Field of the Invention

[0001] The invention generally relates to the installation of surface coverings serving as finishing work on constructions, in particular, of wall coverings or flooring. The invention more specifically relates to the application of decorative surface coverings in the form of panels, tiles or other dimensionally stable surface covering elements and the sealing of the joints between the individual elements.

Technical Background

[0002] When surface covering elements are laid, joints are formed between adjacent surface covering elements, where the lateral faces of the surface covering elements face each other at a small distance. As long as they are unsealed, the joints represent openings, through which water and other liquids may infiltrate the underlayment.

[0003] Various methods are known in the prior art for filling or sealing such joints. The joints between ceramic tiles are typically grouted with mortar. The joints between synthetic (polymer-based) surface covering elements may be sealed using hot-melt. To do this, a welding rod may be weld in situ using a welding machine (welding gun or the like). When the material has cooled down, any excess material is cut away or scraped off, so that a flush continuous surface is obtained. An example of a welding rod suitable for flooring has been disclosed in WO 2009/62962 A1.

[0004] The appearance of the joints after application of the sealant is important.

Although the joints themselves may remain visible, excess sealant protruding from the joint and spreading on the visible surface of the decorative covering is typically perceived as a deficiency. Accordingly, measures have been proposed to avoid that problem.

[0005] US 2017/157840 A1 discloses a joint treatment method for sealing joints between surface covering elements having a protective top layer made of ultraviolet radiation curable resin. The method includes injecting a joint treatment agent, which has no dissolving ability with respect to the protective layer into joint and attaching, before the injected joint treatment agent is cured, an adhesive tape in such a way that it straddles the joint and contacts the joint treatment agent present in joint as well as the protective layer on both sides of the joint. The joint treatment agent is then left to dry. Finally, the adhesive tape is peeled off, whereby any excess joint treatment agent protruding from the joint remains attached to the adhesive tape and is ripped off.

[0006] Another method, disclosed in JP 3388960, consists in attaching an adhesive tape to the flooring surface so as to cover the upper part of a joint, incising the adhesive tape along the joint, injecting seam sealer into the joint while moving the injection nozzle along the incision in the adhesive tape and then peeling off the adhesive tape.

Summary of the Invention

[0007] According to an aspect of the invention, a method for installing a decorative surface covering includes:

o arranging surface covering elements on a substrate,

o applying a UV-curable paste (sealant) into the joints between adjacent surface covering elements,

o removing excess UV-curable paste, if any, and

o curing the UV-curable paste with UV light.

[0008] As used herein, the qualifier“decorative” is used to indicate that the surface covering remains visible after installation but the term is not intended to imply any particular aesthetic design. Decorative surface coverings may constitute the final finishing layer of a wall or floor. It is not excluded, however, that a transparent or at least translucent finishing layer, e.g., wear-resistant varnish, may be applied on top after installation.

[0009] The UV-curable paste preferably includes acrylate resin, e.g. an epoxy acrylate resin, a polyester acrylate resin, or a urethane acrylate resin. As used herein, UV stands for ultra violet radiation, i.e., electromagnetic radiation with a wavelength from 10 to 400 nm. The UV-curable paste may include photo-polymerization initiators (photoinitiators), co-initiators (e.g. photoactivators, spectral sensitizers, reducing agents or the like) and one or more additives (e.g. stabilizers, antioxidants, plasticizers, pigments, compatibilizers, adhesion promoters, processing aids, fillers, dispersing agents, flowing aids, thickening agents, defoaming agents, deaerating agents, flattening agents, matting agents, wetting agents etc.) [0010] According to an embodiment, the UV-curable paste is a UV-curable polymer formulation that includes at least one UV-curable polyurethane acrylate(component A), optionally at least one acidic adhesion promoter (component B), optionally at least one mono- or polyfunctional reactive diluent (component C) and a photoinitiator (component D). Preferably, the radiation-curable polymer formulation further comprises one or more fillers, such as, e.g., calcium carbonate, chalk, colloidal or amorphous silica, magnesium oxide, clay, calcium silicate or other silicate, talc, aluminate or the like (including mixtures of different species of filler material).

[0011] The radiation-curable polyurethane acrylate (component A) preferably has an average molecular weight comprised in the range from 500 to 25 000 g-mol ¹ , more preferably in the range from 1000 to 20 000 g-mol ¹ , and still more preferably in the range from 1500 to 15 000 g-mol ¹ .

[0012] Radiation-curable polyurethane acrylates (component A) are preferably prepared from hydroxyl-containing monomers and/or polymers and compounds, which, at one and the same time, contain at least one isocyanate-reactive group (e.g., alcohol, amine or thiol) and at least one polymerizable acrylate group, by reaction with polyisocyanates. The radiation-curable polyurethane acrylate contains both urethane groups and acrylate groups.

[0013] Suitable hydroxyl containing monomers are preferably chosen from the group consisting of: methanol, ethanol, 1 -propanol, 1 -butanol, 1 -pentanol, 1 -hexanol, 2- propanol, 2-butanol, 2-ethylhexanol, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol, 1 ,2-propanediol, 1 ,3-propanediol, 1 ,4-butanediol, neopentyl glycol, 2-ethyl-2-butylpropanediol, trimethylpentanediol, 1 ,3-butylene glycol, 1 ,4-cyclohexanedimethanol, 1 ,6-hexanediol, 1 ,2- and 1 ,4-cyclohexanediol, 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, hydrogenated bisphenol A (2,2-bis(4-hydroxycyclohexyl)propane), diols derived from dimer fatty acids, 2,2-dimethyl-3-hydroxypropionic acid (2,2-dimethyl-3- hydroxypropyl) ester, glycerol, trimethylolethane, trimethylolpropane, trimethylolbutane and/or castor oil. Neopentyl glycol, 1 ,4-butanediol, 1 ,4- cyclohexanedimethanol, 1 ,6-hexanediol and/or trimethylolpropane are particularly preferred.

[0014] Suitable hydroxyl-containing polymers preferably include polyesters, polyethers, polyether-esters, polycarbonates, polyether carbonate polyols and polycarbonate polyesters having in each case a functionality of from 1 .0 to 3.0, in each case with a weight average molecular weight in the range of from 300 to 4000, preferably 500 to 2500 g-mol ¹ . Hydroxyl functional polyesters and polyether diols are particularly preferred. Polyether diols particular useful are dihydroxy-terminated polyalkylene oxides having 2 to 4 carbon atoms in each alkylene group. Such polyether diols are made by polymerizing ethylene oxide, propylene oxide, or butylene oxide, or mixtures thereof to form block copolymers, with a dihydric initiator. Such initiators are ethylene glycol, diethylene glycol, 1 ,2-propylene glycol, 1 ,4-butanediol (and the like). A preferred polyether diol is made by polymerizing tetrahydrofuran to a polyether diol having 4 carbon atoms in each alkylene group. Preferably, the polyether diols have molecular weights of 800 to 2000 g-mol ¹ . Especially preferably, the polyether diols have a molecular weight of 800 to 1200 g-mol ¹ .

[0015] Hydroxyl-containing polyesters may be prepared by polycondensation of suitable dicarboxylic acids and diols. Condensation may take place in an inert gas atmosphere at temperatures from 180 to 260°C, preferably 200 to 230°C, in the melt, or in azeotropic mode.

[0016] Carboxylic acids that are preferred for polyester preparation may be aliphatic, cycloaliphatic, aromatic and/or heterocyclic in nature and may, if desired, be substituted by halogen atoms and/or be unsaturated. Examples of carboxylic acids include the following: succinic, adipic, suberic, azelaic, sebacic, phthalic, terephthalic, isophthalic, trimellitic, pyromellitic, tetrahydrophthalic, hexahydrophthalic, hexahydroterephthalic, dichlorophthalic and tetrachlorophthalic, endomethylene tetrahydrophthalic, and glutaric acid, 1 ,4-cyclohexanedicarboxylic acid, and -where obtainable- their anhydrides or esters. Adipic acid and 1 ,4-cyclohexanedicarboxylic acid may be found particularly suitable.

[0017] Examples of suitable polyols include monoethylene glycol, 1 ,2- and 1 ,3- propylene glycol, 1 ,4- and 2,3-butylene glycol, di- -hydroxyethylbutanediol, 1 ,5- pentanediol, 1 ,6-hexanediol, 1 ,8-octanediol, decanediol, dodecanediol, neopentyl glycol, cyclohexanediol, bis(hydroxymethyl)tricyclo(5.2.1 0(2,6))decane (Dicidol), 1 ,4- bis(hydroxymethyl)cyclohexane, 2,2-bis-(4-hydroxycyclohexyl)propane, 2,2-bis[4-( - hydroxy-ethoxy)phenyl]propane, 2-methylpropane-1 ,3-diol, 2-methyl-pentane-1 ,5-diol, 2,2,4(2,4,4)-trimethyl-hexane-1 ,6-diol, glycerol, trimethylolpropane, trimethylolethane, hexane-1 , 2, 6-triol, butane-1 ,2, 4-triol, tris( -hydroxyethyl)isocyanurate, pentaerythritol, mannitol, and sorbitol, and also diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, polypropylene glycols, polybutylene glycols, xylylene glycol, and neopentyl glycol hydroxypivalate. Preference is given to monoethylene glycol, neopentyl glycol, Dicidol, cyclohexanedimethanol, trimethylolpropane, and glycerol.

[0018] Polyesters used for obtaining radiation-curable polyurethane acrylates preferably have an OH number of 15 to 750 mg KOH/g. Mixtures of polyesters can be used as well.

[0019] For preparing urethane acrylates, the polyisocyanates used are preferably diisocyanates of (cyclo)aliphatic or aromatic structure. Examples of (cyclo)aliphatic polyisocyanates are: 2-methylpentamethylene 1 ,5-diisocyanate (MPDI), hexamethylene diisocyanate (HDI), trimethylhexamethylene 1 ,6-diisocyanate (TMDI), in particular 2,2,4- and the 2,4,4 isomer and technical mixtures of both isomers, 4,4 ^' - methylenebis(cyclohexyl isocyanate) (H12MDI), norbornane diisocyanate (NBDI), and 3,3,5-trimethyl-1 -isocyanato-3-isocyanatomethyl-cyclohexane (IPDI). Likewise highly suitable are polyisocyanates, which are obtainable by reacting polyisocyanates with themselves via isocyanate groups, such as isocyanurates, which come about through reaction of three isocyanate groups. The polyisocyanates may likewise contain biuret groups or allophanate groups. IPDI and/or IPDI trimer may be found especially suitable.

[0020] Examples of aromatic polyisocyanates are 1 ,4-diisocyanatobenzene (BDI), 2,4-diisocyanatotoluene (2,4-TDI), 2,6-diisocyanatotoluene (2,6-TDI), 1 ,1 '- methylenebis[4-isocyanatobenzene] (MDI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate (TMXDI), 1 ,5-naphthalene diisocyanate (NDI), tolidine diisocyanate (TODI) and p-phenylene diisocyanate (PPDI). Preference may be given to (cyclo)aliphatic polyurethanes.

[0021 ] Examples of suitable polymerizable compounds having at least one free OH group and a polymerizable (meth)acrylate group include the esterification products of aliphatic and/or aromatic polyols with (meth)acrylic acid having a residual average hydroxyl functionality of 1 . The partial esterification products of (meth)acrylic acid with tri-, tetra-, penta- or hexahydric polyols or mixtures thereof may be preferred. In this context, it is also possible to use reaction products of such polyols with ethylene oxide and/or propylene oxide or mixtures thereof, or reaction products of such polyols with lactones, which add to these polyols in a ring-opening reaction. Examples of suitable lactones are g-butyrolactone and, in particular, d-valerolactone and e-caprolactone. These modified or unmodified polyols are partly esterified with acrylic acid, methacrylic acid or mixtures thereof until the desired residual hydroxyl functionality is reached.

[0022] Particularly preferred are compounds comprising at least two (meth)acryl functions such as glycerol diacrylate, trimethylolpropane diacrylate, pentaerythritol triacrylate, ditrimethylolpropane triacrylate, dipentaerythritol pentaacrylate and their (poly)ethoxylated and/or (poly)propoxylated equivalents.

[0023] Other suitable compounds are the (meth)acrylic esters with linear and branched polyols in which at least one hydroxy functional group remains free, like hydroxyalkyl(meth)acrylates having 1 to 20 carbon atoms in the alkyl group. Preferred molecules in this category are hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate.

[0024] Hydroxyl functional polyester (meth)acrylates, polyether (meth)acrylates, polyether-ester (meth)acrylates, polycarbonate (meth)acrylates and polyether carbonate (meth)acrylates, comprising at least one hydroxyl functional group, can be used as well.

[0025] To prepare urethane acrylate from OH-containing monomers and/or polymers, polyisocyanates, and the acrylate component, first of all the polyisocyanate is introduced, a suitable catalyst (e.g., DBTL) and a polymerization inhibitor (e.g., IONOL CP, Shell) are added, and the acrylate component, hydroxyethyl acrylate, for example, is added in an NCO:OH ratio of 2.5 to 1 :1. Thereafter, the OH-containing monomers and/or polymers, preferably the polyester, is added to the reaction product, in a residual NCO:OH ratio of 0.5 to 0.95:1 , and the reaction is completed at 40 to 120°C, so that an NCO content below 0.1 % is obtained.

[0026] The acidic adhesion promoter (component B) preferably comprises one or more acid functional groups and one or more (meth)acrylic functional groups. The one or more acid functional groups are preferably selected from the group consisting of - SO3H, -OSO3H, -COOH, -OPO3H2 and -OPO2HO-. Optionally, the acidic hydrogen could be substituted by an alkali metal or an ammonium base. The acidic adhesion promoter is preferably the reaction product of one or more acid-functional-group- comprising components with one or more functionalized (meth)acrylates. Examples are ethylenically unsaturated polyesters and polyurethanes comprising one or more of -SO3H, -OSO3H, -COOH, -OPO3H2 and -OPO2HO- functional groups.

[0027] Polyesters comprising one or more acid functional groups are preferably prepared from one or more polyol components and one or more polybasic acid components, wherein one or more diol components and/or one or more dibasic acid components contain one or more of -SO3H, -OSO3H, -COOH, and -OPO3H2 functional groups.

[0028] Examples of -SO3H, -OSO3H, -COOH, and -OPO3H2 functional-group- comprising polybasic acid or polyol include: 5-sulfoisophthalic acid, 2-sulfoisophthalic acid, 4-sulfophthalic acid, 3-sulfophthalic acid, dialkyl 5-sulfoisophthalate, dialkyl 2- sulfophthalate, alkyl 4-sulfophthalic acid, alkyl 3-sulfophthalic acid, sodium or potassium salt of these compounds, and dimethylolpropionic acid. Optionally, the sodium, potassium or ammonium salt could be used.

[0029] Polyesters comprising one or more phosphate groups in the polyester chain could be prepared from condensation of one or more polyols and one or more polybasic acids in the presence of phosphoric acid.

[0030] Ethylenically unsaturated polyurethane resin having one or more acid functional groups could be synthesized from reaction of a polyisocyanate compound, a polyol component having one or more acid functional groups, e.g. a polyester polyol having one or more acid functional groups and/or dimethylolpropionic acid, and a compound having a hydroxyl functional group and at least one ethylenically unsaturated double bond, such as, for example, 2-hydroxyethylacrylate.

[0031 ] Hydroxyl functional polyesters having one or more acid functional groups could be converted into ethylenically unsaturated polyesters having one or more acid functional groups through reaction with (meth)acrylic acid.

[0032] Acid functional polyesters having one or more -SO3H, -OSO3H, -COOH, - OPO3H2 and -OPO2HO- functional groups could be converted into ethylenically unsaturated polyesters having one or more acid functional groups through reaction with glycidyl(meth)acrylate or hydroxyethyl(meth)acrylate.

[0033] Other examples of acidic adhesion promoters are: the reaction product of hydroxyethylacrylate and phosphorus pentoxide forming 2-acryloylethylphosphate, the reaction product of 2-hydroxyethylacrylate and succinic anhydride, the reaction product of a polyester oligomer comprising hydroxyl and carboxyl functional groups with acrylic acid and the reaction product of a carboxyl functionalized polyester oligomer with hyd roxyethyl (m eth )acryl ate .

[0034] The acidic adhesion promoter (component B) preferably has a molecular weight of less than 10 000 g-mol ¹ , more preferably of less than 7500 g-mol ¹ and most preferably of less than 5000 g-mol ¹ .

[0035] Reactive diluents (component C) comprise, for example, methanol, ethanol, 1 - propanol, 1 -butanol, 1 -pentanol, 1 -hexanol, 2-propanol, 2-butanol, 2-ethylhexanol, dihydrodicyclopentadienol, tetrahydrofurfuryl alcohol, 3,3,5-trimethylhexanol, octanol, decanol, dodecanol, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol, 1 ,2-propanediol, 1 ,3-propanediol, 1 ,4- butanediol, neopentyl glycol, 2-ethyl-2-butylpropanediol, trimethylpentanediol, 1 ,3- butylene glycol, 1 ,4-cyclohexanedimethanol, 1 ,6-hexanediol, 1 ,2- and 1 ,4- cyclohexanediol, hydrogenated bisphenol A (2,2-bis(4-hydroxycyclohexyl)propane), glycerol, trimethylolethane, trimethylolpropane, trimethylolbutane, pentaerythritol, ditrimethylolpropane, dipentaerythritol, sorbitol esterified completely with (meth)acrylic acid, and the ethoxylated and/or propoxylated derivatives of these alcohols esterified completely with (meth)acrylic acid and the technical grade mixtures obtained during (meth)acrylation of the abovementioned compounds.

[0036] Further suitable reactive diluents (component C) are, for example, epoxy

(meth)acrylates, polyether (meth)acrylates, polyester (meth)acrylates and polycarbonate (meth)acrylates having a number average molecular weight preferable comprised between 500 and 10000 g-mol ¹ .

[0037] Reactive diluents comprising more than one ethylenically unsaturated group may be found particularly well suited.

[0038] The radiation-curable polymer formulation may comprise any suitable photoinitiator (component D). The usual photoinitiators are the type that generate free radicals when exposed to radiation energy. Suitable photoinitiators include, for example, aromatic ketone compounds, such as benzophenones, alkylbenzophenones, Michler's ketone, anthrone and halogenated benzophenones. Further suitable compounds include, for example, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, phenylglyoxylic acid esters, anthraquinone and the derivatives thereof, benzil ketals and hydroxyalkylphenones (e.g. 2-hydroxy-2-methyl-1 -phenyl-propan-1 -one or the like). Other suitable photoinitiators include 2,2-diethoxyacetophenone; 2- or 3- or 4- bromoacetophenone; 3- or 4-allyl-acetophenone; 2-acetonaphthone; benzaldehyde; benzoin; the alkyl benzoin ethers; benzophenone; benzoquinone; 1 - chloroanthraquinone; p-diacetyl-benzene; 9,10-dibromoanthracene; 9,10- dichloroanthracene; 4,4-dichlorobenzophenone; thioxanthone; isopropyl- thioxanthone; methylthioxanthone; alpha, alpha, alpha-trichloro-para-t-butyl acetophenone; 4-methoxybenzophenone; 3-chloro-8-nonylxanthone; 3-iodo-7- methoxyxanthone; carbazole; 4-chloro-4'-benzylbenzophenone; fluorene; fluorenone; 1 ,4-naphthylphenylketone; 1 ,3-pentanedione; 2,2-di-sec.-butoxy acetophenone; dimethoxyphenyl acetophenone; propiophenone; isopropylthioxanthone; chlorothioxanthone; xanthone; maleimides and their derivatives; and mixtures thereof.

[0039] Several suitable photoinitiators are commercially available from Ciba, for instance, Irgacure® 184 (1 -hydroxy-cyclohexyl-phenyl-ketone), Irgacure® 819 (bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide), Irgacure® 1850 (a 50/50 mixture of bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl-phosphine oxide and 1 -hydroxy- cyclohexyl-phenyl-ketone), Irgacure® 1700 (a 25/75 mixture of bis(2,6- dimethoxybenzoyl)-2,4,4-trimethylpentyl-phosphine oxide and 2-hydroxy-2-methyl-1 - phenyl-propan-1 -one), Irgacure® 907 (2-methyl-1 [4-(methylthio)phenyl]-2- morpholinopropan-1 -one), Darocur® MBF (a phenyl glyoxylic acid methyl ester),

Irgacure® 2020 Photoinitiator blend (20% by weight of phenylbis(2, 3, 6-trimethyl benzoyl)phosphine oxide and 80% by weight of 2-hydroxy-2-methyl-1 -phenyl-1 - propanone) and Darocur® 4265 (a 50/50 mixture of bis(2,4,6-trimethylbenzoyl)- phenylphosphineoxide and 2-hydroxy-2-methyl-1 -phenyl-propan-1 -one). There may be other suitable photoinitiators that are not listed.

[0040] Photoactivators can be used in combination with the aforementioned photoinitiators. Photoactivators could e.g. be selected among: methylamine, tributylamine, methyldiethanolamine, 2-aminoethylethanolamine, allylamine, cyclohexylamine, cyclopentadienylamine, diphenylamine, ditolylamine, trixylylamine, tribenzylamine, N-cyclohexylethyleneimine, piperidine, N-methylpiperazine, 2,2- dimethyl-1 ,3-bis(3-N-morpholinyl)-propionyloxypropane, and mixtures thereof. [0041] The radiation-curable polymer formulation could comprise additives, such as , e.g., dispersing agents, flowing aids, thickening agents, defoaming agents, deaerating agents, pigments, fillers, flattening agents, matting agents and wetting agents.

[0042] According to an embodiment, the UV-curable paste is a UV-curable polymer formulation that includes at least one UV-curable polyurethane acrylate (component A), optionally at least one acidic adhesion promoter (component B), optionally at least one mono- or polyfunctional reactive diluent (component C) and a photoinitiator (component D). Preferably, the radiation-curable polymer formulation further comprises one or more fillers, such as, e.g., calcium carbonate, chalk, colloidal or amorphous silica, or the like. Components A, B (optional), C (optional) and D are preferably combined with colloidal silica as a filler.

[0043] According to an embodiment, the UV-curable paste comprises from 50 to 85 wt.% acrylate or urethane acrylate resin, from 10 to 30 wt.% colloidal silica and from 5 to 20 wt.% photoinitiator. A preferred example of an acrylate resin is butylacrylate/methyl methacrylate/methacrylic acid copolymer resin. The acrylate resin preferably has an average molecular weight comprised in the range from 500 to 25 000 g -mol ¹ , more preferably in the range from 1000 to 20 000 g mol ¹ , and still more preferably in the range from 6000 to 19 500 g-moh ¹ . A suitable photoinitiator may comprise, for example, 2-hydroxy-2-methyl-1 -phenyl-propan-1 -one. [0044] The decorative surface covering is preferably a wall covering. Alternatively, the decorative covering may be a floor covering.

[0045] Preferably, the UV-curable paste is transparent (at least in the cured state).

[0046] The edges of the surface covering elements facing each other at the joints are preferably cleaned before the UV-curable paste is applied. Cleaning may include the use of water and/or an organic solvent, such as, e.g. an alcohol (e.g. methanol, ethanol, IPA, etc.)

[0047] Preferably, any excess UV-curable paste is removed by wiping it off with a cloth.

[0048] The surface covering elements are preferably rigid floor or wall covering elements. The term “rigid” is herein used as a qualifier of floor or wall covering elements, or layers thereof, which are significantly more rigid than classical resilient flooring materials. Specifically, as used herein, “rigid” is meant herein to imply a relatively high modulus of elasticity (measured according to EN 310), e.g. greater than 1000 Mpa, preferably greater than 2000 MPa, more preferably greater than 4000 MPa. Preferably, the rigid floor or wall covering elements have a structural core layer, the MOE of which is preferably situated in the range from 1000 MPa to 15000 MPa. Compared to a resilient panels, rigid panels facilitate installation and better bridge slight unevenness as well as local bumps or recesses of the installation surface in a permanent manner (over the lifetime of the surface covering). High stiffness of the core is preferably achieved by incorporating relatively large amounts of mineral filler material and/or by incorporating no or only small amounts of plasticizers into the thermoplastic material of the core layer. Preferably, the rigid floor or wall covering elements have a structural core consisting of a thermoplastic having a plasticizer content less than 5 % by weight. Plasticizers that could be used in the context of the present invention include: organic esters of various acids such as phthalic acid, phosphoric acid, adipic acid, sebacic acid or citric acid. Specific examples of plasticizers include dioctyl phthalate, dioctyl adipate, dibutyl sebacate, and dinonyl phthalate and glyceryl stearate. Alternative plasticizers include so-called

“bioplasticizers”, e.g. plasticizers derived from vegetable oils, such as soybean oil, canola oil, corn oil, linseed oil, rapeseed oil, safflower oil, sunflower oil, tall oil, tung oil, etc.

[0049] The thermoplastic of the structural core is, preferably, polyvinyl chloride.

Alternatives to PVC core layers include layers comprised of PE (polyethylene, including LDPE, HDPE etc.), PU (polyurethane), ABS (acrylonitrile butadiene styrene), PP (polypropylene), polyvinyl acetate (PVA), polyvinyl alcohol (PVOH), other vinyl and vinylidene resins and copolymers, polystyrene (PS), styrene copolymers, propylene copolymers, polyesters, acrylics, polyamide, polycarbonate (PC), polyimide, polysulfone, etc. The thermoplastic material can be virgin, recycled, or a mixture of both. Of all the cited thermoplastics, PVC is currently a preferred choice.

[0050] According to a preferred aspect of the method, application of the UV curable paste is done locally on the joints only (i.e. not throughout the entire surface of the floor or wall covering elements).

[0051] Preferably, the surface covering elements have a thickness in the range from 1 to 20 mm, preferably from 2 to 15 mm. [0052] The joints between adjacent surface covering elements preferably have a maximum visible width of 2 mm, more preferably of 1.5 mm and even more preferably of 1 mm.“Visible width” means the width of the joints at the visible surface of the surface covering elements. It is not excluded that the joint width is greater in portions that are hidden to the eye when the surface covering is in place, e.g., at the back side of the surface covering elements or on any connecting profiles (e.g. tongue and groove, etc.)

[0053] Preferably, the surface covering elements are panels having a length in the range from 1 to 4 m, more preferably from 2 to 3 m, and a width in the range from 0.4 to 1.5 m, more preferably from 0.6 to 1.2 m.

[0054] According to a preferred embodiment of the method, the curing is carried out with a portable UV curing unit, e.g. a handheld UV lamp.

[0055] It is worthwhile noting that the above embodiments and aspects of the method may be combined with one another. Brief Description of the Drawings

[0056] The accompanying drawings illustrate several aspects of the present invention and, together with the detailed description, serve to explain the principles thereof. In the drawings:

Fig. 1 : is a schematic perspective view of a wall covered with wall covering panels; Fig. 2: is an illustration of the application of UV-curable paste on a joint between adjacent panels of the wall of Fig. 1 ;

Fig. 3: is an illustration of the removal of excess UV-curable paste;

Fig. 4: is an illustration of the curing of the UV-curable paste in the joint.

Detailed Description of a Preferred Embodiment

[0057] Fig. 1 shows a wall on which wall covering elements 10 have been fixed. The wall covering elements 10 are relatively large and rigid panels. The panels may have a homogeneous structure (i.e. consist of a single layer of flooring material) or a heterogeneous structure (i.e. be built up from plural individual layers discernable as such). The type of fixture of the wall covering elements 10 to the substrate is not of importance: the wall covering elements 10 could be glued, screwed or otherwise mounted to the substrate. The term“substrate” herein designates the wall material that is located immediately behind the wall covering elements 10.

[0058] Joints 12 extend between adjacent wall covering elements 10. In the shown illustration, the length of the wall covering elements corresponds to the height of the wall surface to be covered, accordingly, only vertically extending joints are visible. Whereas it may be preferable, with regard to ease of installation, to provide wall covering elements 10 long enough to cover the entire height of the wall, it is not excluded that shorter wall covering elements 10 may be used. In the latter case, there will be horizontal joints as well. In general, the shape and orientation of the joints depend on the wall covering elements that are employed. Accordingly, the invention is not limited to horizontal and/or vertical joints but is equally applicable to wall coverings with oblique, curvilinear, zigzag, etc., joints.

[0059] When the wall covering elements 10 are in place, a UV-curable sealant paste 14 is applied onto and into the joints 12. This may be achieved with an applicator gun 16, as illustrated in Fig. 2, or with other tools, e.g. with a putty knife, a brush, a cloth, chosen in accordance with the consistency of the paste 14.

[0060] Any excess sealant paste is thereafter removed (Fig. 3). This may be done with a cloth 18, as illustrated, or a putty knife, or any other suitable tool. After removal of the excess sealant, sealant remains present only in the small interstices between the side edges of the wall covering elements 10 while the visible surfaces of the wall covering elements 10 are substantially free from sealant. In Figs. 3 and 4, the joint 12 filled with uncured sealant paste 14 is indicated by a dashed line.

[0061] Finally, as illustrated in Fig. 4, the UV-curable sealant paste 14 is cured using a handheld UV lamp 20. The latter is moved along the joint 12 filled with sealant paste 14, leaving the joint 12 hardly visible. The UV lamp 20 is moved with a speed that results in the desired irradiation dose and thus a sufficient degree of curing. In Fig. 4, the UV lamp 20 is moved downward but it could also be moved upward. The curing may be carried out in a single pass of the UV lamp or in multiple passes. It may be worthwhile noting that the joint 12 may already be hardly visible when the sealant paste 14 therein is still uncured, so the UV curing as such may have no or only a less important impact on the visibility of the joint than what Fig. 4 may prima facie suggest. The visibility of the joint 12 depends to some extent on its width: the wider the joint, the more visible it may be. Other parameters may have an impact on the joint visibility as well, for example the design of the decorative surface. Discontinuities in the design may lead to perceivable joints. However, it is an advantage of the invention that the sealant does not render the joints of the wall covering more apparent than intended by the decorative surface design. Example

[0062] Rigid PVC wall panels were glued to the substrate using double-sided adhesive tape. The widths of the joints between adjacent panels amounted to between 0.5 and 1.5 mm. The edges along the joints were wiped clean using an aqueous ethanol solution. UV-curable paste was then applied to the joints using a lint-free cloth. The UV-curable paste had the following composition:

[0063] After application of the UV-curable paste, the joints were inspected and excess UV-curable paste was removed with a clean lint-free cloth.

[0064] Curing was carried out using a portable LED UV lamp.

[0065] 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.