Temperature and humidity aging of poly(p-phenylene-2,6-benzobisoxazole) fibers: Chemical and physical characterization

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Abstract

In recent years, poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers have become prominent in high strength applications such as body armor, ropes and cables, and recreational equipment. The objectives of this study were to expose woven PBO body armor panels to elevated temperature and moisture, and to analyze the chemical, morphological and mechanical changes in PBO yarns extracted from the panels. A 30% decrease in yarn tensile strength, which was correlated to changes in the infrared peak absorbance of key functional groups in the PBO structure, was observed during the 26 week elevated temperature/elevated moisture aging period. Substantial changes in chemical structure were observed via infrared spectroscopy, as well as changes in polymer morphology using microscopy and neutron scattering. When the panels were removed to an ultra-dry environment for storage for 47 weeks, no further decreases in tensile strength degradation were observed. In a follow-on study, fibers were sealed in argon-filled glass tubes and exposed to elevated temperature; less than a 4% decrease in tensile strength was observed after 30 weeks, demonstrating that moisture is a key factor in the degradation of these fibers.

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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