516 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 5000[ 2000 EFFECT OF SILICA CONCENTRATION ON VISCOSITY OF GLYCERINE A: VHS SILICA B: HS " C: MS " F: VLS " AA: SAMPLE A CALCI NED @ (600øC) A ,AA 1000 9OO 8OO 7OO 6OO 2.0 4.0 6.0 8.0 CONC. OF SILICA {%) Figure 19. Effect of silica concentration on viscosity ofglycerine I 10.0 cosity increase obtained with the regular Sample A. This substantiates the fact that silica thickener efficiency is reduced when silanol groups are partially removed. Additives have an effect on the viscosity behavior of silicas in various systems. In general, addition of 1 to 2% by weight (based on total weight of system) of a hydrogen- bonding additive (ethylene glycol, amines, etc.) to precipitated silica in a nonhydrogen- bonding or slightly polar system will increase viscosity because addition of the additive will increase the probability of silanol--silanol group interactions by acting as bridging groups. Large addition of additives (5 to 10%) of hydrogen-bonding additive would tend to decrease the viscosity of nonhydrogen-bonding, liquid-silica system. It has been observed that the most favorable pH range for thickening liquid systems capable of hydrogen bonding is between 4 to 8 the maximum viscosity is obtained with silica close to pH 7. In summarizing, for cosmetic applications one needs to choose a very high structure silica with the highest BET surface area and the finest particle size to obtain the maximum viscosity increase in a given system. B. DENTIFRICE SILICA ABRASIVES VERSUS RDA Dentifrice compositions contain an abrasive, liquid humectant, thickener, sudsing agent, flavor and minor amounts of other optional ingredients. Extensive studies have

PROPERTIES AND STRUCTURE OF SILICAS 517 been undertaken by many workers concerning the cleaning, polishing and abrasivity of dentifrices. It is generally recognized in the literature that abrasivity and cleaning properties are more or less directly related, i.e., the higher the abrasivity, the greater the cleaning efficiency (20, 21). It is well recognized that a dentifrice should contain some abrasive agent (21-23), but there has been concern expressed over the possible deleterious effects of excessive abrasiveness (20, 24, 25). The Council on Dental Therapeutics (24) cautions that a dentifrice should be no more abrasive than is necessary to keep teeth free of accessible plaque, debris and superficial stain. We have undertaken extensive research and developed methods by which we can con- trol the abrasivity of precipitated silicas (14, 15). Ideally, dentifrice abrasive should provide maximum cleaning and polishing with minimum abrasion to enamel, dentin and cementum. To do an effective job in a dentifrice, we believe the silica abrasive should exhibit the following properties: lowest humectant demand, controlled abrasivity, controlled particle size and shape, good fluoride compatibility, good clean- ing properties and acceptable surface properties and compatibility behavior. In a series of tests, we have found that low-structure precipitated silicas can be used ef- fectively in dentifrices to provide an acceptable cleaning at low abrasion. The RDA (26) (radioactive dentin abrasion) of precipitated silica abrasives is related to silica structure (15). A plot of silica abrasive RDA versus its oil absorption is shown in Figure 20. 1,000 900 800 700 600 500 400 300 - 200 - 100 6O RDA OF SILICA ABRASIVE VERSUS OIL ABSORPTION I i I i I i 70 80 90 100 110 120 OIL ABSORPTION Figure 20. Plot of silica RDA versus oil absorption I 130