WO1995008596A1

WO1995008596A1 - Flat, dark pigmented radiation curable coating compositions

Info

Publication number: WO1995008596A1
Application number: PCT/US1994/007206
Authority: WO
Inventors: Joseph Randall Friebele
Original assignee: Ppg Industries, Inc.
Priority date: 1993-09-20
Filing date: 1994-06-27
Publication date: 1995-03-30
Also published as: AU7512694A; TR27853A; TW287193B

Abstract

Very low gloss finishes are attained in dark colored (e.g., black), radiation curable coatings by inclusion of black silica pigment in a composition containing a radiation curable binder resin (e.g., polyacrylate) and a photoinitiator (preferably alkoxy substituted acetophonones). Also useful are viscosity control polymers of higher molecular weight than the binder resin and compatible therewith, such as acrylated acrylics. The coating is first exposed to ionizing radiation (e.g., electron beam) in air, then exposed to actinic radiation (ultraviolet light) in an essentially inert atmosphere. The low gloss is achieved by a combination of pigment selection and a fine, uniform surface wrinkling effect.

Description

FLAT, DARK PIGMENTED RADIATION CURABLE COATING COMPOSITIONS

BACKGROUND OF THE INVENTION Coatings of most radiation curable coating compositions when exposed to ultraviolet light or electron beam are cured to glossy, crosslinked coatings. In many instances, however, it is desired to obtain crosslinked coatings of low gloss. One way to achieve low gloss coatings is by adding flatting pigment. However, with black or other dark colors, flatting pigments tend to have an undesirable graying effect on the coating. Also, when the amount of flatting pigment is increased to the extent required to produce coatings of very low gloss, the pigment to binder ratio may be so high that there is insufficient binder to firmly hold the pigment. The result is a low gloss coating which lacks hardness and durability. Furthermore, high pigment to binder ratios may increase viscosity of the coating composition to such an extent that application onto a substrate may be hindered.

Dark colored coatings, particularly black, are particularly difficult to produce in a flat finish by radiation curing because of their sensitivity to showing defects. At gloss levels below about 30% (on a 60° glossmeter) dark colors that have been radiation cured tend to show an unattractive mottled appearance on the surface. It would be desirable to be able to produce mottle- free dark finishes at gloss levels below 30% on a high speed radiation cure line.

Another method heretofore employed to produce low gloss films from radiation curable compositions utilized a two step curing process wherein polymerization of certain coating compositions was inhibited in surface portions in the first step by the presence of oxygen (air) , and curing of the coating was completed in the second step in an inert atmosphere. Shrinkage of underlying layers during the first step caused pigment particles to be driven into the surface portions, whereby the surface contained a larger amount of pigment than the body of the film which reduced the gloss of the film without sacrificing film strength or rheology properties of the coating composition. U.S. Patent Nos. 3,918,393 (Hahn) and 4,048,036 (Prucnal) illustrate this approach. A drawback to the approach of concentrating flatting pigments at the surface of the film is that the surface is subject to physical damage that causes unattractive marking of the surface. One potential form of damage to which these types of low gloss coatings are susceptible is metal marking, whereby contact of certain metals with the surface causes dark markings. It would be desirable to avoid these problems with radiation curable, low gloss coatings as well as to be able to produce coatings of even lower gloss than prior art methods.

Multi-step radiation curing techniques have also been proposed for producing textured finishes. These techniques are disclosed in U.S. Patent Nos. 4,421,784 (Troue) , 3,840,448 (Osborne et al. ) , and 4,411,931 (Duong). The methods of these patents do not appear to be intended to produce the type of flat, low gloss finishes that are the subject of this invention, but are directed to the production of relatively gross, visibly perceivable, surface wrinkle patterns. These patented methods do not involve the use of electron beam radiation. Prior art techniques for producing low gloss or textured radiation cured coatings, including the patents set forth above, have generally been limited to non-pigmented coatings. Typically the low gloss or textured coating is a clear coating that is applied over a pigmented base coat. This has generally been considered necessary in order to assure adequate penetration of radiation during the curing step. It would be highly desirable to produce low gloss coatings by radiation curing directly from pigmented coating compositions.

SUMMARY OF THE INVENTION This invention provides a dark (particularly black) pigmented, radiation curable coating composition adapted to produce a low gloss film, i.e., at gloss levels below 30%. The composition comprises: a resin binder curable by radiation exposure in the presence of a photoinitiator compound, photoinitiator compound, and black silica pigment. Low gloss is achieved in the present invention at least in part by means of a uniform, microscopic surface wrinkling of the film that is produced from the coating composition when the coating is cured by a two step radiation curing process, wherein the first step involves ionizing radiation (e.g., electron beam) and the second step involves actinic radiation (e.g., ultraviolet light). The use of black silica pigment is advantageous in permitting larger amounts of pigment to be included, which assists the attainment of low gloss finish, but also has surprisingly been found to substantially reduce or eliminate the mottling problem that had troubled the prior art. Although not limiting to the invention, a resin binder that has been found effective in preferred embodiments comprises polyacrylate functional resin. Although the resin binders of the present invention are characterized as being radiation cure inhibited by the presence of oxygen, it has been found advantageous in producing flat, dark colors to employ a photoinitiator that has relatively less oxygen inhibition. For this reason a preferred class of photoinitiator constitutes the alkoxy substituted acetophenones.

The mottling problem was found to be further alleviated by the inclusion of a selected viscosity control resin in addition to the primary radiation curable binder resin. The viscosity control resin is reactive with the binder resin and is of relatively high molecular weight. When the binder resin is an acrylic resin, it is preferred that the viscosity control resin be an acrylate functional acrylic polymer for the sake of compatibility.

It is an advantage of the present invention that the coating composition is pigmented, whereby the desired low gloss finish can be attained with one coat rather than requiring a base coat and clear top coat as required by some prior art methods. Another aspect of the invention is the method of using the novel coating composition described above wherein the coating composition is cured in a subsurface portion in a first step by electron beam radiation in the presence of oxygen whereby curing at the surface is partially inhibited. In a subsequent step the curing is completed by means of ultraviolet radiation in a substantially inert atmosphere (less than 1000 ppm oxygen, preferably less than 500 ppm oxygen) .

DETAILED DESCRIPTION The present invention involves a coating composition containing a photoinitiators. Photoinitiators absorb radiation and thereby obtain energy to form free radicals that initiate polymerization of the binder resin. The photoinitiator is one which forms free radicals upon exposure to actinic radiation, viz., ultraviolet light. A particularly suitable class of photoinitiators for this purpose are acetophenone derivatives. Many acetophenone derivatives are known as photoinitiators, a large number of which are disclosed in U.S. Patent No. 4,229,274 (Carlblom) . Acetophenone derivative photoinitiators may be generally characterized by the formula:

O i

II I φC — C — R₂

I R₃

where Ri, R₂, and R3 may, for example, include independently hydrogen, alkyl usually having from 1 to 6 carbon atoms (preferably 1 to 4 carbon atoms) , alkoxy, cycloalkyl, or substituted or unsubstituted phenyl groups, and d is a phenyl group. A particularly useful family of acetophenone derivatives for use in the present invention are those in which at least one (preferably two) of Ri, R₂, and/or R3 is an alkoxy group, for example, methoxy or ethoxy.

The photoinitiator may be present in an amount in the range of from 0.01 percent to 4 percent by weight based on total solids content of the coating composition, preferably at least 0.1 percent to 2 percent.

The primary binder or vehicle in the coating composition of the present invention comprises at least one resin (monomer, oligomer, or polymer) including at least one resin which is curable by exposure to radiation in the presence of one or more of the photoinitiators disclosed above. Binder may constitute 20 to 80, preferably 30 to 70, percent by weight of the total coating composition. Many such resins are known in the art and may be used in the present invention. The resins suitable for use in the present invention are characterized by inhibition of curing by the presence of oxygen (such as in air) . Oxygen inhibition permits maintaining a partially uncured surface layer during the initial curing step, which is important for attaining the wrinkle effect of the present invention. Therefore, radiation curable resins that are adapted to be curable in the presence of oxygen, such as those containing substantial amounts of epoxy acrylate derivatives or allyl groups, are not appropriate for use in the present invention. A particular category of useful radiation curable compounds are characterized by a plurality of acrylyloxy groups and the ability to be free radically addition polymerized by initiation f om the photoinitiators. These are numerous and include difunctional, trifunctional, tetrafunctional and higher polyfunctional organic radicals whose bonds are satisfied with unsubstituted acrylyloxy or a-substituted acrylyloxy groups. The polyvalent radical may be aliphatic, cycloaliphatic, aromatic or other. As used herein, unless otherwise indicated either directly or by context, acrylyloxy is used in its broad sense to mean unsubstituted acrylyloxy or a-substituted acrylyloxy groups such as methacrylyloxy, ethacrylyloxy and a-chloroacrylyloxy. Similarly, unless otherwise indicated either directly or by context, acrylate unsaturation is used in its broad sense to mean the unsaturation provided by unsubstituted acrylyl groups or a-substituted acrylyl groups such as ethacrylyl, ethacrylyl and a-chloroacrylyl. Examples of these compounds are the diacrylates and dimethacrylates of ethylene glycol, 1,3-propanediol, propylene glycol, 2,3-butanediol, 1,4-butanediol, 2-ethylbutane-l,4-diol, 1,5- pentanediol, 1,6-hexanediol, 1,7-heptanediol, 1, 8-octanediol, 1,9- nonanediol, 1, 10-decanediol, 2,10-decanediol, 1,4-cyclohexanediol, 1,4-dimethylolcyclohexane, 2,2-diethylpropane-l,3-diol, 2,2- dimethylpropane-l,3-diol, 3-methylpentane-l,4-diol, 2,2- diethylbutane-1,3-diol, 4, 5-nonanediol, diethylene glycol, triethylene glycol, propylene glycol, neopentyl glycol, 5,5-dimethyl- 3,7-dioxanonane-l,9-diol, 2,2-dimethyl-3-hydroxypropyl 2,2-dimethyl- 3-hydroxypropionate; the triacrylates, trimethacrylates, diacrylates and dimethacrylates of glycerol, 1,1,1-trimethylolpropane and trimethylolethane; and the tetraacrylates, tetramethacrylates, triacrylates, trimethacrylates, diacrylates and dimethacrylates, of pentaerythritol and erythritol. The acrylyloxy groups in each of the molecules are usually the same, but they may be different as exemplified by the compound 2,2-dimethyl-1-acrylyloxy-3- ^** methacrylyloxypropane. Further examples of satisfactory polyacrylyloxy compounds that may be included in the radiation curable resin include polyacrylyloxy functional polyesters, polamides, polyacrylates, polyethers, polycarbonates or polyurethanes as well as polyacrylyloxy functional compounds of mixed functionality such as polyacrylyloxy functional poly(ester-urethanes) , poly(ester-amides) and polyfether- urethanes) . Mixtures of compounds having a plurality of acrylyloxy groups may be used, if desired.

For the sake of viscosity control, it has been found useful in the present invention to include a second polymer reactive with the primary binder resin that is less sensitive to oxygen inhibition than the primary binder resin. When the primary binder resin is an acrylate functional material, the second polymer is preferably also acrylate functional. It is also advantageous for the second polymer to have relatively high molecular weight to serve as a viscosity control agent, whereby fluidity of the uncured coating is decreased. This is believed to contribute to the avoidance of mottling attainable by the present invention. Viscosity of the viscosity control resin is preferably greater than 3000 centipoise at 60°C. A class of materials that have been found to be particularly compatible with the preferred acrylate binder resins are acrylate functional acrylic polymers such as "Ebecryl 1701" from Radcure, Inc., Louisville, KY. The amount of the viscosity control polymer to be included will vary substantially, depending upon the particular polymer used and its viscosity. For the preferred class of acrylated acrylic polymers, the amount may range from 5 to 40 percent by weight on the solids basis.

Monomers having monoacrylate functionality which crosslinks with the compound having polyacrylyloxy functionality may optionally be present in the coating composition. Examples of monoacrylate functional monomers which may be used are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, butyl methacrylate, hexyl acrylate, hexyl methacrylate, hexyl ethyl acrylate, hexyl butyl acrylate, 2-ethyl hydroxy acrylate, octyl acrylate, octyl methacrylate, hydroxy ethyl acrylate, hydroxy butyl acrylate, caprolactone-hydroxyl alkyl acrylate reaction products, and 2-ethyl hydroxy acrylate. The preferred monoacrylate functional monomers are liquid compounds miscible with the polyacrylyloxy compound. A benefit from the use of one or more monoacrylate functional monomers is that the monoacrylate functional monomer may act as a reactive solvent for the polyacrylyloxy functional compound, thereby providing coating compositions having a satisfactory low viscosity without using relatively small amounts or no volatile, nonreactive solvent. The monoacrylate functional monomer, or mixtures of monoacrylate functional monomers, may be employed over a broad range, although none is required. The amount of such monomer when used should be sufficient to provide a liquid, flowable, interpolymerizable mixture. When used, the monomer will ordinarily be present in the coating composition in the range of from about 0 to about 80 percent by weight of the binder of the coating composition. Typically, the monoacrylate functional monomer will be present in the range of from about 0 to about 30 percent by weight of the binder. Other monovalent functional monomers may be employed as known in the radiation curing art, including N-vinyl-2-pyrolidone, vinyl neodecanoate, and other ethylenic unsaturated monomers known to be suitable for radiation curable coatings.

The pigment portion of the compositions of the present invention includes black silica. The term "black silica" describes a naturally occuring mineral substance that includes silica as a major component, and in which the black color is imparted by a small amount of carbon. Other mineral oxides are also usually present, particularly aluminum oxide. A commercial source of black silica is "Ebony Novacite " from Malvern Minerals Company, Hot Springs National Park, Arkansas. A typical chemical analysis of this product is as follows:

Component Percent by weight

Silica 58.00 Carbon 3.09

Sulfur 0.08

Aluminum oxide 21.06 Ferric oxide 2.29

Titanium oxide 1.40 Calcium oxide 6.88

Black silica has low oil absorption, and therefore it can be included in the coating compositions of the present invention in relatively large amounts without having a detrimental effect on coating properties. The relatively high pigment content raises the viscosity of the uncured coatings, which is believed to account at least in part to the avoidance of the mottling problem that was associated with prior art black coatings that employed carbon black, for example. Black silica may be present in amounts of 5 to 20 percent by weight of solids of the coating compositions of the present invention.

Additionally, the pigment portion may optionally contain other coloring or opacifying agents, flatting agents, fillers, and extenders, including titanium dioxide (rutile or anatase) , zinc oxide, zirconium oxide, zinc sulfide and lithopone. Examples of coloring pigments include iron oxides, cadmium sulfide, carbon black, phthalocyanine blue, phthalocyanine green, indanthrone blue, ultramarine blue, chromium oxide, burnt umber, benzidine yellow, tolluidine red, aluminum power and aluminum flakes. Examples of extender pigments include silica, barytes, calcium carbonate, barium sulfate, talc, aluminum silicates, sodium aluminum silicates, potassium aluminum silicates and magnesium silicate. When the pigment is ultraviolet light absorbing, it should be used in amounts which do not preclude curing of the interior of the coating. The maximum amount is therefore related to the thickness of the coating to be cured. Thin coatings may tolerate more ultraviolet light absorbing pigment than thick coatings. When the pigment does not significantly absorb ultraviolet light, there is usually greater latitude in the amounts which may be employed. Total pigment content, including the black silica may range from 10 to 70 percent by weight of solids of the coating composition, preferably 10 to 50 percent. Dyes and tints may be included in the coating composition as replacements for all or some of the optional pigment content.

Another optional ingredient is resinous pigment dispersant, viscosity control agent (e.g., cellulose acetate butyrate) , or grinding vehicle. There are many resinous dispersants which.are commercially available which may be used for that purpose. These dispersants are used in the manner and in amounts known to the art, such as 0 to 20 weight percent of the total composition.

Conventional plasticizers such as dibutyl phthalate, butyl benzyl phthalate, diisooctyl phthalate, decyl butyl phthalate, diisooctyl adipate, dibutyl sebacate, butyl benzoate, triisooctyl trimellitate, n-octyl n-decyl trimellitate, and tricresyl phosphates and flow promoters such as phenyl benzoate, dibenzyl ketone, benzyl methyl ketone and the like may also be optionally included in amounts customary in the art, although they are not considered necessary for the present invention. If any plasticizer is included, it is usually present in amounts no greater than 5 weight percent of the total composition.

Another ingredient which is often included in coating compositions of this type is a non-reactive, volatile organic solvent. However, in preferred embodiments of the present invention, no such non-reactive solvent need be included. In other embodiments of the invention, solvent may be present, but in lesser amounts than conventional. It is generally advantageous to minimize the amount of organic solvent, but if reduction of viscosity is desired for a particular application, the present invention does not preclude adding larger amounts of a non-reactive solvent or mixtures of several solvents. Examples of suitable non-reactive organic solvents are acetone, methyl ethyl ketone, methyl isobutyl ketone, methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, secutyl alcohol, isobutyl alcohol, tert-butyl alcohol, amyl alcohol, hexyl alcohol, 2-ethylhexyl alcohol, cellosolve, ethyl cellosolve, cellosolve acetate, 2-ethylhexyl acetate, tetrahydrofuran, and aliphatic naphtha. When solvent of this type is used it is ordinarily present in the coating composition in the range of from about 0.1 to about 40 percent by weight of the vehicle of the coating composition. From about 0 to about 15 percent is typical. The preferred compositions are solvent-free.

A small amount of wax may be included in the composition as it conventional in the art.

The listing of optional ingredients discussed above is by no means exhaustive. Other ingredients may be employed in their customary amounts for their customary purposes so long as they do not seriously interfere with good coatings practice or the obtaining of cured coatings of low gloss.

The coating compositions of the invention are usually prepared by simply admixing the various ingredients. The compounds comprising the photoinitiator system may be premixed and then admixed with the other ingredients of the coating composition, or they may be added separately. Although mixing is usually accomplished at room temperature, elevated temperatures are sometimes used. The maximum temperature which is usable depends upon the heat stability of the ingredients. Temperatures above about 200°F (93°C) are only rarely employed. The radiation curable coating compositions of the invention are generally used to form cured, adherent coatings on substrates. The substrate is coated with the coating composition using substantially any technique known to the art. These include spraying, curtain coating, dipping, roller application, printing, brushing, drawing and extrusion. Substrates which may be coated with the compositions of this invention may vary widely in their properties. Organic substrates such as wood, fiberboard, particle board, composition board, paper, cardboard and various polymers such as polyesters, polyamides, cured phenolic resins, cured aminoplasts, acrylics, polyurethanes and rubber may be used. Inorganic substrates are exemplified by glass, quartz and ceramic materials. Many metallic substrates may be coated. Exemplary metallic substrates are iron, steel, stainless steel, copper, brass, bronze, aluminum, magnesium, titanium, nickel, chromium, zinc and alloys.

The method of curing the coating composition of the present invention involves a two step radiation exposure wherein the applied coating layer is cured in a subsurface portion in a first step by exposure to ionizing radiation (e.g., electron beam radiation or laser) in the presence of oxygen whereby curing at the surface is at least partially inhibited. In a subsequent step the curing is completed throughout the remainder of the coating thickness by means of ultraviolet radiation in a substantially inert atmosphere.

Suitable electron beam radiation for use in the first curing step may constitute a dose of 2 to 10 megarads, preferably 2 to 5 megarads, at 150 to 300 kiloelectron volts, preferably 170 to 250 kiloelectron volts. The exposure in the first step is chosen so as to substantially cure the portion of the coating closest to the substrate. A portion of the coating thickness nearest to the surface will remain at least partially uncured due to oxygen inhibition.

Any suitable source which emits ultraviolet light, viz., electromagnetic radiation having a wavelength in the range of from about 180 to about 400 nanometers, may be used in the practice of the second curing step. Suitable sources are mercury arcs, carbon arcs, low pressure mercury lamps, medium pressure mercury lamps, high pressure mercury lamps, swirl-flow plasma arc, ultraviolet light- emitting diodes and ultraviolet light emitting lasers. Particularly preferred are ultraviolet light emitting lamps of the medium or high pressure mercury vapor type. Such lamps usually have fused quartz envelopes to withstand the heat and transmit the ultraviolet radiation and are ordinarily in the form of long tubes having an electrode at either end.

The time of exposure to ultraviolet light and the intensity of the ultraviolet light to which the coating composition is exposed may vary greatly. Generally the exposure to ultraviolet light should continue until either the film is thermoset throughout or at least cured to the point where subsequent reactions cause the film to become thermoset throughout. Exposure of the coating to ultraviolet light may be accomplished in the presence of an inert atmosphere, viz., an atmosphere either containing no oxygen or only a concentration of oxygen which insignificantly inhibits polymerization of the coating surface (less than 1000 parts per million, preferably less than 500 parts per million oxygen) . Gases such as nitrogen, argon, carbon dioxide or mixtures thereof are typically the major components of inert atmospheres, although other unreactive gases may be used. Nitrogen is generally employed for this purpose.

Coatings produced in accordance with the present invention exhibit a surface with very fine wrinkles, which are generally smaller than can be seen with the unaided eye. When viewed under magnification, the wrinkles can be seen to be uniformly distributed over the surface in a substantially regular pattern as shown in the drawing. This finely wrinkled surface appears to be responsible for the low gloss attained by the present invention without total reliance on flatting agents. Gloss may conveniently be determined by the Standard Method of Test for Specular Gloss, ASTM Designation D- 523-67 (Reapproval 1971) . Using a Gardner 60θ glossmeter, the gloss of cured coatings of the present invention can be less than 30 percent reflected light, preferably less than 25 percent reflected light. Although not precluded from the present invention, black coatings having less than about 10-15 percent reflected light are generally not considered attractive.

The following Example 1 is a specific embodiment of the invention, which is compared in Table I to a typical prior art commercial composition, Comparative Example A, which had been suggested for flat black, radiation cured coatings prior to the present invention.

EXAMPLE 1

Percent

Component by Weight

Tripropylene glycol diacrylate¹ 35.2

Acrylic monomer² 2.6

Lampblack³ 2.6

Black silica⁴ 11.2

Polyethylene wax⁵ 1.0

Silica flatting pigment⁶ 3.4

•η

Ethoxylated neopentyl glycol diacrylate' 19.7

Acrylated acrylic polymer⁸ 19.7

Silica flatting pigment⁹ 3.4

Photoinitiator¹⁰ 1.2

1 Supplied by Radcure, Inc., Louisville, KY.

2 "SR-802" pigment dispersant from Startomer Corp., Exton, PA.

³ #6 Amorphous carbon black from General Carbon Co., Los Angeles, CA. ⁴ "Ebony Novacite " (25 micron) from Malvern Minerals Co., Hot

Springs, AR.

⁵ "Polymist B-6" wax (6 micron) from Allied Signal Inc., Morristown, NJ.

"OK-412" flatting agent from DeGussa Co., Frankfurt, Germany. ⁷ "SR-9209" from Sartomer Co., Exton, PA. ⁸ "Ebecryl 1701" from Radcure Inc., Louisville, KY.

⁹ "Syloid 378" from W. R. Grace Co., Baltimore, MD*

10 "irgacure 651" 2,2-dimethoxy-2-phenyl acetophenone from Ceiba- Geigy, Hawthorne, NY.

COMPARATIVE EXAMPLE A

Percent

Component by Weight Esterdiol diacrylate¹ 84.0 Celluose acetate butyrate² 2.4 Lampblack³ 2.4 Polyethylene wax⁴ 1.0 Silica flatting pigment⁵ 9.2 Photoinitiator⁶ 1.0

1 Acrylated "Esterdial ED-204" from Startomer Corp., Exton, PA.

2 Thickener from Eastman Chemicals, Kingsport, TN.

³ #6 Amorphous carbon black from General Carbon Co. Los Angeles, CA. "Polymist B-6" (6 micron) from Allied Signal Inc. Morristown, NJ.

⁵ "Syloid 378" from W. R. Grace Co., Baltimore, MD-

6 "Irgacure 651" 2,2-dimethoxy-2-phenyl acetophenone from Ceiba Geigy, Hawthorne, NY.

The coatings of Example 1 and Comparative Example A were each applied by a curtain coater at a thickness of 2.2 mils onto pre- filled substrate (flat stock medium density fiberboard) and cured via electronbeam energy at the following conditions: 250 kilovolts potential (220KV - 275KV) , total dose of 5 megarads achieved at a production line speed of 120 ft./min. in a partially inerted atmosphere by use of nitrogen to 14% oxygen content (10% - 15%) . The second stage cure employed ultraviolet light exposure with 200 watt/in. medium pressure mercury vapor lamps at a speed of 10 ft./min. per lamp in a totally inerted atmosphere by use of nitrogen to 500 ppm of oxygen (0 - 1000 ppm) . The results are set forth in Table I.

Comparative

Example 1 Example A Gloss (60° glossmeter) 15 26

Cross-hatch adhesion Pass Pass

Taber abrasion (mg loss/100 cycles CS-32 -109.2 MG -122.3 MG wheels, 550 gm load) High temperature resistance Pass Pass

Boiling water Pass Pass Impact resistance - (8 oz. falling ball) 20 in. 20 in.

NEMA stains 16 hr. covered Pass Pass

Mottle/appearance Excellent Poor smooth/uniform low gloss spots

(mottled)

The series of trials reported in Table II illustrate the difficulty of producing mottle-free, low gloss black coatings without the viscosity control resin of the present invention. Comparative Example B had the following composition:

COMPARATIVE EXAMPLE B

Percent

Component by Weight Tripropylene glycol diacrylate 76.0 Celluose acetate butyrate¹ 3.0 Lampblack² 3.0 Polyethylene wax³ 1.0 Silica flatting pigment⁴ 1.8

Photoinitiator⁵ 1.2 Black silicaδ 14.0

1 Thickener from Eastman Chemicals, Kingsport, TN.

2 #6 Amorphous carbon black from General Carbon Co. Los Angeles, CA.

³ "Polymist B-6" (6 Micron) from Allied Signal Inc. Morristown, NJ.

⁴ "OK-412" from DeGussa Co., Frankfurt, Germany

5 "Irgacure 651" 2,2-dimethoxy-2-phenyl acetophenone from Ceiba- Geigy, Hawthorne, NY.

6 "Ebony Novacite" (25 micron) from Malvern Minerals Co., Hot Springs, AR Comparative Example C constituted 87.5 percent by weight of the composition of Comparative Example A with the addition of 10 percent by weight hexanediol diacrylate, 1.5 percent additional Irgacure photoinitiator, and 1..0 percent benzophenone photoinitiator.

TABLE II

E.B. Dose Ξ B. 0₂ 60° Gloss

Coatinq (Meσarads) (%) Appearance

Comp. Ex. B 5 6 22 Bad mottle 5 9 20 Mottled 5 12 24 Mottled 8 6 19 Mottled 8 9 21 Mottled 8 12 19 Mottled

Comp. Ex. C - 5 6 17 Mottled-fair 5 9 19 Mottled-fair 5 12 21 Mottled-poor 8 6 15 Mottled 8 9 14 Mottled 8 12 17 Mottled

2. 5 9 29 Mottled

The invention has been disclosed in connection with specific embodiments in order to provide the best mode of the invention, but it should be understood that other variations and modifications as would be known to those of skill in the art can be resorted to within the scope of the invention as defined by the claims which follow:

Claims

THE CLAIMS :

1. A dark, pigmented, radiation curable coating composition adapted to produce a low gloss film, comprising: a resin binder curable by radiation exposure in the presence of at least one photoinitiator compound; photoinitiator compound; and black silica pigment.

2. The composition of claim 1 wherein the black silica pigment comprises silica, carbon, and aluminum oxide.

3. The composition of claim 2 wherein the photoinitiator is an alkoxy substituted acetophenone.

4. The composition of claim 3 wherein the resin binder is a polyacrylate.

5. The composition of claim 1 further including an acrylate functional acrylic polymer viscosity control agent different from the resin binder and having a molecular weight higher than that of the uncured resin binder.

6. The composition of claim 5 wherein the viscosity control agent comprises an acrylate functional acrylic polymer.

7. The composition of claim 5 wherein the viscosity control agent has a viscosity greater than 3000 centipoises at 60°C.

8. The composition of claim 1 comprising:

20-80 weight percent of the binder resin; 0.01-4 weight percent of photoinitiator; and 5-20 weight percent of the black silica pigment.

9. The composition of claim 1 comprising:

30-70 weight percent of the binder; 0.1-2 percent of photoinitiator; 5-20 weight percent of the pigment; and 5-40 weight percent of a viscosity control polymer.

10. The composition of claim 9 wherein the viscosity control polymer comprises an acrylate functional acrylic polymer.

11. The composition of claim 1 wherein the binder resin comprises acrylic monomer.

12. A method of producing a dark, pigmented, low gloss, radiation cured coating comprising: applying to a substrate a coating composition comprising a radiation curable resin binder, a photoinitiator and black silica pigment; in a first exposure step exposing the coating in the presence of oxygen to ionizing radiation so as to at least partially cure a subsurface layer of the coating while leaving an at least partially uncured surface layer; in a subsequent exposure step exposing the coating in a substantially inert atmosphere to actinic radiation sufficient to cure the surface of the coating to produce a low gloss, wrinkled surface.

13. The method of claim 12 wherein the ionizing radiation of the first exposure step is applied by way of an electron beam at a dose of 2 to 10 megarads.

14. The method of claim 12 wherein the actinic radiation of the subsequent exposure step comprises ultraviolet radiation.

15. The method of claim 12 wherein said substantially inert atmosphere contains less than 1000 parts per million of oxygen.

16. The method of claim 12 wherein said substantially inert atmosphere contains less than 500 parts per million of oxygen.