Seawater-immersed glass-polyester composites with optimized mechanical properties

Composites are engineered materials widely used in marine applications due to their excellent durability, strength, practicality, and efficiency. However, prolonged exposure to seawater can cause irreversible degradation. In this study, the maximum stress, strain at break, and tensile modulus of composite materials made from unsaturated polyester reinforced with glass fiber (CMPG) are investigated. Tensile testing in accordance with EN ISO 527 was performed on samples fabricated with 1-layer and 3-layer fiber configurations and varying fiber areal densities (300, 450, 500, and 600 g/m²). Seawater immersion times of 3, 21, and 30 days were applied to assess environmental effects. The results showed that the mechanical properties deteriorated with increased immersion time. For example, the maximum stress decreased by up to 28% after 30 days of seawater immersion compared to dry samples. The 3-layer samples exhibited higher tensile strength than 1-layer samples across all conditions. Additionally, samples with higher fiber content (e.g., 600 g/m²) absorbed less seawater and showed less dimensional swelling than lower-density counterparts. The thickness of 1-layer composites increased from 2.00 mm to 2.81 mm after 30 days, while 3-layer samples increased up to 2.95 mm, primarily due to seawater diffusion into voids. A multiple regression model was developed to predict mechanical properties as a function of fiber number, areal density and immersion time, achieving high accuracy (R² > 0.94). Analysis of variance (ANOVA) confirmed that immersion time had the most statistically significant effect (p < 0.01) on tensile performance. FTIR analysis revealed chemical degradation and shifts in functional groups due to prolonged seawater exposure. These findings confirm that CMPG composites are susceptible to environmental degradation and a careful material selection and surface treatments are essential for long-term marine applications.