GB1566087A - Photosensitive elements and their use in reproduction and printing - Google Patents

Description

(54) PHOTOSENSITIVE ELEMENTS AND THEIR USE IN REPRODUCTION AND PRINTING (71) We, SUBLISTATIC HOLDING S.A., a Swiss Body Corporate of Speilhof 3, 8750 - Glaris - Switzerland, 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 state ment:- The present invention relates to photosensitive elements as well as to a process for reproduction and for thermoprinting by dry transfer.

Numerous reproduction processes are known which employ photosensitive elements to prepare relief printing plates from an original image. The present invention makes it possible to obtain, by a photographic process, printing plates of which the relief can be reproduced by dry thermoprinting, that is to say by simple heating without any staining of the ground, and it is possible to vary the amount of dyestuff transferred from these plates in accordance with the substrate to be printed. Furthermore, the present invention does not damage the original and makes it possible to obtain reproductions consisting solely of dyestuff so as not to affect adversely the properties of the surface of the substrate.

The present invention provides a photosensitive element which comprises, on a waterunswellable sheet of thickness within the range from 100 microns to 1 millimetre, a solid continuous photosensitive layer, the thickness of which is not more than 30 microns, being for example comprised between 1 and 15, preferably 2.5 and 10 microns, and which is composed of a homogeneous mixture comprising at least one sublimable or vaporisable water-insoluble dyestuff and a photosensitive material which, on exposure to actinic radiation, undergoes a change of solubility in an aqueous medium, either by becoming insoluble if it was soluble or by becoming soluble if it was insoluble, and which, in the insoluble state, releases the dyestuff vapours when it is heated to a temperature at which the dyestuff(s) vaporise(s) or sublime(s), the dyestuff(s) being present in an amount of more than 20% by weight in the photosensitive layer It also provides a reproduction process in which a photosensitive element comprising a thin water-unswellable sheet, preferably a metal foil, coated with a solid and continuous photosensitive layer consisting of a homo geneous mixture of at least one sublimable or vaporisable water-insoluble dyestuff and a photosensitive material which, on exposure to actinic radiation, undergoes a change of solubility in an aqueous medium, either by becoming insoluble if it was soluble or by becoming soluble if it was insoluble, and which, in the insoluble state, releases the dye stuff vapours when it is heated to a temperature at which the dyestuff(s) vaporise(s), or sublime(s), is exposed to actinic radiation to define in the layer the pattern to be reproduced, the parts which have remained or become soluble are removed using the said aqueous medium, and the sheet thus obtained is heated in contact with a receiving surface to a temperature at which the dyestuff(s) vaporise(s) or sublime(s).

The copies of the original obtained in accordance with the present invention are sharp, with good definition, and do not show clouding of the ground. In fact, since the dyestuffs are insoluble in water, no staining of the ground occurs during stripping. It is possible to print supports which are as varied as anodised aluminium, synthetic sheets, films, skins or coatings, textile materials of any thickness which may or may not already have been made up, leather, wood and glass if appropriately coated.

The thin sheet carrying the photosensitive layer must be stable to heat, preferably at least up to 250cm and better still up to 3000C.

and must be insoluble in an aqueous medium.

It can equally well be, for example, a metal foil, a brass sheet or an aluminium, zinc or aluminium alloy foil, a sheet of synthetic paper or of some other paper substitute, or even a synthetic film, for example a polyester (polyethylene terephthalate), polyolefin or cellulose ester (acetate, propionate or butyrate) film, or, for example, a glass plate. It can in particular consist of paper or of plastics films which are both impermeable and water-re ceptive, such as, for example, a paper coated with a layer of an appropriate polymer. It must be rather smooth to allow the photosensitive material to form a uniform film and nevertheless sufficiently rough to ensure "the anchoring" of the latter. In fact, the photosensitive layer must be resistant to friction so that the relief formed by the parts which have been rendered insoluble or remained insoluble in aqueous media after exposure will not be damaged during stripping of the soluble parts. On the other hand, it must not be excessively rough, because the material which is soluble in an aqueous medium would persist, in spite of the stripping operation, in certain cracks, which would result in back ground staining on transfer of the image.

It can be advantageous to use a flexible and/or transparent sheet. Using a transparent sheet, it is possible to expose the photosensi tive element through this sheet and to pro duce a relief in accordance with the positive or the negative of the original, but symmetrical with the latter. Hence, transfer gives a right-way-round, positive or negative, image of the original, whilst with an opaque sheet the mirror image or reversed image of the original is formed. However, a transparent sheet is not essential to obtain right-wayround (or direct-viewing) images. These are also easily obtained by means of a suitable optical system, or by double transfer, or by reversing the transparent negative or positive original through which the photosensitive layer is exposed.

It is advantageous to use a thin foil of metal, less than 5 tenths of a millimeter (500 microns) in thickness, for example a zinc, aluminium (anodised or non-anodised) or aluminium alloy foil having a thickness of 100 to 450 a, preferably 190 to 350 a. For printing anodised aluminium by the process of the present invention, the best results are obtained if a foil of a thickness between 190 and 350 thousandths of a millimetre (190 and 350 microns) is used, which carries a photosensitive layer more than 1 thousandths of a millimetre (1 micron) thick, for example a layer of 1 to 15 microns, preferably of 2.5 to 10 microns.

Various photosensitive materials can be applied to the thin foil which forms the base of the photosensitive element of the present invention. They can, for example, be photosensitive resins containing diazo compounds, for example of the type of the naphthoquinone - diazide - 5 - sulpho - esters or photosensitive compositions obtained from novolak resins and esters of naPhthoquinone- diazide - sulphonic acids, or polymers nossess- ing o - quinone - diazide groups, obtained by condensing a polyamide and a halide of an acid containing a - quinone - diazide groups.

These materials are commonly used for photochemical reproduction, and have the property of becoming, under the action of light, easily soluble in, for example, aqueous alkaline solution, whilst the material which has not been exnosed to light remains insoluble.

They can altematively be materials which become insoluble in aqueous media on exposure to actinic radiation, in particular to ultraviolet radiation, such as photoresists, and photocurable, photocrosslinkable, or photopolymerisable compositions. The photosensitive layer can thus consist of monomers or of oligomers and of photoinitiators, optionally distributed in a binder in which the sublimable dyestuff is dispersed. These monomers are derivatives with double bonds (bifunctional or trifunctional), which contain groups of the formula

in which R denotes an aromatic ring, a nitrile radical or a

radical. The polymerisation of these products is brought about by the action of the actinic radiation on carbonyl derivatives (benzoins), azo dyestuffs or organo-metallic dyestuffs present in the photosensitive layer.

At the same time as, or in place of, photopolymerisable monomers, the photosensitive layer can contain photocross-linkable compounds, that is to say compounds which, by photo-dimerisation, form bridges between macromolecular chains, leading to an increase in the molecular weight of the whole and to the formation of an insoluble three-dimensional network. The chains of polymers carrying substituents of the cinnamoyl or cinnamylidene type are easily photodimerisable and constitute very efficient photocrosslinkable polymers. Bridging can also take place between chains of acrylazide groups, which light decomposes into very reactive nitrene groups.

Finally, photocrosslinking can also be achieved with a layer of non-photosensitive polymers, into which molecules of photosensitisers of the diau, type have been introduced, which photosensitizers are crosslinking agents derived from azo compounds or azides, or even simple potassium bichromate.

In particular, it is possible to use, as the photosensitive material, proteins, polysaccharides and other colloids of animal or vegetable origin, such as starch, gelatine, pulluran (a polymer containing maltotriose units), cellulose and its derivatives, such as hydroxypropylcellulose, which compounds contain, for example, an unsaturated ethylenic mono mer and a photoinitiator. It is also possible to use hydrophilic synthetic polymers, in particular containing carboxyl groups, for example of the type of acrylic or methacrylic addition polymers, optionally containing copolymerised ethylenic monomers. Other products to be mentioned are the polyvinyl cinnamates, succinates or phthalates, copolymers of linear polyamides with bifunctional monomers such as glycerol diacrylate or dimethacrylates, or polymers obtained by polymerising piperidine with piperic acid, or oxidisable stilbene derivatives. Others to be mentioned are the products of the polyvinyl alcohol type, for example a product which is between 78 and 90% hydrolised, which contains a watersoluble product, such as a diazonium salt/ formaldehyde condensate, the photodecomposition product of which forms a water-in soluble product with polyvinyl alcohol.

The water-solubility properties of other colloids can thus be transformed by photo decomposable products which exert a curing or tanning action on the colloids, whether these collids are of animal or vegetable origin or whether they are synthetic products of similar structure. They must in particular contain hydroxyl or amino groups.

It is also possible to start from a soluble photopolymerisable composition and obtain a water-insoluble relief which coincides with the zones of the non-exposed element For ex ample, it suffices to add a blocking agent for the free radicals produced by the photo initiator, for example a N-nitroso compound, during a first exposure. It subsequently suffices to deactivate this compound in the parts which have been masked, and to expose the said parts.

The photosensitive composition applied to the base of the element according to the in vention can also consist of heterogeneous mix tures such as a dispersion of a photopoly merisable monomer and of a photoinitiator in a colloid or in an organic polymer or copoly mer which may or may not be swellable, is hydrophilic, film-forming and insensitive to light, but can optionally be cured chemically, and can contain a curing agent. These mix tures can in particular be rather insensitive to oxygen, which is an advantage because it is known that oxygen inhibits photopoly merisations.

In addition to a photopolymerisable mon mer and a photoinitiator, the photosensitive composition can also contain copolymerisable organic compound which may play the role of a solvent.

The photosensitive composition can also consist of a photocurable mixture containing a polymer, a cros-linking agent or a crosslinking monomer, as well as a photosensitiser, or a photocurable resin employed without an associated crosslinking monomer, but mixed with a photocuring catalyst, such as certain resins which cure by photoirradiation, even in the resence of oxygen.

The composition can also be employed as a mixture with another resin which reacts with it by crosslinking, in particular under the action of ultraviolet light It is also possible to use polymers which carry an ammonium salt group, of which the ammonium ion is paired with the anion of a photopolymerisable monomer and of a photoinitiator.

The photopolymerisable monomers or oligomers and the photocrosslinkable resins used in the present invention are known. They can in particular, as already mentioned above, polymerise or crosslink in accordance with a process of photo-addition by oxidation (photooxidation). They can be employed as mixtures with one another or with other monomers which are polymerisable by addition reaction and are soluble in water and in water-alcohol mixtures.

In addition to a photosensitive monomer or polymer, the photosensitive composition must contain at least one sublimable or vaporisable dystuff and, preferably, a binder and a photoinitiator or photosensitiser (or starter, promoter, photorolymerisation catalyst, accelerator or crosslinking agent), which may consist of a system of several compounds.

The nhotosensitiser can be combined with reducing agents. The photosensitive composition can also contain a surface-active agent (in order to facilitate the mixing of the various constituents), a stabiliser (for example to inhibit thermal polymerisation or gelling), or agents which make it possible to control the properties of the coating which is formed on the base of the photosensitive element, for example a plasticiser or an agent which improves the adhesion of the photosensitive composition to the thin sheet which forms the base of the element used in the invention.

As photoinitiators, there are preferably used polvmerisation initiators which liberate free radicals under the action of actinic radiation or which possess free radicals which can be photoactivated, but are insensitive to heat up to temperatures of the order of 850C or even of 1850, for example.

They are capable of co-existing with the photo-sensitive product for a long time without a chemical reaction occurring. They can be used as a mixture with Michler ketones or benzophenones or acetophenones, or with photo-reducing products, for example dyestuffs such as certain cyanines, merocyanines or carbocyanines, or other reducing agents such as N - alkyl - morpholine or N - cyclo alkvl - morpholine.

By way of example of such photoinitiators there may in particular be mentioned substituted or unsubstituted polycyclic quinone derivatives, for example anthraquinones and naphthoquinones, in general substituted by methyl, ethyl, tert. - butyl, phenyl or -SO,H groups, or by halogen atoms, phenanthraquinones, benzanthraquinones, naphthacenequinones and compounds of the type of pivaloin and of benzoin, their ethers or their methylated, xr-allylated or ephenylated derivatives, the benzoin acetals and the monoacetals of aromatic diketones and the benzoyi- phenvldioxolanes. There may be mentioned also bis-azides or compounds containing the system

which are optionally quaternised, or compounds of the type of dimers of 2,4,5 - ui- phenyl - imidazolyl, or organic compounds with free radicals which are electron donor3, such as amines, mercaptans or triarylmethanes.

There may be mentioned also leuco-derivatives or dyestuffs, and their salts, in particular those which carry at least one dialkylamino group; and also complex salts, for example a complex salt of aromatic iodonium of a complex ion containing a halogen; or benzoyl derivatives of diphenyl sulphide used with an organic amine compound carrying at least one -CII- group on the nitrogen of the amineforming group. In some cases the compound may be one which forms, with the photo sensitive monomer and the binder, a photo redox system.

As photosensitiser compound which accelerate the rate of curing or of photo polymerisation, it is advantageously possible to use halogenated benzanthrones, benzo- phenone and acetophenone and their deriva tives, optionally used together with hydro quinone, organic peroxides (used as co-cata lysts with iron dodecarbonyl) and, finally, di azonium salts, diazoketones and other diazo compounds (optionally with the addition of metal ions to reduce the exposure time), as well as combinations of riboflavin or of methylene blue with an organic peroxide.

Any dyestuff which sublimes or vaporises and is insoluble in water can be used in the present invention. It can be selected from the category of the disperse dyestuffs, from the category of the pigments or from the category of the dyestuffs which are soluble in organic solvents and which are classified under the heading "Solvent Dyes" in the Color Index, edited by The Society of Dyers and Colorists, Dean house, Piccadilly, Bradford, Yorkshire, England. For obvious reasons, preferred dye stuffs are those which vaporise below 3000C or, better still, those which vaporise below 2500C or 2200C at atmospheric pressure.

The dye stuffs can equally well be azo dye stuffs as well as anthraquinone dye stuffs, quinophthalone derivatives, styryl derivatives, methines and azomethines.

The most valuable dyestuffs contain a hydroxyl group or at least two different substituents. By way of examples, there may be mentioned 1,4 - dimethylaminoanthraquinone, 1,4- or 1,5 - diisopropylaminoanthraquinone, 1 - amino - 2 - methyl - anthraquinone, 1 amino - 2 - methoxy - 4 - hydroxy - anthraquinone, 1 - amino - 2 - chloro- or 1 - amino - 2 - bromo - 4 - hydroxy ~ anthraquinone, 1 - amino 2 - - hydroxyethoxy) - 4 hydroxy - anthraquinone, brominated or chlorinated 1,5 - dihydroxy - 4,8 - diaminoanthraquinone, 1,4 - diamino - 2,3 - dichloroanthraquinone, 1 - amino - 4 - hydroxy anthraquinone, 1 - amino - 4 - hydroxy 2 - phenoxyanthraquinone, 1,4 - diamino 2 - methoxyanthraquinone, the methyl, ethyl, butyl or propyl ester of 1,4 - diamino anthraquinone 2 - carboxylic acid, 1 amino - 4 - isopropylamino- or 4 - anilido anthraquinone, 1 - amino - 2 - cyano - 4 anilido- or 4 - ethyl - amino - anthraquinone, 1 - hydroxy - 2 - (p - acetaminophenylazo) - 4 - chloro- or 4 - methyl - benzene, 3 methyl - 4 - (nitrophenylazo) - pyrazolone, - (nitrophenylazo) - acetoacetylanilide and 3 - hydroxyquinophthalone.

To prepare the photosensitive layer, the sublimable dyestuffs are mixed with the photosensitive material by any appropriate means, with good stirring to achieve homogeneous distribution. Particularly valuable mixtures (with which it is already possible to produce multiple copies by transfer from one and the same printing plates) are obtained with a dyestuff content of 2060%, preferably 3050%, by weight, calculated relative to the weight of dry material which forms the photosensitive layer.

The binders which can be present in the photosensitive composition applied to the elements of the present invention are preferably film-forming products which are hydrophilic or are soluble in the aqueous medium used to develop the photosensitive elements of the present invention (water or aqueous alkaline, acid or alcoholic solutions containing hydrogen peroxide, ammonium persulphate, sodium carbonate or sodium phosphate, for example).

They are advantageously chosen from amongst those which are stable at the transfer temperatures, which can be as high as 2200C and even 2500C or 300or. They must be capable or giving a film which is as non-tacky as possible and which retains the dyestuff or dyestuffs and the photopolymerisable monomer or oligomer on the base of the photosensitive element, but which is permeable to the vapours of these dyestuffs. It is possible to use natural or synthetic colloids, such as gelatine, dextrin, polyvinyl alcohol, polyvinylpyrrolidone, hydroxyethylcellulose, hydroxypropylcellulose and mixtures thereof.

The addition of a binder is not essential.

The photosensitive material can itself act as a binder, for example if a photosensitive or photosensitised resin or a photosensitised colloid which becomes soluble or insoluble by irradiation with light is used, such as certain vegetable or animal proteins, certain polysaccharides such as starch, polyvinyl alcohol and synthetic materials of a similar structure, which in particular contain hydroxyl or amino groups.

In order to improve the viscosity (or the fluidity) of the photosensitive composition which is applied to the stable base to form the element to be used according to the in vention, it is possible to add, in addition to the abovementioned binders, polyethylene gly cols, glycerol, erythritol and pentitol. These compounds make it possible to increase the uniformity of the thickness of the layer and of the degree of drying. Furthermore, they can assist any possible crosslinking and, to a certain degree, play the role of a sensitiser.

The coating of the base with a thin layer of photosensitive material can be carried out by an appropriate known method, e.g. by pouring, by roller or brush, by dipping, by spraying, by air knife coating or by doctor blade coating. This coating, as well as the subsequent drying, is generally carried out in the absence of light and in the absence of dust. The temperature and duration of drying are chosen to suit the type of photosensitive material and the sublimation or vaporisation temperature of the dyestuff present in the photosensitive composition. The amount of photosensitive composition which has to be applied to the element depends on the type of photosensitive material and also to a large extent on the nature of the base of the ele ment, which can be more or less rough. It is necessary to obtain a coating which is of uni form thickness and has a uniform degree of drying, is compact and covers the points of any possible grains. A thickness of dry var nish greater than 1 micron generally suffices.

Excellent results are obtained with photo sensitive layers of 1 to 30 microns for example of 1 to 15, preferably 2.5 to 10 microns thick ness.

According to the present invention, the photosensitive element is exposed to actinic radiation through a negative or a positive of the original which is transparent or transmits light. Exposure can be carried out, for ex ample, with a line diapositive or a screen diapositive. Provided the base of the element is transparent, it is also possible to carry out reflex exposure, which makes it possible to use an original which is opaque or transmits too little light, and to obtain, after transfer, a direct-viewing copy of the original.

Exposure can be carried out with all the known light sources, preferably sources which are rich in ultraviolet, the original being in close contact with the photosensitive element.

Carbon lamps or mercury vapour lamps, fluorescent lamps, incandescent xenon lamps or argon lamps, electronic flashes and the socalled "floodlights" used in photography may be mentioned as sources of light.

In certain cases, the formation of wrinkles can be avoided, and the crosslinking in depth can be improved, by carrying out an irradiation successively at two different wavelengths.

The thickness and the nature of the photosensitive layer influence the intensity of the radiation, (which diminishes as the radiation passes through the photosensitive layer) which must not be less than one-tenth of the incident radiation after having passed through the layer. Since the absorption coefficient of the layer is approximately equal to the sum of the absorption coefficients of the dyestuff and of the sensitiser used, there is a relationship between the thickness "s" of the layer and the intensity of the radiation, which is given by the equation I, in which Io is the ratio of the intensity of the non-absorbed radiation I to the intensity of the inddent radiation Io, "a" is the coefficient of absorption and "s" is the thickness of the layer. This means that, for the required 1/10 minimum transmission, the thickness of the layer in centimetres must be less than or equal to loge 10 a so that for a coefficient of absorption varying from 750 to 2,300 centimetresl, the thickness of the layer will advantageously vary between about 30 and 10 microns. In order to be able to carry out the photopolymerisation or the photocrosslinking with lamps which are generally found on the market, the desired amount of sensitiser, for example about 5% by weight (relative to the polymerising or crosslinking material) is added. For this purpose it is possible to use chromates or bichromates (which are sensitive to wavelengths ranging from 300 to 550 nanometres), as well as sensitizers of the diazo type which are generally sensitive in the wavelength region ranging from 320 to 450 nanometres, with a maximum at about 375 nanometres. It is for this reason that it is preferred, according to the invention, to use a low pressure mercury lamp, a fluorescent lamp or a halogen lamp to expose the photosensitive layer. Other intense light sources (a xenon arc, a flashlight or even a laser) can also be used, with appropriate adjustment of the exposure time or the layer thickness.

It is thus possible to use mercury lamps, which generally have an intense emission at about 365 nanometres, in order to carry out the photoreaction. For example, Philips lamps of type HPR (125 W) and HPT (2,000 W) can be used. Thus, with two Philips lamps of the HPR type arranged at 40 cm from a size A4 sheet carrying a photosensitive layer having a thickness of S microns, the exposure time will be about 2 minutes, whilst it can be reduced to 30 seconds with a battery of five fluorescent tubes (Philips type TL) of 15 watts, spaced one centimetre from one another and arranged at about 5 cm from the sheet which carries a diazo-sensitised photosensitive layer of about 5 microns. With mercury vapour lamps containing metal halides, which increase the light output in the desired spectral region, the time required for photocrosslinking can be shortened further.

With a system of five Philips type HP/AB 400 W lamps at 20 cm from a 5 microns photocrosslinkable layer, 10 seconds suffice to cure the photosensitive layer.

The development of the irradiated layer is carried out by means of water or an aqueous solution which completely dissolves the zones which have become soluble or remained soluble, after irradiation. Development can be effected by immersion, spraying or wiping with a pad impregnated with the aqueous medium. The method is in particular chosen in accordance with the resistance of the base of the element to a treatment with water.

After exposure and development, the relief printing plate thus obtained can be brought into contact with a cepy-recaving support, and the whole is heated to a temperature at which the dyestuf or dyestuffs contained in the relief vaporise or sublime and transfer onto the copy receiver. This temperature can vary from 120 to 220 C, even to 25000, for example, and is preferably above 15000. If a metallic base is used, there is no reason why heating to higher temperatures, for example to 28030000. should not be used, provided the copy receiver withstands such tempera tures without damage. In this way it is possible to shorten the transfer time and to use, where appropriate, dyestuffs which volatilise above 2500C The copy receiver generally consists of, or is coated or impregnated with, a product which exhibits affinity for the dyestuffs con tained in the printing plate, so that permanent copies which are stable to rubbing, to light and to washing are obtained. Non-textile copy supports of all kinds can be used, namely anodised aluminium surfaces, non-woven fibrous sheets, coated or non-coa ment thus prepared, and the element and film are then placed between two tight glass plates.

A low pressure mercury lamp marketed by Philips under reference HPR 125 watts is placed at 20 cm from these plates.

After an exposure of 1 minute to the light from the lamp, the photosensitive element is separated from the negative and the soluble parts are then removed with the aid of an 0.4% strength aqueous sodium hydroxide solution. Only the zones of the photosensitive layer which have not been exposed to the light from the lamp remain.

Transfer onto aluminium: The printing plate thus obtained is placed on a 1 mm thick plate of 99% pure anodised aluminium, the resin relief being in contact with the anodised surface. This aluminium plate carries a 15 micron thick oxide layer.

The combination of aluminium plate and printing plate is then placed on a heating press and is heated for 1 minute at 200 C.

In this way, a precise reproduction of the original is obtained on the aluminium plate; the reproduction shows good definition and no "clouding" of the ground. The pores of the aluminium oxide layer are then sealed by heat abath of boiling water.

cry satisfactory results are also obtained replacing the anodised aluminium by a piece of clothing which has already been made up, and consists of a knited polyester fabric or 9 cotton fabric containing a polyethylene gly coloran acrylic ruin.

EXAMPLE 2 An 0.30 mum thick aluminium foil is coated with a solution of the following composition: 40 parts of a polyvEjí alcohol, of which a 4% strength solution in water has a viscosity 4 to 6 centipoises, 4 parts of the photo- decamposabledazoniurn salt marketed under the name Diazo RO 110 by Rohner, Basle, 1, parts of water, and 46 parts of a dyeing preparation containing 9 parts of the dyestuff of the formula

4 parts of the dyostuff of the formula

I parts d of 1 - amino - 2 - phenoxy - 4 - hydroxy - anthraqulnone, 0.3 part of an fiionic dispersing agent, 0.6 part of a nonionic dispetsirig agent, 4.1 parts of urea, 9 watts of Water and 1 part of carboxymethyl diluhise.

The coating is dried and then exposed in the apparatus described in Example 1 to ultraviolet light for 2 minutes. The non-exposed zones are developed by spraying with water.

A printing plate is thus obtained, which can be used as described in the preceding example, to reproduce a negative of the original by dry heat transfer. Black copies with good definition are obtained.

EXAMPLE 3 One face of an 0.3 mm thick aluminium foil is coated with a mixture of 40 parts of a polyvinyl alcohol, of which a 4% strength solution in water has a viscosity of 4 to 6 centipoises, 4 parts of the photodecomposable diazonium salt marketed under the name Diazo RO 110 by Rohner, Basle, and 40 parts of very finely ground 1 - amino - 2 - phenoxy 4 - hydroxy - anthraquinone in 1,000 parts of distilled water, to which 0.6 part of a nonionic dispersing agent has been added.

After drying, an approximately 5 microns thick layer is obtained, which is exposed for 2 minutes to the light of a Lw lamp through a transparent positive. The nonexposed zones are developed by spraying with water. A printing plate is thus obtained, which, when heated to 1800C in contact with an anodised aluminium foil containing 1.5% of magnesium and having a 10 microns thick oxide layer, gives, on the latter, a red copy of the original, which it suffices to seal for 30 minutes in a bath of boiling water. The copy obtained is sharp, the definition is excellent, the ground is impeccable and the fastness to light is such that it resists a xenon lamp for more than 500 hours.

WHAT WE CLAIM IS:- 1. A photosensitive element comprising, on a sheet which is unswellable by water and which is within the range from 100 microns to 1 millimetre in thickness, a solid continuous photosensitive layer the thickness of which is not more than 30 microns and which is composed of a homogeneous mixture comprising at least one sublimable or vaporisable waterinsoluble dyestuff and a photosensitive material which, on exposure to actinic radiation, undergoes a change in solubility in an aqueous medium, either by becoming insoluble if it was soluble, or by becoming soluble if it was insoluble, and which, in the insoluble state, releases the dyestuff vapours when it is heated to a temperature at which the dyestuff(s) vaporise(s) or sublime(s), the dyestuff(s) being present in an amount of more than 20% by weight in the photosensitive layer.

2. An element according to claim 1, wherein the sheet is a metal foil.

3. An element according to claim 2, wherein the sheet is a foil of anodised or nonanodised aluminium, of zinc, or of an alumi

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (26)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   ment thus prepared, and the element and film are then placed between two tight glass plates.

   A low pressure mercury lamp marketed by Philips under reference HPR 125 watts is placed at 20 cm from these plates.

   After an exposure of 1 minute to the light from the lamp, the photosensitive element is separated from the negative and the soluble parts are then removed with the aid of an 0.4% strength aqueous sodium hydroxide solution. Only the zones of the photosensitive layer which have not been exposed to the light from the lamp remain.

   Transfer onto aluminium: The printing plate thus obtained is placed on a 1 mm thick plate of 99% pure anodised aluminium, the resin relief being in contact with the anodised surface. This aluminium plate carries a 15 micron thick oxide layer.

   The combination of aluminium plate and printing plate is then placed on a heating press and is heated for 1 minute at 200 C.

   In this way, a precise reproduction of the original is obtained on the aluminium plate; the reproduction shows good definition and no "clouding" of the ground. The pores of the aluminium oxide layer are then sealed by heat abath of boiling water.

   cry satisfactory results are also obtained replacing the anodised aluminium by a piece of clothing which has already been made up, and consists of a knited polyester fabric or 9 cotton fabric containing a polyethylene gly coloran acrylic ruin.

   EXAMPLE 2 An 0.30 mum thick aluminium foil is coated with a solution of the following composition: 40 parts of a polyvEjí alcohol, of which a 4% strength solution in water has a viscosity 4 to 6 centipoises, 4 parts of the photo- decamposabledazoniurn salt marketed under the name Diazo RO 110 by Rohner, Basle, 1, parts of water, and 46 parts of a dyeing preparation containing 9 parts of the dyestuff of the formula

   4 parts of the dyostuff of the formula

   I parts d of 1 - amino - 2 - phenoxy - 4 - hydroxy - anthraqulnone, 0.3 part of an fiionic dispersing agent, 0.6 part of a nonionic dispetsirig agent, 4.1 parts of urea, 9 watts of Water and 1 part of carboxymethyl diluhise.

   The coating is dried and then exposed in the apparatus described in Example 1 to ultraviolet light for 2 minutes. The non-exposed zones are developed by spraying with water.

   A printing plate is thus obtained, which can be used as described in the preceding example, to reproduce a negative of the original by dry heat transfer. Black copies with good definition are obtained.

   EXAMPLE 3 One face of an 0.3 mm thick aluminium foil is coated with a mixture of 40 parts of a polyvinyl alcohol, of which a 4% strength solution in water has a viscosity of 4 to 6 centipoises, 4 parts of the photodecomposable diazonium salt marketed under the name Diazo RO 110 by Rohner, Basle, and 40 parts of very finely ground 1 - amino - 2 - phenoxy 4 - hydroxy - anthraquinone in 1,000 parts of distilled water, to which 0.6 part of a nonionic dispersing agent has been added.

   After drying, an approximately 5 microns thick layer is obtained, which is exposed for 2 minutes to the light of a Lw lamp through a transparent positive. The nonexposed zones are developed by spraying with water. A printing plate is thus obtained, which, when heated to 1800C in contact with an anodised aluminium foil containing 1.5% of magnesium and having a 10 microns thick oxide layer, gives, on the latter, a red copy of the original, which it suffices to seal for 30 minutes in a bath of boiling water. The copy obtained is sharp, the definition is excellent, the ground is impeccable and the fastness to light is such that it resists a xenon lamp for more than 500 hours.

   WHAT WE CLAIM IS:- 1. A photosensitive element comprising, on a sheet which is unswellable by water and which is within the range from 100 microns to 1 millimetre in thickness, a solid continuous photosensitive layer the thickness of which is not more than 30 microns and which is composed of a homogeneous mixture comprising at least one sublimable or vaporisable waterinsoluble dyestuff and a photosensitive material which, on exposure to actinic radiation, undergoes a change in solubility in an aqueous medium, either by becoming insoluble if it was soluble, or by becoming soluble if it was insoluble, and which, in the insoluble state, releases the dyestuff vapours when it is heated to a temperature at which the dyestuff(s) vaporise(s) or sublime(s), the dyestuff(s) being present in an amount of more than 20% by weight in the photosensitive layer.
2. 2. An element according to claim 1, wherein the sheet is a metal foil.
3. 3. An element according to claim 2, wherein the sheet is a foil of anodised or nonanodised aluminium, of zinc, or of an alumi

   nium alloy, the thickness of which ranges from 100 to 450 microns.
4. 4. An element according to claim 3, wherein the thickness of the foil is 190 to 350 microns.
5. 5. An element according to any of the preceding claims, which comprises one or more disperse dyestuffs which are sublimable or vaporisable below 2500C. in an amount of 20 to 60% of the weight of the photosensitive layer.
6. 6. An element according to claim 6, wherein the amount of the dyestuff(s) is 30 to 50%.
7. 7. An element according to any of the preceding claims, which comprises a photosensitive layer, the thickness of which ranges from 1 to 15 microns.
8. 8. An element according to claim 7, which comprises a photosensitive layer, the thickness of which ranges from 2.5 to 10 microns.
9. 9. An element according to any of the preceding claims, wherein the photosensitive layer also contains a sensitiser to ultraviolet radiation.
10. 10. An element according to any of the preceding claims, wherein the photosensitive material is photopolymerizable, photocrosslinkable, photocurable or photosolubilisable in water.
11. 11. An element according to claim 10, wherein the photosensitive material is also insensitive to oxygen.
12. 12. An element according to any of the preceding claims, wherein the photosensitive material is of the type used to prepare offset plates and contains groups which confer solubility in water.
13. 13. An element according to any of the preceding claims, wherein the photosensitive material is a mixture of polyvinyl alcohol and a photodecomposable compound of the diazo type.
14. 14. An element according to any of the preceding claims, wherein the sublimable or vaporisable dyestuff contains at least one hydroxy group.
15. 15. An element according to any of the preceding claims, wherein the sublimable or vaporisable dyestuff is either an anthraquinone carrying two different substituents or a hydroxyquinophthalone derivative.
16. 16. An element according to claim 1 substantially as described in any one of the foregoing Examples 1 to 3.
17. 17. A nrocess for reproduction and printing, wherein 1) a photosensitive element comprising a thin water-unswellable sheet coated with a solid continuous photosensitive layer consisting of a homogeneous mixture of at least one sublimable or vaporisable water-insoluble dyestuff and a photosen sensitive material which, on exposure to actinic radiation, undergoes a change of solubility in an aqueous medium, either by becoming insoluble if it was soluble or by becoming soluble if it was insoluble, and which, in the insoluble state, releases the dyestuff vapours when it is heated to a temperature at which the dyestuff(s) vaporise(s) or sublime(s), is exposed to the actinic radiation to define in the layer the pattern to be reproduced, 2) the parts which have remained or be come soluble are removed using the said aqueous medium, and 3) the sheet thus obtained is heated in con tact with a receiving surface to a tem perature at which the dyestuff(s) vaporise(s) or sublime(s) and is or are thus transferred to the receiving surface.
18. 18. A process according to claim 17, wherein the sheet is a metal foil.
19. 19. A process according to claim 17 or 18, wherein the heating is carried out at above 1500C.
20. 20. A process according to any of claims 17 to 19, wherein the heating is carried out in contact with an anodised aluminium plate.
21. 21. A process according to claim 20, wherein the oxide layer of the plate is less than 20 microns thick.
22. 22. A process according to claim 20 or 21, wherein the anodised aluminium plate contains 0.5 to 3% of magnesium.
23. 23. A process according to claim 20, 21 or 22, wherein the anodised aluminium plate is sealed in a bath of boiling water after the transfer of the dyestuff(s) thereto.
24. 24. A process according to any of claims 17 to 19, wherein the heating is carried out in contact with a polyester surface.
25. 25. A process according to claim 17 substantially as described in any one of the fore going Examples 1 to 3.
26. 26. Dyeings produced by the process of any of claims 17 to 25.