PROCESS FOR PRINTING ON LEATHER

Abstract: A process for printing on leather includes the steps of generating a digital, monochromatic or polychromatic image, printing the image on inkjet transfer paper, pressing the image onto a leather piece, and removing the backing layer of the transfer paper.

Description: PROCESS FOR PRINTING ON LEATHER This invention concerns improvements in or relating to printing on leather. An object of the present invention is to provide an improved process for the printing of leather which obviates the undesirable aspects of known techniques. Accordingly, a process for printing on leather includes the steps of preparing an image of monochromatic or polychromatic character, printing the image using one or more inks or dyes onto a sheet of transfer paper having a backing layer, pressing the transfer paper onto a piece or strip of tanned leather to transfer the said image, and removing the backing layer of the transfer paper. The removal of the backing layer of the transfer paper generally leaves a residue in the form of a film of water-based character which can be removed by the use of a suitable surfactant. One such surfactant which has proved beneficial in this respect is marketed under the trade mark NITROMORS ® . The surfactant tends to re-hydrate the film which enables its removal by simple wiping. It has been found that a degree of heating of the surfactant enhances its removal properties enabling a swifter removal of the film. Advantageously the leather piece or strip carrying the image is heat treated to fix the inks or dyes into the body of the leather piece. It is, however, to be understood that the heat treatment step may not always be necessary. The heat treatment step may be conducted simultaneously with the pressing step. The image may conveniently be digitally generated and stored. The transfer paper may be inkjet transfer paper obtained from a commercial source. The inks or dyes employed in the method of the invention are preferably those produced from naturally occurring sources, such as plants. The inks or dyes may be pigmented dyes. The transfer paper generally includes a surface layer of for example china clay which controls the 'dot gain' of the ink or dye, namely the degree to which the ink or dye is fixed without spreading to an extent causing aberration of the image. The adoption of a paper having a thicker surface layer of this kind will enable heavier inks to be used, thereby enhancing the transfer of the image into the leather. The leather to which the application of the present invention is particularly suited is one which preferably has no mineral salt content and is one produced using naturally occurring dyes and tanning materials. However, the invention is applicable to leathers having a very light mineral salt content which advantageously does not exceed 5%. It is believed that a leather having a salt content greater than the stated upper limit will be less receptive to the printing process of the instant invention, since the salt will resist the acceptance by the leather of the ink or dye. However, it is possible to employ leathers with a mineral salt content in excess of the 5% level. The leather may be provided with a surface coating with the printing being effected on the surface carrying said coating. The surface coating may be a water-based paint. The removal of the backing layer of the inkjet transfer sheet and of the residual film is an important step in terms of restoring the surface of the leather piece to a natural texture both in tactile and visual senses. As indicated supra the surfactant used for this removal step is conveniently one which is capable of removing water-based paints. The removal of the backing layer may be effected either cold or hot. The heating step following the removal of the backing layer of the inkjet transfer paper is intended to fix the inks or dyes in the leather piece, and is preferably effected by the use of a sheet of non-stick material, for example that sold under the trade mark TEFLON ® , which diffuses the heat evenly across the leather piece or strip. The heat may be provided by a conventional heating platen under pressure. It may be convenient to use a heat dispersal medium during this heat treatment step in order to ensure that the heat applied to the leather is dissipated as swiftly as possible consistent with both the efficacy of the fixing of the inks or dyes and the avoidance of heat damage to the leather. The heat dispersal medium may be a silicate which may be in the form of a foam which is disposed beneath the leather piece or strip, namely in contact with a surface of the leather not carrying the image. The duration of the heating step and the temperature at which the heat transfer occurs is dictated by the nature and physical characteristics of the leather piece being printed, eg its thickness. For example, the heating step may last for a period of 15 seconds at a temperature of 180°C, but variations both temporally and in temperature level may be made according to the leather piece or strip being processed. The pressing of the transfer paper onto the piece or strip of leather as indicated above may be carried out simultaneously with the heating step, and one example of press is that of a platen caused to contact the transfer paper which in turn is in contact with the relevant surface of the leather piece or strip. In order to avoid the creation of air pockets and the possible generation of moisture which might adversely affect the process steps, the application of the platen is effected in such manner as to secure that there is substantially no air or moisture trapped between the platen and the backing layer of the paper. In this connection, the platen is caused to address the backing layer in an uneven, asymmetric manner which occasions a gradual application of pressure to the area of the leather piece or strip thereby causing the expulsion of any air or moisture. In contrast a symmetrical rectilinear application of the platen to the layer might give rise to entrapment of air or moisture conceivably resulting in unevenness of image quality. In one alternative method, a swiping action may be adopted whereby pressure is applied to a blade which traverses the backing layer and occasions an even but gradual movement over the area of the leather. In a further alternative method of applying pressure, a roller system may be used with a gradual movement being afforded thereby avoiding the problems of air or moisture pockets forming. The leather piece or strip may be further treated following the heating step in order to restore its drop characteristics, ie its ability to fall easily from the point of suspension, or in essence its pliancy. Such treatment involves a physical massaging of the leather and may be achieved by conventional means used in the leather making industry. A still further treatment of the leather may involve the application of a stain-resistant medium, such as that sold under the trademark SCOTCHGUARD®. By way of example only, one process for printing on leather according to the present invention is described below. A polychromatic image is generated digitally on a computer and is printed using a large format inkjet printer, for example an Encad Nova-Jet Pro 600e, provided with inks or dyes which lay the image on inkjet transfer paper. The inks of dyes may be reactive. The sheet bearing the printed image is then pressed onto a piece of leather which has been tanned. The next step in the process involves the removal of the backing sheet which may merely be stripped away either when cold or hot. The removal of the residual film left after stripping away the backing layer may be effected by means of the application of a surfactant which is a textured water-based paint remover, for example a remover sold under the trade name NITROMORS ® . The action of this remover is to soften the backing layer to an extent that it can easily be wiped gently off the surface of the leather. In an alternative, the film thus produced may be washed off the surface of the leather. The removal of this film may be accelerated by using the surfactant at an elevated temperature. The removal of the film restores the tactile and visual characteristics of the leather, but it is advantageous to apply heat to the leather in order to refix the inks or dyes. In this example, a sheet of non-stick material, such as that sold under the trade mark TEFLON ® , is placed over the leather piece and heat and pressure are applied to the non-stick material which evenly diffuses the heat, thereby to refix and press the inks or dyes back into the body of the leather piece. The resulting leather piece has the normal appearance and tactile qualities of unprinted leather but carries the print image generated digitally by computer, the image being fixed in the fabric of the leather. The printed leather may be further protected by the use of a product such as SCOTCHGUARD ® . It will be understood that the printing of leather may be carried out in a continuous process with the leather being in lengths which could for example be worked over rollers with suitable treatment stations being provided for the printing, image application to the leather, backing layer removal, and heat treatment when applied.

Claims: CLAIMS: 1. A process for printing on leather including the steps of preparing an image of monochromatic or polychromatic character, printing the image using one or more natural inks or dyes onto a sheet of transfer paper having a backing layer, pressing the transfer paper onto a piece or strip of tanned leather, and removing the backing layer of the transfer paper. 2. A process according to Claim 1 in which following removal of the backing layer the residual film is removed from the surface of the leather by the use of a surfactant. 3. A process according to Claim 2 in which the surfactant is heated. 4. A process according to any one of the preceding claims in which the leather piece or strip is heat treated following the removal of the backing layer. 5. A process according to Claim 4 in which a heat dissipating medium is employed during the heat treatment. 6. A process according to Claim 5 in which the heat dissipating medium is silicate. 7. A process according to any one of the preceding claims in which the pressing of the transfer paper onto the piece or strip of tanned leather is effected in such manner as to substantially avoid the creation of air and/or moisture pockets. 8. A process according to Claim 7 in which the pressing of the transfer paper is effected by the use of a platen which is adapted to contact the paper in an irregular, asymmetric manner thereby to apply pressure in an uneven manner to the transfer paper whereby the platen gradually or progressively comes into contact with the paper. 9. A process according to Claim 7 in which the pressing of the transfer paper is effected using a roller whereby the pressure is applied gradually or progressively to the paper. 10. A process according to any one of the preceding claims in which the image is generated and stored digitally. 11. A process according to any one of the preceding claims in which the transfer paper is inkjet transfer paper. 12. A process according to any one of the preceding claims in which the inks or dyes are those produced from naturally occurring sources. 13. A process according to Claim 12 in which the inks or dyes are plant dyes. 14. A process according to Claim 12 in which the inks or dyes are pigmented. 15. A process according to any one of the preceding Claims 1 to 11 in which the inks or dyes are synthetic. 16. A process according to any one of the preceding claims in which the dyes are reactive dyes. 17. A process according to any one of the preceding claims in which the inks or dyes are water-based. 18. A process according to any one of the preceding claims in which the leather of which the strip or piece is made is one which has been tanned using a natural tanning process. 19. A process according to Claim 18 in which the leather no mineral content . 20. A process according to Claim 18 in which the leather has a chromium salt content of up to 5%. 21. A process according to Claim 18 in which the leather has a chromium salt content in excess of 5%. 22. A process according to Claim 18 in which the leather has an alum salt content of up to 5%. 23. A process according to Claim 2 or any claim dependent thereon in which the surfactant used for the removal of the backing layer residue film is one which is capable of removing water-based paints. 24. A process according to Claim 2 or any clam dependent thereon in which the surfactant softens the backing layer residue which is thereby rendered removable. 25. A process according to any one of the preceding claims in which the heating step is effected by employing a heat source in combination with a non-stick pressing sheet in contact with the leather piece or strip. 26. A process according to any one of the preceding claims in which a further treatment step following the heat treatment step involves the restitution of the drop characteristics of the leather. 27. A process according to any one of the preceding claims in which a stain-resistant medium is applied to the leather piece following the heat treatment step.

Inventors: GILHAM ERIC CHARLES, GILHAM, ERIC, CHARLES
Applicants: GILHAM ERIC CHARLES, GILHAM, ERIC, CHARLES
Application year: 2000
Main classification: B41M1
IPC codes: B41J2, B41M1, B41M5, B41M7, B44C1, D06P3, D06P5