Thermomechanical characteristic of benzoxazine-urethane copolymers and their zylon fiber-reinforced composites

Abstract

The purpose of this research is to improve polybenzoxazine properties by broadening its mechanical property i.e. the rigid polybenzoxazine is toughened by alloying with a highly flexible urethane elastomer. Another objective is to utilize the obtained polymer alloy as a matrix for composite material by reinforcing with Zylon fiber for high mechanical property application. Polybenzoxazine (BA-a) possesses several outstanding properties for being used as a composite matrix such as ease of synthesis, self polymerization upon heating, high thermal stability, good mechanical property and rigidity. In addition, polybenzoxazine can be alloyed with various types of resins to further broaden its useful properties. The experimental results revealed that the processing window of the alloys was widened by the amount of the urethane resin. Interestingly, synergism in glass transition temperature (Tg) was clearly observed, i.e. Tg's of the BA-a/PU alloys were significantly greater than those of the parent resins, i.e. BA (Tg = 166oC) and PU (Tg = -70oC). The phenomenon was explained by the ability of the PU fraction to substantially enhanced crosslinked density of the resulting polymer alloys thus created the more connected network structure of the obtained alloys. Furthermore, the degradation temperature of the alloys was systematically increased from 325oC of a neat polybenzoxazine to 336oC with the addition of the PU up to 40% by weight while the char yield of the alloys was steadily enhanced with increasing amount of the benzoxazine fraction. Coefficient of thermal expansion of the polymer alloys were found to show a minimum value at BA-a/PU = 90/10. In addition flexural modulus was found to systemically decrease from 5.4 GPa of the neat polybenzoxazine to 2.1 GPa at 40% by weight of the PU. Flexural strength of the alloys, however, showed a synergistic behavior at the BA-a/PU ratio of 90/10. Furthermore, Zylon fiber-reinforced BA-a/PU composites at the fiber content of about 70% in cross-ply orientation showed that both the flexural strength and flexural modulus of Zylon fiber-reinforced BA-a/PU composites expectedly are higher than polymer matrix. Finally, synergistic behavior in Tg of Zylon fiber-reinforced BA-a/PU composites was observed to be similar to that observed in the matrix alloys