Mechanical robust and thermal insulated rPET microcellular foam via supercritical CO2 foaming cross-linked photovoltaic backsheet

Upcycling of solar panels plays important role in the photovoltaic sustainability. However, their reprocessed foaming was limited due to the low melt strength of the backsheet material after long-term usage. Herein, we utilized chain extender (CE), a copolymer made of glycidyl methacrylate (GMA) and styrene to enhance the melt strength of recycled polyethylene terephthalate (rPET) via the formation of chemical cross-linking network. According to Fourier transform infrared spectroscopy (FTIR) spectrum, the epoxy groups in GMA participated into cross-linking reaction since absorption peaks of 758 cm⁻¹, 847 cm⁻¹, 902 cm⁻¹ and 1248 cm⁻¹ assigned with epoxy group disappeared after reactive extrusion. Consequently, the glass transition temperature increased to 78.5 ℃, and the crystallinity decreased 37.7 % gradually with CE content of 5 %. Rheological results showed that the zero-shear viscosity dramatically increased to 19692.1 Pa·s, almost 100-folds for unmodified rPET, demonstrating the viscoelasticity transition from “liquid” viscos state to “solid” elastic rPET with incorporation of cross-linking agent. With the assistance of two-step supercritical CO₂ foaming technology, the rPET microcellular foam with high compressive strength and thermal insulated performance can be developed. Specifically, the void fraction increased up to 85.6 %, the average cell size could decrease to 9.2 µm, the cell density increased up to 1.98 × 10⁹ cells/cm³, thermal conductivity reduced to 48.5 mW/(m·K), the compressive strength of the rPET foam can reach up to 6.84 MPa. As a proof of concept, this work provides a novel route to develop mechanical robust and thermal insulated rPET microcellular foam for resource utilization of disposed photovoltaic backsheet.