ABSTRACT: A kinetic study of the reaction between a hydroxyl-terminated polybuta-diene (HTPB) and isophorone diisocyanate (IPDI) was carried out in the bulk stateby using quantitative Fourier transform infrared (FTIR) spectroscopy. The reaction isshown to obey a second-order rate law, being first order in both the HTPB and IPDIconcentrations. The activation parameters obtained from the evaluation of kinetic dataare D H ‡ †41.1 {0.4 kJ mol, D S ‡ †0198 {2JK 01 mol 01 and E a †43.8 {0.4 kJ mol 01 ,which are quite different from the solution values. However, they are in agreement withthe results obtained on propellants by torsional braid measurements. The large nega-tive value of the activation entropy is indicative of an associative mechanism, whichis in accord with the second-order rate law for the polyurethane formation. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1979–1983, 1997 Key words: HTPB; IPDI; diisocyanate; kinetics; polyurethane; FTIR spectroscopy INTRODUCTION other binders, such as poly(vinyl chloride). Fur-thermore, the low viscosity of HTPB facilitatesComposite propellants based on hydroxyl-termi- high solid loading in both fuel and oxidizer of thenated polybutadiene (HTPB) have become the propellant. At the final stage of a composite pro-workhorse propellants in present day solid rocket pellant preparation, a curing agent, usually a di-motors. Crystalline ammonium perchlorate (AP) isocyanate, is added to the heterogeneous mixtureand fine aluminum powder are used as oxidizer containing the prepolymer, fuel, oxidizer, andandmetallicfuel, respectively,intheHTPB-based other ingredients. The prepolymer and diisocya-composite rocket propellants. The solid loading, nate react with each other to form the polyure-which contributes to the rheological properties thane network [eq. (1)],which holds the fuel andand therefore affects the processability and me- oxidizerparticulates ina compositematrix havingchanical properties of the propellants, is largely mechanical and ballistic properties suitable fordetermined by the extent of polyurethane forma- rocket applications.tion. In this respect, the binder used in the propel-lant plays an important role. The polybutadienechain, in general, gives higher energy value andO'H 1 OCN O'C'N (1)better mechanical properties compared to the OHThe cure reaction of HTPB with diisocyanate hasbeen found to follow the second-order rate law.