Freeze–Thaw Durability of Basalt Fibre Reinforced Bio-Based Unsaturated Polyester Composite

This paper presents an experimental study of the wet freeze–thaw (FT) durability of a fibre–polymer composite produced by vacuum infusion using an innovative bio-based unsaturated polyester resin (UPR) and basalt fibres. As the benchmark, an equivalent composite produced with a conventional (oil-based) UPR was also tested. The composites were preconditioned in water immersion for 30 days at 20 °C followed by exposure to wet FT for up to 300 cycles; each FT cycle consisted of 3 h in dry freezing condition (−20 °C) and 8 h in thawing condition (23 °C) submerged in water. The composites’ properties were assessed after preconditioning and after 100, 200, and 300 FT cycles, through mechanical (tensile, compressive, in-plane shear, interlaminar shear) and thermomechanical (dynamic mechanical analysis) tests. Gravimetric and scanning electron microscope analyses were also carried out. The results obtained show that the preconditioning stage, involving water immersion, caused most of the damage, with property reductions of 5% to 39% in the bio-composite, while in the oil-composite they ranged between 4% and 22%, being higher for matrix-dominated properties. On the other hand, FT alone had an insignificant effect on the degradation of material properties; after exposure to FT, property recovery was observed, specifically in matrix-dominated properties, such as interlaminar shear strength, which recovered by 12% in the bio-composite during exposure to FT. The overall performance of the bio-composite was inferior to the conventional one, especially during the preconditioning stage, and this was attributed to the hydrophilicity of some of the components of its bio-based resin.