Reference is made to the book by Casson on "Rheology of Disperse Systems," C.C. Mills Edit., Pergamon Press, New York 1959, pp. 84-104. On the basis of an investigation of the formation and disintegration of pigment-particle structures in printing inks subjected to shear stresses the following equation was developed by Casson: √τ = Ko + K1 √D .
In a system of coordinates with the axes √τ and √D, according to past experiences, marble powder plant straight lines were obtained both for offset and book-print varnishes and also for pigmented printing inks and high-concentration flushed or slightly ground pigment varnish pastes. The extrapolated point of intersection of this line with the √τ axis indicates the minimum shear stress τo which is necessary to readily permit a shear movement. The τo value is designated the flow limit. If the straight line that runs through τo is shifted parallel through the point of intersection of the two axes of the graph, the viscosity curve is obtained. The η in this equation is the value of η which results when τ is extremely large. This about expresses the conditions in the printing press.
In FIGS. 3 and 3a, the results are shown of the viscosity measurements with the pigment compositions a to d and the printing inks thus made. The values found for √τ (dyn/cm2) have been entered on the horizontal axis and the values for √D (sec-1) have been entered on the vertical axis. Measurements were carried out with a Laray viscosimeter. For the individual colors, limestone crushing equipment curves a to d resulted which had different gradients and intersected the √τ axis in different places. The gradient of the straight line is a measurement for the viscosity. The viscosity of the printing ink is as much higher as the line is more level. The intersection of the line with the √τ axis determines the flow limit.