Coatings from recycled poly(ethylene terephthalate)-based unsaturated polyester resin with sol-gel modification for hydraulic concrete: Wettability, adhesion, and barrier performance

Surface protection of hydraulic concrete requires coating systems capable of combining stable interaction with the substrate and effective resistance to water and CO2 ingress. In this work, a coating based on an unsaturated polyester (UP) resin (UPR) derived from recycled poly(ethylene terephthalate) (PET) and another based on a hybrid UPR modified with a silane precursor by sol-gel were prepared and applied by brush to concrete in one, two, and three layers. Pure and hybrid UPRs were analyzed by FT-IR, while their surface behavior and barrier performance were evaluated by tests of water contact angle, adhesion, accelerated carbonation, capillary water absorption, and immersion absorption. The FT-IR results were consistent with the sequential transformation of PET to bis(2-hydroxypropyl terephthalate) and UP, while the hybrid UPR exhibited Si—O- and Si—C-related contributions associated with the sol-gel modification. Both coatings maintained hydrophilic surface behavior and were classified as 5B in the cross-cut tape test. Barrier response varied across the evaluated formulation/layer configurations. Within the pure UPR series, the three-layer system showed the best performance, reducing carbonation depth to 7 mm after 28 days and decreasing capillary and immersion-related water absorption. The two- and three-layer H-UPR systems exhibited the best performance, with the latter showing the lowest carbonation depth (6 mm) and the lowest absorption-related parameters. These results demonstrate that UPR coatings derived from recycled PET can effectively protect concrete and that their silane-based sol-gel modification and the evaluated coating configuration provide a route for modifying the concrete surface barrier response against carbonation and water ingress.