Development of benzoxazine-based resins for wood composite application

Abstract

The curing reaction of benzoxazine resin, properties of benzoxazine alloys, and characteristics of wood composites used benzoxazine-epoxy alloy matrices were investigated. Kissinger, Ozawa, Friedman, and Flynn-Wall-Ozawa methods were utilized to determine the kinetic parameters of the curing reaction of the arylamine-based polyfunctional benzoxazine resins. BA-a resin shows only one dominant autocatalytic curing process with the average activation energy of 81-85 kJ/mol, whereas BA-35x exhibits two dominant curing processes signified by the clear split of the curing exotherms. The average activation energies of low-temperature curing (reaction (1)) and high-temperature curing (reaction (2)) were found to be 81-87 and 111-113 kJ/mol, respectively. The reaction (1) is found to be autocatalytic in nature, while the reaction (2) exhibits nth-order curing kinetics. To modify polybenzoxazine properties, effects of a monofunctional benzoxazine diluent (Ph-a) on properties of a bifunctional benzoxazine resin (BA-a) have been studied. The BA-a/Ph-a mixtures are miscible in nature rendering the properties highly dependent on their compositions. The viscosity of the BA-a resin can be reduced to one third using only about 10% by weight the Ph-a diluent. The addition of the Ph-a resin into the BA-a resin can also lower the liquefying temperature of the resin mixtures whereas the gel point is marginally decreased. The gel point, which depends on the BA-a/Ph-a mixtures and the cure temperature, was determined by the frequency independence of loss tangent in the vicinity of the sol-gel transition. The relaxation exponent values of the copolymer were found to be 0.24-0.55, which is dependent on the cure temperature. Gel time of the BA-a/Ph-a systems decreases with increasing temperature according to an Arrhenius relation with activation energy of 60.6 1.5 kJ mol?1. Flexural moduli of the BA-a/Ph-a polymers also increase with the Ph-a mass fraction, however, with the sacrifice of their flexural strength and glass-transition temperature. To systematically investigate the effect of benzoxazine alloy compositions on the interfacial interaction with Hevea braziliensis woodflour filler, wood-substituted composites from highly filled polybenzoxazine-epoxy alloys were investigated. The reinforcing effect of the woodflour shows the enhancement on the stiffness compared to that of the ternary matrices. The outstanding compatibility between woodflour and polybezoxazine matrix can improve the modulus and thermal curability i.e., glass transition temperature of the wood composites with polybenzoxazine fraction increased.