'278 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Yield Value Determination After 7 days of aging, yield value measurements were made with the two 4-oz. samples of each dispersion using a modification of the procedure of Bowles, Davie, and Todd (9). Seven days of aging were allowed to insure that the colloidal structure in thixotropic systems was well established. A single point viscosity at 6 rpm after a 6-min run was made for one sample of each dispersion, with viscosity at 12 rpm after a 6-min run for the other. By using separate samples not previously sheared, and 6-rain runs to allow structure equilibration with the rotating spindle, thixotropy-induced errors in the original computations should be minimized. Better reproducability is therefore ascribed to the yield value computation: (W - Yield Value (dynes/cm 2) = , (2) 100 Where V6' is the viscosity at 6 rpm and V•2' the viscosity at 12 rpm. Due to the nature of the viscometer used and the non-Newtonian rheology of the aqueous systems evaluated, yield value figures are considered relative rather than absolute. Analagous to the VSF figures, yield synergism factors (YSF) were calculated according to: YB (• 3% YSF = (3) YMz+ Y,, (3 - z)%' where YB is the blend yield value, YM is the MAS yield value, Z is the level of the MAS in the blend dispersion, and Yx is the XG yield value. A YSF of 1 indicates an additive yield value is obtained from the blend. Any figure greater than 1 is a measure of the synergism occurring in the blend. RESULTS AND DISCUSSION Apparent synergism in viscosity and yield value have been observed when formulating with magnesium aluminum silicate/xanthan gum combinations. This has occurred with even small additions of the gum. Relatively low levels of the gum have also been used to modify the thixotropic nature of the MAS. Smooth flow on extended storage has in this way been maintained by a reduced increase in viscosity on aging. In order to better characterize this MAS-XG interaction, a series of blends was prepared and evaluated at various concentrations. The 3% level is presented as a matter of convenience for subsequent evaluation of the separate MAS and XG dispersions. Unless otherwise indicated, results at 3% are representative of the general behavior observed even to the lowest practical usage levels. Figure 1 shows the aging curves for the MAS/XG blends compared to MAS alone at 3% solids. These demonstrate the flattening or stabilizing effects on MAS viscosity over time of progressively higher levels of XG. This corresponds to the rheological profile derived from plotting shear stress vs. shear rate as in Figure 2. A blend was considered to provide a thixotropic dispersion if producing a characteristic hysteresis loop as in Curve A of this figure. It was considered to provide pseudoplasticity without thixotropy if producing a curve similar to Curve B. At the 29:1 and 19:1 MAS:XG

RHEOLOGY OF MAS/XANTHAN GUM DISPERSIONS 279 Single Point Viscosity (cps) 2:1 MAS/XG 1:1 MAS/XG o 4:1 MAS/XG O-- • 9:1 MAS/XG 19:1 MAS/XG . 29:1 MAS/XG MAS •o- '- ' I ' I 1do o,1 1 lO Week Month Time (days) Figure 1. Three percent magnesium aluminum silicate/xanthan gum dispersions showing progressive flattening of the MAS aging curve with increasing levels of XG. ratios, thixotropic dispersions are still obtained. With 10% addition of the gum (9:1 ratio), the dispersion is rendered pseudoplastic and nonthixotropic. This is likewise true for the 4:1, 2:1, and 1:1 blends. Due to the inherently high viscosity and yield value of XG alone, it is not obvious that a synergism truly exists. To clarify this, separate MAS and XG dispersions were made at solids corresponding to those used in each blend. These results are presented in Tables I through IV. The calculated viscosity synergism factors (VSF) by Equation 1 and yield synergism factor (YSF) by Equation 3 provide a simple, illustrative indication of actual, synergistic values as opposed to expected additive values. In these tables, a general increase in viscosity synergism is noted as the dispersions age, indicating a colloidal structure becoming progressively more complex. There is, however, an interesting difference in VSF values depending on the type of viscosity determination used. Single point values are derived from a stationary rotating spindle allowed to turn for 6 min. This allows equilibration of colloidal structure breakdown and rebuilding. On a practical basis, the single point values relate to the working viscosity of the system, i.e., while being spread, poured, sprayed, etc. Helipath values are obtained from an inverted-T spindle cutting a slow helical path. The crosspiece constantly encounters new material and leaves that which its previous motion has