Wood-substituted composites from highly-filled polybenzoxazine system

Abstract

Polybenzoxazine, a thermosetting polymer, shows some outstanding properties which are suitable for the utilization as a matrix of composite materials such as low viscosity, low water absorption, near-zero shrinkage, high thermal stability (high Tg, Td), low flammability and no toxic by-products from burning. Wood-substituted composites which use woodflour or sawdust as a filler can increase the value and the utilization of waste materials. In this study, composites made from a polybenzoxazine matrix and woodflour have been prepared and tested. The objectives of the study are to determine the influence of the particle size and percent filler content of woodflour on their thermal, mechanical (flexural test) and some important physical properties. The glass transition temperature and the degradation temperature of these woodflour-filled polybenzoxazine composites were found to have relatively high values up to 200 degree celcius and 273 degree celcius, respectively. The char yield of woodflour-filled polybenzoxazine composite is up to 33.8-36.3 % which is somewhat higher compared to that of the neat resin, i.e. 27.7 %. The mechanical properties of the composites were also strongly affected by the woodflour content, i.e. having the storage modulus of 3.85 GPa in the 75 % by weight filled systems vs. 2.33 GPa of the unfilled system. The storage modulus and flexural modulus were found to increase with the filler content and particle size of woodflour whereas the flexural strength decreases. Water absorption shows the value of about 17 % by weight at saturation when compare to that of natural wood, i.e. between 30 and 200 % by weight. The maximum filler content which the specimen can still support the load at the level comparable to the natural wood is approximately 75 % by weight or 50 % by volume of woodflour. The good interfacial adhesion of woodflour and polybenzoxazine matrix is one key contribution to the desirable high modulus and high thermal stabilityof the resulting composite and was also confirmed by the SEM micrograph.