Differential Absorption of Offset Ink Components on Coated Paper, 2000 International Printing & Graphic Arts Conference Proceedings

Sanna M. Rousu, Dan E. Eklund, Patrick A.C. Gane, Daniel C. Spielmann--In this work offset ink setting on coated paper was investigated, the main emphasis being on the influence of the

chemical interactions between the ink and coating components. The differential absorption of the ink components from different ink formulations on a series of well defined coating layers of various chemical and physical character was studied.

Ink – coating interaction was studied as tack development of the ink during a time period of 3 seconds to 15 minutes after impression on coated paper using the SeGan Ink-Surface-Interaction- Tester (ISIT) device. Inks were developed progressively from a simple oil fluid phase to a pigmented ink by the addition of a known amount of each ink chemical one at a time. The ink compositions and components were characterised for their physical properties, namely for viscosity and surface tension. Model coatings were prepared in the laboratory by hand-coating on synthetic base paper following the same procedure as that of the inks, i.e. starting with binder alone and then progressively adding cobinder together with a variety of pigments, firstly a fine ground calcium carbonate, then comparing this with a coarse ground calcium carbonate and finally with the structure formed using a clay.

The main contributing factors to the ink consolidation and setting behaviour are the release of the fluid components from the ink composition, absorption of the released fluid into the porous structure as well as its diffusion into the polymer matrix of the coating layer and adhesion of the ink resins to the coating surface. Experiments showed that the rate of ink setting is highly dependent on the properties of the fluid phase separating from the ink composition.

The compatibility of ink chemicals for each other, on the other hand, determines the type and quantity of the released fluid or fluid mixture. Preferential interaction of the different oils with coating components was also seen to influence the rate of ink setting. The information gained serves to enhance the understanding of the prevailing mechanisms and importance of the variables influencing the ink – coating behaviour and illustrates the chromatographic separation effects that take place in the pigment and latex structure matrix and how they influence the surface properties of the ink during and after the printing process.