GB2607660A - Printing ink - Google Patents

Abstract

An inkjet ink contains 0.5-15 wt.% vinyl resin dissolved in a liquid medium comprising a radiation-curable monomer, wherein the ink has a viscosity of 30-100 mPa s at 40°C. Typically, the vinyl resin is solid at 25°C and has a molecular weight of 20-200 KDa. The vinyl resin may be present in an amount of 5-12 wt.% and may be a vinyl chloride/hydroxy acrylate copolymer or a terpolymer of vinyl chloride, vinyl acetate and either vinyl alcohol or an unsaturated dicarboxylic acid ester. The ink preferably comprises 20-90 wt.% (meth)acrylate and/or 10-30 wt.% N-vinyl amide as radiation-curable monofunctional monomers. The radiation-curable monomer may be N-vinyl caprolactam, N-acryloylmorpholine, cyclic TMP formal acrylate, tetrahydrofurfuryl acrylate, (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (Medol 10), isobornyl acrylate, 2-(2-vinyloxy ethoxy)ethyl acrylate, or 2-(2-vinyloxy ethoxy)ethyl methacrylate. Additionally, the liquid medium may comprise 1-10 wt.% difunctional monomer. A method of preparing vinyl tiles, planks, or sheets using the ink is also disclosed.

Description

Printing ink This invention relates to a printing ink, and in particular to an inkjet ink for printing onto a vinyl tile, plank or sheet.

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 file/plank/sheet to have a peel strength of 10 N/cm and so the ink must retain adhesion to the substrate and film cohesion for the lifetime of the tile/plank/sheet.

The inks are typically 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 The ink must also retain adhesion to the substrate and film cohesion. For inkjet printing, the ink must also have the required jetting properties. However, it has proven difficult to formulate an inkjet ink with the required jetting reliability, as well as the necessary adhesion to the substrate, film cohesion and lamination bond strength for vinyl tile/plank/sheet applications.

There therefore remains a need in the art for an ink with the required jetting properties for inkjet printing, and good lamination bond strength, adhesion to the substrate and film cohesion for vinyl file/plank/sheeting applications.

Accordingly, the present invention provides an inkjet ink comprising: (a) a liquid medium comprising a radiation-curable monomer; and (b) 0.5 to 15.0% by weight, based on the total weight of the ink, of a vinyl resin dissolved in the liquid medium, wherein the ink has a viscosity of 30 to 100 mPas at 40°C.

It has surprisingly been found that it is possible to provide an inkjet ink, with the required jetting properties for inkjet printing, and good lamination bond strength, adhesion to the substrate and film cohesion for vinyl tile/plank/sheeting applications by providing an inkjet ink comprising: (a) a liquid medium comprising a radiation-curable monomer; and (b) 0.5 to 15.0% by weight, based on the total weight of the ink, of a vinyl resin dissolved in the liquid medium, wherein the ink has a viscosity of 30 to 100 mPas at 40°C.

It has previously been considered that such an inkjet ink having the blend of components and such a high viscosity as being unsuitable for inkjet printing as they provide poor jetting properties.

However, the inventors have found that by using a particular print head, it is possible to use such a high viscosity inkjet ink comprising: (a) a liquid medium comprising a radiation-curable monomer; and (b) 0.5 to 15.0% by weight, based on the total weight of the ink, of a vinyl resin dissolved in the liquid medium, without compromising the jetting properties of the inkjet ink or the lamination bond strength, adhesion to the substrate and film cohesion, for vinyl tile/plank/sheeting applications.

The inkjet ink of the present invention comprises a liquid medium comprising a radiation-curable monomer.

The components of the inkjet ink of the present invention that make up the liquid medium are selected so that the ink provides good lamination bond strength, good adhesion to the substrate and good film cohesion -key requirements in the preparation of vinyl tiles/planks/sheets.

As is known in the art, monomers may possess different degrees of functionality, which include mono, di, tri and higher functionality monomers.

For the avoidance of doubt, mono and difunctional are intended to have their standard meanings, i.e. one or two groups, respectively, which take part in the polymerisation reaction on curing. Multifunctional (which does not include difuncfional) is intended to have its standard meaning, i.e. three or more groups, respectively, which take part in the polymerisation reaction on curing.

Monomers typically have a molecular weight of less than 600, preferably more than 200 and less than 450. Molecular weights (number average) can be calculated if the structure of the monomer is known or molecular weights can be measured using gel permeation chromatography using polystyrene standards.

The radiation-curable monomer is not particularly limited and the formulator is free to include any such radiation-curable monomer in the inkjet ink to improve the properties or performance of the ink, providing the viscosity requirements of the inkjet ink are met. This radiation-curable monomer can include any radiation-curable monomer readily available and known in the art in inkjet inks. By "radiation-curable" is meant a material that contains functional groups that are capable of polymerising upon exposure to radiation.

The amount of radiation-curable monomer is not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. In a preferred embodiment, the inkjet ink comprises 20 to 90% by weight of radiation-curable monomer, preferably 50 to 80% by weight of radiation-curable monomer, based on the total weight of the ink.

In a preferred embodiment, the radiation-curable monomer comprises one or more monofunctional monomers Monofunctional monomers are well known in the art. A radiation-curable monofunctional monomer has one functional group, which takes part in the polymerisation reaction on curing. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation and is preferably selected from a (meth)acrylate group and a vinyl ether group.

The substituents of the monofunctional monomer are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C1-18 alkyl, C3-13 cycloalkyl, C5-10 aryl and combinations thereof, such as C5-10 aryl-or C3-18 cycloalkyl-substituted C1-18 alkyl, any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure.

In a preferred embodiment, the radiation-curable monomer comprises one or more monofunctional monomers present in 20 to 90% by weight, preferably 50 to 80% by weight, based on the total weight of the ink.

In a preferred embodiment, the radiation-curable monomer comprises one or more monofunctional (meth)acrylate monomers, which are well known in the art and are preferably the esters of acrylic acid. Mixtures of (meth)acrylates may also be used.

For the avoidance of doubt, (meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate.

The substituents of the monofunctional (meth)acrylate monomer are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The monofunctional (meth)acrylate monomer may be a cyclic monofunctional (meth)acrylate monomer and/or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

In one embodiment, the monofunctional (meth)acrylate monomer comprises a cyclic monofunctional (meth)acrylate monomer.

The substituents of the cyclic monofunctional (meth)acrylate monomer are typically cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms and/or substituted by alkyl. Non-limiting examples of substituents commonly used in the art include C3-18 cycloalkyl, C6-10 aryl and combinations thereof, any of which may substituted with alkyl (such as C1-18 alkyl) and/or any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure.

The cyclic monofunctional (meth)acrylate monomer may be selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10), 4-tert-butylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA) and mixtures thereof.

In one embodiment, the monofunctional (meth)acrylate monomer comprises an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

The substituents of the acyclic-hydrocarbon monofunctional (meth)acrylate monomer are typically alkyl, which may be interrupted by heteroatoms. A non-limiting example of a substituent commonly used in the art is C1-18 alkyl, which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted.

The acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear or branched C3-C20 group. It may be selected from octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof. In a preferred embodiment, the acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear C6-C20 group.

In a preferred embodiment, the radiation-curable monomer comprises one or more monofunctional (meth)acrylate monomers present in 20 to 90% by weight, preferably 20 to 70% by weight, preferably 30 to 60% by weight, based on the total weight of the ink.

In a preferred embodiment, the radiation-curable monomer comprises an N-vinyl amide monomer and/or an N-(meth)acryloyl amine monomer.

N-Vinyl amide monomers are well-known monomers in the art. N-Vinyl amide monomers have a vinyl group attached to the nitrogen atom of an amide which may be further substituted in an analogous manner to the (meth)acrylate monomers. Preferred examples are N-vinyl caprolactam (NVC), N-vinyl pyrrolidone (NVP), N-vinyl piperidone, N-vinyl formamide and N-vinyl acetamide.

Similarly, N-acryloyl amine monomers are also well-known in the art. N-Acryloyl amine monomers also have a vinyl group attached to an amide but via the carbonyl carbon atom and again may be further substituted in an analogous manner to the (meth)acrylate monomers. A preferred example is N-acryloylmorpholine (ACMO).

In a preferred embodiment, the inkjet ink comprises 10-30% by weight, more preferably 15-25% by weight, of an N-vinyl amide monomer, an N-acryloyl amine monomer or mixtures thereof, based on the total weight of the ink. Most preferably, the ink comprises 10 to 30% by weight, preferably 1525% by weight, of an N-vinyl amide monomer, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises at least one of NVC and/or ACMO. N-Vinyl amide monomers are particularly preferred, and most preferably NVC.

The inkjet ink may also comprise an N-vinyl monomer other than an N-vinyl amide monomer and/or an N-(meth)acryloyl amine monomer. Examples include N-vinyl carbazole, N-vinyl indole and N-vinyl imidazole.

In a preferred embodiment, the inkjet ink comprises 10-30% by weight, more preferably 15-25% by weight, of an N-vinyl monomer other than an N-vinyl amide monomer and/or an N-(meth)acryloyl amine monomer, based on the total weight of the ink.

In a preferred embodiment, the one or more monofuncfional monomers are one or more (meth)acrylate monomers in combination with one or more N-vinyl amide monomers.

Monofunctional monomers are typically added to inkjet inks to reduce the viscosity of the inkjet ink.

The inkjet ink of the present invention has a high viscosity of 30 to 100 mPas at 40°C and as such, the amount and type of monofuncfional monomers are limited to the extent that the viscosity requirements of the inkjet ink are met.

In a preferred embodiment, the radiation-curable monomer comprises one or more difunctional 40 monomers In a preferred embodiment, the radiation-curable monomer comprises one or more difunctional monomer present in 1 to 10% by weight, preferably 5 to 10% by weight, based on the total weight of the ink.

The functional group of the difunctional monomer may be the same or different but must take part in the polymerisation reaction on curing. Examples of such functional groups include any groups that are capable of polymerising upon exposure to radiation and are preferably selected from a (meth)acrylate group and a vinyl ether group.

The substituents of the difunctional monomer are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C1-18 alkyl, C3-18 cycloalkyl, C8-10 aryl and combinations thereof, such as C15-10 aryl-or C3-18 cycloalkyl-substituted C1-18 alkyl, any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure.

Examples of difunctional monomer include difunctional (meth)acrylate monomers, divinyl ether monomers and difunctional vinyl ether (meth)acrylate monomers. Mixtures of difunctional monomers may also be used In a preferred embodiment, the radiation-curable monomer comprises one or more difunctional (meth)acrylate monomers.

Difunctional (meth)acrylate monomers are well known in the art. Examples include hexanediol diacrylate (HDDA), 1,8-octanediol diacrylate, 1,9-nonanediol diacrylate, 1,10-decanediol diacrylate (DDDA), 1,11-undecanediol diacrylate and 1,12-dodecanediol diacrylate, polyethylene glycol diacrylate (for example tetraethylene glycol diacrylate, PEG200DA, PEG300DA, PEG400DA, PEG600DA), dipropylene glycol diacrylate (DPGDA), tripropylene glycol diacrylate (TPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), neopentylglycol diacrylate, 3-methyl-1,5-pentanediol diacrylate (3-MPDDA), propoxylated neopentylglycol diacrylate (NPGPODA), and mixtures thereof. Also included are esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, 1,8-octanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, 1,11-undecanediol dimethacrylate and 1,12-dodecanediol dimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1,4-butanediol dimethacrylate and mixtures thereof. 3-MPDDA is particularly preferred.

Preferably, the inkjet ink of the present invention comprises 1 to 10% by weight, preferably 5 to 10% by weight, of one or more difunctional (meth)acrylate monomers, based on the total weight of the ink.

In a preferred embodiment, the radiation-curable monomer comprises one or more divinyl ether monomers Examples of divinyl ether monomers include Methylene glycol divinyl ether (DVE-3), diethylene glycol divinyl ether, 1,4-cyclohexanedimethanol divinyl ether, bis[4-(vinyloxy)butyl] 1,6- hexanediylbiscarbamate, bis[4-(vinyloxy)butyl] isophthalate, bis[4-(vinyloxy)butyl] (methylenedi- 4,1-phenylene)biscarbamate, bis[4-(vinyloxy)butyl] succinate, bis[4-(vinyloxy)butyl]terephthalate, bis[4-(vinyloxymethyl)cyclohexylmethyl] glutarate, 1,4-butanediol divinyl ether and mixtures thereof.

Preferably, the inkjet ink of the present invention comprises 1 to 10% by weight, preferably 5 to 10% by weight, of one or more divinyl ether monomers, based on the total weight of the ink.

In a preferred embodiment, the radiation-curable monomer comprises one or more divinyl ether (meth)acrylate monomers. 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.

Examples of vinyl ether (meth)acrylate monomers include 2-(2-vinyloxy ethoxy)ethyl acrylate (VEEA), 2-(2-vinyloxy ethoxy)ethyl methacrylate (VEEM) and mixtures thereof.

Preferably, the inkjet ink of the present invention comprises 1 to 10% by weight, preferably 5 to 10% by weight, of one or more divinyl ether (meth)acrylate monomers, based on the total weight of the ink.

Monofunctional monomers are particularly preferred as the radiation-curable monomer. As such, in a preferred embodiment, difunctional monomers are present in an amount of less than 5% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight, and most preferably the inkjet ink is substantially free of difunctional monomers, where the amounts are based on the total weight of the ink.

By substantially free is meant that only small amounts will be present, for example some difunctional monomer may be present as impurities in commercially available inkjet ink components, but such low levels are tolerated. In other words, no difunctional monomer is intentionally added to the ink. For example, the ink may comprise less than 0.5% by weight of difunctional monomers, more preferably less than 0.1% by weight of difunctional monomers, most preferably less than 0.05% by weight of difunctional monomers, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of difunctional monomers.

Multifunctional monomers are also preferably present in an amount of less than 5% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight, and most preferably the inkjet ink is substantially free of multifunctional monomers, where the amounts are based on the total weight of the ink.

By substantially free is meant that only small amounts will be present, for example some multifunctional monomer may be present as impurities in commercially available inkjet ink components, but such low levels are tolerated. In other words, no multifunctional monomer is intentionally added to the ink. For example, the ink may comprise less than 0.5% by weight of multifunctional monomers, more preferably less than 0.1% by weight of multifunctional monomers, most preferably less than 0.05% by weight of multifunctional monomers, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of multifunctional monomers.

Examples of the multifunctional monomer include multifunctional (meth)acrylate monomers, multifunctional vinyl ether monomers and multifunctional vinyl ether (meth)acrylate monomers.

In a preferred embodiment, the radiation-curable monomer of the liquid medium comprises monofunctional monomers as the sole radiation-curable monomers present in the inkjet ink.

Accordingly, the inkjet ink preferably comprises monofunctional monomers as the sole radiation-curable monomer present in the inkjet ink.

The ink of the present invention contains a vinyl resin. The vinyl resin ensures excellent lamination bond strength, as well as good adhesion to the substrate and film cohesion. The resin is present at 0.5 to 15.0% by weight, preferably 5.0-12.0% by weight, based on the total weight of the inkjet ink.

The inclusion of a vinyl resin in an inkjet ink in the amount as claimed, which can result in a high viscosity ink, has previously been seen as unsuitable for inkjet printing as it provides poor jetting properties.

However, the inventors have found that by using a particular print head, it is possible to use such components, without compromising the jetting properties of the inkjet ink.

The resin preferably has a weight-average molecular weight (Mw) of 20 to 200 KDa, and most preferably 20 to 60 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.

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

For the functionalised resins, the vinyl alcohol content is preferably 0 to 30% by weight and more preferably 5 to 20% by weight, based on the total composition of the resin. The unsaturated dicarboxylic acid ester content is preferably 0 to 2% by weight and more preferably 0.1 to 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 is formulated so that the vinyl resin is dissolved in the liquid medium of the ink medium. As such, the liquid medium comprising a radiation-curable monomer must be capable of dissolving the vinyl resin of the inkjet ink.

A suitable test for measuring the solubility of the passive 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.

In a preferred embodiment, the radiation-curable monomer comprises monomers having high solubilising power for the vinyl resin of the inkjet ink. Accordingly, in a preferred embodiment, the radiation-curable monomer comprises at least one of N-vinyl caprolactam, N-acryloylmorpholine, cyclic IMP formal acrylate, tetrahydrofurfuryl acrylate, (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (Medol 10), isobornyl acrylate, 2-(2-vinyloxy ethoxy)ethyl acrylate, 2-(2-vinyloxy ethoxy)ethyl methacrylate, and combinations thereof Such monomers have improved solubilising power for the vinyl resin of the inkjet ink.

In a preferred embodiment, the radiation-curable monomer comprises monomers with a very high solvency power for the substrate. Such monomers are typically required in order to ensure an adequate lamination bond strength for vinyl tile/plank/sheeting applications. Preferred examples include THFA, N-vinyl caprolactam, or mixtures thereof.

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.

In a preferred embodiment, the liquid medium comprises a non-radiation-curable substrate-solvating component. 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 nonradiation-curable substrate-solvating component has a boiling point above 200°C. A preferred non-radiation-curable substrate-solvating component is y-butyrolactone.

The inkjet ink of the present invention may further comprise a radiation-curable (i.e. polymerisable) oligomer, such as a (meth)acrylate oligomer. Any radiation-curable oligomer that is compatible with the other ink components is suitable for use in the ink. In a preferred embodiment however, the amount of radiation-curable oligomers are also to be minimised.

The term "curable oligomer" has its standard meaning in the art, namely that the component is partially reacted to form a pre-polymer having a plurality of repeating monomer units, which is capable of further polymerisation. The oligomer preferably has a molecular weight of at least 600.

The molecular weight is preferably 4,000 or less. Molecular weights (number average) can be calculated if the structure of the oligomer is known or molecular weights can be measured using gel permeation chromatography using polystyrene standards.

The oligomers may possess different degrees of functionality, and a mixture including combinations of mono, di, tri and higher functionality oligomers may be used. The degree of functionality of the oligomer determines the degree of crosslinking and hence the properties of the cured ink.

Oligomers are typically added to inkjet inks to increase the viscosity of the inkjet ink or to provide film-forming properties such as hardness or cure speed. They therefore preferably have a viscosity of 150 mPas or above at 25°C. Oligomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique / 2° steel cone at 60°C with a shear rate of 25s* Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups.

The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. In a preferred embodiment, the radiation-curable oligomer is a (meth)acrylate oligomer. The radiation-curable oligomer may include amine functionality, as the amine acts as an activator without the drawback of migration associated with low-molecular weight amines. In a preferred embodiment, the radiation-curable oligomer is amine modified. In a particularly preferred embodiment, the radiation-curable oligomer is an amine-modified (meth)acrylate oligomer.

More preferably, the radiation-curable oligomer is an amine-modified acrylate oligomer. A suitable amine-modified polyester acrylate oligomer is commercially available as UVP6600. A suitable amine-modified polyether acrylate oligomer is commercially available as CN3715LM.

Other suitable examples of radiation-curable oligomers include epoxy based materials such as bisphenol A epoxy acrylates and epoxy novolac acrylates, which have fast cure speeds and provide cured films with good solvent resistance.

The amount of radiation-curable oligomer, when present, is preferably 0.1 to 10% by weight, based on the total weight of the ink.

In a preferred embodiment however, multifunctional radiation-curable oligomers are present in an amount of less than 5% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight, and most preferably the inkjet ink is substantially free of multifunctional radiation-curable oligomers, where the amounts are based on the total weight of the ink. A radiation-curable oligomer is multifunctional if it contains on average more than one reactive functional group per molecule By substantially free is meant that only small amounts will be present, for example some multifunctional radiation-curable oligomer may be present as impurities in commercially available inkjet ink components, but such low levels are tolerated. In other words, no multifunctional radiation-curable oligomer is intentionally added to the ink. For example, the ink may comprise less than 0.5% by weight, more preferably less than 0.1% by weight, most preferably less than 0.05% by weight of multifunctional radiation-curable oligomers, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of multifunctional radiation-curable oligomers.

In a preferred embodiment, the inkjet ink comprises a combined amount of multifunctional monomer and multifunctional radiation-curable oligomer of less than 5% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight, and most preferably the inkjet ink is substantially free of multifunctional monomers and multifunctional radiation-curable oligomers, where the amounts are based on the total weight of the ink.

In a preferred embodiment, radiation-curable oligomers are present in an amount of less than 5% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight, and most preferably the inkjet ink is substantially free of radiation-curable oligomers, where the amounts are based on the total weight of the ink.

By substantially free is meant that only small amounts will be present, for example some radiation-curable oligomer may be present as impurities in commercially available inkjet ink components, but such low levels are tolerated. In other words, no radiation-curable oligomer is intentionally added to the ink. For example, the ink may comprise less than 0.5% by weight, more preferably less than 0.1% by weight, most preferably less than 0.05% by weight of radiation-curable oligomers, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of radiation-curable oligomers.

The ink may also contain an additional resin, other than the vinyl resin, which will also be dissolved in the liquid medium of the ink. The additional resin is also a passive (i.e. inert) resin, in the sense that it is not radiation-curable and hence does not undergo crosslinking under the curing conditions to which the inkjet ink is subjected. The additional passive resin is typically present at 0.5 to 5.0% by weight, preferably 1.0-4.0% by weight, based on the total weight of the inkjet ink.

Any additional passive resin, other than vinyl resin, that is compatible with the other inkjet ink components is suitable for use in the inkjet ink as long as it is soluble in the liquid medium of the inkjet ink. Thus, the inkjet ink formulator is able to select from a wide range of suitable passive resins, other than vinyl resins. Examples of suitable additional passive resins include an epoxy resin, a polyester resin, a ketone resin, an aldehyde resin, a nitrocellulose resin, a phenoxy resin, an acrylate resin and combinations thereof.

The additional passive resin preferably has a weight-average molecular weight (Mw) of 20 to 200 kDa, and most preferably 20 to 60 kDa, as determined by gel permeation chromatography (GPC) with polystyrene standards. Preferably, the additional passive resin has a glass transition temperature (Tg) of 50 to 90°C. The Tg may be measured by DSC with a heating ramp of 10°C/min.

In a preferred embodiment, the additional passive resin is a (meth)acrylic copolymer such as a copolymer of methyl (meth)acrylate with butyl, isobutyl or isobornyl (meth)acrylate. (Meth)acrylate copolymers may also contain styrene. An example of a preferred passive resin is Diana! BR 113 from Diana! America, Inc., an acrylic copolymer with a weight-average molecular weight of 30 kDa and a Tg of 78°C.

A suitable test for measuring the solubility of the additional resin in the liquid medium is as described above for the vinyl resin of the inkjet ink.

In a preferred embodiment, the inkjet ink of the present invention also includes a colouring agent, which may be either dissolved or dispersed in the liquid medium of the ink. The colouring agent can be any of a wide range of suitable colouring agents that would be known to the person skilled in the art.

Preferably, the colouring agent is a dispersed pigment, of the types known in the art and commercially available such as under the trade-names Paliotol (available from BASF plc), 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.

In a preferred embodiment, the dispersible pigment is in the form of a solid dispersion in an additional vinyl resin, which may be the same or different to the vinyl resin dissolved in the liquid medium of the ink. Such materials are available from BASF under the trade name of Microlith K. 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 Jet magenta 2BC and Cinquasia RT-355D. Black: carbon black pigments such as Pigment black 7.

The total proportion of pigment present is preferably from 0.5 to 15% by weight, more preferably from 1 to 10% by weight The present invention may also provide an inkjet ink set wherein at least one of the inks in the set is an inkjet ink of the present invention. Preferably, all of the inks in the set fall within the scope of the inkjet ink according to the present invention.

Usually, the inkjet ink set of the present invention is in the form of a multi-chromatic inkjet ink set, which typically comprises a cyan ink, a magenta ink, a yellow ink and a black ink (a so-called trichromatic set). This set is often termed CMYK. The inks in a trichromatic set can be used to produce a wide range of colours and tones. Other inkjet ink sets may also be used, such as CMYK+white and light colours.

Other suitable colouring agents include dyes. The dyes include but are not limited to azo dyes, anthraquinone dyes, xanthene dyes, azine dyes, and combinations thereof.

If the ink is cured by exposure to a source of actinic radiation without an inert environment, one or more photoinitiators will be required. If the ink is cured by exposure to a source of low-energy electron beam radiation or a source of actinic radiation in an inert environment, the ink may still contain a photoinitiator, although photoinitiators are not required.

In a preferred embodiment, the ink of the present invention further comprises one or more photoinitiators.

Preferred are photoinitiators which produce free radicals on irradiation (free radical photoinitiators) such as, for example, benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2-benzy1-2- dimethylamino-(4-morpholinophenyl)butan-1-one, benzil dimethylketal, phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide or mixtures thereof Such photoinitiators are known and commercially available such as, for example, under the trade names Omnirad (from IGM) and Esacure (from Lamberti).

Mixtures of free radical photoinitiators can be used and preferably, the ink comprises a plurality of free radical photoinitiators. The total number of free radical photoinitiators present is preferably from one to five, and more preferably, two or more free radical photoinitiators are present in the ink.

Preferably, the photoinitiator if present, is present from 1 to 20% by weight, preferably from 5 to 15% by weight, based on the total weight of the ink.

The presence of a photoinitiator is optional as it is not necessary to include a photoinitiator in the inkjet ink in order to achieve a thorough cure of the ink. This is because the ink can cure without the presence of a photoinitiator by curing with a low-energy electron beam or curing by actinic radiation in an inert environment.

Therefore, in a preferred embodiment, the photoinitiator is present in an amount of less than 20% by weight, preferably less than 5% by weight, more preferably less than 3%, more preferably less than 1%, based on the total weight of the ink.

As such, the inkjet ink may be substantially free of photoinitiator. By "substantially free" is meant that no photoinitiator is intentionally added to the ink. However, minor amounts of photoinitiator, which may be present as impurities in commercially available inkjet ink components, are tolerated.

For example, the ink may comprise less than 0.5% by weight of photoinitiator, more preferably less than 0.1% by weight of photoinitiator, most preferably less than 0.05% by weight of photoinitiator, based on the total weight of the ink. The inkjet ink may also be free of photoinitiator.

An inkjet ink that is substantially free of photoinitiator is advantageous for various applications as there will be no unreacted photoinitiator or unreacted photoinitiator fragments present in the cured inkjet ink film. Photoinitiators create free radicals when exposed to radiation. These radicals react with reactive components of the ink (such as reactive monomers and oligomers). However, some photoinitiator and photoinitiator fragments will remain unreacted in the cured ink film and this is problematic for certain applications, such as food packaging, as such unreacted components can migrate into the substrate.

However, an inkjet ink that is cured with a low-energy electron beam or actinic radiation in an inert environment may still contain a small amount of photoinitiator such as 1 to 5% by weight of a photoinitiator, based on the total weight of the ink. This is required if the ink is for example, first pinned with actinic radiation.

By pinning is meant arresting the flow of the ink by treating the ink droplets quickly after they have impacted onto the substrate surface. Pinning provides a partial cure of the ink and thereby maximises image quality by controlling bleed and feathering between image areas. Pinning does not achieve full cure of the ink. By curing is meant fully curing the ink. Pinning leads to a marked increase in viscosity, whereas curing converts the inkjet ink from a liquid ink to a solid film. The dose of radiation used for pinning is generally lower than the dose required to cure the radiation-curable material fully.

The inkjet ink of the present invention preferably dries primarily by curing, i.e. by the polymerisation of the monomers present, as discussed hereinabove, and hence is a curable ink. The ink does not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink.

Accordingly, the inkjet ink preferably comprises less than 5% by weight of water and volatile organic solvents combined, based on the total weight of the ink. Preferably, the inkjet ink comprises less than 3% by weight of water and volatile organic solvent combined, more preferably less than 2 % by weight combined, more preferably less than 1% by weight combined, and most preferably the inkjet ink is substantially free of water and volatile organic solvents, where the amounts are based on the total weight of the ink.

By substantially free is meant that only small amounts will be present, for example some water will typically be absorbed by the ink from the air and solvents may be present as impurities in the components of the inks, but such low levels are tolerated. In other words, no water or a volatile organic solvent is intentionally added to the ink. However, minor amounts of water or a volatile organic solvent, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight of water or a volatile organic solvent, more preferably less than 0.1% by weight of water or a volatile organic solvent, most preferably less than 0.05% by weight of water or a volatile organic solvent, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of water or a volatile organic solvent In a preferred embodiment, the ink of the present invention comprises a surfactant. The surfactant controls the surface tension of the ink. Surfactants are well-known in the art and a detailed description is not required. An example of a suitable surfactant is BYK307. Adjustment of the surface tension of the inks allows control of the surface wetting of the inks on various substrates, for example, plastic substrates. 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. Surface tension is also critical to ensuring stable jetting (nozzle plate wetting and sustainability). The surface tension is preferably in the range of 18-40 mNm-1, more preferably 20-35 mNm-1 and most preferably 20-30 mNm-1.

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

The ink of the present invention may also include radiation-curable material, which is capable of polymerising by cationic polymerisation. Suitable materials include, oxetanes, cycloaliphafic epoxides, bisphenol A epoxides, epoxy novolacs and the like. Preferably however, the ink of the present invention cures by free radical polymerisation only and hence the ink is substantially free of, preferably free of, radiation-curable material, which polymerises by cationic polymerisation.

By substantially free is meant that only small amounts will be present, for example some radiation-curable material, which polymerises by cationic polymerisation, may be present as impurities in commercially available inkjet ink components, but such low levels are tolerated. In other words, no radiation-curable material, which polymerises by cationic polymerisation, is intentionally added to the ink. For example, the ink may comprise less than 0.5% by weight, more preferably less than 0.1% by weight, most preferably less than 0.05% by weight of radiation-curable material, which polymerises by cationic polymerisation, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of radiation-curable material, which polymerises by cationic polymerisation.

In the embodiment where the ink comprises radiation-curable material, which polymerises by cationic polymerisation, the ink must also comprise a cationic photoinifiator.

In the case of a cationically curable system, any suitable cationic initiator can be used, for example sulfonium or iodonium based systems. Non limiting examples include: Rhodorsil PI 2074 from Rhodia; MC AA, MC BB, MC CC, MC CC PF, MC SD from Siber Hegner; UV9380c from Alfa Chemicals; Uvacure 1590 from UCB Chemicals; and Esacure 1064 from Lamberti spa.

The inks of the invention 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.

The ink of the present invention has a viscosity of 30 to 100 mPas at 40°C. 40°C is a preferred jetting temperature and as such, the inkjet ink of the present invention has a viscosity of 30 to 100 mPas at the jetting temperature of 40°C. Preferably, the ink of the present invention has a viscosity of 30 to 70 mPas at 40°C.

Ink viscosity can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 60 mm diameter /10 aluminium cone at 40°C with a shear rate of 20 s-1. Accordingly, the ink viscosity can be measured using a rotational rheometer having a 60 mm diameter / 1° aluminium cone at 40°C with a shear rate of 20 s* It had previously been considered that such a high viscosity ink was unsuitable for inkjet printing as it was not previously possible to achieve the required jetting properties. However, the inventors have found that by using a particular print head, it is possible to achieve the required jetting properties for such a high viscosity ink.

The ink of the present invention can thus have the required jetting properties, whilst achieving lamination bond strength, adhesion and cohesion for vinyl tile/plank/sheeting applications. Such a high viscosity ink also has the advantage of improving satellite suppression, as well as improved edge definition.

The present invention may also provide a cartridge containing the inkjet ink as defined herein.

The present invention also provides a method of preparing a vinyl tile, plank or sheet, comprising the following steps, in order: (i) providing an opaque vinyl substrate; (ii) jetting the inkjet ink of the present invention on to the surface of the substrate to form an image; (iii) curing the ink; (iv) 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 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 files 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), jetting the inkjet ink of the present invention on to the surface of the substrate to form an image.

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.

Printing is performed by inkjet printing, e.g. on a single-pass inkjet printer, for example for printing (directly) onto the surface of the substrate, on a roll-to-roll printer or a flat-bed printer.

The inventors have found that by using a particular print head, it is possible for the ink of the present invention to be jetted and maintain the required jetting properties. For example, Xaar® provide such high viscosity print heads with very high viscosity capabilities, which include Xaar® 2001 including a printhead with high laydown technology. Xaar® 2001 with high laydown technology allows jetting of inkjet inks up to100 cP at 40°C.

In order to produce a high quality printed image a small jetted drop size is desirable. Preferably the inkjet ink is jetted at drop sizes below 90 picolitres, preferably below 35 picolitres and most preferably below 10 picolitres.

The method of the present invention includes step (iii), curing the inkjet ink.

The ink of the present invention is cured by any means known in the art, such as exposure to actinic radiation and low-energy electron beam radiation.

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 of the inkjet ink from a liquid to solid by polymerisation and/or crosslinking of the radiation-curable material. Herein, however, by "drying" is meant the removal of the water by evaporation and by "curing" is meant the polymerisation and/or crosslinking of the radiation-curable material. Further details of the printing, drying and curing process are provided in WO 2011/021052.

The ink may be cured in an inert atmosphere, e.g. using a gas such as nitrogen.

In a preferred embodiment, the ink is cured by exposing the printed ink to a source of 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 are well-known in the art and a detailed description is not required. These 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.

Preferably, the source of actinic radiation is a mercury discharge lamp and/or LEDs. When LEDs are used, these are preferably provided as an array of multiple LEDs.

The most common UV light source used to cure inkjet inks is a mercury discharge lamp. These lamps operate by creating a plasma between two electrodes in a high pressure mercury gas contained in a quartz envelope. Although these lamps have some drawbacks in terms of their operational characteristics, no other UV light source has yet managed to challenge their position in terms of UV output performance.

LEDs are increasingly used to cure inkjet inks. UV light is emitted from a UV LED light source. UV LED light sources comprise one or more LEDs and are well-known in the art. Thus, a detailed

description is not required.

It will be understood that UV LED light sources emit radiation having a spread of wavelengths. The emission of UV LED light sources is identified by the wavelength which corresponds to the peak in the wavelength distribution. Compared to conventional mercury lamp UV sources, UV LED light sources emit UV radiation over a narrow range of wavelengths on the wavelength distribution. The width of the range of wavelengths on the wavelength distribution is called a wavelength band. LEDs therefore have a narrow wavelength output when compared to other sources of UV radiation. By a narrow wavelength band, it is meant that at least 90%, preferably at least 95%, of the radiation emitted from the UV LED light source has a wavelength within a wavelength band having a width of 50 nm or less, preferably, 30 nm or less, most preferably 15 nm or less.

In a preferred embodiment, at least 90%, preferably at least 95%, of the radiation emitted from the UV LED light source has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less.

LEDs have a longer lifetime and exhibit no change in the power/wavelength output over time. LEDs also have the advantage of switching on instantaneously with no thermal stabilisation time and their use results in minimal heating of the substrate.

In a preferred embodiment, the exposure to a source of actinic radiation is performed in an inert atmosphere, e.g. using a gas such as nitrogen, in order to assist curing of the ink.

In a preferred embodiment, the ink is cured by exposing the printed ink to low-energy electron beam (ebeam).

The source of low-energy electron beam (ebeam) can be any source of low-energy electron beam that is suitable for curing radiation-curable inks. Suitable low-energy electron beam radiation sources include commercially available ebeam curing units, such as the EB Lab from ebeam Technologies with energy of 80-300 keV and capable of delivering a typical dose of 30-50 kGy at line speeds of up to 30 m/min. By "low-energy" for the ebeam, it is meant that it delivers an electron beam having a dose at the substrate of 100 kGy or less, preferably 70 kGy or less.

Ebeam curing is characterised by dose (energy per unit mass, measured in kilograys (kGy)) deposited in the substrate via electrons. Electron beam surface penetration depends upon the mass, density and thickness of the material being cured. Compared with UV penetration, electrons penetrate deeply through both lower and higher density materials. Unlike UV curing, photoinifiators are not required for ebeam curing to take place.

Ebeam curing is well-known in the art and therefore a detailed explanation of the curing method is not required. In order to cure the printed ink, the ink of the invention is exposed to the ebeam, which produces sufficient energy to instantaneously break chemical bonds and enable polymerisation or crosslinking.

There is no restriction on the ebeam dose that is used to cure the inkjet inks of the present invention other than that the dose is sufficient to fully cure the ink. Preferably, the dose is more than 10 kGy, more preferably more than 20 kGy, more preferably more than 30 kGy and most preferably more than 40 kGy. Preferably, the dose is less than 100 kGy, more preferably less than 90 kGy, more preferably less than 80 kGy and most preferably less than 70 kGy. Preferably, the dose is more than 30 kGy but less than 70 kGy, more preferably more than 30 kGy but less than 60 kGy and most preferably, more than 30 kGy but 50 kGy or less. Doses above 50 kGy may cause damage to the substrate and so doses of 50 kGy or less are preferred.

The energy associated with these doses is 80-300 key, more preferably 70-200 key and most preferably 100 key.

The exposure to a source of low-energy electron beam (ebeam) is performed in an inert atmosphere, e.g. using a gas such as nitrogen.

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 method of the present invention includes step (iv) 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. 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 10-60s.

The present invention also provides a vinyl tile, plank or sheet obtainable by the method of the present invention.

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

Examples

Example 1

An inkjet ink was prepared according to the formulation set out in Table 1. Amounts are given as weight percentages based on the total weight of the ink.

Table 1

Component Ink 1 (wt%) CTFA 53.00 NVC 20.00 UV12 0.50 Vinnol® H14/36 10.00 Microlith K pigment dispersion 4.00 Irgacure184 3.00 TPO 9.50 Total 100.00 CTFA and NVC are radiation-curable monomers as defined above. UV12 is a stabiliser. Vinnol® H14/36 is an unfuncfionalised suspension polymerised vinyl chloride/vinyl acetate copolymer (86:14 by weight) having a Mw of 30-40 KDa. Irgacure 184 and TPO are photoinitiators. Microlith K pigment dispersion is a solid pigment dispersion in a vinyl resin.

The ink was prepared by first weighing the monomers into a suitable mixing vessel, placing the vessel under the mixing head of a SiIverson 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.

The viscosity of the ink was 30.21 mPas at 40°C measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 60 mm diameter /10 aluminium cone at 40°C with a shear rate of 20s* The ink was applied to a white 100 micron PVC film substrate using a K2 applicator bar (12 pm wet film). The resulting film was UV cured by passing it through a conveyorised UV cure unit fitted with a 120 W/cm medium pressure mercury lamp at 25 m/min.

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

The bond strength was tested according to the following procedure: The ink was applied to a white 100 micron PVC film substrate using a K2 applicator bar (12 pm wet film). The resulting film was UV cured by passing it through a conveyorised UV cure unit fitted with a 120 W/cm medium pressure mercury lamp at 25 m/min.

Using a credit card punch, a test piece was pressed out from the material having an even coating of ink present thereon. A piece of clear laminate was also cut in the credit card punch and this piece of clear laminate was placed over the coated test piece. A piece of aluminium foil was inserted into one end of the sample to be laminated, leaving a flap to allow for the two materials to be peeled apart.

The sample was then subjected to lamination using an Oasys OLA6 laminating press. The following settings were used on the Oasys OLA6 laminating press for lamination: Laminating temperature: 140°C Low pressure: 29 units Hold timer: 300 s High pressure temperature: 140°C High pressure: 34 units End cycle temperature: 35°C After lamination was complete, the sample was removed, and the backing sheet was removed from the aluminium foil. The sample was then peeled apart by pulling both flaps at an angle of 180°.

The results were recorded according to the following criteria: Pass -substrate snaps with no peeling of laminated surface or failure of ink film Fail -ink delaminates from coated white vinyl surface/the ink film fails leaving ink on both surfaces/the ink delaminates from the clear lamination layer.

Ink 1 of Table 1 passed this test and good lamination properties were achieved. The required jetting properties can also be achieved.

Claims (15)

1. Claims 1. An inkjet ink comprising: (a) a liquid medium comprising a radiation-curable monomer; and (b) 0.5 to 15.0% by weight, based on the total weight of the ink, of a vinyl resin dissolved in the liquid medium, wherein the ink has a viscosity of 30 to 100 mPas at 40°C.
2. 2. An inkjet ink as claimed in claim 1, wherein the ink comprises 20 to 90% by weight of radiation-curable monomer, based on the total weight of the ink.
3. 3. An inkjet ink as claimed in claims 1 or 2, wherein the radiation-curable monomer comprises one or more monofunctional monomers, preferably wherein the one or more monofunctional monomers comprises monofunctional (meth)acrylate monomer and/or N-vinyl amide monomer.
4. 4. An inkjet ink as claimed in claim 3, wherein the ink comprises 20 to 90% by weight of monofunctional (meth)acrylate monomer, based on the total weight of the ink.
5. 5. An inkjet ink as claimed in claims 3 and 4, wherein the ink comprises 10 to 30% by weight of N-vinyl amide monomer, based on the total weight of the ink.
6. 6. An inkjet ink as claimed in any preceding claim, wherein the radiation-curable monomer comprises at least one of N-vinyl caprolactam, N-acryloylmorpholine, cyclic TMP formal acrylate, tetrahydrofurfuryl acrylate, (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (Medol 10), isobornyl acrylate, 2-(2-vinyloxy ethoxy)ethyl acrylate, 2-(2-vinyloxy ethoxy)ethyl methacrylate, and combinations thereof.
7. 7. An inkjet ink as claimed in any preceding claim, wherein the liquid medium comprises one or more difuncfional monomers, preferably in 1 to 10% by weight, based on the total weight of the ink.
8. 8. An inkjet ink as claimed in any preceding claim, wherein the vinyl resin is present in 5.0-12.0% by weight, based on the total weight of the ink.
9. 9. An inkjet ink as claimed in any preceding claim, wherein the vinyl resin has a weight-average molecular weight of 20 to 200 KDa.
10. 10. An inkjet ink as claimed in any preceding claim, wherein the vinyl resin is a solid at 25°C.
11. 11. An inkjet ink as claimed in any preceding claim, wherein the vinyl resin is selected from a poly(vinyl chloride/vinyl acetate/unsaturated dicarboxylic acid ester) terpolymer, a poly(vinyl chloride/vinyl acetate/vinyl alcohol) terpolymer, a poly(vinyl chloride/hydroxy acrylate) copolymer and combinations thereof.
12. 12. An inkjet ink as claimed in any preceding claim, wherein the ink comprises a colouring agent, preferably a solid pigment in an additional vinyl resin, which may be the same or different to the vinyl resin dissolved in the liquid medium of the ink.
13. 13. A method of preparing a vinyl file, plank or sheet, comprising the following steps, in order: (i) providing an opaque vinyl substrate; (ii) jetting the inkjet ink as claimed in any preceding claim on to the surface of the substrate to form an image; (iii) curing the ink; (iv) 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.
14. 14. A method as claimed in claim 13, wherein the opaque substrate and clear vinyl layer are composed of PVC.
15. 15. A vinyl tile, plank or sheet obtainable by the method as claimed in claims 13 or 14.