MULTI-STAGE PROCESSES FOR COATING SUBSTRATES WITH LIQUID BASECOAT AND LIQUID TOPCOAT

Abstract: A process for coating a metal substrate includes: (a) applying a liquid basecoating composition to a surface of the substrate; (b) exposing the basecoating composition to air having a temperature ranging from about 10 DEG C to about 30 DEG C for a period of at least about 30 seconds to volatilize at least a portion of volatile material from the liquid basecoating composition, the velocity of the air at a surface of the basecoating composition being less than about 0.5 meters per second; (c) applying infrared radiation and warm air simultaneously to the basecoating composition for a period of at least about 30 seconds, the velocity of the air at the surface of the basecoating composition being less than about 4 meters per second, the temperature of the substrate being increased at a rate ranging from about 0.02 DEG C per second to about 0.4 DEG C per second to achieve a peak temperature of the substrate ranging from about 20 DEG C to about 60 DEG C; (d) applying infrared radiation and hot air simultaneously to the basecoating composition for a period of at least about 30 seconds, the temperature of the substrate being increased at a rate ranging from about 0.4 DEG C per second to about 1.5 DEG C per second to achieve a peak temperature of the substrate ranging from about 40 DEG C to about 75 DEG C, such that a dried basecoat is formed upon the surface of the substrate; and (e) applying a liquid top coating composition over the dried basecoat. A similar process for coating a polymeric substrate also is provided.

Description: MULTI-STAGE PROCESSES FOR COATING SUBSTRATES WITH LIQUID BASECOAT AND LIQUID TOPCOAT CROSS-REFERENCE TO RELATED APPLICATIONS This patent application is related to U.S. Patent Application Serial No. 09/320,264 entitled "Multi-Stage Processes for Coating Substrates with Liquid Basecoat and Powder Topcoat"; U.S. Patent Application Serial No. 09/320,483 entitled "Processes For Coating A Metal Substrate With an Electrodeposited Coating Composition and Drying the Same"; U.S. Patent Application Serial No. 09/320,484 entitled "Processes For Drying Primer Coating Compositions"; and U.S. Patent Application Serial No. 09/320,522 entitled "Processes For Drying Topcoats And Multicomponent Composite Coatings On Metal And Polymeric Substrates", all of Donaldson J. Emch and each filed concurrently with the present application. Field of the Invention The present invention relates to drying of liquid basecoats for automotive coating applications and, more particularly, to multi-stage processes for drying a liquid basecoat which include a combination of infrared radiation and convection drying for subsequent liquid topcoat application. Background of the Invention Today's automobile bodies are treated with multiple layers of coatings which not only enhance the appearance of the automobile, but also provide protection from corrosion, chipping, ultraviolet light, acid rain and other environmental conditions which can deteriorate the coating appearance and underlying car body. The formulations of these coatings can vary widely. However, a major challenge that faces all automotive manufacturers is how to rapidly dry and cure these coatings with minimal capital investment and floor space, which is valued at a premium in manufacturing plants. Various ideas have been proposed to speed up drying and curing processes for automobile coatings, such as hot air convection drying. While hot air drying is rapid, a skin can form on the surface of the coating which impedes the escape of volatiles from the coating composition and causes pops, bubbles or blisters which ruin the appearance of the dried coating. Other methods and apparatus for drying and curing a coating applied to an automobile body are disclosed in U.S. Patent Nos. 4,771 ,728; 4,907,533; 4,908,231 and 4,943,447, in which the automobile body is heated with radiant heat for a time sufficient to set the coating on Class A surfaces of the body and subsequently cured with heated air. U.S. Patent No. 4,416,068 discloses a method and apparatus for accelerating the drying and curing of refinish coatings for automobiles using infrared radiation. Ventilation air used to protect the infrared radiators from solvent vapors is discharged as a laminar flow over the car body. Fig. 15 is a graph of temperature as a function of time showing the preferred high temperature/short drying time curve 122 versus conventional infrared drying (curve 113) and convection drying (curve 114). Such rapid, high temperature drying techniques can be undesirable because a skin can form on the surface of the coating that can cause pops, bubbles or blisters, as discussed above. U.S. Patent No. 4,336,279 discloses a process and apparatus for drying automobile coatings using direct radiant energy, a majority of which has a wavelength greater than 5 microns. Heated air is circulated under turbulent conditions against the back sides of the walls of the heating chamber to provide the radiant heat. Then, the heated air is circulated as a generally laminar flow along the inner sides of the walls to maintain the temperature of the walls and remove volatiles from the drying chamber. As discussed at column 7, lines 18-22, air movement is maintained at a minimum in the central portion of the inner chamber in which the automobile body is dried. A rapid, multi-stage drying process for automobile coatings is needed which inhibits formation of surface defects and discoloration in the coating, particularly for use with liquid basecoats to be overcoated with liquid topcoat. Summary of the Invention The present invention provides a process for coating a metal substrate, comprising the steps of: (a) applying a liquid basecoating composition to a surface of the metal substrate; (b) exposing the basecoating composition to air having a temperature ranging from about 10°C to about 35°C for a period of at least about 30 seconds to volatilize at least a portion of volatile material from the liquid basecoating composition, the velocity of the air at a surface of the basecoating composition being less than about 0.5 meters per second; (c) applying infrared radiation and warm air simultaneously to the basecoating composition for a period of at least about 30 seconds, the velocity of the air at the surface of the basecoating composition being less than about 4 meters per second, the temperature of the metal substrate being increased at a rate ranging from about 0.02°C per second to about 0.4°C per second to achieve a peak metal temperature of the substrate ranging from about 20°C to about 60°C; (d) applying infrared radiation and hot air simultaneously to the basecoating composition for a period of at least about 30 seconds, the temperature of the metal substrate being increased at a rate ranging from about 0.4°C per second to about 1.5°C per second to achieve a peak metal temperature of the substrate ranging from about 40°C to about 75°C, such that a dried basecoat is formed upon the surface of the metal substrate; and (e) applying a liquid topcoating composition over the dried basecoat. Another aspect of the present invention is a process for coating a polymeric substrate, comprising the steps of: (a) applying a liquid basecoating composition to a surface of the polymeric substrate; (b) exposing the basecoating composition to air having a temperature ranging from about 10°C to about 35°C for a period of at least about 30 seconds to volatilize at least a portion of volatile material from the liquid basecoating composition, the velocity of the air at a surface of the basecoating composition being less than about 4 meters per second; (c) applying infrared radiation having a wavelength ranging from about 0.7 to about 4 micrometers and warm air simultaneously to the basecoating composition for a period of at least about 30 seconds, the velocity of the air at the surface of the basecoating composition being less than about 4 meters per second, the temperature of the polymeric substrate being increased at a rate ranging from about 0.02°C per second to about 0.2°C per second to achieve a peak polymeric temperature ranging from about 30°C to about 50°C; and (d) applying infrared radiation and hot air simultaneously to the basecoating composition for a period of at least about 30 seconds, the temperature of the polymeric substrate being increased at a rate ranging from about 0.4°C per second to about 1.5°C per second to achieve a peak polymeric temperature which is less than a heat distortion temperature of the polymeric substrate and ranges from about 40°C to about 150°C, such that a dried basecoat is formed upon the surface of the polymeric substrate; and (e) applying a liquid topcoating composition over the dried basecoat. Brief Description of the Drawings The foregoing summary, as well as the following detailed description of the preferred embodiments, will be better understood when read in conjunction with the appended drawings. In the drawings: Fig. 1 is a flow diagram of a process for drying liquid basecoat for liquid topcoating according to the present invention; Fig. 2 is a side elevational schematic diagram of a portion of the process of Fig. 1 ; and Fig. 3 is a front elevational view taken along line 3-3 of a portion of the schematic diagram of Fig. 2. Detailed Description of the Preferred Embodiments Referring to the drawings, in which like numerals indicate like elements throughout, there is shown in Fig. 1 a flow diagram of a multi-stage process for coating a substrate according to the present invention. This process is suitable for coating metal or polymeric substrates in a batch or continuous process. In a batch process, the substrate is stationary during each treatment step of the process, whereas in a continuous process the substrate is in continuous movement along an assembly line. The present invention will now be discussed generally in the context of coating a substrate in a continuous assembly line process, although the process also is useful for coating substrates in a batch process. Useful substrates that can be coated according to the process of the present invention include metal substrates, polymeric substrates, such as thermoset materials and thermoplastic materials, and combinations thereof. Useful metal substrates that can be coated according to the process of the present invention include ferrous metals such as iron, steel, and alloys thereof, non-ferrous metals such as aluminum, zinc, magnesium and alloys thereof, and combinations thereof. Preferably, the substrate is formed from cold rolled steel, electrogalvanized steel such as hot dip electrogalvanized steel or electrogalvanized iron-zinc steel, aluminum or magnesium. Useful thermoset materials include polyesters, epoxides, phenolics, polyurethanes such as reaction injected molding urethane (RIM) thermoset materials and mixtures thereof. Useful thermoplastic materials include thermoplastic polyolefins such as polyethylene and polypropylene, polyamides such as nylon, thermoplastic polyurethanes, thermoplastic polyesters, acrylic polymers, vinyl polymers, polycarbonates, acrylonitrile- butadiene-styrene (ABS) copolymers, EPDM rubber, copolymers and mixtures thereof. Preferably, the substrates are used as components to fabricate automotive vehicles, including but not limited to automobiles, trucks and tractors. The substrates can have any shape, but are preferably in the form of automotive body components such as bodies (frames), hoods, doors, fenders, bumpers and/or trim for automotive vehicles. The present invention first will be discussed generally in the context of coating a metallic automobile body. One skilled in the art would understand that the process of the present invention also is useful for coating non- automotive metal and/or polymeric components, which will be discussed below. Prior to treatment according to the process of the present invention, the metal substrate can be cleaned and degreased and a pretreatment coating, such as CHEMFOS 700 zinc phosphate or BONAZINC zinc-rich pretreatment (each commercially available from PPG Industries, Inc. of Pittsburgh, Pennsylvania), can be deposited upon the surface of the metal substrate. Alternatively or additionally, an electrodepositable coating composition can be electrodeposited upon at least a portion of the metal substrate. Useful electrodeposition methods and electrodepositable coating compositions include conventional anionic or cationic electrodepositable coating compositions, such as epoxy or polyurethane-based coatings discussed in U.S. Patent Nos. 4,933,056; 5,530,043; 5,760,107 and 5,820,987, which are incorporated herein by reference. Referring now to Fig. 1 , which presents a flow chart of the process of the present invention, a liquid basecoating composition is applied to a surface of the metal substrate (automobile body 16 shown in Fig. 2) in a first step 110, preferably over an electrodeposited coating as described above or primer. The liquid basecoating can be applied to the surface of the substrate in step 110 by any suitable coating process well known to those skilled in the art, for example by dip coating, direct roll coating, reverse roll coating, curtain coating, spray coating, brush coating and combinations thereof. The method and apparatus for applying the liquid basecoating composition to the substrate is determined in part by the configuration and type of substrate material. The liquid basecoating composition comprises a film-forming material or binder, volatile material and optionally pigment. Preferably, the basecoating composition is a crosslinkable coating composition comprising at least one thermosettable film-forming material, such as acrylics, polyesters (including alkyds), polyurethanes and epoxies, and at least one crosslinking material. Thermoplastic film-forming materials such as polyolefms also can be used. The amount of film-forming material in the liquid basecoat generally ranges from about 40 to about 97 weight percent on a basis of total weight solids of the basecoating composition. Suitable acrylic polymers include copolymers of one or more of acrylic acid, methacrylic acid and alkyl esters thereof, such as methyl methacrylate, ethyl methacrylate, hydroxyethyl methacrylate, butyl methacrylate, ethyl acrylate, hydroxyethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate, optionally together with one or more other polymerizable ethylenically unsaturated monomers including vinyl aromatic compounds such as styrene and vinyl toluene, nitriles such as acrylontrile and methacrylonitrile, vinyl and vinylidene halides, and vinyl esters such as vinyl acetate. Other suitable acrylics and methods for preparing the same are disclosed in U.S. Patent No. 5,196,485 at column 11 , lines 16-60, which are incorporated herein by reference. Polyesters and alkyds are other examples of resinous binders useful for preparing the basecoating composition. Such polymers can be prepared in a known manner by condensation of polyhydric alcohols, such as ethylene glycol, propylene glycol, butylene glycol, 1 ,6-hexylene glycol, neopentyl glycol, trimethylolpropane and pentaerythritol, with polycarboxylic acids such as adipic acid, maleic acid, fumaric acid, phthalic acids, trimellitic acid or drying oil fatty acids. Polyurethanes also can be used as the resinous binder of the basecoat. Useful polyurethanes include the reaction products of polymeric polyols such as polyester polyols or acrylic polyols with a polyisocyanate, including aromatic diisocyanates such as 4,4'-diphenylmethane diisocyanate, aliphatic diisocyanates such as 1 ,6-hexamethylene diisocyanate, and cycloaliphatic diisocyanates such as isophorone diisocyanate and 4,4'- methylene-bis(cyclohexyl isocyanate). Suitable crosslinking materials include aminoplasts, polyisocyanates, polyacids, polyanhydrides and mixtures thereof. Useful aminoplast resins are based on the addition products of formaldehyde, with an amino- or amido-group carrying substance. Condensation products obtained from the reaction of alcohols and formaldehyde with melamine, urea or benzoguanamine are most common. Useful polyisocyanate crosslinking materials include blocked or unblocked polyisocyanates such as those discussed above for preparing the polyurethane. Examples of suitable blocking agents for the polyisocyanates include lower aliphatic alcohols such as methanol, oximes such as methyl ethyl ketoxime and lactams such as caprolactam. The amount of the crosslinking material in the basecoat coating composition generally ranges from about 5 to about 50 weight percent on a basis of total resin solids weight of the basecoat coating composition. The liquid basecoating composition comprises one or more volatile materials such as water, organic solvents and/or amines. Nonlimiting examples of useful solvents included in the composition, in addition to any provided by other coating components, include aliphatic solvents such as hexane, naphtha, and mineral spirits; aromatic and/or alkylated aromatic solvents such as toluene, xylene, and SOLVESSO 100; alcohols such as ethyl, methyl, n-propyl, isopropyl, n-butyl, isobutyl and amyl alcohol, and m- pyrol; esters such as ethyl acetate, n-butyl acetate, isobutyl acetate and isobutyl isobutyrate; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, methyl n-amyl ketone, and isophorone, glycol ethers and glycol ether esters such as ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, ethylene glycol monohexyl ether, propylene glycol monomethyl ether, propylene glycol monopropyl ether, ethylene glycol monobutyl ether acetate, propylene glycol monomethyl ether acetate, and dipropylene glycol monomethyl ether acetate. Useful amines include alkanolamines. The solids content of the liquid basecoating composition generally ranges from about 15 to about 60 weight percent, and preferably about 20 to about 50 weight percent. The basecoating composition can further comprise one or more pigments or other additives such as UV absorbers, rheology control agents or surfactants. Useful metallic pigments include aluminum flake, bronze flakes, coated mica, nickel flakes, tin flakes, silver flakes, copper flakes and combinations thereof. Other suitable pigments include mica, iron oxides, lead oxides, carbon black, titanium dioxide and colored organic pigments such as phthalocyanines. The specific pigment to binder ratio can vary widely so long as it provides the requisite hiding at the desired film thickness and application solids. Suitable waterborne basecoats for color-plus-clear composites include those disclosed in U.S. Patent Nos. 4,403,003; 5,401 ,790 and 5,071 ,904, which are incorporated by reference herein. Also, waterborne polyurethanes such as those prepared in accordance with U.S. Patent No. 4,147,679 can be used as the resinous film former in the basecoat, which is incorporated by reference herein. Suitable film formers for organic solvent-based base coats are disclosed in U.S. Patent No. 4,220,679 at column 2, line 24 through column 4, line 40 and U.S. Patent No. 5,196,485 at column 11 , line 7 through column 13, line 22, which are incorporated by reference herein. The thickness of the basecoating composition applied to the substrate can vary based upon such factors as the type of substrate and intended use of the substrate, i.e., the environment in which the substrate is to be placed and the nature of the contacting materials. Generally, the thickness of the basecoating composition applied to the substrate ranges from about 10 to about 38 micrometers, and more preferably about 12 to about 30 micrometers. Referring now to Fig. 1 , after applying the basecoat, the process of the present invention includes a second step 12, 112 of exposing the basecoating composition to low velocity air having a temperature ranging from about 10°C to about 35°C, and preferably about 20°C to about 30°C, for a period of at least about 30 seconds to volatilize at least a portion of the volatile material from the liquid basecoating composition and set the basecoat. As used herein, the term "set" means that the basecoat is tack-free (resists adherence of dust and other airborne contaminants) and is not disturbed or marred (waved or rippled) by air currents which blow past the basecoated surface. The velocity of the air at a surface of the basecoating composition is less than about 0.5 meters per second and preferably ranges from about 0.3 to about 4 meters per second. The volatilization or evaporation of volatiles from the basecoat 14 can be carried out in the open air, but is preferably carried out in a first drying chamber 18 in which air is circulated at low velocity to minimize airborne particle contamination as shown in Fig. 2. The automobile body 16 is positioned at the entrance to the first drying chamber 18 and slowly moved therethrough in assembly-line manner at a rate which permits the volatilization of the basecoat as discussed above. The rate at which the automobile body 16 is moved through the first drying chamber 18 and the other drying chambers discussed below depends in part upon the length and configuration of the drying chamber 18, but preferably ranges from about 3 meters per minute to about 7.3 meters per minute for a continuous process. One skilled in the art would understand that individual dryers can be used for each step of the process or that a single dryer having a plurality of individual drying chambers or sections (shown in Fig. 2) configured to correspond to each step of the process can be used, as desired. The air preferably is supplied to the first drying chamber 18 by a blower 20 or dryer, shown in phantom in Fig. 2. A non-limiting example of a suitable blower is an ALTIVAR 66 blower that is commercially available from Square D Corporation. The air can be circulated at ambient temperature or heated, if necessary, to the desired temperature range of about 10°C to about 50°C. Preferably, the basecoating composition is exposed to air for a period ranging from about 30 seconds to about 3 minutes before the automobile body 16 is moved to the next stage of the drying process. Referring now to Figs. 1 and 2, the process comprises a next step 114 of applying infrared radiation and low velocity warm air simultaneously to the basecoating composition for a period of at least about 30 seconds (preferably about 30 seconds to about 3 minutes) such that the temperature of the metal substrate is increased at a rate ranging from about 0.02°C per second to about 0.4°C per second to achieve a peak metal temperature ranging from about 20°C to about 60°C and form a pre-dried basecoat upon the surface of the metal substrate. By controlling the rate at which the metal temperature is increased and peak metal temperature, flaws in the appearance of the basecoat and topcoat, such as pops and bubbles, can be minimized. The infrared radiation applied preferably includes near-infrared region (0.7 to 1.5 micrometers) and intermediate-infrared region (1.5 to 20 micrometers) radiation, and more preferably ranges from about 0.7 to about 4 micrometers. The infrared radiation heats the Class A (external) surfaces 24 of the coated substrate which are exposed to the radiation and preferably does not induce chemical reaction or crosslinking of the components of the basecoating. Most non-Class A surfaces are not exposed directly to the infrared radiation but will be heated through conduction through the automobile body and random scattering of the infrared radiation. Referring now to Figs. 2 and 3, the infrared radiation is emitted by a plurality of emitters 26 arranged in the interior drying chamber 27 of a combination infrared/convection drying apparatus 28. Each emitter 26 is preferably a high intensity infrared lamp, preferably a quartz envelope lamp having a tungsten filament. Useful short wavelength (0.76 to 2 micrometers), high intensity lamps include Model No. T-3 lamps such as are commercially available from General Electric Co., Sylvania, Phillips, Heraeus and Ushio and have an emission rate of between 75 and 100 watts per lineal inch at the light source. Medium wavelength (2 to 4 micrometers) lamps also can be used and are available from the same suppliers. The emitter lamp is preferably generally rod-shaped and has a length that can be varied to suit the configuration of the oven, but generally is preferably about 0.75 to about 1.5 meters long. Preferably, the emitter lamps on the side walls 30 of the interior drying chamber 27 are arranged generally vertically with reference to ground 32, except for a few rows 34 (preferably about 3 to about 5 rows) of emitters 26 at the bottom of the interior drying chamber 27 which are arranged generally horizontally to ground 32. The number of emitters 26 can vary depending upon the desired intensity of energy to be emitted. In a preferred embodiment, the number of emitters 26 mounted to the ceiling 36 of the interior drying chamber 27 is about 24 to about 32 arranged in a linear side-by side array with the emitters 26 spaced about 10 to about 20 centimeters apart from center to center, and preferably about 15 centimeters. The width of the interior drying chamber 27 is sufficient to accommodate the automobile body or whatever substrate component is to be dried therein, and preferably is about 2.5 to about 3.0 meters wide. Preferably, each side wall 30 of the chamber 27 has about 50 to about 60 lamps with the lamps spaced about 15 to about 20 centimeters apart from center to center. The length of each side wall 30 is sufficient to encompass the length of the automobile body or whatever substrate component is being dried therein, and preferably is about 4 to about 6 meters. The side wall 30 preferably has four horizontal sections that are angled to conform to the shape of the sides of the automobile body. The top section of the side wall 30 preferably has 24 parallel lamps divided into 6 zones. The three zones nearest the entrance to the drying chamber 27 are operated at medium wavelengths, the three nearest the exit at short wavelengths. The middle section of the side wall is configured similarly to the top section. The two lower sections of the side walls each preferably contain 6 bulbs in a 2 by 3 array. The first section of bulbs nearest the entrance is preferably operated at medium wavelength and the other two sections at short wavelengths. Referring to Fig. 2, each of the emitter lamps 26 is disposed within a trough-shaped reflector 38 that is preferably formed from polished aluminum. Suitable reflectors include aluminum or integral gold-sheathed reflectors that are commercially available from BGK-ITW Automotive, Heraeus and Fannon Products. The reflectors 38 gather energy transmitted from the emitter lamps 26 and focus the energy on the automobile body 16 to lessen energy scattering. Depending upon such factors as the configuration and positioning of the automobile body 16 within the interior drying chamber 27 and the color of the basecoat to be dried, the emitter lamps 26 can be independently controlled by microprocessor (not shown) such that the emitter lamps 26 furthest from a Class A surface 24 can be illuminated at a greater intensity than lamps closest to a Class A surface 24 to provide uniform heating. For example, as the roof 40 of the automobile body 16 passes beneath a section of emitter lamps 26, the emitter lamps 26 in that zone can be adjusted to a lower intensity until the roof 40 has passed, then the intensity can be increased to heat the deck lid 42 which is at a greater distance from the emitter lamps 26 than the roof 40. Also, in order to minimize the distance from the emitter lamps 26 to the Class A surfaces 24, the position of the side walls 30 and emitter lamps 26 can be adjusted toward or away from the automobile body as indicated by directional arrows 44, 46, respectively, in Fig. 3. One skilled in the art would understand that the closer the emitter lamps 26 are to the Class A surfaces 24 of the automobile body 16, the greater the percentage of available energy which is applied to heat the surfaces 24 and coatings present thereon. Generally, the infrared radiation is emitted at a power density ranging from about 10 to about 25 kilowatts per square meter (kW/m 2 ) of emitter wall surface, and preferably about 12 kW/m 2 for emitter lamps 26 facing the sides 48 of the automobile body 16 (doors or fenders) which are closer than the emitter lamps 26 facing the hood and deck lid 42 of the automobile body 16, which preferably emit about 24 kW/m 2 . A non-limiting example of a suitable combination infrared/convection drying apparatus is a BGK combined infrared radiation and heated air convection oven, which is commercially available from BGK Automotive Group of Minneapolis, Minnesota. The general configuration of this oven will be described below and is disclosed in U.S. Patent Nos. 4,771 ,728; 4,907,533; 4,908,231 ; and 4,943,447, which are hereby incorporated by reference. Other useful combination infrared/convection drying apparatus are commercially available from Durr of Wixom, Michigan, Thermal Innovations of Manasquan, New Jersey, Thermovation Engineering of Cleveland, Ohio, Dry- Quick of Greenburg, Indiana and Wisconsin Oven and Infrared Systems of East Troy, Wisconsin. Referring now to Figs. 2 and 3, the preferred combination infrared/convection drying apparatus 28 includes baffled side walls 30 having nozzles or slot openings 50 through which air 52 is passed to enter the interior drying chamber 27 at a velocity of less than about 4 meters per second. During this step, the velocity of the air at the surface 54 of the basecoating composition is less than about 4 meters per second, preferably ranges from about 0.3 to about 4 meters per second and, more preferably, about 0.7 to about 1.5 meters per second. The temperature of the air 52 generally ranges from about 25°C to about 50°C, and preferably about 30°C to about 40°C. The air 52 is supplied by a blower 56 or dryer and can be preheated externally or by passing the air over the heated infrared emitter lamps 26 and their reflectors 38. By passing the air 52 over the emitters 26 and reflectors 38, the working temperature of these parts can be decreased, thereby extending their useful life. Also, undesirable solvent vapors can be removed from the interior drying chamber 27. The air 52 can also be circulated up through the interior drying chamber 27 via the subfloor 58. Preferably, the air flow is recirculated to increase efficiency. A portion of the air flow can be bled off to remove contaminants and supplemented with filtered fresh air to make up for any losses. The automobile body 16 is heated by the infrared radiation and warm air to a peak metal temperature ranging from about 20°C to about 60°C, and preferably about 25°C to about 30°C. As used herein, "peak metal temperature" means the target instantaneous temperature to which the metal substrate (automobile body 16) must be heated. The peak metal temperature for a metal substrate is measured at the surface of the coated substrate approximately in the middle of the side of the substrate opposite the side on which the coating is applied. The peak temperature for a polymeric substrate is measured at the surface of the coated substrate approximately in the middle of the side of the substrate on which the coating is applied. It is preferred that this peak metal temperature be maintained for as short a time as possible. Referring now to Figs. 1 and 2, the process of the present invention comprises a next step 116 of applying infrared radiation and hot air simultaneously to the basecoating composition on the metal substrate (automobile body 16) for a period of at least about 30 seconds, and preferably about 30 seconds to about 3 minutes. The temperature of the metal substrate is increased at a rate ranging from about 0.4°C per second to about 1.5°C per second to achieve a peak metal temperature of the substrate ranging from about 40°C to about 75°C. A dried basecoat 62 is formed thereby upon the surface of the metal substrate. By controlling the rate at which the metal temperature is increased and peak metal temperature, the combination of steps 112, 114 and 116 can provide liquid basecoat and liquid topcoat composite coatings with a minimum of flaws in surface appearance, such as pops and bubbles. Also, high film builds can be achieved in a short period of time with minimum energy input and the flexible operating conditions can decrease the need for repairs. The dried basecoat that is formed upon the surface of the automobile body 16 is dried sufficiently to enable application of the topcoat such that the quality of the topcoat will not be affected adversely by further drying of the basecoat. Preferably, the dried basecoat, after application to the surface of the substrate, forms a film which is substantially uncrosslinked, i.e., is not heated to a temperature sufficient to induce significant crosslinking and there is substantially no chemical reaction between the thermosettable film-forming material and the crosslinking material. For waterborne basecoats, "dry" means the almost complete absence of water from the basecoat. If too much water is present, the topcoat can crack, bubble or "pop" during drying of the topcoat as water vapor from the basecoat attempts to pass through the topcoat. This drying step 116 can be carried out in a similar manner to that of step 114 above using a combination infrared radiation/convection drying apparatus, however the rate at which the temperature of the metal substrate is increased ranges from about 0.4°C per second to about 1.5°C per second and peak metal temperature of the substrate ranges from about 40°C to about 75°C. Preferably, the heating rate ranges from about 0.7°C per second to about 1.3°C per second and the peak metal temperature of the substrate ranges from about 40°C to about 60°C. The infrared radiation applied preferably includes near-infrared region (0.7 to 1.5 micrometers) and intermediate-infrared region (1.5 to 20 micrometers) radiation, and more preferably ranges from about 0.7 to about 4 micrometers. The hot drying air preferably has a temperature ranging from about 50°C to about 110°C, and more preferably about 60°C to about 90°C. The velocity of the air at the surface of the basecoating composition in drying step 116 is less than about 4 meters per second, and preferably ranges from about 1 to about 4 meters per second. Drying step 116 can be carried out using any conventional combination infrared/convection drying apparatus such as the BGK combined infrared radiation and heated air convection oven which is described in detail above. The individual emitters 26 can be configured as discussed above and controlled individually or in groups by a microprocessor (not shown) to provide the desired heating and infrared energy transmission rates. The process of the present invention can further comprise an additional curing step 118 in which hot air 66 is applied to the dried basecoat 62 for a period of at least about 6 minutes after step 116 to hold the coated substrate at a peak metal temperature ranging from about 110°C to about 135°C and cure the basecoat. Preferably, a combination of hot air convection drying and infrared radiation is used simultaneously to cure the dried basecoat. As used herein, "cure" means that any crosslinkable components of the dried basecoat are substantially crosslinked. This curing step 118 can be carried out using a hot air convection dryer, such as are discussed above or in a similar manner to that of step 114 above using a combination infrared radiation/convection drying apparatus, however the target peak metal temperature of the substrate ranges from about 110°C to about 135°C and the substrate is maintained at the peak metal temperature for at least about 6 minutes, and preferably about 6 to about 20 minutes. The hot drying air preferably has a temperature ranging from about 110°C to about 140°C, and more preferably about 120°C to about 135°C. The velocity of the air at the surface of the basecoating composition in curing step 118 can range from about 4 to about 20 meters per second, and preferably ranges from about 10 to about 20 meters per second. If a combination of hot air and infrared radiation is used, the infrared radiation applied preferably includes near-infrared region (0.7 to 1.5 micrometers) and intermediate-infrared region (1.5 to 20 micrometers), and more preferably ranges from about 0.7 to about 4 micrometers. Curing step 118 can be carried out using any conventional combination infrared/convection drying apparatus such as the BGK combined infrared radiation and heated air convection oven which is described in detail above. The individual emitters 26 can be configured as discussed above and controlled individually or in groups by a microprocessor (not shown) to provide the desired heating and infrared energy transmission rates. The process of the present invention can further comprise a cooling step in which the temperature of the automobile body 16 having a cured basecoat thereon from steps 116 and/or 118 is cooled, preferably to a temperature ranging from about 20°C to about 60°C and, more preferably, about 25°C to about 30°C. Cooling the basecoated automobile body 16 can facilitate application of the liquid topcoat. The basecoated automobile body 16 can be cooled in air at a temperature ranging from about 15°C to about 35°C, and preferably about 25°C to about 30°C for a period ranging from about 3 to about 6 minutes. Alternatively or additionally, the basecoated automobile body 16 can be cooled by exposure to chilled, saturated air blown onto the surface of the substrate at about 4 to about 10 meters per second to prevent cracking of the coating. After the basecoating on the automobile body 16 has been dried (and cured and/or cooled, if desired), a liquid topcoating composition is applied over the dried basecoat in a liquid topcoating step 120. The topcoat can be applied by conventional electrostatic spray equipment such as a high speed (about 30,000 to about 60,000 revolutions per minute) rotary bell atomizer at a high voltage (about 60,000 to about 90,000 volts) to a thickness of about 40 to about 65 micrometers in one or two passes. Preferably, the topcoating composition is a crosslinkable coating comprising at least one thermosettable film-forming material and at least one crosslinking material, although thermoplastic film-forming materials such as polyolefms can be used. Suitable waterborne topcoats are disclosed in U.S. Patent No. 5,098,947 (incorporated by reference herein) and are based on water soluble acrylic resins. Useful solvent borne topcoats are disclosed in U.S. Patent Nos. 5,196,485 and 5,814,410 (incorporated by reference herein) and include polyepoxides and polyacid curing agents. The topcoating composition can include crosslinking materials and additional ingredients such as are discussed above but preferably not pigments. The amount of the topcoating composition applied to the substrate can vary based upon such factors as the type of substrate and intended use of the substrate, i.e., the environment in which the substrate is to be placed and the nature of the contacting materials. In a preferred embodiment, the process of the present invention further comprises a curing step 122 (shown in Fig. 1) of curing the liquid topcoating composition after application over the dried basecoat. The thickness of the dried and crosslinked composite coating is generally about 0.2 to 5 mils (5 to 125 micrometers), and preferably about 0.4 to 3 mils (10 to 75 micrometers). The liquid topcoating can be cured by hot air convection drying and, if desired, infrared heating, such that any crosslinkable components of the liquid topcoating are crosslinked to such a degree that the automobile industry accepts the coating process as sufficiently complete to transport the coated automobile body without damage to the topcoat. The liquid topcoating can be cured using any conventional hot air convection dryer or combination convection/infrared dryer such as are discussed above. Generally, the liquid topcoating is heated to a temperature of about 120°C to about 150°C for a period of about 20 to about 40 minutes to cure the liquid topcoat. Alternatively, if the basecoat was not cured prior to applying the liquid topcoat, both the basecoat and the liquid topcoating composition can be cured together by applying hot air convection and/or infrared heating using apparatus such as are described in detail above to individually cure both the basecoat and the liquid coating composition. To cure the basecoat and the liquid coating composition, the substrate is generally heated to a temperature of about 120°C to about 150°C for a period of about 20 to about 40 minutes to cure the liquid topcoat. Another aspect of the present invention is a process for coating a polymeric substrate. The process includes steps similar to those used for coating a metal substrate above. A liquid basecoating composition is applied to a surface of the polymeric substrate as described above. The basecoating composition is exposed to air having a temperature ranging from about 10°C to about 35°C for a period of at least about 30 seconds to volatilize at least a portion of volatile material from the liquid basecoating composition. The velocity of the air at a surface of the basecoating composition is less than about 4 meters per second, and preferably ranges from about 0.3 to about 0.5 meters per second. The apparatus used to volatilize the basecoat can be the same as that used to volatilize the basecoat for the metal substrate. Infrared radiation, having a wavelength ranging from about 0.7 to about 4 micrometers, and warm air are applied simultaneously to the basecoating composition for a period of at least about 30 seconds and preferably about 0.5 to about 2 minutes. The velocity of the air at the surface of the basecoating composition is less than about 4 meters per second, and preferably ranges from about 0.7 to about 1.5 meters per second. The temperature of the polymeric substrate is increased at a rate ranging from about 0.02°C per second to about 0.2°C per second to achieve a peak polymeric substrate temperature ranging from about 20°C to about 60°C, and preferably about 30°C to about 50°C. The apparatus used to dry the basecoat can be the same combined infrared/hot air convection apparatus such as is discussed above for treating the metal substrate. Next, infrared radiation and hot air are applied simultaneously to the basecoating composition for a period of at least about 30 seconds and preferably about 0.5 to about 2 minutes. The velocity of the air at the surface of the basecoating composition is less than about 4 meters per second, and preferably ranges from about 1.5 to about 2.5 meters per second. The temperature of the polymeric substrate is increased at a rate ranging from about 0.4°C per second to about 1.5°C per second to achieve a peak polymeric substrate temperature which is less than the heat distortion temperature of the polymeric substrate and ranges from about 40°C to about 150°C, such that a dried basecoat is formed upon the surface of the polymeric substrate. The heat distortion temperature is the temperature at which the polymeric substrate physically deforms and is incapable of resuming its prior shape. For example, the heat distortion temperatures for several common thermoplastic materials are as follows: thermoplastic olefins about 138°C (280°F), thermoplastic polyurethanes about 149°C (300°F), and acrylonitrile-butadiene-styrene copolymers about 71-82°C (160-180°F). The apparatus used to dry the basecoat can be the same combined infrared/hot air convection apparatus such as is discussed above for treating the metal substrate. The basecoat can be cured, if desired, before the liquid topcoating is applied. The basecoated polymeric substrate is preferably cooled to a temperature of about 20 to about 30°C before the liquid topcoating composition is applied over the dried basecoat. Suitable liquid topcoating compositions and methods of applying the same are discussed in detail above for coating the metal substrate. The present invention will be described further by reference to the following example. The following example is merely illustrative of the invention and is not intended to be limiting. Unless otherwise indicated, all parts are by weight. EXAMPLE In this example, steel test panels were coated with a liquid basecoat and liquid clearcoat as specified below to evaluate drying processes according to the present invention. The test substrates were ACT cold rolled steel panels size 30.48 cm by 45.72 cm (12 inch by 18 inch) electrocoated with a cationically electrodepositable primer commercially available from PPG Industries, Inc. as ED-5000. Commercial waterborne basecoat (HWB90394 which is commercially available from PPG Industries, Inc.) was spray applied (1 coat automated spray (bells) with 30 seconds ambient flash) at 60% relative humidity and 24°C to give a dry film thickness as specified in Tables 1A and 1C below. The basecoat coatings on the panels were dried as specified in Tables 1A-1 C using a combined infrared radiation and heated air convection oven commercially available from BGK-ITW Automotive Group of Minneapolis, Minnesota. The panels were then topcoated with liquid DIAMONDCOAT® DCT-5002 topcoat (commercially available from PPG Industries, Inc.) and cured for 30 minutes at 141 °C (285°F) using hot air convection to give an overall film thickness of about 110 to 130 micrometers. The appearance and physical properties of the coated panels were measured using the following tests: foil solids and appearance (number of pops, orange peel rating and overall rating. The weight percent of foil solids for each sample was determined by measuring the non-volatile coating deposited on a 75 mm by 100 mm foil sheet attached to the sprayed panel. The foil was removed from the panel after the drying process and weighed, then baked until nonvolatiles only are present according to ASTM Method 2369-D at a temperature of 110°C. The number of pops on the surface of the coating of each sample was determined by visual inspection of the entire panel surface. Popping was rated on a scale of 0 to 5, with 0 indicating no popping and 5 indicating severe popping. The orange peel rating, specular gloss and Distinction of Image ("DOI") were determined by scanning a 9375 square mm sample of panel surface using an Autospect QMS BP surface quality analyzer device that is commercially available from Perceptron. The Overall Appearance rating was determined by adding 40% of the Orange Peel rating, 20% of the Gloss rating and 40% of the DOI rating. The following Table 2 provides the measured properties. As shown in Table 2, the coated substrates dried according to the process of the present invention (Run Nos. 1-8) generally exhibited less popping and had comparable appearance to the Control panels in which the coatings were not dried according to the present invention. Table 2 The processes of the present invention provide rapid coating of metal and polymeric substrates, can eliminate or reduce the need for long assembly line ovens can drastically reduce overall processing time. Less popping and good flow and appearance of the basecoat, even at higher thicknesses, provides more operating latitude when applying the basecoat which can lower repairs. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications that are within the spirit and scope of the invention, as defined by the appended claims.

Claims: THEREFORE, I CLAIM: 1. A process for coating a metal substrate, comprising the steps of: (a) applying a liquid basecoating composition to a surface of the metal substrate; (b) exposing the basecoating composition to air having a temperature ranging from about 10°C to about 35°C for a period of at least about 30 seconds to volatilize at least a portion of volatile material from the liquid basecoating composition, the velocity of the air at a surface of the basecoating composition being less than about 0.5 meters per second; (c) applying infrared radiation and warm air simultaneously to the basecoating composition for a period of at least about 30 seconds, the velocity of the air at the surface of the basecoating composition being less than about 4 meters per second, the temperature of the metal substrate being increased at a rate ranging from about 0.02°C per second to about 0.4°C per second to achieve a peak metal temperature of the substrate ranging from about 20°C to about 60°C; (d) applying infrared radiation and hot air simultaneously to the basecoating composition for a period of at least about 30 seconds, the temperature of the metal substrate being increased at a rate ranging from about 0.4°C per second to about 1.5°C per second to achieve a peak metal temperature of the substrate ranging from about 40°C to about 75°C, such that a dried basecoat is formed upon the surface of the metal substrate; and (e) applying a liquid topcoating composition over the dried basecoat. 2. The process according to claim 1 , wherein the metal substrate is selected from the group consisting of iron, steel, aluminum, zinc, magnesium, alloys and combinations thereof. 3. The process according to claim 1 , wherein the metal substrate is an automotive body component. 4. The process according to claim 1 , wherein the volatile material of the liquid basecoating composition comprises water. 5. The process according to claim 1 , wherein the volatile material of the liquid basecoating composition is selected from the group consisting of organic solvents and amines. 6. The process according to claim 1 , wherein the air has a temperature ranging from about 10°C to about 35°C in step (b). 7. The process according to claim 1 , wherein the period ranges from about 30 seconds to about 3 minutes in step (b). 8. The process according to claim 1 , wherein the air velocity ranges from about 0.3 to about 0.5 meters per second in step (b). 9. The process according to claim 1 , wherein the infrared radiation is emitted at a wavelength ranging from about 0.7 to about 20 micrometers. 10. The process according to claim 9, wherein the wavelength ranges from about 0.7 to about 4 micrometers. 11. The process according to claim 1 , wherein the infrared radiation is emitted at a power density ranging from about 10 to about 40 kilowatts per square meter of emitter wall surface. 12. The process according to claim 1 , wherein the air has a temperature ranging from about 25°C to about 50°C in step (c). 13. The process according to claim 1 , wherein the period ranges from about 30 seconds to about 3 minutes in step (c). 14. The process according to claim 1 , wherein the air velocity ranges from about 0.3 to about 4 meters per second in step (c). 15. The process according to claim 1 , wherein the temperature of the metal substrate is increased at a rate ranging from about 0.02°C per second to about 0.2°C per second in step (c). 16. The process according to claim 1 , wherein the peak metal temperature of the metal substrate ranges from about 25°C to about 30°C in step (c). 17. The process according to claim 1 , wherein the air has a temperature ranging from about 50°C to about 110°C in step (d). 18. The process according to claim 1 , wherein the period ranges from about 30 seconds to about 3 minutes in step (d). 19. The process according to claim 1 , wherein the temperature of the metal substrate is increased at a rate ranging from about 0.6°C per second to about 1.3°C per second in step (d). 20. The process according to claim 1 , wherein the peak metal temperature of the metal substrate ranges from about 40°C to about 60°C in step (d). 21. The process according to claim 1 , further comprising an additional step (d') of applying hot air to the dried basecoat for a period of at least about 6 minutes after step (d) to hold at a peak metal temperature ranging from about 110°C to about 135°C, such that a cured basecoat is formed upon the surface of the metal substrate. 22. The process according to claim 21 , wherein additional step (d') further comprises applying infrared radiation to the dried basecoat simultaneously while applying the hot air. 23. The process according to claim 1 , further comprising an additional step (f) of curing the liquid topcoating composition after application over the dried basecoat. 24. The process according to claim 1 , wherein the additional step (f) further comprises curing the basecoating composition and the liquid coating composition after application of the liquid topcoating composition over the dried basecoat. 25. A process for coating a polymeric substrate, comprising the steps of: (a) applying a liquid basecoating composition to a surface of the polymeric substrate; (b) exposing the basecoating composition to air having a temperature ranging from about 10°C to about 35°C for a period of at least about 30 seconds to volatilize at least a portion of volatile material from the liquid basecoating composition, the velocity of the air at a surface of the basecoating composition being less than about 4 meters per second; (c) applying infrared radiation having a wavelength ranging from about 0.7 to about 4 micrometers and warm air simultaneously to the basecoating composition for a period of at least about 30 seconds, the velocity of the air at the surface of the basecoating composition being less than about 4 meters per second, the temperature of the polymeric substrate being increased at a rate ranging from about 0.02°C per second to about 0.2°C per second to achieve a peak polymeric temperature ranging from about 30°C to about 50°C; and (d) applying infrared radiation and hot air simultaneously to the basecoating composition for a period of at least about 30 seconds, the temperature of the polymeric substrate being increased at a rate ranging from about 0.4°C per second to about 1.5°C per second to achieve a peak polymeric temperature which is less than a heat distortion temperature of the polymeric substrate and ranges from about 40°C to about 150°C, such that a dried basecoat is formed upon the surface of the polymeric substrate; and (e) applying a liquid topcoating composition over the dried basecoat.

Inventors: EMCH DONALDSON J, EMCH, DONALDSON, J.
Applicants: PPG IND OHIO INC, PPG INDUSTRIES OHIO, INC.
Application year: 2000
Main classification: B05D
IPC codes: B05D3, B05D7, F26B3