Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2000-11-15
2002-02-26
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S484000, C204S479000
Reexamination Certificate
active
06350359
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for coating three-dimensional, electrically conductive substrates, and to the substrates coated accordingly.
BACKGROUND OF THE INVENTION
Present-day automotive coatings are prepared predominantly by applying an electrodeposition coating primer, a primer surfacer layer and a one-coat top coating or a two-coat top coating comprising a base coat and a clear coat layer.
In the past, a coating process which became known as the “reverse process” was used, wherein initially a primer surfacer layer was applied directly to the metallic outer skin of a motor vehicle body and stoved. Then, an electrodeposition coating primer was applied to the metal surfaces inside the body which had remained uncoated, and stoved. See G. Fetdis, Automotive paints and coatings, Verlag Chemie, Weinheim, 1995, pages 61-63. The motor vehicle bodies coated by the “reverse process” were characterised by an outstanding optical surface quality but had insufficient corrosion protection in the transition area between parts of the body surface coated with primer surfacer and with electrodeposition coating primer.
A process is known from EP-A-0 982 413 which is intended to overcome the disadvantages of the “reverse process” in that a plastics film is bonded to the exterior visible surfaces of a motor vehicle body and electrodeposition coating priming of the metallic surface not provided with plastics film subsequently takes place. The application of the plastics film to achieve an exact fit is complicated, labour-intensive and represents a source of trouble.
The object of the present invention is to avoid the above-mentioned weaknesses in corrosion protection of three-dimensional objects, particularly motor vehicle bodies, coated by the “reverse process” principle.
SUMMARY OF THE INVENTION
The invention provides a process for coating three-dimensional, electrically conductive substrates having interior and exterior surfaces; the process comprising (a) applying a coating layer of a stoving coating composition substantially only to the exterior surfaces of the substrate; (b) stoving the coating layer; and (c) applying an electrodeposition coating composition substantially only to the interior surfaces of the substrate, wherein the stoving coating composition is applied at a coating thickness sufficient to electrically insulate the exterior surfaces of the substrate and wherein the stoving coating composition is characterised by a volume resistivity from 10
8
to 10
11
Ohm·cm of applied and stoved coating layers of said composition.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The references to thickness of a coating composition, such as by the phrases “layer thickness”, “coating thickness” and the like means the dry film thickness of the layer or coating concerned.
In the process according to the invention, three-dimensional, electrically conductive substrates, particularly metal substrates such as motor vehicle bodies and motor vehicle body parts are coated. Examples of metals used in such substrates include galvanised or ungalvanised steel, aluminium and/or magnesium. The metals can be pretreated, for example phosphated and passivated and/or electrically conductive precoated. Composite substrates of such metals and, for example, plastics, are also suitable.
Initially, in the process according to the invention, a coating layer of a stoving coating composition is applied substantially only to the exterior surfaces of the three-dimensional substrates and stoved. The composition is applied at a sufficient thickness to ensure insufficient electrical conductivity for the deposition of an electrodeposition coating layer on the stoved coating layer. In other words, the stoving coating composition is applied at a thickness sufficient to electrically insulate the exterior surfaces of the substrate. The exterior surfaces include, for example, surfaces of a motor vehicle body which are directly visible to the observer from the outside of the vehicle, i.e., body panels, doors, hood, trunk lid, etc.
The stoving coating compositions may be waterborne stoving coating compositions to be applied by electrodeposition, for example, electro powder coatings. Preferably, however, they are waterborne, solvent-based or powder stoving coating compositions to be applied by spraying.
The stoving coating compositions contain conventional cross-linkable binders. These binders may be self cross-linking or may be cross-linked by the use of a separate cross-linking agent. Cross-linkable binder systems known to the skilled person and used in primer surfacer coating compositions are particularly suitable. Examples of binders are polyester, epoxy and polyurethane resins. Examples of cross-linking agents are aminoplastic resins such as melamine resins, benzoguanamine resins and blocked polyisocyanates. The solids weight ratio (parts by weight) between binders and cross-linking agents is preferably 60:40 to 90:10.
It is important for the invention to select the stoving coating compositions such that applied and stoved coating layers of said compositions have a volume resistivity from 10
8
to 10
11
, preferably 10
10
to 8×10
10
Ohm·cm. The volume resistivity may be measured by conventional methods known to the skilled person, as described for example in DIN IEC 93. In order to obtain the volume resistivity of 10
8
to 10
11
Ohm·cm, the stoving coating compositions preferably contain electrical conductive components. Examples of such components are particulate inorganic or organic electrical conductors or semi-conductors, such as, for example, black iron oxide, graphite, conductive carbon black, metal powder, e.g. of aluminium, zinc, copper or refined steel, molybdenum disulfide, in particular conductive pigments such as, for example, doped pearlescent pigments, for example, mica platelets provided with a thin layer of antimony-doped tin oxide, or conductive barium sulfate in which the particle core is coated with a thin layer of antimony-doped tin oxide. Electrically conductive polymers such as, e.g., polyaniline, are also suitable but less preferred. The electrically conductive components may be used alone or in combination. They may be introduced into the stoving coating compositions in the same way as conventional coating pigments or fillers. The amount of electrically conductive components needed in the stoving coating composition is, for example, from 1 to 30 wt. %, based on the solids content. The amount needed for any particular composition may be determined easily by the skilled person and is dependent upon, for example, the specific gravity, the specific electrical conductivity and the particle size of the electrically conductive components.
In addition to binders, cross-linking agents and the electrically conductive components and optionally water and/or organic solvents (for liquid compositions), the stoving coating compositions generally contain pigments and/or fillers. The amount of pigment and/or filler used is typically an amount sufficient to provide a pigment/filler to binder solids weight ratio of 0.1:1 to 2:1. When calculating the pigment/filler to binder solids weight ratio, the electrically conductive components are counted as pigment/filler and the weight of binders solids includes any cross-linking agents.
Examples of pigments and/or fillers used in addition to the electrically conductive components include conventional inorganic and/or organic colored pigments and/or fillers such as carbon black, titanium dioxide, iron oxide pigments, phthalocyanine pigments, quinacridone pigments, azo pigments, barium sulfate, calcium carbonate, kaolin, talc or silica.
Pigments, fillers as well as electrically conductive components may be introduced separately or in mixture into the stoving coating compositions. The nature of introduction is immaterial; the methods known to the skilled person for the preparation of coatings may be used, for example, the addition during the course of a preparation from scratch by way of appropriate pastes or by subsequent addition to an inherently fi
Doebler Klaus-Peter
Klein Klausjoerg
Benjamin Steven C.
E. I. du Pont de Nemors and Company
Mayekar Kishor
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