Coating processes – With pretreatment of the base – Metal base
Reexamination Certificate
2000-08-07
2002-11-12
Dawson, Robert (Department: 1712)
Coating processes
With pretreatment of the base
Metal base
57, 57, 57, 57, 57, 57, 57, 57, C106S014440, C427S386000, C427S404000, C427S405000, C427S406000, C427S410000, C427S388100
Reexamination Certificate
active
06479103
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to conductive and weldable anti-corrosive compositions for coating metal surfaces and to a process for coating metal surfaces with electrically conductive organic coatings.
BACKGROUND OF THE INVENTION
In the metals processing industry, in particular in automotive construction, the metallic components of the products must be protected from corrosion. According to the conventional prior art, this is achieved by initially coating the sheet metal in the rolling mill with anti-corrosive oils and optionally with drawing greases before forming and stamping. In automotive construction, sheet metal components appropriately formed for bodywork or bodywork components are stamped out and formed by deep drawing using such drawing greases or oils and then generally joined together by welding and/or crimping and/or adhesive bonding and then elaborately cleaned. Anti-corrosive surface pretreatment, such as phosphating and/or chromating, is then performed, whereupon a first lacquer layer is applied onto the components by electrocoating. Especially in the case of automotive bodywork, this first electrocoating is followed by the application of further lacquer layers.
There is a need to find simpler production processes which make it possible to weld already precoated sheet metal and to electrocoat it in a proven manner. There is thus a range of processes in which, after phosphating and/or chromating, an organic coating which is conductive to a greater or lesser degree is applied in the so-called coil coating process. These organic coatings should generally be of a composition such that they have sufficient electrical conductivity not to impair the electrical spot welding process. It should moreover be possible to coat these coatings with conventional electrocoating lacquers. It should furthermore be possible to stamp and form the sheet metal coated in this manner with reduced usage of deep drawing greases or oils. Recently, especially in the automotive industry, galvanised and/or alloy-galvanised sheet steel and aluminum and magnesium sheet have increasingly been used in addition to standard sheet steel.
It is in principle known to coat sheet steel with organic coatings which are weldable and are applied directly in the rolling mill using the so-called coil coating process.
DE-C-3412234 thus describes a non-blocking and weldable anti-corrosive primer for electrolytically thin galvanised, phosphated or chromated and formable sheet steel. This anti-corrosive primer consists of a mixture of over 60% zinc, aluminum, graphite and/or molybdenum disulfide and another anti-corrosive pigment and 33 to 35% of an organic binder, together with about 2% of a dispersion auxiliary or catalyst. Polyester resins and/or epoxy resins and derivatives thereof are proposed as the organic binder. It is assumed that this technology forms the basis of the coating composition known in the industry under the name “Bonazinc 2000”. Although this process offers some advantages in comparison with the above-stated method (temporary corrosion protection with anti-corrosive oils, followed by subsequent degreasing once the metal components have been joined), the process described in DE-C-3412234 is still in great need of improvement:
this coating is not sufficiently spot weldable
the stoving temperature for such coatings at a peak metal temperature (PMT) of 250 to 260° C. is still too high. Many recent steels with the “bake-hardening” effect cannot be used at such high stoving temperatures.
lacquer adhesion onto the pretreated substrates, preferably galvanised steels, is still inadequate, especially if the sheets are subjected to relatively severe forming in the press.
According to the teaching of DE-C-3412234, the organic binder may consist of polyester resins and/or epoxy resins and derivatives thereof. An epoxy/phenyl precondensate, an epoxy ester and linear, oil-free terephthalic acid based copolyesters are explicitly mentioned.
EP-A-573015 describes organically-coated composite sheet steel consisting of a surface coated on one or two sides with zinc or zinc alloy, which surface is provided with a chromate film and, thereon, an organic coating having a film thickness of 0.1 to 5 &mgr;m. The organic coating is formed from a primer composition consisting of an organic solvent, an epoxy resin having a molecular weight of between 500 and 10000, an aromatic polyamine and a phenol or cresol compound as accelerator. The primer composition furthermore contains a polyisocyanate and colloidal silica. According to this document, the organic coating is preferably applied to a dry film thickness of 0.6 to 1.6 &mgr;m as layers thinner than 0.1 &mgr;m are too thin to provide corrosion protection. Film thicknesses of above 5 &mgr;m, however, impair weldability. DE-A-3640662 similarly describes surface treated sheet steel comprising sheet steel provided with coating of zinc or zinc alloy, a chromate film formed on the surface of the sheet steel and a layer of a resin composition formed on the chromate film. This resin composition consists of a basic resin, which is produced by reacting an epoxy resin with amines, together with a polyisocyanate compound. This film too may only be applied to dry film thicknesses of less than about 3.5 &mgr;m as weldability is severely reduced at greater film thicknesses.
EP-A-380024 describes organic coating materials based on a bisphenol A type epoxy resin having a molecular weight of between 300 and 100000, together with a polyisocyanate or blocked polyisocyanate, pyrogenic silica and at least one organic colouring pigment. In this process too, pretreatment with chromate to form a thick Cr coating is required. The organic layer cannot be thicker than 2 &mgr;m as sheets having thicker organic coats cannot satisfactorily be spot welded and the properties of the electrocoating lacquer applied onto the organic coating are degraded.
SUMMARY OF THE INVENTION
An object thus arose of providing coating compositions which satisfy the automotive industry's requirements in all respects. In comparison with the prior art, it is intended to improve the following properties of the organic coating compositions suitable for the coil coating process:
lower stoving temperature preferably no higher than 210 to 235° C. PMT
distinct reduction in white rust on galvanised sheet steel in the salt spray test to DIN 50021, i.e. better corrosion protection
improvement in adhesion of the organic coating on the metallic substrate assessed by the T-bend test (ECCA standard) and impact test (ECCA standard)
sufficient corrosion protection even with a thin Cr coating, preferably using Cr-free pretreatment methods
cavity sealing with wax or products containing waxes, which is still conventional, should become superfluous thanks to the improved corrosion protection
suitability for spot welding.
The solution to this problem according to the present invention is stated in the claims. The present solution essentially comprises the provision of coating compositions containing 10 to 40 wt. % of an organic binder, 0 to 15 wt. % of a silicate-based anti-corrosive pigment, 40 to 70 wt. % of powdered zinc, aluminum, graphite and/or molybdenum disulfide, together with 0 to 30 wt. % of a solvent, wherein the organic binder consists of at least one epoxy resin, at least one curing agent selected from guanidine, substituted guanidines, substituted ureas, cyclic tertiary amines and mixtures thereof, together with at least one blocked polyurethane resin.
The solution according to the present invention also comprises the use of the above-stated composition for coating sheet metal in the coil coating process.
The solution according to the present invention furthermore comprises a process for coating metal surfaces with a conductive organic anti-corrosive layer characterised by the following stages:
conventional pretreatment consisting of
cleaning
optionally phosphating
chromating
optionally chromium-free pretreatment
coating with a composition of the above-stated type to a film thickness of 1 to 10 &mgr;m, pref
Krey Wolfgang
Kunz Andreas
Lorenz Wolfgang
Wichelhaus Winfried
Dawson Robert
Harper Stephen D.
Henkel Kommanditgesellschaft auf Aktien
Peng Kuo-Liang
Seifert Arthur G.
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