Background to the Invention Mass transfer printing is a well known technique in which colorant material (commonly carbon black) is transferred by localised application of heat from a mass transfer layer of a thermal melt transfer medium to a receiver material. Mass transfer printing is generally used to print monochrome images, commonly text, bar codes etc.

Mass transfer printing is often used in conjunction with dye diffusion thermal transfer printing of full colour images, using thermally transferable dyes of the three primary colours, yellow, magenta and cyan, with a common application being the printing of identification cards bearing a full colour image of the head of a person and a monochrome (usually black) bar code. Such printing is conveniently carried out using a dye sheet in the form of an elongate strip or ribbon of a heat-resistant substrate, typically polyethylene terephthalate film, carrying a plurality of similar sets of different coloured dye coats and colorant, each set comprising a panel of each dye colour (yellow, magenta and cyan) and a panel of colorant, with the panels being in the form of discrete stripes extending transverse to the length of the ribbon, and arranged in a repeated sequence along the length of the ribbon. The mass transfer layer is capable of being printed directly onto both pre-printed (eg. by thermal dye transfer) and unprinted receiver media. The mass transfer printing can be protected by an optional overlay coating applied as an additional panel or as a separate lamination.

The thermal melt transfer medium comprises a substrate bearing on at least part of one surface thereof a mass transfer layer or coating comprising mass transfer colorant. Early thermal melt transfer media used a coating comprising a mixture of colorant and wax. While these had good printability, with the colorant coating fracturing easily during the printing process and producing clear printed images with well defined sharp edges, the resulting images were of relatively poor durability due to the soft nature of the wax.

Image durability can be improved by use of thermal melt transfer media having a coating comprising a mixture of colorant and polymer binder, eg polyester resins, vinyl chloride/vinyl acetate copolymer resins etc. The use of such binders produces printed images formed of tough, cohesively strong layers of material having good durability. However, the cohesive strength of the materials means that they are difficult to print because the material does not fracture easily during printing; instead the material tends to tear or rupture producing images with jagged or ragged edges, exhibiting a phenomenon known as flashing, rather than forming clear images with well defined sharp edges.

The present invention aims to provide a thermal melt transfer medium capable of producing images of good print quality that have good durability. It has been found that this can be achieved by use in the medium of a resin condensation product of formaldehyde.

Summarv of the Invention According to the present invention there is provided a thermal melt transfer medium comprising a substrate bearing on at least part of one surface thereof a coating comprising colorant, binder and a resin condensation product of formaldehyde.

The resin should be non-reactive, ie not undergoing cross-linking reactions with itself to any significant degree under normal conditions of preparation and use of the medium.

The resin should also be solvent-soluble, ie soluble in one or more of the organic solvents normally used in production of such media, eg butan-2-one, propanone, tetrahydrofuran, toluene, cyclohexanone etc.

Thus, in a further aspect the invention provides a thermal melt transfer medium comprising a substrate bearing on at least one part of one surface thereof a coating comprising colorant, binder and a non-reactive, solvent-soluble resin condensation product of formaldehyde.

The resin is conveniently a condensation product of formaldehyde and toluene sulphonamide, formaldehyde and cyclohexanone or formaldehyde and acetophenone eg hydrogenated acetophenone. The resin may be modified, eg by reaction with urethane. For instance, a urethane modified resin condensation product of formaldehyde and hydrogenated acetophenone may be used.

The resin suitably has a glass transition temperature (Tg) of at least 30°C.

The resin suitably has a softening temperature of at least 45°C, preferably at least 55°C.

Suitable resins may be made by known techniques, or are commercially available.

One preferred commercially available resin is Ketjenflex MH (Ketjenflex MH is a Trade Mark) from Akzo Nobel, which has as its main functional component a resin condensation product of toluene sulphonamide and formaldehyde. Ketjenflex has a softening point of about 62°C, has low melt viscosity and has a glass transition temperature (Tg) of about 40°C.

Other preferred commercially available resins include various formaldehyde condensation resins from Degussa-Huls, particularly a cyclohexanone-formaldehyde condensation resin known as Synthetic Resin CA, a hydrogenated acetophenone- formaldehyde condensation resin, supplied as a 50% by weight solution in ethyl acetate and known as Synthetic Resin SK, and urethane modified Synthetic Resin SK, known as Synthetic Resin 1201. Properties of these materials are as follows:- Material Tg °C Softening Point °C Synthetic Resin CA 75 95-108 Synthetic Resin SK 90 110-120 Synthetic Resin 1201 130 155-170 A mixture of resins may be used.

The resin is suitably present in an amount in the range 5 to 80%, preferably 10 to 50%, typically about 20% by weight of the solids content of the coating.

The resin is found to weaken the cohesive strength of the colorant during printing, enabling production of clear printed images with well defined sharp edges, without having any apparent adverse effect on the durability of the printed image.

The colorant may be selected from materials known for this purpose. As noted above, one commonly used colorant is carbon black. Other preferred colorants have strong absorbances in the red and infra red, and phthalocyanine and substituted phthalocyanines are especially preferred. A mixture of colorants may be used.

Colorant is suitably present in an amount in the range 10 to 70%, preferably 20 to 40%, typically about 35% by weight of the solids content of the coating.

The binder is usually in the form of a thermoplastic resin, preferably having a Tg in the range 50 to 180°C. Suitable binder materials are known in the art, eg as disclosed in EP 0283025, and include vinyl chloride/vinyl acetate copolymers, polyester resins, polyvinyl chloride resins, acrylic resins, polyamide resins, polyacetal resins and vinyl resins. A mixture of binders may be used. One currently preferred binder is Vinylite VYHH (Vinylite VYHH is a Trade Mark) which is a copolymer of vinyl chloride/vinyl acetate at weight proportions of 86/14 having an average molecular weight of 20,000 and a Tg of 72°C.

The binder may be selected bearing in mind, inter alia, compatibility with the intended receiver material, as discussed in EP 0283025.

The binder is suitably present in an amount in the range 10 to 70%, preferably 20 to 40%, typically about 35% of the weight of the solids content of the coating.

It has been found convenient to use carbon black and vinyl chloride/vinyl acetate binder in the form of Noir PVC 512975 black chips which are commercially available from BASF and which consists of a mixture of equal weights of carbon black and Vinylite VYHH which have been hot milled together.

The coating of the thermal melt transfer medium may include surfactant to aid dispersion of the colorant in known manner, although this is not generally necessary if the colorant is used in ready-dispersed form as is the case with Noir PVC 512975.

The coating of the thermal melt transfer medium optionally includes particulate material. Suitable materials include micronised polypropylene. The particulate material suitably has a particle size in the range 0.1 to 20 pm. Good results have been obtained with Micropro 600VF (Micropro 600VF is a Trade Mark) which is micronised polypropylene having a particle size of about 8 pm. Particulate material can improve wear resistance of the printed image without adversely affecting transfer properties.

Particulate material is suitably present in an amount in the range 1 to 20%, typically about 3 to 10% of the weight of the solids content of the coating.

Other materials may optionally be included in the thermal melt transfer medium in known manner.

The substrate may be any suitable heat-resistant material such as those known in the art. Suitable substrate materials include films of polyesters, polyamides, polyimides, polycarbonates, polysulphones, polypropylene and cellophane. Biaxially oriented polyester film, particularly polyethylene terephthalate (PET), is currently favoured for its properties of mechanical strength, dimensional stability and heat resistance.

The substrate suitably has a thickness in the range 1 to 201lu, preferably 2 to 10) 1m, typically about 6pm.

The thermal melt transfer medium is conveniently in the form of a ribbon for use in thermal transfer printing, comprising a substrate having on one surface thereof a plurality of repeated sequences of dye coats and colorants in the form of discrete stripes extending transverse to the length of the ribbon.

Thus in a preferred aspect the invention provides a thermal melt transfer medium, comprising an elongate strip of substrate material having on one surface thereof a plurality of similar sets of thermally transferable dye coats and mass transfer layers, each set comprising a respective coat of each dye colour, yellow, magenta and cyan, and a mass transfer layer, each coat or layer being in the form of a discrete stripe extending transverse to the length of the substrate, with the sets arranged in a repeated sequence along the length of the substrate, wherein each mass transfer layer comprises a coating comprising colorant, binder and a resin condensation product of formaldehyde.

The thermal melt transfer medium is conveniently made by mixing together the coating materials (colorant, binder, resin and any optional ingredients such as particulate material) and dissolving or dispersing the mixture in a suitable solvent as is well known in the art to give a coating liquid. Suitable solvents include butan-2-one, propanone, tetrahydrofuran, toluene, cyclohexanone etc. The coating liquid is then coated on the substrate and dried in known manner eg by bar coating, blade coating, air knife coating, gravure coating, roll coating, screen coating, fountain coating, rod coating, slide coating, curtain coating, doctor coating. The coating suitably has a thickness in the range 0.1 to 5pm, preferably 0.5 to 3pm, typically 1.5 to 2.0pm.

The thermal melt transfer medium may be provided with an optional subcoat between the substrate and colorant-containing coating, and/or an optional heat-resistant backcoat etc in known manner.

In a further aspect the invention provides a method of making thermal melt transfer medium, comprising forming on one surface of a substrate a coating comprising colorant, binder and a resin condensation product of formaldehyde.

The thermal melt transfer medium is used in known manner for printing an image on suitable receiver material. The receiver material is typically in the form of a sheet or card of paper, cardboard, plastics material etc having a suitable image-receiving surface. The thermal melt transfer medium is placed in contact with the receiver material and localised heating effected to cause localised transfer of colorant to produce a desired image, such as text or a barcode, on the receiver material. When used in conjunction with thermal transfer printing of dyes, as discussed above, a full colour image may also be produced on the receiver material. One common use of the thermal melt transfer medium is in production of identification cards, typically formed on a sheet of plastics material such as polyvinyl chloride, ABS and polyester, and which may bear a full colour photograph of the head of an individual, produced by thermal transfer printing, in combination with text and/or a bar code produced by mass transfer printing of colorant.

In a further aspect the invention provides a method of mass transfer printing comprising superposing a thermal melt transfer medium in accordance with the invention and a receiver material : applying localised heating to the thermal melt transfer medium to produce a printed image on the receiver material.

The invention also includes within its scope the receiver material after printing, particularly an identification card bearing a full colour image produced by thermal transfer printing and text and/or a bar code produced by mass transfer printing of colorant.

The invention will be further described, by way of illustration, in the following examples.

Examples Example 1 (Comparative) A coating solution (solution A) was prepared from: Noir PVC 512975 10% by weight Butan-2-one 90% by weight A coating was applied by hand using a Meier bar, to give a wet film thickness of 12go, onto a Gum polyester base film, coated with a back coat to resist the thermal head during printing and a subcoat comprising a crosslinked acrylic system to provide release during printing. The wet coating was dried in an oven at 110°C for 30 seconds.

The subcoat comprises a highly cross-linked acrylic coating in which the cross-linking is achieved by UV-curing using a combination of photoinitiators and synergists included in the subcoat composition, details of which are given below. The subcoat was coated on the polyester to give a dry coat thickness of approximately 0.5p. m. The subcoat composition, expressed as % w, w, was as follows: Chemical % Composition Manufacturer MIBK 47.02% Alcohols LTD Uvecryl E1354 41. 88% UCB Radcure S. A Diakon MG102 5. 98% KDT/Distrupol Irgacure 907 1.68% Ciba Geigy Plastics Uvecryl P101 1. 67% UCB Radcure S. A Quantacure ITX 0.84% Lambson Fine Chemicals Quantacure EPD 0.84% Lambson Fine Chemicals Cyan dye 0.08% MIBK is methyl iso-butyl ketone. This is the solvent from which the subcoat layer is deposited. The solvent is evaporated from the coating before it is subjected to W- curing.

Uvecryl E1354 is a hexafunctional aromatic urethane acrylate oligomer. (Uvecryl is a Trade Mark).

Diakon MG102 is a high molecular weight grade of poly methylmethacrylate.

(Diakon is a Trade Mark).

Irgacure 907, Uvecryl P101, Quantacure ITX & Quantacure EPD catalyse Ut-curling of the Uvecryl E1354. (Irgacure, Uvecryl and Quantacure are Trade Marks.) The resulting thermal melt transfer medium was used to print onto a receiver comprising a card of polyvinyl chloride (PVC). The surface of the PVC card consists predominantly of a vinyl chloride/vinyl acetate copolymer (approximately 95: 5 ratio, respectively). Printing was carried out using a Fargo Pro card printer (Fargo Pro is a Trade Mark) (manufactured by FARGO Electronics Incorporated).

The image was assessed for print quality which was very poor: the bars and characters had ragged edges caused by the black layer sticking in unprinted areas and there was filling in between bars. The printed card was then tested for durability using a tumble test (described below) and gave a good result there being essentially no visible damage to the printed image.

Example 2 (Comparative) A coating solution (solution B) was prepared from: Noir PVC 512975 10% by weight Texicote 1050 4% by weight Butan-2-one 86% by weight Texicote 1050 is a carboxylated polyester resin supplied by Scott Bader Co Ltd.

Texicote 1050 is a Trade Mark.

A coating was applied and printed as described in Example 1.

The image was assessed for print quality which was very good, there being no loose material and no fill in. The printed card was then tested for durability using the tumble test and gave a very poor result, the printed image being almost completely worn away.

Example3 A coating solution (solution C) was prepared from: Noir PVC 512975 10% by weight Ketjenflex MH 4% by weight Butan-2-one 86% by weight A coating was applied and printed as described in Example 1.

The image was assessed for print quality which was good for print sizes down to both 4pt bold text and 4pt standard text, there being no loose material and no fill in. The printed card was the tested for durability using the tumble test and gave a good result, there being essentially no visible damage to the printed image.

Example 4 A coating solution was prepared from: Noir PVC 512975 10% by weight Ketjenflex MH 3% by weight Micropro 600VF 0.65% by weight Butan-2-one 86% by weight A coating was applied and printed as described in Example 1.

The resulting coating was found to give good print quality, with the print having good durability.

Micropro 600VF is a micronised polypropylene which improved wear resistance (as assessed using the eraser test-described below) without adversely affecting transfer properties.

Example5 A series of coating solutions were prepared, each using one of the following resins: Synthetic Resin CA, Synthetic Resin SK and Synthetic Resin 1201, each solution having the following composition: Noir PVC 512975 10% by weight Synthetic Resin 2% by weight Butan-2-one 88% by weight Coatings were applied and printed as described in Example 1.

The images were assessed for print quality which was good for print sizes down to 4 pt bold text, there being no loose material or fill in, but 4 pt standard text showed some loss of detail. The printed cards were tested for durability using the tumble test and all gave goods results, there being essentially no visible damage to the printed image.

Tumble Test for Durabilitv The object of this test is to simulate the sort of everyday wear that a card could be subjected to. This included flexing, handling, high humidity and abrasion by coins, etc.

After printing, samples are flexed 100 times along the length of the card (image extension). After applying Veriderm grease (a hand cream which mimics finger grease, manufactured by Upjohn) to the image surface the samples are placed in a 45°C/85% RH (relative humidity) oven for 24 hours. The cards are then placed around the inside (facing inwards) of a plastic cylindrical container which contains a selection of washers, nuts and bolts. A lid is placed onto the container and the whole is tumbled on a roller at a speed of 20 rpm for two hours. Samples are removed and assessed according to the extent of damage to the image. Good samples show no visual damage and bad samples may be completely worn away.

Eraser Test After printing, the samples are rubbed in one direction using a pencil eraser ('Magic Rub'manufactured by Eberhard Faber) over a small area of the image at a constant rate until the image is completely worn away. The number of strokes is recorded.