The Effects of Different Silane Crosslinking Approaches on Composites of Polyethylene Blends and Wood Flour

Though silane chemistry has been used to crosslink unfilled polyethylene for many years, such crosslinking has only been recently applied to wood plastic composites to improve properties such as creep resistance. However, the presence of wood significantly changes the silane chemistry and a greater understanding is necessary for optimal processing and performance. We investigated three different approaches of adding silane and peroxide during reactive extrusion of wood-plastic composites were investigated and compared to their unfilled counterparts. Up to 2 grams of a 20:1 solution of vinyltrimethoxysilane and dicumyl peroxide by weight were added per 100 grams of plastic. Low density polyethylene, high density polyethylene, and a blend of the two were investigated as matrices in composites containing approximately 25% wood flour. The unfilled plastics crosslinked efficiently and little premature crosslinking occurred during processing. Though only modest improvements in tensile strengths were found in unfilled plastics, the notched Izod impact energy of unfilled HDPE improved nearly 600% when crosslinked. Unlike the unfilled plastics, most of the crosslinking of the composites occurred during processing. This resulted in high viscosities and processing difficulties, negating some of the advantages of a separate crosslinking step and limiting the amount of crosslinking possible. Some reinforcement of the plastics by the wood flour was achieved when sufficient crosslinking solution was added. Adding crosslinking solution also and greatly increased reverse-notched impact energies. The differences between the approaches for preparing the crosslinked composites appeared to largely be a matter of efficiency in terms of the amount of crosslinking solution required to affect processing and performance. The most efficient approach was a two-step procedure where the silane was first grafted to the plastic and then wood flour was compounded with the grafted silane in a second step. Published 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2292–2303, 2011