Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2001-10-25
2003-04-22
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S486000
Reexamination Certificate
active
06551482
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of forming a multilayer coating film utilizing a three-wet coating system which comprises applying a water-borne intermediate coating, a water-borne base coating and a clear coating, in that order, onto an electrodeposited coating film formed on an article to be coated in the wet-on-wet manner and curing simultaneously by heating. More specifically, it relates to a method of forming a multilayer coating film by which the multilayer coating film excellent in chipping resistance and showing no tendency toward yellowing can be obtained, and to a multilayer coating film obtained by this method.
BACKGROUND ART
In recent years it has been urgently demanded that the coating process be curtailed in the field of coatings, in particular in the field of automobile coatings, so that the problems of saving resources, reducing costs and minimizing environmental impacts (VOC and HAPs, etc.) may be solved. In the conventional procedure for finishing coating automobiles, the three-coat three-bake coating technique has been used, namely the electrodeposited coating film, intermediate coating film and top coating film are baked each time after application of each corresponding coating. In recent years, however, it has been demanded that those coating film performance characteristics which can be exhibited by the three-coat films obtained by the conventional three-coat three-bake coating technique be acquired by employing the three-wet coating system according to which the three coating steps, namely intermediate coating, base coating and clear coating, are carried out in the wet-on-wet manner following the step of electrodeposition coating and baking of the electrodeposited coating film and the resulting wet coating films are baked simultaneously, while thereby reducing the number of baking process steps.
Among the coating film performance characteristics referred to above, the shock resistance, in particular the so-called chipping resistance to collision of pebbles or like obstacles with the car body during running, can be secured by the conventional three-coat three-bake coating technique, for example by providing a particular intermediate coating film having chipping resistance. In the three-wet coating system mentioned above, however, the conventional intermediate coatings cannot be used since the coating films obtained tend to be impaired in appearance due to such troubles as blurring or layer inversion. The coating films obtained by the three-wet coating system are disadvantageous in that they are low in shock resistance and chipping resistance.
Japanese Kokai Publication Sho-62-65765 discloses a technique according to which a resin layer capable of absorbing a shock to coating films (the so-called anti-chipping primer layer) is provided during multilayer film formation, in particular between the electrodeposited coating film and intermediate coating film. However, further incorporation of such a step in the car body coating process is against the market needs for curtailment of the process and for cost saving mentioned above.
On the other hand, water-borne coatings have attracted attention in recent years in the field of coatings, in particular in the field of automobile coatings, since they can contribute to reductions in environmental impacts (e.g. VOC). Water-borne coatings are generally prepared by solubilizing, dispersing or emulsifying a coating film-forming resin in water and/or a hydrophilic solvent such as an alcohol. When the coating film-forming resin has an anionic functional group, such as carboxyl group, introduced therein, a basic substance such as an amine is used as the counter ion and, therefore, the use of such water-borne coatings in the above-mentioned three-wet coating system produces the problem of yellowing of the coating films obtained.
Accordingly, it is an object of the present invention to provide a method of forming a multilayer coating film by which the multilayer coating film comparable in shock resistance, in particular chipping resistance, to the conventional three-coat films and resistant to yellowing and having a good appearance can be formed in the three-wet coating system which is intended for coating process curtailment, cost reduction and environmental impact reduction.
SUMMARY OF THE INVENTION
The present invention provides a method of forming a multilayer coating film comprising
the step (I) of coating an article to be coated with an electrodeposition coating followed by curing by heating to form an electrodeposited coating film,
the step (II) of applying a water-borne intermediate coating onto said electrodeposited coating film to form an uncured intermediate coating film,
the step (III) of applying a water-borne base coating onto said intermediate coating film to form an uncured base coating film,
the step (IV) of applying a clear coating onto said base coating film to form an uncured clear coating film and
the step (V) of curing said intermediate coating film, said base coating film and said clear coating film simultaneously by heating to thereby obtain a multilayer coating film,
wherein said electrodeposition coating contains a particle A containing a resin (a) whose solubility parameter is &dgr;a as well as a particle B containing a curing agent and a resin (b) whose solubility parameter is &dgr;b and satisfies that
(1) the value of (&dgr;b −&dgr;a) is not less than 1.0,
(2) as regards the electrodeposited coating film formed from said electrodeposition coating, the resin film formed from said particle A shows a dynamic glass transition temperature of −110 to 10° C. and
the coating film obtained by film formation from said particle A alone shows an elongation percentage of not less than 200% and
(3) as regards the electrodeposited coating film formed from said electrodeposition coating, the resin film formed from said particle B shows a dynamic glass transition temperature of 60 to 150° C., and
wherein the total amount of volatile basic substance in said uncured intermediate coating film and said uncured base coating film prior to carrying out the step (V) is not more than 7×10
−6
mmol per coating film unit area (1 mm
2
).
The invention is also directed to a multilayer coating film which is obtained by the above-mentioned method of forming a multilayer coating film.
DISCLOSURE OF THE INVENTION
The method of forming a multilayer coating film according to the present invention comprises the step (I) of coating an article to be coated with an electrodeposition coating followed by curing by heating to form an electrodeposited coating film, the step (II) of applying a water-borne intermediate coating onto the above electrodeposited coating film to form an uncured intermediate coating film, the step (III) of applying a water-borne base coating onto the above intermediate coating film to form an uncured base coating film, the step (IV) of applying a clear coating on to the above base coating film to form an uncured clear coating film and the step (V) of curing the above intermediate coating film, the above base coating film and the above clear coating film simultaneously by heating to thereby obtain a multilayer coating film.
Step (I)
In the method of forming a multilayer coating film according to the invention, the above step (1) comprises applying an electrodeposition coating on an article to be coated, followed by curing by heating to form an electrodeposited coating film.
Electrodeposition Coating
The above electrodeposition coating contains a particle A containing a resin (a) whose solubility parameter is &dgr;a as well as a particle B containing a curing agent and a resin (b) whose solubility parameter is &dgr;b and in which
(1) the value of (&dgr;b −&dgr;a) is not less than 1.0,
(2) as regards the electrodeposited coating film formed from the above electrodeposition coating, the resin film formed from the above particle A shows a dynamic glass transition temperature of −110 to 10° C.,
and the coating film obtained by film formation from the above pa
Toi Teruzo
Yamamoto Takefumi
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