A process for coil coating aluminium for the manufacture of automotive veneers

Abstract: The present invention is directed to a process for coil coating aluminum comprising the steps of: (a) coating a carrier film with a clearcoat layer; (b) applying a basecoat layer on said clearcoat layer; (c) applying an adhesive layer on said basecoat layer; (d) optionally (i) applying a protective film on said adhesive layer to form a free multilayer coating film; (ii) removing said protective film; and (e) applying an aluminum film on said adhesive layer; for the manufacture of automotive veneers with good appearance and adhesion between coating and aluminum. The new process is very flexible in terms of different colors of coatings and the use of graphics and patterns in the coating.

Description: The present invention is directed to a process for coil coating aluminum for the manufacture of automotive veneers more specifically it is directed to a process for coil coating aluminum with a free multilayer coating superimposed on a carrier film. Background of the Invention Automotive body panels are traditionally made of sheet metal or plastic material painted with layers of pigmented paints. The painting procedure for these panels requires elaborate facilities, and consequently involves heavy expenses. For instance, a large area of floor space must be maintained in a clean room environment for the spraying of paint and for the baking and curing of such paint on the body panels. The paint may include both a pigmented basecoat and transparent clearcoat. Moreover, solvent-based paints have come to be considered undesirable in recent years due to environmental concerns. As a consequence, the evaporation of such solvents must be strictly controlled. There is a great need in the automotive industry to move the coating operation out of the assembly plants. In addition to the problems associated with in-plant coating operations, another problem facing assembly plants is the coating of plastic bodies. There is a movement in the industry to convert from carbon steel to plastic bodied cars. Plastic parts are difficult to coat due to the inherent nature of plastics, which typically have difficult to adhere to surfaces. Plastics are also susceptible to attack and degradation from the solvents that are typically contained in automotive paints. Plastics are typically heat sensitive and require low temperature cure coatings. However, low temperature coatings do not always have optimal coating properties such as durability, gloss and solvent resistance. In addition, it is difficult to produce an automotive quality "glamour" finish on a plastic body part due to the inherent surface distortions and imperfections associated with molded body parts. In order to compensate for surface defects, primers and primer/surfaces must be used on plastic parts, not for corrosion protection, but to mask surface defects. In addition to plastic bodies, alternative substitute materials such as cellulosic materials and ceramic materials as well as other available materials are under consideration for use in automobile bodies. The U.S. Pat. No. 4,769,100 discloses a method of applying carrier films prepainted with metallic paint to automobile body panels by vacuum forming. While such prepainted free films have the advantage of application over flexible substrates, they are subject to certain disadvantages. Such films tend to trap bubbles during application to the substrate; they also tend to "telegraph" imperfections in the surface of the substrate such as bumps, dimples, or reinforcing fiber patterns. In the EP-A-0 374 551 is described a method of manufacturing a coated article which includes coil coating or aluminum with a multilayer coating; cutting, forming and affixing the coated aluminum to a substrate to form automotive veneers. One of the big problems with this veneer coat is the coating of the aluminum. Controlling of dirt, appearance and adhesion to the aluminum is difficult during the multiple coating, drying and curing steps. Another problem is that this process is directed to a limited number of colors. In today's automotive industry there is a need for a broad variety of different colors, and coatings with graphics and patterns because a single car may be coated with different colors and cars in general are offered in a much broader variety of different colors. The object of the present invention was to provide an improved method for coil coating of aluminum for use as automotive veneers with a good appearance and adhesion between coating and aluminum. Another object of the present invention was to provide a process for coil coating of aluminum which is very flexible in terms of changing colors of the coating and use different graphics or patterns in the coating. Another object of the present invention was to reduce the number of process steps for the coating of the aluminum coil. Summary of the Invention The objects of the present invention are achieved with a process for coil coating aluminum comprising the steps of: (a) coating a carrier film with a clearcoat layer; (b) applying a basecoat layer on said clearcoat layer; (c) applying an adhesive layer on said basecoat layer; (d) optionally (i) applying a protective film on said adhesive layer to form a free multilayer coating film; (ii) removing said protective film; and (e) applying an aluminum film on said adhesive layer. Detailed Description of the Invention The coil coating process is well known in the coating arts. Coil coating can be defined as a continuous, automated process for applying coatings to coiled metal stock. Typically the process is initiated by simultaneously unwinding coiled metal and feeding the uncoiled metal stock to a coil coating means. A coil coating means typically consists of a plurality of rollers, feed mechanisms and coating means. As the metal stock from the coil is fed through the coil coating means it is cleaned and then coatings are applied and cured or dried at various stages. The coated metal stock is then rewound as it exits the coil coating means. In the coil coating process of the present invention, first a carrier film is coated with multilayer coatings followed by an application to the aluminum film of an aluminum coil. Optionally, the carrier film with the multilayer coatings could be protected by a protective film to form a free film, which is also coiled and stored. In the coil coating process the protective film is removed followed by the application of the multilayer coating to the aluminum film. The carrier film useful in step (a) of the present invention could be selected from thermoplastic polymers such as polyethylene, polypropylene, polyester or acrylics. Preferred is polyester such as polyethylene terephthalate. The carrier film thickness is in a range of from about 25 to 254 µm (1 to 10 mils), preferably from about 25 to 102 µm (1 to 4 mils) and most preferably from about 51 to 76 µm (2 to 3 mils). Optionally the carrier film has a release coating on that side which is applied to the basecoat layer in step (b). Release coatings are well known to those skilled in the art of making free films on a carrier film and comprise for example acrylics and silicone polymers. Preferred are acrylics such as vinyl acrylic polymers. The clearcoat, in step (a) of the present invention which shall be the topcoat after the carrier film has been removed from the final veneer in step (a) typically contain a resinous film-forming polymer, a cross-linking agent and other additives typically included in coating compositions in this art. The film-forming polymer system includes epoxies, polyesters, polyurethanes, acrylics, fluoropolymers, and the like. Preferred is acrylic polyurethane, polyester polyurethane, acrylic melamine, polyester melamine and most preferred is acrylic polyurethane. The clearcoat will not contain pigments, although slight amounts of pigments could be added as long as the topcoat remains transparent. Optionally, pigmented topcoat coatings can be used instead of pigmented basecoat/clearcoat coatings. The cross-linking agents typically used for the clearcoat will include melamines, amines, urethanes, carboxy resin, and the like. The additives will include acids and salts thereof, silicone flow controls, fumed silicas, and ultraviolet absorbers, e.g. Tinuvin® 328, a benzatriazole derivative from Ciba Geigy. The clearcoat could be applied by spraying, extruding, flow coating, roller coating, preferably by roller coating, optionally followed by a drying or curing step or both at a temperature from about 60°C to about 150°C. The film thickness of the clearcoat is typically from about 38 to about 102 µm (1.5 to about 4 mils), preferably from about 51 to 76 µm (2 to 3 mils) on a dry film basis. The basecoat useful in step (b) of the present invention will typically contain a resinous film-forming polymer, a pigment, a cross-linking agent and other additives typically included in coating compositions in this art. The film-forming polymer includes epoxies, polyesters, polyurethanes, acrylics, fluoropolymers, and the like. Preferred is acrylic polyurethane, polyester polyurethane, acrylic melamine, polyester melamine, and most preferred is acrylic polyurethane. The pigments will include titanium dioxide, phthalocyanines, lamp black, iron oxides, quinacridones, aluminum flake, micas, perylenes, imidazoles, and the like. The cross-linking agents typically used for the basecoat will include melamines, amines, urethanes, carboxy resin, and the like. The additives will include acids and salts thereof, silicone flow controls, fumed silicas, and ultraviolet absorbers, e.g., Tinuvin® 328 which is a benzatriazole derivative from Ciba Geigy. The basecoat could be applied by spraying, extruding, flow coating, roller coating, preferably by roller coating, optionally followed by a drying or curing step or both at a temperature of from about 60°C to about 150°C. The film thickness of the basecoat is typically from about 13 µm (0.5 mils) to about 51 µm (2 mils), preferably from about 20 to 38 µm (0.8 to 1.5 mils). The adhesive layer in step (c) of the present invention will be based upon resinous film-forming polymer systems such as epoxies, polyesters, polyurethanes, acrylics, ureas and the like. The coating will contain sufficient amounts of cross-linking agents, pigments, and conventional additives. The cross-linking agent will include melamine, amines, ureas, carboxy, urethanes and the like. The pigments will include those commonly used for the end application including titanium dioxide, phthalocyanines, quinacridones, iron oxides, carbon blacks and fillers such as clays, talcs, etc. The conventional additives will include acid catalysts such as sulfonic acids and salts thereof, silicone flow control additives and surface slip aids and various waxes for surface slip and mar resistance. The adhesive layer could be applied by spraying, extruding, flow coating, roller coating, preferably by roller coating, optionally followed by a drying or curing step of both, at a temperature of from about 40°C to about 150°C. The optional step (d) (i) provides the application of a protective film on the adhesive layer after step (c). This step makes the whole process more flexible and allows the storage of the so formed free multilayer coating film as a coil which could be used then for the coating of an aluminum film as described in step (e). The protective film is preferably a teflon coated paper. In step (e) an aluminum film is applied on the adhesive layer; this happens in case of the optional step (d) (i) after removing the protective film in step (d) (ii). The aluminum stock is initially cleaned at a cleaning section in the following manner. The coil is passed through a series of solvent baths or baths in combination with solvent vapor degreasers to remove mill oils and other surface contaminants. Solvent selection is determined by contaminants to be removed and metal being treated. Trichlorethylene, perchlorethylene and similar solvents are commonly used. The moving coil is then optionally pretreated with suitable surface protective chemicals. Crystalline and amorphous phosphate or amorphous chromate is used on aluminum substrate. Next, the aluminum is dried in an oven at a sufficient temperature, typically about 140°C. Optionally the dried aluminum stock then moves to a primer coating application section. The primer coating is optionally applied to one or both sides of the aluminum stock to produce a coating having sufficient thickness, typically about 5 µm (0.2 mil) to about 51 µm (2.0 mils), preferably from about 13 µm (0.5 mil) to about 25 µm (1.0 mil). Suitable primer are for example acrylic polyurethane, polyester polyurethane, acrylic melamine, polyester melamine, polyester epoxy, epoxy polyurethane, or alkyd melamine. Preferred is polyester-polyurethane. Next, the primer is moved through a curing oven where it is baked at a temperature sufficient to cure the primer, typically about 125°F (52°C) to about 1500°F (815°C). Then after the printed coil exits the curing oven, it is applied to the adhesive layer from step (c) by roller coating. The coated aluminum is preferably rewound as it exits the coating means. The coated aluminum has an excellent appearance, good adhesion between the aluminum and the coating and is storable or optionally usable without coiling for the next process step for the manufacture of veneers. The manufacture of the veneer is described in the EP-A-0 374 551. In the rewound stage, the coated aluminum is first unwound and then cut into pieces of sufficient size and shape to form a particular contoured veneer. If the coated aluminum has been not rewound, it is used as it exits step (e). The term "veneer" is defined to mean a contoured, coated aluminum article produced by the process of the present invention. The art such as die cutting, laser cutting, saw cutting, and high pressure water cutting apparatuses or by other methods and apparatuses common to the metal fabrication industry. After the coiled metal is cut into pieces, the pieces are inserted into die forming means and processed under sufficient pressure for a sufficient length of time and optionally at elevated temperature for enhanced flexibility of the coating system to produce a contoured veneer having an automotive grade exterior finish overcoated with a removable carrier film. Examples of die-forming means include drawing, ironing, flexible dye forming such as the Verson-Wheelon process, Marform process, stretch forming, stretch-draw forming and other techniques known in the art. The veneers can either be affixed to preformed, contoured automotive body parts or co-molded to a substrate, preferably plastic, to form coated aluminum veneer body parts. The following examples are illustrative of the principles and practice of this invention, although not limited thereto. Example PRODUCTION OF A FINISHED ARTICLE: STEPS (a) to (d)(i) : The carrier film composed of 76 µm (3 mil) thick polyethylene terephthalate coated with a 13 µm (0.5 mils) of vinyl acrylic release coating is loaded onto the feed roll of the film coater. The carrier is fed into the coater and affixed to the take-up roll at the end of the coater. The clear coat composed of an epsilon caprolactone modified hydroxy functional copolymer of Styrene/MMA/nBMA/2-HEA/Acrylic acid blended with 50:50 hexamethylene diisocyanate (Desmodur® N3390, Mobay): isophorone diisocyanate (Desmodur® Z4370, Mobay) is fed onto the carrier directly before a doctor blade which is adjusted to yield a clear film build of 76 µm (3.0 mils). The coated carrier then feeds into the oven where the coating is cured and then it is wound onto the take-up roll. The take-up roll is taken off and then remounted onto the feed roll and fed through the coater back to the take-up roll again. Then the base coat composed of an epsilon-caprolactone modified hydroxy functional copolymer of Styrene/2-EHA/2-HEA/nBMA/Acrylic acid with a molecular weight (Mn) of 7728, pigmented with a carbon black paste and blended with 50:50 hexamethylene diisocyanate (Desmodur® N3390, Mobay): isophorone diisocyanate (Desmodur® Z4370, Mobay) is fed onto the clear coat directly before a doctor blade which is adjusted to yield a base coat film build of 25 µm (1.0 mils). The coated clear then feeds into the oven where the coating is cured and then it is wound onto the take-up roll. The take-up roll is taken off and then remounted onto the feed roll and fed through the coater back to the take-up roll again. The adhesive composed of linear polyester polyurethane based on diphenyl methane diisocyanate (Desmocoll® 530, Mobay) blended with polyisocyanate based on toluene diisocyanate (Mondur®CB-75, Mobay) is fed onto the base coat directly before a doctor blade which is adjusted to yield an adhesive film build of 13 µm (0.5 mils). The coated base coat then feeds into the oven where the adhesive is cured and then when it exits a protective film composed of teflon coated paper is affixed to the surface of the adhesive with a pinch roller and then the whole construction is wound onto the take-up roll. Application of a primer to the aluminum film : The chromated aluminum coil (aluminum alloy 2008-T4, chromate Gardal® 714-A) is fed into a coil coater. The primer composed of a polyester-polyurethane is fed onto the surface directly before a doctor blade which is adjusted to yield a film thickness of 25 µm (1.0 mils). The coated aluminum is fed into an oven where the coating is cured and then the aluminum is rewound into a coil. STEPS (d) (ii) to (e) : The free multilayer coating film of step (d) (i) and the primered aluminum film are fed into a laminator where the protective sheet is removed prior to the adhesive layer meeting the aluminum at a pinch roller. The result is then wound onto a take-up roll to form a coated aluminum coil. Preparation of the veneer : The coated aluminum coil is unwound and cut into blanks suitable for the part being made. The blanks are fed into a press where the shape of the final article is stamped into the blank. Preparation of the automotive body parts : The veneer is fed into a mold together with a plastic automotive body where molding takes place as it is disclosed in EP-A-0 374 551. The carrier film is removed from the coated plastic automotive body part, which shows good appearance and good adhesion of the coating to the automotive body part. MMA = Methylmethacrylate n BMA = n-Butylmethacrylate 2-HEA = 2-Hydroxyethyacrylate 2-HEA = 2-Ethylhexylacrylate

Claims: A process for coil coating aluminum comprising the steps of: (a) coating a carrier film with a clearcoat layer; (b) applying a basecoat layer on said clearcoat layer; (c) applying an adhesive layer on said basecoat layer; (d) optionally (i) applying a protective film on said adhesive layer to form a free multilayer coating film; (ii) removing said protective film; and (e) applying an aluminum film on said adhesive layer. A process according to claim 1, wherein said aluminum film is coated with a primer on at least one side. The process according to claim 2, wherein an uncoated side of said aluminum film is applied on said adhesive layer. The process according to claim 1, wherein said carrier film is selected from the group consisting of polyethylene, polypropylene, polyester and acrylics. The process according to claim 4, wherein said carrier film is polyethylene terephthalate. The process according to claim 1, wherein said carrier film has a thickness of from 25 to 254 µm (1 to 10 mils). The process according to claim 1 , wherein said clearcoat is selected from the group consisting of acrylic polyurethane, polyester polyurethane, acrylic melamine and polyester melamine. The process according to claim 1, wherein said clearcoat is applied in a thickness of from 38 to 102 µm (1.5 to 4 mils). The process according to claim 1, wherein said basecoat is selected from the group consisting of acrylic polyurethane, polyester polyurethane, acrylic melamine, and polyester melamine. The process according to claim 1, wherein said basecoat is applied in a thickness of from 13 to 51 µm (0.5 to 2 mils). The process according to claim 1, wherein said adhesive is selected from the group consisting of acrylic polyurethane and polyester polyurethane. The process according to claim 1, wherein said adhesive is applied in a thickness of from 2,5 to 38 µm (0.1 to 1.5 mils). The process according to claim 1, wherein said aluminum film is a chromated aluminum film. The process according to claim 1, wherein said aluminum film has a thickness of from 2,5 to 127 µm (0.1 to 5 mils). The process according to claim 1, wherein said protective film is a teflon coated paper.

Inventors: GESSNER MICHAEL A, GESSNER, MICHAEL A.
Applicants: BASF CORP, BASF CORPORATION
Application year: 1992
Main classification: B05D7
IPC codes: B05D1, B05D7