Abstract:
A denture adhesive composition contains an adhesive mixture comprising carboxymethyl cellulose and partially neutralized sodium, a salt of an alkylvinyl ether maleic acid copolymer that forms crosslinked polymer complex compounds in situ with zinc cations.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to denture adhesives and methods of making and using denture adhesives. 
     2. Description of Related Art 
     Dentures are usually secured in the mouth using creams or powders having adhesive properties. These powders or creams serve both to adhere the dentures to the gums but also to provide a cushion and sealant between the gums and the dentures to permit a comfortable fit. 
     Denture adhesives must have acceptable organoleptic qualities and need to be sufficiently strong so that a daily application of the adhesive is sufficient for a full day&#39;s use. But, the adhesive must not prevent or hinder denture removal at the end of the day. 
     Most presently available commercial denture adhesives comprise vinyl alkyl ether/maleic acid (or anhydride) copolymers. This class of compounds was set out as a potential denture adhesive in U.S. Pat. No. 3,003,988 to Germann et al., issued Oct. 10, 1961. That patent is directed to synthetic, water sensitized, but water insoluble, materials comprising mixed, partial salts of lower alkyl vinyl ether-maleic anhydride copolymers for stabilizing dentures. The salts mentioned in the patent are a mixture of (a) calcium and (b) alkalis, including sodium, potassium and quaternary ammonium compounds, in a 1:1 to 5:1 mole ratio. The calcium and alkali materials are added to the copolymer to form the mixed salt. 
     Others have adopted the use of calcium in combination with alkali (especially sodium) salts of the copolymer of Germann et al. Examples of formulations using this combination are: U.S. Pat. No. 4,980,391 to Kumar et al., issued Dec. 25, 1990; U.S. Pat. No. 5,037,924 to Tazi et al.; and U.S. Pat. No. 4,910,247 to Haldar et al., issued Mar. 20, 1990. European Patent Application No 406,643, filed Jun. 22, 1990, is directed to aluminum, calcium and sodium salts of a lower alkyl vinyl ether/maleic acid copolymer. 
     These salts, however, have not proven to be fully effective at securing the denture and are susceptible to &#34;washout,&#34; the erosion of the adhesive from under the denture during use. As a result, these formulations provided less than the twelve hours of holding power that denture wearers have begun to demand. 
     In order to provide additional adhesive and cohesive properties, others have turned to zinc and strontium salts. Examples of these salts are found in, for example, PCT application No. WO 92/22280, published Dec. 23, 1992, assigned to Richardson-Vicks, Inc. (the cations may also include calcium cations, sodium cations or both); U.S. Pat. No. 4,758,630 to Shah et al., issued Jul. 19, 1988 (zinc partial salts or strontium partial salts); U.S. Pat. No. 5,073,604 to Holeva et al., issued Dec. 17, 1991 (partial calcium salts in combination with either partial zinc or partial strontium salts); PCT application No. WO 92/10988, published Jul. 9, 1992, assigned to Richardson-Vicks, Inc. (the salts may comprise zinc, strontium, sodium, calcium, magnesium, potassium or ammonium cations and mixtures thereof); and PCT Application No. WO 92/10987, published Jul. 9, 1992, assigned to Richardson-Vicks, Inc. (the salts may include zinc or strontium in combination with sodium and calcium cations). 
     Moreover, PCT Application No. WO 92/10986 is directed to mixed partial salts of lower alkyl vinyl ether-maleic anhydride copolymers. The salts are zinc or strontium cations with calcium cations. 
     Salts of vinyl alkyl ether/maleic acid copolymers have also found use outside the denture adhesive field. U.K. Patent Specification No. 1,492,038 filed Sept. 19, 1974, assigned to Smith &amp; Nephew Research, Ltd., is directed to orthopedic bandages comprising a vinyl alkyl ether/maleic anhydride copolymer with zinc, aluminum or magnesium oxides. U.S. Pat. No. 4,138,477 to Gaffar, issued Feb. 6, 1979, is directed to mouth odor control using zinc salts of vinyl alkyl ether/maleic anhydride copolymers. Begala, A. &amp; Strauss, U., &#34;Dilatometric Studies of Counterion Binding by Polycarboxylates,&#34; J. Phys. Chem., 76, p 254-60 (1972), investigates the volume changes associated with bonding of hydrogen and magnesium ions to, inter alia, a vinyl methyl ether/maleic anhydride copolymer. And Crisp, S, et al., &#34;Zinc Polycarboxylate Cements: A Chemical Study of Erosion and Its Relationship to Molecular Structure,&#34; J. Dental Rsch., 55(2), p. 299-308 (1976), reports on magnesium or zinc salts of lower alkyl vinyl ether/maleic acid copolymers. 
     Despite the effort put into improving the properties of the copolymer and the salts derived from the copolymers, these formulations do not provide the full measure of adhesion, cohesion, and resistance to washout from beneath the denture necessary for a completely successful product. 
     SUMMARY OF THE INVENTION 
     The principal object of the present invention therefore is to provide a denture adhesive composition having both strong adhesive properties and good washout resistance to provide long holding properties. The denture adhesive must exhibit sufficient tack initially upon contact with oral mucosa and build up cohesive strength quickly when hydrated with moisture or saliva and be able to resist stresses such as those that occur upon mastication. The denture adhesive must retain its adhesive properties for prolonged periods of time. 
     Another object of the invention is to provide a method for making the denture adhesive composition to achieve in situ zinc bonding. 
     Still another object of the invention is to provide a method for using the denture adhesive to secure dentures in the mouth. 
     Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from this description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     To achieve the foregoing objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a denture adhesive composition comprising: (a) a salt of a vinyl alkyl ether/maleic anyhdride or acid copolymer and a cation from Group I or Group II of the Periodic Table; and (b) a zinc compound, nonionically associated with the salt. 
     To further achieve the foregoing objects and in accordance with the purpose of the invention, the invention further provides a method for making a denture adhesive compound comprising the steps of: (a) preparing a salt of a vinyl alkyl ether/maleic anhydride or acid copolymer with a cation from Group I or Group II of the Periodic Table; (b) combining the salt with a zinc compound in a nonionic manner in a suitable carrier material to form the denture adhesive compound. 
     To further achieve the foregoing objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method for adhering dentures to gums comprising the steps of: (a) treating the dentures with a denture adhesive composition comprising: (i) a salt of a vinyl alkyl ether/maleic anhydride or acid copolymer and a cation from Group I or Group II of the Periodic Table; and (ii) a zinc compound, nonionically associated with the salt; and (b) placing the dentures in close proximity to gum tissue thereby engaging the denture adhesive composition. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the presently preferred embodiments of the invention. 
     The denture adhesive composition of the invention is most suitable in liquid and cream forms. The composition preferably contains both active and nonactive ingredients. 
     The active ingredients are cations, alkyl vinyl ether/maleic anhydride copolymer salts, preferably with 70-40% unreacted carboxylic acid groups, carboxyl methyl cellulose gum, zinc oxide and/or zinc salts (organic or inorganic zinc salts). 
     The copolymer of the invention is a vinyl alkyl ether/maleic acid or anhydride copolymer. The preferred copolymer is a copolymer available from GAF sorporation, Wayne, N.J., under the trademark &#34;GANTREZ&#34; in two forms: the acid form is sold under the trademark GANTREZ S&#34; Series, and the preferred type of acid is identified as &#34;GANTREZ S95&#34; or &#34;GANTREZ S97. &#34; The anhydride form is sold as the &#34;GANTREZ AN&#34; Series, and the preferred anhydride is &#34;GANTREZ AN169. &#34; 
     The preferred alkali cations are alkali cations, although cations from Group I and Group II of the Periodic Table may be selected. Acceptable cations include sodium, potassium, magnesium, calcium, and strontium. Preferred are sodium and potassium, and most preferred is sodium. 
     When the salt is prepared, the cations associate with ionic bonding sites on the copolymer and are neutralized. Preferably less than all the ionic bonding sites on the copolymer are used. More preferably, the cations occupy from about 5% to about 60%, and most preferably from about 20% to about 60% of the bonding sites on the copolymer, leading to a &#34;partially neutralized&#34; copolymer. 40% partial neutralization is most preferred. 
     Preferably, the salt comprises from about 10% to about 50% by weight of the total denture adhesive composition. More preferably the salt comprises from about 20% to about 40% of the final denture adhesive composition and most preferably about 30% by weight of the denture adhesive composition. 
     Carboxymethyl cellulose gum is used for sensitizing the adhesive to moisture, enhancing the cohesive properties of the formulation and improving gel strength. Acceptable substitutes may be selected from carboxyethyl cellulose and carboxypropyl cellulose materials. The carboxymethyl cellulose gum preferably comprises from about 10% to about 30% by weight of the denture adhesive composition, more preferably from about 15% to about 25% and most preferably about 20% of the composition. The carboxymethyl cellulose may be present in the form of a full or partial salt, preferably a sodium salt. During use, zinc from the added salt of the invention will replace at least some of the sodium and from crosslinks in the carboxymethyl cellulose or crosslinks with the copolymer. 
     Acceptable zinc compounds include oxides, carbonates and halides. While other anions of Group VI of the Periodic Table may be used in place of oxides, oxides are preferred for their ease of handling and organoleptic qualities. Preferred halides include fluorides, chlorides and bromides. Chlorides are most preferred. 
     Nonactive ingredients present in the denture adhesive include thickening agents and carriers, such as petrolatum and mineral oil, flavors, colors, preservatives, thickeners and non-toxic anticaking agents such as silica, magnesium stearate and talc. Acceptable nonactive ingredients, and the appropriate levels at which the ingredients should be present in a particular formulation for a specific purpose are well known in the art. 
     For a cream base, the mineral oil or other carrier preferably comprises from about 10% to about 30% by weight of the denture adhesive, preferably from about 15% to about 25% by weight. Petrolatum or some other thickening agent comprises from about 15% to about 30% by weight of the denture adhesive composition, preferably from about 20% to about 30% by weight. 
     Other nonactive ingredients comprise colorants, flavors and preservatives and may comprise up to about 10% by weight of the denture adhesive composition. 
     In use, the partially neutralized copolymer salt will provide the initial tack quickly when it comes in contact with moisture in the oral mucosa. When it is hydrated with moisture or saliva, zinc oxide and/or the other zinc salts present in the oil phase of the adhesive become available to form zinc copolymer complexes. Zinc from zinc oxide will react more favorably with acidic carboxylic acid groups by means of an acid-base interaction while the zinc from zinc halide salts would be more effective in replacing sodium cations from carboxylate groups. 
     The method of making the denture adhesive composition is a two-step process of making the copolymer salt and then incorporating the salt into the denture adhesive composition. 
     The copolymer salt is made by heating a suitable solvent, such as water (deionized or distilled water is preferable) to a temperature sufficient to hasten dissolution of the copolymer in the solvent. In the case of methyl vinyl ether/maleic anhydride copolymer being dissolved in water, the temperature is about 90° C. 
     Separately, a donor solution for the cation can be prepared. Preferably the solvent for the solution is identical to, or compatible with, the solvent for the copolymer. Preferably the donor solution comprises the cation or cations to be used to form the salt and an initiator, such as a Lewis acid or base, to facilitate the partial substitution of the cation for the hydrogen in the copolymer. In the event that the cation is sodium or potassium, suitable solutions would include solutions of sodium or potassium hydroxide, compounds that provide both the cation and a Lewis base. 
     After the copolymer solution has cooled sufficiently, the cation donor solution and the copolymer solution are allowed to react. The product is dried in a drier to form a dried material that is then milled to a small size (about 100 mesh) and further processed to form the denture adhesive of the invention using techniques well known in the art. 
     The following non-limiting examples will serve to illustrate the invention. 
     EXAMPLE 1 
     3.25 Kg of purified water were heated in a reaction vessel equipped with a high speed stirrer to 90° C. 260 grams of methyl vinyl ether/maleic anhydride copolymer were slowly added to the reaction vessel and mixing was continued until a clear solution was obtained. The solution was then cooled to 65°-75° C. 
     7.41% (by weight) NaOH solution was prepared in a separate container. When the temperature of the copolymer solution reached 65°-75° C., 539.8 grams of 7.41% NaOH solution was added while mixing at high speed (to prevent localized reaction). 
     The combination was then mixed for 15 minutes after the addition of the NaOH solution. The sodium salt of methyl vinyl ether/maleic anhydride copolymer was then transferred to a series of shallow steel drying trays, and the trays were placed in a hot air convection oven at 75° C. for 18-20 hours. The dried sodium salt was then milled through a suitable mill. The milled salt is then screened through a 100 mesh sieve. The powder had a bulk density of about 0.5-0.9 g/cc and the pH of one percent solution is 4-5. This salt was identified as a 30% Na partial salt of MVE/MA copolymer. 
     EXAMPLE 2 
     3.25 kg purified water was heated in a reaction vessel equipped with a high speed stirrer to 90° C. 248.9 g methyl vinyl ether/maleic anhydride copolymer was slowly added to the reaction vessel, and mixing was continued until a clear solution was obtained. The heat was then turned off and the solution was cooled to 65°-75° C. 
     In a separate container, a 9.26% NaOH solution was prepared. 551.1 g of the NaOH solution were slowly added to the cooled solution while mixing. Mixing was continued for 15 minutes. 
     The sodium salt/copolymer solution was then transferred to shallow stainless steel drying trays and placed in a hot air convection oven at 75° C. for 18-20 hours. The dried sodium Gantrez salt was then milled through a suitable mill. The milled salt is then screened through a 100 mesh sieve. The powder had a bulk density of about 0.5 to 0.9 g/cc, and the pH of a one percent solution was 4-6. The salt was a 40% Na partial salt of MVE/MA copolymer. 
     EXAMPLE 3 
     3.25 kg purified water was heated in a reaction vessel equipped with a high speed stirrer to 90° C. 238.8 g methyl vinyl ether/maleic anhydride copolymer was slowly added to the reaction vessel and mixing was continued until a clear solution was obtained. The heat was then turned off and the solution was cooled to 65°-75° C. In a separate container, a 10.91% NaOH solution was prepared. 561.2 g of the NaOH solution were slowly added to the cooled solution while mixing. Mixing was continued for 15 minutes. 
     The sodium salt/copolymer solution was then transferred to shallow stainless steel drying trays and placed in a hot air convection oven at 75° C. for 18-20 hours. The dried sodium salt is then milled through a suitable mill. The milled salt is then screened through a 100 mesh sieve. The powder would have a bulk density of about 0.5 to 0.9 g/cc, and the pH of a one percent solution is 5-7. The salt is a 50% Na partial salt of MVE/MA copolymer. 
     EXAMPLE 4 
     3.25 kg purified water was heated in a reaction vessel equipped with a high speed stirrer to 90° C. 229.4 g methyl vinyl ether/maleic anhydride copolymer was slowly added to the reaction vessel and mixing was continued until a clear solution was obtained. The heat was then turned off and the solution was cooled to 65°-75° C. In a separate container, a 12.37% NaOH solution was prepared. 570.4 g of the NaOH solution were slowly added to the cooled solution while mixing. Mixing was continued for 15 minutes. 
     The sodium salt/copolymer solution was then transferred to shallow stainless steel drying trays and placed in a hot air convection oven at 75° C. for 18-20 hours. The dried sodium salt is then milled through a suitable mill. The milled salt is then screened through a 100 mesh sieve. The powder would have a bulk density of about 0.5 to 0.9 g/cc, and the pH of a one percent solution is 6-8. The salt is a 60% Na partial salt of MVE/MA copolymer. 
     EXAMPLE 5 
     Denture adhesive creams were prepared using the sodium salts of Examples 1-4. The respective salts were blended with the other ingredients in Table 1 in the following manner. 
     Mineral Oil was heated to a temperature of about 165° F. Petrolatum of about the same temperature was added to the mineral oil and blended until a uniform consistency was reached. Sodium carboxymethyl cellulose (CMC), colorant, zinc oxide, zinc chloride and flavoring were added to the mixture, as appropriate, while the mixture slowly cooled to about 120° F. The respective salts were added at about 120° F. The combination was then cooled to room temperature and tested for shear strength. 
     The shear strength was measured as follows: three homogeneously mixed samples of adhesive/water combinations were prepared at weight ratios of 1:1, 1:0.75 and 1:0.5. Each combination was spread on about 1 square inch of a separate transparent acrylic slide. A second slide with a 3/8&#34; diameter hole in one end was placed over the first slide. 20 lbs. pressure was placed on the slide and was held at that level for 5 seconds. The slides were set aside for hydration for about 1 or about 5 minutes. 
     20 lbs. pressure was again applied for 5 seconds, and the shear strength was measured on an Instron Model 1122 machine. The results of the measurements were averaged. The results at 1 minute and at 5 minutes hydration are reported in Table 1. 
     EXAMPLE 6 
     As a comparison to Example 5, commercial denture adhesives were tested for shear strength. The results are shown in Table 2. 
     
                       TABLE 1______________________________________Test Formulation NumberIngredient      1       2       3     4     5     6______________________________________Mineral Oil     23.0    19.0    16.5  16.5  16.5  20.0Petrolatum     22.0    26.0    27.0  27.0  28.0  26.0Colorant   0.05    0.05    0.05                            0.05                                  0.05                                        0.05CMC       19.0    19.0    20.0  20.0  19.0  19.0(sodium)30% Na    29.5    --      --    --    --    --Gantrez Salt40% Na    --      29.5    --    --    30.0  29.5Gantrez Salt50% Na    --      --      30.0  --    --    --Gantrez Salt60% Na    --      --      --    30.0  --    --Gantrez SaltZinc Oxide      6.0     6.0     6.0   6.0    3.0                                        5.0Zinc      --      --      --    --     3.0  --ChloridePreservative      0.05    0.05    0.05                            0.05                                  0.05                                        0.05Flavor     0.4     0.4     0.4   0.4   0.4   0.4Shear     715     953    1180  1816  1135  1249(1 minute)g/in.sup.2Shear    1498    2088    2361  2542  1385  2043(5 minutes)g/in.sup.2______________________________________ 
    
     
                       TABLE 2______________________________________      Commercial Commercial Formulation      Formulation                 U.S. Pat. Nos.Shear      U.S. Pat. No.                 4,758,630Rate       3,003,988  5,073,604______________________________________Shear      397        1158(1 minute)g/in.sup.2Shear      658        1271(5 minutes)g/in.sup.2______________________________________ 
    
     The purpose of the above description is to illustrate some embodiments of the present invention without implying a limitation. It will apparent to those skilled in the art that various modifications and variations may be made in the apparatus or procedure of the invention without departing from the scope or spirit of the invention.