Metal working – Method of mechanical manufacture – Assembling or joining
Reexamination Certificate
2001-12-19
2004-04-06
Vo, Peter (Department: 3726)
Metal working
Method of mechanical manufacture
Assembling or joining
C029S527200, C427S405000, C427S189000, C427S202000, C427S388100, C427S379000
Reexamination Certificate
active
06715196
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an improved method of producing visually attractive weldable parts, in particular automotive parts, with attractive metallic-like appearance, without the need of expensive electrodeposition baths, using weldable prepainted metal substrate to which are applied essentially zero VOC powder coatings. The powder coatings comprise flake pigments, which give the metallic-like appearance. The invention also relates to parts prepared by this method.
BACKGROUND OF THE INVENTION
Light gauge continuous sheet metal is produced by rolling mill lines in various thickness and widths. In the case of steel sheet metal, it may be coated at the mill with a thin layer of zinc or zinc alloy in order to provide steel sheet with improved corrosion resistance. After production of the sheet, mill oil is applied in the case of steel sheet and the sheet metal is wound into a coil for shipment to a customer for further processing. Such sheets are used by customers for a number of industrial and automotive applications. At the customer, the metal sheet is unwound and cleaned to remove any mill oil and dirt and to reduce the amount of metal oxide on the surface of the metal, after which the metal is coated with one or more layers of coating. The coatings usually include at least one primer to provide improved corrosion protection as well as adhesion of subsequent coating layers to the substrate. One common and very effective method of applying primer to metal substrates is the electrodeposition method in which a primer with an ionic, often cationic, species on the polymer backbone, is attracted to and deposits itself on a metal part which has the opposite charge, after which the coated parts are baked to cure the primer. Following the application of the primer, other layers of coating such as primer-surfacer can be applied for improved adhesion and smoothness. The final layers of coating to be applied are what is generally seen by the end user of the part, and these coatings, in addition to providing protection, such as hardness, weathering protection, and the like to the part, provide a visually attractive finish.
In the production of parts for automobile and other vehicle bodies, sheet metal from the mill, usually galvanized steel, is generally formed into the desired shape. The forming oil is then cleaned from the sheet. Following the cleaning step, the metal is pretreated with a phosphate pretreatment. The phosphated metal parts are then assembled into an automobile body with various forms of attachment such as clenching, gluing, and particularly spot welding.
The vehicle body is then primed with a cationic electrodeposition primer. The application of the electrodeposition primer (ED primer) at the automotive manufacturer requires large immersion baths. Such baths require large capital investment and continuous monitoring during production and occupy large areas of plant space. Moreover, the ED primer often does not form a film of sufficient thickness to be effective in confined or partially enclosed areas. Such areas may be seen where one piece of metal is bent over and clenched to another piece of metal to connect the two pieces of metal. In such a configuration, the ED primer often fails to deposit adequately in the region of the bend, leaving an area of metal relatively unprotected against corrosion. Another area in which an adequate layer of ED primer may not form is the interior of enclosed parts such as doors.
The process of applying a weldable anticorrosive primer to the metal sheet after cleaning and prior to forming of the metal sheet into an automotive part ensures the presence of an adequate thickness of anticorrosive primer in enclosed or confined areas of vehicle assemblies. Furthermore, application of the primer to the continuous sheet of metal can be done by roll coating in which the primer is applied by a roll moving in the same direction, or more commonly the opposite direction, as the moving sheet of metal. After the weldable primer is applied and dried and/or cured, the continuous sheet of primed metal can be wound into a coil. Roll coat application of primer to a continuous strip of metal has the advantage that it is nearly 100% efficient, that is, virtually all of the liquid primer is applied to the metal strip and cured, and the volatiles emitted during the baking process are commonly collected and burned as fuel for the oven, leading to low atmospheric emissions. The roll coat application and cure of the weldable primer can be done at a location separate from the vehicle manufacturing plant. Typically it is done at a company specializing in coil coating application, but it may even be done at the steel mill itself. Removal of the priming step from the vehicle manufacturing plant can eliminate the need for the large expensive ED immersion tanks and can lead to more efficient use of space and resources in the vehicle plant.
Although use of such coating processes are well known to those practicing the coil-coating art, conventional coil coating primers generally can not be used because the steel sheet, after being cut and formed into parts in a stamping press, is usually assembled into assemblies and vehicle bodies by spot welding. Conventional coil coating primers do not allow sufficient electric current to pass during the spot welding process to cause a weld to form in the metal. If conventional coil coatings are applied at very low dry film thickness enough current may pass to form a weld, but at such low thickness corrosion protection is inadequate. The weldable primer of the current invention avoids such limitations by inclusion of electrically conductive pigments as well as anticorrosive pigments to give a weldable formable primer with good corrosion protection. Because the primer is electrically conductive, additional corrosion protection can be realized, if needed, by coating the parts formed from the prepainted metal with ED primer after they are assembled.
SUMMARY OF THE INVENTION
In the current invention, after assembly of the parts formed from the metal sheet coated with weldable primer, the parts may optionally be given an additional phosphate pretreatment. The parts are then coated with a colored powder basecoat and optionally a powder clearcoat. Powder basecoats and clearcoats are desirable because they provide: superior appearance and chip resistance vs. liquid primers; essentially zero VOC vs. liquid primers; and 98 to 99% utilization in most facilities vs. 70 to 80% maximum for liquids.
The colored powder basecoat comprises metallic or non-metallic flake pigments. The pigments themselves may be colored, or uncolored. The parts are baked for a period of time sufficient to melt and coalesce the powder coating and to allow the flakes to align with the surface. The use of the flake pigments, especially colored flake pigments, in colored basecoats allows a wide range of striking visual effects.
The powder basecoat may be used without further coatings, but improved hardness, weathering and UV resistance, and visual appeal will be realized with application of a powder clearcoat. These powder clearcoats provide similar VOC and utilization advantages as those gained with powder basecoats with appearance and durability comparable to liquid clear coats. U.S. Pat. No. 5,407,707 describes the preparation of powder clear coats with excellent physical and chemical properties prepared from epoxy functional copolymers and polycarboxylic acid curing agents.
The advantages of the invention are the ability to produce panels and parts, particularly for automotive applications, with striking visual effects, good hardness, and weather and UV resistance by a method that does not require the use of large expensive electrodeposition baths. Although the conductive coating is positioned beneath the powder basecoat, primer surfacers are not needed, and preferably are not used. A further advantage of the invention is the ability to produce these panels and parts using a virtually zero VOC topcoat system with the same high utilization rates as th
Isger Ronald J.
McCollum Gregory J.
Reising John
Tansey Richard S.
Cozart Jermie E.
Uhl William J.
Vo Peter
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