Reaction kinetics dominated by melt decomposition mechanism: Intrinsic pyrolysis of insensitive HTPE polyurethane and the efficient inter-reaction with AP

The novel insensitive HTPE (hydroxyl terminated polyether) adhesive holds a great potential application to develop the insensitive solid propellants. However, the pyrolysis kinetics and reaction mechanism of HTPE polyurethane are remained unclear. In this experimental investigation, the DTG curve of HTPE polyurethane was effectively deconvoluted into two main reaction stages via Gaussian peak fitting method, and the kinetic parameters for each pyrolysis stage were calculated. The calculation of the reaction mechanism functions indicated that both reaction stages follow an n-order reaction model with a very close n value. The overall pyrolysis process can be expressed as f(α) = (1 - α)ⁿ (n = 1.8 or 1.9). The TG-FTIR-GCMS results of online-collected gaseous products demonstrated that the pyrolysis of HTPE polyurethane is gradually decomposed from the outer layer to the inner layer, rather than being completely dominated by the kinetics of different functional groups. Localized melting of HTPE polyurethane was observed at 150 °C, moreover, it would almost completely transform into the liquid phase before the decomposition reaction occurred. Thus, it is suggested that the distinctive melt decomposition process of HTPE polyurethane alters the chemical environment even turns heat and mass transfer models of internal molecules, ultimately leading to its unique pyrolysis kinetics and reaction mechanisms. Furthermore, HTPE polyurethane could delay the first-stage pyrolysis of ammonium perchlorate (AP), but significantly promote its second-stage pyrolysis process. Therefore, HTPE polyurethane is beneficial in reducing the sensitivity of AP under thermal stimulation as well as promoting its concentrated heat release process.