NO141804B - PHOTOPOLYMERIZABLE PREPARATION. - Google Patents

Description

The present invention relates to a new and improved foot polymerizable preparation. More particularly, the present invention relates to a light-sensitive preparation which can be easily developed

by means of an alkaline aqueous solution so that they give a product which is usable for printing plates and photoresists.

These preparations are particularly useful in the production of printed circuits due to the fact that the resists formed therefrom are superior to ordinary plating solutions.

The desire for available light-sensitive preparations that can be developed without the need for the usual organic solvents has long been recognized. Organic solvents are expensive, dangerous in terms of toxicity and flammability, and they pollute air and water.

The advantages of such aqueous systems were noted as far back as 1956 in US Patent No. 2,760,863; however, no particular preparation was described, and it is reasonable to assume that the applicant was unable to find a satisfactory preparation. Other references to such water-soluble preparations are mentioned in US patent no. 2,927,022 and US Patent No. 2,893,868. Whatever advantage the preparations according to these patents may have, it appears that such preparations, in so far as they could be developed with aqueous alkaline solutions, were either not sufficiently insoluble in the exposed portions, or they were victims of attack by the ordinary plating- and etching solutions where the photoresists were used in a printed circuit.

According to the invention, it has now been discovered that it is possible to produce photopolymerizable preparations which can be developed with aqueous alkaline solutions, and where the exposed parts have excellent resistance to alkaline solutions, including the alkaline etching solutions and alkaline plating solutions which are frequently used in the production of printed circuits and chemically manufactured parts.

The present invention relates to a photopolymerizable preparation which contains

a) an addition polymerizable material,

b) a photoactivatable initiation system for addition polymerization,

c) a thermal addition polymerization inhibitor and

d) a polymeric binder,

and optionally up to 1% by weight of a triazole, and the preparation is

characterized in that the addition polymerizable material a)

essentially consists of 10-60 parts by weight of one or more non-gaseous compounds with a boiling point above 100°C at atmospheric pressure and containing at least two terminal ethylenic groups; that the photoactivatable initiation system for addition polymerization b) consists of 0.001-10 parts by weight of a system that works by forming free radicals, that the inhibitor c) is used in an amount of 0.001-5 parts by weight and that the polymeric binder

d) consists of 40-90 parts by weight of a formed macromolecular binder which is a polymer of a first monomer material which

contains one or more non-acidic vinyl compounds selected from those of the formula

or

where R is hydrogen, an alkyl group with 1-6 carbon atoms or a halogen atom, and where X is hydrogen and Y is OOCR^, OR^, OCR^,

COOR , CN, CH=CH2,

or Cl, or X is methyl and Y is COOR1, CN, CH=CH2, or

or both X and Y are Cl; and where R 1 is an alkyl group of 1-12 carbon atoms, a phenyl group or a benzyl group, and R 1 and R 2 are the same or different and are hydrogen, an alkyl group of 1-12 carbon atoms or a benzyl group; and a second monomer material consisting essentially of one or more alpha, beta-unsaturated carboxyl-containing monomers of 3-15 carbon atoms; where the ratio between the first and the second monomer material is sufficient to make the binder soluble in a 2% aqueous solution of trisodium phosphate.

The advantages of the invention are achieved by selecting a preformed, compatible macromolecular polymer binder which is a copolymer of (1) a styrene type or a non-acidic vinyl monomer and (2) an unsaturated carboxylic monomer. The use of the preparation as defined herein completely eliminates the need for organic solvents and provides a resistance with high resistance to solvents.

The preparations may also contain suitable dyes and pigments and other additives, such as plasticizers and adhesion promoters, as these may be necessary to improve the physical and chemical properties of the photopolymerizable preparation.

The ethylenically unsaturated compound must contain at least

two terminal ethylenic groups (CH2=CC), have a boiling point above 100°C at atmospheric pressure and be capable of forming a high polymer by a free-radical photoinitiated chain-promoting addition polymerization. Such compounds are described in US Patent No. 2,760,863.

Suitable compounds which can be used alone or in combination include an alkylene or a polyalkylene glycol diacrylate prepared from the alkylene glycols, with from 2 to 15 carbon atoms or polyalkylene ether glycols with 1-10 ether linkages.

Because of their generally more rapid insolubilization upon exposure, presumably due to relatively rapid establishment of a polymeric network structure, those compounds represent a superior class of low molecular weight addition polymerizable compounds having multiple addition polymerizable ethylenic linkages, especially when present as terminal bonds, and in particular where at least one and preferably most of these bonds are conjugated to a double-bonded carbon, including carbon double-bonded to carbon and to such heteroatoms as nitrogen, oxygen and sulfur. Those substances are superior in which the ethylenically unsaturated groups, especially the vinylidene groups, are conjugated with ester or amide structures.

The following particular compounds are further illustrative of this class: unsaturated esters of polyols, especially such esters of the methylene carboxylic acids, e.g. ethylene diacrylate; diethylene glycol diacrylate; glycerol diacrylate; glycerol triacrylate; ethylene dimethacrylate; 1,3-propylene dimethacrylate; 1,2,4-butanetriol trimethacrylate; 1,4-benzenediol dimethacrylate; pentaerythritol tetramethacrylate; 1,3-propanediol diacrylate; 1,5-pentanediol dimethacrylate; the bis-acrylates and methacrylates of polyethylene glycols with a molecular weight of 200-500 o.l.; unsaturated amides, especially those of the methylene carboxylic acids, and especially those of alpha, omega-diamines and oxygen-interrupted omega-diamines, such as methylene bisacrylamide; methylenebismethacrylamide; 1,6-hexamethylenebisacrylamide; diethylene triamine trismethacrylamide; bis-(methacrylamidopropoxy)ethane; B-methacrylamidoethyl methacrylate; N-(B-hydroxyethyl)-oxyethyl)-acrylamide; vinyl esters such as divinyl succinate, divinyl adipate, divinyl phthalate, divinyl terephthalate, divinylbenzene-1,3-disulfonate and divinylbutane-1,4-disulfonate; and unsaturated aldehydes, such as sorbaldehyde (hexadienal).

The preferred monomeric compounds are di- or poly-functional, but monofunctional monomers can also be used. The amount of added monomer varies with the particular thermoplastic monomer.

The styrene-type component of the polymeric binder may

have the general formula:

where R means hydrogen or an alkyl group with from 1 to 6 carbon atoms or a halogen group. The benzene ring can be ring substituted with from 1 to 5 other functional groups,

such as hitro, alkoxy, acyl, carboxyl, sulfo, hydroxyl

or halogen groups. Preferably, the substituent is a single alkyl group such as a methyl or t-butyl group. Most preferred of these compounds are styrene, alpha-methylstyrene, para-methylstyrene and para-t-butylstyrene.

The non-acidic vinyl monomer component of the polymeric binder has the general formula:

where X is hydrogen and Y is OOCP^, OR^, OCR^ COOR^ CN, CH2=CH2,

CNR3R4 or Cl, or X is methyl and Y is COOR^ CN, CH2=CH2,

or CNR^R^; or both X and Y are Cl; and where R 1 is an alkyl group of from 1 to 12 carbon atoms or phenyl group or a benzyl group, and R 1 and R 4 are the same or different and are hydrogen, an alkyl group of 1 to 12 carbon atoms or a benzyl group.

Examples of these vinyl monomers are vinyl acetate, vinyl butyrate, vinyl benzoate, vinyl chloride, vinylidene chloride, methyl methacrylate and methyl acrylate, acrylonitrile and methacrylonitrile, meth-acrylamide and alkyl-substituted acrylamides, vinyl methyl ketone, vinyl propyl ketone, vinyl methyl ether, vinyl ethyl ether and vinyl hexyl ether.

The second comonomer may be one or more unsaturated carboxylic monomers having from 3 to 15 carbon atoms, preferably from 3 to 6. The most preferred compounds are acrylic acid and methacrylic acid. Other acids that can be used are cinnamic acid, crotonic acid, sorbic acid, itaconic acid, propargylic acid, maleic acid and fumaric acid, or the corresponding half-esters or, if possible, the corresponding anhydride.

The ratio between the styrene or vinyl component and the acidic comonomer is chosen so that the copolymer is soluble in the aqueous alkaline medium. If the amount of styrene or vinyl monomer is too high, the unexposed portion would not be sufficiently soluble; if, on the other hand, the amount of styrene or vinyl monomer is too low, the exposed area will be sticky, swollen or dissolved in the aqueous alkaline solution. As a convenient criterion, the binder copolymer should be such that a 40% solution in ketones or alcohols has a viscosity of from 100-50,000 centipoise.

Representative comonomer ratios are 70:30 to 85:15 for styrene acrylic acid or methacrylic acid; 35:65 to 70:30 for styrene monobutyl maleate and 70:30 to 95:5 for vinyl acetate crotonic acid. The degree of polymerization of the binder copolymer is such that the binder forms a non-tacky continuous film after exposure and development. Usually the molecular weight is from 1000 to 500,000. The ranges of the copolymer ratios and of the degree of polymerization for the particular binder can be easily investigated by testing the solubility in the dilute alkaline solution of representative polymers. These represent a molecular weight of from about 1,000 to 500,000.

As mentioned above, the cover agents according to the invention are resistant to common plating and etching solutions. Most surprising is the resistance to copper pyrophosphate

which is used in pattern plating and which has an extremely high alkalinity. Other solutions that do not affect the covering agents include ferric chloride, ammonium persulfate and chromic sulfuric acid.

The photoinitiators used in the preparations are preferably

those which are activated by actinic light and which are thermally inactive at 185°C or below. These include substituted or unsubstituted polynuclear quinones, 9,10-anthraquinone; 1-chloroanthraquinone, 2-chloroanthraquinone, 2-methylanthraquinone; 2-ethylanthraquinone; 2-tert-butylanthraquinone; octamethylanthraquinone; 1,4-naphthoquinone; 9,10-phenanthrenequinone; 1,2-benzanthraquinone; 2,3-benzanthraquinone; 2-methyl-1,4-naphthoquinone; 2,3-dichloronaphthoquinone; 1,4-dimethylanthraquinone; 2.3-Dimethylanthraquinone; 2-phenylanthraquinone; 2,3-diphenylanthraquinone; the sodium salt of anthraquinone-alpha-sulfonic acid; 3-chloro-2-methylanthraquinone; retenquinone; 7,8,9,10-tetrahydronaphthacenequinone; 1,2,3,4-tetrahydrobenz(a)anthracene-7,12-dione.

The following photoinitiators, described in US Patent No. 2,760,863, some of which may be thermally active at temperatures down to 85°C, may also be used: vicinal ketaldonyl compounds such as diacetyl and benzyl; alpha-ketaldonyl alcohols such as benzoin and pivaloin; acyloin ethers, e.g. benzoinmethyl and ethyl ethers; and alpha-hydrocarbon substituted aromatic acyloins such as alpha-methylbenzoin; alpha-allylbenzoin; and alpha-phenylbenzoin.

Silver persulphate is also useful as a free-radical-forming initiator that can be activated by actinic radiation. Special aromatic ketones, eg benzophenone and 4,4<1->bis-dialkylamino-benzophenones are also usable.

Thermal polymerization inhibitors are also present in the preparations. These include p-methoxyphenol, hydroquinone and alkyl and aryl substituted hydroquinones and quinones, tert-butylcatechol, pyrrogallol, copper resinate, naphthylamines, beta-naphthol, copper(I) chloride, 2,6-di-tert-butyl-p- cresol, 2,2-methylene-bis-(4-ethyl-6-t-butylphenol), phenothiazine, pyridine, nitrobenzene, dinitrobenzene, p-toluquinone, chloranil, aryl phosphites and aryl alkyl phosphites.

If desired, the preparations may contain dyes and pigments. Suitable dyes must be able to be used together with the photosensitive preparations and not affect the photosensitivity of these preparations to a significant extent. The following special compounds are illustrative: "Fuchsine" (Color Index No. 42510); "Auramin Base" (color index No. 4100B); "Calcocide Green S"

(color index no. 44090); "Para Magenta" (color index no. 42500); "Tryparosan" (color index 42505); "New Magenta" (color index 42520); "Acid Violet RRH" (color index 42425); "Red Violet 5RS"

(color index 42690); "Nile Blue 2B" (color index 51185); "New Methylene Blue GG" (color index 51195); "Basic Blue 20" (color index 42585); "Iodone Green"(color index 42556) ; "Night Green B"

(color index 42115); "Direct Yellow 9" (color index 19540); "Acid Yellow 17" (color index 18965); "Acid Yellow 29" (color index 18900); "Tartrazine" (color index 19140); "Supramine Yellow G" (color index 193O0); Buffalo Black 10B" (Color Index No. 27790); "Naphthalene Black 12"R" (Color Index No. 20350); "Fast Black L" (color index no. 51215); "Ethyl Violet" (color index No. 42600) ; "Pontacyl Wool Blue BL" (Colour Index No. 50315); "Pontacyl Wood Blue GL" (color index no. 523201. INxutimers from the second edition of the Color Index) .

The photopolymerizable elements are exposed to a

source of actinic radiation. This can be done through a half-tone image or a process transparency, e.g. a process negative or positive, a stencil or a mask. The exposure can also be through a negative or a positive image with continuous tone. The exposure can take place by the contact or projection method, with or without a cover sheet over the photopolymerizable layer, or by projection using a cover sheet. These methods are all well known to those skilled in the art.

Since free-radical-forming addition polymerization initiators which can be activated by actinic radiation usually show the maximum sensitivity in the ultraviolet region, the radiation source should provide an effective amount of this radiation. Both point radiation sources and broad radiation sources are effective. Such sources include carbon arcs, mercury vapor arcs, fluorescent lamps with phosphors emitting ultraviolet radiation, argon incandescent lamps, electronic flash units and photographic lighting lamps. Of these, the mercury vapor arcs, especially "mountain solar", are most suitable. Under special circumstances, it may be advantageous to expose with visible light, using a photoinitiator that is sensitive in the visible region of the spectrum, e.g. 9,10-phenanthrenequinone. In such cases, the radiation source must provide an effective amount of visible radiation. Many of the radiation sources listed above provide the desired amount of visible light.

The photopolymerizable preparations can be developed after exposure, e.g. by treatment with spray jets, with rinsing or washing by agitated immersion to obtain the desired images with aqueous bases, i.e. aqueous solutions of water-soluble bases in concentrations usually in the range from 0.01-10% by weight.

Suitable bases for the development include alkali metal hydroxides, e.g. lithium; sodium and potassium hydroxide; the basic reacting alkali metal salts of weak acids, e.g. lithium, sodium and potassium carbonates and bicarbonates; amines with a base ionization constant greater than about lx 10 ^, e.g. primary amines such as benzyl, butyl and allyl amines; secondary amines, e.g. dimethylamine and benzylmethylamine;

tertiary amines, e.g. trimethylamine and triethylamine; primary, secondary and tertiary hydroxyamines, e.g. propanol, diethanol and triethanolamines, and 2-amino-2-hydroxymethyl-1,3-propanediol; cyclic amines, e.g. morpholine, piperidine, piperazine and pyridine; polyamines such as hydrazine, ethylene and hexamethylenamines; water-soluble basic salts, e.g. carbonates and bicarbonates of the above-mentioned amines; ammonium hydroxide and tetrasubstituted ammonium hydroxides. e.g. tetramethyl, tetraethyl, trimethylbenzyl and trimethylphenylammonium hydroxides, sulfonium hydroxides, e.g. trimethyl-, diethylmethyl-, dimethyl-

benzylsulfonium hydroxides and the basic soluble salts thereof, e.g. the carbonates, bicarbonates and sulphides; alkali metal phosphates and pyrophosphates, e.g. sodium and potassium triphosphates and sodium and potassium pyrophosphates; tetra-substituted (preferably all-alkyl) phosphonium, arsonium and stibonium hydroxide, e.g. tetramethylphosphonium hydroxide.

The photopolymerized preparations can usually be removed

by immersion in heated aqueous solutions of strong alkalis or, if desired, in commercially available removal preparations, well known in the art.

To further illustrate the invention, attention is drawn to the following examples.

Example 1

The following solution was coated on a 0.025 mm thick polyester film and dried in air. The dry thickness of the sensitive layer was about 0.025 mm. The dried layer was covered with a 0.025 mm thick polyethylene film.

A piece of copper-clad, epoxy fiberglass was cleaned

by scrubbing with an abrasive, sanding and thorough rinsing in water. It was then given a 20 second immersion in a dilute HCl solution (2 volumes of water + 1 volume of concentrated HCl), a second rinse with water and then dried with air jets.

The polyethylene cover film was removed from a portion of the polymerizable element. The exposed masking agent coating with the polyester carrier was laminated onto the pure copper with the surface of the polymerizable layer in contact with the copper surface.

The lamination was carried out using rubber coated rollers operating at 120°C with a pressure of 535.7 g/linear cm in the gap

and at a speed of 0.6 m/min. The resulting sensitized copper clad plate could, if necessary, be stored for later use as it was protected by the polyester film. In this case, it was exposed to light through a high-contrast transparent image

in which the conductive pattern appeared as transparent areas on a light opaque background. The exposure was made by placing the sensitized copper-clad plate (with the polyester film still intact) and the transparency in a photographic print frame. The exposure was for a period of 45 seconds to a 400 watt 50 amp mercury vapor lamp at a distance of 0.3 meters. The polyethylene terephthalate carrier film was peeled off and the exposed coating layer developed by agitating the plate in a vessel containing 2% sodium carbonate in water for 3 min, followed by water rinsing. The resulting plate had a colored cover layer pattern from the clear areas of the exposure transparency.

The plate was now etched with a 45° Baume solution of ferric chloride, then rinsed and dried. The masking agent was removed from the remaining copper by dipping for two minutes in a 3% solution of sodium hydroxide in water at 70°C. The result was a high-quality printed circuit board.

Example 2

The following solution was coated on 0.025 mm thick polyester film and allowed to dry under ambient conditions for 30 min.

The dry thickness of the sensitized layer was about 0.033 mm. The coated fabric was then laminated onto a cleaned copper-coated plate as in ex. 1. The exposure took place through a transparent which had an opaque pattern of a known area for 1^ min with an exposure unit as described in ex. 1. The polyester carrier film was peeled off and the exposed masking agent layer developed by agitating the plate in a vessel containing 2% trisodium phosphate in water for 2 min, followed by a water rinse.

The surface of the exposed copper was further cleaned after development by dipping the plate in a 20% ammonium persulfate bath for 30 seconds, washing copiously with water, dipping for 30 seconds in a 20% solution of HC1 in water, rinsing with water and then drying the plates in air currents. This cleaned plate was then plated for 45 min at 322.8 amps per m<2>

in a copper pyrophosphate plating bath of 55°C.

Example 3

A copper-coated piece of an epoxy fiberglass plate was cleaned as described in ex. 1. The cleaned, dried plate was sensitized by pouring the following solution over the surface of the plate:

The excess solution was poured off the plate at room temperature for 2 min. The coating was further dried by heating in an air flow oven at 60°C for 5 min. After cooling, the coated plate was exposed as described in ex. 1. The resist was developed by agitating the plate in a solution

of 2% trisodium phosphate in water for 1 min, followed by a water rinse. The plate was now etched in iron (III) chloride as described in ex. 1. After etching, the exposed resist was removed from the protected one

copper by immersing the plate in a 3% solution of sodium hydroxide in water at 50°C for 2 min. The result is a high-quality printed circuit board.

Example 4

The following solution was coated on a 0.025 mm thick polyester film and dried in air.

The dry thickness of the sensitized layer was about 0.025 mm. A copper-coated plate was prepared, the resist coating laminated on this and the resulting element exposed exactly as in ex. 1. The support film was peeled off and the exposed resist layer developed by agitating the plate in a vessel containing 2% trisodium phosphate for 2 min, followed by a water rinse.

The plate was now etched as in ex. 1, so that it gave

a high quality printed circuit board.

Example 5

The photosensitive solution described in ex. 1, was coated on zinc, magnesium and copper printing plates. After drying in hot air to a dry thickness of about 0.025 mm, the photosensitive layer was coated with a dilute aqueous solution of polyvinyl alcohol and dried again with hot air. The water-soluble polymer formed a thin protective layer against oxygen. These presensitized metal sheets can be stored for extended periods.

By exposure to actinic light through a suitable photographic negative, the unexposed photosensitive layer and the water-soluble top layer can be developed simultaneously, and the metal plate is ready for etching. The photopolymerized image area then serves as an excellent masking agent for deep etching processes as usual in the production of metal printing plates. The results are resistant to common etchants, e.g. ferric chloride and nitric acid, film-forming agents and agents usually added to the etching mixture to control the geometry of the etch.

Example 6

The procedure in ex. 5 was followed, except that a 0.025 mm thick film of polyester is used as the protective layer instead of a water-soluble polymer. After the exposure to actinic light, the protective layer is removed before developing in an aqueous alkaline solution. As in ex. 5, the photopolymerized image area serves as an excellent resist for deep etching printing plates.

Example 7

The photosensitive solution described in ex. 1, is coated on a 0.025 mm thick polyester film, dried in air and covered with a 0.025 mm thick polyethylene film. This three-layer film can be stored in sheets or rolls light-proof for an unlimited period of time. Before use, the polyethylene film is pulled off and the photosensitive layer is placed in contact with a metallic plate of the type described in ex. 5 and attached by lamination. When exposed to actinic light, the protective polyester layer is removed, and the development takes place in an aqueous alkaline solution. As in ex. 5, the photopolymerized image area is an excellent cover layer for deep etching printing plates.

Example 8

The photosensitive solution as described in ex. 1, are coated on thin aluminum plates as they are usually used in offset lithography, by methods described in ex. 5, 6 and 7. After exposure to actinic light and development in aqueous alkaline solution, the photopolymerized image area serves as an excellent printing ink receiver. It also has superior abrasion resistance. The resulting plates can be used with excellent results for offset lithographic printing.

Example 9

The photosensitive solution as described in ex. 1, is coated on a woven net-like substrate by methods described in ex. 5, 6 and 7. After exposure to actinic light and development in aqueous alkaline solution,

the photopolymerized image area serves as an excellent mask.

This application has shown its usefulness when printing screen-printed

the type.

Claims (9)

1. Photopolymerizable preparation containing a) an addition polymerizable material, b) a photoactivatable addition polymerization initiation system, c) a thermal addition polymerization inhibitor, and d) a polymeric binder, as well as possibly up to 1% by weight of a triazole, characterized in that the addition polymerizable material a) essentially consists of 10-60 parts by weight of one or more non-gaseous compounds with a boiling point above 100°C at atmospheric pressure and containing at least two terminal ethylenic groups; that the photoactivatable initiation system for addition polymerization b) consists of 0.001-10 parts by weight of a system that works by forming free radicals, that the inhibitor c) is used in an amount of 0.001-5 parts by weight and that the polymeric binder d) consists of 40-90 parts by weight of a formed macromolecular binder which is a polymer of a first monomer material containing one or more non-acidic vinyl compounds selected from those with the formula or where R is hydrogen, an alkyl group with 1-6 carbon atoms or a halogen atom, and where X is hydrogen and Y is OOCR^, OR^, OCR^, COORlf CN, CH=CH2, or Cl, or X is methyl and Y is COOR^ CN, CH=CH2, or or both X and Y are Cl; and where R^ is an alkyl group of 1-12 carbon atoms, a phenyl group or a benzyl group, and R^ and R^ are the same or different and are hydrogen, an alkyl group of 1-12 carbon atoms or a benzyl group; and a second monomer material consisting essentially of one or more alpha, beta-unsaturated carboxyl-containing monomers of 3-15 carbon atoms; where the ratio between the first and the second monomer material is sufficient to make the binder soluble in a 2% aqueous solution of trisodium phosphate. 2. Composition according to claim 1, characterized in that the binder is a copolymer of styrene and an unsaturated carboxyl-containing monomer of the specified type. 3. Composition according to claim 1, characterized in that the binder is a copolymer of vinyl acetate and an unsaturated carboxyl-containing monomer of the specified type. 4. Composition according to any one of the preceding claims, characterized in that the unsaturated carboxyl-containing monomer is methacrylic acid or acrylic acid. 5. Composition according to any one of claims 1-3, characterized by. that the unsaturated carboxyl-containing monomer is maleic anhydride, maleic acid, a half-ester of maleic acid or a mixture of two or more of these compounds. 6. Composition according to any one of the preceding claims, characterized in that the addition polymerizable material is a mixture of trimethylpropane triacrylate and tetraethylene glycol diacrylate. 7. Compound according to any one of claims 1-5, characterized in that the addition polymerizable material is a mixed acrylate ester of pentaerythritol. 8. Composition according to any one of claims 1-5, characterized in that the addition polymerizable material is a mixture of mixed acrylate esters of pentaerythritol and tetraethylene glycol diacrylate. 9. Composition according to any one of the preceding claims, characterized in that the free-radical-forming addition polymerization initiation system b) contains benzophenone, 4,4<1->bis-(dimethylamino)-benzophenone or a mixture thereof.