Source: {"pile_set_name": "USPTO Backgrounds"}

This invention relates to rinse additive compositions containing insoluble inorganic zinc salts which are useful for inhibiting glassware corrosion which can occur in an automatic dishwasher.
Corrosion of glass in automatic dishwashers is a well known phenomenon. A paper by D. Joubert and H. Van Daele entitled "Etching of Glassware in Mechanical Dishwashing" in Soap and Chemical Specialties, Mar. 1971, pp. 62, 64, and 67, discusses the influence of various detergent components, particularly those of an alkaline nature. This subject is also discussed in a paper entitled "The Present Position of Investigations Into the Behavior of Glass During Mechanical Dishwashing" presented by Th. Altenschoepfer in April, 1971, at a symposium in Charleroi, Belgium, on "The Effect of Detergents on Glassware in Domestic Dishwashers". See also another paper delivered at the same symposium by P. Mayaux entitled "Mechanism of Glass Attack by Chemical Agents".
It has been determined that the glassware corrosion problem actually consists of two separate phenomena; one is corrosion due to the leaching out of minerals from the glass composition itself together with hydrolysis of the silicate network, and the second is deposition and redeposition of silicate material onto the glass. It is a combination of the two that can result in the cloudy appearance of glassware that has been washed repeatedly in an automatic dishwasher. This cloudiness often manifests itself in the early stages as an iridescent film that becomes progressively more opaque with repeated washings. The harsh washing conditions of the automatic dishwashing process, particularly the use of detergency builders and the high alkalinity, are believed to cause glassware corrosion.
Use of zinc, in general, in automatic dishwashing detergent compositions to prevent glass corrosion is known, See for example, U.S. Pat. No. 3,677,820, Rutkowski, issued July 18, 1972, which discloses hanging a strip of metallic zinc in the dishwasher to prevent corrosion of glassware. U.S. Pat. No. 3,255,117, Knapp et al, issued June 7, 1966, discloses the use of soluble zinc salts in automatic dishwashing detergent compositions to prevent glassware corrosion. This reference states that introducing soluble metal salts (alkali aluminate, zincate, or berylliate) in automatic dishwashing detergent compositions can result in precipitation out of insoluble material. Such material is said to be very undesirable as it can adhere to dishwasher parts and dishware during the washing cycle. This precipitation is said to be avoided by carefully adjusting the levels and proportions of the various components in product formulation.
U.S. Pat. No. 3,350,318, Green, issued Oct. 31, 1967, also describes the use of soluble zinc salts (sodium aluminate, sodium zincate) to prevent attack by automatic dishwashing detergent compositions of overglaze colors and decorations on fine china and the aluminum of pots and pans. The problem of precipitate formation is discussed and said to be avoided by spraying a solution of the soluble zinc salt onto granular polyphosphate particles.
U.S. Pat. No. 2,575,576, Bacon et al, issued Nov. 20, 1951, describes the use of a water-soluble zinc or aluminum salt to prevent the corrosion of vitreous and ceramic surfaces. It is stated that the problem of compounding alkali metal salts such as sodium carbonates, -phosphates, -silicates, or -sulfates with water-soluble zinc or aluminum compounds is that an undesirable precipitate is formed. This problem is said to be overcome by the careful choice of particular components at particular ranges and proportions.
U.S. Pat. No. 3,755,180, Austin, issued Aug. 28, 1973, describes use of a precipitated silico-aluminate compound for inhibiting overglaze attack in china. Again, the problem of precipitate formation when soluble zinc and aluminum salts are utilized for this purpose is discussed. (See also U.S. Pat. No. 3,966,627, Gray, issued June 29, 1976.)
U.S. Pat. No. 4,443,270, Baird et al, issued Apr. 17, 1984, discloses a rinse additive formulation containing a low-foaming nonionic surfactant, a chelating agent, a hydrotrope-water solubilizing system, and a soluble magnesium, zinc, or bismuth salt. The metal salt is said to be present for protection against glassware corrosion caused in the rinse. See also U.S. Pat. No. 4,416,794, to Barrat et al, issued Nov. 22, 1983. More specifically, the water-soluble zinc salts of chloride, sulfate, or acetate are taught.
Despite these disclosures, there is a continuing need for methods of providing protection against glassware corrosion in the dishwasher without causing the formation of insolubles.
Accordingly, it is an object of the present invention to provide compositions which protect glassware against corrosion in the dishwasher without causing the formation of insolubles which can adhere to dishwasher parts and dishware.
It has been surprisingly discovered that by utilizing certain insoluble inorganic zinc salts in rinse additive compositions, the above objectives can be attained.