Diffusion Effect on Octogen Coating-Curing Kinetics with Polyurethane Using Infrared Spectroscopy

The kinetic analysis of octogen coating with a polyurethane base containing hydroxyl-terminated polybutadiene (HTPB) was investigated using infrared spectroscopy. The coating process involved a solvent method, where octogen and liquid polyurethane were mixed, the solvent was evaporated, and curing took place at an elevated temperature. The ratio of HTPB to diisocyanate was equimolar. About 200 g octogen was coated with mixture of 2 mL HTPB, 50 mL ethyl acetate, and 0.2 mL TDI in the glass beaker at 30 °C for 1 h. The filtrated ethyl acetate was then evaporated, and the residue was dried in a vacuum oven for 15 min at 30 °C. The resulting film-coated octogen was cast into a KBr pellet and cured in the oven for 7 days at 40 °C, then infrared-analyzed every hour during the curing process. After curing, the shape of the coated octogen particles was analyzed using SEM Initially, the curing process occurred in the solvent system, followed by further curing in the bulk system. The kinetic analysis was performed using a modified diffusion-autocatalytic model, which includes noncatalytic, autocatalytic, and diffusion components. This model was compared with others during the bulk reaction and proved to be effective in correcting errors, particularly in the gel time region. Thermodynamic parameters were evaluated using the Arrhenius and Eyring equations. The reaction rate was initially controlled by chemical reactivity, but after the gel time, diffusion became the controlling factor. In the HTPB-TDI system, both the noncatalytic and autocatalytic parts decreased with increasing temperature, while diffusivity increased. It is worth noting that diffusivity is temperature-dependent. Different di-isocyanates, namely toluene diisocyanate (TDI), iso-phorone diisocyanate (IPDI), and hexamethylene diisocyanate (HMDI), were studied, revealing that HMDI exhibited higher reactivity than TDI and IPDI. The catalyst effect on reaction rate of the HTPB-TDI system was investigated. The addition of catalysts (0.1%) to the HTPB-TDI system decreased their activation energy in the order DBTL > FeAA > TPB. Catalysts did not change their diffusivity.