University of Massachusetts - Amherst Amherst United States
Polyp-phenylene-2,6-benzobisoxazole PBO is an engineering fiber of extraordinary properties, but has been found to degrade quickly under moisture and ultraviolet light, which is a fatal flaw for soft body armor applications. The present report uses 31P solid-state nuclear magnetic resonance ssNMR spectroscopy to investigate the chemical structure and physical state of the residual phosphorous in PBO fiber, which has been long suspected to hydrolyze the benzoxazole bonds of PBO chains and cause the degradation. The residual phosphorous is found to be in the form of phosphoric acid PA and its various forms of hydrated or dehydrated derivatives. Moisture would drive the reaction along the direction of P2O5 - polyphosphoric acid - PA - water-bound PA, while increasing temperature would reverse the direction. Hydration increases the mobility of PA molecules and increases its damaging power, while higher temperature removes water and immobilizes PA. The residual P species are easily accessible by gas molecules but inaccessible by aqueous media. These studies provide direct evidence to confirm the proposed environmental degradation mechanisms in the literature, and point out future directions for mitigation of this crippling problem for PBO fiber, the otherwise superior fiber of choice for soft body armor applications.