RHEOLOGY OF HYDROPHILIC POLYMER SOLUTIONS 145 The use of agitation to prevent settling is very effective as a temporary expedient. Removal of agitation always results in a reappearance of the problem. This method is commonly associated with the very undesirable commercial warning on a product--"Shake Well before Using." Those products which have achieved commerical success have employed a specific favorable combination of these factors at the sacrifice of a con- siderable degree of compounding freedom. Many products could be successful if a new method of obtaining permanent suspension was available which functioned even under adverse conditions of density difference and particle size. A detailed study of the rheology of various suspending media has revealed yield value to be an effective method for accomplishing this objective (Fig. 1). GEI•ERAL PROCEDURE A system was devised to permit detailed study of the theology of the suspending medium free from the influence of particle size distribution and particle interaction such as flocculation. This was accomplished by the use of large spherical particles of a unifmm size. An Ottawa flint silica sand with an average grain diameter of 0.06 cm. served this purpose. Addition of a few grains of sand to the suspending medium allows visual settling rate measurements which can be directly related to the theology of the suspending medium. The Brookfield Viscometer is used throughout this evaluation since it is widely available in industry and since detailed methods of relative yield value analysis with this instrument have been developed in our laboratory (1). Discussion Brook field Rheolo•y The Brookfield Viscometer is a rotational instrument which measures the torque required to revolve a spindle through a sample at a given rate of speed. The Brookfield RVT instrument has a range of spindle speeds from 0.5 to 100 r.p.m. An increase in r.p.m. produces a corresponding increase in rate of shear. The torque at any constant spindle speed is indicated by a dial reading of from 0 to 100 and is a measure of shearing stress. Char- acteristic curves for a sample are obtained by plotting the rate of shear (spindle r.p.m.) rs. shearing stress (Dial Reading). Figure 2 classifies typical flow plots for several different materials to demonstrate the characteristic differences in flow properties which are found in synthetic and natural polymer "solutions." You may consider these flow curves to be in effect x-ray photographs since they pictorialize the internal structure of liquids. A Newtonian liquid is characterized by flow that is directly proportional

146 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS $#•R/N• ( ......... o) Figure 2.--Types of flow. to shearing force and is depicted as a straight line through the origin. Most pure liquids or solutions of' low molecular weight exhibit Newtonian flow properties. This property is rare in hydrophilic polymer "solutions." Almost all hydrophilic polymer "solutions" are characterized by a pseudoplastic flow curve. These materials flow more readily as the rate of shear is increased to give a curve convex to the shearing stress axis and which goes through the origin. By contrast, materials exhibiting plastic flow characteristics will flow only after the applied shearing stress exceeds a critical minimum value. This minimum shearing stress is designated as yield value and is measured as the intercept on the stress axis (2). Apparent viscosity values can be calculated for any shear range as demonstrated in Fig. 2 in the case of the plastic flow curve. Viscosity for any range of shear is shown to be proportional to the slope of the flow curve in that area of shear. Refer now to Fig. 3 in which a typical plastic flow curve has been super- imposed upon a series of Newtonian flow curves. In common use, the Brookfield instrument expresses apparent viscosity of any liquid based on a single r.p.m. determination. Thus at 40 r.p.m., the viscosity of the plastic sample is represented in terms of a NewtonJan fluid with a viscosity of 2000 cps. At 5 r.p.m. the viscosity is represented in terms of a New- tonian fluid with a viscosity of 8000 cps. It will be noted in Fig. 3 that the plastic flow curve is characterized by