Personal Body Armor

Mechanical Degradation

This year, data on poly(benzoxazole)(PBO) ballistic fibers obtained with NIST's automated fold tester have demonstrated that repeated folding decreases the material properties governing ballistic performance. Initial tests designed to simulate six months and 10 years of wear indicate performance reductions of 15% and 40%, respectively.  Preliminary small angle x-ray scattering(SAXS) data from these damaged fibers suggest that the transition from fibrillar failure to brittle failure (shown below) that accompanies repeated folding is associated with the formation of micro-cracks perpendicular to the fiber axis.

These results were supported by modified-single fiber test results from new, hydrolytically aged, and field returned vests. The back panel of a compromised vest indicates that in addition to the environmental degradation experienced by PBO ballistic fibers, folding of these fibers results in localized regions whose fiber properties are weaker relative to the fibers in the non-folded regions by 10% to 15%. In addition to this degradation mechanism, the fibers in the hoop direction of used vests were found to be even weaker, with strength losses between 20% to 30% relative to those in the non-folded regions. The source of this latter degradation mechanism is under investigation. 

Chemical Degradation

Residual phosphoric acid, left over from the ballistic fiber spinning process, has long been targeted as a potential root cause for chemical degradation of PBO fibers. To date, proof of this mechanism does not exist. However, key findings from our hydrolytic study indicate that the phosphorus detected by many researchers using simple aqueous or supercritical fluid extractions are phosphate esters added by the manufacturer to aid in processing of the fibers. We have further demonstrated by fluorescence spectroscopy, that 75% of the phosphorus remains in the fiber after aqueous extraction on fibers that have been cut and pounded.  

This hypothesis has been supported by model compound studies of the PBO reaction process that indicate that the phosphoric acid can react with the diamino-dihydroxy benzene compounds used to prepare PBO and form a stable aromatic phosphate ester (see figure below). 

These results suggest that the remaining phosphorus is not trapped in microvoids, but is chemically bound to the PBO chain structure and therefore not easily leachable. However, the bound phosphorus provides regions of very localized acidity that can accelerate fiber degradation.