Ks. Sastri et al., MODELING THE EFFECT OF FUNCTIONALITY DISTRIBUTION ON THE CROSS-LINKING CHARACTERISTICS OF HYDROXY-TERMINATED POLYBUTADIENE LIQUID PREPOLYMERS, Polymer, 35(21), 1994, pp. 4555-4561
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.