GB1571060A - Actinic light polymerizable coating compositions - Google Patents

Description

(54) ACTINIC LIGHT POLYMERIZABLE COATING COMPOSITIONS (71) We, PPG INDUSTRIES, INC., a corporation organised and existing under the laws of the Commonwealth of Pennsylvania, United States of America, of One Gateway Center, Pittsburgh, Commonwealth of Pennsylvania 15222, United States of America, (assignee of GERALD WILLIAM GRUBER), do hereby declare the invention, for which-we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The use of actinic light polymerizable compositions is becoming more widespread. This growing interest is primarily occasioned by the low power requirements of actinic light sources as compared to thermal ovens, by the low levels of environmental pollution which can be obtained, and by the minimal space required for actinic light curing equipment. Nonetheless, several problems have arisen which have retarded the use of actinic light curable compositions in 'certain areas.

One problem is that the presence of a photopolymerization activator often imparts an unacceptable degree of instability to the compositions during storage prior to use.

Another problem is that during polymerization by exposure to actinic light, the photopolymerization activator or the reaction products of the photopolymerization activator often imparts an undesirable color to the polymerized composition.

In some end uses, as for example in the coating of food and beverage containers, it is frequently desirable to apply an actinic light polymerizable composition to one side of a substrate and cure it by exposure to actinic light. The other side of the substrate is then coated with a heat curable coating composition and polymerized by baking at elevated remperatures in an oven. Often the presence of the photopolymerization activator or its reaction products in the actinic light cured coating causes the exterior coating to discolor.

Another problem that has arisen regarding the use of actinic light polymerizable coating compositions is that of achieving adequate hiding in coating systems where hiding of the substrate is desired. In theory, one method of obtaining hiding is to incorporate pigments into the coating composition. Unfortunately, many of the pigments known to provide opacity to non-radiation curable coatings absorb strongly in most areas of the ultraviolet light region, viz., electromagnetic radiation having wavelengths in the range of from about 180 nanometers to about 400 nanometers. This absorption prevents adequate quantities of ultraviolet light from penetrating very far into the interior of the film. The result is inadequate polymerization of the interior region of the coating, or using the terminology of the art, inadequate "through cure". Ultraviolet light absorbing hiding pig ments which are desirably used in coating compositions due to their excellent hiding characteristics include, but are not limited to, titanium dioxide (including rutile and anatase), zinc sulfide, zinc oxide, antimony tri oxide and lithopone. The preferred pigment is titanium dioxide. Rutile is especially preferred.

It has now been found that the use of 9,10phenanthrenequinone in actinic light polymerizable coating compositions containing ultraviolet light absorbing hiding pigments allows the interior of coating of the com position to be adequately polymerized.

Although it is not desired to be bound by any theory, it is believed that the reason phenanthrenequinone performs satisfactorily is as follows: Although the pigments absorb light strongly in the ultraviolet region at a wavelength of about 200 nanometers, the absorption diminishes as the wavelength of light is in creased. At about 400 nanometers, the absorp tion has diminished to a small value so that a significant portion of such light is able to reach the interior of the coating. The absorption of phenanthrenequinone, however, remains at a high value well into the visible spectrum before it too diminishes to a small value. Phenanthrenequinone is, therefore, suit able for pigmented systems because it absorbs actinic light having a wavelength in a region of substantial pigment transparency and uses the energy of the absorbed photons to produce free radicals capable of causing polymerization of organic polymerizable material containing a plurality of sites of ethylenic unsaturation.

The use of phenanthrenequinone alone is not without disadvantage, however. Since phenanthrenequinone absorbs photons, as indeed it must in order to produce free radicals, and since much of the adsorption is in the violet and blue regions of the visible spectrum, phenanthrenequinone is a highly colored compound having a yellow to orange hue.

Although the photoreaction of phenanthrenequinone produces compounds which do not significantly absorb in the visible region and hence are not colored, some of the phenanthrenequinone remains unreacted at the end of the polymerization process and imparts a yellow or orange color to the polymerized coating. Yellow and orange coatings are not desirable where opaque white coatings or opaque coatings of colors other than yellow or orange are desired. A further disadvantage is that phenanthrene quinone does not materially reduce the oxygen inhibition of the polymerization process.

It is now been found that the presence of at least one aromatic ketone photopolymerization activator which has a triplet energy in the range of from 54 kilocalories per mole to n kilocalories per mole causes more complete reaction of the phenanthrenequinone in the thin surface layer resulting in a much whiter appearance of the coating.

Accordingly the present invention provides an actinic light polymerizable coating composition comprising: a. at least one aromatic ketone photopoly merization activator having a triplet energy in the range of from 54 kilo calories per mole to 72 kilocalories per mole; b. phenanthrenequinone; c. organic polymerizable material contain ing a plurality of sites of ethylenic un saturation and capable of being free radically addition polymerized by inter action with said aromatic ketone photo polymerization activator and said phen anthrenequinone upon exposure to actinic light, said organic polymerizable material comprising ethylenically un saturated polyester containing a plurality of sites of ethylenic unsaturation, poly mer having a plurality of sites of acrylic unsaturation, monomer having a plurality of sites of acrylic unsaturation or mix ture thereof; and d. ultraviolet light absorbing hiding pigment The present invention also provides a method comprising: A) coating a substrate with an actinic light polymerizable coating composition com prising: a. at least one aromatic ketone photopoly merization activator having a triplet energy in the range of from 54 kilo calories per mole to 72 kilocalories per mole; b. phenanthrenequinone; c. organic polymerizable material contain ing a plurality of sites of ethylenic un saturation and capable of being free radically addition polymerized by inter action with said aromatic ketone photo; polymerization activator and said phen anthrenequinone upon exposure to actinic light, said organic polymerizable material comprising ethylenically un saturated polyester containing a plurality of sites of ethylene unsaturation, poly mer having a plurality of sites of acrylic unsaturation, monomer having a plurality of sites of acrylic unsaturation or mixture thereof; and d. ultraviolet light absorbing hiding pig ment; and B) exposing said coated substrate to actinic light of the first kind and to actinic light of the second kind, said actinic light of the first kind (1) having a wavelength in the ultraviolet region of the spectrum such that said ultraviolet light absorbing hiding pig ment is substantially opaque thereto, and (2) being absorbable by said photopoly merization activator to produce free radicals capable of causing polymeriza tion of acrylic groups, said actinic light of the second kind (3) having a wavelength longer than that of said actinic light of the first kind and such that said ultraviolet light absorbing pigment is substantially transparent thereto, and (4) being absorbable by said phen anthrenequinone to produce free radi cals capable of causing polymerization of acrylic groups to thereby polymerize said coating into a hard, infusible film throughout its thickness.

It is preferred that the sites of ethylenic unsaturation of the organic polymerizable material be sites of acrylic unsaturation.

As used throughout the present specification and claims, unless otherwise indicated, the term "acrylic unsaturation" is used in its broad sense to mean the unsaturation provided by unsubstituted acrylyl groups or asubstituted acrylyl groups such as methacrylyl, ethacrylyl and er-chloroacrylyl groups.

As used throughout the specification and daims, unless otherwise indicated, the term "binder" identifies the primary resin which forms a film on a substrate and which functions to hold together other optional ingredients of the coating compositions, such as pigment, dispersants or viscosity control agents.

Examples of photopolymerization activators which may be used in the present invention are: benzil 3,4-benzofluorene 1 -acetylnaphthalene 1-benzoylnaphthalene 9-acetyiphenanthrene 3-acetylphenanthrene 2-acetylnaphthalene 2-benzoylnaphthalene 4-phenylbenzophenone 4-phenylacetophenone anthraquinone 2-methylanthraquinone thioxanthone 2-chlorothioxanthone 3,4-methylenedioxyacetophenone 4-cyanoberizophenone 4-benzoylpyridine 2-benzoylpyridine 4,4'-dichlorobenzophenone 4-trifiuoromethylbenzophenone 3 -methoxybenzophenone 4-chlorobenzophenone 3 -chlorobenzophenone 3 benzoylpyridine 4-methoxybenzophenone 3,4-dimethylbenzophenone 4-methylbenzophenone benzophenone 2-methylbenzophenone 4,4'-dimethylbenzophenone 2,5-dimethylbenzophenone 2,4-dimethylbenzophenone 4-cyanoacetophenone 4- luorobenzophenone o-benzoylbenzophenone 4,4'-dimethoxybenzophenone 4-acetylpyridine 3,4,5-trimethylacetophenone 3 ,5 -dimethylacetophenone 4-bromoacetophenone 4-methoxyacetophenone 3,4-dimethylacetophenone triphenylmethylacetophenone anthrone 4-chioroacetophenone 4-trifluoromethylacetophenone 2-chloroanthraquinone o-benzoylbenzoic acid ethyl benzoylbenzoate dibenzosuberone o-benzoylbenzophenone acrylyloxyethyl benzoylbenzoate 4-acrylyloxybenzophenone 2-acrylyloxyethoxybenzophenone A preferred class of photopolymerization activator comprises the aromatic ketones represented by the formula:

wherein R is hydrogen, alkyl containing from one to twenty-two carbon atoms, benzyl, phenyl, hydroxy-alkyl containing from one to ten carbon atoms, chioroalkyl containing from one to ten carbon atoms, bromoalkyl containing from one to ten carbon atoms, alkoxyalkyl where the alkoxy portion contains from one to four carbon atoms and where the alkyl portion contains from one to ten carbon atoms, or phenoxyalkyl where the alkyl portion contains from one to ten carbon atoms; X is hydrogen, halogen, alkoxy containing from one to four carbon atoms or alkyl containing from one to four carbon atoms; R is preferably alkyl containing from one to twelve carbon atoms, benzyl or phenyl. Most preferably, R is alkyl con taining from one to four carbon atoms, alkoxyalkyl where the alkoxy portion contains from about one to four carbon atoms and where the alkyl portion con tains from one to six carbon atoms or phenyl. X is preferably hydrogen or chloro. X is preferably located in the ortho position although the meta and para positions are satisfactory.

Additional minor substituents which do not render the compound unsuitable for its intended purpose may be placed on the phenyl ring.

Examples of these photopolymerization activators are: methyl phenylglyoxylate ethyl phenylglyoxylate butyl phenylglyoxylate benzyl phenylglyoxylate butoxyethyl phenylglyoxylate phenoxyethyl phenylglyoxylate dodecyl phenylglyoxylate phenyl phenylglyoxylate ethyl chlorophenylglyoxylate phenylglyoxylic acid.

The preferred photopolymerization activators are methyl phenylglyoxylate, ethyl phenylglyoxylate, butyl phenylglyoxylate and butoxyethyl phenylglyoxylate. The esters of phenylglyoxylic acid may be prepared by reacting phenylglyoxylol chloride (Kharasch and Brown, Jounl of the American Chemical Society, vol. 64, page 329 et seq. [1942]) with the appropriate alcohol.

These photopolymerization activators are useful in the photopolymerization of organic polymerizable material containing ethylenic unsaturation. The polymerization reaction may be used to form polymers, solutions of polymers, polymer emulsions coatings3 adhesives, and photoresists. A particularly preferred use is in the formation of hard, infusible thermoset coatings.

The amount of phenanthrenequinone present in the coating composition may be widely varied. Ordinarily, the phenanthrenequinone is present in an amount in the range of from 0.005 percent to 5 percent by weight based on the weight of the binder of the coating composition. Most often, an amount in the range of from 0.01 percent to 3 percent is used. From 0.1 percent to 1 percent by weight based on the weight of the binder is preferred.

The ultraviolet light absorbing hiding pigment constitutes for example from 5 percent to 70 percent by weight of the polymerizable coating composition. From 20 percent to 70 percent is typical. An amount in the range of from 33 percent to 50 percent by weight is preferred.

The ethylenically unsaturated polyesters constitute a useful class of organic, polymerizable material. These polyesters are ordinarily esterification products of ethylenically unsaturated polycarboxylic adds and polyhydric alcohols. Usually, the ethylenic unsaturation is in the alpha, beta position.

The ethylenically unsaturated polycarboxylic acids include maleic acid, fumaric acid, aconitic acid, itaconic acid, citraconic acid, mesaconic acid, muconic acid and dihydromuconic acid and halo and alkyl derivatives of such acids. The preferred acids are maleic acid and fumaric acid. Especially preferred is maleic acid. Mixtures of ethylenically unsaturated polycarboxylic acids may be used or only a single such acid may be employed.

The anhydrides of these acids, where the anhydrides exist, are, of course, embraced by the term "acid", since the polyesters obtained therefrom are essentially the same whether the acid or anhydride is used in the reaction.

One or more saturated polycarboxylic acids may optionally be utilized in combination with the ethylenically unsaturated acid or anhydride in the preparation of unsaturated polyesters.

Such acids, especially the saturated dicarboxylic acids, increase the length of the polyester without adding additional crosslinking sites, which is a desired feature in some polyesters. Saturated tricarboxylic acids and saturated acids of higher carboxylic functionality may be used to provide branching where this is desirable.

For purposes of the present invention, the aromatic nuclei of aromatic acids such as phthalic acid are generally regarded as saturated since the double bonds do not ordinarily react by addition as do ethylenic groups. Therefore, wherever the term "saturated" is -utilized, it is to be understood that such term includes aromatic unsaturation or other form of unsaturation which does not react by addition, unless otherwise qualified.

Examples of useful saturated polycarboxylic acids include oxalic acid, malonic acid, succinic acid, methylsuccinic acid, 2,2-dimethylsuccinic acid, 2,3 -dimethylsuccinic acid, hexylsuccinic acid, glutaric acid, 2-methylglutaric acid, 3methylglutaric acid, 2,2-dimethylglutaric acid, 3,3-dimethylglutaric acid, 3,3-diethylglutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebaccic acid, phthalic acid, isophthalic acid, terephthalic acid, tetrachlorophthalic acid, I,2-hexahydrophthalic acid, 1,3hexahydrophthalic acid, 1,4-hexahydrophthalic acid, 1,1-cyclobutanedicarboxylic add and mms-1,4-cyclohexanedicarboxylic acid. As in the case of the ethylenically unsaturated polycarboxylic acids, the anhydrides of the saturated acids, where anhydrides exist, are embraced by the term "acid" since the polyesters obtained therefrom are essentially the same.

The ethylenically unsaturated polycarboxylic acids are usually present in an amount in the range of from 10 mole percent to 100 mole percent of the polycarboxylic acids employed.

Preferably, they are present in the range of from 50 mole percent to 100 mole percent.

The polyhydric alcohols useful in preparing ethylenically unsaturated polyesters include saturated polyhydric alcohols such as ethylene glycol, 1,3-propanediol, propylene glycol, 2,3butanediol, 1,4-butanediol, 2-ethylbutane-1,4diol, 1,5-pentanediol, 1,6-hexanediol, 1,7heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 2,10-decanediol, 1,4-cyclohexanediol, 1,4-dimethylolcyclohexane, 2,2-di ethylpeopane - 1,3 - diol, 2,2 - dimethyl propane - 1,3 diol, 3 - methylpentane - 1,4diol, 2,2 - diethylbutane - 1,3 - diol, 4,5nonanediol, diethylene glycol, triethylene glycol, dipropylene glycol, glycerol, pentaerythritol, erythritol, sorbitol, mannitol, 1,1,1trimethylolpropane, trimethylolethane, and 2,2 - dimethyl - 3 - hydroxypropyl 2,2 - dimethyl - 3 - hydroxypropionate. Ethy- lenically unsaturated polyhydric alcohols such as 2-butene-1,4-diol may be used alone or in admixture with the saturated polyhydric alcohols. Of course, mixtures of saturated polyhydric alcohols or mixtures of unsaturated polyhydric alcohols may be employed. If unsaturated polyhydric alcohols are used to introduce ethylenic unsaturation into the poly ester, the preparation of ethylenically un saturated polycarboxylic acid may be reduced correspondingly, if desired.

A mixture of ethylenically unsaturated poly esters containing a plurality of sites of ethylenic unsaturation may be used, if desired.

Another useful class of organic polymeriz able material is polymer having a plurality of sites of acrylic unsaturation. The sites of acrylic unsaturation may. be provided by acrylyl groups or a-substituted acrylyl groups such as methacrylyl, ethacrylyl and a-chloro- acrylyl. The sites of acrylic unsaturation may be terminal groups of the polymer, they may be in sidechains attached to the molecular backbone of the polymer or both.

Polymers having acrylic unsaturation in sidechains attached to the molecular backbone are usually prepared by including one or more monomers which, when interpolymerized with other monomers, to form the polymer, pro vides reactive sites attached to the polymer along the backbone. Acrylically unsaturated compounds having at least one functional group which will react with the reactive sites on the polymeric backbone are then used to introduce the acrylic unsaturation into the molecule. The usual reactive sites attached directly or indirectly to the polymer are hydroxy, amino, carboxy, carbamyl, isocyanato or epoxy. Hydroxy or carboxy are most often used. When the reactive sites are hydroxy, the acrylically unsaturated compound usually has carboxy, haloformyl (most often chloro formyl) or isocyanato functionality. When the reactive sites on the polymer are amino, the acrylically unsaturated compound usually - has isocyanato, - haloformyl (again, most often chloroformyl) or epoxy functionality. When the reactive sites on the polymer are carboxy, the acrylically unsaturated compound generally has hydroxy, epoxy or isocyanato functionality.

When the reactive sites are carbamyl, they are usually reacted with formaldehyde to pro duce N-methylol carbamyl groups. When the reactive sites are isocyanato, the acrylically unsaturated compound ordinarily contains hydroxy or carboxy functionality. When the reactive sites are epoxy (usually glycidyl), the acrylically unsaturated compound generally has carboxy functionality. The acrylically un saturated compound ordinarily contains carboxy, haloformyl or isocyanato functionality.

The polymer having reactive sites attached thereto can itself be any of many types, as for example, polyacrylates, polyamides, poly esters, polyethers or polyurethanes.

The term polyacrylate is used in its broadest sense to include not only polymerized unsub stituted acrylates, but also polymerized a substituted acrylates, such as methacrylates, ethacrylates and a-chloroacrylates. Compounds from any of these subclasses may be used alone, but most often, compounds from two or more subclasses are interpolymerized.

Examples of suitable monomers which may be used in the preparation of the polyacrylate polymer include methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, isobutyl acrylate, sec-butyl acrylate, tert-butyl acrylate, amyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, butyl methacrylate, isobutyl methacrylate, sec-butyl methacrylate, tert-butyl methacrylate, amyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, dodecyl methacrylate, methyl achloroacrylate, ethyl a-chloroacrylate, propyl a-chloroacrylate, hexyl a-chloroacrylate, octyl a-chloroacrylate5 decyl a-chloroacrylate and dodecyl a-chloroacrylate. Esters of unsubstituted acrylic acid and methacrylic acid are most often used.

Acrylic monomers which introduce reactive sites to the polymer molecule include acrylic acid, 2-hydroxyethyl acrylate3 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, glycidyl acrylate, acrylamide, 2-aminoethyl acrylate, methacrylic acid, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, glycidyl methacrylate, methacrylamide, Z-aminoethyl methacrylate, 3-aminopropyl methacrylate and a-chloroacrylic acid.

Other ethylenically unsaturated monomers are often included. Examples of these compounds are styrene and a-methylstyrene.

The amount of acrylic monomers which are used to introduce reactive sites to the polymer molecule may vary widely, but they are ordinarily present in the range of from 3 percent to 50 percent by weight of the ethylenically unsaturated monomers interpolymerized. An amount in the range of from 4 percent to 25 percent is most often the case.

Addition polymerization may be effectuated by combining the ethylenically unsaturated monomers with a free radical initiator and heating the mixture. Exemplary free radical initiators are organic peroxides such as ethyl peroxide and benzoyl peroxide; hydroperoxides such as methyl hydroperoxide, certain azo compounds such as a,a'-azobisisobutyronitrile and yiy'-azobis(yicyanavaleric acid); persulfates; peracetates such as methyl peracetate and tert-butyl peracetate; peroxalates such as dimethyl peroxalate and di(tert-butyl) peroxalate; disulfides such as dimethyl thiuram disulfide and ketone peroxides such as methyl ethyl ketone peroxide. The polymerization may be accomplished in the presence or absence of an inert solvent. Temperatures in the range of from 75"F. to 400"F. are generally employed. More often, temperatures in the range of from 1000F. to 3000F. are used.

When the polymer is a polyamide, polyester, polyether or polyurethane, the principles are analogous to those given for the polyacrylates. The known reactions for forming such polymers will, of course, be used instead of the addition polymerization reaction illustrated above for the polyacrylates.

Other examples of satisfactory polymers having a plurality of sites of acrylic unsaturation are acrylic polyester and acrylic polyamide molecules represented by the formulae:

wherein n is an integer in the range of from 1 to 4; each R independently represents a divalent aliphatic, cycloaliphatic or aromatic hydro carbon radical having from 1 to 10 carbon atoms; each R' independently represents hydro; methyl or ethyl; and each A independently represents 0 or NH.

It is preferred that every A represent 0. The polyester and polyamide oligomers represented by formula (I) may be prepared by reacting dicarboxylic acids or acid amides and dihydric alcohols or diamines and then reacting the product with an unsubstituted acrylic acid or an a-substituted acrylic acid. The acrylic poly ester and polyamide oligomers represented by formula (II) may be prepared by reacting a hydroxy functional monocarboxylic acid, a dimer, trimer or a tetramer of such acid, an amino functional monocarboxylic acid or a dimer, trimer or tetramer of such acid with an unsubstituted or a-substituted acrylic acid.

Where desired, the lactone may be used n lieu of the hydroxy functional monocarboxylic acid and the lactam may be used in place of the amino functional monocarboxylic acid.

A mixture of polymers having a plurality of sites of acrylic unsaturation may be used, if desired.

Another useful class of organic polymeriz able material is monomer having a plurality of sites of acrylic unsaturation. Such mono mers generally comprise divalent, trivalent or tetravalent organic radicals whose bonds are satisfied with unsubstituted acrylyloxy or (z- substituted acrylyloxy groups. The polyvalent radical may be aliphatic, cycloaliphatic or aromatic. Usually, the molecular weight of the monomer is in the range of from 170 to 1000.

Examples of such monomers are the diacrylates and dimethacrylates of ethylene glycol, 1,3 -propanediol, propylene glycol, 2,3butanediol, 1,4-butanediol, 2-ethylbutane 1,4- diol, 1,5-pentanediol, 1,6-hexanediol, 1,7heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 2,10-decanediol, 1,4-cyclohexanediol, 1,4-dimethylolcyclohexane, 2,2-dimethylpropane - 1,3 - diol, 3 - methylpentane1,4 - diol, 4,5 - nonanediol, diethylene glycol, triethylene glycol, propylene glycol, 5,5dimethyl - 3,7 - dioxanonane - 1,9 - diol, 2,2 - dimethyl - 3 - hydroxypropyl 2,2 - dimethyl - 3 - hydroxypropionate, Bisphenol A-diglycidyl ether, 1,4-butanediol diglycidyl ether and neopentyl glycol diglycidyl ether; the triacrylates, trimethacrylates, diacrylates and dimethacrylates of glycerol, 1,1,1-trimethylolpropane and trimethylolethane; and the tetracrylates, tetramethacrylates, triacrylates, trimethacrylates, diacrylates and dimethacrylates of pentaerythritol and erythritol.

The acrylic groups on the monomer molecules are usually the same, but they may be different as exemplified by the compound 2,2 dimethyl - 1 - acrylyloxy - 3 - methacrylyloxypropane.

A mixture of monomers having a plurality of sites of acrylic unsaturation may be used, if desired.

Additional monomers having one or more vinyl groups which crosslink with the organic polymerizable material containing a plurality of sites of ethylenic unsaturation heretofore described may optionally be present in the composition. Examples are N-vinyl-2-pyrrolidone, styrene, a-methylstyrene, divinyl benzene, vinyl toluene, vinyl benzoate, vinyl acetate, vinyl propionate and diallyl phthalate. Particularly preferred are monomers having monoacrylic functionality which crosslink with the resin having acrylic unsaturation which may optionally be present in the coating composition. Examples of monoacrylic 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, octyl acrylate and octyl methacrylate. The preferred vinyl functional monomers are liquid compounds miscible with the resin. The use of one or more vinyl functional monomers is desirable. A benefit is that the vinyl functional monomer usually acts as a reactive solvent for the resin thereby providing compositions having a satisfactorily low viscosity without using an inordinate amount, if any at all, of volatile, nonreactive solvent.

The vinyl functional monomer, or mixtures of vinyl functional monomers, may be em ployed over a broad range. At the lower end of the range, no vinyl functional monomer need be used. At the upper end of the range, about 80 percent by weight of the binder can be vinyl functional monomer. Often, the vinyl functional monomer will be present in the coating composition in the range of from 1 to 80 percent by weight of the binder of the coating composition. Ordinarily when used, the vinyl functional monomer will be in the range of from 15 to 30 percent by weight of the binder.

Extender pigments which are generally transparent to ultraviolet light are optional ingredients which are often included in the composition. Examples of suitable extender pigments are finely divided particles of silica, barytes, calcium carbonate, talc, magnesium silicate and aluminum silicate. When used, extender pigment is generally present in an amount in the range of from 1 to 70 percent by weight of the composition. An amount in the range of from 1 to 50 percent is more often employed. Most often, it is present in the range of from 1 to 35 percent by weight of the composition. Although a single extender pigment is ordinarily used, mixtures of several extender pigments are satisfactory.

Ultraviolet light absorbing pigments as used in amounts which do not preclude polymerization of the interior of the actinic light curable composition. The maximum amount is therefore related to the thickness of the composition to be polymerized. Thin coatings may tolerate more ultraviolet light absorbing pigment than thick coatings. When ultraviolet light absorbing pigment is used, it is usually present in the range of from 1 percent to 70 percent by weight based on the weight of the binder. For thicker sections, from 1 percent to 50 percent are ordinarily satisfactory.

Examples of suitable ultraviolet light absorb ing pigments are titanium dioxide, antimony oxide, zinc oxide, zirconium oxide, zinc sul fide and lithopone. Mixtures of pigments may be used.

Another optional ingredient which is often included in the composition is an inert volatile organic solvent. Mixtures of several inert volatile organic solvents may be used when desired. Examples of suitable inert volatile organic solvents are acetone, methyl ethyl ketone, methyl isobutyl ketone, methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, sec-butyl alcohol, iso butyl alcohol, tert-butyl alcohol, amyl alcohol, hexyl alcohol, 2-ethylhexyl alcohol, cellosolve, ethyl cellosolve, cellosolve acetate, 2-ethylhexyl acetate, tetrahydrofuran, and aliphatic naphtha. ("Cellosolve" is a Trade Mark).

When inert volatile organic solvent is used, it is usually present in the range of from 1 to 15 percent by weight of the vehicle.

Another optional ingredient is resinous pigment dispersant or grinding vehicle. There are many resinous dispersants which are commer cially available for that purpose. These resins are often low molecular weight resins which have a high carboxyl content. Illustrative of such pigment dispersants are the so-called acrysol dispersants such as Acrysol 1-94, a copolymer of butyl acrylate, methyl meth acrylate, styrene and acrylic acid, available commercially from the Rohm and Haas Com pany. These dispersants are used in the man ner and in amounts known to the art.

Conventional plasticizers such as dibutyl phthalate, butyl benzyl phthalate, diisooctyl phthalate, decyl butyl phthalate, diisooctyl adipate, dibutyl sebacate, butyl benzoate tri isooctyl trimellitate, n-octyl n-decyl trimel litate, and tricresyl phosphates and flow pro moters such as phenyl benzoate, dibenzyl ketone and benzyl methyl ketone may also be optionally included in amounts customary in the art.

Various conventional chain modifying agents or chain-transfer agents may be included in the mixture. The preferred chaintransfer agents are the mercaptan compounds such as dodecyl mercaptan, tertiary-dodecyl mercaptan, octyl mercaptan and hexyl mer captan. The quantity and manner of use are also known in the an.

Any of the conventional viscosity control agents may be optionally employed in the composition. The preferred materials are resinous or polymeric viscosity control agents.

Many of these resinous materials are available. Illustrative of such materials are cellulose acetate butyrate and sodium carboxymethyl cellulose. The use of such resinous or polymeric viscosity control agents is advantageous in that it permits the mixture to be prepared in the form of a viscous mass or syrup having sufficient viscosity to remain in place on the substrate until polymerization is effected. These viscosity control agents are used in the manner and in amounts known to the art.

The amount of aromatic ketone photopolymerization activator present in the actinic light polymerizable compositions of the irrven- tion may be widely varied. Usually, the photopolymerization activator is present in an amount in the range of from 0.01 percent to 50 percent based on the weight of the binder of the coating composition. More often, an amount in the range of from 0.1 percent to 20 percent is employed. From 0.3 to 10 percent by weight based on the weight of the binder is preferred.

The amount of organic polymerizable material having a plurality of sites of ethylenic unsaturation present in the polymerizable composition is subject to wide variation. The material is ordinarily present in an amount in the range of from 20 to 100 percent by weight of the binder of the composition. An amount in the range of from 50 to 100 per cent is typical. From 80 to 100 percent by weight of the binder is preferred.

The polymerizable compositions of the invention are usually prepared by simply admixing the various ingredients. The compounds comprising the photocatalyst 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 200"C. are only rarely employed.

The actinic light polymerizable compositions of the invention are generally used to form cured adherent coatings on substrates.

The substrate is coated with the polymerizable composition using substantially any technique known to the art. These include spraying, curtain coating, dipping, roller application, printing, brushing, drawing and extrusion.

The coated substrate is then exposed to ultraviolet light to cure (viz., C-stage) the coating into a hard, infusible film throughout its thickness. t1 The thicknesses of polymerized coatings of the actinic light polymerizable composition of the invention are subject to wide variation.

Usually, such thicknesses are in the range of from 0.001 millimeter to 1 millimeter. More often, they are in the range of from 0.005 millimeter to 0.3 millimeter. Typically, they are in the range of from 0.012 millimeter to 0.15 millimeter. When the actinic light polymerizable composition is an actinic light polymerizable printing ink, the polymerized coatings usually have thicknesses in the range of from 0.001 millimeter to 0.03 millimeter.

The pigmented coating compositions containing phenanthrenequlnone may be polymerized by sequentially exposing the coated substrate to actinic light of the first kind and then to actinic light of the second kind. Polymerization may also be accomplished by sequentially exposing the coated substrate to actinic light of the second kind and then to actinic light of the first kind. Preferably, however, the coated substrate is exposed simultaneously to actinic light of the first kind and to actinic light of the second kind.

Any suitable sources of actinic light of the first kind and actinic light of the second kind may be used in the practice of this invention.

Separate sources for the two kinds of actinic light may be used, but it is preferred to employ a source which emits both actinic light of the first kind and actinic light of the second kind.

Suitable sources are mercury arcs, carbon arcs, low pressure mercury lamps, medium pressure mercury lamps, high pressure mercury lamps, swirl-flow plasma arc, radio frequency induced mercury lamps and ultraviolet light emitting xenon flash lamps. 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. Examples of these lamps are PPG Models 602032/604393, 600197 and 6S2031 and Hanovia (Registered Trade Mark) Models 6512A431, 6542A431, 6565A431 and 6577A431.

The wavelengths of electromagnetic radiation which are suitable for use as actinic light of the first kind and actinic light of the second kind may be ascertained from the general principals and definitions of the two kinds of actinic light heretofore set out. Usually, but not necessarily, actinic light of the first kind has a wavelength in the range of from 185 to 380 nanometers and actinic light of the second kind has a wavelength in the range of from 380 to 500 nanometers.

The times of exposure to actinic light and the intensity of the actinic light to which the polymerizable composition is exposed may vary greatly. Generally, the exposure to actinic light is continued until the C-stage is reached where the film is hard and infusible throughout its thickness, although in some instances, as for example in the formation of pressure sensitive adhesives, the exposure is continued only to form a gel (viz., B-stage).

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 resigns, 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. Strippable substrates or substrates including a release coating are sometimes used, particularly when the polymerized composition is an adhesive.

The photopolymerization of compounds containing acrylyl or a-substituted acrylyl groups such as methacrylyl groups is often inhibited by the presence of oxygen. The oxygen content of air is, in many instances, sufficient to preclude curing the thin layer of the coating having a surface which is adjacent to the air. In many cases, the interior of the coating may be adequately cured, but oxygen inhibition causes the surface to remain tacky and unsuitable for most applications. This phenomenon is known in the art as inadequate surface cure". Although it is not desired to be bound by any theory, it is believed that the inhibition is due to the formation of per oxide at the site of chain propagation which quenches the reaction and thereby terminates chain growth.

It has now been found that the oxygen inhibition of the photopolymerization of resins containing acrylic groups may be substantially reduced by employing at least one aromatic ketone photopolymerization activator which has a triplet energy in the range of from 54 to 72 kilocalories per mole. Accordingly, substrates coated with the polymerizable compositions of the present invention may not only be exposed to actinic light in the presence of an inert atmosphere, viz., an atmosphere either containing no oxygen or only a concentration of oxygen which produces an insignificant degree of polymerization inhibition, but also in the presence of an atmosphere containing a polymerization inhibiting concentration of oxygen, such as air.

The actinic light polymerizable compositions of the present invention are particularly useful for coating steel and aluminum food and beverage cans.

In the illustrative examples which follow, all parts are parts by weight and percentages are percent by weight unless otherwise specified.

Example I.

A reactor equipped with a thermometer, a heater, a cooler, an agitator, a condenser set for total reflux, a source of air and a source of nitrogen is charged with 380.8 parts acrylic acid, 1.87 parts 2,6-di-tert-butyl-4-methyl phenol and 1.86 parts triphenyl phosphine and an air sparge is applied. The charge is then heated to 1100 C. A mixture comprising 385 parts epichlorohydrin and 166.6 parts Epon 828 bisphenol A-diglycidyl ether is preheated to about 1100 C. Over a period of 4 hours, 551.6 parts of the preheated mixture is added to the reactor while maintaining the temperature of the reaction mixture in the range of from llO"C. to 111.70C. Upon completion of the addition, the temperature of the reaction mixture is held in the range of from 1100 C. to 113"C. for 75 minutes. At the end of this period (temperature: 112.2"C.), heat is shut off and cooling is applied. Fifteen minutes later (temperature: 96.1"C.), the condenser is set for distillation, a slight vacuum of 12 kilopascals (1 pascal= 1 newton per square meter) is applied while maintaining an air sparge, and distillation is begun. Two hours later (temperature: 97.8"C.), 21 parts distillate has been removed and the vacuum is removed. Thirty minutes later (temperature: 97.20 C.), a slight vacuum of 10.7 kilopascals is applied while maintaining an air sparge and distillation is again begun. Two hours later (temperature: 97.2"C.), 7 additional parts distillate has been removed and the vacuum and air sparge are removed. Fifteen minutes later (temperature: 97.80C.), the vacuum and air sparge are reapplied. Thirty minutes later (temperature: 97.80C.), the vacuum and air sparge are removed, heat is shut off and cooling is applied. Forty-five minutes later when the temperature has reached 54.4"C., the product is discharged through a filter into containers. This intermediate product, which is a mixture of 3chloro - 2 - hydroxypropyl acrylate, 2chloro - 1 - (hydroxymethyl)ethyl acrylate and the diacrylate of Epon 828 bisphenol Adiglycidyl ether, is found to have an acid number of 3.9, a Gardner-Holdt viscosity of K, a hydroxyl number of 242 and to contain 0.02 percent water and 14.1 percent chlorine.

A reactor equipped with an agitator, a heater, a packed distillation column, a condenser, thermometers and a source of nitrogen is charged with 272.8 parts ethylene glycol, 296 parts phthalic anhydride and 0.57 part butyl stannoic acid. A nitrogen sparge is applied, the contents of the reactor are heated to 195"C. and water is removed from the system. Fifty minutes later, the temperature has risen to 2100C. The temperature is then held in the range of from about 209"C. to about 2100 C. for 40 minutes while water is removed. At the end of this time, the distillation column is bypassed so that the vapor from the reactor is vented through a condenser in a manner such that condensate is not returned to the reactor. The liquid is maintained at temperatures in the range of from about 209"C. to about 211"C. for l- hours and then discharged into containers. The product polyester resin has an acid number of 0.84, a Gardner-Holdt viscosity of Z2+ and a total solids content greater than 99 percent.

A reactor equipped with a heater, a cooler, an agitator, a distillation column, condenser, phase separator, a vacuum source, a source of air and a source of nitrogen is charged with 777 parts of the above polyester resin, 475 parts acrylic acid, 173 parts toluene and 9.6 parts hydroquinone. The condenser and phase separator are set for total reflux. The reaction mixture is heated to 490 C. at an absolute pressure of about 80 kilopascals and 28.7 parts sulfuric acid is added. The absolute pressure is reduced to about 67 kilopascals and refluxing is observed. The condenser and phase separator are set for azeotropic distillation. One hour later, (temperature: 89"C.; absolute pressure: 43 kilopascals), 52 parts water has been removed. After another hour, (temperature: 94"C.; absolute pressure: 36 kilopascals), a total of 98 parts water has been removed. After another 45 minutes, (temperature: 85"C., absolute pressure: 24 kilopascals), a total of 102 parts water has been removed. Heat is then removed and cooling is applied. When the temperature reaches 24"C., the vacuum is broken with nitrogen, 2210 parts toluene and 170 parts normal hexane are added and the mixture is well agitated. The mixture is then washed with 340 parts 20 percent aqueous sodium hydroxide solution using agitation while maintaining the temperature below 270 C. Agitation is stopped and the phases are allowed to separate. The aqueous layer is removed and 25 parts sodium sulfate is added and admixed with the organic phase. The mixture is filtered into containers to remove solid material.

The filtrate (3188 parts) is charged back into the reactor. A solution is prepared by admixing 1.6 parts hydroquinone and 13.3 parts acetone and the solution (14.9 parts) is added to the reactor. The condenser is set for vacuum distillation. A vacuum is applied to reduce the absolute pressure to about 46.7 kilopascals, the contents of the reactor are heated to 600C. and the removal of distillate is begun. The temperature of the liquid is maintained in the range of from about 520 C. to about 600 C. for 8i hours while the absolute pressure is gradually reduced to 13.3 kilopascals and 1835 parts distillate is removed. During the next 3 hours 25 minutes, the absolute pressure is increased to 16 kilopascals and the temperature is increased to 61"C. At this time, a total of 2263 parts distillate has been removed. Distillation is stopped, heat is removed and cooling is begun. When the temperature reaches 270 C., the vacuum is broken with nitrogen. The stripped product, amounting to 896 parts, is admixed with 180 parts methanol and the mixture is subjected to strip ping by batch vacuum distillation until the temperature of the remaining liquid is 600C. at an absolute pressure of 3.3 kilopascals. The product, amounting to 769 parts, is cooled to about 270 C., the vacuum is broken with nitrogen and the product is discharged into containers. This product, a polyester diacrylate composition, has a solids content of greater than 99 percent, an acid number of 0.84 and a hydroxyl number of 13.

A pigment paste is prepared by grinding 200 parts rutile (RTC 2, Tioxide of Canada) with 100 parts of the above polyester diacrylate composition. ("Tioxide" is a registered Trade Mark).

A coating composition is prepared by admixing 300 parts of the above pigment paste, 3 parts phenanthrenequinone, 300 parts of the above intermediate product and 6 parts benzo phenone.

The coating composition is spread onto an aluminum substrate with a number 024 wire wound bar to provide a film having a thickness of about 0.03 millimeter. The coated substrate is passed once at 6.1 meters per minute, in air, under four medium pressure mercury vapor lamps, each operating at 78.7 watts per centimeter and emitting both ultraviolet light and visible light.

The lamps are 8.9 centimeters above the plane of the substrate surface and are spaced apart at intervals of about 20.3 centimeters.

Passage of the coated substrate under the lamps causes polymerization of the film and produces a hard, adherent, white coating.

Example II.

A reactor equipped with a thermometer, a heater, a pressure equalizing dropping funnel, an. agitator and an air sparge is charged with 725 parts acrylic acid, 4 parts 2,6-di-tert butyl-pcresol, 10 parts N,N-dimethylcyclo hexylamine and 0.04 part hydroquinone. The charge is then heated to 100"C. and 1300 parts neopentyl glycol diglycidyl ether (XD 7114, Dow Chemical Co.) is added dropwise over 3.67 hours. After the addition is completed, the mixture is held at about 1000 C. for 5.83 hours and then cooled to produce a diacrylate of neopentyl glycol diglycidyl ether product having an acid value of 14.

One hundred fifty parts of the above diacrylate of neopentyl glycol diglycidyl ether and 300 parts RTC 2 rutile are ground using a Cowles blade to form a fine intermediate paste. Three hundred parts of the above diacrylate of neopentyl glycol diglycidyl ether is added to and admixed with the intermediate paste to produce a pigment paste.

Coating Composition A is prepared by admixing 50 parts of the pigment paste and 1 part methyl phenylglyoxylate.

Coating Composition B is prepared by admixing 700 parts of the above pigment paste and 7 parts of phenanthrenequinone using a Cowles blade until the phenanthrenequinone dissolves.

Coating Composition C is prepared by admixing 50 parts of Composition B and 2 parts methyl phenylglyoxylate.

Coating Compositions D through H are each prepared by admixing 50 parts of Coating Composition B with 1 part of a photo polymerization activator additive, the identities of which are shown in Table 1.

Each coating composition is drawn down onto separate aluminum substrates with a number 014 wire wound bar to provide films having thicknesses of about 0.02 millimeter.

The coated substrates are each passed once at 15.2 meters per minute, in air, under the four lamps of Example I. The results are shown in Table 5, which follows.

TABLE 5 Coating Phenanthrenequinone Additive Content Through Surface 60 Composition Content, Percent Additive Percent Cure Cure Color Gloss A 0 methyl phenylglyoxylate 1.96 poor mar free white 83 B 0.99 none 0 excellent easily scratched yellow 76 C 0.95 methyl phenylglyoxylate 3.85 excellent mar free white 83 C 0.97 methyl phenylglyoxylate 1.96 excellent mar free white 81 E 0.97 benzophenone 1.96 excellent mar free white 86 F 0.97 benzil 1.96 excellent mar free slightly yellow 83 G 0.97 2-methylanthraquinone 1.96 excellent mar free slightly yellow 82 H 0.97 2-chlorothioxyanthrone 1.96 excellent mar free slightly yellow 81 Example III.

One hundred parts of the diacrylate of neopentyl glycol diglycidyl ether is admixed with 200 parts titanium dioxide (R960; E. I. duPont de Nemours and Co.) and ground to a fine paste with a Cowles blade. Three parts phenanthrenequinone is then added and ground into the paste with a Cowles blade.

To the resulting mixture are added 300 parts of the diacrylate of neopentyl glycol dyglycidyl ether, 20 parts methyl ethyl ketone and 9 parts ethyl phenylglyoxylate. The whole is admixed to form an intermediate composition.

To 90 parts of the above intermediate composition are added 10 parts of methyl ethyl ketone and 10 parts of a 5% dispersion of phthalo blue pigment in the diacrylate of neopentyl glycol diglycidyl ether. After mixing, the resulting composition is filtered to produce a sprayable coating composition.

The sprayable coating composition is sprayed onto a metal substrate to form a film having a thickness of about 0.05 millimeter.

The coated substrate is passed once at 3.05 meters per minute, in air, under the four lamps of Example I to polymerize the film into a hard, adherent, blue coating having a high gloss.

Example IV.

A reactor equipped with a thermometer, a heater, an addition funnel and an agitator is charged with 918 parts propylene carbonate.

The charge is then heated to about 60"C.

Over a period of one hour, 675 parts Nmethyl-ethanolamine is added while maintaining the temperature of the mixture at 60 to 70"C. Upon completion of the addition, the mixture is held at 60 to 700 C. for one hour and cooled to produce a first polyol intermediate.

A loop reactor equipped with a steam heated heater on one leg, a cooler on the other leg, a thermometer, a pressure gauge and a pump for circulating liquid in the loop is charged with 1210 parts of the first polyol intermediate and 14 parts crushed sodium hydroxide. The charge is heated to 99"C. and maintained in a range of from 99"C. to 108 C. at a pressure in the range from about 206 to about 290 kilopascals gauge for about 4i hours during which time 970 parts propylene oxide is added. The reactor is cooled and vented and 14 parts crushed sodium hydroxide is added. The reaction mixture is heated to 99"C. and maintained in the range of from 99"C. to 107"C. at a pressure of from about 138 to about 311 kilopascals gauge for about 14 hours during which time 350 parts propylene oxide is added. The reactor is cooled. The next morning, the reactor is heated to 102"C. and maintained in the range of from 1010C. to 1070C. at a pressure of from about 206 to about 345 kilopascals gauge for about one hour during which time 270 parts propylene oxide is added. The reactor is held at a temperature of from 105 C. to 1100C. at a pressure of from about 311 to about 345 kilopascals gauge for about i hour.

The reactor is then cooled and vented, and the liquid contents of the reactor are drained into a container. The container is found to contain 2688 parts of product which is a second polyol intermediate.

To 2688 parts of the second polyol intermediate is added 28 parts of 86% phosphoric acid at 600 C. After mixing, a sample of the reaction mixture is taken and diluted with an equal weight of water. The pH of the diluted sample is measured and found to be less than 7.0. A vacuum is applied to the reaction mixture and the reaction mixture is heated to 1200 C. and held at that temperature for one-hauf hour. The vacuum is then released, and one percent diatomaceous earth filter aid (Hy-Flo), based on the weight of total charge, is added. The mixture is then filtered in a pressure filter to form a third polyol intermediate.

A reactor equipped with a thermometer, a heater, an addition funnel, an agitator and an air sparge is charged with 222 parts l-iso- cvanatomethyl - 5 - isocyanato - 1,3,3 - trimethylcyclohexane, 132 parts phenylcellulose acrylate, 1 part N,N-dimethylcyclohexylamine and 0.2 part dibutyl tin dilaurate. The charge is heated to SO"C. Over a period of one hour, 178 parts of the third polyol intermediate is added. Upon completion of the addition, the reaction mixture is held at 50"C. for one hour and then at 70"C. for two hours. Over a period of 5 to 10 minutes at 70"C. under an air sparge, 0.5 part 2,6-di-tert-btityl- cresol and 130 parts 2-hydroxyethyl acrylate are added. The reaction mixture is held at 70"C. for six hours. Twenty-five parts 2hydroxyethyl acrylate is added. The reaction mixture is then held at 85"C. for two hours and cooled to produce a polyurethane terminated with acrylyl groups.

A reactor equipped with a thermometer, a heater, a pressure equalizing addition funnel and an agitator is charged with 275 parts acrylic acid, 4 parts N,N-dimethylcyclohexylamine, 1.6 parts t,6-di

Claims (24)

WHAT WE CLAIM IS:

1. An actinic light polymerizable coating composition comprising: a. at least one aromatic ketone photopoly merization activator having a triplet energy in the range of from 54 kilo calories per mole to 72 kilocalories per mole; b. phenanthrenequinone; c. organic polymerizable material contain ing a plurality of sites of ethylenic un saturation and capable of being free radically addition polymerized by inter action with said aromatic ketone photo polymerization activator and said phen anthrenequinone upon exposure to actinic light, said organic polymerizable material comprising ethylenically un saturated polyester containing a plurality of sites of ethylenic unsaturation, poly mer having a plurality of sites of acrylic unsaturation (as hereinbefore defined), monomer having a plurality of sites of acrylic unsaturation or mixture thereof; and d. ultraviolet light absorbing hiding pigment.

2. A composition as claimed in claim 1 wherein said aromatic ketone photopolymerization activator is represented by the formula:

wherein (1) R is hydrogen, alkyl containing from one to twenty-niro carbon atoms, benzyl, phenyl, hydroxyalkyl contain ing from one to ten carbon atoms, chloroalkyl containing from one to ten carbon atoms, bromoalkyl containing from one to ten carbon atoms, alkoxy alkyl where the alkoxy portion contains from one to four carbon atoms and where the alkyl portion contains from one to ten carbon atoms, or phenoxy alkyl where the alkyl portion contains from one to ten carbon atoms; and (2) X is hydrogen, halogen, alkoxy con taining from one to four carbon atoms or alkyl containing from one to four carbon atoms.

3. A composition as claimed in claim 2 wherein the photopolymerization activator is selected from methyl phenylglyoxylate, ethyl phenylglyoxylate, butyl phenylglyoxylate and butoxyethyl phenylglyoxylate.

4. A composition as claimed in claim 1, 2 or 3 wherein: a. said photopolymerization activator is present in an amount in the range of from 0.01 percent to 50 percent by weight based on the weight of the binder (as hereinbefore defined) of said coating composition; b. said phenanthrenequinone is present in an amount in the range of from 0.005 percent to 5 percent by weight based on the weight of the binder of said coating composition; c. said organic polymerizable material is present in an amount in the range of from 20 to 100 percent by weight of the binder of said coating composition; d. said ultraviolet light absorbing hiding pigment is present in an amount in the range of from 5 percent to 70 percent by weight of said coating composition.

5. A composition as claimed in claim 4 wherein the photopolymerization activator is present in an amount of from 0.1 to 20 percent by weight based on the weight of the binder of said composition.

6. A composition as claimed in claim 5 wherein the photopolymerization activator is present in an amount of from 0.5 to 10 percent by weight based on the weight of the binder of said composition.

7. A composition as claimed in claim 4, 5 or 6 in which the phenanthrenequinone is present in an amount of from 0.01 to 3 percent by weight based on the weight of the binder of said coating composition.

8. A composition as claimed in claim 7 wherein the phenanthrenequinone is present in an amount of from 0.1 to 1 percent by weight based on the weight of the binder of said composition.

9. A composition as claimed in any one of claims 4 to 8 wherein the organic polymerizable material is present in an amount of from 50 to 100 percent by weight based on the weight of the binder of said composition.

10. A composition as claimed in claim 9 wherein the organic polymerizable material is present in an amount of from 80 to 100 percent by weight of the binder of said composition.

11. A composition as claimed in any one of claims 4 to 10 wherein the ultraviolet light absorbing hiding pigment is present in an amount of from 20 to 70 percent by weight of said composition.

12. A composition as claimed in claim 11 wherein the ultraviolet light absorbing hiding pigment is present in an amount of from 33 to 50 percent by weight of said composition.

13. A composition as claimed in any one of claims 1 to 12 wherein said ultraviolet light absorbing hiding pigment is selected from titanium dioxide, zinc sulfide, zinc oxide, antimony trioxide and lithopone.

14. A composition as claimed in any one of claims 1 to 12 wherein said light absorb ing hiding pigment is rutile.

15. A composition as claimed in any one of claims 1 to 14 containing at least one additional monomer containing one or more vinyl groups.

16. A composition as claimed in claim 15 wherein said additional monomer is present in an amount of from 1 to 80 percent by weight of the binder of said composition.

17. A composition as claimed in claim 16 wherein said additional monomer is present in an amount of from 15 to 30 percent by weight of the binder of said composition.

18. A method comprising: a. coating a substrate with an actinic light polymerizable coating composition as claimed in any one of claims 1 to 17; and b. exposing said coated substrate to actinic light of the first kind and to actinic light of the second kind, said actinic light of the first kind (1) having a wavelength in the ultra violet region of the spectrum such that said ultraviolet light absorbing hiding pigment is substantially opaque thereto, and (2) being absorbable by said photo polymerization activator to produce free radicals capable of causing polymerization of acrylic groups, said actinic light of the second kind (3) having a wavelength longer than that of said actinic light of the first kind and such that said ultra violet light absorbing pigment is substantially transparent thereto, and (4) being absorbable by said phen anthrenequinone to produce free radicals capable of causing poly merization of acrylic groups to thereby polymerize said coating into a hard, infusible film throughout its thickness.

19. A method as claimed in claim 18 wherein said exposure is conducted in an atmosphere containing a sufficient concentration of oxygen to inhibit polymerization of the organic polymerizable material in the absence of said photopolymerization activator from said composition.

20. A method as claimed in claim 18 or claim 19 wherein said exposure is conducted in air.

21. A method as claimed in any one of claims 18 to 20 wherein said actinic light of the first kind has a wavelength in the range of from 185 to 380 nanometers and wherein said actinic light of the second kind has a wavelength in the range of from 380 to 500 nanometers.

22. A method as claimed in any one of claims 18 to 21 wherein said coated substrate is exposed to both actinic light of the first kind and to actinic light of the second kind simultaneously.

23. A composition as claimed in claim 1 and substantially as hereinbefore described with reference to any one of the Examples.

24. A method as claimed in claim 18 and substantially as hereinbefore described with reference to any one of the Examples.