Coating compositions containing 1,3-dialkylimidazole-2-thione catalysts

Abstract: The present invention describes the use of 1,3-dialkylimidazole-2-thiones as a catalyst for epoxy/anhydride reactions (and other reactions) and a method of making these catalysts. Epoxy/anhydride coatings containing these improved catalysts offer the following advantages: improved appearance (especially since even weeks after application the coatings still have a "wet" look); better pot-life; improved resistance to alkaline substances which will improve durability; improved solubility; and coatings which have a reduced volatile organic content (VOC).

Description: Field of the Invention We have discovered that 1,3-dialkylimidazole-2-thione compounds are useful for catalyzing crosslinking reactions between polymers with pendent anhydrides and polymers with hydroxyl or epoxy functionality. Previously, most crosslinking reactions between anhydride compounds and compounds with hydroxyl or epoxy functionality have been catalyzed with amines or amine boranes. The use of 1,3-dialkylimidazole-2-thione catalysts is especially useful in the ambient cure of coatings containing a polymer having anhydride groups, and a polyepoxide or polyhydroxyl crosslinking agent. These novel catalysts form epoxy/anhydride resins useful in coatings with a number of significant advantages. Prior Art Coating compositions containing epoxy/anhydride resins are known in the art as shown by U.S. Patent No. 4,816,500 assigned to Du Pont This disclosure does not show or suggest the use of 1,3-dialkylimidazole-2-thiones to catalyze the crosslinking reaction between the epoxy and the anhydride. An unrelated reference found in Organic Syntheses , 1986, Vol 64, page 92 shows the preparation of 1,3-dimethylimidazole-2-thione. However this reference does not disclose other 1,3-dialkylimidazole-2-thiones, nor does it disclose or suggest the use of 1,3-dimethylimidazole-2-thione as a crosslinking catalyst. Summary of the Invention The present invention is concerned with the use of a 1,3-dialkylimidazole-2-thione as a catalyst for epoxy/anhydride reactions (and other reactions). Epoxy/anhydride coatings containing these improved catalysts offer the following advantages: improved appearance (especially since even weeks after application the coatings still have a "wet" look); better pot-life; improved resistance to alkaline substances which will improve durability; improved solubility; and coatings which have a reduced volatile organic content (VOC). The 1,3-dialkylimidazole-2-thiones of the present invention have the following structure: where R and R₁ are alkylene C₁ to C₁₄ (straight chain or branched) and Y and Y₁ are H, OH, CO₂H, aryl, ether, amine, perfluoroalkyl, amide, nitrile, or olefin. (Typically, Y and Y₁ are H thus the nitrogens usually bear unsubstituted alkyl groups). Detailed Description of the Invention The present invention relates to the use of 1,3-dialkylimidazole-2-thione as a catalyst for epoxy/anhydride resins which are used in paints, coatings, laminates, moldings, castings and adhesives. These catalysts improve appearance (especially since even weeks after application the coatings still have "wet" look); pot-life; resistance to alkaline substances which will improve durability; solubility; and VOC. According to the present invention, therefore, there is provided a coating composition comprising: a) a polymer having anhydride functionality which contains at least two reactive anhydride groups and an average molecular weight of less than 100,000; b) a compound having epoxy or hydroxyl functionality containing at least two epoxy groups or hydroxyl groups and having an average molecular weight of less than 100,000, wherein the composition contains from 20 to 80% by weight of polymer (a) and 80 to 20% by weight of compound (b); and c) a catalyst which catalyses a cross-linking reaction between compound a) and compound b), said catalyst being a 1,3-dialkylimidazole-2-thione compound with the structure: where R and R₁ are alkylene C₁ to C₁₄ (straight chain or branched) and Y and Y₁ are H, OH, CO₂H, aryl, ether, amine, perfluoroalkyl, amide, nitrile, or olefin, and being present in an amount from 1 to 10% by weight of the total binder. Typically, the reactive components used for making an epoxy/anhydride coating or hydroxyl/anhydride coating are an anhydride bearing polymer, copolymer or monomer and an epoxy or hydroxyl bearing polymer, copolymer or monomer. The anhydride, epoxy or hydroxyl bearing reactants may be in a solvent. These components along with a catalyst are often provided to the user in separate containers and are mixed on demand. We have also found that 1,3-dialkylimidazole-2-thiones can be used to catalyze the crosslinking reaction between acid functional compounds and compounds with epoxy functionality. This is shown in Example 5 where an acid functional polymer is used in conjunction with an anhydride functional polymer and both are crosslinked with an epoxy functional compound. The anhydride component of the epoxy/anhydride resins which are useful for the curing agents of this invention, may be any polymer or copolymer with a weight average molecular weight of less than 100,000 containing at least two reactive anhydride groups. Also low molecular weight monomeric anhydrides may be used. Prefered anhydride components are copolymers prepared from one or more of the monomers of styrene, methacrylate, or acrylates with one or more of the monomers of itaconic acid, itaconic anhydride, maleic anhydride or isobutenyl succinic anhydride. (After formation of the polymer the itaconic acid which is contained in the polymer is converted to the anhydride). As will be apparent to one skilled in the art there are a number of other monomers which could also be used to form the anhydride polymer. Some of these different monomers are disclosed in U.S. Patent No. 4,816,500 which is incorporated herein by reference. The epoxy component of the epoxy/anhydride coatings may be any polymer, copolymer or compound with a weight average molecular weight of less than 100,000 containing at least two epoxy groups. Also low molecular weight monomeric epoxies may be used. Prefered epoxy components are copolymers prepared from alkyl (meth)acrylates [hereinafter "(meth)acrylates" refers to either acrylates or methacrylates] with glycidyl (meth)acrylates. These prefered polymers may be used by themselves or in combination with the polyglycidylethers of sorbitol. Other preferred epoxies are Araldite CY-184® (from Ciba-Geigy Corporation) and epoxies based on Bisphenol A such as Epon 1001® (available from Shell Chemical Company). As will be apparent to one skilled in the art there are a number of different monomers which could also be used to form the epoxy polymer. Some of these different monomers are also disclosed in U.S. Patent No. 4,816,500. The hydroxyl containing compounds which can be used as a substitute or in addition to the epoxy containing compounds are polymers or copolymers containing hydroxy functionality with a weight average molecular weight of less than 100,000 containing at least two hydroxly groups. Low molecular weight monomeric hydroxyl compounds may also be used. Prefered hydroxyl compounds are copolymers containing alkyl (meth)acrylates and hydroxy functional (meth)acrylates. Also hydroxy functional polyesters can be used. The acid functional compounds that may be used are monomeric, oligomeric, or polymeric which may or may not contain hydroxyl functionality. If polymeric, the compounds are typically formed by polymerizing monomers of alkyl (meth)acrylates where the alkyl groups have 1-12 carbon atoms and ethylenically unsaturated acids. Optionally, the acid functional polymer can also contain other components such as styrene, acrylonitrile, methacrylonitrile in amounts of about 0.1-50% by weight. The coating composition formed using the components described above contains 20% to 80% of the polymer or copolymer having at least two anhydride groups and 80% to 20% of the polymer or copolymer having at least two epoxy groups (or hydroxyl groups). The coating composition may also contain components such as monomeric anhydrides; acid functional monomeric, oligomeric or polymeric components which may or may not contain hydroxyl functionality (if there are acid functional groups present there must also be epoxy groups present in order to get cure); hydroxy functional polymers; and self-stabilized dispersed resins. Examples of monomeric anhydrides are methyl hexahydrophtalic anhydride and the like. Examples of such acid functional components are glycolic acid and acrylate/methacrylic acid copolymer and the like. None of the components should contain any substituents which would interfere with the curing process. As discussed above the catalyst for the epoxy/anhydride coating is 1,3-dialkylimidazole-2-thione. The catalyst is present at a level of 1-10% by weight, preferably about 4-6%, of the total binder. The 1,3-dialkylimidazole-2-thione is preferably made in two reaction steps. Although another method of making the 1,3-dialkylimidazole-2-thiones through a potassium carbonate route was disclosed in Organic Syntheses , 1986, Vol 64, page 92, this prior method provides yields of only about 60-70% compared with the current 2-step method which gives yields of as high as 90%. Furthermore, the two step method is simpler to operate than the previous potassium carbonate method. (A comparative example of the potassium carbonate method is shown in Example 2). The first step in the two step process is reacting 1-alkylimidazole and alkyl halide (substituted or unsubstituted) to yield a N-alkyl-N′-alkylimidazolium halide. The 1-alkylimidazole and alkyl halide are preferably added in a 1:1 molar ratio. However, an excess of either may be used, but will result in a waste of starting material. The reaction may be done in the presence of an organic solvent, however, it is preferred that the reaction be done neat so that no seperation step is needed to isolate the N-alkyl-N′-alkylimidazolium halide. If a sovent is used, the solvent chosen should not react with the starting materials or product. Preferred solvents are toluene, methylene chloride, and methanol. The reactants and solvents must be substantially dry. Most preferably they should contain less than 0.01% water, more preferably less than 0.1% water and preferably less than 0.5% water. The reaction temperature changes as the reactants change. If a heavy halide (i.e. iodide) is used then the reaction can proceed at room temperature. If a lighter halide is used then the reaction should be heated to between 40°C to 120°C (depending on the reactants). The time it takes for the reaction to go to completion also varies according to reactants from 2 to 48 hours. In the second reaction step the product from the first reaction step, N-alkyl-N′-alkylimidazolium halide, is reacted with a suspension of sulfur and a base in a solvent to give the final catalyst 1,3-dialkylimidazole-2-thiones. In this second reaction step, the preferred ratio of N-alkyl-N′-alkylimidazolium halide to sulfur to base is 1:1:1. Excesses of any of the three may be used but will result in a waste of starting material. The reactants and solvents must be substantially dry. Most preferably they should contain less than 0.01% water, more preferably less than 0.1% water and preferably less than 0.5% water. Preferably, the addition of the ingredients should be such that the base and N-alkyl-N′-alkylimidazolium halide are not in contact without sulfur. Usually the sulfur and base are premixed in solvent and then the N-alkyl-N′-alkylimidazolium halide is added to that mixture. The solvent is preferably a volatile alcohol. Most preferably the solvent is methanol. The base can be any alkali metal alkoxide. Preferably the base is sodium methoxide. Preferably the reaction takes place at room temperature. However, it is possible for the reaction to take place anywhere from 20° to 80°C and it normally takes between two to forty-eight hours. After the reaction has gone to completion, it is possible to seperate the alkali metal halide from the 1,3-dialkylimidazole-2-thiones by either solvent evaporation, water extraction or filtration. Our most preferred 1,3-dialkylimidazole-2-thione is 1- n -butyl-3-methylimidazole-2-thione. This compound is made as shown in Example 1. Other preferred 1,3-dialkylimidazole-2-thiones are 1-n-octyl-3-methylimidazole-2-thione (see Example 2), 1- n -propyl-3-methylimidazole-2-thione (see Example 3), and N,N'-diethylimidazole-2-thione. Typical solvents used to prepare the anhydride acrylic polymer, and used as a diluent for the coating composition are as follows: toluene, xylene, butyl acetate, ethylbenzene, higher boiling aromatic hydrocarbons, amyl acetate, ethyl acetate, propyl acetate, ethylene or propylene glycol mono alkyl ether acetates and so forth. In addition to the solvents listed above, certain alcoholic solvents are also useful. The alcoholic solvents under certain use conditions convert portions of the anhydride to a half ester also useful as reactants in this system. Examples of such alcohols are propanol, isobutanol, methanol, isopropanol, tertiary butanol, n-butanol, propylene glycol monomethyl ether, ethylene glycol monobutyl ether, and other alcoholic solvents. Generally, the composition is applied by conventional techniques such as spraying and electrostatic spraying. The resulting coating can be dried and cured at ambient temperatures or can be cured at elevated temperatures of 60 to 200°C. The coating can be applied in pigmented or nonpigmented (clear) form. At ambient temperatures, the coating dries to a tack free condition in about 180 minutes and in about 24 hours the coating is substantially cured. In about 5-7 days, the coating is completely cured. Coatings are applied to form a finish about 1.27 to 12.7 x 10⁻⁵m (0.5-5 mils) thick, and preferably 2.54 to 5.08 x 10⁻⁵m (1-2 mils) thick. The finish has excellent gloss, retention of the "wet" look even weeks after application, good pot-life, good resistance to alkaline substances, good adhesion to substrate, good hardness and fast dry times. To improve weatherability of the clear finish of the coating composition, about 0.1-5%, by weight, based on the weight of the binder, of an ultraviolet light stabilizer or a combination of ultraviolet light stabilizers can be added. These stabilizers include ultraviolet light absorbers, screeners, quenchers and specific hindered amine light stabilizers. Also, about 0.1-5% by weight, based on the weight of the binder, of an antioxidant can be added. Typical ultraviolet light stabilisers that are useful are disclosed in U.S. Patent 4,816,500. Generally, when the coating composition of this invention is used as a clear coating, it is applied by conventional spraying techniques to a color or base coat of an automobile or truck. The coatings are baked at about 60° to 140°C for about 10 to 40 minutes. In refinishing automobiles and trucks, the clear coating is applied to a color coat and then can be dried at ambient temperatures or baked to form a clear finish. The resulting clear coat or finish is about 2.54 to 12.7 x 10⁻⁵m (1-5 mils) thick, preferably 2.54 to 5.08 x 10⁻⁵m (1-2 mils) thick, and has excellent gloss, good adhesion to the color coat, good hardness, and good retention of the "wet" look even weeks after application. The composition can be pigmented to form a colored finish or primer. About 0.1-200% by weight, based on the weight of the binder, of conventional pigments can be added using conventional techniques in which a mill base containing pigment, dispersant and solvent is first formed. The mill base is then mixed with the composition to form a colored composition. This composition can be applied and cured as shown above. The following examples illustrate the invention. All parts and percentages, unless otherwise stated, are on a weight basis. The examples are merely illustrative and should not be read in any way to limit the scope of the invention. EXAMPLE 1 PREPARATION OF 1-n-BUTY-METHYLIMIDAZOLE-2-THIONE A 3 litre single neck round bottom flask is equipped with a condenser and nitrogen bubbler. The flask is charged with 500 grams (6.1 mole) 1-methylimidazole and 650 grams (7.0 mole) n-butyl chloride. While stirring, the mixture is heated to reflux, 80 °C (pot temp). The mixture is refluxed for 16 hrs (the pot temperature is gradually increased to 110 °C over this time). The product is allowed to cool to room temperature and may crystallize on cooling. The N-methyl-N′-n-butylimidazolium chloride, from the previous reaction, is dissolved into 500 ml anhydrous methanol. The methanol solution is cannulated into a 10 litre single neck flask containing a suspension of sulfur and sodium methoxide in anhydrous methanol (prepared by dissolving 140.3 grams (6.1 mol) sodium metal in 5.0 litre anhydrous methanol then adding 196 grams (6.1 mol) lac sulfur). The reaction is stirred for 24 hrs at 23°C and then checked by ¹H nuclear magnetic resonance (NMR) to assure completion. The reaction mixture is then pressure filtered through Celite® to remove sodium chloride. The clear solution is acidified with 6 N HCL to pH 5.0. A small amount of yellow solids precipitate out of the solution and are filtered off through Celite®. The solution is concentrated on a rotary evaporator to leave a dark oil. The oil is vacuum distilled, 105 °C/10 ⁻² torr, from KOH pellets to give a clear to slightly yellow colored oil. The yield is 886.1 grams (85.3 %). The ¹H NMR (CD₃CN) δ 0.93 (t,3H,CH₂CH₃); 1.30 (tq,2H,CH₂CH₃); 1.63(tt,2H,NCH₂CH₂); 3.49(s,3H,NCH₃); 3.96(t,2H,NCH₂); 6.84(s,2H,NCH). Density - 1.04 g/ml. EXAMPLE 2 PREPARATION OF 1-n-OCTYL-3-METHYLIMIDAZOLE-2-THIONE USING THE POTASSIUM CARBONATE METHOD In the hood, a 0.5 litre round bottom flask equipped with a condenser and nitrogen bubbler, is charged with 65.70 grams (0.8 mol) 1-methylimidazole, 154.5 grams (0.8 mol) 1-bromooctane, and 100 ml acetonitrile. While stirring, the light yellow colored reaction mixture is heated to reflux, 97 °C, and refluxed for about 1.5 hour. The mixture, now slightly darker in color, is then cooled to room temperature. The NMR is checked to assure completion and then the acetonitrile is removed under reduced pressure. This amber colored imidazolium bromide (220.0 grams, 0.8 mol) is placed in a 2 litre round bottom flask charged with 76.9 grams (2.4 mol) sulfur, and one 1 litre methanol. To this 221.38 grams (2.0 mol) of potassium carbonate is added in a single portion. A Drierite® drying tube is attached to the flask and the reaction mixture is allowed to stir for 24 hours. The reaction mixture is checked by ¹H NMR to assure completion. The bright yellow mixture is then filtered through a course fritted glass funnel. The filter cake is washed with methanol to extract any remaining product. The methanol is removed on a rotary evaporator. The dark red oil is then acidified with 1 N HCl to pH 5.0 and filtered to remove excess sulfur. The solution is concentrated via rotary evaporator, then vacuum distilled from zinc powder to give 125 grams (a yield of 69%) of a dark red oil. The ¹H NMR (CD₃)₂CO- 7.00(m,NCH,2H); 4.02(t,NCH₂,2H), 3.52 (s,NCH₂,3H); 1.74(m,NCH₂C,H₂,2H); 1.31(m,(CH₂)₅,10H); 0.87(m,(CH₂)₅CH₃,3H). EXAMPLE 3 PREPARATION OF 1-n-PROPYL-3-METHYLIMIDAZOLE-2-THIONE In the hood, a 200 ml single neck round bottom flask is equipped with a condenser and nitrogen bubbler. The flask is charged with 33.6 grams (0.41 mole) 1-methylimidazole and 55.5 grams (0.45 mol) 1-bromopropane. While stirring the mixture is heated to reflux, 70 °C (pot temperature). The mixture is refluxed for 16 hrs (the pot temperature is gradually increased to 110 °C over this time). The product is allowed to cool to room temperature. The 1H NMR is checked to assure completion. The NMR shows a single product with resonances in the expected regions. The N-methyl-N'-n-propylimidazolium bromide from the previous reaction is dissolved in 50 ml anhydrous methanol. The methanol solution is cannulated into a 500 ml single neck flash containing a suspension of sulfur and sodium methoxide in 200 ml anhydrous methanol (prepared by dissolving 9.43 grams (0.41 mol) sodium metal in anhydrous methanol then adding 13.15 grams (0.41 mol) lac sulfur). The reaction is stirred for 24 hrs at 23°C and then checked by ¹H NMR to assure completion. The reaction mixture is then pressure filtered through Celite® to remove sodium chloride. The clear brown solution is acidified with 1 N HCl to pH 5.0. The solution is concentrated via rotary evaporator to give a dark brown oil. The oil is then vacuum distilled (85 - 100 °C/0.1 mm Hg) from KOH to give 51.82 grams (a yield of 81%) of colorless oil. 1H NMR (CD₂Cl₂)δ 0.93 ppm (t,CH₂CH₃,3H); 1.77(dq,CH₂CH₃,2H); 3.55(s,NCH₃,3H; 3.95(t,NCH₂,2H); 6.71(s,NCH,2H). EXAMPLE 4: PREPARATION OF AN AMBIENT CURE COATING USING A THIONE CATALYST Preparation of a Polyanhydride Part Ingredient Parts by Weight 1 Xylene 232.10 2 Styrene 91.70 Butylmethacrylate 122.20 Butylacrylate 232.2 Xylene 50.20 3 Itaconic acid 191.60 Xylene 60.00 4 75%Tert-Butylperoxyacetate Initiator ("Lucidol 70" from the Lucidol Division of Penwalt Corporation) 30.50 Propyleneglycolmonomethyletheracatate 12.10 Xylene 57.50 5 Propyleneglycolmonomethyletheracetate 102.10 6 Propyleneglycolmonomethyletheracetate 102.10 Part 1 was added to a reactor equipped with a thermomether, stirrer, dropping funnel, water separater, nitrogen purge and condenser and heated to reflux. Parts 2 and 3 were premixed and added to the reactor over 3 hours simultaneously. Part 4 was premixed and added to the reactor over 3 hours and 20 minutes beginning with the start of Parts 2 and 3. The batch was then maintained at reflux until 25.2 parts of water were collected in the water separater. Part 5 was then added to the batch and 341.3 parts of solvent were removed by distillation. Part 6 was then added to the batch; it was cooled. This polymer solution had a Garner-Holdt viscosity of Z1+1/2 and a measured solids of 69.7%. The anhydride content was determined to be 0.91 Meq/gm and the acid content to be 0.19 Meq/gm. The molecular weight was measured by gel permeation chromatography to be Mn = 2074 and Mw = 5093. Preparation of a Polyol Part Ingredient Parts by Weight 1 Methyl N-amyl ketone 170.85 2 Methyl N-amyl ketone 61.90 75% t-Butylperacetate Initiator ("Lupersol 70" from the Lucidol Division of Pennwalt Corporation) 27.74 3 Styrene 143.88 Ethylmethacrylate 143.88 Laurylmethacrylate 115.12 Hydroxyethylacryate 172.63 Total 836.00 Part 1 was added to a reactor equipped with a stirrer, thermometer, condenser, nitrogen purge and feed funnel and heated to reflux. Part 2 was premixed and added to the reactor over 165 minutes. Part 3 was premixed and added to the reactor over 150 minutes beginning with the start of Part 2. The batch was then held at reflux for 2 additional hours and then cooled. The resulting polymer had a weight percent solids of 70% and a hydroxyequivalent weight on solids was 386.7 gms per equivalent. Preparation of a Polyepoxide Part Ingredient Weight 1 Butylacetate 244.67 Toluene 25.51 Ethylacetate 41.39 2 Glycidylmethacrylate 233.12 Butylacetate 62.29 Butylmethacryalete 155.41 "Vazo 67" Initiator (from DuPont) 18.68 3 Butylacetate 14.32 Propyleneglycolmonomethyletheracetate 48.32 "Vazo 67" Initiator (from DuPont) 4.77 Total 848.48 Part 1 was added to a reactor equipped with a stirrer, thermometer, condenser, nitrogen purge, and feed funnel and heated to reflux. Part 2 was premixed and added to the reactor over 2 hours maintaining reflux. Part 3 was premixed and added over 30 minutes after part 2 was finished. The batch was held at reflux for 30 more minutes then cooled and fillled out. The resulting polymer's weight percent solids was 45.7% and the epoxyequivalent weight on solids was 236.7 gms per equivalent. Preparation of Coatings Using Various Catalysts Coating compositions were prepared by thoroughly blending the following components. As can be seen from the table below, six different coating compositions were prepared using the polymers described above, other additives and a catalyst or mixture of catalysts. Coatings A and E contained only "thione" catalyst, while coatings B,C and D contained a mixture of "thione" and amine catalyst. Coating U contained only amine catalyst. Films of the above compositions were cast using a .010 inch draw down blade onto panels made of glass and thermoplastic polyolefin. Descriptions of the tests performed on the coatings are described at the end of the Example Section. Test Results After One Day Cure at 25°C Coating Geltime (hours) Persoz Hardness 1 Day Dry Film Thickness metre⁻⁶ (µm) A 9.00 90 75 B 3.25 129 65 C 4.33 114 75 D 8.08 115 65 E 5.08 154 65 U 1.67 117 65 Test Results After Seven Days Cure at 25°C Coating Persoz Hardness Swell Ratio in CH₂Cl₂ 5%Naoh 10%Sulf Ford#24 A 178 1.64 3 5 4 B 193 1.58 3 5 4 C 183 1.6 3 5 4 D 207 1.61 4 5 4 E 200 1.54 4 4 4 U 168 1.67 3 5 4 Description of various tests in Example 4 Gel Time T₀= The clock time when all the ingredients are mixed to form a complete solution of reactive binders and catalyst. T₁= The clock time when this mixture forms a tail on the bubble in a Gardner-Holdt tube. Gel time= T₁ - T₀. Persoz Hardness The solution of mixed ingredients is drawn down on a 101,6 x 304.8 mms (4" by 12") glass. The solids are normally 45% or 50% in our experiments. We use a 0.25 mm (.010") doctor blade. Film thickness will be in the range 45 to 60 (µm). In 24 hrs (+/- 2 hrs) the hardness of the film is measured by the Byk Mallinkrodt Pendulum Hardness Tester with a Persoz Pendulum. Swelling Ratio The solution of mixed ingredients is drawn down on a panel of TPO (Thermoplastic Polyolefin). On test day a free film is lifted off the TPO with a single edge razor. A circular specimen is punched from the film with a 3.1mm grid punch. We usually sandwich the film in between aluminum foil for punching so that the specimen has enough weight to fall down the barrell of the punch into the catch pan. The 3.1mm specimen is then mounted on a slide and its diameter measured in filar units using a microscope equipped with a filar micrometer. This diameter is D₀. Methylene chloride is dropped onto the specimen; swelling starts; a cover glass is put over the swelling specimen; swelling reaches its equilibrium value. The swollen diameter is D s . The swelling ratio is calculated as follows: This is the ratio of the swollen area to the unswollen area. Swelling Ratio Comments We have used swelling ratio as measure of cure in hundreds of screening tests and have adopted the following guideline for describing cure... Cure Swelling Ratio CH2CL2 7 - 8 Days Excellent <1.55 Very Good 1.55 - 1.65 Good 1.65 - 1.75 Fair 1.75 - 1.85 Poor 1.85 - 2.00 Very Poor >2.00 Chemical Spot The film drawn down on glass is exposed to drops of various chemicals such as 5%NAOH and 10%H2SO4. After one hour the spot is wiped away and the exposed coating is checked for softening, swelling, discoloration and wrinkling. Using 5 as a perfect rating a point is deducted for each of the aforementioned defects found. For example a coating that turns yellow and softens loses 2 points and gets a rating of 5 - 2 = 3. Summary of Results As can be seen from the above results, Coating E with "thione" catalyst at the same level as the coating with only amine catalyst (Coating U) has longer gel time, better one day hardness, better seven day hardness and better cure (as indicated by a lower swelling ratio) than the amine based system. (Coating E and Coating U both have catalyst levels of .0357 equivalents per hundred grams of total binder). Even Coating A with only half the level of "thione" catalyst has only slightly less hardness after one day's cure and comparable hardness and cure after seven days when compared to Coating U. Significantly, the gel time, indicative of useful pot life, is much longer for Coating A than Coating U. Coating B,C and D, which are blends of the "thione" catalyst with various amine catalysts show the possibility for getting better balances of pot-life and cure by mixing the thione catalyst and amine catalysts. EXAMPLE 5: COATING COMPOSITION THAT REQUIRES BAKING TO CURE Preparation of Acid Polymer Component A butyl acrylate/methacrylic acid copolymer was prepared as follows: A reactor was charged with the following: Propyleneglycolmonomethyletheracetate (PM acetate) 1604.0 parts Butyl acetate 441.0 parts Xylene 1243.0 parts This mixture was heated with agitation under nitrogen to reflux. Then the following mixture was added at a uniform, linear rate over three hours while maintaining reflux. Butyl acrylate monomer 2427.0 parts Methacrylic acid monomer 1630.0 parts Tertiary butyl peroxyacetate ("Lucidol 70" from Pennwalt) 224.0 parts Then the following mixture was added over ten minutes while maintaining reflux temperature: Xylene 200.0 parts Tertiary butyl peroxyacetate 19.0 parts The mixture was maintained for one hour at reflux temperature and then diluted with the following: PM Acetate 692.0 parts Total 8480.0 parts This polymer had a Garner-Holdt viscosity of Z1+1/2 and a measured solids of 52.3%. The acid content was determined to be 2.28 Meq/gm and molecular weight by gel permeation chromatography was Mn = 2762, Mw = 6108. A coating composition using 1-n-butyl-3-methylimidazole-2-thione catalyst was prepared by thoroughly blending the following constituents: Parts by Weight Polyanhydride (from Example 4) 453.2 PM acetate 38.2 Methyl hexahydrophthalic anhydride 57.5 Resiflow S® (acrylic flow additive from Estron Chemical Co.) 3.4 Butyl acetate 118.7 Araldite CY-184® (Epoxy resin from Ciba-Geigy) 238.1 Acid polymer (as prepared above) 174.3 Butanol 38.8 Tinuvin 292® (Ciba-Geigy hindered amine light stabilizer) 13.2 Tinuvin 1130 (Ciba-Geigy UV screener) 20.1 25% 1-n-butyl-3-methylimidazole-2-thione in Butyl Acetate 66.9 Total 1204.4 The resulting coating composition was reduced to a spray viscosity of 35 seconds measured with a No. 2 Zahn cup accomplished by adding butyl acetate. The coating composition was sprayed onto a primed metal panel coated with a waterborne basecoat and cured at 180-200°F and provided a clear coat with excellent color, durability, humidity resistance, chemical resistance, and other film properties. The coating composition was also sprayed over solvent borne melamine cured basecoat and cured at 240-284°F. The resulting coating exhibited excellent color, durability, humidity resistance, chemical resistance, and other film properties. EXAMPLE 6: PIGMENTED COATING WITH THIONE CATALYST Preparation of hydroxy functional polymer "B" Part Ingredient Weight 1 Methyl n-amyl ketone 339.830 2 Methylmethacrylate 94.970 Styrene 94.970 Butylacrylate 253.230 Hydroxyethylacryate 189.940 Methyl n-amyl ketone 16.728 3 Methyl n-amyl ketone 47.345 75% t-butylperacetate Initiator ("Lupersol 70" from Pennwalt Corporation) 21.080 4 Methyl N-Amyl Ketone -233.240 5 Methyl N-Amyl Ketone 40.147 Yield 865.000 Part 1 was added to a reactor equipped with a stirrer, thermometer, condenser, nitrogen purge and feed funnel and heated to reflux. Part 2 was premixed and added to the reactor over 225 minutes. Part 3 was premixed and added to the reactor over 240 minutes beginning with the start of part 2. The batch was then held at reflux. For 1 additional hour, part 4 was then removed by distillation, the batch was cooled and pumped from the reactor. Part 5 was used to rinse the reactor and lines. The weight percent solids was 75% and the hydroxyequivalent weight on solids were 386.9 gms per equivalent. Preparation of Polyepoxide Polymer "C" Part Ingredient Weight 1 Xylene 137.408 2 Glycidyl Methacrylate 209.080 Butylacrylate 179.170 Methyl Methacrylate 209.080 Xylene 14.256 3 Xylene 40.992 75% T-Butylperacetate Initiator ("Lupersol 70" from Pennwalt Corporation) 33.160 4 Xylene 18.951 5 Xylene 37.903 Total 880.000 Part 1 was added to a reactor equipped with a stirrer, thermometer, condenser, nitrogen purge, and feed funnel and heated to reflux. Part 2 was premixed and added to the reactor over 5 hours while maintaining reflux. Part 3 was premixed and added over 5 hours simultaneously with part 2. The batch was held at reflux for 45 more minutes then pumped to a prefilter tank. Part 4 is then added to the reactor, stirred to rinse the vessel, and pumped to the prefilter tank. Following filtration, part 5 is used for viscosity/solids adjustment. The weight percent solids was 70.0%. Preparation of a White Dispersion A white dispersion was prepared by mixing the following constituents into a blend tank: Part Ingredient Parts by Weight 1 Hydroxy functional Polymer "B" (prepared above) 7.500 Methyl n-amyl ketone 16.670 2 Titanium dioxide white pigment 69.990 3 Hydroxy functional Polymer "B" (prepared above) 5.840 Total 100.000 Part 1 was added to the blend tank and mixed for 15 minutes. Part 2 was then added to the blend tank while mixing and then mixed for 30 minutes. Part 3 was then added to the blend tank while mixing and mixed for an additional 10 minutes. The resulting blend was put through a sand mill for fineness. Preparation of Rheology Control Dispersion A rheology control dispersion was prepared by mixing the following constituents into a blend tank: Part Ingredient Parts by Weight 1 Mixed methyl esters of succinic, glutaric and adipic acids (Dibasic Ester-DBE from E. I. duPont deNemours) 7.500 Ethyleneglycolmonobutyletheracetate 20.240 Denecol EX622® (Epoxy Resin from Nagase) 65.800 2 Colloidal Silica 7.210 Total 100.000 Part 1 was added to the blend tank and mixed for 15 minutes. Part 2 was then added to the blend tank slowly while mixing and then mixed for an additional 10 minutes. The resulting blend was put through a sand mill for fineness. Formulation of a White Enamel Part Ingredient Parts by Weight 1 White Dispersion (described above) 818.280 Denecol EX622® (Epoxy Resin from Nagase) 106.380 Araldite CY184® (Epoxy Resin from Ciba-Geigy) 121.000 Polyepoxide Polymer "C" (prepared above) 87.020 Silicone SF-69® (from General Electric) 0.092 Xylene 4.508 Rheology control dispersion (described above) 171.170 Various color dispersions (for controlling final color made like the white dispersion descibed above) 2.550 Total 1311.000 Add all of the Part 1 ingredients in order while mixing. Mix dispersions before use. The ingredients are then mixed for 1 hour. Preparation of an Activator Mixture Part Ingredient Parts by Weight 1 Polyanhydride Polymer (from Example 4) 832.750 Methylethylketone 51.250 Total 884.000 The two ingredients from Part 1 were added in order while mixing. The mixture was mixed for 1 hour. Coating compositions were prepared by thoroughly blending the following ingredients. Code 1 2 3 4 5 6 White Enamel Formulation 131.1 131.1 131.1 131.1 131.1 131.1 Activator Mixture 88.4 88.4 88.4 88.4 88.4 88.4 Ethanol 18.5 18.5 18.5 18.5 18.5 18.5 n-Butanol 1.9 1.9 1.9 1.9 1.9 1.9 Diethyleneglycolmonobutylether 12.2 12.2 12.2 12.2 12.2 12.2 Dimethylethanolamine 1.5 1.5 1.5 1.5 1.5 1.5 Tinuvin 292 R (Ciba Geigy) 0.5 0.5 0.5 0.5 0.5 0.5 Tinuvin 328 R (Ciba Geigy) 0.5 0.5 0.5 0.5 0.5 0.5 10% TEDA (Air Products & Chemicals) 1.5 0 0.7 0 0 0 30% MTEDA (Air Products & Chemicals) 0 1.7 0 0.8 0.8 0 30% TEDA:BH3 0 0 0.8 0.8 0 0 30% BTI 0 0 0 0 2.3 7.4 Films of the above compositions were spray applied to treated aluminum panels. A dry film `c thickness of approximately 5.08 x 10⁻⁵m (2 mils) and allowed to air dry for at least 2 weeks prior to testing chemical resistance. The tests performed on the coatings are described following the test results. TEST RESULTS Chemical Spot Test Chemical 1 2 3 4 5 6 H3P04-1% NE VSlSw NE NE NE NE -10% VSlSw VSlSw NE InSw InSw NE HNO3-1% NE NE NE NE NE NE -10% Mod-Sev Blst Mod-Sev Blst Sev Blst Mod-Sev Blst Sev Blst Sev Blst Morpholine-1% NE NE NE NE NE NE -10% NE NE InSw InSw InSw NE H2SO4-1% InSw NE NE InSw NE NE -10% InSw InSw InSw InSw InSw NE NaOH-1% NE NE NE NE NE NE -5% FR FR FR FR FR NE -10% FR FR FR FR FR FR NH4OH-10% NE NE NE NE NE NE Rating Code NE-No Effect IN-Insignificant VSl-Very Slight Sl-Slight Mod-Moderate Sev-Severe Blst-Blistering FR-Film Removed Sw-Swelling Description of Chemical Spot Test in Example 6 The cured film on the aluminum panel is exposed to drops of various chemicals such as 5%NAOH and 10%H2SO4 under individual watch glasses. After overnight exposure (18 hours) the spot is washed away and the exposed coating is checked for softening, swelling, discoloration, wrinkling, or complete removal. The condition of the spots that had undergone testing is then verbally described as above.

Claims: A coating composition comprising: a) a polymer having anhydride functionality which contains at least two reactive anhydride groups and an average molecular weight of less than 100,000; b) a compound having epoxy or hydroxyl functionality containing at least two epoxy groups or hydroxyl groups and having an average molecular weight of less than 100,000, wherein the composition contains from 20 to 80% by weight of polymer (a) and 80 to 20% by weight of compound (b); and c) a catalyst which catalyses a cross-linking reaction between compound a) and compound b), said catalyst being a 1,3-dialkylimidazole-2-thione compound with the structure: where R and R₁ are alkylene C₁ to C₁₄ (straight chain or branched) and Y and Y₁ are H, OH, CO₂H, aryl, ether, amine, perfluoroalkyl, amide, nitrile, or olefin, and being present in an amount from 1 to 10% by weight of the total binder. A coating composition as claimed in claim 1 wherein Y and Y₁ are hydrogen. A coating composition as claimed in claim 1 wherein said 1,3-dialkylimidazole-2-thione is selected from the group consisting of 1- n -butyl-3-methylimidazole-2-thione, 1-n-octyl-3-methylimidazole-2-thione, 1- n -propyl-3-methylimidazole-2-thione, or N,N'-diethylimidazole-2-thione. The coating composition as claimed in any one of claims 1 to 3 wherein compound b) is epoxy functional and the coating composition also contains an acid functional compound. A coating composition as claimed in any one of claims 1 to 3 wherein compound a) is a copolymer prepared from one or more of the monomers of styrene, methacrylate, or acrylates with one or more of the monomers of itaconic acid, itaconic anhydride, maleic anhydride, maleic acid or isobutenyl succinic anhydride. A coating composition as claimed in any one of the preceding claims wherein compound b) are epoxy copolymers prepared from alkyl acrylates or alkyl methacrylates with glycidyl acrylates or glycidyl methacrylates. A coating composition as claimed in claim 6 wherein the epoxy copolymers are used in combination with the polyglycidylethers of sorbitol. A coating composition as claimed in claim 1 wherein compound b) is a copolymer containing alkyl acrylates or alkyl methacrylates and hydroxy functional acrylates or hydroxy functional methacrylates. A coating composition as claimed in any one of the preceding claims wherein the 1,3-dialkylimidazole-2-thione catalyst is used in conjunction with amine catalysts. Use of a 1,3-dialkylimidazole-2-thione having the formula: where R and R₁ are alkylene C₁ to C₁₄ (straight chain or branched) and Y and Y₁ are H, OH, CO₂H, aryl, ether, amine, perfluoroalkyl, amide, nitrile, or olefin, as a catalyst to catalyse the cross-linking of an acid functional polymer and an anhydride functional polymer with an epoxy functional compound.

Inventors: ARDUENGO III ANTHONY JOSEPH, BARSOTTI ROBERT J, CORCORAN PATRICK H, ARDUENGO, III, ANTHONY JOSEPH, BARSOTTI, ROBERT J., CORCORAN, PATRICK H.
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1990
Main classification: C09D7
IPC codes: C09D167, C09D133, C08G63, C08G59, C09D163, C08K5, C07D233