'FINE LINE PHOTOGRAPHIC TRANSFERS'

TECHNICAL FIELD

This invention seeks to provide diazo photographic films suitable for use in transfer foils, and the like, and with methods of producing the films and foils. It is concerned particularly with hot-stamp transfer foils having heat-activated adhesives and bearing high-resolution images. BACKGROUND ART Hot-stamp transfer foils usually comprise a flexible carrier sheet of a high melting point plastic material having a coating of a heat-softening release agent such as wax, a transfer layer on the release agent bearing printed or decorative matter thereon and a heat- ctivated adhesive coating on the transfer layer by which the layer may be stuck to an article under the pressure of a hot die and then pulled from the carrier sheet.

Because photographic techniques allow a much higher optical resolution than printing, there are occasions where it would be desirable to produce a transfer foil in which the transfer layer was a photographic film (whether unexposed, exposed but undeveloped or both exposed and developed). An example is in the production of Moire patterns for use in security tokens of the type described in our prior Australian Patent No. 488652.

For reasons of resolution, cost, ease of production and processing, the diazo process would be preferred over the silver halide photographic process.

The well-known diazo reproduction process is based on two reactions of aromatic diazonium salts:

(i) The reaction, usually in the presence of alkali, with organic species known as "couplers", to form highly-coloured dye-like derivatives.

(ii) The decomposition of the diazonium salts on exposure to light, to form species which cannot react with the couplers.

Thus, a thin film containing a suitable diazonium salt

(the "sensitizer") when illuminated with actinic light passing through a master film bearing a pattern of opaque lines or other pictorial matter on a transparent background and when subsequently developed by treatment with an alkaline solution of suitable couplers, will have reproduced on it a positive image of the original master, the colour of the reproduced image depending on the choice of sensitizer and couplers.

In a variation of this process, the couplers may be incorporated into the sensitizer-containing composition, premature coupling being inhibited by the co-addition of various acidic stabilizers. After exposure with actinic light, the image is developed by exposure of the film to alkali, usually in the form of warm, moist ammonia vapour.

Familiar examples of articles incorporating a diazo process include:

(i) Plan-printing papers, consisting of a sized opaque paper substrate, the surface of which contains the sensitizer formulation.

(ii) Drafting film or photographic film, consisting of a thick, transparent or translucent plastics film on which is coated a thin, tightly-adherent layer of a polymeric composition containing the sensitizer formulation, or alternatively, coated with a tightly adherent composition capable of adsorbing the sensitizer formulation, (iii) Labelling material consisting of a thick metal, plastics, or metallised plastics sheet, coated on one side with a pressure-sensitive adhesive, and coated on the other side with a tightly-adherent composition containing the sensitizer formulation. These examples do not, however, provide diazo films suitable for use in transfer foils, particularly hot-stamped transfer foils, because:

1. The compositions containing the sensitizer formulation are selected for maximum adhesion to the paper, plastics, or metal carrier; that is, the compositions are designed for direct application to the substrate, not for indirect application as in the case of a transfer foil.

2. Where the element of transfer is involved, as in example (iii) above, the carrier is transferred to the substrate and forms an integral part of the transfer. This technique differs from that employed in a transfer foil, in which the carrier is necessary to maintain mechanical integrity of the transfer device only until its application to the substrate, after which it is discarded.

3. Prior art diazo systems using the element of transfer have used pressure sensitive adhesives or liquid adhesives, not a dry, thermally-activated adhesive.

4. Prior art diazo systems using the element of transfer have an intervening carrier or non-adhesive layer between the image-bearing layer and the adhesive; they do not use an adhesive layer applied directly to the image-bearing layer as in the case of a transfer foil. DISCLOSURE OF THE PRESENT INVENTION

Now, the present invention is based upon the discovery that diazo compositions can be dispersed in a solution of polymeric material which can be applied to a carrier to form a coherent and transferrable photographic film which will be compatible with heat activated adhesives and will adhere sufficiently to the carrier to maintain integrity. In accordance with this invention, a diazo composition is dispersed within a transparent polymer matrix and cast or

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coated in the form of a film from an organic solvent onto a transparent polymeric carrier. For many purposes a film thickness of less than 10 microns, preferably less than 5 microns, is desirable. Thus, another aspect of the present invention is a method for producing a transfer foil bearing a photographic image in the transfer layer, comprising the steps of: coating a polymeric carrier sheet with a light-sensitive diazo composition dispersed within a solution of a matrix polymer in an organic solvent, removing the solvent by evaporation to form a transfer layer on the carrier, exposing the transfer layer to light to produce a photographic image therein, - developing said image in a manner appropriate for the diazo composition, and coating the exposed surface of the transfer layer with heat-activated adhesive.

There are a number of factors governing the selection of the matrix polymer. First, the diazo film (or transfer layer) should be permeable to the developer employed - that is, to aqueous alkaline solutions or ammonia vapour. Matrix polymers found suitable in this regard are the cellulose esters, vinyl acetate co-polymers and partially-hydrolysed co-polymers. Secondly, since any release agent adhering to the polymer matrix after separation from the carrier may cause degradation of the image, it is preferable to avoid

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the use of a release agent, and select a matrix polymer capable of adhering directly to the carrier sheet sufficiently to preserve the mechanical integrity of the . transfer layer without inhibiting the release of that layer from the carrier during hot-stamping. It has been found that certain cellulose esters or vinyl acetate copolymers can be ^' used as a matrix polymer for the transfer layer and that, by choice of a suitable solvent mixture, solutions of the polymers can be applied to thin carrier films of poly(ethylene terephthalate) and dried to form a layer which has the desired controlled adhesion to the carrier.

A cellulose acetate propionate having a relatively large proportion of non-esterified hydroxyl groups is a preferred matrix polymer, having good resistance to aromatic solvents used in subsequent processing of the foil. This particular cellulose ester could be dissolved in a mixture of alcohol plus aromatic solvent. A suitable solvent mixture giving solutions of low viscosity and allowing rapid evaporation at low temperatures without the development of "blushing", or separation of components of the diazotype formulation, and also resulting in acceptable adhesion between the carrier and the transfer layer might consist of a mixture of low and moderate boiling-point aliphatic esters and alcohols, diluted with an aromatic non-solvent, such as toluene. Suitable alcohols and esters would be ethanol, propanol or butanol, ethyl acetate, ethyl propionate, propyl acetate, butyl acetate.

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Many diazonium salts are available as sensitizers for diazo formulations, and the invention is not confined to the use of sensitizers disclosed in the examples.

We have discovered that commercial diazonium hemizincichloride salts, although normally insoluble in the toluene-based solvent mixture used for the cellulose ester coating, may be rendered soluble and suitable for the process described herein by the addition of a small amount of zinc chloride or other metal halide capable of forming a coordination complex with the diazonium salt and the polymer matrix. The optimum amount of additional zinc chloride lies in the range 0.45 to 0.7 mole per mole of diazonium salt; The addition of zinc chloride also greatly enhances the solvent resistance of the developed image compared to that of coatings not containing the additional salt. This improvement in solvent resistance was particularly apparent in compositions containing hydroxyl-rich cellulose esters as the matrix polymer.

A wide range of couplers which are soluble in the coating composition can be used as components of the diazo formulation, and the invention is not confined to the use of couplers disclosed in the examples. While the examples largely use a diazotype formulation containing incorporated couplers for ease of processing using conventional high-speed "dry process" diazo reprographic machinery, the invention does not exclude "wet process" methods of development, such as development of the exposed diazo film

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by treatment with an aqueous alkaline solution which may also contain the coupler or couplers.

Contrary to what might have been expected from the prior art, by choice of a suitable combination of diazonium salt, acidic stabilizer, and a metal halide salt as a complexing agent, the diazo formulation, including couplers, can be dissolved in the matrix polymer/solvent composition of choice, and after coating on the carrier, drying, photographic exposure and development with warm, moist ammonia vapour, the composition yields images of high optical density and contrast on a colourless, transparent background, and that these images are resistant to moisture and to the solvents to be used in the adhesive coating and subsequent treatments. The images formed by conventional diazo compositions become diffuse and lose resolution in contact with such solvents.

The solvent resistance of the transferred images may be enhanced if necessary by the incorporation of a reactive poly-functional substance into the diazo formulation which, after coating of the carrier with the composition and evaporation of the solvent, reacts with reactive groups on the matrix polymer and components of the diazo formulation, insolubilising the coating and any subsequently-formed diazo images. Such a crosslinking agent may be a reactive polymethylol-alkyl-ether aminoplast capable of acid catalysed condensation with hydroxyl groups in the matrix polymer.

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For example greatly improved solvent resistance of the developed diazo image can be obtained by the addition of a small amount (0.5-20% w/w cellulose ester) of an alkylated oligomeric urea-formaldehyde adduct to the diazo/transfer layer composition. Coating solutions containing this adduct are stable prior to, and during the coating process, but on drying and removal of the alcoholic co-solvents, the coating becomes insoluble in organic solvents. Although a butylated urea-formaldehyde adduct is used in the examples attached to this specification, other alkylated aminoplasts may be useful, the reactivity of which can be modified by the choice of type or extent of the alkylation.

A significant improvement in solvent resistance of the diazo image may be obtained by the incorporation of other organic crosslinking agents which can react with the matrix polymer and thereby restrict the rate of solvent-induced diffusion of the dye. These crosslinkers include dialdehydes, such as glutaraldehyde.

Improved solvent resistance can also be obtained by the incorporation of hydrolyzable, reactive, organosilicon derivatives. These reagents include silicate and oligosilicate esters which are soluble in the organic solvent mixtures used for dissolution of the matrix polymers, and which, on removal of the co-solvent, alcohols, can condense with, and thus cause the crosslinking of the matrix polymers via the residual hydroxyl groups. The

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homogeneous siliceous residues formed in the condensation reaction, or by subsequent hydrolysis, can also improve the solvent resistance of the diazo image by adsorption of the dye and adsorptive bonding of the matrix polymer. Improvement in adhesion of the transfers to the product substrate can be obtained by the use of a combination of adhesive layers, one with optimised affinity for the image-bearing transfer layer, and the other with optimised affinity for the intended substrate,the two adhesive layers also having good mutual affinity. The first of these, the priming layer, may be applied to the developed transfer layer in the form of an aqueous emulsion of the polymer. Suitable polymer emulsions are those of partially-hydrolysed vinyl acetate or acrylic-vinyl acetate copolymers, but emulsions or solutions of other polymers having affinity for the transfer layer may be useful. The second adhesive layer, applied over the priming layer, may be an acrylic polymer or copolymer, but it is not necessarily confined to this class of materials, and may include other heat- ctivated thermoplastic or thermoset adhesive compositions having affinity for both the transfer/priming layers of the transfer foil and the surface of the intended substrates. DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Manufacture of a sensitized film suitable for wet or dry-process diazo reproduction of high-resolution images and conversion to a transfer foil might follow the steps below.

A solution is prepared by dissolving a suitable matrix polymer in an appropriate solvent. A diazo salt sensitizer is added to the solution together with the requisite amount of zinc chloride and stabilizing acid. When these have dissolved, the coupler(s) are added, plus a crosslinking additive. The resultant solution is then applied, as a coating to a carrier film using, for example, gravure or wire-wound roll coating apparatus of the types known in the art. The solvents are evaporated with hot-air, and the dry coated film stored in darkness, at say 15-20°C for 2-7 days, for optimum curing of the coating prior to photographic exposure and development.

The printed, developed film will require the application of an adhesive layer if it is to be used as a hot-stamp transfer foil. Suitable adhesive polymers are acrylic polymers or copolymers and partially-hydrolyzed vinyl ester polymers and copolymers, but the invention is not necessarily confined to these classes of polymers. If desired, the adhesion of the adhesive coating to the surface of the diazo film may be enhanced by employing a thin priming coating of a polymeric material having affinity for both the matrix polymer and the adhesive.

Hot-stamp transfer foils prepared according to the following examples can be used to transfer diazo photographic images to acrylic, polyvinyl chloride, or cellulose substrates, or to polyolefin substrates having a

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surface layer of a chlorinated polyolefin or similar surface energy-enhancing priming layer.

Example 1 Cellulose acetate propionate (Eastman Kodak "CAP 504-0.2") (150 g) was dissolved in a solvent mixture consisting of 96% ethanol (420 ml), ethyl acetate (190 ml), n-butanol (200 ml), n-butyl acetate (390 ml), and toluene (800 ml). To this solution was added: 2,5-di-butoxy-4-(l- morpholinyl)benzene diazonium hemi(tetrachlorozincate) (30 g), hereafter abbreviated as DBMZ, anhydrous zinc chloride (15 g), sulfosalicyclic acid (5 g), 2,3-dihydroxynaphthalene (22 g), hereafter abbreviated as DHN, and thiourea (0.5 g). The resultant sensitizing solution was coated on a thin poly(ethylene terephthalate) (PET) film, to a wet-thickness of 25 micron, and dried in air at 45°C for 3 minutes. The sensitized film formed a deep blue-colored diazo positive image when contact-printed by exposure to actinic light passing through a photographic master, and developed by exposure to moist ammonia vapor at 75°C. The printed film, bearing the diazo images, was converted to a foil suitable for hot-stamp transfer by first priming the cellulose ester layer surface by coating with a 12 micron wet-thickness layer of an acrylic copolymer plasticized poly(vinyl acetate) emulsion (A.C. Hatrick "Acropol RE258", diluted 1:1 with water), drying at 65°C for 3 minutes, and then coating the primed surface with a

25 micron wet-thickness layer of an acrylic copolymer solution (Albright and Wilson "Durasol 326", diluted with toluene to give a Zahn II cup drainage time of 25 sec), followed by drying at 85°C for 4 minutes. The resultant image-bearing, adhesive-coated cellulose ester layer could be transferred to a poly(vinylidene chloride)-coated polypropylene substrate by conventional hot-stamp transfer techniques, using die temperatures of 125°C. The transferred image of a Fresnel zone-plate, having line densities up to 40 lines/mm showed no opical deterioration after washing with toluene or with 50% aqueous ethanol.

Example 2 Disodium phosphate (50 mg) was dissolved in 37% formalin (70 g), and the mixture adjusted to pH 8.5 by the addition of dilute alkali. To this was added urea (23 g) and n-butanol (44 g), and the mixture heated at 70°C for 1 hour. The mixture was acidified by the addition of phthalic acid (0.8 g), plus toluene (30 g) and butanol (30 g), and then refluxed for 3 hours, with azeotropic removal of water (56 g) during this period. The product was then filtered to remove insoluble salts, then concentrated by vacuum evaporation at 80°C, to yield a butylated urea-formaldehyde resin (82 g) having a solids content, after drying at 120°C, of 65%. The procedure of Example 1 was then followed, except that cellulose acetate propionate (128 g) plus butylated

urea- ormaldehyde resin (34 g) was substituted for the 150 g of cellulose ester. The dried, sensitized film was stored at 20°C for 3 to 5 days, to allow for adequate curing of the crosslinked matrix polymer, prior to photographic exposure and development. The transferred image of a 40 lines/mm. Fresnel zone-plate showed no significant diffusion of the dye-lines after washing with 96% ethanol. The image-bearing crosslinked cellulose ester layer was swollen by acetone, without drastic leaching of the dye-lines, or destruction of the image.

Example 3 The procedure of Example 1 was followed, except that a cellulose acetate butyrate (Eastman Kodak "CAB 381-2") (100 g) was substituted for the cellulose acetate propionate. This composition provided a more robust transfer than that of Example 1.

Example 4 A copolymer of vinyl acetate and vinyl chloride containing 1% of dibasic acid (Union Carbide "VMCH") (10 g) was dissolved in a mixture of toluene (90 ml), ethanol (12 ml), and n-butyl acetate (12 ml). To this was added DBM2 (1 g), zinc chloride (0.5 g), sulfosalicyclic acid (0.2 g), and DHN (1 g). The solution was coated at 24 micron wet-thickness on a PET film, and then dried in air at 40°C for 5 minutes. The sensitized film provided a blue image after exposure and development as described in Example 1. The image-bearing vinyl copolymer film was then coated with a 12 micron wet-thickness of "Acropol RE258" emulsion, and

dried at 70°C, to produce an adhesive-coated foil suitable for hot-stamp transfer of the images onto the PVDC-primed polypropylene.

Example 5 The procedure of Example 1 was followed, except that 2,5-dibutoxy-4-(l-morpholinyl)benzene diazonium borofluoride (25 g) was used as the sensitizer, instead of DBMZ, and that the zinc chloride was replaced with an ethyl oligosilicate (Union Carbide "Ethyl Silicate 40") (35 g). The diazo images of a 40 lines/mm. Fresnel zone-plate were not degraded on washing with toluene or 50% aqueous ethanol. The above composition without ethyl oligosilicate is a conventional organic soluble diazo composition; the lines of a Fresnel zone plate image made with such a prior art composition could be destroyed by washing with toluene, ethanol or acetone.

Example 6 The procedure of Example 1 was followed, except that glutaraldehyde (40 g of a 50% aqueous solution) was added to the cellulose ester solution. The diazo images showed improved resistance to diffusion of the dye on treatment with concentrated ethanol solutions.

Example 7 The procedure of Example 1 was followed, except that a phthalate-plasticized, partially-hydrolyzed poly(vinyl acetate) emulsion was used as the foil adhesive. The emulsion (A.C. Hatrick, "Acropol 63-404") was diluted with 25% by volume of with water, and then coated to 12 micron wet-thickness on the surface of the image-bearing

layer; the adhesive layer was dried in air at 85°C. The images could be transferred to a PVDC-primed polypropylene by a hot-stamp method using die temperatures of 125° to 130°C. Example 8 The procedure of Example 1 was followed, except that the image-bearing layer was transferred, by a hot-stamp process, on to a cellulose acetate sheet.

Example 9 The procedure of Example 1 was followed, except that the image-bearing layer was transferred, by a hot-stamp process, on to a smooth-surfaced paper sheet.

Example 10 The procedure of Example 1 was followed, except that the DHN coupler was replaced with N-acetoacetyl-benzamide (18 g). The sensitized film formed a yellow-coloured image on photographic exposure and development with ammonia vapor.

Example 11 The procedure of Example 1 was followed, except that the DNH coupler was replaced with a mixture of N-(2-hydroxyethyl)-3-hydroxy-2-naphthoamide (20 g) and acetoacetanilide (4 g). The sensitized film formed a neutral-coloured (black) image on photographic exposure and development with ammonia vapor.

Example 12 The procedure of Example 1 was followed, except that DBMZ sensitizer was replaced with 4-diethylamino- benzene diazonium hemitetrachlorozincate (20 g). Example 13 A sensitized film was prepared by coating, then drying, a PET film with a 24 micron wet-thickness layer of

the sensitizing formulation of Example 1, from which the DNH coupler was omitted. The film was contact-printed using actinic light, and an intense purplish-black coloured image developed by immersion for 1 minute in an aqueous solution of phloroglucinol (5%) and ammonium carbonate (10%). The water rinsed air-dried image-bearing surface was then coated with 12 micron wet-thickness layer of an acrylic copolymer plasticized poly(vinyl acetate) (A.C. Hatrick "Acropol RE258"). After drying in air at 65°C, the image-bearing layer was transferred, using a hot-stamp method, on to a PVDC-primed polypropylene sheet. INDUSTRIAL APPLICABILITY

It will be seen that this invention provides readily-applied transfer foils beaming high-resolution images. While, in particular, this facilitates the incorporation of detailed designs, such as those of Moire patterns and Fresnel zone plates into e.g. banknotes, it will be appreciated that the invention also extends the scope of the art of transfer printing in many other areas.