The present invention is directed to water-reducible coating compositions containing a modified polyamine resin and/or epoxy resin.
Organic solvents used in coating compositions have been identified as a contributor to air pollution. Since 1966 legislation and regulations on volatile organic compounds have been directed to limiting the use of organic compounds in coatings. Regulations have been issued to limit the volatile organic content (VOC) of organic coatings to a maximum of 350 grams per liter, where ##EQU1## where VOM=pounds of volatile organic material per gallon of coating material as applied, and W=the volume of water per gallon in the coating as applied.
The volatile organic material is defined as any volatile compound of carbon, excluding methane, carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, ammonium carbonate, methylene chloride, 1,1,1-trichloroethane, and trichlorotrifluoroethane.
In order to achieve low VOC content, much attention is being directed toward waterborne coatings. Advantages of waterborne coatings include reduced air pollution, limited fire and health hazards, and reduced usage of costly organic solvents.
Although a number of waterborne coatings are presently available, none has the high performance properties required to match existing catalized, ambient temperature curing, solvent-borne coatings such as two package epoxy coatings. Such high performance properties are vital for protection of metal surfaces on ships, aircraft, trucks, bridges, tanks and the like. Although each high performance coating has numerous unique requirements dictated by its end use, all high performance coatings must be insensitive to water, solvents, and other fluids.
Water sensitivity is largely due to hydrophilic groups present in the polymer binder and/or hydrophilic paint additives. Most water-borne coatings have a preponderance of hydrophilic groups and hence are water sensitive. Solvent and fluid resistance properties are only obtained when the coating binder is highly crosslinked, as is the case with catalized systems. Unfortunately, most water-borne, ambient temperature, crosslinked systems such as latexes and water-soluble alkyds and acrylics, have insufficient crosslink densities for high performance applications.
Two component amino-epoxy coating systems have been considered for use in high performance, ambient temperature curing, water-borne coating systems. Because the most suitable epoxy resins and amino functional curing agents are not directly water reducible, various systems have been devised to overcome this technical difficulty.
One system that has been devised uses an emulsified epoxy resin. The epoxy resin is either emulsified by a surfactant type emulsifying agent prior to use or emulsified during catalization by amino-functional curing agents which have been modified to have emulsifying properties. These emulsifier type curing agents are of two types: aminofunctional curing agents which are neutralized by acids to form salts, such as disclosed in U.S. Pat. Nos. 2,899,397 and 4,013,601; and aminofunctional curing agents (generally, special polyamide types) which are chemically modified with hydrophilic ether groups so they can be water reduced and emulsified, such as disclosed in U.S. Pat. Nos. 3,998,771 and 4,179,418.
These emulsified epoxy resins systems suffer from poor water resistance. Curing agents of the salt type remain water sensitive after curing due to residual salt. Curing agents modified with the hydrophilic ether groups remain permanently water sensitive. Furthermore, only low molecular weight curing agents can be used because of the very high viscosities which develop when the high molecular weight curing agents are neutralized.
Curing agents useful in curing epoxy resins preemulsified using surfactant type emulsifiers are disclosed in U.S. Pat. Nos. 4,086,179; 4,104,223; 4,152,285; 3,816,366; and 3,956,208. All of these curing agents are low molecular weight polyamide types specially modified to make them more water compatible by using low molecular weight carboxylic acids and/or hydrophilic polyamines in their makeup. Most have amine values greater than 300. Almost all polyamide type resins with amine values of greater than about 400 are inherently water reducible without modification. The amine value is the number of milligrams of KOH equivalent per free amine groups in a one (1) gram sample of resin. In general, the higher the amine value of the resin the shorter the pot life of the catalized mixture and the more brittle the resulting coating film. Because of the high amine value of these modified polyamide type curing agents they have limited utility due to their short pot life.
A different approach to water-borne epoxy coatings is disclosed in U.S. Pat. Nos. 3,719,629 and 3,945,963. These patents disclose systems which are water-reducible acrylic copolymers with pendant amino groups. When catalized, these pendant amino groups react with the epoxy resin to form a crosslinked acrylic coating film. Unfortunately, the resulting coatings are not flexible. Also, the films are water and solvent sensitive.
From the foregoing, it is evident that there is a need for a water-borne coating composition that has a long pot life and that produces high performance coatings which have solvent and water resistance, and are durable and flexible.