Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2001-05-10
2003-07-08
Mayekar, K. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S493000, C204S507000, C204S509000
Reexamination Certificate
active
06589411
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for coating electrically conductive substrates with aqueous electrodeposition coating agents.
BACKGROUND OF THE INVENTION
Electrodeposition coating agents are in particular used for the production of corrosion protective primers on metallic substrates. After electrodeposition, the electrodeposition coating layers are usually baked.
Electrodeposition primers should exhibit good mechanical properties, especially on external surfaces facing towards an observer, as these surfaces are exposed to external mechanical influences. Electrodeposition primers are intended to protect not only the surfaces, but also the edges of substrates from corrosion. Edge corrosion on edges visible to the observer is particularly aesthetically troublesome, for example, taking the form of visible rust spots and streaks that develop on the coated substrates during use.
There is a requirement for an electrodeposition coating process that produces electrodeposition coated substrates which, in addition to good surface corrosion protection, a) exhibit good edge corrosion protection and/or b) the electrodeposition coating of which has improved resistance to mechanical stresses.
It has now been found that electrodeposition coating layers exhibit surprisingly improved mechanical properties if they have been cured by irradiation with near infrared radiation (NIR radiation) instead of by conventional baking. Surprisingly, improved edge corrosion protection may also be achieved with an electrodeposition coating cured in this manner. Both effects may simultaneously be achieved on substrates comprising edges if the electrodeposition coating layer on the surfaces and on the edges of the substrates are cured by means of NIR-irradiation.
The term “curing” used in the description and in the patent claims means curing in the sense of chemical cross-linking of the electrodeposition coating layer by the formation of covalent bonds between the constituents of the thermally curable electrodeposition coating binder system.
SUMMARY OF THE INVENTION
One general embodiment of the present invention is an electrodeposition coating process consisting of the successive process steps:
1) electrodeposition of a coating layer of an electrodeposition coating agent that contains a thermally curable binder system onto the surface of an electrically conductive substrate and
2) thermal curing of the electrodeposited coating layer by irradiation with near infrared radiation.
DETAILED DESCRIPTION OF THE EMBODIMENTS
In the case of substrates having zones both accessible and inaccessible to near infrared radiation, after electrodeposition coating and completion of the NIR-irradiation, a thermal curing step is performed by conventional methods of heat input in order to cure completely uncured or incompletely cured portions of the electrodeposition coating layer. In the case of such substrates, another embodiment of the present invention consists in an electrodeposition coating process consisting of the successive process steps:
1) electrodeposition of a coating layer of an electrodeposition coating agent that contains a thermally curable binder system onto the surface of an electrically conductive substrate comprising zones accessible and inaccessible to near infrared irradiation,
2) thermal curing by near infrared irradiation of the zones of the electrodeposited coating layer accessible to irradiation with near infrared radiation and
3) thermal curing of hitherto uncured portions of the electrodeposition coating layer by means of heat input without NIR-irradiation.
In the event that the substrates comprise edges and it is only desired to achieve improved edge corrosion protection, it is also possible to proceed in such a manner that, after electrodeposition coating, the electrodeposition coating layer is cured by NIR-irradiation only on edges or in the zone of edges, including relevant edges, before a thermal curing step subsequently proceeds by means of conventional heat input methods. The edges may comprise all the edges or only selected edges or parts of edges. This further embodiment of the invention comprises an electrodeposition coating process consisting of the successive process steps:
1) electrodeposition of a coating layer of an electrodeposition coating agent that contains a thermally curable binder system onto the surface of an electrically conductive substrate comprising edges,
2) thermal curing of the electrodeposited coating layer on edges or in the zone of edges, including relevant edges, by near infrared irradiation and
3) thermal curing of hitherto uncured portions of the electrodeposition coating layer by means of heat input without NIR-irradiation.
Another embodiment of the invention is an electrodeposition coating process consisting of the successive process steps:
1) electrodeposition of a coating layer of an electrodeposition coating agent that contains a thermally curable binder system onto the surface of an electrically conductive substrate comprising edges with zones accessible and inaccessible to near infrared irradiation,
2) thermal curing by near infrared irradiation of the electrodeposited coating layer on edges or in the zone of edges, including relevant edges, and of zones of the electrodeposited coating layer accessible to irradiation with near infrared radiation and
3) thermal curing of hitherto uncured portions of the electrodeposition coating layer by means of heat input without NIR-irradiation.
Per se known electrodeposition coating agents are used in the process according to the invention. These are aqueous coating agents with a solids content of, for example, 10 to 30 wt. %. The solids content consists of the resin solids, optionally, together with fillers, pigments and conventional non-volatile paint additives. The resin solids content comprises the sum of the solid constituents of the thermally curable binder system. The binder systems contained in the electrodeposition coating agents comprise binder systems conventional in electrodeposition coatings that are thermally curable by free-radical polymerization of olefinically unsaturated double bonds and/or by condensation reactions and/or addition reactions. Binder systems curable by condensation reactions and/or addition reactions are preferred. The binder systems may contain self-cross-linking binders or combinations of externally cross-linking binders and cross-linking agents. Nonionic additive resins and paste resins serving to disperse pigments may optionally also be present, as may microgels. For example, the composition of the electrodeposition coating binder system comprises solids contents proportions by weight adding up to 100 wt. % of 50 to 100 wt. % of electrodeposition coating binders, 0 to 50 wt. % of cross-linking agents, 0 to 30 wt. % of nonionic additive resins and 0 to 20 wt. % of paste resin. The sum of the solids content by weight of cross-linking agent, nonionic additive resin and paste resin is at most 50 wt. % of the resin solids content of the electrodeposition coating binder system.
The electrodeposition coating agents may for example comprise conventional anodically electrodepositable (AED) coating agents. AED coating agents contain, for example, binders based on polyesters, epoxy resin esters, (meth)acrylic copolymer resins, maleinate oils or polybutadiene oils with a weight-average molecular mass (Mw) of, for example, 300 to 10,000 and an acid value from 35 to 300 mg KOH/g. The binders carry —COOH, —SO
3
H and/or —PO
3
H
2
—groups and, after neutralization of at least a part of the acid groups with bases, particularly amines, may be converted into the aqueous phase. The binders may be self-cross-linking or externally cross-linking. The AED coating agents may therefore also contain conventional cross-linking agents, e.g., triazine resins, cross-linking agents containing groups capable of transesterification or blocked polyisocyanates.
Conventional cathodically electrodepositable (CED) coating agents based on CED binders may also be used as electrodeposition coating agents
Kimpel Matthias
Moelzer Christoph
Pech Gerhard
Reis Oliver
Wulf Martin
Deshmukh Sudhir G.
E.I. du Pont de Nemours and Company
Mayekar K.
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