Freeze-thaw durability of a partially bio-based unsaturated polyester composite reinforced with glass fibres

The pursuit for more sustainable fibre-polymer composites has motivated the development of renewably sourced bio-based resins. Although these can provide short-term performance similar to that of conventional composites, uncertainties about their long-term durability are still an important limitation. This study investigates the freeze-thaw (FT) durability of a partially bio-based unsaturated polyester resin (UPR) composite, incorporating fumaric acid, 1,3-propanediol, and isosorbide, and reinforced with glass fibres, comparing its performance to that of an oil-based counterpart with identical reinforcement. The composites were preconditioned by immersion in water at 20°C for 30 days, followed by exposure to 300 FT cycles. Each FT cycle consisted of 3 h of dry freezing at −20°C, 8 h of thawing submerged in water at 23°C, and 2.5 h for temperature transitions. Water uptake at 20°C was monitored for up to 180 days. The materials were assessed through mechanical (tensile, compression, in-plane shear, interlaminar shear) and thermomechanical (dynamic mechanical analysis) testing. Most degradation occurred during preconditioning, with property reductions of 20%– 40%, with higher incidence in the bio-based composite. FT ageing led to further but slower degradation in both composites. In-plane shear strength retention of the bio- and oil-based composite was 59% and 93%, respectively, after the preconditioning, which further reduced to 38% and 75% after 300 FT cycles. In comparison, the tensile modulus of both composites remained almost unaffected (∼4-8% reductions). The findings highlight the need for further optimisation of bio-based resin formulations to improve their environmental durability and enable wider structural application.