Interfacial contributions in lignocellulosic fiber-reinforced polyurethanecomposites

Whereas lignocellulosic fibers have received considerable attention as a re inforcing agent in thermoplastic composites, their applicability to reactiv e polymer systems remains of considerable interest. The hydroxyl-rich natur e of natural lignocellulosic fibers suggests that they are particularly use ful in thermosetting systems such as polyurethanes. To further this concept , urethane composites were prepared using both unused thermomechanical pulp and recycled newsprint fibers. In formulating the materials, the fibers we re considered as a pseudo-reactant, contributing to the network formation. A di-functional and tri-functional poly(propylene oxide)-based polyol were investigated as the synthetic components with a polyol-miscible isocyanate resin serving as a crosslinking agent. The mechanical properties of the com posites were found to depend most strongly on the type of fiber, and specif ically the accessibility of hydroxy functionality on the fiber. Dynamic mec hanical analysis, swelling behavior, and scanning electron micrographs of f ailure surfaces all provided evidence of a substantial interphase in the co mposites that directly impacted performance properties. The functionality o f the synthetic polyol further distinguished the behavior of the composite materials. Tri-functional polyols generally increased strength and stiffnes s, regardless of fiber type. The data suggest that synthetic polyol functio nality and relative accessibility of the internal polymer structure of the fiber wall are dominant factors in determining the extent of interphase dev elopment. Considerable opportunity exists to engineer the properties of thi s material system given the wide range of natural fibers and synthetic poly ols available for formulation. (C) 2001 John Wiley & Sons, Inc.