Chemical Coupling in Wood Fiber and Polymer Composites: a Review of Coupling Agents and Treatments'

Coupling agents in wood fiber and polymer composites (WFPC) play a very important role in improving the compatibility and adhesion between polar wood fibers and non-polar polymeric matrices. In this paper, we review coupling agents, pretreatment, and mixing technology for wood fiber and polymer currenrly used in the manufacture of WFPC. So far, over forty coupling agents have been used in production and research. These agents are classified as organic, inorganic, and organic-inorganic groups, among which organic agents are better than inorganic agents because of stronger interfacial adhesion. The most popular coupling agents currently being used include isocyanates, anhydridcs, silanes, imd anhydride-modified copolymers. Coupling agents are usually coated on the surface o i wood fiber, polymer or both by compounding, blending, soaking, spraying, or other methods. Three basic processes suitable for coupling treatment are discussed: directly coating during mixing and fully or partly pretreating before mixing. The pretreatment of wood fiber and polymer by coating or grafting is the preferred method to improve the mechanical properties of WFPC.2 Kuvwords: Coating, coupling agents, grafting, polymer, thermoplastic matrix, WFPC, wood fiber. INTRODUCTION fiber as filler in a polymer matrix, and pressing wood fiber and polymer composites or molding under high pressure and tempera(WFPC) are normally produced by mixing ture. Most polymers, especially thermoplaswood fiber with polymer, or by adding wood tics, are non-polar (hydrophobic) substances that are not compatible with polar (hydrophilt Member of SWST. ic) wood fibers and, therefore, poor adhesion I This paper (No. 98-:!2-0578) is published with the apbetween polymer and wood fiber in WFPC proval of the Director of the Louisiana Agricultural Experiment Station. can result (Geottler 1983; Klason et al. 1984). I See nomenclature fc~IIowing references. In order to improve the affinity and adhesion Wood ond trhc,r S c r r ~ ~ < p , 1?(1) 2000. pp. 88-104 C 2000 hy the Society of Wood Sclencc and Technology LLI et a/.-CHEMICAL COUPLING IN WOOD FIBER AND POLYMER COMPOSITES 819 between wood fibers and thermoplastic matrices in production, chemical coupling agents have been employed (Chun and Woodhams 1984; Woodhams et al. 1984; Dalvag et al. 1985; Schneider and Brebner 1985). Coupling agents are substances that are used in small quantities to treat a surface so that bonding occurs between it and other surfaces, e.g., wood and thermoplastics (Pritchard 1998). Generally, coupling agents comprise bonding agents and surfactants (surface-active agents), including compatibilizers and dispersing agents (Stepek and Daoust 1983; Radian Corporation 1987; Clint 1998). Bonding agents act as bridges that link wood fibers and thermoplastic polymers by one or more of the following mechanisms: covalent bonding, polymer chain entanglement, and strong secondary interactions as in the case of hydrogen bonding (Raj et al. 1988; Maldas et al. 1989a). Compatibilizers are used to provide compatibility between otherwise immiscible polymers through reduction of the interfacial tension (Pritchard 1998). Some compatibilizers, such as acetic anhydride and methyl isocyanate, are monofunctional reactants. They lower the surface energy of the fiber, and make it non-polar, more similar to the plastic matrix. Some bonding agents, such as maleated polypropylene (MAPP), maleated styrene-ethylenefbutylenestyrene (SEBS-MA) and styrene-maleic anhydride (SMA), also act as compatibilizers in WFPC (Oksman and Lindberg 1998; Oksman et al. 1998; Simonsen et al. 1998). Dispersing agents reduce the interfacial energy at the wood fiber-matrix interface to help uniform dispersion of wood fiber in a polymer matrix without aggregation and thereby facilitate the formation of new interfaces (Rosen 1978; Porter 1994). For example, stearic acid and its metallic salts are used to improve the dispersibility of wood fibers in the matrix. In general, compatibilizers and dispersing agents do not form strong adhesion at the fiber-matrix interface (Stepek and Daoust 1983). Thus, a functional distinction between bonding agents, compatibilizers, and dispersing agents should be noticed. In this paper, however, all bonding agents and surfactants are lumped together as coupling agents for the purpose of the review. With the development of coupling agents, a number of pretreatment (i.e., fiber coating and graft co-polymerization) and mixing processes for improving mechanical properties of WFPC have been introduced. For example, Youngquist and colleagues (Krzysik et al. 1990; Krzysik and Youngquist 199 1) conducted successful experiments on the bonding of airformed wood fiber-polypropylene composites using MAPP as a coupling agent. They developed an excellent coating method to spray the emulsified Epolene E-43 on wood fiber before formation. As a result of these efforts, WFPC have been developed very rapidly during the last decade. Several review articles on wood polym~zr composites have been published (e.g., Hamed and Coran 1978; Meyer 1981, 1982, 1984; Rowell and Konkol 1987; Schneider 1994; Youngquist 1995). These reviews cover from chemical modification and treatment of wood and plastics to production technologies and applications for various types of wood pokymer composites. However, none of these r~eviews systematically dealt with couplir~g agents and treatments currently used in this field. The objective of this work is to provitle a state-of-the-art review on coupling agents, pretreatment of wood fiber and polymer, arid mixing technology for the manufacture of WFPC. The adhesion mechanism and coupling performance of various coupling agents will be discussed in future publications.