Hydrolytic stability of polybenzobisoxazole and polyterephthalamide body armor

Previous work conducted at the National Institute of Standards and Technology (NIST) to investigate the field failures of soft body armor containing the material poly(p -phenylene-2,6-benzobisoxazole), or PBO, revealed that this material was susceptible to hydrolysis, and a mechanism of this hydrolysis was proposed. In this work, viscometric estimations of the molar mass of environmentally conditioned PBO are used to support a previously proposed mechanism of PBO hydrolysis. Results with PBO were compared with poly(p-phenylene terephthalamide), or PPTA, which has been used in body armor applications for more than 30 years. Losses in tensile strength were found to correspond to a reduction in molar mass for PBO. This indicates that chain scission due to complete hydrolysis is occurring in this material. Similar trends were observed for PPTA, but the relationship between molar mass reduction and losses in tensile strength was not as evident for this material. Confocal microscopy, mechanical properties measurements, and molecular spectroscopy are used to further investigate the degradation of both PBO and PPTA.