Coating processes – With pretreatment of the base – Etching – swelling – or dissolving out part of the base
Reexamination Certificate
2000-08-28
2002-03-19
Beck, Shrive P. (Department: 1762)
Coating processes
With pretreatment of the base
Etching, swelling, or dissolving out part of the base
C427S140000, C427S290000, C427S292000, C427S307000, C216S096000, C216S108000
Reexamination Certificate
active
06358564
ABSTRACT:
This invention pertains to a process for refinish coating of porcelain and ceramic surfaces, and more particularly to process steps for preparing old porcelain and ceramic substrates and refinish coating the substrate with epoxy enamel protective coatings.
BACKGROUND OF THE INVENTION
Repainting projects ordinarily require good preparation of the old substrate prior to applying a new paint coating. Substrate preparation is particularly important prior to refinish coating an existing porcelain or ceramic surfaces such as bathroom tubs. The refinish process ordinarily requires a tedious series of steps including cleaning, etching, sanding, washing, drying and otherwise preparing the old surfaces prior to refinish coating being applied. In the past, potentially harmful and dangerous chemicals were used to prepare the surfaces for refinish coating, such as muriatic acid or hydrochloric acid, which were used to etch the surfaces prior to refinish coating. These chemicals however are known to be difficult to use, especially for a non-professional, and frequently expel noxious odors and fumes which can be poisonous and perhaps even fatal in some instances in a non-ventilated area. The acids are corrosive and require protective clothing when using to avoid contact with skin and to keep the vapors from causing a burning sensation of the eyes.
It now has been found that a new simplified and much safer method of refinish coating old porcelain and ceramic substrates can be achieved by using citric acid to etch the substrate surfaces in the surface preparation process. Citric acid is a naturally occurring organic acid found in citrus fruits and commonly is used in foods and pharmaceuticals. Citric acid for instance is often used as an additive in fruit flavored beverages. A major advantage of this process is that citric acid is a rather strong organic acid but not corrosive as muriatic or hydrochloric acids and can be easily handled without health or environmental concerns and without special handling precautions. Since citric acid is widely used in foods, and the toxicity in use is negligible, while the process of preparing the substrate including etching the substrate surface can be done by an ordinary craftsman without elaborate protective precautions. Disposal of citric acid does not create a hazardous chemical disposal problem. Citric acid is a dry solid and can be easily mixed with water to form an aqueous solution for use. In preferred aspects of this invention, aqueous sodium carbonate solution is used to clean the substrate prior to the step of etching with citric acid solution. After the porcelain or ceramic substrate has been properly prepared in accordance with this invention, an epoxy enamel can be applied and cured to form a hard painted surfaces comparable to original substrate surface with excellent adhesion to the former porcelain or ceramic substrate. With old porcelain sink and tubs, particularly those produced before 1980, the citric acid treatment resulted in equal or better adhesion of the epoxy finish coat when compared to hydrochloric acid treatment. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and illustrative examples.
SUMMARY OF THE INVENTION
Briefly, the invention pertains to a process of preparing and coating existing porcelain and ceramic substrates with a refinish coating, the process steps including cleaning, etching, sanding, and washing, where the improvement comprises etching the substrate with aqueous citric acid prior to the step of refinish coating the substrate with epoxy coating. The preferred process includes the step of cleaning old ceramic or porcelain substrate with an aqueous cleaning solution of sodium carbonate prior to the step of etching with citric acid.
DETAILED DESCRIPTION OF THIS INVENTION
The invention pertains to using citric acid to etch porcelain or ceramic substrates with citric acid prior to the step of refinish coating the substrate with epoxy coating. Citric acid is a naturally occurring organic tricarboxylic acid compound commonly found in most fruit and some plants. In dry form, citric acid can by anhydrous or monohydrate crystalline solid ordinarily translucent and colorless in form. Citric acid and the anhydrous or salt forms of citric acid are ordinarily solvated in water to typically form a 50% by weight aqueous solution of citric acid. The water content of the aqueous solution is not critical but preferably is between about 30% and 70% by weight water, with the remainder being citric acid.
In accordance with the process of this invention, an existing porcelain or ceramic surface substrate can be refinish coated by first etching the substrate surface with an aqueous mixture of citric acid and then coating the prepared and etched surface with epoxy coating. Ordinarily the old substrate surface is cleaned, etched, sanded, rinsed and dried prior to applying refinish epoxy coating to the substrate, although the order of steps is not critical. The etching treatment of the substrate surface with citric acid is washed off and the surface preferably is dried prior to coating the substrate surface with epoxy. The citric acid solution is preferably applied to the substrate at ambient temperature and conveniently between about 40 and 95° F. The citric acid solution need not be warmed although solution temperatures between about 60° F. and 85° F. have been found to be particularly helpful.
In preferred aspects of this invention, the old ceramic or porcelain substrate is first cleaned with an aqueous solution of sodium carbonate prior to the step of etching with citric acid. An aqueous sodium carbonate solution is strongly alkaline or ordinarily can contain about 10% to 30% by weight sodium carbonate in water. Sodium carbonate is a white crystalline solid known in the trade as ash, soda ash, soda, and calcined soda.
Prior to the discovery of the use of sodium carbonate cleaning solution in accordance with the invention, substrate cleaning solutions were based on trisodium phosphate solutions. However, trisodium phosphate solution, a common cleaning solution for many purposes, is a hazardous chemical and, in fact, has been banned in several states in the United States because of the phosphate content. The phosphate causes serious environmental hazard associated with phosphates in the water supply. In contrast, sodium carbonate solutions useful in accordance with this invention are not an environmental nor a health hazard and, hence, can be safely used without extraordinary safety precautions.
After the old substrate surface has been properly prepared in accordance with this invention, the prepared and etched substrate can then be coated with epoxy coating. Preferred epoxy coatings are two component coreactive epoxy coatings based on epoxy binder comprising a reactive epoxy terminated component and a coreactive crosslinking component. Epoxy coatings ordinarily comprise resin binder, water or organic solvent, curing agents, and pigments.
Epoxy resins are predominantly linear chain molecules comprising the coreaction product of polynuclear dihydroxy phenols or bisphenols with halohydrins to produce epoxy resins containing at least one and preferably two epoxy groups per molecule. The most common bisphenols are bisphenol-A, bisphenol-F, bisphenol-S, and 4,4′dihydroxy bisphenol, with the most preferred being bisphenol-A. Halohydrins include epichlorohydrin, dichlorohydrin, and 1,2-dicloro 3-hydroxypropane with the most preferred being epichlorohydrin. Preferred epoxy resins comprise the coreaction product of excess molar equivalents of epichlorohydrin and bisphenol-A to produce predominately an epoxy group terminated linear molecular chain of repeating units of diglycidyl ether of bisphenol-A containing repeating copolymerized units of diglycidyl ether of bisphenol-A. In practice, excess molar equivalents of epichlorohydrin are reacted with bisphenol-A to produce epoxy resins where up to two moles of epichlorohydrin coreact with one mole of bisphenol-A,
Beck Shrive P.
Calcagni Jennifer
Schmitz Thomas M.
The Glidden Company
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