GB2545084A

GB2545084A - A printing ink

Info

Publication number: GB2545084A
Application number: GB1618914.4A
Authority: GB
Inventors: Caras-Quintero Dolores; Phillips Andrew; Gould Nigel; Kite Matthew
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

An inkjet ink comprising a liquid medium composed of a radiation-curable diluent containing a difunctional monomer having one (meth)acrylate group and one vinyl ether group, optionally one or more monofunctional monomers and optionally a solvent; a vinyl resin dissolved in the liquid medium; a dispersed pigment and a photoinitiator. The difunctional monomer is preferably 2-(2-vinyloxyethoxy)ethyl acrylate (VEEA). The vinyl resin may be a solid at room temperature and may be a poly(vinyl chloride/vinyl acetate) copolymer, poly(vinyl chloride/vinyl acetate/hydroxyl acrylate) copolymer or a poly(vinyl chloride/vinyl acetate/unsaturated dicarboxylic acid ester) copolymer. Preferably, the one or more monofunctional monomers are N-vinyl caprolactam, N-acryloylmorpholine, cyclic formal acrylate, tetrahydrofurfuryl acrylate, (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate or mixtures thereof. Also disclosed is a method of preparing a vinyl tile, plank or sheet, comprising providing an opaque vinyl substrate; jetting the inkjet ink onto the surface of the substrate to form an image; curing the ink and, when a solvent is present, drying the ink; applying a clear vinyl layer over the image; and applying heat and/or pressure to the substrate to form the vinyl tile, plank or sheet. The vinyl tiles, planks or sheets are typically used for flooring applications.

Description

A printing ink

This invention relates to a printing ink and in particular to an ink that is suitable for preparing 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 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. However, more versatile techniques which are susceptible to digital printing, such as inkjet printing, do not provide inks which are sufficiently robust to meet the stringent specification for vinyl tile/plank/sheeting applications. There remains a need in the art for an ink which meets the stringent specification for vinyl tile/plank/sheeting applications, yet can be applied by inkjet printing.

Accordingly, the present invention provides an inkjet ink comprising: (i) a liquid medium composed of a radiation-curable diluent containing a difunctional monomer having one (meth)acrylate group and one vinyl ether group, optionally one or more monofunctional monomers and optionally a solvent; (ii) a vinyl resin dissolved in the liquid medium; (iii) a dispersed pigment; and (iv) a photoinitiator.

Thus, the present invention provides an ink simultaneously meeting the specification for vinyl tile/plank/sheeting applications, whilst being susceptible to inkjet printing.

The inkjet ink of the present invention contains a vinyl resin. 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 WackerChemie AG.

The ink 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 solvent and a hybrid inkjet ink contains solvent and a radiation-curable diluent. This defines the liquid medium of the ink.

When present, the solvent (i.e. an 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 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 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 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 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 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 contains 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 molecular weight is preferably less than 400. The 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: / W-ν ' ; COO ^ O ''

This diluent optionally contains one or more monofunctional monomers. By “monofunctional” is meant that the monomers have only one functional group which takes part in a polymerisation reaction during curing. A preferred example is a (meth)acrylate monomer and/or an N-vinyl amide.

The difunctional (meth)acrylate/vinyl ether monomer and, when present, the monofunctional monomers preferably have a combined glass transition temperature (Tg) of 20-100°C, more preferably 40-80°C. That is, the Tg of the cured difunctional (meth)acrylate/vinyl ether monomer and optimally monofunctional monomers when cured together to form a single polymer film must be within this range. The measurement is performed with the monofunctional monomers solely in the presence of a photoinitiator. Thus, the other components of the ink are not included when measuring the Tg of the monomers. The Tg may be measured by DSC with a heating ramp of 10°C/min.

The ink is preferably formulated so that the vinyl resin is dissolved in the liquid medium of the ink medium. To this end, the ink typically contains from 20 to 80% by weight of the difunctional (meth)acrylate/vinyl ether monomer, more preferably 30 to 60% by weight, based on the total weight of the ink. The ink also contains 0 to 70% by weight of monofunctional monomer which is capable of dissolving the vinyl copolymer resin, more preferably 10 to 60% by weight, based on the total weight of the ink. This range is particularly suitable for solvent-free inks. Examples of suitable monomers include N-vinyl caprolactam, N-acryloylmorpholine, cyclic formal acrylate, tetrahydrofurfuryl acrylate (including the low-THF-alcohol grade available as Viscoat 150 from Osaka Organic Chemical Co), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (Medol 10), and mixtures thereof. 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 may additionally contain monofunctional monomers with low solubilising 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.

Other multifunctional monomers (i.e. other than the difunctional (meth)acrylate/vinyl ether monomer) 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 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 4 to 10% by weight, of the ink.

The ink-jet ink of the present invention also includes a colouring agent, which may be either dissolved or dispersed in the liquid medium of the ink. Preferably the colouring agent is a dispersible pigment, of the types known in the art and commercially available such as, 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 monofunctional 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 ink of the present invention is suitable for application by ink-jet printing. 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 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 is formulated particularly for preparing vinyl tiles planks or sheets.

In this regard, the present invention further 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 as defined herein on to the surface of the substrate to form an image; (iii) curing the ink and, when a solvent is present, drying the ink; (iv) applying a clear vinyl layer over the image; and (v) applying heat and/or pressure to the substrate to form the vinyl tile, plank or sheet.

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.

In step (i), an opaque vinyl substrate is provided. 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.

In steps (ii) and (iii), the ink is jetted onto the substrate to form the required image and cured by exposure to actinic radiation. When a solvent is present, the substrate is solvent is allowed to evaporate, usually by heating the ink.

The ink of the present invention is 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, in step (iv), a clear vinyl layer is applied over the image. The clear vinyl layer is preferably PVC. Preferably the opaque substrate and clear vinyl layer are both composed of PVC.

In step (v), the laminate tile/plank/sheet is treated with heat and/or pressure, 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

Four inks containing resins were prepared. The inks had formulations as shown in Tables 1-4.

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

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

Table 3. Formulation of ink 3 (not of the invention)

Table 4. Formulation of ink 4 (not 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. Vinnol® E15/H45 is an unfunctionalised emulsion polymerised vinyl chloride/vinyl acetate copolymer (85:15 by weight) having a Mw of 45-55 KDa. Dianal BR113 is an acrylic copolymer resin with a Mw of 30KDa. Microlith blue 4GK contains 50% PVC copolymer resin and 50% Pigment Blue 15.3. The cyan dispersion contains 20.00 wt% DISPERBYK 168, 50.00 wt% RAPI-CURE DVE3 and 30.00 wt% HELIOGEN BLUE D 7110 F. Irgacure 819/2959/184, BAPO and Speedcure ITX are photoinitiators. BYK 331 and BYK307 are surfactants.

The inks were 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.

Each of the above ink formulations was coated on to a vinyl PVC/titanium dioxide opaque tile substrate using a K 2 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, 2 passes at 25 m/min providing a total cure dose of approximately 900 mJ/cm2.

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

Example 2

The peel strength of the tiles prepared using inks 1-4 were measured by a 180 degree peel test using an Instron 5544 test unit. The tiles using inks 1 and 2 had a peel strength above the regulation value of 10 N/cm whereas the tiles using inks 3 and 4 had a peel strength below 10 N/cm.

Claims

1. An inkjet ink comprising: (i) a liquid medium composed of a radiation-curable diluent containing a difunctional monomer having one (meth)acrylate group and one vinyl ether group, optionally one or more monofunctional monomers and optionally a solvent; (ii) a vinyl resin dissolved in the liquid medium; (iii) a dispersed pigment; and (iv) a photoinitiator.

2. An inkjet ink as claimed in claim 1, wherein the difunctional monomer is a vinyloxyalkyl (meth)acrylate.

3. An inkjet ink as claimed in claim 2, wherein the vinyloxyalkyl (meth)acrylate is 2-(2-vinyloxy ethoxy)ethyl acrylate.

4. An inkjet ink 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.

5. An inkjet ink as claimed in any preceding claim, wherein the resin has a weight-average molecular weight of 20-200 KDa.

6. An inkjet ink as claimed in any preceding claim, wherein the resin is a solid at 25°C.

7. An inkjet ink as claimed in any preceding claim, wherein the resin is a poly(vinyl chloride/vinyl acetate) copolymer.

8. An inkjet ink as claimed in any preceding claim, wherein the resin is a poly(vinyl chloride/vinyl acetate/hydroxyl acrylate) copolymer.

9. An inkjet ink as claimed in any of claims 1 to 6, wherein the resin is a poly(vinyl chloride/vinyl acetate/unsaturated dicarboxylic acid ester) copolymer.

10. An inkjet ink as claimed in any preceding claim, wherein the difunctional monomer and, when present the one or more monofunctional monomers, have a combined glass transition temperature (Tg), when cured solely in the presence of a photoinitiator, of 20-100°C.

11. An inkjet ink as claimed in any preceding claim, wherein the one or more monofunctional monomers are present and include N-vinyl caprolactam, N-acryloylmorpholine, cyclic formal acrylate, tetrahydrofurfuryl acrylate, (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate, or mixtures thereof.

12. An inkjet ink 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 inkjet ink.

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

14. 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 as claimed in any preceding claim on to the surface of the substrate to form an image; (iii) curing the ink and, when a solvent is present, drying the ink; (iv) applying a clear vinyl layer over the image; and (v) applying heat and/or pressure to the substrate to form the vinyl tile, plank or sheet.

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