Ultraviolet-curable cycloaliphatic polyesters containing spiroacetal moieties for application as powder coatings

Abstract

Thermosetting cycloaliphatic powder coatings that exhibit good weatherability, corrosion resistance, and mechanical properties have been desired for some time. Unfortunately, most cycloaliphatic resins have glass transition temperatures (Tgs) that are too low for powder coating applications. In this study, a series of ultraviolet (UV)-curable, cycloaliphatic polyesters was synthesized from 3,9-bis(1,1-dimethyl-2-hydroxyethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane (spiroglycol), 1,4 cyclohexanedimethanol, and 1,4-cyclohexanedicarboxylic acid. The oligomers were characterized by 1 H nuclear magnetic resonance spectroscopy, Fourier-transform infrared spectroscopy, gel-permeation chromatography, and differential scanning calorimetry. The Tg value, the tensile strength, and the modulus of the crosslinked polyesters systematically increased with the spiroglycol (SPG) loading level. Dynamic mechanical analysis experiments highlighted structure−property relationships and showed evidence of secondary relaxations at around −44 °C. These β-relaxations were attributed to conformational transitions of the cycloaliphatic rings. The oligomers that contained 30 and 45 mol % SPG had Tgs that were suitable for powder coating applications. As such, they were formulated into UV-curable powder coatings. Coated test panels were evaluated in a salt spray chamber (ASTM B117) and a QUV weatherometer. The impact resistance, adhesion, and pencil hardness properties of the coatings were also evaluated. When compared to conventional (aromatic-based) controls, the cycloaliphatic powder coatings exhibited comparable weatherability, corrosion resistance, and Tgs along with superior impact resistance, substrate adhesion, and resistance to yellowing. These findings suggest that the cycloaliphatic powder coatings would be good alternatives to aromatic-based systems that are used in exterior durable clear coat applications.