Pretreatment of hydroxy-terminated polybutadiene (HTPB)/toluene diisocyanate (TDI) binder system for biodegradation

Hydroxy-terminated polybutadiene/toluene diisocyanate (HTPB/TDI) binder system is widely used in composite solid propellants. Expiration of weapons and ammunitions results in a significant amount of abandoned propellants. After recovery of energetic components, biodegradation is a potential treatment technology for the remaining material to convert into a harmless substance with low molecular weight. Due to high molecular weight and the presence of carbon-carbon long chain segments lacking electronegative elements, this binder system is difficult to be biodegraded. Nevertheless, some pretreatments will improve biodegradation. A two-step pretreatment of the binder system was performed using CH3-ONa/CH3-OH as a depolymerization reagent and HCOOH/H2O2 as an epoxidizing reagent so as to obtain an initial substrate for biodegradation. FT-IR and GPC analyses show that depolymerization breaks the urethane bonds in the binder system, and the hydroxyl-terminated depolymerization product has a structure similar to that of HTPB and significantly reduced molecular weight close to that of HTPB. Subsequent epoxidation makes some carbon-carbon double bonds in the hydroxyl-terminated depolymerization product convert into epoxy groups. The biodegradability tests show that there are richer potential degrading microorganisms for the epoxidation product as the sole carbon source vs. the original binder system, and the epoxidation product has a higher weight loss in biodegradation. As a pretreatment, depolymerization and epoxidation can reduce the molecular weight of substrate, improve the hydrophilicity, and make the biodegradability of pretreatment product superior to the HTPB/TDI binder system.