Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2000-10-31
2003-02-11
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C423S415100
Reexamination Certificate
active
06517695
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to cathodic electrocoats which are thermally curable at low baking temperatures. More particularly, the invention relates to cathodic electrocoats comprising an epoxy-based resin and an oxime-blocked isocyanate crosslinker capable of curing at temperatures as low as 93° C.
BACKGROUND AND SUMMARY OF THE INVENTION
Electrodeposition coatings have found utility for the corrosion protection of metallic objects ranging from automobile parts to appliances, to a wide range of industrial fixtures. Such coatings are typically thermally cured at bake temperatures ranging from 150° C. to 200° C. Factors which would make it desirable to cure at significantly lower temperatures include energy savings and the ability to electrocoat heavy metal castings or metal configured with plastic parts without risk of deformation of the plastic due to heating at high temperatures.
The present invention relates to a low-temperature curing cathodic electrocoat composition which comprises an epoxy-based resin containing primary amine and hydroxyl functionality, an acid solubilizer in an amount greater than the stoichiometric equivalent of amine functionality in the resin, and an oxime-blocked aromatic isocyanate crosslinker and a method for application of this composition. Surprisingly, this composition and method result in high degrees of cure at baking temperatures as low as 93° C. without use of external catalysts and yet provide long term stability of the isocyanate crosslinker in the presence of alcoholic co-solvents in the electrocoat bath prior to application of the electrocoat. The coating composition of the present invention exhibits excellent cure response, impact resistance, and hardness.
Numerous U.S. Patents disclose the use of blocked isocyanates as crosslinkers for film forming compositions in the cathodic electrodeposition process. U.S. Pat. Nos. 3,894,922; 3,947,339; 3,984,299; 3,959,106; 4,017,438; 4,038,232; 4,031,050, 4,101,486; 4,134,816; 4,260,697; 4,297,255; 4,310,646; 4,393,179; 4,176,221; 4,182,831; 4,182,833; 4,225,478; 4,225,479; 4,339,369; 4,452,681; 4,452,930; and 4,452,963 disclose various blocked isocyanate crosslinkers which will deblock at temperatures below 160° C., such as oxime-blocked isocyanates. U.S. Pat. No. 4,596,744 discloses the use of an oxime-blocked isocyanate crosslinker in which the oxime blocking agents are the reaction products of acyclic aliphatic carbonyl-containing compounds having at least seven carbon atoms and a hydroxyl amine. U.S. Pat. Nos. 3,935,087; 3,947,338; 4,007,154; 4,009,133; 4,040,924; 4,081,343; 4,134,865; and 4,134,866 disclose the use of partially blocked isocyanate crosslinkers which will deblock at temperatures below 160° C. and which are reacted with base cationic resins that are useful in cathodic electrodeposition formulations.
Other patents disclose low temperature cured cathodic electrodeposited coatings which do not cure via oxime-blocked isocyanates. U.S. Pat. No. 4,435,559 discloses the use of beta-hydroxy urethane crosslinkers which are appreciably cured at 121° C. and produce excellent cures at 163° C. U.S. Pat. No. 4,440,612 discloses a coating comprising a polymeric polyol with a polyester crosslinking agent having at least two beta-alkoxyester groups per molecule which cures from about 150° C. to about 205° C. in 10 to 45 minutes. U.S. Pat. No. 4,452,930 teaches the use of glycolamides as blocking agents for low temperature curing (121° C. or greater) systems based on blocked isocyanates.
It has been desired for many years to develop cathodic electrodeposition products which will cure at low baking temperatures. The principal challenge in developing such electrocoat compositions is the inherent tendency of crosslinkers which deblock at low temperatures to undesirably prematurely react with other film-forming resins in the bath composition, with co-solvents in bath composition or with water used in the electrocoat bath composition. Many different approaches have been taken to overcome this problem, each with limitations. For example, U.S. Pat. Nos. 4,575,523 and 4,575,529 teach the use of oxime-blocked isocyanates in low temperature cure electrocoat bath compositions, but those compositions cure at higher temperatures than the compositions of the present invention. U.S. Pat. No. 4,872,961 teaches that a reduced curing temperature can be achieved using an isocyanate crosslinker that deblocks below 160° C., but the use of solvents which are non-reactive with the crosslinker is required. In the present invention, the presence of alcoholic co-solvents within the uncured deposited film does not inhibit cure of the coating such that films with good solvent resistance are provided even when formulated with significant levels of glycol ethers such as ethylene glycol monobutyl ether. Thus, the compositions of the present invention exhibit a high degree of cure at low temperature and provide long-term stability of the crosslinker in the electrocoat solution, even in the presence of alcoholic co-solvents, avoiding the limitations associated with conventional methods including other methods employing oxime-blocked isocyanates.
Accordingly, important aspects of the invention are that curing can be accomplished at low baking temperatures and in the presence of alcoholic co-solvents, such as glycol ethers, which are frequently highly desirable. Such co-solvents are desirable in that they are useful in the manufacture of resin intermediates, they improve the stability of many cationic resin dispersions, and they generally positively influence the appearance of the final cured paint film by improving smoothness and glossiness. A disadvantage of conventional methods of application of electrodeposition products containing blocked isocyanates, is that baking temperatures ranging from 163° C. to 200° C. are used to unblock the blocked isocyanates and begin crosslinking. At these temperatures alcoholic co-solvents are driven from the film via evaporation. Furthermore, in conventional methods the alcoholic co-solvent can compete with the polymer for reaction with the crosslinker to effectively reduce crosslink density. In contrast, in the present invention the primary amine functionality of the resin is highly salted reducing its reactivity and resulting in stabilization of the crosslinker which prevents reduction of crosslink density by the co-solvent. Thus, in the present invention alcoholic co-solvents are retained and inhibition of cure of the coating is minimized in the presence of alcoholic co-solvents resulting in films with good solvent resistance even when formulated with significant levels of commonly used glycol ethers such as ethylene glycol monobutyl ether.
In one embodiment of the present invention a low temperature curing cathodic electrodeposition coating composition is provided. The composition comprises an aqueous dispersion of an epoxy-based resin having primary amine and hydroxyl functionality, an acid solubilizer in an amount greater than the stoichiometric equivalent of amine functionality in the resin, and an oxime-blocked aromatic polyisocyanate crosslinker. In a preferred embodiment of the invention the epoxy resin is a bis-phenol A-based epoxy resin reacted with the ketimine of diethylene triamine. The pH of the electrodepositable coating composition is preferably maintained at a pH of from about 3.5 to about 5. The primary amine equivalent weight based on resin solids is from about 280 to about 4000 grams/equivalents of primary amine. The electrodepositable coating composition may further comprise alcoholic co-solvents, a monofunctional phenolic compound for use as a capping agent, an external catalyst to promote curing, a pigment, other cationic pigment dispersion resins, and/or other formulating additives known in the art.
Exemplary of aromatic polyisocyanate crosslinkers are such crosslinkers as diphenyl methane diisocyanate or toluene diisocyanate, and such oxime-blocking agents as methyl ethyl ketoxime or acetone oxime may be used. Lactic acid and acetic acid ar
Garner Archie W.
McGee John D.
Barnes & Thornburg
Lilly Industries, Inc.
Mayekar Kishor
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