Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
1999-12-28
2002-08-27
Mayekar, K. (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S486000, C204S508000, C204S506000
Reexamination Certificate
active
06440286
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for forming double-layer coatings, to a method for forming multilayer coatings, and to the multilayer coatings obtained thereby.
PRIOR ART
In recent years, methods for forming double-layer coatings by two-coat one-bake technique have been developed particularly from the coating step curtailment, energy saving, resources saving and/or environmental pollution control viewpoint. These methods for giving double-layer coatings comprise applying a cationic electrodeposition coating composition to form an uncured cationically electrodeposited coating, applying an intermediate coating composition thereon to form an uncured intermediate coating in the so-called wet-on-wet manner, and finally heating both uncured coatings simultaneously.
Several problems are often encountered in forming double-layer coatings by such two-coat one-bake technique. When heating both uncured coatings simultaneously, large amounts of basic compound(s) and/or low-molecular-weight compound(s) in the lower uncured cationically electrodeposited coating tend to volatilize and transfer into the upper uncured intermediate coating, with the results that smoothness of the double-layer coating may decrease and/or yellowing may occur. When a top coating composition is further applied to such a defective double-layer coating, there arises the problem that the resulting multilayer coating may have an unfavorable appearance. Thus, there is no established method available for forming double-layer coatings by two-coat one-bake technique using an electrodeposition coating composition and an intermediate coating composition.
SUMMARY OF THE INVENTION
The present invention solves the above problems and provides a method for forming double-layer coatings, which comprises electrodepositing a cationic electrodeposition coating composition to form an uncured cationically electrodeposited coating, applying an intermediate coating composition to said uncured coating to form an uncured intermediate coating, and heating both uncured coatings simultaneously, said cationic electrodeposition coating composition containing a sulfonium group- and propargyl group-containing resin composition.
The present invention also provides a method for forming multilayer coatings, which comprises further applying a top coating composition to the double-layer coating obtained by the above method for forming double-layer coatings.
The present invention further provides a multilayer coating obtained by the above method for forming multilayer coatings.
DETAILED DESCRIPTION OF THE INVENTION
Cationic Electrodeposition Coating Composition
The cationic electrodeposition coating composition to be used in the practice of the present invention comprises a sulfonium- and propargyl-containing resin composition. The resin constituting said resin composition may have both the sulfonium and propargyl groups per molecule. That is not always necessary, however. Thus, for instance, one molecules may have either of the sulfonium and propargyl groups. In this latter case, the resin composition as a whole has these two curing functional group species. Thus, said resin composition may comprise a resin having both sulfonium and propargyl groups or a mixture of a sulfonium-containing resin and a propargyl-containing resin, or a mixture composed of resins of all the three types. The resin composition contained in the cationic electrodeposition coating composition to be used according to the present invention has both sulfonium and propargyl groups in the above sense.
Said sulfonium group is a hydratable functional group in the resin composition mentioned above. When a voltage or current is applied at a certain level or above during electrodeposition coating, the sulfonium group can be irreversibly converted to a passive state as a result of its electrolytic reduction on the electrode, hence loss of its ionicity. This is supposedly the reason why the cationic electrodeposition coating composition to be used according to the present invention can show a high level of throwing power.
It is also presumable that, during electrodeposition coating, electrode reaction be induced to form hydroxide ion, and the sulfonium ion hold this hydroxide ion to form an electrolytically generated base in the electrodeposited coating. Said electrolytically generated base can convert propargyl group, which occurs in the electrodeposited coating and is low in reactivity upon heating, to an allene bond, which is higher in reactivity upon heating.
The resin that constitutes the backbone of the resin composition contained in the cationic electrodeposition coating composition to be used according to the present invention is not particularly restricted but is preferably an epoxy resin. Suited for use as the epoxy resin are those having at least two epoxy groups per molecule. More specifically, there may be mentioned epi-bis type epoxy resins, modifications thereof as obtained by chain extension with a diol, a dicarboxylic acid, a diamine or the like; epoxidized polybutadiene; novolak phenol type polyepoxy resins; novolak cresol type polyepoxy resins; polyglycidyl acrylate; polyglycidyl ethers of aliphatic polyol or polyether polyol; polybasic carboxylic acid polyglycidyl esters; and like polyepoxy resins. Among them, novolak phenol type polyepoxy resins, novolak cresol type polyepoxy resins and polyglycidyl acrylate are preferred because they can easily be polyfunctionalized for curability improvement. Said epoxy resins may partly comprise a monoepoxy resin.
The resin composition contained in the cationic electrodeposition coating composition to be used according to the present invention comprises a resin having the above epoxy backbone. It has a number average molecular weight of 500 to 20,000. When the number average molecular weight is below 500, the coating efficiency of the cationic electrodeposition coating is poor. If it exceeds 20,000, no good coating can be formed on the surface of a substrate or article to be coated. Said number average molecular weight can be selected within a more preferred range, which depends on the resin backbone. In the case of novolak phenol type epoxy resins and novolak cresol type epoxy resins, for instance, it is preferably within the range of 700 to 5,000.
The sulfonium group content in the above resin composition is 5 to 400 millimoles per 100 grams of the solid matter in said resin composition, on condition that the requirement relative to the total content of sulfonium and propargyl groups, which is to be mentioned later herein, is satisfied. If it is less than 5 millimoles per 100 grams, sufficient throwing power or curability cannot be attained and the hydratability and bath stability will be poor. If it exceeds 400 millimoles per 100 grams, the deposition of coatings on substrate surfaces becomes poor. Said sulfonium group content can be selected within a more preferred range, which depends on the backbone employed. In the case of novolak phenol type epoxy backbone and novolak cresol type epoxy backbone, for instance, it is preferably within the range of 5 to 250, further preferably 10 to 150 millimoles, per 100 grams of the solid matter in the resin composition.
The propargyl group in said resin composition functions as a curing functional group in the cationic electrodeposition coating composition to be used according to the present invention. For unknown reasons, the throwing power of the cationic electrodeposition coating composition can be further improved when said group coexists with the sulfonium group.
The propargyl group content in the above-mentioned resin composition is 10 to 495 millimoles per 100 grams of the solid matter in said resin composition, on condition that the requirement relative to the total content of sulfonium and propargyl groups, which is to be mentioned later herein, is satisfied. If it is less than 10 millimoles per 100 grams, sufficient throwing power or curability cannot be obtained. If it exceeds 495 millimoles per 100 grams, hydration stability of the cationic elec
Kawakami Ichiro
Saito Takao
Sakamoto Hiroyuki
Takeshita Sho
Uchidoi Satoru
Connolly Bove & Lodge & Hutz LLP
Mayekar K.
Nippon Paint Co. Ltd.
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