SELECTION OF SUSPENDING AGENTS 127 suspension vehicles used in pharmacy and the cosmetic industry. Neverthe- less, we can adopt the unique features of ideal plastic flow as criteria by which to evaluate suspension vehicles. Any approach to this type of flow behavior, therefore, could be considered a step toward attaining the ideal suspension vehicle. In Fig. 3 are shown the flow curves of three suspension vehicles which differ in type of flow: (A) a pseudoplastic vehicle (B) a thixotropic vehicle and (C) a Bingham plastic vehicle. The curve for ideal plastic flow is included merely for comparison. It can be seen that the flow curve ? P6eudopla6tic ? .... Thixotropic •,,•,•,•Bingham Plastic B Ideal Plastic ½ Rate of Shear Figure 3.--Typical flow curves of non-Newtonian suspension vehicles. of vehicle C, the Bingham plastic, most closely approximates that of the ideal plastic. Thus, according to our criteria, the Bingham plastic vehicle C should be the best of the three. The thixotropic vehicle B should be rated second, and the pseudoplastic vehicle A last. There may be some question as to whether vehicle C or vehicle B should be placed first, be- cause of the relative positions of the flow curves and the presence of a hysteresis loop (in vehicle B) which has been considered important for good suspension properties (4). The reason for this disagreement may be based on the interpretation of the thixotropic flow curve. Thixotropic flow is described by two flow curves, an upcurve and a downcurve. The two curves meet at a point of maximum shear. The

128 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS upcurve describes the breakdown of internal structure as the vehicle is being sheared up to the maximum value. The downcurve, on the other hand, represents the flow characteristics of the vehicle after it has been subjected to maximum shear. In determining the suspension quality of the vehicle, we are concerned mainly with the conditions soon after the maximum shear has been applied, since this is when the particles begin to settle. If we assume, therefore, that the maximum shear described in the flow curves corresponds to the high shear of mixing during preparation of the suspension or to the shear induced by shaking and pouring the suspension during its use, it seems reasonable to consider only the down- curve for the measure of suspension quality. The upcurve may also represent the gel state of the vehicle. For many thixotropic vehicles, this state may take hours or even days to reach. Therefore, if the upcurve is used to measure suspension quality, some of the measuring would be done long after the particles have begun to settle. By this time, and if the downcurve is such that it denotes poor suspension qualities, the particles may have settled markedly. The upcurve, then, could be used as a measure of suspension quality only if the thixotropic vehicle recovers its gel state within a very short time. The flow curve of such a vehicle, however, would appear similar to that of a Bingham plastic vehicle. Before proceeding, I would like to clarify a point. It may seem that, from my foregoing discussion, thixotropic vehicles, in general, are inferior to Bingham plastic vehicles. This is not always true. I merely wish to point out that in the specific example shown, the Bingham plastic (Fig. 3, curve C) appears more desirable than the thixotrope (Fig. 3, curve B). Certainly, if a more concentrated thixotropic vehicle was used, such that its downcurve was positioned above the flow curve of the Bingham plastic, the thixotrope would be the vehicle of choice, provided, of course, that in its gel state it is not too thick to shake and pour. Bingham plastic flow is distinguished by the need of a finite yield stress required to initiate flow. It differs from ideal plastic flow in that after the yield stress has been exceeded, the rate of shear is not a linear function of the shearing stress. Instead, the flow curve is concave to the rate of sk ear axis. This is similar to pseudoplastic flow except that at low rates of shear the curve appears to intersect the stress axis, indicating the presence of a yield stress, and at higher rates of shear the slope of the curve is usually small, indicating low viscosity. Unlike the ideal plastics, Bingham plastics do exist among the suspension vehicles which have potential use in pharmaceutical and cosmetic products. There may be some difficulty in finding them, however, because of the lack of reliable methods for detecting this type of flow behavior.