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Methodology for Detecting Residual Phosphoric Acid in Polybenzoxazole Fibers

NCJ Number
Analytical Chemistry Volume: 81 Issue: 23 Dated: 2009 Pages: 9607-9617
Eun Su Park; John Sieber; Charles Guttman; Kirk Rice; Kathleen Flynn; Stephanie Watson; Gale Holmes
Date Published
11 pages

This paper reports on a study to investigate the presence of phosphoric acid in soft-body armor containing the ballistic fiber in order to determine the reasons for the material’s failure while in service, and to develop testing methodologies to ensure that those failures cease to occur.


Because of the premature failure of in-service soft-body armor containing the ballistic 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 this failure and to develop testing methodologies and protocols to ensure that these types of failures do not reoccur. In another report that focused on the stability of the benzoxazole ring that is characteristic of PBO fibers, the authors 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 may cause degradation of the benzoxazole ring resulting in a reduction of ballistic performance. Prior to this work, no definitive data have indicated the presence of phosphoric acid since the residual phosphorus is not easily extracted and the processed fibers are known to incorporate phosphorus containing processing aids. Methods to efficiently extract phosphorus from PBO are described in this article. Further, characterization determined that the majority of the extractable phosphorus in PBO was attributed to the octyldecyl phosphate processing aid with some phosphoric acid being detected. Analysis by matrix assisted laser desorption ionization of model PBO oligomers indicates that the nonextractable phosphorus is attached to the PBO polymer chain as a monoaryl phosphate ester. The response of model aryl phosphates to NaOH exposure indicates that monoaryl phosphate ester is stable to NaOH washes used in the manufacturing process to neutralize the phosphoric acid reaction medium and to extract residual phosphorus impurities. (Published Abstract Provided)