Rheokinetic Analysis of HTPB-TDI Based Polyurethane Binder System

Composite propellants involve powder fuel and oxidizer along with a polymeric binder. Hydroxyl Terminated Poly Butadiene (HTPB) is a well-known binder for composite propellant industry that reacts with Toluene Di-Isocynate (TDI)to give polyurethane (PU) network. The characteristics of polyurethane network is highly dependent on the ratio of cyanate/hydroxyl (NCO/OH) groups, temperature and time A stoichiometric ratio of NCO and OH groups was used along-withDioctylSebacate (DOS) plasticizer in the PU binder system. Current study explains the effects of temperature on rheokinetics of the system. Isothermal rheological studies have been performed by using cone and plate rheometer and the sample temperature was controlled using peltier heaters. The study was carried out at three different temperatures (55°C, 70oC and 85°C at a constant shear rate and viscosity buildup was measured. It was further used for the calculation of reaction rate constant (k). activation energy also has been calculated by Arrhenius relationship while, activation enthalpy and entropy can be calculated by eyring relationship. It was observed that the polyurethane linkages are formed with the conversion of isocyanate groups which builds up the viscosity and result in an increased shear stress. A direct relationship between the temperature and reaction rate was observed and the increase in viscosity was attributed to this effect. Moreover, the shear rate also plays a significant role in the curing kinetics. To study this effect, the shear stress was kept constant. It was concluded that as the reaction proceeds, the shear rate was decreased with an increase in viscosity. Hence the effect of temperature and shear rate both are substantial on the reaction kinetics.