The results of following the oxidative degradation of a plastic-bonded explosive (PBX 9501) are reported. Into over 1100 sealed containers were placed samples of PBX 9501 and combinations of its components and aged at relatively low temperatures to induce oxidative degradation of the samples. One of the components of the explosive is a poly(ester urethane) polymer and the oxidative degradation of the samples were following by measuring the molecular weight change of the polymer by gel permeation chromatography (coupled with both differential refractive index and multiangle laser light scattering detectors). Multiple temperatures between 40 and 64 degrees C were used to accelerate the aging of the samples. Interesting induction period behavior, along with both molecular weight increasing (cross-linking) and decreasing (chain scissioning) processes, were found at these relatively mild conditions. The molecular weight growth rates were fit to a random crosslinking model for all the combinations of components. The fit rate coefficients show Arrhenius behavior and activation energies and frequency factors were obtained. The kinetics of molecular weight growth shows a compensatory effect between the Arrhenius prefactors and activation energies, suggesting a common degradation process between PBX 9501 and the various combinations of its constituents. An oxidative chemical mechanism of the polymer is postulated, consistent with previous experimental results, that involves a competition between urethane radical crosslinking and carbonyl formation. (C) 2009 Elsevier Ltd. All rights reserved.

The results of the constituent aging study (CAS) are given, where low-temperature (T < 64 degrees C) aging experiments were performed on over 1100 closed-container samples of various combinations of the components of the plastic-bonded explosive PBX 9501. Experiments were performed on the various combinations both in the absence and presence of free-radical stabilizers. The product gases were identified and quantified as a function of aging time. The gas data show diverse chemistry between CAS samples and initial linear increases in product gas formation. Temperature analysis of the initial production rates of gas products shows straight Arrhenius plots. The extracted activation energies and frequency factors for the formation of the individual gas products yield a single linear kinetic compensation plot suggesting a common degradation pathway for PBX 9501 and combinations of constituents that contained nitroplasticizer (a eutectic mixture of bis-2,2-dintropropyl acetal and bis-2,2-dintropropyl formal).

Estane (R) 5703 is a segmented poly(ester urethane) that binds HMX explosive crystals together and provides mechanical integrity to the composite plastic-bonded explosive PBX 9501. A mixture of bis-2,2-dinitropropyl acetal and formal is used to plasticize the Estane. Upon heating, this nitroplasticizer (NP) loses NO2 groups and produces oxidizing species that can react with the urethane links of Estane. We report on aging studies of the oxidative degradation of Estane/NP/HMX mixtures for multiple temperatures between 40 degrees and 85 degrees C. Product gases (such as N2O, NO, N-2 and CO2) were identified and quantified as a function of aging time. Changes in molecular weight of the Estane as a function of aging time were measured by gel permeation chromatography (GPC). Oxidation products of both non-enriched and isotopically enriched Estane and Estane analogues were identified by H-1 NMR and LC/MS techniques and quantified as a function aging time. The gas product evolution rates, Estane crosslinking rates from GPC, and oxidation rates of Estane from NMR are compared.

Hydrolytic degradation data from a wide variety of experiments on Estane 5703, a commercial poly(ester urethane), are modeled with an A(AC)2 mechanism of reversible esterification and hydrolysis together with a cluster model for water concentration. The experimental conditions spanned a range of temperatures from 21 to 95degreesC, relative humidities from 0 to 100%, times up to more than 30 years, and different initial molecular weights and acidities. In addition, the experiments were performed in several different laboratories with a number of different instruments and techniques. All the experimental data, both for molecular weight and acidity, are modeled with a single set of concentration-independent rate coefficients, and the kinetic model fits the data well and constitutes a robust model of the hydrolytic degradation of this polymer. The results also show that ester hydrolysis is the dominant cause of the molecular weight loss of Estane 5703 in indoor storage at ambient temperatures and humidities. (C) 2003 Wiley Periodicals, Inc.