COATING COMPOSITION OF AN ACRYLIC POLYMER, A DISPERSED ACRYLIC POLYMER AND AN ALKYLATED MELAMINE CROSSLINKING AGENT

Abstract: A coating composition that is used primarily as a clear finish over a colored or pigmented finish that are applied to automobile and truck bodies; the composition contains as the film forming binder (a) an acrylic polymer (b) a self stabilized dispersed resin formed by poly­ merizing in solution the following constituents: (1) an acrylic solution polymer, (2) monomers of alkyl methacrylate or acrylate and hydroxy ethyl acrylate or methacrylate, wherein either the acrylic solution polymer contains post reacted glycidyl methacrylate or acrylate or the monomers contain glycidyl methacrylate or acrylate, and (c) an alkylated melamine formaldehyde crosslinking agent.

Description: This invention relates to a coating composition useful for finishing automobiles and trucks. One particular finish that provides an excellent aesthetic appearance is being used widely on automobiles and trucks has a coloured base coat which contains pigments and a clear topcoat which is unpigmented. There is a need for a clear top-coating composition that has excellent durability, weatherability and adhesion to a coloured base coat and appearance. GB 2 074 588 (Du Pont) discloses a high solids coating composition comprising a binder having the following constituents: A 5-45 wt% medium molecular weight acrylic polymer obtained from C₁₋₁₈ alkyl methacrylate C₂₋₁₈ alkyl acrylate or styrene, B 10-50 wt% low molecular weight acrylic polymer obtained from the above starting materials but a molecular weight of at least 3000 less and C 25-45 wt% alkylated melamine formaldehyde cross linking agent. According to the invention there is provided a coating composition comprising 20-85% by weight of film forming binder and 15-80% by weight of an organic liquid carrier; wherein the binder comprises an acrylic polymer I having a weight average molecular weight of 3,000 to 20,000 and comprising polymerised monomers of alkyl methacrylate, alkyl acrylate and hydroxy alkyl acrylate or methacrylate prepared by heating to 90-250 o C in the presence of polymerisation initiators and a self stabilised dispersed resin formed by polymerising at 80-250 o C in the presence of polymerisation initiators the following constituents: (1) an acrylic polymer II comprising monomers of alkyl methacrylate, alkyl acrylate, hydroxy alkyl acrylate or methacrylate and an ethylenically unsaturated carboxylic acid and having a weight average molecular weight of 3,000 to 20,000 and (2) monomers comprising alkyl methacrylate, alkyl acrylate and hydroxy alkyl acrylate or methacrylate; wherein either the acrylic polymer II contains glycidyl methacrylate or acrylate monomers post reacted with the carboxylic acid moities or the monomers contain glycidyl methacrylate or acrylate, and an alkylated melamine formaldehyde crosslinking agent. According to the invention there is also provided a method of preparing the heat-curable coating composition claimed in any one of claims 1 to 9 which comprises forming a non-aqueous, self-stabilised resin dispersion by polymerising in a solvent a monomer mixture comprising an alkyl acrylate, an alkyl methacrylate, an hydroxy alkyl acrylate or methacrylate and a glycidyl acrylate or methacrylate in the presence of an acrylic polymer (II) comprising polymerised monomers of alkyl acrylate, alkyl methacrylate, hydroxyalkyl acrylate or methacrylate and an ethylenically unsaturated carboxylic acid; blending the resulting resin dispersion with an alkylated melamine formaldehyde cross-linking agent and an acrylic polymer (I) comprising monomers of alkyl methacrylate, alkyl acrylate, and hydroxyalkyl acrylate or methacrylate polymerised at 90-250 o C in the presence of polymerisation initiator, the acrylic POLYMERS (I) and (II) having average molecular weights of 3,000 to 20,000. DETAILED DESCRIPTION OF THE INVENTION The coating composition has a film forming binder content of 20-85% by weight and correspondingly 15-80% by weight of a liquid carrier. Preferably, the coating composition is a high solids composition that contains 50-80% by weight of the binder and 20-50% by weight of organic solvent for the binder. The binder of the composition preferably is a blend of 5-75% by weight of an acrylic polymer I, and 5-45% by weight of a self stabilized dispersed resin and 20-50%, by weight of an alkylated melamine formaldehyde crosslinking agent. The acrylic polymer I is of polymerized monomers of an alkyl methacrylate, an alkyl acrylate, hydroxy alkyl acrylate or methacrylate and has a weight average molecular weight of 3,000-20,000. preferably, styrene is used with the above monomers in acrylic polymer I. Molecular weight is measured by gel permeation chromatography using polymethyl methacrylate as a standard. Typical alkyl methacrylates used to form the acrylic polymer are methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, isobutyl methacrylate, pentyl methacrylate, hexyl methacrylate, octyl methacrylate, nonyl methacrylate, lauryl methacrylate and the like. Typical alkyl acrylates used to form the acrylic polymer are methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, isobutyl acrylate, pentyl acrylate, hexyl acrylate, octyl acrylate, nonyl acrylate, lauryl acrylate and the like. Typical hydroxy alkyl acrylates and methacrylates are hydroxy ethyl acrylate, hydroxy propyl acrylate, hydroxy isopropyl acrylate, hydroxy butyl acrylate, hydroxy ethyl methacrylate, hydroxy propyl methacrylate, hydroxy isopropyl methacrylate, hydroxy butyl methacrylate and the like and mixtures of the above monomers. Preferably, the acrylic polymer I is composed of 5-30% by weight styrene, 10-40% by weight butyl methacrylate 10-40% by weight butylacrylate, 15-50% by weight of hydroxyethyl acrylate or hydroxy propyl acrylate and has a weight average molecular weight of 5,000-15,000. Optionally, the polymer can contain 0.1-5% by weight of acrylic acid or methacrylic acid. The acrylic polymer 1 is prepared by solution polymerization in which the monomers, conventional solvents, polymerization initiators, such as peroxy acetate are heated to 90-250°C for 1-6 hours. The self-stabilized dispersed resin is formed by polymerizing in solution the following constituents: (1) an acrylic polyer II: (2) monomers of alkyl acrylate, alkyl methacrylate, and hydroxy acrylate or methacrylate. Conventional polymerization initiators such as t-butyl peracetate, and solvents are used. The constituents are heated to 80-250°C for 1-6 hours to form the resin. The solution acrylic polymer II is composed of polymerized monomers of alkyl methacrylate, alkyl acrylate, hydroxy alkyl acrylate or methacrylate and an ethylenically unsaturated carboxylic acid and optionally, monomers of styrene. Any of the aforementioned alkyl acrylates, methacrylates, hydroxy alkyl acrylates or methacrylates can be used to prepare the polymer. Typical ethylenically unsaturated carboxylic acids that can be used are acrylic acid and methacrylic acid. Preferably, styrene is used to form the polymer with the above monomers. One preferred polymer contains 5-25% by weight styrene, 20-40% by weight butyl acrylate, 20-40% by weight butyl methacrylate, 5-15% by weight hydroxy ethyl acrylate, 1-5% by weight acrylic acid and 10-20% by weight ethyl methacrylate and has a weight average molecular weight of 7,000-15,000. Another preferred polymer contains 5-25% by weight styrene, 25-35% by weight butyl methacrylate, 20-40% by weight butyl acrylate, 5-15% by weight hydroxy ethyl acrylate, 1-5% by weight acrylic acid, 10-20% by weight ethyl methacrylate and 1-5% by weight glycidyl methacrylate post reacted with acrylic acid and has a weight average molecular weight of 7,000-15,000. Conventional polymerization techniques as described above for acrylic Polymer I are used to prepare acrylic Polymer II. Glycidylmethacrylate or acrylate is either post reacted with the carboxyl groups of acrylic polymer II or is one of the monomers that are polymerized with acrylic polyer II. Any of the aforementioned alkyl methacrylates, alkyl acrylates, hydroxy alkyl acrylates or methacrylates, ethylenical unsaturated acids are monomers that can be used to prepare the self stabilized/dispersed resin. Other useful monomers are styrene, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, perfluoro alkyl methacrylates or acrylates, dicarboxylic acids such as itaconic acid and the like. Typical solvents and diluents are used to form the above polymers and the resulting coating composition. Appropriate solvents are chosen to form polymer solutions or dispersions and appropriate diluents are chosen to form coating compositions. Typical solvents and diluents are toluene, xylene, butyl acetate, acetone, methyl isobutyl ketone, methyl ethyl ketone, methanol, isopropanol, butanol, hexane, acetone, ethylene glycol monoethyl ether, VM and P naphtha, mineral spirits, heptane and other aliphatic, cycloaliphatic, aromatic hydrocarbons, esters, ethers and ketones and the like. The crosslinking agent used in the composition is a fully alkylated melamine formaldehyde resin that preferably is a methylated and butylated melamine formaldehyde resin that has a degree of polymerization of 1-3. Generally, this melamine formaldehyde resin contains about 50% butylated groups and 50% methylated groups. Typically these crosslinking agents have a number average molecular weight of 300-600 and a weight average molecular weight of 500-1500. It is possible to use other alkylated melamine formaldehyde crosslinking agents. Typically lower alkyl alcohols are used to form these crosslinking agents such as methanol, ethanol, propanol, isopropanol, butanol, isobutanol, hexanol and the like. Also, urea formaldehyde, benzoquanamine formaldehyde and polyisocyanates may be used as crosslinking agents. One preferred coating composition contains 5-75%, by weight of a solution acrylic polymer I of 5-30% by weight styrene, 10-40% by weight butyl methacrylate, 10-40% by weight butyl acrylate, 15-50% by weight hydroxy ethyl acrylate or hydroxy propyl acrylate and has a weight average molecular weight of 5,000-15,000; 5-45% by weight of a self stabilized dispersion resin formed by polymerizing the following constitutents: (1) an acrylic solution Polymer II composed of 5-25% by weight styrene, 20-40% by weight butyl acrylate, 20-40% by weight butyl methacrylate, 5-15% by weight hydroxy ethyl acrylate, 1-5% by weight acrylic acid 10-20% by weight ethyl methacrylate and having a weight average molecular weight of 7,000-15,000 and (2) monomers of styrene, methyl methacrylate hydroxy ethyl acrylate, methacrylic acid, methyl acrylate and glycidyl methacrylate: (3) an acrylic solution polymer II wherein the carboxyl groups of the acrylic acid are post reacted with glycidyl methacrylate and 25-50% by weight of a fully methylated and butylated melamine formaldehyde crosslinking agent. Generally, an acid catalyst is used in the coating composition to enhance crosslinking of the components on curing. 0.1-2% by weight, based on the weight of the composition, of catalyst can be used. Typically, blocked aromatic sulfonic acids are used. One preferred blocked acid catalyst is dodecyl benzene sulfonic acid blocked with dimethyl oxazolidine. Other acid catalysts that can be used are sulfonic acid, para-toluene sulfonic aid, dinonyl naphthalene sulfonic acid and the like. To improve weatherability of the clear finish of the coating composition, 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, 0.1-5% by weight, based on the weight of the binder, of an antioxidant can be added. Typical ultraviolet light stabilizers that are useful are as follows: Benzophenones such as hydroxydodecylbenzophenone, 2,4-dihydroxybenzophenone, hydroxy-benzophenones containing sulfonic acid groups, 2,4-dihydroxy-3',5'-di- t -butylbenzophenone, 2,2',4'-trihydroxybenzophenone esters of dicarboxylic acids, 2-hydroxy-4-acryloxyethoxybenzophenone, aliphatic mono-esters of 2,2',4-trihydroxy-4'-alkoxybenzophenone, 2-hydroxy-4-methoxy-2'-carboxybenzophenone; Triazoles such as 2-phenyl-4-(2'-4'-dihydroxybenzoyl)triazoles, substituted benzotriazoles such as hydroxyphenyltriazoles such as 2-(2'hydroxy-5'-methylphenyl) benzotriazole, 2-(2'hydroxyphenyl)benzo-triazole, 2-(2'-hydroxy-5'-octylphenyl)naphthotriazole; Triazines such as 3,5-dialkyl-4-hydroxyphenyl derivatives of triazine, sulfur-containing derivatives of dialkyl-4-hydroxyphenyltriazines, hydroxyphenyl-1,3,5-triazines and such triazines containing sulfonic acid groups, aryl-1,3,5-triazines, orthohydroxyaryl- s -triazine; Benzoates such as dibenzoate of diphenylolpropane, t -butyl benzoate of diphenylolpropane, nonyl phenyl benzoate, octyl phenyl benzoate, resorcinol dibenzoate. Other ultraviolet light stabilizers that can be used include lower alkyl thiomethylene-containing phenols, substituted benzenes such as 1,3-bis(2'-hydroxybenzoyl)benzene, metal derivatives of 3,5,-di- t butyl-4-hydroxyphenylpropionic acid, asymmetrical oxalic acid diarylamides, alkylhydroxyphenylthioalkanoic acid esters, dialkylhydroxyphenylalkanoic acid esters of di- and tri- pentaerythritol, phenyl- and naphthalene-substituted oxalic acid diamides, methyl-β-(3,5-di- t -butyl-4-hydroxyphenyl)propionate, α,α'-bis(2-hydroxy-phenyl)diisopropylbenzene, 3,5'-dibromo-2'-hydroxy-acetophenone, ester derivatives of 4,4-bis(4'-hydroxy-phenyl)pentaonic acid wherein there is at least one unsubstituted position ortho to the aromatic hydroxyl groups, organophosphorus sulfides such as bis(diphenyl-phosphinothioyl)monosulfide and bis(diphenyl-phosphinothioyl)disulfide, 4-benzoyl-6-(dialkylhydroxybenzyl)resorcinol, bis(3-hydroxy-4-benzoylphenoxy)diphenylsilane, bis(3-hydroxy-4-benzoylphenoxy)dialkylsilane, 1,8-naphthalimides, α-cyano-β,β-diphenylacrylic acid derivatives, bis(2-benzoxazolyl)alkanes, bis(2-naphthoxazolyl)alkanes, methylene malonitriles containing aryl and heterocyclic substitutes, alkylenebis(dithio)carbamate, 4-benzoyl-3-hydroxyphenoxyethyl acrylate, 4-benzoyl-3-hydroxyphenoxyethyl methacrylate, aryl or alkyl-substituted acrylonitriles, 3-methyl-5-isopropylphenyl-6-hydroxycourmarone, 8-acetyl-3-dodecyl-7,7,9,9-tetramethyl-1,3,8-triazaspiro(4,5)decanol-2,4-dione. Particularly useful ultraviolet light stabilizers that can be used are hindered amines of piperidyl derivatives such as those disclosed in Murayama et al., U.S. Patent US-A-4,061,616, issued December 6, 1977, column 2, line 65, through column 4, line 2, and nickel compounds such as [1-Phenyl-3-methyl-4-decanoylpyrazolate(5)]-Ni, bis[phenyldithiocarbamato]-Ni(II), and others listed in the above Patent, column 8, line 44 through line 55. The following blend of ultraviolet light stabilizers is particularly preferred 2-[2'-hydroxy-3',5'-1(1-1-dimethyl-propyl)phenyl]benzotriazole and bis-[4-(1,2,2,6,6-pentamethylpiperidyl)] 2-butyl-2-[(3,5-t-butyl-4-hydroxyphenyl)methyl] propanedioate. The stabilizers can be used in any ratio however, a 1:1 ratio of benzotriazole to propanedioate is preferred. Another useful blend of ultraviolet light stabilizers is 2-(benzotriazole-2-VL)-4,6-bis(methylethyl-1-phenyl ethyl)phenol, and 2(3 hydroxy-3,5'-ditert amyl phenyl) benzotriazole. Generally, the clear coating composition of this invention is applied by conventional spraying techniques to a color or base coat of an automobile or truck and then is baked. Preferably, electrostatic spraying is used to apply the composition. The oatings are baked at 80 to 200 o C for 10 to 60 minutes. The resulting clear coat is (1-5 mils) 25µm-127µm thick preferably 25-50µm (1-2 mils) thick and has excellent gloss, good adhesion to the colour coat and excellent weatherability. The following examples illustrate the invention. All parts and percentages are on a weight basis unless otherwise indicated. The weight average molecular weight of polymers was determed by GPC (gel permeation chromatography) using polymethyl methacrylate as a standard. EXAMPLE 1 Preparation (A) A non-aqueous, self-stabilised acrylic resin dispersion was prepared by charging the following constituents into a reaction vessel equipped with a stirrer, a heating source and a reflux condenser: Portion 1 is charged into the reaction vessel and heated to its reflux temperature Portion 2 is added to the reaction vessel mixed and held at reflux temperature for 2 minutes. Then portions 3 and 4 are added simultaneously with portion 5 over a 210 minute period to the reaction vessel while maintaining the resulting reaction mixture at its reflux temperature. Then the mixture is held at its reflux temperature for an additional 45 minutes. Portion 6 is added over a 90 minute period while maintaining the reaction mixture at its reflux temperature and then held at this temperature for an additional 90 minutes. Portion 7 is added and excess solvent is stripped off to give a 60% solids self-stabilised acrylic resin dispersion (A). Preparation B A coating composition was prepared by blending together the following constituents: The constituents of Portion 1 were added in the order shown to a mixing vessel and agitated until in solution. Portion 2 was added to the vessel and mixed for 30 minutes. Portion 3 was added and mixed for 30 minutes. The resulting clear coating composition had 70% solids content. The resulting composition was sprayed onto primer coated phosphatized steel panels that were coated with a pigmented acrylic base coat. The composition was sprayed onto the panels before the base coat was baked. The panels were baked at 120 o C for 30 minutes and a clear coat 50µm (2 mils) thick was formed on each panel. The clear coating had a hardness of 8 knoops, a gloss measured at 20 degrees of 95. The coating had excellent outdoor weatherability. EXAMPLE 2 A coating composition was prepared that was identical to the coating composition of Example 1 except the following acrylic resin solution was used in the preparation of the coating composition: Acrylic resin solution (71% solids in xylene and aromatic hydrocarbon solvent mixture of an acrylic resin of 15% styrene, 30% butyl methacrylate, 17% butyl acrylate and 38% hydroxy propyl acrylate and has a weight average molecular weight of 6,000). The resulting clear coating composition was sprayed onto primer coated phosphatized steel panels that were coated with a pigmented acrylic base coat as in Example 1 and baked as in Example 1. The resulting coating was 50µm (2 mils) thick and had the same hardness and gloss as the coating composition of Example 1 and had excellent outdoor weatherability.

Claims: A coating composition comprising 20-85% by weight of film forming binder and 15-80% by weight of an organic liquid carrier; wherein the binder comprises an acrylic polymer I having a weight average molecular weight of 3,000 to 20,000 and comprising polymerised monomers of alkyl methacrylate, alkyl acrylate and hydroxy alkyl acrylate or methacrylate prepared by heating to 90-250 o C in the presence of polymerisation initiators and a self stabilised dispersed resin formed by polymerising at 80-250 o C in the presence of polymerisation initiators the following constituents: (1) an acrylic polymer II comprising monomers of alkyl methacrylate, alkyl acrylate, hydroxy alkyl acrylate or methacrylate and an ethylenically unsaturated carboxylic acid and having a weight average molecular weight of 3,000 to 20,000 and (2) monomers comprising alkyl methacrylate, alkyl acrylate and hydroxy alkyl acrylate or methacrylate; wherein either the acrylic polymer II contains glycidyl methacrylate or acrylate monomers post reacted with the carboxylic acid moities or the monomers contain glycidyl methacrylate or acrylate, and an alkylated melamine formaldehyde crosslinking agent. A coating composition according to claim 1 comprising 50-80% by weight of the film forming binder and 20-50% by weight of organic solvent for the binder. A coating composition according to claim 2 wherein the binder comprises: (a) 5-75% by weight, based on the weight of the binder, of the acrylic polymer I, (b) 5-45% by weight, based on the weight of the binder, of the self-stabilised dispersion resin and (c) 20-50% by weight, based on the weight of the binder, of an alkyated melamine formaldehyde crosslinking agent. A coating composition according to any one of claims 1 to 3, in which the acrylic polymer I is formed from polymerised monomers of 5-30% by weight styrene, 10-40% by weight of butyl methacrylate, 10-40% by weight of butyl acrylate and 15-50% by weight of hydroxy ethyl acrylate or hydroxy propyl acrylate and has a weight average molecular weight of 5,000-15,000. A coating composition according to any one of the preceding claims in which the acrylic polymer I contains additionally 0-1.5% by weight of acrylic acid or methacrylic acid. A coating composition according to any one of the preceding claims in which the acrylic polymer II is post reacted with glycidyl methacrylate. A coating composition according to claim 6 in which the acrylic polymer II is formed from 5-25% by weight styrene, 25-35% by weight butyl methacrylate, 20-40% butyl acrylate, 5-15% by weight hydroxy ethyl acrylate, 1-5% by weight acrylic acid, 10-20% by weight ethyl methacrylate and 1-5% by weight glycidyl methacrylate, post reacted with acrylic acid and has a weight average molecular weight of 7,000 to 15,000. A coating composition according to any one of the preceding claims which additionally contains 0.1-2% by weight, based on the weight of the coating composition, of an acid catalyst. A coating composition according to any one of the preceding claims in which the alkyated melamine formaldehyde crosslinking agent is approximately 50% methylated and 50% butylated. A substrate coated with a cured layer of the composition claimed in any one of the preceding claims. A substrate coated with a layer of a pigmented paint composition and having adhered thereto a cured layer of an unpigmented composition claimed in any one of claims 1 to 9. A method of preparing the heat-curable coating composition claimed in any one of claims 1 to 9 which comprises forming a non-aqueous, self-stabilised resin dispersion by polymerising in a solvent a monomer mixture comprising a alkyl acrylate, an alkyl methacrylate, an hydroxy alkyl acrylate or methacrylate and a glycidyl acrylate or methacrylate in the presence of an acrylic polymer (II) comprising polymerised monomers of alkyl acrylate, alkyl methacrylate, hydroxyalkyl acrylate or methacrylate and an ethylenically unsaturated carboxylic acid; blending the resulting resin dispersion with an alkylated melamine formaldehyde cross-linking agent and an acrylic polymer (I) comprising monomers of alkyl methacrylate, alkyl acrylate, and hydroxyalkyl acrylate or methacrylate polymerised at 90-250 o C in the presence of polymerisation initiator, the acrylic POLYMERS (I) and (II) having average molecular weights of 3,000 to 20,000.

Inventors: ANTONELLI, JOSEPH ALBERT, HAZAN, ISIDOR
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1985
Main classification: C09D133
IPC codes: C08L61, C09D133, C09D151, C08L33