MEASUREMENT AND INTERPRETATION OF DENTIFRICE ABRASIVENESS 395 cause the wear resistance/hardness plot to reduce to the proportional relationship characteristic of many other materials. The distortion of the linear characteristic is a measure of the discriminating power of the denti- frice to abrade tissues of different hardness and is clearly a factor to con- sider in selecting an abrasive for dentifrice purposes. In order to study this problem in greater detail, a number of test slurries were prepared from a range of abrasives of different hardness. A particle size of approximately 101• diameter was chosen for the abrasive and the slurries were made up with a water-glycerine mixture to which a small amount of carboxymethylcellulose (CMC) was added to give a con- sistency similar to that of a diluted commercially available dentifrice. Testpieces of three dental tissues were prepared: Ivory, human dentine and human enamel. The ivory and human dentine specimens were given identical geometry, but the shape of the enamel testpiece differed from the others as it was considered advantageous to preserve the natural tooth contours. The possibility of a proportional relationship existing between the wear resistance and hardness of the dental tissues was first studied with the aid of a dilute slurry of SiC particles (5% w/w). The results of these experiments indicated a very slight upward curvature to the wear-resistance/hardness plots when suitable corrections for specific activity and nominal area of the specimens were taken into account. This curvature could not have arisen from any lack of hardness in the abrasive, but it could quite easily have arisen from the slight differences in specimen geometry. To investigate this point, additional experiments were made with a standard pin-on-disc machine employing a fixed abrasive cloth lubricated with water. The specimens consisted of small 2.5 mm pins machined from dental tissues and mounted so that the ends traversed the wet abrasive cloth in a spiral path. The change in length of the pins after passing over 3 m of cloth was measured with a micrometer and used to obtain the wear resistance of the tissue. With this machine, which is normally used for studying the abrasive wear of metals, the wear rates are higher than those usually recorded with a loose abrasive and brush loading. However, the load applied to the pins was adjusted so that the load on any abrasive particle was commensurate with that normally experienced with fibre loading. The results obtained from these experiments are shown in Table I. It will be seen that slight variations in the wear resistance of ivory arise with different orientations of the tissue, but if one accepts the mean value obtained for this material, then the wear resistances of the three tissues are

396 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I Wear resistance and hardness of dental tissues Ivory Dental tissue Enamel Dentine Grain Grain End parallel perpendicular grain to track to track -- Relative wear 0.71 resistance with 4.25 1.00 0.67 0.45 respect to dentine Mean 0.61 Hardness, HV2.5 280 57 28 closely proportional to their indentation hardnesses. As there was no reason to suppose that the employment of a brush was the cause of the curvature shown by the first set of results, it was decided to correct all subsequent brushing experiments so that the wear resistances of the tissues, when using SiC as an abrasive, were strictly proportional to their hardnesses. The wear resistance values obtained with the other abrasives, A1203, SiO 2 and CaHPO4.2H20 are shown in FiR. $ after correction in this manner. Although the test dentifrices have all been shown with a dentine rating of unity, the actual wear figures varied widely, being dependent upon such additional factors as particle size and concentration. However, by reducing all the data to a common dentine rating, it is possible to observe the differ- ences in discriminating power more clearly. Of particular significance is the strong discriminating power of the CaHPO 4.2H20 abrasive, which in these experiments recorded an enamel wear resistance rating 40 times that of the much harder SiC abrasive. The inherent dangers in routine dentifrice testing with dentine specimens are immediately apparent and this practice cannot be justified if any major changes are contemplated in the composition of a product. Owing to the limited number of dental tissues available, it is not possible to describe the exact shape of the wear-resistance/hardness curve for the dicalcium phosphate (DCP) type dentifrice, but it is clear that slight variations in the hardness of an enamel specimen will have a pronounced effect on the wear resistance recorded. The curves, given in FiR. 8, suggest that a 10% change in hardness of enamel might cause its wear resistance to change by as much as 50%, whereas for the harder abrasives, such as SiC and A120 3, a hardness variation of this magnitude would produce only a 10% change in wear resistance. A comparison of two dentifrices of very different discriminating power in a range of hardnesses which includes that of the test tissue, will thus be highly sensitive to the actual hardness of the