164 JOURNAL OF COSMETIC SCIENCE Kinoshita et al. (4) examined several dentifrices by the methods of surface analyzer and scanning electron microscopy. Among the several abrasives (CaHPO 4 ß 2H20, a mix of CaHPO 4 ß 2H20 + CaHPO4, and CaSiO3) , CaSiO 3 appeared to be more abrasive than the other two. Also, Panzeri et al., (5) examined by scanning electron microscopy the particles of abrasives used in 17 dentifrices and concluded that the majority of their particles present irregular forms and heterogeneous arrangements of the particles. In this work, the abrasiveness of the samples of silica and calcium carbonate was evaluated by radiometric method. This method consisted of brushing the irradiated teeth (dentine) with an abrasive agent and reference material slurties, one at each time. The beta radioactivity of 3•p transferred from the dentine to slurties was measured using a plastic scintillatot detector. The abrasiveness index or RDA (radioactive dentine abra- sion) is the ratio of 3•p counting rates obtained for abrasive and reference material slurties (6). MATERIALS AND METHODS MATERIAL SAMPLE The materials silica and calcium carbonate were provided from industries. PROCEDURE The radiometric method applied was based on a paper by Hefferren (6). The experi- mental conditions were established in order to use the available facilities. This experi- mental procedure consisted of the following steps: Selection of the teeth. The substracts to be abraded were roots from extracted permanent human teeth. After extraction, the teeth were stored in 4% formaldehyde solution. They were cleaned by stirring in a domestic detergent solution and then were washed with water and cut, separating the crown from the root. Irradiation of roots of the teeth. The roots of the teeth immersed in formaldehyde solution were irradiated in plastic vials during a one-hour period under a thermal neutron flux of 10 •2 n/cm-2/s -• at the IEA-R1 nuclear research reactor and in a position where the temperature was lower than 40øC. After irradiation, the tooth samples were removed from the core of the reactor to avoid damage from gamma radiation. During the irradiation a part of 3•p present in the hydroxyapatite of teeth was converted to radio- active 32p. Brushing operation of the teeth. After about one week of decay time, these irradiated teeth were fixed in a mold made by dental methacrylate resin that was fitted in a sample holder (reservoir for slurry) of a brushing machine. The brushing machine was manu- factured at IPEN/CNEN-SP and was equipped with two toothbrushes made of nylon bristles of medium hardness and a stroke counter with 125 strokes/min. A pressure corresponding to 150 g could be applied on each toothbrush. Before the first brushing, the irradiated dentins were brushed with a slurry containing reference material of calcium pyrophosphate for 6000 strokes in order to reduce irregular patterns of abrasion on the surfaces of the newly mounted teeth. The number of strokes applied to each toothbrushing operation was 1000.

EVALUATION OF DENTAL ABRASIVES 165 The slurries of reference material calcium pyrophosphate supplied by Monsanto Co. (St. Louis, MO) or of abrasives were prepared using a mass of 10 g of the material and 50 ml of diluent. In the case of thickeners, the slurries were prepared using a mass of 5 g of the material and 50 ml of diluent because of the large volume of the thickener. The diluent was prepared by adding 5 g of carboxymethylcellulose in 50 ml of glycerin heated to 60øC while stirring to obtain a homogenous mixture. Another 50 ml of heated glycerin was added to the mixture, and then 900 ml of distilled water was added. The stirring was continued at room temperature to obtain a clean solution of diluent. Each radioactive slurry was stirred, and three aliquots of 3 ml were pipetted onto separated planchets. These slurries were dried in an oven with air circulation, at 60øC, carefully to avoid cracking in the dried samples. The beta radiation of 1.71 MeV 3•p (with a half life of 14.3 days) of the dried samples was measured using a plastic scintillator detector. Calculation of abrasivehess indices. To calculate the abrasiveness indices, known as RDA, the 32p counting rate obtained for abrasive material was compared to that obtained for the reference material. A score of 100 for calcium pyrophosphate RDA was considered according to an ADA (American Dental Association) committee (6). Correction factors were also applied in this calculation because different abrasives may present distinct self-absorption and backscatterring radiation characteristics. Particle size and microscopy analysis of abrasives. The particle size of the silica and calcium carbonate samples was determined by sedigraphic method and their particle forms were examined using scanning electron microscopy at the Metallurgy Department of the IPEN/CNEN-SP. RESULTS AND DISCUSSION Table I shows RDA values obtained for six samples of silica and three samples of calcium carbonate, together with their particle sizes determined by sedigraphic method. The RDA results for raw materials used as abrasive agents (silica 1 and calcium car- bonate) presented in Table I varied from 136 to 19. The relative standard deviations of these RDA results, in general, varied from 5.9% to 11.8%, showing a good precision in Table I RDA and Particle Size Obtained for Raw Materials, Silica and Calcium Carbonate Silica 1 (abrasive) Silica 2 (thickener) Calcium carbonate Samples A B C D E F G H I RDA_+ s 136 + 8 94-+ 6 85 -+ 10 7 _+ 1 6.6_+ 1.0 5.5 -+ 2.1 54 + 4 24_+ 2 19-+ 2 s r (%) (5.9) (6.4) (11.8) (14.3) (15.2) (38.2) (7.4) (8.3) (10.5) n 7 8 8 8 5 6 8 8 8 Mean diameter (t•m) 4.26 3.21 2.54 1.20 0.31 0.32 3.13 1.77 1.49 RDA _+ s: RDA arithmetic mean values and standard deviation. st: relative standard deviations of the RDA values. n: number of determinations.