Mechanical PropertiesPolypropylene-Wood

KRISTIINA OKSMAN,1 CRAIG CLEMONS2

and Morphology of Impact ModifiedFlour Composites

1 Luleå University of Technology, Division of Wood Technology, Skellefteå Campus, Skeria 3, S-93187 Skellefteå,Sweden

2 USDA Forest Service, Forest Products Laboratory, 1 Gifford Pinchot Drive, Madison, Wisconsin 53705

Received 24 February 1997; accepted 4 June 1997

ABSTRACT: The mechanical properties and morphology of polypropylene/wood flour(PP/WF) composites with different impact modifiers and maleated polypropylene(MAPP) as a compatibilizer have been studied. Two different ethylene/propylene/dieneterpolymers (EPDM) and one maleated styrene-ethylene/butylene-styrene triblockcopolymer (SEBS-MA) have been used as impact modifiers in the PP/WF systems.All three elastomers increased the impact strength of the PP/WF composites but theaddition of maleated EPDM and SEBS gave the greatest improvements in impactstrength. Addition of MAPP did not affect the impact properties of the composites buthad a positive effect on the composite unnotched impact strength when used togetherwith elastomers. Tensile tests showed that MAPP had a negative effect on the elonga-tion at break and a positive effect on tensile strength. The impact modifiers were foundto decrease the stiffness of the composites. Scanning electron microscopy showed thatmaleated EPDM and SEBS had a stronger affinity for the wood surfaces than did theunmodified EPDM. The maleated elastomers are, therefore, expected to form a flexibleinterphase around the wood particles giving the composites better impact strength.MAPP further enhanced adhesion between WF and impact-modified PP systems. EPDMand EPDM-MA rubber domains were homogeneously dispersed in the PP matix thediameter of domains being between 0.1-1 µm. © 1998 John Wiley & Sons, Inc. J Appl PolymSci 67: 1503-1513, 1998

Key words: polypropylene-wood flow composites; mechanical properties; morphol-ogy; MAPP

INTRODUCTION

The main purpose of adding cellulose-based fillerto thermoplastics is to reduce the cost per unitvolume and improve stiffness. Low price cellulosebased fillers such as wood flour, wood fibers, andcellulose fibers have high stiffness, low density,and are recyclable. However, there is poor interfa-cial adhesion between the hydrophobic matrixand the hydrophilic filler, which usually resultsin decreased toughness. 1-3

Correspondence to: K. Oksman.Journal of Applied Polymer Science, Vol. 67, 1503-1513 (1998)© 1998 John Wiley & sons, Inc. CCC 0021-8995/98/91503-11

The toughness of filled polymers can be im-proved in several ways: 1) increase the matrixtoughness; 2) optimize the interface (or in-terphase) between the filler and the matrixthrough the use of copling agents, comptibilizer,and sizes; 3) optimize the filler-related propertiessuch as filler content, particle size, and disper-sion; 4) aspect ratio and orientation distributionsalso play a role in toughness of composites con-taining more fiberous materials.4 This investiga-tion focused on the first two approaches.

Toughening of PP Matrix

The matrix material plays an important role infilled polymer systems. If the polymer matrix has

1503

1504 OKSMAN AND CLEMONS

high ductility, it can increase the toughness of thecomposite.4 PP itself has poor impact properties,especially at low temperatures. The impact prop-erties of PP can be improved through the use ofblock copolymers of polypropylene, addition ofelastomers, or with copolymerization with ethyl-ene. The most commonly used and most effitiveimpact modifiers for PP are ethylene/propylenecopolymers (EPM) or ethylene/propylene/dieneterpolymer (EPDM) .5-7 Blends with 10-40%elastomers are called “impact polypropylenes”;the elastomeric phase being dispersed in the poly-mer matrix. Elastomer domain size and distribu-tion also have an effect on the impact propertiesof PP. Uniform distribution and small domains,< 1 µm, are expected to improve the impact prop-erties of PP.8,9 Jang et al.8 studied the dependenceof the impact behavior of the PP on the rubberparticle size and found that small particles weremore effective than larger particles for tough-ening PP. Dao9 reported that the optimum parti-cle size for impact improvement using EPDM rub-ber in the PP matrix is between 0.1-1 µm.

Toughening of Filled Polymers

Many elastomers have been used as impact mod-ifiers for filled/reinforced PP systems. The elasto-meric phase improves the impact strength butreduces the E-modulus.10-14 The mechanism oftoughening is more complicated in a compositesystem than in a single-phase system becausestress concentrations, interactions between com-ponents and heterogeneity, etc., provide addi-tional complications. Generally, the role of theelastomer particles is to act as stress concentra-tors that initiate the local yielding of the matrix,which avoids brittle catastrophic failure of thematerial. 4 Impact modifiers can affect the filledPP morphology in three different ways: (a) theelastomer phase can exist as a third separatephase in the PP matrix, (b) the elastomer canpartially or completely encapsulate the filler(form a interphase), or (c) a mixed condition of(a) and (b).10-13 Stamhuis 10,11 used styrene/buta-diene/styrene rubber (SBS), styrene-ethylene/butylene-styrene (SEBS), acrylonitrile/butadi-ene rubber (NBR), ethylene vinylacetate (EVA),and EPDM as impact modifiers in tale-filled PPcomposites. He reported improved impact proper-ties with all additives but obtained the best re-sults when the additives partially coated (encap-sulated) the filler surfaces.

Earlier investigations have shown that the in-teraction between PP and cellulose-based fillers

can be improved with the addition of a maleatedpolypropylene (MAPP).3

,15-19 Dalväg et al.3 re-

ported improvements in tensile strength, elonga-tion at break, and Charpy impact properties whenMAPP was used as a compatibilizer in PP/WFcomposites. Felix and Gatenholm15 and Gaten-holm et al. 16 used MAPP in a PP/cellulose fiber(CF) system and reported increases in tensilestrength and impact strength. Good adhesion be-tween the cellulose fibers and PP matrix withMAPP was shown using scanning electron micros-copy. The fracture surface of the compositeshowed that the failure occurred in the matrixand not in the interface between the CF and PP.Myers et al.17,18 studied the influence of MAPPand extrusion temperature on PP/WF composites.MAPP had a positive effect on composite tensilestrength and stiffness, but a negative effect onnotched impact strength. They believed that theloss of impact strength may be caused by the in-creased reinforcement in the composites and alsoby increased wood filler brittleness at higher ex-trusion temperatures. Han et al.19 used MAPP asa compatibilizer in different PP/wood compositesystems. They reported improved mechanicalproperties with the addition of which dependsupon the reactive size of hydroxyl groups on thefiller surfaces. Cellulose kraft pulp fibers and dis-solving pulp fibers had higher concentrations ofhydroxyl groups on the surface and, therefore,demonstrated better mechanical properties thanPP/WF composites.

Dalväg et al.3 tested several elastomer addi-tives in cellulose filler/thermoplastic compositesystems to improve the impact properties. Theelastomers used were ethylene/vinylacetate(EVA), chlorinated polyethylene (CPE), polyiso-butylene (PIB), ethylene/propylene thermoplas-tic elastomer (TPO), ionomer-modified polyethyl-ene (syrlyn), and acrylonitrile/butadiene elasto-mer (NBR). They reported an impact strength forthe PP/WF (30%) composite of about 27 kJ/m2,increasing to about 41 kJ/m2 with the addition of10% NBR, 30 kJ/m2 with 10% PIB, and 29 kJ/m2 with 10% TPO. The other additives did notimprove the impact strength.

Scott et al.20 reported that the addition of un-modified EPDM and maleated EPDM gave goodIzod impact strength properties in calcium car-bonate and oxidized silicon powder-filled polyeth-ylene (PE) composite systems—the EPDM-MAshowing superior impact properties. Oksman21

used SEBS-MA in a PE/WF composite systemand reported an improvement in impact proper-ties and also a compatibilizing effect between the

MORPHOLOGY OF PP/WF COMPOSITES 1505

wood filler and the PE. Long and Shank’s12 studyof filled PP systems showed that SEBS-MA andEP-MA (ethylene-propylene) act not only as atoughening agent but also as compatibilizer be-tween filler particles (talc, CaCO3, nylon-12) andthe PP matrix. The maleated elastomers encapsu-lated the filler particles (core-shell), resulting inimproved impact properties. Gupta et al.14 studiedglass fiber (GF)-reinforced PP/EPDM composite.They reported that GF increased impact strength,which was further increased with a high EPDMcontent (20–30 wt %). The reinforcing effects ofGF was increased with the addition of EPDM.They explained that EPDM played a dual role inGF/PP composites. It helps maintain the align-ment of the GF during tensile loading and by in-duces shear yielding of the matrix. Morphologicalstudy of the composites showed the fibers werecoated, probably with EPDM. The GF was treatedwith a coupling agent compatible with PP, but thepossible effects of this were not discussed.

The size and dispersion of filler particles inthe matrix can effect the composites properties.Small, well-dispersed particles generally give bet-ter properties.5 Small particles can block crackpropagation, resulting in impact toughening. How-ever, it is often difficult to disperse very fine parti-cles because of their tendency to agglomerate.Particles with higher aspect ratios (e.g., wood fi-bers) have high stresses at the fiber ends that cancause failure under impact.22 Stamhuis10 foundthat smaller particles were coated more com-pletely than larger ones.

An initial study was proposed to investigate theeffects of elastomer-compatibilizer combinationson the morphology and mechanical properties ofWF filled PP. The impact modifiers are believedto improve the PP impact strength and elongationat break but to decrease the E-modulus of thecomposites. Maleated impact modifiers are be-lieved also to act as compatibilizere; MAPP mayincrease this effect further. Combinations of im-pact modifier and MAPP should result in goodadhesion between the WF particles and PP matrixand increase the toughness of the PP matrix.

MATERIALS AND METHODS

Materials

The following commercially available materialswere used:

Matrix

Polypropylene PP, Fortilene PP 9200 (Solvaypolymers ), density 900 kg/m3, MFI 4 g/10 min,(230°C/2160 g).

Impact Modifiers and Compatibilizer

Ethylene/propylene/diene terpolymer EPDM,Royalene IM-7200 (Uniroyal Chemical) density870 kg/m3, ethylene to propylene (E/P) ratio 75/25. Maleated ethylene/propylene/diene terpoly-mer EPDM-MA, Royaltuf 465 (Uniroyal Chemi-cal) total maleic anhydride/acid content of 1%,ethylene to propylene (E/P) ratio 55/45. Male-ated styrene-ethylene/butylene-styrene triblockcopolymer SEBS-MA, Kraton FG 1901X (ShellChemicals ), PS content 28% by weight, function-ality 2% by weight as bound maleic anhydride,density 910 kg/m3. Maleated polypropyleneMAPP, Unite MP880 (Aristech Chemical Corpo-ration) average molecular weight of 90,000.

Filler

Wood flour WF, number 402, Western pine, parti-cle size 420 µm (nominal 40 mesh) (AmericanWood Fibers, Inc, Schofield, WI).

Compositions of various PP/WF composites areshown in Table I.

Processing

The PP, WF, and additives were preblended in aMarion mixer and then compounded in a Davis-Standard (Pawcutuck, CT) corotating twin-screwextruder (screw diameter 32 mm). The barreltemperatures were: 193°C, for zones 1, 2, and171°C for zones 3-7 and melt temperature at thedie was 200°C. The screw speed was 220 rpm,pressure at the die was 4.5 MPa, and materialoutput was 24 kg/h. The extruded strands werecooled in a water slide system, pelletized, anddried at 105°C. The compounded pellets were in-jection molded using a conventional CincinnatiMilacron (Batavia, OH) 33 ton reciprocatingscrew injection molder into standard ASTM testspecimens.

Mechanical Testing

Tensile testing of the specimens were performedaccording to ASTM D 638 on a MTS 810 materialtest machine using an model 632, 12F-20 straingauge (MTS, Systems, Minneapolis, MN). Cross-head speed was 5 mm/min. The E-modulus and

1506 OKSMAN AND CLEMONS

Table I Composition of the Various PP/WF Composites

Composite Compositions for Polymers (%-by Weight), Impact Modifiersand Compatibilizer

SampleCode PP WF MAPP EPDM EPDM-MA SEBS-MA

maximum tensile strength and elongation atbreak were calculated from the tensile test data.At least 10 test specimens of every compositionwere tested.

Impact testing was performed using the ASTMD 256 Izod Impact method. Both notched and un-notched impact energies were determined for atleast 10 test samples of every composition.

Conditions during testing were 23°C and 50%relative humidity.

Scanning Electron Microscopy

The fracture surfaces from room temperature andliquid nitrogen Izod impact test specimens wereexamined using JEOL JSM-840 and CamScan S4-80DV scanning electron microscopes (SEM) atan acceleration voltage of 15 kV. The elastomerparticles on the fracture surfaces were relevatedby etching with n -heptane vapour for 20 s. Sev-eral authors have used n -heptane as an etchingmedium for EPDM rubber in PP.23,24 Particle sizedistributions were determined using an imageanalysis system for scanning electron microscopes(SemAfore, JEOL). All specimens were sputtercoated with gold.

RESULTS AND DISCUSSION

Mechanical Properties

Table H summarizes the mean and standard devi-ation of the mechanical properties of PP/WF com-posites with different impact modifiers and MAPPas compatibilizer. The results are also presentedin separate figures.

Figure 1 shows the tensile strength of the PP/WF composites (bar 1) compared with composites

with different impact modifiers and MAPP. Thetensile strength is highest for the PP/WF compos-ite with MAPP as a compatibilizer, but the com-posites with both SEBS–MA and SEBS–MA withMAPP also have increased tensile strength. TheMAPP compatibilizer has a positive effect on thetensile strength in all composites. An increase intensile strength means that the stress has beentransferred from the PP matrix to the WF parti-cle. The tensile strength of many filled polymerscan be improved using adhesion promoters (com-patibilizers ), which improve the adhesion and thenature of the filler/matrix interface.25 It is, there-fore, excepted that MAPP and SEBS-MA will im-prove the interracial bonding between WF and PPresulting in improved tensile strength. Bothgrades of EPDM had a negative effect on the ten-sile strength but were improved slightly withMAPP.

Figure 2 shows the elongation at break for mod-ified PP/WF composites. Bar 1 is the referencePP/WF composite. The elongation at break isgreater for both composites with EPDM andEPDM– MA rubber (bar 3, 5) but decreases whenthe MAPP compatibilizer is added (bar 4, 6). TheSEBS–MA and SEBS-MA with MAPP compositesystems (bar 7, 8) have the highest elongation atbreak. Combining the data from Figures 1 and 2shows that the addition of MAPP makes PP/WFcomposites stronger but less tough, except com-posites with SEBS - MA.

Fillers with higher stiffness than the matrixcan increase the modulus of the composites, butgenerally fillers cause a dramatic decrease in theelongation at break. Almost all of the elongationoccurs in the matrix if the filler is rigid. If thereis good adhesion between the filler and the matrix,a decrease of the elongation at break, even with

MORPHOLOGY OF PP/WF COMPOSITES 1507

Table II Mechanical Properties of WF/PP Composites (± Values are Standard Deviations)

Tensile Properties Izod Impact Properties

Strength Modulus Elongation NotchedSample Code

Unnotched(MPa) (GPa) at break (%) (J/m) (J/m)

small amounts of filler, can be expected. If theadhesion is poor, the elongation at break may de-crease more gradually.25 This seems to agree withthe results obtained. The PP/WF composite elon-gation at break is dramatically decreased com-pared to unfilled PP (590% at a crosshead speedof 2 in/min, reported by the manufacturer). Theaddition of MAPP decreases the elongation atbreak further while impact modifiers increased it.The positive effect of impact modifiers may de-pend on more a flexible matrix (addition ofEPDM) and maybe further affected by the devel-opment of a flexible interphase around the woodparticles (addition of EPDM-MA and SEBS-MA).

Figure 3 shows that the stiffness of the WFfilled PP without impact modifiers is 2.4 GPa ( bar1) compared with 1.4 GPa (bar 0) for unfilled PP(reported by the manufacturer). All impact mod-ifiers decrease the stiffness of the composites, asexpected due to the low E -modulus of the elasto-mer. The E -modulus of elastomer-filled PP can be

Figure 1 Tensile strength of the composites with dif-ferent impact modifiers and MAPP as a compatibilizer.Tested in the dry condition.

calculated using the Lewis and Nielsen equa-tion, 25 as a result, a reduction of E-modulus about19% with elastomer content of 10% can be ex-pected. The E-modulus of the PP/WF composite(2.4 GPa) decreases with the addition of EPDMand SEBS-MA to 1.9 GPa (21%) and withEPDM-MA to 1.6 GPa (33%). The results fromthe addition of EPDM and SEBS-MA agree quitewell with the theoretical calculations, but for theaddition of EPDM-MA, they do not. If the stiff-ness of the composites containing EPDM andEPDM-MA are compared, it can be seen thatboth elastomers are very soft, but that theEPDM–MA decreased the stiffness more. Be-cause of the greater affinity of the EPDM–MA,we can speculate that the elastomer form a in-terphase around the WF particles to some degree.This type of morphology has been shown to causea greater reduction in E-modulus than a morphol-ogy where the elastomer exists as discrete do-mains in the matrix.11,12 The composites con-taining SEBS-MA do not show as large reduc-

Figure 2 Elongation at break of the composites withdifferent impact modifiers and MAPP as a compatbi-Iizer.

1508 OKSMAN AND CLEMONS

tions as composites containing EPDM-MA,possibly as a result of the higher modulus ofSEBS-MA. However, because of the complexityof the interphase, further study is necessary tofully explain the composite properties.

A better balance of composite properties maybe achieved at lower concentrations of the impactmodifier, depending upon the required productperformances, for example, higher E-moduluswhile retaining good impact and tensile strength.Oksman21 reported that the addition of 5 wt % ofSEBS-MA to PE/WF composites improved the E-modulus by almost 10%, the tensile strength 28%,and impact strength 58%.

Figure 4 shows the impact strength of eachcomposition. Improvements in impact strengthwere seen in all impact-modified systems. The ad-dition of EPDM-MA resulted in greater improve-ments in impact performance but a lower E-modu-lus when compared to composite systems con-taining unmodified EPDM. Stamhuis 10,11 notedsimilar findings in talc-filled PP systems usingSBS as an impact modfier. Impact modifiers withan affinity for the particles partially encapsulatedor coated the talc particles rather than simplyexisting as separate domains in the bulk matrix.This encapsulation reduces stress concentrationsat the particle-polymer interface, leading to bet-ter impact performance, but also resulting inlower E -moduli. The composite systems con-taining SEBS-MA gave the highest impact energ-ies. The MAPP compatibilizer had little or no ef-fect on the notched impact strength but seemsto increase the unnotched impact strength of thecomposites when used together with elastomers.Myers et al.17,18 reported that the MAPP did notimprove the impact strength of PP/WF compos-ites but instead reduced it with increasing MAPPcontent. Dalväg et al.3 reported increased impact

strength using MAPP in PP/WF composites. Theaddition of 2% MAPP increased the unnotchedimpact strength about 30%.

A summary of the effects of elastomers andMAPP on the WF filled PP is shown in Table III(“+” is positive, “-” negative, and “O” is no ef-fect ).

Table III shows that both composites con-taining SEBS-MA gave the best properties. Thenotched Izod impact strength of PP/WF/SEBS-MA + MAPP composites were increased about100% and the unnotched impact strength 95%,the elongation at break was increased by 35%,and tensile strength was improved 9% over thePP/WF composite. The E-modulus decreased 35%compared to PP/WF composites (see Table II) butwas still 35% greater than unfilled PP. The tensilestrengths was increased in composites with theaddition of MAPP or SEBS-MA. The reasons forthe absence of this behavior in composites withEPDM and EPDM-MA with and without MAPP

MORPHOLOGY OF PP/WF COMPOSITES 1509

are difficult to explain. It is possible that EPDMand EPDM-MA do not form an interphase strongenough for stress transfer from the matrix to thefiller to take place.

Morphology

Examination of the fracture surfaces of the com-posites by scanning electron microscope gave in-formation about how impact modifiers and MAPPaffect the morphology of the composite. The rub-ber particle sizes and the interracial region be-tween the PP matrix and the wood filler were in-vestigated. Figure 5 shows the microstructure ofthe composite without impact modifier and com-patibilizer (blend 1) showing a wood particle em-bedded in the polymer matrix. The wood particleis not broken and there are voids around the parti-cle indicating poor interaction between the woodsurface and the PP matrix. The WF particles werewell dispersed on the PP matrix.

Figure 6 shows microstructure of the compositewith MAPP as compatibilizer (blend 2). The mi-crostructure is different to that in Figure 5. Gen-erally, it is more difficult to differentiate woodparticles from the PP matrix. This may suggestthat the wood particles are coated, probably bythe matrix, and that the failure most commonlyoccurs in the matrix and not at the filler surfaces.There are places where the adhesion between thePP and the WF is good, as broken wood particlescan be seen with no gap between the PP matrixand WF surfaces. When adhesion is not so good,there are voids around the wood particles andplaces where WF particles have pulled out. Goodadhesion between the filler and the matrix wasexpected because composite 2 had improved ten-sile strength, which means that loads can betransferred from the matrix to the filler. This sug-gests that there is some kind of interracial contactbetween the WF and the PP. The majority of thestudies of compatibilizing using MAPP were onthe PP/CF composite system. Other authors have

1510 OKSMAN AND CLEMONS

b)

Figure 7 SEM micrograph of fractured specimen. (a)PP/WF composite with EPDM, (b) PP/WF with EPDMand MAPP.

shown3,15-19 that MAPP has a compatibilizing ef-fect on PP/CF and PP/WF composites, The com-posite system in Figure 6 showed the highest stiff-ness and tensile strength, which suggest im-proved adhesion between the WF filler and thePP matrix, but the lowest impact properties andelongation at break, which signify embrittlementof the filler or the matrix. The MAPP may alsoact as a dispersing agent between polar fillers andunpolar matrix, resulting in better dispersion ofthe filler. 19 In this investugation a good dispersionof WF particles in PP matrix was found. Differ-ences in particle dispersion between the differentcomposite microstructure with addition of MAPPwere not observed.

Figure 7 shows micrographs the PP/WF com-posite with (a) EPDM and (b) with both EPDMand MAPP. Figure 7(a), there is poor adhesionbetween the PP matrix and the WF particle sur-

face, as there are voids around the wood particle.In Figure 7(b), better adhesion between WF par-ticle and PP matrix can be seen. Fracture pathsoften passed through the wood particles (brokenWF particles are visible), which indicates goodadhesion between the WF filler and the PP matrixand also that the interphase is stronger than theWF particle. Tensile strength is improved in thelatter case from 22 to the 27.7 MPa, which sug-gests improved adhesion. The E-modulus is unaf-fected, but elongation at break and the notchedimpact strength are decreased. It could be ex-pected that the better interface is only formed inthe composite containing the MAPP, which ex-plains the decreases in elongation at break andimpact strength. The improved impact strengthcompared to composites without elastomer addi-tives can be explained by better toughness of thematrix itself. In both composites, the EPDM elas-tomer is uniformly distributed in the PP matrixand has a domain size between 0.1 – 1µm.

Figure 8 shows the morphology of the PP/WFcomposite with (a) EPDM-MA and (b) EPDM -MA with MAPP. There is good adhesion betweenthe PP matrix and the wood particle surfaces inboth composites; fracture paths have passedthrough the wood particles in both cases, whichindicates good adhesion. The adhesion betweenthe rubber particles and PP matrix is improvedwhen compared to composites containing unmodi-fied EPDM. As far as the mechanical propertiesare concerned, the tensile strengths and E-modu-lus are the lowest of all composites but elongationat break and impact strength are better. The mi-crographs and mechanical properties indicatethat the EPDM–MA forms an interphase that ef-fectively increases the impact strength and de-creases the E -modulus. but that the interphase istoo soft for stress transfer from the matrix to thefiller. It is difficult to see the rubber particles be-cause of the good interaction between the matrixand EPDM–MA, which causes the failure in thePP matrix. The EPDM elastomer particles areuniformly distributed in the PP matrix and havea size between 0.1 to 1 µm.

Figure 9 shows the morphology of the compos-ite with (a) SEBS-MA and (b) the same withMAPP. There is good adhesion between the woodparticle and PP matris in both micrographs. Novoids around the wood particle surfaces are pres-ent, and the fracture paths have often passedthrough the wood particles. Almost all mechanicalproperties were improved compared to compositeswith EPDM and EPDM- MA elastomers. The ten-sile strength of (a) was 29.3 MPa, which is second

MORPHOLOGY OF PP/WF COMPOSITES 1511

highest after PP/WF with MAPP and is furtherincreased to 30.5 MPa when MAPP was added.Elongation at break and impact strength were thehighest of all composites tested and the E-modu-lus was the same as the composites with EPDM.It is difficult to measure the particle size of theelastomer phase because of the good adhesion be-tween the matrix and the SEBS–MA.

Figure 10 shows EPDM and EPDM-MA elas-tomer particles in the PP matrix. The fracturesurfaces have been etched with n -heptane vapor;the holes in these micrographs represent theEPDM particle size that was between 0.1 and 1µm with an average of 0.3 µm. A more accuratedetermination of the particle size distributionfrom the fracture surfaces is very difficult, espe-cially if there is strong interracial adhesion be-

b)

Figure 8 SEM micrograph of fractured specimens.(a) PP/WF composites with EPDM-MA as a impactmodifier, and (b) same composite but with MAPP addedas a compatibilizer.

a)

b)

Figure 9 (a) PP/WF composite with SEBS–MA asimpact modifier, and (b) the same blend when MAPPis added as a compatibilizer.

tween the matrix and the elastomer. The fracturepropagates through the bulk matrix and the elas-tomer particles do not pull out. The quality of theEPDM-MA particle size micrographs can be dueto particle/matrix adhesion and etching condi-tions.

Figure 11 shows schematic representations ofdifferent microstructures showing unmodifiedPP/WF composite and different modifications. InFigure 11( a) the unmodified PP filled with WFshows poor adhesion between the WF and PP sur-faces, and there are voids around the WF particle.In Figure 11(b) the composite with MAPP as acompatibilizer, the MAPP is expected to enhanceinterracial adhesion between the WF and PP. Fig-ure 11(c) shows the microstructure for the com-posite with EPDM elastomer. There is poor adhe-sion between the WF particle and the PP matrix.When MAPP is added as a compatibilizer [ Fig.11(d) ], the interracial adhesion between the WFparticle and the PP matrix is improved. TheMAPP improves interracial adhesion but does not

1512 OKSMAN AND CLEMONS

form a soft interphase around the WF particle.Figure 11(e) EPDM-MA shows good adhesionwith WF particles and is expected to form a softinterphase around them. A similar morphology isfound when MAPP is added. Figure 11(f) showsthe microstructure when SEBS-MA is added.Good adhesion between the surfaces is expected,and the SEBS-MA forms a strong interphase be-tween the WF and PP. When MAPP is added, theadhesion is even stronger.

CONCLUSIONS

The objective of this study was to investigate theeffects of different combinations of impact mod-ifiers and MAPP as compatibilizer on the mechan-ical properties and morphology of WF-filled PP.

The E-modulus was improved by 70% when WFwas added to pure PP but reductions were found

when elastomers were added to these compositesystems.

Addition of MAPP did not affect the notchedimpact strength but improved the unnotched im-pact strength when added with elastomers. MAPPhad a positive effect on the tensile strength form-ing an interphase between the WF and PP, but anegative effect on the elongation at break. MAPPgenerally had a positive effect on the mechanicalproperties when it used together with SEBS-MAand EPDM.

EPDM increased toughness, elongation atbreak and impact strength were improved com-pared to unmodified the composites.

The SEBS-MA and the EPDM-MA both havehigher affinity for wood flour than EPDM, andgave the largest increases in impact performance.The results indicate that SEBS-MA and EPDM-MA act both as compatibilizer and impact mod-ifier, resulting in better interracial adhesion, bet-ter impact strength, and higher elongation atbreak. However, the lower E-modulus and a in-ability to transfer stress born the matrix to theWF particle confirm that the EPDM-MA in-terphase between the WF and PP is softer thanthe interphase that SEBS-MA forms.

Composites containing both SEBS-MA andMAPP show 107% better notched impact strengthcompared to PP/WF composites, but the stiffnesswas reduced by 35% compared to unmodifiedcomposites. The addition of both EPDM-MAand MAPP to PP/WF composites increased thenotched impact strength by 83%, but decreasedthe composites stiffness by 28%.

Scanning electron microscopy showed that ma-leated EPDM and SEBS had a stronger affinity forthe wood surfaces than did the unmodified EPDM,and that the MAPP further improved the adhe-sion between WF and PP.

The particle size distributions were between

MORPHOLOGY OF PP/WF COMPOSITES 1513

0.1 to 1 µm for both EPDM and EPDM-MA. Themaleated EPDM particle sizes were more difficultto measure because of good adhesion between theimpact modifier and the PP matrix.

These results can be summarized as follows:Maleated elastomers make effective impact mod-ifiers in the PP/WF composite systems and theaddition of MAPP compatibilizer has a positiveeffect on composite stiffness, tensile strength, andunnotched impact strength. Morphological studyshowed that the MAPP-enhanced adhesion be-tween the WF and the impact-modified PP sys-tems. Composites with SEBS-MA showed thegreatest impact strength, elongation at break,and tensile strength compared to composite sys-tems with EPDM and EPDM-MA. SEBS-MAshows the best potential to be used as an impactmodifier in the PP/WF composite system, and theuse of MAPP can further increase mechanical per-formance.

This work was carried out during K. Oksman’s stay asa visiting scientist at the USDA Forest Service, ForestProducts Laboratory in Madison, WI.

REFERENCES

J o u r n a l o f

A P P L I E DP O L Y M E RS C I E N C EVOLUME 67 / NUMBER 9 / FEBRUARY 28, 1998

CONTENTS

Mechanical Properties and Morphology of Impact Modified Polypropylene–Wood Flour CompositesK. Oksman and C. Clemens

Use of Chitosan to Improve Dyeability of DP-Finished Cotton (II)Y. Shin and D. I. Yoo

Amphiphilic Beechwood Glucuronoxylan DerivativesA. Ebringerová, I. Sroková, P. Talába, M. Katurakova, and Z. Hromádková

Morphology and Mechanical Properties of NylonY.-J. Cha, C.-H. Lee, and S. Choe

Fracture Energy Release Rate in Nylon FibersS. Michielsen

6 Toughened with Waste Poly (vinyl butyral) Film

A GC-MS Study of the Addition Reaction of Aryl Amines with Acrylic MonomersM. Farahani, J. M. Antonucci, and L. R. Karam

Simulations of Primary and Secondary Gas Penetration for a Gas-Assisted Injection-Molded ThinPart with Gas ChannelS. C. Chen, N.-T. Cheng, and S.-Y. Hu

Impact Behavior of Glass-Epoxy Composites Containing Foam MaterialKishore and S. Venkatraman

Modeling of Industrial Nylon-6,6 Polycondensation Process in a Twin-Screw Extruder Reactor. I.Phenomenological Model and Parameter AdjustingR. Giudici, C. A. Oller Do Nascimento, I. C. Beiler, and N. Scherbakoff

Reaction Rate Enhancement During Swollen-State Polymerization of Poly (ethylene terephthalate)M. K. Parashar, R. P. Gupta, A. Jain, and U. S. Agarwal

Differential Scanning Calorimetry and Wide-Angle X-ray Scattering Studies of Bread StalingJ. H. Jagannath, K. S. Jayaraman, S. S. Arya, and R. Somashekar

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