Protective coatings are often used on many mechanical or protective surfaces to prevent corrosion, abrasion, and other wear on the surface. High performance coatings have been developed which exhibit high resistance to abrasion, corrosion, impact, and other characteristics of high performance. These coatings include powder comprising finally divided inert particles dispersed in a resin which is then mixed with a hardening catalyst and applied.
The resin of the present coating should be selected for its wear resistant properties and by the strength of its adhesion to the surface being protected. Some commonly used resins include epoxy (e.g., Bisphenol A and Bisphenol F based epoxies), polyester, vinyl-ester, phenolic, novolac (phenol/novolac), and polyglycol resin. Epoxy resins are known to those skilled in the art to have the highest adhesion values of all resins.
The adhesion strength of coatings is measured according to an international industry standard referred to as American Standard Testing Materials (ASTM). ASTM adhesion testing involves applying and curing a coating on to a surface substrate prepared according to National Association of Corrosion Engineers (NACE) specifications. For example, a steel test panel prepared to an anchor profile of 1 to 21/2 mils (white to near white metal) and cleaned of substantially all contaminates. The coating is applied and cured at room temperature (72.degree. F.). Adhesion is tested according to ASTM protocol by pulling off the cured coating and measuring the force required to pull the coating of the surface. Adhesion is measured under ASTM protocol in metric units known as Megapascals (Mpa). For convenience, Mpas are often converted herein to approximate English units of pounds per square inch (psi).
In addition, or alternatively to ASTM testing, adhesion may also be measured by resistance to hydroblasting, i.e., measuring the force (in psi) of a pressurized stream of water to dislodge a coating from a standard prepared surface. Typically, adhesion strengths are the same whether measured by ASTM pulling or hydroblasting. One drawback of ASTM testing is that the surface substrate may fail by mechanical breakage or some other means before the coating being tested fails.
Epoxy resins are known to those skilled in the art to have the highest adhesion strength of commonly available resins, epoxy having adhesion strength measured in the range of approximately 800 to 1,200 psi.
Phenol/novolac resins are thermoset plastic materials offering alternatives to Bisphenol A-based epoxies and Phenolic resins, particularly when formulators and fabricators seek good strength and good chemical resistance at high temperatures. Phenol/novolac epoxy combines in one molecule the stability of a phenolic backbone with the reactivity and versatility of an epoxy resin. The resulting resins have multi-epoxy functionality. The additional reactive sites, as compared to a Bisphenol A-type resin, produce tightly cross-linked cured systems with improved resistance to acids, bases, and solvents; retention of good mechanical properties at high temperatures; minimal shrinkage for accurate reproduction; acceptance of a wide range of modifiers, fillers, and pigments; and improved high temperature adhesive properties. The phenol/novolac resin is usually received from the manufacturer dissolved in 15% acetone by weight of phenol/novolac.
The resin carrying the inert particles must be mixed with an appropriate hardening catalyst or curing agent to form a base composition before application. Generally both the resin and catalyst are supplied from the coating manufacturer with instructions as to mixing. A coating commercially available is manufactured by Owens-Corning and marketed under the name Owens-Corning Abrasion Resistant Coating (ARC). The Owens-Corning coating contains finally divided ceramic particles dispersed in an epoxy resin, and a compound that adds flexibility to the coating.
Another coating commercially available is manufactured by Freecom, Inc. under the name "Ceram-Kote 54." The Freecom product is sold with instructions that allow the user to dilute or thin the resin and catalyst mixture with an appropriate solvent to provide for ease of application and for various desired surface finishing and coating qualities.
To the base composition mixture various additives may be combined to greatly enhance the qualities of the final product. Such additives may include pigments for color and thixatropes to inhibit running and sagging, a variety of inert ceramic powers added to enhance the abrasion resistant capabilities of the coating, and selected solvents to dilute the coating for enhanced uniformity of coverage of the surface being coated. The solvent or solvents should be selected on the basis of their lack of reactivity with any other components of the coating, particularly the base composition and any other additives. Isopropyl alcohol (isopropanol) and methyl-ethyl-ketone (MEK) have shown to be very good solvents in the present invention.
U.S. Pat. No. 4,789,567, issued Dec. 6, 1988, to Freeman, hereinafter referred to as the '567 patent, and which is incorporated herein by reference, discloses a protective coating of finely divided abrasions resistant inerts carried in a corrosion resistant epoxy resin that is diluted with a solvent and applied to a surface. The coatings of the '567 patent may be cured to achieve either a glossy or a mat finish. The '567 patent, however, does not disclose nor teach the performance characteristics of the coating of the present invention. Nor does the '567 patent teach the use of additives in the manufacture of the coating to achieve desired performance characteristics.
U.S. Pat. No. 4,968,538, issued Nov. 6, 1990, to Freemen, a continuation-in-part of the '567 patent, hereinafter referred to as the '538 patent, and which is incorporated herein by reference, discloses a protective coating of finely divided abrasion resistant inerts in a corrosion resistant epoxy resin diluted with a solvent, and further disclosed the addition of novolac resin dissolved in methyl-ethyl-ketone (MEK) and polyglycol di-epoxide resin to the epoxy resin carrying inert particles. The '538 patent, however, the '567 patent, however, does not disclose nor teach the performance characteristics of the coating of the present invention. Nor does the '567 patent teach the use of additives in the manufacturer of the coating to achieve desired performance characteristics.