Microcalorimetric study applied to the comparison of compatibility tests (VST and IST) of polymers and propellants

Abstract

A few years ago, a discussion was started to update the test method for determining the chemical compatibility of ammunition components with explosives and propellants. According to this test and also a military test method, a reactivity test to establish the amount of compatibility of single- or double-base propellants with other materials by the vacuum stability test (VST) has been standardised. For several decades, the VST was the most commonly used test to determine the compatibility of propellants with other materials. However, the VST has some well-recognised drawbacks. For example, only processes that result in gas evolution can be measured. Processes such as migration and absorption, which are important for safety and performance considerations for propellants, are not taken into account in the test criteria. Therefore it was decided at the Prins Maurits Laboratory TNO to start a research project to obtain more information about aspects related to compatibility. As deterioration of materials is accompanied by the release or absorption of heat, the microcalorimeter test (IST) was a logical choice. Several combinations of polymers (to obtain a quick fingerprint) and propellants were tested in the VST and in the microcalorimeter, simultaneously, under isothermal conditions. Six propellants (single-base, double-base and DEGN propellants) and polymers were combined. The compatibility tests in the VST for the single-base propellants were performed over 40 h at a temperature of 100°C, while 40 h at 90°C was used for double-base propellants. In the microcalorimeter, the propellants were measured for a period of 168 h at a temperature of 85°C. The results of these tests present a few problems concerning the interpretation of the test results. It turns out that the absence of extra gas evolution in the VST gives no guarantee of compatibility as the heat generated in the microcalorimeter test can be very high for the same combination of materials. It is therefore recommended that an extra test in addition to the VST be performed to determine compatibility.