In this study, we investigated the effect of heat treatment on the mechanical properties of high-tenacity polypropylene (PP) fibers. An application field of versatile polypropylene as fibers and tapes is the reinforcement of single-polymer composites. During consolidation at an elevated temperature, typically near the melt temperature of PP, the heat causes molecular relaxation of the strongly oriented molecular chains, which impairs mechanical properties. We investigated the shrinkage of PP single fibers isothermally and anisothermally, and heat-treated PP single fibers and multifilament rovings in a temperature range of 120-190 °C for 5-20 min in a constrained and an unconstrained arrangement. The heat-treated fibers and rovings were then tensile tested and their residual mechanical properties were determined and compared to the as-received rovings. We analyzed the tensile characteristics mathematically, applying the statistical fiber-bundle-cell modeling method, and described the measured and averaged stress-strain curves with fitted E-bundles having fibers with nonlinear tensile characteristics. The tensile modulus of the constrained fibers treated for 5 min decreased less in the whole heat treatment temperature range but considerably decreased further with increasing treatment time. Conversely, their tensile strength decreased only slightly, and treatment time had a minor effect up to 180-190 °C (above the melting temperature of the fiber). The results proved that constraining is a useful tool for preserving the reinforcing ability of high-tenacity polymer fibers.
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