Compositions: coating or plastic – Coating or plastic compositions – Corrosion inhibiting coating composition
Reexamination Certificate
2000-03-31
2001-09-11
Moore, Margaret G. (Department: 1712)
Compositions: coating or plastic
Coating or plastic compositions
Corrosion inhibiting coating composition
C106S623000, C106S286600, C106S286800, C106S287160, C106S287110, C106S014210
Reexamination Certificate
active
06287372
ABSTRACT:
COPYRIGHT NOTICE AND AUTHORIZATION
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
FIELD OF THE INVENTION
The present invention relates in general to an improved zinc coating composition for use as a primer to prevent corrosion of metal substrates, and pertains more particularly to a coating made of siloxane polymer resin in combination with zinc dust and ceramic microspheres. The zinc coating produced is economical, highly effective, and is a great improvement over prior art coatings in regards to mud cracking, even when coated thickly.
BACKGROUND OF THE INVENTION
The use of zinc containing coatings to prevent corrosion of steel substrates is well known. The zinc and the metal substrate set up an electrochemical reaction in which the zinc particles are reduced, acting as electron donors. For this reason these types of coatings are often called cathodic coatings. Traditionally, it has been taught that very high concentrations of zinc particles must be used in order for the ions flow freely. This teaching has limited attempts to use fillers in order to lower the cost by reducing the amount of zinc present.
In U.S. Pat. No. 4,891,394 issued to Ronald R. Savin, an attempt was made to lower the percentage of zinc in a coating by replacing a portion of the zinc with hollow ceramic spheres. The spheres are lightweight and economical, but would theoretically reduce conductivity in the coating, as the spheres would separate the zinc particles from one another. For this reason, the spheres were coated with zinc prior to being incorporated into the coating in order to enhance conductivity.
U.S. Pat. No. 5,252,632 issued to Savin discloses the use of uncoated hollow glass spheres as filler. By using lightweight spheres Savin '632 teaches that a significant amount of filler can be added while maintaining the necessary conductivity. Savin '632 discloses the use of microspheres, specifically microspheres having diameters up to 150 microns.
The polymers disclosed in the patent are defined in three groups. The first group includes epoxy resins, vinyl chloride resins copolymerized with isocyanates, polyurethane resins, polyester resins, oil-modified polyester resins or alkyds, polymers of acrylic and methacrylate esters and mixtures thereof. The second group includes lattices prepared by the emulsion polymerization of monomers selected from the group consisting of acrylic, methacrylic or vinyl monomers, water-borne urethanes and mixtures thereof. The third group includes polymers which are suited for use in powder coatings applied by electrostatic means or by fluidized bed immersion techniques, including polyester powders, epoxy powders, acrylic powders, and urethane powders.
Disadvantages of the Savin '632 coating include that the addition of the spheres results in pitting of the coating surface as well as splitting or cracking of the coating, known as mud-cracking, which occurs as the solvent evaporates and the coating dries. These cracks can result in loss of anti-corrosion protection in these areas. Mud cracking increases significantly with the thickness of the coating film.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an improved zinc cathodic coating utilizing ceramic microspheres as filler to reduce the amount of zinc without lessening the conductivity necessary to prevent corrosion.
Another object of the present invention is to provide a coating in which the added spheres are uniformly dispersed throughout the dried coating.
Still another object of the present invention is to provide a cathodic coating which resists mud cracking, even when coated thickly.
A further object of the present invention is to provide a cathodic coating which is easily prepared and applied.
Still a further object of the present invention is to provide a coating which exhibits improved physical characteristics.
Yet another object of the present invention is to provide a coating which is economical while being highly effective.
To accomplish the foregoing and other objects of this invention there is provided a cathodic zinc coating which provides good corrosion protection at low cost, while resisting mud- cracking.
The cathodic coating of the present invention consists essentially of a polymer binder, zinc, and ceramic microspheres. The polymer material ties the pigment together and provides adhesion to the surface being coated. Zinc powder acts as a sacrificial ion to prevent corrosion of the metal substrate. The ceramic microspheres act as spacers for the zinc dust, thereby reducing the quantity of zinc necessary in the composition. This reduction results in a lighter, less expensive coating.
In the presently preferred composition, the polymer component is a liquid alkyl silicate inorganic binder, the zinc is in the form of zinc dust, and the ceramic spheres have diameters less than 45 microns. It has been discovered that the use of this class of polymer, siloxanes, in combination with microspheres of controlled and reduced diameter, results in the superior product disclosed and claimed herein.
These and other objects and features of the present invention will be better understood and appreciated from the following detailed description of the following embodiments, selected for the purposes of illustration.
DETAILED DESCRIPTION
Unless otherwise specified, all percents are percent total volume of the coating as prepared prior to application.
The presently preferred polymer component is liquid prehydrolyzed alkyl silicate inorganic binder. Alkyl silicate binder resins are typically derived from tetraethyl orthosilicate to produce liquid polysiloxane resins. Silbond XHT33 produced by Silbond Corp., at about 20 to 40% is preferred, most preferred about 40%.
A two component solvent system is utilized to lower the viscosity of the coating to a point where application is possible. Ethyl alcohol (EtOH) and propylene glycol methyl ether acetate (PGME acetate) are presently preferred. EtOH is a relatively low density solvent and therefore helps to minimize the volatile organic compounds of the coating. In addition, the evaporation rate of EtOH causes the film to dry faster. About 1 to 10% is preferred, about 5% most preferred.
PGME acetate has a somewhat slower evaporation rate than EtOH. This allows the applied film to flow together to form a more continues film and helps minimize dry spray to which inorganic zinc primers are prone. Dry spray occurs when the solvent evaporates before the paint spray reaches the surface. About 5 to 15 % is preferred, about 8.5% most preferred.
A wetting agent is added to help with dispersion of the pigments in the binder solution improving storage stability and preventing settling. Presently preferred is Anti Terra 204, a solution of a carboxylic acid salt and a polyamine amide, by BKY Chemic. About 0.1 to 2.0% is preferred, about 0.5% most preferred.
Yellow iron oxide pigment is added as a colorant. The zinc dust utilized in the present coating renders the coating gray in color. This coloring is often difficult to distinguish from the metal substrate being coated. This pigment imparts a slight yellow-green color to the coating. Bayferrox Type iron oxide by Bayer Corp. is presently preferred, at about 0.1 to2.0%, about 0.6% most preferred.
Mica is added to the coating as an inert filler. Mica 3X by Mineral Mining Company Inc. is preferred due to the particle size and low oil absorption, at about 0.5 to 5.0%, about 1.0% most preferred.
Fumed silica is added to act as an anti-sag agent. The silica helps to hold the other pigments and components of the coating is suspension during storage. It also prevents the newly applied coating from sagging off the surface until the film dries and cures. Aerosil R805 fumed silica
Briand Remi
Freeman Hylen Keith
Blackwell Sanders Peper Martin LLP
Elliott Kyle L.
Moore Margaret G.
Themec Company Incorporated
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