GB2545085A

GB2545085A - A method of inkjet printing

Info

Publication number: GB2545085A
Application number: GB1618919.3A
Authority: GB
Inventors: Caras-Quintero Dolores; Gould Nigel
Original assignee: Fujifilm Speciality Ink Systems Ltd
Current assignee: Fujifilm Speciality Ink Systems Ltd
Priority date: 2015-11-09
Filing date: 2016-11-09
Publication date: 2017-06-07

Abstract

A method of preparing a vinyl tile, plank or sheet, comprising the following steps, in order (i) providing an opaque vinyl substrate; (ii) optionally pre-heating the substrate to a temperature of at least 40 °C; (iii) heating the substrate at a temperature of at least 40 °C whilst jetting an inkjet ink on to the surface of the substrate to form an image, wherein the ink comprises a liquid medium composed of a radiation-curable diluent containing a difunctional monomer having one (meth)acrylate group and one vinyl ether group, a substrate-solvating component other than tetrahydrofurfuryl acrylate, and optionally a volatile organic solvent; a vinyl resin dissolved in the liquid medium; a photoinitiator; and optionally a pigment, wherein the ink contains 5% or less of tetrahydrofurfuryl acrylate; (iv) curing the ink and, when a solvent is present, drying the ink; (v) applying a clear vinyl layer over the image; and (vi) applying heat and/or pressure to the substrate to form the vinyl tile, plank or sheet. Preferably, the ink is substantially free from tetrahydrofurfuryl acrylate. The difunctional monomer is preferably 2-(2-vinyloxyethoxy)ethyl acrylate (VEEA). Also claimed is the inkjet ink. The vinyl tiles, planks or sheets are typically used for flooring applications.

Description

A method of inkjet printing

This invention relates to a method of inkjet printing and in particular to a method of inkjet printing that is suitable for preparing a vinyl tile, plank or sheet, and a printing ink which may be used in the method.

Vinyl tiles, planks and sheets are popular materials for flooring, and other surface coverings. They are prepared by printing an image onto an opaque (usually white) vinyl substrate and then applying a clear vinyl layer over the image. The resulting laminate is then bonded by applying heat and/or pressure to the substrate.

Particular demands are placed on the ink in this process. The ink must bind to the vinyl layers and be robust enough to withstand the application of heat and pressure. Industry regulations require the vinyl tile/plank/sheet to have a peel strength of 10 N/cm and so the ink must retain adhesion and cohesion for the lifetime of the tile/plank/sheet.

The inks typically contain acrylate polymers suspended in water or an organic solvent. The inks are applied using gravure printing. Gravure printing involves engraving an image onto a cylindrical image carrier. The substrate is passed between the cylindrical image carrier and an impression roller. During the process, the cylindrical image carrier is continually wetted with the ink and the image is thereby transferred onto the substrate. An advantage of this technique is that few constraints are placed on the ink formulator. A disadvantage is that the image is limited to a repeating pattern corresponding to the circumference of the drum.

There is a desire in the art to have more control over the image formation and to increase the productivity of the process using more versatile techniques which are susceptible to digital printing, such as inkjet printing. Inks that are suitable for vinyl tile/plank/sheeting applications need to form strong bonds between the layers of the vinyl tile/plank/sheet and have the required lamination bond strength. The lamination bond strength is the bond strength between the layers of laminated material in the vinyl tile/plank/sheet. Further, to meet the production speed requirements for inkjet printing, the method of inkjet printing needs to be “single pass” where the inkjet printhead moves relative to the substrate and the whole image is formed in a single pass. Where only part of the image is formed and further passes of the printhead are required, the technique is termed “multi-pass” printing. Single-pass printing places greater demands on the ink than multi-pass printing in terms of jetting reliability and sustainability of the inks as blocked or deviated nozzles in the printhead are more obvious in the printed image formed using single-pass printing than multi-pass printing.

Inks with the required jetting reliability and lamination bond strength are known in the art. However, to achieve the required lamination bond strength, components with a very high solvency power for the vinyl substrates have to be used in the inks, e.g. tetrahydrofurfuryl acrylate (THFA). Ink formulations containing such components are corrosive and carry a H314 hazard statement (causes severe burns and eye damage).

There remains a need in the art for an ink and printing process with the required jetting reliability for inkjet printing and lamination bond strength for vinyl tile/plank/sheeting applications without recourse to hazardous components.

Accordingly, the present invention provides a method of preparing a vinyl tile, plank or sheet, comprising the following steps, in order: (i) providing an opaque vinyl substrate; (ii) optionally preheating the substrate to a temperature of at least 40°C; (iii) heating the substrate at a temperature of at least 40°C whilst jetting an inkjet ink on to the surface of the substrate to form an image, wherein the ink comprises: (a) a liquid medium composed of a radiation-curable diluent containing a difunctional monomer having one (meth)acrylate group and one vinyl ether group, a substrate-solvating component other than tetrahydrofurfuryl acrylate, and optionally a volatile organic solvent; (b) a vinyl resin dissolved in the liquid medium; (c) a photoinitiator; and (d) optionally a pigment; wherein the ink contains 5% or less of tetrahydrofurfuryl acrylate; (iv) curing the ink and, when a solvent is present, drying the ink; (v) applying a clear vinyl layer over the image; and (vi) applying heat and/or pressure to the substrate to form the vinyl tile, plank or sheet.

The present invention further provides an ink which may be used in the method of the present invention comprising: (a) a liquid medium composed of a radiation-curable diluent containing a difunctional monomer having one (meth)acrylate group and one vinyl ether group, a substrate-solvating component other than tetrahydrofurfuryl acrylate, and optionally a volatile organic solvent; (b) a vinyl resin dissolved in the liquid medium; (c) a photoinitiator; and (d) optionally a pigment; wherein the ink contains 5% or less of tetrahydrofurfuryl acrylate.

Thus, the present invention provides a method of inkjet printing in which inks with the required jetting reliability and lamination bond strength for vinyl tile/plank/sheeting applications can be used without recourse to hazardous components and an ink which may be used in such a method.

The present invention provides a method of preparing vinyl tiles, planks or sheets. Vinyl tiles, planks or sheets are typically used for flooring applications, but they can also be used for covering other surfaces, such as walls. The tiles or planks are for the high-end or luxury markets. The vinyl tiles, planks and sheets are typically composed of a plasticised white PVC layer which is decorated with the printed image, often the images are wood patterns or stone effects. The printed layer is protected from wear by a thicker clear PVC layer. This can be gloss or matt in appearance and have patterns embossed in the surface to give a more natural appearance. The current gravure print process means that regular repeats occur in the pattern dependent on the diameter of the gravure roller, which can lead to an unnatural appearance with poor aesthetics. Digital printing can give a fully random pattern giving a more pleasing effect.

The method of the present invention includes step (i), providing an opaque vinyl substrate. Such substrates are known and widely used in the art. They are composed of PVC and include a pigment, usually titanium dioxide, to make the substrate opaque. The substrate is usually white.

The method of the present invention includes step (ii), optionally pre-heating the substrate to a temperature of at least 40°C. In one embodiment, step (ii) is present.

The method of the present invention includes step (iii), heating the substrate at a temperature of at least 40°C whilst jetting an inkjet ink on to the surface of the substrate to form an image, wherein the ink comprises: (a) a liquid medium composed of a radiation-curable diluent containing a difunctional monomer having one (meth)acrylate group and one vinyl ether group, a substrate-solvating component other than tetrahydrofurfuryl acrylate, and optionally a volatile organic solvent; (b) a vinyl resin dissolved in the liquid medium; (c) a photoinitiator; and (d) optionally a pigment; wherein the ink contains 5% or less of tetrahydrofurfuryl acrylate.

The printer is fitted with independently controlled heaters. The heaters are positioned so that the substrate can be subjected to different temperatures as it passes through the printer. The substrate is heated during printing. In order to heat the substrate during printing (i.e. during ink deposition), the substrate is heated using a platen heater. When the substrate is pre-heated before printing (i.e. before ink deposition), the substrate is heated using a pre-heater. The substrate is optionally heated after printing. When the substrate is heated after printing (i.e. after ink deposition), the substrate is heated with a post-heater. In one embodiment, the substrate is heated before and during printing at a temperature of at least 40°C. In one embodiment, the substrate is heated during and after printing at a temperature of least 40°C. In another embodiment, the substrate is heated before, during and after printing at a temperature of least 40°C.

In a preferred embodiment, the substrate is heated during printing at a temperature of 40°C to 100°C, more preferably 40°C to 80°C. In a further preferred embodiment, the substrate is heated before and during printing at a temperature of 40°C to 100°C, more preferably 40°C to 80°C. In a further preferred embodiment, the substrate is heated during and after printing at a temperature of 40°C to 100°C, more preferably 40°C to 80°C. In a further preferred embodiment, the substrate is heated before, during and after printing at a temperature of 40°C to 100°C, more preferably 40°C to 80°C.

The substrate is therefore heated during jetting. Heating ensures that good lamination bond strength is achieved in the resultant vinyl tiles/planks/sheets. The heating step is crucial to the method in which inks containing substrate-solvating components other than THFA are used. Compared with THFA, inks containing substrate-solvating components other than THFA typically produce vinyl tiles/planks/sheets with inadequate lamination bond strength but by heating the substrate during printing and optionally before printing, adequate lamination bond strength is achieved. The method of the present invention therefore allows the use of less hazardous substrate-solvating components which are free from the H314 hazard statement, i.e. substrate-solvating components other than THFA, in inks that have the required jetting reliability for inkjet printing and the required lamination bond strength for vinyl tile/plank/sheet applications.

An inkjet ink is jetted on to the surface of the substrate to form an image. In inkjet printing, minute droplets of black, white or coloured ink are ejected in a controlled manner from one or more reservoirs or printing heads through narrow nozzles on to a substrate which is moving relative to the reservoirs. The ejected ink forms an image on the substrate. For high-speed printing, the inks must flow rapidly from the printing heads, and, to ensure that this happens, they must have, in use, a low viscosity, typically below 100 mPas at 25°C (although in most applications the viscosity should be below 50 mPas, and often below 25 mPas). Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5-15 mPas and ideally 10.5 mPas at the jetting temperature, which is often elevated to about 40°C (the ink might have a much higher viscosity at ambient temperature). The inks must also be resistant to drying or crusting in the reservoirs or nozzles.

The ink of the present invention, which is jetted in step (iii) of the method of the present invention, comprises: (a) a liquid medium composed of a radiation-curable diluent containing a difunctional monomer having one (meth)acrylate group and one vinyl ether group, a substrate-solvating component other than tetrahydrofurfuryl acrylate, and optionally a volatile organic solvent; (b) a vinyl resin dissolved in the liquid medium; (c) a photoinitiator; and (d) optionally a pigment; wherein the ink contains 5% or less of tetrahydrofurfuryl acrylate.

Such an ink may be prepared by known methods such as, for example, stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill. Further, the ink exhibits a desirable low viscosity, less than 100 mPas, preferably 50 mPas or less and most preferably 30 mPas or less at 25°C. The ink most preferably has a viscosity of 20 to 30 mPas at 25°C. Viscosity may be measured using a digital Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as model LDV1+.

The ink of the present invention, which is jetted in step (iii) of the method of the present invention, is either a solely radiation-curable inkjet ink (often termed a “UV ink”) or a radiation-curable/solvent-containing inkjet ink (often termed a “hybrid ink”). Thus, a radiation-curable inkjet ink is typically free of volatile organic solvent and a hybrid inkjet ink contains volatile organic solvent and a radiation-curable diluent. This defines the liquid medium of the ink.

When present, the solvent (i.e. a volatile organic solvent) is in the form of a liquid at ambient temperature and is capable of acting as a carrier for the remaining components of the ink. The volatile organic solvent component of the ink may be a single solvent or a mixture of two or more solvents. As with known solvent-based inkjet inks, the volatile organic solvent used in the ink of the present invention is required to evaporate from the printed ink, typically on heating, in order to allow the ink to dry. The volatile organic solvent has a boiling point of up to 200°C such that it is not retained in the dried and cured ink film and as such does not contribute to the adhesion process between the printed ink film and the base substrate. The volatile organic solvent can be selected from any solvent commonly used in the printing industry, such as glycol ethers, glycol ether esters, alcohols, ketones, esters, organic carbonates, lactones and pyrrolidones. The organic solvent, when used, may be present in an amount of 5 to 60% by weight, more preferably 10 to 50% by weight and most preferably 20 to 40% by weight, based on the total weight of the ink. The upper limit is typically 85% or 75% by weight based on the total weight of the ink.

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 alcohols, glycol ethers, glycol ether acetates, lactones and mixtures thereof.

Other solvents may be included in the volatile organic solvent component. A particularly common source of other solvents is derived from the way in which the colouring agent is introduced into the inkjet ink formulation. The colouring agent may be prepared in the form of a pigment dispersion in a solvent, e.g. 2-ethylhexyl acetate. The solvent tends to be around 40 to 50% by weight of the pigment dispersion based on the total weight of the pigment dispersion and the pigment dispersion typically makes up around 5 to 15% by weight of the ink and sometimes more.

The ink is preferably substantially free of water, although some water will typically be absorbed by the ink from the air or be present as an impurity in the components of the inks, and such levels are tolerated. For example, the ink may comprise less than 5% by weight of water, more preferably less than 2% by weight of water and most preferably less than 1% by weight of water, based on the total weight of the ink.

The ink of the present invention, which is jetted in step (iii) of the method of the present invention, contains a radiation-curable diluent. 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 diluent comprises a difunctional monomer having one (meth)acrylate group and one vinyl ether group. The (meth)acrylate group cures fully on exposure to actinic radiation. However, the vinyl ether group is not very responsive to free radical cure, although some reaction will occur. This monomer provides good solvency for the vinyl resin, combined with resistance to heat and pressure during lamination, and a low Tg in the cured film. The cured film has a sufficiently low Tg such that under heat and/or pressure, the ink film softens and “wets out” over the underside of the clear vinyl layer. “Wetting out” means the adhesive flows and covers a surface to maximize the contact area and the attractive forces between the adhesive and bonding surface. This process creates a strong bond between the ink film and the clear vinyl layer. In a preferred embodiment, the cured ink film has a glass transition temperature (Tg) of 20-100°C, more preferably 40-80°C. The Tg may be measured by DSC with a heating ramp of 10°C/min.

The difunctional monomer having one (meth)acrylate group and one vinyl ether group has a molecular weight of preferably less than 400. The difunctional monomer is preferably a vinyloxyalkyl (meth)acrylate, wherein the alkyl group is typically C^-alkyl, optionally interrupted by 1-5 ethereal oxygen atoms. The alkyl group may be linear or branched, but is preferably linear. In a particularly preferred embodiment, the monomer is 2-(2-vinyloxy ethoxy)ethyl acrylate (“VEEA”) or 2-(2-vinyloxy ethoxy)ethyl methacrylate (“VEEM”), and most preferably VEEA. VEEA has the formula:

The ink typically contains from 20 to 80% by weight of difunctional (meth)acrylate/vinyl ether monomer, more preferably 50 to 80% by weight, based on the total weight of the ink. This range is particularly suitable for solvent-free inks.

The radiation-curable diluent also contains a substrate-solvating component other than tetrahydrofurfuryl acrylate.

Components with a very high solvency power for the substrate are typically required in order to ensure an adequate lamination bond strength for vinyl tile/plank/sheeting applications. An example of a widely used component in the industry is THFA. Although THFA in combination with the vinyl resin yields inks with excellent lamination properties, THFA is corrosive in its uncured form and carries a H314 hazard statement (causes severe burns and eye damage). The present invention therefore uses less hazardous components which are free from the H314 hazard statement, whilst still providing an ink which is suitable for vinyl tile/plank/sheet applications and inkjet printing. In this regard, the ink contains a substrate-solvating component other than THFA. Although the substrate-solvating component other than THFA has a lower solvency power for the substrate than THFA, the substrate-solvating component other than THFA is able to sufficiently solvate the substrate when the ink is applied to a substrate that is heated during printing and optionally before printing. This ensures that the printed ink film has excellent adhesion to the substrate. The substrate-solvating component therefore solvates the substrate worse than tetrahydrofurfuryl acrylate at room temperature (25°C) but

sufficiently solvates the substrate at 40°C. Further, the ink contains 5% or less of THFA, preferably 3% or less of THFA and most preferably, the ink is free from THFA.

Substrate solvency power may be determined by placing a small drop of the fluid under test on the substrate. The fluid is left in contact with the surface for 60 seconds. The fluid is then gently wiped from the surface with an adsorbent cloth. The surface is then examined for signs of swelling, softening and changes to the appearance of the surface. The presence of any or all of these features confirms solvency of the fluid under test for the substrate.

The substrate-solvating component other than THFA may or may not be radiation-curable.

In one embodiment, the substrate-solvating component other than THFA is a radiation-curable monofunctional monomer. By “monofunctional” is meant that the monomers have only one functional group which takes part in a polymerisation reaction during curing. Preferred monofunctional monomers include an N-vinyl amide and/or an N-acryloyl amine. In a preferred embodiment, the ink contains N-vinyl caprolactam, N-acryloylmorpholine or mixtures thereof.

In an alternative embodiment, the substrate-solvating component other than THFA is non-radiation-curable. In this embodiment, the non-radiation-curable substrate-solvating component is non-volatile in the sense that it remains in the printed film. Preferably, the non-radiation-curable substrate-solvating component has a boiling point above 200°C. A preferred non-radiation-curable substrate-solvating component is y-butyrolactone.

The ink typically contains 1 to 20% by weight of the substrate-solvating component other than THFA, more preferably 5 to 15% by weight, based on the total weight of the ink. This range is particularly suitable for solvent-free inks.

In a preferred embodiment, the ratio of the substrate-solvating component other than THFA to the difunctional monomer is preferably 1:2 to 1:10, more preferably 1:4 to 1:8 by weight, based on the total weight of the ink.

The ink of the present invention, which is jetted in step (iii) of the method of the present invention, contains a vinyl resin. The vinyl resin ensures that the ink film has good cohesion. The resin is typically present at 0.5 to 7.0% by weight, preferably 1.0-6.0% by weight, based on the total weight of the inkjet ink.

The resin preferably has a weight-average molecular weight (Mw) of 20-200 KDa, and most preferably 30-100 KDa. The Mw may be measured by known techniques in the art, such as gel permeation chromatography (GPC), using a polystyrene standard. The resin is preferably a solid at 25°C. It is preferably soluble in the liquid medium (or “phase”) of the ink (the radiation-curable diluent and, when present, additionally the solvent).

The resin is a passive (i.e. inert) resin, in the sense that it is not radiation curable and hence does not undergo cross-linking under the curing conditions to which the ink is subjected.

The resin is preferably a poly(vinyl chloride) copolymer, more preferably a poly(vinyl chloride/vinyl acetate) copolymer. The resin may also contain hydroxy or carboxyl functionality. These resins are termed “functionalised resins”. However, although they contain functional groups, principally to assist adhesion to the substrate, they do not take part in the ink curing reaction and hence are still passive resins within the meaning of the present invention.

The resin preferably contains 60-90% by weight of vinyl chloride, based on the total composition of the resin. The vinyl acetate content is preferably 0-40% by weight and more preferably 10-30% by weight, based on the total composition of the resin.

For the functionalised resins, the vinyl alcohol content is preferably 0-30% by weight and more preferably 5-20% by weight, based on the total composition of the resin. The unsaturated dicarboxylic acid ester content is preferably 0-2% by weight and more preferably 0.1-1.5% by weight, based on the total composition of the resin.

Preferred functionalised resins include a poly(vinyl chloride/vinyl acetate/unsaturated dicarboxylic acid ester) terpolymer, a poly(vinyl chloride/vinyl acetate/vinyl alcohol) terpolymer and a poly(vinyl chloride/hydroxy acrylate) copolymer. Such resins are commercially available as Vinnol® from Wacker Chemie AG.

The ink of the present invention, which is jetted in step (iii) of the method of the present invention, is formulated so that the vinyl resin is dissolved in the liquid medium of the ink medium. The difunctional (meth)acrylate/vinyl ether monomer is capable of dissolving the vinyl resin. A suitable test for measuring the solubility of the vinyl resin in a monomer is as follows. The monomer, e.g. 500 g, is weighed into a suitable container. The monomer is stirred using a Silverson disperser at 5,000 rpm for 15 minutes to achieve a temperature of 40°C. The resin is slowly added into to the vortex. The stirrer speed is reduced to 3,000-3,500 rpm such that the temperature of the blend is maintained at 35-40°C. The stirring is maintained for 1 hour. After this period the mixture is checked for residual undissolved resin, if none is present solution is removed from the stirrer, the container sealed with a lid and is allowed to stand for 12 hours at temperature. The resin/monomer combination is suitable for use in the invention if, after the standing period, there is no precipitation of the resin. The test is also applicable to monomer combinations, and hybrid inks.

The ink of the present invention, which is jetted in step (iii) of the method of the present invention, may also contain monofunctional monomers with low solvating power for the vinyl resin, such as phenoxy ethyl acrylate, ethoxylated phenoxy ethyl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate, tertiary-butyl cyclohexyl acrylate, 3,3,5-trimethyl cyclohexyl acrylate, isophoryl acrylate, isodecyl acrylate, octyl/decyl acrylate, tridecyl acrylate and mixtures thereof.

Multifunctional monomers, other than the difunctional (meth)acrylate/vinyl ether monomer which is present in the ink, are preferably present in an amount of no more than 10% by weight, more preferably no more than 5% by weight and most preferably no more than 2% by weight based on the total weight of the ink. The other multifunctional monomers which are limited in amount may be any multifunctional monomer other than the difunctional (meth)acrylate/vinyl ether monomer which could be involved in the curing reaction, such as a multifunctional (meth)acrylate monomer.

Examples of the other multifunctional acrylate monomers whose level is minimised 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) hexa-acrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, ethoxylated trimethylolpropane triacrylate, and mixtures thereof. In addition, multifunctional acrylate monomers include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, trimethylolpropane triacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1,4-butanediol dimethacrylate, and mixtures thereof. (Meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate. Mono and multifunctional are also intended to have their standard meanings, i.e. one and two or more groups, respectively, which take part in the polymerisation reaction on curing.

The amount of radiation-curable oligomers are also to be minimised. They are preferably present in an amount of no more than 10% by weight, more preferably no more than 5% by weight and most preferably no more than 2% by weight based on the total weight of the ink. Such materials typically have a molecular weight above 3,000 and 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 polymerisable group can be any group that is capable of polymerising upon exposure to radiation.

The ink of the present invention, which is jetted in step (iii) of the method of the present invention, also includes a photoinitiator which under irradiation, for example using ultraviolet light, initiates the polymerisation of the radiation-curable diluent. Preferred are photoinitiators which produce free radicals on irradiation (free radical photoinitiators) such as, for example, benzophenone, 1- hydroxycyclohexyl phenyl ketone, 2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1-one, benzil dimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide or mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Irgacure, Darocur (from Ciba) and Lucerin (from BASF). The ink of the present invention is preferably cured by ultraviolet irradiation. In a preferred embodiment the radiation-curable material polymerises by free-radical polymerisation.

Preferably the photoinitiator is present from 1 to 20% by weight, preferably from 2 to 10% by weight, of the ink.

The ink of the present invention, which is jetted in step (iii) of the method of the present invention, optionally contains a pigment, which may be either dissolved or dispersed in the liquid medium of the ink. Preferably the pigment is a dispersible pigment, of the types known in the art and commercially available such as, for example, under the trade-names Paliotol (available from BASF pic), Cinquasia, Irgalite (both available from Ciba Speciality Chemicals) and Hostaperm (available from Clariant 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. Often, pigments are commercially available as dispersions in monomer or solvent. If the dispersion contains a monomer, it should be taken into account when assessing the Tg of the monomer combination.

In one embodiment, the dispersible pigment is in the form of a solid dispersion in a vinyl resin. Such materials are available from BASF under the trade name of Microlith K.

The total proportion of pigment present is preferably from 0.5 to 15% by weight, more preferably from 1 to 10% by weight.

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 and identifying tracers. The surfactant assists with wetting of the substrate surface by the ink, but it can be detrimental to the bonding process and so is preferably present at no more than 0.5% by weight, based on the total weight of the ink.

The method of the present invention includes step (iv), curing the ink and, when a solvent is present, drying the ink.

The ink is therefore cured by exposure to actinic radiation. 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.

Any means that is suitable for evaporating solvent from known solvent-based inkjet inks may be used for the present ink. Examples include dryers, heaters, air knives and combinations thereof.

After the ink has been cured/dried to a solid film, step (v) of the method of the present invention involves applying a clear vinyl layer over the image. The clear vinyl layer is preferably PVC. Preferably the opaque substrate and clear vinyl layer are both composed of PVC.

The method of the present invention includes step (vi), applying heat and/or pressure to the substrate to form the vinyl tile, plank or sheet, and usually both, to bond the layers together. The temperature is preferably 90-180°C, more preferably 100-150°C. The pressure is preferably 0.5-2.0 MPa, more preferably 0.8-1.2 MPa. Bonding is usually performed for 10-60 s.

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

Examples

Example 1

An ink of the invention having the formulation as shown in Table 1 was prepared.

Table 1. Formulation of ink 1 (of the invention)

UV12 is a stabiliser. Vinnol® H14/36 is an unfunctionalised suspension polymerised vinyl chloride/vinyl acetate copolymer (86:14 by weight) having a Mw of 30-40 KDa. Microlith blue 7080 KJ

is a pigment dispersion containing 50 wt% P.B. 15:3, predispersed in a vinyl chloride/vinyl acetate copolymer. Irgacure 819 and Irgacure 2959 are photoinitiators. BYK 331 is a surfactant.

The ink was prepared by first weighing the monomers into a suitable mixing vessel, placing the vessel under the mixing head of a Silverson stirrer and starting the stirrer. The resin was added and the mixture stirred until all the resin particles had dispersed. The temperature was monitored throughout to ensure that the temperature did not exceed 60°C. The remaining components were added to the mixture and the mixture stirred for a further five minutes.

Example 2

To investigate the heating step, a white 100 pm PVC substrate and ink 1 were loaded into a Mimaki JV400 SUV printer. The printer is fitted with three independently controllable heaters. The heaters are positioned such that as the substrate is fed through the printing zone, it can be subjected to different temperatures. The heaters are positioned as follows:

Pre-heater - allows the substrate to be heated before ink deposition;

Print platen heater - allows the substrate to be heated during ink deposition; and Post-heater - allows the printed substrate to be heated after ink deposition.

The substrate was exposed to a range of different heater settings during printing. The resulting films had 100% coverage using print mode/resolution 1200x900 6 pass bidirectional at approximate print speed 18 m2/h.

The resulting films were cured using a conveyorised UV cure unit fitted with 1x120 W/cm medium pressure mercury lamp at 25 m/min.

The cured films are then coated with a clear 300 pm PVC film and the resulting laminate bonded by compressing at 140°C and 1 MPa (10 bar) for 30 seconds.

The lamination bond strengths of the products were measured by a 180 degree peel test using an Instron 5544 test unit. The results are shown in Table 2.

Table 2

It can be seen that without heating the substrate, the product has a lamination bond strength below the minimum value of 10 N/cm whereas heating the substrate during printing and optionally before and after printing, results in a product with a lamination bond strength above the minimum value of 10 N/cm, as long as the temperature of the heater is at least 40°C. Further, it can be seen that heating the substrate before and after printing is not required in order to achieve a product with the required lamination bond strength.

Example 3

Additional inks were prepared in order to further investigate the heating step. The inks were prepared in the same way as ink 1 in Example 1 above. The inks had formulations as shown in Tables 3-5.

Table 3. Formulation of ink 2 (of the invention)

Table 4. Formulation of ink 3 (of the invention)

Table 5. Formulation of ink 4 (not of the invention)

Example 4

To further investigate the heating step, inks 1-4 were coated on to a white 100 pm PVC substrate using a K2 applicator bar (12 pm wet film). The resulting films were cured using a conveyorised UV cure unit fitted with 1x120 W/cm medium pressure mercury lamp at 25 m/min, i.e. the substrate was not heated before, during or after printing.

The cured films were then coated with a clear 300 pm PVC film and the resulting laminate bonded by compressing at 140°C and 1 MPa (10 bar) for 30 seconds.

The lamination bond strengths of the products prepared using inks 1-4 were measured by a 180 degree peel test using an Instron 5544 test unit. The results are shown in Table 6.

Table 6

The products using inks 1-3 all failed, showing that without the heating step of the invention, the products do not have the required lamination bond strength. This is in contrast to ink 1 as used in Example 2, where heating the substrate to a temperature of at least 40°C during jetting resulted in a product with the required lamination bond strength.

The product using comparative ink 4 passed, showing that a heating step is not necessary to produce a product with the required lamination bond strength when the ink contains THFA. However, THFA is hazardous, making its formulation undesirable.

Claims

1. A method of preparing a vinyl tile, plank or sheet, comprising the following steps, in order: (i) providing an opaque vinyl substrate; (ii) optionally pre-heating the substrate to a temperature of at least 40°C; (iii) heating the substrate at a temperature of at least 40°C whilst jetting an inkjet ink on to the surface of the substrate to form an image, wherein the ink comprises: (a) a liquid medium composed of a radiation-curable diluent containing a difunctional monomer having one (meth)acrylate group and one vinyl ether group, a substrate-solvating component other than tetrahydrofurfuryl acrylate, and optionally a volatile organic solvent; (b) a vinyl resin dissolved in the liquid medium; (c) a photoinitiator; and (d) optionally a pigment; wherein the ink contains 5% or less of tetrahydrofurfuryl acrylate; (iv) curing the ink and, when a solvent is present, drying the ink; (v) applying a clear vinyl layer over the image; and (vi) applying heat and/or pressure to the substrate to form the vinyl tile, plank or sheet.

2. A method as claimed in claim 1, wherein step (ii) is present.

3. A method as claimed in claims 1 or 2, wherein the ink contains 3% or less of tetrahydrofurfuryl acrylate, more preferably wherein the ink is free from tetrahydrofurfuryl acrylate.

4. A method as claimed in any preceding claim, wherein the difunctional monomer having one (meth)acrylate group and one vinyl ether group is a vinyloxyalkyl (meth)acrylate, preferably 2-(2-vinyloxy ethoxy)ethyl acrylate.

5. A method as claimed in any preceding claim, wherein the substrate-solvating component is a radiation-curable monofunctional monomer, preferably selected from an N-vinyl amide and/or an N-acryloyl amine, more preferably selected from N-vinyl caprolactam, N-acryloylmorpholine or mixtures thereof.

6. A method as claimed in claims 1 to 4, wherein the substrate-solvating component is nonradiation-curable and has a boiling point of above 200°C, and is preferably y-butyrolactone.

7. A method as claimed in any preceding claim, wherein the ratio of the substrate-solvating component to the difunctional monomer having one (meth)acrylate group and one vinyl ether group is 1:2 to 1:10 by weight, based on the total weight of the ink.

8. A method as claimed in any preceding claim, wherein the resin is present at 0.5 to 7.0% by weight based on the total weight of the inkjet ink.

9. A method as claimed in any preceding claim, wherein the resin has a weight-average molecular weight of 20-200 KDa.

10. A method as claimed in any preceding claim, wherein the resin is a solid at 25°C.

11. A method as claimed in any preceding claim, wherein the resin is selected from a poly(vinyl chloride/vinyl acetate) copolymer, a poly(vinyl chloride/vinyl acetate/hydroxyl acrylate) copolymer or a poly(vinyl chloride/vinyl acetate/unsaturated dicarboxylic acid ester) copolymer.

12. A method as claimed in any preceding claim, wherein the ink contains 10% by weight or less of other multifunctional monomers, based on the total weight of the ink.

13. A method as claimed in any preceding claim, wherein the ink contains 10% by weight or less of an oligomer, based on the total weight of the ink.

14. A method as claimed in any preceding claim, wherein the opaque substrate and clear vinyl layer are composed of PVC.

15. An inkjet ink comprising: (a) a liquid medium composed of a radiation-curable diluent containing a difunctional monomer having one (meth)acrylate group and one vinyl ether group, a substrate-solvating component other than tetrahydrofurfuryl acrylate, and optionally a volatile organic solvent; (b) a vinyl resin dissolved in the liquid medium; (c) a photoinitiator; and (d) optionally a pigment; wherein the ink contains 5% or less of tetrahydrofurfuryl acrylate.