Long Term Stability and Implications for Performance of High Strength Fibers Used in Body Armor
Forster, Amanda Lattam
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The objective of this work is to examine the relationship between structure (both molecular and morphological structure) and properties of high strength fibers. The superior performance of the high strength fibers is predicated on the development of a highly aligned molecular structure that allows the polymer to exhibit a superior strength in the axial direction of the fiber. Armor manufacturers have exploited the inherent strength of these materials to develop body armor that continues to defeat ever-increasing threats. However, even an ideal molecular structure will be subjected to a potentially hydrolytic or oxidative environment during use, which can reduce the high strength of these fibers, and impact their ability to protect the wearer. The effect of the wear environment on the molecular structure, which is responsible for the high strength of these fibers, has not been well understood by the scientific community. In this work, the chemical mechanisms of degradation were investigated at the molecular level to understand the effect of the environmental conditions on crystallinity, orientation, and molecular weight. The chemical mechanism and kinetics elucidated from these measurements are used to understand the reduction in strength of these materials after degradation. Hydrolysis was found to be the predominant mechanism of degradation for polybenzobisoxazole and goes to irreversible chain scission. Hydrolysis is also the primary mechanism of degradation for aramid fibers. Ultra-high molecular weight polyethylene (UHMWPE) fibers undergo an oxidative mechanism of degradation, and the activation energy for this mechanism was calculated. Additionally, the release of acids from aramid copolymer fibers, and the performance of these fibers in hydrolytic and thermooxidative environments were studied to determine that hydrolytic degradation is the predominant degradation mechanism for these fibers. Exploratory research was also performed in an effort to improve the stability of UHMWPE fibers by using radiation to crosslink the UHMWPE fibers and increase the temperature of their alpha relaxation. However, this radiation treatment was still found to reduce the overall tensile strength of these fibers. In summary, the wear environment and vulnerabilities of a material to degradation are essential when selecting materials or developing new materials for use in body armor.