Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2002-03-28
2004-05-18
Mayekar, Kishor (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S500000, C204S506000, C204S507000, C204S509000, C204S493000, C428S416000
Reexamination Certificate
active
06736950
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of lacquering electrically conductive substrates with electrically depositable aqueous electro-dipcoating lacquers (EDL). The invention relates also to a method of preventing edge migration on the stoving of electrically deposited EDL coating layers.
Electro-dipcoating lacquers are used especially to produce anti-corrosive primer coats on metallic substrates. However, they may, for example, also be deposited and stoved on any desired electrically conductive substrates as a single-layer finishing lacquer, a clear lacquer, or as a lacquer layer that is arranged within a multilayer lacquer coating. An EDL coating layer arranged within a multilayer lacquer coating may be, for example, a lacquer layer having a decorative effect, which serves as a finishing lacquer or may be covered with a clear lacquer layer.
A problem in the case of lacquering with electro-dipcoating lacquers is edge migration on stoving of an EDL coating layer deposited beforehand on an electrically conductive substrate. The EDL coating layer pulls away from the edge, with a reduction in the layer thickness at or in the immediate vicinity of the edge. In the extreme case, the edge is insufficiently covered after stoving. While this is noticeable, for example, in the case of decorative EDL coatings as a difference in colour because the substrate shows through in the region of the edge, it results in the case of anti-corrosive EDL primer coats in an impairment or in the loss of the anti-corrosive action at or in the region of the edge. Apart from the technical disadvantages of edge corrosion, corrosion at edges that are accessible to an observer is troublesome in particular from a visual point of view, for example in the form of visible rust spots and runs that form during use of the lacquered substrates. EDL coating compositions that are curable by irradiation with ultraviolet light, and the curing by UV irradiation of EDL coating layers deposited electrically from such EDL coating compositions, are known inter alia from U.S. Pat. Nos. 4,040,925 and 4,039,414.
SUMMARY OF THE INVENTION
The object of the invention is to provide a method of electro-dipcoating with which EDL coatings with good edge coverage can be obtained on electrically conductive substrates having edges, that is to say the deposited EDL coating layer is to exhibit no or only slight edge migration behaviour on stoving. In particular, edge migration at edges that are accessible to an observer is to be avoided, in order to avoid the undesirable occurrence of visually disturbing corrosion phenomena at such edges during subsequent use of the lacquered substrates.
It has been found that that object can be achieved if a heat-curable EDL coating composition containing a binder system having a content of radically polymerisable olefinically unsaturated double bonds is used for the electro-dipcoating of an electrically conductive substrate having edges, and if, before stoving of the coating layer deposited electrically from the EDL coating composition, UV irradiation of the electrically deposited coating layer is carried out at least at edges of the substrate and the wholly or partially UV-irradiated EDL coating layer is then cured by stoving. The term “curing” used here and hereinbelow means curing in the sense of a chemical crosslinking of the EDL coating layer by formation of covalent bonds between the constituents of the heat-curable EDL binder system.
Only so-called dual-cure systems have hitherto been known from the literature. In such systems, curing takes place in the opposite manner. For example, U.S. Pat. No. 4,066,523 and U.S. Pat. No. 4,070,258 describe electro-dipcoating lacquers and combinations of polymers with tertiary amino groups, polymers with lateral mercapto groups, a bismaleimide crosslinking agent or an ethylenically unsaturated carbonyl crosslinking agent, which are cured after deposition first by means of heat and finally by UV irradiation.
Accordingly, the present invention provides a method of electro-dipcoating, consisting of the successive steps:
1) electro-deposition of a coating layer from an EDL coating composition containing a heat-curable binder system having a content of olefinically unsaturated double bonds that are radically polymerisable under UV irradiation, on an electrically conductive substrate having edges,
2) UV irradiation of the electrically deposited coating layer, at least in the region of the edges, and then
3) curing of the EDL coating layer by stoving.
A preferred embodiment of the invention comprises a method of electro-dipcoating consisting of the successive steps:
1) electro-deposition of a coating layer from an EDL coating composition containing a heat-curable binder system having a content of olefinically unsaturated double bonds that are radically polymerisable under UV irradiation, on an electrically conductive substrate having edges,
2) UV irradiation of the electrically deposited coating layer in the region of the edges, especially of the electrically deposited coating layer at the edges,
3) curing of the EDL coating layer by stoving.
The method according to the invention can advantageously be used for the electro-dipcoating of three-dimensional, electrically conductive, especially metallic, substrates having edges and having regions that are directly accessible and regions that are not accessible to an observer, especially having edges that are directly accessible and edges that are not accessible to an observer. “Directly accessible to an observer” means “accessible to the eye of an observer from the outside without particular technical or optical aids”. Only regions or edges that are directly accessible to an observer are also directly accessible to UV irradiation. Examples of substrates having regions that are directly accessible and regions that are not accessible to an observer, especially having edges that are directly accessible and edges that are not accessible to an observer, are especially motor vehicle bodies with their hollow spaces, joins and other construction-related undercuts. Examples of edges of motor vehicle bodies that are directly accessible to an observer are externally visible cut edges of individual body components, the edges of holes, for example of clip holes or of openings provided for components that are to be inserted, such as windows, head lamps, door locks or door handles, and the edges, of roof rails.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly, the invention relates in a particular embodiment to a method of electro-dipcoating consisting of the successive steps:
1) electro-deposition of a coating layer from an EDL coating composition containing a heat-curable binder system having a content of olefinically unsaturated double bonds that are radically polymerisable under UV irradiation, on a three-dimensional, electrically conductive substrate having edges and having regions that are accessible and regions that are not accessible to an observer,
2) UV irradiation of the electrically deposited coating layer in regions of the substrate surface that are directly accessible to an observer,
3) curing of the EDL coating layer by stoving.
The particular embodiment of the method according to the invention is preferably a method of electro-dipcoating consisting of the successive steps:
1) electro-deposition of a coating layer from an EDL coating composition containing a heat-curable binder system having a content of olefinically unsaturated double bonds that are radically polymerisable under UV irradiation, on a three-dimensional, electrically conductive substrate having edges that are directly accessible and edges that are not accessible to an observer,
2) UV irradiation of the electrically deposited coating layer in the region of edges of the substrate that are accessible to an observer, especially the EDL-coated edges that are directly accessible to an observer,
3) curing of the EDL coating layer by stoving.
The electro-dipcoating lacquers used in the method according to the invention are aqueous coating com
Klein Klausjörg
Kühhiri Walter
Kuphal Siegfried
Benjamin Steven C.
E. I. du Pont de Nemours and Company
Mayekar Kishor
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