The present invention relates to an aqueous coating composition comprising synthetic polymers and inorganic fillers for coating and/or impregnating paper and nonwoven fabrics in general, in particular wallpapers as well as the use of such compositions.

Coating compositions for papers and nonwoven fabrics are generally known. However, known coating composition for such substrates, including for example wallpapers, include environmentally harmful substances, such as PVC and/or melamine (1 ,3,5-triazine-2,4,6-triamine) resins as well as volatile organic compounds. It is thus desirable to replace such compositions by more environmentally friendly water-based coating compositions. In addition, the compositions should provide for improved surface characteristics of the substrate, i.e. increase quality and performance, high embossing stability and barrier functions to improve water resistance.

The present invention meets this object and provides a water-based synthetic polymer containing coating composition for paper and nonwovens that provides for improved surface characteristics, good embossing stability, high water resistance, and increased production speeds. The invention is based on the inventors' surprising finding that by combining an aqueous dispersion of a synthetic polymer with a particle size d50 of < 2 μιη with an inorganic filler having a similar particle size, a high performance coating composition is obtained that upon application on paper or other nonwoven provides for a high quality substrate suitable for high speed digital printing, high embossing stability and high water resistance. It has further been found that the smaller the particle sizes of the fillers are the more stable the compositions get. In addition, smaller particle sizes have the advantage that the surface of the coated substrate gets smoother. d50 values of > 2 μιη decrease storage stability and increase roughness of the surface of the coated substrates.

In a first aspect, the present invention thus relates to an aqueous coating composition for paper or nonwoven fabrics, wherein the composition includes:

(a) 20 to 80 wt.-%, preferably 35 to 55 wt.-%, of an aqueous dispersion of one or more synthetic polymers; and

(b) 20 to 70 wt.-%, preferably 35 to 55 wt.-%, of an inorganic filler.

The composition is characterized in that the particle size d50 of the synthetic polymer particles in the dispersion is < 2 μιη and the ratio of the particle size d50 of the synthetic polymer particles and the particle size d50 of the inorganic filler particles is in the range of 2:1 to 1 :1

Another aspect of the invention is a method for coating a substrate, wherein the substrate is partially or completely coated with a coating composition as described herein. In still another aspect, the present invention also encompasses the coated substrate obtainable according to the methods described herein.

In a further aspect, the invention also relates to the use of a coating composition as described herein for coating or impregnating a substrate.

The coating compositions of the present invention are aqueous systems, more particularly dispersions, with the term "dispersion", as used herein, encompassing dispersions, emulsions and solutions with water as the main solvent. These dispersions comprise solids in form of synthetic polymers and inorganic filler particles, but can further comprise emulsifiers and/or surfactants, pigments and/or dyes and optionally further additives. The aqueous dispersions may comprise small amounts of organic solvents, but are preferably essentially free of such organic solvents. The polymers may be non-reactive, but can alternatively also be crosslinking systems.

The polymers are preferably free of halogen-containing components, such as PVC, or melamine. Preferred synthetic polymers in the compositions of the present invention are polymers on the basis of (meth)acrylates, polyvinyl esters, and polyvinyl alcohols. The synthetic polymers may be homopolymers, copolymers or mixtures of distinct polymers. "Synthetic polymer", as used herein, relates to a non- naturally occurring polymer that is produced synthetically from building blocks, such as monomers or prepolymers.

"One or more", as used herein, relates to at least one and comprises 1 , 2, 3, 4, 5, 6, 7, 8, 9 or more of the referenced species.

In various embodiments, suitable synthetic polymers are (co)polymerisates of olefinic unsaturated monomers. Preferred are water insoluble (co)polymers that are obtainable by radical polymerization of unsaturated monomers, such as vinyl ester polymers as homopolymers or copolymers, polyacrylic acid esters or polymethacrylic acid esters or copolymers with other monomers containing polymerisable double bonds.

Polyvinyl esters may be directly produced in form of dispersions or emulsions and are a preferred embodiment of the synthetic polymers of the present invention. In the polymers of the invention, the polymers predominantly comprise vinyl ester monomers, but can additional include polymerisable monomers that bear anionic functional groups, such as carboxy groups, unpolar, copolymerizable monomers, such as aromatic monomers or unsaturated carboxylic acid esters, or monomers with polar groups.

Copolymers are composed of different copolymerizable monomers. They can contain esters of vinyl alcohols and C2-C6 mono carboxylic acids, such as vinyl acetate, vinyl propionate, and vinyl n-butyrate. Such polymers may additionally comprise copolymerizable monomers that include acidic groups. Such acidic groups may be inorganic acid groups, such as sulfonic acid, phosphoric acid or phosphonic acid groups, or preferably carboxylic acid groups. Such monomers contribute to the dispersibility in water.

Exemplary polyacrylates are those obtainable by (co)polymerization of at least one (meth)acrylate monomer. The monomers may be polar or unpolar and may comprise additional functional groups. Also possible is the use of additional copolymerizable monomers. By means of the selection of monomers, molecular weight, glass transition temperature, degree of crosslinking, hydrophobicity and/or solubility may be controlled.

Suitable monomers include, without limitation, (meth)acrylate esters, such as alkyl (meth)acrylates of straight chain, branched chain or cycloaliphatic alcohols with 1 to 40 carbon atoms, such as, for example, methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, cyclohexyl (meth)acrylate, aryl (meth)acrylate, such as benzyl (meth)acrylate and phenyl (meth)acrylate, mono (meth)acrylate ethers, polyethylene glycols, polypropylene glycols or mixtures thereof with 5-80 carbon atoms, such as furfurylmethacrylate, 2- butoxyethylmethacrylate, poly(ethylene glycol)methyl ether-(meth)acrylate and poly(propylene glycol)methyl ether-(meth)acrylate. Also suitable are hydroxy-functionalized (meth)acrylates, such as hydroxyalkyl (meth)acrylates of straight chain, branched chain, or cycloaliphatic diols with 2-36 carbon atoms, for example 3-hydroxy propyl (meth)acrylate, 3,4-dihydroxybutyl mono (meth)acrylate, 2- hydroxyethyl (meth)acrylate, and 4-hydrocybutyl (meth)acrylate.

In addition to the afore-mentioned (meth)acrylates, the polymerizable compositions may comprise further unsaturated monomers which are copolymerizable with the (meth)acrylates disclosed above. Such copolymerizable monomers include, without limitation, acrylonitrile, vinyl esters, such as vinyl acetate, styrene, substituted styrenes, such as alpha-methyl styrene, vinyl toluene and p-methyl styrene, heterocyclic compounds, such as 2-vinyl pyridine, 3-vinyl pyridine, vinloxolane, vinyl furane, maleinic acid derivatives, such as maleinic acid anhydride, methyl maleinimide, and dienes, such as divinyl benzene, olefins, such as ethylene, butadiene, chlorobutadiene, isoprene, and other olefins without further functionalities, 1-alkenes, such as 1-hexene, branched alkenes, such as vinyl cyclohexane, itaconic acid, crotonic acid, maleinic acid, fumaric acid or monoesters thereof, (meth)acrylic acid, acryl amide, methacryl amide, epoxy acrylates or acrylates with other functional groups.

The afore-mentioned acrylates provide for a good water resistance of the coatings.

In various embodiments, the one or more synthetic polymers comprise at least one polyvinyl ester, preferably polyvinyl acetate. Even more preferred are combinations of such polyvinyl esters with acrylates or methacrylates, in particular an acrylate-styrene copolymer. Such polymer dispersions may additional comprise small amounts of polyvinyl alcohols. The polymers may be non-reactive, but it is equally possible to use (self-)crosslinking polymers that crosslink at a later stage, for example by an oxidative reaction.

All above described polymers are commercially available. The skilled person is readily in a position to select suitable polymers based on the desired molecular weight, polarity, hydrophobicity, crosslinking moieties, and dispersibility.

The synthetic polymers are used in the form of aqueous dispersions that have a solid content of 30 to 75 wt.-%, commonly 40-60 wt.-%. Such dispersions are contained in amounts of 20 to 80 wt.-%, preferably 35 to 55 wt.-%, in the aqueous coating composition.

In various embodiments, the aqueous coating composition comprises an aqueous dispersion of one or more synthetic polymers which includes 20 to 60 wt.-%, preferably 30 to 40 wt.-% relative to the complete coating composition of an aqueous dispersion of a polyvinyl ester homo- or copolymer, preferably a polyvinyl acetate homo- or copolymer; and 0 to 20 wt.-%, preferably 5 to 15 wt.-% relative to the complete coating composition of an aqueous dispersion of a (meth)acrylic polymer, preferably styrene(meth)acrylate polymer. In even more preferred embodiments, the aqueous dispersion of one or more synthetic polymers further comprises polyvinyl alcohol, in an amount of up to 20 wt.-%, preferably 1 to 5 wt.-% relative to the complete coating composition.

The coating compositions described herein further include at least one inorganic filler, preferably in form of a solid, such as a powder. The inorganic filler may be selected from all suitable filler substances known to those skilled in the art. The filler may also be a pigment. Suitable fillers include, but are not limited to, oxides, phosphates, sulfates or carbonates of aluminum, silicon, zirconium, titanium, zinc, iron, manganese, or the earth alkaline metals, in particular calcium and magnesium. Exemplary fillers include calcium carbonate, titanium dioxide, zinc oxide, iron oxide, magnesium oxide, silica, alumina, barium sulfate, calcium sulfate and kaolin, or mixtures thereof, preferably calcium carbonate. Also included are hydrates of the afore-mentioned substances

As already mentioned above, in the compositions described herein the particle size d50 of the synthetic polymer particles in the dispersion is < 2 μιη, preferably < 1 ,5 μιη. The particle size d50 means that 50% of the particles satisfy the given requirement, i.e. their diameter in the biggest dimension is 2 μιη or smaller and 1 ,5 μιη or smaller, respectively.

In various embodiments, the particle size d90 of the synthetic polymer particles is < 3,5 μιη, preferably < 3 μιη. Similar to d50 above, the particle size d90 means that 90% of the particles satisfy the given requirement, i.e. their diameter in the biggest dimension is 3,5 μιη or smaller and 3 μιη or smaller, respectively. I ne panicie size οτ me inorganic filler material is preferably in the same range as the particle sizes of the synthetic polymers. It has been found that a particularly high quality product can be obtained if both, the polymer particles and the filler particles have a similar size and size distribution, with d50 < 2 μιη, preferably < 1 ,5 μιη and/or d90 < 3,5 μιη, preferably < 3 μιη. Accordingly, in the aqueous coating composition described herein, the ratio of the particle size d50 of the synthetic polymer particles and the particle size d50 of the inorganic filler particles is in the range of 2:1 to 1 :1 . Optionally, also the ratio of the particle size d90 of the synthetic polymer particles and the particle size d90 of the inorganic filler particles is in the range of 2:1 to 1 :1.

According to the invention, particle sizes are determined by laser diffraction spectroscopy using a laser diffraction particle size analyzer, preferably the Beckman Coulter LS 13 320.

The coating compositions can further comprise one or more additives. Such additives include stabilizers, antioxidants, photo stabilizers, wetting agents. pH regulators, plasticizers, pigments, dyes, catalysts, surfactants, emulsifiers, preservatives, fragrances and biocides. The amounts of volatile organic compounds in the compositions are however as low as reasonably achievable. Preferably, the compositions are free of volatile organic compounds and/or PVC and/or melamine. A "volatile organic compound" or "VOC", as used herein, refers to organic chemicals that have a high vapor pressure at ordinary, room-temperature conditions and can do damage to visual or audible senses. Their high vapor pressure results from a low boiling point of less than or equal to 250 °C measured at standard atmospheric pressure (1 bar), which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and enter the surrounding air. One example for such a compound is, without limitation PVC.

The compositions may comprise a surface active agent. These include compounds that influence surface tension, such as foam stabilizers, defoamers, surfactants and wetting agents. Such compounds generally include hydrophilic and hydrophobic groups. Such compounds can be added already during production of the polymer dispersion or are later added upon production of the coating composition. They are used for controlling foaming, increasing wettability, and stabilizing water insoluble polymer and filler particles. Such agents may include anionic, nonionic or ampholytic surfactants.

Suitable anionic surfactants include, but are not limited to alkyl sulfates, alkyl or aryl ether sulfates, such as alkyl phenol ether sulfates, sulfonates, in particular fatty alcohol sulfonates, alkyl sulfonates, alkyl aryl sulfonates, mono- and diesters of sulfo succinic acid, all optionally ethoxylated, alkaline metal or ammonium salts of carboxylic acids, such as fatty acids, partial esters of phosphoric acid and alkaline metal and ammonium salts thereof. ¾unaoie ampnoiyiic surfactants include long chain substituted amino acids, such as N-alkyl- di(aminoeihyl)glycine or N-Alkyl-2-aminopropionic acid salts, betains, such as N-(3-acylamidopropyl)-N,N- dimethylammonium salts or alkyl imidazolium betains.

Exemplary nonionic surfactants include, but are not limited to alkyl, aryl and fatty alcohol polyglycol ethers, ethyleneoxide/propyleneoxide (EG7PO)-block copolymers, -fatty alcohols, and alkyl phenol adducts, preferably those with 8 to 50 EO/PO units; addition products of alkyl amines, fatty acids and resin acids, alkyl polyglycosides with linear or branched, saturated or unsaturated alkyl groups with 8 to 24 carbon atoms and an oligoglycoside moiety, natural materials and their derivatives, such as lecithin, lanolin or sarcosine, in particular those with alkoxy groups of up to 10 carbon atoms and up to 30 EO or PO units.

In various embodiments, the coating composition comprises at least one surface active agent, such as an anionic or nonionic surfactant, in an amount of 0,01 to 5,0 wt.-%, for example 0,1 to 2.5 wt.-% relative to the complete composition.

As preservatives benzoates, fluorides, such as sodium fluoride, 5-chloro-2-methyl-4-isothiazolin-3-one (CMIT), 2-Methyl-4-isothiazolin-3-one (MIT), Bronopol (2-bromo-2-nitropropane-1 ,3-diol), amidic substances or hydroxy benzoic acid ester can be used. Such agents are used in amounts of 0, 1 to 2.0 wt - %. Further suitable additives are photo stabilizers in amounts of up to 2 wt.-%, preferably 0, 1 to 1 wt.-%. Particularly useful are UV stabilizers, such as the so-called HALS (hindered amine light stabilizer) compounds.

Plasticizers are commonly used for controlling viscosity and flexibility and are usually present in amounts of 0 to 20 wt.-% , preferably up to 10 wt.-%, in particular below 2 wt.-%. Generally, it is desirable to keep the content of plasticizers as low as possible and the coating compositions described herein are advantageous in that they reduce the amount of plasticizer needed compared to common PVC-based compositions by more than 90 %. Exemplary plasticizers that may be used include, but are not limited to medicinal white oils, naphthenic mineral oils, polypropylene, polybutylene and polyisoprene oligomers, hydrogenated polyisoprene and/or polybutadiene oligomers, benzoate esters, phthalates, adipates, vegetable and animal oils and their derivatives. Hydrogenated plasticizers may for example be selected from the group of paraffin hydrocarbons. Polyproyplene glycol and polybutylene glycol as well as polymethylene glycol are also suited. Also esters can be used, such as liquid poly esters and glycerol esters or plasticizers on the basis of aromatic dicarboxylic acid esters. It is preferred that the compositions do not contain organic solvents.

The compositions may also include pigments, for example also in form of a dye paste. The amount of such pigments is usually below 30 wt.-%, preferably in the range of 5 to 20 wt.-%. Suitable pigments include those disclosed as fillers above. i ne compositions aescnoea herein are preferably flee flowing liquids. In various embodiments they have a viscosity in the range of 500 to 10.000 mPas, preferably 1 .000 to 8.000 mPas at 25°C. (Brookfield, EN, ISO 2555).

The coating compositions described herein can be produced from the described components by methods generally known in the art. The synthetic polymer is generally used in form of an aqueous dispersion in which all further components are dispersed. To obtain good storage stability, it is crucial to ensure a good distribution of the filler and pigments particles. Accordingly, one exemplary method for the production of an aqueous coating composition as described herein method comprises the steps of:

(a) providing an aqueous dispersion of one or more synthetic polymers;

(b) providing the inorganic filler in form of a powder;

(c) disperging the inorganic filler in the aqueous dispersion of one or more synthetic polymers by means of an automatic rotor-stator disperger.

Suitable automatic dispergers are known and commercially available. One exemplary disperger suited for this production method is the ystral Conti-TDS (Ystral, DE).

The composition should generally have a solid content in the range of 30 to 70 wt.-% (DIN 53189 at 105°C). The dispersing agent is predominantly water. The pH can be controlled by neutralizing agents and is usually in the range of 5 to 9.

The density of the composition is for example in the range of 1 ,0 to 1.7 g/cm ³ , as measured by a pycnometer.

In various embodiments, the invention also relates to a method for coating a substrate, wherein the substrate is partially or completely coated with a coating composition, the method comprising

(i) applying an aqueous coating composition as described herein to said substrate to obtain a coated substrate, and

(ii) drying said coated substrate.

For the methods of coating a substrate as described herein, the substrates are usually provided in shape of a web. The coating composition is then continuously applied on this substrate, either partially or completely. The application can be performed by a roll, nozzle, coating knife, screen or flexo printing. The aqueous coating composition can be applied in an amount of 10 to 100 g/m ² , preferably 20 to 40 g/m ² . After application, the applied coating composition is usually smoothened to ensure an even distribution and uniform thickness of the coating. The coated substrate may be dried, for example in an oven. Drying can be performed by warm air or irradiation with IR. The temperature in the drying step is usually in the range of 70 to 220°C and is selected such that a rapid drying is obtained. However, the temperature is also selected such that it does not adversely influence the structural integrity of the coating or the suDsiraie. rreierrea arying temperatures are in the range of 150 to 200°C and the drying time is 1 to 60 seconds, preferably 2 to 30 seconds.

The coated substrate can then be further processed. For example, it can be rolled up and stored. Alternatively, it may be subjected to printing, embossing and/or further coating. The coated substrates described herein are particularly suited for high speed digital printing due to their outstanding surface characteristics. After this processing, it can be cut to a desired length and stored. Suitable techniques are known in the art.

The substrate coated with the compositions of the present invention is preferably composed of hydrophilic polymers selected from the group consisting of polyesters and/or natural fibers, preferably cellulose or lignin. In various embodiments, the substrate is paper or a nonwoven fabric, preferably in form of a web. The nonwoven fabrics preferably comprise or consist of cellulose fibers.

Alternatively, the coating compositions described herein can be used as a binder that is directly added to the substrate, such as paper or nonwoven fabric, during its production.

In various embodiments, the present invention also encompasses the substrates obtainable by the above described coating methods or by adding the composition as a binding agent during production of the substrate.

The substrates obtained by the methods described herein include a base material that is usually flexible and is on one or both sides coated with a coating obtained by applying and drying a coating composition as described herein. The base material is preferably paper or a cellulose-based nonwoven fabric and is preferably used as a wallpaper. In one embodiment, the substrate, the coating or the surface of the coating is thus printed or dyed. In alternative or additional embodiments, the substrate or the coating is partially or completely embossed, preferably by cold embossing.

Also encompassed by the present invention is the use of the coating compositions described herein for coating or impregnating a substrate. The substrate may be as defined above. The coating or impregnating may be achieved by the above-described methods. txampies

Example 1 : Preparation of coating compositions

Coating compositions according to the present invention were prepared by dispersing fine powder filler materials, namely calcium carbonate, in an aqueous dispersion of polyvinyl acetate by means of an automatic rotor-stator disperger (ystral Conti-TDS) and the particle sizes of the different educts as well as the thus produced coating compositions were measured by laser diffraction in a Beckman Coulter LS 13 320 or in a Mastersizer S by Malvern Instruments. The results are shown in Table 1.

Table 1 : Particle sizes and particle size distribution of different compositions and educts

Product d10 (μιη) d50 (μιη) d90 (μιη) Particles < 2 μιη

(%)

Coating composition 1 (according to 0,304 1 ,235 3,610 70

the invention: ) Figure 1A

Micral 1 14 %

Micral Opac extra 14 %

Acrylate Dispersion 7 %

KaMin 70C 14 %

Wormalit PM 5208 35 %

Water 16 %

Coating composition 2 (according to 0,270 1 ,122 3,490 75

the invention: ) Figure 1A

Micral 1 1 1 %

Micral Opac extra 1 1 %

Micral 95 T 6 %

Acrylate Dispersion 7 %

KaMin 70C 14 %

Wormalit PM 5208 35 %

Wasser 16 %

Polyvinyl acetate dispersion 0,494 1 ,367 2,809 74

(Wormalit PM 5208 Henkel AG &

Co.KGaA, Porta Westfalica, DE)

Acrylate dispersion 0,562 1 ,647 3,432 70

KaMin 70 C (Calcined Kaolin Clay) - 1 ,3 - -

Micral 1 (calcium carbonate filler; 1 ,0 70

S.A. Reverte, ES)

Micral Opac extra (calcium 0,9

carbonate filler; S.A. Reverte, ES) Microcarb 95 T (calcium carbonate - 0,65 - 94

filler; H. Heller GmbH, Wuppertal,

DE)

Example 2: SEM analysis of paper coated with the coating composition

The surface of paper coated with the coating composition of the invention (see Example 1 , composition 1 ) was analyzed by scanning electron microscopy (SEM). Images of the surface of the coating according to the invention are shown in Figure 1A. As a reference, SEM images of paper coated with known coating compositions (Table 2) are provided in Figures 1 B and 1 C. It is clearly apparent that the coating compositions of the present invention provide for a much smoother surface, which is desirable for high speed printing purposes.

Table 2: Particle sizes and particle size distribution of different compositions and educts as standard reference

Product d 10 (μιη) d50 (μιη) d90 (μιη) Particles < 2 μιη

(%)

Omycarb 2 - GU ( Calcium 2,5 40

carbonate filler; Omya GmbH

Cologne )

Polyvinyl acetate dispersion 0,494 1 ,367 2,809 74

(Wormalit PM 5208 Henkel AG &

Co.KGaA, Porta Westfalica, DE)

Reference 1 ( Figure 1 B ) 0,562 1 , 178 7,206 45

Polyvinyl acetate dispersion 72 %

Omycarb 2 - GU 28 %

Reference 2 ( Figure 1 C ) 0,347 1 ,647 5,413 45

Polyvinyl acetate dispersion 80 %

Omycarb 2 - GU 20 %