Stock material or miscellaneous articles – Composite – Of polyamidoester
Reexamination Certificate
1998-12-21
2001-04-10
Gorgos, Kathryn (Department: 1741)
Stock material or miscellaneous articles
Composite
Of polyamidoester
C204S501000, C204S505000, C524S901000
Reexamination Certificate
active
06214470
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns thermosetting electrocoat primer compositions that have curing agents based on polyisocyanates.
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
We have now invented electrocoat coating compositions that have lower emissions upon curing of the coating and that typically cure at lower temperatures than current blocked-isocyanate electrocoat compositions. The compositions of the present invention comprise a curing agent having at least one allophanate group. The electrocoat coating compositions of the invention also have unexpectedly improved throwpower properties.
The present invention further provides 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. A potential of electric current is applied between the conductive substrate and an electrode of the opposite charge to deposit a coating layer onto the conductive substrate. The deposited coating is cured by reaction between the ionic resin and the curing agent having at least one allophanate group.
DETAILED DESCRIPTION OF THE INVENTION
The electrocoat compositions of the present invention comprise, in an aqueous medium, an ionic principal resin and a curing agent having at least one allophanate group. The allophanate crosslinkers of the invention may be prepared by a method that includes a first step of preparing an allophanate compound that has isocyanate functionality and an optional second step of reacting at least some of the residual isocyanate groups with a blocking agent and/or an isocyanate-reactive extender compound to produce a blocked isocyanate and/or higher functionality crosslinker.
In the first step, the allophanate group or groups of the allophanate compound are formed by reacting an excess of equivalents of organic polyisocyanate with a mono- or polyhydric compound in the presence of a catalyst. The reaction is understood to involve formation of an initial urethane group which then, in the presence of an appropriate catalyst, further reacts to form allophanate. The amount of mono- or polyhydric compound employed should usually not exceed one-half equivalent of mono- or polyhydric compound per equivalent of isocyanate to avoid products having an unusably high viscosity. In general, there should be about 0.01 to about 0.5 equivalents of hydroxyl per equivalent of isocyanate. A more preferred range would be from about 0.1 to about 0.3 equivalents of hydroxyl per equivalent of isocyanate. Although reaction conditions may be varied, typically the reaction may continue for 3 to 10 hours at temperatures of perhaps about 60° 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.
Organic polyisocyanates that may be employed to prepare the allophanate containing compound include aromatic, aliphatic, and cycloaliphatic polyisocyanates and combinations thereof. Representative of useful polyisocyanates are diisocyanates such as m-phenylene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, mixtures of 2,4- and 2,6-toluene diisocyanate, hexamethylene diisocyanate, tetramethylene diisocyanate, cyclohexane-1,4-diisocyanate, any of the isomers of hexahydrotoluene diisocyanate, isophorone diisocyanate, any of the isomers of hydrogenated diphenylmethane diisocyanate, naphthalene-1,5-diisocyanate, 1-methoxyphenyl-2,4-diisocyanate, any of the isomers of diphenylmethane diisocyanate, including 2,2′-diphenylmethane diisocyanate, 2,4′-diphenylmethane diisocyanate, and 4,4′-diphenylmethane diisocyanate, isomers of biphenylene diisocyanate including 2,2′-, 2,4′-, and 4,4′-biphenylene diisocyanates, 3,3′-dimethoxy-4,4′-biphenyl diisocyanate and 3,3′-dimethyl-diphenylmethane-4,4′-diisocyanate; triisocyanates such as 4,4′,4″-triphenylmethane triisocyanate and toluene 2,4,6-triisocyanate; and the tetraisocyanates such as 4,4′-dimethyldiphenylmethane-2,2′,5,5′-tetraisocyanate; and polymeric polyisocyanates such as polymethylene polyphenylene polyisocyanate. Especially useful due to their availability and properties are the various isomers of toluene diisocyan
December Timothy S.
Narayan Thirumurti
Tazzia Charles L.
BASF Corporation
Budde Anna M.
Gorgos Kathryn
Keehan Christopher M
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