Agentes compatibilizantes para funcionalizar nps

CHAPTER 2. A REVIEW OF COUPLING AGENTS AND TREATMENTS* 2.1 INTRODUCTION Wood fiber and polymer composites (WFPC) are normally produced by mixing wood fiber with polymer, or by adding wood fiber as filler in a polymer matrix, and pressing or molding under high pressure and temperature. Most polymers, especially thermoplastics, are non-polar (hydrophobic) substances, which are not compatible withpolar (hydrophilic) wood fibers and, therefore, poor adhesion between polymer and wood fiber in WFPC can result (Geottler 1983; Klason et al. 1984). In order to improve the affinity and adhesion between wood fibers and thermoplastic matrices in production, chemical coupling agents have been employed (Chun and Woodhams 1984; Woodhams et al. 1984; Dalväg 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 (surfaceactive agents), including compatibilizers and dispersing agents (Štepek and Daoust 1983; Radian Corporation 1987; Clint 1998). Bonding agentsact 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

*Reprinted in part with permission from Wood Fiber and Science, 2000, Vol. 32, No. 1, Pages 88-104; J. Z. Lu; Q. Wu; and H. S. McNabb, Jr.; Chemical Coupling in Wood Fiber and PolymerComposites: A Review of Coupling Agents and Treatments. Society of Wood Science and Technology State-of-the-Art-Review. Copyright 2000 by the Society of Wood Science and Technology.

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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). Somecompatibilizers, 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-ethylene/butylene-styrene (SEBS-MA) and styrene-maleic anhydride (SMA), also act as compatibilizers in WFPC (Oksman and Lindberg1998; 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 fibersin the matrix. In general, compatibilizers and dispersing agents do not form strong adhesion at the fibermatrix interface (Štepek 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 thedevelopment 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 1991) conducted successful experiments on the bonding of air-formed wood fiber-polypropylene composites using MAPP as acoupling agent. They developed an excellent coating method to spray the emulsified 12

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-polymer composites have been published (e.g., Hamed and Coran 1978; Meyer 1981, 1982, 1984; Rowell and Konkol 1987; Schneider 1994; Youngquist...