[0001] The present invention relates to a PVC-based surface covering, more specifically to a PVC-based surface comprising a varnish layer as a top coating, and to a method to produce such a surface covering.

Prior art and related technical background

[0002] Surface coverings, particularly PVC-based surface coverings, are known as "resilient" coverings and are widely used to decorate buildings or houses surfaces.

[0003] Such resilient surface coverings offer a good compromise in aesthetic and performances aspects, but they do not provide extra mechanical and chemical resistance properties. Generally the mechanical, chemical, moisture and stain resistance properties are commonly provided by surface treatments, and more particularly by chemical or radiation curing treatments .

[0004] Radiation curing surface treatments may commonly be used to enhance aesthetic aspect of surface coverings, but more important they are generally used to enhance resistance to mechanical stress, to chemicals and moisture.

[0005] For example, EP 1 164 149 discloses a varnish layer of acrylate-based composition which is cured by UV radiations .

[0006] However, PVC-based surface coverings, present the drawback of releasing VOCs (Volatile Organic Compounds) which are, for many of them, toxic components. Indeed, PVC-based surface coverings comprise PVC resins, plasticizers and stabilizers which contribute for part of them to VOC emission. Furthermore, for PVC-based surface coverings produced by coating technology, which represent the main part of produced articles, special additives, such as viscosity reducers, are used as processing helps, the most efficient viscosity reducers being low molecular weight molecules (lower then Ci ₆ ) . Such special additives contribute also to VOCs emission.

[0007] To address this issue, it was suggested to replace the volatile materials by non-volatile materials. However this leads to an important decrease of stiffness of the surface covering as the non-volatile materials have low volatility at process temperature while stiffness is an important requirement for resilient surface coverings, especially for floor coverings.

[0008] It was also suggested to incorporate in PVC-based surface coverings a gas barrier layer made of PVOH, EVOH or PVdC to inhibit or reduce the VOCs release. However, such gas barrier layer provides no or poor mechanical or stain resistance to surface covering and furthermore adhesion of these barrier layers to PVC-based substrate is main issue.

[0009] An acrylate-based photocured varnish layer applied on the top surface of a surface covering may prevent the emissions of a part or the majority of VOCs; however, it is difficult to apply such a coating uniformly on finished products having sharp or deep surface embossing. Moreover, when the covering is installed in heavy traffic areas for a long time, the varnish layer can be worn out and VOCs may be released again. Furthermore, to offer a good protection in an aggressive environment (heavy traffic, chemical attack, soiling...) , a thick varnish layer (thickness higher than 20 μηι) is generally required. The consequence is an increase of production costs and brittleness of the surface covering.

[0010] Gloss is an important parameter for aesthetic appearance of surface covering. It is generally measured at an angle of incidence from a line drawn perpendicular to the surface of 60° .

[0011] Gloss varies from very low gloss (1 to 5 at 60°) to regular gloss (15 to 20 at 60°) and high gloss (higher than 70 at 60°) . To achieve the desired level of gloss, a matting agent, mainly silica powder, is added to the varnish layer formulation. Generally a high amount of the matting agent allows to obtain a very low gloss, but leading in return to a decrease of the mechanical properties of the varnish layer. In particular, regarding to photocured acrylate-based formulations, because of the very high curing speed, low gloss is very difficult to obtain.

[0012] US 5888617 describes a method for the production of a matted surface by using acrylate or epoxide formulation cured by UV radiation of a wavelength of 172 nm. However even if very low gloss (lower than 5) can be obtained, higher gloss cannot be obtained with such technique.

[0013] Acoustic properties, such as for example sound absorption and dampening, are desired for PVC-based surface coverings. Good acoustic performances can be obtained with special back foam layer in a multilayer surface covering. However a good acoustic is obtained to the expense of lower indentation performance. This phenomenon is known as the indentation-acoustic compromise. A solution to come to this compromise is to combine a foamed layer with a layer of high modulus, which may for example be a cured varnish layer known to present a high modulus. However with existing curing technologies, only moderate high modulus could be obtained.

[0014] Therefore, there is no solution to provide a surface covering, economic to produce and having mechanical, stain and chemicals resistance properties, having gas barrier properties and having a wide range of gloss, and in particular no solution exists to provide a top coating providing such properties . Aims of the invention

[0015] The present invention aims to provide a PVC-based surface covering and a method to produce such a surface covering that does not have the drawbacks of the prior art.

[0016] The invention aims to provide a PVC-based surface covering having mechanical, stain and chemicals resistance properties, having gas barrier properties and having a controlled gloss.

[0017] The invention aims to provide a PVC-based surface covering which reaches a compromise between good acoustic and good indentation performances.

[0018] The invention aims to provide a PVC-based surface covering comprising a top coating which is an alternative to conventional UV cured acrylate-based coatings.

Summary of the invention

[0019] The present invention relates to a method to produce a PVC-based surface covering comprising a varnish layer, said method comprising the steps of providing a PVC-based surface covering, providing a varnish composition comprising a polyester acrylate resin and a polyester polyol resin, or a polyester resin comprising acrylate and polyol functions, or a polyester acrylate resin with grafted hydroxyl groups, a reactive thinner and at least one photo-initiator, applying said varnish composition on said PVC-based surface covering to form a varnish layer having a thickness of between 5 and 15μηι, partially curing said varnish composition over a thickness of 0 . 1 to 0 . 3 ]im using a monochromatic radiation having a wavelength comprised between 1 70 to 1 74 nm to form a partially cured varnish layer, and further curing said varnish composition over the remaining thickness of said varnish layer using a ultraviolet radiation having a wavelength comprised between 20 0 and 320 nm. [0020] According to particular embodiments, the method according to the invention may comprise one, or a combination, of any of the following characteristics:

- the varnish composition comprises a least one reticulation activator of the varnish composition,

- the varnish composition comprises a least one additive to adjust the gloss level of the varnish layer,

- the varnish composition comprises at least one filler and/or at least one wetting agent,

- the polyester acrylate resin and the polyester polyol resin, or the polyester resin comprising acrylate and polyol functions, or the polyester acrylate resin with grafted hydroxyl groups represent between 40 and 60% in weight of the total weight of the varnish composition,

- the reactive thinner represents between 40 and 60% in weight of the total weight of the varnish composition,

- the photo-initiator represents 0.1 and 20% in weight of the total weight of the varnish composition,

- the varnish composition has a viscosity of between 0.2 to 1.0 Pas,

- the varnish composition is applied on said PVC-based surface covering at a weight comprised between 5 to 20 g/m ² ,

- the step of further curing the varnish composition over the remaining thickness is performed using a ultraviolet radiation having a wavelength of around 240 nm,

- the PVC-based material is a multiple layer material comprising a PVC-based support layer, a PVC-based wear layer, and preferably a decorative layer.

[0021] The present invention also relates to a PVC-based surface covering comprising a varnish layer overlying said PVC- based surface covering, said varnish layer comprising a network of polyester acrylate and a polyester polyol resin, or of a polyester resin comprising acrylate and polyol functions, or of a polyester acrylate resin with grafted hydroxyl groups, said varnish layer having a reticulation level of between 9 0 to 95 % over a thickness of 0 . 1 to 0 . 3μηι, and having a gloss level, measured at 6 0 degrees, of between 3 to 30 .

[0022] According to particular embodiments, the surface covering according to the invention may comprise one, or a combination, of any of the following characteristics:

- the PVC-based surface covering comprises a PVC-based support layer, a decorative layer overlying said support layer and a PVC-based wear layer overlying said decorative layer,

- the PVC-based support layer is a foamed layer.

Short description of the drawings

[0023] Figure 1 represents schematically the varnish layer curing step of the method according to the invention.

Detailed description of the invention

[0024] The ability to produce a surface covering having mechanical, stain and chemicals resistance properties, having gas barrier properties, having a controlled gloss, and which reaches a compromise between good acoustic and good indentation performances is very desirable.

[0025] According to the invention, a PVC-based material

1 , preferably a PVC-based surface covering, is coated using any suitable method, with a varnish composition to form a varnish layer 2 , having a thickness of between 5 and 15 μηι, over the upper surface of a PVC-based material.

[0026] The varnish composition may be applied using spray coating, Mayer bar coating, roll coating, screen printing, flexo printing or rotogravure printing.

[0027] The coating weight of the varnish composition applied on a wear layer of a surface covering is preferably comprised between 5 to 20 g/m ² . [0028] The PVC-based material 1 is any suitable material to produce a surface covering. The PVC-based material may be a unique layer, but preferably, it comprises a PVC-based support layer and a PVC-based wear layer overlying said PVC-based support layer. More preferably the PVC-based material comprises a decorative layer overlying the support layer and/or a decorative layer between the PVC-based support layer and the PVC-based wear layer .

[0029] In preferred embodiments, the PVC-based material 1 may further comprise a backing layer onto which the PVC-based support layer is overlaid.

[0030] In preferred embodiments, the PVC-based support layer is a foamed layer.

[0031] The varnish composition according to the invention comprises a polyester acrylate resin and a polyester polyol resin, or more preferably a polyester resin comprising acrylate and polyol functions, or polyester acrylate resin with grafted hydroxyl groups. Preferably the polyester resin or polyester resins represent between 40 and 60% in weight of the total weight of the varnish composition.

[0032] The varnish composition further comprises a reactive thinner, also known as compound suitable to adjust the viscosity .

[0033] In a preferred embodiment, the reactive thinner is an acrylate monomer, mono or polyfunctional, like for example isobornyl acrylate, hexanedioldiacrylate, propoxylated glycerol triacrylate (OTA ^® 480 from Cytec) , triacrylate or tetraacrylate like alkoxylated pentaerythritol tetraacrylate (Ebecryl ^® 40 from Cytec) . The reactive thinner represents between 20 and 40% of the total weight of the varnish composition.

[0034] Preferably, the final viscosity of the varnish composition is comprised between 0.2 to 1.0 Pas. [0035] The varnish composition further comprises a photo- initiator which represents between 0.1 and 20% in weight of the total weight of the varnish composition.

[0036] The varnish composition may further comprise at least one reticulation activator to adjust the curing speed of the varnish composition, or at least one additive to adjust the gloss level of the varnish layer. The at least one reticulation activator or at least one additive may represent in weight between 20 to 40% of the total weight of the varnish composition .

[0037] In a preferred embodiment, the reticulation activator of the varnish composition is a polyfunctional acrylate monomer, like for example PETA (pentaerythritol triacrylate) or PETTA (pentaerythritol tetracrylate ) .

[0038] In a preferred embodiment, the additive to adjust the gloss level of the varnish layer is of the same nature as the reactive thinner. Therefore, it can be an acrylate monomer mono or polyfunctional like isobornyl acrylate, hexaneDioldiacrylate , propoxylated glycerol triacrylate (OTA ^® 480 from Cytec), triacrylate or tetraacrylate like alkoxylated pentaerythritol tetraacrylate (Ebecryl ^® 40 from Cytec).

[0039] The varnish composition may further comprise at least one filler, or a mixture of filler, and/or a wetting agent, which may represent in weight between 3 and 15% of the total weight of the varnish composition.

[0040] The at least one filler is preferably silica, alumina or polyamide particles .

[0041] Compositions used as comparative examples, being polyurethane-based varnish compositions, are given in table 1. The proportion of each compound is given in percentage in weight of the total weight of the composition. Table 1: Urethane Acrylate based compositions.

[0042] In table 1, as an example, the urethane acrylate oligomer is Desmolux ^® U100 from Bayer, the reactive thinner is an hexane diol diacrylate, for example Sartomer SR 238 from Cray Valley, the curing speed adjuster is an ethoxylated pentaerythritol tetraacrylate, for example Miramer 4004 from Rahn, the photoinitiator is Irgacure ^® 184 from Ciba, the wetting agent is silicone, for example Byk 307 from Byk, and the matting agent is silica, for example Syloid ^® RAD 2005 from Grace.

[0043] Examples of varnish compositions according to the invention are given in table 2.

Polyester-based varnish composition

[0044] In table 2, as an example, the polyester acrylate is Desmolux ^® XP 2744 from Bayer, or a polyester acrylate with grafted hydroxyl groups and is for example Desmolux ^® FW 5094 from Bayer, the polyester polyol is Desmophen ^® FW 1652 from Bayer, the reactive thinner is an hexane diol diacrylate, for example Sartomer SR 238 from Cray Valley, the curing speed adjuster is an ethoxylated pentaerythritol tetraacrylate, for example Miramer 4004 from Rahn, the photoinitiator is Irgacure ^® 184 from Ciba, the wetting agent is silicone, for example Byk 307 from Byk, and the matting agent is silica, for example Syloid ^® RAD 2005 from Grace.

[0045] The polyester acrylate, having a molecular weight of around 1000 and hard segment domains in its molecular chains, in combination with a polyester polyol and with a reactive thinner, like the hexane diol diacrylate (HDDA), allows getting a gloss which may be adapted to cover a large gloss range, i.e. from 3 to 30.

[0046] The varnish composition is applied on the PVC- based material 1 and is partially cured under Nitrogen using a first curing device 3 with a monochromatic radiation having a wavelength comprised between 170 and 174 nm, preferably a wavelength of 172 nm from a Xenon lamp. Due to limited penetration of this radiation, only the upper layer of the varnish composition is cured, over a thickness of 0.1 to 0.3μηι. The rest of the thickness of the varnish composition still remains liquid. As a consequence, the upper surface of the varnish layer 2 presents a shrinkage pattern which diffuses the light and generates low gloss aspect.

[0047] The partially cured varnish layer 2 is then cured through the remaining thickness of said varnish layer 2 using a second curing device 4 with an ultraviolet radiation having a wavelength comprised between 200 nm and 320 nm.

[0048] Preferably the second curing of the varnish layer

2 is performed using a high pressure Hg/UV radiation, preferably produced by a mercury arc lamp .

[0049] Compared to polyurethane-based varnish composition comprising an urethane acrylate oligomer (table 1), submitted to the same curing conditions, varnish compositions according to the invention offer additional performance in the form of a better gloss control, a better VOCs barrier layer property and a better indentation-acoustic compromise.

[0050] The gloss, the mechanical and stain properties, the VOCs barrier property were assessed for compositions used as comparative examples (table 1) and compositions according to the invention (table 2) .

[0051] Gloss comparison between compositions used as comparative examples (table 1) and compositions according to the invention (table 2) is given in table 3. The gloss is measured with a Byk/Gardner glossmeter at an angle of incidence of 60°. Table 3: Gloss, measured at 60°, of the varnish layer having the composition of tables 1 and 2.

[0052] From table 3, it appears that for the compositions used as comparative examples, the first curing using a monochromatic radiation having a wavelength of around 172 nm followed by the second curing using ultraviolet radiation having a wavelength comprised between 200 nm and 320 nm allows to obtain very low gloss level. While for compositions according to the invention the two step curing allows to get a large range gloss by adapting the amount of reactive thinner.

[0053] The stain resistance properties were assessed using a test reproducing household or hospital soiling and synthetic pigments soiling. A drop of soiling (liquid tar, shoe polish, marker and gouache paint for household soiling, iodine, betadine, picric acid based-liquid, i.e. Bouin liquid for hospital soiling and pigments in oil (Yellow 146 / Blue 670 / Red 500), in alcohol (Yellow 105 / Blue 644 / Red 335) and in water (Yellow 094 / Blue 762 / Red 316) for pigments soiling) is laid on the upper surface of the a surface covering 1 comprising the varnish layer 2 according to the invention. After a certain time depending on the type of the stain (table 4), the stain is removed with a paper tissue and washed with alcohol ( isopropanol ) . The residual soiling is rated for seventeen soiling agents with a mark comprised between 1 and 5, "1" being the best result obtained (no or few soiling is remaining) and "5" being the worst result obtained (a lot of soiling is remaining) . Individual ratings for each soiling agents are added, giving for soiling a total mark of "17" being the best result obtained and "85" being the worst. Table 4 : contact time of soiling

[0054] The mechanical resistance properties (scuff) were assessed using a test wherein the upper surface of a surface covering 1 with, or without, a 7μηι thick varnish layer 2 according to the invention is scratched by means of a rubber pad. The residual marking is rated between 0 and 15, "0" being the best result obtained (no or few marks on the varnish layer), "15" being the worst result obtained (a lot of marks on the varnish layer) .

[0055] The stain resistance and the mechanical resistance properties are given in table 5 for compositions of table 1 and table 2 submitted to the first and the second curing according to the invention.

Table 5: Stain and scuff comparison.

PVC PVC + PVC + without varnish layer varnish layer varnish layer composition Fl composition F4

Stain 60 19 19

Scuff 3 13 13 [0056] From table 5, it appears that for varnish compositions of tables 1 and 2, the first curing at 172 nm followed by the second curing between 200 nm and 320 nm gives very good stain and scuff performance in comparison to a PVC- based material without varnish layer.

[0057] The VOCs barrier property was assessed using a measure of VOCs emission according to ISO 16000-10. Surface covering samples with, or without, a 7 μηι thick varnish layer according to the invention are put under the FLEC (Field and Laboratory Emission Cell) and fluxed with dry air. After 24h, the volatile compounds are collected in a Tenax tube, thermal desorbed and analysed by GC/MS. The VOCs emission is classified as TVOC (compounds having a carbon number between C ₆ and Ci ₆ ) and SVOC (compounds having a carbon number between Ci ₆ and C ₂₂ ) . The results are given in table 6.

Table 6 : VOCs emission measurements

[0058] From table 6, it appears that the first curing at

172 nm followed by the second curing between 200 nm and 320 nm of compositions used as comparative examples (table 1) does not offer to the surface covering a complete protection towards VOC emission, while the surface covering comprising the varnish layer according to the invention presents improved protection.

[0059] The residual indentation performances of a surface covering with, or without, a 7 μηι thick varnish layer according to the invention is assessed by measuring according to NF EN 433 the residual indentation after having pressed the upper surface of the surface covering 1 with a stamp of 1 cm ² under a pressure of 50 kg. After 150 minutes, the pressure is released and the depth of the residual indentation is measured.

[0060] The residual indentation performances of the varnish layer according to the invention are given in table 7.

Table 7: Residual indentation

[0061] From table 7, it appears that, in comparison to a surface covering 1 without a varnish layer 2, a urethane acrylate-based varnish layer 2 improves the residual indentation of the surface covering due to the higher modulus of the varnish layer. However, a surface covering 1 comprising the varnish layer 2 according to the invention presents a higher modulus and thus an improved indentation property.

[0062] The surface covering 1 according to the invention has a gloss controlled curable varnish layer 2 as top coating and presents outstanding performances in terms of surface protection, scratch and stains resistance, combined with excellent barrier properties to chemicals and gases, especially to VOCs .