434 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS crease in shearing stress with time at a constant rate of shear was logarith- mic and therefore follows a first order rate. From the slope of the lines in Fig. 6 it can be seen that the rate of breakdown of 2% Laponite is similar to that of 8ø//0 bentonite. As soon as the Laponite is broken down the dispersion becomes ex- tremely fluid and although the static yield value of bentonite is appreciably 1.8 1.4 I-0 0.6 x -- x• Bentonire 8% xN x x __ •nife 2 % \ 2 4 6 8 I0 Time of sheo½ing, s Figure 6 Thixotropic breakdown of bentonire and Laponite gels at constant rate of shear (17.59 lower it exhibits a much higher plastic viscosity than Laponite. Examination of the down curve of Laponite gels showed linearity between 1 759 and approximately 500 s-• but at lower rates of shear there was evidence of shear thickening. The curvature may be due to slippage of the gel between the cone and plate. For this reason the "dynamic yield value" (6) was determined by extrapolation of the down curve. The effect of concentration of gelling material on the plastic viscosity and dynamic yield value of some Laponite and bentonire gels is shown in Tabl• I. The low plastic viscosity of the 4% Laponit• gel may be due to slip but in general the values are lower than those recorded by Neumann (11) using a concentric cylinder viscometer. However, a good measure of agreement was found between the

SOME APPLICATIONS OF RIGIDITY AND YIELD VALUES 435 Table I Plastic viscosity and yield values of Laponite and bentonire gels Material Laponite Solid content 1.5 4 Plastic viscosity (ce) 4.6 5.5 6.2 5.7 Dynamic yield value (Nm -2 x 10) 18 82 226 382 Bentonire 8 32 73 10 59 400 Weissenberg rheogoniometer values of Cheng (12) for a 2% gel and those recorded here. The increase in the static yield value can be used as a means of assessing the fairly rapid reformation of the Laponite gel structure. Fig. 7 shows 2.7 4 8 12 .16 Time, rain Figur• ?' Build up of yield value with time. Laponit• 2• ¸ ¸ 2 days old Laponite 9, •o • 9 days old Laponit• $•o U [] 2 days old Laponit• $ •o ffi [] 9 days old Laponit• 4 •o A A 2 days old Laponite 4 % ß ß 9 days old Bentonire 8•o x--x 2 days old