J. Soc. Cosmetic Chemists 17 439-465 (1966) ¸ 1966 Society of Cosmetic Chemists of Great Britain Techniques for assessing the rheological properties of toiletry and cosmetic products P. SHERMAN* Presented at the Symposium on "Physical Methods," organised by the Society of Cosmetic Chemists of Great Britain, in Bristol on 77th November 7965. 1tYn0I}Sis--Techniques for assessing textural properties of materials ranging from liquids through semi-solids to solids are considered. Methods described in some detail, and reviewed critically, are viscometry at low and high rates of shear for liquids and semi-solids, penetro- meters and cohesion for semi-solids, and shear or compression between parallel plates and modulus of rigidity for solids. Choice of method for a particular consistency depends on whether information is required on sample structure or on its performance under normal usage conditions. In the latter case slow deformation should be used when measuring firmness, and rapid deformation when measuring ease of spreadability. INTRODUCTION Consistency limits for cosmetic products range from the solid state at one end of the scale, to the fluid state at the other end. Rheology is usually defined as the science of deformation and flow, so that in order to study the rheological properties of cosmetics it is necessary to select methods which will measure these attributes over a wide range of con- sistencies. Many of the methods to be described have been in use for some time, while others have been introduced recently. A high proportion of these methods is used on products other than cosmetics, but of similar consistency, so that there is no reason why they should not be applied to testing the latter. When selecting a suitable method for rheological examination of any material the following points should be considered. *Unilever Research Laboratory, Welwyn, Iterts. 439

44O JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (a) Should the test provide information about the static structure of the product, i.e. under conditions involving only slight structural change ? (b) Should the test conform to practical usage conditions, i.e. conditions which often involve more than slight structural breakdown ? (c) Do we require an empirical method which merely simulates the mechanical action involved when the product is used ? Under (a) and (b) behaviour is defined in terms of physical values which have mathematical significance, whereas under (c) arbitrary values are derived which cannot easily be inter-related with one another. Discussion will be restricted to (a) and (b) since each method falling into category (c) is specific for a single process. Tests which cause little alteration in sample structure are usually time consuming, so that they are not suitable for routine control purpose. They should be restricted to fundamental study of structure. Category (b)tests are more readily applied to routine examination, since many tests can be made daily without difficulty. If the maximum value is to be derived from the latter test, however, the test conditions should be such that the product is sheared to approximately the same extent as when used. These conditions cannot usually be estimated accurately. Table I Consistency Test Methods Quantitative Qualitative Fluid Viscometry {Tabl• II) Semi-solid Viscometry {Table II) Penetrometers- rod, cone, sphere Forces of cohesion Modulus of rigidity Solid Penetrometers- rod, cone, sphere Parallel plate viscoelastometer (Creep behaviour under constant stress) Compression between parallel plates Torsion of hollow cylinder Torsional vibration of solid cylinder Resonance techniques Weissenberg rheogoniometer sample response to stress varying with time Sectilometer Indentation by falling sphere BI.I.R.D. - B.F.M.R.A. extruder For ease of discussion consistencies will be classified as fluid, semi-solid, or solid, although there is obviously no sharp demarcation between them. Methods used to examine these three types of consistency are summarized in Table I. It is immediately apparent that one technique can often be