TOWARDS AN UNDERSTANDING OF THE DEGRADATION MECHANISMS OF UHMWPE-BASED SOFT BALLISTIC INSERTS
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The objective of this work is to advance the field of lightweight and soft ultra-high molecular weight polyethylene (UHMWPE) inserts used in ballistic resistant-body armor, through the evaluation of chemical and physical degradation, and provide critical insight into the mechanisms involved. These inserts are comprised of non-woven UHMWPE fibers, foil-matrix low density polyethylene (LDPE), and a binder resin. Degradation of these components can be initiated by mechanical stress induced by routine use of the armor, thermal exposure due to storage and wear, and exposure to humidity and oxygen. Degradation of this system may include C-C and C-H bond ruptures resulting in C-centered radicals, thermo-oxidative reactions, as well as changes in the degree of crystallinity and the crystalline morphology of the UHMWPE fibers. This is the first comprehensive study on degraded UHMWPE-fibers extracted from body armor that have been subjected to accelerated aging. Previous studies have only focused on oxygen uptake and changes in the tensile strength of virgin UHMWPE fibers as markers of degradation. This work extends beyond oxygen uptake, to examine changes in the topography, the degree of crystallinity, and the crystal phases of UHMWPE fibers. Mechanical stress was found to be the main cause of kink band formation in UHMWPE fibers. Additionally, oxidation products and molecular oxygen were found to be at higher concentrations in the kink bands compared to other parts of the fiber. This suggests a synergistic effect between mechanical stress induced kink bands and oxidative degradation. The degree of crystallinity of the fibers did not change significantly, however morphological changes of the crystalline phases and changes in the orientation of the crystals were observed. Finally, this study investigates, for the first time, the degradation of the binder material that retains the fibers together in the laminates. The binder resin used in the laminates was identified to be a copolymer of polystyrene and polyisoprene, which undergoes oxidative degradation accompanied by a decrease in the weight-average molecular weight.