Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2003-01-10
2004-04-20
Sellers, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S404000, C525S504000, C525S524000, C525S528000
Reexamination Certificate
active
06723765
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to autodeposition coating compositions based on epoxy resins and the use of such compositions to form autodeposited coatings on metallic substrates.
BACKGROUND OF THE INVENTION
Autodeposition has been in commercial use on steel for about thirty years and is now well established for that use. For details, see for example, U.S. Pat. Nos. 3,592,699 (Steinbrecher et al); 4,108,817 and 4,178,400 (both to Lochel); 4,180,603 (Howell. Jr.); 4,242,379 and 4,243,704 (both to Hall et al.); 4,289,826 (Howell, Jr.); and 5,342,694 (Ahmed) as well as U.S. Pat. No. 5,500,460 (Ahmed et al.). The disclosures of all of these patents are hereby incorporated by reference.
Autodeposition compositions are usually in the form of liquid, usually aqueous, solutions, emulsions or dispersions in which active metal surfaces of inserted objects are coated with an adherent resin or polymer film that increases in thickness the longer the metal object remains in the bath, even though the liquid is stable for a long time against spontaneous precipitation or flocculation of any resin or polymer, in the absence of contact with active metal. “Active metal” is defined as metal that spontaneously begins to dissolve at a substantial rate when introduced into the liquid solution or dispersion. Such compositions, and processes of forming a coating on a metal surface using such compositions, are commonly denoted in the art, and in this specification, as “autodeposition” or “autodepositing” compositions, dispersions, emulsions, suspensions, baths, solutions, processes, methods, or a like term. Autodeposition is often contrasted with electrodeposition, which can produce very similar adherent films but requires that metal or other objects to be coated be connected to a source of direct current electricity for coating to occur. No such external electric current is used in autodeposition.
One way in which autodeposition coatings have traditionally been prepared is by emulsifying a mixture containing an epoxy resin, a crosslinker, any optional additives and a solvent. The emulsified mixture is then subjected to distillation to remove solvent, resulting in the formation of an autodeposition composition made of resinous particles dispersed in water. When used in an autodeposition process, the composition when cured forms a polymeric coating of high gloss. Unfortunately, such coatings are sensitive to coating defects such as craters and edge pullback and other defects caused by the presence of particulate matter on the substrate surface.
It would be desirable to have a composition and method of use that would form an autodeposition coating that is more resistant to pinholes, blemishes and other forms of surface defects, particularly on zinciferous surfaces, more particularly galvanized steel or some variation.
Additionally, conventional epoxy resin based autodeposition compositions typically produce a glossy surface film when cured. Not every end user of this technology, however, desires a gloss surface. Additional processing steps or additives are generally needed to convert a conventional glossy coating to a matte or semi-gloss surface, but it would be preferable to avoid the need to use such additional steps or additives.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an autodeposition coating composition that is resistant to surface defects, including cratering, edge pullback, and defects caused by the presence of particulate matter.
It is another object of the invention to provide an autodeposition process that forms an autodeposited coating, when cured, of reduced gloss.
In accordance with these and other objects of the invention, an autodeposition composition according to the invention comprises, preferably in emulsion or dispersion form, (a) a crosslinkable reaction product made by the reaction at a first temperature of a resin containing both reactive hydroxyl groups and reactive epoxide groups and a reactive first crosslinking agent containing free isocyanate groups; and (b) a second crosslinking agent that is reactive with said reaction product only at a second higher temperature. The second temperature is preferably at least 50° C. greater than the first temperature, more preferably at least 75° C. greater than the first temperature. Hybrid crosslinking agents may also be used which have both free isocyanate groups and moieties capable of reacting with hydroxy and/or epoxy groups only at elevated temperature, thereby serving the function of both the aforedescribed first and second crosslinking agents.
The autodeposition composition of the invention can be used to form a reduced-gloss resinous film that is durable and resistant to defects due to cratering, edge pullback, and the presence of particulate matter on the treated surface. Such an autodeposition composition can provide manufacturers of metal goods with a durable, corrosion resistant surface for use with or without additional coatings, paints, and the like.
DETAILED DESCRIPTION
The autodeposition composition and method of use according to the invention are based on resins that contain within the same molecule: (i) one or more epoxide groups as well as (ii) one or more hydroxyl groups that are reactive with free isocyanate groups at ambient to slightly elevated temperatures. The use of both types of functionalities allows the formation of a reactive prepolymer component of increased molecular weight relative to the starting resin component through preliminary reaction of the hydroxyl groups in a low temperature reaction with a first crosslinking agent. In one embodiment of the invention, this reactive prepolymer is mixed or emulsified with a second crosslinking agent (which contains functional groups capable of reacting with the prepolymer only at elevated temperatures) and used to form an autodeposited resin coating under autodeposition conditions, which is then cured by heating. In another embodiment of the invention, a hybrid crosslinking agent is employed which has characteristics of both the first crosslinking agent and second crosslinking agent. The elevated temperatures in the curing step induce crosslinking reactions between the reactive prepolymer and second crosslinking agent (or between individual molecules of the reactive prepolymer where a hybrid crosslinking agent is used), thereby forming an autodeposited coating of reduced gloss which is resistant to minor defects in surface cleaning and which also exhibits low shrinkage and edge pullback.
Resins
The reactive prepolymers useful in the present invention can be based on one or more epoxy resins which contain one or more hydroxyl groups that are reactive to isocyanate groups. Such epoxy resins are well known substances and are described, for example, in the chapter entitled “Epoxy Resins” in Volume 6 of The Encyclopedia of Polymer Science and Engineering (Second Edition). Preferably, the epoxy resin used contains one or more hydroxyl groups (for low temperature reactivity) and approximately two epoxide groups per molecule.
Epoxy resins are often described by the type of central organic moiety or moieties to which the 1,2-epoxy moieties are attached. Non-exclusive examples of such central moieties are those derived from bisphenol A, bisphenol F and their analogs in which one or two —NH
2
moieties are substituted for an equal number of —OH moieties in the bisphenol; novolak condensates of formaldehyde with phenol and substituted phenols and their amino analogs, the condensates containing at least two aromatic nuclei; triazine; hydantoin; and other organic molecules containing at least two hydroxyl and/or amino moieties each, in each instance with as many hydrogen atoms deleted from hydroxy and/or amino moieties in the parent molecule as there are epoxy moieties in the molecules of epoxy resin. Optionally, the 1,2-epoxide moieties may be separated from the central moieties as defined above by one or more, preferably only one, methylene groups. Oligomers of such monomers, either with themselves or with other organic molecules contai
Cameron Mary K.
Harper Stephen D.
Henkel Corporation
Sellers Robert
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