Abstract and subjects
The plastic-bonded explosive PBX 9501 is 94.9% by weight HMX explosive, 2.5% nitroplasticizer (NP), 2.5% Estane 5703, and 0.1% Irganox 1010 stabilizer (anti-oxidant). Estane 5703 is a poly(ester urethane) segmented copolymer which serves as a glue that binds the HMX crystals together to allow for machining of high precision high explosive parts. The urethane units segregate (phase separate) into hard domains that act as physical crosslinks between the soft polyester domains. (See Figure 1.) The NP softens the Estane, and together this markedly decreases the mechanical sensitivity of the PBX. However, the Estane slowly degrades in time, and the corresponding effect on the mechanical properties of the PBX can potentially raise safety and reliability concerns. Currently, we are studying the chemical mechanisms of degradation processes in PBX 9501 and developing kinetics models. An overarching goal is to relate the mechanical properties of the PBX to the chemical information gathered in our current studies. We are interested in understanding all possible degradation mechanisms (thermolysis, photolysis, hydrolysis, oxidation). The hydrolytic aging of the ester links in Estane 5703 and a chemical kinetic model has been reported previously. 1 In this paper we describe recent progress in the study of the free-radical oxidation of the urethane segments. Free-radical oxidation may be as important as hydrolysis, especially as the PBX dries out, since the storage environment has both low moisture and oxygen content. The nitroplasticizer (NP) is composed of a mixture of bis-2,2-dinitropropyl acetal (BDNPA) and formal (BDNPF). Various research efforts at Los Alamos and Pantex are showing that upon heating NP loses nitro (NO 2 ) groups and produces oxidizing species that can degrade Estane, (for example, the oxidation of the methylene bridge, left-hand dashed circle in Figure 1 ). A likely mechanism for NP decomposition is that nitro (NO 2 ) groups dissociate from the NP molecules and subsequently oxidize the carbon atoms in the backbone of the NP molecules as well as oxidize the hard segments in Estane. The oxidation of the Estane polymer can cause chain scission (decreased MW) and crosslinking (increased MW and gel formation), while small molecule remnants of the reactions can be detected in gas analyses. Such large changes in MW can have drastic effects on the mechanical properties of the elastomer.