Stock material or miscellaneous articles – Composite – Of epoxy ether
Utility Patent
1999-07-22
2001-01-02
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C428S414000, C428S416000, C428S418000, C523S414000, C525S502000, C525S529000
Utility Patent
active
06168864
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a multilayer coating film in particular suited for two-coat coating of automobiles.
PRIOR ART
Automotive coating films generally have the multilayer structure as comprised of an electrodeposition coat, an intermediate coat and a top coat. These multilayer coating films require at least three coating steps. The so-called “two-layer coating” in which said intermediate coating step is omitted is being put into practical use as an economically advantageous process for solid color top coating of cars for commercial use, light cars, trucks and the like.
However, it has been pointed out that when, in this two-coat coating process with the intermediate coating step being omitted, a top coat is formed directly on an electrodeposition coat, the phenomenon of peeling may possibly occur at the interface between the electrodeposition coat and the top coat.
This is considered to be due to the following phenomenon: in the case of ordinary multilayer coating films formed from an electrodeposition coat, an intermediate coat and a top coat, light rays which have entered the coating film are intercepted by the intermediate coat and can hardly penetrate further into the depth, but in the case of multilayer coating films having no intermediate coat, light rays that have entered the coating film can partly reach the electrodeposition coat and cause photodegradation of the electrodeposition coat surface. This phenomenon is remarkable in the case of blue-based pale solid colors and metallic colors, which show high light transmissivity.
Generally, the addition of an ultraviolet absorber or light stabilizer or the like means is employed to prevent ultraviolet degradation of coating films. However, photodegradation of such an additive itself or release thereof from the coating film with the lapse of time results in lessening of the ultraviolet degradation preventing effect, so that the durability of the effect is not so satisfactory.
Accordingly, it is urgently demanded that the multilayer coating film in particular having no intermediate coat show improved weather resistance as the whole multilayer coating film including an electrodeposition coat.
SUMMARY OF THE INVENTION
In view of the current situation mentioned above, the present invention has its primary object to provide a multilayer coating film whose electrodeposition coat can show increased weather resistance to secure sufficient weather resistance as a whole despite no intermediate coat, without allowing the interfacial peeling phenomenon between the top coat and the electrodeposition coat.
The present invention provides a multilayer coating film which comprises:
a cationic electrodeposition coat formed from an epoxy-based cationic electrocoating composition containing 5 to 300 millimoles of sulfonium group and 50 to 2,000 millimoles of carbon-carbon unsaturated bond per 100 g of the resin solid content, with carbon-carbon triple bond accounting for at least 15% of said carbon-carbon unsaturated bond; and
a top coat formed thereon.
DETAILED DESCRIPTION OF THE INVENTION
The multilayer coating film of the present invention comprises a cationic electrodeposition coat formed from an epoxy-based cationic electrocoating composition, and a top coat formed thereon. Said cationic electrodeposition coat is formed by electrodeposition of said epoxy-based cationic electrocoating composition, followed by baking for curing. Said top coat is formed by application of a top coating composition, followed by baking for curing.
The epoxy-based cationic electrocoating composition (hereinafter referred to as “the above cationic electrocoating composition” for short), which is to be used in the formation of the multilayer coating film of the present invention, contains sulfonium group and carbon-carbon unsaturated bond.
Said sulfonium group is a hydratable functional group in the above cationic electrocoating composition. When applying a voltage or current at or above a certain level during the electrocoating process, the sulfonium group undergoes electrolytic reduction on an electrode, whereupon the ionic group disappears to form a sulfide, so that it can irreversibly become nonconductor, as illustrated below. It can be considered that owing to this fact the above cationic electrocoating composition displays a high level of throwing power.
It can also be considered that, during this electrocoating process, electrode reaction is induced and the resulting hydroxide ion is kept by the sulfonium group, thus electrolytically generating a base in the electrodeposition coat. This electrolytically generated base can convert the propargyl group occurring in the electrodeposition coat, which has low thermal reactivity, to allene bond, which has high thermal reactivity.
The sulfonium group content is 5 to 300 millimoles per 100 g of the resin solid content of the above cationic electrocoating composition. When it is less than 5 millimoles per 100 g, sufficient throwing power or curability cannot be obtained and, further, the hydratability and bath stability will be poor. When it is above 300 millimoles per 100 g, the deposition of coats on the surface of articles becomes poor. A preferred range is 10 to 250 millimoles, more preferably 10 to 150 millimoles, per 100 g of the resin solid content of the above cationic electrocoating composition.
Said carbon-carbon unsaturated bond is carbon-carbon double bond or triple bond. This carbon-carbon unsaturated bond may occur terminally in the molecule of the basic resin of the above cationic electrocoating composition, or somewhere within the molecular chain constituting the skeleton of said basic resin. Said carbon-carbon unsaturated bond functions as a curing functional group and can also lead to improvement in weather resistance. The definite reason therefor is not so clear. However, it is conceivable that it shows radical trapping action against radicals formed in the coating film. When it coexists with sulfonium group, it can improve the throwing power of the above cationic electrocoating composition further, although the reason is not known.
The content of said carbon-carbon unsaturated bond is 50 to 2,000 millimoles per 100 g of the resin solid content of the above cationic electrocoating composition. When it is less than 50 millimoles per 100 g, sufficient weather resistance or curability cannot be obtained and the throwing power will also be insufficient. When it is above 2,000 millimoles per 100 g, the hydration stability will be adversely affected when used as a cationic electrocoating composition and the deposition of coats on the surface of articles will become poor. A preferred range is 80 to 1,000 millimoles, more preferably 80 to 500 millimoles, per 100 g of the resin solid content of the above cationic electrocoating composition.
At least 15%, in number, of said carbon-carbon unsaturated bond should be accounted for by carbon-carbon triple bond. This condition is critical since the curability and weather resistance become insufficient when the carbon-carbon triple bond is less than 15%, in number, of the carbon-carbon unsaturated bond.
For example, even when introducing a compound having a plurality of carbon-carbon double bonds per molecule, such as a long-chain unsaturated fatty acid, the content of the carbon-carbon unsaturated bond is expressed in terms of the content of said compound itself having a plurality of carbon-carbon double bonds. This is because even if a compound having a plurality of carbon-carbon double bonds per molecule is introduced, substantially only one carbon-carbon double bond among them is presumably involved in radical trapping or curing reactions, rather than the plurality of carbon-carbon double bonds being equally involved.
The above cationic electrocoating composition is of the epoxy type. In the present specification, the term “epoxy type” means that the basic resin of the electrocoating composition has a skeleton of epoxy resin. Said epoxy resin is not particularly restricted but is preferably a polyepoxide having at least two
Kawakami Ichiro
Saito Takao
Sakamoto Hiroyuki
Nippon Paint Co. Ltd.
Paulraj Christopher
Pollock Vande Sande & Amernick
Thibodeau Paul
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