Rheokinetic Modeling of HTPB–TDI and HTPB–DOA–TDI Systems B. M. Bandgar, 1 K. C. Sharma, 1 T. Mukundan, 2 V. N. Krishnamurthy 3 1 Department of Space Sciences, University of Pune, Pune-411 007 2 High Energy Materials Research Laboratory, Sutarwadi-411021 3 DRDO-ISRO Cell, University of Pune, Pune- 411 007 Received 11 March 2002; accepted 18 October 2002 ABSTRACT: A study of the effect of temperature on a mixture of polymer and curative in the processing of rocket propellants is reported. Experimental viscosity of a hydrox- yl-terminated polybutadiene–toluene diisocyanate (HTPB– TDI) system was measured using a Brookfield viscometer model DV III. Viscosity showed dependence on temperature as well as time. The viscosity data of the HTPB–TDI system showed a linear relationship with temperature, with a change in slope at 45°C. The time dependence model showed a fourth-order curve fit, which gave better results over the exponential model fit. The activation energy re- quired for flow of the HTPB–TDI system was found to be 15.5 kJ/mol. Experimental viscosity measurements at vari- ous temperatures was also carried out on a hydroxyl-termi- nated polybutadiene– dioctyl adipate –toluene diisocyanate (HTPB–DOA–TDI) system. The temperature dependence showed a decrease in viscosity with an increase in temper- ature up to 60 min, beyond which the viscosity increased. Viscosity showed a linear relation with temperature, with a change in the slope at 50°C instead of at 45°C for HTPB–TDI system. Beyond 50°C the data followed a polynomial model similar to that of the HTPB–TDI system, and the results matched well with the experimental data. The activation energy of the HTPB–DOA–TDI system increased with an increase in the binder weight ratio. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1331–1335, 2003 Key words: rheology; modeling; curing of polymers; poly- butadiene INTRODUCTION Currently hydroxyl-terminated polybutadiene (HTPB) is considered a workhorse propellant binder and is used all over the globe. Dioctyl adipate (DOA) has been found to be an excellent plasticizer for HTPB. When a plasticizer is added to the polymer, it acts as a lubricant and helps the polymer molecules move freely, thereby reducing the viscosity of the system. The plasticizer helps the easy processing of the pro- pellant mix and also improves low-temperature me- chanical properties like tensile strength and elonga- tion of propellant grain. Low viscosity of the binder system helps in achieving more solid loading, which leads to a significant increase in the total thrust or the specific impulse of the propellants. In propellant formulations curative is added to transform the propellant slurry into a solid grain. The trifunctional curative system in the presence of cata- lyst builds up a three-dimensional network. Toluene diisocyanate (TDI) is a difunctional curative com- monly used for HTPB. A curing catalyst like dibutyl tin– dilaurate (DBTDL) is employed. For HTPB with a functionality of more than two, toluene diisocyanate (TDI) curative and dibutyl tin dilaurate (DBTDL) are used as curative and cure catalyst, respectively. Sekkar et al. 2 studied the effect of catalyst (DBTDL) concentration on the viscosity of HTPB curative sys- tems. They observed that, based on viscosity data, the cure reaction between toluene diisocyanate (TDI) or isophoron diisocyanate (IPDI) with HTPB polymer took place in two stages because of the difference in reactivity between the isocyanate groups present in IPDI and TDI at a lower temperature (30°C), whereas with HMDI it took place in a single stage. This may be because of (1) the reactivity difference of the func- tional groups of the curatives and (2) high viscosity buildup, decreasing the rate of reaction. The NCO functional group at the para position was more reac- tive than that at the ortho position because of steric hindrance factor. Yamaguchi et al. 5,6 studied the effect of crosslinked linear and low-density polyethylene on rheological properties like strain hardening behavior in elongation viscosity and steady shear viscosity, and they also studied the weight fraction effect on the blending of linear and crosslinked polymer gels on elongational viscosity and shear viscosity. They found that the stretching of the chain section between the crosslink points was responsible for the hardening behavior. Eom et al. 7 proposed a chemorheological Correspondence to: T. Mukundan (guruv@hotmail.com). Contract grant sponsor: DRDO. Journal of Applied Polymer Science, Vol. 89, 1331–1335 (2003) © 2003 Wiley Periodicals, Inc.