Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2000-11-21
2002-11-05
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S506000, C523S415000
Reexamination Certificate
active
06475366
ABSTRACT:
The present invention relates to a cationically electrodepositable coating composition, more specifically to a cationically electrodepositable coating composition which comprises a specific blocked polyisocyanate cross-linking agent and has a low heating loss, a low gum soot property and a low temperature-curing property and which can form an electrodeposited coating film having a good coated surface smoothness.
Known as a cationically electrodepositable coating material is a thermosetting type cationically electrodepositable coating material comprising an amine-added epoxy resin and a blocked polyisocyanate as a cross-linking agent, and those obtained by blocking aromatic or aliphatic polyisocyanates with ether alcohol base blocking agents such as ethylene glycol butyl ether have so far been used as the blocked polyisocyanate from the viewpoints of a corrosion resistance of the coating film formed and a stability of the coating material. However, a cationically electrodepositable coating material using a polyisocyanate blocked with an ether alcohol base blocking agent as a cross-linking agent has the problems that a lot of gum and soot due to a heating loss content (a weight loss proportion of a coating film in curing by baking) are produced in a line oven and that the baking temperature is high.
On the other hand, a cationically electrodepositable coating material using a polyisocyanate blocked with a blocking agent of an oxime base such as methyl ethyl ketoxime and hexanone ketoxime as a cross-linking agent provides a coating film which can be cured at a relatively low temperature but has the problem that an aging stability of the coating material and a corrosion resistance of the coating film are inferior.
An object of the present invention is to provide a cationically electrodepositable coating material which has a low heating loss property and a low temperature-curing property and which is excellent in a stability as a coating material and a corrosion resistance of the coating film.
The present inventors have found that the object described above can be achieved by using a blocked isocyanate as an external or internal cross-linking agent component, which is formed by using some kind of a specific amide compound as a blocking agent, and they have come to complete the present invention.
Thus, the present invention provides a cationically electrodepositable coating composition (hereinafter referred to as the coating composition A of the present invention) comprising as a cross-linking agent, a blocked polyisocyanate having at least 0.1 blocked isocyanate group represented by Formula (I):
wherein R
1
represents a hydrogen atom, methyl, ethyl or propyl, and R
2
represents methyl or ethyl.
Further, the present invention provides a cationically electrodepositable coating composition (hereinafter referred to as the coating composition B of the present invention) comprising as a base resin, an active hydrogen-containing cationic resin having at least 0.1 blocked isocyanate group per molecule on the average represented by Formula (I) described above.
The coating compositions of the present invention shall be explained below in further details.
Coating Composition A of the Present Invention
The coating composition A of the present invention is a cationically electrodepositable coating composition comprising a cationic resin having an active hydrogen group capable of reacting an isocyanate group as a base resin and the blocked polyisocyanate having the blocked isocyanate group represented by Formula (I) described above as an external cross-linking agent.
Capable of being used as the active hydrogen-containing cationic resin used as the base resin in the coating composition A of the present invention are, for example, conventional cationically electrodepositable coating resins having an active hydrogen group such as a primary or secondary amino group and a hydroxyl group and a cationic group such as a primary amino group, a secondary amino group, a tertiary amino group and a quaternary ammonium group (when this cationic group contains active hydrogen, it can double as the active hydrogen group), which is required for making the resin water-soluble or water-dispersible, for example, resins of an epoxy base, an acryl base, a polybutadiene base, an alkid base and a polyester base. Among them, amine-added epoxy resins are particularly suitable.
The above amine-added epoxy resin includes, for example, (1) adducts of polyepoxide compounds to primary mono- and polyamines, secondary mono- and polyamines or primary and secondary mixed-polyamines (refer to, for example, U.S. Pat. No. 3,984,299); (2) adducts of polyepoxide compounds to secondary mono- and polyamines having a primary amino group which is reduced to ketimine (refer to, for example, U.S. Pat. No. 4,017,438); and (3) reaction products obtained by etherification reaction of polyepoxide compounds with hydroxyl compounds having a primary amino group which is reduced to ketimine (refer to, for example, Japanese Patent Application Laid-Open No. 43013/1984).
The polyepoxide compound used for producing the amine-added epoxy resin described above is a compound having at least two epoxy groups in a molecule and is suitably a compound having a number average molecular weight falling in a range of usually at least 200, preferably 400 to 4000 and more preferably 800 to 2000. In particular, a compound obtained by reacting a polyphenol compound with epichlorohydrin is preferred. The polyphenol compounds which can be used for producing the above polyepoxide compounds include, for example, bis(4-hydroxyphenyl)-2,2-propane, 4,4-dihydroxy-benzophenone, bis(4-hydroxyphenyl)-1,1-ethane, bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxy-tert-butyl-phenyl)-2,2-propane, bis(2-hydroxynaphthyl)methane, tetra(4-hydroxyphenyl)-1,1,2,2-ethane, 4,4-dihydroxy-diphenylsulfone, phenol novolak and cresol novolak.
The above polyepoxide compounds may be those reacted partly with polyols, polyetherpolyols, polyesterpolyols, polyamideamines, polycarboxylic acids and polyisocyanate compounds. Further, they may be those graft-polymerized with &egr;-caprolactone and acryl monomers.
The blocked polyisocyanate used as a cross-linking agent in the coating composition A of the present invention can be produced by blocking at least one isocyanate group of a polyisocyanate compound having at least two isocyanate groups (NCO) in a molecule with an amide compound represented by the following Formula (II):
R
1
—CO—NH—R
2
(II)
wherein R
1
and R
2
are synonymous with those defined above.
The polyisocyanate described above includes, for example, aromatic, aliphatic or alicyclic polyisocyanate compounds such as tolylenediisocyanate, xylilenediisocyanate, phenylenediisocyanate, bis-(isocyanatemethyl)cyclohexane, tetramethylenediisocyanate, hexamethylenediisocyanate, methylenediisocyanate and isophoronediisocyanate, and terminal isocyanate group-containing compounds obtained by reacting excess amounts of these polyisocyanate compounds with low molecular active hydrogen-containing compounds such as ethylene glycol, propylene glycol, trimethylolpropane, hexanetriol and castor oil. Among these polyisocyanate compounds, suited are aromatic diisocyanates, particularly diphenylmethane-2,4′-diisocyanate, diphenylmethane-4,4′-diisocyanate (usually called MDI) and a mixture (usually called crude MDI) of diphenylmethane-4,4′-diisocyanate with diphenylmethane-2,4′-diisocyanate and polymethylenepolyphenylisocyanate.
The amide compound represented by Formula (II) described above which is used for blocking these polyisocyanate compounds includes, for example, N-methylacetamide, N-ethylacetamide, N-methylpropionamide and N-methylformamide, and they can be used alone or in combination of two or more kinds thereof. Substantially all isocyanate groups of the polyisocyanate compound are preferably blocked with the above amide compound in order to reduce a heating loss, but a part of the isocyanate groups may be blocked, if necessary, with other conventional block
Kamikado Koji
Nishiguchi Shigeo
Kansai Paint Co. Ltd.
Mayekar Kishor
Wenderoth , Lind & Ponack, L.L.P.
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