MEASUREMENT AND INTERPRETATION OF DENTIFRICE ABRASIVENESS 405 It is equally apparent that, in dentifrice testing, proper consideration must be given to the range of concentrations covered during oral usage. The common practice of comparing dentifrices at some arbitrary concentration level cannot be justified in the light of the present experimental data. For example, examination of Fig. 7(b) clearly shows that although two denti- frices with characteristics such as 'a' and 'b' are directly comparable at any concentration level, this situation will rarely arise and, in most cases, one will be attempting to compare dentifrices with characteristics such as 'b' and 'c'. Furthermore, although many of the dentifrices investigated show a gradually saturating wear characteristic, some dentifrices were found to exhibit a sharp falling off in abrasivehess at high concentration levels. This effect could have arisen from the interplay of a mixture of hard and soft abrasives, but it is generally felt to be due to the stiff paste-like qualities of these dentifrices preventing full tissue coverage during brushing. The only satisfactory solution to this problem is to resort to an integrated wear test, which takes full account of the changes in dentifrice abrasiveness over the range of concentrations normally experienced during oral brushing. As the dentifrice concentration is not itself linearly related to the number of brush strokes, this test must take the dentifrice dilution function into account. If one assumes that saliva is produced at a constant rate under the stimulation of the brushing action, then the relationship between con- centration, C, and number of brush strokes, S, will be of the form C=a/ (l+bS), where a and b are constants. a is determined by the initial con- centration of the dentifrice, which is generally less than 100% owing to the habit of wetting the brush before use, whilst b reflects the rate of saliva production. For practical testing purposes, the integrated dentifrice abrasiveness is best obtained by simply allowing the dentifrice to be continuously diluted in a fashion that reflects the oral condition, but adjusting the rate of dilution to give the required final concentration, usually about 20% (w/w) (9). The initial concentration is similarly adjusted to correspond to a wetted brush. As the continuous addition of diluent is not easy to arrange in practice, a satisfactory alternative is to add the liquid at a few set times during a test. A suitable liquid, having a similar viscosity to human saliva, is obtained by adding a small quantity of CMC to water (10). The level of radioactivity of the slurry at the end of the test then yields a measure of the integrated dentifrice abrasiveness under oral conditions of usage.

406 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS CONCLUSIONS 1. It has been shown that the radiotracer technique can provide an extremely rapid assessment of the abrasiveness of a dentifrice and can be made so sensitive that it is possible to measure transient wear rates arising from changes in surface contour or environment of the dental tissue. If necessary it is possible to measure the wear caused by one or two strokes of a brush on dentine polished with a commercially available dentifrice. 2. The results obtained with different tissues suggest that their wear behaviour is similar to pure metals or other materials in that their abrasive wear resistance is proportional to indentation hardness when very hard abrasive compounds are used. In other cases where the dentifrice abrasive has a hardness comparable to that of the tissues, the wear resistance increases rapidly as the tissue hardness approaches that of the abrasive and the dentifrice exhibits a strong discriminating action towards different tissues. This situation applies to most proprietary dentifrices and accounts for the large dentine/enamel wear ratios and the sensitivity of enamel wear resistance to its hardness. The possibility of much harder abrasive compounds being used in dentifrice formulation makes it imperative to incorporate into any test specification the requirement that enamel as well as dentine be used as a test material. Failure to do this could lead to the production of dentifrices which were excessively abrasive to enamel. 3. A detailed examination of the tissue wear/dentifrice concentration curves suggests that the form of this relationship is closely associated with the brush fibre tip geometry. In the case of mono-disperse systems the controlling factor appears to be the ratio of the volume of the active col- lection region of the fibre tip to that of an abrasive particle. However, in most practical cases, the former increases with particle diameter and the sensitivity of the wear/concentration curves to the size of the abrasive particles is reduced. As a result the level of dentifrice concentration at which tissue wear saturation occurs varies but slowly with particle diameter. The effect could be enhanced by a change in tip geometry, but most commonly used geometrical forms tend to minimise it. This variability stresses the need to standardise brush fibre geometry in dentifrice testing and to employ an integrated test to take account of the wide range of dentifrice concentrations that occur in oral use. The fact that some dentifrices often show a marked falling off in abrasiveness when used at high concentrations strengthens the case for this form of assessment.