Abstract:
A curing agent for fluorocarbon polymer coating compositions selected from the following group: ##STR1## where R is R 1  NH, R 1  is an aliphatic or cycloaliphatic hydrocarbon radical, R 2  is C or an aliphatic hydrocarbon group, R 3  is H or CH 3 , where x is 3 when R 2  is a hydrocarbon group and x is 4 when R 2  is C; the curing agents form ambient curing fluorocarbon polymer coating compositions; 
     These compositions are used to provide corrosion and abrasion resistant coatings for large structures in chemical plants, oil refineries, oil drilling platforms, and the interior of smoke stacks of large utility companies.

Description:
BACKGROUND OF THE INVENTION 
     This invention is related to a curing agent and in particular to a curing agent for a fluorocarbon polymer coating composition. 
     Fluorocarbon polymers are inert to strong acids such as sulfuric acid, nitric acid, hydrochloric acid and strong bases such as sodium hydroxide and are resistant to weathering and salt water corrosion and are tough and abrasion resistant. Coatings of these polymers would be useful in chemical plants and oil refineries to coat pipes, vessels and other equipment, on off shore oil well platforms, on ships, and as protective coatings for the interior of smoke stacks of utility companies. Fluorocarbon polymer coatings would be particularly useful for metal smoke stack interiors which are subjected to abrasion from fly ash and corrosion by acids resulting from combustion products such as SO x  and NO x  and halogen ions. However, conventional fluorocarbon polymer coatings require curing at elevated temperatures which could not be used on the aforementioned structures. An ambient curing fluorocarbon polymer coating composition is required. 
     The curing agent of this invention is used in fluorocarbon polymer coating composition and provides a composition that cures at ambient temperatures. 
     SUMMARY OF THE INVENTION 
     A curing agent for fluorocarbon polymer coating compositions selected from the following group: ##STR2## where R is R 1  NH 2 , R 1  is an aliphatic or cycloaliphatic hydrocarbon radical, R 2  is C or a hydrocarbon, R 3  is H or CH 3  ; where x is 3 when R 2  is a hydrocarbon group and X is 4 when R 2  is C. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Curing agent (1) is prepared by reacting 1 mole of dialkyl maleate with 3 moles of an aliphatic or cycloaliphatic polyamine at about 100°-150° C. for about 1-6 hours. Alkanol formed in the reaction is removed. Preferred reaction conditions are about 110°-130° C. for about 2-4 hours. Preferably, the above constituents are in an alkanol solvent such as methanol, propanol isopropanol and the like. 
     Typical dialkyl maleates that can be used to form the curing agent are dimethyl maleate, diethyl maleate, dipropyl maleate, diisopropyl maleate, methyl ethyl maleate, methyl propyl maleate, ethyl propyl maleate, dibutyl maleate and the like. Dimethyl maleate is preferred. 
     Typical polyamines used to form the curing agent are isophorone diamine which is 3-aminomethyl-3,5,5-trimethylcyclohexylamine, hexamethylene diamine, ethylene diamine, 1,4-cyclohexane bis(methylamine), 1,2-diaminopropane, propylene diamine, diethyl ether diamine, trimethylhexamethyl methylene diamine. 
     One preferred curing agent is the reaction product of isophorone diamine and dimethyl maleate and has the following structural formula: ##STR3## 
     Curing agent (2) is prepared through a Michael&#39;s reaction in which a multifunctional acrylate or methacrylate is reacted with a polyamine. The polyamine is heated to about 100°-150° C. and then the multifunctional acrylate or methacrylate is reacted with the polyamine for a 1-6 hour period to form an amine terminated curing agent. 
     Typical multifunctional acrylates or methylacrylates that can be used to form curing agent (2) are trimethylol propane acrylate, trimethylol propane methacrylate, pentaerythritol acrylate, pentaerythritol methacrylate and the like. Any of the aforementioned polyamines can be used to form the curing agent (2). 
     The curing agents are used in fluorocarbon polymer coating compositions or primer compositions. Generally, about 5-25% by weight, based on the weight of the binder of the composition, of curing agent is used. 
     One of the advantages of compositions that contain the curing agent is that these compositions cure at ambient temperatures and baking is not required. Therefore, the compositions can be used on large structures such as chemical storage tanks, chemical reactors the interior of smoke stacks and the like which could not be subjected to baking temperatures using conventional techniques. 
     Typical coating composition contains about 10-70% by weight binder and 30-90% by weight of an organic solvent, in which the binder is 
     a fluorocarbon polymer of vinylidene fluoride and hexafluoropropylene and has a weight average molecular weight of about 50,000-300,000 and contains 
     a metallic oxide such as magnesium oxide which is an acid acceptor. 
     Usually the composition contains a reinforcing pigment such as titanium dioxide or carbon black. 
     Molecular weight, as used herein, is determined by gel permeation chromatography using polymethylmethacrylate as a standard. 
     Preferably, the polymer used in the coating composition contains about 50-70% by weight of vinylidene fluoride and 30-50% by weight of hexafluoropropylene. The polymer can contain up to 40% by weight of other monomers such as tetrafluoropropylene. One useful polymer contains about 20-30% by weight of tetrafluoroethylene. 
     The metallic oxide which is an acid acceptor is used in the composition to react with the hydrofluoric acid which is generated during the curing or crosslinking reaction. Typical metallic oxides are magnesium oxide, lead oxide, calcium oxide, lead hydrogen phosphate and a mixture of calcium oxide and magnesium oxide. Magnesium oxide is preferred. 
     Generally, the binder of a coating composition contains about 55-90% by weight, of the fluorocarbon polymer, 5-25% by weight of one of the above amine curing agents and 5-20% by weight of a metallic oxide which is an acid acceptor. 
     The coating composition also can contain dispersed fluorocarbon polymers such as polytetrafluoroethylene, fluorinated ethylene/propylene polymers, polyvinyl fluoride, polyvinylidene fluoride, copolymer of tetrafluoroethylene/perfluoroalkoxy vinyl ether and the like. These dispersed fluorocarbon polymers are present in amounts of about 5-20% by weight, based on the weight of the binder of the composition. 
     Preferably, the coating composition contains a reinforcing agent such as titanium dioxide pigment usually in a pigment to binder weight ratio of about 20:100 to 200:100. Other inert pigments can be used such as barytes, barium sulfate, fibrous calcium silicate and the like. Carbon black, bone black or lamp black can also be used as a reinforcing pigment in a pigment to binder weight ratio of about 20:100 to 50:100. 
     Typical organic solvents that are used in the coating composition are acetone, tetrahydrofuran, methyl ethyl ketone, ethyl acetate, propyl acetate, butyl acetate, isobutyl acetate, methyl isobutyl ketone, methyl amyl acetate, diisobutyl ketone, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether and mixtures of the above. These solvents are used to make the compositions and can be used to reduce the compositions to an application viscosity. 
     To decrease curing time and increase toughness of the resulting finish of the coating composition about 0.01-3% by weight based on the weight of the binder, of a bicyclic amidine can be added. One preferred bicyclyic amidine is 1,8-diaza-bicyclo(5,4,0)undecene-7. 
     Generally, the coating composition is sold in two components. The solvents, fluorocarbon polymer, pigments such as titanium dioxide and metallic oxide acid acceptor are the first component and the amine curing agent is the second component. The second component is blended with the first component by the user to form a coating composition. The composition is then reduced with one of the aforementioned solvents to an application viscosity and then applied to a substrate. 
     The coating composition can be applied directly over a wide variety of substrates and provide a fluorocarbon polymer coating. Typical substrates are treated or primed steel, phosphatized steel, grit blasted steel, galvanized steel, aluminum, copper, brass, cement and cementitious surfaces such as fire brick, mortar used for fire brick and the like. 
     Generally, the coating composition is sprayed applied to the substrate and the solvent is allowed to flash off between coatings then the resulting coating composition is cured at ambient temperatures. The coating can be cured in about 4 to 48 hours or longer or can be heated to 80° C. to 120° C. for 0.5 to 2.0 hours for rapid curing. Cured coatings are about 75-1500 microns thick. 
     For some substrates such as untreated steel, a fluorocarbon primer is first applied and then while the primer is still wet the coating composition is applied and dried at ambient temperatures or at the above elevated temperatures. 
     One useful primer contains the aforementioned fluorocarbon polymer, a metallic oxide acid acceptor such as magnesium oxide and an aminoalkyl-alkoxy silane such as amino-propyl trimethoxysilane or amino-propyl triethoxysilane. 
     The ambient curing characteristics of the coating composition provided by the novel amine curing agent of this invention allows for the application of coatings on large vessels and reactors in chemical plants, and oil refineries, large structures and equipment and pipes, heat risers, i.e., pipes which are used to transport oil from the underground well to the structure, off shore oil well platforms, and on the interior of smoke stacks used by large utility companies. It is practical to use compositions that cure at ambient temperature for the above applications. 
     The following examples illustrate the invention. All parts and percentages are on a weight basis unless otherwise indicated. 
     EXAMPLE 1 
     A curing agent was prepared by charging the following constituents into a reaction vessel equipped with a stirrer, a heating mantel and a condenser: 
     
         ______________________________________          Parts by          Weight______________________________________Portion 1Isophorone diamine            535.5Portion 2Dimethyl maleate 144.0Portion 3Isopropanol      516.0Total            1195.5______________________________________ 
    
     Portion 1 was charged into the reaction vessel and heated to about 120° C. and then portion 2 was added at a uniform rate over a 4 hour period while the temperature was held at 130° C. and methanol was removed from the vessel. After the 4 hour period heating was stopped and a sample was removed and tested in an infrared spectrophotometer which showed the reaction was complete. Then portion 3 was added to give a 55% solids solution. 
     About 5 parts of the above curing agent was added to about 300 parts of each of the following coating compositions: 
     Coating Composition A 
     Fluorocarbon polymer solution containing 33% solids in ethyl acetate, of a copolymer of 40% hexafluoropropylene and 60% vinylidine fluoride having a weight average molecular weight of about 100,000 and about 5% by weight magnesium oxide. 
     Coating Composition B 
     Fluorocarbon polymer solution identical to composition A except the copolymer has a molecular weight of about 200,000. 
     Coating Composition C 
     Fluorocarbon polymer solution identical to composition A except the composition contains additionally about 25% by weight of finely divided polyvinylidene fluoride. 
     Each composition was diluted with methyl ethyl ketone to a spray viscosity and sprayed onto aluminum and steel panels to form a coating having a dry film thickness of about 1000 microns. After drying under ambient temperature conditions for 7 days, each of the coatings is resistant to sulfuric acid, sodium hydroxide, steam and methyl ethyl ketone which indicates that the coatings were fully cured and crosslinked. 
     EXAMPLE 2 
     The following amine curing agents were prepared: 
     
         ______________________________________          CURING AGENT          Parts by Weight          D     E       F       G______________________________________Isophorone diamine            510     --      562   --Hexamethylene diamine            --      321     --    388Trimethylol propane acrylate            296     296     --    --Pentaerythritol acrylate            --      --      298   298Isopropanol      659     505     703   562Total            1465    1122    1563  1248______________________________________ 
    
     In the preparation of each of the above curing agents D-G, the amine was charged into a reaction vessel and heated to 120°-140° C. and then the acrylate was slowly added at a uniform rate over a 4 hour period and then the reaction mixture was cooled and isopropanol added. 
     Trimethylol propane acrylate and pentaerythritol acrylate were prepared by conventional techniques well known to the skilled in the art in which an acrylate moiety was attached to trimethylol propane and with pentaerythritol. 
     Separate coating compositions were prepared with each of the curing agents D-G using the same constituents as in Example 1 for the coating composition A except the above curing agent was substituted for the curing agent of Example 1. In each case the resulting coating composition was reduced to a spray viscosity with methyl ethyl ketone as in Example 1 and sprayed onto grit blasted steel panels allowing the coating to flash dry between each application to provide a 1000 micron thick dry coating. After 7 days, the coatings were fully cured but were softer and more elastic than the coating of Example 1. The coatings were resistant to sulfuric acid, sodium hydroxide and methyl ethyl ketone.