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 effe ctive chain length (L(X)) of polyurethanes based on hydroxy-terminated polybutadiene (HTPB) prepolymers and toluene diisocyanate were calcul ated using the models of Marsh et al. and of Macosko and Miller. The i nput parameters required are the functionality distribution and equiva lent weight of the polymer, extent of reaction (P(A)) and density of t he cured network. It was found that the functionality distribution of HTPB can be adequately described by di- and trifunctional species esti mated from C-13 nuclear magnetic resonance spectra and non-functional species obtained from sol content at r = [NCO]/[OH] = 1.0. The value o f P(A)) was shown to be nearly 1.0 when r < 0.9. Satisfactory correlat ions were obtained between calculated and observed crosslink densities of HTPB prepolymers of widely varying functionality distribution. The se studies were extended to mixed systems containing HTPB, trimethylop ropane and 1,4-butanediol and to propellant systems containing 86% sol id loading. Relationships were derived between V(e) and tensile streng th and modulus, and between L(X) and elongation at break (E(b)) of gum stocks and propellants. These relationships were used to predict the m echanical properties of several propellant formulations, with very goo d agreement between observed and predicted properties.