Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2002-04-05
2003-11-11
Seidleck, James J. (Department: 1711)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S471000, C204S484000, C204S487000
Reexamination Certificate
active
06645363
ABSTRACT:
The present invention relates to a method in which a cationic electrodeposition paint and a water based intermediate coat paint are applied by wet-on-wet and then heated to cross-link and cure both paint films at the same time, that is, coating is carried out by a so-called “2 coat 1 bake” system (2C1B) to form a multi-layer paint film, particularly to a method for forming a multi-layer paint film which is improved in a curing property, a chipping resistance, a corrosion resistance, a finishing appearance (smoothness, gloss feeling and the like) and an interlayer adhesive property between both paint films.
It is known to apply a cationic electrodeposition paint containing a block polyisocyanate compound as a cross-linking agent and a water based intermediate coat paint containing an amino resin as a cross-linking agent in order on a conductive coated article such as an outside plate of a car body by wet-on-wet and then heat and cure both paint films at the same time by heating to form a multi-layer paint film. However, this multi-layer paint film does not have a satisfactory finishing appearance such as smoothness, gloss feeling and the like, and it is difficult to solve this defect even by further applying a top coat paint. Further, automobiles using an outside plate on which such multi-layer paint film is formed have the problem that when small stones sent flying during running hit against such multi-layer paint film, chipping peeling is liable to be caused in a layer between both paint films.
An object of the present invention is to solve the problems described above in a method for applying a cationic electrodeposition paint and a water based intermediate coat paint by 2C1B to form a multi-layer paint film.
Intensive researches repeated by the present inventors have resulted in finding that the object described above can be achieved by using a block polyisocyanate compound as a cross-linking agent for both cationic electrodeposition paint and water based intermediate coat paint and adding a specific amount of a tin base catalyst to the cationic electrodeposition paint, and they have come to complete the present invention.
Thus, according to the present invention, provided is a method for forming a multi-layer paint film, comprising:
applying a cationic electrodeposition paint (A) containing a block polyisocyanate compound as a cross-linking agent on a surface of a substrate,
applying a water based intermediate coat paint (B) containing a block polyisocyanate compound as a cross-linking agent on an electrodeposition paint film surface without curing the electrodeposition paint film to form an intermediate coat paint film, and then
heating to cure both of the electrodeposition paint film and the intermediate coat paint film at the same time to thereby form a multi-layer paint film,
wherein the above cationic electrodeposition paint (A) contains a tin base catalyst of at least 10 parts by weight per 100 parts by weight of the resin solid matter, and a cross-linking curing reaction in the electrodeposition paint film is controlled so that it starts earlier than a cross-linking curing reaction in the intermediate coat paint film.
In the present invention, a cross-linking curing-starting time in the paint films of the cationic electrodeposition paint (A) and the water based intermediate coat paint (B) can be determined by means of a pendulum type visco-elasticity measuring device (LEOVI-BRON DDV-OPA type, manufactured by Toyo Boldwin Co., Ltd.). To be specific, a pendulum having a weight of 22 g and an inertia moment of 850 g·cm
2
is used, and this pendulum is put on an uncured paint film of a paint applied on a steel plate so that the cured paint film has a film thickness of 30 &mgr;m. The above paint film is heated at a prescribed temperature (for example, 140 to 180° C.) for cross-linking and curing the paint film while vibrating the pendulum, and the time when a value of a logarithmic decrement of the pendulum starts going up is referred to as “cross-linking curing-starting time”. In this case, the time spent from commencement of heating up to cross-linking curing-starting time is referred to as “curing-starting time”, and the shorter time thereof means that “a cross-linking curing reaction starts earlier”. The cross-linking curing-starting times of both paint films of the cationic electrodeposition paint (A) and the water based intermediate coat paint (B) are compared on the basis of results obtained by measuring at he same heating temperature.
The method of the present invention for forming a multi-layer paint film shall be explained below in further details.
Cationic Electrodeposition Paint (A):
The cationic electrodeposition paint (A) contains a block polyisocyanate compound as a cross-linking agent and further contains a tin base catalyst of at least 10 parts by weight per 100 parts by weight of the resin solid matter contained in the above paint. The cationic electrodeposition paint containing a base resin (A-1) having a hydroxyl group and a cationic group, a block polyisocyanate compound (A-2) and a tin base catalyst (A-3) can suitably be used.
The hydroxyl group contained in the base resin (A-1) takes part in a cross-linking reaction with the block polyisocyanate compound (A-2), and the cationic group takes part in the formation of a stable aqueous dispersion.
Such base resin (A-1) includes, for example, (1) reaction products of epoxy resins with cationizing agents; (2) products obtained by protonating polycondensation products (refer to U.S. Pat. No. 2,450,940) of polycarboxylic acids and polyamines with acids; (3) products obtained by protonating polyaddition products of polyisocyanate compounds, polyols and mono- or polyamines with acids; (4) products obtained by protonating copolymers of acryl base or vinyl base monomers containing a hydroxyl group and an amino group with acids (refer to Japanese Patent Publication No. 12395/1970 and Japanese Patent Publication No. 12396/1970); and (5) products obtained by protonating adducts of polycarboxylic acids to alkylene-imines with acids (refer to U.S. Pat. No. 3,403,088).
Among them, particularly a base resin, which is included in the category (1), obtained by reacting a cationizing agent with an epoxy resin obtained by reacting a polyphenol compound with epichlorohydrin is preferred because of an excellent corrosion resistance thereof.
Capable of being used as the epoxy resin described above are conventionally known resins which have at least two epoxy groups in a molecule and which have a number average molecular weight falling in a range of 400 to 4000, preferably 800 to 2000 and an epoxy equivalent falling in a range of 190 to 2000, preferably 400 to 1000.
The polyphenol compound used for preparing the above epoxy resin includes, for example, bis(4-hydroxyphenyl)-2,2-propane, 4,4′-dihydroxybenzophenone, bis(4-hydroxyphenyl)-1,1-ethane, bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxy-tert-butyl-phenyl)-2,2-propane, bis(2-hydroxybutyl)methane, 1,5-dihydroxynaphthalene, bis(2,4-dihydroxyphenyl)methane, tetra(4-hydroxyphenyl)-1,1,2,2-ethane, 4,4′-dihydroxydiphenyl ether, 4,4′-dihydroxydiphenyl sulfone, phenol novolak and cresol novolak. Further, capable of being used as such epoxy resin are resins modified with polyhydric alcohols, polyether polyols, polyester polyols, polyamideamines, polycarboxylic acids and polyisocyanate compounds, and resins subjected to graft polymerization with acryl base monomers.
Capable of being used as the cationizing agent are, for example, amine compounds such as primary amines, secondary amines, tertiary amines and polyamines, and they are reacted with epoxy groups to introduce cationic groups such as a secondary amino group, a tertiary amino group and a quaternary ammonium base into the epoxy resins, whereby cationic group-containing resins can be prepared.
A hydroxyl group can be introduced into the cationic group-containing resin by a reaction with alkanolamines as a cationizing agent and a reaction with a ring-opening product of caprolactone which is sometimes intr
Hiraki Takayoshi
Kasari Akira
Tomizaki Yasuhiro
Kansai Paint Co. Ltd.
Seidleck James J.
Tran Thao
Wenderoth , Lind & Ponack, L.L.P.
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