Eliminating the Need for Branching and Chain Extender Additives in PET Foaming Process by Exploiting CO₂-Induced Crystals with 3D-Foam Printing

Polyethylene terephthalate (PET) is a versatile and widely utilized thermoplastic with a significant presence in various industries, including textiles, packaging, construction, membrane filtration, and biomedical applications. This study demonstrates the feasibility of an innovative additive-free 3D foam printing technology for PET, utilizing preinduced crystals from CO2 impregnation to produce high-quality foam structures. By optimizing extrusion speed and hot-end geometry, the process effectively controls foam morphology, eliminating the need for branching agents or chain extenders. The mechanical properties of the foamed strands were systematically analyzed, revealing that the mechanical properties are strongly influenced by the degree of foaming, crystallinity, and relative density. To account for this effect, a modified scaling law was introduced, incorporating the influence of crystals on stiffness. The model accurately describes the elastic modulus across a wide range of densities and provides a useful tool for predicting mechanical properties. The methodology is readily applicable to commercially available PET and standard 3D printing technology, making it a scalable and practical solution for lightweight structured materials in diverse industries. The findings highlight the critical role of process-induced crystallinity in enhancing the mechanical properties of foamed PET, offering a scalable, cost-effective, and sustainable method for lightweight polymer manufacturing. This approach eliminates the reliance on chemical additives while optimizing foam performance, making it highly relevant for industrial applications requiring high-performance lightweight materials.