Morphology-property relationships in wood-fibre-based polyurethanes

Many of the advances in material performance over the last decade can be attributed to developments in multicomponent polymer systems and specifically, to multiphase materials such as incompatible polymer blends and fibre-reinforced composites. In these types of material, performance properties are often dominated by the interphase that defines the transition region from one pure phase to another. Although typically a minor component, the interphase effectively determines the mechanism and efficiency of stress transfer between the components that make up the material system. With the potential for chemical reaction across the interface, complications exist when viewing the material as an incompatible polymer blend or a reinforced polymer composite. One example of this type of composite is polyurethanes incorporating a lignocellulosic fibre. This class of natural fibre is characterized by a hydroxy-rich surface that may react with the isocyanate group of the urethane formulation. A considerable volume of research has addressed the use of isocyanate resins as binders for traditional wood composites (e.g., particleboard, mediumdensity fibreboard and oriented strandboard) and has attributed the dramatic improvements in strength and moisture resistance to possible covalent binding [l-3]. Also, research has investigated the use of isolated wood polymers, such as lignin, as the polyol component in polyurethanes [4-61. Considering the wood fibre as a reactive polyol-like component is a relatively recent development [7]. (propylene oxide)-based polyol, and a polyolmiscible isocyanate prepolymer (Blendur-5000) cross-linking agent (Miles Inc., Pittsburgh, PA). Composites of various fibre contents were prepared by first dispersing the required volume of polyisocyanate in the polyol and then spraying it on the oven-dried fibre. The hydroxyl number of the synthetic polyol was provided as 56, and a value of 190 was used for the wood fibres [7]. The component formulations were calculated using an isocyanate index of 1.25. The 6 in X 6 in fibre mat was manually formed and pressed for approximately 4 min at 175 “C in a Carver laboratory press. All panels were produced to a nominal thickness of 0.25 in and a target density of 45% fibre. The cured polyurethane composites were cooled under ambient conditions and stored over PzOs in a vacuum desiccator until testing. Dynamic mechanical properties of the composites were measured from -75 to 200 “C at 5 “C intervals in three-point bending at a frequency of 6.28 cycles SK’ (Rheometrics, Inc.).