Stock material or miscellaneous articles – Structurally defined web or sheet – Physical dimension specified
Reexamination Certificate
1998-05-27
2001-04-17
Short, Patricia A. (Department: 1712)
Stock material or miscellaneous articles
Structurally defined web or sheet
Physical dimension specified
C428S458000, C428S460000, C427S327000, C427S385500, C427S409000, C525S443000, C525S519000
Reexamination Certificate
active
06217994
ABSTRACT:
This invention relates to a polyester resin-based paint composition which is capable of forming paint film of favorable hardness, formability, curability and adhesion, exhibits high resistance to pinholes, and enables thick layer coating; in particular, relates to a paint composition suitable to be applied to produce precoat-painted steel sheet; and relates also to coated metal sheets onto which the paint composition was applied.
Conventionally, as finish paint for precoated steel sheet for use as a construction material or for into processing furnitures, coating compositions in which ployester resins are used as the base and a methyl etherified and/or butyl etherified methylolated melamine resin or blocked polyisocyanate compound is blended as crosslinking agent, are frequently used because of their well balanced properties such as hardness, processability and the like. However, they are subject to such a problem that the dry thickness of the finish paint film normally is no more than 20 &mgr;m, because baking of the paint film in the occasion of finish painting precoated sheet steel is conducted at high temperatures for a short time, and when the thickness exceeds said limit, popping occur due to foaming in the film. If thick film can be formed without popping, such thick film is advantageous in respect of durability properties such as corrosion resistance, weather resistance and the like. Thus, a paint composition capable of forming a thick film by single application has been in demand.
The object of the present invention is to provide a polyester resin-based paint composition which is capable of forming a paint or coating film of favorable hardness, formability, curability and adhesion; exhibits high resistance to popping and enables thick layer painting.
We have discovered that the above object can be achieved by a polyester resin-based paint composition in which, as a crosslinking agent, a methylolated melamine resin, which has been etherified with a specific alkyl group, and a curing catalyst are combined with a specific polyester resin, and whereupon completed the present invention.
Namely, the present invention provides a paint composition which is characterized by comprising a film-forming resin component which comprises
(A) a hydroxyl-containing polyester resin having a number average molecular weight of from 1,500 to 35,000, a glass transition temperature of from −30° C. to 60° C. and a hydroxyl value of from 3 to 100 mg KOH/g 65-95 parts by weight
(B) a melamine resin crosslinking agent which is a methylolated melamine resin whose methylol groups have been alkyl etherified, at least 20% of the total number of methylol groups having been etherified with a C
6
-C
12
alkyl group 5-35 parts by weight and
(C) 0.1-2.0 parts by weight of a curing catalyst, per 100 parts by weight of total sum of said resin component.
The invention also provides coated metal sheet which is characterized by being coated with the above-specified paint composition on a metal sheet, with or without a primer film therebetween, in such a manner that the thickness of the cured paint film ranges from 21 to 40 &mgr;m.
The paint composition of the present invention is explained in further details hereinafter.
Polyester Resin (A):
Polyester resin (A) which is the main constituent of the film-forming resin component in the composition of the present invention is a polyester resin containing hydroxyl groups, examples of which including oil free polyester resin, oil modified alkyd resin, their modifications, e.g., urethane modified polyester resin, urethane modified alkyd resin, epoxy modified polyester resin, acryl modified polyester resin, and the like.
Said oil free polyester resin is composed of a polycondensation product of a polybasic acid component and polyhydric alcohol component. As the polybasic acid component, one or more dibasic acids selected from, for example, phthalic anhydride, isophthalic acid, terephthalic acid, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, succinic acid, fumaric acid, adipic acid, sebacic acid and maleic anhydride; and their reactive derivatives such as lower alkyl esters thereof, are mainly used, and if necessary monobasic acid such as benzoic acid, crotonic acid, p-t-butylbenzoic acid or the like and trivalent or higher polybasic acid such as trimellitic anhydride, methylcyclohexene tricarboxylic acid, pyromellitic anhydride, and the like, are concurrently used. As the polyhydric alcohol component, dihydric alcohol such as ethylene glycol, diethylene glycol, propylene glycol, 1,4-butanediol, neopentyl glycol, 3-methylpentanediol, 1,4-hexanediol, 1,6-hexanediol and the like are mainly used, and if necessary trihydric or higher alcohol such as glycerin, trimethylolethane, trimethylolpropane, pentaerythritol and the like can be concurrently used. These polyhydric alcohols can be used either singly or as mixtures of more than one kind. Esterification or ester interchange reaction of the two components can be conducted by the means known per se. As the acid component, isophthalic acid, terephthalic acid, and lower alkyl esters of these acids are particularly preferred.
A hydroxyl-containing oil free polyester resin can be obtained through a polycondensation reaction of the polybasic acid component with a polyhydric alcohol component at such a ratio, eg., that the hydroxyl groups in the polyhydric alcohol component become excessive to the carboxyl groups (an acid anhydride group is calculated as two carboxyl groups, as done throughout in this specification) in the polybasic acid component.
Oil modified alkyd resin is formed by reacting, in addition to said polybasic acid component and polyhydric alcohol component of above oil free polyester resin, an oil fatty acid by a method known per se. Examples of usable oil fatty acid include coconut oil fatty acid, soy bean oil fatty acid, linseed oil fatty acid, safflower oil fatty acid, tall oil fatty acid, dehydrated castor oil fatty acid and tung oil fatty acid. It is generally preferred to keep the oil length in the alkyd resin to be not more than 30%, in particular, within a range of 5 to 20%.
Hydroxyl-containing oil modified alkyd resin can be obtained, for example, by reacting the three components at such ratios that the hydroxyl groups in the polyhydric alcohol component become excessive to the total carboxyl groups in the polybasic acid component and the oil fatty acid.
As urethane modified polyester resin, one formed by reacting above-described oil free polyester resin or a low molecular weight oil free polyester resin which has been formed by reacting the acid component and the alcohol component used in the occasion of making the oil free polyester resin, with a polyisocyanate compound by a method known per se. Said urethane modified alkyd resin, furthermore, encompasses reaction products of said alkyd resin or a low molecular weight alkyd resin which has been obtained by reacting the named components used in the occasion of making the alkyd resin, with a polyisocyanate compound by a method known per se. As examples of polyisocyanate compound useful in the occasion of making said urethane modified polyester resin and urethane modified alkyd resin include hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, 4,4′-methylenebis (cyclohexylisocyanate) and 2,4,6-triisocyanatotoluene. As the urethane modified polyester resin and alkyd resin, generally those whose extent of modification is such that the amount of the polyisocyanate compound forming the urethane modified resin is not more than 30% by weight, in particular, 2 to 10% by weight, to the urethane modified resin, are preferred.
Hydroxyl-containing urethane modified polyester resin and hydroxyl-containing urethane modified alkyd resin can be prepared by, for example, having a low molecular weight oil free polyester or alkyd resin which is to be reacted with polyisocyanate compound contain hydroxyl groups in advance by aforesaid means, and reacting such low
Kansai Paint Co. Ltd.
Short Patricia A.
Wenderoth , Lind & Ponack, L.L.P.
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