Low-temperature oxidative degradation of PBX 9501 and its components determined via molecular weight analysis of the Poly[ester urethane] binder
Introduction
This paper is the sixth in a series attempting to understand the aging and degradation of the Plastic-Bonded eXplosive (PBX) 9501. PBX 9501 is an explosive that is by weight nominally 94.9% HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane), 2.5% Estane® 5703 [a poly(ester urethane), subsequently referred to as Estane and shown in the top of Fig. 1], 2.5% nitroplasticizer (NP, a 1:1 mix of bis-2,2-dintropropyl acetal (BDNPA) and bis-2,2-dintropropyl formal (BDNPF), shown in the bottom of Fig. 1), and 0.1% stabilizing agent, typically either diphenyl amine (DPA) or Irganox 1010. The NP/Estane mixture is known as the binder and is used to decrease the mechanical sensitivity of the explosive so that it may be pressed and machined into parts. PBX 9501 has many military uses and, thus, is subjected to many storage environments. Our work has been to perform experiments submitting PBX 9501 and its components to diverse environmental conditions (temperature and relative humidity) in order to induce aging of the samples and model the aging [1], [2], [3], [4], [5]. Studies from very low to very high relative humidities, room temperature to 70 °C, have been performed in order to elicit differing degradative pathways that attempt to get as close as possible to ambient conditions. To our knowledge, these studies are the lowest temperature oxidation/hydrolysis degradation studies of HMX-based explosives. The results of the previous studies show that Estane may degrade by both molecular weight decreasing and increasing pathways. Hydrolysis was explored as a means of decreasing the molecular weight. In these studies, the sorption and diffusion of water in PBX 9501 and its components were measured and modeled [4]. An ester hydrolysis mechanism was fit to the aged Estane molecular weight data. Both the water sorption and hydrolysis models were put into a master chemical kinetics code for modeling multi-temperature hydrolysis and the repairing esterification reverse reaction. The model fit well the hydrolysis accelerated aging experiments of Estane and another poly(ester urethane) under diverse temperature and relative humidity regimes [2], [3].
Recently, we have been investigating the molecular weight increasing (crosslinking) and decreasing (chain scissioning) mechanism of chemical oxidation. In principle, this means of degradation is much more complex than hydrolysis. Very reactive gas-phase species are released by the nitroplasticizer as it undergoes the initiating degradative processes [6]. These gas-phase species may undergo free radical reactions with the heterogeneous polymer environment or they may diffuse through the polymer and undergo reactions in the gas phase. The chemistry of the oxidation of PBX 9501 and its components were studied by examining the gas-phase products released during oxidation. These results are reported in Ref. [5] and show relatively few numbers of gas-phase products were formed at significant and reproducible concentrations, that the nitroplasticizer-containing samples release much larger amounts of these gas-phase products at the low temperatures used, and that the multi-temperature kinetics can be connected by a simple kinetic relation, thus suggesting a common degradative pathway of formation. This is significant since these studies were performed in an oxygen-free environment, which was designed to mimic the environment that the explosive experiences in practice. Usually polymer oxidation is thought to proceed through the propagation of a peroxy chain reaction; however, in the case of these experiments, the nitroplasticizer provides the oxidizing species and the degradation can be thought of as “NP-driven”. Recent electronic structure calculations investigated the decomposition of NP [6]. A likely mechanism for NP decomposition is that NO2 groups dissociate from the NP molecules producing oxidizing species, which can oxidize Estane.
Several studies over the last decade have been initiated to better understand the oxidation chemistry of PBX 9501 [6], [7], [8], [9], [10], [11], [12]. Electron spin resonance (ESR), Gel permeation chromatography (GPC), Nuclear magnetic resonance (NMR), and Infrared (IR) techniques have been employed to follow the X-ray irradiated and thermally induced oxidation of the Estane/NP binder of PBX 9501. In these studies it has become clear that the diphenylmethylene of the Estane polyurethane hard segment (CH2 group in upper part of Fig. 1) readily loses a hydrogen to form a relatively stable diphenylmethyl radical. This radical may then undergo chemical crosslinks with surrounding hard segments to rapidly increase the molecular weight of the Estane. The rate of the crosslinking was measured by proton NMR at temperatures between 65 and 85 °C by comparing the growth of a new N–H NMR peak and the collapse of the phenyl protons in Estane's methyl diisocynate (MDI) hard segment [12]. The multi-temperature rates were fit using 1st order kinetics and an activation energy of 25.5 kcal/mol was obtained. Estane's polyester soft segments were also shown to undergo oxidation chemistry; however, these tend to show the formation of peroxy radicals when exposed to oxygen and result in molecular weight decreasing chain-scission reactions. Thus, the results of these studies are that oxidation is more complex than hydrolysis and may lead to both molecular weight increasing or decreasing processes that are dependent on oxygen concentration.
Section snippets
Overview
The Constituent Aging Study (CAS) was performed at the Pantex Plant in Amarillo, TX with the aim of performing a systematic study of the oxidative aging of PBX 9501 at low temperatures in order to provide data for the development of an oxidative aging chemical model. The temperature dependence of the degradation chemistry is also needed; thus, CAS samples containing PBX 9501 and mixtures of its components were submitted to multiple temperatures. Experiments were begun on March 10, 2000 when
Results
Shown in Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6 as a function of aging time are average values of the Mw(t)/Mw(0) ratio at 64 °C. The ratio of Mw(t)/Mw(0) is reported rather than Mw(t) in order to normalize the data so that both the RI and MALS data can be displayed in a single figure for the same CAS combinations. Fig. 2, Fig. 3, Fig. 4, Fig. 5 contain data from both the unstabilized and the Irganox-stabilized CAS combinations and Fig. 6 contains data for the Irganox-stabilized PBX 9501 CAS
Unstabilized
The neat Estane CAS 1 and Estane/HMX CAS 6 samples shown in Fig. 2, Fig. 3, respectively, show some initial growth in Mw and Mw decline after about 1 year at 64 °C, which suggests that both Mw increasing crosslinking and Mw decreasing chain scission reactions are taking place. All unstabilized NP-containing CAS samples show significant early Mw growth. Comparing the rates at 64 °C shows that the CAS 1 and CAS 6 samples have a much lower rate of Mw growth than either of the NP-containing CAS 9
Conclusions
The CAS experiments reported herein represent a systematic study of the low-temperature oxidation of PBX 9501 and its components. In this study it is seen that for most samples Mw undergoes early growth, reaches a maximum value and then may either decline quickly or remain around this maximum. The results reported herein corroborate the product gas formation results of Ref. [5] in that the Estane and Estane/HMX results are similar, but for those samples in which NP was present, much larger
Acknowledgments
We would like to thank Russ Pack and Pat Foster for help in the experimental design of the CAS studies. We also thank Gordon Osborn for the CAS sample formulations, Gail Watson, Jerry Bishop for the sample containers, and Judy Pitts for coordinating sample removal. For the analytical results, we thank Debbie Jones, George Howard IV, Susan Britten, Rebecca Kennon, and Michele McWilliams. We thank Denise Pauler, Sheldon Larson, Debra Wrobleski, Bruce Orler, and David Hanson for very helpful
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