Viscoelastic Properties of Coatings During Formation of Network Structure

Abstract

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.