Polyethylene Terephthalate-Based Composites with Recycled Flakes and Chemically Resistant Glass Fibres for Construction

This study aims to evaluate the influence of glass fibre (GF) type and content on the thermal, mechanical, and morphological properties of polyethylene terephthalate (PET)-based composites containing post-consumer PET flakes, intended for mouldings exposed to cementitious environments (e.g., anchors). Two chemically resistant GFs were compared: alkali-resistant (AR) fibres with soft sizing (SGFs) and electrical-/chemical-resistant (ECR) fibres with hard sizing (HGFs). Composites with fibre contents of 10-60 wt.% were prepared, with detailed analysis focused on 30 to 50 wt.%-the range typical for practical applications. AR fibres experienced greater shortening during processing, and their actual fibre content was lower than the intended value. Differential scanning calorimetry (DSC) revealed enhanced crystallisation kinetics for SGF composites, with higher crystallinity of the injection-moulded samples and elevated crystallisation temperatures (206-208 °C for SGF vs. 196-197 °C for HGF). Thermogravimetric analysis (TGA) indicated that thermal stability was primarily governed by fibre content; both SGF and HGF composites showed improved resistance compared to neat PET. Mechanical tests demonstrated that both fibre types effectively reinforced the matrix: HGF composites exhibited a higher elastic modulus (20.6 GPa for HGF-50 vs. 18.3 GPa for SGF-50), whereas SGF composites exhibited roughly 10-20% higher tensile, flexural, and impact strength, despite slightly lower fibre content. SEM analysis confirmed more uniform fibre distribution and fewer voids in SGF composites. These results highlight the importance of GF selection for PET composites in alkaline environments, taking into account the properties of the sizing film former and balancing trade-offs between mechanical performance, thermal stability, processability, and economic factors.