Wood-fibre composites: Stress transfer and hygroexpansion Karin M. Almgren

Wood-fibre composites: Stress transfer and hygroexpansion Karin M. Almgren
Wood-fibre composites:
Stress transfer and hygroexpansion
Karin M. Almgren
Doctoral Thesis
No. 9
KTH Fibre and Polymer Technology
School of Chemical Sciences and Engineering
Royal Institute of Technology
SE-100 44 Stockholm
Sweden
2010
KTH accounts for one-third of Sweden’s technical research and engineering
education capacity at university level. Education and research cover a broad
spectrum – from natural sciences to all the branches of engineering.
Part of this work was performed a Innventia AB. Innventia AB is a world
leader in research and development relating to pulp, paper, graphic media,
packaging and biorefining.
© Karin M. Almgren, Stockholm, 2010
TRITA-CHE-Report 2010:9
ISSN 1654-1081
ISBN 978-91-7415-583-9
SUMMARY
Wood fibres is a type of natural fibres suitable for composite applications.
The abundance of wood in Swedish forests makes wood-fibre composites a
new and interesting application for the Swedish pulp and paper industry.
For large scale production of composites reinforced by wood fibres to be
realized, the mechanical properties of the materials have to be optimized.
Furthermore, the negative effects of moisture, such as softening, creep and
degradation, have to be limited. A better understanding of how design
parameters such as choice of fibres and matrix material, fibre modifications
and fibre orientation distribution affect the properties of the resulting
composite material would help the development of wood-fibre composites.
In this thesis, focus has been on the fibre-matrix interface, wood-fibre
hygroexpansion and resulting mechanical properties of the composite. The
importance of an efficient fibre-matrix interface for composite properties is
well known, but the determination of interface properties in wood-fibre
composites is difficult due to the miniscule dimensions of the fibres. This is
a problem also when hygroexpansion of wood fibres is investigated. Instead
of tedious single-fibre tests, more straightforward, macroscopic approaches
are suggested. Halpin-Tsai’s micromechanical models and laminate analogy
were used to attain efficient interface characteristics of a wood-fibre
composite. When Halpin-Tsai’s model was replaced by Hashin’s concentric
cylinder assembly model, a value of an interface parameter could be derived
from dynamic mechanical analysis. A micromechanical model developed by
Hashin was used also to identify the coefficient of hygroexpansion of wood
fibres. Measurements of thickness swelling of wood-fibre composites were
performed. Back-calculation through laminate analogy and the
micromechanical model made it possible to estimate the wood-fibre
coefficient of hygroexpansion. Through these back-calculation procedures,
information of fibre and interface properties can be gained for ranking of
e.g. fibre types and modifications.
Dynamic FT-IR (Fourier Transform Infrared) spectroscopy was investigated
as a tool for interface characterization at the molecular level. The effects of
relative humidity in the test chamber on the IR spectra were studied. The
elastic response of the matrix material increased relative to the motion of
the reinforcing cellulose backbone. This could be understood as a stress
transfer from fibres to matrix when moisture was introduced to the system,
e.g. as a consequence of reduced interface efficiency in the moist environment. The method is still qualitative and further development is potentially
very useful to measure stress redistribution on the molecular level.
PREFACE
This work has been carried out at KTH Solid Mechanics (2005-2007), KTH
Fibre and Polymer Technology (2007-2010) and at Innventia AB. It has
formed an integrated part of, and been financially supported by, the cluster
program “New Fibres for New Materials III” sponsored by Södra, Mondi,
Billerud, Korsnäs, Stora, M-real, BASF and Hartmann. The industrial parties
are all gratefully recognized for their support.
My advisor, associate professor Kristofer Gamstedt, is thanked for his
competent guidance during these years; I especially appreciate that you have
encouraged me to visit conferences around the world, enabling broader
insight in the different research fields connected to our research. Associate
professor Mikael Lindström is acknowledged for giving me the opportunity
to work at Innventia. Thank you for sharing your visionary ideas and
optimism, to you everything is possible! Also, thanks to Professor Lars
Berglund, for letting me into his group. My co-authors are greatly
acknowledged for their contributions to this thesis and for sharing their
expertise with me: Dr. Fredrik Berthold for providing composite materials
and valuable input on processing times, temperatures etc., associate
professor Lennart Salmén and Dr. Margaretha Åkerholm for conveying
insights in the dynamic FT-IR technique, professor Janis Varna for his
contribution to and improvement of modelling sections, and the image
analysis team at CBA, Filip Malmberg, Dr. Joakim Lindblad and Dr.
Catherine Östlund, for their work with the tomography data. I would also
like to express my appreciation to the colleagues at KTH and Innventia,
especially to my “roomies” Dr. Fredrik Wredenberg and Eva-Lisa Lindfors,
who brightened my days at work.
Slutligen, tack till min familj, till Martin och Hilda, för att ni delar mitt liv
och finns vid min sida, för all glädje, kärlek och trygghet ni ger mig.
LIST OF APPENDED PAPERS
Paper A
Dynamic-mechanical properties of wood fibre reinforced polylactide:
Experimental characterisation and micromechanical modelling
Bogren, K.M., Gamstedt, E.K., Neagu, R.C.,
Åkerholm, M. and Lindström, M., (2006)
Journal of Thermoplastic Composite Materials 19(6): 613-637
Paper B
Effects of moisture on dynamic mechanical properties of wood-fibre
composites studied by dynamic FT-IR spectroscopy
Almgren, K.M., Åkerholm, M., Gamstedt, E.K. and Salmén, L., (2008)
Journal of Reinforced Plastics and Composites 27(16-17): 1709-1721
Paper C
Characterization of interfacial stress transfer ability by dynamic
mechanical analysis of cellulose fiber based composite materials
Almgren, K.M. and Gamstedt, E.K., (2010)
Manuscript submitted for publication
Paper D
Role of fibre-fibre and fibre-matrix adhesion in stress transfer in
composites made from resin-impregnated paper sheets
Almgren, K.M., Gamstedt, E.K., Nygård, P., Malmberg, F.,
Lindblad, J. and Lindström, M., (2009)
International Journal of Adhesion and Adhesives 29(5): 551-557
Paper E
Moisture uptake and hygroexpansion of wood fiber composite materials
with polylactide and polypropylene matrix materials
Almgren, K.M., Gamstedt, E.K., Berthold, F. and Lindström, M., (2009)
Polymer Composites 30(12): 1809-1816
Paper F
Contribution of wood fiber hygroexpansion to moisture induced
thickness swelling of composite plates
Almgren, K.M., Gamstedt, E.K. and Varna, J., (2009)
Polymer Composites In Press
CONTRIBUTION REPORT
Contribution of the author to the appended papers:
Paper A
Experimental work
Modelling
Writing of paper
Paper B
Experimental work
Writing of paper
Paper C
Experimental work
Modelling
Writing of paper
Paper D
Joint efforts in experimental work
Interpretation of data
Writing of paper
Paper E
Joint efforts in experimental work
Joint efforts in interpretation of data
Writing of paper
Paper F
Experimental work
Joint efforts in modelling
Writing of paper
In addition to this thesis, the work has resulted in the following
publications
Dynamic-mechanical properties of wood fibre/polylactide
Bogren, K., Gamstedt, K., Neagu, C., Åkerholm, M. and Lindström, M.,
Proceedings of EcoComp 3rd International Conference on Eco-Composites,
Stockholm, June 2005, poster
Dynamic-mechanical properties of wood-fibre reinforced polylactide:
Experimental characterization and micromechanical modelling
Bogren, K.M., Gamstedt, E.K., Neagu, R.C., Åkerholm, M. and
Lindström, M., Proceedings of Progress in Wood and Bio Fibre Plastic
Composites 2006, Toronto, May 2006, 10 p
Micromechanical approaches to development of improved wood-fibre
biocomposites
Gamstedt, E.K., Neagu, R.C., Bogren, K. and Lindström, M., Proceedings
of the International Conference on Progress in Wood and Bio-Fibre Plastic
Composites 2006, Toronto, 2006, 10 p
Effects of relative humidity on load redistribution in cyclic loading of
wood-fibre composites analysed by dynamic Fourier transform
infrared spectroscopy
Bogren, K.M., Gamstedt, E.K., Åkerholm, M., Salmén, L. and Lindström,
M., Proceedings of Fifth Plant Biomechanics Conference, Stockholm,
August 2006, 6 p
Stress transfer and failure in pulp-fiber reinforced composites: Effects
of microstructure characterized by X-ray microtomography
Bogren, K. Gamstedt, E. K., Berthold, F., Lindström, M., Nygård, P.,
Malmberg, F., Lindblad, J., Axelsson, M., Svensson, S. and Borgefors, G.,
Proceedings of Progress in Paper Physics – A Seminar, Oxford, October
2006, 4 p
Measuring fibre-fibre bonds in 3D images of fibrous materials
Malmberg, F., Lindblad, J., Östlund, C., Almgren, K.M. and
Gamstedt, E.K., Proceedings of the 14th International Conference on
Image Analysis and Processing, International Association on Pattern
Recognition, Modena, 2007
Hygroexpansion of wood-fibre composite materials: Effects of cellwall cross-linking and composition of thermoplastic matrix
Almgren, K.M, Gamstedt, E.K., Berthold, F. and Lindström, M.,
Proceedings of the 13th European Conference of Composite Materials,
Stockholm, 2008, 10 p.
Measuring fibre-fibre contact in 3D images of fibrous materials
Malmberg, F., Lindblad, J., Östlund, C., Almgren, K.M. and Gamstedt,
E.K., Proceedings of the 13th European Conference of Composite
Materials, 2008, 10 p.
Inverse modelling to identify the fibre hygroexpansion coefficient
from experimental results of wood-fibre composites swelling
Almgren, K.M., Berthold, F., Varna, J. and Gamstedt, E.K., Proceedings
of ICTAM XXII International Congress of Theoretical and Applied
Mechanics, Adelaide, August 2008, poster
TABLE OF CONTENTS
1 INTRODUCTION ............................................................................. 1
1.1 FIBRE-MATRIX INTERFACE ....................................................... 5
1.1.1 Stress transfer in paper and board .................................... 6
1.1.2 Stress transfer in composite materials ............................... 7
1.2 WOOD FIBRES AND THEIR HYGROEXPANSION .......................... 10
1.2.1 Structure of wood fibres ................................................. 10
1.2.2 Wood fibre hygroexpansion ............................................ 12
2 MATERIALS AND METHODS ........................................................... 14
2.1 MATERIALS AND MANUFACTURING .......................................... 14
2.2 EXPERIMENTAL CHARACTERISATION ....................................... 15
2.2.1 Material characterisation ................................................ 15
2.2.2 Mechanical properties .................................................... 16
2.2.3 Bonds and bond strength ............................................... 16
2.2.4 Hygroexpansion and vapour and water sorption ................ 18
2.3 MODELLING TOOLS ............................................................... 18
3 RESULTS AND DISCUSSION .......................................................... 23
3.1 MATERIALS AND EXPERIMENTAL CHARACTERISATION ............... 23
3.1.1 Materials and manufacturing ........................................... 23
3.1.2 Mechanical properties .................................................... 25
3.1.3 Bonds and bond strength ............................................... 26
3.1.4 Hygroexpansion and vapour and water sorption ................ 30
3.2 MODELLING TOOLS ............................................................... 33
4 SUMMARY OF PAPERS .................................................................. 37
PAPER A .................................................................................... 37
PAPER B .................................................................................... 38
PAPER C .................................................................................... 39
PAPER D .................................................................................... 40
PAPER E..................................................................................... 41
PAPER F ..................................................................................... 42
5 FUTURE WORK ............................................................................ 43
6 LITERATURE................................................................................ 44
Karin M. Almgren
1
1 INTRODUCTION
During the last decade, environmental awareness has led to a considerably
increased interest in developing sustainable materials to replace materials
made from fossil-based resources. Polymers and reinforcing fibres from
renewable resources, e.g. annual plants or wood, is one way to produce
renewable and biodegradable composite materials for packaging and
structural applications.
Wood and wood fibres are commonly used as structural material, e.g. as
particle and flake board. In the pulp and paper industry, wood fibres are
used to produce a wide variety of products with different properties: paper
for printers, newsprint, paper for magazines, packaging materials such as
board and corrugated board, paper for tissues and fluff products for diapers,
to mention a few.
Due to their mechanical properties, wood fibres are also suitable as
reinforcement in composite materials. The low density of natural fibres
makes their specific properties comparable to those of commonly used glass
fibres [1]. One application of natural fibre composites is interior panels in
cars, where flax and hemp fibres are used as reinforcement in synthetic
resins. Other promising applications for wood-fibre composites are
packaging materials, furniture and non-structural building components.
Wood and other natural fibres offer many advantages compared to synthetic
fibres, e.g. glass and carbon fibres. They are relatively inexpensive, and the
cost of wood fibres lies in the lower region. They are derived from
renewable resources and are biodegradable, and they are also less abrasive
than the traditionally used synthetic fibres to equipment used in the
manufacturing processes. These advantages have led to an increased interest
in natural fibre composites. Many sources of natural fibres are utilized, e.g.
wood [2-7], jute [8], flax [9], hemp [1], sisal [10], cotton [11], oil palm [12]
and bamboo [13]. Compared to other natural fibres, wood has the advantage
of around-the-year harvest, and a well-developed infrastructure for cutting,
pulping, fibre treatment and preforming manufacture could essentially be
provided by the well established pulp and paper industry. The benefits of
wood fibres have led to intense research on wood-fibre composites;
different types of fibre and pulping processes have been investigated [14-16]
as well as suitable matrix materials [2] and methods for modification of both
fibres and reinforced polymers to improve the interface properties [17, 18].
There is a general agreement that wood-fibre composites offer an important
contribution to the composite field, and better understanding of the effects
of the mentioned design parameters (types of fibres and fibre preprocessing, polymer and polymer modification as well as process conditions
2
Wood-fibre composites: Stress transfer and hygroexpansion
such as process temperature, pressure and shear flow) on composite
properties would lead to improvement of the composite material properties.
Adequate adhesion between fibres and matrix is crucial to achieve optimal
mechanical properties of wood-fibre composites and to make them
appropriate as structural materials. The fibre surface is suitable for chemical
modifications aiming to improving the interface properties. Fibre
modifications can also be used to improve dimensional stability, i.e. reduce
fibre hygroexpansion and absorption of moisture. The dimensional stability
of wood fibres subjected to moisture is an Achilles’ heel of wood-fibre
composites, since contact with moisture leads to softening and swelling of
the fibres, and thereby to softening and deformation of the composite
material. The wood-fibre hygroexpansion and interface properties hence
have in common that they can readily be improved by chemical
modifications of the wood fibres, but also that they are quite difficult to
measure or determine experimentally. Single-fibre tests have been
performed [e.g. 19-22], but these tests are time consuming and the variability
is large. Measurements of composite samples are straightforward compared
to single-fibre tests and can be considered to reflect the effective average
behaviour of the fibres, since all fibres in the composite contribute to the
composite properties. The scope of this thesis is to investigate ways of
determining hygroexpansion properties of wood fibres and fibre-matrix
interface properties from such wood-fibre composite measurements.
Micromechanical modelling is a powerful tool to predict wood-fibre
composite properties or, if used backwards, to quantify properties of wood
fibres and fibre-matrix interface. Micromechanical models are commonly
and successfully used to predict thermal and elastic properties of both
synthetic and natural fibre composite materials. The composite theory
developed by Hashin and Rosen [23] as well as Halpin-Tsai’s [24], TsaiHahn’s [25] micromechanical models and Halpin and Pagano’s laminate
approximation for short fibre composites [26] are commonly used to link
the elastic properties of the composite constituents and composite
microstructure to the elastic properties of the composites.
Some models are extended to thermoelastic properties, e.g. [27, 28], but the
literature on hygroelastic properties is not that extensive, since the effect of
moisture on dimensional stability of glass- and carbon fibre composites is
small. The mechanics of linear thermo- and hygroelasticity are, however,
essentially the same, and if the variation in moisture content in the different
phases in the composite are accounted for, the models developed for
thermoelasticity are valid also for hygroelasticity. Models of this type are
originally developed with continuous synthetic fibres in mind, e.g. carbon
and glass fibres, but are also applicable on natural fibre composites [29].
Karin M. Almgren
3
Micromechanical models of this type are used in Papers A, C and F, to
evaluate the not so easily measured properties of fibre-matrix interface
(Papers A and C) and wood fibre hygroexpansion (Paper F). Wood
tracheids have high aspect ratios despite their short length [30, 31] and the
composites studied in this thesis are based on intact, slender and wellseparated wood fibres, which have not been broken down during a forceful
manufacturing process such as injection moulding. For this reason,
micromechanical models for continuous fibres are chosen over short-fibre
approaches. Papers B, D and E focus on experimental techniques (studies of
stress transfer in Papers B and D and of hygroexpansion in Paper E). The
content of the different papers and how they are correlated is visualized in
Figure 1.
This introduction is followed by a brief inventory of how stress is
transferred in paper and composites and of how stress transfer and interface
properties are commonly measured. Modelling approaches used by other
authors to quantify interface properties are discussed. A description of the
wood fibre ultrastructure is given, since the rather complex configuration
and composition of wood fibres affect their physical and mechanical
properties, making them anisotropic. The anisotropy of the mechanical
properties and hygroexpansion of wood fibres, and simplifications thereof,
are then discussed. Fibres and matrix materials used are presented in the
section “Materials and Methods” where material preparations as well as
experimental methods used are described. This is followed by the modelling
strategies adopted for characterization of interface efficiency and wood fibre
hygroexpansion. Results are presented and discussed and suggestions for
future investigations are given.
4
Wood-fibre
fibre composites: Stress transfer and hygroexpansion
Figure 1: Schematic illustration of the app
appended papers.
5
Karin M. Almgren
1.1 FIBRE-MATRIX INTERFACE
In composite materials, stress is transferred between matrix and fibres.
Several bonding mechanisms between fibres and matrix are possible, e.g.
interdiffusion, chemical bonding and mechanical locking, as illustrated in
Figure 2.
FIBRE
FIBRE
A
A
B
MATRIX
FIBRE
A
B
A
B
MATRIX
B
MATRIX
Figure 2: Illustration of interdiffusion, chemical bonding through covalent bonds and
mechanical locking.
Interdiffusion is a bond formed by diffusion of polymer chains on one
surface into the polymer network of the other phase. If groups on the
surfaces of fibre and matrix form bonds, different kinds of chemical or
physical bonding can occur. The bonds could be covalent, dipole or
hydrogen bonds, or van der Waal forces. Coupling agents are commonly
used to form chemical bonds between fibres and matrix. The strength of the
bonds depends on type and amount of the chemical bonds or on the degree
of entanglement and the amount of entangled chains.
Mechanical locking or keying occurs when the fluid matrix solidifies on a
rough fibre surface. For synthetic fibres, which generally are rather smooth,
this type of frictional bond is considered to give only a small contribution to
the fibre-matrix interface strength. More than one bonding mechanism may
occur, e.g. chemical bonding and mechanical locking, and types of bonding
in the composite is naturally depending on the types of fibres and matrix.
Also depending on the characteristics of the material, the bonded zone is
described either as an interface, i.e. as a surface between fibres and matrix,
or as an interphase, i.e. as a third material phase with properties between
those of fibres and matrix.
In wood-fibre composites, however, the fibre-matrix interface is not the
only possible stress transfer mechanism. Depending on types and amount of
matrix and fibre-fibre contacts, stress transfer is possible also between
fibres. This is how stress is transferred in paper and board, as illustrated in
Figure 3.
6
Wood-fibre composites: Stress transfer and hygroexpansion
While wet pressure is used to improve stress transfer between fibres, various
methods, e.g. fibre modifications and polymer grafting, are used to improve
fibre-matrix interface. Several methods and their effects on natural-fibre
composite properties are described in the reviews by Bledzki and Gassan
and Nabi Saheb and Jog [32, 33]. Methods applied to wood-fibre
composites are described by e.g. Bledzki [17].
Figure 3: Illustration of stress transfer regions: fibre-matrix interface in composite
material and fibre-fibre bond in paper application.
1.1.1 Stress transfer in paper and board
In paper and board, where stress is transferred between fibres, both the
amount of fibre-fibre bonds and bond strength are of importance. In the
literature, the amount of fibre bonds is commonly related to the relative
bonded area (RBA), defined as the fibre to fibre bonded area divided by
total fibre area. Several authors have correlated RBA to paper strength [3436] and stiffness [37, 38]. A light scattering technique is commonly used to
measure RBA [35, 39]. Free fibre surfaces reflect more light than bonded
fibre segments and hence paper sheets with few fibre-fibre bonds, i.e. small
RBA, reflect more light than a well consolidated sheet with many fibre-fibre
bonds and large RBA. The reflected light of the total fibre area is found by
measurements of unbonded reference sheets, and the RBA is then
determined by comparing the scattered light from investigated samples and
unbonded reference sheets. The study of hydrogen gas absorption has been
suggested to give more accurate results since the nitrogen molecules are
smaller than the wavelength of light [40, 41]. Surfaces inside lumens and
micro cracks in the fibre wall will however also be identified as free surfaces,
making these indirect methods imprecise [42].
A direct method to determine RBA has been presented by Yang et al [43]
who used image analysis to study thin cross sections of paper sheets. Their
study suggests that one fibre cross-section could be bonded to as many as
five different fibres if lumen was not collapsed, compared to a maximum of
two as suggested by other authors [44]. The method is limited by resolution
and the fact that examination of the cross sections is done manually, which
could introduce an error to the method since definition of fibre contacts
may differ between operators. An improved version of this direct but
approximate method is developed in Paper D, where X-ray tomography is
Karin M. Almgren
7
used for enhanced resolution and computerized image analysis tools are
used to identify fibre-fibre contacts.
Bond strength has been studied in single-fibre tests [19-22, 45]. Different
set-ups, e.g. fibre crossings and fibres pressed against thin shrives or
cellophane, have been used to study shear strength of fibre bonds. However,
these tests are difficult to perform; sometimes only about 30-50 % of the
prepared specimens are deemed good enough to test and the variability of
the test results is large, in some cases over 100 % [20]. Experiments of this
type provide information of the strength of fibre joints, but to determine the
strength of the actual fibre-to-fibre bonds, the bonded area in the joint must
be determined. The surface of wood fibres is sometimes rough, and
overlapping fibres are not necessarily bonded, or even in contact, over the
entire overlapping area [46]. In the studies mentioned above, the bonded
area is determined by light scattering techniques or simply by studying
samples in a microscope.
Indirect test methods to characterise fibre-fibre bonding have been applied
to avoid the difficulties of single fibre tests. Z-strength is such an indirect
measure of the fibre-to-fibre bonding in paper sheets [42, 47]. If the fibres
in a paper sheet have an in-plane orientation, the strength in the out-ofplane direction, i.e. z-direction, is mainly dependent on the strength and
amount of fibre-fibre bonds. However, if some of the fibres have an out-ofplane orientation, which is commonly the case especially for commercial
sheets, the z-strength will be dependent also on the axial fibre strength. A
difference between the z-strength test and single-fibre methods is that the zstrength determines the bond strength normal to the fibre surface, while
single-fibre tests are used to determine shear strength of the fibre to fibre
bond. Shear strength and failure are believed to be the most important
mechanisms in paper fracture. Despite this, and due to the simplicity of the
z-strength tests, these methods for determination of fibre-fibre bonding
strength are far more common than single-fibre tests.
1.1.2 Stress transfer in composite materials
The properties of the fibre matrix interface contribute to stiffness and
strength of the composite and control the fracture behaviour of the material
to a large extent. A weak interface gives poor stress transfer between matrix
and fibres and the reinforcing abilities of the fibres are hence not fully
utilized. At failure, fibres will be pulled out of the matrix, giving a ductile
fracture. Strong interface gives stiffer and more brittle materials. At failure,
the fibres will rupture rather than being pulled out of the matrix. It is thus
important to control interface properties when designing a new material to
obtain a combination of good mechanical properties.
8
Wood-fibre
fibre composites: Stress transfer and hygroexpansion
Direct evaluation of stress transfer abilities is difficult due to the complex
nature and the small scale of the fibre-matrix
fibre matrix interface. For composite
applications, single-fibre
fibre tests are used, e.g. fibre fragmentation and fibre
pull-out,, both illustrated in Figure 4.
Figure 4: Illustration of single fibre tests for testing of interface properties: fibre
fragmentation test and fibre pull
pull-out test.
information
ion assessed from these tests has been
The evaluation of thee informat
discussed by several authors. Désamont and Favre [[48] and Favre and
Merienne [49]] pointed at the operational difficulties of the single-fibre
single fibre pullpull
out test, but concluded that even though the single-fibre
single
pull-out
out test offers
a coarse simplification of the complex structure of the composite, important
information about fibre-matrix
matrix interaction can be derived from the tests.
Preparation of test specimens is considered cumbersome
cumbersome, even for synthetic
fibres, which
ch show less variability and are much longer, and thus easier to
handle, than wood fibres. In their review on single-fibre
single fibre fragmentation test
test,
Tripathi and Jones [50]] highlight the problem of correct interpretation of
single-fibre
fibre fragmentation tests. Thei
Theirr review shows that it is difficult to
predict macroscopic composite properties if the knowledge of the
interphase is based solely on results from single-fibre
single fibre fragmentation tests.
This is explained by the somewhat unrealistic test conditions and the
difficulty
culty of separating interphasial strength from other mechanisms
activated during the test, e.g. matrix yielding and cracking. However, for the
ranking of different surface treatments with respect to interfacial shear
strength, the single-fibre
fibre fragmentatio
fragmentation test is very useful [51].]. A different
approach of single fibre tests is offered by Raman spectroscopy [[52].
Through the study of changes in peak wavenumber shifts in the Raman
spectra of stretched fibres embedded in polymeric matrix, information of
interface
face efficiency is attained. The difficulties of handling and mounting
single wood fibres are, however,
however the same as for other single fibre tests.
To avoid cumbersome single fibre tests, the dynamic mechanical analyser
(DMA) technique has been suggested for
f evaluation of interface properties
during cyclic loading, i.e. not at interfacial failure as in the single-fibre
fibre tests
described above.. Kubát et. al [[53] used dynamic mechanical thermal analysis
(DMTA) to investigate the effects of a coupling agent in a glass sphere
spherepolymer composite. They suggested
suggest that the coupling agent creates an
9
Karin M. Almgren
interface between fibres and matrix, and derived a parameter to compare
efficiency of interfaces. Simple models were used to derive an interface
parameter and the authors concluded
concluded that the method was convenient, albeit
approximate. Afaghi-Khatibi
Khatibi and Mai [54] used DMTA to study the effect
of cyclic fatigue on interface properties in a carbon fibre-epoxy
fibre epoxy resin
composite. They concluded, in agreement with Kubát et al., that th
the DMA
technique can be used to detect the presence of different interfaces in
composite systems, but that results should be interpreted with care. In this
thesis interface is studied through DMA-measurements
DMA measurements in Papers A and C.
In Paper C, a more sophisticated
sophisti
model for interface studies developed by
Hashin [55], was employed
employed. The model is based on his previous composite
models where perfect fibre
fibre-matrix
matrix interface is assumed, i.e. on composite
cylinder assembly (CCA) and generalized self consistent scheme (GSCS)
(GSCS), but
extended with parameters for interface properties to investigate the effect of
imperfect interface on composite properties. In the CCA model, concentric
composite cylinders (fibres coated with matrix) of same fibre
fibre-matrix
fractions but different,
t, smaller and smaller, radius, are stacked together to
form a composite. The microstructure is shown in Figure 5a. In the GSCS
model, a fibre surrounded by matrix is embedded in a composite substrate
with the effective
ctive properties of the composite, Figure 5b. In [55],], where the
effects of an imperfect interface on mechanical properties of composite
materials are studied, both models are used in parallel and deliver the same
results. The matrix is assumed to be isotropic while the fibres have
anisotropic material properties. The interface is assumed to be elastic and is
represented by three interface parameters as shown in Figure 5c. For
derivation of the interface parameter µR only the GSCS model is used. In a
following publication, the model was extended to cover linear viscoelastic
properties of a fibre matrix interphase [56].
[
(a)
(b)
(c)
Figure 5: Fibre arrangement in (a) CCA-model: Fibres (black) coated
ted with matrix
(white), (b) GSCS-model:
model: Fibre (black
(black)) coated with matrix (white) embedded in
composite substrate (grey) and (c) elastic interface parameters µA, µT and µR as described
in [55].
10
Wood-fibre composites: Stress transfer and hygroexpansion
Nairn [57] used shear-lag analysis of concentric cylinders to investigate
effects of an elastic, imperfect interface. Data from fragmentation studies of
a carbon fibre-epoxy system were used to exemplify how an imperfect
interface parameter can be derived from experimental data with the shearlag analysis derived. The shear-lag analysis presented in his work describes
axial interface properties, compared to Hashin’s model which also includes
interface parameters tangential and normal to the fibre surface. Wu et al [58]
predicted the transverse shear modulus of a three phase system consisting of
coated spheres or fibres embedded in a matrix. The inclusion-coating
interface was assumed to be perfect, while the coating-matrix interface was
imperfect and elastic, like a spring layer of vanishing thickness. Their results
showed the same trends as presented by Hashin [55] for the investigated
transverse shear modulus of the fibre composite.
1.2 WOOD FIBRES AND THEIR HYGROEXPANSION
The physical and mechanical properties of wood fibres, e.g. hygroexpansion,
stiffness and strength, depend on various factors like species, growth
conditions and pulping process. The influence of these parameters on wood
fibre properties is briefly described in the following.
1.2.1 Structure of wood fibres
Both softwoods (e.g. pine and spruce) and hardwoods (e.g. birch) are used
in the Swedish pulp and paper industry. Softwoods generally offer long
(~2-3 mm) and flexible fibres, while hardwood fibres are shorter (~1 mm)
and stiffer. Differences are also seen between fibres grown during
springtime (earlywood) and during the summer (latewood), where earlywood
fibres are larger and have thinner cell walls than the denser latewood fibres
[31]. The properties of pulp fibres are highly dependent on the method of
fibre extraction, i.e. the pulping process. In chemical pulp, wood is separated
into fibres chemically, while mechanical treatment, i.e. grinding, is used to
extract the fibres in mechanical pulp. Mechanical pulping renders fibres that
are stiff and straight with high bending stiffness. The fibres are short and
thick and less collapsed compared to fibres from chemical pulps and contain
relatively high amounts of lignin and hemicellulose. Mechanical pulp is used
for e.g. newsprint, while the slender and flexible fibres from chemical
pulping processes are used in e.g. copy paper [31].
Wood fibres are hollow and have a layered structure with primary and
secondary walls, as illustrated in Figure 6a. The primary wall is thin
compared to the secondary walls and consists to a large extent of lignin. In
the secondary walls, lignin and hemicellulose are reinforced by cellulosic
fibrils. In the thick S2 layer the fibril orientation is close to parallel to the
fibre axis, while fibrils in the thinner S1 and S3 layers have a clear off-axis
orientation [59]. The inner (S3) layer surrounds lumen, the hollow centre of
Karin M. Almgren
11
the fibre. As a consequence of the fibril orientation in the layered structure,
the wood fibres are anisotropic with different properties in longitudinal,
radial and tangential directions, illustrated
illus
in Figure 6b.
b. The off-axis
off
orientation of the microfibrils gives a spiral structure, with a resulting
coupling between twist and extension.
(a)
(b)
Figure 6: (a) Schematic
hematic illustration of the cell wall of a softwood fibre with different
orientation of cellulose microfibrils in the layers [60]. (b) Illustration of simplification
fication of
wood fibre to transversely isotropic cylinder.
Several authors have investigated the longitudinal Young’s modulus of
longer natural fibres, i.e. hemp and flax, by single fibre tests [29].
[ ]. Transverse
Young’s modulus and shear properties are not
not available in the literature to
the same extent due to the difficulties in experimental determination of
these properties. The values found are often obtained through studies of the
natural-fibre
fibre composite or of the cell wall structure of the fibre [[29, 61].
Wood fibres are small also in the longitudinal direction,
direction which makes single
singlefibre measurements to determine longitudinal Young’s modulus
cumbersome and sometimes uncertain [62].
[ ]. Transverse properties are found
in similar manners as for longer natural fibres, with studies of the fibre cell
wall. In the determination of transverse properties, wood fibres are often
regarded as transversely isotropic and the anisotropy ratio, i.e. the ratio of
longitudinal to transverse Young’s modulus is determined [62,
[62, 663]. The
properties in radial direction are hence assumed to be equal to those in the
tangential direction. This simplification,, employed in Papers A, C and F, is
considered reasonable, since differences between properties in radial and
tangential directionss are small compared to the properties in the longitudinal
direction. For small microfibril angles and for constrained fibres in
composites, the helical structure and twist-extension
twist
n coupling is generally
ignored.
12
Wood-fibre composites: Stress transfer and hygroexpansion
1.2.2 Wood fibre hygroexpansion
Hygroexpansion and moisture induced softening of the hydrophilic wood
fibres are perhaps the most severe drawbacks of wood-fibre composites.
Hygroexpansion of the fibres leads to deformation of the composite
component and the stiffness and strength of the composite are decreased by
the moist induced softening of the fibres. If hydrogen bonds between fibres
and in fibre-matrix interfaces are reduced by the absorbed moisture, stiffness and strength are further diminished and in the presence of moisture,
cellulose is also more susceptible to microbial attacks.
In papermaking, where additives are used to increase the wet-strength of
paper, focus has been on preserving the fibre-fibre bonds. In the field of
natural fibre composites, efforts have been made to reduce the hygroexpansion of the reinforcing material, and different approaches have been
suggested. Treatment with acetylation is one of the most studied methods
presented in the literature. It is reported to increase dimensional stability and
decrease hygroexpansion of wood and natural-fibre based materials.
Unfortunately, stiffness and strength are also decreased by the acetylation
treatment [64-68]. Various cross-linking reactions have been used to
improve the dimension stability and wet-strength of paper. According to the
review by Caulfield and Weatherwax [69], formaldehyde has been of primary
interest for fibre cross-linking since it has been reported to increase wetstrength and decrease moisture sorption of paper. Wood is however not
completely stabilized by the reaction with formaldehyde according to the
later review by Bledzki et al. [17]. Instead of formaldehyde, the effects of
cross linking with butanetetracarboxylic acid (BTCA) on hygroexpansion of
wood-fibres for composite applications are investigated in Paper E.
The hygroexpansion of wood fibres shares a common trait with fibre-matrix
interface properties in that it is difficult to measure the effects through
single-fibre tests. The small dimensions, natural variability and the
anisotropic swelling of the fibre make single-fibre tests of fibre swelling
cumbersome. Attempts to assess information of wood fibre swelling from
the swelling of paper sheets have been made [70, 71]. This is, as discussed by
the authors, not without difficulty, since paper is a heterogeneous and
porous material. Some of these difficulties can be avoided if back-calculation
is performed from well consolidated wood-fibre composites instead of
paper sheets, which is the approach employed in Paper F. Modelling of
hygroexpansion of the fibre cell wall, where hygroexpansion of the
constituents cellulose, lignin and hemi-cellulose are used as input
parameters, is an alternative way to asses information about wood-fibre
hygroexpansion [72].
Karin M. Almgren
13
Similar to the mechanical properties, the hygroexpansion and moistureinduced deformation of fibres is complex due to the complex structure of
the wood fibre; the hygroexpansion in radial, tangential and longitudinal
directions vary and the fibres tend to twist since the fibrils are not parallel to
the fibre axis [73]. The anisotropic swelling also leads to other changes of
the form of the fibre. Transverse swelling of the fibre cell wall straightens
buckled fibres making them longer and opens the fibre cross section from
the elliptic or rectangular cross section of dry fibres, to the more circular
cross section. These form changes are not directly linked to the fibre
coefficient of hygroexpansion. In the case of fibre elongation, generally only
a minor part of the elongation is caused by actual elongation of the fibre
correlated to the longitudinal coefficient of hygroexpansion. The main part
of the elongation is explained by straightening of the buckled fibre, initiated
by the radial swelling of the fibre cell wall [74]. When determining the
coefficients of hygroexpansion, which are material parameters, it is therefore
important to separate actual swelling of the fibre and other geometrical
effects that might contribute to changing the form of the fibre.
14
Wood-fibre composites: Stress transfer and hygroexpansion
2 MATERIALS AND METHODS
In this thesis, interface properties in wood-fibre composites are studied and
new methods for interface characterisation are investigated. The aim has
been to avoid the cumbersome single-fibre tests and coarse DMA methods
for the benefit of simpler and more straightforward methods, including
more suitable micromechanical models with a balance of simplicity and
accuracy. Hygroexpansion of wood-fibres and wood-fibre composites have
been studied with the same goals: to simplify measurements by avoiding
single fibre tests. Single-fibre tests give, however, well-defined, local
measurements, and could be used to validate macroscopic characterization
techniques.
2.1 MATERIALS AND MANUFACTURING
Three different polymers were used as matrix material. The composites
investigated in the interface studies in Papers A, B and C were wood-fibre
reinforced polylactide (PLA). Polylactide is a thermoplastic and
biodegradable polymer derived from starch-rich plants like maize and wheat.
Polylactide is rather brittle, but the adhesion to wood and natural fibres is
good compared to more hydrophobic, non-polar polymers, and several
studies have shown that it is suitable as a matrix material in natural-fibre
composites [3, 75]. In Paper E, where hygroexpansion of wood fibres and
wood-fibre composites was investigated, both polylactide and polypropylene
(PP) were used as matrix material. Polypropylene is petroleum-based,
inexpensive and commonly used in both natural- and synthetic-fibre
composites. Composites with a polylactide matrix, polypropylene matrix and
a mixed polylactide-polypropylene matrix (50 wt % of each) were used, but
only well consolidated composites with a polylactide matrix were further
investigated in Paper F. In the study of stress transfer in Paper D, the matrix
material was an epoxy vinyl ester. Like polypropylene, epoxy vinyl ester is
petroleum-based and commonly used in composite applications, but
contrary to polylactide and polypropylene that are thermoplastic, epoxy
vinyl ester is a thermoset polymer.
The reinforcing wood fibres were softwood pulp fibres (fully bleached in
Papers A-C and unbleached in Paper D) or bleached birch pulp from
industrial pulp (Papers E and F). The fibre treatment studied was butanetetracarboxylic acid (BTCA) modification. BTCA modification prevents the
fibres from swelling by cross-linking of the hydrogen groups in the fibre cell
wall [76]. Both modified and untreated reference fibres were used for
composite manufacturing and the results were compared.
The thermoplastic polymers, polylactide and polypropylene, were delivered
as short fibres. Since both matrix material and reinforcement - wood fibres -
Karin M. Almgren
15
were delivered as fibres, a wet-forming technique much similar to the
method used to make paper sheets for laboratory investigations was applied:
Wood and polymer fibres are mixed in water and when the water is
removed, a commingled wood- and polymer-fibre sheet is formed. Sheets
with even, in-plane fibre distribution (Papers A, C, E and F) and oriented
fibre sheets (for the FT-IR study in Paper B) were produced. After drying,
the sheets were hot pressed, which caused the polymer fibres to melt and
form a void-free matrix. The sheets were pressed individually in
Papers A, B and C to obtain thin composites, while in Papers E and F
sheets were stacked before pressing to render thicker composite plates.
In Paper D, where a thermoset resin was used, a resin transfer moulding
(RTM) was used to manufacture the composites. Wood-fibre sheets were
formed and vacuum suction was used to remove air and fill the sheets with
the resin. The filled sheets were then cured in an oven to solidify the resin,
and thereby stiff composites were prepared. Both RTM and hot-press
moulding result in composites with slender fibres, enabling improved
mechanical properties as compared to injection-moulded wood-fibre
composites, where the fibre length is degraded in the severe shear flow
during processing.
2.2 EXPERIMENTAL CHARACTERISATION
Experimental work was performed to study fibre-matrix interface properties
and wood-fibre hygroexpansion. Some of the collected data was used as
input to the modelling approaches for interface characterization and
determination of wood-fibre coefficient of hygroexpansion, as described in
section “2.3 Modelling tools” and presented in detail in Papers A, C and F.
The subjects of interest for experimental characterisation in this thesis are
given below, together with experimental methods used.
2.2.1 Material characterisation
Polylactide, the most employed matrix in this thesis, is a semi-crystalline
polymer and its mechanical properties depend on the degree of crystallinity.
When mechanical properties of polylactide film are used as input in the
modelling sections, and the predicted results are compared to measured
composite data, it is important to establish whether the degree of
crystallinity of polylactide film and of composite samples are the same.
Therefore, the crystallinity of pure polylactide film and composite samples
(of the type studied in Papers A-C) was studied through DSC
measurements.
To evaluate the effect of BTCA modification on water absorption of wood
fibres the water retention value of modified and untreated reference fibres
was determined. A standardized centrifuging method was used; after
16
Wood-fibre composites: Stress transfer and hygroexpansion
centrifuging, the pulp is weighed, dried and reweighed. The water content
corresponds to the weight loss and the water retention value is expressed as
a percentage of water content to the dry weight of the sample [77].
A microscopy survey was performed to study the microstructure of selected
composite materials and the presence of voids, cracks, fibre agglomeration
and filled lumens was observed. Small samples were embedded in epoxy and
the cross sections were gently polished before the electron microscope
(ESEM) examination.
2.2.2 Mechanical properties
Linear viscoelastic mechanical properties, i.e. Young’s modulus, E, and loss
factor, tan δ, of the composites, wood-fibre sheets and pure polylactide film
used in Papers A and C were determined with dynamic mechanical analyzer
(DMA). Cyclic testing was performed in dry and humid conditions to study
the influence of moisture on the materials and generate data needed for the
micromechanical models presented for interface characterization. A smaller
DMA-equipment connected to the FT-IR was used to generate Young’s
modulus and tan δ of the composite, wood-fibre and neat polylactide
samples tested in Paper B.
In Paper D the stiffness and strength of composite plates, wood-fibre sheets
and pure resin samples were determined with quasistatic tensile tests. In
Paper E, where the effects of fibre treatment, choice of matrix and fibre
fraction were studied, three-point-bending tests were performed to compare
stiffness and strength of the samples. Tensile tests are preferable due to a
uniform and uniaxial stress field in the gauge section, although the flexural
tests were chosen for practical reasons in cases where the manufactured
composite plates were too small to machine standard dog-bone specimens.
2.2.3 Bonds and bond strength
In Paper B, thin wood-fibre polylactide composites were studied with
Fourier Transform Infrared technique (FT-IR). Stretching and bending of
molecular bonds in cellulose and polylactide could be observed as the
samples were subjected to cyclic loading. Comparison of these motions
when samples were tested under dry and humid conditions was performed.
Observed differences could be interpreted in terms of fibre-matrix stress
transfer-ability. FT-IR spectroscopy was used in Paper A and in Paper B,
where a more detailed evaluation of the technique was performed.
In Paper D, the degree of consolidation in wood-fibre sheets was studied
with z-strength tests, i.e. strength in the out-of-plane direction, as described
in section “1.1.1 Stress transfer in paper and board”. This was further
studied with X-ray microtomography at beamline ID19 at the European
17
Karin M. Almgren
Synchrotron Radiation Facility (ESRF)
(ESRF) in Grenoble, France. The tomograph
gives three-dimensional
dimensional (3D) images of the samples with high resolution
(0.7 µm × 0.7 µm × 0.7 µm). Image analysis was performed over the tested
volumes to obtain a measure of the fibre-fibre
fibre
contact area.
The procedure
re of the image analysis method used is presented in Figures 77
8. In the tomography of a sample volume, Figure 7,, wood fibres are clearly
seen. Figure 8a shows a cross section of the sample volume. Fibre voxels are
white and surrounding non-fibre
non fibre voxels (matrix material) are black. The
lumen voxels are then identified, grey colour in Figure 8b, and fibre
ibre contacts
are identified by letting rays run through the image in the z-direction,
direction, i.e.
thickness direction, of the sample. Any time a ray passes between two
separate lumen areas without touching any non-fibre
non fibre voxels, a contact is
considered to be found.
Det går inte att v isa bilden. Det finns inte tillräckligt med ledigt minne för att kunna öppna bilden eller så är bilden skadad. S tarta om datorn och öppna sedan filen igen. O m det röda X:et fortfarande v isas måste du kanske ta bort bilden och sedan infoga den igen.
Figure 7: Wood-fibre
fibre composite sample studied by tomography.
(a)
Det går inte att v isa bilden. Det finns inte tillräckligt med ledigt minne för att kunna öppna bilden eller så är bilden skadad. S tarta om datorn och öppna sedan filen igen. O m det röda
X:et fortfarande v isas måste du kanske ta bort bilden och sedan infoga den igen.
(b)
Figure 8: Illustration of image analysis procedure.
procedure (a) Cross section of wood-fibre
fibre
composite sample. (b) A ray (dark grey) passes through identified lumens (grey
grey)
surrounded by fibre wall (white). Average
Ave
fibre diameter ~30 µm.
For each x- and y-coordinate,
coordinate, a ray is computed and the total contact area is
defined as the number of identified contacts for all rays. Similarly, the total
fibre area is defined as the total number of times the material changes
chang
between fibre wall and non-fibre
non fibre along the rays. The measure of the relative
amount of fibre contact area is obtained by dividing the total contact area by
18
Wood-fibre composites: Stress transfer and hygroexpansion
the total fibre area. A more detailed description of the image analysis tools
used is given in [78]. The method presented will not detect collapsed fibres,
which leads to an underestimation of the fibre contact area, in particular for
chemical pulps. Furthermore, if two fibres are close enough (0.7 µm), a fibre
contact will be identified whether the fibres are bonded or in contact or
neither. This may lead to an overestimation of the fibre contact area. The
aim of the study was not, however, to attain an exact value of fibre contact
area, but to compare the degree of bonding of different composite samples
and the corresponding effect on composite stiffness and strength. For that
purpose, the developed method should suffice.
2.2.4 Hygroexpansion and vapour and water sorption
Dynamic vapour sorption system (DVS) was used to study vapour
absorption of the materials used in Papers A and C. The temperature was
kept constant and the moisture content of small samples of composite,
wood-fibre mat and PLA was determined. Every fifth hour the relative
humidity in the test chamber was increased in 10 % relative humidity steps
from the initial dry condition to the final 90 % relative humidity. The
samples used for the studies of hygroexpansion in Papers E and F were too
big for the DVS to be used. Instead, samples were dried in an oven to
obtain the dry weight and kept in a sealed humidity chamber for the
moisture absorption test. Weight and thickness of the samples was
continuously and manually collected during the time of the test.
2.3 MODELLING TOOLS
In Papers A and C, the fibre-matrix interface was studied through DMAmeasurements and linear viscoelastic material properties were used. In
Paper F, where the wood fibre coefficient of hygroexpansion was
determined, only the hygroelastic properties were considered. In all three
papers (A, C and F), the modelling section is divided into micromechanical
models and laminate analogy as illustrated in Figure 9.
Mechanical properties of
wood fibre and matrix
material
matrix material
Mechanical properties of
unidirectional composite
lamella
Micromechanical model
Mechanical properties of
unidirectional composite
lamella
+
+
+
+
Laminate analogy
Mechanical properties of
composite laminate with
arbitrary fibre orientation
Figure 9: Illustration of the length scales used in the models.
=
19
Karin M. Almgren
The micromechanical model links the mechanical properties of the
constituents, fibres and matrix, and interface properties to the mechanical
properties of a hypothetical, unidirectional composite lamella. Laminate
analogy is then used to predict the mechanical properties of a composite
material with any given in-plane fibre orientation. This is made through a
summation of auxiliary, unidirectional lamellas that are added together to a
composite of given fibre distribution using classic laminate theory.
The stress-strain correlation of purely elastic materials is given by Young’s
modulus, E. For viscoelastic materials, storage modulus, E', and loss
modulus, E'', are used. Young’s modulus can then be expressed as a
complex value, E*, where the storage modulus corresponds to the real
component and the loss modulus to the imaginary component,
E * = E′ + iE′′ .
(1)
The energy loss in a material can be expressed as the loss angle, δ. The loss
angle is defined as the time delay between stress and strain, and is related to
the loss modulus and the storage modulus through
tan δ =
E ′′
E′
(2)
and commonly referred to as the loss factor, η,
η = tan δ .
(3)
Micromechanical models, as Halpin-Tsai’s [24] and Hashin’s [55] models
used in this study, are generally derived for purely elastic materials. The
correspondence principle, however, can easily be used to extend the
validation of the equations to the linear viscoelastic case [79-81]. The rule of
mixtures, used to predict the longitudinal Young’s modulus of a
unidirectional composite lamina, EL is transformed from
E L = Vf E f1 + V m E m
(4)
where E is Young’s modulus, V is volume fraction, ‘f’ and ‘m’ denote fibre
and matrix properties, respectively, and f1 means in axial fibre direction,
coincideing with the longitudinal direction, L, of the composite lamella, to
EL* = Vf Ef1* + Vm Em*
(5)
20
Wood-fibre composites: Stress transfer and hygroexpansion
where E L* , E f1* and E m* contain both storage (real) and loss (imaginary)
components according to Equation 1.
Many micromechanical models, as Halpin-Tsai’s model [24] used in
Paper A, assume perfect interface between fibres and matrix. A comparison
between predicted energy loss in a material and measured actual energy loss,
when a material with imperfect interface is subjected to loading, could hence
give indications on the efficiency of the interface. An imperfect interface
results in stress redistribution from fibre to matrix, which affects the
dissipation during cyclic loading.
In Paper A, where this model was used for the transverse and shear
properties at the level of a unidirectional ply, viscoelastic material properties
of wood-fibres and polylactide were used as input data. Using a laminate
analogy, the damping of a wood-fibre-polylactide composite with perfect
interface could hence be predicted by the model. The predicted damping
could then be compared to experimentally determined data, and since tests
were performed under both dry and humid conditions, the effect of
humidity on energy losses in the material could be analyzed.
As in Paper A, where composite damping and Young’s modulus are
predicted, micromechanical models are commonly used to predict
composite properties from known properties of its constituents. Due to the
difficulty of determining wood-fibre and interface properties, it can be
useful to employ the micromechanical models the other way around, starting
with experimentally determined composite properties and then predicting
contributing properties of wood-fibres or interface. This approach was used
in Paper C, where Hashin’s micromechanical model for imperfect elastic
interface [55] was used in the search for a quantitative measure of the
interface. The model with an elastic interface was chosen over the
viscoelastic interphase, since the fibre-matrix interface in the investigated
material is considered to be thin and hence the damping effects of the
interface should be small. Hashin’s model includes three interface parameters µA, µT, and µR, as discussed earlier and illustrated in Figure 5. The
relations between the interface parameters are not known. To reduce the
number of unknowns, the three interface parameters were assumed to be
equal and were replaced with one interface parameter, µ.
In Paper F, a method similar to the back-calculation approach used for
interface studies in Paper C was utilized to determine the coefficient of
hygroexpansion of untreated reference fibres and BTCA modified fibres.
The fibres were used as reinforcement of a polylactide matrix and the
thickness swelling of the composites was monitored as the samples were
allowed to reach equilibrium in a humid environment. The out-of-plane
Karin M. Almgren
21
hygroexpansion of the composites was determined through thickness
measurements. If the mechanical properties of the constituents, fibre
volume fraction, fibre distribution and hygroexpansion of the matrix
material are known, the only material parameter left is the coefficient of
hygroexpansion of the wood-fibres, which could be found through backcalculation from composite properties.
The micromechanical model was constituted by Hashin’s [27, 82]
thermoelastic expressions, here used to describe hygroelastic behaviour. A
difference between temperature and moisture content is that the former
generally is known and constant in the constituent materials at equilibrium
conditions, while moisture content in the constituents is most likely to differ
depending on the hydrophilicity of the materials. Therefore, the relative
humidity, which, similar to temperature, is constant, was used to describe
hygroexpansion.
Hashin’s model was employed by McCartney and Kelly [28], who studied
the out-of-plane thermal expansion of composite laminates. They found that
constraints from neighbouring plies had a significant effect on the out-ofplane thermal expansion. The influence of in-plane stresses in the composite
laminate should therefore be addressed when the out-of-plane
hygroexpansion is considered. As in the previous studies, the fibres were
assumed to be transversely isotropic in stiffness, but also proportionally in
hygroexpansion, which was supported by the results of Schulgasser [83].
Only linear elastic and linear hygroexpansion properties were considered,
and the fibre-matrix interface was assumed to be perfect.
The material parameters required as input to the micromechanical models,
and hence to determine interface properties or fibre hygroexpansion
through back-calculation, are the complex Young’s modulus of composite
material, wood fibres and polylactide, the fibre volume fraction and
distribution and the anisotropy ratio of the fibres. For the studies of
hygroexpansion, the coefficients of hygroexpansion of composites and
polylactide are required. For the composite material and the isotropic
polylactide, these properties are simply determined through dynamic
mechanical analysis and thickness measurements. Since direct measurements
of wood fibres are difficult and uncertain, the properties of the wood fibres
were determined through measurements on well-consolidated wood-fibre
mats and back-calculation from laminate analogy and micromechanical
models. The approach, derived by Neagu et al. [71], was similar to
procedures used in Papers C and F.
In Papers C and F, as well as in the wood-fibre stiffness back-calculation,
the summation over the lamellas in the laminate analogy was made with the
22
Wood-fibre composites: Stress transfer and hygroexpansion
assumption of constant in-plane strain and zero stress in the out-of-plane
direction. In Paper A, the summation was performed under constant stress
over the lamellas. In paper technology, both methods are used; uniform
stress tends to underestimate the stiffness of paper, while the assumption of
uniform strain leads to an overestimation of the stiffness [84]. The latter is
considered the most accurate for these dense composites, albeit the
difference in predicted stiffness is not believed to be large.
Karin M. Almgren
23
3 RESULTS AND DISCUSSION
In this chapter, the results from experimental work and modelling tools will
be presented and discussed. The quality of the manufactured composites
was evaluated with scanning electron microscope (ESEM). Observations
from the microscopy survey correlated well with trends seen in Young’s
modulus of the samples, as discussed in “3.1.2 Mechanical properties”. The
importance of an efficient interface is emphasized and results from the
dynamic FT-IR spectroscopy are presented in“3.1.3 Bonds and bond
strength”, followed by results from the experimental work on
hygroexpansion and moisture absorption. Finally, the results obtained from
the modelling tools, where interface properties and wood-fibre
hygroexpansion were investigated, are presented and discussed.
3.1 MATERIALS AND EXPERIMENTAL CHARACTERISATION
3.1.1 Materials and manufacturing
In this work, four different polymer systems were used as matrix material:
polylactide, polypropylene, a mixture of polylactide and polypropylene and
epoxy vinyl ester. As reinforcement, bleached fibres (Papers A, B, C, E, F)
and unbleached softwood fibres (Paper D), are used. In Papers E and F the
effects of BTCA modification on wood-fibre hygroexpansion and
absorption was studied. The BTCA modification was considered successful,
with lower water retention value and less moisture absorption than the
reference fibres. The weight gain due to moisture sorption was 27 % for the
untreated fibres and 18 % for the BTCA modified fibres. A negative effect
however, was an increased tendency to form aggregates in the composite
manufacturing process. This was a problem particularly in combination with
the polypropylene matrix, resulting in poor adhesion and void-filled
composites.
Differential scanning calorimetry (DSC) was performed on polylactide
composites (of the type studied in Papers A-C) and on pure polylactide
samples to investigate whether the degree of cristallinity was the same in the
polylactide films studied to generate input data to the micromechanical
models (used in Papers A and C) as in the composite samples. No difference
could be seen between the polylactide film and the composite sample, and
the crystallinity was determined to approximately 43 %. Polylactide has
previously been suggested for wood/natural-fibre composite applications [3,
85] and its affinity to wood fibres has been reported to be high. The
adhesion of the hydrophobic polypropylene to wood-fibre is not as good as
that of the hydrophilic polylactide, which was supported by the microscopy
surveys performed in this thesis.
24
Wood-fibre composites: Stress transfer and hygroexpansion
Figure 10a: Micrograph of polylactide composite with 70 % reference fibres. The darker
regions are voids
Figure 10b: Micrograph of polylactide-polypropylene composite with 40 % reference
fibres. Fibres are coated with polylactide (light grey); only few fibres are surrounded by
polypropylene (dark grey).
Figure 10c: Micrograph of epoxy composite with 18 % (volume fraction) fibres. High
affinity between epoxy and fibres, resin filled lumens and no voids are seen.
Karin M. Almgren
25
Micrographs of some of the different materials studied are presented in
Figure 10a-c. The wetting of the polylactide matrix on the wood fibres was
good as was fibre dispersion. Lumens of uncollapsed, thick-walled latewood
fibres were filled. However, for high fibre fractions, as in Figure 10a where
approximately 70 % reference fibres were used, a substantial amount of
voids are seen (since the density of polylactide, approximately 1.3 g/cm3
(data provided from manufacturer), is close to that of the wood fibres
1.5 g/cm3 [86], weight and volume fractions of fibres in polylactide
composites are nearly the same). With only 30 % matrix material, larger fibre
regions remained unimpregnated, which resulted in cracks and air pockets.
When polypropylene was used as matrix material, the tendency for the fibres
to agglomerate was increased and cracks between fibres and matrix were
seen also at lower fibre contents. The result of the 50/50 mixture of
polylactide and polypropylene is shown in Figure 10b, where the polylactide
surrounds the fibres like a coating. This gave a material with fewer voids and
cracks compared to the case where pure polypropylene was used. Similar
results have been presented by Huda et al. [3] in their study of wood fibres
in polylactide/polypropylene blends. The micrograph of one of the
composites manufactured in Paper D shows that epoxy vinyl ester
impregnation using RTM led to excellent wetting. Filled lumens were
observed and no voids were seen, Figure 10c.
3.1.2 Mechanical properties
Young's modulus (GPa)
The mechanical properties of the composites studied showed clear
improvement from polymer properties to composite properties when the
polymers were reinforced, Figure 11.
8
7
6
5
4
3
2
1
0
Neat matrix material
Composite
Epoxy
PLA
PP
Figure 11: Young’s modulus of neat and reinforced polymers.
26
Wood-fibre composites: Stress transfer and hygroexpansion
This confirms that the manufacturing of the composites were successful and
that fibres and matrix interact well. The trends from the microscopy survey
can also be found in the stiffness of the composite samples. The wood-fibre
reinforced epoxy resin (Figure 10c), where filled lumens but no voids or
cracks could be observed, shows the highest improvement in Young’s
modulus when fibres are added to the matrix. The Young’s modulus of
polylactide is also considerably improved when reinforced with fibres (40 %
fibres added to polylactide and polypropylene in Figure 11). When
hydrophobic polypropylene was used, the composite showed less
improvement of mechanical properties, especially for higher fibre contents
and BTCA-modified fibres, which correlates well with the observations of
agglomerated fibres and voids found e.g. in the micrograph in Figure 10b.
Mechanical testing was performed on the well consolidated, low fibre
fraction polylactide composites. No effect of the BTCA modification could
be seen. This is an advantage of BTCA modification as compared to e.g.
acetylation, where mechanical properties have been reported to be impaired
by the fibre treatment [65-68]. Although these results are promising, they
should be treated with care, since few samples were used and the scatter was
considerable. The highest modulus was found for composites with 40 %
fibres, followed by those with 30 % fibres. The results for higher fibre
fractions were lower due to insufficient wetting, in agreement with results
presented by other authors [87].
3.1.3 Bonds and bond strength
The importance of good fibre-matrix adhesion was emphasized by the
results in Paper D where effects on fibre-matrix interface and fibre-fibre
bonds on composite properties were investigated. Wood-fibre mats with
different levels of consolidation were manufactured through solvent
exchange and subsequent pressing. Composites were then made by epoxy
resin impregnation of the wood-fibre sheets, Table 1. The mechanical
properties of composites and sheets were then determined. In paper
mechanics, where the thickness of the sometimes fluffy low basis-weight
samples is difficult to measure, the properties specific stiffness and strength
are commonly used. These are commonly referred to as tensile stiffness
index and tensile strength index in the paper research community. Such
properties are also used in composite mechanics when thin composite plates
are designed, and are simply defined as Young’s modulus and strength,
respectively, divided by density. For the unimpregnated wood-fibre sheets,
z-strength was determined, and both unimpregnated sheets and composites
were studied by microtomography.
27
Karin M. Almgren
Table 1: Notation and characteristics of wood-fibre sheets and composites studied in
Paper D.
Description of sheet
Consolidation
Sheet
Apperence
Composite
low
S1
Fluffy
C1
Iso-propanol,
wet pressure: 0 bar
Iso-propanol,
wet pressure: 0.5 bar
Iso-propanol,
wet pressure: 2 x 0.5 bar
low
S2
Fluffy
C2
high
S3
Intermediate
C3
Reference
higest
S4
Thin
C4
The z-strength tests confirmed that different levels of consolidation had
been reached, Figure 12. After visual examination of the samples, this result
was not surprising. Two of the sheets, here denoted S1 and S2, were fluffy
with few fibre-to-fibre bonds, while the remaining two sheets, S3 and S4,
were better consolidated.
6
Z-strength (kNm/kg)
5
4
3
2
1
0
S1
S2
Figure 12: Z-strength of sheets S1-S4.
S3
S4
The different amounts of fibre contacts in the sheets were visible in image
analysis studies of the X-ray computer tomography images of the woodfibre sheets. It was also suggested that these differences were maintained
during the RTM process. This implies that composites with different
amounts of fibre contacts between the reinforcing fibres had been
manufactured. No effect of these fibre contacts could be seen on the
composite properties. In Figure 13, C1 and C2 represent composites made
from the fluffy sheets S1 and S2, respectively, and C3 and C4 are
composites made from the better consolidated sheets S3 and S4,
respectively.
28
Wood-fibre composites: Stress transfer and hygroexpansion
140
120
100
Strength (MPa)
100
80
80
60
60
40
40
20
20
0
0
Matrix
C1
C2
C3
Specific strength (kNm/kg)
120
C4
Figure 13: Strength of pure matrix material and composites C1-C4 (manufactured with
RTM from paper sheets with the same degree of consolidation as sheets S1-S4,
respectively).
The improved matrix properties suggest good stress transfer between the
polymeric matrix and the reinforcing fibres, and since no influence of the
amount of fibre contact could be seen, the fibre-fibre bonding is not
considered to be an important stress transfer mechanism in this type of
wood-fibre composites. Fibre bonds could play a more important role in
wood-fibre composites, where the fibre fraction is high or where the fibrematrix interface is weak. High fibre contents and poor fibre-matrix interface
are however known to result in poor mechanical properties of wood-fibre
composite materials, hence making them less interesting for structural
applications.
Dynamic FT-IR spectra show small changes in spectral intensity induced by
the straining of the tested sample. Instant and time delayed responses can be
seen in the in-phase and out-of-phase spectra, respectively. The wavenumbers on the horizontal axis of the dynamic FT-IR spectra correspond to
energy levels related with molecular vibrations characteristic for molecular
bonds. Molecular bonds affected by the straining of the sample, i.e. bonds
subjected to stretching or bending, can thereby be identified. The evaluation
of dynamic FT-IR spectra of composite materials requires information of
the absorbance of the constituent materials, i.e. the reinforcing cellulose in
the wood fibres and polylactide, the matrix material. In-phase spectra of
wood-fibre mat, polylactide film and composite material are presented in
Figure 14.
29
Karin M. Almgren
Dynamic FT-IR response
0.0015
Fiber
PLA matrix
Composite material
0.001
1435
1323
1377
1389
1369
0.0005
1265
0
1447
-0.0005
-0.001
1500
1450
1400
1350
1300
1250
Wavenumber (cm-1)
Figure 14:. In-phase spectra of wood-fibre mat, polylactide film and composite material
at 0 %relative humidity.
The signals at 1447, 1389, 1369 and 1265 cm-1 (corresponding to CH3
asymmetric bend, CH3 symmetric bend, CH and CH3 symmetric bend and a
combination band with contributions from CH bend and COC stretching,
respectively) [88] were considered characteristic of the PLA matrix. The
signals at the split peak 1423/1435, 1377 and 1323 cm-1 (COH bend, CH
bend and CH2 bend, respectively) [89] were considered characteristic of the
wood fibres. Previous studies by Hinterstoisser and Salmén have shown that
it is mainly the cellulose in the wood fibres that contribute to the absorption
at 1423/1435 cm-1 and 1323 cm-1 [90].
To visualize the relative response of the two constituents as moisture was
induced, a normalization of the in-phase spectra was done with respect to
the peak at 1435 cm-1, reflecting the backbone deformation of cellulose [91].
The normalized in-phase spectra for the composite material at different
levels of relative humidity are presented in Figure 15, where shifts from
cellulose to polylactide is seen when the relative humidity increases. In the
1400 to 1450 cm-1 region, where the dry and 60 % relative humidity spectra
(light grey and grey lines) have their maxima at the cellulose peak 1435 cm-1,
the 80 and 90 % relative humidity spectra show responses from the
polylactide peak at 1447 cm-1. At dry conditions (light grey curve) the
composite peak between 1350 and 1400 cm-1 is dominated by the peak at
1377 cm-1, which is characteristic for the fibre. However, when the relative
humidity is increased (grey, dark grey and black lines), the peak at 1389 cm-1,
characteristic for polylactide, becomes visible as a shoulder. Furthermore,
increased response is seen at wavenumber 1323 cm-1, representing response
30
Wood-fibre composites: Stress transfer and hygroexpansion
from CH2 bend in a side group in the cellulose molecule. This motion of the
side-group is believed to be affected by the behaviour of the surrounding
matrix rather than directly representative for the backbone movement of
cellulose. Increased response is also seen in the region around 1265 cm-1,
where the main contribution origins from polylactide. The relative stress
redistribution from the fibres to the matrix, as seen in the dynamic FT-IR
spectra when humidity was increased, could be explained by reduced stress
transfer ability as the interface absorbs moisture, and the matrix hence has
to carry a larger part of the load.
6
Dynamic FT-IR response
1389
1323
1377
0 %RH
5
60 %RH
80 %RH
4
90 %RH
3
2
1447
1265
1435
1
0
1500
-1
1450
1400
1350
1300
1250
Wavenumber (cm-1)
-2
Figure 15: In-phase spectra of composite material at different levels of relative humidity,
italic for PLA peaks.
3.1.4 Hygroexpansion and vapour and water sorption
Effects of moisture on the interface properties of a wood-fibre/polylactide
composite were analysed. To separate effects of moisture-induced softening
and swelling from changes in interface properties, the influence of moisture
on constituents and composite material were analyzed. Moisture absorption
of wood-fibre mat, polylactide film and composite material is shown in
Figure 16a, thickness swelling is presented for wood-fibre mat and
composite material, Figure 16b. The thickness swelling of the polylactide
film is not included since it was too small to measure using standard
procedures.
31
Karin M. Almgren
(a)
PLA film
Composite material
Wood-fibre mat
Moisture Content (%)
20
16
12
8
4
0
Thickness fibre mat (mm)
(b)
0.24
20 40 60 80
Relative humidity (%)
100
0.1
Wood-fibre mat
Composite material
0.23
0.095
0.22
0.09
0.21
0.085
0.2
0.08
0
5
10
Thickness composite (mm)
0
15
Young's modulus (GPa)
(c)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Young's modulus
0
20
Loss factor
0.1
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
Loss factor
Moisture content (%)
40
60
80
100
Relative humidity (%)
Figure 16: Effects of moisture on (a) mass, (b) thickness, (c) Young’s modulus and
loss factor.
32
Wood-fibre composites: Stress transfer and hygroexpansion
Experimentally determined absorption of wood fibres and polylactide was
used to predict composite mass change through the linear rule of mixture.
The predicted data, 7.4 %, was somewhat higher than the experimentally
determined value, 6.7 %. This small difference is more likely explained by
material heterogeneity of the small composite samples than by constrained
hygroexpansion of the wood fibres in the thin composite. The mass increase
of polylactide was less than 1 % when humidity was increased from dry
conditions to 80 % relative humidity. It was therefore not surprising that the
thickness increase of polylactide film was small. The effects of moisture on
Young’s modulus and loss factor of composite material are seen in
Figure 16c.
Out-of-plane hygroexpansion (%)
The BTCA modification was considered successful at fibre level since both
moisture absorption and water retention values were reduced for BTCA
modified fibres. The modified fibres showed potential as reinforcement in
polylactide composites, but were not compatible with polypropylene,
resulting in poorly consolidated composites with high swelling. Figure 17
shows hygroexpansion of composite materials tested under humid and
immersed conditions.
50
30% Fibres
60% Fibres
40% Fibres
70% Fibres
40
30
20
10
0
Ref/PLA
BTCA/PLA
Ref/PP
BTCA/PP
Ref/PLA-PP
BTCA/PLA-PP
Figure 17: Out-of-plane hygroexpansion for composites tested under humid conditions.
The moisture induced swelling of polylactide and polylactide-polypropylene
composites is reduced when BTCA modified fibres are used, most
noticeable for higher fibre contents. For polypropylene composites, the
effect of BTCA modified fibres is the opposite, with higher hygroexpansion
than when the reference fibres are used. It is also seen that for reference
fibres the choice of matrix is not crucial to moisture induced composite
swelling, suggesting that the more effective wood-fibre/polylactide interface
33
Karin M. Almgren
does not prevent swelling of the composites. For composites where BTCA
modified fibres are used, increased swelling is seen when polypropylene is
used as matrix material. This could be explained by the increased
agglomeration of BTCA modified fibres in polypropylene, leading to
concentrations of the fibres and microcracks, and thereby also to high local
strains.
The remaining thickness change after redrying of the samples follows the
same trends, Figure 18. BTCA modification of fibres in polylactide
composites leads to smaller residual out-of-plane strains, explained by less
damage caused by the restrained swelling of the modified fibres. The change
from polylactide to polypropylene when reference fibres were used led to an
increased remaining thickness change, suggesting that a stronger interface
could lead to reduced residual strains. This effect was also seen for BTCA
modified fibres.
Residual out-of-plane hygroexpansion
(%)
35
30
30% Fibres
60% Fibres
40% Fibres
70% Fibres
25
20
15
10
5
0
Ref/PLA
BTCA/PLA
Ref/PP
BTCA/PP
Ref/PLA-PP BTCA/PLA-PP
Figure 18: Remaining thickness change after drying of moisture saturated samples.
3.2 MODELLING TOOLS
In the first modelling approach in this thesis, Halpin-Tsai’s micromechanical
model for transverse and shear stiffness [24] was used, which assumes
perfect interface between fibres and matrix. A laminate analogy was used to
account for the in-plane fibre orientation distribution. Measurements were
performed under dry and humid conditions on composites and their
constituents, polylactide film and wood-fibre mat, and the predicted results
of Young’s moduli and loss factors were compared to experimentally
determined data for the composites. The elastic prediction, i.e. of Young’s
34
Wood-fibre composites: Stress transfer and hygroexpansion
1.2
1
0.8
0.6
0.4
0.2
0
tan (δ)
Young's modulus (GPa)
modulus, was close to the measured data (92 % of the experimentally
determined values for both dry and humid conditions), while the prediction
of the specific damping was only ≈ 65 % of the measured data for both dry
and humid conditions, Figure 19. The larger mismatch in damping
prediction could be subscribed to losses induced by stress redistribution in
the fibres and matrix. Moisture induced softening and swelling of both
wood-fibre mat and composite material was observed and accounted for in
the model, but no effects of moisture on the fibre-matrix interface could be
proved.
Dry
Humid
0.3
0.25
0.2
0.15
0.1
0.05
0
Measured
Predicted
Dry Humid
Figure 19: Measured and predicted Young’s modulus and tan (δ).
When Halpin-Tsai’s micromechanical model was replaced with Hashin’s
micromechanical model for an imperfect interface [55], an interface
parameter could be derived. In the materials investigated in this thesis, no
fibre coatings or other surface modifications are used to improve the fibrematrix interface. Interdiffusion from fibres to matrix is therefore not
believed to contribute to the strength of fibre-matrix interface in these
materials. Instead, chemical bonding through hydrogen bonds on fibre
surfaces and in polylactide matrix and mechanical locking are considered
more important to the interface strength and thereby in the stress transfer
between matrix and fibres. The assumption of elastic interface is therefore
deemed to be reasonable and the interface is hence described as a surface
with vanishing thickness. As in the previous study, measured mechanical
data of polylactide and wood-fibres were used as input to the model. The
interface parameter included in the micromechanical model was adjusted to
fit the predicted composite properties to experimentally determined data
and thereby a value of the interface parameter was found. This was
performed for samples tested under both dry and humid conditions.
Figure 20 shows predicted values of Young’s modulus (absolute value of the
complex stiffness) for both dry (solid grey line) and humid (solid black line)
conditions. As the humid prediction is lower than the dry prediction, the
softening of the material is seen. For low and high values of the interface
parameter, plateau values are seen, corresponding to interfacial debonding
35
Karin M. Almgren
and strong interface, respectively. In the intermediate region, the predicted
curves cross the experimentally determined values (dashed line, grey and
black for dry and humid conditions, respectively). It is seen that the
intersection occurs at the same value of the interface parameter for both dry
and humid conditions. Hence, no effect of moisture on the interface could
be seen, supporting the results from the previous study.
1.6
│E*│ (GPa)
1.4
1.2
Predicted, dry
1.0
Measured, dry
0.8
Predicted, moist
0.6
Measured, moist
0.4
Help line
0.2
0.0
8
10
←poor interface
12
14
log(μ) (log(Pa/m))
16
18
good interface→
Figure 20: Predicted Young’s modulus (absolute value of the complex stiffness) as a
function of the interface parameter and experimentally determined values under dry and
humid conditions: Grey line for dry conditions and black line for humid conditions,
predicted and measured values are represented by solid and dashed lines, respectively.
The results are hence complementary to those indicated by molecular effects
characterized by dynamic FT-IR spectroscopy. The dynamic FT-IR data
reflect changes on the molecular level, whereas the micromechanical model
offers a link between micro- and macroscale. Molecular groups might locally
have a different time-dependent behaviour than the polymer chain as a
whole, which determines the macroscopic behaviour of the polymer.
A back-calculation strategy resembling the method used to back out the
interface parameter was also used to determine the coefficient of
hygroexpansion of single wood fibres. Again, Hashin’s micromechanical
model [23, 82] was used. As in the previous studies, the properties of the
known constituents were used as input to the model, and the sought
parameter was identified by comparing the experimental macroscopic
property with the corresponding predicted value. The wood-fibre hygroexpansion was thus determined by fitting the predicted value as closely as
36
Wood-fibre composites: Stress transfer and hygroexpansion
possible to experimentally determined data of the out-of-plane hygroexpansion. This procedure was carried out for composites reinforced with
BTCA modified and untreated reference fibres. It was found that the BTCA
modification reduced the wood-fibre coefficient of hygroexpansion in the
transverse direction from 0.28 to 0.12 % strain / % relative humidity.
The great advantage of modelling approaches, such as the methods
developed in Papers A, C and F, is the simplicity of measurements and the
fast and easy evaluation of tests. The models are however dependent on
input data on mechanical properties of the composite constituents, fibres
and matrix. Finding reliable input data and making realistic simplifications of
the complex ultra- and microstructure of the fibre and composite,
respectively, are hence considered to be the main issues. The lack of material
data for different fibre types and feasible ways to experimentally determine
fibre modulus and anisotropy make assumptions necessary. Reliable fibre
data would increase the accuracy of the predicted results, and thereby of the
sought interface parameter or coefficient of hygroexpansion.
37
Karin M. Almgren
4 SUMMARY OF PAPERS
Young's modulus (GPa)
PAPER A
Dynamic-mechanical properties of wood-fibre reinforced
polylactide: Experimental characterization and micromechanical modelling
To avoid single-fibre tests for evaluation of interface properties in woodfibre polymer composites, new methods for an indirect study of stress
transfer and interface efficiency were investigated. Halpin-Tsai’s micromechanical model was used together with laminate analogy to predict
mechanical properties, i.e. Young’s modulus and loss factor, of a wood-fibre
composite material. The model, extended to viscoelastic material properties,
assumes perfect interface between fibres and matrix. Energy loss in the
cyclically loaded composite material caused by imperfections in the fibrematrix interface will hence be neglected in the model. The predicted data
were then compared to experimentally determined data. For the elastic
property, Young’s modulus, the predicted and experimentally determined
data correlated well, while the experimentally determined loss factor was
significantly larger than the predicted one. Energy losses due to the
imperfections of the interface are an important reason for this mismatch,
which therefore can be used as a measure of interface efficiency. The study
was performed under both dry and moist conditions. Softening of the
materials was observed, as illustrated in Figure 21, where Young’s modulus
in dry and humid conditions is presented. No effect of moisture on the
fibre-matrix interface could be seen.
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Dry
Humid
Wood-fibre mat
PLA film
Composite
Figure 21: Experimentally determined Young’s modulus in dry (0 % RH) and humid
(80 % RH) conditions.
Dynamic FT-IR (Fourier Transform Infrared) spectroscopy was tried to
investigate changes of molecular losses pertaining to fibres and matrix. The
results are promising, but the technique is not yet fully developed for stress
transfer evaluation of composite materials.
38
Wood-fibre composites: Stress transfer and hygroexpansion
Dynamic FT-IR response
PAPER B
Effects of moisture on dynamic mechanical properties of
wood-fibre reinforced polylactide studied by dynamic FTIR spectroscopy
Dynamic FT-IR (Fourier Transform Infrared) spectroscopy was evaluated
for stress transfer investigations in a wood-fibre/polylactide composite
material. With this technique, specific molecular bonds are detected and
studied as the sample is subjected to cyclic load. The purpose of the study
was to investigate whether this method could be used to evaluate stress
transfer between fibres and matrix in a wood-fibre composite material.
When a polymer sample is strained, the polymer structure is reoriented. This
can be seen as an increase or decrease of absorbed IR radiation at a given
energy. Stretching of covalent bonds is seen as frequency shifts and changes
in width of the absorption maxima in the dynamic FT-IR spectra. Timeresolved spectra make it possible to obtain instant and time-delayed
response of each individual molecular vibration. Dynamic FT-IR spectra can
hence be divided into in-phase (instant) and out-of-phase (time-delayed)
spectra. In-phase and out-of-phase spectra for the composite in the region
1500 to 1250 cm-1 are shown in Figure 22. The dominance of the in-phase
spectrum is a result of the elastic nature (with low loss factor) of the
composite material under dry test conditions.
0.0015
0.001
in-phase
out-of-phase
0.0005
0
1500
-0.0005
1450
1400
1350
1300
1250
Wavenumber (cm-1)
Figure 22: Dynamic FTIR spectra of composite material, in-phase response,
90° polarization.
As the environment in the test chamber was changed from dry to humid,
shifts from cellulose to polylactide peaks were seen. These could be
explained by a weakening of the interface, resulting in decreased stress
transfer, and thereby increased load carrying by the matrix. Softening in the
transverse direction of the fibres could also result in a larger proportion of
the load being carried by the matrix and thereby contributing to the
observed shift.
39
Karin M. Almgren
PAPER C
Characterization of interfacial stress transfer ability by
dynamic mechanical analysis of cellulose fiber based
composite materials
The modelling approach from Paper A was developed from comparison of
predicted and measured energy loss to derivation of an interface parameter.
As in Paper A, a two step model including micromechanical model and
laminate theory was used. Hashin’s micromechanical model for elastic
interface was utilized and expanded to cover viscoelastic material properties
of reinforcing fibres and matrix through the correspondence principle. The
assumption of elastic interface was chosen over the viscoelastic interface,
since the interface in the composite studied is deemed to be thin and hence
lack significant damping abilities, Figure 23.
Composite substrate
Fibre
Interface
Matrix
Figure 23: Illustration of fibre, interface and matrix imbedded in a composite substrate,
as described by the general self consistent scheme.
The wood-fibre reinforced polylactide studied in Paper A was used also in
this study. The mechanical properties of the composite and its constituents
were hence known from dynamic mechanical analysis, and the interface
parameter was thus the only unknown parameter. This interface parameter
could consequently be determined through back calculation. To further
investigate the effect of moisture on interface efficiency, the interface
parameter was derived for composite samples saturated in dry and humid
environments.
When interface parameters were chosen to fit the experimental data as
closely as possible, no difference between dry and humid states was seen.
This suggests that moisture absorption leads to softening and mechanical
dissipation in the hydrophilic wood fibres and thermoplastic matrix, rather
than in loss of interfacial stress transfer ability, confirming the findings in
Paper A.
40
Wood-fibre composites: Stress transfer and hygroexpansion
PAPER D
Role of fibre-fibre and fibre-matrix adhesion in stress
transfer in composites made from resin-impregnated
paper sheets
While stiffness and strength of paper are highly dependent on the degree of
fibre-fibre bonding, stress is transferred through the fibre-matrix interface in
synthetic composites. In wood-fibre composites, it is not obvious through
which of these mechanisms stress is mainly transferred and how they
contribute to stiffness and strength of the composite. Therefore, an
investigation of the importance of fibre-fibre bonds and fibre-matrix
interface in wood-fibre composites was performed.
Stiffness and strength of paper sheets and wood fibre composites with
varying degree of fibre-fibre bonding were experimentally determined. The
results showed that in contrast to paper properties, composite properties are
not dependent on the degree of fibre-fibre bonding, Figure 24.
8
7
6
6
5
5
4
4
3
3
2
2
1
1
0
0
Matrix
C1
C2
C3
Speciffic stiffness (MNm/kg)
Young's modulus (GPa)
7
C4
Figure 24: Young’s modulus or specific stiffness of neat matrix material and composite
materials with different degree of fibre-fibre bonding.
Image analysis tools were used to study the paper sheets and composites
used in the study. The results suggest that the fibre network in the paper
sheets remained intact through the manufacturing of the composites (resin
transfer moulding, RTM, was used). The degree of fibre-to-fibre bonding is
therefore believed to be the same in the investigated paper and composite
samples. It is hence suggested that the stress in thermoset wood-fibre
composites is mainly transferred from the matrix to the fibres and not from
fibre to fibre.
41
Karin M. Almgren
PAPER E
Moisture uptake and hygroexpansion of wood fiber
composite materials with polylactide and polypropylene
matrix materials
The effects of the material parameters matrix composition, fibre volume
fraction and fibre modification on material properties connected to moisture
and swelling, i.e. hygroexpansion, moisture and water uptake and time of
diffusion were investigated. A total of 24 different types of composites, see
Table 1, were manufactured. The matrix material was polylactide, polypropylene or a 50-50 mixture of the two. Fibre loading was 30, 40, 60 or
70 %, and both butanetetracarboxylic acid (BTCA) modified fibres and
untreated reference fibres were used. Changes in thickness and weight were
monitored as the composites were allowed to reach equilibrium in a humid
environment or immersed in water.
Table 2: Composition of samples tested in Paper E.
Constituents
Polymer Wood fibre
Fibre volume fraction (%)
100
70
60
40
30
PLA
Untreated
•
•
•
•
•
PLA
BTCA modified
•
•
•
•
•
PP
Untreated
•
•
•
•
PP
BTCA modified
•
•
•
•
PLA-PP
Untreated
•
•
•
•
PLA-PP
BTCA modified
•
•
•
•
The BTCA modification reduced the moisture uptake and the water
retention value of the fibres by 33 and 69 %, respectively. Also for the
results for polylactide and polylactide-polypropylene composites, where
polylactide coated the wood fibres, the BTCA modification showed positive
effects with a clear decrease of out-of-plane hygroexpansion. However, the
treatment led to increased fibre agglomeration and poor results for polypropylene composites where increased swelling and damage accumulation
were observed.
42
Wood-fibre composites: Stress transfer and hygroexpansion
PAPER F
Contribution of wood fibre hygroexpansion to moisture
induced thickness swelling of composite plates
One of the main drawbacks of wood fibre based composite materials is their
tendency to swell due to moisture uptake in wet and moist environments.
The main contribution to the hygroexpansion usually comes from the
hydrophilic wood fibres. In order to compare and rank different types of
fibres, quantification of the coefficient of hygroexpansion of wood fibres
would be of interest. Single-fibre tests are cumbersome and tedious due to
the miniscule dimensions of the fibres. Therefore, a method based on
macroscopic measurements on wood-fibre composite materials is suggested.
An inverse method to estimate the transverse coefficient of hygroexpansion
of wood fibres is investigated. The outlined method is based on backcalculation from measured moisture induced thickness swelling of
composite plates, through laminate theory and composite micromechanics
derived by Hashin.
The transverse coefficient of hygroexpansion was determined for BTCA
modified, cross linked fibres and untreated reference fibres. The two types
of fibres were used for the manufacturing of thin composite plates. Polylactide, used as matrix material, was reinforced with 30 and 40 % wood
fibres. Thickness swelling of the composite materials was monitored as
composite samples were allowed to saturate in a humid environment.
Samples were then re-dried and the reversible swelling was used as input to
the model, since the model is limited to reversible hygroexpansion.
Table 3: Transverse coefficient of hygroexpansion for reference and BTCA modified
fibres. Details on estimated values found in the literature are given in Paper F.
Type
β (ε/RH)
Reference (40 wt %)
0.28
BTCA (30 and 40 wt %)
0.12
Estimated from paper properties
0.2
Estimated by FEM simulations
0.12-0.13
Estimated from wood samples
0.21
The estimated values corresponded well with values found in the literature,
Table 3. The transverse coefficient of hygroexpansion of the BTCA
modified fibres is lower than the earlier reported values, suggesting that the
modification was successful.
Karin M. Almgren
43
5 FUTURE WORK
The aim of this thesis has been to find more straightforward and simple
methods to measure the fibre-matrix interface properties in wood-fibre
composites as well as wood-fibre hygroexpansion. Micromechanical models
have been used to characterise fibre-matrix interface efficiency and woodfibre hygroexpansion from macroscopic measurements of composite
materials. In the studies of interface efficiency, comparisons between dry
and humid conditions were made. Further testing, e.g. determination of the
interface parameters and wood-fibre coefficient of hygroexpansion of
composites with treated fibres vs. composites with untreated fibres, would
validate and demonstrate the practical use of the models. The models could
then be used to rank surface modifications and fibre treatments and thereby
serve as a useful tool in the development of wood fibre composite materials.
The proposed methods should be compared with other microscopic
methods to characterize fibre hygroexpansion and interfacial stress-transfer,
to validate whether the macroscopic methods could be used not only for
ranking different materials, but also in an absolute quantitative sense. The
next step would be to put the methods into practical use in more
rationalized materials development.
Dynamic FT-IR was used to study stress transfer at the molecular level as
thin composite samples were subjected to cyclic loading. Interesting
differences between dry and humid test conditions were seen. Shifts from
reinforcing cellulose to the matrix were observed and could be interpreted
as stress redistribution from fibres to matrix, possibly explained by a
weakening of the interface and the constituents as moisture was introduced
to the system. The scatter in e.g. molecular loss factor was however,
considerable, and the method remained qualitative. A quantification of the
dynamic FT-IR technique for interface characterisation would significantly
simplify the evaluation of stress transfer ability in wood-fibre composite
materials.
44
1
Wood-fibre composites: Stress transfer and hygroexpansion
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