Revisiting Unsaturated Polyesters: Recyclable-by-Design Vinylester Resins for the Circular Economy

Unsaturated polyesters (UPs) are a family of thermoset polymers largely used for glass fiber reinforced composite (GFRC) manufacturing, but their recycling is very challenging due to their crosslinked structure. Here, novel recyclable-by-design model unsaturated polymer resins were redesigned, positioning the C=C acrylic unsaturations as end groups and replacing styrene with alternative reactive diluents bearing –OH groups. The crosslinked polymers showed thermomechanical properties in line with conventional UPs, and the corresponding GFRCs containing 30 %vol of GFs were successfully prepared. Both the crosslinked unsaturated polymers and their composites were effectively solvolyzed in mild conditions (T = 200 °C, atmospheric pressure, t = 1–3 h) using different alcohols and glycols as solvent media, and 1,5,7-trazabicyclo[4.4.0]dec-5-ene as transesterification catalyst. Liberated GFs were found to be clean (as assessed by thermogravimetric analysis) and retained nearly 100 % of their original modulus and strength. The recovered organic recyclate showed a high –OH functional group reactivity and could be reused as hydroxylated binder in the formulation of bicomponent polyurethane coatings. These materials were found to exhibit excellent optical clarity, high Tg (> 100 °C), good substrate adhesion (3.45 MPa on glass), and moderate hydrophilicity (water contact angle of 65°). Finally, the same unsaturated polymer resins were prepared with the addition of the transesterification catalyst directly in the liquid prepolymer formulation. While the base physical properties of the crosslinked material remained unchanged, the resulting network showed a dynamic behavior, with thermally-induced stress-relaxation response clearly dependent on both hydroxyl group content and catalyst concentration, and easy post-cure thermoformability.