J. Soc. Cosmetic Chemists 16 591-606 (1965) ¸ 1965 Society of ½osmeti• Chemists of Great Britaiv A changes aged method for predicting rheological in emulsion products when P. SHERMAN Presented at the Symposium on "Emulsions", organized by the Society of Cosmetic Chemists of Great Britain at Harrogate, Yorks, on $Ist March 1965. Synopsis--The relationship between viscosity at a given rate of shear and mean globule size is the same for both freshly prepared and aged pseudoplastic emulsions. Consequently the fall in viscosity when an emulsion is aged, due to globule coalescence, can be established from viscosity curves constructed for fresh emulsions of the same composition but with variable mean globule size. The only restriction is that the limits of size distribution should not change too drastically during ageing. The rate constant governing the increase in mean globule size can be determined from simple tests extending over a few days. This enables one to predict what the globule size will be at any future time. In turn the corresponding viscosity can be derived from the viscosity-mean globule size curves. Experimental and theoretical data show good agreement, thus eliminating the necessity to resort to accelerated ageing techniques of questionable value. INTRODUCTION The rheological properties of cosmetic emulsion products affect their practical performance. Following manufacture, several months may elapse before the cosmetic is used. Any changes in rheological properties during this storage period may impair their effectiveness. Consequently, to facilitate correct formulation, one must be able to predict rheological changes over long periods of time using a fairly simple and rapid procedure. Hitherto, estimates of ageing behaviour have been based on accelerated ageing tests at elevated or reduced temperatures (1), stability to high speed centrifugation (1), and more recently on stability to ultracentrifuga- tion at about 40,000 rpm (2-6). These methods are applicable only to relatively unstable, low viscosity, emulsions in which the disperse phase *Unilever Research Laboratory, Welwyn, Herts. 591

592 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS separates readily in bulk. The rate of separation is assumed to be a measure of stability. Unfortunately the conditions prevailing during accelerated testing are quite different from those to which the cosmetic is normally exposed during storage, so the results are of questionable value. Furthermore, the aforementioned tests provide information only about the final stages of coagulation. Emulsion coagulation is a three-stage process. Initially, globules. flocculate giving rise to aggregates which increase their size with time. Flocculation is a reversible process since the aggregates can be broken down, and the constituent globules redispersed, if adequate shearing forces are applied. Within the aggregates, globules are separated by a thin film of continuous phase. Before globules can coalesce this film must drain away. The rate of drainage depends on the nature of the electrical double layer, viscosity of the continuous phase, temperature, etc. The final stage in coagulation is globule coalescence. This is an irreversible process which leads to a progressive decrease in the interfacial area/' unit volume, and eventually to bulk separation of disperse phase. Both flocculation and coalescence influence rheological properties long before there is visible separation of the disperse phase. This paper describes a method for predicting viscosity changes in emulsions during storage, provided globule coalescence is the only irre- versible structural change. It cannot be applied, as yet, to predict changes in emulsions which contain finely divided pigments and/or hydro- colloids which may swell as a consequence of liquid absorption. The method is based upon the fundamental relationship between viscosity and globule size. PRINCIPLES OF METHOD FOR PREDICTING VISCOSITY CHANGES WHEN EMULSIONS ARE AGED Most emulsions, apart from very dilute ones, exhibit pseudoplasticity if not plasticity, so that flow properties depend on the applied rate of shear. At high rates of shear, viscosity becomes independent of shear rate and reaches a steady value, but viscosity at low shear rate depends partially on the extent to which the shearing forces have disrupted the. globule aggregates. Globules are rarely of equal size, so size distribution is defined in terms of mean globule size (Dm). Nevertheless, the spread of sizes affects, viscosity in addition to the effect exerted by Din. Very little is known about the influence of the former factor. In general, the relative increase