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 a solvent, a vinyl and/or polyester resin dissolved in the solvent and a dispersed pigment, wherein the ink contains no more than 10% by weight of hyperdispersant, based on the total weight of the ink absent the solvent.

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 and/or polyester resin. The ink preferably contains a vinyl or polyester resin, and most preferably a vinyl resin. The resin is typically present at 0.5 to 5.0% by weight, preferablyl .0-4.5% by weight, based on the total weight of the inkjet ink. The resin is a passive (i.e. inert) resin, in the sense that it is not radiation curable.

The vinyl 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 vinyl resin preferably has a viscosity of 5-200 mPas at 25°C. It is preferably soluble in the liquid medium (or "phase") of the ink (i.e. the solvent).

The vinyl resin is preferably a polyvinyl chloride) copolymer, more preferably a polyvinyl chloride/vinyl acetate) copolymer. The vinyl resin may also contain hydroxy or carboxyl functionality. These vinyl resins are termed "functionalised resins". However, although they contain functional groups, principally to assist adhesion to the substrate, they are still termed "passive resins" as they do not undergo cross-linking. The vinyl resin preferably contains 60-90% by weight of vinyl chloride, based on the total composition of the vinyl 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 vinyl resin.

For the functionalised vinyl 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 vinyl 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 vinyl resin.

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

The polyester resin is typically a copolyester resin having aromatic diacids and saturated diols. Examples include terephthalic acid and ethylene glycol with other diacids such as isophthalic acid, or other diols, such as cyclohexane dimethanol. The polyester resin is preferably amorphous. It is also preferably a thermoplastic resin.

The polyester resin preferably has a weight-average molecular weight (Mw) of 30-100 KDa, more preferably 50-80 KDa. Again, the Mw may be measured by known techniques in the art, such as gel permeation chromatography (GPC), using a polystyrene standard. The polyester resin preferably has a viscosity of 200-400 Pas and more preferably 250-350 Pas, measured at 215°C. The viscosity may be measured using a Brookfield viscometer. The polyester resin may have a glass transition temperature of -50 to 100°C, more preferably 30-70°C and most preferably 40-60°C. The glass transition temperature may be measured using differential scanning calorimetry (DSC). The polyester resin preferably has an acid number of 1 -2 and a hydroxyl number of 2-5. Again, the polyester resin is preferably soluble in the liquid medium (or "phase") of the ink (i.e. the solvent). Such resins are commercially available as Vitel 2700B available from Bostik. The solvent provides the liquid medium of the ink. The solvent (also referred to in the art as a "volatile organic solvent") is in the form of a liquid at ambient temperature and is capable of acting as a carrier for the remaining components of the ink. The 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 may be selected from glycol ethers, glycol ether esters, alcohols, ketones, esters, organic carbonates, lactones and pyrrolidones. The organic solvent may be present in an amount of 60-98% by weight, more preferably 80-95% by weight and most preferably 85 to 93% 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.

The ink is preferably formulated so that the resin is dissolved in the solvent. A suitable test for measuring the solubility of the resin is as follows. The solvent, e.g. 500 g, is weighed into a suitable container. The solvent 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 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, the solution is removed from the stirrer, the container sealed with a lid and is allowed to stand for 12 hours at temperature. The solvent/resin combination is suitable for use in the invention if, after the standing period, there is no precipitation of the resin.

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-jet ink of the present invention also includes a dispersible pigment, of the types known in the art and commercially available. The pigment may be of any desired colour such as, for example, Pigment Yellow 13, Pigment Yellow 83, Pigment Red 9, Pigment Red 184, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7. Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used.

In one embodiment the dispersible pigment is in the form of a solid dispersion in the resin. Such materials are available from BASF under the trade name of Microlith®-K, or from Penn Color, Inc. under the name Copolymer vinyl chip dispersions.

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

Inks currently supplied for the printing of vinyl substrates for the production of laminated flooring and other surface coverings are solvent or water-based inks applied by gravure printing. A key feature of the inks is that they not only provide the means of decoration of the substrate but also act as the laminating adhesive to bond the printed image to a clear protective top sheet. The inventors have found that have found that a key component of an ink to yield adequate lamination bond strength is a vinyl and/or polyester resin. It has found that unpigmented solvent-based systems based on vinyl and/or polyester resins give excellent lamination bond strength. However, when pigment dispersions are added to such systems the lamination bond strength is greatly diminished and falls far below acceptable levels. Nevertheless, pigments are required to provide the image.

In order to pass through the fine nozzles of the inkjet print head, the pigment must be milled to micron-sized particles (typically having a diameter of less than 5 microns). In order to disperse a pigment of this size, and to maintain resistance to flocculation, relatively large amounts of pigment hyperdispersants are commonly used. A hyperdispersant is defined as a polymer having an anchor group capable of adsorbing on to the surface of a particle in a colloidal system and a polymeric chain providing steric stabilisation so as to hold the particles apart and prevent flocculation of the particles. The definition in the context of an inkjet ink would therefore be a polymer having an anchor group capable adsorbing on to the surface of the dispersed pigment particles in a inkjet ink and a polymeric chain providing steric stabilisation so as to hold the pigment particles apart and prevent flocculation of the pigment particles. An example is an amine anchor group and a polyester chain. The hyperdispersant may have one or more anchor groups and one or more polymeric chains. Where the hyperdispersant contains multiple anchor groups and multiple polymer chains, it can form a so-called comb-structured dispersant. So-called "comb" polymers are a subset of branched polymers formed of a main chain with two or more three-way branch points defining linear side chains, i.e. it has the appearance of a comb. When the hyperdispersant contains amine anchor groups, the hyperdispersant preferably has an amine value of 1 .7-37.5 mg KOH/g. Examples of hyperdispersants include Solsperse® hyperdispersants available from Lubrizol.

The anchor groups adsorb to the pigment particles forming a layer over the surface of the pigment particles with the polymer chains presented to the external environment (i.e. the liquid medium of the ink). The surface is thereby effectively coated with the polymer chains. As the pigment particles approach one another, the polymer chains intermingle, which is entropically unfavourable, providing a thermodynamic barrier to further attraction. Hyperdispersants are typically present in a final pigmented ink composition at about 2-3% by weight based on the total weight of the ink. Solvent-based inks commonly contain about 80-90% by weight of solvent based on the total weight of the ink, and this is evaporated to yield the dried film the nonvolatile dispersant forms a significant part of the final ink film. A typical the final film may contain, for example, 40% resin, 40% pigment and 20% hyperdispersant (percentages are by weight and based on the total weight of the ink, absent the solvent). Whilst this is not problematic when the inks are being used for standard graphic printing, the inventors have found that the presence of a relatively large proportion hyperdispersant is detrimental to the lamination bond strength. Accordingly, it is has been found important to minimise the amount of hyperdispersant in the ink when preparing laminated vinyl substrates.

Thus, the ink of the present invention contains no more than 10% by weight of hyperdispersant, more preferably no more than 5% by weight of hyperdispersant, and most preferably no more than 2% by weight of hyperdispersant, based on the total weight of the ink absent the solvent. The amount is determined in relation to the dried film, and hence the weight percentage is quoted in the absence of the solvent. In a preferred embodiment, the ink is substantially free, ideally free, of hyperdispersant.

The ink dries by evaporation of the solvent and so it does need to contain any radiation-curable material. As a consequence, there is also no requirement for a photoinitiator. Accordingly, the ink is preferably substantially free, ideally free, of both radiation-curable material and photoinitiator. 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.

Radiation-curable materials to be excluded include mono-, di- and multifunctional monomers and oligomers, such as (meth)acrylates. Photoinitiators typically produce free radicals on irradiation (free radical photoinitiators) and include 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.

Other components of types known in the art may be present in the ink to improve the properties or performance. These components include, for example, surfactants, defoamers, reodorants, flow or slip aids, biocides and identifying tracers. Surfactants assist 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 present invention also provides an inkjet ink consisting of a solvent, a vinyl and/or polyester resin dissolved in the solvent, a dispersed pigment, and optionally one or more of a surfactant, a defoamer, a reodorant, a flow or slip aid, a biocide and an identifying tracer. The present invention further provides an inkjet ink set (e.g. CMYK), wherein the inks in the set are inks as defined herein. The present invention additionally provides a container containing the ink of the present invention.

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) 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 the solvent is allowed to evaporate, usually by heating the ink.

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 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 tile/plank/sheet is identifiable as being specific to the ink/method described herein because the ink film contains finely divided pigment (on account of its application by inkjet printing) and the low level of hyperdispersant. Accordingly, the present invention also provides a vinyl tile, plank or sheet obtainable by the method defined herein.

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 Table 1 .

Table 1 . Formulation of inks 1 -4

BGA, cyclohexanone and methoxypropanol acetate are solvents. Vinnol® E15/H45 is an unfunctionalised emulsion polymerised vinyl chloride/vinyl acetate copolymer (85:15 by weight) having a Mw of 45-55 KDa.

Microlith blue 7080 KJ and Microlith black 0066 KJ are pigment dispersions containing 50 wt% P.B. 15:3 and P.B. 7, respectively, predispersed in a vinyl chloride/vinyl acetate copolymer. The cyan dispersion contains 1 .6 wt% SOLSPERSE 5000, 16.0 wt% SOLSPERSE 32000, 40.0 wt% HELIOGEN BLUE D 71 10 F and 42.4 wt% BUTYL CELLOSLVE ACETATE. The black dispersion contains 0.2 wt% SOLSPERSE 5000, 5.75 wt% EFKA PX 4700, 19.2 wt% ELFTEX 415 CARBON BLACK, 5.57 wt% VINNOL E15/45 TCE FREE, 57.73 wt% BUTYL CELLOSLVE ACETATE and 1 1 .55 wt% CYCLOHEXANONE

The inks were prepared by first weighing the solvents into a suitable mixing vessel, placing the vessel under the mixing head of a Silverson stirrer and starting the stirrer. In the case of the solid vinyl pigment based inks (Microlith) the pigment chips were gradually added into the vortex and the mixture stirred until all the pigment chips had dispersed. Once the pigment chips had dispersed the resin was gradually added into the vortex and stirred until fully dissolved.

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. In the case of the pigment dispersion inks (inks 3 and 4) a similar process was used except the resin was dissolved into the solvents and the additives and liquid pigment dispersion added once the resin fully solubilised. Example 2

Each of the above ink formulations was coated on to a vinyl PVC/titanium dioxide opaque tile substrate using a K2 applicator bar (12 μηι wet film). The resulting films were dried in an oven at 60°C for three minutes.

The dried 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.

The relative bond strengths were assessed using a 180 degree peel test on an Instron 5544 test unit. The results are set out in Table 2. The target pass value was 10 N/cm.

Table 2. Peel strengths