GB2551810A

GB2551810A - A method of printing

Info

Publication number: GB2551810A
Application number: GB1611463.9A
Authority: GB
Inventors: Gould Nigel; Caras-Quintero Dolores
Original assignee: Fujifilm Speciality Ink Systems Ltd
Current assignee: Fujifilm Speciality Ink Systems Ltd
Priority date: 2016-06-30
Filing date: 2016-06-30
Publication date: 2018-01-03
Also published as: GB201611463D0

Abstract

A method of preparing a vinyl tile, plank or sheet, comprises: providing an opaque vinyl substrate; jetting an inkjet ink; curing the inkjet ink; applying a clear inkover the inkjet ink; curing the clear ink; applying a clear vinyl layer over the clear ink and applying heat and/or pressure to the substrate to form the vinyl, tile, plank, or sheet. The inkjet ink includes one or more monofunctional monomers, a passive resin dissolved in the one or more monofunctional monomers, a dispersed pigment and a photoinitiator and the clear ink includes one or more monofunctional monomers including tetrahydrofurfuryl acrylate, a vinyl resin dissolved in the one or more monofunctional monomers and a photoinitiator, wherein the clear ink contains less than 2% by weight of hyperdispersant, based on the total weight of the ink. An inkjet ink and a clear ink having the above compositions are also disclosed.

Description

(54) Title of the Invention: A method of printing

Abstract Title: Preparing vinyl substrate with inkjet ink and clear vinyl layer.

(57) A method of preparing a vinyl tile, plank or sheet, comprises: providing an opaque vinyl substrate; jetting an inkjet ink; curing the inkjet ink; applying a clear inkover the inkjet ink; curing the clear ink; applying a clear vinyl layer over the clear ink and applying heat and/or pressure to the substrate to form the vinyl, tile, plank, or sheet. The inkjet ink includes one or more monofunctional monomers, a passive resin dissolved in the one or more monofunctional monomers, a dispersed pigment and a photoinitiator and the clear ink includes one or more monofunctional monomers including tetrahydrofurfuryl acrylate, a vinyl resin dissolved in the one or more monofunctional monomers and a photoinitiator, wherein the clear ink contains less than 2% by weight of hyperdispersant, based on the total weight of the ink. An inkjet ink and a clear ink having the above compositions are also disclosed.

A method of printing

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

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

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

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

There is a desire in the art to have more control over the image formation and to increase the productivity of the process using more versatile techniques which are susceptible to digital printing, such as inkjet printing. To meet the production speed requirements for inkjet printing, the method of inkjet printing needs to be “single pass” where the inkjet printhead moves relative to the substrate and the whole image is formed in a single pass. Where only part of the image is formed and further passes of the printhead are required, the technique is termed “multi-pass” printing. Single-pass printing places greater demands on the ink than multi-pass printing in terms of jetting reliability and sustainability of the inks as blocked or deviated nozzles in the printhead are more obvious in the printed image formed using single-pass printing than multi-pass printing. Inkjet inks with the required jetting reliability are known in the art. Radiation-curable inks are particularly reliable as these inks contain no volatile species that can evaporate and lead to clogging of the printhead nozzles.

Inks that are suitable for vinyl tile/plank/sheet 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. Inkjet inks with the required lamination bond strength are known in the art. In order to achieve the required lamination bond strength, the ink components need to be carefully selected, such as a monomer with a strong solvency for the substrate, a vinyl copolymer resin for film cohesion and the amount of pigment hyperdispersant needs to be minimised.

However, it has proven difficult to formulate a pigmented radiation-curable inkjet ink with the required jetting reliability, printed image quality and lamination bond strength for vinyl tile/plank/sheet applications.

Hyperdispersants are typically present in a final pigmented ink composition at about 2-3% by weight based on the total weight of the ink. Whilst this is not problematic when the inks are being used for standard graphic printing, 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. To minimise the amount of pigment hyperdispersant that is present in such inks, solid vinyl chip pigments have been investigated as an alternative to conventional pigments. However, whilst these solid vinyl chip pigments are commonly used in solvent-based inkjet inks, their use is not known in radiation-curable inkjet inks. In fact, solid vinyl chip pigments are poorly soluble in radiation-curable monomers, leading to poor ink viscosity stability.

There therefore remains a need in the art for a method of preparing a vinyl tile/plank/sheet with the required jetting reliability, printed image quality and lamination bond strength in which pigmented radiation-curable inkjet inks can be used.

Accordingly, the present invention provides a method of preparing a vinyl tile, plank or sheet, comprising the following steps, in order: providing an opaque vinyl substrate; jetting an inkjet ink comprising one or more monofunctional monomers, a passive resin dissolved in the one or more monofunctional monomers, a dispersed pigment and a photoinitiator; curing the inkjet ink; applying a clear ink over the inkjet ink, wherein the clear ink comprises one or more monofunctional monomers including tetrahydrofurfuryl acrylate, a vinyl resin dissolved in the one or more monofunctional monomers and a photoinitiator, wherein the clear ink contains less than 2% by weight of hyperdispersant, based on the total weight of the ink; curing the clear ink; applying a clear vinyl layer over the clear ink; and applying heat and/or pressure to the substrate to form the vinyl, tile, plank, or sheet.

The present invention further provides two ink compositions for use in the method. The first is an inkjet ink comprising one or more monofunctional monomers, a passive resin dissolved in the one or more monofunctional monomers, a dispersed pigment and a photo initiator, wherein the passive resin is not a vinyl resin. The second is a clear ink comprising one or more monofunctional monomers including tetrahydrofurfuryl acrylate, a vinyl resin dissolved in the one or more monofunctional monomers and a photoinitiator, wherein the clear ink contains less than 2% by weight of hyperdispersant, based on the total weight of the ink.

Thus, the present invention provides a method of preparing a vinyl tile/plank/sheet in which pigmented radiation-curable inks can be used whilst still meeting the requirements of jetting reliability, printed image quality and lamination bond strength, and inks which can be used in such a method.

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

The inventors have found that unpigmented radiation-curable inks based on vinyl 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 of adsorbing on to the surface of the dispersed pigment particles in an 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.

The inventors have found that applying a clear ink wherein the clear ink contains a vinyl resin and less than 2% by weight of hyperdispersant, based on the total weight of the ink, over a pigmented ink that contains a dispersed pigment, can restore excellent lamination bond strength. The pigmented ink and the clear ink contribute different required properties to the vinyl tile/plank/sheet. In this regard, the pigmented ink, whilst not having excellent lamination bond strength owing to the presence of a dispersed pigment, provides the image, good adhesion to the substrate and good film cohesion. The clear ink, which contains a vinyl resin but less than 2% by weight of hyperdispersant, based on the total weight of the ink, provides excellent lamination bond strength. Therefore, the ink formulator has greater flexibility when preparing such a pigmented ink - they can select from a wider range of dispersed pigments and resins without having to worry about lamination bond strength. Similarly, the ink formulator has greater flexibility when preparing such a clear ink - they do not have to worry about the effect of hyperdispersants on lamination bond strength and can instead focus on preparing inks with good application characteristics and viscosity stability.

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 an inkjet ink comprising one or more monofunctional monomers, a passive resin dissolved in the one or more monofunctional monomers, a dispersed pigment and a photoinitiator.

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

The inkjet ink of the present invention, which is jetted in step (ii) of the method of the present invention, comprises one or more monofunctional monomers, a passive resin dissolved in the one or more monofunctional monomers, a dispersed pigment and a photoinitiator.

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

The inkjet ink of the present invention, which is jetted in step (ii) of the method of the present invention, is a radiation-curable inkjet ink (often termed a “UV ink”). By “radiation-curable” is meant a material that polymerises or crosslinks when exposed to actinic radiation, commonly ultraviolet light, in the presence of a photoinitiator. The ink does not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink. Thus, a radiation-curable inkjet ink is typically free of volatile organic solvent and water. Preferably, the inkjet ink comprises less than 5% by weight of volatile organic solvent and water combined, preferably less than 3% by weight combined, more preferably, less than 2% by weight combined and most preferably less than 1% by weight combined, based on the total weight of the inkjet ink. 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.

The inkjet ink of the present invention, which is jetted in step (ii) of the method of the present invention, 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.

The one or more monofunctional monomers must be capable of dissolving the passive resin of the inkjet ink. To this end, the inkjet ink preferably contains 60-95% by weight of the one or more monofunctional monomers, more preferably 70-90% by weight of the one or more monofunctional monomers, based on the total weight of the ink. Examples of suitable monomers include N-vinyl caprolactam, N-acryloylmorpholine, cyclic TMP formal acrylate, tetrahydrofurfuryl acrylate, (2-methyl2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (Medol 10), isobornyl acrylate and mixtures thereof. In a preferred embodiment, the one or more monofunctional monomers are (meth)acrylate monomers in combination with an N-vinyl amide.

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.

The inkjet ink may additionally contain monofunctional monomers with low solubilising power for the passive resin, such as phenoxyethyl acrylate, ethoxylated phenoxyethyl acrylate, 2-(2ethoxyethoxy)ethyl acrylate, tertiary-butyl cyclohexyl acrylate, 3,3,5-trimethyl cyclohexyl acrylate, isophoryl acrylate, isodecyl acrylate, octyl/decyl acrylate, tridecyl acrylate and mixtures thereof.

Multifunctional monomer is 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 inkjet ink. The multifunctional monomer which is limited in amount may be any multifunctional monomer which could be involved in the curing reaction, such as a multifunctional (meth)acrylate monomer or a multifunctional vinyl ether.

Examples of the 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 inkjet ink of the present invention, which is jetted in step (ii) of the method of the present invention, contains a passive resin dissolved in the one or more monofunctional monomers. The resin is 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 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 passive 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 one or more monofunctional monomers of the inkjet ink. Preferably, the passive resin is not a vinyl resin. Whilst vinyl resins yield cured inkjet ink films with excellent lamination bond strength, these inkjet inks often suffer from poor jetting performance and therefore, excluding vinyl resins from the list of suitable passive resins, gives the ink formulator more flexibility. Thus, the inkjet ink formulator is able to select from a wide range of suitable passive resins. Examples of suitable 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 passive resin preferably has a weight-average molecular weight of 3,000 Da or above, as determined by gel permeation chromatography (GPC) with polystyrene standards. The weightaverage molecular weight is preferably below 200,000 Da. In a particularly preferred embodiment, the passive resin has a weight-average molecular weight of 5,000 to 60,000 Da. Preferably, the 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 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 Dianal BR 113 from Dianal America, Inc., an acrylic copolymer with a weight-average molecular weight of 30 kDa and a Tg of 78°C.

The inkjet ink of the present invention, which is jetted in step (ii) of the present invention, may also contain a radiation-curable 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. Thus, the ink formulator is able to select from a wide range of suitable oligomers.

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 weight-average molecular weight of at least 600 Da. The weight-average molecular weight is preferably below 3,000 Da. Molecular weights (weight-average) can be calculated if the structure of the oligomer is known or molecular weights can be measured using gel permeation chromatography (GPC) 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. The oligomer is preferably multifunctional meaning that it contains on average more than one reactive functional group per molecule. The average degree of functionality is preferably from 2 to 6.

Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups. The oligomer preferably comprises a urethane backbone. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. Preferably the oligomers are (meth)acrylate oligomers.

Particularly preferred radiation-curable oligomers are urethane acrylate oligomers as these have excellent adhesion and elongation properties. Most preferred are di-, tri-, tetra-, penta- or hexafunctional urethane acrylates, particularly difunctional urethane acrylate or hexafunctional urethane acrylates as these yield films with good solvent resistance.

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.

In one embodiment, the radiation-curable oligomer polymerises by free-radical polymerisation.

The radiation-curable oligomer cures upon exposure to radiation in the presence of a photoinitiator to form a crosslinked, solid film.

The inkjet ink may comprise 1 to 12 wt%, preferably 2 to 5 wt%, of a radiation-curable oligomer, based on the total weight of the inkjet ink.

The inkjet ink of the present invention, which is jetted in step (ii) of the method of the present invention, contains a dispersed pigment. Dispersed pigments are 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.

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 weight of the inkjet ink.

The inkjet ink of the present invention, which is jetted in step (ii) of the method of the present invention, contains a photoinitiator which under irradiation, for example using ultraviolet light, initiates the polymerisation of the radiation-curable material. Preferred are photoinitiators which produce free radicals on irradiation (free radical photoinitiators) such as, for example, benzophenone, 1hydroxycyclohexyl 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 Lucirin (from BASF). The inkjet 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 inkjet ink.

Other components of types known in the art may be present in the inkjet 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 inkjet 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 inkjet ink.

The components of the inkjet ink of the present invention, which is jetted in step (ii) of the method of the present invention, are selected so that the ink provides good adhesion to the substrate and good film cohesion - two essential requirements in the preparation of vinyl tiles/planks/sheets.

The present invention also provides an inkjet ink, which can be used in step (ii) of the method of the present invention, comprising one or more monofunctional monomers, a passive resin dissolved in the one or more monofunctional monomers, a dispersed pigment and a photoinitiator, wherein the passive resin is not a vinyl resin.

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

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

The method of the present invention includes step (iv), applying a clear ink over the inkjet ink, wherein the clear ink comprises one or more monofunctional monomers including tetrahydrofurfuryl acrylate, a vinyl resin dissolved in the one or more monofunctional monomers and a photoinitiator, wherein the clear ink contains less than 2% by weight of hyperdispersant, based on the total weight of the ink.

The clear ink can be applied using inkjet printing, flexography, gravure printing, rotary screen printing or roller coat printing. Inks containing vinyl resins often have poor jetting reliability and therefore, the clear ink is preferably applied using flexography, gravure printing, rotary screen printing or roller coat printing. In a preferred embodiment, the clear ink is applied using roller coat printing.

The clear ink of the present invention, which is jetted in step (iv) of the method of the present invention, comprises one or more monofunctional monomers including tetrahydrofurfuryl acrylate, a vinyl resin dissolved in the one or more monofunctional monomers and a photoinitiator, wherein the clear ink contains less than 2% by weight of hyperdispersant, based on the total weight of the ink.

Clear is a term in the art and by “clear” is meant colourless and that the ink contains no colouring agent. The clear ink contains no pigment and therefore, the clear ink is typically free of hyperdispersant. This ensures excellent lamination bond strength as discussed hereinabove. Accordingly, the clear ink contains less than 2% by weight of hyperdispersant, based on the total weight of the ink. Preferably, the clear ink contains less than 1% by weight of hyperdispersant, based on the total weight of the ink, and more preferably, the clear ink is substantially free, ideally free, of hyperdispersant.

The clear ink of the present invention, which is applied in step (iv) of the method of the present invention, comprises one or more monofunctional monomers including tetrahydrofurfuryl acrylate (THFA).

The one or more monofunctional monomers including THFA must be capable of dissolving the vinyl resin of the clear ink. To this end, the clear ink preferably contains 60-95% by weight of one or more monofunctional monomers including THFA, more preferably 70-90% by weight of one or more monofunctional monomers including THFA, based on the total weight of the clear ink.

The one or more monofunctional monomers of the clear ink are selected in order to provide a cured ink which can withstand the bonding process. The monofunctional monomers present preferably have a combined glass transition temperature (Tg) of 20-100°C, more preferably 40-80°C. That is, the Tg of the cured 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 clear 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.

THFA is an essential monofunctional monomer of the clear ink. THFA has a high solvency power and therefore, it ensures good adhesion to the underlying inkjet ink and it is capable of dissolving the vinyl resin. In a preferred embodiment, the clear ink contains 50-80% by weight of THFA, preferably 6070% by weight, based on the total weight of the clear ink. Examples of suitable monomers in addition to THFA for use in the clear ink include N-vinyl caprolactam, N-acryloylmorpholine, cyclic TMP formal acrylate, (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (Medol 10), isobornyl acrylate and mixtures thereof. In a preferred embodiment, the one or more monofunctional monomers including THFA are (meth)acrylate monomers in combination with an N-vinyl amide.

A suitable test for measuring the solubility of the vinyl resin in a monomer is as described above for the passive resin of the inkjet ink.

The clear ink may additionally contain monofunctional monomers with low solubilising power for the vinyl resin, such as phenoxyethyl acrylate, ethoxylated phenoxyethyl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate, tertiary-butyl cyclohexyl acrylate, 3,3,5-trimethyl cyclohexyl acrylate, isophoryl acrylate, isodecyl acrylate, octyl/decyl acrylate, tridecyl acrylate and mixtures thereof.

Multifunctional monomer is 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 clear ink. The multifunctional monomer which is limited in amount may be any multifunctional monomer which could be involved in the curing reaction, such as a multifunctional (meth)acrylate monomer or a multifunctional vinyl ether. In a preferred embodiment, the clear ink is substantially free, ideally free, from multifunctional (meth)acrylate monomers.

Examples of the multifunctional acrylate monomers whose level is minimised in the clear ink are as described for the inkjet ink.

The amount of radiation-curable oligomers in the clear ink is 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 clear ink. In a preferred embodiment, the clear ink is substantially free, ideally free, from multifunctional (meth)acrylate oligomers.

The clear ink of the present invention, which is applied in step (iv) of the method of the present invention, comprises a vinyl resin dissolved in the one or more monofunctional monomers. The vinyl resin ensures that the clear ink film has good cohesion and good lamination bond strength. The vinyl resin is typically present at 1-12% weight, preferably 2-10% by weight, based on the total weight of the clear 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 vinyl 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 vinyl resin is preferably a poly(vinyl chloride) copolymer, more preferably a poly(vinyl chloride/vinyl acetate) copolymer. The vinyl 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 inkjet ink, they do not take part in the ink curing reaction and hence are still passive resins within the meaning of the present invention.

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 1030% 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 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 vinyl resins are commercially available as Vinnol® from Wacker Chemie AG.

The clear ink of the present invention, which is applied in step (iv) of the method of the present invention, contains a photoinitiator which under irradiation, for example using ultraviolet light, initiates the polymerisation of the radiation-curable material. Preferable photoinitiators and amounts of photoinitiator of the clear ink are discussed hereinabove with reference to the inkjet ink.

Other components of types known in the art may be present in the clear 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 inkjet 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 clear ink.

The components of the clear ink of the present invention, which is applied in step (iv) of the method of the present invention, are selected so that the ink provides good adhesion to the underlying inkjet ink and excellent lamination bond strength.

The present invention also provides a clear ink, which can be used in step (iv) of the method of the present invention, consisting of one or more monofunctional monomers including tetrahydrofurfuryl acrylate, a vinyl resin dissolved in the one or more monofunctional monomers, a photoinitiator and optionally one or more of a surfactant, a defoamer, a reodorant, a flow or slip aid, a biocide and an identifying tracer.

The method of the present invention includes step (v), curing the clear ink. Curing the clear ink is as described hereinabove with reference to the inkjet ink.

The method of the present invention includes step (vi), applying a clear vinyl layer over the clear ink.

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

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

Examples

Example 1

A pigmented ink was prepared. The ink had the formulation as shown in Table 1.

Table 1. Formulation of pigmented ink 1

Component	Weight %
2-Phenoxyethyl acrylate	28.78
Cyclic TMP formal acrylate	22.90
Isobornyl acrylate	11.90
N-Vinylcaprolactam	16.50
Florstab UV-12	0.27
Dianal BR113	2.40
Genomer 5695	2.00
Cyan pigment dispersion	8.30
Irgacure 184	2.85
Lucirin TPO	2.00
Irgacure 819	2.00
BYK-307	0.10

2-Phenoxyethyl acrylate, cyclic TMP formal acrylate, isobornyl acrylate and N-vinylcaprolactam are monofunctional monomers. Florstab UV-12 is a stabiliser. Dianal BR113 is an acrylic copolymer resin with a Mw of 30 kDa and a Tg of 78°C. Genomer 5695 is an amine-modified urethane acrylate oligomer. The cyan pigment dispersion contains 20.00 wt% DISPERBYK 168, 50.00 wt% RAPICURE DVE3 and 30.00 wt% HELIOGEN BLUE D 7110 F. Irgacure 184, Lucirin TPO and Irgacure 819 are photoinitiators. BYK-307 is a surfactant.

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

Example 2

A clear ink was prepared. The ink had the formulation as shown in Table 2.

Table 2. Formulation of clear ink 2

Component	Weight %
N-Vinylcaprolactam	20.00
Tetrahydrofurfuryl acrylate	67.40
Florstab UV-12	0.50
Vinnol® H14/36	6.00
Irgacure 819	4.00
Irgacure 2959	2.00
BYK-331	0.10

N-Vinylcaprolactam and tetrahydrofurfuryl acrylate are monofunctional monomers. Florstab UV-12 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. Irgacure 819 and Irgacure 2959 are photoinitiators. BYK-331 is a surfactant.

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

Example 3

Pigmented ink 1 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 pigmented 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.

Example 4

A second ink film was prepared as in Example 3 but the cured pigmented ink film was further coated with clear ink 2 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 clear 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.

Example 5

The lamination bond strengths of the laminated ink films of Examples 3 and 4 were measured using an Instron 5544 test unit. The results are set out in Table 3. The target pass value was 10 N/cm.

Table 3. Lamination bond strengths

Laminated ink film	Lamination bond strength (N/cm)	Pass/fail
Example 3	< 10 N/cm	Fail
Example 4	> 10 N/cm	Pass

Example 3, containing only the pigmented ink film, fails the lamination bond strength test whereas Example 4, containing both the pigmented ink film and the clear ink film, passes the lamination bond strength test.

Claims

1. A method of preparing a vinyl tile, plank or sheet, comprising the following steps, in order:

(i) providing an opaque vinyl substrate;

(ii) jetting an inkjet ink comprising one or more monofunctional monomers, a passive resin dissolved in the one or more monofunctional monomers, a dispersed pigment and a photoinitiator;

(iii) curing the inkjet ink;

(iv) applying a clear ink over the inkjet ink, wherein the clear ink comprises one or more monofunctional monomers including tetrahydrofurfuryl acrylate, a vinyl resin dissolved in the one or more monofunctional monomers and a photoinitiator, wherein the clear ink contains less than 2% by weight of hyperdispersant, based on the total weight of the ink;

(v) curing the clear ink;

(vi) applying a clear vinyl layer over the clear ink; and (vii) applying heat and/or pressure to the substrate to form the vinyl, tile, plank, or sheet.

2. A method as claimed in claim 1, wherein the passive resin is a (meth)acrylic copolymer.

3. A method as claimed in claims 1 or 2, wherein the clear ink is substantially free of hyperdispersant.

4. A method as claimed in any preceding claim, wherein the clear ink is applied using inkjet printing, flexography, gravure printing, rotary screen printing or roller coat printing.

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

6. A vinyl tile, plank or sheet obtainable by the method as claimed in any preceding claim.

7. An inkjet ink comprising one or more monofunctional monomers, a passive resin dissolved in the one or more monofunctional monomers, a dispersed pigment and a photoinitiator, wherein the passive resin is not a vinyl resin.

8. An inkjet ink as claimed in claim 7, wherein the passive resin is a (meth)acrylic copolymer.

9. A clear ink comprising one or more monofunctional monomers including tetrahydrofurfuryl acrylate, a vinyl resin dissolved in the one or more monofunctional monomers and a photoinitiator, wherein the clear ink contains less than 2% by weight of hyperdispersant, based on the total weight of the ink.

10. A clear ink as claimed in claim 9, wherein the clear ink is substantially free of hyperdispersant.

11. A clear ink consisting of one or more monofunctional monomers including tetrahydrofurfuryl acrylate, a vinyl resin dissolved in the one or more monofunctional monomers, a photoinitiator and optionally one or more of a surfactant, a defoamer, a reodorant, a flow or slip aid, a biocide and an identifying tracer.

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12. A clear ink as claimed in any one of claims 9-11, wherein the ink contains 60-95% by weight of one or more monofunctional monomers including tetrahydrofurfuryl acrylate, based on the total weight of the ink.

13. A clear ink as claimed in any one of claims 9-12, wherein the ink contains 50-80% by weight of tetrahydrofurfuryl acrylate, based on the total weight of the ink.

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14. A clear ink as claimed in any one of claims 9-13, wherein the ink contains 1-10% by weight of vinyl resin, based on the total weight of the ink.

15. A clear ink as claimed in any one of claims 9-14, 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 and a poly(vinyl chloride/hydroxy acrylate) copolymer.

Intellectual

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Office

Application No: GB1611463.9 Examiner: Dr David Palmer