Manufacture of decorative natural leather

Abstract

A method for manufacturing decorated natural leather comprises the steps of: a) providing a fixture layer (11) having holes (12) corresponding to the outer contour of a leather piece (14); b) attaching an adhesive backing layer (13) to the fixture layer (11) having the holes (12); c) positioning the leather piece (14) in the holes (12) with the genuine leather side attached to the adhesive backing layer; and d) inkjet printing a decorative image on the leather piece (14) to deliver a decorated leather piece (15).

Description

Technical Field

[0001] The present invention relates to a method for producing decorated natural leather by inkjet. Background Art

[0002] The manufacture of natural leather products is well known and can be generally divided into five stages. Preparatory stage 1 usually occurs partly in slaughterhouses and partly in tanneries, while stages 2 to 4 occur in tanneries, and stage 5 occurs at leather product manufacturers. In the first stage, i.e. the preparatory stage, the skin is removed (skinned) from the animal and pretreated for the second stage of tanning. Pretreatment can include processes such as soaking, liming, depilation, splitting and pickling (adjusting pH to assist the penetration of tanning agents). In the tanning stage, the protein of the hide or skin is converted into a stable material that will not corrupt. Chromium is most commonly used as a tanning agent, and the tanned product thus obtains a light blue color, so it is generally referred to as "wet blue". In the third stage of semi-nitre treatment (crusting), the tanned leather is dried and softened. Semi-nitre treatment generally includes processes such as stripping (removing surface fixed tannins), fatliquoring (fixing fats, oils and waxes to leather fibers), dyeing, whitening, physical softening and polishing (wearing the leather surface to reduce grain defects). In the fourth stage, called the finishing stage, the leather is prepared for sale to leather goods manufacturers. Finishing operations can include lacquer coating, polishing and embossing. In the fifth stage, leather goods are manufactured, including processes that can include cutting, perforating, sewing, leather wrapping, decoration and embossing.

[0003] Natural leather has been decorated in the past by screen printing. However, screen printing is labor intensive because a separate screen is required for each color. This is expensive and time consuming, especially when personalization or customization is desired. Digital printing technology has entered the manufacture of decorated natural leather. For example, GB 2510696 (SERICOL) and EP

[0004] 3202581A (CODUS / AGFA GRAPHICS) each discloses a method of printing onto natural leather.

[0005] Inkjet printing is usually performed on substrates with standardized shapes and qualities. Due to its origin, natural leather has irregular shapes and non-standard qualities. For example, leather made from different cow hides can have different physical properties and can contain creases, fecal marks, warts and scars from barbed wire. The latter can result in poor image quality of the inkjet printed image.

[0006] Another problem with inkjet printing on natural leather is the difficulty in providing it as a flat substrate into the inkjet printer so that it does not hit or damage the print head.

[0007] Some mechanical solutions to this problem are disclosed in 3450574A (AGFA).Chemical solutions to this problem are disclosed in EP 3572234A (AGFA) and EP 3572235A (AGFA), where an adhesive is used to attach the leather to a temporary carrier.

[0008] WO 2020 / 225046 A (AGFA) discloses another method for keeping leather flat for inkjet printing, in which a breathable support (e.g. felt) with a fiber layer is used in combination with vacuum suction.

[0009] For printing on three-dimensional products such as pens, golf balls, mobile phone cases, lighters and other three-dimensional objects, a jig or fixture is a key tool to achieve high print quality. A jig is a printing tool used to hold a three-dimensional product in a fixed orientation for printing. They are commonly used for screen printing and pad printing, but are also used for inkjet printing as exemplified by EP 3272542 A (OCE) for printing on smartphone cases.

[0010] For the proposed solutions for inkjet printing on natural leather, there are still trade-offs in terms of image quality, manufacturing speed and printing reliability. This is especially the case when a series of identical leather pieces of irregular shape are inkjet printed with a personalized image, such as for a certain type of handbag or for leather seats in a car. Summary of the invention

[0011] In order to overcome the above-mentioned problems, a preferred embodiment of the present invention has been achieved with a method for manufacturing decorative natural leather according to claim 1 .

[0012] It was found that by using a specific sequence of steps in the manufacturing method and a two-part fixture, images of high image quality can be printed on natural leather in a reliable manner at high production rates.

[0013] The use of a jig allows for quick and precise registration in an inkjet printer so that the image can be printed edge to edge on the leather piece without ink spillage.

[0014] Get a reusable clamp after choosing a repositionable pressure-sensitive adhesive.

[0015] These and other objects of the present invention will become apparent from the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1The invention relates to a leather handbag (1) having a flap (2), a front part (3), a back part (not visible), sides (4) and a leather strap (5). The handbag (1) is assembled by sewing together leather pieces (6) to (9), some or all of which are decorated by inkjet.

[0017] Figure 2 is an embodiment of the present invention, which shows a method for producing decorative leather by inkjet. In step A, a jig layer (11) is provided with holes (12) corresponding to the contour of the leather piece to be inkjet printed. In step B, an adhesive backing layer (13) is attached to the jig layer (11) with holes (12). In step C, leather pieces (14) are positioned in their corresponding holes. In an inkjet printing step D, a decorated leather piece (15) is obtained, preferably also with printing of an identification code (16) on the jig layer (11).

[0018] Figure 3 Schematically shown is a cross section of an animal skin comprising a grain (31) and dermis (33) separated by a grain-dermis junction (32). Different leathers made from animal skin include full grain leather (34), top grain leather (35) and split leather (36). DETAILED DESCRIPTION

[0019] definition

[0020] The term "crust leather" refers to leather that has been tanned and tanned but not finished.

[0021] The term "pore" refers to the space between two surfaces of a layer having pore openings on both surfaces.

[0022] The term "cavity" refers to the empty space between two surfaces of a layer, the layer having pores at only one surface.

[0023] Manufacturing method

[0024] The method for manufacturing decorative natural leather according to a preferred embodiment of the present invention comprises the following steps:

[0025] a) providing a fixture layer (11) having holes (12) corresponding to the outer contour of a leather piece (14); b) attaching an adhesive backing layer (13) to the fixture layer (11) having the holes (12); c) positioning the leather piece (14) in the holes (12) with the genuine leather side attached to the adhesive backing layer; and d) inkjet printing a decorative image on the leather piece (14) to deliver a decorated leather piece (15).

[0026] In a preferred embodiment, the thickness of the clamp layer is greater than the thickness of the leather piece. This has the advantage that it can be automatically checked whether a non-flat part of the leather piece is likely to hit the print head of the inkjet printer. Furthermore, a contact device can be provided in the inkjet printer to prevent a collision between the leather piece and the print head. Such a contact device can be a roller located just before the reciprocating print head module of the inkjet printer. The roller contacts the clamp and presses the leather piece (14) back into the hole (12) of the clamp layer (11) so that it does not hit the print head or cause image artifacts.

[0027] In order to achieve both reliable inkjet printing and good image quality, the difference in thickness between the jig layer and the leather piece is preferably not more than 0.6 mm, more preferably in the range of 0.3-0.5 mm. This allows a distance of at least 1.3 mm, preferably 1.5 mm, between the nozzle plate of the print head and the top surface of the jig, which is considered a safe distance when the jig is transported on a conveyor belt under the print head module of the inkjet printer, thus ensuring high printing reliability. In order to obtain good image quality, the distance between the nozzle plate of the print head and the top surface of the leather piece (14) in the hole (12) is preferably less than 3 mm, more preferably not more than 2 mm.

[0028] Cavities are usually made in the jig rather than holes to hold three-dimensional objects. In the present invention, it was found that making holes in the jig layer and then closing one of the holes with an adhesive backing layer is faster than making cavities. At the same time, the adhesive of the backing layer not only allows the backing layer (13) to be attached to the jig layer (11), but also allows the leather piece (14) to be attached to the backing layer (13).

[0029] For good image quality, the printing accuracy on the leather piece should preferably be at least 0.5 mm. This means that the clamp layer (11) is provided with holes (12) having an additional 0.5 mm added to the outer contour of the leather piece (14).

[0030] The preferred material for the clamp layer is PVC foam board because it is relatively light weight compared to other materials and has sufficient strength and flexibility. Such PVC foam board is readily available in a wide range of thicknesses at low cost. Suitable commercially available PVC foam board is available from VINK PVC board and PVC board.

[0031] There is no practical limit to the size of the jig layer, but the size of the jig layer is preferably no larger than 1.3m x 1.3m, which is convenient for manual handling and is less likely to break when using PVC foam sheets. The thickness of the jig layer is determined by the thickness of the leather that must be inkjet printed.

[0032] The outer contour of the leather piece (14) defines the size of the hole (12) in the fixture layer (11). The leather piece can be scanned to obtain the outer contour, which is particularly advantageous when the outer contour has a non-rectangular or very irregular shape because it improves reliability and productivity. This scan information is then used to cut the holes in the fixture layer, preferably using a laser cutter, as they are generally more precise than knife cutters. The same scan information can also be used to define an image layout for inkjet printing. The image layout corresponds to the location of one or more holes in the fixture layer so that a decorative image can be printed edge to edge on the leather piece in these holes.

[0033] This is usually done when the leather piece is a so-called full side or half side when there is only one hole in the jig. In this case, after inkjet printing, the leather piece is then further cut into smaller leather pieces.

[0034] The cutting of the holes is preferably carried out on an automatic cutting table, such as the one from ESKO. Cutting table or from AGFA Cutting table.

[0035] When one or more holes (12) are present in the jig layer (11), the adhesive backing layer (13) is attached to the jig layer (11) with adhesive, thereby closing one of the openings of the hole (12) and thus forming a "closed hole", i.e., a cavity. In this cavity, the unused adhesive is used to fix the leather piece (14) when it is positioned in the hole. The leather piece (14) is located in the hole (12) with its genuine leather side attached to the adhesive backing layer to obtain excellent image quality.

[0036] Fixture

[0037] The clamp of the preferred embodiment of the present invention comprises a clamp layer (11) including holes (12) corresponding to the outer contour of the leather piece (14) and having an adhesive backing layer (14) attached to the clamp layer (11) and the leather piece (14).

[0038] In a preferred embodiment, the clamp layer (11) is a PVC foam board.

[0039] Adhesive backing

[0040] There is no limitation on the type of adhesive used for the adhesive backing layer (13) as long as the leather piece (14) and the adhesive backing layer (13) can be separated from each other after inkjet printing.

[0041] The adhesive minimizes the risk that the leather piece may hit the print head during inkjet printing.

[0042] The adhesive is preferably a pressure-sensitive adhesive. Pressure-sensitive adhesives have been discussed in detail by Creton (MRS Bulleting, 434-439 (2003); Materials Science and Technology, A comprehensive treatment, Volume 18, Processing of Polymers, Chapter 15, Material Science of Pressure-Sensitive Adhesives, Wiley-VCH (2012))).

[0043] In a preferred embodiment, the adhesive backing layer (13) uses a repositionable pressure sensitive adhesive. Such repositionable pressure sensitive adhesives are well known, even to the wider public, for example, through the available note.

[0044] A repositionable pressure sensitive adhesive is preferred because the jig can be reused after the inkjet printed leather piece (14) is removed from the hole (12) in the jig layer (11). This is economically advantageous for inkjet printing a series of identical leather pieces (possibly with a personalised decorative image).

[0045] Del Campo and Fernández-Blázquez have described bioinspired repositionable adhesives (Biomimetic Approaches for Biomaterial Development, Chapter 11, ed. F. Mano, Wiley-VCH Verlag GmbH & Co. KGaA, first edition, 2012).

[0046] Preferred polymers for pressure sensitive adhesives are selected from polyacrylates, silicone polymers, polydienes or copolymers thereof, such as natural rubber and styrene-butadiene type rubbers and block copolymers of styrene and diene. Typical formulations of pressure sensitive adhesives also include tackifying resins to customize properties for applications. Pressure sensitive adhesives may also include additives such as waxes, plasticizers and antioxidants.

[0047] The pressure-sensitive adhesive is applied to the surface of the backing layer by water, solvent or as a hot melt adhesive. Hot melt adhesives and water-based adhesives are preferred. Water-based adhesives are particularly preferred due to their environmental friendliness. Water-based pressure-sensitive adhesives are based on emulsions and have been and Dubé (Journal of Macromolecular Science, Part C-Polymer Reviews, C44(1), 1-55 (2004)).

[0048] Suitable adhesives are available from KIWO (KISSEL+WOLF GmbH).

[0049] There is no restriction on the manner in which the adhesive is applied to the surface of the backing layer, which may be, for example, by coating or by spraying. Repositionable adhesives are preferred, since they cause no or only very limited damage to the leather side when separating the leather piece from the backing layer. Commercial examples of suitable spray adhesives are Repositionable 75 Spray Adhesive. Spray adhesives are also known as aerosol adhesives.

[0050] Ready-to-use adhesive backing layers are also commercially available. Suitable products include self-adhesive foils from GRAFITYP and adhesive backings from AGFA as PS can provide pressure sensitive PET foil. PS AR models use a repositionable adhesive.

[0051] Identification Code

[0052] In a preferred embodiment of the method for manufacturing decorative natural leather, one or more identification codes are used, which are connected to the leather piece with the inkjet printed decorative image. Such a connection can be, for example, information about which customer should receive the leather piece or the leather product made from the leather piece. Another example of a connection is that it identifies which leather product should be made from it.

[0053] The identification code may comprise alphanumeric data, such as a name and / or a number, but preferably it is a machine-readable identification code. The identification code is preferably machine-readable, since this generally speeds up the manufacturing process and / or the logistics process, since it allows faster identification without errors. There is no restriction on the type of machine-readable identification code. It may be a simple barcode, but also a so-called 2D code. Preferred 2D codes include barcodes, QR codes, data matrix codes, cool-data-matrix codes, Aztec codes, upcodes, trillcodes, quickmark codes, shot codes, mcodes, beetaggs, etc.

[0054] Before inkjet printing, one or more identification codes may be present on the backing layer. For example, the identification code can then identify which decorative image(s) will be printed on the leather piece.

[0055] The identification code may also be present on the fixture layer (11), preferably on the side opposite to the backing layer, unless the backing layer comprises a transparent foil.

[0056] In a preferred embodiment of the manufacturing method, the identification code is inkjet printed on the jig layer (11) at the same time as the decorative image is printed. This minimizes the risk of errors in, for example, connecting the correct customer to the printed leather piece. The jig containing the printed leather piece can also be sent as a whole to the leather goods manufacturer without further identification of the sent jig.

[0057] Alternatively, there is an identification code on the backing layer, and after the jig layer is removed, only the backing layer with the attached printed leather pieces is sent to the leather goods manufacturer. The jig layer can then be reused with a new backing layer to print a new set of leather pieces.

[0058] Rather than having the identification code present on the backing layer prior to inkjet printing, the identification code may also be inkjet printed through a hole in the fixture layer, wherein no leather piece is applied to the backing layer.

[0059] Decorative images

[0060] There is no practical limit to the type of decorative image that is inkjet printed on leather. The decorative image may consist of a single color, or it may include multiple colors, such as black, white, cyan, magenta, yellow, red, orange, violet, blue, green, and brown.

[0061] A decorative image may be purely decorative, but it may also contain alphanumeric data or other types of information, such as logos or pictograms.

[0062] natural leather

[0063] There are different grades of natural leather (see Figure 3 ), such as full grain (34), top grain (35) which is essentially full grain but part of the grain layer is sanded and the underlying separation layer is removed, and split leather (36). For the latter, the lower layer of the raw hide is removed and used to produce a split leather. Depending on the thickness of the lower layer, many pieces of leather can be produced. Split leather has a rough appearance and is generally used to make suede leather. When printing on the grain side of natural leather, high image quality is obtained.

[0064] To prevent grain damage and weakening, the hides or skins are preferably chrome tanned, but other tanning methods, such as vegetable tanning, may also be used. After tanning, the leather is dried and softened into a so-called crust leather. Tanning may include processes such as stripping (removal of surface-fixed tannins), fatliquoring (fixation of fats, oils and waxes to the leather fibers), dyeing, whitening, physical softening and polishing (abrasion of the leather surface to reduce grain defects).

[0065] Decorated natural leather can be used to make a wide range of leather products. Preferred leather products include footwear, furniture, upholstery, bags and luggage, gloves, belts, wallets, clothing, automotive leather (e.g., train, airplane, boat and car seats), interior decoration, books and stationery, packaging, equestrian products, etc.

[0066] Ink receiving layer

[0067] In order to improve the image quality, especially when the leather piece is intended for luxury leather goods, it is preferred to apply an ink receiving layer on the crust leather.The low viscosity of inkjet inks usually causes them to penetrate quickly into the leather, resulting in reduced image quality.

[0068] The ink receiving layer may be applied as a single layer, or may be applied as multiple layers. The multiple layers may even have different compositions to improve properties such as adhesion or flexibility.

[0069] The ink receiving layer preferably comprises a polyurethane based polymer or copolymer as this has been found to improve the flexibility of the printed leather. The ink receiving layer preferably also comprises a polyamide polymer or copolymer as polyamide has been found to improve compatibility with crust leather and improve the strength of the ink receiving layer.

[0070] Suitable polyurethanes include Urepal from CHEMPAL SpA TM PU147 and PU181; Melio from STAHL TM Promul 61; Astacin from BASF TM Finish PS; Ecrothan from MICHELMAN TM 4075, 4078 and 4084; Incorez from INCOREZ TM CS8073 and CS065-195. The dry weight of polyurethane in the ink receiving layer is preferably 1-6 g / m 2 within the range.

[0071] Suitable polyamides include PA emulsion types ED310 and 161148CX from MICHELMAN. The dry weight of the polyamide in the ink receiving layer is preferably less than 7 g / m 2 , more preferably less than 5g / m 2 .

[0072] Although polyurethane and / or polyamide are preferred as polymers for the basecoat, other polymers may be preferably used in combination with polyurethane and / or polyamide. Such polymers preferably have an elongation at break of greater than 200%, more preferably 300%. The elongation at break is measured according to ISO 527-2, for example using MTS Exceed from MTS Systems Corporation. TM Test setup measurements.

[0073] Another preferred class of polymers for the ink receiving layer is polyacrylates. Polyacrylates provide good flexibility and stabilize the pigment dispersion in the ink receiving layer.

[0074] In a preferred embodiment, the ink receiving layer preferably comprises a polyurethane-based polymer or copolymer and a polyacrylate-based polymer or copolymer. This combination brings excellent flexibility even in the presence of pigments.

[0075] Preferred polyacrylates are Roda TM Base 5514 and Primal from LANXESS TM HPB 980. A suitable polymerized acrylate emulsion is Bioflex™ KGA from LMF Biokimica.

[0076] A crosslinking agent may be added to the ink receiving layer to improve the strength of the ink receiving layer and the adhesion to the crust leather. Preferred crosslinking agents include aldehyde-based crosslinking agents such as formaldehyde, melamine formaldehyde derivatives, urea-formaldehyde resins, glyoxal and glutaraldehyde, epoxides, oxazolines, carbodiimides and isocyanates, with isocyanates being particularly preferred. The dry weight of the crosslinking agent in the ink receiving layer is preferably less than 1.4 g / m 2 , more preferably less than 1.0 g / m 2 .

[0077] The ink receiving layer is preferably applied by spraying, but may be applied by any known coating technique, such as knife coating, extrusion coating, slide hopper coating, and curtain coating.

[0078] The ink receiving layer may be transparent, but an opaque ink receiving layer is preferred because this masks defects in the natural leather.

[0079] The ink receiving layer can be a white ink receiving layer to enhance the color vibrancy of the inkjet printed image, but preferably the ink receiving layer has a similar color to the grain and leather (if present). Any desired color can be selected for the leather or grain and the ink receiving layer, such as red, green, brown, black, blue.

[0080] Inkjet ink

[0081] The inkjet ink printed on the leather item contains a colorant. The colorant can be a dye, but is preferably a pigment. Color pigments are not susceptible to light fading.

[0082] The pigmented inkjet ink for inkjet printing can be selected from aqueous pigmented inkjet inks, solvent-based pigmented inkjet inks and radiation curable pigmented inkjet inks. However, the one or more pigmented inkjet inks are preferably radiation curable inkjet inks, most preferably UV curable inkjet inks.

[0083] The pigmented inkjet inks preferably contain organic colored pigments, as they allow a high color gamut to be obtained on natural leather.Carbon black and titanium dioxide are inorganic pigments which can be advantageously used in the present invention to constitute black and white pigmented inkjet inks, respectively.

[0084] The organic colored pigments may be selected from those disclosed by HERBST, Willy et al. Industrial Organic Pigments, Production, Properties, Applications. 3rd edition. Wiley-VCH, 2004. ISBN 3527305769. Suitable colored pigments are disclosed in paragraphs

[0128] to

[0138] of WO 2008 / 074548 (AGFA GRAPHICS).

[0085] In a preferred embodiment, the pigmented inkjet inks form a CMYK(W) or CRYK(W) inkjet ink set.

[0086] The pigment particles in inkjet inks should be small enough to allow the ink to flow freely through the inkjet printing device, especially at the nozzles. It is also desirable to use small particles to obtain maximum color strength and slow down sedimentation.

[0087] The number average pigment particle size of the organic color pigment and the inorganic black pigment is preferably between 0.050 and 1 μm, more preferably between 0.070 and 0.300 μm, and most preferably between 0.080 and 0.200 μm. The number average particle size is preferably determined by photon correlation spectroscopy at a wavelength of 633 nm using a 4 mW HeNe laser on a diluted sample of the pigmented inkjet ink. A suitable particle size analyzer used is the Malvern TM Nano-S, purchased from Goffin-Meyvis. For example, a sample can be prepared by adding a drop of ink to a cuvette containing 1.5 mL of ethyl acetate and mixing until a homogenous sample is obtained. The measured particle size is the average of 3 consecutive measurements consisting of 6 20-second runs.

[0088] White pigments preferably have a number average pigment particle size greater than 180 nm, so as to have a strong hiding power. Suitable white pigments are given in Table 2 of

[0116] of WO 2008 / 074548 (AGFA GRAPHICS). White pigments are preferably pigments having a refractive index greater than 1.60. White pigments can be used alone or in combination. Preferably, titanium dioxide is used as a pigment having a refractive index greater than 1.60. Suitable titanium dioxide pigments are those disclosed in WO 2008 / 074548 (AGFA GRAPHICS)

[0117] and

[0118] .

[0089] When a UV curable pigmented inkjet ink is used, a polymerizable compound and at least one photoinitiator are present in the inkjet ink.

[0090] Any polymerizable compound commonly known in the art can be used. The polymerizable compound can be any monomer or oligomer found in Polymer Handbook Vol 1+2, 4th edition, edited by J. BRANDRUP et al., Wiley-Interscience, 1999. The oligomer in the present invention is understood to contain 2 to 8 repeating monomer units. Preferred monomers and oligomers are those listed in

[0106] -

[0115] of EP 1911814 A (AGFA).

[0091] Preferably, monomers or oligomers capable of free radical polymerization are used as polymerizable compounds. Combinations of monomers, oligomers and / or prepolymers may also be used. Monomers, oligomers and / or prepolymers may have different functionalities, and mixtures comprising combinations of monomers, oligomers and / or prepolymers of single, two, three and higher functionality may be used. However, preferably, at least 60-100 wt % of monofunctional polymerizable compounds are used in UV curable pigmented inkjet inks, wherein the wt % is based on the gross weight of the inkjet ink. The viscosity of the radiation curable inkjet ink may be regulated by changing the amount of specific monomers and oligomers.

[0092] The UV curable inkjet ink comprises one or more photoinitiators, preferably one or more free radical photoinitiators.Free radical photoinitiators are compounds that initiate polymerization of monomers and oligomers by forming free radicals when exposed to actinic radiation.

[0093] Suitable photoinitiators are disclosed in CRIVELLO, JV et al. Volume III: Photoinitiators for Free Radical Cationic. 2nd edition. Edited by BRADLEY, G. London, UK: John Wiley and Sons Ltd, 1998. pp. 287-294.

[0094] The inkjet ink printed in step d of the manufacturing method is most preferably a UV LED curable pigmented inkjet ink. Such an inkjet ink allows reliable and cost-effective inkjet printing.

[0095] In a particularly preferred embodiment, the one or more photoinitiators include an acylphosphine oxide photoinitiator and a thioxanthone photoinitiator. This combination allows for rapid UV curing with UV LEDs emitting above 370 nm. UV LEDs are more economical than mercury lamps in operating inkjet printing.

[0096] To further increase the photosensitivity, the UV curable inkjet ink may additionally contain a coinitiator. Suitable examples of coinitiators can be classified into three groups: (1) tertiary aliphatic amines, such as methyldiethanolamine and N-methylmorpholine; (2) aromatic amines, such as 2-(dimethylamino)ethyl benzoate; and (3) (meth)acrylated amines, such as dialkylaminoalkyl (meth)acrylates and N-morpholinoalkyl (meth)acrylates. Preferred coinitiators are aminobenzoates.

[0097] The preferred amount of photoinitiator and co-initiator is 0.1-30 wt %, and most preferably 5-10 wt %, based on the total weight of the UV curable inkjet ink.

[0098] The pigmented inkjet ink or inks may contain other additives such as surfactants, polymerization inhibitors and dispersants for the color pigments.

[0099] The preparation of pigmented UV curable inkjet inks is well known to the person skilled in the art. Preferred preparation methods are disclosed in paragraphs

[0076] to

[0085] of WO 2011 / 069943 (AGFA).

[0100] Top coating

[0101] A topcoat may be applied over the decorative image to enhance the scratch resistance of the decorative image.

[0102] The topcoat layer can be applied as a single layer, or can be applied as multiple layers. The multiple layers can even have different compositions to improve properties such as flexibility.

[0103] The protective top coating may have the same or similar composition as the ink receiving layer. Typically, the protective top coating is optimized to a certain extent depending on the leather application. For example, in contrast to leather shoes, flexibility does not play an important role for leather book covers. Therefore, the protective top coating for book covers may be optimized towards scratch resistance.

[0104] The topcoat layer preferably comprises a crosslinking agent and a polyurethane and / or polyamide based polymer or copolymer.

[0105] The top coating preferably comprises a polyurethane based polymer or copolymer as this contributes to the flexibility of the printed leather. If improved scratch resistance is required, it is preferred to include a polyamide polymer which has been found to have high compatibility with polyurethane binders.

[0106] Suitable polyurethanes include Urepal from CHEMPAL SpA TM PU147 and PU181; Melio from STAHL TM Promul 61; Astacin from BASF TM Finish PS; Ecrothan from MICHELMAN TM 4075, 4078 and 4084; Incorez from INCOREZ TM CS8073 and CS065-195. The dry weight of polyurethane in the top coating is preferably 1 to 6 g / m 2 within the range.

[0107] Suitable polyamides include PA emulsion types ED310 and 161148CX from MICHELMAN. The dry weight of the polyamide in the topcoat is preferably less than 7 g / m 2 , more preferably less than 5g / m 2 .

[0108] Although polyurethane and / or polyamide are preferred as polymers for the top coat, other polymers may be preferably used in combination with polyurethane and / or polyamide. Such polymers preferably have an elongation at break of greater than 200%, more preferably 300%. The elongation at break is measured according to ISO 527-2, for example using MTS Exceed from MTS Systems Corporation. TM Test setup measurements.

[0109] Another type of preferred polymer for the topcoat is polyacrylate.Polyacrylates provide good flexibility to the topcoat.

[0110] In a preferred embodiment, the top coating preferably comprises a polyurethane-based polymer or copolymer and a polyacrylate-based polymer or copolymer. This combination brings excellent flexibility.

[0111] Preferred polyacrylates are Roda TM Base 5514 and Primal from LANXESS TM HPB980. A suitable polymerized acrylate emulsion is Bioflex from LMF Biokimica TM KGA.

[0112] A crosslinking agent may be introduced into the protective topcoat to improve scratch resistance. Preferred crosslinking agents include those described above for the ink receiving layer. The dry weight of the crosslinking agent in the protective topcoat is preferably less than 1.4 g / m 2 , more preferably less than 1.0 g / m 2 .

[0113] The topcoat is preferably applied by spraying, but may also be applied by the same coating techniques as described above for the ink receiving layer.

[0114] The topcoat is most preferably a transparent topcoat, but can be a translucent topcoat. By having a transparent topcoat, the inkjet printed image is clearly visible through the topcoat. By using a translucent topcoat, a special aesthetic effect is produced.

[0115] If a matte top surface is desired for inkjet printed leather, a matting agent may be included. Any suitable matting agent may be used. Preferred matting agents include silica. A preferred commercial example of a silica dispersion is Euderm® from LANXESS. TM SN2.

[0116] Inkjet printing device

[0117] One or more pigmented inkjet inks are jetted by one or more print heads which eject small droplets through nozzles in a controlled manner onto natural leather which is moving relative to the print heads.

[0118] The preferred print head for the inkjet printing system is a piezoelectric head. Piezoelectric inkjet printing is based on the movement of a piezoelectric ceramic transducer when a voltage is applied to it. The application of voltage changes the shape of the piezoelectric ceramic transducer in the print head, creating a void that is then filled with inkjet ink or liquid. When the voltage is removed again, the ceramic expands to its original shape, ejecting ink drops from the print head.

[0119] A preferred piezoelectric print head is a so-called through-flow piezoelectric drop-on-demand print head. By using a through-flow print head, reliability is enhanced and thus a more efficient and economical method of manufacturing high quality decorative natural leather products is obtained. Such a print head is available, for example, from TOSHIBA TEC as a CF100 print head.

[0120] However, the inkjet printing method according to the present invention is not limited to piezoelectric inkjet printing. Other inkjet printing heads may be used, and include various types, such as a continuous type printing head.

[0121] The inkjet print head usually scans back and forth across the moving leather surface in the transverse direction, which is called a multi-pass printing mode. Sometimes the inkjet print head does not print on the way back. Bidirectional printing is preferred for obtaining high area production.

[0122] Another preferred printing method is by a "single pass printing process" which can be performed by using a page-wide inkjet print head or multiple staggered inkjet print heads that cover the entire width of the leather surface. In a single pass printing process, the inkjet print head generally remains stationary and the leather surface receiving the ink is transported below the inkjet print head.

[0123] The advantage of using a multi-pass printing mode is that the liquid UV-curable inkjet ink is cured in successive passes rather than in a single pass requiring a curing device with a high UV output. The print head life is also longer with multi-pass printing. While in single-pass printing one side gun is sufficient to replace the entire print head, in multi-pass printing side guns and even failures can be tolerated. Moreover, the cost of a multi-pass printing press is generally much lower, especially for large-format crust leather.

[0124] Curing device

[0125] If water-based or solvent-based pigmented inkjet inks are used, some curing means for evaporating water and organic solvents is required. Suitable curing means include heat radiation devices, such as hot air dryers, ovens or infrared light sources, such as infrared lasers, one or more infrared laser diodes or infrared LEDs.

[0126] An effective infrared radiation source for aqueous inkjet inks has an emission maximum between 0.8 and 2.5 μm, preferably between 0.8 and 1.5 μm. Such an infrared radiation source is sometimes referred to as a NIR dryer.

[0127] If UV-curable pigmented inkjet inks are used, curing is usually carried out only by UV radiation. If so-called hybrid UV-curable pigmented inkjet inks are used which also contain water or organic solvents, the curing device preferably also comprises a curing device for water-based or solvent-based pigmented inkjet inks.

[0128] In inkjet printing, the curing device may be arranged in combination with the print head of the inkjet printer, travelling therewith, so that the inkjet ink is exposed to curing radiation soon after being ejected.

[0129] Any UV light source, as long as part of the emitted light can be absorbed by the photoinitiator or the photoinitiator system, can be used as the radiation source, such as high-pressure or low-pressure mercury lamps, cold cathode tubes, black lights, UV LEDs, UV lasers and flash lamps. Among them, the preferred light source is a light source with a relatively long wavelength UV contribution of 300-400 nm as the main wavelength. In particular, a UV-A light source is preferred because its reduced light scattering leads to more efficient internal curing.

[0130] UV radiation is usually classified as UV-A, UV-B and UV-C as follows:

[0131] UV-A: 400nm-320nm

[0132] UV-B: 320nm-290nm

[0133] UV-C: 290nm-100nm.

[0134] In addition, two light sources of different wavelengths or illumination can be used to cure the image, either sequentially or simultaneously. For example, a first UV source rich in UV-C can be selected, particularly in the range of 260nm-200nm. The second UV source can then be rich in UV-A, such as a gallium-doped lamp, or a different lamp high in both UV-A and UV-B. It has been found that using two UV sources has advantages such as fast curing speed and high degree of cure.

[0135] In a particularly preferred embodiment, UV curing is performed using UV LEDs emitting at wavelengths greater than 360 nm.

[0136] To facilitate curing, the inkjet printer may include one or more oxygen depletion units. The oxygen depletion unit places a blanket of nitrogen or other relatively inert gas (e.g., CO2) with adjustable position and adjustable inert gas concentration to reduce the oxygen concentration in the curing environment. Residual oxygen levels are typically maintained as low as 200 ppm, but are typically in the range of 200 ppm to 1200 ppm.

[0137] Example

[0138] This embodiment shows one way of how to implement the invention.

[0139] Preparation of the fixture

[0140] Leather pieces with a thickness of 1.7 mm were cut to A4-size from chrome-tanned cowhide, dyed black, with a black ink receiving layer and textured embossing, available from ECCO.

[0141] Use 2mm thick PVC foam board is used to prepare a 1200mm×1200mm fixture layer. The bench cutter cuts three holes in the jig layer that are 0.5mm larger than the A4 size at each edge.

[0142] A piece of 0.1 mm thick self-adhesive foil (obtained from GRAFITYP as POLY 75WGPT, 152.5 cm x 50 m) was placed on the table, adhesive side up. The fixture layer with holes was then placed on the self-adhesive foil. Excess foil was trimmed around the fixture layer.

[0143] Three A4 size leather pieces were then positioned in the holes of the clamp layer by pressing them against the repositionable pressure sensitive adhesive. It was observed that even when the leather had contacted the adhesive, the leather could still be moved into the corner. It was also easily observed that there was no portion of the leather piece above the top surface of the clamp layer.

[0144] Inkjet printing on leather goods

[0145] Load the fixture with three leather pieces into On the conveyor belt of the eTU25UV inkjet printer, at the (0,0) position relative to the printer's fixed rod. Set the gap between the print head and the top surface of the fixture to 1.7 mm. Use iUL 10 inkjet ink prints a different multi-color decorative image on each of the three leather pieces.After UV LED curing of the printed samples, the jig is removed from the conveyor belt of the printing press and the cured leather samples are disassembled from the jig.

[0146] The jig is then reused by positioning the three new A4 size leather pieces into the holes of the jig. eTU25UV inkjet printer and The three leather pieces were inkjet printed in the same manner using iUL 10UV inkjet ink. It was found that the fixture could be reused at least 10 times to print leather samples in a fast and reliable manner.

[0147] No residues of adhesive were detected on the leather side of the leather piece.

[0148] Reference Mark List

[0149] exist Figure 1 and 2 The following reference signs are used.

[0150] Table 1

[0151] 1 handbag 2 Flip 3 Front 4 Side 5 bring 6 Leather flap 7 Front leather panel 8 Rear leather parts 9 Side and bottom leather panels 10 With leather parts 11 Fixture layer 12 hole 13 Adhesive backing 14 Leather Goods 15 Decorative leather pieces 16 Identification Code 31 Grain 32 The junction between grain and leather 33 Genuine Leather 34 full grain leather 35 Top grain leather 36 Split Leather

Claims

1. A method for manufacturing decorative natural leather, comprising the following steps: a) providing a clamp layer (11) having holes (12) corresponding to the outer contour of the leather piece (14); b) attaching an adhesive backing layer (13) to a fixture layer (11) having holes (12); c) positioning the leather piece (14) in the hole (12) with the leather side attached to the adhesive backing; as well as d) inkjet printing a decorative image on the leather piece (14) to deliver a decorated leather piece (15).

2. The method according to claim 1, wherein the thickness of the clamp layer (11) is greater than the thickness of the leather piece.

3. The method of claim 1, wherein the adhesive backing layer (13) uses a repositionable pressure sensitive adhesive.

4. The method according to claim 1, wherein the outer contour is obtained by scanning the leather piece (14).

5. The method of claim 1, wherein the hole (12) is cut by a laser cutter.

6. The method of claim 1, wherein the image layout for inkjet printing is defined on a fixture layer (11) comprising one or more holes (12).

7. The method of claim 1, wherein the clamp layer (11) or the adhesive backing layer comprises an identification code (16).

8. The method of claim 7, wherein the identification code (16) is machine-readable.

9. The method of claim 1, wherein in step d, one or more UV LED curable pigmented inkjet inks are inkjet printed.

10. The method of claim 1, wherein the leather piece (14) is a crust leather piece having an ink receiving layer containing polyurethane.

11. The method of claim 1, wherein the fixture layer (11) is a PVC foam board.

12. A clamp comprising a clamp layer (11) including holes (12) corresponding to the outer contour of a leather piece (14) and having an adhesive backing layer (14) attached to the clamp layer (11) and the leather piece (14), wherein the adhesive backing layer (13) uses a repositionable pressure sensitive adhesive.

13. The clamp according to claim 12, wherein the clamp layer (11) is a PVC foam board.

14. A clamp as claimed in claim 12 or 13, wherein the clamp layer comprises an identification code (16).

15. A fixture as claimed in claim 14, wherein the identification code (16) is machine readable.

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