Microcellular Foamed Wood-Plastic Composites by Different Processes: a Review Omar Faruk, Andrzej K. Bledzki, * Laurent M. Matuana Introduction The use of natural and wood fibers as a reinforcement of polymer materials is becoming more and more interesting, thanks to their fibrous structure. A technique commonly employed to improve the mechanical properties of polymers is to add short or long reinforcing fibers. In this way stresses on the composite material are transmitted by the fiber-polymer matrix interface to the reinforcing fibers, which enhances the stiffness and strength of the material. Review Wood fiber reinforced polymer composites represent a relatively small but rapidly growing material class, extensively applied in interior building applications and in the automotive industry. The polymer-wood fiber composites utilize fibers as reinforcing filler in the polymer matrix and are known to be advantageous over the neat polymers in terms of the materials cost and mechanical properties such as stiffness and strength. Wood fiber reinforced polymer com- posites are microcellularly processed to create a new class of materials with unique properties. Most manufacturers are evaluating new altern- atives of foamed composites that are lighter and more like wood. Foamed wood composites accept screws and nails like wood, more so than their non-foamed counterparts. They have other advantages such as better surface definition and sharper contours and corners than non- foamed profiles, which are created by the internal pressure of foaming. This paper represents a review on microcellular wood fiber reinforced polymer composites obtained by different processes (batch, injection molding, extrusion, and compression molding process) and includes an overview of foaming agents (physical and chemical) and the foaming of wood fiber- polymer composites (changes in phase morphology, formation of polymer-gas solution, cell nucleation, and cell growth control). O. Faruk, A. K. Bledzki Institut fu ¨r Werkstofftechnik, Kunststoff- und Recyclingtechnik, University of Kassel, Mo¨nchebergstr. 3, D-34109 Kassel, Germany. E-mail: [email protected]O. Faruk, L. M. Matuana Department of Forestry, Michigan State University, East Lansing Michigan, MI-48824, USA Macromol. Mater. Eng. 2007, 292, 113–127 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mame.200600406 113
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Review
Microcellular Foamed Wood-PlasticComposites by Different Processes:a Review
Omar Faruk, Andrzej K. Bledzki,* Laurent M. Matuana
Wood fiber reinforced polymer composites represent a relatively small but rapidly growingmaterial class, extensively applied in interior building applications and in the automotiveindustry. The polymer-wood fiber composites utilize fibers as reinforcing filler in the polymermatrix and are known to be advantageous overthe neat polymers in terms of the materials costand mechanical properties such as stiffness andstrength. Wood fiber reinforced polymer com-posites are microcellularly processed to create anew class of materials with unique properties.Most manufacturers are evaluating new altern-atives of foamed composites that are lighter andmore like wood. Foamed wood compositesaccept screws and nails like wood, more so thantheir non-foamed counterparts. They have otheradvantages such as better surface definition and sharper contours and corners than non-foamed profiles, which are created by the internal pressure of foaming. This paper represents areview on microcellular wood fiber reinforced polymer composites obtained by differentprocesses (batch, injectionmolding, extrusion, and compressionmolding process) and includesan overview of foaming agents (physical and chemical) and the foaming of wood fiber-polymer composites (changes in phase morphology, formation of polymer-gas solution, cellnucleation, and cell growth control).
O. Faruk, A. K. BledzkiInstitut fur Werkstofftechnik, Kunststoff- und Recyclingtechnik,University of Kassel, Monchebergstr. 3, D-34109 Kassel, Germany.E-mail: [email protected]. Faruk, L. M. MatuanaDepartment of Forestry, Michigan State University, East LansingMichigan, MI-48824, USA
Macromol. Mater. Eng. 2007, 292, 113–127
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Introduction
The use of natural and wood fibers as a reinforcement of
polymermaterials is becomingmore andmore interesting,
thanks to their fibrous structure. A technique commonly
employed to improve the mechanical properties of
polymers is to add short or long reinforcing fibers. In this
way stresses on the composite material are transmitted by
the fiber-polymermatrix interface to the reinforcing fibers,
which enhances the stiffness and strength of the material.
DOI: 10.1002/mame.200600406 113
O. Faruk, A. K. Bledzki, L. M. Matuana
114
Wood fiber reinforced plastic composites represent an
emerging class of materials that combine the favorable
performance and cost attributes of both wood and
thermoplastics.[1–6] The forest product companies see
plastics as a way to make new construction materials
with attributes thatwood does not have, such as resistance
to moisture and insects. Plastic processors see wood as
readily available, relatively inexpensive filler that can
lower their resin costs, add stiffness, and increase
profile extrusion rates because wood cools faster than
plastics.[7] Most all-wood-plastic composites can be
fastened, sanded, stained, and machined in the same
way as wood without the need to invest in new
equipment.
Within the last 15 to 20 years, the field of natural and
wood fiber research has experienced an explosion of
interest, particularly with regard to the comparable
properties of natural and wood fibers to glass fibers
within composite materials. The main area of increasing
usage of these composite materials is the automotive
industry, predominantly in interior applicationswhere the
need is greatest.[8–10]
Dr. Omar Faruk completed his B.Sc. (Hons) and M.Sc.DAAD (German Academic Exchange Service) scholarsRecycling Technology, Department of Mechanical Engresearch project ‘‘Natural Fiber and Wood Reinforcecontinued to work there as a Research Assistant to puntil February 2006 he worked there as a Post DoctoraUniversity, East Lansing Michigan USA as a Visiting R(including a book) to his credit which have been pubProf. Andrzej K. Bledzki was born in Torun/Poland. Heand at the University Halle-Merseburg/Germany (Ph.DUniversity of Szczecin/Poland (habilitation 1987). Durinuniversities and research institutes in Germany (DAAstadt), France (Centre de recherches sur la physico-cheSlovakia, Russia, and Latvia. Since 1988 he has beeEngineering, Germany. From 1988 until 1994 he workedhead of a Professorship entitled ‘Polymer and RecyclinDean of the Faculty of Mechanical Engineering, chairmthe extended steering committee at the University otechnical papers. In 1993 he was awarded i.a. the scientWalesa, and in 1998 he received the Polish Medal of MTechnical University Riga, Latvia.Dr. Laurent Matuana is an Associate Professor of Engaddition, he is the coordinator of the Wood Products Mat MSU. He earned his Ph.D. at the University of ToroUniversite Laval, Quebec, Canada. Dr. Matuana’s researengineered wood-based composites. Microcellular anstrong interest in his research program, with the overaand growth during the foaming process in order tofoamed composites. He has received numerous awardscientific papers, 4 book chapters, holds 4 US patents,Dr. Matuana is an active member of the Vinyl Division (2002) and the Technical Program Committee (TPC) (s
Macromol. Mater. Eng. 2007, 292, 113–127
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Nowadays, the filler-reinforced plastic composites
market is dominated by calcium carbonate (40%) and
glass fiber (31%) and some other inorganic fillers such as
talc, mica, clay, and so on.[11–14] In comparison to glass
fibers, natural fibers have lower tensile strength and
comparable modulus, but their specific modulus (mod-
ulus/specific gravity) shows values that are often greater
than that of glass fibers.[15–18]
Although natural and wood fiber reinforced polymer
composites have been commercialized, their potential for
use in many industrial (mainly automotive and decking)
applications has been limited because of their brittleness,
lower impact resistance, and mainly higher density
compared to neat plastics.[19] The potential range of uses
for these materials in innovative applications would be
expanded if these shortcomings could be improved. The
concept of creating microcellular foamed structures in the
composites as a means to improve these shortcomings has
successfully been demonstrated.[20] Foamed plastics can
often be stronger than their non-foamed analogues and
because of the reduced weight can achieve outstanding
cost-to-performance and favorable strength-to-weight
in Chemistry at the University of Chittagong, Bangladesh. With ahip, he moved to the Institute for Materials Science, Polymer &ineering, University of Kassel, Germany in 1999 to work on thed Composites’’. After completion of the scholarship (2001), he
ursue his Ph.D.. In 2005 he achieved his Ph.D. in Engineering andl researcher. He joined the Department of Forestry, Michigan Stateesearch Associate on March 2006. He has around 50 publicationslished in different international journals and at conferences.studied material science at the Technical University of Lodz/Poland. thesis). Between 1971 and 1988 he was employed at the Technicalg this time he was able to spend periods of various length at severalD and Humboldt-Stiftung, Deutsches Kunststoff-Institut in Darm-mie des surfaces solides, CNRS in Mullhouse) and also in Hungary,n a member of the University of Kassel, Faculty of Mechanicalas the head of the Chair for Plastics Processing and since 1994 as theg Technology’, co-founded by industry. Since 2002 he has been the
an of the Dean’s Conference Engineering Faculty, and member off Kassel. Prof. Bledzki has published more than 230 scientific andific title ‘Professor of the Technical Science’ by the polish presidenterit in Gold and in 2004 was named Doctor honoris causa by the
ineered-Wood-Based Composites at Michigan State University. Inanufacturing and Marketing Program in the Department of Forestrynto, Ontario, Canada. He also holds B.Sc. and M.Sc. degrees fromch interests are in the areas of design, process and manufacture ofd conventional foaming of wood–plastic composites are also of
ll goal of understanding the basic mechanisms of bubble formationestablish process–cell morphology and property relationships fors for his teaching and research and has published more than 100and has supervised several graduate and undergraduate students.Society of Plastic Engineers) serving in the Board of Directors (since
ince 2000, chair from May 2005 to May 2006).
DOI: 10.1002/mame.200600406
Microcellular Foamed Wood-Plastic Composites by . . .
ratios. The foaming of wood fiber reinforced composites
improves their ability to withstand repeated nailing and
screwing operations compared to un-foamed products of
the same composition. Foaming also results in better
surface definition, and sharper contours and corners than
un-foamed profiles.[21] Foaming reduces the material
requirement with the associated economic benefits.
Because of the plasticizing effects of gas, the foamed
composites run at a lower temperature and at faster
speeds than their un-foamed counterparts, and thus the
production cost is reduced.[21] When microcellular wood
fiber reinforced composites generate a finer microcellular
structure, the specific mechanical properties of the
composites are significantly improved.[20]
Despite the flurry of commercial and development
activity, the process of microcellular foamed composites is
still a poorly understood black art. The field of micro-
cellular plastic technology is in some ways in the early
stages of research and development, notwithstanding its
relatively long history.
Based on the concept of microcellular plastics, the first
microcellular plastics technology in a form of batch
processing was developed by Martini and Suh.[22–24]
Microcellular foams were initially produced in a batch
process and later in continuous extrusion and injection
and compression molding systems.[4]
In the batch process, a polymer sample is first placed in a
high-pressure chamber where the sample is saturated
with an inert gas (such as CO2 or N2) under high pressure at
ambient temperature. Because of the low rate of gas
diffusion into the polymer at room temperature, a very
long time is required for the saturation of the polymer with
gas, which is the major disadvantage of the batch process.
Injection molding is one of the most commercially
important fabrication processes for molding a broad
spectrum of thermoplastics. A great deal of attention has
been given to defining the engineering aspects of the
operation for maximizing production rates and for control-
ling part strength, brittleness, shrinkage, and appearance
characteristics.[25,26] The advantages of the injectionmolding
process for microcellular composites is that the injection
pressure decreases because of the presence of dissolved gas,
which lowers the viscosity. The cycle time is also reduced
because of the elimination of the ‘hold and pack’ time and an
approx. 25% reduction in cooling time. The microcellular
composites in injection molding process are more advanta-
geous because of minimization of distortion/deformation
and also clamp force reduction.
Microcellular plastics have been developed as an
extension of the extrusion application while extruder
evolution is primarily based on its function optimization.
Extrusion is one of themost widely used plastic processing
techniques, in which a plastic resin is heated, melted,
compressed, and conveyed by the motion of a rotating
Macromol. Mater. Eng. 2007, 292, 113–127
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
screw/screws in a barrel for further processing down-
stream. It is also one of the most cost-effective processes
for the production of plastics. The commercial utility
would be further elevated, if the production was a
continuous extrusion-based technology.
The compression molding technique proved suitable for
the production of profiles with any thermoplastic prepreg
used. Compression molding brings the thermoplastic
prepreg gently to the required shape without over
compressing the material. The different layer orientations
are thus retained after molding.
Wood–plastic composites are gaining growing accep-
tance for structural applications. For these applications,
extrusion, injection, and compression molding processes
are the preferred methods of production.
Foaming of Wood Fiber-Polymer Composites
The generation of high cell density for foaming becomes
possible by inducing a sudden thermodynamic instability
in a polymer/gas solution. After their formation, the cells
should be preserved by controlling their growth until the
gas bubbles are stabilized.[27,28] The microcellular foaming
system should have the following essential processing
mechanisms to successfully achieve these conditions: A
mechanism for completely dissolving a large, soluble
amount of a blowing gas into a polymer, under a high
processing pressure; a mechanism for inducing a thermo-
dynamic instability in the homogeneous polymer/gas
solution formed earlier; and a mechanism for controlling
the growth of bubbles, while preventing them from
coalescing and collapsing. Based upon the successful
implementation of microcellular foaming,[27–31] the sys-
tem requirements for microcellular foaming of wood fiber
reinforced polymer composites can be established.
Changes in Phase Morphology
Typically the cell density ranges from 103–104 cells � cm�3
in conventional foam processes. But the microcellular
process requires nucleation control where the nuclei
density is larger than 109 cells � cm�3 so that the fully
grown cell size will be less than 10 mm. The key to
producing the required cell density is to induce a very high
rate of cell nucleation in the polymer/gas solution.[32] High
nucleation rates could be achieved by using the thermo-
dynamic instability of the gas and polymer system. In
order to make use of the thermodynamic instability, a
rapid drop in the gas solubility must be induced in the
polymer/gas solution. The solubility of gas in a polymer
changes with pressure and temperature.[33–37]
Formation of Polymer/Gas Solution
The sudden change in solubility is the driving force for
microcellular foaming and the dissolution of a large
www.mme-journal.de 115
O. Faruk, A. K. Bledzki, L. M. Matuana
116
amount of gas in polymer provides that opportunity.
Wood fiber retains the solid phase and is not plasticized
during processing, which is an obvious difference in
wood fiber reinforced foamed composites. The gas does not
dissolve in thewood fiber[38] and, therefore, the dissolution
is restricted only to the polymer. This limits the amount of
gas that can be dissolved in the mixture and utilized for
homogeneous nucleation. Moreover, the interfaces of the
solid wood fibers provide a dominant force for hetero-
geneous nucleation.[39] The following factors affect the gas
dissolution, and its effects on nucleation.
The interfacial regions between natural/wood fibers
and polymers (especially poly(propylene) (PP) and poly-
ethylene (PE)) are not wetted and these interfaces may
provide channels for fast gas movement.[40] Consequently,
the apparent/effective diffusion is enhanced. Some
undissolved gases may be retained in the micro-voids at
the interfaces of natural/wood fibers and polymers,[39]
which may lead to an impression of higher solubility.
In reality some portion of the blowing gases may remain in
a separate phase, instead of being dissolved. Thesemay also
add to the already dominant contribution of heterogeneous
nucleation as a result of the solid wood fibers.
Cell Nucleation
The fundamental principles of foam formation are bubble
nucleation formation, bubble growth, and bubble stability.
The first step in producing foam is the formation of gas
bubbles in a liquid system. If the bubbles are formed in an
initially truly homogeneous liquid, the process is called
‘self-nucleation’.
During foaming of wood fiber reinforced polymer
composites, because of the presence of solid wood fibers,
there is a much higher potential for heterogeneous
nucleation at the solid melt interfaces than for homo-
geneous nucleation. The heterogeneous nucleation can
occur either as a result of an increase in the free energy of
the system caused by reduced surface tension at the
interface of the liquid polymer and the solid fiber, or
because of entrapped gas in the micro-voids at the
interfaces.[39]
At higher processing temperatures, wood fiber releases
volatiles that affect the cell nucleation. These volatiles
contain H2O, CO2, and other constituents.[41–45]
Although CO2 is soluble in polymer, H2O has very low
solubility and nothing is known about the solubility of the
other volatiles.
Cell Growth Control
The final step of microcellular foaming, that of control
of the cell growth, is dependent on te following factors:[46]
i) the use of an appropriate amount of blowing gas, ii)
minimal diffusion of gas, and iii) the suppression of cell
coalescence, cell coarsening, and cell collapse.
Macromol. Mater. Eng. 2007, 292, 113–127
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Microcellular Polymer
Suh introduced the theory that microcellular polymers are
closed cell plastic foams with bubble densities in excess
of 109 bubbles per cm3 and diameters of 1 to 10 mm.
Nowadays, other investigators are presenting that a cell
size of 10 to 300/400 mm is also called amicrocellular foam
and a cell size from 1–10 mm is called a MuCell foam.[47]
The classical work on bubble nucleation and growth,
that of the pioneering modeling of the growth of a single
gas bubble in a polymer matrix, was carried out by Street
et al.[48] Other authors then introduced the concept of a
finite influence volume around each bubble. They also
considered the effects of heat, temperature, pressure,mass,
and mass transfer on bubble growth.[49–62]
Colton and Suh[63–67] developed a nucleation theory for
microcellular thermoplastic foam. Three possible mechan-
isms for the nucleation of gas in polymeric systems are
considered: homogeneous, heterogeneous, and mixed
mode nucleation.
Typically, microcellular plastics exhibit a high Charpy
impact strength,[68–75] high toughness,[76–78] high fatigue
life,[79–81] high thermal stability,[82,83] high light reflect-
ability, low dielectric constant, and low thermal con-
ductivity[84,85] over the neat plastic. These improvements
are a result of the presence of bubble cells, which inhibit
crack propagation by blunting the crack tip and increasing
the amount of energy needed to propagate the crack.[24]
Foaming Agents
Physical Foaming Agents
Physical foaming agents are compounds that liberate gases
as a result of physical processes (evaporation, desorption) at
elevated temperatures or reduced pressures. Because of the
environmental benefits, carbon dioxide and nitrogen are
nowadays becoming more and more in demand for use as
physical foaming agents.[86–93] Physical foaming agents that
have been reported[94] to be used in microcellular processing
include water, argon, nitrogen, and carbon dioxide.
Chemical Foaming Agents
Chemical foaming agents (CFAs) are substances that de-
compose at processing temperatures thus liberate gases
like CO2 and/or nitrogen. Solid organic and inorganic
substances (such as azodicarbonamide and sodium
bicarbonate) are used as CFAs. In general, CFAs are divided
by their enthalpy of reaction into two groups including
exothermic and endothermic foaming agents. The reaction
that produces the gas can either absorb energy (endother-
mic) or release energy (exothermic). Nowadays, a combi-
DOI: 10.1002/mame.200600406
Microcellular Foamed Wood-Plastic Composites by . . .
nation of exothermic and endothermic CFAs is also used
for foaming.
The effect of CFAs on the processing and properties of
wood plastic composites has gained interest because
properties such as insulation values, shrinkage and
distortion, and stiffness can be influenced positively.
The benefits of using CFAs include consistent process
control, and nucleating effects, which can solve the
moisture problems and improvement of mechanical
properties.[95–108]
Processing of Microcellular Natural and WoodFiber-Polymer Composites
Many new innovative technologies are now being
introduced and re-introduced for foam processing. Often
called microcellular foaming, the new technologies utilize
a number of approaches to achieve fine cellular structures
with double digit weight and cycle time reductions. The
key to the innovative technologies is computerized process
control, good tool design (including counter pressure),
static melt mixing, and new CFAs. Natural and wood
fiber-polymer composites are mainly produced by the
following methods.
Figure 1. Effect of foaming temperature on cell density and cellsize (PVC-wood fiber composites, foaming time 15 s).
Batch Processing
During the batch process, a polymer sample is first placed
in a high-pressure chamber where the sample is saturated
with an inert gas (such as CO2 or N2) under high pressure at
ambient temperature. A thermodynamic instability is
then induced by rapidly lowering the solubility of the gas
in the polymer. This is accomplished by releasing the
pressure and heating the sample. This expansion drives
the nucleation of a large number of microcells, and the
nucleated cells grow to produce the foam expansion.
Because of the low rate of gas diffusion into the polymer at
room temperature, a very long time is required for the
saturation of the polymer with gas, which is the major
disadvantage of the batch process.
Matuana et al.[38,109–113] investigated the processing of
microcellular-foamed structures in poly(vinyl chloride)
(PVC)-wood fiber (silane treated) composites by a batch
foaming process. They have established the relationships
between cell morphology and processing conditions, as
well as between the cell morphology and mechanical
properties. The effects of foaming temperature on the cell
size and cell density of PVC-wood fiber composites in a
batch process are presented in Figure 1.[38]
It is seen that cell densities show a decreasing tendency
with the increase of foaming temperatures. The cell
density decreased significantly after a foaming tempera-
ture above 90 8C because of the activated cell coalescence
Macromol. Mater. Eng. 2007, 292, 113–127
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
by the lowered melt strength at elevated temperatures.
The cell size increased with the increase of foaming
temperature because of the increase of void fraction and
cell coalescence.
Matuana et al.[114,115] also investigated also microcel-
lular foam of polymer blends of high-density polyethylene
(HDPE)/PP with wood fiber in a batch process. Figure 2
represents the influence of foaming time and blend
composition on the void fraction of HDPE/PP blend–wood
fiber composites. HDPE-wood composites had a reasonably
high void fraction at high foaming time compared to
PP-wood composites and HDPE/PP blend-wood compo-
sites.
The batch foaming process used to generate cellular
foamed structures in the composites is not likely to be
implemented in the industrial production of foams
because it is not cost effective. The microcellular batch
foaming process is time consuming because of the
multiple steps in the production of foamed samples.[115]
In order to overcome the shortcomings of the batch
process, a cost-effective, continuous microcellular process
(injection molding, extrusion, and compression molding
process) was developed based on the same concept of
thermodynamic instability as in the batch process.
Injection Molding Processing
Microcellular wood fiber reinforced PP composites were
processed by an injection molding process where several
variables were considered when operating an injection
molding machine. Some of these variables can affect the
physical properties of the foam. It is well established, for
example, that the mold temperature and cooling time are
important variables in this regard. However, there are
many other factors that can be adjusted, including such
variables as front flow speed and filling quantity, which
might also have an effect on one or more foam properties.
The microcellular foaming of wood fiber-PP composites
in an injection molding process was investigated.[116–124]
The advantage of this injection molding process is the fact
that microcellular composites can be prepared with a
www.mme-journal.de 117
O. Faruk, A. K. Bledzki, L. M. Matuana
Figure 2. Effect of foaming time and blend composition on thevoid fraction (HDPE/PP-wood fiber composites, wood fiber con-tent 30 phr, foaming temperature 160 8C).
Figure 4. Effect of exothermic foaming agent content on thestructure of hard wood fiber-PP microcellular composites(exothermic foaming agent, wood fiber content 30 wt.-%.(a): 2 wt.-%, (b): 5 wt.-%).
118
sandwich structure using a conventional injection mold-
ing machine using different CFAs. The microscopic
observations, as well as microcell classifications, of the
performance when considering cell size, diameter, dis-
tance, and polydispersity compared to endothermic and
endo/exothermic CFAs.
The light micrograph (Figure 3) illustrated that the
foamed structure, near the injecting point, had a three
layer sandwich structure. It contained a middle layer with
distributed cells and identified a compact outer hull.
Between the foaming area and surface layers there was a
transition zone where microcells ride from the injection
point to the boundary area. The microcells were distorted
in this transition zone along the direction of flow at the
boundary layer to the cooled edge skin.
It is also revealed that[125] foaming with exothermic
CFAs (2 wt.-%) produces a finer cellular structure compared
to other contents (5 wt.-%) at same wood fiber content
(Figure 4). An optimum CFA content depends on the type
of CFA, wood fiber type, and also the wood fiber content
used.
Figure 3. Light micrograph of hard wood fiber-PP microcellular compo30 wt.-%).
Macromol. Mater. Eng. 2007, 292, 113–127
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
The solubility of the decomposition gases in the matrix
is also related to the optimum amount of CFA. A CFA
content above 2 wt.-% could not produce a better structure
because of the inhomogeneity of the initial material. From
micrographs (Figure 4), it is also seen that the microcells
can orient themselves depending on the direction of flow.
From the light micrographs, the cumulative fraction of
cell diameter and cell distance of the wood fiber-PP
microcellular composites were measured with the com-
puter software Digitrace.[119]
Figure 5 illustrates the cumulative fraction of cell
diameter and cumulative fraction of cell distances with
sites (wood fiber content
different CFAs. The exothermic
foaming agents show a better
spatial cell distribution and form
compared to other CFAs (Figure 5).
It is also notable that the max-
imum cell diameters are between
100 to 200 mm.
The density of the microcellular
composite foamed with exother-
mic foaming agent is reduced up to
30% (from 1.0 to 0.71 g � cm�3)
compared to non-foamed compo-
sites.[120]
DOI: 10.1002/mame.200600406
Microcellular Foamed Wood-Plastic Composites by . . .
The specific tensile strengths of hard wood fiber-PP
microcellular composites show only small differences
between all types of CFAs. Specific mechanical properties
of the composites were calculated by taking the ratio of
tensile or flexural properties to the density. Specific tensile
strength follows a trend in different wood fiber contents in
that the tensile strength is reduced with increasing the
fiber content as illustrated in Figure 6.[120]
The addition of coupling agent MAH-PP5% to the
microcellular composites had a great influence on the
Figure 5. Cumulative fraction of cell diameter and cell distance of hardmicrocellular composites: a) Cell diameter, b) Cell distance, wood fibwt.-%, chemical foaming agent content 4 wt.-%.
Macromol. Mater. Eng. 2007, 292, 113–127
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
fiber-matrix adhesion, cell size distribution, as well as on
the mechanical characteristics, which improved the
specific tensile strength of around 80%. It is remarkable
that hard wood fiber (30 wt.-%) composites with exother-
mic foaming agent and MAH-PP showed the lowest
density and finest cellular structure. So it can be concluded
that the mechanical properties are improved more by a
homogeneous and finer cellular effect.
Bledzki et al.[126] reported that the melt flow index of PP
and a variation of injection parameters (mold temperature,
wood fiber-PPer content 30
front flow speed, and filling quantity) have
a great influence on the properties and
structure of the microcellular wood fiber-PP
composites. The surface roughness of the
microcellular composites decreased nearly
70% when the mold temperature increased
from 80 to 110 8C (Figure 7). Since the
temperature difference between the
foamed core and surface is reduced, the
gas expands with the mass against the
smooth mold wall. It is also important to
mention that PP exhibits a smooth surface
with rising mold temperature.
The influence of filling quantity on the
specific flexural strength is illustrated in
Figure 8. It is observed that because of
the increase of filling quantity, the speci-
fic flexural strength decreases gradually,
which suggests that a suitable injected
mass should be selected. This confirms that
with this production process of the micro-
cellular materials, a material saving and an
improvement of the specific mechanical
characteristics can be obtained at the same
time.
Bledzki et al.[127] also demonstrated that
the wood fiber type and length strongly
affect the microcellular structures. Finer
wood fibers are correlated with a finer
microcellular structure. The microcellular
structure of finer soft wood fiber-PP com-
posites is illustrated in Figure 9. From the
optical light and scanning electron micro-
scopy (SEM) micrographs, it can be clearly
seen that the cell size and shape are finer,
similar, and distributed more uniformly
compared to hard wood fiber microcellular
composites (Figure 3 and 4).
The maximum cell size is nearly 50 mm. It
is possible that the bulk density (170–230
g � L�1) of the soft wood fibers affects the
structure, which is lower than the hard
wood fibers bulk density (190–270 g � L�1). It
seems that the small size of the wood fiber
www.mme-journal.de 119
O. Faruk, A. K. Bledzki, L. M. Matuana
Figure 6. Specific tensile strength of non-foamed and foamed microcellular hard woodfiber-PP composites (chemical foaming agent content 4 wt.-%).
120
particles provides a greater possibility for the expansion of
gas. It indicates that the finer wood fibers are more
amenable to foaming and can reduce the CFA content to
obtain finer microcellular composites.
Microcellular wood fiber reinforced composites also
exhibit a smoother surface compared to non-foamed
composites obtained by an injection molding process.[128]
This is reached by an outer non-foamed zone because of
the smaller surface structuring of the microcellular foam
and the resulting internal pressure of the microfoam.
Figure 10 shows that an endothermic foaming agent
reduces the maximum surface roughness (around 70%)
compared to other chemical foaming agents. This is
Figure 7. Influence of mold temperature on surface roughness (irregularity and arithmetiroughness mean deviation) of the hard wood fiber-PP microcellular composites (endothermfoaming agent 4 wt.-%, wood fiber content 30 wt.-%).
Macromol. Mater. Eng. 2007, 292, 113–127
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
because of the slow nucleation pro-
cess of an endothermic foaming
agent and to the thickness of the
outer zone of the composites.
Microcellular soft wood fiber-PP
composites were also prepared in a
box part by an injection molding
process.[129,130] A comparative study
of cell morphology, weight reduction,
and mechanical properties was con-
ducted between the box part and a
panel shape by considering different
processing temperatures. The micro-
cellular injection molded box part
and panel of soft wood fiber rein-
forced composites are illustrated in
Figure 11. The composites show a
finer microcellular structure at a
lower temperature, which suggests
that the processing temperature should be below 170 8C.The microcellular injection molded box part showed a
weight reduction of around 15%, whereas the panel
composites reduced the weight by nearly 25% because
of the different cellular structure and the composites’ wall
thickness. The cell morphology of the injectionmolded box
part differed from part area to part area, with the area near
the injection point showing a finer cellular structure than
the areas far from the injection point area. As a result, the
mechanical properties also differed from part area to part
area.
Extrusion Processing
The extrusion process requires a polymer with a higher
calic
molecularweight for bettermelt
strength, whereas the injection
molding process requires a
polymer with a low mole-
cular weight and low viscosity.
Twin- screw extruders dominate
today’s market because of their
compounding capability and
functional versatility, and they
are widely used for wood fiber–
plastic composites.[131–140] The
large number of patents[141–152]
of wood fiber–plastic compo-
sites obtained by the extrusion
process indicates the speed of
commercialization of that pro-
cess. Extrusion processes con-
tinuously devolatizewood fibers
and other natural cellulosic
materials and mix with plastics.
A suitable combination of pro-
DOI: 10.1002/mame.200600406
Microcellular Foamed Wood-Plastic Composites by . . .
Figure 8. Influence of filling quantity variation on the specific flexural strength ofhard wood fiber-PP microcellular composites (exothermic foaming agent 4 wt.-%content, wood fiber content 30 wt.-%).
Figure 9. The microcellular structure of finer soft wood fiber-PPcomposites: a) optical light micrograph and b) SEM micrograph,exothermic foaming agent 2 wt.-%, wood fiber content 30 wt.-%.
Macromol. Mater. Eng. 2007, 292, 113–127
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
cess variables was necessary for limiting the
thermal degradation of the wood and natural
fibers.
Rigid PVC-wood fiber composites foamed in
a continuous extrusion process were investi-
gated by Matuana et al.[153–157] The effects of
wood fiber moisture content, all-acrylic foam
modifier content, CFA content, and extruder die
temperature on the foamed composites struc-
ture and properties were studied. Figure 12
illustrates the influence of CFA type and
content on the cell size of rigid PVC-wood fiber
composites.[156] Exothermic foaming agents
produced smaller average cell sizes compared
to endothermic foaming agents regardless of
the CFA content. This trend is because of the
lower solubility and higher diffusivity of N2
(exothermic) in the PVC matrix compared to
that of CO2 (endothermic).
Park et al.[158] have experimented with two
system configurations (tandem extrusion sys-
tem vs. single extruder system) for wood
fiber-polymer composites to demonstrate the system
effect on the cell morphology and foam properties. The
system configuration had a strong effect on the cell
morphology, and the tandem extrusion system is highly
effective for fine-celled foaming of HDPE-wood fiber
composites compared to a single screw extruder system.
Polystyrene (PS)-wood fiber foamed composites were
investigated using moisture as a foaming agent.[159–161]
HDPE-wood fiber foamed composites were also investi-
gated by considering the effect of CFA (endothermic and
exothermic) and the influence of critical processing
temperature on the cell morphology.[162,163] The effect of
CFA type and content on the void fraction of the HDPE-
wood fiber composites are presented in Figure 13.[162] The
void fraction was significantly affected by the level of
foaming agent. The void fraction increased to 1% of
foaming agent, and above that concentration the void
fraction decreased regardless of CFA type.
Matuana et al.[164] examined the extrusion foaming of
PP-wood fiber composites using a factorial design approach
to evaluate the statistical effects of materials used and
processing conditions on the void fraction. It was revealed
that statistical analysis of void fraction datawas best fitwith
a linear model and the void fraction of foamed composites is
a strong function of the extruder’s die temperature.
Nowadays, nanoparticles (i.e., clay) are used in micro-
[1] R. Marutzky, Fifth Global Wood and Natural Fiber Compo-sites Symposium, April 27–28, Kassel Germany 2004.
[2] Kunststoffe 2004, 94, 38.[3] K-Zeitung 2003, 20, 10.[4] A. K. Bledzki, V. E. Sperber, O. Faruk, Rapra Rev. Rep. 2002, 13,
1.[5] V. E. Sperber, Fourth International Wood and Natural Fiber
Composites Symposium, April 10–11, Kassel, Germany 2002.
DOI: 10.1002/mame.200600406
Microcellular Foamed Wood-Plastic Composites by . . .
[6] U. Riedel, J. Nickel, Seventh International Conference onWoodfiber-Plastic Composites, May 19–20, Madison, Wiscon-sin, USA 2003.
[7] J. Patterson, J. Vinyl Addit. Technol. 2001, 7, 138.[8] B. C. Suddel,W. J. Evans, Seventh International Conference on
Woodfiber-Plastic Composites, May 19–20, Madison, Wiscon-sin, USA 2003.
[9] Annual report of the Government-Industry Forum on Non-Food Uses of Crops, Department of Environment, Food andRural Affairs Publications, EU, August 2002.
[10] J. Morton, J. Quarmley, L. Rossi, Seventh International Con-ference on Woodfiber-Plastic Composites, May 19–20, Madi-son, Wisconsin, USA 2003.
[11] W. Storck, Chem. Eng. News 2002, 65, 15.[12] Opportunities for Natural Fibers in Plastic Composites, 2000,
Study by Kline & Company, Inc, presented at 6th Inter-national Conference on Woodfiber-Plastic Composites, May14, Madison WI 2001.
[13] R. G. Raj, B. V. Kokta, G. Groleau, C. Daneault, Plast. RubberProcess. Appl. 1989, 11, 4.
[14] J. J. Balatinecz, R. T. Woodhams, Journal of Forestry 1993, 91,22.
[15] S. Nabi, D. Saheb, J. P. Jog, Adv. Polym. Technol. 1999, 18, 351.[16] A. K. Mohanty, M. Misra, G. Hinrichsen, Macromol. Mater.
Eng. 2000, 276, 1.[17] S. Thomas, Second International Workshop on Green Com-
posites, January 14–15, Yamaguchi, Japan 2004.[18] T. Peijs, Second International Workshop on Green Compo-
sites, January 14–15, Yamaguchi, Japan 2004.[19] L. M. Matuana, P. A. Heiden, ‘‘Wood Composites’’, in: Ency-
clopedia of Polymer Science and Technology, J. I. Kroschwitz,Ed., John Wiley & Sons, Inc., New York 2004.
[20] L. M.Matuana, C. B. Park, J. J. Balatinecz, SPE ANTEC TechnicalPapers 1996, 1900.
[21] J. H. Schut, Plastics Technol. 2001, July.[22] J. E. Martini, M.Sc. Thesis Cambridge, MA, USA 1982.[23] US 4 473665 (1984), invs.: J. E. Martini, N. P. Suh,
F. A. Waldman.[24] J. E. Martini, N. P. Suh, F. A. Waldman, SPE ANTEC Technical
Papers 1982, 674.[25] E. Baer, ‘‘Engineering Design for Plastics’’, in: SPE Polymer
Science and Engineering Series, Reinhold, New York 1964.[26] E. C. Bernhardt, ‘‘Processing of ThermoplasticsMaterials’’, in:
SPE Plastics Engineering Series, Reinhold, New York 1959.[27] C. B. Park, N. P. Suh, Polym. Eng. Sci. 1996, 36, 34.[28] C. B. Park, D. F. Baldwin, N. P. Suh, Cell. Microcell. Mater. 1994,
53, 109.[29] C. B. Park, A. H. Behravesh, R. D. Venter, ‘‘Polymeric Foams:
Science and Technology’’, K. Khemani, Ed., American Chemi-cal Society, Washington 1996, Ch. 8.
[30] C. B. Park, A. H. Behravesh, R. D. Venter, Cell. Polym. 1998, 17,309.
[31] A. H. Behravesh, Ph.D. Thesis University of Toronto, 1998.[32] C. B. Park, D. F. Baldwin, N. P. Suh, Polym. Eng. Sci. 1995, 35,
432.[33] J. J. Shim, K. P. Johnston, AIChE J. 1991, 37, 607.[34] P. L. Durril, R. G. Griskey, AIChE J. 1966, 12, 1147.[35] P. L. Durril, R. G. Griskey, AIChE J. 1969, 15, 106.[36] D. W. Krevelen, ‘‘Properties of Polymers’’, Elsevier, New York,
USA 1980.[37] C. B. Park, ‘‘Foam Extrusion’’, Technomic Publishing co.,
Pennsylvania, USA 2000, p. 263.[38] L. M. Matuana, C. B. Park, J. J. Balatinecz, Polym. Eng. Sci.
[40] L. M.Matuana, C. B. Park, J. J. Balatinecz, SPE ANTEC TechnicalPapers 1998, 1968.
[41] A. K. Mohanty, M. Misra, Polym. Plastic Technol. Eng. 1995,34, 729.
[42] J. J. M. Orfao, F. L. A. Antunes, J. L. Figueiredo, Fuel 1999, 78,349.
[43] M. M. Tang, R. Bacon, Carbon 1964, 2, 211.[44] W. F. Degroot, W. P. Pan, M. D. Rehman, G. N. Richards,
J. Anal. Appl. Pyrolysis 1988, 13, 221.[45] J. Scheirs, G. Camino, W. Tumiatti, Eur. Polym. J. 2001, 37,
933.[46] C. B. Park, A. H. Behravesh, R. D. Venter, Polym. Eng. Sci. 1998,
38, 1812.[47] S. T. Lee, ‘‘From Cellular to Microcellular Foam–What’s Up
and Coming’’, in: Trends in Plastics, May 2004 (www.plastic-strends.net/articles/microcellular.htm.)
[48] J. R. Street, A. L. Fricke, L. P. Reiss, Ind. Eng. Chem. Res. Fund.1971, 10, 54.
[49] N. Ramesh, S. Rasmunssen, G. A. Campbell, Polym. Eng. Sci.1991, 31, 1657.
[50] N. Ramesh, S. Rasmunssen, G. A. Campbell, Polym. Eng. Sci.1994, 34, 1685.
[51] N. Ramesh, S. Rasmunssen, G. A. Campbell, Polym. Eng. Sci.1994, 34, 1698.
[52] N. Ramesh, S. Rasmunssen, G. A. Campbell, SPE ANTECTechnical Papers 1992, 1078.
[53] M. Amon, C. D. Denson, Polym. Eng. Sci. 1984, 21, 1026.[54] A. Arefmanesh, S. G. Advani, E. E. Michalelides, Polym. Eng.
Sci. 1990, 30, 1330.[55] D. E. Rosner, M. Epstein, Chem. Eng. Sci. 1972, 27, 169.[56] R. D. Patel, Chem. Eng. Sci. 1980, 35, 2352.[57] C. D. Han, C. A. Villamizar, Polym. Eng. Sci. 1978, 18,
687.[58] C. D. Han, C. A. Villamizar, Polym. Eng. Sci. 1978, 18, 699.[59] S. K. Goel, E. J. Beckman, Polym. Eng. Sci. 1994, 34, 1137.[60] S. K. Goel, E. J. Beckman, Polym. Eng. Sci. 1994, 34, 1148.[61] D. F. Baldwin, C. B. Park, N. P. Suh, Cell. Microcell. Mater. 1994,
53, 85.[62] R. K. Upadhyay, Adv. Polym. Technol. 1984, 5, 55.[63] J. S. Colton, N. P. Suh, Polym. Eng. Sci. 1987, 27, 500.[64] J. S. Colton, N. P. Suh, Polym. Eng. Sci. 1987, 27, 485.[65] J. S. Colton, N. P. Suh, Polym. Eng. Sci. 1987, 27, 493.[66] J. S. Colton, Plastics Eng. 1998, August, 53.[67] J. S. Colton, Mater. Manuf. Processes 1989, 4, 1253.[68] S. Doroudiani, C. B. Park, M. T. Kortschot, Polym. Eng. Sci.
1998, 38, 1205.[69] L. M.Matuana, C. B. Park, J. J. Balatinecz, Cell. Polym. 1998, 17,
1.[70] D. I. Collias, D. G. Baird, R. J. M. Borggreve, Polymer 1994, 25,
3978.[71] V. Kumar, R. P. Juntunen, C. C. Barlow, Cell. Polym. 2000, 19,
25.[72] R. P. Juntunen, V. Kumar, J. E. Weller, W. R. Bezubic, J. Vinyl
Addit. Technol. 2000, 6, 93.[73] C. C. Barlow, V. Kumar, B. Flinn, R. K. Bordia, J. E. Weller,
J. Eng. Mater. Technol., 2001, 123, 229.[74] C. C. Barlow, V. Kumar, J. E. Weller, R. K. Bordia, B. Flinn, Cell.
Microcell. Mater. 1998, 78, 45.[75] A. K. Bledzki, H. Kirschling, C. Barth, SPE ANTEC Technical
Papers 2001, 1737.[76] D. I. Collias, D. G. Baird, SPE ANTEC Technical Papers 1992,
1532.
www.mme-journal.de 125
O. Faruk, A. K. Bledzki, L. M. Matuana
126
[77] D. F. Baldwin, N. P. Suh, SPE ANTEC Technical Papers 1992,1503.
[78] G. Wing, A. Pasricha, Polym. Eng. Sci. 1995, 35, 673.[79] V. Kumar, K. A. Seeler, J. Reinf. Plast. Compos. 1993, 12, 359.[80] V. Kumar, K. A. Seeler, Cell. Polym. 1992, 38, 93.[81] V. Kumar, K. A. Seeler, SPE ANTEC Technical Papers 1993,
1823.[82] D. F. Baldwin, N. P. Suh, M. Shimbo, Polym. Eng. Sci. 1995, 35,
1387.[83] D. F. Baldwin, N. P. Suh, M. Shimbo, Polym. Mater. Sci. Eng.
1992, 37, 512.[84] V. Kumar, R. Juntunen, T. Fidale, K. Nix, Foams 2000, 117.[85] N. P. Suh, Macromol. Symp. 2003, 201, 187.[86] O. Schoenherr, Kunststoffe 2003, 10, 22.[87] U. Schroder, Kunststoffe 2003, 10, 30.[88] W. Michaeli, O. Pfannschmidt, S. H. Naini, KU Spritzgiessen
2002, 92, 48.[89] W. Michaeli, E. Krampe, S. H. Naini, Kunststoffe 2003, 10,
34.[90] P. Egger, First Workshop Polymere Mikroschaume, 27th
November, Kassel, Germany 2003.[91] A. Sahnoune, J. Tatibouet, R. Gendron, A. Hamel, L. Piche,
J. Cell. Plastics 2001, 37, 429.[92] T. M. Pontiff, P. M. Techmer, Blowing Agents 99, December
9–10, Manchester, United Kingdom 1999.[93] S. Pahlke, Blowing Agents and Foaming Processes, May
19–22, Munich, Germany 2003.[94] S. Pahlke, Thermoplastische Schaumstoffe (Thermoplastic
Foam Material), February 4–5, Aachen, Germany 2003.[95] G. Luebke, Blowing Agents and Foaming Processes, March
13–14, Frankfurt, Germany 2001.[96] H. Helberg, Kunststoffe 1985, 75, 342.[97] G. Luebke, T. Holzberg, SKZ–6. Fachtagung (SKZ 6th Technical
[104] G. Lubke, Blowing Agents and Foaming Processes, May27–28, Heidelberg, Germany 2002.
[105] M. E. Reedy, Blowing Agents 99, December 9–10, Manche-ster, United Kingdom 1999.
[106] N. Lippel, Blowing Agents 99, December 9–10, Manchester,United Kingdom 1999.
[107] M. Kearns, Blowing Agents and Foaming Processes, May27–28 Heidelberg, Germany 2002.
[108] R. Benker, Blowing Agents and Foaming Processes, May19–20, Munich, Germany 2003.
[109] L. M. Matuana, C. B. Park, J. J. Balatinecz, Polym. Eng. Sci.1998, 38, 1862.
[110] L. M. Matuana, C. B. Park, J. J. Balatinecz, J. Cell. Plastics 1996,32, 449.
[111] L.M.Matuana, F.Mengeloglu, J. Vinyl Addit. Technol. 2001, 7,67.
Macromol. Mater. Eng. 2007, 292, 113–127
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
[112] L.M.Matuana, C. B. Park, J. J. Balatinecz, SPE ANTEC TechnicalPapers 1995, 2394.
[113] L. M. Matuana, C. B. Park, J. J. Balatinecz, Cell. Microcell.Mater. 1996, 76, 1.
[114] L. M. Matuana, R. Rachtanapun, S. E. M. Selke, SPE ANTECTechnical Papers 2003, 1762.
[115] L. M. Matuana, R. Rachtanapun, S. E. M. Selke, J. Appl. Polym.Sci. 2003, 88, 2842.
[116] A. K. Bledzki, O. Faruk, SPE ANTEC Technical Papers 2002,1897.
[117] A. K. Bledzki, O. Faruk, W. Zhang, Fifth International AVK-TVConference for Reinforced Plastics and Thermoset MouldingCompounds, September 17–18, Baden-Baden, Germany2002.
[118] A. K. Bledzki, O. Faruk, Cell. Polym. 2002, 21, 417.[119] A. K. Bledzki, O. Faruk, Seventh International Conference on
Woodfiber-Plastic Composites, May 19–20,Madison,Wiscon-sin, USA 2003.
[120] A. K. Bledzki, O. Faruk,Workshop on Innovative Materials onthe Base of Modified Wood Fiber and Polyolefins, February14–16, Kassel, Germany 2002.
[121] A. K. Bledzki, O. Faruk, Fourth International Wood andNatural Fiber Composites Symposium, Poster, April 10–11,Kassel, Germany 2002.
[122] A. K. Bledzki, O. Faruk, ECOCOMP 2003, Second InternationalConference on Eco-Composites, September 4–5, London, Uni-ted Kingdom 2003.
[123] A. K. Bledzki, O. Faruk, SPE ANTEC Technical Papers 2004,2665.
[124] A. K. Bledzki, O. Faruk, Fifth Global Wood and Natural FiberComposites Symposium, Poster, April 27–28, Kassel,Germany 2004.
[125] A. K. Bledzki, O. Faruk, J. Cell. Plastics 2006, 42, 63.[126] A. K. Bledzki, O. Faruk, J. Appl. Polym. Sci. 2005, 97,
1090.[127] A. K. Bledzki, O. Faruk, J. Cell. Plastics 2006, 42, 77.[128] A. K. Bledzki, O. Faruk, J. Cell. Plastics 2005, 41, 539.[129] A. K. Bledzki, O. Faruk, Macromol. Mater. Eng. 2006, 291,
1226.[130] A. K. Bledzki, O. Faruk, Progress in Wood & Bio Fiber Plastic
Composites, May 1–2, Toronto, Canada 2006.[131] Y. Wang, H. C. Chan, S. M. Lai, H. F. Shen, Int. Polym. Proc.
2001, 16, 100.[132] Br. Plast. Rubber 2000, November, 13.[133] P.W. Balasuriya, L. Ye, Y.W.Mai, Composites Part A. 2001, 32,
619.[134] R. D. Leaversuch, Mod. Plastics Int. 2000, 30, 62.[135] Plast. Rubber Wkly. 2000, November, 18.[136] C. Smith, Plast. Rubber Wkly. 2000, July, 10.[137] L. J. Yong, H. C. Myung, Int. Polym. Proc. 1999, 14, 10.[138] U. Berghaus, Plastverarbeiter 1995, 46, 18.[139] Y. Wang, H. C. Chan, S. M. Lai, H. F. Shen, Y. K. Hsiao, SPE
ANTEC Technical Papers 2001, 1789.[140] R. Colvin, Mod. Plastics Int. 2000, 30, 26.[141] US 6 153293 (2000), invs.: M. E. Dahl, R. G. Rottinghaus,
A. H. Stephans.[142] US 6 066680 (2000), invs.: C. W. Cope.[143] US 5 997784 (1999), invs.: W. Karnoski.[144] US 6 015612 (2000), invs.: M. J. Deaner, J. Puppin,
K. E. Heikkila.[145] US 6 015611 (2000), invs.: M. J. Deaner, J. Puppin,
K. E. Heikkila.[146] EP 976 790 (2000), invs.: V. W. Taverne, H. Simka,
H. Feil.
DOI: 10.1002/mame.200600406
Microcellular Foamed Wood-Plastic Composites by . . .
[147] US 5 932334 (1999), invs.: M. J. Deaner, J. Puppin,K. E. Heikkila.
[148] US 5 866641 (1999), invs.: C. P. Ronden, J. C. Morin.[149] US 5 847016 (1998), invs.: C. W. Cope.[150] US 5 725939 (1998), invs.: S. Nishibori.[151] EP 807510 (1997), invs.: C. W. Cope.[152] US 5 882564 (1999), invs.: G. Puppin.[153] F.Mengeloglu, L.M.Matuana, J. Vinyl Addit. Technol. 2002, 8,
264.[154] F. Mengeloglu, L. M. Matuana, SPE ANTEC Technical Papers
2001, 2997.[155] F. Mengeloglu, L. M. Matuana, SPE ANTEC Technical Papers
2001, 3003.[156] F.Mengeloglu, L.M.Matuana, J. Vinyl Addit. Technol. 2001, 7,
142.[157] F.Mengeloglu, L.M.Matuana, J. Vinyl Addit. Technol. 2003, 9,
26.[158] H. Zhang, G. M. Rizvi, W. S. Lin, G. Guo, C. B. Park, SPE ANTEC
Technical Papers 2001, 1746.[159] C. B. Park, G. M. Rizvi, H. Zhang, Fifth International Con-
ference on Woodfiber–Plastic Composites, May 26–27,Madison, Wisconsin, USA 1999, p. 105.
[160] L. M. Matuana, C. B. Park, J. J. Balatinecz, Fifth InternationalConference on Woodfiber–Plastic Composites, Poster, May26–27, Madison, Wisconsin, USA 1999, p. 318.
Macromol. Mater. Eng. 2007, 292, 113–127
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
[161] G. M. Rizvi, L. M. Matuana, C. B. Park, Polym. Eng. Sci. 2000,40, 2124.
[162] L. M. Matuana, Q. Li, J. Appl. Polym. Sci. 2003, 88, 3139.[163] G. Guo, G. M. Rizvi, C. B. Park, W. S. Lin, J. Appl. Polym. Sci.
2004, 91, 621.[164] L. M. Matuana, Q. Li, Cell. Polym. 2001, 20, 115.[165] G. Guo, K. H. Wang, C. B. Park, Y. S. Kim, G. Li, SPE ANTEC
Technical Papers 2004, 2620.[166] L. S. Turng, M. Yuan, H. Kharbas, Seventh International
Conference on Woodfiber–Plastic Composites, May 19–20,Madison, Wisconsin, USA 2003, p. 217.
[167] O. Faruk, Ph.D. Thesis University of Kassel, Germany2005.
[168] Y. H. Lee, T. Kuboki, C. B. Park, M. Sain, Conference of Progressin Wood & Bio Fiber Plastic Composites, May 1–2, Toronto,Canada 2006.
[169] D. Rodrigue, S. Souici, E. Twite-Kabamba, SPE ANTEC Tech-nical Papers 2005, 2679.
[170] A. K. Bledzki, O. Faruk, Int. Polym. Proc. 2006, 21, 256.[171] A. K. Bledzki, O. Faruk, Blowing Agents and Foaming Pro-
cesses, May 10–11, Stuttgart, Germany 2005.[172] A. K. Bledzki, O. Faruk, SPE ANTEC Technical Papers 2004,
2665.[173] G. M. Rizvi, C. B. Park, G. Guo, K. Wang, SPE ANTEC Technical