Degradation mechanisms of fibre reinforced composites manufactured using different resin and fibre types under solar ultraviolet radiation

This research examines the degradation mechanisms of single-yarn composites manufactured with three resin systems (epoxy, bio-epoxy, and vinylester) and four fibre types (carbon, glass, basalt, and flax) after 2000 h of solar ultraviolet (UV) exposure at 60°C. Neat resins, bare fibres, and composites were characterised through physical, thermal, chemical, mechanical, and microstructural analyses, including surface morphology, tensile testing, differential scanning calorimetry, FTIR, EDS, and SEM. UV exposure caused mass loss, surface smoothing, and yellowing in all resin systems, accompanied by increased glass transition temperature, indicating oxidative stiffening. The most severe degradation occurred in bio-epoxy, which retained only 28% of its original tensile strength, followed by epoxy (40%), whereas vinylester showed the highest resistance with 85% strength retention. FTIR and SEM revealed pronounced chain scission, ether bond cleavage, and extensive cracking in bio-epoxy, while EDS indicated the photo-oxidative degradation in all resins. Among fibres, carbon exhibited the highest tensile strength retention (97%), followed by basalt (75%) and glass (66%), whereas flax was the most susceptible to degradation (43%). Interfacial shear strength was governed primarily by resin type rather than fibre type, with vinylester composites retaining 65–76% of their interfacial strength compared with 28–49% for bio-epoxy and 35–46% for epoxy. Overall, within the investigated single-yarn composite configuration, matrix degradation exerted a stronger influence on IFSS retention than fibre type. Among the specific grades of commercial resin systems, the selected vinylester formulation exhibited the best overall durability under UV exposure, whereas selected bio-epoxy systems showed the highest susceptibility to degradation under present exposure conditions.