Rheological properties of coatings during drying processes

Coatings such as paints and printing inks transferred on substrates change from viscous liquids to viscoelastic solids during drying processes and finally become a hardened film. In order to investigate more precisely the viscoelastic properties of coatings during liquid‐to‐solid conversion, we developed the same type of ultrasonic rheometer as employed by Mason et al., in which the reflection coefficient technique is utilized. The complex modulus can be determined by the reflection coefficient and the phase shift. Using this rheometer, we can obtain the viscoelastic properties of coatings at various stages of drying at infinitesimal shear strain at the frequency of 3 MHz. The viscoelastic properties of a commercial offset litho ink which was dried by oxidative polymerization were measured at various temperatures. From the results of viscoelastic measurements, the drying of ink seems to be accelerated remarkably with increasing temperature. As the drying processes progress, a three‐dimensional network structure may be formed in the ink film with the aid of oxidative polymerization. The drying‐time dependence of G′ is different from that of G″ . From the drying‐time dependences of G′ and G″ , we can quantitatively deduce the internal structure of printing ink films during drying processes and provide a physical explanation for various drying stages. The dispersed pigments in a coating seem to play an important role in film formation and affect the mechanical properties of the drying film.