Research On Catalytic Curing Of Casting PBX Explosives

Hydroxyl terminated polybutadiene (HTPB) is extensively applied as binder in solidpropellant and casting PBX (polymer bonded explosive) due to its convenience for castingand manufacturing.The binder imparts dimensional stability and structural integrity to thegrain for appropriate mechanical properties.Different curatives effect curing reaction rate andmechanical properties of HTPB elastomer.The low reaction rate of the urethane formingcuring reaction of HTPB binder with isocyanate curative causes the high curing temperatureand long manufacturing period.Catalysts are widely used for reducing the curing temperatureand curing time for different curatives.The main objective of the present study is selecting the high reaction rate curing system withappropriate pot life by comparing the curing periods of different curing systems composed bydifferent curatives and catalysts,investigating the influence of catalysts on curing reactionkinetic by using differential scanning calorimetry (DSC) and exploring the reactionmechanism.In order to investigate the influence of different curatives on curing reaction,toluenediisocyanate (TDI) was used as curative.Curing period was3days of HTPB-TDI curingsystem without catalyst.The effect of diphenylmethane diisocyanate (MDI),isophoronediisocyanate (IPDI) and their mixture of different proportions on curing reaction wereinvestigated too.The experimental results show that the pot life of the slurry which using thecurative mixed according to the proportion of IPDI with MDI in75%:25%reaches6h.It meetsthe production requirement.The curing time of the slurry is4days and it’s shorter than theslurry using IPDI as the curing agent.The curing agent mixture of this proportion and TDI areboth ideal curative of HTPB.The influence of catalysts on curing reaction with different curatives were investigatedincluding dibutyltin dilaurate (DBTDL),ferric tris (acetyl acetonate)(Fe(AA)3), triphenyl bismuth(TPB) and acetylacetone (HAA).The experimental results show that reaction rates ofHTPB-IPDI curing systems using DBTDL and Fe(AA)3as catalysts are too high.The pot lifeof the curing systems cannot meet the production requirement.TPB shorten the curing periodof the curing system and insure the pot life meeting the requirement.But the curingtemperature is still60℃.HAA is used as inhibitor in the HTPB-MDI curing system.Theexperimental results show that the reaction rate is high at earlier stage and low at last stage.Itcannot meet application requirement.HTPB-TDI curing system can be catalyzed by TEPB atambient temperature and its pot life can meet the production requirement.The effect of TPB and tri (exhoxyphenyl) bismuthine (TEPB) on kinetics of HTPB-TDIcuring reaction were studied by DSC.Characteristic temperature of curing systems weremeasured for calculating kinetic parameters by Kissinger and Crane method and the curingreaction kinetic equations was established. Results show that catalysts decrease the curingtemperature and shorten the curing temperature gradient. Activation energy (Ea) ofuncatalyzed HTPB-TDI curing system is51.29kJ·mol-1,the Ea of HTPB-TDI are decreased to46.43and40.14kJ·mol-1by using TPB and TEPB.Both catalysts lower reaction temperatureand curing time through decreasing activation energy of curing reaction and acceleratingreaction rate.The reaction rate constant value of HTPB-TDI catalyzed by TEPB at34℃issimilar to the value of HTPB-TDI catalyzed by TPB at50℃.The catalytic activity of TEPB ishigher and it’s available for ambient catalyst.HTPB-TDI-TEPB is the ideal ambient curingsystem.In order to investigate the influence of PBX filler on curing reaction,Al powder and RDXwere added in HTPB-TDI curing system.The experimental result show that the Ea isincreased from51.29kJ·mol-1to73.97kJ·mol-1after Al powder was added.The curing periodis increased from3days to4.5days after RDX was added.The filler slows down the curingreaction rate.