Viscoelasticity of Water/Ink Emulsions at Low and High Frequency, 1999 Coating Fundamentals Symposium Proceedings

P. Aurenty, J.F. Palierne, A. Gandini

The deformation frequencies in a rolling Nip and during cavitation and filamentation correspond to 10^3 -10^4 Hz. The viscoelastic properties of neat inks have been relatively well characterized only in the range of low frequencies, ie less than 50 Hz. On a conventional lithographic printing press, inks are emulsified with fountain solution and the viscoelastic parameters of the ensuing emulsions also constitute a relevant information. The rheology of both neat and emulsified inks in the high frequency domain is mostly unexplored because the conventional cone/plate rheometers are not reliable above 50 Hz. An experimental rheometer able to measure viscoelastic parameters in the range 0.1-10^4 Hz was therefore used to characterize model ink/water emulsions. Three distinct rheological responses

were obtained at 25°C as a function of the frequency range applied, viz.: (i) at low frequencies (0.1-10 Hz), increasing the volume fraction f of the emulsified droplets drastically enhanced the elastic modulus G’ of the emulsion; (ii) in the 10-100 Hz range, a relaxation plateau, corresponding to the shape relaxation of the droplets, was observed on the G’ modulus; and (iii) at the higher frequencies (100-10^4 Hz), both the elastic and viscous moduli decreased when f was increased. These trends are discussed and related to the various events related to the history of the emulsified ink on a printing press.