J. Soc. Cosmet. Chem. 27, 247-256 (1976) ¸ 1976 Society of Cosmetic Chemists of Great Britain Psycho-rheology the relevance of rheology to consumer acceptance JEAN V. BOYD* Presented at the Symposium on ',4 Sensory Approach to Cosmetic Science' organized by the Society of Cosmetic Chemists of Great Britain at Manchester on 7-9th April 1975 Synopsis--The RHEOLOGICAL properties commonly found in COSMETIC materials are discussed. Since they are in general, SHEAR sensitive VISCOELASTIC materials, the impor- tance of using test conditions (temperature, shear rate, etc) relevant to the usage situation is discussed. In order to quantify the desirable physical properties in a product, the results of SENSORY and rheological tests have to be correlated. Types of PANEL test frequently used are briefly discussed. A method for pictorially representing TEXTURE is presented. The use of this TEXTUREGRAM to illustrate a range of rheological properties is demonstrated. INTRODUCTION Cosmetics have been used by men and women the world over since earliest times. Early cosmetics consisted largely of the natural oils, sesame olive and almond, rendered fragrant with extracts of herbs and flowers. The earliest records on the use of cosmetics come from Ancient Egypt, where kings were buried surrounded by the luxuries of life. The tomb of Tutankhamun, who ruled in around 1350 Bc, contained 50 alabaster vases, many of them exquisitely carved they would have contained 400 1. of fragrant oils, had it not been stolen by thieves. In 1770 a bill was introduced into the British Parliament (1) saying that 'All women... that shall impose upon, seduce, and betray into matrimony * Mars Foods Limited, c/o Dornay Foods, Hansa Road, King's Lynn, Norfolk. 247

248 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS any of his Majesty's subjects by means of the scents, paints, cosmetic washes, artificial teeth, false hair, Spanish wool, iron stays, hoops, high-heeled shoes or bolstered hips, shall incur the penalty of the law in force against witchcraft and like misdemeanours, and that marriage, upon conviction, shall stand null and void'. A similar Act was passed in America in 1770. The author is unaware whether the Act has ever been repealed! We can conclude that a great deal of time and expertise have been devoted to the art and science of cosmetic formulation for several hundred years. In this century many new and improved raw materials have been introduced, while many, some of them highly toxic, have disappeared. The very size of the business has rendered it imperative that it be put on a much more scientific basis than that on which it had existed for so long. Cosmetics today consist of oils, gels, emulsions, suspensions, or a com- bination of the last three. Physical forms range through solids, pastes, creams, gels, lotions and liquids. They will usually contain thickening and/or stabilizing agents. Hence they show complicated rheological pro- perties. It is the function of the cosmetic rheologist to quantify the physical properties so that a set of numbers are available which are related to what the consumer will see and feel when he or she buys and uses the product. To be useful the numbers must relate to the conditions of use. TYPES OF RHEOLOGICAL TEST The physical conditions which are important in a description of a pro- duct are the stress likely to be applied, the rate at which it will be applied, and the temperature during use or storage. Every product will have a unique set of numbers describing its physical properties, and each type of product a typical set. Shear stress-shear rate data Practically all cosmetics have one property in common they are shear sensitive, i.e. they break down when disturbed. This breakdown is charac- terized by the i-• (shear stress-shear rate) diagram. Some typical curves are shown in P•g. 1. Fig. l(a) describes a simple Newtonian fluid--not likely to be encountered by many cosmetic rheologists. Fig. l(b) describes a non- NewtonJan fluid without a yield stress, also uncommon in cosmetics. Fig. l(c) describes a fluid where the yield stress is a very important factor,