d. Soc. Cosmet. Chem. 26, 189-204 (1975) ¸ 1975 Society of Cosmetic Chemists of Great Britain Four methods for the characterization of dentifrices and other semisolids M. BLOCK* Presented on I2-I4th November 1973 at Nottingham at the Symposium on 'Evaluation of Product Performance' organized by the Society of Cosmetic Chemists of Great Britain. Synopsis--During the course of development work, the need arose for characterization and com- parison of batches of DENTIFRICES. Four methods used in other technologies are reported for their utility in the evaluation of these and other SEMISOLIDS. Force-time traces were recorded on a modified lnstron TENSIOMETER for a piston moving at constant speed down a full tube of toothpaste and again down the emptied one. The difference was the corrected initial FORCE for EXTRUSION. At the same time, a number of extruded drops were weighed. The mean weight of a drop was divided by the diameter of the orifice to obtain the TENSILE STRENGTH of the paste. A polyethylene disc resting on the surface of a jar of dentifrice was withdrawn at constant speed on an lnstron tensiometer. The force-time curve showed a characteristic maximum co- hesive force. The final value obtained on separation of paste and disc was a measure of STRINGINESS. A notched doctor blade was used to apply 10 parallel stripes of uniform width but increasing height upon a surface. Rotation through 90 ø permitted sag of the semisolid, giving a stripe number characteristic of RESISTANCE to SAG. INTRODUCTION Numerous semisolids are in household use to-day, ranging from medicinal ointments, skin creams and dentifrices in the bathroom, to sandwich spreads, mayonnaise, whipped cream and jellies in the kitchen, * Unilever Research Laboratory, 455 London, Isleworth, Middlesex. 189

190 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS and to putty, polishes, grease and paint in the workroom. Thermoplastics, ceramics, glass and other materials appear rigid at room temperature but are processed in the semisolid state. The measurement and control of theological properties of such materials presents great difficulties. Historic- ally, each craft and later each industry has tried to solve its particular rheological problems in isolation, so that to-day a vast amount of knowledge is available but dispersed among numerous specialized sectors of technology. During the course of product development on dentifrices, the need arose for characterization and comparison of batches of pastes. Four methods were transferred from other technologies and are reported for their potential utility in the evaluation of semisolids. The first two methods utilize the principle of extrusion, the next measures values related to co- hesion and adhesion and the fourth ascribes a number to the property of sag. EARLIER WORK ON EXTRUSION A variety of simple rheometers has been used to measure the extrusion of butter (1), fats (2-4), molten lead and wax (5), clay pastes (6), mastics (7) and foods (8). Prentice (9, 10) and later Wiedermann (11) found a good correlation between the ease of extrusion of fats and their spreadability. Further measurements on the extrusion of foods through simple devices have been made (12-16). The force for extrusion of ointments and pastes from tubes and bottles has also been measured (17-20). The ductility of petrolatum has been measured by extruding a column vertically downwards, permitting it to break under its own weight and measuring the length of the residual cone adhering to the orifice (21). Food texture has been assessed by back extrusion through the concentric annulus between a cup and a metal plunger (22-24). EXPERIMENTAL Materials A number of viscous liquids, suspensions, gels and pastes were prepared for this study. Most mucilages consisted of a dispersion in 70•o sorbitol syrup of gums and other hydrocolloids, such as Carbopol 940, a carboxy-vinyl polymer