Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Reexamination Certificate
2000-02-16
2002-02-19
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
C427S195000, C427S201000, C427S375000, C427S393000, C427S485000
Reexamination Certificate
active
06348242
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to ultraviolet (UV) radiation curable powder coatings. More particularly, it relates to a method for producing cured coatings with a low to medium gloss appearance from UV curable powder coatings. This invention is an improvement over U.S. Pat. No. 6,017,593, which discloses a method for producing low gloss coatings from UV curable powders.
BACKGROUND OF THE INVENTION
Thermosetting powder coatings have gained considerable popularity in recent years over liquid coatings foe a number of reasons. Powder coatings are virtually free of harmful fugitive organic solvents normally present in liquid coatings, and, as a result, give off little, if any, volatiles to the environment when cured. This eliminates solvent emission problems and dangers to the health of workers employed in coating operations. Powder coatings also improve working hygiene, since they are in dry solid form and have no messy liquids associated with them to adhere to workers' clothes and coating equipment. Furthermore, they are easily swept up in the event of a spill without requiring special cleaning and spill containment supplies. Another advantage is that they are 100% recyclable. Over sprayed powders are normally recycled during the coating operation and recombined with the original powder feed. This leads to very high coating efficiencies and minimal waste generation.
Despite many advantages, powder coatings traditionally have not been used foe coating heat sensitive substrates, such as wood and plastic articles, due to the rather high temperatures demanded for flow and cure. Recently, the powder coating industry has concentrated its efforts on developing low temperature curable powders. These new generation powders permit polymerization or curing at much lower temperatures, reducing the potentially damaging and deforming heat loads imposed on sensitive substrates.
One class of low temperature curable powder recently developed are the UV curable powders. UV curable powders have the ability to flow and 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 powders are formulated from solid unsaturated base resins with low Tg, such as unsaturated polyesters, unsaturated co-polymerizable crosslinker resins, such as vinyl ethers, photoinitiators, flow and leveling agents, performance-enhancing additives, and, if necessary, pigments and fillers. It is also common
to replace all or part of the base resins or crosslinkers with crystalline materials to provide powders with lower melt viscosity and better flow out behavior.
During coating operations, UV curable powders are applied to a substrate in the usual fashion, using electrostatic spray techniques. The coated substrate is then heated for as long as it takes to drive out substrate volatiles and fuse the powders into a smooth molten Ad coating. Immediately following fusion, the molten coating is exposed to UV light, which, in an instant, cures and hardens the film into a durable, extraordinarily smooth, attractive coating.
One drawback of UV curable powders is that it is very hard to produce a low gloss (i.e., matte) coating. The coatings formed tend to have a relatively high glossy appearance. For reasons of aesthetic preference, it would be desirable to have UV curable powder coatings which provide low gloss coatings. Gloss reduction can normally 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.
It would be desirable to provide a method for producing cured coatings with a low gloss appearance from UV curable powders without having to employ the cooling down step of U.S. Pat. No. 6,017,593. U.S. Pat. No. 6,017,593 discloses allowing the heat fused coating to cool for a period of at least one minute to recrystallize and form a matte finish prior to UV curing.
SUMMARY OF THE INVENTION
It is, therefore, a primary object of this invention to provide a method for producing cured coatings with a low to medium gloss appearance from UV curable powders.
In accordance with the invention, low gloss coatings having 60° Gardner Haze-Gloss levels of about 50 or below, preferably about 30 or below and medium gloss coatings having a 60° gloss level of above about 50 to 70, are achieved with UV curable powders by including in the powder composition crystalline resins or blends of crystalline and amorphous resins, and then allowing the molten coating time to cure at a temperature of about the melting temperature of the crystalline resins to form a matte finish before curing with UV light to the desired hard, chemical resistant, smooth, low or medium gloss coating film.
It is a related object of this invention to provide a method for producing both medium and low gloss cured coatings from substantially identical UV curable powders.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Throughout this specification, all parts and percentages specified herein are by weight unless otherwise stated. Herein, the resin of the powder coating is considered to be the base resin and crosslinker resin. Levels of other components are given as parts per hundred resin (phr). Further herein, the term “low gloss” or “matte” means gloss levels of about 50 or below on a 60° Gardner-Haze Gloss scale. A medium gloss means gloss levels of about 50 to 70 on a 60° Gardner-Haze Gloss scale.
In UV curable powders, the base resins are typically unsaturated polyesters to impart desired weatherability to the coating. Unsaturated polyesters are formed in a conventional manner from di- or polyfunctional carboxylic acids (or their anhydrides) and di- or polyhydric alcohols. The unsaturation is typically supplied by the carboxylic acid, although it is possible to supply it through the alcohol. Often, monohydric alcohols or monofunctional carboxylic acids (or their esters) are employed for chain termination purposes.
Examples of typical ethylenically unsaturated di- or polyfunctional carboxylic acids (or their anhydrides) include maleic anhydride, fumaric acid, itaconic anhydride, citraconic anhydride, mesaconic anhydride, aconitic acid, tetrahydrophihalic anhydride, nadic anhydride, dimeric methacrylic acid, etc. Maleic anhydride, fumaric acid, or their mixtures are generally preferred because of economic considerations. Often, aromatic and saturated acids are employed in conjunction with the unsaturated acids to reduce the density of the ethylenic unsaturation and provide the desired chemical and mechanical properties. Examples of typical aromatic or saturated di- or polycarboxylic acids (or their anhydrides) include adipic acid, succinic acid, sebacic acid, malonic acid, glutaric acid, cyclohexane dicarboxylic acid, dodecane dicarboxylic acid, phthalic anhydride, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, trimellitic acid, pyromellitic anhydride, etc. Examples of typical monofunctional acids for chain termination include acrylic acid, methacrylic acid, etc.
Examples of typical di- or polyhydric alcohols include ethylene glycol, diethylene glycol, triethylene glycol, propanediol, butanediol, neopentyl glycol, cyclohexanedimethanol, hexanediol, 2-n-butyl-2-ethyl-1,3-propanediol, MP Diol, dodecanediol, bisphenol A, hydrogenated bisphenol A, trimethylol propane, pentaerythritol, etc.
The un
Daly Andrew T.
Haley Richard P.
Mill Gregory R.
Reinheimer Eugene P.
Morton International Inc.
Parker Fred J.
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