The rheology of printing inks. II. temperature control studies in the rotational viscometer

Abstract

The use of the rotational viscometer has become well accepted in fundamental studies of the rheological properties of non-Newtonian materials such as printing inks. It is very important, therefore, that the behavior characteristics of this instrument be well known. One of the most important variables in rheological studies is the temperature since small temperature changes greatly affect viscosities. The present studies indicate that in previous work in this field this point has not been adequately treated. At low energy inputs, the viscometer bath itself is found to be effective in keeping the sample at constant temperature. In the medium range of 60,000–300,000 ergs/sq. cm/sec., the temperature can be controlled by lowering the bath temperature to keep the bob temperature constant. In the high range above 250,000 ergs./sq. cm./sec., the temperature can best be controlled by keeping the mean temperature of the cup and bob constant. These energy input ranges are given per square centimeter of bob surface so that they can be related to various precision rotational viscometers. Contrary to previous findings reported in the literature, ordinary mineral oils, vegetable oils, and litho varnishes show no evidence of thixotropy when measured by the new technique. Furthermore, the equilibrium flow curves of several oil-based printing inks were found to be linear above the low stress range. Therefore, well-defined values of the plastic viscosity and yield value can be reported for such printing inks.