Plastisol ink composition

Abstract

A radiation-gellable plastisol ink composition (24) comprises PVC resin having a D90 particle size of ≤5 μm, and, per 100 parts of PVC: 60 to 130 parts of a plasticizer or a mixture of plasticizers; 140 to 225 parts of reactive (meth)acrylate monomer and / or oligomer; 3 to 18 parts of photoinitiator; 1 to 5 parts of a heat stabilizer for PVC. The plasticizer or the mixture of plasticizers, as well as the reactive polyfunctional (meth)acrylate monomer and / or oligomer or the mixture of one or more the reactive polyfunctional (meth)acrylate monomers and / or oligomers have a (dynamic) viscosity at 25°C of 20 mPa.s or less. In a further aspect, the invention relates to a printing method, wherein the radiation-gellable plastisol ink (24) composition is printed on a printing substrate (22), gelled by irradiation with UV light (28) and then heat-cured (30).

Description

PLASTISOL INK COMPOSITIONBackground of the Invention

[0001] The invention generally relates to a plastisol ink, e.g., for digital printing. More specifically, a plastisol ink composition is proposed, which is “hybrid” in the sense that it comprises a thermoplastic component and a crosslinkable component, in contrast to entirely thermoplastic compositions.

[0002] In a more specific aspect, the invention relates to the production of finishing materials for constructions, in particular of decorative surface coverings such as, for instance, floorings, wallcoverings or ceiling coverings, wherein a surface-textured coating is produced.

[0003] US 4634562 discloses a shapable and photogellable liquid or fluid polyvinyl chloride (PVC) composition comprising a blend of PVC with plasticizer and about 13 to 50 phr (weight parts per hundred weight parts of resin) of a photopolymerizable polymer to give a liquid or fluid composition. The composition may contain a photoinitiator and a PVC stabilizer. The shapable and photogellable composition can be spread on a shaping surface such as a moving belt or a mold. The composition can then be gelled by exposure to ultraviolet (UV) light and, thereafter, heat fused.

[0004] US 4929171 relates to a composite plastisol composition for coating a metal sheet. The composite plastisol composition essentially consists of PVC powder, an acrylic resin powder and an acrylic monomer and / or oligomer. The coating may be cured with ultraviolet rays after heating and embossing. The PVC powder has a specific particle size distribution with two peaks in the distribution curve: 20 to 60 wt.% of the particles have sizes in the range from 0.5 to 5 pm and 35 to 75 wt.% of the particles have sizes in the range from 5 to 40 pm.

[0005] While photogellable liquid or fluid PVC compositions have been investigated in the past, none of the described compositions meets the requirements of modem printing equipment.Summary of the Invention

[0006] According to a first aspect of the invention, a radiation-gellable plastisol ink composition is proposed. The composition comprises a PVC resin having a D90 particle size of 5 pm or less, and, per 100 parts of PVC resin:- between 60 and 130 parts of a plasticizer or a mixture of plasticizers;- between 140 and 225 parts, preferably between 160 and 210 parts, more preferably from 165 to 190 parts, of a) a reactive monofunctional or polyfunctional (meth)acrylate monomer or oligomer or b) a mixture of one or more reactive monofunctional or polyfunctional (meth)acrylate monomers and / or oligomers;- between 3 and 18 parts of photoinitiator, under the proviso that the wt. / wt. ratio of photoinitiator to the reactive monofunctional or polyfunctional (meth)acrylate monomer or oligomer or to the mixture of one or more reactive monofunctional or polyfunctional (meth)acrylate monomers and / or oligomers lies between 2% and 8%;- between 1 and 5 parts of a heat stabilizer for PVC.The plasticizer or the mixture of plasticizers, as well as the reactive (meth)acrylate monomer and / or oligomer or the mixture of one or more reactive (meth)acrylate monomers and / or oligomers have a viscosity at 25°C of 20 mPa.s or less. When the plastisol ink composition comprises a mixture of one or more reactive (meth)acrylate monomers and / or oligomers, each one of one or more reactive (meth)acrylate monomers and / or oligomers may have a viscosity at 25°C of 20 mPa.s or less. When this is not the case, i.e. , when one or more of the monomers and / or oligomers of the mixture have a higher viscosity, the proportion of these monomers and / or oligomers in the mixture relative to the other monomers and / or oligomers has to be chosen low enough to ensure that the viscosity of the mixture of reactive (meth)acrylate monomers and / or oligomers at 25°C is 20 mPa.s or less.

[0007] Viscosity may be measured with a rotational viscometer using a standardized absolute measuring geometry (measuring system), according to ISO 3219 (2021 ) such as, e.g., a coaxial cylinder (CC) system, preferably at a shear rate of 100 s-1(see, in particular, Part 2 of ISO 3219 (2021 ). When a mixture of plasticizers is used, preferably, each one of the plasticizers individually also has a viscosity at 25°C of 20 mPa.s or less. Examples of plasticizers having a viscosity at 25°C of 20 mPa.s or less include, e.g., isodecyl benzoate (e.g., Jayflex MB10 (commercial designation)), trimethyl pentanyl diisobutyrate (Eastman TXIB (commercial designation)), triethylene glycol bis(2-ethylhexanoate) (e.g., Eastman TEG-EH (commercial designation)), bis(2-ethylhexyl) sebacate (sometimes referred to as dioctyl sebacate or “DOS”), or bis(2- ethylhexyl) succinate. Combinations of the foregoing plasticizers may be used in the context of the present disclosure. Furthermore, one or more of the foregoing plasticizers may be used in combination (mixture) with one or more plasticizers having a viscosity at 25°C higher than 20 mP.s, provided that the resulting mixture of plasticizers has a viscosity at 25°C of 20 mPa.s or less.

[0008] The at least one reactive (meth)acrylate monomer and / or oligomer may have a molecular weight of at least 250 g / mol when the reactive (meth)acrylate monomer and / or oligomer comprises two methacryloyl groups. Preferably, in the plastisol ink composition, any reactive (meth)acrylate monomer and / or oligomer that comprises two methacryloyl groups and that is present in an amount of at least 5 parts per 100 parts of PVC resin has a molecular weight of at least 250 g / mol.

[0009] When reference is made to particle size, this means the particle diameter measured in accordance with ISO Standard 13320:2020 using the Mie scattering model. The D90 particle size corresponds to the 90thpercentile of the cumulative undersize distribution (weighted by volume). More generally, a “DX” particle size, where X is an integer from 1 to 99 designates the Xthpercentile of the cumulative undersize distribution (weighted by volume).

[0010] It is worthwhile noting that the radiation-gellable plastisol ink composition comprises a first component, which is a plastisol with PVC resin and plasticizer as the main ingredients, and a second component, which is a radiation-curable crosslinkable (pre)polymer composition of which the main ingredients are the reactive (meth)acrylate monomer(s) and / or oligomer(s) and the photoinitiator. In the radiation-gellable plastisol ink composition, a significant part, e.g., between 40 to 60% by volume, preferably between 45 and 55% by volume, of the ink composition is made of the radiation-curable crosslinkable (pre)polymer composition, while the balance is or comprises a plastisol consisting mainly of the PVC resin and the plasticizer.

[0011] The radiation-gellable plastisol ink composition may have a solid content in the range from 10 to 23% (vol. / vol.), preferably in the range from 15 to 21 % (vol. / vol.)

[0012] The heat stabilizer may comprise a liquid Ca / Zn stabilizer, a liquid Ba / Zn stabilizer and / or a liquid Sn stabilizer. Additionally, or alternatively, the heat stabilizer may comprise a solid stabilizer having a D90 particle size of 5 pm or less. Preferably,any solid heat stabilizer used in the composition has a D90 particle size not exceeding the D90 particle size of the PVC resin. Examples of solid stabilizers include zinc laurate, zinc stearate, calcium laurate, calcium stearate, or hydrotalcite. Calcium laurate, calcium stearate, or hydrotalcite may, preferably, be used in combination with zinc laurate or zinc stearate. Other solid heat stabilizers may be used. If necessary, the particle size of the solid heat stabilizer may be reduced through micronization.

[0013] Apart from the above-mentioned ingredients, the radiation-gellable plastisol ink composition preferably comprises at most 10 parts per 100 parts of PVC resin of (other) additives, such as, e.g., one or more of a co-stabilizer, a surfactant (surface additive, wetting agent), a processing aid, a modifying resin, or the like. Examples of costabilizers may include epoxidized soybean oil (e.g., Drapex HSE), etc. Examples of surfactants may include silicone-based surface additives (e.g., polyether-modified polydimethylsiloxane, such as, BYK 307), etc. Examples of modifying resins may include ABS (acrylonitrile butadiene styrene), MBS (methacrylate butadiene styrene), acrylic rubber, chlorinated polyethylene, EVA (ethylene-vinyl acetate), etc. Any solid additives preferably have a D90 particle size of 5 pm or less, more preferably a D90 particle size not exceeding the D90 particle size of the PVC resin.

[0014] Preferably, the PVC resin may consist of emulsion PVC (also “paste PVC” or “P-PVC”.

[0015] The photoinitiator may preferably have an absorption peak in the IIV-A wavelength range, i.e. , in the wavelength range from 315 to 400 nm.

[0016] The PVC resin may preferably have a D90 particle size of 3 pm or less, more preferably of 2 pm or less.

[0017] The radiation-gellable plastisol ink composition may preferably have a viscosity in the range from 10 to 60 mPa.s, more preferably in the range from 15 to 25 mPa.s, at the temperature of the printing (printing temperature). The printing temperature may lie in the range from 20°C (ambient temperature) to 40°C, preferably in the range from 30°C to 40°C, e.g., 35°C. Heating the plastisol ink composition up to the printing temperature may be necessary to guarantee sufficient fluidity of the ink composition in the printheads. Heating the plastisol ink composition to higher temperatures or storing the plastisol ink composition at higher temperatures should be avoided as that may reduce the shelf life of the plastisol ink composition.

[0018] According to a preferred embodiment, the radiation-gellable plastisol ink composition has a solid content in the range from 10 to 23% (vol. / vol.), preferably in the range from 15 to 21 % (vol. / vol.), comprises an emulsion PVC resin having a D90 particle size of 3 pm or less, preferably of 2 pm or less, and, per 100 parts of PVC resin:- between 60 and 130 parts of a plasticizer or a mixture of plasticizers;- between 140 and 225 parts, preferably between 160 and 210 parts, more preferably from 165 to 190 parts, of a) a reactive monofunctional or polyfunctional (meth)acrylate monomer and / or oligomer or b) a mixture of one or more reactive monofunctional or polyfunctional (meth)acrylate monomers and / or oligomers;- between 3 and 18 parts of photoinitiator having an absorption peak in the IIV-A wavelength range (from 315 to 400 nm), under the proviso that the wt. / wt. ratio of photoinitiator to the reactive monofunctional or polyfunctional (meth)acrylate monomer or oligomer or to the mixture of one or more reactive monofunctional or polyfunctional (meth)acrylate monomers and / or oligomers oligomer lies between 2% and 8%;- between 1 and 5 parts of a heat stabilizer for PVC, the heat stabilizer comprising a liquid Ca / Zn stabilizer, a liquid Ba / Zn stabilizer and / or a liquid Sn stabilizer;- at most 10 parts of additives, such as, e.g., one or more of a co-stabilizer, a surfactant, a processing aid, a modifying resin, or the like;The plasticizer or the mixture of plasticizers, as well as the at least one reactive (meth)acrylate monomer and / or oligomer or the mixture of one or more reactive (meth)acrylate monomers and / or oligomers have a viscosity at 25°C of 20 mPa.s or less. The radiation-gellable plastisol ink composition may preferably have a viscosity in the range from 10 to 60 mPa.s, preferably in the range from 15 to 25 mPa.s, at the printing temperature.

[0019] In a second aspect, the invention relates to a printing method, comprising: printing the radiation-gellable plastisol ink composition as described herein on a printing substrate;gelling the plastisol ink composition by irradiating the ink composition with UV light; and heat-curing the printed ink composition.

[0020] The radiation-gellable plastisol ink composition may preferably be applied on the printing substrate by digital printing, e.g., by inkjet printing. The printing of the radiation-gellable plastisol ink composition may be implemented as 3D-printing or digital embossing. According to an embodiment, the printing step may be performed at a temperature in the range from ambient temperature to 40°C, at atmospheric pressure.

[0021] Thanks to the radiation-curable crosslinkable (pre)polymer part of the plastisol ink composition, the latter can be pinned with UV light within seconds after application on the substrate, e.g., after jetting. Regular plastisol inks (not containing crosslinkable moieties) need to be gelled (fused) by heat-curing, which requires a relatively high amount of energy and a relatively long time. Accordingly, regular plastisol inks are likely to suffer from too high dot gain for certain applications, in particular, when larger droplets need be jetted. The plastisol ink composition according to the invention can be gelled within shorter times, allowing lower dot gain and thus a sharper texture. Irradiation with UV light induces polymerization of the crosslinkable (pre)polymer part, which very quickly leads to the formation of larger molecules, possibly macromolecules, and to a significant increase of the viscosity of the composition (the initial gelling under UV light). The plastisol part of the ink composition, i.e., the suspension of PVC particles in plasticizer, is thereby prevented from spreading and can thereafter be heat cured under no or at least significantly less time pressure. The method gives better control over the curing. The heat curing is significantly less time- critical than for a regular plastisol ink. The overall curing process is also less energy consuming. Furthermore, it may be possible to dynamically control the dot gain by varying the delay between the jetting and the gelling by irradiation with UV light.

[0022] According to an embodiment of the method, the gelling of the ink composition with UV light may be carried out within 5 seconds, preferably withing 3 seconds, more preferably within 2 seconds and most preferably within 1 second, from completion of the printing (jetting).

[0023] The printing method using the radiation-gellable plastisol ink composition may generate a surface texture, i.e., a three-dimensional surface topography, such as a relief or an embossing.

[0024] The printing method may comprise printing plural layers of radiation-gellable plastisol ink composition to generate a three-dimensionally structured surface.

[0025] After printing each layer of radiation-gellable plastisol ink composition, the layer may be gelled by irradiating the layer with UV light.

[0026] Optionally, each layer may be heat-cured after gelling with UV light. Alternatively, two or more layers of the plastisol ink composition may be superposed without intermediate heat-curing. These layers may thereafter be heat-cured in a common heat-curing step.

[0027] The heat-curing of the printed ink composition may comprise heat-curing the three-dimensionally structured surface.

[0028] The radiation-gellable plastisol ink composition may be transparent or translucent.

[0029] The printing substrate may comprise a printed decor layer and, optionally, a transparent or translucent protective layer covering the printed decor layer, and the printing of the radiation-gellable plastisol ink composition may preferably be carried out in register with the printed decor layer.

[0030] A more specific aspect of the invention relates to a method of manufacturing a decorative surface covering, e.g., flooring or wallcovering, comprising:- providing a core structure including one or more core layers and a printed decor layer, the printed decor optionally covered with a transparent or translucent wear layer;- carrying out the printing method according to the second aspect of the invention, using a printed decor layer, optionally covered with the transparent or translucent wear layer, as the printing substrate.

[0031] The method of manufacturing a decorative surface covering may further comprise application and curing of a crosslinkable topcoat on the printed ink composition after the heat-curing thereof. The topcoat may, preferably, be continuous, so as to seal off the underlying layers. The topcoat preferably comprises athermosetting (pre-)polymer resin, or a radiation curable (pre-)polymer resin (e.g., a UV curable or an electron-beam curable crosslinkable polymer composition). The crosslinkable resin composition forming the topcoat may optionally comprise at least one photoinitiator. According to a preferred embodiment, a UV-curable topcoat is applied. Applying such UV-curable topcoat may comprise generating a microfolded skin layer in the topcoat by exposing the UV-curable topcoat to V-UV light (vacuum ultraviolet light, i.e., UV light with a wavelength in the range from 100 nm to 200 nm) and then curing the topcoat in depth. In-depth curing of the topcoat may include irradiation with UV light of longer wavelengths (> 200 nm), capable of penetrating further into the topcoat layer than the V-UV light, which is quickly absorbed in the superficial skin layer (typically not thicker than 200 nm). The topcoat composition may comprise (poly)urethane (meth)acrylate, epoxy (meth)acrylate, polyester (meth)acrylate, polyether (meth)acrylate, amino (meth)acrylate, silicone (meth)acrylate, and / or any mixtures thereof.

[0032] As used herein, “(meth)acrylate” means “methacrylate” or “acrylate” or a combination of both. Such compounds may comprise at least one acrylate (CH2=CHCOO-) and / or methacrylate (CH2=CCH3COO-) group. In particular, the acrylated form is preferred.

[0033] Suitable (meth)acrylate monomers may be monofunctional, difunctional, or trifunctional, tetrafunctional, pentafunctional or hexafunctional (meth)acrylate monomers. Representative examples of such monomers include but are not limited to: acrylate monomer with carboxylic acid function such as 2-carboxyethyl acrylate, (meth)acrylic acid, tris(2-hydroxyethyl) isocyanurate tri(meth)acrylate as well as the di(meth)acrylate, alkyl (such as isobornyl, isodecyl, isobutyl, n-butyl, t-buyl, methyl, ethyl, tetrahydrofurfuryl, cyclohexyl, n-hexyl, iso-octyl, 2-ethylhexyl, n-lauryl, octyl or decyl) or hydroxy alkyl (such as 2-hydroxyethyl and hydroxy propyl) esters of acrylic acid or methacrylic acid, 2-(-2-ethoxyethoxy)ethyl(meth)acrylate, 2- butoxyethyl(meth)acrylate, ethoxylated and / or propoxylated hexanediol di(meth)acrylate, tricyclodecanedi(meth)acrylate, tricyclodecanedimethanol di(meth)acrylate, propylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, ethoxylated and / or propoxylated neopentylglycol di(meth)acrylate, hexamethylene glycol di(meth)acrylate, the (meth)acrylates obtained from the esterification with (meth)acrylic acid of aliphatic glycidyl ethers.

[0034] According to an embodiment, the (meth)acrylate oligomers may be composed of only a few monomer units such as a dimer, trimer, tetramer etc. They may typically be defined as being composed of repeating monomer units and as having a molecular weight (MW) between 500 and 20,000 Daltons. Examples of (meth)acrylated oligomers that can be used in the present invention include polyester (meth)acrylates, polyether (meth)acrylates), polycarbonate (meth)acrylate, (poly)urethane (meth)acrylates, epoxy (meth)acrylates and amino (meth)acrylate oligomers, or mixtures thereof. In particular, the acrylated forms are preferred. The (meth)acrylate oligomers are preferably having a molecular weight of from 500 to 5000 Daltons.

[0035] According to an embodiment, the reactive (meth)acrylate monomer and / or oligomer may preferably be aliphatic. More preferably, all the reactive (meth)acrylate monomers and / or oligomers may be aliphatic. Preferably, the reactive (meth)acrylate monomer and / or oligomer or the mixture of reactive (meth)acrylate monomers and / or oligomers may comprise or consist of difunctional or trifunctional (meth)acrylate monomer(s) and / or oligomer(s).

[0036] According to an embodiment, the radiation-gellable plastisol ink composition comprises a mixture of reactive acrylate monomers.

[0037] According to an embodiment, the reactive (meth)acrylate monomer and / or oligomer is selected from the group consisting of: tri propylene diacrylate (TPGDA), ethoxylated-3-trimethylolpropane triacrylate (TMP(3EO)TA), dipropylene glycol acrylate (DPGDA), tertiobutyl cyclohexyl acrylate (TBCHA), Cs-Cio octyl decyl acrylate (ODA), lauric acrylate (LA), isodecyl acrylate (IDA) and any mixtures thereof. More preferably the reactive (meth)acrylate monomer may be Tri propylene diacrylate (TPGDA) .and / or ethoxylated-3-trimethylolpropane triacrylate (TMP(3EO)TA).

[0038] According to a specific embodiment, the mixture of one or more reactive (meth)acrylate monomers and / or oligomers may be a mixture of:- ethoxylated-3-trimethylolpropane triacrylate (TMP(3EO)TA) and / or tri propylene diacrylate (TPGDA), and- and at least one compound selected from the group consisting of: tertiobutyl cyclohexyl acrylate (TBCHA), lauric acid (LA), and isodecyl acrylate (IDA), Cs- C10 octyl decyl acrylate (ODA), and dipropylene glycol acrylate (DPGDA).

[0039] According to a specific embodiment, the mixture of one or more reactive (meth)acrylate monomers and / or oligomers may be a mixture of:- ethoxylated-3-trimethylolpropane triacrylate (TMP(3EO)TA) and / or tri propylene diacrylate (TPGDA), and- at least one compound selected from the group consisting of: tertiobutyl cyclohexyl acrylate (TBCHA), lauric acid (LA), Cs-Cio octyl decyl acrylate (ODA), and isodecyl acrylate (IDA).

[0040] The expressions “transparent” and “translucent” herein refer to transparency and translucency, respectively, in the visible part of the electromagnetic spectrum, i.e., in the wavelength range from 400 nm to 700 nm.

[0041] Unless otherwise indicated, the expression “parts” means “parts by weight” or “parts in units of mass” with respect to a certain reference quantity (reference mass).

[0042] When it is specified herein that a value lies between a lower numerical bound and an upper numerical bound, the lower and upper numerical bounds shall be considered as included in the range. In other words, when a numerical range is indicated as “between numerical value A and numerical value B”, this means the same as “from numerical value A to numerical value B”. When the lower and upper numerical bounds of a range are to be excluded, it may be said that a value lies strictly between the lower and upper numerical bounds.

[0043] The expressions “decor” and “decorative” are used herein to indicate that the corresponding item, layer, or surface remains visible in the final product when in use as intended and contributes to the outer appearance of the surface covering.

[0044] The abbreviation “IR” stands for infrared (light). As used herein, “infrared light” (or IR light) designates electromagnetic radiation with wavelengths from 750 nm to 15 pm, i.e., belonging to the near-infrared (0.75 pm - 1.4 pm), the short-wavelength infrared (1 .4 pm - 3 pm), the mid-wavelength infrared (3 pm - 8 pm) and / or the long- wavelength infrared (8 pm - 15 pm) ranges. Preferably, the IR light comprises wavelengths from 1 pm to 8 pm.

[0045] The abbreviation “UV” stands for ultraviolet (light), i.e., electromagnetic radiation with wavelengths between 10 and 400 nm.

[0046] In the present document, the verb “to comprise” and the expression “to be comprised of’ are used as open transitional phrases meaning “to include” or “to consist at least of’. Unless otherwise implied by context, the use of singular word form is intended to encompass the plural, except when the cardinal number “one” is used: “one” herein means “exactly one”. Ordinal numbers (“first”, “second”, etc.) are used herein to differentiate between different instances of a generic object; no particular order, importance, or hierarchy is intended to be implied by the use of these expressions. Furthermore, when plural instances of an object are referred to by ordinal numbers, this does not necessarily mean that no other instances of that object are present (unless this follows clearly from context). When this description refers to “an embodiment”, “one embodiment”, “embodiments”, etc., this means that the features of those embodiments can be used in the combination explicitly presented but also that the features can be combined across embodiments without departing from the invention, unless it follows from context that features cannot be combined.Brief Description of the Drawings

[0047] By way of example, preferred, non-limiting embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which:Fig. 1 : is a schematic illustration of the manufacturing of a decorative surface covering according to a first preferred embodiment of the invention;Fig. 2: is a schematic illustration of a variant of the embodiment of Fig. 1 .

[0048] It should be noted that the drawings are strictly schematic and not to scale. This holds, in particular, for the inserts showing the cross-sections of the decorative surface covering in the making. In particular, it should be noted that these inserts do not necessarily show the thicknesses of the different layers in the actual proportions.Detailed Description of Embodiments

[0049] It will be understood that the following description and the drawings to which it refers describe, by way of example, different embodiments for illustration purposes. This description of embodiments shall not limit the scope, nature, or spirit of the claimed subject matter. The skilled person will appreciate that features of the different embodiments may be combined into further embodiments without departing from the scope of the present invention.

[0050] Fig. 1 illustrates a first embodiment of the proposed method for producing a decorative surface covering, e.g., a floor covering.

[0051] A surface covering substructure (also: core structure) 10 is provided as a printing substrate in a digital printing stage 12. The surface covering substructure may be of a monolayer or multilayer configuration. Fig. 1 shows a multilayer core structure comprising a backing layer and two structural core layers. The core structure may comprise one or more further layers, but these are not shown in the drawing.

[0052] An industrial digital printer 14 prints a decorative motif 18 onto the surface covering substructure 10. The printed decorative motif 18 may be considered as two- dimensional to the extent that any height variations in the layer of ink(s) may be regarded as insignificant. The surface covering substructure 10 may comprise a primer layer or base coat for printing thereon or it may comprise a printable layer that is part of the structural layers. The layer that receives the ink directly thereon may be referred to as the decor-carrying layer 16. The digital printer 14 may comprise printheads that project ink droplets onto the decor-carrying layer 16 in a very precise manner, in terms of position and volume of the droplets. The digital printer 14 may comprise a singlepass industrial printer, which uses several printheads aligned side by side in several rows that cover the entire width of the multilayer surface covering substructure. Each row of printheads may print one or more colours. During the printing process, the surface covering substructure 10 advances in the machine direction under the printheads. The digital printer 14 may be custom-made for the application in accordance with the requirements in terms of capacity and print quality. The digital printer 14 could use thermal printhead technology, wherein a current pulse passing through a heating element vaporizes a tiny quantity of ink in a chamber, so as to form a bubble, and this bubble propels an ink droplet through the printhead nozzle onto the printing substrate. Additionally, or alternatively, the digital printer 14 could also use piezoelectric printheads, wherein a piezoelectric element, on application of a voltage, generates a pressure pulse that drives an ink droplet through the nozzle. The ink is chosen in accordance with the printhead technology, the decor-carrying layer 16, the subsequent processing steps as well as quality and price constraints.

[0053] Various types of ink could be used for printing the decorative motif 18. Inks may comprise one or more colorants, a binder that bonds the colorants to the surface and a carrier liquid. Colorants may comprise dyes or pigments or a combination ofboth. Pigments are solid colorant particles that are suspended or dispersed throughout the carrier liquid. Pigment-based inks may be more light-stable and more fade-resistant than dye-based inks. Furthermore, dye-based inks often comprise organic solvents which may lead to higher VOC emissions than pigment-based inks, especially when water is the carrier liquid of the latter. Carrier liquids may include solvents, oil(s), water, and polymeric resins. For certain surface coverings, radiation-curable inks may be considered as particularly advantageous. The digital printing stage 12 may include one or more drying or curing devices (not shown in the drawing), wherein the printed decorative motif 18 is solidified and bonded to the decor-carrying layer 16. Such drying or curing devices could comprise one or more heaters and / or one or more blowers and / or one or more radiation sources, depending on the type of ink used in the printing of the decorative motif 18.

[0054] After application of the printed decorative motif 18, a transparent, or at least translucent, wear layer 20, e.g., a PVC wear layer, may be applied. Application of the wear layer 20 may be carried out by calendaring. (The calendaring process is not shown realistically in Fig. 1 .) The thickness of the wear layer 20 could be situated, e.g., in the range from 100 pm to 800 pm, more preferably in the range from 150 pm to 770 pm, and still more preferably in the range from 200 pm to 600 pm.

[0055] The assembly of the substructure 10, the printed decorative motif 18 and the wear layer 20 serves as the printing substrate 22 in a subsequent printing stage, in which a radiation-gellable plastisol ink composition 24 is applied.

[0056] The radiation-gellable plastisol ink composition comprises a PVC resin having a D90 particle size of 5 pm or less, and, per 100 parts of PVC resin: between 70 and 130 parts of a plasticizer or a mixture of plasticizers; between 150 and 225 parts, preferably between 160 and 210 parts, more preferably from 165 to 190 parts, of a) a reactive monofunctional or polyfunctional (meth)acrylate monomer or oligomer or b) a mixture of one or more reactive monofunctional or polyfunctional (meth)acrylate monomers and / or oligomers; between 3 and 18 parts of photoinitiator, under the proviso that the wt. / wt. ratio of photoinitiator to the reactive monofunctional or polyfunctional (meth)acrylate monomer or oligomer or to the mixture of one or more reactive monofunctional or polyfunctional (meth)acrylate monomers and / or oligomers oligomer lies between 2% and 8%; between 1 and 5 parts of a heat stabilizer for PVC. The plasticizer or the mixture of plasticizers, as well as the at least one reactive(meth)acrylate monomer and / or oligomer or the mixture of one or more reactive (meth)acrylate monomers and / or oligomers have a viscosity at 25°C of 20 mPa.s or less. The radiation-gellable plastisol ink composition may have a solid content in the range from 10 to 23% (vol. / vol.), preferably in the range from 15 to 21 % (vol. / vol.)

[0057] The application of the radiation-gellable plastisol ink composition 24 may be effected by digital printing. Other printing techniques, although less preferred, could be used as well. In the embodiment illustrated in Fig. 1 , a digital printer 26 is used to jet the radiation-gellable plastisol ink composition 24 onto the printing substrate 22. The digital printer 26 may comprise a single-pass or a multi-pass printer. The digital printer 26 preferably comprises piezoelectric printheads. Directly after the digital printing, preferably within 5 s, more preferably withing 3 s, still more preferably within 2 s and most preferably within 1 s from the jetting, the printed radiation-gellable plastisol ink pattern is gelled with a UV light source 28. The gelling could be performed, e.g., using a UV-A irradiation of between 100 and 500 mJ / cm2, preferably between 150 and 350 mJ / cm2, and still more preferably between 200 and 300 mJ / cm2, e.g., 250 mJ / cm2. The energy may be adapted to the thickness of the printed layer. The gelling with UV light is possible because the radiation-gellable plastisol ink composition comprises a significant part, e.g., between 40 to 60% by volume, preferably between 45 and 55% by volume, of a radiation-curable crosslinkable (pre)polymer composition, which polymerizes when exposed to UV light. The formation of macromolecules (polymer molecules) translates into a significant increase of the viscosity of the composition and prevents the droplets of ink composition 24 to (further) spread on the printing substrate 22. At this stage of the process, the UV-gelled ink composition 24 comprises a yet uncured plastisol part, i.e. , a dispersion of PVC particles in liquid plasticizer.

[0058] The radiation-gellable plastisol ink composition 24 is preferably printed in register with the decorative motif 18. The printing of the radiation-gellable plastisol ink composition 24 may produce a three-dimensional surface structure (surface texture). Such a surface texture may improve the realism of the decorative surface covering, e.g., when it mimics a particular type of surface, such as wood, bamboo, stone, textile, or the like.

[0059] In a subsequent step, the plastisol ink composition 24 is heat-cured. In the embodiment illustrated in Fig. 1 , the gelled plastisol ink composition 24 is heated under one or more IR lamps 30 (only one is shown in Fig. 1 ). Additionally, or alternatively,the heat-curing could be carried out in an oven. Heat-curing the plastisol ink composition 24 comprises the gelling and fusing of the plastisol part of the ink composition 24.

[0060] The completely cured ink composition 24 may thereafter be coated with a topcoat 32. The topcoat 32 may comprise a radiation curable crosslinkable composition, e.g., a radiation-curable crosslinkable (pre-)polymer resin, preferably a (poly)urethane acrylate. The coating may be effected by any suitable technique, e.g., using a roller-coating stage 34 (as illustrated in Fig. 1 ) or printing (e.g., digital printing), etc. The topcoat 32 may comprise a UV-curable topcoat. In this case, the topcoat 32 may be cured (crosslinked) by exposing it to an appropriate dose of UV light provided by UV light source 36. A matte surface finish may be obtained by generating a microfolded skin layer in the topcoat by first exposing the topcoat to V-UV light (from a V-UV light source) and then curing the topcoat in depth. In-depth curing of the topcoat may include irradiation with UV light of longer wavelengths (> 200 nm), capable of penetrating further into the topcoat layer than the V-UV light, which is quickly absorbed in the superficial skin layer. The UV light of longer wavelengths may be provided by one or more UV lamps. The topcoat is preferably comprised of one or more continuous layers, so as to completely seal off the underlying layers.

[0061] Fig. 2 illustrates a second embodiment of the proposed method for producing a decorative surface covering, e.g., a floor covering.

[0062] The embodiment illustrated in Fig. 2 differs from the embodiment in Fig. 1 only in that the radiation-gellable plastisol ink composition is printed in plural layers in order to produce a three-dimensionally structured surface with greater amplitude (height variations). In the illustrated embodiment, each layer of radiation-gellable plastisol ink composition is gelled by irradiating the layer with UV light directly after its being printed. The layers of plastisol ink composition may be superposed without intermediate heatcuring. The three-dimensionally structured surface formed by the different layers is heat-cured in a common heat-curing step.Examples

[0063] A first example of a radiation-gellable plastisol ink composition is provided inTable 1. The quantities of the ingredients or components are indicated in parts perweight. The radiation-gellable plastisol ink composition according to this example comprises a solids content of 20% (vol. / vol.)

[0064] In the example according to Table 1 , P-PVC was Vinnolit™ P 70 F or Inovyn™ P1510. Low-viscosity plasticizer was di-(2-ethylhexyl)adipate (also known as: di-octyladipate) Plastomoll™ DOA. Reactive monomer was tripropyleneglycol diacrylate, specifically: Laromer™ TPGDA or Sartomer™ SR 306. The photoinitiator was Omnirad™ 1173. Epoxidized soybean oil was Drapex™ HSE or Novasol™ E50. The stabilizer was liquid Ca / Zn stabilizer, e.g., Lankromark™ LZC 723. The surface additive was silicone-containing surface additive BYK™ 307. To achieve a uniform dispersion of the PVC resin in the plasticizer, a paste was first made by mixing half of the plasticizer and the PVC resin. The plasticizer and the PVC were mixed until a homogeneous paste was obtained. The remainder of the plasticizer was then added to the mixture which was stirred until a uniform dispersion was formed.

[0065] Digital printing tests with the radiation-gellable plastisol ink composition according to Table 1 were conducted with a Seiko™ RC1536A printhead at a controlled temperature of 35°C. It was demonstrated that satisfactory surface texture depths could be achieved with significantly improved sharpness when compared with regular plastisol ink. The entire surface texturing process, including the printing, gelling with UV light and subsequent heat-curing proved to be fast and energy efficient.

[0066] Further examples of radiation-gellable plastisol ink compositions are provided in Table 2. The quantities of the ingredients or components are indicated in parts per weight.

[0067] In examples 2-7, TPGDA was TPGDA 300 g / mol (Sartomer SR306), TMP(3EO)TA was TMP(3EO)TA 428 g / mol (Sartomer SR454), DPGDA was DPGDA 252 g / mol (Sartomer SR 508), C8-C10 ODA was C8-C10 ODA 200 g / mol (Sartomer SR484), epoxidized soybean oil was Drapex™ HSE or Novasol™ E50. Lankromark™ LZC723 was used as a stabilizer. To achieve a uniform dispersion of the PVC resin in the plasticizer, a paste was first made by mixing 50 parts of the plasticizer and the 100 parts of PVC resin. The plasticizer and the PVC were mixed until a homogeneous paste was obtained. The remaining 43 parts of the plasticizer were then added to the mixture which was stirred until a uniform dispersion was formed. In examples 2, 3, 4, 6 and 7, a mixture of reactive monomers was used. It may be worthwhile noting that TMP(3EO)TA has a viscosity of 65 mPa.s, which is compensated by its mixture partner so that the viscosity of the mixtures of reactive acrylate monomers is at most 20 mPa.s at 25°C.

[0068] Examples 1 to 7 were subjected to aging tests to assess the evolution of the viscosity at increased temperatures. After compounding, a first sample of each radiation-gellable plastisol ink composition according to the examples was subjectedto initial viscosity measurements at 25°C and then at 40°C. A second sample of each composition was sealed in an opaque container and stored for 24 hours at 40°C. After the storage time, the second samples were let cool down to 25°C and further viscosity measurements were carried out at 25°C and then at 40°C. Table 3 summarises the measured viscosities. The viscosities were measured according to ISO 3219 (2021 ) using a standard coaxial cylinder measuring geometry (shear rate of 100 s-1) in an Anton Paar rheometer. Relative aging corresponds to the ratio of the final to the initial viscosity at 40°C. A relative aging less than 130% in one day is considered acceptable.

[0069] Table 4 provides further examples of radiation-gellable plastisol ink compositions. The quantities of the ingredients or components are indicated in parts per weight.

[0070] In examples 8-15, TPGDA was TPGDA 300 g / mol (Sartomer SR306), TMP(3EO)TA was TMP(3EO)TA 428 g / mol (Sartomer SR454), TBCHA (tertiobutyl cyclohexyl acrylate) was Sartomer SR217, LA (lauryl acrylate) was Sartomer SR335, IDA (isodecyl acrylate) was Sartomer SR395. Expoxidized soybean oil was Drapex™ HSE or Novasol™ E50. Lankromark™ LZC 723 was used as a stabilizer. To achieve a uniform dispersion of the PVC resin in the plasticizer, a paste was first made by mixing 50 parts of the plasticizer and the 100 parts of PVC resin. The plasticizer and the PVC were mixed until a homogeneous paste was obtained. The remaining 43 parts of the plasticizer were then added to the mixture which was stirred until a uniform dispersion was formed. In examples 8-15, a mixture of reactive monomers was used.

[0071] Examples 8 to 15 were subjected to the same viscosity measurements and aging tests as examples 1 -7 to assess the evolution of the viscosity at increased temperatures. Table 5 shows the results of these measurements. Examples 8-15 exhibited less than 130% relative aging at 40°C.

[0072] While specific embodiments have been described herein in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.

Claims

Claims1. A radiation-gellable plastisol ink composition, comprising a PVC resin having a D90 particle size of 5 pm or less, and, per 100 parts of PVC resin: between 60 and 130 parts of a plasticizer or a mixture of plasticizers; between 140 and 225 parts, preferably between 160 and 210 parts, still more preferably from 165 to 190 parts, of a) a reactive monofunctional or polyfunctional (meth)acrylate monomer or oligomer or b) a mixture of one or more reactive monofunctional or polyfunctional (meth)acrylate monomers and / or oligomers; between 3 and 18 parts of photoinitiator, under the proviso that the wt. / wt. ratio of photoinitiator to the reactive monofunctional or polyfunctional (meth)acrylate monomer or oligomer or to the mixture of one or more reactive monofunctional or polyfunctional (meth)acrylate monomers and / or oligomers oligomer lies between 2% and 8%; between 1 and 5 parts of a heat stabilizer for PVC; wherein the plasticizer or the mixture of plasticizers, as well as the at least one reactive (meth)acrylate monomer and / or oligomer or the mixture of one or more reactive (meth)acrylate monomers and / or oligomers have a viscosity at 25°C of 20 mPa.s or less.

2. The radiation-gellable plastisol ink composition as claimed in claim 1 , having a solid content in the range from 10 to 23% (vol. / vol.), preferably in the range from 15 to 21 % (vol. / vol.)3. The radiation-gellable plastisol ink composition as claimed in claim 1 or 2, wherein the heat stabilizer comprises a liquid Ca / Zn stabilizer, a liquid Ba / Zn stabilizer and / or a liquid Sn stabilizer.

4. The radiation-gellable plastisol ink composition as claimed in any one of claims 1 to 3, wherein the heat stabilizer comprises a solid stabilizer, the solid stabilizer having a D90 particle size not exceeding the D90 particle size of the PVC resin.

5. The radiation-gellable plastisol ink composition as claimed in any one of claims 1 to 4, comprising at most 10 parts per 100 parts of PVC resin of additives, such as, e.g., one or more of a co-stabilizer, a surfactant, a processing aid, a modifying resin, or the like.

6. The radiation-gellable plastisol ink composition as claimed in any one of claims 1 to 5, wherein the PVC resin consists of emulsion PVC.

7. The radiation-gellable plastisol ink composition as claimed in any one of claims 1 to 6, wherein the photoinitiator has an absorption peak in the IIV-A wavelength range from 315 to 400 nm.

8. The radiation-gellable plastisol ink composition as claimed in any one of claims 1 to 7, wherein the PVC resin has a D90 particle size of 3 pm or less, preferably of 2 pm or less.

9. The radiation-gellable plastisol ink composition as claimed in any one of claims 1 to 8, having a viscosity in the range from 10 to 60 mPa.s, preferably in the range from 15 to 25 mPa.s, at a temperature in the range from 20°C to 40°C, at atmospheric pressure.

10. The radiation-gellable plastisol ink composition as claimed in any one of claims 1 to 9 wherein the at least one reactive (meth)acrylate monomer and / or oligomer is an aliphatic compound.11 . The radiation-gellable plastisol ink composition as claimed in claims 2, 3, 5, 6, 7, 8 and 9 taken in combination.

12. The radiation-gellable plastisol ink composition as claimed in any one of claims 1 to 11 , wherein the reactive (meth)acrylate monomer and / or oligomer has a molecular weight of at least 250 g / mol when the reactive (meth)acrylate monomer and / or oligomer comprises two methacryloyl groups.

13. A printing method, comprising: printing radiation-gellable plastisol ink composition as claimed in any one of claims 1 to 12 on a printing substrate; gelling the plastisol ink composition by irradiating the ink composition with UV light; and heat-curing the printed ink composition.

14. The printing method as claimed in claim 13, wherein the radiation-gellable plastisol ink composition is applied on the printing substrate by digital printing, e.g., by inkjet printing.

15. The printing method as claimed in claim 13 or 14, wherein the gelling of the ink composition with UV light is carried out within 5 seconds from completion of the printing.

16. The printing method as claimed in any one of claims 13 to 15, comprising printing plural layers of radiation-gellable plastisol ink composition to generate a three- dimensionally structured surface.

17. The printing method as claimed in claim 16, wherein after printing each layer of radiation-gellable plastisol ink composition, the layer is gelled by irradiating the layer with UV light.

18. The printing method as claimed in claim 17, wherein after gelling each layer with UV light, the layer is heat-cured.

19. The printing method as claimed in claim 17, wherein two or more layers of the plastisol ink composition are superposed without intermediate heat-curing.

20. The printing method as claimed in any one of claims 17 to 19, wherein heat-curing the printed ink composition comprises heat-curing the three-dimensionally structured surface.

21. The printing method as claimed in any one of claims 13 to 20, wherein the radiation-gellable plastisol ink composition is transparent or translucent, wherein the printing substrate comprises a printed decor layer and, optionally, a transparent or translucent protective layer covering the printed decor layer, and wherein the printing of the radiation-gellable plastisol ink composition is carried out in register with the printed decor layer.

22. The printing method as claimed in any one of claims 13 to 21 wherein the printing step is conducted at a temperature in the range from 20°C to 40°C, at atmospheric pressure.

23. A method of manufacturing a decorative surface covering, e.g., flooring or wallcovering, comprising: providing a core structure including one or more core layers and a printed decor layer, the printed decor optionally covered with a transparent or translucent wear layer;carrying out the printing method as claimed in any one of claims 13 to 22 using a printed decor layer, optionally covered with the transparent or translucent wear layer, as the printing substrate.

24. The method of manufacturing a decorative surface covering as claimed in claim 23, comprising application and curing of a crosslinkable topcoat on the printed ink composition after the heat-curing thereof.

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