In the metalworking industry, particularly in automobile construction, the metal parts of the products have to be protected against corrosion. According to the conventional prior art, the sheet metal is first coated with anti-corrosion oils in the rolling mill and optionally coated with drawing compounds prior to forming and stamping. In the vehicle construction sector, appropriately shaped sheet metal parts are stamped out for vehicle bodies or body parts and shaped using such drawing compounds or oils in a deep-drawing process, then they are assembled, generally by means of welding and/or flanging and/or bonding, and finally cleansed in a costly procedure. Anti-corrosion surface pretreatments, such as phosphating and/or chromating, are applied followed by a first paint layer applied to the structural parts by electro-deposition. In general, this first electro-deposition process, especially in the case of car bodies, is followed by the application of several more layers of paint. For less demanding applications, such as for example domestic appliances, a powder coating can be applied directly after the mentioned anti-corrosion pretreatment.
In the metal processing industry, such as in the vehicle and domestic appliance construction sectors, in order to simplify the process, there is a need to reduce the cost of chemical anti-corrosion treatment. This may be achieved by providing the raw material in the form of metal sheets or metal strips which have already been provided with an anti-corrosion layer. There is, therefore, a need to find simpler methods of production in which pre-coated sheets may be welded and then painted in an electro-deposition process in a well-proven manner. Thus, there are a number of processes in which an organic, more or less conductive coating is applied, subsequent to phosphation and/or chromation in a so-called coil coating process. As a rule, these organic coatings should be made up in such a way that they are sufficiently electrically conductive so as not impair the typical welding processes used in the car industry, for example electrical spot-welding. In addition, these coatings should be overpaintable with conventional electro-deposition paints.
Particularly in the car industry, in addition to normal steel sheeting, steel sheeting and/or aluminum sheeting, which have been galvanised and/or alloy galvanised in a variety of processes, have also been used to an increasing extent recently.
The coating of steel sheeting with organic coatings that are weldable and which are applied directly in the rolling mill by the so-called coil coating process is known in principle. German patent DE-C-3412234 describes a conductive and weldable anti-corrosion primer for electrolytically thin-layer galvanized, phosphatized or chromatized and drawable steel sheeting. This anti-corrosion primer consists of a mixture of more than 60% zinc, aluminum, graphite and/or molybdenum disulfide and also another anti-corrosion pigment and 33 to 35% of an organic binder and about 2% of a dispersion auxiliary or catalyst. Polyester resins and/or epoxy resins and derivatives thereof are proposed as organic binders. It is assumed that this technology represents the basis of the coating agent known in the industry by the name “Bonazinc® 2000”. Although this process provides some advantages as compared with the procedure described above (temporary corrosion protection with anti-corrosion oils followed by subsequent degreasing after assembling the metallic components), the process described in DE-C-3412234 still requires much improvement where:                The coating is not sufficiently spot-weldable.        The baking temperature for this type of coating ranges from 250 to 260° C. peak metal temperature (PMT) and is still too high. Many new types of steel with a “bake hardening effect” cannot be employed for this type of high baking temperature.        
In accordance with the disclosure in the German patent DE-C-3412234, the organic binder may consist of polyester resins and/or epoxy resins and derivatives thereof. Specific examples mentioned are an epoxide/phenyl precondensate, an epoxy ester and linear, oil-free mixed polyesters based on terephthalic acid.
European patent application EP-A-573015 describes an organic-coated steel composite sheet, consisting of surfaces coated with zinc or a zinc alloy on one or two sides, provided with a chromate film and an organic coating applied thereto with a layer thickness of 0.1 to 5 μm. The organic coating is formed from a primer composition that consists of an organic solvent, an epoxy resin having a molecular weight between 500 and 10,000, an aromatic polyamine and a phenol or cresol compound as accelerator. In addition, the primer composition comprises a polyisocyanate and colloidal silica. In accordance with the teaching of this document, the organic coating is preferably applied in a dry film of layer thickness 0.6 to 1.6 μm, since layers thinner than 0.1 μm are too thin to provide any corrosion protection. Layer thicknesses greater than 5 μm, however, impair weldability.
In an analogous manner, German patent application DE-A-3640662 describes a surface-treated steel sheet comprising a zinc-coated or zinc alloy-coated steel sheet, a chromate film produced on the surface of the steel sheet and a layer of a resin composition produced on the chromate film. This resin composition consists of a basic resin that is prepared by reacting an epoxy resin with amines, and a polyisocyanate compound. This film should also be used with a dry film thickness of less than about 3.5 μm, because thicker layers greatly reduce the weldability.
European patent application EP-A-380 024 describes organic coating materials based on a bisphenol-A-type epoxy resin having a molecular weight between 300 and 100,000 and also a polyisocyanate or blocked polyisocyanate, pyrogenic silica and at least one organic coloring pigment. A chromate-containing pretreatment with a high application of Cr is also required in this process. The organic layer should be not thicker than 2 μm, since sheets with thicker organic layers cannot be satisfactorally spot-welded and the properties of the electro-dipcoated paint applied to the organic coating are negatively affected.
International patent application WO 99/24515 discloses a conductive and weldable anti-corrosion composition for coating metal surfaces, characterised in that it comprises:    (a) 10 to 40 wt. % of an organic binder comprising:            (aa) at least one epoxy resin;        (ab) at least one hardener selected from guanidine, substituted guanidines, substituted ureas, cyclic tertiary amines and mixtures thereof;        (ac) at least one blocked polyurethane resin;            (b) 0 to 15 wt. % of an anti-corrosion pigment based on silicate;    (c) 40 to 70 wt. % of powdered zinc, aluminum, graphite and/or molybdenum sulfide, carbon black, iron phosphide; and    (d) 0 to 30 wt. % of a solvent.
The international patent application WO 03/089530 describes a mixture for applying a polymeric corrosion resistant, electrically weldable coating, whose electrically conductive particles should be more electrically conductive than that of zinc particles. Furthermore, these conductive particles should have a Moh hardness greater than 4. According to this publication, a photoinitiator can be present in the composition. It can also comprise water and/or an organic solvent. In addition, this document teaches that coating compositions of this type that are rich in zinc particles are undesirable primer coatings for welding as the tooling would be heavily contaminated from abrasion.
The international patent application WO 03/062328 describes corrosion resistant mixtures comprising corrosion resistant pigments, amorphous silicon dioxide that is modified with metal ions, such as compounds of the general formula Mn (X)m, in which M are a central atom from the group of Lewis acceptors and Lewis donor ligands, which comprise at least one bridging atom from the elements of the 5th or 6th main group of the periodic table of the elements. The added binders should be preferably (meth)acrylate copolymers, partially saponified polyvinyl esters, polyesters, alkyd resins, polylactones, polycarbonates, polyethers, epoxy resin amine adducts, polyureas, polyamides, polyimides or polyurethanes. This document further discloses that the coating materials can be crosslinked with actinic radiation, particularly UV radiation, or thermally crosslinked. They are suitable for use in coil coating processes.
The international patent application WO 00/75250 describes a process for applying a weldable anti-corrosion coating on a metallic substrate, especially an autobody sheet steel in the automobile industry, as well as coating mixtures for carrying out this process. The coating mixture should comprise a solid polymeric organic binder, a low molecular weight, liquid, radically polymerizable compound and a compound that forms radicals when irradiated with actinic radiation. In addition, these compositions should comprise a conductive, inorganic pigment from the group of the oxides, phosphates or phosphides of iron or aluminum or graphite mica pigments. According to the teaching of this document, coating mixtures that comprise zinc powder are disadvantageous as they often tend to instances of corrosion, which initiates between the pigmented layer and the metallic or galvanized substrate.
The international patent applications WO 03/089507 and WO 03/089529 describe coating mixtures for the application of a thin, polymeric, corrosion resistant, low abrasion, formable and electrically conductive coating on a metallic substrate. These compositions should comprise at least one binder as well as optionally a reactive diluent and at least one crosslinker and/or at least one photoinitiator as well as optionally a crosslinkable compound such as e.g. isocyanates, blocked isocyanates, isocyanurates and the like as well as an organic solvent and/or water. In addition, the mixture should comprise electrically conductive and/or semi-conductive elements/compounds that have a particle size distribution with a d80 diameter of <6 μm, alternatively electrically conductive or semi-conductive amine-containing and/or ammonium-containing compounds are proposed. Alternatively, mixtures of electrically conductive hard particles and very soft or soft organic, lubricious, electrically conductive or semiconductive particles are proposed as pigment fractions. Specific examples of lubricious, very soft or soft particles are: graphite, sulfide, selenide and/or telluride.
There is a need to provide improved coating compositions that satisfy the requirements of the automobile industry in all respects. In comparison to the prior art, the following properties of organic coating compositions suitable for the coil coating process should be improved:                a clear reduction in white rust on galvanised steel sheeting in the spray test according to DIN 50021, i.e. better corrosion protection;        an improvement in adhesion of the organic coating to the metallic substrate in accordance with an evaluation by the T-bend test (ECCA standard) and impact test (ECCA standard);        fit for external use (i.e. capacity for use as the external sheet in a car body;        adequate corrosion protection, also with Cr-free pretreatment processes;        today's still conventional cavity sealing with wax or wax-containing products may become unnecessary due to the improved corrosion protection;        adequate suitability for typical car welding processes;        resistance towards process chemicals used in automobile manufacturing; and        paintability.        
In order for these coatings to be applicable in the coil coating process, they must have a suitable viscosity. For highly pigmented systems this usually requires the presence of solvents such as water or organic solvents, which evaporate when the coating is cured. The use of water requires additional energy costs, the use of organic solvents requires additional technical measures in order to prevent their escape into the atmosphere.
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