ACCELERATED STORAGE TESTS* By F. GROSSMITH, B.Sc., F.R.I.C. Macleans, Ltd., Brentford, Middlesex, England Is SPEAKING of accelerated storage tests one has in mind procedures which w•11 give a rapid indication of the shelf life characteristics of cosmetic preparations. Possible types of deterioration cover a wide field, and in- formation may be required covering some, or all of the following character- istics: emulsion stability freedom from phase separation or other change in structure of the prod- uct colour stability freedom from clouding and deposition resistance to spoilage by microbiological or oxidative processes avoidance of drying out freedom from undesirable corrosive action between product and con- tainer etc., etc. AVAILABLE TECHNIQUES--ACCELERATED SHELF TESTS At the outset let it be said that there is no complete substitute for the long term shelf test under practical conditions through a range of seasons, and for the new product, whenever the possibility exists, such tests should be carried out. Next to long term shelf tests are shelf tests involving an intensification of the environmental factor of interest, for example, high humidity, ultraviolet radiation, exposure of the unit pack to the vapour of odorous materials, and, perhaps most important of all, testing the stability of the preparation when exposed to variations of temperature. During practical conditions of transit and storage, products are exposed to temperatures which are rarely static for long, and so temperature tests should include alternating temperatures. It is usually accepted that twelve-hour cycles alternating between -3-2 and 50øC. duplicate the effects of most temperature extremes. For the home market 30øC. alternating * Presented at the April 9, 1954, Meeting, London, England. 257

258 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS with 2øC. is an adequate (although quite conservative) range. It is also desirable that the product should be able to withstand periods of exposure to It may be considered that the temperature ranges quoted are narrower than are likely to be encountered in practice, but it should be remembered that the tests involve stringent conditions in the form of regular, rapid temperature changes covering the extremes of the range. If, under such conditions, no phase separation, flocculation, or crystallization occurs for a one- to two-week period, then it is probable, but not certain, that the prod- uct will be stable under actual conditions of use. The fact has to be recognized, however, that, particularly in the case of emulsion products and other colloidal systems, the behaviour of a new product cannot be reliably established without a field trial. In any case, a new product should be studied by laboratory tests for Several months, as some types of changes (for example of consistency) can be much delayed. Further, with certain creams, such as lather shaving creams, a greater number of alternating temperature tests covering various ranges are neces- sary, as consistency changes are frequently specific to a particular range. MORE INDIRECT TECHNIQUES In addition to accelerated shelf tests, there are available more indirect techniques which may be regarded as rather specialised analytical proce- dures. Such tests are best illustrated by quoting examples. Emulsion Stability Subjecting an emulsion to a few minutes of centrifugal action is a useful method of assessing proneness to phase separation. In some cases of emulsion creams and pastes, particularly for comparing alternative formu- lations, dilution of an emulsion with an equal weight of water and observ- ing rates of sedimentation or creaming is a useful test. Emulsions may break down as a result of evaporation losses any such tendency can be assessed by a simple oven test using a tightly closed vessel. Closure The efficacy of closure of a jar or other container to prevent drying out can be examined by enclosing a suitable volatile solvent and determin- ing weight losses at intervals, and similarly, a desiccant can be used in the container to estimate efficacy to prevent ingress of moisture. Corrosion Many products packed in collapsible tubes involve a corrosion hazard, leading to breakdown of the tube, product, or both. Corrosion propensities and the efficacy of inhibitors, can be assessed by measuring corrosion po- tentials against a standard electrode employing a suitable potentiometer.