Synergistic Effects of UV Radiation and Water Immersion on the Photo-Oxidative Degradation and Thermal Stability of CFRP Composites

Abstract The photo-oxidative degradation of the epoxy matrix is a factor limiting the long-term environmental stability of carbon fiber reinforced polymer (CFRP) composites. This study investigates the physicochemical and thermal degradation mechanisms of DGEBA-based CFRP laminates exposed to prolonged (1000 h) ultraviolet radiation. To evaluate the synergistic effects of environmental factors, standard photoaging (PN-EN ISO 4892-2) was compared with a novel in-house method utilizing narrow-band UV radiation (UVA, UVB, UVC) in both dry and continuous underwater immersion conditions. ATR-FTIR and Raman spectroscopy revealed extensive photo-oxidation of the epoxy matrix, evidenced by the formation of carbonyl-containing moieties (ketones and carboxylic acids) and the deterioration of the aromatic skeleton. While the flexural and tensile strengths remained largely unaffected due to the shielding effect of the carbon fibers, the chemical degradation of the matrix led to severe surface damage, resin microcracking, and subsequent fiber exposure. Crucially, dynamic mechanical analysis (DMA) demonstrated a significant decrease in the glass transition temperature (Tg), from approximately 98 to 92 °C (a 6% drop) after 1000 h of exposure, indicating a reduction in the polymer network’s cross-linking density and thermal stability. Under the coupled action of UV radiation and water immersion, a specific evolution mechanism of material property degradation was identified: UV-induced chain scission caused severe surface microcracking, which compromised the matrix barrier and facilitated deeper moisture penetration into the composite. This synergistic effect deteriorated the fiber-matrix interface, leading to a noticeable reduction in interlaminar shear strength (ILSS). While the macroscopic flexural and tensile strengths remained largely unaffected due to the shielding effect of the carbon fibers, the chemical degradation of the matrix led to severe surface damage and fiber exposure. These findings highlight that UV exposure fundamentally limits the high-temperature operational range of CFRPs, proving that Tg and interfacial properties are significantly more sensitive and reliable indicators of early-stage photodegradation than conventional static mechanical testing.