RHEOLOGIC AGING OF COSMETIC LOTIONS 151 I O0 - 40- I0 4- I -- 0.4- 0.1 0.1 0.4 I 4 I0 40 I00 400 ELAPSED TIME (DAYS) Figure 3.--The change in shear stress with time for three different shear rates for an antiperspirant lotion. lations. In each case, the linearity is quite satisfactory. In this example, we have arbitrarily used the viscosity at one second -• shear rate as read graphically from a rheogram. Other choices a•e open, and in Fig. 3, for one lotion, aging curves at three different shear rates are shown for the actual shear stress measurement. It is apparent from these curves that the 100,000 -- 60,000- 40,1300- zo,000- 10,1300 -- 4,000. - i000-- 6130- 0.01 0.04 0.1 0.4 I 4 i0 ELAPSED TIME (DAYS) i I 40 I O0 I 40O Figure 4.--Aging curves for various formulation and process variants of a lotion subject to age hardening.

152 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS low shear yield value term is building at a faster rate than the plastic vis- cosity. Thus a series of lines may summarize for a lotion any desired shear properties. In some cases, after an initial period of linearity, the slope of the line shows an increase with time (see Figs. 4 and 5). Such an increase is fre- quently followed by a marked drop. At this time we are inclined when using these plots to consider that as long as the linear holds, regardless of the slope, the lotion or paste is behaving normally. Of course, it is ideal if the aging curve is flat, but this is seldom seen. If a tendency toward I0,0001" 6,00ol- •""4' ...... 0.04 0.• 0.• 0.4 • • 4 •0 •0 40 •00 •00 4oo •000 ELAPSED TIME (DAYS) Figure 5.--Effect of delayed filling on the aging viscosity of a lotion. (-- -- -- filled the same day as making ---- filled the following day.) marked upward curving becomes apparent after some variable induction period, we consider that excessive "hardening" is possible. This abnormal increase often is the forerunner of an eventual drop in viscosity signifying phase separation. IMPI•CAT•OSS oF Tm• Us]• oF Lo• T•m• PI•OTS First it is of interest to note that the average time-doubling scale so fre- quently used for cosmetic lotion observation is close to the logarithm time choice favored here. Secondly, the logarithmic scale of viscosity has been shown by Fryk16f (14) to be directly related to a linear scale of subjective or ocular judgment of consistency. What in practice is gained by the logarithm plot of property and time? When there exists a simple and essentially linear relation, and this is reason- ably common, there are two major advantages which can be of great value to a stability program. The first is that as long as a reasonably linear relation holds there is considerable confidence that aging is not inducing any unanticipated complications. The other, and possibly the most im- portant, is the tentative long range prediction available from early meas-