ACRYLIC POWDER COATING INCLUDING HIGH HOMOPOLYMER GLASS TRANSITION TEMPERATURE CYCLIC (METH)ACRYLATE MONOMER AS VISCOSITY MODIFIER

Abstract: A powder paint coating composition comprising a binder which is comprised of two components wherein: (a) one of said components includes one or more acrylate polymers derived from two or more monomers with the proviso that at least 15 percent of said two or more monomers comprises a cyclic acrylate monomer having a homopolymer glass transition temperature greater than about 75 DEG C; and (b) the other of said components includes one or more cross-linking agents; wherein upon application to a surface and heating thereafter, said coating cross-links and forms a thin irreversible film is provided.

Description: ACRYLIC POWDER COATING INCLUDING HIGH HOMOPOLYMER GLASS TRANSITION TEMPERATURE CYCLIC (METH)ACRYLATE MONOMER AS VISCOSITY MODIFIER Background of the Invention 1 Field of the Invention The present invention relates to a coating composition More specifically, the composition is an acrylic powder coating, which includes a high homopolymer glass transition temperature cyclic (meth)acrylate monomer as a viscosity modifier The modifier functions to either reduce the melt temperature of the final coating composition or reduce the viscosity so that a thin uniform film is obtained upon curing of the coating 2 Technology Description Continued improvements in formulation, application and process economics have established thermoset powder coatings as a reliable and affordable finishing technology A number of different technologies have been developed within the finishing industry to address a diversity of end use applications based upon polyester, epoxy and acrylic polymer chemistry The initiative of the automotive industry to apply thermoset powder more widely in critical application areas has led to a resurgence of activity in the area of acrylic powder coatings, particularly because of their established reputation for excellent outdoor durability and hardness Acrylic powder coatings currently being used and developed for use in thermoset powder coatings are based on glycidyl, hydroxyl or carboxyl functional acrylic resins Typically glycidyl functional acrylics are cured with either long-chain dicarboxylic acids or acid anhydrides, while the hydroxyl functional acrylics can be cross nked by both blocked isocyanates and glycolunls Carboxyl functional acrylics are capable of being crosshnked by a number of different chemistries, namely epoxy and hydroxy alkylamides When making such coatings, it is desirable to provide a coating that either has a low melt temperature or can achieve an extremely low melt viscosity The primary incentive for the former property is to reduce energy costs associated with the application of the coating to a substrate The primary incentive for the latter property is to provide as thin and continuous a coating as possible Discussion of acrylic powder coatings and/or the use of viscosity modifiers may be potentially found in one or more of the following documents Yeates et al , Rheology of Acrylic Powder Coatings, Journal of Coatings Technology, Vol 68, No 861 , October 1996, U S Patent Nos 4,131 ,572, 4,286,021 , 4,023,977, 5,098,956, 3,787,340, 5,510,444, 5,492,955, 5,407,707 and 5,153,252 Despite the above teachings, there still exists a need in the art for acrylic powder coatings that can cure at relatively low temperatures or are capable of forming continuous thin films upon curing Bπef Summary of the Invention In accordance with the present invention novel acrylic powder coatings which can cure at relatively low temperatures or are capable of forming continuous thin films upon curing are provided The coatings are particularly characterized by having as a binder an acrylate polymer derived from two or more monomers wherein at least 15 percent by weight of said two or more monomers is a cyclic acrylate monomer having a homopolymer glass transition temperature greater than about 75°C More specifically, one embodiment of the present invention provides a powder paint coating composition comprising a binder which is comprised of two components wherein (a) one of said components includes one or more acrylate polymers derived from two or more monomers with the proviso that at least 15 percent by weight of said two or more monomers comprises a cyclic acrylate monomer having a homopolymer glass transition temperature greater than about 75°C, and (b) the other of said components includes one or more crosslinking agents, wherein upon application to a surface and heating thereafter, said coating crosslinks and forms a thin irreversible film In preferred embodiments, component (a) of the binder includes a polymer of a hydroxyl functional (meth)acrylate monomer and isobornyl methacrylate as the cyclic acrylate and component (b) preferably includes a polyisocyanate crosslinking agent, or component (a) of the binder includes a polymer of a carboxy functional (meth)acrylate monomer and isobornyl methacrylate as the cyclic acrylate and component (b) preferably includes an epoxy crosslinking agent, or component (a) of the binder includes a polymer of a glycidyl functional (meth)acrylate monomer and isobornyl methacrylate as the cyclic acrylate and component (b) preferably includes a carboxyl functional crosslinking agent In still additional preferred embodiments, component (a) may also be derived from additional ethylenically unsaturated monomers Another embodiment of the present invention comprises a process for coating a substrate comprising the steps of coating the above defined composition onto a surface and thereafter heating the surface to crosslink the binder and form a thin irreversible film In preferred embodiments, the curing temperature is between about 80°C and about 220°C Still another embodiment of the present invention comprises a surface having a coating thereon which comprises the end product of the process defined above An object of the present invention is to provide a novel acrylic powder coating having excellent film forming and viscosity properties Still another object of the present invention is to provide a process for coating a substrate with an acrylic powder coating and thereafter curing the coating onto the substrate. A further object of the present invention is to provide a substrate having cured thereon a thin film of an acrylic powder coating. These, and other objects, will readily be apparent to those skilled in the art as reference is made to the detailed description of the preferred embodiment. Detailed Description of the Preferred Embodiment In describing the preferred embodiment, certain terminology will be utilized for the sake of clarity. Such terminology is intended to encompass the recited embodiment, as well as all technical equivalents which operate in a similar manner for a similar purpose to achieve a similar result. The coating composition of the present invention includes a binder which is comprised of two components wherein: (a) one of said components includes one or more acrylate polymers derived from two or more monomers with the proviso that at least 15 percent by weight of said two or more monomers comprises a cyclic acrylate monomer having a homopolymer glass transition temperature greater than about 75°C; and (b) the other of said components includes one or more crosslinking agents. The first component comprises the acrylate polymer. This polymer must be derived from two or more monomers, one of which is a cyclic acrylate monomer having a homopolymer glass transition temperature greater than about 75°C, more preferably greater than 100°C and most preferably greater than 115°C. This cyclic acrylate monomer must be present in an amount by weight of at least 15 percent of the polymer component, more preferably between about 15 and about 50 percent by weight of the polymer component and most preferably between about 15 and about 30 percent by weight of the polymer component The cyclic acrylate monomer may be selected from isobornyl methacrylate, cyclohexyl methacrylate, tπmethyl cyclohexyl (meth)acrylate, isobornyl acrylate, 4-t- butyl cyclohexyl methacrylate, and mixtures thereof, with isobornyl methacrylate being particularly preferred At least one of the other monomers preferably used to form the binder is either a hydroxy functional (meth)acrylate, glycidyl functional (meth)acrylate, carboxyl functional (meth)acrylate or a carbamate functional (meth)acrylate Mixtures of the above materials are expressly contemplated as falling within the scope of the invention Suitable hydroxyalkyl acrylates or methacrylates include hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, and mixtures thereof A preferred hydroxyalkyl acrylate is hydroxyethyl acrylate In practice, when present these monomers comprise between about 10 to about 85 percent by weight of the acrylic polymer, more preferably between about 15 to about 50 percent by weight and most preferably between about 20 to about 40 percent by weight of the acrylic polymer Suitable glycidyl functional acrylates or methacrylates include glycidyl acrylate or glycidyl methacrylate and mixtures thereof In practice, when present these monomers comprise between about 10 to about 85 percent by weight of the acrylic polymer, more preferably between about 15 to about 50 percent by weight and most preferably between about 20 to about 40 percent by weight of the acrylic polymer Suitable carboxyl functional acrylates or methacrylates include acrylic acid, methacryhc acid, fumaπc acid, crotonic acid, itaconic acid, maleic acid, cinnamic acid, 2,3-bιs-(para-methoxyphenyl)-acrylιc acid, meta-phenylene diacry c acid, oleic acid, and the like, and mixtures thereof In practice, when present these monomers comprise between about 10 to about 85 percent by weight of the acrylic polymer, more preferably between about 15 to about 50 percent by weight and most preferably between about 20 to about 40 percent by weight of the acrylic polymer Suitable carbamate functional acrylates or methacrylates include hydroxypropyl carbamoyl(meth) acrylate In practice, when present these monomers comprise between about 10 to about 85 percent by weight of the acrylic polymer, more preferably between about 15 to about 50 percent by weight and most preferably between about 20 to about 40 percent by weight of the acrylic polymer In addition to the above-defined monomers, it is possible that the acrylic polymer include other ethylenically unsaturated monomers not having the above functionalities These monomers are present in amounts ranging from about 0 to about 75 percent by weight of the acrylic polymer, more preferably between about 30 to about 70 percent by weight of the acrylic polymer and most preferably between about 35 to about 65 percent by weight of the acrylic polymer These monomers include ethylenically unsaturated aromatic hydrocarbons, and alkyl (meth)acrylates wherein alkyl represents a group containing between one and about thirty carbon atoms and mixtures thereof Examples of ethylenically unsaturated aromatic hydrocarbons include styrene, ortho- methyl styrene, para-methyl styrene, and mixtures thereof A preferred comonomer is styrene, present in an amount of between about 15 to about 30, more preferably between about 20 to about 25 percent by weight of the acrylic polymer Examples of alkyl (meth)acrylates wherein alkyl represents a group containing between one and about thirty carbon atoms include, but are not limited to, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, sec-butyl acrylate, tert-butyl acrylate, n-pentyl acrylate, neopentyl acrylate, n-hexyl acrylate, cyclohexyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, lauryl (n- dodecyl) acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, sec-butyl methacrylate, tert-butyl methacrylate, n-pentyl methacrylate, neopentyl methacrylate, n-hexyl methacrylate, cyclohexyl methacrylate, n-octyl methacrylate, 2-ethylhexyl methacrylate, lauryl (n-dodecyl) methacrylate and mixtures thereof Preferred alkyl acrylate monomers include ethyl acrylate, n-butyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate and mixtures thereof When present, such acrylate monomers are present in an amount of between about 1 to about 30, more preferably between about 5 to about 15 percent by weight of the acrylic polymer Preferred alkyl methacrylate monomers include methyl methacrylate, n-butyl methacrylate, ethyl methacrylate, and mixtures thereof When present, such acrylate monomers are present in an amount of between about 25 to about 60, more preferably between about 30 to about 45 percent by weight of the acrylic polymer In a preferred embodiment, the acrylate polymer is a copolymer of isobornyl methacrylate (20 percent by weight), hydroxyethyl acrylate (20 percent by weight), styrene (20 percent by weight), methyl methacrylate (35 percent by weight) and n- butyl acrylate (5 percent by weight) The polymer is prepared by means known in the art such as free-radical polymerization in bulk, solution, emulsion or suspension form Preferably, the reaction is conducted in the presence of a free radical initiator such as benzoyl peroxide, tert-butyl peroxide, decanoyl peroxide, azo compounds such as azobisisobutyronitπle, and the like Useful initiators are present in amounts ranging from about 0 1 to about 5 percent by weight of the total monomers The use of heat to improve reaction between the monomers is also clearly contemplated as falling within the scope of the present invention The second component of the binder includes one or more crosslinking agents which reacts with the functional group(s) of the acrylic polymer These compounds contain reactive groups capable of reacting at elevated temperatures with the functional groups of the basic polymer Preferably, the crosslinking agent is selected from compounds having the following groups alkoxy, blocked isocyanate, carboxy, epoxy and hydroxylamide groups The crosslinking agent can be selected from the following compounds dicyandiamide, hydroxyalkylamides, hexamethoxy-melamine, tetramethoxymethylglycoluni, an aliphatic dicarboxylic acid, 1 ,-12-dodecanedιonιc acid, sebacic acid, 1 ,4-cyclohexanedιcarboxylιc acid, blocked isocyanates, such as caprolactam blocked isophorone diisocyanate and its ohgomers, blocked toluene diisocyanate and its ohgomers, tπglycidyl isocyanurate, low molecular weight glycidyl methacrylate copolymers, and others The selection of crosslinking agent should be performed to match the functional group with that of the acrylic polymer so that sufficient crosslinking occurs upon thermosetting Preferred crosslinking agents for thermosetting acrylic polymers having carboxyhc group functionality include tetramethoxymethyl- glycoluπl, tnglycidyl isocyanurate and hydroxyalkyl amides Preferred crosslinking agents for thermosetting acrylic polymers having hydroxyl functionality include blocked dnsocyanates such as caprolactam blocked meta-tetramethyl xylylene diisocyanate, methyl ethyl ketone blocked isophorone diisocyanate, blocked tnmer of hexamethylene diisocyanate, including but not limited to caprolactam blocked hexamethylene diisocyanate tnmer, or caprolactam blocked isophorone diisocyanate For thermosetting acrylic copolymers having glycidyl methacrylate functionality preferred crosslinking agents are 1 ,12-dodecanedιonιc acid, 1 ,3,5-trιscarboxyethyl isocyanurate and 1 ,4- cyclohexane dicarboxylic acid In accordance with a preferred embodiment wherein the acrylate polymer has hydroxy functionality, a blocked polyisocyanate, and more preferably caprolactam blocked isophorone dnsocyanates are selected for use In practice, the binder comprises between about 25 to about 95 percent by weight of the acrylic polymer and between about 75 to about 5 percent by weight of the component including a crosslinking agent, more preferably between about 50 to about 90 percent by weight of the acrylate polymer to about 50 to about 10 percent by weight of the component including a crosslinking agent, and most preferably between about 70 to about 90 percent by weight of the acrylate polymer to about 30 to about 10 percent by weight of the component including a crosslinking agent The two different components are typically maintained in separate containers prior to use to prevent premature reaction between the functional groups of the acrylic polymer and the functional groups of the crosslinking agent Optionally, plasticizers, stabilizers, primers, catalysts, antistatic agents, degassing additives such as benzoin, ultraviolet light absorbers, fillers, extenders and other conventional additives and mixtures thereof can be included in the binder composition These additives can be added in amounts from about 0 1 to about 30 percent by weight of the final acrylic powder coating composition Of the two component system, these optional additives are typically maintained in the container which includes the acrylic polymer The composition of the present invention can additionally contain flow control agents used to produce a smooth, uniform coating different than the high T g cyclic (meth)acrylate monomer melt viscosity modifier of the present invention The flow control agents are added in amounts ranging from about 0 1 to about 30 percent by weight of the binder composition to eliminate surface imperfections, such as poor flow, orange peel effect and crateπng Useful flow control agents include sihcone ohgomers, fluoπnated polyolefins, polyvinyl butyral, polyacrylates and others known in the art Flow control agents function by lowering the surface tension of the coating system The melt viscosity modifier of the present invention in contrast lowers the melt viscosity of the powder coating composition during the initial step of the baking process The above composition can be used as a clear powder acrylic coating However, in a preferred embodiment, the composition can also include pigments and/or colorants Examples of suitable pigments include titanium dioxide, carbon black, iron blues, phthalocyanine blues and greens, metal oxides, hydroxides, sulfides, sulfates, silicates and chromates, organic maroons, aluminum flake, bronze powders, pearl essence, and various fillers or extenders such as talc, barytes, china clay and diatomaceous earth It will be obvious to those skilled in the organic coating art that the amount of pigment may be varied widely, depending on the effect desired The amount of pigment, by weight based on the weight of the coating composition, may vary from between about 1 percent by weight of the final composition for light, high- hiding pigments, such as carbon black, to about 50 percent by weight of the final composition (equal amounts of pigment and binder) for heavy, low-hiding pigments such as lead chromate The method of dispersing the pigment in the powdered resins is not critical provided a uniform dispersion is produced The pigment may be present as separate particles or may be dispersed in the acrylic polymer, preferably the latter Powder coating compositions can be prepared by dry blending all ingredients (i e , both components of the binder, optional additives and pigments), followed by melt blending in an internal mixer, or in an extruder at room temperatures ranging from 80°-120° C Other methods such as solution mixing may be selected as is would be understood by one skilled in the art The homogeneous composition is then cooled to room temperature, comminuted by crushing or ground in a mill and screened Useful product is a free-flowing powder having a particle size of less than about 500 microns and preferably from about 50 to about 250 microns The powder composition of the present invention is physically and chemically stable at room temperature for prolonged periods of time up to 2 years The powder coating is preferably free flowing and resistant to sintering to form agglomerates at the temperatures used The compositions can be applied using appropriate spraying apparatus as known in the art as a dry coating on a substrate, such as on a metallic object, and then baked within an oven at about 80°-220°C, more preferably at about 100-140°C for 5-30 minutes to obtain a crosshnked film having excellent mechanical strength, thermal stability, solvent resistance, adhesion, mechanical strength, and durability against weathering An advantage when using the present invention is the lowering of the baking temperature when curing onto a substrate Whereas prior art compositions typically require baking temperatures exceeding 150°C to cure, the use of the cyclic acrylate monomer having a homopolymer glass transition temperature greater than about 75°C can reduce the film forming (cure) temperature by 10 to 50°C, resulting in preferred curing temperatures of between about 100 and about 140°C The reduction in cure temperature can result in significant energy savings when applying the coating composition onto a substrate Alternatively, it is further hypothesized that the coating composition may have a reduced viscosity upon curing, enabling the formation of extremely thin coatings This can have the effect of providing an extremely continuous defect-free coating Prior art powder paint coatings can suffer from not be directly applied in a smooth fashion This results in the presence of an "orange peel" type texture and is an indication of inefficient flow following application of the powder The use of the cyclic acrylate monomer in the coating composition can help prevent the orange peel effect and yield a high gloss, continuous thin film coating The invention is described in greater detail by the following non-limiting examples Example 1 The following monomers are mixed together in liquid form, dissolved in xylene as a solvent and 2 parts of benzoyl peroxide catalyst are added to the mixture The amounts listed are in parts by weight Hydroxyethyl Acrylate 20 Styrene 20 Methyl Methacrylate 35 n-Butyl Acrylate 5 The catalyzed monomer solution is added to xylene under nitrogen which is heated to 140°C over a 4-hour period After completion of the monomer addition, an additional 0.12 parts of benzoyl peroxide are added to the heated reaction vessel. The polymerization continues for an additional 3 hours. At the end of the polymerization, the material is cooled, spread onto aluminum sheets and the solvents are removed to provide a solid polymer resin (Resin A). A second container contains a caprolactam-blocked isocyanate (Vestagon BF 15- 40). 80 parts of Resin A are mixed with 20 parts of the second container composition and the resulting formulation is extruded via a Gay's twin screw extruder with a barrel temperature profile of 80/90/1 OOoC at 130-150 rpm and 70 torque. The extrudate is cooled to room temperature and is milled and classified to a size (70-micron mesh Tyler screen). The extrudate is applied onto steel panels at a dry film thickness of about 70-90 microns and the panel is heated to between 100 and 150°C for a time period of between about for 5-30 minutes to thermoset the coating composition and obtain a clear and colorless crosshnked film having excellent mechanical strength, thermal stability, solvent resistance, adhesion, mechanical strength, and durability against weathering. Example 2 The experiment of Example 1 is repeated except that prior to coating, 50 parts by weight of titanium dioxide are added. The resulting steel panels have a glossy white color. Having described the invention in detail and by reference to the preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the appended claims.

Claims: WHAT IS CLAIMED IS 1 A powder paint coating composition comprising a binder which is comprised of two components wherein (a) one of said components includes one or more acrylate polymers derived from two or more monomers with the proviso that at least 15 percent of said two or more monomers comprises a cyclic acrylate monomer having a homopolymer glass transition temperature greater than about 75°C, and (b) the other of said components includes one or more crosslinking agents, wherein upon application to a surface and heating thereafter, said coating crosslinks and forms a thin irreversible film 2 The composition according to claim 1 wherein said cyclic acrylate monomer comprises a bicychc alcohol of an ethylenically unsaturated (meth)acrylιc acid 3 The composition according to claim 2 wherein said cyclic alcohol of an ethylenically unsaturated acrylic acid comprises isobornyl methacrylate 4 The composition according to claim 1 wherein at least one of said two or more monomers comprises a cross nkable (meth)acrylate monomer 5 The composition according to claim 4 wherein said crosshnkable (meth)acrylate monomer is selected from the group consisting of hydroxy functional (meth)acrylates, glycidyl functional (meth)acrylates, carboxyl functional (meth)acrylates, carbamate functional (meth)acrylates and mixtures thereof 6 The composition according to claim 4 wherein at least one of said two or more monomers further comprises an additional ethylenically unsaturated monomer 7 The composition according to claim 6 wherein said additional ethylenically unsaturated monomer is selected from the group consisting of ethylenically unsaturated aromatic hydrocarbons, alkyl (meth)acrylates wherein alkyl represents a group containing between one and about thirty carbon atoms and mixtures thereof 8 The composition according to claim 7 wherein said additional ethylenically unsaturated monomer is selected from the group consisting of styrene, methyl methacrylate, butyl acrylate and mixtures thereof 9 The composition according to claim 3 wherein the amount of said isobornyl methacrylate comprises between about 15 and about 50 percent by weight of said two or more monomers 10 The composition according to claim 9 wherein the amount of said isobornyl methacrylate comprises between about 15 and about 30 percent by weight of said two or more monomers 11 The composition according to claim 1 wherein said crosslinking agent in component (b) of said binder is selected from the group consisting of compounds including isocyanate, amide, carboxy, epoxy or ammo functional groups and mixtures thereof 12 The composition according to claim 11 wherein said acrylic polymer is derived from hydroxy functional (meth)acrylates and said crosslinking agent comprises one or more blocked polyisocyanates 13 The composition according to claim 1 1 wherein said acrylic polymer is derived from carboxyl functional (meth)acrylates and said crosslinking agent comprises an epoxy functional crosslinking agent 14 The composition according to claim 11 wherein said acrylic polymer is derived from glycidyl functional (meth)acrylates and said crosslinking agent comprises a carboxy functional crosslinking agent 15. The composition according to claim 1 further comprising an additive material selected from the group consisting of pigments, colorants, flow control agents, plasticizers, stabilizers, primers, catalysts, antistatic agents, degassing additives, ultraviolet light absorbers, fillers, extenders and mixtures thereof 16. The composition according to claim 1 wherein said cyclic acrylate monomer is selected from the group consisting of cyclohexyl methacrylate, trimethyl cyclohexyl (meth)acrylate, isobornyl acrylate, 4-t-butyl cyclohexyl methacrylate, and mixtures thereof. 17. The composition according to claim 1 wherein said acrylate polymer of component (a) is a copolymer of isobornyl methacrylate, hydroxyethyl acrylate, styrene, methyl methacrylate and n-butyl acrylate; and wherein component (b) contains a caprolactam blocked isocyanate. 18. A process for forming a thin irreversible film coating onto a substrate comprising the steps of : (1) coating onto said substrate a powder paint coating composition comprising a binder which is comprised of two components wherein: (a) one of said components includes one or more acrylate polymers derived from two or more monomers with the proviso that at least 15 percent of said two or more monomers comprises a cyclic acrylate monomer having a homopolymer glass transition temperature greater than about 75°C; and (b) the other of said components includes one or more crosslinking agents; and (2) heating said substrate to crosslink the binder and form a thin irreversible film onto said substrate. 19. The process according to claim 18 wherein said heating step occurs at a temperature of between about 80°C and about 220°C. 20. The process according to claim 19 wherein said heating step occurs at a temperature of between about 100°C and about 140°C. 21. A substrate having a powder paint coating formed thereon by: (1) coating onto said substrate a powder paint coating composition comprising a binder which is comprised of two components wherein: (a) one of said components includes one or more acrylate polymers derived from two or more monomers with the proviso that at least 15 percent of said two or more monomers comprises a cyclic acrylate monomer having a homopolymer glass transition temperature greater than about 75°C; and (b) the other of said components includes one or more crosslinking agents; and (2) heating said substrate to crosslink the binder and thereby form a thin irreversible film onto said substrate.

Inventors: HOPKINS KIMBERLY D, ZEZZA CHARLES, HOPKINS, KIMBERLY, D., ZEZZA, CHARLES
Applicants: RHODIA, RHODIA INC.
Application year: 1998
Main classification: C08G18
IPC codes: C08F20, C08G18, C09D5, C09D133