Long-term natural ageing effects on the structural, mechanical, and ballistic performance of UHMWPE-based soft ballistic inserts

• Aging of ballistic vests in natural use for a period of 16 and 18 years. • Polyethylene inserts retain ballistic properties for up to 10 years in usage. • Tensile strength and dimensions stayed within tolerance during natural ageing. • Investigations revealed progressive oxidative changes in polyethylene over time. • Electron microscopy showed only minor microstructural changes in aged fibres. The aim of the study was to evaluate the degradation mechanisms of ultra-high molecular polyethylene (UHMWPE) fibres used in soft ballistic inserts manufactured in 2003 and 2005 and used and stored in conditions typical for this type of products. The research focused on the identification of changes in the structure of polymers (using FT-IR, DSC, SEM techniques), mechanical behavior and verification of ballistic resistance. Structural studies showed degradation of the polymer based on FT-IR and DSC analysis. Progressive oxidative changes in the polymer matrix were shown (carbonyl index, which is the absorption coefficient of C=O and C-H bonds, raised to approx. 0.4). SEM analysis of UHMWPE fibers showed microstructural changes, whereas mechanical tests have shown stable elongation at maximum strength of individual ballistic inserts layers, and ballistic tests have confirmed that UHMWPE fiber based interests provide effective protection over more than 10 years of standard use or storage. The research provided new insights into the ageing effects of fibrous UHMWPE ballistic materials based on chemical, microstructural, mechanical and ballistic data on ballistic systems subjected to long-term exposure to environmental agents