Viscosity behavior during the baking of melamine resin cured acrylic high solids coatings was measured by an oscillating plate rheometer with heating unit. Viscosity behavior was greatly influenced by molecular weight of acrylic resin, and the linear relationship between the molecular weight and the logarithmic value of amount of flow, F, which can be regarded as an index of levelling and sagging properties was observed. The influences of acrylic resin/melamine resin ratio, blocking agents for acid catalyst, setting time and the baking condition on the viscosity behavior were also studied. From these results, high solids coatings generally give larger F values than conventional type coatings, and the influences of baking conditions of high solids coatings on the F value were greater than that of conventional coatings.
Viscoelastic properties of coatings during drying processes were measured in order to investigate kinetic properties of network strands at various drying stages and to clarify the mechanism of formation of three-dimensional network structures. Measurements were carried out at 3 MHz by the same type of ultrasonic rheometer as used by Mason et al., in which the reflection coefficient technique was utilized. Disperse medium which is based on linseed oil and its suspension of chrome yellow 10 G were used as coatings. With the progress in the drying processes, three dimensional network structure may be formed in the coatings by the aid of oxidative polymerization. At an initial drying stage, drying time dependence of storage modulus G′is different from that of loss modulus G”due to kinetic properties of internal structure. At lightly cross-linked stages, the coatings behaves as an elastomer. Therefore, the value of G′increases with increasing drying time. On the other hand, the value of G”is almost constant irrespective of drying time. Particularly for coatings containing resin, G”decreases tentatively with increasing drying time. The three-dimensional networks in the coatings become closer with increasing drying time. At highly cross-linked stages, the effect of internal viscosity between network strands becomes a dominant factor, and the value of G” increases rapidly with the progress in film drying. For linseed oil-chrome yellow suspensions, the dispersed particles behave as multiple crosslinks or semicrystalline linkages.Therefore, the viscoelastic quantities increase remarkably and drying of film is accelerated. From the temperature dependence of G′and G” at different drying stages, the viscoelastic transition region for coating films shifts to higher temperature with progress in film drying.