Polymer, Vol.35, No.21, 4555-4561, 1994
Modeling the Effect of Functionality Distribution on the Cross-Linking Characteristics of Hydroxy-Terminated Polybutadiene Liquid Prepolymers
Network properties like sol content, crosslink density (V(e)) and effective chain length (L(X)) of polyurethanes based on hydroxy-terminated polybutadiene (HTPB) prepolymers and toluene diisocyanate were calculated using the models of Marsh et al. and of Macosko and Miller. The input parameters required are the functionality distribution and equivalent weight of the polymer, extent of reaction (P(A)) and density of the cured network. It was found that the functionality distribution of HTPB can be adequately described by di- and trifunctional species estimated from C-13 nuclear magnetic resonance spectra and non-functional species obtained from sol content at r = [NCO]/[OH] = 1.0. The value of P(A)) was shown to be nearly 1.0 when r < 0.9. Satisfactory correlations were obtained between calculated and observed crosslink densities of HTPB prepolymers of widely varying functionality distribution. These studies were extended to mixed systems containing HTPB, trimethylopropane and 1,4-butanediol and to propellant systems containing 86% solid loading. Relationships were derived between V(e) and tensile strength and modulus, and between L(X) and elongation at break (E(b)) of gumstocks and propellants. These relationships were used to predict the mechanical properties of several propellant formulations, with very good agreement between observed and predicted properties.