Review article

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Polymer composites are widely used in indoor and outdoor applications owing to their high strength-to-weight ratio, corrosion resistance, and design flexibility. However, exposure to environmental factors such as ultraviolet (UV) radiation, humidity, temperature fluctuations, and biological agents significantly affect their long-term durability and performance. In this review, we comprehensively examine the degradation mechanisms of polymer composites under both natural and accelerated weathering, with particular emphasis on degradation mechanisms, characterization techniques, and durability assessment. We critically discuss the fundamental degradation mechanisms, including photo-oxidation, hydrolysis, polymer chain scission, crosslinking and interfacial debonding. We also evaluated accelerated weathering techniques, conducted under standardized testing conditions, for their ability to simulate long-term outdoor exposure within significantly shorter timeframes, by controlling key environmental parameters such as UV radiation, humidity, and temperature. We also examine the influence of reinforcements, including natural fibres, synthetic fibres, and nanofillers, on the weathering resistance of polymer composites. Attention is given to natural fibre–reinforced composites because their inherent hydrophilicity promotes moisture absorption, accelerating degradation and weakening the fibre–matrix interface. Furthermore, we critically evaluate the effects of weathering on the mechanical properties, crystalline structure and barrier performance of polymer composite films, and discuss recent advances in enhancing weather resistance through hybridization, surface modification, and the incorporation of stabilizing additives. Finally, the review highlights the key challenges and future research directions for the development of durable, high-performance, and sustainable polymer composite systems for long-term outdoor applications.