Autodeposition compositions

Details Autodeposition compositions Autodeposition compositions

2001-09-25

2003-11-11

Dawson, Robert (Department: 1712)

Stock material or miscellaneous articles

Composite

Of epoxy ether

C428S413000, C428S523000, C427S386000, C525S107000, C525S403000, C525S404000

Reexamination Certificate

active

06645633

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to autodeposition coating compositions based on epoxy resins in combination with acrylic 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. No. 3,592,699 (Steinbrecher et al.); U.S. Pat. Nos. 4,108,817 and 4,178,400 (both to Lochel); U.S. Pat. No. 4,180,603 (Howell. Jr.); U.S. Pat. Nos. 4,242,379 and 4,243,704 (both to Hall et al.); U.S. Pat. No. 4,289,826 (Howell, Jr.); and U.S. Pat. No. 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 is more active than hydrogen in the electromotive series, i.e., that spontaneously begins to dissolve at a substantial rate (with accompanying evolution of hydrogen gas) when introduced into the liquid solution, emulsion 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. The composition when autodeposited on a metal surface and 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 has been discovered that the incorporation of dispersed acrylic resins into epoxy resin-based autodeposition compositions provides a number of unexpected advantages. Coverage of the metal parts being coated is more uniform, especially where welds are present on the surfaces of the metal parts. Leveling, flow and film formation of the coating deposited during operation of an autodeposition process are improved as compared to an autodeposition composition containing only epoxy resin. Surface gloss is reduced and may be readily controlled as desired by varying the amount of acrylic resin present as compared to the epoxy resin. Film coverage is also improved by incorporation of both an acrylic resin and an epoxy resin in an autodeposition composition. If particular reaction rinses are used to treat the uncured autodeposition film, physical properties of the cured coating may also be enhanced by the incorporation of acrylic resin.
The invention also provides an improved method of preparing an autodeposition composition containing dispersed particles of both epoxy resin and acrylic resin. If an epoxy resin dispersion and acrylic resin dispersion are simply blended together, the resulting autodeposition composition typically will initially provide autodeposited coatings of satisfactory quality. However, as the autodeposition composition is used repeatedly over a long period of time, bath instability problems are often encountered. It has unexpectedly been discovered that bath stability can be markedly improved by first contacting the acrylic resin dispersion with either a surfactant or a coalescing solvent (most preferably both a surfactant and a coalescing solvent) prior to being combined with the epoxy resin dispersion.
DETAILED DESCRIPTION OF THE INVENTION
The epoxy resins employed as one component of the autodeposition compositions of the present invention may be generally described as organic substances containing one or more (and preferably at least two) epoxide groups per molecule which are capable of reacting with a suitable curing agent to form a thermoset polymer. 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).
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 containing at least two hydroxyl and/or amino moieties each, may also serve as the central organic moiety.
Epoxy resins useful for the present invention include glycidyl ethers of a polyhydric phenol, such as bisphenol A (a particularly preferred species of polyhydric phenol), bisphenol F, bisphenol AD, catechol, resorcinol, and the like.
Primarily for reasons of economy and commercial availability, it is generally preferred to utilize epoxy resins derived from bisphenol A in this invention. More particularly, epoxy moiety containing molecules utilized in this invention preferably conform to the general chemical formula:
and “n” is an integer from 0 to 50. Commercially available epoxy resins of this type are normally mixtures of molecules having somewhat different n values and different numbers of epoxy groups. Preferably, the epoxy resin mixture used has a number average molecular weight in the range of from about 350 to about 5000, more preferably in the range from about 400 to about 3000. Preferably, the average number of epoxide groups per molecule in the epoxy resin mixture is in the range from 1.7 to 2.5, more preferably in the range from 1.9 to 2.1.
Particularly useful as the epoxy resin component of the present invention are chain-extended ep

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