GB2540011B

GB2540011B - Method of printing

Info

Publication number: GB2540011B
Application number: GB1607513.7A
Authority: GB
Inventors: Gould Nigel
Original assignee: Fujifilm Speciality Ink Systems Ltd
Current assignee: Fujifilm Speciality Ink Systems Ltd
Priority date: 2015-04-30
Filing date: 2016-04-29
Publication date: 2019-09-25
Anticipated expiration: 2036-04-29
Also published as: GB201607513D0; GB201507494D0; GB2540011A

Description

Method of printing

The present invention is concerned with a method of printing and more particularly to a method of printing onto a leather or synthetic leather substrate.

The printing of ink images onto permeable leather or synthetic leather substrates for the production of sportswear and sports equipment, such as footballs and football boots, is a significant challenge. Leather or synthetic leather presents an impervious (often thermoset) surface that is highly flexible and it is difficult to gain adhesion thereto. It is also essential that the finished printed substrate retains excellent flexibility, has high resistance to abrasion and is resistant to water.

Synthetic leather substrates such as those based on PVC and polyurethane resins are particularly challenging owing to their surface chemistry and in particular, their low surface energy. During production, the leather grain effect is commonly added by casting the liquid resin onto grain-patterned release paper. After drying, this grain effect layer is then backed with carrier foam to give the substrate its final structure. Finally, the release paper is removed to reveal the grain effect surface. Residual release aids such as silicone fluids contribute to the low surface energy of these substrates.

The current process for printing ink onto a leather or synthetic leather substrate, for sportswear and sports equipment, is a screen printing process. Screen printing is currently used because such a process allows for the use of an ink having a wide viscosity range, allows for the alteration of film thickness and provides tough films. However, such a screen printing process is labour intensive. A large number of screens are required as individual screens are required for each colour and for each size of print. This is costly and time consuming.

Therefore an improved process for printing on such difficult substrates is required.

Inkjet printing is desirable and therefore an inkjet ink process has been contemplated in order to produce a printed image onto such difficult substrates because colours can be printed concurrently and any image size can be printed simply by varying the data file used. Examples of inkjet printing processes on leather or synthetic leather substrates can be found in GB 2510486, GB 2510693, GB 2512429, GB 2512430, GB 2510694, GB 2510695, GB 2510696 and GB 2511606. However, a drawback of these processes is that they all require solvents and these can be irritants.

Hence there exists a requirement in the art for a method of printing onto a leather or synthetic leather substrate, which eliminates or reduces the aforementioned problems.

Accordingly, the present invention provides a method of printing onto a leather or synthetic leather substrate having a primer layer obtainable by providing the substrate, depositing a primer onto the substrate to form a primer layer, wherein the primer comprises an aqueous polyurethane dispersion, which is redispersible in water after thermal drying and before curing, a water-dispersible or water-soluble photoinitiator, a surfactant, and at least partially drying the primer; the method comprising each ofthe following steps in order: (i) inkjet printing a hybrid water/radiation-curable ink onto the primer layer to form a hybrid water/radiation-curable ink layer; (ii) drying the hybrid water/radiation-curable ink layer; and (iii) (a) optionally depositing a protective layer onto the hybrid water/radiation-curable ink layer, wherein the protective layer comprises: a thermoplastic resin selected from polyurethane, polymethacrylate, polyvinyl, poly(cellulose acetate butyrate), polyester, epoxy resin, hydrocarbon resin, ketone or aldehyde resins, amino resins, and copolymers and blends thereof, a solvent and/or radiation-curable diluent, and, when a monofunctional radiation-curable diluent is present, a photoinitiator, and a crosslinking agent; and (b) curing the hybrid water/radiation-curable ink layer; wherein when (iii)(a) is present, (iii)(a) and (iii)(b) can be in either order, with the proviso that when step (iii)(b) occurs before step (iii)(a), the method further comprises the step of drying and/or curing the protective layer.

The present invention further provides a method of printing onto a leather or synthetic leather substrate comprising each ofthe following steps in order: (i) providing the substrate; (ii) depositing a primer onto the substrate to form a primer layer, wherein the primer comprises: an aqueous polyurethane dispersion, which is redispersible in water after thermal drying and before curing, a water-dispersible or water-soluble photoinitiator and a surfactant; (iii) at least partially drying the primer; (iv) inkjet printing a hybrid water/radiation-curable ink onto the primer layer to form a hybrid water/radiation-curable ink layer; (v) drying the hybrid water/radiation-curable ink layer; and (vi) (a) optionally depositing a protective layer onto the hybrid water/radiation-curable ink layer, wherein the protective layer comprises: a thermoplastic resin selected from polyurethane, polymethacrylate, polyvinyl, poly(cellulose acetate butyrate), polyester, epoxy resin, hydrocarbon resin, ketone or aldehyde resins, amino resins, and copolymers and blends thereof, a solvent and/or radiation-curable diluent, and, when a monofunctional radiation-curable diluent is present, a photoinitiator, and a crosslinking agent; and (b) curing the hybrid water/radiation-curable ink layer; wherein when (vi)(a) is present, (vi)(a) and (vi)(b) can be in either order, with the proviso that when step (vi)(b) occurs before step (vi)(a), the method further comprises step of drying and/or curing the protective layer.

The present invention is particularly suited to synthetic leather substrates.

The present invention may provide a printed substrate and sportswear or sports equipment obtainable by the methods ofthe present invention.

The present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows a schematic ofthe printed substrate obtainable by the method ofthe present invention; Fig. 2 shows a photograph of a printed substrate having a primer layer and an inkjet ink ofthe present invention, in contrast with the printed substrate having comparative primer layers and the inkjet ink of the present invention;

Fig. 3 shows a photograph ofthe printed substrate obtainable by the method ofthe present invention, in contrast with the uncoated substrate; and

Fig. 4 shows a photograph of a printed substrate having a primer layer ofthe present invention which has been both dried and cured prior to the printing of an inkjet ink ofthe present invention, in contrast with the uncoated substrate.

The low viscosity of inkjet inks make them highly sensitive to de-wetting on low energy surfaces such as synthetic leather. Water-based inkjet inks are particularly attractive for use in inkjet printing. Low film thickness ofthe dried printed film gives a smooth, even appearance to the printed surface and good flexibility. Further, compared to solvent-based inks, water-based inkjet inks are less irritating and exhibit low or no solvent emission during the printing and drying process. However, water-based inkjet inks are particularly sensitive to de-wetting on low energy surfaces owing to the large difference in polarity between the inks and the surface.

It has been found that depositing a primer as defined herein onto the substrate to form a primer layer aids the pinning ofthe hybrid water/radiation-curable ink. In the method ofthe invention the primer is deposited onto the substrate to form a primer layer. The primer layer is then at least partially dried. The resultant at least partially dry primer layer is then deposited with the desired inkjet image by inkjet printing a hybrid water/radiation-curable ink onto the primer layer to form a hybrid water/radiation-curable ink layer.

Without wishing to be bound by theory, it is believed that the primer layer absorbs some of the water from the inkjet droplets resulting in a rise in viscosity of the hybrid water/radiation-curable ink, thus pinning the ink droplets. The water is pulled into the primer and results in an increase in viscosity of the hybrid water/radiation-curable ink, preventing reticulation (or pooling) ofthe ink. This results in an ink image of excellent quality. The resulting film also has excellent flexibility and water, chemical and abrasion resistance. Thus, the method of the present invention allows for the production of a printed leather or synthetic leather substrate having an improved image quality, flexibility and abrasion, water and chemical resistance.

The substrate is a leather or synthetic leather substrate, preferably a synthetic leather substrate. Leather or synthetic leather substrates are non-porous, non-elongatable, but highly flexible which makes them difficult to print onto. The substrate is, however, typically found in the sportswear or sports equipment field. Sportswear and sports equipment require the printed substrate to have excellent flexibility, have high resistance to abrasion and be resistant to water.

Leather is a well-known material, created by the tanning of animal rawhide and skin. The tanning processes available to transform hides and skins into leather are well known and do not require further discussion. The leather may be natural leather, where the surface to be printed is obtained solely by tanning the rawhide and skin. Alternatively, the leather may be coated leather, where the leather is split (to provide two batches from one hide) and coated with polyurethane.

Synthetic leather is produced from a fibrous base layer, typically polyester, coated with polyurethane or polyvinyl chloride. Again, synthetic leathers are well known in the art.

Thus, the substrate will be natural leather, polyurethane-coated leather, or synthetic leather (a material having a fibrous base layer coated with polyurethane or polyvinyl chloride). Thus, the surface which contacts the primer is either composed of leather, polyurethane or polyvinyl chloride.

The primer comprises an aqueous polyurethane dispersion (PUD), which is redispersible in water after thermal drying and before curing. It should be noted that resolubility and resoluble are terms often used in the art to mean redispersibility and redispersible, respectively.

The PUDs are chosen such that the resultant primer layer which has been at least partially dried is resoluble in the water of the ink that will be deposited thereon and will soften with heat. It is important that the primer layer is not thermoset. If the primer layer were not resoluble in the water of the ink, the hybrid water/radiation-curable ink layer would fail to be pinned by the primer layer.

When it is stated that the primer is “at least partially” dried, it means that the drying is performed to a sufficient extent that the primer layer retains its resolubility properties as determined by the below-described water rub test.

In the present invention, redispersibility of the primer in water after thermal drying and before curing is controlled by the selection of the PUD. The test to measure the suitability of an aqueous PUD for use in the primer of the present invention involves measuring the redispersibility of a PUD in water after thermal drying and before curing. In order to measure the redispersibility of a PUD in water after thermal drying and before curing, the aqueous PUD under test is blended with an aqueous pigment dispersion, such as ProJet (RTM) APD 1000 cyan pigment dispersion (available from Fujifilm (RTM) imaging colorants) or Diamond D71C cyan pigment dispersion (available from Diamond dispersions), to facilitate observation of film redispersion and removal. A surfactant, such as fluoro surfactants Capstone (RTM) FS31, Capstone (RTM) FS30 or Capstone (RTM) FS34 (available from Dupont(RTM)), is added to reduce surface tension and to allow wetting onto a suitable test substrate. After mixing the components, the test composition is coated onto a suitable test substrate to produce a wet film. The wet film is thermally dried and then cooled to room temperature. Redispersibilty of the thermally dried ink film in water can then be assessed by a water rub test.

The water rub test is well known in the art. One takes a lint-free (cotton) cloth saturated in water. One then carries out a single rub where the saturated cloth is applied to one side of the dried PUD film and under light pressure, traverses the length of the dried PUD film in a single stroke. In order for the PUD to be suitable for use in the present invention, the PUD must be dispersible in water after thermal drying and before curing. Put another way, the PUD film should be cleanly removed from the substrate surface leaving no residual staining visible to the naked eye after a single rub. The presence of the pigment in the film helps to determine if this requirement has been met as the substrate should become visible when the colour is removed. Further, no particulate matter visible to the naked eye should be transferred to the wiping cloth or to the substrate at the end of the wiping area.

Therefore, in a preferred embodiment, the PUD of the primer, after thermal drying and before curing, can be redispersed in water in three rubs or fewer, preferably two rubs or fewer and most preferably a single rub of the water rub test. A PUD is water-dispersible after thermal drying and before curing if it maintains its water sensitivity/compatibility after thermal drying and before curing. In order to maintain this water sensitivity/compatibility in a PUD after thermal drying and before curing, it is necessary to maintain a water-sensitive functionalised PUD after thermal drying and before curing. Such functionality must be water-sensitive and therefore hydrophilic, and often includes ionic groups. An example of a PUD having ionic functionality which is maintained after thermal drying and before curing is a PUD which has carboxylic acid functional groups which are neutralised with an alkali metal hydroxide, such as NaOH, to produce a metal salt. Such a PUD maintains water dispersibility after thermal drying and before curing because the PUD salt is stable and compatible with water. An example of a PUD having non-ionic functionality which maintains oxygen functionality after thermal drying and before curing is a PUD having non-ionic blocks in the PUD chain ofthe polymer, such as polyether blocks. A PUD is not water-dispersible after thermal drying and before curing if it has reduced water sensitivity/compatibility after thermal drying. This occurs if the water-sensitive functional groups are lost during thermal drying of the ink to produce a thermally dried film. For example, in the case where a PUD has carboxylic acid functional groups and is neutralised with an amine salt as opposed to for example an alkali metal hydroxide, on thermal drying of an ink comprising such a functionalised PUD, this results in the breaking down of the amine salt. This amine salt is driven off during thermal drying and hence the ionic character of the PUD is lost and a PUD with carboxylic acid groups remains, which is not redispersible in water.

In the event that the PUD is not redispersible in water after thermal drying and before curing and therefore has lost its water sensitivity/compatibility after thermal drying, the dried film will not redisperse in water. This results in the production of shards of film.

Such PUDs for the present invention, which are redispersible in water after thermal drying and before curing, are available commercially, for example, from BASF (RTM). An example of preparing such a PUD is known in the art, see C.Y. Bai et al. “A new UV curable waterborne polyurethane: Effect of C=C content on the film properties”, Progress in Organic Coatings, 2006, 55, 291-295.

The PUD which is redispersible in water after thermal drying and before curing preferably has a number average molecular weight of over 1,200 Daltons. In a preferred embodiment, the PUD has a number average molecular weight of 1,200 to 20,000, preferably 1,500 to 10,000, and most preferably 2,500 to 5,000, as measured by Infinity 1260 supplied by Agilent technologies, using gel permeation chromatography calibrated against polystyrene standards.

Further, the aqueous PUD which is redispersible in water after thermal drying and before curing is in dispersed form and preferably has a particle size of less than 200 nm as measured by Zeta PALS provided by Brookhaven Instruments Corporation.

The aqueous PUD which is redispersible in water after thermal drying and before curing with actinic (preferably UV) radiation, is non-dispersible in water after curing with actinic (preferably UV) radiation.

The aqueous PUD is crosslinkable when exposed to UV radiation as it is contains radiation-curable functional groups and is preferably acrylate functionalised. This helps to provide the physical film properties required, such as chemical and scratch resistance. The PUD of the invention is preferably a radiation-curable (meth)acrylate PUD. A particularly preferred PUD is Laromer (RTM) UA9122 available from BASF (RTM).

Preferably, the primer of the present invention comprises 80% by weight or more, preferably 90% by weight or more and most preferably 95% by weight or more, based on the total weight of the primer, of aqueous PUD which is redispersible in water after thermal drying.

The primer further comprises a water-dispersible or water-soluble photoinitiator. The free-radical, water-dispersible or water-soluble photoinitiator can be selected from any of those known in the art.

Examples of water dispersible photoinitiators include Esacure (RTM) DP250, Irgacure (RTM) 819 DW and Lucerin (RTM) TPO-L. Examples of water-soluble photoinitiators include Darocur (RTM) 1173, Irgacure (RTM) 2959 and 2-hydroxy-1-(4-(2-(2-hydroxyethoxy)ethoxy)phenyl)-2-methylpropan-1-one.

Preferably the photoinitiator is present in the primer an amount of 1 to 20% by weight, preferably 2 to 5% by weight.

In the present invention, the surface tension of the primer is controlled by the addition of one or more surface active materials such as commercially available surfactants. Therefore, the primer of the present invention further comprises a surfactant. Surfactants are well known in the art and a detailed description is not required. Adjustment of the surface tension of the primer allows control of the surface wetting of the primer. Too high a surface tension can lead to ink pooling and/or a mottled appearance in high coverage areas of the print. Too low a surface tension can lead to excessive ink bleed between different coloured inks. The surface tension of the primer is preferably in the range of 20-40 mNm'1 and more preferably 25-35 mNm’1. A preferred surfactant is Capstone (RTM) FS31, a fluoro surfactant available from Dupont (RTM).

The primer layer can optionally contain inorganic materials such as extender powders and the like, waxes and matting agents

The primer layer may optionally further comprise a humectant/water-based retarder. Preferred examples include mono propylene glycol, mono ethylene glycol and mixtures thereof.

The primer layer may also optionally comprise precipitated calcium carbonate to reduce the finish.

The primer layer may optionally contain a thickener, such as an anionic polyacrylate copolymer or a hydrophobicaliy modified ethylene oxide urethane rheology modifier.

To achieve good wetting onto the substrate, the viscosity of the primer needs to be higher than is suitable for inkjet printing applications. The primer may be applied to the substrate using a range of analogous printing techniques such as screen printing, flexographic printing, bar coating spraying, curtain coating, slit coating or wipe on by hand. Preferably, the primer is deposited onto the substrate by screen printing flatbed or rotary, flexographic printing, bar coating, spraying, curtain coating or slit coating. It is thought that the dewetting processes are hindered by the high viscosity of the primer, resulting in an evenly wetted surface. In a preferred embodiment, the viscosity of the primer is 500 to 1000 mPa-s, more preferably 600-900 mPa-s and most preferably 700-800 mPa-s at 25°C. Primer viscosity can be measured using a Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as a DV1 running at 20 rpm at 25°C with a UL-A spindle and adapter kit.

The primer is then at least partially dried to form a primer layer.

It should be noted that the terms “dry” and “cure” are often used interchangeably in the art when referring to radiation-curable inkjet inks to mean the conversion ofthe inkjet ink from a liquid to solid by polymerisation and/or crosslinking ofthe radiation-curable material. Herein, however, by “drying” is meant the removal ofthe solvent by evaporation and by “curing” is meant the polymerisation and/or crosslinking ofthe radiation-curable material.

The primer layer is at least partially thermally dried and is preferably fully dried to evaporate the water before the hybrid water/radiation-curable ink is deposited on the surface ofthe primer layer. A key factor for the pinning efficiency ofthe primer layer is the resolubility of the primer layer in the water ofthe hybrid water/radiation-curable ink which is to be deposited onto the primer layer.

In a preferred embodiment, the at least partially dried, preferably fully dried, films are readily resoluble by the water ofthe ink. The at least partially dried, preferably fully dried, films will still be receptive to water from the overlaying ink because it leaves behind a thermoplastic resin that is resoluble. The resolubility of the primer by the water of the ink results in excelling wetting of the ink film on the primer. Consequently, the printed ink film has a smooth appearance with no reticulation of ink droplets. In this way, the primer and inkjet ink ofthe present invention are compatible.

The primer ofthe invention facilitates both the wetting ofthe substrate (as a result of its high viscosity) and the wetting ofthe inkjet ink on the primer (as a result of its redispersability in water after thermal drying but before curing). This is achieved by the PUD of the present invention, which has a high molecular weight and hence a high viscosity, and a redispersible nature.

The thermally dried water-based UV crosslinkable thermoplastic resin dispersion primer must not be UV crosslinked before the ink is deposited as this would render the primer layer insoluble and non-receptive in the water and hence it could not perform its pinning role.

The primer layer preferably has a thickness of 40 microns or less, more preferably 20 microns or less and most preferably 10 microns or less, based on the dry film thickness. The dry film thickness means the thickness ofthe film after it is at least partially dried, i.e. when the primer layer receives the ink. It is surprising that such a small amount of primer provides the required image quality combined with the necessary robustness and flexibility. The thickness is preferably 1 micron or above, more preferably 5 microns or above. Film thicknesses can be measured using a confocal laser scanning microscope.

The inkjet ink is a hybrid water/radiation-curable ink jet ink which preferably comprises a radiation-curable material, a photoinitiator, a colorant and water.

The ink comprises a modified ink binder system. The preferred binder system ofthe ink is a radiation-curable polyurethane dispersion (PUD) with good redispersibility after thermal drying but before UV cure. The PUD suitable for the binder system of the ink is discussed hereinabove in relation to the PUD of the primer. Preferably, the binder system of the ink after thermal drying and before curing can be redispersed in three rubs or fewer, more preferably two rubs or fewer and most preferably a single rub of the water rub test. The water rub test is described hereinabove in relation to the PUD of the primer. The presence of a radiation-curable material and a photoinitiator in the ink means that crosslinked polymers can be formed in the dried ink film, leading to improved adhesion to a range of substrates and improved resistance to solvents. The presence of water means that the advantageous properties of water-based inkjet inks are maintained.

Preferably the ink comprises 30 to 80% by weight of radiation-curable material based on the total weight of the ink, preferably at least 40-70% by weight, more preferably 50 to 60% by weight, based on the total weight of the ink.

By “radiation-curable” is meant a material that polymerises or crosslinks when exposed to actinic radiation, commonly ultraviolet light, in the presence of a photoinitiator.

The radiation-curable material may optionally comprise a radiation-curable oligomer. The oligomers may possess different degrees of functionality, and a mixture including combinations of mono, di, tri and higher functionality monomers/oligomers may be used.

Radiation-curable oligomers suitable for use in the present invention comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation polymerisable groups. The oligomer preferably comprises a urethane backbone.

The radiation-curable material may optionally comprise a radiation-curable monomer. Suitable free-radical polymerisable monomers are well known in the art and include (meth)acrylates.

The monomers may possess different degrees of functionality, and a mixture including combinations of mono, di, tri and higher functionality monomers/oligomers may be used.

The ink includes one or more photoinitiators. The ink includes a free-radical polymerisable material and hence the photoinitiator system includes a free-radical photoinitiator. Preferably, the photoinitator is water-dispersible or water-soluble. Examples of water dispersible photoinitiators include Esacure (RTM) DP250, Irgacure (RTM) 819 DW and Lucerin (RTM) TPO-L. Examples of water-soluble photoinitiators include Darocur (RTM) 1173, Irgacure (RTM) 2959 and 2-hydroxy-1 -(4-(2-(2-hydroxyethoxy)ethoxy)phenyl)-2-methylpropan-1-one.

Preferably the photoinitiator is present in the ink an amount of 1 to 20% by weight, preferably 2 to 5% by weight.

Preferably the photoinitiator is present in an amount of 1 to 20% by weight, preferably 2 to 10% by weight, based on the total weight of the ink.

The inkjet ink of the present invention is preferably substantially free of volatile organic solvents. Preferably, the inkjet ink of the present invention comprises less than 5% by weight of volatile organic solvent, preferably less than 3% by weight, more preferably, less than 2% by weight by weight and most preferably less than 1 % by weight, based on the total weight of the ink. Some solvents may be present as impurities in the components of the inks, but such low levels are tolerated.

The coloured inks comprise at least one colouring agent. The colouring agent may be either dissolved or dispersed in the liquid medium of the ink. Preferably the colouring agent is a dispersible pigment, of the types known in the art and commercially available such as under the trade-names Paliotol (RTM) (available from BASF (RTM) pic), Cinquasia (RTM), Irgalite (RTM) (both available from Ciba (RTM) Speciality Chemicals) and Hostaperm (RTM) (available from Clariant (RTM) UK). The pigment may be of any desired colour such as, for example, Pigment Yellow 13, Pigment Yellow 83, Pigment Red 9, Pigment Red 184, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7. Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used. in one aspect the following pigments are preferred. Cyan: phthalocyanine pigments such as Phthalocyanine blue 15.4. Yellow: azo pigments such as Pigment yellow 120, Pigment yellow 151 and Pigment yellow 155. Magenta: quinacridone pigments, such as Pigment violet 19 or mixed crystal quinacridones such as Cromophtal (RTM) Jet magenta 2BC and Cinquasia (RTM) RT-355D. Black: carbon black pigments such as Pigment black 7.

Pigment particles dispersed in the ink should be sufficiently small to allow the ink to pass through an inkjet nozzle, typically having a particle size less than 8 pm, preferably less than 5 pm, more preferably less than 1 pm and particularly preferably less than 0.5 pm.

The colorant is preferably present in an amount of 20% by weight or less, preferably 15% by weight or less, based on the total weight of the ink. A higher concentration of pigment may be required for white inks, however, for example up to and including 30% by weight, or 25% by weight based on the total weight of the ink.

The inkjet ink exhibits a desirable low viscosity (200 mPa.s or less, preferably 100 mPa.s or less, more preferably 25 mPa.s or less and most preferably 10 mPa.s or less, at 25°C).

Other components of types known in the art may be present in the ink to improve the properties or performance. These components may be, for example, surfactants, defoamers, dispersants, synergists for the photoinitiator, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides, humectants and identifying tracers.

The ink may be prepared by known methods such as stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill.

The hybrid water/radiation-curable ink is deposited onto the at least partially dried primer layer to form a hybrid water/radiation-curable ink layer or deposited inkjet image by inkjet printing. Inkjet printing is well known in the art. The image may be deposited using a roll-to-roll solvent UV hybrid printer. Alternatively, the inks may be deposited using a flatbed printer where the print head scans across a stationary substrate before it is advanced ready for the next print swathe to be deposited. In this case there is an optional top down heater or air knife to assist the efficiency of the pinning process by removal of the water carrier from the water UV hybrid ink. Following the pinning process, in one embodiment the print is removed manually from the printer to a separate thermal drier where the remainder ofthe water carrier is removed from the ink before finally curing the image by exposure to a source of actinic radiation. This final curing stage can either be completed in-line by using a combination drier fitted with both a source of heat and a UV curing system or using separate thermal drying and UV curing systems.

In a further embodiment the printing pinning, thermal drying and final UV curing can be completed inline. After deposition of the image with the water UV hybrid ink and pinning on the primed material, the print is transferred to a thermal drying apparatus before finally passing under a source of actinic radiation to effect the final cure process. The transfer ofthe print through the combination printer can either be accomplished using sheets ofthe substrate held in grippers that move it through each stage ofthe process or alternatively via a roll-to-roll process where the primed substrate is moved as a web through each stage by the use of rollers. In a further embodiment the roll-to-roll web production process above can be modified to apply the primer layer in-line using a suitable method, for example rotary screen printing, flexographic printing, roller coating, bar coating or slit coating

The hybrid water/radiation-curable ink layer may then be dried and cured.

The printing is preferably all performed by inkjet printing, e.g. on a roll-to-roll printer or flat-bed printer. Evaporation of the water can occur simply by exposure of the inks to the atmosphere, but the inks may also be heated to accelerate evaporation. In addition, the inks are exposed to actinic radiation to cure the ink.

The ink can be printed using inkjet printers that are suitable for use with water-based inkjet inks, in combination with a source of actinic radiation. The features of printers that are suitable for printing water-based inkjet inks are well known to the person skilled in the art and include the features described below.

The printing apparatus may comprise a means for evaporating water from the ink at the appropriate time after the ink has been applied to the substrate. Any means that is suitable for evaporating water from known water-based inkjet inks can be used in the apparatus. Examples are well known to the person skilled in the art and include dryers, heaters, air knives and combinations thereof.

In one embodiment, the water is removed by heating. Heat may be applied through the substrate and/or from above the substrate, for example by the use of heated plates (resistive heaters, inductive heaters) provided under the substrate or radiant heaters (heater bars, IR lamps, solid state IR) provided above the substrate. In a preferred embodiment, the ink can be jetted onto a preheated substrate that then moves over a heated platen. The apparatus may comprise one or more heaters.

Typical materials used for the production ofthe sportswear and sports equipment have relatively high film thickness and hence have a high heat capacity. The high heat capacity means that it is not possible to increase the temperature of the substrate quickly enough to effect the required viscosity increase in the ink droplets. The heat cannot penetrate through the substrate sufficiently, meaning that the pinning process is disabled, leading to excessive ink flow, yielding very poor image quality. In other words, the time needed to pin the ink droplets is too long owing to the high heat capacity ofthe substrates, which results in the pooling of ink droplets and a poor quality image.

Accordingly, the primer layer absorbs some ofthe water from the inkjet droplets resulting in a rise in viscosity of the hybrid water/radiation-curable ink, thus pinning the ink droplets. The water is pulled into the primer and results in an increase in viscosity of the hybrid water/radiation-curable ink, preventing reticulation (or pooling) ofthe ink.

When printing the ink, a significant portion ofthe water is preferably allowed to evaporate before the ink is cured. Preferably substantially all ofthe water is evaporated before the ink is finally cured. This is achieved by subjecting the printed ink to conditions that would typically dry conventional water-based inkjet inks. In the case of the ink, such conditions will remove most of the water but it is expected that trace amounts of water will remain in the film given the presence of the radiation-curable component in the ink.

Unlike standard water-based inks, once the water has evaporated, the ink is not expected to be completely solid. Rather, what remains on the surface is a high viscosity version of a radiation-curable ink. The viscosity is sufficiently high to inhibit or significantly hinder ink flow and prevent image degradation in the timescale that is needed to post-cure the ink. Upon exposure to a radiation source, the ink cures to form a relatively thin polymerised film. The ink of the present invention typically produces a printed film having a thickness of 1 to 20 pm, preferably 1 to 10 pm, for example 2 to 5 pm. Film thicknesses can be measured using a confocal laser scanning microscope.

The source of actinic radiation can be any source of actinic radiation that is suitable for curing radiation-curable inks but is preferably a UV source. Suitable UV sources include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof. One or more mercury discharge lamps, fluorescent tubes, or flash lamps may be used as the radiation source. When LEDs are used, these are preferably provided as an array of multiple LEDs.

Preferably the source of actinic radiation is a source that does not generate ozone when in use.

The source of UV radiation could be situated off-line in a dedicated conveyor UV curing unit, such as the SUVD Svecia UV Dryer. Preferably, however, the source of radiation is situated in-line, which means that the substrate does not have to be removed from the printing apparatus between the heating and curing steps.

The radiation source can be mobile, which means that the source is capable of moving back and forth across the print width, parallel with the movement of the printhead.

In one embodiment, one or more sources of actinic radiation are placed on a carriage that allows the source of actinic radiation to traverse the print width. The carriage may be placed up and downstream of the printer carriage to allow irradiation before and after evaporation of the solvent. In this embodiment the source of actinic radiation moves independently of the printer carriage and movement of the printhead does not therefore have to be slowed in order to provide adequate time for solvent evaporation. Thus, overall productivity can be improved.

When the source of radiation is provided on separate carriage, it is necessary to provide an additional carriage rail, motor and control systems. This adaptation can lead to large increases in equipment costs.

Preferably the source of radiation is static. This means that the source does not move backwards and forwards across the print width of the substrate when in use. Instead the source of actinic radiation is fixed and the substrate moves relative to the source in the print direction.

When the source of actinic radiation is provided in the print zone of the printer, light contamination at the printhead, which could lead to premature curing in the nozzle, must be avoided. Adaptations to prevent light contamination, such as lamp shutters, give rise to additional costs. The source of radiation is therefore preferably located outside the print zone of the printing apparatus. By print zone is meant the region of the printing apparatus in which the printhead can move and therefore the region in which ink is applied to the substrate.

Static curing units preferably span the full print width, which is typically at least 1.6 m for the smaller wide format graphics printers. Fluorescent tubes, mercury discharge lamps, and light emitting diodes can be used as static curing units.

Any of the sources of actinic radiation discussed herein may be used for the irradiation of the inkjet ink. A suitable dose would be greater than 200 mJ/cm2, more preferably at least 300 mJ/cm2 and most preferably at least 500 mJ/cm2. The upper limit is less relevant and will be limited only by the commercial factor that more powerful radiation sources increase cost. A typical upper limit would be 5 J/cm2.

The wavelength of the pinning source is typically 200-700 nm, preferably 300-500 nm and most preferably 350-450 nm.

The delay between evaporating the solvent and providing a final cure ofthe ink is typically at least 1 minute after jetting.

High and medium pressure mercury discharge lamps may be used.

In another embodiment of the invention the source of radiation comprises one or more flash lamps. Flash lamps operate by discharge breakdown of an inert gas, such as xenon or krypton, between two tungsten electrodes. Unlike mercury discharge lamps, flash lamps do not need to operate at high temperature. Flash lamps also have the advantage of switching on instantaneously, with no thermal stabilisation time. The envelope material can also be doped, to prevent the transmission of wavelengths that would generate harmful ozone. Flash lamps are therefore economical to operate and therefore suitable for use.

The PUD ofthe primer layer ofthe present invention remains uncured after the at least partial drying step. Therefore, after deposition and thermal drying ofthe hybrid water/radiation-curable inkjet ink, both the primer layer and the hybrid water/radiation-curable ink layer are cured concurrently. A protective layer may optionally be deposited onto the hybrid water/radiation-curable ink layer. The protective layer may be deposited before or after the step of curing the hybrid water/radiation-curable ink layer.

The protective layer comprises a resin selected from polyurethane, polymethacrylate, polyvinyl, poly(cellulose acetate butyrate), polyester, epoxy resin, hydrocarbon resin, ketone or aldehyde resin, amino resin, and copolymers and blends thereof. Preferably, the protective layer comprises polyurethane.

The protective layer further comprises an organic solvent and/or radiation-curable diluent.

In a preferred embodiment the organic solvent is a low toxicity and/or a low odour solvent. Solvents that have been given VOC exempt status by the United States Environmental Protection Agency or European Council are also preferred.

The most preferred solvents are selected from glycol ethers and organic carbonates and mixtures thereof. Cyclic carbonates such as propylene carbonate and mixtures of propylene carbonate and one or more glycol ethers are particularly preferred.

Alternative preferred solvents include lactones, which have been found to improve adhesion ofthe ink to PVC substrates. Mixtures of lactones and one or more glycol ethers, and mixtures of lactones, one or more glycol ethers and one or more organic carbonates are particularly preferred. Mixtures of gamma butyrolactone and one or more glycol ethers, and mixtures of gamma butyrolactone, one or more glycol ethers and propylene carbonate are particularly preferred.

In another embodiment ofthe invention, dibasic esters and/or bio-solvents may be used.

Dibasic esters are known solvents in the art. They can be described as di(C1-C4 alkyl) esters of a saturated aliphatic dicarboxylic acid having 3 to 8 carbon atoms having following general formula:

in which A represents (CH2)i-6, and R1 and R2 may be the same or different and represent (N-C4 alkyl which may be a linear or branched alkyl radical having 1 to 4 carbon atoms, preferably methyl or ethyl, and most preferably methyl. Mixtures of dibasic esters can be used.

Bio-solvents, or solvent replacements from biological sources, have the potential to reduce dramatically the amount of environmentally-polluting VOCs released in to the atmosphere and have the further advantage that they are sustainable. Moreover, new methods of production of bio-solvents derived from biological feedstocks are being discovered, which allow bio-solvent production at lower cost and higher purity.

Examples of bio-solvents include soy methyl ester, lactate esters, polyhydroxyalkanoates, terpenes and non-linear alcohols, and D-limonene. Soy methyl ester is prepared from soy. The fatty acid ester is produced by esterification of soy oil with methanol. Lactate esters preferably use fermentation-derived lactic acid which is reacted with methanol and/or ethanol to produce the ester. An example is ethyl lactate which is derived from corn (a renewable source) and is approved by the FDA for use as a food additive. Polyhydroxyalkanoates are linear polyesters which are derived from fermentation of

sugars or lipids. Terpenes and non-linear alcohols may be derived from corn cobs/rice hulls. An example is D-limonene which may be extracted from citrus rinds.

Other solvents may be included in the organic solvent component.

The solvent should not be selected such that they will react with the crosslinker. For example, in the case where an isocyanate crosslinker is used, hydroxyl solvents that will react with this should be avoided, e.g. water, alcohols and glycol mono ethers. The radiation-curable diluent is not so limited, although when a radiation-curable diluent is present, a photoinitiator is required. The monofunctional monomers described with reference to the primer layer may be used. Alternatively, or in addition, other radiation-curable materials may be used, including multifunctional monomers.

Suitable multifunctional (meth)acrylate monomers include di-, tri- and tetra-functional monomers. Examples of the multifunctional acrylate monomers that may be included in this layer include hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethylene glycol diacrylate (for example tetraethylene glycol diacrylate), dipropylene glycol diacrylate, tri(propylene glycol) triacrylate, neopentyl glycol diacrylate, bis(pentaerythritol) hexaacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, ethoxylated trimethylolpropane triacrylate, and mixtures thereof.

Suitable multifunctional (meth)acrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, trimethylolpropane trimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1,4-butanediol dimethacrylate. Mixtures of (meth)acrylates may also be used.

Suitable photoinitiators are the same as those described with reference to the ink.

The protective layer further comprises a crosslinking agent. The crosslinking agent should be capable of reacting with the resin system used in the protective top coat. Depending on the resin selected suitable crosslinkers include amines, acids, aziridines, carbodimides and isocyanates. Preferably, the crosslinking agent is isocyanate as these have the widest range of reactivity.

The protective layer may comprise formulations used for the primer, together with a crosslinking agent. The crosslinker may optionally also crosslink with the primer to provide additional toughness to the film.

The protective layer is applied by any means known in the art but is preferably applied by traditional printing process such screen printing, flexographic printing or roller coating.

In the event that the step of curing the hybrid water/radiation-curable ink layer occurred before the deposition of the protective layer, the protective layer is then thermally dried and/or cured. In the event that the step of curing the hybrid water/radiation-curable ink layer did not occur before the deposition ofthe protective layer, the protective layer is then thermally dried and radiation cured. The hybrid water/radiation-curable ink layer is cured through the protective layer. The crosslinkable PUD ofthe primer remains uncured after the at least partial drying step. In this case, after deposition and thermal drying of the hybrid water/radiation-curable ink jet ink, and optional deposition of the protective layer, both the primer layer and the hybrid water/radiation-curable ink layer are cured concurrently through the protective layer.

The present invention may provide a printed substrate obtainable by the method of the present invention. Preferably, the substrate is sportswear or sports equipment.

The invention will now be described with reference to the following examples, which are not intended to be limiting.

Examples

Example 1 (PUD test composition')

To determine the solubility ofthe PUD for use in the method ofthe present invention, the following method was used.

First, the PUD under test is blended with an aqueous pigment dispersion to facilitate observation of film redispersion and removal. A surfactant is added to reduce surface tension and to allow wetting onto a suitable test substrate. The PUD test composition therefore comprises the components as set out in Table 1.

Table 1

The components of Table 1 are accurately weighed into a mixing vessel and stirred with a fiat bladed impeller stirrer at 800 rpm for 20 minutes to ensure the composition is fully homogeneous. After mixing, the composition is allowed to stand for 24 hours to deaerate.

The PUD test composition is then coated onto a 220 micron gloss PVC (Genotherm, as supplied by Klockner (RTM) Pentaplast) using a number 2 K bar. A wet film is deposited of approximately 12 microns.

The ink film is then dried by placing in an oven set at 60°C for three minutes.

When the ink film has cooled to room temperature, the redispersibility ofthe thermally dried ink film is assessed. In this respect, the corner of a sheet of E Tork paper towel (supplied by Tork UK) is wetted with 1 ml of deionised water and placed over the tip of the index finger. The wetted corner of the paper towel is brought into contact with the thermally dried ink film at the left hand side of the printed film and drawn across the printed film in single stroke with a light pressure. The stroke is continued until the wetted paper towel has completely traversed the printed film

The number of rubs required to cleanly remove the ink film from the substrate surface leaving no residual staining visible to the naked eye, and transferring no particulate matter to the wiping paper or to the substrate at the end of the wiping area, is shown in Table 2.

Table 2

Laromer (RTM) UA9122 is an acrylate functional polyurethane dispersion available from BASF(RTM). Bayhydrol (RTM) 2282 and Bayhydrol (RTM) 2689/1 are polyurethane dispersions available from Bayer (RTM) Chemicals.

For a PUD for use in the method of the present invention, excellent redispersibility in water after thermal drying but before curing is required. As can be seen from Table 2, the PUD test composition containing a PUD of the invention, Laromer (RTM) UA9122, requires only a single rub to cleanly remove the ink film from the substrate whereas the PUD test composition containing comparative PUDs, Bayhydrol (RTM) 2282 and Bayhydrol (RTM) 2689/1, require eight rubs, indicating a lower level of resolubility.

Example 2 (primers)

Primers, as detailed in Table 3, were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the primer. Primers 2 and 3 were prepared with a target viscosity of 800 mPa-s at 25°C.

Table 3

The measured viscosity of primer 1 was 762 mPa-s at 25°C.

The primer layers were drawn down onto a synthetic leather polyurethane substrate (Insqin (RTM) supplied by Bayer (RTM)) using a K2 bar depositing 12 microns wet film weight. The primer layers were then thermally dried at 60°C for three minutes.

Example 3 (ink)

After thermal drying, the primer layers were over coated with a hybrid water/radiation-curable ink using a K2 bar depositing 12 microns wet film weight. Hybrid water/radiation-curable ink formulations shown in Table 4 were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 4

The ink had a viscosity of 18.9 mPa-s and a surface tension of 38.75 mNm'1.

The films were thermally dried at 60°C for three minutes before being UV cured using a conveyorised UV cure unit fitted with 1x120 w/cm medium pressure mercury lamp and a belt speed of 25 m/min.

Example 4 (optical density and visual appearance)

The relative wetting of the ink composition over primer 1 (invention) and primers 2 and 3 (comparative) was assessed in the following manner.

The optical density of the printed ink films was measured using a Vipdens C5 colour densitometer from Viptronic. The better the wetting ofthe printed ink, the less the substrate can be seen through the printed ink and hence the higher the optical density. The ink printed ink films were also visually assessed. The optical densities and the visual appearances ofthe printed ink films are given in Table 5.

Table 5

As can be seen from the results in Table 5, all ofthe ink films that had been printed onto a primer had a higher optical density and showed less reticulation than an uncoated substrate. However, the ink film that had been printed onto a primer of the invention showed the highest optical density and the least reticulation.

The results are evident in Fig. 2. The left-hand section ofthe photograph shows the ink drawdown over primer 1, the middle section of the photograph shows the ink drawdown over primer 2 and the right-hand section of the photograph shows the ink drawdown over primer 3. The highest quality image corresponds to the area ofthe substrate that has been printed with primer 1 (left-hand section of Fig. 2). This clearly shows the importance ofthe redispersibility ofthe PUD ofthe primer and the compatibility of this primer with the hybrid water/radiation-curable ink. Preferably, the PUD of the primer, after thermal drying and before curing, can be redispersed in a single rub ofthe water rub test.

Example 6 (optional protective layer) A protective layer, as detailed in Table 6, can optionally be applied to the hybrid water/radiation-curable ink layers of Example 5. The protective layer was prepared by mixing the components of

Table 6 in the given amounts. Amounts are provided as weight percentages based on the total weight of the ink.

Table 6

Table 7

If it is desirable to apply the protective layer, there are two processes that can be used - process A or process B. The protective layer is optional depending on the level of resistance required.

Process A

The protective layer is applied over the thermally dried hybrid water/radiation-curable ink layer (printed image) without UV curing the hybrid water/radiation-curable ink layer. The protective layer is applied to the hybrid solvent/radiation-curable ink layer by screen printing through a 90.48 PW screen.

The protective layer is then thermally dried for three minutes at 60°C before being exposed to UV radiation. The UV radiation readily passes through the protective layer to cure the ink layer beneath. The UV cure process is highly efficient as the top coat excludes air from the UV components thus avoiding inhibition by atmospheric oxygen.

Process B

The thermally dried hybrid water/radiation-curable ink layer is UV cured. The protective layer is then applied over the image by screen printing through a 90.48 PW screen and dried at 70°C for 3 minutes to produce the composite.

Example 7 (treatment of primer)

The resolubility of primer 1 was investigated after thermal drying but before curing, and after thermal drying and curing. The results are shown in Fig. 3 and 4.

Primer 1 was deposited onto a first and second synthetic leather substrate by drawing down the composition onto synthetic leather using a K2 bar applicator, depositing approximately 12 microns wet film weight. The first and second printed substrates were then dried in an oven at 60°C for three minutes. The second printed substrate was then UV cured on a conveyorised UV dryer fitted with 1x120 w/cm medium pressure mercury lamp and a belt speed of 25 m/min.

The ink of Example 3 was applied to the dried first printed substrate and the dried and cured printed second substrate using a K2 bar applicator, depositing approximately 12 microns wet film weight. The ink films of the first and second substrates were then dried in an oven at 60°C for three minutes and cured on a conveyorised UV dryer fitted with 1x120 w/cm medium pressure mercury lamp and a belt speed of 25 m/min.

Fig. 3 shows the printed ink film of the first substrate, in contrast with the uncoated substrate. Fig. 4 shows the printed ink film of the second substrate, in contrast with the uncoated substrate. Primer 1 is no longer resoluble after curing and therefore, as can be seen from Fig. 4, the printed ink film is of a poorer quality than the printed ink film of Fig. 3.

Comparing the visual optical density and smoothness of the ink films on the primed areas of the substrates (the right-hand section of the photographs) for Fig. 3 and 4, it can be seen that UV curing the primer layer before ink application has reduced the pinning efficiency of the primer. The density and smoothness of the print in Fig. 3 is superior than the equivalent area in Fig. 4.

Claims

1. A method of printing onto a leather or synthetic leather substrate having a primer layer obtainable by providing the substrate, depositing a primer onto the substrate to form a primer layer, wherein the primer comprises an aqueous polyurethane dispersion, which is redispersible in water after thermal drying and before curing, a water-dispersible or water-soluble photoinitiator, a surfactant, and at least partially drying the primer; the method comprising each of the following steps in order: (i) inkjet printing a hybrid water/radiation-curable ink onto the primer layer to form a hybrid water/radiation-curable ink layer; (ii) drying the hybrid water/radiation-curable ink layer; and (iii) (a) optionally depositing a protective layer onto the hybrid water/radiation-curable ink layer, wherein the protective layer comprises: a thermoplastic resin selected from polyurethane, polymethacrylate, polyvinyl, poly(cellulose acetate butyrate), polyester, epoxy resin, hydrocarbon resin, ketone or aldehyde resins, amino resins, and copolymers and blends thereof, a solvent and/or radiation-curable diluent, and, when a monofunctional radiation-curable diluent is present, a photoinitiator, and a crosslinking agent; and (b) curing the hybrid water/radiation-curable ink layer; wherein when (iii)(a) is present, (iii)(a) and (iii)(b) can be in either order, with the proviso that when step (iii)(b) occurs before step (iii)(a), the method further comprises the step of drying and/or curing the protective layer.

2. A method of printing onto a leather or synthetic leather substrate comprising each of the following steps in order: (i) providing the substrate; (ii) depositing a primer onto the substrate to form a primer layer, wherein the primer comprises: an aqueous polyurethane dispersion, which is redispersible in water after thermal drying and before curing, a water-dispersible or water-soluble photoinitiator and a surfactant; (iii) at least partially drying the primer; (iv) inkjet printing a hybrid water/radiation-curable ink onto the primer layer to form a hybrid water/radiation-curable ink layer; (v) drying the hybrid water/radiation-curable ink layer; and (vi) (a) optionally depositing a protective layer onto the hybrid water/radiation-curable ink layer, wherein the protective layer comprises: a thermoplastic resin selected from polyurethane, polymethacrylate, polyvinyl, poly(cellulose acetate butyrate), polyester, epoxy resin, hydrocarbon resin, ketone or aldehyde resins, amino resins, and copolymers and blends thereof, a solvent and/or radiation-curable diluent, and, when a monofunctional radiation-curable diluent is present, a photoinitiator, and a crosslinking agent; and (b) curing the hybrid water/radiation-curable ink layer; wherein when (vi)(a) is present, (vi)(a) and (vi)(b) can be in either order, with the proviso that when step (vi)(b) occurs before step (vi)(a), the method further comprises step of drying and/or curing the protective layer.

3. A method according to claims 1 or 2, wherein the primer comprises a radiation-curable (meth)acrylate polyurethane dispersion.

4. A method according to any preceding claim, wherein the primer layer has a viscosity of 500 to 1000 mPa sat 25°C.

5. A method according to any preceding claim, wherein the polyurethane dispersion of the primer layer, after thermal drying and before curing, can be redispersed in water in three rubs or fewer ofthe water rub test.

6. A method according to any preceding claim, wherein the primer layer has a thickness of 40 microns or less based on the dry film thickness.

7. A method according to any preceding claim, wherein the hybrid water/radiation-curable ink comprises a radiation-curable material, a photoinitiator, a colorant and water.

8. A method according to any preceding claim, wherein the protective layer comprises polyurethane.

9. A method according to any preceding claim, wherein the primer layer is deposited onto the substrate by screen printing, flexographic printing, roller coating, bar coating or wipe on by hand.

10. A method according to any preceding claim, wherein the substrate is sportswear or sports equipment.