Polycarbonate-modified acrylic polymers for coating materials

Abstract

Reactivities of six-membered cyclic carbonates ( 1) were studied and a carbonate-modified acrylic polymer was synthesized, which was expected to show good water resistance. The carbonates ( 1) reacted with several alcohols under acidic conditions giving the corresponding alkyl 3-hydroxypropyl carbonates, quantitatively. The reactivities of 1 with alcohol depended on their steric hindrance, on the acidity of the catalyst and on the basicity of the solvents. 13C NMR analysis indicated that the reaction proceeded by interaction between the CO group of 1 and the acid catalyst, followed by ring-opening of 1 via the nucleophilic attack by the alcohol on the CO group. This reaction of 5,5-dimethyl-1,3-dioxane-2-one ( 1a) with n-butanol produced polymers ( 2) at 50°C in toluene, using methane sulfonic acid as a catalyst. The molecular weights ranged from 2530 to 9310 and 1H NMR showed that the polymerization was initiated from n-butanol. Also the ring-opening reaction enabled the production of both a carbonate-modified acrylic polymer ( 4a) containing carbonate units in their side chains by the reaction of 1a with poly(styrene-co-2-hydroxyethyl methacrylate) ( 3a) and methacrylic monomers (HEMAC) containing 3-hydroxyneopentyl carbonate units by reaction of 1a with 2-hydroxyethyl methacrylate (HEMA). Free radical copolymerization of HEMAC also produced carbonate-modified acrylic polymer ( 4c). A carbonate-modified acrylic polymer showed excellent hydrolysis resistance when compared with ε-caprolacton-modified acrylic polymer ( 5a). Waterborne coatings, consisting of a carbonate-modified acrylic polymer ( 4c) and a melamine resin, showed excellent storage stability at 40°C when compared with a similar coatings based on an ε-caprolactone-modified acrylic polymer ( 5b), and the 4c melamine coating showed nearly the same curability to that of the 5b melamine coating when baked at 140°C for 30 min.