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木纖維增強(qiáng)聚丙烯復(fù)合材料：
壓縮和注塑成型工藝
摘要
含有不同種類的木質(zhì)纖維（硬木纖維和軟木纖維）的木纖維增強(qiáng)聚丙烯復(fù)合材料經(jīng)過注射成型和擠壓成型工藝而成。人們針對不同處理系統(tǒng)和相容劑對復(fù)合材料的力學(xué)性能的影響進(jìn)行過調(diào)查。當(dāng)今研究是針對在處理系統(tǒng)和相容劑作用下木纖維聚丙烯復(fù)合材料的拉伸，彎曲，沖擊和影響性能。從研究結(jié)果中發(fā)現(xiàn)，相對于壓縮成型工藝，注塑成型工藝具有較好的拉伸和彎曲性能（拉伸強(qiáng)度和彎曲強(qiáng)度大約分為155%和60%）。簡支梁沖擊增加了壓縮硬木材纖維聚丙烯（PP）復(fù)合材料成型的最值，在30%木材纖維含量的70％左右。相比較于壓縮成型工藝，注射成型過程的耐沖擊力更有優(yōu)勢。在注射成型過程中含50%木質(zhì)纖維的硬木材纖維聚丙烯復(fù)合材料阻尼指數(shù)最多可以降低60%。為了更好理解在這兩個加工系統(tǒng)中木纖維和聚丙烯之間的相互作用，本研究對復(fù)合材料的掃描電子顯微鏡也有所探討。
關(guān)鍵詞：木纖維聚丙烯復(fù)合材料；注塑成型工藝；壓縮成型工藝；機(jī)械性能
引言
聚丙烯纖維復(fù)合材料的木材成為昂貴和缺乏環(huán)保材料的替代品。聚丙烯纖維復(fù)合材料是一種可回收聚合物，來自可再生能源派生的木纖維，且宜生物降解。在聚丙烯基體使用木材纖維擁有很多優(yōu)勢，如改善復(fù)合材料的尺寸穩(wěn)定性，加工溫度較低，熱變形溫度較高，改善木表面外觀，質(zhì)輕，體積小，縮短了高達(dá)30％的注射成型產(chǎn)品的周期時間和易于生產(chǎn)高性能材料的產(chǎn)品。
相比較傳統(tǒng)的補(bǔ)強(qiáng)劑（如玻璃纖維和礦物顆粒），如今復(fù)合技術(shù)的進(jìn)展提高了他們的競爭力。木粉填充物都是現(xiàn)成的木磨。作為磨削工藝的功能，它可以控制大小，粒徑分布，形狀，以及木粉顆粒的縱橫比。木粉通常包括一個破碎纖維的混合物，尤其是原纖化纖維和纖維束。與礦物顆粒相比，用聚丙烯復(fù)合的木粉強(qiáng)力地集合了比剛度和強(qiáng)度增高，加工過程中減少磨損，密度降低和價格低廉的特點(diǎn)。
注塑和擠出是用來生產(chǎn)柱狀或片狀形式的木纖維熱塑性復(fù)合材料的。注塑需要具有低分子量聚合物維持低粘度。《約翰遜自控汽車》對最新近的塑料天然纖維復(fù)合材料在汽車內(nèi)部零部件和木纖維聚丙烯復(fù)合材料產(chǎn)品生產(chǎn)技術(shù)（如注塑成型，低壓注塑成型，共注射成型）的使用提出了一個概述。與對幾種自然和木纖維（黃麻，亞麻，紅麻，桉樹）被應(yīng)用到半成品，即注射成型過程中顆粒（簡稱天然纖維）的，幾種線路進(jìn)行重點(diǎn)研究。
（亞麻纖維，[3,4]麻，[5]黃麻，[6]稻殼，[7]和劍麻纖維[8,9]）自然屬性和木纖維[10]增強(qiáng)聚合物復(fù)合材料注塑成型工藝中有所研究。
熱塑性纖維增強(qiáng)復(fù)合材料區(qū)別于熱固性復(fù)合材料主要在于斷裂伸長率較高，周期短和具有回收的可能性。壓縮成型技術(shù)被證明對任何熱塑性預(yù)浸的型材產(chǎn)品都適用。
壓縮成型技術(shù)無需壓縮材料就可溫和地將熱塑性預(yù)浸塑成所需的形狀。不同層取向也就此成型后保留。
約翰遜自控[11]比較了汽車門板制造的新材料和新工藝。該材料是一種天然纖維氈（亞麻，劍麻，大麻，紅麻）,表面噴有壓縮成型產(chǎn)出的聚氨酯樹脂。
麻纖維，[12-14]木纖維[15]和黃麻纖維[16]增強(qiáng)復(fù)合材料也由壓縮成型工藝制備。
最近的一項審查報告[17]描述了在不同處理系統(tǒng)中（擠出，注射成型，壓縮成型，混合機(jī)和擠壓過程）聚合物中自然木纖維的加固?；鞜掃^程的一個重要特征是加入了相容劑，這種相容劑是用來克服極地木和非極地?zé)N聚合物之間的不相容。兼容性不足通常使沖擊和拉伸強(qiáng)度的下降。
這些研究旨在比較注射成型和壓縮成型工藝中木纖維（聚丙烯）復(fù)合材料的力學(xué)性能。
實(shí)驗
材料：
高聚物基聚丙烯是由德國蓋爾森基興帝斯曼提供的一種顆粒（Stamylan P17M10），它的熔點(diǎn)是173°C，熔融指數(shù)為230度下10.5/10min。它在室溫下密度為0.905g/cm3。
木纖維
標(biāo)準(zhǔn)硬木纖維和軟木纖維，150-500mm大小，由德國J. Rettenmaier and So¨ hne GmbH t有限公司提供，值得關(guān)注的是硬木纖維和軟木纖維的纖維組織分別是纖維狀和立體狀的。
相容劑
市面上銷售的馬來酸酐共聚聚丙烯共聚物（Licomont，氬氣504 FG），一種用于纖維處理相容劑，這可以從德國法蘭克福科萊恩公司買到。用量大概是木纖維含量的5%，用來改善木纖維和聚丙烯基之間的相容性和粘著力。
配制和樣品制備
注塑成型工藝
硬木材纖維和軟木材纖維（30％和50％重量比）聚丙烯顆粒經(jīng)雙螺桿擠出機(jī)（哈克擠出機(jī)，RheomexPTW 25/32）混合，加或不加相容劑?；旌锨八械哪静睦w維在空氣循環(huán)烘箱中以80°C溫度烘干24小時。注塑成型工藝制成備用樣品前，擠壓后顆粒需在80°C下再烘干24小時。在熔化溫度150 °C - 180 °C，模具溫度80°C-100°C，注塑壓力20kN/mm下，注塑成型工藝將干燥的顆粒制成實(shí)驗樣品。
壓縮成型工藝
聚丙烯顆粒被轉(zhuǎn)換成粉末，然后與木材纖維混紡。木纖維和聚丙烯粉混合物放置于汽缸壓縮塊成型機(jī)，機(jī)內(nèi)壓力為20kN/mm，溫度達(dá)到190°C。氣缸維持20kN/mm5分鐘，然后在另一個裝有制冷設(shè)施的模子里冷卻（10 _C /分鐘）。準(zhǔn)備好的薄板放入3kN/mm壓力，180°C的壓縮成型機(jī)中5-10分鐘，使薄板厚度達(dá)到2mm。然后將壓片根據(jù)各種機(jī)械試驗需要的DIN數(shù)字剪切成各個矩形標(biāo)本。
測量
根據(jù)EN ISO 527或EN ISO178，拉伸和彎曲強(qiáng)度（茲維克機(jī)，芬歐匯川1446年）在2毫米/分鐘的測試速度進(jìn)行了測試；EN ISO 527和EN ISO178是對于不同木纖維聚丙烯復(fù)合材料（加入相容劑或沒有加）而定的。所有測試是在室溫（23 _C）和相對濕度為50％進(jìn)行研究的。
簡支梁沖擊試驗（標(biāo)準(zhǔn)EN ISO 179）進(jìn)行了10個無缺口樣品試驗。每個系列中，標(biāo)準(zhǔn)差（<15％）是用來衡量擺錘沖擊能量的。
為了測量特征值的影響，標(biāo)本是在室溫下以非滲透模式運(yùn)用低速落錘沖擊試驗機(jī)（標(biāo)準(zhǔn)EN ISO 6603 - 2）進(jìn)行測試的。沖擊機(jī)有0.75千克，沖擊能量為0.96。
瀏覽顯微鏡掃描
利用掃描電子顯微鏡（SEM）（織女星TESCAN）對這兩個工藝生產(chǎn)得到的木纖維聚丙烯復(fù)合材料的形態(tài)進(jìn)行觀察研究。然而，彎曲試樣斷口需鍍上金后再在電子顯微鏡下進(jìn)行研究。
結(jié)果與討論
以重量為30%和50%纖維負(fù)載的木纖維聚丙烯復(fù)合材料來研究處理系統(tǒng)對力學(xué)性能的影響，如復(fù)合材料的拉伸強(qiáng)度和彎曲強(qiáng)度，彎曲電子模量，沖擊強(qiáng)度和沖擊性能。我們已經(jīng)報道[18]過，含（馬來酸酐）-聚丙烯相容劑（濃度5%，相當(dāng)于木纖維含量）的木纖維聚丙烯復(fù)合材料性能最佳。這就是為什么在我們目前的工作中，兩個工藝中所有種類的木纖維聚丙烯復(fù)合材料中馬來酸酐聚丙烯相容劑的含量都是5%。這些復(fù)合材料的各種性能進(jìn)行了以下的討論。
圖一顯示了兩個工藝中隨著木纖維（硬木纖維和軟木纖維）的變化和加入和沒加入相容劑情況下木纖維聚丙烯復(fù)合材料的拉伸試驗結(jié)果。在一般情況下，加入了相容劑的木纖維聚丙烯復(fù)合材料的機(jī)械性能顯示了一個增加的趨勢。圖1表明，由注塑工藝制備復(fù)合材料的拉伸強(qiáng)度高于由壓縮成型工藝制備的復(fù)合材料，它也說明，加入相容劑后，注塑工藝制備硬木材纖維增強(qiáng)聚丙烯復(fù)合材料的拉伸強(qiáng)度最高，幾乎在155％的升幅，壓縮成型工藝在50％木纖維含量。
從圖2和圖3中可以很容易看出一個處理系統(tǒng)對木纖維聚丙烯復(fù)合材料的彎曲性能的影響。據(jù)觀察，該復(fù)合材料的抗彎強(qiáng)度（圖2）顯示了隨著相容劑的加入而不斷增大。將兩個工藝進(jìn)行比較，30%的木纖維含量（硬木纖維和軟木纖維），結(jié)果相差不是很明顯。但是50%的木纖維含量時，注塑成型工藝?yán)鞆?qiáng)度更好，此時壓縮成型工藝需要60%的木纖維。圖3表明，兩個工藝中硬木纖維和軟木纖維聚丙烯復(fù)合材料的彎曲電子隨著拉伸強(qiáng)度變化相同。這意味著在30％木纖維含量（硬木材纖維和軟木材纖維）這差異不是很顯著。但在50％木纖維含量，相比較壓縮成型工藝，注塑成型工藝具有較好的彎曲強(qiáng)度，并具有增加的趨勢。圖4顯示了兩個工藝中加入相容劑的木纖維聚丙烯復(fù)合材料的沖擊強(qiáng)度的變化過程。從這些數(shù)字可以看出，注塑成型工藝制備的硬木纖維和軟木纖維聚丙烯復(fù)合材料的沖擊強(qiáng)度比壓縮成型工藝的要高。在復(fù)合材料中加入相容劑后壓縮成型工藝中的硬木聚丙烯復(fù)合材料的沖擊強(qiáng)度提高到最大值。大約是30%木纖維含量的70%。
圖5中描述的撞擊試驗的結(jié)果可以說是由兩個獨(dú)立問題來描述。他們是：
（a）力撓度曲線: 力撓度曲線是指所有材料反應(yīng)
(b)特征值: 作為耗能衡量的失能（Wv），作為儲能衡量的應(yīng)變能（Ws）和失能和應(yīng)變能比值稱作為阻尼指數(shù)（A*）
圖6顯示了兩個工藝中硬木纖維和軟木纖維聚丙烯復(fù)合材料的耐沖擊性。圖6說明了在兩個工藝中有和沒有相容劑的硬木纖維聚丙烯復(fù)合材料的耐沖擊性，注塑成型工藝的耐沖擊性更好，壓縮成型工藝中可以看到大量的起爆損壞。但隨著相容劑的加入，硬木纖維聚丙烯復(fù)合材料在壓縮成型工藝中的耐沖擊性能最好，沒有大量的起爆損壞。在軟木纖維聚丙烯復(fù)合材料的試驗中（Fig. 6b），可清晰看出注塑成型工藝中的耐沖擊性比壓縮成型工藝的更好，沒有大量的起爆損壞。
所有樣品的阻尼指數(shù)，即采取的耗能（失能）與存儲的能量（應(yīng)變能）的比例，是用來來衡量特征值的影響。失能包括不可逆轉(zhuǎn)變形的能量，和由于基體裂變產(chǎn)生，傳播，分層，直到最后纖維斷裂所消耗的能量。
圖7顯示了兩個工藝中加入相容劑的硬纖維聚丙烯復(fù)合材料的阻尼指數(shù)。可以看出注塑成型工藝的阻尼指數(shù)比壓縮成型工藝相對較好，但不是很明顯。
顯而易見，所有情況下，相容劑的加入大大降低了阻尼指數(shù)，在含50%木纖維含量情況下，注塑成型工藝制備的硬木纖維聚丙烯復(fù)合材料最高，近60%。
圖8-10顯示的是在電子顯微鏡下觀察注塑成型工藝和壓縮成型工藝制備的木纖維聚丙烯復(fù)合材料彎曲斷口情況。圖8（a）和（b）顯示了在含有30%木纖維含量情況下，壓縮成型工藝制備的硬木纖維和軟木纖維聚丙烯復(fù)合材料的情況。圖8（a）和（b）都顯示了壓縮成型工藝中硬木纖維和軟木纖維聚丙烯復(fù)合材料，在壓縮成型工藝中現(xiàn)有纖維拔出，脫粘，微纖化，就像一層到另一層。眾所周知，這些結(jié)構(gòu)（層與層）構(gòu)成瀏覽更高沖擊強(qiáng)度，從圖4中可知，壓縮成型工藝制備的復(fù)合材料的簡支梁強(qiáng)度比注塑成型工藝更高。
但圖9和圖10顯示兩個工藝中通過加入相容劑，木纖維和基體之間的相互作用更好。
圖9（a）和(b)展示了注塑成型工藝中加入和未加入相容劑的軟木纖維聚丙烯復(fù)合材料的微觀結(jié)構(gòu)，其中木纖維含量為50%。圖10（a）和（b）展示的是與注塑成型工藝情況相同下的壓縮成型工藝制備的復(fù)合材料的微觀結(jié)構(gòu)。很明顯，在兩個工藝比較中，相比較壓縮成型工藝，注塑成型工藝制備木纖維聚丙烯復(fù)合材料的木纖維和基體相互作用更好。為了更好的理解，兩個工藝制備的復(fù)合材料的密度也都計量了。對于木纖維聚丙烯復(fù)合材料（30%木纖維含量），注塑成型工藝中復(fù)合材料密度是1.06g/cm3，而壓縮成型工藝制備的復(fù)合材料密度是0.98g/cm3。壓縮成型工藝制備的復(fù)合材料密度低表明此孔隙度大，也就是說木纖維和聚丙烯之間的粘著力和相互作用力小。從這兩個工藝制備木纖維聚丙烯復(fù)合材料的機(jī)械力學(xué)也能推測出這樣的結(jié)果。
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POLYMERPLASTICS TECHNOLOGY AND ENGINEERING Vol. 43, No. 3, pp. 871888, 2004 Wood Fiber Reinforced Polypropylene Composites: Compression and Injection Molding Process Andrzej K. Bledzki * and Omar Faruk Institut fur Werkstofftechnik, Kunststoff- und Recyclingtechnik, University of Kassel, Kassel, Germany ABSTRACT Wood fiber reinforced polypropylene composites containing differ- ent types of wood fiber (hard and softwood fiber) were prepared by an injection molding and a compression molding process. Influence of different processing systems and compatibilizer on the composite mechanical properties was investigated. The present study investi- gated the tensile, flexural, charpy impact, and impact properties of wood-fiber reinforced polypropylene composites as a function of processingsystemandcompatibilizer.Fromtheresults,itisobserved that injection molding process showed better tensile and flexural properties comparative with compression molding process, which is about 155% and 60% for tensile strength, and flexural strength, *Correspondence: AndrzejK.Bledzki,InstitutfurWerkstofftechnik,Kunststoff- und Recyclingtechnik, University of Kassel, Monchebergstr.3, D-34109 Kassel, Germany; Fax: 49-561-8043692; E-mail: kutechuni-kassel.de. 871 DOI: 10.1081/PPT-120038068 0360-2559 (Print); 1525-6111 (Online) Copyright Injection molding; Compression molding; Mechanical properties. INTRODUCTION Polypropylene-wood fiber composites are used as substitutes for more expensive and less environmentally friendly materials. Polypropyl- ene is a recyclable polymer and wood fibers derive from a renewable source and are biodegradable. The use of wood fibers in a polypropylene matrix includes many benefits, such as improved dimensional stability of composites, lower processing temperatures, increased heat deflection temperature, improved wood surface appearance, lighter products, low volumetric cost, up to 30% reduced cycle time for injection molded products, and production of good performance materials. 1 Recent progress in compounding technology improves their compe- titiveness against conventional reinforcing agents such as glass fibersand mineral particles. Wood flour fillers are readily available by grinding of wood. As a function of the grinding processes, it is possible to control size, size distribution, shape, andthe aspect ratioof wood flourparticles. Typically wood flour comprises a mixture of broken fibers, partially fibrillated fibers, and fiber bundles. Compounding wood flour together with polypropylene can afford an attractive combination of high specific stiffness and strength, less abrasion during processing, low density, and low price with respect to mineral fillers. Injection molding and extrusion are established processes for manufacturing wood fiber-thermoplastic composites in prismatic or sheet forms. Injection molding requires a polymer with a low molecular weight to maintain low viscosity. Johnson Controls Automotive 2 presented a overview of the state of the art of the use of plastic-natural fiber composite materials for interior car parts and the technologies to 872 Bledzki and FarukORDER REPRINTS producesuchparts(injectionmolding,lowpressureinjectionmolding,and co-injection molding). With emphasis on the research lines performed on severalkinds ofnatural andwood fibers(jute, flax,kenaf,eucalyptus)to be applied to semi-finished products: granules (short natural fiber) for injection molding process. The properties of natural (flax fiber, 3,4 hemp, 5 jute, 6 rice hull, 7 andsisalfiber 8,9 )andwoodfiber 10 reinforcedpolymercompositeswere investigated by the injection molding process. Thermoplastic fiber-reinforced composites are distinguished from thermosetreinforcedcompositesprimarilybyahighelongationatbreak, short cycle times, and the possibility of recycling. The compression moldingtechniqueprovedsuitablefortheproductionofprofileswithany thermoplastic prepreg used. Compression molding brings the thermo- plastic prepreg gently to the required shape without overcompressing the material.The different layerorientations are thus retained aftermolding. Johnson Controls 11 compared new materials and processes for the manufacture of automotive door panels. The material is Fibropur, a natural fiber mat (flax, sisal, hemp, kenaf) sprayed with PU-resin produced by compression molding. Flax fiber, 1214 wood fiber, 15 and jute fiber 16 reinforced compo- sites also were prepared by compression molding process. A recent review report 17 describes the reinforcement of natural and woodfibersintopolymerconsideringdifferentprocessingsystems(extru- sion,injectionmolding,compressionmolding,mixerandexpressprocess). Animportantfeatureofthecompoundingprocessesistheadditionof compatibilizers, which are required to overcome incompatibility between the polar wood and the nonpolar hydrocarbon polymer. Inadequate compatibility frequently is accompanied by significantly reduced impact and tensile strength. The objective of these studies is to compare the mechanical properties of wood-fiber(PP) composites between the injection molding and the compression molding process. EXPERIMENTAL Materials Polymeric Matrix Polypropylene (Stamylan P17M10) was provided as granules by DSM, Gelsenkirchen, Germany. Its melting temperature was 173 Cand Wood Fiber Reinforced Polypropylene Composites 873ORDER REPRINTS melting index was 10.5g/10min at 230 C. Its density at room temperature was 0.905g/cm 3 . Wood Fibers Standard hard-wood fiber (Lignocel HBS 150-500) and soft-wood fiber(LignocelBK40-90)withparticlesizeof150500mm,weresupplied by J. Rettenmaier and Sohne GmbHCo. Germany. It is also notable that fiber structure of hard- and soft-wood fiber is fibrous and cubic, respectively. Compatibilizer A commercially available maleic anhydride-polypropylene copoly- mer (Licomont, AR 504 FG) was used as a compatibilizer for fiber treatment, and it was obtained from Clariant Corp., Frankfurt, Germany. It was used 5% by weight relative to the wood fiber content andwasexpectedtoimprovethecompatibilityandadhesionbetweenthe wood fiber and the PP matrix. Compounding and Sample Preparation Injection Molding Process Polypropylene granules with hard-wood fiber and soft-wood fiber (30% and 50% by weight) were mixed by twin-screw extruder (Haake extruder, Rheomex PTW 25/32) with and without compatibilizer. All the wood fibers were initially dried at 80 C in an air-circulating oven for 24hr before mixing. The extruded granules were dried again 80 C for 24hr(watercontent1%)beforethesamplepreparationbytheinjection molding process. Test samples were prepared from dried granules by the injection molding process at melting temperature 150 C180 C, mold temperature of 80 C100 C, and under a injection pressure 20kN/mm 2 . Compression Molding Process Polypropylene granules were converted into powder and then mixed with wood fibers. The wood fiber and PP powder mixture were placed 874 Bledzki and FarukORDER REPRINTS into a block cylinder compression molding machine under a pressure 20kN/mm 2 till the temperature reached at 190 C. Then the cylinder pressed for 5min under a pressure 20kN/mm 2 , and then it was followed by cooling (10 C/min) in another press equipped with refrigeration facilities. The prepared sheet (7mm) then was placed into a compression molding machine at 180 C for 510min under a pressure 3kN/cm 2 to bringthesheettoa2mmthickness.Rectangularspecimenswerecutfrom the pressed sheets according to a DIN number for various mechanical experiments. Measurements The tensile and flexural strength (Zwick Machine, UPM 1446) were testedatatestspeedof2mm/minaccordingtoENISO527andENISO 178 for different wood fiberPP composites with and without a com- patibilizer in both processes. All the tests were investigated at room temperature (23 C) and at a relative humidity of 50%. A charpy impact test (EN ISO 179) was carried out with 10 unnotched samples. In each series standard deviation (15%) was used to measure charpy impact energy. To measure the impact characteristics values, the specimens were tested by using a low-velocity falling weight impact tester (EN ISO 6603- 2) at room temperature in nonpenetration mode. The impactor had a mass of 0.75kg, and the impact energy was 0.96J. Scanning Election Microscope The morphology of the wood-fiberPP composites prepared in both processes were investigated by using a scanning electron microscope (SEM) (VEGA TESCAN), whereas, fractured surfaces of flexural test samples were studied with SEM after being sputter coated with gold. RESULTS AND DISCUSSION WoodfiberPPcompositeswith30and50wt%offiberloadingwere prepared to investigate the effect of processing systems on mechanical properties, like tensile and flexural strength, flexural E-modulus, charpy impact strength, and impact properties of composites. We have repor- ted 18 earlier that wood-fiberPP composites, containing (MAH)PP Wood Fiber Reinforced Polypropylene Composites 875ORDER REPRINTS compatibilizer showed the best performance in the concentration of 5% (relativetothewood-fibercontent).Thatiswhy,inourpresentwork,the content of MAHPP was used at 5% for all types of wood fiberPP composites in both processes. The various properties of these composites are discussed below. Results of tensile test of the wood-fiberPP composites are shown in Fig. 1 with the variation of wood fiber (hard-wood fiber and soft-wood fiber) and with and without a compatibilizer for both processes. In general, the wood-fiberPP composites show an increasing trend in the mechanical properties with the addition of a compatibilizer. Figure 1 showed that the tensile strength of the composites prepared by the injection molding process is higher compared to the composites prepared by the compression molding process, and it also illustrated that hard- wood-fiber-reinforced PP composites prepared by the injection molding process showed highest tensile strength with the addition of a com- patibilizer, which is nearly at 155% increase to the compression molding process at the 50% wood-fiber content. The effect of a processing system on the flexural properties of wood- fiberPP composites can be readily assessed from the Figs. 2 and 3. It is observed that the flexural strength (Fig. 2) of the composites showed an 0 5 10 15 20 25 30 35 40 WF30% WF30%+MAHPP5% WF50% WF50%+MAH-PP5% HW (injection molding) HW (compression molding) SW (injection molding) SW (compression molding) Tensile strength MPa Figure 1. Tensile strength of hard- and soft-wood-fiberPP composites with and without compatibilizer in both processes. (View this art in color at .) 876 Bledzki and FarukORDER REPRINTS 0 10 20 30 40 50 60 70 WF30% WF30%+MAH-PP5% WF50% WF50%+MAH-PP5% HW (injection molding) HW (compression molding) SW (injection molding) SW ( compression molding) Flexural strength MPa Figure 2. Flexural strength of hard- and soft-wood-fiberPP composites with and without compatibilizer in both processes. (View this art in color at .) 0 1 2 3 4 5 6 WF30% WF30%+MAH-PP5% WF50% WF50%+MAH-PP5% HW (injection molding) HW (compression molding) SW (injection molding) SW ( compression molding) Flexural E-modulus GPa Figure 3. Flexural E-modulusofhard- andsoft-wood-fiberPPcomposites with and without compatibilizer in both processes. (View this art in color at .) Wood Fiber Reinforced Polypropylene Composites 877ORDER REPRINTS increasing tendency with the addition of a compatibilizer. With the comparison between both processing systems, at the 30% wood fiber content (both hard-wood fiber and soft-wood fiber) it is not a very significant difference. But at the 50% wood fiber content the injection molding process showed better flexural strength, with an increase about 60% to compression molding process. Figure 3 showed that the flexural E-modulus of the hard wood fiber andsoft wood-fiberPPcomposites in both processing system followed the same trend as flexural strength. It means at the 30% wood-fiber content (both hard-wood fiber and soft- wood fiber) it is not very significant in difference. But at the 50% wood fiber content, the injection molding process showed better flexural strength,withanincreasingtendencytothecompressionmoldingprocess. Figure 4 shows the variation of charpy impact strength of wood- fiberPP composites in both processes with the addition of a com- patibilizer. From the figures, it is seen that the charpy impact strength of the hardwood fiber and soft-wood-fiberPP composites are found to be more, prepared by the compression molding process than by the injection molding process. With the addition of compatibilizer in com- posites, charpy impact strength increased the maximum in the compres- sionmoldingprocessforhard-wood-fiberPPcomposites,anditisabout 70% at the wood fiber content 30%. Theresultsoftheimpacttestcanbedescribedbytwoseparateissues, described in Fig. 5. They are: (a) Force-deflection curve: the force-deflection curve refers to all the materials behaviors, including the damageinitiation defined by the first significant drop of the force. (b) Characteristic values: loss energy (Wv) as a measure of dissipated energy and strain energy (Ws) as a measure of the stored energy, and the damping index ( *) as a ratio of loss energy to strain energy. Impact resistance of hard-wood fiber and soft-wood-fiberPP composites in both processes is shown in Fig. 6. Figure 6a illustrated the impact resistance of hard-wood-fiberPP composites with and without a compatibilizer in both processes, and impact resistance in the injection molding process shows better performance, where in the compression molding process, a large amount of damage of initiation wasobserved.Butwiththeadditionofacompatibilizer,impactresistance of hard-wood-fiberPP composites shows highest performance in the compression molding process, without having a large amount of damage of initiation. In the case of soft-wood-fiberPP composites (Fig. 6b), it is 878 Bledzki and FarukORDER REPRINTS clearly observed that impact resistance in the injection molding process, shows the better performance comparative to the compression molding process, without having a large amount of damage of initiation, as with the compression molding process. (a) (b) 0 2 4 6 8 10 12 14 Charpy impact strength mJ/mm 2 Injection molding Compression molding 0 2 4 6 8 10 12 14 Charpy impact strength mJ/mm 2 Injection molding Compression molding SW30%+PP70% SW30%+PP70%+MAH-PP5% SW50%+PP50% SW50%+PP50%+MAH-PP5% HW30%+PP70% HW30%+PP70%+MAH-PP5% HW50%+PP50% HW50%+PP50%+MAH-PP5% Figure 4. Charpy impact strength of hard-wood-fiberPP composites (a) and soft-wood-fiberPP composites (b) with and without compatibilizer in both processes. (View this art in color at .) Wood Fiber Reinforced Polypropylene Composites 879ORDER REPRINTS Thedampingindexforallsampleswascalculatedbytakingtheratio of dissipated energy (loss energy) to the stored energy (strain energy) to measurethe impact characteristic values. The lossenergy involves energy that is based on irreversible deformations, energy dissipation due to the creation of matrix cracks and their propagation, delaminations, and, finally, fiber fracture. Thedampingindexofhard-andsoft-wood-fiberPPcompositesasa function of having a compatibilizer in both processes is shown in Fig. 7. It is seen that the damping index in the injection molding process is comparatively better than the compression molding process, but this is not very significant. It is clearly evident that more damping index is decreased with the addition of a compatibilizer in all cases and it is highest for hard-wood-fiberPP composites (Fig. 7a) in the injection molding process at the wood fiber content 50%, which is nearly 60%. The flexural fractured surface of wood-fiberPP composites in both injection and compression molding processes examined with SEM are presented in Figs. 810. Figure 8a, b shows the hard- and soft-wood- fiberPP composites containing 30% wood fiber content in the compression molding process. Both Figs. 8a and 8b show the hard- and soft-wood-fiberPP composites in the compression molding process, where present fiber pullout, debonding, fibrillation, and just, like a layer to layer. As we know, these structures (layer to layer) are responsible for higher charpy impact strength, and we observed that at Fig. 4 where composites made from the compression molding process showed Deflection Force loss energy (Wv) strain energy (Ws) damage initiation Figure 5. Typical impact force-deflection curve for fiber reinforced polymer composites including definition of the characteristic values used. 880 Bledzki and FarukORDER REPRINTS better charpy strength in comparison with injection molding process composites. But with the addition of a compatibilizer indicates much better interaction between the wood fiber and the matrix in both processing systems, which is represented in Figs. 9 and 10. Figures 9a and 9b (a) (b) 100 0 100 200 300 400 500 600 700 01234567 Deflection mm Force N HW30% (Injection molding) Hw30%+MAH-PP5% (Injection molding) HW30% (Compression molding) HW30%+MAH-PP5% (Compression molding) 100 0 100 200 300 400 500 600 700 0123456 7 Deflection mm Force N SW30% (Injection molding) SW30%+MAH-PP5% (Injection molding) SW30% (Compression molding) SW30%+MAH-PP5% (Compression molding) Figure 6. Impact resistance (maximum force) of hard-wood-fiberPP compos- ites and soft-wood-fiberPP composites (b) with and without compatibilizer in both processes. (View this art in color at .) Wood Fiber Reinforced Polypropylene Composites 881ORDER REPRINTS (a) (b) 0 0.5 1 1.5 2 2.5 3 Damping index - Injection molding Compression molding HW30%+PP70% HW30%+PP70%+MAH-PP5% HW50%+PP50% HW50%+PP50%+MAH-PP5% 0 0.5 1 1.5 2 2.5 3 Damping index - Injection molding Compression molding SW30%+PP70% SW30%+PP70%+MAH-PP5% SW50%+PP50% SW50%+PP50%+MAH-PP5% Figure 7. Dampingindexofhard-wood-fiberPPcomposites(a)andsoft-wood- fiberPPcomposites(b)withandwithoutcompatibilizerinbothprocesses.(View this art in color at .) 882 Bledzki and FarukORDER REPRINTS (a) (b) Figure 8. SEM micrograph of hard- (a) and soft- (b) wood-fiberPP composites in compression molding process (wood-fiber content 30%). Wood Fiber Reinforced Polypropylene Composites 883ORDER REPRINTS (a) (b) Figure 9. SEM micrographs of fractured surface of soft-wood-fiberPP composites in injection molding process (a) without MAHPP 5%, (b) with MAHPP 5%, wood-fiber content 50%. 884 Bledzki and FarukORDER REPRINTS (a) (b) Figure 10. SEM micrographs of fractured surface of soft-wood-fiberPP composites in compression molding process (a) without MAHPP 5%, (b) with MAHPP 5%, wood fiber content 50%. Wood Fiber Reinforced Polypropylene Composites 885ORDER REPRINTS represent the microstructure of soft-wood-fiberPP composites contain- ing 50% wood-fiber content with and without a compatibilizer prepared by the injection molding process and Figs. 10a and 10b represent the before stated composites in compression molding process. It is also notable that in comparison between both processes, wood-fiberPP composites have a better interaction between wood fiber and the matrix in the injection molding process than the compression molding process; to better understand, the density of the composites prepared by both processes also was measured. It was observed for hard-wood-fiberPP composites (30% wood-fiber content). In the injection molding process, compositedensitywas1.06g/cm 3 ,whichis0.98g/cm 3 inthecompression molding process. The lower density of the composites in the compression molding process refers to more void content, which represents, finally, poor bonding and interaction between wood fiber and PP. This was expected also from the mechanical properties of wood-fiberPP composites in both molding processes. CONCLUSIONS The influence of the processing systems (injection molding and compression molding) on the mechanical properties of the hard-wood- and soft-wood-fiber-reinforcedPP composites were investigated in this