Coating processes – Solid particles or fibers applied
Reexamination Certificate
2002-10-08
2004-08-17
Tucker, Philip (Department: 1712)
Coating processes
Solid particles or fibers applied
C522S100000, C522S102000, C522S103000, C522S104000, C522S107000, C427S580000
Reexamination Certificate
active
06777027
ABSTRACT:
BACKGROUND
This disclosure relates to ultraviolet radiation curable powders and, more particularly, to coating powders that give powder coatings having a smooth, low gloss finish, and powder coatings formed thereby.
Thermosetting coating powders are dry, finely divided, free-flowing solid materials at room temperature. Coating powders find particular utility in industrial coating applications because they are readily applied to a variety of conductive substrates, they use very little (or no) organic solvents, and excess coating powders can be readily recycled.
One class of curable coating powders is ultraviolet (UV) radiation curable powders. UV curable powders have the ability to flow, cure, and produce smoother coatings at much lower temperatures than previously possible with traditional thermosetting chemistry. This is primarily due to the curing reaction being triggered by photoinitiated radiation rather than heat. Typically, UV curable powders are formulated from solid unsaturated base resins with low glass transition temperatures (T
g
), such as unsaturated polyesters, unsaturated co-polymerizable cross linking resins such as vinyl ethers, photoinitiators, flow and leveling agents, performance-enhancing additives, and, optionally, pigments and fillers.
During coating operations, the parts are preferably preheated to drive out substrate volatiles, then UV curable powders are applied to a substrate, usually using electrostatic spray techniques. The coated substrate is then heated to fuse the powders into a smooth molten coating. The coating is then exposed to UV light, which cures and hardens the coating into a durable, extraordinarily smooth, attractive coating. However, because of the very rapid cure of UV curable coatings, it has been difficult to obtain a low gloss, smooth UV cured coating and the coatings so formed tend to have a relatively high glossy appearance. For reasons of aesthetic preference and commercial application, low gloss coatings are desirable for certain applications.
Generally, gloss reduction can be obtained in traditional powder coatings through the introduction of matting agents, such as fillers or waxes, which rise to the surface during curing and cause matting through disruption of the surface of the coating. However, because UV curable powders cure so quickly, there is not adequate time for the fillers and waxes to flocculate to the surface, and they become trapped within the coating. There is reduction in flow in the coating but little matting takes place. Higher amounts of filler or waxes may be used, but this tends to cause the powders to block or cake during normal storage and/or produce coatings with severe orange peel, limiting the amount of gloss reduction that could be attained.
U.S. Pat. No. 6,348,242 discloses UV curable powder coatings containing crystalline components, wherein the heat fused powder is further heated to flow out the crystalline components, followed by cooling to recrystallize them to form a low gloss finish prior to UV curing the coating. However, the coating powders thus formed do not include cationic curable resins and, accordingly, the film continuity and smoothness or appearance properties of the coatings formed from the powders of U.S. Pat. No. 6,348,242 should desirably be improved.
Accordingly, there remains a need for UV curable powders that form powder coatings with a low gloss. In accordance with the present invention, the present inventors have found powders that are resistant to blocking and that give surprisingly durable, low gloss, smooth UV cured powder coatings.
STATEMENT OF INVENTION
In a first aspect, the present invention provides a coating powder comprising one or more than one or more than one cationic curable resin; one or more than one cationic photoinitiator; one or more than one free radical curable resin; and one or more than one free radical photoinitiator, wherein the cationic curable resin and the free radical curable resin are each present in a ratio of 95:5 to 5:95. In a second aspect, the present invention provides a method of making a coating, comprising disposing a layer of the coating powder according to the first aspect of the present invention onto a substrate; fusing the disposed powder layer with heat to form a powder coating; and curing the powder coating to achieve a gloss level of below 60 on a Gardner Gloss scale. In a third aspect, a powder coated article is made according to the method of the second aspect of the present invention.
DETAILED DESCRIPTION
For purposes of better defining the coating powder and powder coating, the coating powder, powder or powder coating composition refers herein to the particulate material, and the powder coating refers to the coating applied to a substrate or article. An improved coating powder that provides coatings having a low gloss appearance, preferably below 60 on a 60° Gardner Gloss scale, comprises a blend of a cationic curable resin with a free radical curing resin, together with photoinitiators effective to initiate polymerization, wherein the weight ratio of the cationic curable resin to free radical curable resin is 95:5 to 5:95. Additional components such as heat-activated catalysts, pigments, fillers, flow control agents, dry flow additives, anticratering agents, surfactants, texturing agents, light stabilizers, matting agents, photosensitizers, wetting agents, anti-oxidants, plasticizers, opacifiers, stabilizers, and degassing agents can also be present. More particularly, low gloss luster can be achieved without the use of additives and/or fillers that can cause the resultant coating to cake and/or peel.
The coating powder composition employs a combination of resins having different curing mechanisms: cationic and free radical. The resin itself is typically a polymer, oligomer, or monomer that has at least two unreacted functional groups capable of crosslinking, polymerizing, or other reaction that leads to the coating. In cationic curing mechanisms, the reactive functionality of the resin reacts in the curing step by means of positively charged chemical species. In free radical curing mechanisms, the reactive functionality of the resin reacts during cure by means of free radical (uncharged) intermediate species.
Cationic curable resins may generally comprise, for example, epoxides, vinyl ethers, oxetanes, oxolanes, cyclic acetals, cyclic lactones, thiiranes, or thiotanes, or combinations comprising at least one of the foregoing resins. Preferably, the cationic curable resin comprises a polyglycidyl compound, a cycloaliphatic polyepoxide, an epoxy cresol novolac, or an epoxy phenol novolac compound, having, on average, at least two epoxy groups (oxirane rings) in the molecule. Such resins may have an aliphatic, aromatic, cycloaliphatic, araliphatic or heterocyclic structure; they contain epoxide groups as side groups, or these groups form part of an alicyclic or heterocyclic ring system. Epoxy resins of these types are known in general terms and are commercially available.
Polyglycidyl esters and poly(&bgr;-methylglycidyl) esters are one example of suitable epoxy resins. Polyglycidyl esters can be obtained by reacting a compound having at least two carboxyl groups in the molecule with epichlorohydrin or glycerol dichlorohydrin or &bgr;-methylepichlorohydrin. The reaction is expediently carried out in the presence of bases. The compounds having at least two carboxyl groups in the molecule can be, for example, aliphatic polycarboxylic acids, such as glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid or dimerized or trimerized linoleic acid. It is also possible to employ cycloaliphatic polycarboxylic acids, for example tetrahydrophthalic acid, 4-methyltetrahydrophthalic acid, hexahydrophthalic acid, or 4-methylhexahydrophthalic acid. It is also possible to use aromatic polycarboxylic acids such as, for example, phthalic acid, isophthalic acid, trimellitic acid or pyromellitic acid, or else carboxyl-terminated adducts, for example of trimellitic acid and polyols, for example glycerol or 2,2-bis(4-h
Daly Andrew T.
Haley Richard P.
Kraski, Jr. Richard A.
Reinheimer Eugene P.
Shah Navin B.
Aylward D.
Merriam Andrew E. C.
Rohm and Haas Company
Tucker Philip
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