Evaluation of criteria for blending hydroxy terminated polybutadiene (HTPB) polymers based on viscosity build-up and mechanical properties of gumstock

The reaction of HTPB with TDI was studied by bulk viscosity build-up and was found to occur in two stages, the rate constant for the first stage (K1=2.318−3.448×10−2 min−1) being greater than for the second stage (K2=1.040−2.296×10−2 min−1). The rate constant for blends of polymers was shown to be equal to the weighted average of rate constants of the individual components. The bulk viscosity of the system at the point of intersection (ηi) of the two stages was observed to be independent of r ([NCO]/[OH]), the functionality distribution and temperature for polymers with very close molecular weights whereas the ratio of ηi to the initial viscosity (η0) was found to be constant for polymers of different molecular weights. Initial modulus and elongation at break of gumstocks of blends of HTPBs were found to be given by the weighted average of the individual components, but tensile strength remained constant. These observations lead to criteria for blending polymers to obtain desired properties.