Effects of diol molecular weight on properties of benzoxazine-urethane polymer alloys for ballistic armor applications

Abstract

This study aims to develop a light weight ballistic armor from Kevlar (TM)-reinforcing fiber having polybenzoxazine alloys as a matrix. Polybenzoxazine (BA-a), a novel class of phenolic resin, possesses several outstanding properties, for example, ease of synthesis, low viscosity, near-zero shrinkage, lack of by-product upon curing, high thermal stability and high mechanical property. Moreover, the benzoxazine resins can be alloyed with various types of resins because of the various function groups in its structure. In this work, urethane elastomer (PU) at various polyol molecular weights is uded to enchance toughness of the polybenzoxazine and its effects on properties of ba:pu polymer alloys and also the ballistic characteristics of their Kevlar –reinforced composite are investigated. The experiment reveals that the similar curing peaks of the matrices at various polyol molecular weights, with the same urethane mass fraction, in the resin mixtures were obtained. The glass transition temperature increased from 165 degree Celsius of polybenzoxazine to 240-245 degree Celsius in the 70:30 BA:PU system and the degradation tempaerature also increased from 330 degree Celsius to 344 degree Celsius in the same alloy for all molecular weights of the polyol. In addition, the char yield increased when the higher molecular weight of polyol was added. The flexural modulus of polybenzoxazine also decreased from 6.2 GPa to be in range of 2.0-3.0 GPa with 30wt% of PU at every molecular weight of the polyol. Furthermore, the synergism with ultimate flexural strength in 90:10 BA:PU was obtained at all molecular weights used. The firing test suggested that the TDI-polyol (MW 3000) based urethane prepolymer had the suitable chain length for alloying with the benzoxazine resin particularly at the mass ratio of 80:20 BA:PU to yield a composite of best ballistic performance.