Ultraviolet-Induced Surface Modification of Virgin and Recycled Polypropylene Fibers: Modification Technology, Functionalization Mechanism and Its Application in Cement-Based Materials

Virgin and recycled polypropylene fibers (PPF and rPPF) can reinforce cement-based materials, but their surface inertness limits interfacial bonding. This review examines ultraviolet (UV)-induced surface modification, linking changes in surface chemistry, morphology and wettability with fiber integrity, interfacial behavior and composite performance. UV-induced photo-oxidation introduces polar functional groups and modifies surface morphology, potentially improving fiber–matrix compatibility and mechanical performance. However, these benefits depend on irradiation conditions, fiber characteristics and matrix composition. In one cement-paste study using 2 vol.% PPF, UV treatment increased 28-day compressive strength at room temperature by 24% and residual compressive strength after exposure to 200 °C by 18%, relative to specimens containing the same dosage of untreated PPF. For rPPF, pre-existing oxidation and recycling-related variability require treatment conditions tailored to its initial state. Excessive irradiation may degrade fiber properties, highlighting the need to balance surface functionalization with mechanical integrity. Limited mechanistic evidence, long-term performance data and standardized protocols remain key challenges. This review identifies the potential and limitations of UV modification and outlines research needs for its application to virgin and recycled polypropylene fibers in cement-based materials.