Hot Spot Interaction with Hydroxyl-Terminated Polybutadiene Binder in Energetic Composites
K Joshi and S Chaudhuri, JOURNAL OF PHYSICAL CHEMISTRY C, 122, 14434-14446 (2018).
Binders play a pivotal role in solid propellants and polymer-bonded energetic composites by providing the means for safe transport, storage, and use. The fundamental role of binders is known qualitatively, but the exact role in the modification of chemistry and thermal transport is hard to quantify. Thermochemical response of a commonly used binder, hydroxyl-terminated polybutadiene (HTPB), in a model composite comprising of cyclotrimethylene trinitramine (RDX) in the presence of a thermal hot spot was studied using reactive molecular dynamics (RMD) simulations. The results from model interfaces between RDX and HTPB were analyzed to identify the unique role played by the binder phase. It was found that the binder phase provides a safety buffer by extending the time required for the hot spot to undergo ignition-to-deflagration transition. This delay is mainly due to the disparity in the heat capacities of HTPB and RDX. HTPB, which has a higher heat capacity than RDX, absorbs energy from the hot spot at a rate that is at least 4 times higher than the corresponding rate for RDX. RMD trajectory was mapped onto Eulerian control volumes to capture energy flux across a moving deflagration front. It was found that a deflagration wave created in RDX moves in HTPB at least 3 times slower than in RDX. Because of deflagration in RDX, HTPB is exposed to high temperatures (>2000 K) and high pressures (>10 GPa). At such high temperatures and pressures, thermal degradation of confined HTPB primarily occurs through chain- branching bimolecular reactions. The primary products of thermal degradation include heavy branched polymers (heavier than HTPB), cyclic molecules such as cyclohexene, cyclohexadiene, and cyclopentadiene, and linear molecules such as butadiene and transbutadiene oligomers. The results show that RMD simulations are capable of providing an in-depth understanding and qualitative picture of the events after thermal initiation in polymer-bonded explosives (PBXs). There is still scope for enhancing the quantitative accuracy of RMD predictions by further improving the force field parameters. Thus, RMD can play a significant role in understanding the role of binders in the intergranular regions of PBXs.
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