Effect types of isocyanates on mechanical and thermal properties of benzoxazine and urethane polymer alloys

Abstract

This research aims to investigate the effect of types of isocyanates on mechanical and thermal properties of benzoxazine and urethane polymer alloys. Polybenzoxazine, a new thermoset plastics, possesses various intriguing properties including high thermal stability, easy processability, low water absorption, near zero shrinkage, excellent mechanical properties, as well as ability to alloy with various types of resins. However, one shortcoming of this resin is its rather high rigidity making the polymer unsuitable for some applications. This work aims to develop an approach to further improve properties of polybenzoxazine by alloying with urethane elastomer based on various types of isocyanates such as isocyanates such as toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), and isophorone diisocynate (IPDI). The experimental results showed that glass transition temperature (T [subscript g]) of the alloys at various types of isocyanates increased from 160 degrees celsius of the neat polybenzoxazine to 253 degrees Celsius in the 60:40 BA : PU system and the degradation temperature increased from 321 degrees Celsius to about 330 degrees Celsius with increasing the urethane content for all evaluated compositions. In addition, flexural modulus of the polybenzoxazine decreased from 5.8 to 1.7-2.0 GPa when adding 40 wt% of PU for all BA:PU alloys. Furthermore, the flexural strength showed a synergistic behavior at the BA:PU ratio of 90:10 for all three types of the alloys. The effect of urethane prepolymer based on TDI provided the highest T [subscript g ], flexural modulus, and flexural strength of the alloys among the three isocyanates used (i.e. in comparison at the same urethane content). The optimum isocyanate of the binary systems for making high processable carbon fiber composites was BA:PU based on TDI. The flexural strength of the carbon fiber-reinforced BA:PU based on TDI at 80wt% of the fiber in cross-ply orientation rendered relatively high values of about 490 MPa. The flexural modulus slightly decreased from 51GPa for polybenzoxazine to 48 GPa for the alloys having 40 wt% of the PU fraction.