microcellular foamed wpc

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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

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Page 1: Microcellular Foamed Wpc

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.

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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).

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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

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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-

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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

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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

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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

microcellular wood fiber reinforced PP composites[119]

showed that exothermic foaming agents give better

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]

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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

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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-

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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-

cellular wood fiber reinforced polymer composites[165,166]

in extrusion processes. The results indicate that the addi-

tion of nanoparticles generally decreases the cell size,

increases the cell density, and facilitates foam expansion.

For pure PE, the addition of clay has a small effect on the

crystallinity. However, for the PE-wood fiber composites,

www.mme-journal.de 121

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O. Faruk, A. K. Bledzki, L. M. Matuana

Figure 10. Surface roughness (irregularity and arithmetical roughness mean deviation) of thenon-foamed and foamed microcellular composites.

122

the addition of nanoclay significantly reduces the crystal-

linity of PE. The PE-nanoparticle composite did not show a

distinctively low diffusivity compared to that of PE.

Moreover, when the wood fiber was added to the PE or

PE/nanoparticle matrix, there was no distinct difference

between the wood-PE or wood-PE-nanoparticle compo-

sites. The addition of clay did not significantly change the

diffusivity of CO2 in the composites, and the foammaterial

with nanoparticles showed good char formation when it

was burned.

Bledzki et al.[167] prepared microcellular wood fiber

reinforced PP composites as a profile by a continuous

Figure 11. Injection molded box part (geometry: 150� 100� 70 mm3) and panel (geometry:200�90�4 mm3) of soft wood fiber-PP composites.

Macromol. Mater. Eng. 2007, 292, 113–127

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

extrusion process. The effects of

different CFA types and the

variation of their concentrations

on the cell morphology and

physico-mechanical properties

of microcellular wood fiber-PP

composites were studied. Effects

of fiber type (hard and soft wood

fiber), fiber length (finer soft

wood fiber), and fiber contents

on the properties were also

investigated. It was observed

that the screw pressure should

be constant to get a good

foamed profile (optically in sur-

face) and that with high pres-

sure, it may be better. The

residence time of the materials

in the barrel plays a big role in

microfoaming, especially in the

extrusion process, as does the

inherent moisture content of

the wood fibers.

Park and co-workers[168] also studied the extrusion

foaming of rice hull–PP composites with a physical

foaming agent. It was found that the coupling agent

SEBS-MA increases the impact strength, and the desired

foam density of 0.6–0.8 g � cm�3 was successfully achieved

with rice hull as filler.

Rodrigue et al.[169] investigated the effect of wood

powder on polymer foam nucleation of wood-low

density PE composites in an extrusion process and

reported that the wood particles act as nucleating

agents to substantially reduce cell size and increase cell

density.

Compression MoldingProcessing

The hard wood fiber reinforced

PP microcellular composites

with CFA obtained by a com-

pression molding process were

studied.[167] The influence of

different CFA types and content

on the cell morphology, density,

and mechanical properties of

the composites were investi-

gated. In processing, the mold-

ing temperature, pressure, and

foaming time are very sensitive

and important to obtain well-

foamed composites.

A comparative study of

different foaming techniques

DOI: 10.1002/mame.200600406

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Microcellular Foamed Wood-Plastic Composites by . . .

Figure 12. Influence of chemical foaming agent type and contents on thecell size of rigid PVC-wood fiber composites (wood fiber content 30 phr).

(extrusion, injection, and compression molding) with

wood fiber-PP composites was also investigated.[170–172]

The injection molding process showed better achievement

compared to extrusion and compression molding pro-

cesses for most of the properties, especially in cell size,

shape, and distribution (Figure 14).

CFA content strongly influenced the density reduction

in the extrusion and compression molding process,

whereas it did not affect the density reduction of the

composites produced in the injection molding process. As

compared to the non-foamed composites, the maximum

density reduction was 30% (0.741 g � cm�3), 20% (0.837

g � cm�3) and 22% (0.830 g � cm�3) for the injectionmolding,

extrusion, and compression molding process, respec-

tively.[171]

Figure 13. Influence of chemical foaming agent type and contents on thevoid fraction of HDPE-wood fiber composites (wood fiber content 40wt.-%).

Macromol. Mater. Eng. 2007, 292, 113–127

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Odour concentration of extruded and injection

molded microcellular hard wood fiber–PP and non-

foamed composites are presented in Figure 15,[172] as

measured by olfactometry. As a result of foaming,

microcellular composites show the lowest odor

concentration. During foaming, the CFA decomposes

into gas and some portion of this comes out of the

composites simultaneously with the odor. However, it

is important to note that the odor concentration levels

of extruded composites are twice those of the injection

molded ones. This is because of the longer residence

time of the composites inside the barrel in extru-

sion, which provides more time for material decom-

position, with the end effect of higher odor concen-

tration.[173]

Conclusion

There is a growing trend to usewood fibers as fillers and/or

reinforcers in plastics compared to mineral fillers. Their

flexibility during processing, high-specific stiffness, and

low cost (on a volumetric basis) make wood fibers attrac-

tive to manufacturers. This century has witnessed ever-

increasing consumption of plastics as important raw

materials, more than 80% ofwhich are thermoplastics, as a

result of the increased demand. Creating microcellular

structures to produce foamed thermoplastics, and utilizing

inexpensive fillers to manufacture thermoplastic compo-

sites are two effective ways of addressing the reduced

weight, while maintaining or improving the material

properties.

Microcellular foams and their processing may change

the fundamental cost structure through the following

sources of economic benefits. The operating costs can

be reduced through cycle time reductions, reduced

scrap and reject rates, and lower energy consump-

tion. Through the component density reduction,

thinner design, and material substitution, the

material costs can be reduced. It is also of benefit

to use thermoplastic materials, which are flatter,

straighter, free of sink marks, and dimensionally

improved.

In general, microcellular wood-plastic composites

can reduce the density from 10 to 50% depending on

the material selection and composition compared to

non-foamed composites. With the use of a coupling

agent, the mechanical properties can be improved

significantly. Microcellular wood fiber reinforced

composites show lower surface roughness and odor

concentration compared to non-foamed composites.

The influence of different CFAs depends on their

decomposition rate, nature of decomposition gas,

average gas yield, effective components, and on their

www.mme-journal.de 123

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O. Faruk, A. K. Bledzki, L. M. Matuana

Figure 14. Microcellular structure of hard wood fiber–PP compo-sites: a) injection molding process, b) extrusion process, and c)compression molding process, exothermic foaming agent2 wt.-%, wood fiber content 30%.

Figure 15. Odor concentration of foamed and non-foamed micro-cellular hard wood fiber–PP composites extruded and injectionmolded (exothermic foaming agent 4 wt.-%, wood fiber content30 wt.-%).

124

carrier polymer. The microcellular structures are strongly

affected by the cell size, form, and distribution. The

mechanical performance of the microcellular composites

depends on the finer size and distribution of the cells,

which show significant improvement in the properties.

The length, geometry, and content of the wood fibers

strongly affect the microcellular structures of the compo-

sites. Finer wood fibers are correlated with a finer

microcellular structure.

Macromol. Mater. Eng. 2007, 292, 113–127

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

It should also be mentioned that with the same content

and type of all materials (wood fiber, PP and CFA), a finer

microcellular structure would be obtained by the injection

molding process compared to the other processes. In the

injection molding process, an optimum filling quantity

confirms that, with this production process of the

microcellular materials, a raw material saving and an

improvement of the specific mechanical characteristics

can be obtained at the same time. Because of being a

continuous-based technology, the extrusion process is a

time saving process.

The microcellular wood fiber reinforced polymer com-

posites represent a specific group of wood-polymer

composites, which find versatile applications in various

sectors, and its specific characteristics are more favourable

in comparison to the non-foamed composites.

Received: October 27, 2006; Revised: December 11, 2006;Accepted: December 13, 2006; DOI: 10.1002/mame.200600406

Keywords: cell morphology; compression molding; extrusion;foamed wood plastic composites; foaming agent; injectionmolding; mechanical properties; microcellular foaming; odorconcentration; surface roughness

[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

Page 13: Microcellular Foamed Wpc

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.

1997, 37, 1137.

Macromol. Mater. Eng. 2007, 292, 113–127

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

[39] J. Throne, ‘‘Thermoplastic Foams’’, Sherwood Publishers,Hinckley, Ohio 1996, Ch. 6.

[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

Page 14: Microcellular Foamed Wpc

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

Conference), Festung Marienberg, Wurzburg, Germany2001.

[98] G. Luebke, Seminare zur Kunsstoffverarbeitung (Seminar ofPlastic Processing), Aachen, Germany 2002.

[99] ‘‘Amending Directive 2002/72/EC’’, in: Official Journal of theEuropean Union, 2004, pp. L7/45–L7/46.

[100] A. Franc, Sixth International Conference on Woodfiber-Plastic Composites, May 15–16, Madison, Wisconsin, USA2001 p. 249.

[101] J. Kosin, A. Tice, L. Christopher, J. Cell. Plastics 1990, 26, 6.[102] L. Wahlen, First Workshop, Polymere Mikroschaume, 27th

November, Kassel, Germany 2003.[103] T. Holzberg, G. Lubke, SKZ-7, Fachtagung (SKZ 7th Technical

Conference), Festung Marienberg, Wurzburg, Germany2002.

[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

Page 15: Microcellular Foamed Wpc

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

Papers 2003, 2039.

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