interfacial Contributions in Lignocellulosic Fiber-Reinforced Polyurethane Composites

ABSTRACT: Whereas lignocellulosic fibers have received considerable attention as a rein-forcing agent in thermoplastic composites, their applicability to reactive polymer systems remains of considerable interest. The hydroxyl-rich nature of natural lignocellulosic fibers suggests that they are particularly useful in thermosetting systems such as polyurethanes. To further this concept, urethane composites were prepared using both unused thermo-mechanical pulp and recycled newsprint fibers. In formulating the materials, the fibers were considered as a pseudo-reactant, contributing to the network formation. A di-func-tional and tri-functional poly(propy1ene oxide)-based polyol were investigated as the syn-thetic components with a polyol-miscible isocyanate resin serving as a crosslinking agent. The mechanical properties of the composites were found to depend most strongly on the type of fiber, and specifically the accessibility of hydroxy functionality on the fiber. Dynamic mechanical analysis, swelling behavior, and scanning electron micrographs of failure surfaces all provided evidence of a substantial interphase in the composites that directly impacted performance properties. The functionality of the synthetic polyol further distin-guished the behavior of the composite materials. Tri-functional polyols generally increased strength and stiffness, regardless of fiber type. The data suggest that synthetic polyol functionality and relative accessibility of the internal polymer structure of the fiber wall are dominant factors in determining the extent of interphase development. Considerable opportunity exists to engineer the properties of this material system given the wide range of natural iibers and synthetic polyols available for formulation. 0 2001 John Wlley & Sons