Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
1998-12-21
2001-05-08
Gorgos, Kathryn (Department: 1741)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C204S493000, C204S501000, C204S505000, C524S901000
Reexamination Certificate
active
06228472
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns processes for preparing allophanate compounds and thermosetting coating compositions that have allophanate curing agents.
BACKGROUND OF THE INVENTION
Electrodeposition coating compositions and methods are widely used in industry today. One of the advantages of electrocoat compositions and processes is that the applied coating composition forms a uniform and contiguous layer over a variety of metallic substrates regardless of shape or configuration. This is especially advantageous when the coating is applied as an anticorrosive coating onto a substrate having an irregular surface, such as a motor vehicle body. The even, continuous coating layer over all portions of the metallic substrate provides maximum anticorrosion effectiveness.
Electrocoat baths usually comprise an aqueous dispersion of a principal film-forming resin, such as an acrylic or epoxy resin, having ionic stabilization. For automotive or industrial applications for which hard electrocoat films are desired, the electrocoat compositions are formulated to be curable compositions. This is usually accomplished by including in the bath a crosslinking agent that can react with functional groups on the principal resin under appropriate conditions (such as with the application of heat) and thus cure the coating. During electrodeposition, coating material containing an ionically-charged resin 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 then applying an electrical potential between the substrate and a pole of opposite charge, for example, a stainless steel electrode. The charged coating material migrates to and deposits on the conductive substrate. The coated substrate is then heated to cure the coating.
One curing mechanism utilizes a melamine formaldehyde resin 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 (perhaps 130° C.), but the crosslink bonds contain undesirable ether linkages and the resulting coatings provide poor overall corrosion resistance as well as poor chip and cyclic corrosion resistance.
In order to address some of the problems with melamine cross-linked electrocoats, many commercial compositions employ polyisocyanate crosslinkers to react with hydroxyl or amine functional groups on the electrodeposited resin. This curing method provides desirable urethane or urea crosslink bonds, but it also entails several disadvantages. In order to prevent premature gelation of the electrodepositable coating compositions, the highly reactive isocyanate groups on the curing agent must be blocked. Blocked polyisocyanates, however, require high temperatures, typically 175° C. or more to unblock and begin the curing reaction. In the past, the isocyanate crosslinkers have been blocked with a compound such as an oxime or alcohol, which unblocks and volatilizes during cure, in order to provide the lowest temperatures for the unblocking and curing reactions. The volatile blocking agents released during cure can cause other deleterious effects on various coating properties, however, and increase organic emissions. There is thus a need for electrodepositable coating compositions that could provide desirable urethane or urea crosslink linkages but that avoid the problems that now accompany compositions having polyisocyanate curing agents blocked with volatilizing agents.
SUMMARY OF THE INVENTION
I have now invented a process for making an allophanate compound that includes reacting a urethane group-containing precursor with an isocyanate group-containing compound in the presence of a metal catalyst and a tertiary amine. The process of the invention offers an advantage of reduced time for the allophanate reaction, even for reduced levels of metal catalyst as compared to allophanate synthesis without the presence of a tertiary amine. The present invention further provides a novel allophanate compound produced according the method just described.
In another embodiment, the invention offers a method of producing a compound containing at least one allophanate group which employs copper acetate monohydrate as catalyst. While metal acetylacetonate catalysts have previously been used, copper acetate monohydrate offers advantages in improved reaction and reduced cost of manufacture.
The invention further provides a coating composition, in particular an electrocoat coating composition, that includes a compound prepared according to the processes of the invention, in which the compound has at least one allophanate group. Electrocoat coating compositions that include the allophanate-containing compound formed by the process of the invention have unexpectedly improved throwpower properties. Electrocoat coating compositions that include the present allophanate compound also have unexpectedly improved cure at lower temperatures compared to current blocked-isocyanate electrocoat compositions.
The present invention further furnishes a method of coating a conductive substrate. In the method of the invention, a conductive substrate is immersed in an electrodeposition coating composition comprising, in an aqueous medium, an ionic resin and a curing agent having at least one allophanate group; then, a potential of electric current is applied between an electrode and the conductive substrate (which is then an electrode of the opposite charge) to deposit a coating layer onto the conductive substrate. The deposited coating layer is cured by reaction between the ionic resin and the curing agent having at least one allophanate group.
DETAILED DESCRIPTION OF THE INVENTION
The allophanate compounds of the invention are formed by reacting an excess of equivalents of organic polyisocyanate with a mono- or polyhydric compound in the presence of a tertiary amine compound and a catalyst. The reaction is understood to involve formation of an initial urethane group which then, in the presence of the tertiary amine and the catalyst, further reacts with an isocyanate to form the allophanate group. In one embodiment of the invention, the tertiary amine compound includes one or more hydroxyl groups that may react with isocyanate to form a urethane group. The urethane group may then react with more isocyanate to form allophanate functionality.
The equivalents of hydroxyl of mono- or polyhydric compound employed may range from about 0.01 to about 0.95 equivalents of hydroxyl per equivalent of isocyanate. A more preferred range would be from about 0.3 to about 0.75 equivalents of hydroxyl per equivalent of isocyanate, and even more preferred is from about 0.4 to about 0.6 equivalents of hydroxyl per equivalent of isocyanate. In a preferred embodiment, the tertiary amine compound is an aminoalcohol. The aminoalcohol may be included in an amount of from about 0.01 to about 0.5 equivalents hydroxyl per equivalent of isocyanate. More preferably, the aminoalcohol is used in an amount of from about 0.01 to about 0.1 equivalents hydroxyl per equivalent isocyanate, and even more preferred is from about 0.01 to about 0.07 equivalents of hydroxyl per equivalent of isocyanate. Although reaction conditions may be varied, the reaction may continue for 3 to 10 hours at temperatures of perhaps about 50° C. to about 150° C. Progress of the reaction can be monitored by any of the usual methods, such as titration, infrared spectroscopy, or viscosity measurement. A catalyst deactivator may optionally be added to stop the allophanate formation at a point where the desired isocyanate content or viscosity has been obtained. Addition of a deactivator is also desirable for storage stability of the product with unreacted isocyanate content. Typically, the reaction may be allowed to continue to completion so that the product has substantially no residual isocyanate functionality.
Organic polyisocyanates that may be employed to prepare the allophanate containing compo
BASF Corporation
Budde Anna M.
Gorgos Kathryn
Keehan Christopher
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