Abstract

The unexpected failure of soft body armor composed of polybenzoxazole (PBO) fibers ushered in an intense investigation to identify the mechanisms associated with the apparent reduction in ballistic performance of PBO fibers that comprised the vest. For the case of PBO fibers acid catalyzed hydrolytic degradation, associated with the presence of residual phosphoric acid from the manufacturing process, has long been suspected of reducing the mechanical properties of the PBO fibers by attacking the benzoxazole ring found in the polymer chain. However, this suspicion was only supported by data very recently. It has been found for the first time that the 25 % by mass extractable phosphorus detected by previous researchers was a phosphorus processing aid used in the manufacturing process. Furthermore, using model compound studies, the remaining non-extractable phosphorus was found to be chemically bound to the PBO polymer structure as a mono-aryl phosphate ester. This type of phosphate ester was found to be resistant to caustic hydrolysis that is usually used in the manufacturing process.

Degradation associated with folding that occurs during normal wear was also found to be a factor. Using a device designed to impart localized folding damage, the strain to failure and ultimate tensile strength of PBO fibers were found to be reduced. In tests designed to simulate 6 months and 10 years of wear, the strain to failure of PBO fibers were reduced by 14 % and 40 %, respectively (Kim 2008). These results coupled with analyses of field returned vests showed that the folding damage creates localized regions in the vest that are weaker than the damage caused by hydrolytic action (Holmes 2009). Similar tests on poly-paraphenylene terephthalamide (Kevlar type) fibers indicate that folding damage is much less severe than observed in PBO fibers.

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