In recent years, hybrid polymer/POSS (Polyhedral Oligomeric Silsesquioxane) systems have attracted particular attention, combining the advantages of organic and inorganic components. This paper reports on the thermal and thermo-oxidative degradation and weathering processes of these materials, as well as their impact on mechanical, chemical, and morphological properties. The paper discusses the physical and chemical changes occurring during degradation, the mechanisms of autoxidation, and the influence of environmental factors such as UV radiation, temperature, and humidity. Particular attention is paid to the role of POSS nanoparticles in polymer stabilization—their barrier function, free radical scavenging, and oxygen diffusion limitation. Methods for analyzing ageing processes are presented, including thermogravimetry coupled with infra-red spectroscopy (TG-FTIR), mechanical property testing, and yellowness index assessment. Material durability prediction models and their importance in designing composite lifespans in the context of the circular economy are also discussed. It is demonstrated that the appropriate type and concentration of POSS (typically 2–6 wt.%) can significantly improve polymer composites’ resistance to heat, radiation, and oxidizing agents, extending their service life and enabling more sustainable lifecycle management of products.

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.