Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-12-15
2001-10-16
Short, Patricia A. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S123000, C525S142000, C525S157000, C525S328200, C525S484000, C525S528000, C525S533000, C204S501000, C204S504000, C204S505000, C204S506000
Reexamination Certificate
active
06303690
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to cathodic electrocoat coating compositions and methods of cathodic electrodeposition using the compositions of the invention. More particularly, the invention provides cathodic electrocoat compositions having a carbamate functional resin having at least one cationic salting site, a curing agent, and a particular carbamate functional reactive additive.
BACKGROUND OF THE INVENTION
Coating compositions are widely in use today which utilize a variety of cure mechanisms. Among these are anodic and cathodic electrodeposition coating compositions and methods wherein a film-forming composition is deposited on a substrate under the influence of an applied electric potential. “Electrodeposition” as used herein refers to electrophoretic deposition. “Electrocoat” as used herein refers to both coating compositions used in electrophoretic deposition processes and to coating films obtained from electrophoretic deposition processes.
During electrodeposition, an ionically-charged polymer having a relatively low molecular weight is deposited onto a conductive substrate by submerging the substrate in an electrocoat bath having dispersed therein the charged resin, and applying an electrical potential between the substrate and a pole of opposite charge, usually a stainless steel electrode. This produces a relatively soft coating of low molecular weight on the substrate. This coating is usually converted to a hard high molecular weight coating by curing or crosslinking of the resin, usually upon exposure to elevated temperatures. In cathodic electrocoat, the workpiece being coated serves as the cathode.
One curing mechanism utilizes a melamine formaldehyde polymer-curing agent in the electrodepositable coating composition to react with hydroxyl functional groups on the electrodeposited resin. This curing method provides good cure at relatively low temperatures (e.g., 132° C.), but the crosslinked bonds contain undesirable ether linkages and the resulting coatings provide poor overall corrosion resistance.
For example, U.S. Pat. No. 4,501,833 discloses aminoplast curable cationic coating compositions that contain an onium salt-group containing polymer and a specific aminoplast curing agent. However, the performance of the cured coating is believed to be less than that desired by current commercial conditions.
In order to address some of the problems with melamine-crosslinked electrocoats, many users employ polyisocyanate crosslinkers to react with hydroxyl functional groups on the electrodeposited resin. This curing method provides desirable urethane crosslink bonds, but it also entails several disadvantages. In order to prevent premature gelation of the electrodepositable coating composition, the highly reactive isocyanate groups on the curing agent must be blocked (e.g., with an oxime, lactam, or alcohol).
Blocked polyisocyanates,.however, require high temperatures (e.g., 176° C. or more) to unblock and begin the curing reaction. The resulting electrocoats can also be susceptible to yellowing. Moreover, the volatile blocking agents released during cure can cause other deleterious effects on various coating properties, as well as increasing VOC. In addition, use of some the volatile blocking agents may give rise to environmental concerns. Finally, the volatile blocking agents account for significant and disadvantageous weight loss upon crosslinking.
In addition to the foregoing problems, prior art electrocoat compositions have sometimes lacked good flow and/or leveling. Ideally, an electrocoat coating composition will possess good flow and leveling at a desirable dip viscosity and solids. It is thus desirable to obtain an electrocoat coating composition which would is free of the foregoing problems but also exhibits good flow and leveling at a desirable dip viscosity and desirably high solids. It would also be advantageous to obtain such improvements with the add of a component which resulted in advantageous urethane linkages upon crosslinking, rather than undesirable ether linkages.
Pending U.S. Pat. No. 6,165,338 discloses cathodic electrocoat compositions having carbamate functional resins. Although such compositions provide advantages over the prior art, further improvements are desired.
U.S. Pat. Nos. 4,814,382, 5,114,015, and 5,158,808 describe the use of certain N-alkyl carbamate compounds as reactive diluents in coating compositions having OH— functional curable polymer resins. These compounds, however, may require excessively-high catalyst or temperature levels in order to fully react into the crosslink matrix during cure of the film.
WO 87/00851 describes the use of certain reactive carbamate derivatives in an effort to minimize the emission of volatile organic compounds (VOC). U.S. Pat. No. 5,744,550 describes the use of primary carbamate additives. However, further reductions in VOC are desireable without loss of desirable performance properties such as etch resistance and the like.
There is thus a need in the art for electrodepositable coating compositions that can provide desirable urethane crosslink linkages, but avoid the problems that accompany the use of blocked polyisocyanate curing agents. In particular, it is desireable to provide a cathodic electrodeposition coating composition capable of providing urethane linkages at low bake temperatures of 121° C. or less with decreased weight loss upon crosslinking, while being free of isocyanates and the volatile blocking agents used with isocyanates.
SUMMARY OF THE INVENTION
It has been unexpectedly discovered that the foregoing objects can be achieved with the use of the cathodic electrocoat coating composition of the invention. the electrocoat coating composition of the invention requires (A) a polymer comprising at least one primary carbamate group and at least one cationic salting site, (B) a curing agent having groups that are reactive with said functional groups on (A), and (C) a reactive additive comprising at least one compound having a molecular weight of from 131 to 2000 and comprising at least one primary carbamate group and at least one alkyl group selected from the group consisting of branched alkyl groups of from 5 to 30 carbons, straight chain alkyl groups of more than 10 carbons, and mixtures thereof, wherein one or both of (A) and (B) comprise groups that are reactive with the primary carbamate group of (C).
The invention further provides a method of electrocoating a substrate, the composition comprising 1) immersing a conductive substrate in a coating composition comprising, in an aqueous medium: (A) a polymer comprising at least one primary carbamate group and at least one cationic salting site, (B) a curing agent having groups that are reactive with said functional groups on (A), and (C) a reactive additive comprising at least one compound having a molecular weight of from 131 to 2000 and comprising at least one primary carbamate group and at least one alkyl group selected from the group consisting of branched alkyl groups of from 5 to 30 carbons, straight chain alkyl groups of more than 10 carbons, and mixtures thereof, wherein one or both of (A) and (B) comprise groups that are reactive with the primary carbamate group of (C), 2) applying a voltage between an anode and the conductive substrate, and 3) removing the substrate from the coating composition.
DETAILED DESCRIPTION OF THE INVENTION
The electrocoat coating composition of the invention requires (A) a polymer comprising at least one primary carbamate group and at least one cationic salting site, (B) a curing agent having groups that are reactive with said functional groups on (A), and (C) a reactive additive comprising at least one compound having a molecular weight of from 131 to 2000 and comprising at least one primary carbamate group and at least one alkyl group selected from the group consisting of branched alkyl groups of from 5 to 30 carbons, straight chain alkyl groups of more than 10 carbons, and mixtures thereof. One or both of (A) and (B) must contain one or more groups which are reactive with
December Timothy S.
Ohrbom Walter H
BASF Corporation
Golota Mary E.
Short Patricia A.
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