Preparation and Morphology of Polypropylene/WoodFlour Composite Foams via Extrusion

Shuwen Zhang, Denis RodrigueDepartement de Genie Chimique, Universite Laval, Quebec City, Quebec G1K 7P4, Canada

Bernard RiedlDepartement des Sciences du Bois et de la Foret, Universite Laval, Quebec City, Quebec G1K 7P4, Canada

In this study, polypropylene (PP)/wood flour compositesfoams with an average cell size lower than 100 � weresuccessfully produced by extrusion. The effects of thecoupling agent (maleated PP), wood flour content (20%,30%, and 40% weight), initial wood moisture, and blow-ing agent content on the cell structure are reported. Itwas found that the addition of a coupling agent in-creases the blowing agent efficiency and helps to re-duce the density of the composites. Moreover, woodmoisture appears to be an effective secondary blowingagent under specific conditions. POLYM. COMPOS., 26:731–738, 2005. © 2005 Society of Plastics Engineers

INTRODUCTION

In general, wood-based polymer composites are superiorto neat polymers in terms of material costs and stiffness.This is why the use of wood-filled thermoplastics is becom-ing more and more accepted by the plastics industry [1–3].However, these improvements are usually accompanied bya loss in ductility and impact resistance of these composites.Moreover, wood/plastic composites have higher densitiescompared to unfilled thermoplastics and natural wood [4].These shortcomings have limited the use of wood thermo-plastic composites in the early years.

To overcome these shortcomings, the introduction of auniform cellular structure into the composite was proposedbecause small bubbles (�100 �) can effectively inhibitcrack propagation by blunting the crack tip and increasingthe amount of energy needed to propagate the crack [4]. Itis also known that neat microcellular polymers exhibithigher impact strength, toughness, fatigue life, and thermalstability compared to conventional polymer foams [4].

Over the past decade, several studies have examined theproduction of composite foams with fine cell structures, andsome investigated the relations between cell morphologyand macroscopic properties. One of the first microcellularPVC/wood composite preparation technologies, in the formof a batch process, was developed by Matuana et al. [5]. Thebasic approach of this process is to saturate a compositesheet with an inert gas in a high-pressure chamber andinduce thermodynamic instability by rapidly dropping thesolubility of the gas in the polymer. It was found that a highsaturation pressure favors a high nucleation rate by a rapiddrop in gas solubility (i.e., a high saturation pressure ben-efits the formation of a fine cell structure). With thismethod, microcellular structures were successfully pro-duced in PVC/wood fiber composites. As expected, thenotched Izod impact strength of the samples increased withthe void fraction [6].

Unfortunately, batch and semicontinuous processes areusually associated with high processing costs because oftheir long cycle times. Industries are more interested incontinuous technologies to manufacture microcellularwood/thermoplastic composite foams at high output [7].The concept of good extrusion foaming is to produce foamsvia a thermodynamic instability created by a rapid pressuredrop in the extruder die. For that purpose, Park and Suh [8,9] designed a filamentary extrusion processing and con-ducted a series of experiments to study the effects of pres-sure drop rates on cell morphology. They concluded that theprocessing pressure and pressure drop rate strongly affectthe final structure of the foams, with higher rates benefitingthe development of finer cell structures. Nevertheless, apartfrom the extrusion processing parameters (temperature pro-file, screw speed, etc.), several other variables should alsobe considered regarding wood/polymer composite extrusionfoaming. For instance, one must totally understand theeffects of the foaming matrix, blowing agents (type andconcentration), coupling agents (type and concentration),and finally the wood itself (type, size, loading and moisturecontent).

Correspondence to: D. Rodrigue; e-mail: Denis.Rodrigue@gch.ulaval.caContract grant sponsor: National Science and Engineering Research Coun-cil of Canada.DOI 10.1002/pc.20143Published online in Wiley InterScience (www.interscience.wiley.com).© 2005 Society of Plastics Engineers

POLYMER COMPOSITES—2005

Polypropylene is one of the most commonly used poly-mers to produce composites because of its low cost, goodmechanical properties, and commercial availability [1]. Toavoid the problem of coalescence and rupture resulting fromthe weak melt strength of polypropylenes (PPs), branchedPPs with high melt strength were developed [10, 11].

Blowing agents also play an important role in the pro-duction of extruded foamed thermoplastics. They can bedivided into two types: physical blowing agents (PBA) andchemical blowing agents (CBAs). Since high-pressureequipment is required for PBA foam production, the equip-ment investment is higher than that for the CBA. Currently,PBAs are used for low-density foams and CBAs are usedfor high-density foams [12]. Aside from the gas given offduring their thermal decomposition, CBAs used in the ex-trusion foaming process can be in either powder (standalone) or masterbatch forms (i.e., without or with polymercarriers). It has been reported that polymer carriers used toproduce CBA pellets can improve compatibility betweenthe active foaming agent and the polymer matrix, whichmay favor the production of foamed samples with fine cellmorphology [13].

There is also the problem of incompatibility betweenhydrophilic wood and hydrophobic thermoplastics. This hasbeen an issue for the production of wood-based composites.Over the last two decades substantial research has beencarried out on the modification of the wood surface usingcoupling agents [14, 15]. One simple and efficient way toenhance wood adhesion and dispersion in a polyolefin ma-trix is to add copolymer as a coupling agent. Maleatedpolyolefins have received considerable attention because oftheir effectiveness in improving the mechanical propertiesof polyolefin/wood fiber composites [14, 16–21]. For ex-ample, Kazayawoko et al. [17] studied the mechanical prop-erties at 30% wood loading and examined the effect ofmaleated PP (MAPP) (Epolene E-43 and Epolene G-3002)on bleached kraft pulp (BKP) and unbleached thermome-chanical pulp (TMP) PP composites. For 3% Epolene E-43(by weight of wood), increases in tensile strength of about28% and 27% for the TMP-PP and BKP-PP composites,respectively, were obtained. On the other hand, the tensilemodulus was not significantly affected. Similar results werealso reported for HDPE/wood composites [21].

Nunez et al. [22] found that none of the mechanicalproperties (except Young’s modulus) of wood flour/PPcomposites were significantly improved by either the woodflour chemical modification or the use of MAPP. However,wood dispersion in the PP matrix was significantly im-proved.

In the work of Douglas et al. [23], PP and PE compositesat 40% wood loading were prepared. Two different couplingagents—maleic anhydride with the appropriate base poly-mer (Polybond 3109 for PE, and Polybond 3200 for PP) andneoalkoxy titanate powder—were incorporated into thecomposites at concentrations of 1% and 2%. The resultsindicate that the addition of coupling agents had little effecton the impact strength of the PE composites, but had an

adverse effect on the PP composites. The tensile strength ofthe composites undergoes no significant change with theaddition of coupling agents, with the exception of titanate,which increases tensile strength in the case of PP-basedcomposites. The stress at break increases with the additionof maleic anhydride for PE-based composites. In the case oftitanate and PP-based composites with both couplingagents, no substantial change was observed. The elongationat break increases for PE-based composites and remainsrelatively unchanged for PP-based composites regardless ofthe coupling agent used.

The second issue in processing wood-based compositefoams is the effect of moisture and volatiles present inwoods, because these molecules can participate in the foam-ing process [5, 6, 20, 24]. During the heating stage ofextrusion (plastication), a substantial amount of moisturecan be released from the wood. Since the solubility of waterin hydrophobic thermoplastics is very low, dispersion ofwater vapor in the polymer matrix is difficult to achieve [25,26]. As a result, moisture causes significant deterioration ofthe cell structure (i.e., nonuniform and large cell size dis-tribution). To obtain a uniform and fine cell foam structure,it is usually suggested to remove most of the wood moisturebefore processing the composite foams. For extrusion, stan-dard drying techniques such as online devolatilization, ovendrying, hot air convective drying, drying in K-mixers, andthe like have been used [27, 28]. However, even dried woodcan release additional volatiles when it is further heated tohigher processing temperatures. These volatile emissionsmust be considered when fine cell structures are required[28].

Another way to decrease the influence of moisture oncell structure is to improve wood-particles dispersion (andat the same time moisture) by using an appropriate couplingagent and intensive mixing. This is why preparation of awood/polymer masterbatch using a twin-screw extruder ispreferred [29].

Recently, some studies have been published on the effectof processing parameters and composite compositions onthe mechanical properties of wood/polymer compositefoams [30, 31]. Unfortunately, the cell morphology was notdirectly reported. Since the mechanical properties of a ma-terial are directly related to its morphology, it was thepurpose of the present study to determine the effects ofwood content, wood moisture, blowing agent content, andcoupling agent content on the morphology (cell size distri-bution) of these popular materials.

EXPERIMENTAL

Material

The polymer used in this study was PP (Pro-Fax PF 814)supplied by Basell Canada. It has a density of 900 kg/m3

and a melt flow index of 3.0 g/10 min (230°C/2.16 kg). Oakflour from Mafor Inc. (Quebec, Canada) was used as the

732 POLYMER COMPOSITES—2005

wood filler. The flour was sieved and the fraction between120 mesh (125 �) and 140 mesh (106 �) was retained.MAPP Epolene E-43 (Mw � 9100 g/mol) from EastmanChemical Co. (USA) was used as the coupling agent andFoamazol R71 supplied by Bergen International (USA) wasused as the CBA. This endothermic CBA was supplied in amasterbatch form with 18% active agent. The recommendedconcentration for extrusion foaming is between 0.4% and2.5%. Finally, Irganox B215 from Ciba Fine Chemicals(USA) was added as an antioxidant and stabilizer.

Extrusion

The extruder used in this study was a HAAKE counter-rotating twin-screw extruder (model Rheocord TW-100), asin a previous study [32]. The extruder was fitted with acylindrical die with an inside diameter of 2 mm and a lengthof 5 mm (L/D � 2.5).

Thermogravimetric Analyses (TGAs)

TGAs were conducted to study the thermal devolatiliza-tion of moisture contained in the wood flour. A thermo-gravimeter from Mettler (TG 50 � TC11TA processor) wasused. A typical amount of material used for a TGA exper-iment was 15 mg. Both isothermal and dynamic TGA ex-periments were carried out. Isothermal tests were performedat 150°C, 175°C, 200°C, and 225°C for 60 min with undriedwood flour samples, while dynamic experiments were per-formed at a heating rate of 20°C/min. For all of the exper-iments the oven was continuously purged with nitrogen toremove the released moisture and maintain dry and inertconditions.

Mixing and Foaming

As a first step, a wood masterbatch was prepared withoutblowing agent. PP, antioxidant, and MAPP were first dryblended in a 2-L beaker for 5 min. In all cases the concen-tration of MAPP was maintained at 2% by weight of thewood flour, and the antioxidant at 0.02% by weight of PP.In the case of dry wood flour, the flour was initially dried at80°C in a vacuum oven for 24 hr before mixing. Then boththe preblended mix of PP and wood flour were fed into theextruder via two calibrated volumetric feeders. The feederswere set to obtain a 50% wood loading. In all cases theextruder screw speed was maintained at 25 rpm and thetemperatures of the barrel zones were maintained at 170°C,180°C, and 190°C, respectively. Finally, the composite ex-trudate was collected and later pelletized as the masterbatch.

All designed experiment formulations were divided intotwo groups: predried and undried masterbatches. For thefirst group, the masterbatch obtained was predried in avacuum oven at 80°C for 24 hr. Then the masterbatch,additional PP (according to the desired concentrations ofcomposite (0%, 20%, 30%, or 40% wood loading), andFoamazol R71 (0%, 0.75%, 1.0%, 1.25%, 1.5%, or 2.5%

concentration by weight of PP) were dry-blended in abeaker for 10 min. The blended mixture was then fed intothe corotating twin-screw extruder by one feeder. The fourzone temperatures of the extruder were maintained at170°C, 180°C, 190°C, and 190°C (die), respectively. Thescrew speed was constant at 25 rpm.

Density Measurement

According to the definition of the apparent density, thevolume and weight of each sample were carefully deter-mined with a micrometer caliper and an electronic balanceat 1 mg precision. Seven replicates were used for eachsamples and the mean value was taken. One standard devi-ation (SD) is used here as an estimation of the experimentalerror. The density is given by:

� � m/V (1)

where � is the density, m is the weight of the sample, and Vis its volume.

Scanning Electron Microscopy (SEM)

SEM observations of the foam fractured surface weremade at a magnification of 30� and 100�. Each samplewas fractured after cooling in liquid nitrogen (–196°C) forat least 30 min in order to avoid damage to the surfacemorphology and obtain a clean break. The samples werethen coated with a gold-platinum alloy and scanned. Quan-titative image analysis was used to assess the cell structureand measure the cell size using Image-Pro Plus imageanalysis software from Media Cybernetics.

RESULTS AND DISCUSSION

TGA of Wood

Wood particles are subjected to high temperatures andpressures during the extrusion foaming process. This is whywater molecules present in wood (free water, bound water,water of constitution), and volatile organic substances willprogressively release from the wood upon heating. It isknown that a certain amount of moisture in the wood candestroy the cell structure of composite foams because of thelow solubility of water in polymers [29]. To obtain a uni-form and fine cell structure, it is necessary to remove mostof the moisture from the wood before processing. Moreover,dried wood flour releases additional volatiles when it isfurther heated to the processing temperatures of thermoplas-tics [26]. When the operating temperature is over a certainlimit, wood flour can be seriously degraded [33].

A TGA was used to study the effect of temperature onwood. It was interesting to note that weight loss did notincrease uniformly with time or with increasing tempera-ture. For all of the temperatures tested (150°C, 175°C,

POLYMER COMPOSITES—2005 733

200°C, and 225°C), weight loss was significant during theinitial minutes (around 4%). This is related to the unboundwater. Figure 1 shows that weight loss after 5 min wasalmost constant for temperatures lower than 200°C. Thisresult suggests that most of the moisture present in the woodflour can be removed in less than 5 min in a static mode (nomixing) even at relatively low temperatures (150–175°C). Itis suspected that for temperatures of 150°C or lower, woodflour degradation can be effectively prevented, while fortemperatures above 175°C, more volatile components arereleased, which explains the higher losses. For temperatureshigher than 200°C, weight loss increases again due todevolatilization and thermal degradation of wood compo-nents, and the wood flour begins to be severely degraded.Similar TGA results were reported recently by Rizvi et al.[26, 27] and Rizvi and Park [29].

Figures 2 and 3 show dynamic thermograms of undriedand oven-dried wood flour being subjected to a heating rateof 20°C/min. Figure 2 shows that weight loss is significantin the 80–110°C range where the unbound water is released.Then the weight is almost constant up to 180°C, where asecond inflexion point, related to the release of volatiles, isobserved. This confirms the results obtained from the iso-

thermal tests. It can be concluded that the adsorbed moisturecan be very rapidly (�3 min) removed at a low temperature(lower than 110°C) in a few minutes and the processingtemperature should be kept below 200°C to prevent thermaldegradation of wood and release of volatiles. This conclu-sion is better illustrated in Fig. 3 for the dried wood flour.The weight is almost constant up to 180°C, where a sub-stantial weight reduction is observed. Since 3–5 min repre-sents the average residence time of the material in theextruder, it is recommended that the temperature should bekept below 180°C for this foam processing.

Foam Density

One important objective in foaming composites is den-sity reduction, which is related to the void content achievedin the polymer matrix. In this study, the effects of thecoupling agent, wood flour content, and moisture on foamdensity were investigated. Figure 4 shows that the additionof coupling agents is efficient in increasing density reduc-tion for wood/polymer composite foams, and an appropriatecoupling agent improves the wood–polymer interaction andoverall properties. Matuana et al. [5, 6] reported that woodsurface modification with a coupling agent has strong ef-fects on both the concentration of gas molecules absorbed

FIG. 1. Weight loss as a function of temperature (isothermal conditions).E: 2.5; ●: 5; : 10; �: 20 min.

FIG. 2. Weight of wood flour as a function of time at 20°C/min (undried).

FIG. 3. Weight of wood flour as a function of time at 20°C/min (oven-dried).

FIG. 4. Foam density as a function of foaming agent content for 20%wood content with ( ) and without (�) coupling agent.

734 POLYMER COMPOSITES—2005

by the composites and the cell morphology of the foamedcomposites produced through a batch foaming process.They found that the addition of a coupling agent into theformulation helped to encapsulate the gas in the compositesfor cell growth. This appears to also be the case for extru-sion.

Figure 5 presents the effect of wood content on thedensity of a composite foamed with 0.75% R71. As ex-pected, the density of the composite increases with woodcontent because the density of neat PP is 900 kg/m3 whilethe compacted density of wood is about 1.3–1.4 kg/m3 dueto the high pressures encountered in polymer processing(e.g., extrusion and injection molding) [2]. Figure 5 alsoshows some interaction between the wood content and thecoupling agent, since density reduction is more effectivewith a low wood content. It is suggested that the couplingagent has a significant effect on foam density by preventinggas loss when the system has a limited amount of gas. Thisconclusion was also reported by Matuana et al. [5, 6, 24]. Asthe amount of gas available for foaming (wood moisture,volatiles, or blowing agent) increases, the capacity of thecoupling agent to retain the gas in the composite becomesweaker. As a consequence, the final foam density is defi-nitely affected by wood moisture and foaming agent contentdespite the use of a coupling agent.

Figure 6 depicts the effect of moisture on foam density.Even if the wood flour is dried before the masterbatch isproduced, it will reabsorb some moisture from the environ-ment. Wood/PP composites (masterbatch) are known tohave different moisture adsorption behaviors with differentlevels of wood content. Stark [33] stated that at 90% relativehumidity (RH), the amount of moisture absorbed by woodflour itself was more than the amount absorbed by woodflour in composites. It is known that a 20% wood flourcomposite absorbs around 3.6% moisture, while 40% woodflour can absorb up to 5.5% moisture (i.e., composites withhigher wood content absorb more moisture). In the extru-sion foaming process, moisture absorbed by the masterbatch

must be considered. Figure 6 also shows that foam densityas a function of wood content for oven-dried masterbatchesincreases with wood flour content, while it decreases forundried masterbatches. It has been reported that wood mois-ture can act as a blowing agent and definitely affects cellnucleation and cell growth [29]. The density of 40% woodcomposite foam prepared with undried wood flour is thelowest, indicating that moisture effectively participates inthe composite foaming process as a secondary blowingagent.

Composite Foam Morphology

The morphology of the composite foams was analyzed interms of cell size distribution. Typical scanning electronmicrographs are presented in Figs. 7 and 8. From thesefigures it is clear that wood content and moisture have adefinite effect on cell size and cell size distribution. Byanalyzing the micrographs, the effect of each parameter canbe determined.

Figure 9 shows the effect of wood content on the cell sizedistribution for 0.75% R71. Under similar processing con-ditions, the cell size of wood composite foams is remarkablysmaller than that of neat PP foams. For all composite foams,the average cell size is smaller than 100 �m. The reducedcell size as wood loading increases can be explained by twoeffects. First, the melt viscosity of the matrix increases withwood content, generating higher resistance to cell growth inthe foaming process [24]. Figure 10 shows the dynamicviscosity of the composites at 180°C. It is known thatincreasing wood content increases viscosity [34, 35]; how-ever, in our case the small viscosity increase with theaddition of 20% wood cannot explain the reduction inaverage cell size from 150 to 75 �m.

The improvement in cell morphology of compositefoams can be attributed to the dispersed wood flour in thepolymer matrix. These particles are believed to act likenucleating agents, thus promoting heterogeneous nucleation[36]. At a constant blowing agent concentration, the amountof gas available for foaming is constant. Increasing the

FIG. 5. Foam density as a function of wood content for 0.75% R71 with( ) and without (�) coupling agent.

FIG. 6. Effect of moisture on foam density (2% coupling agent and0.75% R71).

POLYMER COMPOSITES—2005 735

number of cell through heterogeneous nucleation leads tosmaller sizes.

The cell structure of foamed composites at 20% and 40%wood flour content and 1.5% R71 is presented in Fig. 7, inwhich it can be seen that while all of the composite foamshave mostly a closed cell structure, for wood loading inexcess of 30% the cells have irregular and complex shapes.

As discussed above, the presence of wood moisture isusually the cause of cell structure deterioration in compositefoams because dispersion of water in a thermoplastic melt isdifficult to achieve due to mass transfer properties and highsurface tension [29]. Composite foams prepared by themasterbatch (dried and undried wood) are effective in im-proving wood and moisture dispersion in the polymer ma-trix. Moreover, the masterbatch absorbs less moisture fromthe environment than wood alone [33]. Both of these ad-vantages are expected to reduce cell deterioration caused bymoisture. This can be seen in Fig. 8, in which the cellmorphologies of composite foams prepared via undriedmasterbatches are similar to the oven-dried masterbatches.In other words, the cell morphologies of these samples werenot destroyed by moisture. In addition, the foam density ofsamples prepared via undried masterbatches (about 0.80

g/cm3 at 40% wood loading) is lower than that obtained inoven-dried masterbatches (about 1.03 g/cm3 at 40% woodloading). Thus we can conclude that the small amount ofmoisture absorbed by masterbatches can participate in thefoaming process as a secondary blowing agent. Less mois-ture absorbed by the masterbatches than wood flour, andbetter wood flour dispersion in the matrix promoted by themasterbatch can explain the fine cell structure in these cases.It is assumed that both the finely dispersing moisture bub-bles in the matrix and less moisture reabsorbed by themasterbatch than wood alone contributed to a fine cellstructure in these composite foams. This is shown in Fig. 11,in which a reverse trend is observed between dried andundried wood (i.e., the average cell size for the undriedwood increases while it decreases for the dried samples).This is expected in relation to the composite foam densitypresented in Fig. 6.

Finally, the effect of the coupling agent on cell sizedistribution is presented in Fig. 12 for 20% wood contentfoamed with 0.75 and 2.5% R71. At low blowing agentconcentration (0.75% R71), the mean cell size with thecoupling agent is 73 �m, while it is 64 �m without thecoupling agent. The difference in average cell size decreases

FIG. 7. Micrographs of composite foams (1.5% R71) at 100�. a: 20%. b: 40% wood content.

FIG. 8. SEM of composite foams with40% wood content. a: Undried master-batch. b: Oven-dried masterbatch.

736 POLYMER COMPOSITES—2005

as the blowing agent concentration increases up to a point(2.5% R71) at which the coupling has almost no effect onthe cell size distribution.

This is also related to the density with and withoutcoupling agent (presented in Fig. 5). It is believed that thecoupling agent not only enhances the wood/polymer adhe-sion, but also prevents the gas from escaping. All otherthings being equal, the addition of the coupling agentseemed to be more efficient in retaining the gas, whichresulted in a slightly larger cell size for foam samples withcoupling agent than that without coupling agent. Li andMatuana [37] obtained similar results for HDPE-based com-posites in a study examining the effects of chemical foam-ing agents as well as coupling agents on density reductionand cell morphology. They found a small but significantimprovement in composite foam morphology when cou-pling agents were present.

CONCLUSIONS

After performing a systematic study of the preparation ofPP/wood flour composite foams with a twin-screw extruder

and analyzing their morphology, we can draw several gen-eral conclusions.

PP/wood flour composite foams with a mean cell sizesmaller than 100 �m can be obtained by a continuousextrusion process via masterbatches and CBAs. The foamshad a relatively uniform and close cell structure, but increas-ing wood content over 30% led to composite foams withirregular cell geometry.

FIG. 11. Cumulative cell size distribution as a function of wood contentwith 0.75% R71 for dried (open) and undried (close) wood.

FIG. 12. Cumulative cell size distributions for 20% wood content. �:with 2% coupling agent, : without coupling agent. a: 0.75%. b: 2.5%R71.

FIG. 9. Cumulative cell size distributions for PP and WPC foams withoutcoupling agent.

FIG. 10. Dynamic viscosity of PP and wood/PP composites withoutcoupling agent.

POLYMER COMPOSITES—2005 737

The addition of a coupling agent (Epolene E-43) has aslight but significant effect on reducing sample densities byimproving the adhesion between wood flour and PP whenthe system has limited gas. However, as the amount of gasincreases, its capacity to keep the gas inside the polymermelt seems to decrease.

The production of a masterbatch can effectively over-come the problems related to moisture in the cell structure.There appears to be a suitable amount of moisture that canparticipate in the cell nucleation/growth processes, thusreducing density. An optimal amount of moisture for form-ing fine cell size composite foams is expected; however,further investigations are warranted.

ACKNOWLEDGMENTS

The authors are grateful to Basell Canada, Mafor Inc.,Eastman Chemical Company, and Bergen International forproviding the samples used in this study.

REFERENCES

1. H. Chtourou, B. Riedl, and A. Aıt-Kadi, J. Reinf. Plast.Compos., 11, 372 (1992).

2. B. English, C.M. Clemons, N.M. Stark, and J.P. Schneider,Waste-Derived Fillers for Plastics, Forest Products Labora-tory General Technical Report FPL-GTR-91, Forest ProductsLaboratory, Madison, WI, 15 (1996).

3. F.M.B. Coutinho and T.H.S. Costa, J. Polym. Test., 18, 581(1999).

4. H. Zhang, G.M. Rizvi, W.S. Lin, G. Guo, and C.B. Park,ANTEC 2001, 47, 1746 (2001).

5. L.M. Matuana, C.B. Park, and J. Balatinecz, ANTEC 1996, 54,1900 (1996).

6. L.M. Matuana, C.B. Park, and J. Balatinecz, ASME, MD-76,1 (1996).

7. C.B. Park, “Continuous Production of High-Density and Low-Density Microcellular Plastics in Extrusion,” in Foam Extru-sion, S.T. Lee, ed., Technomic Publications, Lancaster, PA,263 (2000).

8. C.B. Park and N.P. Suh, J. Manuf. Sci. Eng., 118, 639 (1996).

9. C.B. Park and N.P. Suh, Polym. Eng. Sci., 36, 34 (1996).

10. M.B. Bradley and E.M. Philips, ANTEC 1990, 36, 717 (1990).

11. H.E. Naguib, J.X. Xu, and C.B. Park, ANTEC 2001, 47, 1623(2001).

12. T. Pontiff, “Foaming Agents for Foam Extrusion,” in FoamExtrusion, S.T. Lee, ed., Technomic Publications, Lancaster,PA, 251 (2000).

13. M.E. Reedy, Plast. Eng., 56(5), 47 (2000).

14. B. Liao, Y. Huang, and G. Cong, J. Appl. Polym. Sci., 66,1561 (1997).

15. S. Godavarti, ANTEC 2003, 61, 2047 (2003).

16. M. Kazayawoko and J.J. Balatinecz, J. Reinf. Plast. Compos.,16, 1383 (1997).

17. M. Kazayawoko, J.J. Balatinecz, and L.M. Matuana, J. Mater.Sci., 34, 6189 (1999).

18. T.H.S. Costa, D.L. Carvalho, D.C.S. Souza, F.M.B. Coutinho,J.C. Pinto, and B.V. Kokta, J. Polym. Test., 19, 419 (2000).

19. Q.X. Li and L.M. Matuana, ANTEC 2002, 60, 2174 (2002).

20. A.K. Bledzki and O. Faruk, Cell. Polym., 21, 417 (2002).

21. S.M. Lai, F.C. Yeh, Y. Wang, H.C. Chan, and H.F. Shen,J. Appl. Polym. Sci., 87, 487 (2003).

22. A.J. Nunez, P.C. Sturm, J.M. Kenny, M.I. Aranguren, N.E.Marcovich, and M.M. Reboredo, J. Appl. Polym. Sci., 88,1420 (2003).

23. P. Douglas, W.R. Murphy, G. McNally, and M. Billham,ANTEC 2003, 61, 2408 (2003).

24. L.M. Matuana, C.B. Park, and J.J. Balatinecz, Polym. Eng.Sci., 38, 1862 (1998).

25. G.M. Rizvi, C.B. Park, and L.M. Matuana, ANTEC 1999, 57,2040 (1999).

26. G.M. Rizvi, W.S. Lin, C.B. Park, and G. Guo, “VolatileEmissions from Wood-Fibre in Extrusion of Plastic/Wood-Fibre Composite,” Polymer Processing Society Conference(PPS-18), Guimaraes, Portugal, paper 220 (2002).

27. G.M. Rizvi, W.S. Lin, G. Guo, R. Pop-Iliev, and C.B. Park,“Effect of Moisture on the Cellular Structure of HDPE/WoodFiber Composite Foams,” SPE Topical Conference, Montreal,35–49 (2001).

28. G.M. Rizvi, C.B. Park, G. Guo, and K. Wang, ANTEC 2003,61, 2039 (2003).

29. G.M. Rizvi and C.B. Park, “A Novel Extrusion System Designfor Production of Fine-Celled Plastic/Wood-Fiber CompositeFoams,” ASME, IMECE, Porous, Cellular and MicrocellularMaterials 2000, 83 (2000).

30. S. Doroudiani and M.T. Kortschot, J. Thermoplast. Compos.Mater., 17, 13 (2004).

31. L.M. Matuana and Q. Li, J. Thermoplast. Compos. Mater., 17,185 (2004).

32. D. Rodrigue and R. Gosselin, “The Effect of NucleatingAgents on Polypropylene Foam Morphology,” RAPRA Blow-ing Agents and Foaming Processes Conference 2003, Munich,169 (2003).

33. N. Stark, J. Thermoplast. Compos. Mater., 14, 421 (2001).

34. K. Xiao and C. Tzoganakis, ANTEC 2003, 61, 975 (2003).

35. S.N. Maiti, R. Subbarao, and N.M. Ibrahim, J. Appl. Polym.Sci., 91, 644 (2004).

36. D. Rodrigue, C. Woelfle, and L.E. Daigneault, “The Effect ofNucleating Agent Particle Size and Specific Surface Area ofFoam Morphology: A New Descriptor,” RAPRA BlowingAgents and Foaming Processes 2001, Frankfurt, paper 22(2001).

37. Q.X. Li and L.M. Matuana, ANTEC 2002, 60, 2174 (2002).

738 POLYMER COMPOSITES—2005