RHEOLOGY OF HYDROPHILIC POLYMER SOLUTIONS 147 0 • 20 • • •/• Fig. 3.•Plastic flow related to Newtonian flow. a rapidly increasing apparent viscosity as the r.p.m. is decreased. Although the absolute shear stress at 0 r.p.m. cannot be measured, extrapolation of the curve to 0 r.p.m. results in an estimated yield value which is acceptable for comparative purposes. This is approximated in the case of the Brook- field RVT instrument by (1) Brookfield Yield Value= (Apparent Viscosity at 0.5 r.p.m. -- Apparent Viscosity at 1 r.p.m.) 100 Polymer "Solution" Evaluation In Fig. 4 the specific flow curves for a series of representative synthetic polymer "solutions" are depicted. In this plot, a 25 per cent "solution" of low molecular weight polyacrylic acid is shown to be NewtonJan. Two per cent methyl cellulose is shown to be slightly pseudoplastic, whereas 1.5 per cent polyethylene oxide "solutions" are definitely pseudoplastic in nature. A plastic flow curve is found among the synthetic polymer "solutions" only in the case of 0.20 per cent Carbopol 934 (which will be referred to hereafter as a carboxy vinyl polymer). This polymer "solution" with plastic flow characteristics has been com- pared to a series of natural polymer "solutions" in Fig. 5. It is shown here that 3 per cent gum tragacanth prepared in a special manner exhibits a plastic flow curve. The curves for 2.5 per cent locust bean gum, 2.5 per cent sodium alginate, 1.5 per cent guar gum, and 3 per cent gum tragacanth (freshly prepared by low shear mixing) all are pseudoplastic in character.

148 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS YIGLL• V•L UG Figure 4.--Flow diagrams--synthetic polymer "solutions." $ #EA/?IN&, $/'/•'• $S Figure 5.--Flow diagrams--natural polymer "solutions." •guspension Evaluation Using this pictorial means of classifying the flow properties of a polymer "solution," an insight into the suspending ability of the "solutions"