Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
1999-05-11
2001-06-19
Mayekar, K. (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S493000, C204S500000, C204S509000, C524S591000, C524S840000, C524S901000
Reexamination Certificate
active
06248225
ABSTRACT:
BACKGROUND
The present invention is directed to a process for electrodepositing a two-coat composite coating on a substrate where the first electrodeposited coating protects against corrosion and the second electrodeposited coating protects against chipping of the coated composite.
Multilayered coating composites find use in various industries including the coating and/or painting of various motor vehicles. In several of these industries and in the automotive industry in particular there can be from 2 up to 6 or more coating layers in the multilayered coating composites. These coating layers can serve to protect the substrate and/or to provide a decorative finish.
Multilayered coating composites for metal substrates like those in the automobile industry have involved electrodeposition coatings as an initial resinous coating layer to protect the metal substrate from corrosion. Cationic electrodeposition coatings have become the coatings of choice for corrosion protection. Electrodeposition has become increasingly important in the coatings industry because by comparison with non-electrophoretic coating means, electrodeposition offers higher paint utilization, outstanding corrosion protection, low environmental contamination, and a highly automated process.
Two-coat application by the electrodeposition process is known in the art. For example in U.S. Pat. Nos. 4,988,420; 4,840,715; and 5,275,707 different types of electroconductive pigments are added to a first electrodeposited acrylic resinous coating, and a second electrodeposition coating is applied over the conductive first electrodeposited acrylic coating. Typically, these second electrodeposition coatings have been applied for durability and decorative purposes.
Also in multilayered coating composites for motor vehicles another coating layer that can be present is a spray applied chip resistant coating layer. Such a layer protects the surface of the substrates from losing paint through chipping when the substrate of the vehicle is hit with solid debris such as gravel and stones. The art for achieving chip resistance from spray applied primer coatings has postulated that reducing the differential in impact energy between multiple coating layers should improve chip resistance of the coating. This is especially the situation for those coating layers with excessive difference of hardness between them. This reduction in the differential would lessen delamination between the coatings such as between the undercoat and an intermediate coat and a top coat or an undercoat and an intermediate coat.
In U.S. Pat. No. 5,674,560, this differential is reduced through a chip resistant polyolefin type of primer that is spray applied over a cationic or anionic electrodeposited coated film before application of a soft intermediate polyester film. The reduction of the differential in impact energy is reportedly maximized when the polyolefin primer is applied over the softer anionic electrodeposited film as opposed to a cationic electrodeposited film.
Therefore, even though the art recognizes that cationic electrodeposited coatings provide better corrosion resistance than anionic electrodeposited coatings, further improvements in chip resistance in a multilayered coating system may be at odds with or involve sacrificing some corrosion resistance by using the anionic electrodeposited coating for corrosion protection.
An object of the present invention is to provide a process and system for the improved multiple coatings with both good chip resistance and corrosion protection while additionally providing efficiencies in application and processing. These include higher paint utilization, low environmental contamination, and a highly automated process.
SUMMARY OF THnE INVENTION
The present invention provides a process of electrocoating electrically conductive substrates with two electrodeposited layers, comprising the steps of:
(a) electrodepositing on the substrate an electrically conductive coating that is deposited from a composition comprising a curable ionic resin and an electrically conductive pigment;
(b) at least partially curing the electrodeposited coating so as to make the coating electrically conductive;
(c) electrodepositing a second coating on the electrically conductive coating from step (b); the second coating being deposited from a composition comprising a curable ionic polyurethane resin that contains a polymeric segment derived from an active hydrogen-containing polymer having a glass transition temperature of 0° C. or less and a number average molecular weight of 400 to 4000;
(d) curing the second coating from step (c).
An article coated by the process of the invention is also provided.
Also provided is an aqueous electrodepositable composition comprising a curable polyurethane resin containing ionic salt groups dispersed in an aqueous medium which is the reaction product of:
(a) a polyisocyanate and
(b) an active hydrogen-containing polymer having a glass transition temperature of 0° C. or less and a number average molecular weight of 400 to 4000.
DETAILED DESCRIFION OF THE INVENTION
The use of numerical values in the various ranges specified in this application, unless expressly indicated otherwise, are stated as approximations as though the minimum and maximum values within the stated ranges were both preceded by the word “about”. In this manner slight variations above and below the stated ranges can be used to achieve substantially the same results as values within the ranges. Also, the disclosure of these ranges is intended as a continuous range including every value between the minimum and maximum values.
The first step in the process of the invention is electrodepositing an electrically conductive coating onto the surface of an electrically conductive substrate. The coating is deposited from a composition comprising a curable ionic resin and an electrically conductive pigment. The composition may be an anionic electrodepositable composition or a cationic electrodepositable composition, which is preferred. The anionic and cationic electrodepositable compositions which can be used are those which provide high throwpower and good corrosion resistance. These compositions are well known in the art.
Examples of ionic resins suitable for use in anionic electrodepositable coating compositions are base-solubilized, carboxylic acid containing polymers such as the reaction product or adduct of a drying oil or semi-drying fatty acid ester with a dicarboxylic acid or anhydride; and the reaction product of a fatty acid ester, unsaturated acid or anhydride and any additional unsaturated modifying materials which are further reacted with polyol. Also suitable are the at least partially neutralized interpolymers of hydroxy-alkyl esters of unsaturated carboxylic acids, unsaturated carboxylic acid and at least one other ethylenically unsaturated monomer. Still another suitable electrodepositable resin comprises an alkyd-aminoplast vehicle, i.e., a vehicle containing an alkyd resin and an amine-aldehyde resin. Yet another anionic electrodepositable resin composition comprises mixed esters of a resinous polyol. These compositions are described in detail in U.S. Pat. No. 3,749,657 at col. 9, lines 1 to 75 and col. 10, lines 1 to 13, all of which are herein incorporated by reference. Other acid functional polymers can also be used such as phosphatized polyepoxide or phosphatized acrylic polymers as are well known to those skilled in the art.
Examples of ionic resins suitable for use in cationic electrodepositable coating compositions include amine salt group-containing resins such as the acid-solubilized reaction products of polyepoxides and primary or secondary amines such as those described in U.S. Pat. Nos. 3,663,389; 3,984,299; 3,947,338; and 3,947,339. Usually, these amine salt group-containing resins are used in combination with a blocked isocyanate curing agent. The isocyanate can be fully blocked as described in the aforementioned U.S. Pat. No. 3,984,299 or the isocyanate can be partially blocked and reacted with the resin backbone such
Corrigan Victor G.
Eswarakrishnan Venkatachalam
Faucher Philippe
McCollum Gregory J.
Palaika Thomas
Altman Deborah M.
Mayekar K.
PPG Industries Ohio Inc.
Uhl William J.
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