Comparative study on flexibility enhancement of polybenzoxazine using epoxy and urethane prepolymer

Abstract

The flexibility of polybenzoxazine (BA) can substantially be improved by alloying with IPDI-based urethane prepolymers (PU) or with flexible epoxy (EPO732). The experimental results, i.e. flexural testing and dynamic mechanical analysis, reveal that flexibility of the rigid polybenzoxazine and the PU or the EPO732 alloys systematically increases with the amount of both tougheners due to the added flexible molecular segments in the polymer hybrids. The curing temperature of the benzoxazine resin at about 225 degree celcius shifts to a higher value when the fraction of BA in both resin mixtures decreases. Interestingly, the synergism in glass transition temperature (Tg) obtained from the peak of loss modulus in the dynamic mechanical analysis of BA/PU alloys is clearly observed, i.e. Tg of the BA/PU alloys are significantly greater (Tg beyond 200 degree celcius) than those of the parent resins, i.e. BA (Tg = 165 degree celcius) and PU (Tg = -70 degreecelcius). However, this synergistic characteristic is not observed in the BA/EPO732 alloy systems. The synergistic Tg of BA/PU alloy at 70/30 mass ratio is found to be 221 degree celcius while that of BA/EPO732 at the same mass ratio is reported to be of only 95 degree celcius and further decreases with the amount of epoxy fraction. The phenomenon is explained by the ability of the PU fraction to substantially enhance the crosslinked density of the polymer alloys thus create the more connected network structure of the resulting alloys whereas the EPO732 shows almost negligible effects on the alloys crosslinked density or even reduce the alloy's crosslinked density when the amount of this flexible epoxy is greater than 50wt%. Furthermore, the degradation temperature based on the 5% weight loss of the TGA thermograms of BA/PU alloys is found to be improved with the presence of the PU while the opposite trend is observed in BA/EPO732 systems. Finally, the char yield of both alloy systems is steadily enhanced with increasing the amount of the benzoxazine fraction due to the inherent high char yield of the benzoxazine resin comparing with the other two tougheners. The toughened polybenzoxazine using IPDI-based urethane prepolymer, e.g. 70/30 BA/PU, is thus the promising system for a tough and high thermal stability polymeric network for bulk or composite matrix applications.