Abstract:
The present invention relates to a peroxydisulfate oxidizing system for toothpaste compositions that provides excellent bleaching effect together with oral care compositions containing the oxidizing system and methods and uses therefor.

Description:
FIELD 
       [0001]    The present invention relates to an oxidizing system for toothpaste compositions that provides excellent bleaching effect together with oral care compositions containing the oxidizing system and methods and uses therefor. 
       BACKGROUND 
       [0002]    There is a general desire for people to have white teeth. Such white teeth are an indication of good health and in particular good oral care health. A problem is that various foods and the use of tobacco and red wine will discolor teeth. Beverages such as coffee, tea and cola beverages can discolor teeth. As a result various products and procedures have been developed to whiten teeth. These products and procedures are either purchased and/or used directly by the consumer or are applied by a dentist or other professional. 
         [0003]    Current home treatment methods include abrasive toothpastes, toothpastes that produce oxides, whitening gels for use with a dental tray and whitening strips. The effectiveness of such techniques depends on a variety of factors including the type and intensity of the stain, the type of bleaching agent, contact time of the bleaching agent on the teeth, the amount of available bleaching active in the composition the ability of the bleaching agent to penetrate the tooth enamel, and consumer compliance. Effectiveness is also dependant on the amount of bleaching active in the composition, the ability of the active to be released during use, and the stability of the active in the product. However, the effectiveness of many of these treatments is adversely affected because of deficiencies in one or more factors relating to the composition and consumer compliance. 
         [0004]    Thus there is a need for improved whitening (i.e. bleaching) compositions for inclusion in oral care compositions, and in particular, toothpaste compositions. 
       SUMMARY 
       [0005]    Unless otherwise indicated, the terms “%” or “percent” when used in connection with an ingredient of the toothpaste compositions of the invention is intended to refer to the percent by weight of the indicated ingredient in the toothpaste composition. 
         [0006]    Amounts of peroxydisulfate are given by weight of peroxydisulfate without its counterion, unless otherwise indicated. Amounts of specific salts, e.g., sodium peroxydisulfate, are given by weight of the salt, unless otherwise indicated. 
         [0007]    In some embodiments, the present invention provides oral care compositions comprising a peroxydisulfate bleaching agent, wherein the composition has a pH in the oral cavity of less than 7; or from 4 to 7; or from 5 to 6; or from 5.5 to 6. In some embodiments, the oral care composition is a toothpaste. 
         [0008]    In some embodiments of the oral care compositions of the invention, the peroxydisulfate bleaching agent is present in an amount of up to 25% by weight of the composition; or from 1% to 9% by weight of the composition; or from 5% to 9% by weight of the composition; or 7% by weight of the composition. 
         [0009]    In some embodiments of the oral care compositions of the invention, the pH of the composition is from 4 to 7. 
         [0010]    In some embodiments of the oral care compositions of the invention, the peroxydisulfate is sodium peroxydisulfate. 
         [0011]    In some embodiments of the oral care compositions of the invention, the composition comprises one or more humectants; one or more thickeners; one or more abrasives; one or more sources of fluoride; and one or more detergents or surfactants. 
         [0012]    In some embodiments of the oral care compositions of the invention, the composition comprises the bleaching system described herein, and: polyethylene glycol in an amount of from 0% to 23%; glycerin (99.0%-101.0%) in an amount of from 0% to 32%; propylene glycol in an amount of from 1% to 40%; polyethylene glycol/polypropylene glycol 116/66 copolymer in an amount of from 1% to 10%; polyvinyl pyrrolidone in an amount of from 0.5% to 10%; calcium pyrophosphate in an amount of from 5% to 30%; silica, for example fumed silica, in an amount of from 0.1% to 5%; sodium lauryl sulfate in an amount of from 1% to 3%; and tetrasodium pyrophosphate in an amount of from 0% to 3%, by weight of the composition. In some embodiments, the composition further comprises butylated hydroxytoluene in an amount of form 0.01% to 0.05% by weight of the composition. In some embodiments, the composition comprises sodium peroxydisulfate in an amount of from 6% to 9%; or 7% or 8% by weight of the composition. 
         [0013]    In some embodiments of the oral care compositions of the invention, the composition further comprising one or more adjuvants selected from sweetening agents flavoring agents and coloring agents. 
         [0014]    In some further embodiments, the invention provides a method for whitening teeth comprising contacting teeth with an oral care composition according to the invention. 
         [0015]    In some further embodiments, the invention provides the use of a peroxydisulfate bleaching agent in the manufacture of an oral care composition according to the present invention. 
     
    
     DETAILED DESCRIPTION 
       [0016]    It has been discovered in accordance with the present invention that a bleaching or oxidizing system comprising a peroxydisulfate, for example sodium peroxydisulfate (PDS), wherein the pH of the bleaching system is less than 7, preferably from 5 to 7, provides whitening effects that are superior to comparable bleaching systems having higher pH. 
         [0017]    Sodium peroxydisulfate, sometimes referred to as sodium persulfate or PDS, has the molecular formula Na 2 S 2 O 8 . PDS is a strong oxidant with oxidation potential around 2.12 v, as compared to 1.77 v for HP. It has been discovered in accordance with the present invention that PDS performs best as a bleaching agent under acidic conditions (i.e., at a pH less than 7). Thus, PDS can be formulated into an oral care composition, for example a toothpaste, at a pH less than or equal to neutral (e.g., 4 to 7) which increases efficacy, and results in the composition possessing whitening power superior to PDS at more alkaline pH. 
         [0018]    Thus, in one embodiment, the present invention provides an oral care composition comprising a peroxydisulfate bleaching agent, wherein the composition has a pH in the oral cavity of less than 7; or from 4 to 7; or from 5 to 6; or from 5.5 to 6. In some embodiments, the oral care composition is a toothpaste. 
         [0019]    As used herein, “oral care composition” is intended to mean a product, which in the ordinary course of usage, is not intentionally swallowed for purposes of systemic administration of particular therapeutic agents, but is rather retained in the oral cavity for a time sufficient to contact substantially all of the dental surfaces and/or oral tissues for purposes of oral activity. The oral care composition may be in various forms including toothpaste, dentifrice, tooth gel, subgingival gel, mouthrinse, mousse, foam, mouthspray, lozenge, tooth powder, chewable tablet, chewing gum or denture product. In one embodiment, the oral care composition is in a form selected from toothpaste, dentifrice, tooth gel, mouth rinse or denture product, with toothpaste being particularly preferred. The oral care composition may also be incorporated onto strips or films for direct application or attachment to oral surfaces. 
         [0020]    The peroxydisulfate (also known as persulfate) of the bleaching or oxidizing systems of the oral care compositions of the invention can be in the form of a metal salt, for example and not limitation the sodium, ammonium, potassium, calcium, zinc, or magnesium salt. In one preferred embodiment, the peroxydisulfate is sodium peroxydisulfate. Generally, the peroxydisulfate is present in an amount of up to 25% by weight of the composition; or from 1% to 15%; or from 5% to 9%; or 6%; or 7%; or 8% by weight of the oral care composition. 
         [0021]    As mentioned above, the pH of the composition in the oral cavity is at or below neutral (i.e., 4 to 7) which increases the efficacy of the PDS oxidant. Thus, in some embodiments, the pH of the oral care composition is from 4 to 7; or from 5 to 6; or from 5.5 to 6. 
         [0022]    The oral care compositions of the invention, for example toothpaste compositions of the invention, can include a variety of components and ingredients known in the art for toothpaste compositions. For example, the oral care compositions of the invention can include one or more humectants; one or more binders or thickeners; one or more abrasives; one or more sources of fluoride; and one or more detergents or surfactants. 
         [0023]    Examples of suitable humectants include polyhydric alcohols (polyols) such as propylene glycol, glycerin, sorbitol, xylitol or low molecular weight polyethyleneglycols (PEGs), or polyethylene glycol/polypropylene glycol copolymers, such as for example polyethylene glycol/polypropylene glycol 116/66 copolymer. In various embodiments, humectants can prevent hardening of paste or gel compositions upon exposure to air, and also function as sweeteners. In some embodiments, the humectant system consists of one or more of polyethylene glycol in an amount of from 0% to 23%; glycerin (99.0%-101.0%) in an amount of from 0% to 32%; propylene glycol in an amount of from 1% to 40%; and polyethylene glycol/polypropylene glycol 116/66 copolymer in an amount of from 1% to 10%. However, the presence of other humectants still providing satisfactory toothpaste properties is also contemplated. 
         [0024]    In some embodiments, the toothpaste compositions of the invention include one or more binding and/or thickening agents. Binding agents may include polymers include polyethylene glycols, polysaccharides (e.g., cellulose derivatives, for example carboxymethyl cellulose, or polysaccharide gums, for example xanthan gum or carrageenan gum). Acidic polymers, for example polyacrylate gels, may be provided in the form of their free acids or partially or fully neutralized water soluble alkali metal (e.g., potassium and sodium) or ammonium salts; and include synthetic anionic polymeric polycarboxylates, such as 1:4 to 4:1 copolymers of maleic anhydride or acid with another polymerizable ethylenically unsaturated monomer, preferably methyl vinyl etherimaleic anhydride having a molecular weight (M.W.) of about 30,000 to about 1,000,000, most preferably about 300,000 to about 800,000. In some embodiments, the binder or thickener system contains polyvinylpyrolidone, fumed silica, carrageenan or one or more binding agents derived from cellulose, preferably a cellulose ether, for example carboxymethylcellulose (CMC), e.g. having a medium to high degree of polymerization, e.g. 1000 to 3000, for example about 2000, e.g., in sodium salt form, e.g., CMC 2000s, in an amount effective to provide the desired viscosity and stability. Generally, the thickeners are present in the composition in an amount of up to 10%, for example from 0.1% to 10%. Examples of thickening agents include, without limitation, the binding agents described above, which also modify viscosity, for example carboxyvinyl polymers, cellulosic polymers such as hydroxyethylcellulose, carboxymethylcellulose (carmellose) and salts thereof (e.g., carmellose sodium), natural gums such as karaya, xanthan, gum arabic and tragacanth, colloidal magnesium aluminum silicate, colloidal silica, and mixtures thereof. In some preferred embodiments, the binder or thickener system comprises polyvinylpyrolidone, for example in an amount of from 0.5% to 10%, and fumed silica, for example in an amount of up to 5% by weight of the composition. 
         [0025]    The toothpaste compositions further comprise one or more abrasives, e.g. selected from abrasive silica and/or calcium salts, e.g. calcium carbonate and/or a calcium phosphate or pyrophosphate abrasive, e.g., tricalcium phosphate (Ca 3 (PO 4 ) 2 ), hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ), or dicalcium phosphate dihydrate (CaHPO 4 .2H 2 O, also sometimes referred to herein as DiCal) or calcium pyrophosphate. In a particular embodiment, the abrasive includes one or both of calcium pyrophosphate in an amount of, for example from 5% to 30%, and silica, for example fumed silica, in an amount of, for example of up to 5%, for example from 1% to 3%. 
         [0026]    The toothpaste compositions of the invention can further include one or more detergents or surfactants. Surfactants useful for the present invention include, without limitation: anionic, nonionic, and amphoteric surfactants. Surfactants may be used, for example, to provide enhanced stability of the formulation, to help in cleaning the oral cavity surfaces through detergency, and to increase foaming of the composition upon agitation, e.g., during brushing. Suitable anionic surfactants include, for example, water-soluble salts of C 8-20  alkyl sulfates, sulfonated monoglycerides of C 8-20  fatty acids, sarcosinates and taurates; for example sodium lauryl sulfate, sodium coconut monoglyceride sulfonate, sodium lauryl sarcosinate, sodium lauryl isoethionate, sodium laureth carboxylate and sodium dodecyl benzenesulfonate, and mixtures thereof. Suitable nonionic surfactants include, for example, poloxamers, polyoxyethylene sorbitan esters, fatty alcohol ethoxylates, alkylphenol ethoxylates, tertiary amine oxides, tertiary phosphine oxides, dialkyl sulfoxides, and mixtures thereof. In one embodiment, the toothpaste comprises sodium lauryl sulfate, for example in an amount of from 1% to 3%, or about 2%. The toothpaste may also or alternatively contain one or more nonpolar surfactants, for example polymers and co-polymers of ethylene glycol and propylene glycol, e.g., poloxamers, i.e., nonionic triblock copolymers composed of a central hydrophobic chain of polyoxypropylene (poly(propylene oxide)) flanked by two hydrophilic chains of polyoxyethylene (poly(ethylene oxide)). The approximate lengths of the two PEG blocks is, in some embodiments, an average of about 50-150 repeat units, e.g., about 100 repeat units while the approximate length of the propylene glycol block is an average of about 25-75 repeat unties, e.g., about 50-60 repeat units. In one embodiment, the poloxamer is poloxamer 407, also known by the BASF trade name Pluronic F127, e.g., in an amount of from 0.5% to 2%, for example about 1%. For example, in certain embodiments, the toothpaste compositions of the invention may contain both sodium lauryl sulfate and a poloxamer such as poloxamer 407. In some embodiments, the sodium lauryl sulfate is present in an amount of from 1% to 2%, for example about 2%, and the poloxamer such as poloxamer 407 is present in an amount of from 0.5% to 2%, for example about 1%, by weight of the toothpaste composition. In a preferred embodiment, the detergent or surfactant is sodium lauryl sulfate, in an amount of from 1% to 3% by weight of the composition. 
         [0027]    The oral care compositions of the present invention may also contain a fluoride source—i.e., a fluoride-containing compound having a beneficial effect on the care and hygiene of the oral cavity, e.g. diminution of enamel solubility in acid and protection of the teeth against decay. Examples of suitable fluoride sources include sodium fluoride, stannous fluoride, potassium fluoride, potassium stannous fluoride (SNFZ-KF), potassium fluorozirconate, sodium hexafluorostannate, stannous chlorfluoride, and sodium monofluorophosphate (MFP). Where present, the fluoride source would provide fluoride ion in amounts sufficient to supply about 25 ppm to about 25,000 ppm of fluoride ions, generally at least about 500 ppm, e.g., about 500 to about 2000 ppm, e.g., about 1000 to about 1600 ppm, e.g., about 1450 ppm. The appropriate level of fluoride will depend on the particular application. A toothpaste for general consumer use would typically have about 1000 to about 1500 ppm, with pediatric toothpaste having somewhat less. A dentifrice or coating for professional application could have as much as about 5,000 or even about 25,000 ppm fluoride. The amount by weight of these materials, which dissociate or release fluoride or fluorine-containing ions, will depend on the molecular weight of the counterion as well as on the particular application, but suitably may be present in an effective but non-toxic amount, usually within the range of 0.1 to 2% by weight. In some embodiments, a fluoride source selected from sodium fluoride, stannous fluoride, sodium monofluorophosphate and mixtures thereof, is used, for example the toothpaste of the invention may comprise an effective amount of sodium monofluorophosphate. In some embodiments, the fluoride source is sodium fluoride, which is present in an amount of from 0.15% to 0.5%, or from 0.2% to 0.3% by weight of the toothpaste composition. 
         [0028]    The oral care compositions of the present invention may also contain one or more tartar control agents, for example pyrophosphates such as tetrasodium pyrophosphate, zinc citrate, and antibiotic compounds such as triclosan. In some embodiments, the oral care compositions of the invention comprise tetrasodium pyrophosphate, in an amount of up to 3% (i.e., from 0% to 3%) by weight of the composition. 
         [0029]    As will be evident to one of skill in the art, some components of the invention may perform multiple functions, and the identification of a compound as having one function herein is not meant to exclude its use for other functions in a particular composition. For example, a compound such as carboxymethylcellulose or polyethylene glycol may act as a binder, but also has humectant and thickening properties. 
         [0030]    It is also understood that compounds in formulation may naturally react, disassociate, and/or form complexes with one another. Accordingly, certain ingredients may be formed in situ (for example, it is understood that sodium chloride may be formed by reacting sodium hydroxide with hydrochloric acid), and also may in formulation exist in different forms (for example, to the extent the sodium chloride is dissolved, it will naturally disassociate into separate sodium and chloride ions, as opposed to a solid salt). As is usual in the art, the compositions of the invention are described in terms of exemplary formulation ingredients, without intending to exclude combinations of other ingredients that result in the same final compositions, or to exclude the natural reaction products of the described ingredient combinations. 
         [0031]    In some embodiments described above, the toothpaste compositions of the invention can further include one or more sweetening agents, flavoring agents and coloring agents. Any suitable flavoring or sweetening material may be employed. Examples of suitable flavoring constituents include flavoring oils, e.g. oil of spearmint, peppermint, wintergreen, clove, sage, eucalyptus, marjoram, cinnamon, lemon, and orange, and methyl salicylate. Suitable sweetening agents include sucrose, lactose, maltose, xylitol, sodium cyclamate, perillartine, AMP (aspartyl phenyl alanine methyl ester), saccharine and the like. Suitably, flavor and sweetening agents may each or together comprise from about 0.1% to 5% more of the oral care composition. In some embodiments, the toothpaste compositions of the invention include one or more flavoring agents in an amount of from about 0.5% to about 3.0%; about 0.8% to about 1.6%; or about 1.2%. In some further embodiments, the toothpaste contains one or more sweetening agents in an amount of up to 0.5% by weight of the toothpaste composition; for example up to 0.3%; or from 0.1% to 0.2%. 
         [0032]    Various other materials may be incorporated in the oral preparations of this invention such as whitening agents, including urea peroxide, calcium peroxide, titanium dioxide, hydrogen peroxide, complexes of polyvinylpyrolidone (PVP) and hydrogen peroxide, preservatives such as butylated hydroxytoluene, vitamins such as vitamin B6, B12, E and K, silicones, chlorophyll compounds, potassium salts for the treatment of dental hypersensitivity such as potassium nitrate as well as antitartar agents such as sodium tripolyphosphate and di- and tetra-alkali metal pyrophosphate salts such as di- and tetrasodium pyrophosphate. These agents, when present, are incorporated in the compositions of the present invention in amounts which do not substantially adversely affect the properties and characteristics desired. 
         [0033]    In general, each of the foregoing adjuvants may be typically incorporated in the instant toothpastes in amounts up to 5% provided they do not adversely affect the stability and cleaning properties of the non-bleeding striped dentifrice of present invention. 
         [0034]    In some embodiments of the oral care compositions of the invention, the composition comprises the bleaching system described herein, and: 
         [0035]    polyethylene glycol in an amount of from 0% to 23%; 
         [0036]    glycerin (99.0%-101.0%) in an amount of from 0% to 32%; 
         [0037]    propylene glycol in an amount of from 1% to 40/%; 
         [0038]    polyethylene glycol/polypropylene glycol 116/66 copolymer in an amount of from 1% to 10%; 
         [0039]    polyvinyl pyrrolidone in an amount of from 0.5% to 10%; 
         [0040]    calcium pyrophosphate in an amount of from 5% to 30%; 
         [0041]    silica, for example fumed silica, in an amount of from 0.1% to 5%; 
         [0042]    sodium lauryl sulfate in an amount of from 1% to 3%; and 
         [0043]    tetrasodium pyrophosphate in an amount of from 0% to 3%, by weight of the composition. 
         [0044]    In some embodiments, the invention provides oral care compositions comprising a peroxydisulfate bleaching agent, wherein the composition has a pH in the oral cavity of less than 7; or from 4 to 7; or from 5 to 6; or from 5.5 to 6 (Composition 1). For example, the invention provides: 
         [0045]    1.1. Composition 1, wherein the composition is a toothpaste. 
         [0046]    1.2. Any foregoing composition, wherein the peroxydisulfate bleaching agent is present in an amount of up to 25% by weight of the composition; or from 1% to 15% by weight of the oral care composition; or from 5% to 9% by weight of the composition; or 7% by weight of the composition. 
         [0047]    1.3. Any foregoing composition, wherein the pH of the oral care composition is from 4 to 7; or from 5 to 6; or from 5.5 to 6. 
         [0048]    1.4. Any foregoing composition, wherein the peroxydisulfate is sodium peroxydisulfate. 
         [0049]    1.5. Any foregoing composition wherein the composition comprises: 
         [0050]    one or more humectants; 
         [0051]    one or more thickeners; 
         [0052]    one or more abrasives; 
         [0053]    one or more sources of fluoride; and 
         [0054]    one or more detergents or surfactants. 
         [0055]    1.6. Any foregoing composition wherein the composition comprises: 
         [0056]    polyethylene glycol in an amount of from 0% to 23%; 
         [0057]    glycerin (99.0%-101.0%) in an amount of from 0% to 32%; 
         [0058]    propylene glycol in an amount of from 1% to 40%; 
         [0059]    polyethylene glycol/polypropylene glycol 116/66 copolymer in an amount of from 1% to 10%; 
         [0060]    polyvinyl pyrrolidone in an amount of from 0.5% to 10%; 
         [0061]    calcium pyrophosphate in an amount of from 5% to 30%; 
         [0062]    silica, for example fumed silica, in an amount of from 0.1% to 5%; 
         [0063]    sodium lauryl sulfate in an amount of from 1% to 3%; and 
         [0064]    tetrasodium pyrophosphate in an amount of from 0% to 3%, by weight of the composition. 
         [0065]    1.7. Any foregoing composition further comprising butylated hydroxytoluene in an amount of form 0.01% to 0.05% by weight of the composition. 
         [0066]    1.8. Any foregoing composition wherein the composition comprises sodium peroxydisulfate in an amount of 6% to 9% by weight of the composition. 
         [0067]    1.9. Any foregoing composition wherein the composition comprises one or more adjuvants selected from sweetening agents flavoring agents and coloring agents. 
         [0068]    The invention further provides, in another embodiment, a method for whitening teeth comprising contacting teeth with an oral care composition according to any of Compositions 1, et seq. 
         [0069]    The invention further provides, in another embodiment, the use of a peroxydisulfate bleaching system, e.g., sodium peroxydisulfate, in the manufacture of an oral care composition, e.g., according to any of Compositions 1, et seq. 
         [0070]    The following examples are further illustrative of the nature of the present invention, but it is understood that the invention is not limited thereto. All amounts and proportions referred to herein and in the appended claims are by weight, unless otherwise indicated. 
       Example 1 
     Dentifrice Formulation 
       [0071]    PDS was formulated into a commercial toothpaste. A typical composition is shown in Table 1. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Dentifrice Composition 
               
             
          
           
               
                   
                 Ingredient 
                 Percent of Composition 
               
               
                   
                   
               
               
                   
                 Polyethylene Glycol 600 
                  8.5-22.5 
               
               
                   
                 99.0%-101.0% Glycerin 
                 18-32 
               
               
                   
                 Propylene Glycol USP 
                 15-22 
               
               
                   
                 Polyethylene Glycol/Polypro- 
                 7.5 
               
               
                   
                 pylene Glycol 116/66 Copolymer 
               
               
                   
                 Sodium Peroxydisulfate 
                 7   
               
               
                   
                 Polyvinyl Pyrrolidone 
                 2.5-10  
               
               
                   
                 Sodium Saccharin 
                 0.6 
               
               
                   
                 Sucralose USP 
                   0-0.05 
               
               
                   
                 Calcium Pyrophosphate 
                 10-20 
               
               
                   
                 Fumed Silica 
                     0-2.5 
               
               
                   
                 Sodium Lauryl Sulfate 
                 2   
               
               
                   
                 Butylated Hydroxytoluene 
                  0.03 
               
               
                   
                 85% Syrupy Phosphoric Acid or 
                 0-1 
               
               
                   
                 Sodium Hydroxide - 50% Soln. 
               
               
                   
                 Tetrasodium Pyrophosphate 
                 0-2 
               
               
                   
                 Flavor 
                     1-2.5 
               
               
                   
                   
               
             
          
         
       
     
       Example 2 
     In Vitro Brushing Test 
       [0072]    An in vitro brushing test was conducted on bovine teeth using a brushing machine. Whiteness of the bovine teeth was measured using a Spectroshade from MHT Optic Research, AG, Italy. Stained bovine teeth were prophied first to have L value within 60-65. The dentifrice was made into 1:1 slurry using DI water. The prophied bovine teeth were mounted in molding clay in a tray which could be assembled in a brushing machine. The teeth were brushed using the dentifrice slurry for 2 minutes with 120 stroke/minute under 250 gram weight load. The whiteness of teeth, L*a*b*, after each brushing cycle was measured using the Spectroshade. The Whiteness Index, W, was calculated as W*=√{square root over (a* 2 +b* 2 +(L*−100) 2 )}, and delta W (compared to baseline W) was used to record the change of whiteness after brushing. A greater change on whiteness of teeth is reflected in a more negative value for delta W. 
       Example 3 
     Efficacy of Dentifrice 
       [0073]    Three dentifrices, all containing 7% PDS, were made using the formulation in Table 1 with different pH (1:1 slurry). The three dentifrices had pH values of 5.9, 7.2 and 8.5, respectively. The brushing test was conducted using these dentifrices for total 14 cycles. Delta W was calculated after 7 and 14 cycles brushing, and the results are shown in Table 2. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Delta W after Brushing Cycles for Three Dentifrices 
               
             
          
           
               
                   
                 pH = 5.9 
                 pH = 7.2 
                 pH = 8.5 
               
               
                   
                   
               
             
          
           
               
                   
                 Cycle 7 
                 −8.28 
                 −5.60 
                 −3.24 
               
               
                   
                 Cycle 14 
                 −10.23 
                 −7.25 
                 −4.26 
               
               
                   
                   
               
             
          
         
       
     
         [0074]    The results clearly show that PDS is most effective when the pH of the dentifrice is acidic. When slurry pH increases from 5.9 to 7.2 to 8.5, the efficacy of PDS consequently declines. This trend was maintained through 14 cycles brushing. The results indicate that PDS performs better in acidic conditions, which is quite different from other peroxide bleaching systems such as hydrogen peroxide, which is more effective at alkaline pH. 
         [0075]    While the present invention has been described with reference to embodiments, it will be understood by those skilled in the art that various modifications and variations may be made therein without departing from the scope of the present invention as defined by the appended claims.