The viscoelastic properties of printing ink films during drying processes were examined with ultrasonic attenuation and sound velocity instead of visoelastic functions such as dynamic modulus.
Two kinds of commercial offset litho inks were used as samples. One is ordinary offset ink (NC ink) and the other is the offset ink for printing on flexible plastic films (POP ink).
With increasing drying time, the sound velocity in ink film coated on Al base becomes faster and the temperature dependence of sound velocity becomes larger. These tendency are more remarkable for NC ink films. This phenomenon seems to be attributed to the formation of the three-dimensional networks in the systems. Therefore, it is thought that with the progress in ink drying the network structure of ink film becomes denser, and in NC ink films denser network structure may be formed compared with POP ink films throughout drying process.
Since the coefficient of ultrasonic attenuation (α) can be related to the mechanical loss function such as tan δ, the molecular motions in the three-dimensional networks of ink films can be examined by the temperature dependences of α. In the plots of α vs. temperature, the values of α for wet ink films decrease monotonously with increasing temperature, however with increasing drying time, the maximum is observed, and the values of α become larger. With the progress in ink drying, the α maximum shifts to higher temperature and the values of α become smaller. The values of α for NC ink films are smaller than those for POP ink films in the corresponding drying stage, and the maximum of α for NC ink films is not definitely observed compare with that for POP ink films. Therefore, NC ink films seem to act as if they are highly cross-linked or semi-crystalline. It is deduced that NC ink films with a high degree of cross-linking behave as more rigid film comparing with POP ink films, and these differences in mechanical properties seem to be fit to the purpose for which both inks are used.

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An intaglio ink is easy to cause set-off when ink film is comparably thick. In order to prevent set-off, drying properties, interfacial characteristics and flow properties of ink have been improved. For example, thixotropic ink seems to depress the initial adhesion of ink film and wax additive weakens surface adhesive energy.
In this study, we investigate the effects of ink drying and rheological properties on set-off. The drying processes of printing inks can be considered as a viscoelastic transition phenomena and can be discussed quantitatively with the aid of ultrasonic attenuation instead of loss tangent. The coefficient of ultrasonic attenuation can be determined using an ultrasonic spectrometer. The dynamic shear rigidity and viscosity, and steady shear viscosity were measured with co-axial double cylinder rheometer. The experiments were carried out for sample inks which show set-off or not, and following conclusions are obtained.
1. The relations between set-off and ink drying properties cannot be recognized.
2. The inks which show structural viscosity and remarkable thixotropy are available for non-set-off.
3. Viscoelastic behavior of non-set-off intaglio inks seem to depend on the formation of the temporary structural networks due to the association of the suspended particles.

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