This work demonstrates the synergy between the thermo-mechanical induced degradation and the oxidation reactions in the kink-banded areas of ultra-high molar mass polyethylene (UHMMPE) fiber-based laminates used in body armor. While the EDS and FTIR results reveal high concentrations of oxygen containing products, the EPR results demonstrate the presence of the peroxyl radicals (RO2) and alkoxyl radicals (RO), in the kink-banded areas. The very long- lived RO2 and RO radicals were observed one year after the ageing experiment was discontinued. The total percentage of crystallinity, as measured by differential scanning calorimetry, of the kink-banded fibers was unchanged, indicating that the degradation occurs only in the amorphous region, and may also involve recrystallization processes of the degraded chains. However, the most abundant orthorhombic crystalline phase decreases significantly from 62 % to 46 %. This decrease in the orthorhombic structure leads to more diffusion of oxygen into the kink-banded region, enhancing the oxidation processes. No changes are observed in the monoclinic phase of the kinked fibers, which remained constant and constituted ~2 % of the total crystallinity.
Polymer Degradation and Stability
UHMMPE fibers, kink bands, body armor, free radicals, oxidation, degradation