CHEMISTRY AND KINETICS OF HYDROXYL-TERMINATED POLYBUTADIENE (HTPB) AND DIISOCYANATE HTPB POLYMERS DURING SLOW DECOMPOSITION AND COMBUSTION-LIKE CONDITIONS

The decomposition chemistry and kinetics of hydroxyl-terminated polybutadiene (HTPB), hydroxyl-terminated polyethyleneglycol adipate (HTPA), and diisocyanate cross-linked HTPB (diisocyanates = TDI, IPDI, and DDI) were determined by TGA, DSC, and IR spectroscopy at 10-degrees-C/min and by SMATCH/FTIR at 250-degrees-350-degrees-C/s. At 10-degrees-C/min, the first step (300-degrees-400-degrees-C) for diisocyanate-HTPB is fission of the urethane cross-link bonds. The diisocyanate cross-linking agent is vaporized to an extent that is controlled by its vapor pressure. The HTPB polymer then exothermically cross-links and cyclizes, but also depolymerizes to yield volatile products. The overall exothermicity of these steps is proportional to the number of double bonds in the parent polymer. Above 400-degrees-C, the final major stage is endothermic decomposition of the mostly saturated residue of HTPB formed in the 300-degrees-400-degrees-C range. The kinetics of this final step are independent of whether the original polymer contained urethane cross-links or not. SMATCH/FTIR spectroscopy simulates the conditions in the condensed phase during combustion at 15 psi, as evidenced by the fact that the regression rates calculated for HTPB from the SMATCH/FTIR kinetics at 250-degrees-350-degrees-C/sec fall within the range of combustion-like regression rates of IPDI-HTPB and similar polymers (0.05-0.2 mm/s at 15 psi). SMATCH/FTIR spectroscopy establishes that under combustion-like conditions, the urethane cross-links cleave in the first step, but that less cyclization of the residual HTPB occurs because of the shorter time available. Hence, no initial exotherm appears. Instead, HTPB skips directly to depolymerization, giving up butadiene, butadiene oligomers, and CH2O to the first stage of the flame zone.