Precipitated Silicas Having Improved Dentifrice Performance Characteristics and Methods of Preparation

1. Field of the Invention

This invention relates to improved precipitated silica compositions. More particularly, the present invention relates to improved precipitated silica compositions that impart improved cleaning and abrasive performance to dentifrice formulations. Further, the present invention relates to methods for preparation of the improved precipitated silica compositions of this invention.

2. Description of the Related Art

The function of an abrasive substance in formulations intended for use in dentifrice compositions is to remove various deposits including pellicle film from the surface of teeth. Pellicle film is tightly adherent and often contains brown or yellow pigments which impart an unsightly appearance to the teeth. While cleaning is important the abrasive should not be so abrasive as to damage the teeth. Thus, an effective dentifrice abrasive material should maximize film removal without causing undue abrasion to the hard tooth tissue. Accordingly, dental researchers are continually seeking dentifrice abrasives that demonstrate satisfactory levels of cleaning without being unduly abrasive and damaging to oral tissues.

Precipitated silica (silicon dioxide) abrasive compositions (also referred to simply as "silicas" herein) and their use in dentifrice formulations are well known. In known silicas, however, there is a nearly linear relation relationship between abrasiveness and cleaning ability. As a result, the ability to develop dentifrice compositions with improved cleaning ability is limited by the tendency of the silicas to become more abrasive as their cleaning properties improve. Thus, there is a need for silicas for use in dentifrice compositions that impart improved cleaning ability to the toothpaste compositions without being overly abrasive and, therefore, damaging to oral tissues.

The cleaning and abrasiveness properties of dentifrice compositions are typically expressed in terms of Pellicle Cleaning Ratios ("PCR") and Radioactive Dentin Abrasion ("RDA") values, respectively. A PCR test measures the ability of a dentifrice composition to remove pellicle film from a tooth under fixed brushing conditions. A PCR test is described generally in "In Vitro Removal of Stain With Dentifrice", G. K. Stookey, T. A. Burkhard and B. R. Schemehorn, J. Dental Research, 61, 1236-9, 1982, which is incorporated herein by reference. The RDA test measures the abrasiveness of dentifrice compositions by measuring the amount of radio-labeled dentin removed from a tooth under fixed brushing conditions. Both PCR and RDA results vary depending upon the nature and concentration of the components of the dentifrice composition.

One key variable affecting PCR and RDA results is the nature and quantity of silica abrasive loaded into the dentifrice composition. It is generally believed that higher concentrations of a given silica abrasive will increase PCR results. Thus, it is desirable to load silica abrasives into dentifrice compositions at relatively high concentrations to improve cleaning. It is also generally believed that relatively hard or abrasive silicas tend to impart relatively high (poor) RDA values to dentifrice compositions. Thus, it is desirable to provide relatively soft or low abrasiveness silicas for use in making dentifrice compositions with low RDA values without sacrificing cleaning properties.

Unfortunately, however, known silicas tend to build viscosity relatively rapidly in dentifrice compositions. As a result most commercial dentifrice compositions contain only approximately 15 to 25% by weight of silica abrasive. Higher loading levels of known silicas tend to make the dentifrice compositions too viscous for their intended purpose. Thus, there is a need for silicas with lower viscosity build characteristics that can be loaded into dentifrice compositions at greater concentrations.

Also, known silicas tend to have a relatively large reactor slurry average particle size (APS). APS is believed to be correlated to abrasivity. While it is possible to dry and mill a silica to almost any desired final, or milled APS, the milling process is itself time consuming and expensive and can have undesired side effects, such as causing discoloration of the silica. More importantly, it is now believed that silicas with lower reactor slurry APS are softer than similar silicas with higher reactor slurry APS. As a result, it is now believed that if two silicas are milled to the same dry milled APS, the silica with the smaller reactor slurry APS will be softer than the silica with the higher reactor slurry APS, will have a lower functional APS and will, therefore, be less abrasive. Accordingly, there is a need for silicas with smaller reactor slurry APS.

It is accordingly an object of the present invention to provide novel precipitated silica compositions that impart improved performance in dentifrice formulations.

It is a further object of the present invention is to provide precipitated silica compositions that impart improved cleaning and abrasive characteristics to dentifrice formulations.

It is a further object of the present invention to provide precipitated silica compositions that have relatively low viscosity build characteristics compared to known silicas.

It is a further object of the present invention to provide precipitated silica compositions that have relatively low reactor slurry APS compared to known silicas.

It is a further object of the present invention is to provide methods for the preparation of the novel precipitated silica compositions of the invention by acidulation of an alkali metal silicate by a mineral acid in the absence or presence of an electrolyte.

Further objects and advantages of the present invention will become apparent from the detailed description of the preferred embodiments which follows.

The invention provides a precipitated silica composition that, when incorporated into a suitable dentifrice composition, the dentifrice composition exhibits an mPCR of about 103 to 124 and an RDA of about 88 to 102. The term "mPCR" refers to a modified pellicle cleaning ratio value determined by the method detailed below.

The invention also provides a precipitated silica composition that, when incorporated into a suitable dentifrice composition, the dentifrice composition exhibits an mPCR to RDA ratio of greater than about 1.1.

The FIGURE is a graphical depiction of the relationship between particle size and 10% brass Einlehner abrasion values for a prior art silica and a silica in accordance with the invention.

The present invention provides novel amorphous precipitated silica compositions which impart improved cleaning and abrasive characteristics to dentifrice formulations. The silicas of the present invention are preferably characterized as synthetic hydrated amorphous silicas, also known as silicon dioxides or SiO.sub.2. In particular, the silicas of the present invention have smaller reactor slurry average particle sizes (APS) than known silicas. Silicas of the present invention also have a significantly lower viscosity build than known silicas of comparable abrasiveness. Accordingly, silicas of the present invention may be loaded into dentifrice compositions in greater concentrations than known silicas, resulting in dentifrice compositions with improved cleaning properties and without undue abrasiveness.

Reactor slurry APS is defined as the APS of the precipitated silica compositions as measured after processing in the reactor or washed slurry but before drying, milling, and/or use. Reactor slurry APS is measured using a Microtrac II Particle Analyzer manufactured by Leeds and Northrup. Silicas of the present invention preferably have a reactor slurry APS of approximately 10 to 50 .mu.m, and more preferably approximately 10 to 20 .mu.m. All reactor slurry APS values provided herein are median values ("50%") unless otherwise indicated.

Without limiting themselves to any particular theory, Applicants believe that the relatively low reactor slurry APS results in the preparation of silicas with relatively low average functional particle size, where functional particle size is defined as the particle size of the silica while it is in use (e.g., during the brushing process). In other words, Applicant believes that the silicas of the present invention are softer than silicas with higher reactor slurry APS. As a result, Applicant believes that, in use, the silicas of the present invention (i.e., with relatively low reactor slurry APS) break up into smaller and, therefore, less abrasive particles more readily than silicas with higher reactor slurry APS. The silicas of the present invention are, therefore, less abrasive than known silicas with the same milled APS. In addition, silicas of the present invention have improved milling properties because they are initially smaller and require less milling, and because they are softer and, therefore, tend to break up into smaller particles more readily. As a result, silicas of the present invention are less likely to suffer from the graying caused by excessive milling.

Silicas according to the present invention are also relatively less abrasive than known silicas of approximately the same milled APS and viscosity build. Several tests are used to measure abrasiveness of silicas. The most direct measure is the Brass Einlehner Abrasion test. In the Brass Einlehner Abrasion test, an Einlehner AT-1000 Abrader is used as follows: (1) a Fourdrinier brass wire screen is weighed and exposed to the action of a 10% aqueous silica suspension for a fixed length of time; (2) the amount of abrasion is then determined as milligrams brass lost from the Fourdrinier wire screen per 100,000 revolutions. 10% Brass Einlehner (10% BE) results are expressed in milligrams loss/100,000 revolutions. Silicas in accordance with the present invention have 10% BE values of approximately 2.5 to 20.0 mg loss/100,000 revolutions, and more preferably approximately 2.5 to 8.0 mg loss/100,000 revolutions, and most preferably approximately 3 to 6 mg loss/100,000 revolutions.

Silicas in accordance with the present invention also tend to have a relatively small impact on viscosity of dentifrice compositions compared to known silicas. The tendency of a silica to increase the viscosity of a fluid is referred to as "viscosity build." Viscosity may be measured by a viscosimeter and may be expressed in centipoise. In a 40% slurry test, silicas in accordance with the present invention, having a moisture content of 6 to 8% and where moisture is measured by weight loss at 105.degree. C. for 2 hours, have a viscosity of less than approximately 20,000 centipoise, and preferably less than approximately 12,000 centipoise, and most preferably less than 7,000 centipoise. Detailed procedures for the 40% slurry test are set forth in the Examples.

Silicas in accordance with the present invention are preferably Low Structure silicas in accordance with the definitions set forth in the J. Soc.Cosmet. Chem. 29, 497-521 (August, 1978), and Pigment Handbook: Volume 1, Properties and Economics, Second Edition, Edited by Peter A. Lewis, John Wiley & Sons, Inc., 1988, p. 139-159.

Silicas in accordance with the present invention preferably have an oil absorption in the range of approximately 60 to 120 cc/100 g and more preferably approximately 80 to 100 cc/100 g, and most preferably approximately 80 to 95 cc/100 g. In the present specification oil absorption is measured using the ASTM rub-out method D281.

Silicas in accordance with the present invention preferably have a BET surface area in the range of approximately 50 to 250 m.sup.2 /g. Surface area is determined by the BET nitrogen adsorption method of Brunaur et al, J. Am. Chem. Soc., 60, 309 (1938).

Silicas in accordance with the present invention also preferably exhibit fluoride availability and compatibility values in the range of approximately 90-100%, as defined in U.S. Pat. No. 4,340,583, which is incorporated by reference herein.

Silicas in accordance with the present invention preferably have mercury intrusion void volume values in the range of 1.0 to 4.0 cc/g and more preferably 1.2 to 2.0 cc/g. The pore volumes (mercury pore volume) are determined using an Autopore II 9220 Porosimeter (Micromeritics Corporation). This instrument measures the void volume and pore size distribution of various materials. Mercury is forced into the voids as a function of pressure and the volume of mercury intruded per gram of sample is calculated at each pressure setting. Total pore volume expressed herein represents the cumulative volume of mercury intruded at pressures from vacuum to 60,000 psi. Increments in volume (cc/g) at each pressure setting are plotted against the pore radius corresponding to the pressure setting increments. The peak in the intruded volume versus pore radius curve corresponds to the mode in the pore size distribution and identifies the most common pore size in the sample.

Silicas in accordance with the present invention preferably have a pH of approximately 4.0 to 8.5 and more preferably from 6.5 to 8.5, as measured in a 5% aqueous slurry.

Silicas in accordance with the present invention preferably have a pour density of approximately 15-25 lb./ft.sup.3 and a pack density of approximately 25-35 lb./ft.sup.3. Bulk density is measured by measuring the volume in liters occupied by a given weight of the silica, and is reported in pounds per cubic foot.

Silicas in accordance with the present invention preferably have a brightness value of approximately 90 to 100. To measure brightness, fine powder materials are pressed into a smooth surfaced pellet and are evaluated using a Technidyne Brightimeter S-5/BC. The Technidyne Brightimeter S-5/BC has a dual beam optical system where the sample is illuminated at an angle of 450.degree., and the reflected light viewed at O.degree.. This method conforms to TAPPI test methods T452 and T646, and ASTM Standard D985.

Silicas in accordance with the invention preferably have a moisture content of about 6-8% as measured by weight loss on drying (L. O. D. %) at 105.degree. C. for 2 hours.

When incorporated into a suitable dentifrice composition, silicas in accordance with the present invention produce a dentifrice composition having an mPCR in a range of about 103 to 124, plus or minus an mPCR standard error of 5-7% (n=no. of trials=16 for the mPCR standard error). When incorporated into a suitable dentifrice composition, silicas in accordance with the present invention also produce a dentifrice composition having an RDA in a range of about 88 to 102, plus or minus an RDA standard error of 3-7% (n=no. of trials=8 for the RDA standard error). When incorporated into a suitable dentifrice composition, silicas in accordance with the present invention preferably produce a dentifrice composition with a mPCR to RDA ratio greater than about 1.1.

Suitable dentifrice compositions can include any dentifrice vehicle suitable for use in the oral cavity. Such vehicles include the usual components of toothpastes, tooth powders, prophylaxis pastes, lozenges, gums, oral gels and the like and are more fully described hereinafter. Toothpastes are the preferred vehicles.

One of the preferred optional agents in suitable dentifrice compositions is a surfactant, preferably one selected from the group consisting of sarcosinate surfactants, isethionate surfactants and taurate surfactants. Most preferred herein are the sodium and potassium salts of the following: lauroyl, sarcosinate, myristoyl sarcosinate, palmitoyl sarcosinate, stearoyl sarcosinate and oleoyl sarcosinate.

The surfactant can be present in the suitable dentifrice compositions at a content of about 0.1% to about 2.5%, preferably from about 0.3% to about 2.5% and most preferably from about 0.5% to about 2.0% by weight of the total composition.

Other compatible surfactants can optionally be used along with a sarcosinate surfactant in suitable dentifrice compositions. Optional surfactants are described more fully in U.S. Pat. No. 3,959,458, May 25, 1976 to Agricola et al.; U.S. Pat. No. 3,937,807, Feb. 10, 1976 to Haefele; and U.S. Pat. No. 4,051,234, Sep. 27, 1988 to Gieske et al. These patents are incorporated herein by reference.

Preferred anionic surfactants include the water-soluble salts of alkyl sulfates having from 10 to 18 carbon atoms in the alky radical and the water-soluble salts or sulfonated monoglycerides of fatty acids having from 10 to 18 carbon atoms. Sodium lauryl sulfate and sodium coconut monoglyceride sulfonates are examples of anionic surfactants of this type. Mixtures of anionic surfactants can also be utilized.

Preferred cationic surfactants can be broadly defined as derivatives of aliphatic quaternary ammonium compounds having one long alkyl chain containing from about 8 to 18 carbon atoms such as lauryl trimethylammonium chloride; cetyl pyridinium chloride; cetyl trimethylammonium bromide, di-is-obutylphenoxyethyl-dimethylbenzylammonium chloride; coconut alkyltrimethylammonium fluoride, etc. Preferred compounds are the quaternary ammonium fluorides described n U.S. Pat. No. 3,535.421, Oct. 20, 1970, to Briner et al., herein incorporated by reference, where said quaternary ammonium fluorides have detergent properties. Certain cationic surfactants can also act as germicides in the compositions disclosed herein. Cationic surfactants such as chlorhexadine, although suitable for use in the current invention, are not preferred due to their capacity to stain the oral cavity's hard tissues. Persons skilled in the art are aware of this possibility and should incorporate cationic surfactants only with this limitation in mind.

Preferred nonionic surfactants that can be used in suitable dentifrice compositions can be broadly defined as compounds produced by the condensation of alkylene oxide groups (hydrophilic in nature) with an organic hydrophobic compound which may be aliphatic or alkylaromatic in nature. Examples of acceptable nonionic surfactants include the Pluronics, polyethylene oxide condensates of alkyl phenols, products derived from the condensation of ethylene oxide with the reaction product of propylene oxide and ethylene diamine, ethylene oxide condensates of aliphatic alcohols, long chain tertiary amine oxides, long chain tertiary phosphine oxides, long chain dialkyl sulfoxides and mixtures of such materials.

Preferred Zwitterionic synthetic surfactants can be broadly described as derivatives of aliphatic quaternary ammonium, phosphonium, and sulfonium compounds, in which the aliphatic radicals can be straight chain or branched, and wherein one of the aliphatic substituents contains from about 8 to 18 carbon atoms and one contains an anionic water-solubilizing group, e.g. carboxy, sulfonate, sulfate, phosphate or phosphonate.

Preferred betaine surfactants are disclosed in U.S. Pat. No. 5,180,577 to Poletka et al., issued Jan. 19, 1993. Typical alkyl dimethyl betaines include decyl betaine or 2-(N-decyl-N,N-dimethylammonio) acetate, coco betaine or 2-(N-coc-N, N-dimethyl ammonio) acetate, myristyl betaine, palmityl betaine, lauryl betaine, cetyl betaine, cetyl betaine, stearyl betaine, etc. The amidobetaines are exemplified by cocoamidoethyl betaine, cocoamidopropyl betaine, lauramidopropyl betaine and the like. The betaines of choice are preferably the cocoamidopropyl betaine and, more preferably, the lauramido propyl betaine.

Another preferred optional agent in suitable dentifrice compositions is a chelating agent selected from the group consisting of tartaric acid and pharmaceutically-acceptable salts thereof, citric acid and alkali metal citrates and mixtures thereof. Chelating agents are able to complex calcium found in the cell walls of the bacteria. Chelating agents can also disrupt plaque by removing calcium from the calcium bridges which help hold this biomass intact. However, it is possible to use a chelating agent which has an affinity for calcium that is too high. This results in tooth demineralization and is contrary to the objects and intentions of the present invention.

Sodium and potassium citrate are the preferred alkali metal citrates, with sodium citrate being the most preferred. Also preferred is a citric acid/alkali metal citrate combination. Preferred herein are alkali metal salts of tartaric acid. Most preferred for use herein are disodium tartrate, dipotassium tartrate, sodium potassium tartrate, sodium hydrogen tartrate and potassium hydrogen tartrate. The amount of chelating agent suitable for use in the present invention are about 0.1% to about 2.5%, preferably from about 0.5% to about 2.5% and more preferably from about 1.0% to about 2.5%. The tartaric acid salt chelating agent can be used alone or in combination with other optional chelating agents.

Other optional chelating agents can be used. Preferably these chelating agents have a calcium binding constant of about 10.sup.1 to 10.sup.5 to provide improved cleaning with reduced plaque and calculus formation.

Another group of agents suitable for use as chelating agents are the soluble pyrophosphates. The pyrophosphate salts used in the present composition can be any of the alkali metal pyrophosphate salts. Specific salts include tetra alkali metal pyrophosphate, dialkali metal diacide pyrophosphate, trialkali metal monoacid pyrophosphate and mixtures thereof, wherein the alkali metals are preferably sodium or potassium. The salts are useful in both their hydrated and unhydrated forms. An effective amount of pyrophosphate salt useful in the present composition is generally enough to provide at least 1.0% pyrophosphate ion, preferably from about 1.5% to about 6%, more preferably from about 3.5% to about 6% of such ions. It is to be appreciate that the level of pyrophosphate ions is that capable of being provided to the composition (i.e. the theoretical amount at an appropriate pH) and that pyrophosphate forms other than P.sub.2 O.sub.7.sup.-4 (e.g. (HP.sub.2 O.sub.7.sup.-3)) may be present when a final product pH is established.

The pyrophosphate salts are described in more detail in Kirk & Othmer, Encyclopedia of Chemical Technology, Second Edition, Volume 15, Interscience Publishers (1968), incorporated herein by reference.

Still another possible group of chelating agents suitable for use in suitable dentifrice compositions are the anionic polymeric polycarboxylates. Such materials are well known in the art, being employed in the form of their free acids or partially or preferably fully neutralized water soluble alkali metal (e.g. potassium and preferably sodium) or ammonium salts. Preferred are 1:4 to 4:1 copolymers of maleic anhydride or acide with another polymerizable ethylenically unsaturated monomer, preferably methyl vinyl ether (methoxyethylene) having a molecular weight (M.W.) of about 30,000 to about 1,000,000. These copolymers are available for example as Gantrez AN 139 (M.W. 500,000), AN 119 (M.W. 250,000) and preferably S-97 Pharmaceutical Grade (M.W. 70,000), of GAF Chemicals Corporation.

Other operative polymeric polycarboxylates includes those such as the 1:1 copolymers of maleic anhydride with ethyl acrylate, hydroxyethyl methacrylate, N-vinyl-2-pyrollidone, or ethylene, the latter being available for example as Monsanto EMA NO. 1103, M.W. 10,000 and EMA Grade 61, and 1:1 copolymers of acrylic acide with methyl or hydroxyethyl methacrylate, methyl or ethyl acrylate, isobutyl vinyl ether or N-vinyl-2-pyrrolidone.

Additional operative polymeric polycarboxylates are disclosed in U.S. Pat. No. 4,138,477, Feb. 6, 1979 to Gaffar and U.S. Pat. No. 4,183,914, Jan. 15, 1980 to Gaffar et al. both patents are incorporated herein by reference. The disclosed polymeric polycarboxylates include copolymers of maleic anhydride with styrene, isobutylene or ethyl vinyl ether, polyacrylic, polyitaconic and polymaleic acids, and sulfoacrylic oligomers of M.W. as low as 1,000 available as Uniroyal ND-2.

Flavoring agents can also be added to suitable dentifrice compositions. Suitable flavoring agents include oil of wintergreen, oil of peppermint, oil of spearmint, oil of sassafras, and oil of clove. Sweetening agents which can be used include aspartame, acesulfame, saccharin, dextrose, levulose and sodium cyclamate. Flavoring and sweetening agents are generally used in dentifrices at levels of from about 0.005% to about 2% by weight.

Suitable dentifrice compositions can also contain emulsifying agents. Acceptable emulsifying agents are those which are reasonably stable and foam throughout a wide pH range, including non-soap anionic, nonionic, cationic, zwitterionic and amphoteric organic synthetic detergents. Many of these suitable surfactants are disclosed by Gieske et al., in U.S. Pat. No. 4,051,234, Sep. 27, 1977, incorporated herein by reference.

It is common to have an additional water-soluble fluoride compound present in suitable dentifrice compositions in an amount sufficient to give a fluoride ion concentration in the composition at 25.degree. C., and/or when it is used from about 0.0025% to about 5.0% by weight, preferably from about 0.005% to about 2.0% by weight, to provide additional anticaries effectiveness. A wide variety of fluoride ion-yielding materials can be employed as sources of soluble fluoride in the present composition. Examples of suitable fluoride ion-yielding materials are found in U.S. Pat. No. 3,535,421, Oct. 20, 1980 to Briner et al. and U.S. Pat. No. 3,678,154, Jul. 18, 1972 to Widder et al., both being incorporated herein by reference. Representative fluoride ion sources include: stannous fluoride, sodium fluoride, potassium fluoride, sodium monofluorophosphate and many others. Stannous fluoride and sodium fluoride are particularly preferred as well as mixtures thereof.

Water is also present in suitable dentifrice compositions. Water employed in the preparation of commercially suitable toothpastes should preferably be deionized and free of organic impurities. Water generally comprises from about 10% to 50% preferably from about 20% to 40%, by weight of suitable toothpaste compositions. These amounts of water include the free water which is added plus that which is introduced with other materials such as with sorbitol.

In preparing toothpastes, it is necessary to add some thickening material to provide a desirable consistency. Preferred thickening agents are carboxyvinyl polymers, carrageenan, hydroxethyl cellulose and water soluble salts of cellulose ethers such as sodium carboxymethyl cellulose and sodium carboxymethyl hydroxethyl cellulose. Natural gums such as gum karaya, xanthan gum, gum arabic and gum tragacanth can also be used. Thickening agents in an amount from 0.5% to 5.0% by weight of the total composition can be used.

It is also desirable to include some humectant material in a toothpaste to keep it from hardening. Suitable humectants include glycerin, sorbitol, and other edible polyhydric alcohols at a level of from about 15% to about 70%.

Also desirable for inclusion in suitable dentifrice compositions are other stannous salts such as stannous pyrophosphate and stannous gluconate and antimicrobials such as quaternary ammonium salts, such as cetyl pyridinium chloride and tetradecylethyl poyridinium chloride, bis-biquanide salts, copper bisglycinate, nonionic anti-microbial salts and flavor oils. Such agents are disclosed in U.S. Pat. No. 2,946,725, Jul. 26, 1960, to Norris et al. and U.S. Pat. No. 4,051,234, Sep. 27, 1977 to Gieske et al., incorporated herein by reference. Other optional components include buffering agents, bicarbonates, peroxides, nitrate salts such as sodium and potassium nitrate. These agents, if present, are included at levels of from about 0.01% to about 30%.

Other useful carriers include biphasic dentifrice formulations such as those disclosed in U.S. Pat. Nos. 5,213,790, issued May 23, 1993, 5,145,666, issued Sep. 8, 1992, and 5,281,410 issued Jan. 25, 1994 all to Lukacovic et al. and in U.S. Pat. Nos. 4,849,213 and 4,528,180 to Schaeffer the disclosures of which are incorporated by reference herein.

Suitable lozenge and chewing gum components are disclosed in U.S. Pat. No. 4,083,955, Apr. 11, 1978 to Grabenstretter et al., incorporated herein by reference. A particular example of a suitable gum composition follows:

The silica abrasive content in suitable dentifrice compositions may range from about 6 wt % to about 70 wt %, preferably from about 15 wt % to about 35 wt %, if the dentifrice is a toothpaste. Silica contents as high as 95% may be used if the dentifrice composition is a toothpowder.

Detailed descriptions of the mPCR and RDA tests are set forth in the Examples.

The precipitated silicas of the present invention may be prepared by a fresh water, or electrolyte solution, acidulation process wherein silica (silicon dioxide or SiO.sub.2) is precipitated by reaction of an alkali metal silicate and a mineral acid in aqueous solution. The alkali metal silicate may be any alkali metal silicate, but sodium silicate is preferred. While any mineral acid may be used in the process, sulfuric acid is a preferred reactant. In the preferred fresh water process, no electrolyte such as alum, Na.sub.2 SO.sub.4, or NaCl, is present during the reaction.

In the preferred process, an aqueous sodium silicate solution is provided wherein the sodium silicate is present in a concentration of approximately 8.0 to 35 weight percent, preferably 8.0 to 15 weight percent. The Na.sub.2 O:SiO.sub.2 ratio in the silicate solution should range from about 1 to 3.5 and preferably from about 2.5 to 3.4. The sulfuric acid reactant will preferably have a concentration of about 6 to 35% in water, more preferably about 9.0 to 15 weight percent.

Silicas in accordance with the present invention are prepared as follows:

1. A portion of the sodium silicate solution is charged to a reactor comprising a reaction chamber and a means for subjecting the reactor contents to shearing forces. In the preferred embodiment, about 5 to 15% of the total stoichiometric amount of sodium silicate solution, preferably about 8 to 12%, should be placed in the reactor to serve as initiating nuclei for the silica. In a preferred embodiment, the shear means is a line blender, such as the Lightnin Mixer model 5-LBC-500 (5 hp, 1750 rpm) or model 1-LBDS-75 (3/4 hp, 1750 rpm), a recirculation pump, such as the Labour DZT 30 (25 hp, 1750 rpm), a high shear reactor agitator blade, steam sparging apparatus, air sparging apparatus, a steam jacket, bead milling, or a combination thereof.

2. The aqueous solution of sodium silicate in the reactor is then preheated to a temperature in the range of about 160.degree. to 210.degree. F., preferably about 185.degree. to 195.degree. F. Prior to introduction into the reactor, the remaining sodium silicate solution is preferably preheated to approximately 150.degree. to 185.degree. F., and the acid solution is preferably preheated to approximately 90.degree. to 95.degree. F.

3. Once the reactor solution and the remaining reactants have reached the desired temperatures, the remaining reactants are added simultaneously to the reactor. The sodium silicate solution and sulfuric acid are metered into the sodium silicate solution in the reactor over an addition time of about 40 to 60 minutes, but preferably over approximately 50 minutes. Rates of addition of the reactants depend upon the Mol. ratio and concentration of the silica and the concentration of the acid. Generally, 1 Mol. sodium silicate is neutralized with one Mol. sulphuric acid. Thus, the ratio of sodium silicate addition rate to acid addition rate are as follows given the reactants set forth below:

4. At the end of this addition time, the silica has precipitated and the sodium silicate addition is terminated. Addition of the acid may be continued until the reactor slurry reaches the desired pH. The preferred slurry pH is approximately 5.4 to 6.4, and is more preferably approximately 5.5 to 5.7. At this stage, the silica has precipitated to provide a mixture of the precipitated silica and the reaction liquor.

5. Once the desired final slurry pH is reached, digestion or cure time begins and the reaction temperature is raised to approximately 190.degree. to 210.degree. F. and preferably to approximately 199.degree. F..+-.4.degree. F. Digestion at elevated temperature is continued to for approximately 5 to 60 minutes, and preferably for approximately 10 minutes.

6. Sometime prior to drying, the reaction products are subjected to shearing forces. In a preferred embodiment, the contents of the reactor are subjected to shearing forces during the reaction and digestion steps. In a preferred embodiment of the present invention, shear is applied to the contents of the reactor from the time the initial silicate reaches the desired reaction temperature until the digestion is complete. Alternatively, shear may be applied to the reactor contents for only a portion of the reaction time. In an alternative preferred embodiment of the present invention, shear is applied to the contents of the reactor beginning approximately 30 minutes after the reactants are first added to the initial nuclei and continued for approximately 45 minutes until digestion is complete. The amount of shear can be expressed in terms of the number of times the entire reactor contents are recirculated during the reaction time. The minimum shear rate should be 0.75. Preferably the shear rate is greater than 2.68 (e.g., 450 g.p.m. for 45 min. in 7,545 gal reactor). Most preferably, the shear rate is approximately 4.5 (e.g., 450 g.p.m. for 75 min. in a 7,545 gal. reactor). Alternatively, the reaction product may be subjected to shearing forces after the reaction is complete, but prior to drying.

7. After the digestion is complete, the pH may again be adjusted. In a preferred embodiment, the pH is adjusted to about 5.6.

8. The reaction mixture may then be filtered and washed with water to remove salts from the filter cake.

9. The filter cake may then be dried, preferably by conventional spray drying to produce a precipitated silica containing about 3 to 10% moisture.

10. The precipitated silica may then be milled to the desired particle size.

The following examples are presented to illustrate the invention, but the invention is not to be considered as limited thereto. In the following examples, parts are by weight unless otherwise indicated.

434 gal. of sodium silicate solution (3.73 percent Na.sub.2 O, 9.57 percent SiO.sub.2) of specific gravity 1.12 was added into a 7,545 gallon stainless steel reactor jacketed for steam heating. The reactor was provided with a Labour DZT30 450 g.p.m. pump and a 5-LBC-500 Lightnin Mixer line blender. The reaction medium was preheated to 195.degree..+-.5.degree. F. The remaining silicate was heated to approximately 185.degree..+-.5.degree. F. and the acid was heated to approximately 92.degree. F. After the initial nuclei silicate temperature reached 195.degree..+-.5.degree. F., the pump and line blender were turned on. Thereafter, sulfuric acid, 11.4% concentration (specific gravity 1.079), and the remainder of the stoichiometric amount of the sodium silicate solution were separately simultaneously metered into the reaction medium at the rate of 41.3.+-.0.5 g.p.m. acid and 92.7.+-.0.5 g.p.m. sodium silicate while maintaining the reaction temperature at 195.degree..+-.5.degree. F. The silicate addition was discontinued after 48 minutes. The acid addition was thereafter continued until the slurry pH was between 5.8 to 6.0. The reaction slurry was then digested for 10 minutes at 199.degree..+-.4.degree. F. The reaction pH was adjusted again to between 5.5 to 5.7. Shearing of the reactor slurry was then stopped. The resulting silica slurry was filtered and washed to remove most of the reaction by-product (sodium sulfate). The filter cake was dried and the dry product milled to a milled APS of approximately 10 .mu.m (50%).

The basic procedure of Example 1 was repeated with only those variations set forth in Table 1.

The basic procedure of Example 1 was repeated without applying shear to the reactor contents and with only those variations set forth in Table 1.

The dry silicas of examples 1-3 were subjected to various tests, the results of which are set forth in Table 1. The procedures for the tests are set forth above and/or are generally known to one of ordinary skill in the art.

Silica prepared in accordance with the procedures of Example 1 were milled to varying particle sizes and tested for abrasiveness as shown in Table 2.

Silica prepared in accordance with the procedures of Example 3 were milled to varying particle sizes and tested for abrasiveness as shown in Table 3.

The results of Examples 4 and 5 are shown in the FIGURE. As shown in the FIGURE, for a given dry, milled APS (MV), silicas prepared in accordance with the present invention, i.e., prepared by applying shear to the silica prior to drying and milling, are less abrasive than silicas prepared without shear.

434 gal. of sodium silicate solution (3.73 percent Na.sub.2 O, 9.57 percent SiO.sub.2) of specific gravity 1.12 was added into a 7,545 liter stainless steel reactor jacketed for steam heating. The reactor was provided with a Labour DZT30 450 g.p.m. pump and a 5-LBC-500 Lightnin Mixer line blender. The reaction medium was preheated to 195.degree..+-.5.degree. F. The remaining silicate was heated to approximately 185.degree..+-.5.degree. F. and the acid was heated to approximately 92.degree. F. After the initial nuclei silicate temperature reached 195.degree..+-.5.degree. F., the pump and line blender were turned on. Thereafter, sulfuric acid, 11.4% concentration (specific gravity 1.079), and the remainder of the stoichiometric amount of the sodium silicate solution were separately simultaneously metered into the reaction medium at the rate of 41.3.+-.0.5 g.p.m. acid and 92.7.+-.0.5 g.p.m. sodium silicate while maintaining the reaction temperature at 195.degree..+-.5.degree. F. The silicate addition was discontinued after 48 minutes. The acid addition was thereafter continued until the slurry pH was between 5.8 to 6.0. The reaction slurry was then digested for 10 minutes at 199.degree..+-.4.degree. F. The reaction pH was adjusted again to between 5.5 to 5.7. Shearing of the reactor slurry was then stopped. The resulting silica slurry was filtered and washed to remove most of the reaction by-product (sodium sulfate). The filter cake was dried and the dry product milled to a milled APS of approximately 10 .mu.m (50%).

The basic procedure of Example 6 was repeated with only those variations set forth in Table 4.

The silicas of examples 6-12 were loaded at 34% into a dentifrice composition comprising the following:

The mPCR and RDA of the dentifrice compositions were then determined using the procedures set forth below. The results of the PCR and RDA tests are set forth in Table 5.

In the preceding examples, the 40% slurry test was conducted as follows:

A. Using a tachometer, mark setting on rheostat that provides 300 rpm to mixing shaft. Repeat for 600 rpm.

B. In a large weighing pan, weigh out 340.0.+-.0.05 g of silica.

C. Tare a 4000 ml metal beaker, and add 510.0.+-.0.05 g of deionized water.

D. Position mixing blade and shaft in center of metal beaker so that blade is close to bottom of beaker, but does not touch.

E. Start mixer at 300 rpm. Add silica over a 2-3 minute period.

F. Scrape sides of beaker to thoroughly mix all silica in the beaker.

G. Increase rpm to 600, and time mixing for 5 minutes. Periodically scrape sides of beaker and mixing blade.

H. At 5 minutes, stop mixing and transfer slurry into 800 ml plastic beaker.

I. Set Brookfield viscometer (model 1/2 RVDV-II+) at 5 rpm.

J. Fit the Brookfield viscometer with the appropriate spindle. The T-B spindle may be used with moderate viscosities, but it will become necessary with higher viscosities to select a spindle that will produce a mid-scale reading. Lower spindle so that it is barely covered by slurry, and is centered in the beaker.

K. Turn on viscometer and helipath stand. Record measurement after 2 complete revolutions. Using a Brookfield chart, convert scale reading into centipoise.

Note: Mixing speeds may need to be altered slightly for slurries of very high viscosity. It is important to impart as little shear as possible to the slurry, therefore, use of the least amount of mixing speed necessary to produce a homogeneous slurry is suggested.

In the preceding examples, the Brass Einlehner abrasion test was conducted as follows:

A. Equipment: Einlehner AT-1000; Wire Screen for Einlehner (Phosphor Bronze P.M. wire disc); 10 PVC tubing from Einlehner; Top loading balance (minimum 2000 g, weighing to 0.1 g); Analytical balance (weighing to 0.01 mg); drying oven (convection type); 2 liter metal beaker; mixer equipped with propeller type agitator and rheostat; tweezers; lint free toweling; liquid dishwasher detergent.

B. Preparation of wire screens, PVC tubing and backing plates: New screens, PVC tubing, and backing plates may have residual oil left from their manufacture which must be removed as follows: (1) prepare solution of liquid soap and hot tap water; (2) soak screens, tubing, and plates for 1/2 hour in soap solution; (3) manually wash screens, tubing, and plates (do not use any utensil that may abrade the screens); (4) rinse thoroughly with hot water; (5) place in convection drying oven for 15-20 minutes at 60.degree. C.; (6) remove from oven and cool on lint-free surface.

C. Einlehner Preparation: (1) on analytical balance weighing to 0.01 mg, weigh screen; (2) prepare test cylinder by seating rough edge of wearing plate into male thread portion of testing cylinder; (3) invert Einlehner cylinder and seat the clamping ring onto the base of the cylinder; (4) place screen on top of clamping ring, with smooth side against the ring; (5) invert the bottom section of cylinder containing wearing plate onto top portion with clamping ring and screen, then assemble; (6) place one piece of PVC tubing onto each end of barrel (testing body), notches on the PVC tubing face inward and are placed on the barrel so that the notches line up with the number 4; (7) barrel is placed onto carrier clamps so that flat sides of barrel are held loosely by carrier clamps.

D. Test Procedures: (1) weigh 900.+-.0.05 g of deionized water into a 2 liter metal beaker; (2) into same beaker, add 100.+-.0.05 g of homogenous silica sample; (3) mix sample for 5 min., set rheostat to produce a moderate vortex around agitator; (4) remove sample and without allowing sample to settle, place in Einlehner cylinder; (5) place cylinder onto Einlehner base, seat teeth at top of agitator shaft into their slots; (6) turn on power switch and set program selector for 87,000 rotations, push start button; (7) after test cycle has been completed, remove test cylinder from Einlehner apparatus; (8) discard excess sample and disassemble test cylinder; (9) wash cylinder with hot tap water; (10) flush screen under hot running water and rub briskly to remove excess sample from screen; (11) place screen in 60.degree. C. convection oven and dry for 20-25 min., remove screen and wrap in lint-free towel, return to oven for additional 5 min. to drive moisture from towel; (12) remove from oven and place in desiccator, cool to ambient temperature, about 10 min.; (13) weigh screen on analytical balance.

The mPCR (modified Pellicle Cleaning Ratio) values were determined by a slightly modified version of the PCR test described in "In Vitro Removal of Stain With Dentifrice", G. K. Stookey, T. A. Burkhard and B. R. Schemehorn, J. Dental Research, 61, 1236-9, 1982. This test is identical to that described by Stookey et al with the following modifications: (1) a clear artificial pellicle film is applied to bovine chips prior to application of the stained film, (2) solution heating is used rather than radiative during film application, (3) the number of brush strokes is reduced to 200 strokes and (4) the slurry concentration is 1 part dentifrice to 3 parts water.

The RDA (Radioactive Dentin Abrasion) values were determined according to the method set forth by Hefferren, Journal of Dental Research, July-August 1976, pp 563-573, and described in the Wason U.S. Pat. Nos. 4,340,583, 4,420,312 and 4,421,527, which publication and patents are incorporated herein by reference.

It will be understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the principle and scope of the invention as expressed in the following claims.