Temperature Effects in a Rotation Viscometer C. C. 3IILL 4 h D E. R. G.iTE.5 Printing, Packaging, and ..lllied Trades Research 'issociation, Leatherhead, Surrey, England I n the earl: stages of work designed to study the rheology of printing inks using a rotation wiscometer, it was observed that even with simple oils the viscosity tended to decrease at higher shear rates. A thermocouple inserted in the bob indicated a rise of temperature when the power input exceeded a critical value, and experiments were undertaken to determine the magnitude of the heating effect, the manner in which it affects the rheological data, and a simple method of correction. The data recorded in an experiment can be corrected simply if the temperature coefficient of viscosity and the equilibrium temperature of the bob are known, the method applying to pigmented systems as well as to simple oils. Use of the technique shows that part of the hysteresis loop previously reported w-ith suspensions is due to temperature changes during the experiment.
T
HE use of a rotation viscometer for studying the rheological properties of thixotropic suspensions is well established. A
narrow annulus enables a nearly uniform shearing stress to be applied to the sample, and the effect of amount of shear on its consistency may be studied by continuing the shearing for any desired time. When dealing with materials like printing inks, which may be subjected during use to shear rates ranging from 0 to 106 sec.-1 {approximately), it is necessary to obtain flow curves-i.e., torque us. rate of shear curves-over a large range of rates of shear in order to apply the results to each part of the printing process. However, the heat developed a t high shear rates modifies the curves obtained in such an experiment, and for this reason a n investigation of the heating effect in a rotation viscometer was undertaken. In general, two techniques have been employed when using a rotation viscometer. The first is to run the viscometer a t each shear rate until an equilibrium torque value is reached, and the second is to obtain the whole curve as rapidly as the apparatus will permit. The work of Goodeve and Rhitfield ( 3 ) is an example of the first method, and their results are in good agreement with their theory of thixotropy. They did not consider the effect of a temperature rise, but as they used comparatively low shear rates (
