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Development and morphological characterization of wood pulp reinforced biocomposite fibers

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Development and morphological characterization of wood pulp reinforced biocomposite fibers

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Biocomposite fiber has been developed from wood pulp and polypropylene (PP) by an extrusion process and the generated biocomposite fibers were characterized to understand the nature of interaction between wood pulp reinforcement and PP matrix. The use of maleated polypropylene (MAPP) as a compatibilizer was investigated in relation to the fiber microstructure. Fiber length analysis showed that most of the fiber lengths lie within the range of 0.2–1.0mm. Changes in absorption peaks were observed in Fourier transform infrared spectroscopy of biocomposite fibers as compared to the virgin wood pulp, which indicated possible chemical linkages between the fiber and polymer matrix. SEM study was carried out to observe fiber–matrix adhesion at the interface within the composite and MAPP treatment was found to be effective in increasing reinforcing fibers–matrix compatibility. X-ray computed tomography was conducted to understand the internal architecture of the biocomposite fiber and the results showed that with incorporation of additional wood pulp content, the fiber becomes more aligned along length axis possibly due to compression and die geometry of the extruder.
Content may be subject to copyright.
Development and morphological characterization of wood pulp
reinforced biocomposite fibers
A. Awal ÆS. B. Ghosh ÆM. Sain
Received: 2 January 2009 / Accepted: 2 March 2009 / Published online: 25 March 2009
ÓSpringer Science+Business Media, LLC 2009
Abstract Biocomposite fiber has been developed from
wood pulp and polypropylene (PP) by an extrusion process
and the generated biocomposite fibers were characterized to
understand the nature of interaction between wood pulp
reinforcement and PP matrix. The use of maleated poly-
propylene (MAPP) as a compatibilizer was investigated in
relation to the fiber microstructure. Fiber length analysis
showed that most of the fiber lengths lie within the range of
0.2–1.0 mm. Changes in absorption peaks were observed in
Fourier transform infrared spectroscopy of biocomposite
fibers as compared to the virgin wood pulp, which indicated
possible chemical linkages between the fiber and polymer
matrix. SEM study was carried out to observe fiber–matrix
adhesion at the interface within the composite and MAPP
treatment was found to be effective in increasing reinforcing
fibers–matrix compatibility. X-ray computed tomography
was conducted to understand the internal architecture of the
biocomposite fiber and the results showed that with incor-
poration of additional wood pulp content, the fiber becomes
more aligned along length axis possibly due to compression
and die geometry of the extruder.
Introduction
Growing awareness for an eco-friendly environment has
revived the interest to develop composite fibers from bio-
based products. Lately, natural fibers have drawn much
attention of scientists as they are biodegradable, renewable,
inexpensive and readily available from natural resources.
Besides, these fibers have relatively light weight, high
strength, and stiffness. Several studies have been conducted
to generate reinforced composites based on the potential of
cellulose-based fibers [112]. However, it is important to
understand that while fully renewable bio-based materials
are more eco-friendly, such materials may lack some per-
formance attributes designed for specific industrial
applications. One alternative solution is to use polymers
and materials derived from mixed renewable and fossil fuel
sources, which not only alleviate the fossil fuel dependency
but also have an added advantage of delivering the desired
performance from a more sustainable stock material.
However, with such composites, there are some associated
problems including poor wettability, high moisture
absorption, and less dispersion of natural fibers within the
polymeric matrices. The fiber–matrix adhesion is a vital
problem among all disadvantages. The hyprophilic nature
of fibers unfavorably affects adhesion to a hydrophobic
matrix; eventually resulting into poor mechanical proper-
ties. A good interfacial adhesion between fiber and matrix
is therefore required to enhance the mechanical perfor-
mances of a composite. These properties may be achieved
by modifying the natural fiber surface; graft copolymeri-
zation of polymers onto the fiber surface, physical
treatments (cold plasma treatment, corona treatment) etc.
[1113]. Many researchers have reported that coupling
agent such silanes, maleic anhydride, titanates, zirconates,
triazine compounds can also increase the fiber–matrix
adhesion [13]. There are several methods available for fiber
manufacturing of which melt spinning, solution spinning,
and electro-spinning are the most commonly employed
methods. Extrusion is a modified version of melt spinning
in which polymer chips are fed through the hopper and the
A. Awal (&)S. B. Ghosh M. Sain
Centre for Biocomposites and Biomaterials Processing,
University of Toronto, 33 Willcocks St., Toronto, ON,
Canada M5S 3B3
e-mail: a.awal@utoronto.ca
123
J Mater Sci (2009) 44:2876–2881
DOI 10.1007/s10853-009-3380-4
extrudate is obtained after passing through different zones
of the extruder. It has the advantages of being a simple
process, and technically elegant method and solidification
of the melt thread involves only heat transfer. Another
potential merit of extrusion approach is that no solvents are
required to produce polymer solution.
However, the relevance of all these concepts within the
context of development of continuous biocomposite fiber
from wood pulp and a suitable polymer has not been studied
to date. Related experimental data are therefore difficult to
find, and characterization results are virtually non-existent.
Therefore, development of a novel method to produce wood
pulp–polypropylene (PP) biocomposite fibers in an extru-
sion process seems to be a promising technique, which
provides a simple and low processing route. In this study,
PP was used as matrix because it is inexpensive, recyclable
and has high thermal stability. Wood pulp was used as a
reinforcing fiber. MAPP was used as a compatibilizer with
PP to enhance the interfacial adhesion between fiber and
matrix. Scanning electron microscopy (SEM) studies were
performed to investigate the fiber–matrix adhesion at the
interface. The internal structures of biocomposite fibers
were also studied by X-ray computed tomography. Fourier
transform infrared spectroscopy (FTIR) was used to inves-
tigate the nature of functional group in the composite fiber,
which in turn suggested the compatibility between rein-
forcement and matrix.
Experimental
Materials
Matrix
Polypropylene resin (PP3622) was supplied by Arkema,
Canada. The melt flow index (MFI) of supplied PP was
12 g/10 min.
Reinforcements
Bleached kraft wood pulp was collected from the Erving
Paper Inc., New Brunswick, Canada.
Coupling agent
Maleic anhydride (in the form of maleated polypropylene,
MAPP) was supplied by Atofina, Canada.
Fabrication of biocomposite fibers
A twin screw extruder with a screw nominal diameter of
25 mm, screw center distance 21.2 mm, and L/D of 40 was
used to produce the biocomposite fibers. Wood pulp was
initially crushed by grinder. The crushed wood pulp was
then kept at ambient temperature for 24 h. Biocomposite
fibers were manufactured by two steps.
First, wood pulp and polymer were mixed together
manually and placed in the extruder main feeder (Hopper).
The compounded materials were passed through the dif-
ferent zones of the extruder and finally extruded through
the spinneret holes (diameter of hole *6 mm). The
extrudates coming out of the extruder were cooled down by
using cold water for a better dimensional stability and
wound up manually. Finally, the biocomposite material
was pelletized by a pelletizer. A schematic representation
of twin screw extruder was shown in Fig. 1. The temper-
ature profile of the extruder in the respective zones is
shown in Table 1.
In the second step, the pelletized material was fed into
twin screw extruder and the biocomposite fibers were
generated by using same processing parameters of first step
except the spinneret hole. In this case, the diameter of
spinneret hole utilized was 1.5 mm instead of 6 mm to
reduce the end diameter of biocomposite fibers.
Fiber length determination
The mean length of short fiber (wood pulp) was determined
by the fiber quality analyzer (FQA). FQA was used to
analyze the medium density fiber length. The FQA has a
fully integrated sensor unit containing the optics, control,
and measurement electronics. It suspends the fibers in
water, uptakes the fibers into the machine itself, and ana-
lyzes them using the imaging sensor. The FQA reports
average length with the accuracy to 70 lm. Small amount
of fibers were well separated and stirred into approximately
500 ml of water in a beaker. The beaker was then placed
under the stirring device, which allows the fibers to be fully
suspended during test, and the suction tube, which uptakes
the fibers into the machine for analysis. In this technique,
10,000 short fibers were counted to determine an average
fiber length of wood pulp.
Fig. 1 A schematic diagram of twin screw extruder
J Mater Sci (2009) 44:2876–2881 2877
123
Fourier transform infrared spectroscopy (FTIR)
Bruker FT-IR model TENSOR 27 using 32 scans in ATR
mode was used to investigate the nature of adhesion
between fiber and matrix. Both wood pulp and biocom-
posite fibers were used to identify the functional groups
and also corresponding changes that appeared in biocom-
posite fibers with and without coupling agent.
Scanning electron microscopy (SEM)
Scanning electron microscopy was used to determine the
topography of the composite fibers and also to investigate
the effect of MA at the fiber–matrix interface. The samples
were broken down in liquid nitrogen to observe fractured
surface of the biocomposite fibers. In order to achieve the
required electrical conductivity, the samples were sputtered
with Au/Pd for 3–4 min in SC7620 Mini Sputter Coater
machine. Fractured surfaces of the specimens were studied
with a JEOL JSM-840 SEM with an acceleration voltage of
12 kV.
X-ray computed tomography
The samples were examined using the high-resolution
micro-CT system SkyScan 1172 (SkyScan, Belgium). The
X-ray source was an air-cooled, sealed microfocus X-ray
tube with a focal spot size \8lm. The X-ray tube was
operated at 40 kV and 150 lA (no filter). The X-ray CCD
(charge-coupled device) camera is based on a 2000 91048
12-bit cooled CCD sensor with fiber optic coupling to the
X-ray scintillator. The system was controlled by a PC
workstation. Scanning of the specimens was done with
180°rotation around the vertical axis and a single rotation
step of 0.4. The cross-sectional pixel size was 4.01 lm.
After a half circle (180°) was completed, the entire set of
radiographs was synthesized by computer software. The
raw dataset was reconstructed with the software NRecon.
An automatic filter changer for beam-hardening compen-
sation during reconstruction was used at a level of 35%.
The reconstructed 2D images were saved as a stack of
uncompressed 16-bit TIFF files. These TIFF files were then
used to create a three-dimensional (3D) image of micro-
structure with the software Image-Pro Plus 6.1.
Results and discussion
Fiber length analysis
During the extrusion process, the shear stress applied by
the screw will break the fibers. The resultant fiber lengths
will affect the ultimate mechanical properties and in
addition to the influence of fiber damage and breakage
during processing, the final fiber lengths are determined
also by the initial fiber length in the feedstock. It is
therefore important to analyse the initial fiber length dis-
tribution. The fiber length distribution is presented by a
histogram curve in Fig. 2. The arithmetic, length weighted,
and weight weighted values of the fiber length were found
to be 0.43, 0.98, and 1.52 mm, respectively. Most of the
fiber lengths lie within the range of 0.2–1.0 mm since
crushed wood pulp was used in this study. 10,000 single
fibers were taken into consideration to obtain these average
fiber lengths.
FTIR investigation
The nature of interaction between fiber and matrix was
explored using FTIR. The C–H stretching vibrations were
observed at about 2,800–3,000 cm
-1
in pure wood pulp,
and all composite fibers, which are shown in Fig. 3
Table 1 Processing parameters for extrusion
Processing settings
Material PP/wood pulp
Motor rpm 40
Feeder rpm 13
Vacuum vent Yes
Temperature profile (°C)
Zone 1 (PP, wood pulp) 180
Zone 2 180
Zone 3 180
Zone 4 180
Zone 5 180
Zone 6 180
Zone 7 180
Zone 8 180
Zone 9 180
Zone 10 185
Zone 11 185
Fig. 2 Length histogram of crushed wood pulp
2878 J Mater Sci (2009) 44:2876–2881
123
[1416]. FTIR spectra of composite fibers were signifi-
cantly more intense, compared to a spectrum of pure wood
pulp. It is assumed that PP and MA would react with pure
wood pulp, and eventually the intensity of C–H stretching
of pure wood pulp might be affected in the composite
system.The mechanical properties of wood pulp–PP bio-
composite fibers depend upon how strongly the fiber and
matrix are linked by chemical bonds. The infrared (IR)
absorption peak of free –OH groups of wood fiber was
observed in Fig. 3at 3,324 cm
-1
. As shown in Fig. 3, the
hydroxyl absorption peak in all biocomposite fibers has
shifted to lower wave numbers (3,214–3,324 cm
-1
).
MAPP may potentially be grafted to the wood fiber sur-
faces by hydrogen bonds or ester linkage. The hydroxyl
absorption peak will move to lower wave number when
hydrogen bonds are generated due to reaction of MAPP
with wood fiber. If ester linkages are established between
MAPP and the hydroxyl groups, an indication of new
ester group stretch would be viewed at about 1,740 cm
-1
,
which was not observed from the FTIR spectra. The shift
of hydroxyl absorption was, however, noticed for all
biocomposite fibers. The higher the spectral shift, the
more hydrogen bonds created. The greater spectral shift
corresponds to a greater extent of MA. These results are
in good agreement with the studies of Luo et al. [17]. A
considerable changes in IR absorption peak of free –OH
groups of wood pulp–PP biocomposite fibers was also
observed as compared to the cellulose fiber (wood pulp).
A significant spectral movement is detected in wood
pulp–PP biocomposite fibers owing to use of various
percentage of MA. Mechanical properties of biocomposite
fibers can therefore be controlled with varying content of
MA, which in turn would influence the interfacial bonding
between wood fiber and MAPP due to the generation of
hydrogen bonds.
Analysis of microstructure by SEM
Scanning electron microscopy was used to investigate the
fracture surface of neat PP and wood pulp–PP biocom-
posite fibers with and without MA. SEM observations
indicate substantial differences across the surfaces among
neat PP, wood pulp-PP biocomposite fiber with and with-
out MA. The fracture surface of pure PP appears smoother
than any other samples of biocomposite fibers because of
the homogeneous system. Figure 4a–f shows the fracture
surfaces of the pure PP and biocomposite fibers with
increasing fiber content. The fibers tend to agglomerate
when MA was not used as a coupling agent, which is
presented clearly in Fig. 4c–e. It can also be observed that
the wood fibers are dispersed within the matrix. Fiber
distribution seems to be more uniform in the PP matrix
with MA. Figure 4f represents fracture surface of wood
pulp–PP biocomposite fibers with MA (30%F/65%PP/
5%MA). As compared to Fig. 4c, the reinforcing fibers
appeared to be impregnated due to the presence of MA
with PP, which should have increased the reinforcing
fibers–matrix compatibility. Overall, this would affect the
mechanical and thermal performance of the substrate
where the fibers will potentially be used.
X-ray computed tomography studies
The fiber orientation plays a crucial role in obtaining the
behavior of short-fiber composites. Although, majority of
the processing method involving short-fiber reinforced
composites produces random fiber orientation, fabrication
kinetics can lead to semi orientation, which may increase
the anisotropic properties. Fabrication method can alter the
degree of fiber orientation in composite system slightly.
During extrusion process, progressive and continuous
changes in fiber orientation take place in composite system.
Several other factors are also responsible for changes in the
orientation of fibers including the flow behavior of polymer
matrix, the size and amount of fibers, the processing
parameters, and die geometry. Mechanical properties
changes due to directional differences, which is correlated
with the orientation of fibers. Knowledge of fiber orienta-
tion of is, therefore, important in composite part because it
controls the mechanical properties of the finished product.
The composite is stronger and stiffer in the direction in
which most short fibers are oriented. The mechanical per-
formance of composite will be poor in the direction of less
oriented. An understanding of fiber orientation, nature of
distribution is therefore important in terms to processing
and mechanical performance of biocomposite fibers.
Fiber–matrix networking microstructures were investi-
gated by X-ray computed tomography. Figure 5a–c reveals
the internal architecture of biocomposite fibers. With
Fig. 3 FTIR spectra of 100% wood pulp and 30% wood pulp–PP
biocomposite fiber with variation of MA percentage
J Mater Sci (2009) 44:2876–2881 2879
123
increasing fiber content, the reinforcing fibers became more
prominent, leaving less void spaces. The reinforcing fibers
were found to be aligned along the length axis in bio-
composites fibers possibly due to compression and
geometry of the die of extruder. It is expected that the
desired orientation and in turn the mechanical properties of
biocomposite fibers can be achieved by carefully optimiz-
ing the fiber content, die geometry, and processing
parameters.
Conclusions
In this study, biocomposite fibers have been generated from
wood pulp and PP using a twin screw extruder. The
developed fiber has been characterized using several
techniques. Analysis of fiber length provided the data for
average fiber length and the fiber length distribution within
the wood pulp reinforcement. Results show that most of the
fiber lengths lie within the range of 0.2–1.0 mm. FTIR
spectroscopy analysis indicated considerable changes in
absorption of wood pulp–PP biocomposite fibers as com-
pared to the virgin wood pulp, which in turn relates to a
possible chemical linkages between the fiber and polymer
matrix. SEM study also indicated fiber–matrix interaction
within the composite, which was found to be enhanced on
treatment with MA and as a result increased reinforce-
ment–matrix compatibility. X-ray computed tomography
has been applied as a useful tool for the non-destructive
investigation of the wood fiber orientation in a composite
system. X-ray computed tomography studies were con-
ducted to understand the internal microstructure of the
biocomposite and it has been clearly shown that with
increasing fiber content, the reinforcing wood fibers
become more aligned along length axis possibly due to
compression and die geometry of the extruder.
Fig. 4 Fractured surface of
pure PP and short fiber/PP–
composite fibers (9250): apure
PP (9190), b10%F/90%PP
(9230), c30%F/70%PP
(9230), d50%F/50%PP
(9230), e70%F/30%PP
(9230), and f30F/65%PP/5%
MA (9250)
2880 J Mater Sci (2009) 44:2876–2881
123
Acknowledgements The authors would like to gratefully
acknowledge financial support of this study given by the Ontario
Centres of Excellence (OCE), Canada.
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Fig. 5 3D image of aPure PP,
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J Mater Sci (2009) 44:2876–2881 2881
123
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Cellulose is one of the oldest natural polymers; it is renewable, biodegradable, and can be derivatized to manufacture useful products. The electrospinning, a technique for the manufacture of nanofibers, has garnered attention in recent years due to its versatility and potential for applications in various fields such as biomedicine, tissue engineering and also filtration. In this study, cellulose acetate fibers have been obtained by electrospinning. To achieve these results, the cellulose was initially obtained from the sugarcane bagasse of local plantations in Moniquira, Boyaca, then cellulose was modified to obtain cellulose acetate, which has enhanced properties for electrospinning. Yarn parameters were determined, such as needle-manifold distance, flow rate, voltage and polymer concentration, among others. Instrumental analyses were carried out including Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC), and Thermogravimetric Analysis (TGA). As a result, cellulose acetate fibers were obtained with an average diameter of 258 nm, with excellent properties such as temperature resistance. This was done in order to continue the work on nanofiber functionalization, as for example, cationization of cellulose and further addition of reactive dyes.
... Hence, biopolymer-based materials can have a new direction in designing of greener materials and could widen the spectrum of applications in different sectors such as automobiles, furniture, drugs, carpet and fire protective suits and construction of industrial parts [7][8][9][10][11]. Out of various natural fibres, jute fibre has attracted much attention being less expensive and readily available worldwide [12][13][14]. In recent years, the focus of research regarding jute composites has been on the modification of flame retaerdance [15][16][17][18][19][20]. ...
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A range of bio-nanocomposites were prepared by incorporation of organo modified montmorillonite nanoclay (OMMT) with or without use of aluminum hydroxide (Al(OH)3) within polylactic acid (PLA) solution. Furthermore, the solution was employed for modification of ligno-cellulosic (jute) fabric structural reinforcements. The successful incorporation of nanofillers within the host polymer, polylactic acid (PLA) was confirmed by Fourier-transform infrared spectroscopy (FT-IR). Water uptake and swelling behaviour studies revealed that the water uptake and swelling ratio of bio-composites reduced significantly as compared to pristine jute fabric, whereas upon incorporation of OMMT and Al(OH)3, the water barrier properties reduced even further in the developed bio-nanocomposites. The flexural strength of the bio-nanocomposites also showed improved mechanical and dimensional stability. Synergistic effects of OMMT and Al(OH)3 were observed in enhancing the aforementioned physico-mechanical properties. Scanning electron microscopy (SEM) studies revealed microstructural details of developed samples. Similarly, the thermo-gravimetric analysis and linear burning rate studies of Al(OH)3 treated bio-nanocomposite materials revealed enhanced thermal resistance and reduced flammability respectively compared to both pristine woven jute fabric and fabrics treated with PLA alone or those without Al(OH)3. From the above results it can safely be said that the bio-nanocomposite material can be a prospective candidate for development of flame retardant biopackaging.
... Natural fibers have attracted much attention from researchers because they are inexpensive, abundant, biodegradable, renewable, strong and light weight. Cellulose fibers are valuable in a wide range of fields such as filtration and biomedical applications [4], [5]. ...
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Poly(propylene) (PP) wood composites were prepared in a wide composition range from 0 to 70 wt% wood content. Matrix/wood adhesion was improved by the introduction of two maleinated polypropylenes (MAPPs) with different molecular weights and functionality. MAPP/wood ratio changed from 0 to 0.25 in 0.05 steps. Mechanical properties of the composites were characterized by tensile testing, while their fracture resistance was determined with instrumented impact measurements. Micromechanical deformation processes were followed by acoustic emission and volume strain measurements, which were supported by scanning electron microscopy done on the broken surface of fractured samples. The results show that stiffness increases with wood content and it does not depend very much either on the type or the amount of the functionalized polymer used. However, ultimate tensile properties are strongly influenced by the amount and properties of MAPP; larger molecular weight and smaller functionality are more advantageous both for strength and impact resistance. The optimum MAPP/wood ratio was found to be around 0.05 in accordance with some literature data. Because of their large size, wood particles debond very easily from the matrix leading to volume increase and catastrophic failure at small deformations. When adhesion is improved by the introduction of MAPP, large wood particles fracture thus also contributing to the failure of the composite. At large wood content considerable aggregation of the particles may take place leading to inferior strength. Copyright © 2006 John Wiley & Sons, Ltd.
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Kudzu fiber-reinforced polypropylene composites were prepared, and their mechanical and thermal properties were determined. To enhance the adhesion between the kudzu fiber and the polypropylene matrix, maleic anhydride-grafted polypropylene (MAPP) was used as a compatibilizer. A continuous improvement in both tensile modulus and tensile strength was observed up to a MAPP concentration of 35 wt %. Increases of 24 and 54% were obtained for tensile modulus and tensile strength, respectively. Scanning electron microscopy (SEM) showed improved dispersion and adhesion with MAPP. Fourier transform infrared (FTIR) spectroscopy showed an increase in hydrogen bonding with an increase in MAPP content. Differential scanning calorimetry (DSC) analysis indicated little change in the melting temperature of the composites with changes in MAPP content. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1961–1969, 2002
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The surface of wood flour used as reinforcement in PP/wood composites was successfully modified by benzylation in NaOH solution of 20 wt% concentration at 105 °C. The time of the reaction was changed between 5 and 360 min in several steps. The progress of modification was followed by the measurement of weight increase and by diffuse reflectance infrared spectroscopy (DRIFT). The structure of the wood was characterized by X-ray diffraction (XRD) and its surface tension was determined by inverse gas chromatography (IGC). PP composites containing 20 wt% filler were prepared from a PP block copolymer and the modified wood flour. The mechanical behavior of the composites was characterized by tensile testing. The majority of the active hydroxyl groups at the surface were replaced by benzyl groups in about 2 h under the conditions used. Further increase in reaction time did not influence the properties of the filler. Both the structure of the wood flour and its surface tension changed as an effect of modification. The reduction of surface tension led to significant changes in all interactions between the wood flour and other substances resulting in a considerable decrease of water absorption, which is the major benefit of this modification. All measured mechanical properties of the composites decreased slightly with increasing degree of modification. A detailed analysis of the results proved that the dominating micromechanical deformation process of these PP/wood composites is debonding, which is further facilitated by the decrease in the surface tension of the filler. Chemical modification of wood flour slightly improved processability and the surface appearance of the composites prepared with them and considerably decreased the water absorption of these latter.
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Natural fiber reinforced composites is an emerging area in polymer science. These natural fibers are low cost fibers with low density and high specific properties. These are biodegradable and non-abrasive. The natural fiber composites offer specific properties comparable to those of conventional fiber composites. However, in development of these composites, the incompatibility of the fibers and poor resistance to moisture often reduce the potential of natural fibers and these draw backs become critical issue. This review presents the reported work on natural fiber reinforced composites with special reference to the type of fibers, matrix polymers, treatment of fibers and fiber-matrix interface. © 1999 John Wiley & Sons, Inc. Adv in Polymer Techn 18: 351–363, 1999
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The microstructure and mechanical properties of polypropylene composites containing flax and wheat straw fibres are discussed. Particular emphasis has been given to determining the nature and consequences of fibre damage induced during melt-processing operations, fibre orientation occurring in mouldings, and possible interfacial adhesion between the matrix and fibres. Compared to unfilled polypropylene, addition of flax and wheat straw caused a significant increase in tensile modulus, particularly, in the case of flax fibres, which also gave higher tensile yield strength and Charpy toughness, despite a lack of interfacial bonding. Tensile strength was increased further through inclusion of 5% by weight of maleic anhydride-modified polypropylene, which was shown to promote adhesion between fibres and matrix.