Molecular design of UV curable polyester methacrylate powder coatings for heat sensitive substrates: Linking resin structure to film properties

Abstract

UV-curable powder coatings are solvent-free and energy-efficient systems with strong potential for heat-sensitive substrates, yet achieving sufficient melt flow and film formation at low temperatures remains challenging. To address this limitation, methacrylated polyester resins were synthesized by using different linear diol monomers and varying amounts of trimethylolpropane (TMP) and trimellitic anhydride (TMA) from 0.00 to 0.75 mol each. This approach systematically increased branching and methacrylate functionality. The two-step synthesis was confirmed by FT-IR, ¹H NMR, GPC, and acid-value measurements. The methacrylated resins exhibited glass-transition temperatures (T_g) of 19.3–34.0 °C. Melt viscosity measurements identified 60 °C as a suitable processing temperature, with viscosities of 2417–3910 mPa.s. Higher methacrylate functionality increased network formation in the UV-cured films, evidenced by increases in gel fractions (GF) (85–95%) and DMTA-derived crosslink densities (1132.58–2140.27 mol.m⁻³). However, the highest methacrylate content was not optimal, as excessive network formation reduced pull-off adhesion and direct impact resistance. The UV-cured coating based on the resin prepared with 0.50 mol each of TMP and TMA provided the best overall balance, with a GF of 92.5%, Vickers microhardness of 11.9, pull-off adhesion of 3.05 MPa, cross-hatch adhesion of 5B, and 94.2% thickness retention after 25 acetone double rubs. The optimal coating was successfully applied on heat-sensitive substrates such as medium-density fiberboard (MDF) and polycarbonate, melted at 60 °C, and UV-cured. These results demonstrate that controlling polyester backbone structure, branching, and methacrylate content enables UV-curable powder coatings capable of low-temperature processing on heat-sensitive substrates.