Synthesis and characterisation of a novel hydroxylated polyester resin system for coil coatings

The continuous coating of steel and galvanised steel coils is referred to as coil coating. Coil coating is considered the most efficient coating method with regard to the amount of coated area per unit of time. Coil coatings include primer coating, top coating and backside coating. In general, the primer coating and top coating need to balance effectively among flexibility, adhesion, and film hardness. As such, saturated polyester resins with the capability of being molecularly engineered have found applications in coil coatings as the key binder. Nowadays, a combination of up to five polyester resins with different molecular structures are employed in coil primer coatings and coil top coat coatings to derive hard yet flexible coatings. The aim of this study was to synthesise and characterise a novel and standalone polyester resin system that would provide multiple properties simultaneously, thus, eliminating the need to use mixtures of polyesters in coil coatings. The multiple properties concerned included high flexibility, excellent adhesion and sufficient chemical resistance. For development of the polyester resin system concerned, a variety of polyols and dicarboxylic acids were polymerised to form polymeric polyester binders. Direct esterification polymerisation technique was employed for synthesis of the polyesters. The results obtained at an early stage indicated that in order to develop such a single resin system, achievement of an optimum balance among flexibility, adhesion and glass transition temperature was essential. Differential scanning calorimetry (DSC) showed that the augmentation or reduction of the glass transition temperature of the saturated polyester led to the augmentation or reduction of the film hardness of the resulting coil coating. Thus, a unique blend of neopentyl glycol, 1,4 cyclohexanedimethanol,1,6 hexanediol, adipic acid, isophthalic acid, and terephthalic acid was polymerised. The resulting product performed efficiently as a standalone binder system in a coil primer coating and a coil top coating, providing all of the required properties. Mglecular structures of the saturated polyesters synthesised were confirmed by carbon-13 and proton CH) nuclear magnetic resonance (NMR) and Fourier transformed infrared (FTIR). Alterations made to molecular structure of the polyesters through partial replacement of their diots with new ones were visible in expected frequency ranges of the resulting 13CNMR and 1HNMR spectra. According to size exclusion chromatography (SEC), molecular weight characteristics of the pilot scale and the industrial scale reproductions of the novel, single component coil coating polyester resin system developed were comparable with the initial product synthesised at the lab scale.