Because of the premature failure of soft-body armor that contains the active fiber poly [(benzo-[1,2-d:5,4-d]-benzoxazole-2,6-diyl)-1,4-phenylene] (PBO), the Office of Law Enforcement Standards (OLES) at the National Institute of Standards and Technology (NIST) initiated a research program to investigate the reasons for the failures do not reoccur. In a report that focused on the stability of the benzoxazole ring that is characteristic of PBO fibers, Holmes et al. (2006) showed that the benzoxazole ring was susceptible to hydrolytic degradation under acid conditions. Because of the processing conditions for the fibers, it is suspected by many researchers that residual phosphoric acid causes the degradation of the benzoxazole ring that result in a reduction in ballistic performance. Prior to this work, no definitive data has been given to indicate the presence of phosphoric acid since the residual phosphorus is not easily extracted and the processed fibers are known to be laced with phosphorus containing processing aids. In this report, the majority of the extractable phosphorus in PBO was found to be due to the octadecyl phosphate processing aid with some phosphoric acid being detected. The phosphorus that is not easily extracted is attached to the PBO polymer chain as an aryl phosphate ester that is stable to the NaOH washes that neutralize the phosphoric acid reaction medium and attempts to remove phosphorus impurities.
Citation: NIST Interagency/Internal Report (NISTIR) - 7573
NIST Pub Series: NIST Interagency/Internal Report (NISTIR)
Pub Type: NIST Pubs
Ballistic fibers, Polybenzoxazole, Phosphoric acid