Limonene-derived polycarbonates as biobased UV-curable (powder) coating resins

The evaluation of poly(limonene carbonate)s (PLCs), derived from orange oils and carbon dioxide, as UV-curable (powder) coating binders is described. PLCs with moderate molecular weight were prepared by copolymerization of limonene oxide with CO₂ and a subsequent molecular weight reduction step by transcarbonation with 1,10-decanediol (1,10-DCD). These PLCs were cured with a trifunctional thiol monomer in the presence of a photoinitiator via thiol-ene chemistry to form poly(thioether-co-carbonate) networks. The UV curing of the resins following solvent casting was studied at 130 °C using ATR-FTIR, revealing a fast curing and a quick thiol-ene network (TEN) formation, realized by the addition reactions of thiol groups of the curing agent onto pendant isopropenyl groups of PLC. An ene addition enhanced by thiol-ene crosslinking was also observed, known as a ‘cage effect’, which was discovered for the first time in a UV-curable thiol-ene system. The efficient TEN formation was also evidenced by the high sol fractions in those UV-cured samples and their dynamic mechanical thermal analysis (DMTA). The DMTA results of these TENs showed high T_gs (up to 125.9 °C) and a wide range of thermomechanical properties, including rubbery moduli from 4.4–27.5 MPa. The UV-cured powder coating employing PLCs as binders showed outstanding properties such as high transparency, good acetone resistance, high pencil hardness (H-2 H) and high König hardness (174−199 s), suggesting their potential as very promising biobased alternatives to conventional powder coating resins.