Thermomechanical Pulp Fiber Surface Modification for Enhancing the Interfacial Adhesion with Polypropylene

Chemical coupling on the thennomechanical pulp (TMF') fiber improved tensile shrength of the TMP fiber handsheet and isotactic polypropylene film laminates (TPL). For the maleic anhydride W) with benzoyl peroxide (%PO) as an initiator, tensile strength i n d 52% with the TMP fiber treatment over untreated laminates. The optimum strength jmperties were obtaiaed with an MA and BPO ratio of 21. Scanning electron m i ~ p y (SEM) images also showed the effectiveness of MA loading on the surface of TMP fibers due tb increased fiber failure without fiber pullout h m the polypropylene matrixes. Crystallinity and heabflow from DSC, as expected, decreased with the addition of MA on the TMP fiber surface. These results were also in accordance with the morphological observations at the fractu~. surface, Fourier-transform inffared spectroscopy (FTIR) spectra, and thermal analysis. Based on the high correlation between tensilk strength and 'ihe number of fibers counted at the point of failures, the number of fibers proved to be a sensitive m& of the effectiveness of surface treatment. t Member of SWST This paper (No. 064b0296) is published with the approval of the Dhctor of the Louisiana Agricultmd Experiment Station. Wood a d Fiber Science, 39(3). uXn. pp. 4%-433 O#mbytheSociecyofWoodScicllccacdTechmlogy Kwords: Themmechanical pulp. isotactic polypropylene, d e i c anhydride, benzoyl peroxide, surface modification. The potential use of themmechanical pulp (TMP) fibers as a reinforcing component for thermoplastics opens up further possibilities for use of low-cost lignocellulosic fiber resources. Researchers have shown an increasing interest in natural fiber-reinforced polymer composites due to the inherent properties of bio-based fibers, such as low density, relatively high toughness, high strength and stiffness, and good thermal properties and biodegradability (Rowel1 and Clemons 1992; Rezai and Warner 1997; Wu et al. 2000; Joseph et al. 2003). However, a main disadvantage of natural fibers is their poor compatibility with hydrophobic thermoplastics. Therefore, chemical modifications are necessary to increase interfacial adhesion at the wood fiber and semicrystalline polymer interface. Surface modification with multifunctional monomers increases interfacial adhesion by modifying surface chemistry, surface roughness, and surface free energy of the wood fibers (Gray 1974; Liu et al. 1994; Wang and Hwang 1996). Anhydrides as a coupling agent have received a great amount of interest (Maldas et al. 1989; Maldas and Kokta 1991; Khan and Idriss Ali 1993; Batpford and Al-Lamee 1994; Lu and Chung 1998; Hill and Cetin 2000; Li et al. 2001). Anhydride groups react chemically with the hydroxyl groups of wood fibers to form ester bonds. These bonds between the treated wood fiber and PP provide a good interfacial adhesion for wood fiber and thermoplastic composites (WPC). Maleic anhydride (MA), polymethylene polyphenyl isocyanate (PMPPIC), and maleic anhydride polypropylene (MAPP) are recognized as well-known coupling agents in WPC (LU et al. 2000). MA and MAPP have been shdwn to be relatively effective in improving both physical and mechanical properties of TMP fiber and PP composites. Both the chemical structure and the modification procedure improved mechanical properties of WPC through their coupling actions at the interface. The reaction rate and the number of anhydride groups reacted with the lignocellulosic fibers differenced from t h w of MA due to differences in their chemical structure which affected their reactivity (Clemons et al. 1992; Felix and Gatenholm 1993; Grell 2001; Lu et al. 2002). Benzoyl peroxide (BPO) and dicumyl peroxide (DCP) were commonly used as initiators for the modification of wood fibers and thermoplastics to graft MA onto the wood fiber surfaces (Keener et al. 2004; Demir et a]. 2005; Denac et al. 2005a; 200%; Minisini and ~sobn& 2005). Initiators have been used to improve interfacial adhesion by adding them directly to the fiberplastic mixture (Cousin et al. 1989; Sapieha et al. 1990). However, the BPO impregnation was much less effective than the direct blending process with an exception of some improvement in the yield stress of low density polyethylene (LDPE) composites (Raj et al. 1990). The lower effectiveness of the impregnation method was caused by fiber surface adsorption which reduced the amount of peroxide available when the fibers were treated with MA (Bataille et al. 1990). A small amount of BPO or DCP sharply increased the yield stress of PE composites. The BPO or DCP concentration, defined as a peroxide weight fraction at which the yield stress reaches 95% of its maximum, depended on the fiber content (Sapieha et al. 1990; Gassan and Bledzki 1997). DCP was more effective than BPO at low levels of peroxide addition due to the lower decomposition rate at high temperatures ensuring a better dispersion in the polymer. However, therinodynamic mechanical properties of polyvinyl chloride (PVC) based composites increased 20% by using 4.12% MAPP (Raj and Kokta 1995; Matuana et al. 1998; Lu et al. 2004). The MA has two different functional groups, an unsaturated carbon-carbon double bond and an anhydride group. Both groups attack wood fiber surfaces and build crosslinking reactions via hydroxyl groups. This characteristic makes 426 WOOD AND mBER SCrWC JULY 207. V. 39(3) MA an attractive coupling agent in WPC. EsSuppliers, Inc., Columbus, OH) were used to terification reactions between wood fiber and fabricate TPL. MAPP also proved to be beneficial in increasing strength proberties (Marcovich et d. 2001). The esterification reaction can be optimized by using catalysts, sodium hypophosphite hydrate, for esterification between the MA and the bleached wood fibers (Kazayawoko et al. 1997). However, little effect on the esterification of chemical-thennomechanical pulp (CTMP) fibers was reported. Tensile strength properties of PP based composites were improved with 25% MA addition (Mohanakrishnan et al. 1993). With regard to mechanical properties of WPC, interfacial property enhancement is important at the TMP fiber-thermoplastic interface. The maleation of the wood fiber surfaces provided a high variation on the mechanical prop erties of WPC depending on the wood or plastic materials, copolymer types, and processing conditions. Therefore, this study was conducted to evaluate the effects of MA-grafted TMP fiber surfaces-in contact with PP melts. It also evaluated the porosity of TMP fiber handsheets and number of fibers exposed on the fracture surfaces to determine its relationship to tensile strength properties of TMP fiber handsheets and isotactic polypropylene laminates (TPL). MATERIALS AND METHODS