wabash 50 ton hydraulic press. The processing conditions are given in Table 10.
Table 10: Compression moulding process conditions
Composite system Temperature (°C) Pressure (MPa) Time (mins)
ePLA-KF and KFC 165 35 10
pPLA-KF and lignin 180 15 4
HDPE-lignin 155 2-3.5 2
29
3.4 Characterization
3.4.1 Water uptake
Water uptake of each different composite system was determined by the immersion technique. Film
samples were cut into square specimens and oven dried at 70°C for 24 hours, cooled and weighed on an
analytical scale to obtain the base weight. The specimens were then immersed in distilled water at room
temperature and monitored for weight gain for 24 hours or until the change in mass was less than ±0.01%.
Water uptake (Wabs) for each specimen was calculated as follows:
Equation 13
Where Wi represents the initial dry weight of the specimen taken before immersion, and Wt is the weight
of the specimen at time t. A total of five specimens per composite system were tested to obtain an average
value of water absorption.
3.4.2 Mechanical testing
Tensile tests were carried out according to ASTM method D638 on an Instron machine model 3367 with
a 2kN load cell, pneumatic grips and a strain rate of 2 mm/min to determine the tensile modulus, strength
and elongation at break. The tensile modulus of each specimen was calculated as the initial slope of the
stress-strain curve , and the tensile strength was calculated from the maximum load, F, carried
by the composite divided by the specimen cross-sectional area, A; . Elongation at break was
calculated from the displacement, ΔL, at the point of composite fracture, divided by the initial length of
the specimen, Lo; .
30
3.4.3 Dynamic mechanical analysis
Dynamic mechanical analysis (DMA) was conducted on a TA Instruments Q800 DMA instrument in
tension mode. The storage modulus, loss modulus and loss tangent curves were obtained at amplitude 15
μm and a heating rate of 3°C/min to 100°C. The curves were analyzed to determine the transition region
for each composite system. The onset point of the storage modulus curve was taken as the point at which
material passes from a glassy to a transition region, followed by a peak in the loss tangent curve, which
marks the midpoint between the material’s glassy and rubbery regions. A total of five specimens were
tested for each composite system to determine average transition temperatures, loss tangent peak value,
and initial and final storage modulus values. A sample curve is shown in Figure 9.
Figure 9: Schematic of typical DMA storage modulus and loss tangent curves
3.4.4 Scanning electron microscopy
A Hitachi S-2500 Scanning Electron Microscopy was used to analyze gold sputtered fracture surfaces of
composite tensile bars. Images were obtained at an acceleration voltage of 15kV and a magnification
range of 300 to 1200. The SEM micrographs at lower magnification were used to identify general failure
mechanisms, while images at higher magnification were used to identify specific characteristics such as
cracks, fibre pull-out and fibre-matrix interactions for each composite system.
31
3.4.5 Differential Scanning Calorimetry
In order to study the melt behavior of the composites, differential scanning calorimetry (DSC) was carried
out on a TA Instrument Q2000 DSC instrument. Composite samples in aluminum pans were heated from
room temperature to 200°C at a rate of 5°C/min, and cooled back to room temperature at 10°C/min. Glass
transition and melt temperatures were determined by endothermic peaks in the DSC melt scans.
Crystallinity of composites was determined by the following equation:
Equation 14
Where % χc is the percent crystallinity, ΔHm and ΔHc are heats of melting and cold crystallization of the
composite and ΔHm* is a the melting enthalphy of 100% crystalline polymer. A value of 75.57 J/g was
used as the melting enthalpy of 100% crystalline PLA, as determined by Kantoglu and Guven in 2002. A
total of 4 speciments were tested for each composite system.
32
4 RESULTS AND DISCUSSIONS
4.1 Kraft pulp fibre in emulsion type and extrusion grade PLA
In this section, the potential impact of processing method on composite properties was explored. Two sets
of Kraft pulp fibre-reinforced composites systems were studied
ePLA-KF composites were prepared using an emulsion type PLA and aqueous processing
pPLA-KF composites were prepared using extrusion grade PLA and thermal processing
Figure 10 shows the composite films after compression moulding. Distinct phase separation and poor
fibre dispersion was evident in ePLA-KF composites after visual inspection of the films. In this
composite system, the majority of the fibre had agglomerated in the centre of the film as seen in Figure
10(a). This phase separation was not evident after aqueous processing and only was observed after
application of heat and pressure. The pPLA-KF composites, however, showed no signs of phase
separation and appeared to have even distribution of fibres in the matrix. It is speculated that the bond
strength between the emulsion type PLA and Kraft pulp fibre was not sufficient to prevent separation of
two phases. A series of tests was carried out in order to understand the underlying reason for this phase
separation, and to determine the extent to which poor fibre dispersion impacts composite properties.
Figure 10: (a) ePLA-15KF and (b) pPLA-15KF after compression moulding
33
4.1.1 Mechanical properties
The mechanical properties of both composite systems were analyzed using stress-strain curves (sample
calculation given in Appendix A) obtained through tensile testing, the results of which are summarized in
Table 11. The tensile modulus, strength and elongation at break of each neat polymer were compared to
composites with 5, 15 and 20 wt% fibre content. The experimental value of tensile modulus for neat
emulsion type PLA, 3.3 GPa, was consistent with literature values (Satyanarayana, 2009; Funabashi,
2005; Bodros, 2007; Bledzki, 2009) and was found to be higher than that of the extrusion grade polymer,
which only displayed a tensile modulus of 2.3 GPa. In order to determine the impact of processing
method on tensile modulus, the data was normalized based on the neat polymer modulus values, shown in
Figure 11. Density calculations were carried out and the composites were found to have a density range of
0.9 to 1.3 g/cm3. Statistical analysis of the data revealed that the differences in density were not
statistically significant, and an average density value of 1.1 ± 0.1 was assumed for the composites. The
density data is shown in Appendix B of this thesis.
Table 11: Summary of tensile properties for ePLA and pPLA composites, shown with standard
deviations (samples sizes given in Appendix B of this thesis)
Sample ID Tensile Strength (MPa) Tensile Modulus (GPa) Tensile Elongation (%)
ePLA 53.1 ±11.7 3.3 ±0.6 1.3% ±0.7%
ePLA-5KF 53.4 ±1.8 3.2 ±0.3 2.1% ±0.1%
ePLA-15KF 70.4 ±9.3 4.8 ±0.3 1.9% ±0.3%
ePLA-20KF 50.5 ±7.5 3.0 ±0.4 1.9% ±0.2%
pPLA 60.9 ±4.8 2.3 ±0.3 5.8% ±1.1%
pPLA-5KF 60.0 ±3.3 2.6 ±0.1 3.1% ±0.3%
pPLA-15KF 59.9 ±3.9 2.9 ±0.2 2.9% ±0.3%
pPLA-20KF 61.5 ±4.6 2.9 ±0.2 2.9% ±0.4%
34
Both ePLA and pPLA composites displayed tensile strengths approximately the same as the neat
polymers (within error) with the exception of ePLA-15KF, which had a tensile strength 1.3 times greater
than neat ePLA. Statistical analysis of the data proved this result to be statistically significant, meaning
that addition of 15% Kraft pulp fibre to ePLA using aqueous processing successfully strengthened the
polymer (Figure 11b). Improvements in tensile strength are difficult to achieve with natural fibre
reinforcement without the use of a compatibilizer, as was demonstrated in the literature study summarized
in Table 5, which shows that short fibre composites with randomly aligned fibres seldom achieve
improved tensile strengths compared to the neat polymer (Oksman et al., 2003; Garcia et al., 2007;
Mathrew et al., 2006; Huda et al., 2005; Ludvik et al., 2007; Thunwall et al., 2008). The tensile strength
of composites is highly dependent on the interactions of constituents on molecular scale, and an
improvement at 15% fibre in the ePLA composite shows that the unmodified Kraft pulp fibres were able
to form a strong interfacial bond with PLA polymer chains. However, no improvements were seen at fibre
contents above 15% by weight.
Furthermore, slight improvement to the pPLA composite modulus was seen with the addition of fibre. At
5%, 15%, and 20% fibre content, an improvement of 1.1, 1.2 and 1.3 times, respectively, was seen in the
pPLA composites. This is consistent with the literature and the Rule of Mixtures, where addition of fibre
results in restricted polymer chain mobility, and increased tensile modulus. However, ePLA composite
modulus only improved with the addition of 15% fibre. At a fibre content higher than 15%, the composite
displayed a modulus similar to that of the neat polymer (Figure 11a). This drop-off signifies a threshold
for fibre loading, and is associated with the poor fibre dispersion and phase separation seen at higher fibre
content.
35
Figure 11: Normalized tensile modulus (a) tensile strength (b) and elongation at break (c) for Kraft
pulp fibre-reinforced ePLA and pPLA composites. Error bars represent standard deviation.
The normalized elongation for the composite systems is shown in Figure 11(c) and it can be seen that
pPLA-KF composites have a significantly lower elongation at break than neat pPLA, a trend that is also
observed in other studies that use natural fibre as a reinforcing agent (Mathew et al., 2005, Pan et al.,
2007). Elongation of fibre-reinforced polymer composite is mostly attributed to the polymer elongation;
the presence of fibre prevents polymer from stretching freely, hence reducing the overall pPLA-KF
composite elongation. At the same time, a slight improvement can be seen in ePLA-KF composites
compared to neat ePLA. As was seen earlier, fibre dispersion in ePLA composites was not ideal, and fibre
agglomerations with coiled and kinked fibre were present. When a tensile load is applied to the ePLA
composites, energy is taken up not only to stretch the polymer, but also to remove kinks and twists in
fibre and it is possible that this contributes to the improvement of elongation of the composites compared
to neat ePLA.
Tensile data treatment and statistic analyses are available in Appendix B of this thesis.
36
Figure 12: Comparison of theoretical normalized composite elastic modulus (calculated using HT
model) and experimental tensile moduli for Kraft pulp fibre-reinforced ePLA and pPLA
composites. Ec,random is the theoretical composite elastic moduls with randomly aligned fibres, EL
and ET are the longitudinal and transverse moduli, respectively. Error bars represent standard
deviation.
Figure 13: Comparison of theoretical composite tensile strength (HT model) to experimental values
at corresponding elongation for each composite system. Error bars represent standard deviation.
37
Experimental values of composite tensile modulus and strength were compared to both the theoretical
values calculated using the HT model (Figures 12 and 13, respectively). For both ePLA and pPLA
composite systems, the experimental values were found to be lower than the theoretical and this
difference was most pronounced in the pPLA composites, where the experimental values were 52-56%
less than what was predicted by the HT model. In order to explain the discrepancies, the assumption of
the model must be revisited. The mechanics theory assumes that fibre aspect ratio is constant, and that
perfect bonding is achieved between the fibre and matrix. Furthermore, composite theories assume that a
perfect bond is achieved between the fibre and matrix. It is important to note that uniform fibre dispersion
was not achieved in the ePLA-KF composites, which can induce stress concentration in the materials
when a load is applied. In materials with high stress concentration, less energy is required to create new
surfaces, and there is a possibility that the critical crack length in these materials is lower than those with
ideal fibre dispersion. In pPLA-KF composites, however, no phase separation was apparent, suggesting
that the discrepancy is attributed to either poor interfacial bonding or variation in the aspect ratio of the
fibre. It is possible that fibre length is compromised during material processing, resulting in fibres with a
lower aspect ratio than expected, and consequently a lower tensile modulus.
4.1.2 Fracture surface morphology (SEM)
Figure 14(a) depicts the fracture surface of an ePLA composite reinforced with 15% untreated Kraft fibre.
This SEM micrograph shows a polymer rich fracture surface, with no apparent fibre-matrix interaction.
The smooth surface, along with absence of fibres suggests mechanical failure in this sample was caused
by brittle matrix failure. In comparison, the fracture surface of the pPLA-KF composite, shown in figures
14(b) is distorted with the presence of fibres and evidence of fibre pull-out (voids in surface). As was
shown previously, the apparent fibre dispersion in pPLA composites had improved, and the SEM
micrograph also shows signs of fibre-matrix interaction. However, the tensile test results do not give the
indication that improved fibre dispersion also led to better bonding between the fibre and matrix.
38
Figure 14: Fracture surface of tensile bars for (a) ePLA-15KF and (b) pPLA-15KF
4.1.3 Dynamic mechanical properties (DMA)
The storage modulus and loss tangent of neat polymer films obtained through DMA in tension mode are
shown in Figure 15, where a distinct difference between ePLA and pPLA properties can be observed. The
DMA curves of the neat polymers show that the extrusion type pPLA reaches the transition region at a
lower temperature, as seen by the loss modulus onset point at approximately 55°C and loss tangent peak
at 65.8°C, compared to 62.0°C and 70.2°C, respectively, in neat emulsion type ePLA.
Figure 15: Storage modulus and loss tangent curves for neat ePLA and pPLA films, obtained through DMA.
Voids (a)
Fibre
(b)
39
Furthermore, DMA results also showed that pPLA-KF composites reach the transition region at a lower
temperature than the ePLA-KF composites, as shown in Table 12. Aside from the shift in transition
regions, general trends were observed in both composite systems with respect to the impact of fibre
content on initial storage modulus and loss tangent peak values. The values initial storage modulus
increased as a function of fibre content while the converse is true for the loss tangent peak values. Loss
tangent peak values are related to the amorphous nature of a material and the amount of energy lost as
heat due to frictional forces. Figure 16 demonstrates that in imperfectly elastic materials, the area under
the stress-strain curve is not conserved; the material sustains some permanent changes during unloading,
leading to lost work. In the composite systems, as the fibre content in the polymers increases, the extent to
which heat is dissipated is reduced because the mobility of polymer chains is hindered by the fibres.
Figure 16: Work lost during unloading in imperfectly elastic materials
An unexpected result was observed for the ePLA composite filled with 20% untreated fibre (ePLA-
20KF). This specimen showed reduced thermal stability, as seen by a shift in storage modulus onset point
and loss tangent peak to a lower temperature. All other specimens in this composite system displayed tanδ
peak temperatures around 70°C, while the curve for ePLA-20KF peaked at 62.92°C. The evidence for
reduced thermal stability is also consistent with the reduced mechanical properties seen in section 4.1.1,
where the tensile modulus and strength of the ePLA-20KF specimens also fell short compared to other
specimens in the composite system.
40
Table 12: DMA results for ePLA and pPLA composite systems
Sample ID tan δ
peak
tan δ peak Temp
(°C)
E' Offset Point
(°C)
E'initial
(MPa)
E'final
(MPa)
E" peak
(°C)
ePLA 2.05 70.5 62.2 1727 1.3 65.6
ePLA-5KF 0.63 71.0 63.8 1768 41.3 67.2
ePLA-15KF 0.42 72.4 61.7 2451 115.6 66.1
ePLA-20KF 0.20 62.9 57.0 3615 419.4 60.7
pPLA 2.63 64.9 54.5 3009 2.8 57.2
pPLA-5KF 1.91 66.1 56.8 3427 13.2 59.1
pPLA-15KF 1.48 65.1 55.1 3512 21.7 58.2
pPLA-20KF 1.29 65.6 56.4 3619 32.8 59.1
Detailed DMA results for each composite system are included in Appendix C of this thesis.
4.1.4 Water absorption properties
Water uptake of neat polymer and composite films was monitored over a 24 hour period and it was
observed that both the initial rate of water absorption, as well as overall water uptake for the entire test
period were greater in neat ePLA than neat pPLA (Figure 17) with an overall weight gain of 2.47% and
0.77%, respectively.
Figure 17: Water uptake of neat ePLA and pPLA over a 24 hour interval
41
This observed trend can be explained by considering mechanisms of water absorption in composite
materials. Water uptake can occur through mass transfer, which is related to polymer permeability.
Analysis of the base case showed that ePLA absorbs more water than pPLA, and this could be due to the
presence of surfactants in the emulsion type PLA. Therefore, it is possible that the higher permeability of
emulsion type PLA results in composites that are more susceptible to water uptake through mass transfer.
Another mechanism of water absorption is capillary flow of water through fibre lumen, which is
associated with fibre content and accessibility (Hashemi et al., 2003; Lima et al., 2000). In order to
determine the specific impact of fibre content on water absorption, the data presented in Table 13 were
normalized based on neat polymer equilibrium water uptake, shown in Figure 18. While both composite
systems demonstrated increased water uptake as a function of fibre content, ePLA-KF composites at the
highest fibre content absorbed more than twice the amount of waters as the pPLA-KF composite at the
same fibre loading. This suggests that during thermal compounding process, the accessibility of the fibre
lumen is reduced through improved dispersion, thereby hindering water uptake by capillary flow at higher
fibre loadings.
Table 13: Water absorption results for ePLA and pPLA composites over a 24-hour period, shown
with standard deviations (samples size, N = 5)
Sample ID Time (hours)
1 2 12 24
ePLA 1.4% ±0.4% 1.7% ±0.3% 2.2% ±0.2% 2.5% ±0.2%
ePLA-5KF 2.7% ±1.0% 2.8% ±1.0% 3.3% ±1.2% 2.8% ±1.2%
ePLA-15KF 3.2% ±0.4% 5.2% ±0.8% 7.9% ±0.9% 7.2% ±0.6%
ePLA-20KF 12.6% ±3.1% 12.9% ±2.9% 17.1% ±6.5% 20.5% ±7.7%
pPLA 0.5% ±0.2% 0.6% ±0.3% 0.7% ±0.1% 0.8% ±0.1%
pPLA-5KF 0.7% ±0.2% 0.9% ±0.3% 1.3% ±0.4% 1.4% ±0.1%
pPLA-15KF 0.9% ±0.2% 1.1% ±0.2% 2.0% ±0.03% 2.2% ±0.3%
pPLA-20KF 1.1% ±0.4% 1.6% ±0.7% 2.7% ±0.01% 3.0% ±0.3%
42
Figure 18: Normalized water uptake of ePLA-KF and pPLA-KF composites after 24 hours. Error
bars represent standard deviation.
Full water absorption results are included in Appendix D.
4.1.5 Melting and crystallization behavior
The DSC thermograms were used to determine the glass transition temperature, Tg, melt temperature, Tm,
and crystallization temperature, Tc, of ePLA and pPLA composite systems. The glass transition
temperature, Tg, of each specimen was determined from the first endothermic peak, while the second
endothermic peak denoted the melting temperature, Tm. The area under the exothermic peak was
calculated to determine the percent crystallinity. The DSC thermograph in Figure 19 shows the curves for
neat ePLA and pPLA. It can be seen that Tg for both polymers are approximately the same (56.39°C ±
0.33°C). The addition of fibre to each polymer resulting in a slight increase in Tg, as shown in Figure 20;
however, the shift in temperature was not significant enough to denote an improvement in the glass
transition temperature. Furthermore, the absence of crystallization and melt peaks in the ePLA curves
indicate that the amorphous nature of ePLA does not change as a function of fibre content. In comparison,
it is shown that the addition of fibre to pPLA improves the crystallinity of the composites. An
improvement of 5% and 32% were seen with the addition of 5% and 15% fibre to the polymer, as can be
43
seen in Table 14. The improvement can be attributed to the thermal treatment and fibre dispersion, which
enable better packing and alignment of the polymer chains.
Figure 19: DSC thermograph of neat pPLA and ePLA
Figure 20: Glass transition temperatures of ePLA-KF and pPLA-KF composites
Table 14: Melt and crystallization data for pPLA-KF composites, shown with standard deviations
(sample size N = 4)
Sample ID Crystallinity (%) Tc (°C) Tm (°C)
pPLA - - 155.5 ±0.9
pPLA-5KF 5.2 ±1.2 134.0 ±0.2 155.1 ±0.2
pPLA-15KF 31.7 ±2.6 104.2 ±1.9 157.3 ±0.3
44
4.1.6 Summary
The comparison study presented above aimed to determine the impact of processing method on fibre-
reinforced PLA composites. The major findings are summarized below:
SEM images suggest the fibre-matrix bond strength was not sufficient to prevent composite
failure through interfacial debonding and fibre pull-out in both composite systems.
Analysis of tensile data showed that:
o σePLA-KF and EePLA-KF: ePLA-15KF > ePLA = ePLA-5KF = ePLA-20KF
o ε ePLA-KF : ePLA < ePLA-5KF = ePLA-15KF = ePLA-20KF
o σpPLA-KF : fibre content had no significant impact on tensile strength
o EpPLA-KF : pPLA < pPLA-5KF < pPLA-15KF = pPLA2-20KF
o ε pPLA-KF : pPLA > pPLA-5KF = pPLA-15KF = pPLA-20KF
Water absorption studies showed that ePLA is more susceptible to water uptake by mass transfer
than pPLA, resulting in composites with greater overall water uptake. Furthermore, thermal
compounding effectively reduced water uptake associated with fibre content at higher fibre
loadings.
Reduced thermal stability was observed emulsion type PLA at the highest fibre loading, a trend
that was consistent with reduced mechanical properties due to poor fibre dispersion.
Analysis of DSC results showed an increase in crystallinity of pPLA as a function of fibre loading
45
4.2 Modified Kraft Pulp Fibre in Emulsion-type PLA
It was shown in section 4.1 that fibre dispersion remains a challenge in composites with emulsion-type
PLA, prepared using aqueous processing. In order to address this issue, a fibre surface modification
technique developed by our group (Chen and Yan, 2012) was used to change the hydrophobicity of Kraft
pulp fibre surfaces. The modified fibres were then used as reinforcement in ePLA in a comparison study
of the following composite systems:
ePLA-KF: emulsion type PLA filled with untreated Kraft pulp fibre using aqueous processing
ePLA-KFC: emulsion type PLA filled with organoclay-treated Kraft pulp fibre using aqueous
processing
Figure 21 shows the composite films after compression moulding. As previously mentioned, ePLA-KF
composites showed poor fibre dispersion as was evident by distinct zones of phase separation and fibre
agglomeration in the centre of the composite film (Figure 21a). The treatment of fibre with an organically
modified nanoclay improved the apparent fibre dispersion in the composites, as shown in Figure 21(b).
This composite lacks major fibre-rich regions in the centre, and KFC fibres seem to be distributed more
evenly throughout the entire film than KF fibres.
This could be explained by considering the potential bonding mechanisms between fibre and matrix. In a
Chemical Force Microscopy study of cellulosic fibre in an aqueous medium, Bastidas et al. found that
there is a small and consistent adhesion force between a model surface with OH groups and a CH3-
functionalized tip15
, suggesting interactions between cellulosic fibres with OH surface functionality and
hydrophobic polymers such as PLA is possible in an aqueous medium. However, no adhesion forces were
observed between a CH3-functionalized tip and native cellulose fibre surfaces in this study, leading to the
stipulation that there is reduced OH group accessibility in the extracted cellulose fibre surface. It is
possible that the solvent exchange and clay-treatment of Kraft pulp fibres increases OH accessibility in
the ePLA-KFC systems, allowing a weak adhesive bond to form between fibre and PLA. The strength of
46
this adhesive bond may be sufficient to withstand the pressure applied during compression moulding and
maintain the dispersion in the composite film. A series of tests was carried out to determine whether an
effective bond was formed between clay-modified Kraft pulp fibre and emulsion type PLA.
Figure 21: Composite films after compression moulding (a) ePLA-5KF and (b) ePLA-5KFC
4.2.1 Mechanical properties
The tensile properties of ePLA-KF and ePLA-KFC composites were determined from stress-strain curves
and are given in Figure 22. As was seen in the previous section, the tensile modulus and strength of
ePLA-KF composites did not follow an expected trend improvement with increasing fibre content.
Instead, only the addition of 15% fibre effectively strengthened and increased stiffness of the composite.
However, a steady increase of both tensile modulus and strength were seen in ePLA-KFC composites,
suggesting that the fibre surface modification with an organo-nanoclay has effectively improved the
interfacial bond strength. More importantly, this improvement becomes more apparent at higher fibre
content; at the highest fibre loading, a significant drop was observed in the tensile strength and modulus
of ePLA-20KF. In fact, at strength of 50.3 MPa and modulus of 3.0 GPa, the properties of ePLA-20KF
were the same as the neat polymer (within error) suggesting that no effective reinforcement took place at
this fibre content. Conversely, the most improvement to mechanical properties in the ePLA-KFC
composite systems was seen at 20% fibre loading, at which strength of 73.6 MPa and modulus of 5.2 GPa
were achieved.
47
Figure 22: Tensile properties of ePLA-KF and ePLA-KFC composites (a) Tensile modulus; (b)
tensile strength and (c) elongation at break. Error bars represent standard deviation
The experimental mechanical properties were compared to theoretical values calculated using the HT
model. As was seen in section 4.1.1, the experimental tensile modulus and strength of both ePLA-KF and
pPLA-KF composite systems fell short of the theoretical values. This discrepancy was associated with
poor fibre dispersion, potential decrease in fibre aspect ratio, and weak bond strengths between fibre and
matrix. Figures 23 and 24 show the theoretical tensile modulus and strength, respectively, of the ePLA-
KFC composite systems. It can be seen that the experimental values of modulus are within the upper and
lower limits of the theoretical values (shown as the longitudinal and transverse modulus in Figure 23).
48
Figure 23: Theoretical elastic modulus of ePLA-KFC composites (HT model) compared to
experimental values
Furthermore, the difference between the theoretical and experimental tensile strength of the ePLA-KFC
composites was found to be less than 30% (Figure 24), whereas the ePLA-KF composite showed a
difference of up to 45%. Since the processing method for both composite systems was the same, it can be
assumed that the fibre aspect ratio remained was the same and was constant for all fibre loadings. It is
possible that the organo-nanoclay treatment of the Kraft pulp fibres improves the fibre-matrix interaction
such that the bond strength achieved between the modified fibre and PLA is closer to the theoretical
perfect bond that is assumed by the HT model.
Figure 24: Comparison of theoretical and experimental tensile strength of ePLA-KFC composites.
Error bars represent standard deviation.
49
4.2.2 Fracture surface morphology
The tensile test results suggest that the fibre-matrix interaction in ePLA-KFC composites had improved;
therefore it was expected that SEM imaging would reflect this as well. The fracture surface of ePLA
reinforced with 15% organo-clay modified Kraft pulp fibre was studied and is shown in Figure 25. It has
been stipulated that if good fibre-matrix interaction is achieved, it is difficult to distinguish between the
two phases in SEM images because the fibres will be coated by the matrix (Mathew et al., 2005).
Another indication of a strong interfacial bond between the fibre and matrix would be evidence of fibre
breakage, or lack of fibre pull-out. Analysis of the fracture surface of an ePLA-15KFC specimen showed
both fibre breakage, as well as fibre pull out. This suggests that the fibre-matrix interaction has improved
to a certain extent, but is not consistent throughout the whole composite.
Figure 25: Fracture surface of an ePLA-15KFC tensile bar specimen
4.2.3 Dynamic mechanical properties
As was seen in section 4.1.3 with the ePLA-KF and pPLA-KF composites, the storage modulus of ePLA-
KFC composites increased as a function of fibre content. Conversely, the tanδ peak values decreased,
50
signifying a reduction in energy dissipation due to movement of polymer chains. The DMA results are
summarized in Table 15.
Table 15: DMA results for ePLA-KFC composites
Sample ID
tan δ
peak
tan δ peak
Temp (°C)
E' Offset Point
(°C)
E'initial
(MPa)
E'final
(MPa)
E" peak
(°C)
ePLA 2.05 70.5 62.2 1727 1.3 65.6
ePLA-5KFC 0.529 71.4 64.2 2177 68.1 67.4
ePLA-15KFC 0.489 70.8 62.0 2459 93.6 66.3
ePLA-20KFC 0.276 71.3 63.8 2466 348.4 68.1
In section 4.1.3, it was observed that at the highest fibre content, the transition region of ePLA-KF
composite shifted to a lower temperature, signifying a reduction in thermal stability. However, no shifts in
transition region were seen in the ePLA-KFC composites, even at the highest fibre content (Figure 26).
To explain this, the molecular motion of the matrix phase at the onset of the transition region should be
considered. The onset of the glass transition curve is associated with main chain gradual motion, followed
by large scale chain movement reflected in the peaks of loss modulus and loss tangent curves (Li, 2000).
It is possible that improved fibre dispersion and the formation of stronger interfacial bonds in ePLA-KFC
prevent the onset main chain movement that was seen in ePLA-KF composites. Furthermore, it is
important to note that while increased fibre content leads inevitably to increased moisture content due to
the hydrophilic nature of fibre, care is taken to avoid moisture adsorption during processing. This is done
to avoid potential hydrolytic degradation of the PLA, which could theoretically result in a lower MW and
more brittle matrix phase.
51
Figure 26: Storage modulus and loss tangent curves of ePLA-20KF and ePLA-20KFC composites
4.2.4 Water absorption properties
The impact of fibre surface treatment on water absorption was determined by comparing the ePLA-KF
and ePLA-KFC composites. In this comparison, the polymer permeability was the same for both
composite systems; therefore, any major differences in water absorption properties can be associated with
fibre content and lumen accessibility. Past studies have shown that water absorption through capillary
flow can be reduced through surface modification of the fibres (Sreekala and Thomas, 2003; Lu e al.,
2004) and it was expected that organoclay treatment of the Kraft fibres would have a similar outcome.
The result of water absorption studies showed that clay-treatment reduces both the initial rate of water
absorption, as shown in Figure 27, as well as the overall water uptake, shown in Table 16. This trend was
most pronounced at the highest fibre content, where the untreated fibre composite gained 21% water after
4 hours, while the clay-treated composite only gained 11% water.
52
Figure 27: Water absorption curves of ePLA-KF and ePLA-KFC composites monitored over 2 hours
Table 16: Equilibrium water absorption for ePLA-KF and ePLA-KFC composites with standard
deviations (Sample sizes given in Appendix D)
Fibre Content Overall Water absorption (wt%)
KF KFC
Neat polymer 2.47%
5% 2.8% ± 1.2% 6.0% ± 1.4%
15% 7.2% ± 0.6% 7.1% ± 0.5%
20% 20.5% ± 7.7% 11.3% ± 1.5%
53
4.2.5 Summary
It was expected that the treatment of Kraft pulp fibre will lead to enhanced fibre-matrix interaction, and a
stronger interfacial bond. The major findings of the comparison study between ePLA-KF and ePLA-KFC
composites are summarized below:
Modification of Kraft pulp fibre with an organonanoclay resulted in composites with
improved fibre dispersion compared to composites with unmodified Kraft pulp fibre
Analysis of tensile data showed that both strength and modulus improved with the addition
of fibre; however, no significant changes were seen in tensile elongation as a function of
fibre volume fraction.
Comparison of experimental and theoretical mechanical properties revealed that ePLA-KFC
composite strength and modulus are closer to the ideal case (perfect bonding between fibre
and matrix) than ePLA-KF composites
Reduced overall water absorption in ePLA-KFC composites suggests that water uptake
through capillary flow was effectively reduced through organoclay treatment
Improved fibre-matrix interactions in ePLA-KFC composites prevented a shift in transition
temperatures that were seen in ePLA-KF composites.
54
4.3 Lignin as a filler in HDPE and PLA
This section explores the viability of lignin by-product from a biorefinery as a high value constituent in
composite materials. The following composite systems were studied:
1. HDPE-USL: extrusion grade HDPE thermally compounded with 10-40% unscreened lignin
2. HDPE-FSL: extrusion grade HDPE thermally compounded with 10 & 20% fine-screened lignin
3. PLA-USL: extrusion grade PLA thermally compounded with 10 and 20% unscreened lignin
4. PLA-KF-USL: extrusion grade PLA thermally compounded with 10% USL with 5% and 20%
Kraft pulp fibre
4.3.1 Lignin as biofiller in HDPE
The comparison of mechanical properties between HDPE-USL and HDPE-FSL composites in Table 17
serves to answer the question of whether homogeneity in particle size is required to improve mechanical
properties in the composite. It was observed that the addition of 10% of fine screened and unscreened
lignin to HDPE both resulted in an increase in tensile properties, compared to the neat polymer. The
values of tensile modulus and strength achieved by both composites were the same, within error,
suggesting that the strengthening and stiffening capability of lignin is not highly dependent on particle
size. A major observation, however, was that the variation in particle sizes of unscreened lignin (USL)
results in non-uniform properties in composites, as is evident by the large standard deviation of tensile
strength for HDPE-10USL. Therefore, if lignin is to be used as a constituent in biocomposites for
consumer products, further processing is needed to reduce particle size variations in order to meet quality
control requirements.
55
Table 17: Influence of particle size on mechanical properties with standard deviations (sample size,
N = 12)
Sample ID Modulus (GPa) Strength (MPa)
Neat HDPE 1.1 ± 0.1 22.2 ± 0.5
HDPE with 10% USL 1.6 ± 0.2 31.4 ± 6.4
HDPE with 10% FSL 1.6 ± 0.2 32.5 ± 3.0
In order to reduce energy consumption associated with further processing of the lignin by-product,
unscreened lignin (USL) was used in the remainder of studies. Figure 28 shows the tensile properties of
HDPE filled with 10-40% USL. Since the lignin by-product contained equal amounts of lignin and
cellulosic matter, a combination of strengthening and stiffening was projected for the HDPE-USL
composites. As expected, the tensile modulus of the composites increased as a function of lignin content,
a result that is consistent with the Rule of Mixtures. The tensile strength of these composites, however,
depends on the reinforcing mechanism and the interaction between each phase. In this case, lignin
particles can potentially strengthen the composite by restricting the movement of the matrix phase, and
this restriction in movement is dependent on how well the lignin particles can bond to HDPE. An
improvement in the tensile strength was seen with 10% and 20% USL content, while no significant
increase was seen with 30% and 40% USL content, compared to the neat polymer. It has been suggested
that increasing fibre or filler content in composites can lead to agglomeration and fibre-fibre interactions
that hinder the properties of composites (Idicula et al., 2005). It is possible that at higher lignin content,
both cellulosic and lignin particles have higher interaction, leading to stress concentration and reduced
tensile strength.
56
Figure 28: Tensile properties of HDPE-USL composites (a) tensile modulus; (b) tensile strength.
Error bars represent standard deviation. Sample size is given in Appendix B.
4.3.2 Lignin as a compatibilizer in PLA-KF composites
In nature, interactions between cellulose, hemicelluloses and lignin provide strength and structural
integrity to trees. Although the lignin product obtained from biorefineries has been modified, it still
contains both phenolic and cellulosic compounds with varying degrees of OH-functionalities; it is
possible that interactions between KF and oligomers and PLA and lignin clusters occur during thermal
processing. Therefore, given the nature and composition of the lignin by-product, it was expected that it
may act as a compatibilizer between PLA and Kraft pulp fibre. Tensile tests and dynamic mechanical
analysis were carried out to test this hypothesis.
57
4.3.2.1 Mechanical Properties
The tensile modulus, strength and elongation of pPLA composites with 5% and 20% Kraft pulp fibre as
reinforcement, and 10% unscreened lignin as a compatibilizer are shown in Figure 29. These results are
compared to pPLA-KF composites with no compatibilizer. As expected, the tensile modulus of pPLA-KF-
USL increased as a function of fibre content, and these composites also showed greater tensile modulus
than those without USL (Figure 29a). In order to identify the constituent responsible for the increase in
tensile modulus, the composition of the lignin by-product were studied in more detail. The by-product
contains both phenolic content and cellulosic matter, and it was speculated that the improvement in
tensile modulus was due to the fibre content. In order to test this, further work can be done on isolating
the lignin content of the by-product and analyzing its impact on the tensile modulus of the PLA-fibre
composites.
In addition to an improvement to tensile modulus, a significant reduction in tensile elongation of both
neat pPLA and pPLA-KF composites was seen as a result of lignin inclusion, seen in Figure 29(c). These
changes in tensile properties can be attributed to the restriction of movement in the amorphous regions
of pPLA by lignin particles. Finally, Figure 29(b) shows that the addition of USL to both neat pPLA and
pPLA-KF composites does not result in any significant changes to tensile strength.
These results suggest that USL fails to create an effective bond between Kraft pulp fibre and pPLA, but
rather acts as a filler in the matrix. Given these results, lignin can be used as a potential displacing
material that can help offset the high cost of PLA. The current cost estimation of the biorefinery lignin
values the by-product at $350 per metric ton (Mascoma Canada), while PLA is sold at $2000 per metric
ton (Endres & Siebert-Raths, 2011). The use of lignin as a displacing agent translates to an 8.25% cost
savings for every 10% polymer displaced by weight, and further tests can help determine the optimal
lignin content in a PLA-KF composite without compromising the composite performance.
58
Figure 29: pPLA-KF-USL tensile properties (a) tensile modulus; (b) tensile strength; (c) elongation
at break. Error bars represent standard deviation. Sample sizes are given in Appendix B.
59
4.3.2.2 Dynamic mechanical properties
DMA studies were done to determine the impact of lignin addition on thermal stability of pPLA-KF
composites. As previous mentioned, the transition region of composites is marked by the onset point of
the storage modulus curve (E’ offset), followed by peaks in the loss modulus (E” peak) and loss tangent
(tanδ peak), respectively. Improvements to thermal stability in composites would manifest themselves as
shift in the transition region to a higher temperature. Figure 30 shows the transition regions of neat pPLA,
pPLA-KF and pPLA-KF-USL composites. The addition of lignin to pPLA and pPLA-KF did not have a
major impact on the storage modulus onset point, which is associated with mechanical failure (Turi,
1997). However, the loss modulus and loss tangent peak temperatures shifted to higher temperatures
compared to composites without lignin. This is also consistent with the increase in tensile modulus and
decrease in tensile elongation seen in the previous section. Shifts in the glass transition temperature can
be a function of changes in molecular weight, presence of inherently stiff materials such as phenylenes,
cross-linking or formation of hydrogen bonds, as well as the presence of plasticizers in the material
(Nicholson, 2006).
The interaction between composite constituents can be further explained by considering
structure/property relationships. The structure of a composite at every scale has an impact on the material
properties. For example, molecular structure and packing of polymer chains and fibres will have an
impact on the hydrogen bonding potential and tensile modulus and strength, whereas the microstructure
of a composite will dictate the viscoelastic properties. In these composite systems, the molecular weight
and extend of crosslinking is assumed to be constant and the tensile strength results reveal that the
constituents are not interacting well on a molecular scale. However, changes in the viscoelastic properties
of the composite, like the shift of the transition region to a higher temperature, can be associated with
interactions at the micro-structural scale and the restriction of mobility in the amorphous regions of pPLA
by lignin particles. These particles are 250-500 μm in diameter, and consist of high molecular weight and
highly cross-linked lignin, as well as low molecular weight sugar oligomers with hydroxyl functionality
that can potentially form bonds with both phenolic and aliphatic hydroxyl groups. These interactions can
60
restrict movement of pPLA polymer chains and increase the overall stiffness of the composite. This is
also reflected in the loss tangent peak values, which decreased as a function of both fibre and lignin
addition, as seen in Table 18.
Figure 30: Transition region temperatures of neat pPLA, pPLA-KF and pPLA-KF-USL
composites. Error bars represent standard deviation.
Table 18: Dynamic mechanical properties of pPLA-KF-USL composites compared to neat pPLA
and pPLA-KF composites
Sample ID tan δ peak E'initial (MPa) E'final (MPa)
neat pPLA 2.639 3009 2.79
pPLA-10USL 2.131 3261 13.28
pPLA-5KF-10USL 1.697 2590 17.41
pPLA-20KF-10USL 1.114 3386 87.95
pPLA-5KF 1.914 3427 13.21
pPLA-20KF 1.295 3619 32.81
61
4.3.3 Summary
This section explored the use of a biorefinery lignin by-product as a natural filler in HDPE, as well as a
compatibilizer in PLA-KF composites. The major findings are summarized below:
Addition of up to 20 wt% lignin by-product successfully improved both tensile modulus and
strength of HDPE.
Analysis of tensile test results of pPLA-KF-USL composites showed no improvement in tensile
strength, suggesting that lignin by-product is not an effective compatibilizer for Kraft pulp fibre
and PLA. However, an increase in tensile modulus revealed that lignin acts as a filler instead.
DMA results showed that the addition of lignin by-product to PLA-KF composites effectively
increases the glass transition temperature compared to both neat PLA, and PLA-KF composites
without lignin. An increase in thermal stability can enable higher temperature applications for
this composite system.
62
5 Conclusion and Recommendations
Characterization of four composite systems developed in this thesis provided some insight into the
interaction between Kraft pulp fibre and polylactic acid, and the factors that have an impact fibre-matrix
adhesion and compatibility. The criteria for good fibre-matrix interaction included improvements to the
tensile strength, glass transition temperature, water absorption, and evidence of fibre fracture as a major
failure mechanism in the composites. Based on these indicating factors, the following conclusions can be
made about each composite system:
The use of aqueous processing and emulsion type PLA resulted in composites with a low
fibre content threshold and poor fibre dispersion. Although the tensile strength of the
emulsion type PLA with 15% fibre content was effectively improved, increasing the fibre
content beyond this threshold resulted in thermal instability, a drop-off in the mechanical
properties and significant increase in moisture sensitivity.
Thermal compounding reduced water absorption by effectively coating fibres and reducing
fibre lumen accessibility. However, the fibre-matrix interfacial bond and interactions at a
molecular scale were not sufficiently strong to improve the tensile strength and thermal
stability of these composites.
Treatment of Kraft pulp fibre with an organonanoclay resulted in improvements to tensile
strength and water absorption in this composite system. Furthermore, the deterioration of
material properties at higher fibre content that was seen in emulsion type PLA composites
with untreated Kraft pulp fibre was successfully prevented by the surface treatment of fibres.
Inclusion of a biorefinery lignin by-product in PLA-fibre composites did not improve the
tensile strength of composites, signifying that lignin fails to act as an interface compatibilizer.
However, lignin’s use as a filler is still viable, as improvements to other material properties
such as tensile modulus and glass transition temperature were observed in this composite
system.
63
In addition to the studies on PLA-fibre composites, the viability of the lignin by-product as a biofiller in
high density polyethylene was also proven through a series of mechanical tests. It was shown that lignin
has the ability to both strengthen and stiffen HDPE at up to 20% inclusion (by weight). Therefore, it was
shown that it is possible to displace up to 20% of a petroleum-derived polymer by a renewable material,
while maintaining and even improving the polymer’s initial properties. Currently, HDPE is priced at
$1800 per metric ton, while the lignin by-product is valued at $350 per metric ton. Therefore, the result of
this exploratory study provides a positive outlook for the application of biorefinery lignin by-products as
a displacing material in polymers with a significant cost saving opportunity. These results warrant further
studies on the use of lignin in biocomposites and the following recommendations can be made for
subsequent research on lignin biomateirals:
A study of lignin by-product structure and reactivity in order to determine mechanisms by
which HDPE is strengthened and PLA is made more thermally stable:
o Determine the contribution of phenolic and cellulosic content on improvements in
tensile properties by isolating each constituent and performing comparative tests
o Determine the impact of the soluble lignin on viscoelastic properties of the composite
using dynamic mechanical analysis
Research and development of possible fibre surface treatment by the lignin by-product:
o Determine the hydrogen bonding potential between the fibre and by-product
cellulosic content through AFM studies and FTIR analyses.
Determining lignin by-product compatibility with other commodity plastics (PP, PS, PVC,
etc)
64
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7 Appendices
Appendix A : Calculation of Tensile Modulus
Instron software calculates tensile modulus values from the stress-strain curves obtained during tensile
testing. The curve for specimen 2 of the pPLA-15KF series is shown below.
The tensile modulus of this specimen is calculated as the slope of the linear region of the curve, as
demonstrated in the figure below. The tensile modulus of this specimen is 2.67 GPa.
73
Appendix B : Statistical Analysis
B-1: Elimination of Outliers
The modified three-sigma test for small samples sizes of n < 10 was used to determine the presence of outliers in
raw data obtained from different characterization methods. Sample calculations for tensile strength data for ePLA-
KF composites are given in Table I. The analysis was performed as follows:
The value calculated for z* is used to determine α
* associated with the corresponding zα from the z-PDP. The outlier
is discarded if (the value falls outside of the 3σ range).
Table 19: Elimination of tensile strength ouliers in ePLA-KF series
ePLA ePLA-5KF ePLA-15KF ePLA-20KF
n Tensile
Strength z* α*
Tensile Strength
z* α* Tensile
Strength z* α*
Tensile Strength
z* α*
1 52.61 0.24 0.4013 55.23 0.27 0.3936 85.45 1.49 0.0681 34.73 1.49 0.0681
2 49.15 0.04 0.4840 53.09 0.52 0.3015 71.75 0.30 0.3821 43.77 0.50 0.3085
3 71.03 1.35 0.0885 57.94 1.27 0.1020 44.82 2.04 0.0207 40.79 0.82 0.2061
4 50.92 0.14 0.4404 51.72 1.02 0.1539 75.47 0.62 0.2709 54.78 0.72 0.2358
5 57.79 0.55 0.2877
81.65 1.16 0.1230 56.3 0.88 0.1893
6 57.53 0.54 0.2946
57.03 0.98 0.1635 59.59 1.25 0.1056
7 32.3339 0.97 0.1685
59.38 0.77 0.2206 47.94 0.04 0.4840
8 16.9156 1.89 0.0287
76.27 0.69 0.2451
9
66.93 0.12 0.4522
10
60.53 0.67 0.2514
11
74.42 0.53 0.2981
12
65.78 0.22 0.4129
48.53
54.50
68.29
48.27
S 16.72
2.71
11.53
9.08
n 8
4
12
7
αcrit 0.0625
0.125
0.0083
0.0714
Pα,crit 0.9375
0.875
0.992
0.929
53.05
53.35
70.42
50.53
S* 11.65
1.77
9.29
7.49
74
B-2: Analysis of Variance (ANOVA) of tensile properties
To determine whether the addition of fibre or filler has a significant effect on composite tensile
properties, analysis of variance was carried out with the tensile data after the elimination of outliers. The
null hypothesis was that changes in properties were not statistically significant. The Hypothesis and
rejection criteria were set out as follows:
Accept Ho if: P*α,max < 85%, meaning addition of fibre or filler has no significant impact on properties.
Reject Ho if: P*α,max > 90%, meaning at least one of the treatments has a significant impact on properties.
ANOVA results are presented in Tables 20, 22, 24, 26, 28 and 30 of this appendix.
In the case of null hypothesis rejection, the Least Significant Diffference (LSD) method was used to
establish a hierarchy of treatments. Pα of 90% was used the rejection criterion. The LSD hierarchies are
shown in Tables 21, 23, 25, 27, 29 and 31 of this appendix.
75
B-2: Analysis of Variance (ANOVA) of tensile properties (continued)
Table 20: ePLA-KFC tensile test raw data and ANOVA
ePLA-KFC
Property → Tensile Strength Tensile Modulus Tensile Elongation
Rep N0. ↓ Treat. N0. → 1 2 3 4 1 2 3 4 1 2 3 4
1 52.61 53.5 45.99 75.72 3.651 3.81 3.11 5.67 1.10% 1.80% 2.21% 1.67%
2 49.15 48.51 57.45 74.77 3.601 3.33 3.2 5.24 1.01% 1.56% 2.25% 2.08%
3 71.03 51.91 44.01 82.96 4.117 3.57 3.38 5.9 1.26% 1.44% 1.96% 2.08%
4 50.92 52.78 56.53 68.32 3.209 3.23 3.4 3.72 1.52% 2.00% 2.58% 2.08%
5 57.79 52.93 60.21 66.33 3.021 3.32 3.47 5.32 2.27% 1.84% 2.42% 1.75%
6 57.53 51.18 52.17
3.423 4.11 4.21
1.60% 1.24% 1.46%
7 32.33
63.83
2.329
4.64
0.00%
2.04%
8
54.82
4.65
1.42%
9
58.63
4.9
1.46%
10
59.28
4.96
1.67%
11
63.8
5.07
1.63%
12
63.67
5.17
1.50%
13
62.23
5.18
1.21%
14
64.43
5.77
1.13%
53.05 51.80 57.65 73.62 3.34 3.56 4.37 5.17 0.01 0.02 0.02 0.02
b 7 6 14 5 7 6 14 5 7 6 14 5
N 32 32 32
58.04 4.12 1.66E-02
SST 1623.51 12.53 1.74E-04
SSR 1556.18 15.65 6.21E-04
vt 3 3 3
vr 28 28 28
ST2 541.17 4.18 5.81E-05
SR2 55.58 0.56 2.22E-05
FT 9.74 7.47 2.62E+00
α < 0.001 0.001 < α < 0.005 0.05 < α < 0.1
Table 21: ePLA-KFC comparison of treatments using LSD method
t0.025, 28 = 2.048 Tensile Strength Tensile Modulus Tensile Elongation
i j |xi-xj| LSD95% Stat
Significance |xi-xj| LSD95%
Stat Significance
|xi-xj| LSD95% Stat
Significance
ePLA ePLA-5KFC 1.25 8.49 No 0.23 0.85 No 3.95E-03 5.36E-03 No
ePLA-5KFC ePLA-15KFC 5.84 7.45 No 0.80 0.75 Yes 1.33E-03 4.70E-03 No
ePLA-15KFC ePLA-20KFC 15.97 7.95 Yes 0.81 0.80 Yes 1.54E-03 5.02E-03 No
76
B-2: Analysis of Variance (ANOVA) of tensile properties (continued)
Table 22: ePLA-KF tensile test raw data and ANOVA
ePLA-KF
Property→ Tensile Strength
Tensile Modulus Tensile Elongation
Rep. NO. ↓ Treat. NO.→ 1 2 3 4 1 2 3 4 1 2 3 4
1 52.61 55.23 85.45 43.77 3.651 3.52 4.98 2.758 1.10% 2.12% 2.20% 1.72%
2 49.15 53.09 71.75 40.79 3.601 2.9 5.17 2.64 1.01% 2.16% 1.68% 1.87%
3 71.03 51.72 75.47 54.78 4.117 3.18 4.94 3.238 1.26% 1.96% 2.24% 1.70%
4 50.92
81.65 56.3 3.209
5.35 3.477 1.52%
2.20% 2.00%
5 57.79
57.03 59.59 3.021
4.55 3.258 2.27%
1.52% 2.17%
6 57.53
59.38 47.94 3.423
4.33 2.604 1.60%
1.76% 2.05%
7 32.3339
76.27
2.329
4.88
0.00%
2.32%
8
66.93
4.53
1.72%
9
60.53
4.37
2.04%
10
74.42
4.9
1.88%
11
65.78
5.17
1.40%
12
13
14
53.05 53.35 70.42 50.53 3.34 3.20 4.83 3.00 1.25% 2.08% 1.91% 1.92%
b 7 3 11 6 7 3 11 6 7 3 11 6
N 27 27
27
59.60 3.86 1.76E-02
SST 2199.88 18.14 2.50E-04
SSR 1964.09 3.98
4.05E-04
vt 3 3 3
vr 23 23 23
ST2 733.29 6.05 8.32E-05
SR2 85.40 0.17 1.76E-05
FT 8.59 34.95 4.720069742
α < 0.001 < 0.001 0.01 < α < 0.025
Table 23: ePLA-KF comparison of treatments using LSD method
t0.025, 23 = 2.069 Tensile Strength Tensile Modulus Tensile Elongation
i j |xi-xj| LSD95% Stat
Significance |xi-xj| LSD95%
Stat Significance
|xi-xj| LSD95% Stat
Significance
ePLA ePLA-5KFC 0.29 13.19 No 0.14 0.59 No 8.28E-03 5.99E-03 Yes
ePLA-5KFC ePLA-15KFC 17.08 12.45 Yes 1.63 0.56 Yes 1.75E-03 5.66E-03 No
ePLA-15KFC ePLA-20KFC 19.90 9.70 Yes 1.84 0.44 Yes 1.13E-04 4.41E-03 No
77
B-2: Analysis of Variance (ANOVA) of tensile properties (continued)
Table 24: pPLA-KF tensile test raw data and ANOVA
pPLA-KF
Property→ Tensile Strength Tensile Modulus Tensile Elongation
Rep. NO. ↓ Rep. NO. ↓ 1 2 3 4 1 2 3 4 1 2 3 4
1 61.57 58.99 64.77 62.24 2.673 2.63 2.68 3.13 4.9% 3.36% 3.1% 3.1%
2 61.89 62.57 54.96 67.22 2.493 2.49 2.92 2.87 7.0% 3.28% 2.6% 3.1%
3 60.83 57.02 55.79 61.05 2.233 2.52 2.66 2.79 4.8% 3.04% 2.6% 2.9%
4 65.1 61.49 62.13 63.93 2.071 2.46 3.1 3.03 6.8% 3.52% 3.3% 3.1%
5 64.15 60.85 65.06 67.7 1.994 2.51 3.05 3.3 4.8% 3.08% 3.3% 3.0%
6 51.56 58.79 64.14 60.86 2.373 2.39 2.58 2.75 6.5% 2.84% 3.3% 3.1%
7
56.3 62.39
2.47 3
2.96% 3.2%
8
61.19 56.97 63.27
2.53 2.69 2.73
3.00% 2.8% 2.8%
9
63.91 55.87 63.78
2.46 2.73 2.79
3.44% 2.6% 3.3%
10
62.11 60.25 54.66
2.64 3.05 2.91
3.20% 2.4% 2.5%
11
55.88 56.99
2.49 2.92
2.84% 2.4%
12
65.56
53.62
2.89
3.02
3.52%
2.2%
13
54.76
57.57
2.69
3.04
2.52%
2.4%
14
60.85 59.96 59.94 61.45 2.31 2.55 2.85 2.94 0.06 0.03 0.03 0.03
b 6 13 11 11 6 13 11 11 6 13 11 11
N 41 41
41
60.48146341 2.701146
3.38%
SST 17.88155275 2.116923
0.004163
SSR 612.9707594 1.213796
0.10%
vt 3 3
3
vr 37 37
37
ST2 5.960517585 0.705641
0.001388
SR2 16.56677728 0.032805
2.57E-05
FT 0.359787392 21.50998
53.90855
α > 0.1 < 0.001
< 0.001
Table 25: pPLA-KF comparison of treatments using LSD method
t0.025, 37 = 2.0273 Tensile Strength Tensile Modulus Tensile Elongation
i j |xi-xj| LSD95% Stat
Significance |xi-xj| LSD95%
Stat Significance
|xi-xj| LSD95% Stat
Significance
pPLA pPLA-5KF 0.89 4.07 No 0.25 0.18 Yes 2.67E-02 5.08E-03 Yes
pPLA-5KF pPLA-15KF 0.02 3.38 No 0.30 0.15 Yes 2.54E-03 4.21E-03 No
pPLA-15KF pPLA-20KF 1.51 3.52 No 0.09 0.16 No 3.64E-05 4.39E-03 No
78
B-2: Analysis of Variance (ANOVA) of tensile properties (continued)
Table 26: pPLA-KF-USL tensile test raw data and ANOVA
pPLA-KF-10USL
Property→ Tensile Strength
Tensile Modulus Tensile Elongation
Replication number ↓
Treatment Number →
1 2 3
1 2 3
1 2 3
1 54.2 54.44 58.61
3.11 3.8 3.85
1.76% 1.84% 1.60%
2 52.01 67 53.46
3.02 4.55 4.08
1.68% 1.28% 1.28%
3 63.43 45.36 60.05
3.4 4.2 4.81
1.52% 0.76% 0.92%
4 46.44 39.02 57.28
3.04 3.98 4.42
0.92% 0.44% 1.28%
5 52.53 61.64 49.32
3.43 4.86 4.08
1.00% 1.08% 1.08%
6 70.11 49.53
3.67 3.68
1.52% 1.28%
7 61.18 54.21 61.16
3.35 3.87 3.47
2.12% 1.44% 2.44%
8 61.34 48.87 57.96
3.39 3.77 4.73
1.92% 1.16% 1.08%
9 57.58 46.1 52.32
2.91 3.64 4.2
2.20% 1.08% 1.00%
10 68.83 53.59
4.18 3.83
1.20% 1.20%
11 40.85 37.47 61.79
3.53 3.81 4.23
0.92% 0.60% 1.68%
12 54.19 58.9
3.08 3.9
1.92% 1.20%
13 52.97
3.53
1.08%
56.59 51.34 56.88
3.36 3.99 4.21
0.02 0.01 0.01
b 13 12 9
13 12 9
13 12 9
N 34
34
34
54.81588
3.805882
1.34%
SST 223.9848
4.484499
0.000105
SSR 1831.392
4.279324
0.06%
vt 2
2
2
vr 31
31
31
ST2 111.9924
2.24225
5.24E-05
SR2 59.07715
0.138043
1.9E-05
FT 1.895698
16.24316
2.760885
α > 0.1
< 0.001
0.05 < α < 0.1
Table 27: pPLA-KF-USL comparison of treatments using LSD method
t0.025, 31 = 2.0399 Tensile Strength Tensile Modulus Tensile Elongation
i j |xi-xj| LSD95% Stat Sig |xi-xj| LSD95% Stat Sig |xi-xj| LSD95% Stat Sig.
pPLA-10USL pPLA-10USL-5KF
5.25 6.28 No 0.63 0.30
Yes 4.07E-
03 3.56E-
03 Yes
pPLA-10USL-5KF
pPLA-10USL-20KF
5.54 6.91 No 0.22 0.33
No 2.60E-
03 3.92E-
03 No
79
B-2: Analysis of Variance (ANOVA) of tensile properties (continued)
Table 28: HDPE-USL tensile test raw data and ANOVA
HDPE-USL
Property→ Tensile Strength Tensile Modulus Tensile elongation
Rep. no ↓ Treat No → 1 2 3 4 1 2 3 4 1 2 3 4
1 25.48 23.34 21.56 26.17 1.5 1.6 1.95 2.23 4.76% 1.68% 1.00% 1.20%
2 28.75 29.65 20.7 20.5 1.51 1.81 1.81 2.07 3.44% 1.68% 2.52% 1.12%
3 36.95 26.43 24.41 16.85 1.83 1.54 1.93 1.59 1.68% 1.52% 0.92% 0.84%
4 40.74 27.61 21.92 16.55 1.65 1.92 2.04 2.03 1.84% 2.00% 1.28% 1.12%
5 35.26 29.03 24.06 16.69 1.94 1.93 2.19 1.95 2.36% 2.36% 0.76% 0.76%
6 43.58 26.69 19.85 25.2 1.68 1.93 1.84 2.2 3.36% 2.36% 1.76% 1.20%
7 31.88 30.11 23.92 18.39 2.05 1.75 1.87 2.22 1.36% 1.60% 1.60% 1.44%
8 27.82 28.45 21.87 19.91 1.43 1.55 1.9 2.45 3.48% 2.00% 1.28% 1.20%
9 29.58 27.57 20.45 18.85 1.71 1.77 1.93 1.95 1.92% 2.44% 0.76% 0.84%
10 25.26
19.04
1.48 1.93
2.92% 0.84%
11 24.85
20.15 1.5 1.66 1.92% 0.92%
12 26.63
19.11 1.59 2.00 2.60% 0.92%
31.40 27.65 21.42 19.90 1.66 1.76 1.92 2.08 0.03 0.02 0.01 0.01
b 12 9 12 9 12 9 12 9 12 9 12 9
N 42 42
42
25.28119048 1.843095
1.75%
SST 939.0895849 1.053292
0.001733
SSR 617.8072556 1.28380
0.15%
vt 3 3
3
vr 38 38
38
ST2 313.0298616 0.351097
0.000578
SR2 16.25808567 0.033784
4E-05
FT 19.2537958 10.39231
14.43995
α < 0.001 < 0.001
< 0.001
Table 29: HDPE-USL comparison of treatments using LSD method
t0.025, 38 = 2.0252 Tensile Strength Tensile Modulus Tensile Elongation
i j |xi-xj| LSD95% Stat
Significance |xi-xj| LSD95%
Stat Significance
|xi-xj| LSD95% Stat
Significance
pPLA-10USL pPLA-20USL 3.75 3.60 Yes 0.10 0.16 No 6.77E-03 5.65E-03 Yes
pPLA-20USL pPLA-30USL 6.23 3.60 Yes 0.17 0.16 Yes 7.47E-03 5.65E-03 Yes
pPLA-30USL pPLA-40USL 1.52 3.60 No 0.16 0.16 No 1.33E-03 5.65E-03 No
80
B-2: Analysis of Variance (ANOVA) of tensile properties (continued)
Table 30: Density raw data of all composites and ANOVA
Density
ePLA-KF ePLA-KFC pPLA-KF
Replication number ↓
Treatment Number → 1 2 3 4 1 2 3 4 1 2 3 4
1 1.17 1.04 1.05 0.89 1.17 1.02 0.92 1.10 1.25 1.22 0.77 1.20
2 1.17 0.98 0.97 0.90 1.17 0.96 1.12 1.18 1.21 1.20 1.02 0.69
3 1.08 0.96 0.93 0.90 1.08 0.96 1.32 1.29 0.92 1.11 1.35 1.84
4 1.19
0.86 1.19
0.95 1.24 0.92 1.42
5 0.99
1.02 0.99
1.10 1.22 1.37 1.19
6
0.90
1.12 0.99 0.98 0.91 1.12 0.98 1.12 1.19 1.09 1.20 1.09 1.27
b 1.25 0.98 0.97 0.83 1.25 0.96 1.25 1.42 1.18 1.44 1.18 1.60
N 5 3 3 6 5 3 3 3 5 5 5 5
6.26 2.95 2.90 5.00 6.26 2.89 3.75 4.25 5.89 7.18 5.90 8.02
SST 17 14 20
SSR 1.00 1.10 1.16
vt 1.00 1.22 1.34
vr 0.12 0.07 0.12
ST2 0.06 0.13 1.08
SR2 3 3 3
FT 13 10 16
α 0.04 0.02 0.04
0.00 0.01 0.07
b 8.97 1.76 0.59
N 0.001< α < 0.005 >0.1 >0.1
Table 31: Density comparison of treatments using LSD method
t0.025, 13 = 2.16 ePLA-KF ePLA-KFC pPLA-KF
i j |xi-xj| LSD95% Stat
Significance |xi-xj| LSD95%
Stat Significance
|xi-xj| LSD95% Stat
Significance
ePLA ePLA-5KFC 0.13 0.10 Yes 0.14 0.19 No 0.11 0.35 No
ePLA-5KFC ePLA-15KFC 0.01 0.12 No 0.14 0.21 No 0.11 0.35 No
ePLA-15KFC ePLA-20KFC 0.07 0.10 No 0.07 0.21 No 0.18 0.35 No
81
Appendix C : DMA Curves
C-1: ePLA-KF vs pPLA-KF
82
C-2: ePLA-KF vs ePLA-KFC
83
Appendix D : Water Absorption Data
D-1: ePLA-KF water absorption data
Sample ID Mass of composite (g) % Weight gain
t=0 t=1hr t=2 hr t= 12hr t=24 hr 0 1 2 12 24
ePLA
1 0.1012 0.1020 0.1033 0.1032 0.1034 0 0.79% 2.08% 1.98% 2.17%
2 0.1562 0.1588 0.1584 0.1598 0.1603 0 1.66% 1.41% 2.30% 2.62%
3 0.1447 0.1463 0.1468 0.1481 0.1484 0 1.11% 1.45% 2.35% 2.56%
4 0.0879 0.0895 0.0895 0.0898 0.09 0 1.82% 1.82% 2.16% 2.39%
5 0.1201 0.1219 0.1222 0.1228 0.1232 0 1.50% 1.75% 2.25% 2.58%
Mean 0.12202 0.1237 0.12404 0.12474 0.12506 0 0.01376 0.017007 0.022081 0.024652
SD 0.02865305 0.029097 0.028832 0.029425 0.029531 0 0.004218 0.002759 0.001474 0.001857
ePLA-5KF
1 0.4469 0.4574 0.4582 0.4603 0.458 0 2.35% 2.53% 3.00% 2.48%
2 0.3977 0.4126 0.4131 0.4159 0.4141 0 3.75% 3.87% 4.58% 4.12%
3 0.2527 0.2574 0.2575 0.2582 0.2571 0 1.86% 1.90% 2.18% 1.74%
Mean 0.36576667 0.3758 0.376267 0.378133 0.3764 0 2.65% 2.77% 3.25% 2.78%
SD 0.10096144 0.104956 0.105298 0.106211 0.105623 0 0.98% 1.01% 1.22% 1.22%
ePLA-15KF
1 0.5174 0.532 0.5473 0.5631 0.5577 0 2.82% 5.78% 8.83% 7.79%
2 0.4488 0.4631 0.4678 0.4807 0.4789 0 3.19% 4.23% 7.11% 6.71%
3 0.4335 0.4492 0.4574 0.4668 0.464 0 3.62% 5.51% 7.68% 7.04%
Mean 0.46656667 0.481433 0.490833 0.503533 0.5002 0 3.21% 5.18% 7.87% 7.18%
SD 0.0446827 0.04434 0.049177 0.052052 0.050351 0 0.40% 0.83% 0.88% 0.55%
ePLA-20KF
1 0.1048 0.1148 0.1171 0.1195 0.1235 0 9.54% 11.74% 14.03% 17.84%
2 0.1111 0.1259 0.1267 0.1335 0.1361 0 13.32% 14.04% 20.16% 22.50%
3 0.1063 0.1192 0.1209 0.1268 0.1294 0 12.14% 13.73% 19.29% 21.73%
4 0.1158 0.1321 0.1327 0.139 0.1443 0 14.08% 14.59% 20.03% 24.61%
5 0.0937 0.1022 0.1027 0.1027 0.1046 0 9.07% 9.61% 9.61% 11.63%
6 0.1022 0.1171 0.117 0.1242 0.1281 0 14.58% 14.48% 21.53% 25.34%
Mean 0.11855 0.119517 0.124283 0.127667 0 12.58% 12.89% 17.06% 20.53%
SD 0.010213 0.01022 0.012622 0.013419 0 3.05% 2.85% 6.50% 7.71%
84
D-2: pPLA-KF water absorption data
Sample ID Mass of composite (g) % Weight gain
t=0 t=1hr t=2 hr t= 12hr t=24 hr 0 1 2 12 24
pPLA
1 0.2514 0.2523 0.2536 0.2531 0.2534 0 0.36% 0.88% 0.68% 0.80%
2 0.2382 0.2388 0.2391 0.2398 0.2401 0 0.25% 0.38% 0.67% 0.80%
3 0.2009 0.2018 0.2029 0.2025 0.2026 0 0.45% 1.00% 0.80% 0.85%
4 0.1613 0.1625 0.1622 0.1623 0.1623 0 0.74% 0.56% 0.62% 0.62%
5 0.1299 0.1305 0.1303 0.1305 0.1309 0 0.46% 0.31% 0.46% 0.77%
Mean 0.19634 0.19718 0.19762 0.19764 0.19786 0 0.004527 0.006229 0.006452 0.007658
SD 0.05110463 0.05109 0.051645 0.05156 0.051574 0 0.001832 0.003025 0.001212 0.000861
pPLA-5KF
1 0.2539 0.2556 0.2556 0.2564 0.2572 0 0.67% 0.67% 0.98% 1.30%
2 0.202 0.2033 0.2037 0.2043 0.2048 0 0.64% 0.84% 1.14% 1.39%
3 0.1401 0.1406 0.1421 0.1415 0.1421 0 0.36% 1.43% 1.00% 1.43%
4 0.2145 0.2161 0.2158 0.2171 0.2174 0 0.75% 0.61% 1.21% 1.35%
5 0.1742 0.1758 0.1761 0.1776 0.1764 0 0.92% 1.09% 1.95% 1.26%
Mean 0.19694 0.19828 0.19866 0.19938 0.19958 0 0.67% 0.93% 1.26% 1.35%
SD 0.04279688 0.043188 0.04263 0.043052 0.043319 0 0.20% 0.34% 0.40% 0.07%
pPLA-15KF
1 0.1609 0.1624 0.1631 0.164 0.1651 0 0.93% 1.37% 1.93% 2.61%
2 0.1711 0.1724 0.1729 0.1744 0.1747 0 0.76% 1.05% 1.93% 2.10%
3 0.1772 0.1783 0.1789 0.1807 0.1811 0 0.62% 0.96% 1.98% 2.20%
4 0.1611 0.1629 0.1629 0.1643 0.1642 0 1.12% 1.12% 1.99% 1.92%
5 0.1972 0.1989 0.1993 0.2011 0.2017 0 0.86% 1.06% 1.98% 2.28%
Mean 0.1735 0.17498 0.17542 0.1769 0.17736 0 0.86% 1.11% 1.96% 2.22%
SD 0.01494707 0.01495 0.014977 0.01526 0.015305 0 0.19% 0.15% 0.03% 0.25%
pPLA-20KF
1 0.2494 0.2518 0.2523 0.2556 0.2561 0 0.96% 1.16% 2.49% 2.69%
2 0.1151 0.117 0.1173 0.1183 0.1188 0 1.65% 1.91% 2.78% 3.21%
3 0.2425 0.2447 0.2453 0.2485 0.2491 0 0.91% 1.15% 2.47% 2.72%
4 0.2471 0.2491 0.25 0.2537 0.2551 0 0.81% 1.17% 2.67% 3.24%
5 0.1713 0.1736 0.1749 0.1759 0.1761 0 1.34% 2.10% 2.69% 2.80%
Mean 0.20508 0.20724 0.20796 0.2104 0.21104 0 1.08% 1.64% 2.68% 3.02%
SD 0.05993265 0.060033 0.060083 0.061323 0.061541 0 0.38% 0.66% 0.01% 0.31%
85
D-3: ePLA-KFC water absorption data
Sample ID Mass of composite (g) % Weight gain
t=0 t=1hr t=2 hr t= 12hr t=24 hr 0 1 2 12 24
ePLA
1 0.1012 0.1020 0.1033 0.1032 0.1034 0 0.79% 2.08% 1.98% 2.17%
2 0.1562 0.1588 0.1584 0.1598 0.1603 0 1.66% 1.41% 2.30% 2.62%
3 0.1447 0.1463 0.1468 0.1481 0.1484 0 1.11% 1.45% 2.35% 2.56%
4 0.0879 0.0895 0.0895 0.0898 0.09 0 1.82% 1.82% 2.16% 2.39%
5 0.1201 0.1219 0.1222 0.1228 0.1232 0 1.50% 1.75% 2.25% 2.58%
Mean 0.12202 0.1237 0.12404 0.12474 0.12506 0% 1% 2% 2% 2%
SD 0.028653 0.029097 0.028832 0.029425 0.029531 0% 0% 0% 0% 0%
ePLA-5KFC
1 0.305 0.3127 0.314 0.3212 0.321 0 2.52% 2.95% 5.31% 5.25%
2 0.3027 0.3102 0.315 0.3273 0.3257 0 2.48% 4.06% 8.13% 7.60%
3 0.3095 0.3156 0.3192 0.3255 0.3251 0 1.97% 3.13% 5.17% 5.04%
Mean 0.305733 0.312833 0.316067 0.324667 0.323933 0 2.32% 3.38% 6.20% 5.96%
SD 0.003459 0.002702 0.002759 0.003134 0.002558 0 0.31% 0.60% 1.67% 1.42%
ePLA-15KFC
1 0.404 0.4201 0.4278 0.4315 0.4345 0 3.99% 5.89% 6.81% 7.55%
2 0.6369 0.6501 0.6593 0.6766 0.6787 0 2.07% 3.52% 6.23% 6.56%
3 0.752 0.7735 0.7841 0.8014 0.8067 0 2.86% 4.27% 6.57% 7.27%
Mean 0.597633 0.614567 0.623733 0.6365 0.639967 0 2.97% 4.56% 6.54% 7.13%
SD 0.177292 0.17936 0.180793 0.188182 0.189099 0 0.96% 1.21% 0.29% 0.51%
ePLA-20KFC
1 0.4665 0.4934 0.4935 0.5139 0.5124 0 5.77% 5.79% 10.16% 9.84%
2 0.4914 0.5226 0.5234 0.5538 0.5543 0 6.35% 6.51% 12.70% 12.80%
3 0.5112 0.535 0.5391 0.569 0.5691 0 4.66% 5.46% 11.31% 11.33%
Mean 0.4897 0.517 0.518667 0.545567 0.545267 0 5.59% 5.92% 11.39% 11.32%
SD 0.022398 0.021358 0.023166 0.028458 0.02941 0 0.86% 0.54% 1.27% 1.48%
