Macro- and Micromechanical Deformations and Failure of Recycled PET/Flax Polyester Biocomposites

Environmental sustainability has become a global priority aimed at reducing waste generation and dependence on fossil resources. This study reports on the development of sustainable biocomposites by combining flax woven fabrics (2, 4, and 6 plies) with UPRs synthesized from recycled polyethylene terephthalate (rPET) via glycolysis promoting resource recycling and renewability. The curing kinetics of the rPET resin were optimized by thermal analysis, correlating glass transition temperature with the degree of crosslinking using the Prout–Tompkins and DiBenedetto models. Laminates were fabricated via the vacuum bagging technique and their macro- and micro-mechanical behavior was characterized thoroughly. Acoustic emission, dilatometric tests, and SEM analysis revealed debonding as one of the main failure mechanisms of the composites, explained by poor fiber–matrix adhesion. Besides the debonding of fibers oriented parallel to the external load, the fracture of fibers oriented perpendicularly also influences mechanical properties, mostly strength and impact resistance. This novel experimental approach describes deformation processes in flax/rPET composites by coupling interactions and micromechanical deformations with macroscopic properties reaching stiffness values of around 7 GPa for the six plies composite. The results prove that rPET UPRs are a viable solution for reducing the use of fossil resources without compromising end-use properties.