Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Forming nonmetal coating
Patent
1996-05-23
1998-10-13
McGinty, Douglas J.
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Forming nonmetal coating
205159, 205324, 205162, 205322, 205164, 205323, 205414, 205431, 205435, 205440, 205434, 252500, 427487, 427 58, 4273855, 4274301, 524600, 528188, 525199, C09D17908, C09D 544, C09D12712, C08L 7908, C08L 2712
Patent
active
058207422
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an electrodeposition coating method of a water soluble polyimide composition and an electrodeposition coating agent comprising a water soluble polyimide composition.
BACKGROUND OF THE INVENTION
A polyimide composition is used in various fields, such as for a coating agent, adhesive, sealant and fiber for the reason of being excellent in thermal resistance, chemical stability and electrical property, and a large commercial success has been obtained. Particularly, there are many cases where the polyimide composition is used for coating by utilizing its excellent film forming property, and in general the coating has been carried out through spray coating and dip coating methods. In case of spray coating, there are problems such that an adhesion efficiency is lowered by about 30% as compared with other coating methods and unevenness of the coating is easy to occur. In case of dip coating, there are problems such that cracks of the coating occur on the thickened parts and that sharp-pointed portions (edge, end of pin) are difficult to be coated.
Electrodeposition coating method is an effective method to solve the above-mentioned problems. As a method to use a polyimide composition for electrodeposition coating, there can be considered a method to use, as a water soluble composition, a polyamic acid which is a polyimide precursor and is prepared usually by using an aromatic tetracarboxylic acid such as pyromellitic acid anhydride and a diamine. However particularly in case of solubilizing the aromatic polyimide precursor in water through the polymerization reaction, it is difficult to increase a molecular weight due to imidation reaction. Therefore the baked film has neither strength nor elongation and thus suffers damage easily.
Also though there are examples of electro-deposition coating of a polyimide polymer in some literatures and patent specifications, present situation is such that those coating methods have not yet reached practicable level because, even if electrodeposition is successfully carried out, a film cannot be obtained after baking, or has problems in continuity, flexibility and thermal resistance.
The present inventors have made intensive studies to solve the above-mentioned problems, and as a result, have found that to make it possible to enhance physical properties of the coating film of a water soluble polyimide in electrodeposition coating much more than that of the conventional coating methods and to enhance stability of an aqueous solution, it is preferable to use a composition in which a polyamic acid is prepared by using 1,2,3,4-butanetetracarboxylic acid or its imide-forming derivative and a nearly equivalent mole of diamine and neutralized with ammonia or the like to be water-solubilized. However though the thus prepared polyamic acid can be used for electrodeposition coating, there are drawbacks such that, after baking, the baked coating becomes in the form of mottle or spot, thus the baked film cannot be continuous, and is lacking in uniformity, adhesivity to a substrate and toughness.
The present invention can provide an electrodeposition coating method of a water soluble polyimide composition free of the above-mentioned drawbacks and an electrodeposition coating agent comprising a water soluble polyimide. Therefore objects of the present invention are to provide an electrodeposition coating method being practicably useful for a water soluble polyimide composition which makes it possible to obtain a continuous coating film and form a uniform baked film being excellent in adhesivity to a substrate and toughness, and to provide an electrodeposition coating agent.
DISCLOSURE OF THE INVENTION
(1) An electrodeposition coating method, characterized in that in electrodeposition coating of a water soluble composition of a polyimide precursor which is obtained from 1,2,3,4-butanetetracarboxylic acid or its imide-forming derivative and a diamine and has a percentage of residual acid value of 30 to 3%, there is added previously a water
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Hamabuchi Kazufumi
Hayashi Kenjiro
Mizuno Toshio
Oda Shin-i-ti
Okamoto Toshihide
Daikin Industries Ltd.
McGinty Douglas J.
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