JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (d) The speed of foam production (e) Flavour clarity (f) Rate of flavour release in the mouth (g) Appearance of the extruded ribbon (h) Pressure required to extrude the ribbon (i) Feel in the mouth. Attributes (a), (b), (c), (d), (e), (f) and (i) are all related to the rate of dispersion of toothpaste in water. It is obvious that a dentifrice which disperses rapidly in water, will disperse rapidly in the mouth. This is very important because of the very short time in which the toothpaste must do its work. In Great Britain the average brushing time is about 30 s, and the prime requirements are speed of foam build up, immediate flavour release, high impact and rapid rinsing. Fast dispersibility must also be a factor in the contact time of anti-caries ingredient with the tooth surface. The fluorine compound cannot react with the tooth if it is locked in the dentifrice. It is perhaps not so obvious that the maximum volume of foam pro- duced is greater, in general, when high thixotropy thickeners are used than when low thixotropy materials, giving viscous liquids rather than gels, are used. In dentifrices, there is a maximum foam volume which can be achieved for any particular thickener, the type of thickener having more influence than the level of detergent. To go above this, it is necessary to change to another thickener having faster dispersion and greater thixotropy. Fast dispersion is normally linked with gel structure and thixotropy, but of course these are not the only factors, as non-thixotropic gums of different types disperse at different rates. Good dispersion can also be achieved by using very low levels of thickener in the aqueous phase and relying upon absorbent fillers to give a suitable consistency. This latter method must be treated with some caution as such products may exhibit syneresis on ageing. The SCMCs, and particularly those of high thixotropy, give remarkable clarity of flavour. This may be related to dispersion rate, or may be linked with the physical chemistry of the detergent/flavour micelie in the presence of SCMC. The feel of the toothpaste in the mouth is obviously important. A product of slow dispersibility, for example with polyoxyethylene resin,

SYNTHETIC HYDROCOLLOIDS AND DENTIFRICES 463 gives a slimy feel due to the undiluted mucilage, whereas SCMC, which disperses quickly, does not give this effect. The presence of a gel structure does however present certain difficulties. The gel must be partially broken in order to extrude the ribbon from the tube. The pressure required is therefore greater and in the larger size tubes extrusion may be difficult unless a fairly wide nozzle is provided. The surface of the gel becomes disturbed on extrusion and if the product does not flow, these imperfections are slow to fill, and the ribbon appears rough. This can sometimes present a problem to the formulafor. THE IDEAL THICKENER FOR DENTIFRICES The perfect dentifrice thickener does not of course exist, but if it did, it would confer the following properties on a toothpaste: Rapid dispersibility and rinsibility Good foam characteristics High flavour impact with good clarity Clean mouth feel Smooth extruded ribbon with adequate rigidity and good white colour Consistency changes small with changes of temperature No hardening or syneresis on ageing for periods of up to 18 months in all climates Relatively little thixotropy. The hydrocolloid itself should be: Non hygroscopic Not subject to breakdown either by micro-organisms or enzymes Able to retain its viscosity for periods of up to 2 years Compatible with other toothpaste ingredients Easy to incorporate into the dentifrice Of low cost relative to the level required. THE CELLULOSICS Methyl cellulose Methyl cellulose may give toothpastes of rough texture, and poor stability, in the presence of even moderate levels of glycerine. This can be explained by its very poor solubility. The hydrophobic methoxyl groups merely open up the chains to permit ingress of water to