Temperature and humidity aging of poly(p-phenylene-2,6-benzobisoxazole) fibers: Chemical and physical characterization
Section snippets
Introduction and objectives
In recent years, high performance fibers based on poly(p-phenylene-2,6-benzobisoxazole) (PBO) have become prominent in high strength applications such as body armor, ropes and cables, and recreational equipment. Problems with the performance and durability of PBO-based body armor became apparent to the law enforcement community in the summer of 2003, when ballistic penetration of PBO-based body armor occurred in two separate incidents.
Data from the manufacturer and the published literature have
Materials
Commercial, non-deployed body armor manufactured for this study consisted of a ballistic panel constructed from 20 layers of woven PBO fabric stacked and diagonally stitched together, inserted in a water vapor-permeable fabric liner, and placed in a cotton fabric carrier. For testing and analyses, PBO yarns were extracted from the exposed vest panels at predetermined inspection times.
Environmental exposure
Twenty-four ballistic panels enclosed in liners and carriers (shown in Fig. 1) were aged in a Cincinnati
Results and discussion
PBO is a member of the benzazole polymer family and is characterized by the heterocyclic benzobisoxazole group in its main chain structure. The chemical structure of the PBO repeat unit is shown in Fig. 4. The conjugated benzobisoxazole and phenyl rings in the PBO repeat unit contribute to extended π-electron delocalization and molecular rigidity, which provides high thermal stability and outstanding mechanical property. PBO fibers, as are the majority of the polybenzazole-based fibers, are
Summary and conclusions
Tensile strength of yarns extracted from ballistic panels aged at elevated temperature and relative humidity exhibited approximately 40% loss in strength over a 157 d period. FTIR analysis of yarns exposed to elevated temperature/relative humidity indicated benzoxazole ring opening to yield amide functionality, and subsequent hydrolysis of amide to carboxylic acid and aminophenol groups. Analysis of the fiber surface morphology indicated a surface roughening as a function of exposure time. SANS
Acknowledgements
The authors gratefully acknowledge project funding provided by the National Institute of Justice (NIJ) as well as the assistance of Ron Rise and Steve Lightsey of the Tekne Group in tensile testing, Nathaniel Waters of NIST's Office of Law Enforcement Standards (OLES) in setting up the temperature/humidity chamber exposures, and Jeff Anderson of the Fabrication Technology Division for preparing the sealed tube samples.
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