Effect of moisture on copolymer fibers based on 5-amino-2-(p-aminophenyl)-benzimidazole

In recent years, there has been concern in the soft body armor community that copolymer fibers based on 5-amino-2-(p-aminophenyl)-benzimidazole can release hydrochloric acid, which is present in these fibers as a by-product of the manufacturing process. The presence of acids could potentially be detrimental to other fibers that might come in contact with these materials. In an effort to examine this issue, a study was designed to investigate the release of acid in different environments from these fibers. During the first phase of the study, fibers were exposed to water and pH decreases were observed. While immersed in deionized water, two of the fiber samples studied released a sufficient amount of acid to drop the pH of the solution from approximately pH 6.0 to approximately pH 3.0 in less than 10 d at room temperature. Further ion-selective electrode studies of chloride ion released from these fibers indicated that hydrochloric acid may not be the species responsible for this pH reduction. In a second phase of the investigation, fibers were exposed to water vapor in an elevated temperature environment (conditions were 65 °C, 80% RH). While the pH reduction released by the water vapor exposure was substantially less than observed in the submersion phase, a reduction in the yarn tensile strength of some of the fibers was observed during this phase of the study. In a third phase, fibers were exposed in a dry oven (less than 5% RH) at 65 °C. Almost no pH reduction or strength reduction was observed. Molecular spectroscopy was also performed to better understand the effect of elevated temperature and moisture environments on these fibers.