US20140251563A1

US20140251563A1 - Use in paper coatings of a mixture of a secondary polymeric dispersion and of a primary dispersion of an emulsion polymer

Info

Publication number: US20140251563A1
Application number: US14/355,430
Authority: US
Inventors: Dirk Lawrenz; Hubertus Kroener
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2011-11-09
Filing date: 2012-11-07
Publication date: 2014-09-11
Also published as: CN103917565A; KR20140095536A; WO2013068363A1; EP2776473A1

Abstract

The use in paper coatings is described of a mixture of an aqueous secondary dispersion of a polymer selected from polyalkylene carbonates, polyesters and polyethylene and an aqueous primary dispersion of an emulsion polymer having a glass transition temperature of below 50° C.

Description

The present invention relates to the use in paper coatings of a mixture of an aqueous secondary dispersion of certain polymers and of an aqueous primary dispersion of an emulsion polymer useful as binder in paper coatings.
Paper coating binders typically utilize emulsion polymers based on suitable monomers such as, for example, styrene, butadiene, acrylic esters, acrylonitrile, vinyl acetate and other monomers polymerizable using emulsion polymerization technology. These binders are produced as so-called primary dispersions, i.e., directly in the form of aqueous polymeric dispersions, but are frequently inconvenient and costly to produce.
There is a whole series of polymers which, owing to their polymer properties, might in principle likewise be useful as binders for paper coating, but they cannot be produced by emulsion polymerization. Examples include polyesters, polypropylene carbonate, polyethylene and other substances which are typically polymerized in organic solvents or in the absence of any solvent and are obtained as mostly high-viscosity resins or solutions in organic solvents. They also include some natural-based resins as well as chemically modified natural resins, for example modified rosins. Converting these resins into an aqueous form of dispersion requires the technology of secondary dispersal, which typically involves emulsifiers and water being added under high shear to obtain a secondary dispersion. However, this manner of production is scarcely able to produce polymer particles on the order of below 200 nm which are particularly suitable for paper coating. The secondary dispersions are generally too coarse for paper coatings.
Furthermore, appreciable quantities of emulsifier are often needed to stabilize the secondary dispersion, adversely affecting paper coating performance characteristics. Therefore, secondary dispersions have hitherto achieved practically no industrial significance as binders for paper coating. The coarse and emulsifier-rich secondary dispersions have a distinctly weaker pigment-binding capacity than the finer and emulsifier-lean emulsion polymers.
The problem addressed by the present invention was that of expanding the spectrum of polymers useful for paper coating and more particularly of polymers which are inexpensive and convenient to produce, and of providing alternative binders for paper coating compositions having very high binding power.
It was found that mixtures of secondary dispersions of certain polymers not produced by emulsion polymerization, with primary dispersions of certain emulsion polymers do not have the typically expected disadvantage of lower binding power of secondary dispersions to the expected degree and therefore are useful as binders for paper coating. As described in the examples hereinbelow, synergistic effects can be evidenced over a wide mixing range.
The present invention provides for the use of a mixture of

(a) an aqueous secondary dispersion of at least one polymer selected from the group consisting of polyalkylene carbonates, polyesters and polyethylene, and
(b) an aqueous primary dispersion of at least one emulsion polymer having a glass transition temperature of below 50° C.,
in paper coatings.

The present invention also provides a paper coating composition comprising

(a) an aqueous secondary dispersion of at least one polymer selected from the group consisting of polyalkylene carbonates, polyesters and polyethylene,
(b) an aqueous primary dispersion of at least one emulsion polymer having a glass transition temperature of below 50° C., and preferably in the range from −10 to +30° C., and
(c) inorganic pigments.

The present invention also provides paper or card coated with a paper coating composition of the present invention.
The average diameter of polymeric particles can be measured by hydrodynamic chromatography (HDC). In HDC, a colloidal sample elutes from a size-exclusion separation column sorted according to hydrodynamic radius. The eluent comprises salt, nonionic surfactants and anionic surfactants. Elution time is calibrated with PS calibration latices. Measurement range extends from 15 nm to 1200 nm—larger components are filtered out and not detected. Diameter and weight fractions can be measured to an accuracy of 3%. The fractions are weighted using the UV absorption at 254 nm.
The glass transition temperature can be determined as differential scanning calorimetry midpoint temperature (ASTM D 3418-08).
The weight ratio of emulsion polymer to secondary dispersion polymer is preferably in the range from 1:2 to 2:1.
The glass transition temperature of secondary dispersion polymers is preferably in the range from −50 to +50° C., while any partial crystallinity of polymers can lead to special effects (melting point). Suitable combinations of primary and secondary dispersions (e.g., low glass transition temperature of primary dispersion and high glass transition temperature of secondary dispersion) here expand the spectrum of possible polymers.
The average particle size of secondary dispersion polymers is preferably below 1 μm and more preferably below 400 nm, but not less than 50 nm.
Useful secondary dispersions include for example secondary dispersions based on polyalkylene carbonates and preferably based on polypropylene carbonate. Unlike aqueous polymeric dispersions where polymer chains comprise a backbone constructed of carbon atoms, aqueous dispersions of polyalkylene carbonates are not obtainable via emulsion polymerization. On the contrary, polymers of this type are generally prepared via polycondensation of aliphatic diols and phosgene or via polyaddition of aliphatic oxiranes onto CO₂in the presence of suitable catalysts in a nonaqueous medium and after solvent removal are typically obtained as solids. To obtain aqueous dispersions, they have to be dispersed in water. Several processes are known for dispersing polyalkylene carbonates in water. There is one type of process where high shearing forces are applied while the molten polymer is emulsified in the aqueous dispersing medium, which comprises surface-active substances, and subsequently cooled down. Processes of this type are described in U.S. Pat. No. 4,320,041, DE 4115531, EP 1302402, EP 1514891 and WO 97/49762. In the other type of process, the polymer dissolved in an organic, preferably water-miscible solvent is mixed with the aqueous dispersing medium and subsequently the organic solvent is removed. Processes of this type are described in U.S. Pat. No. 3,238,173, U.S. Pat. No. 3,726,824 and WO 2007/074042. Further processes for producing secondary dispersions of polyalkylene carbonates are described in WO 2006/136555, PCT/EP2011/054471 and EP application numbered 11162705.5.
Preference for use as polyalkylene carbonates is given to aliphatic polyalkylene carbonates that are predominantly constructed of repeat units of formula (I). They may additionally further comprise repeat units of formula (II):
—[O—(C═O)—O-A]— (I)
—[O-A]— (II)
where A is alkane-1,2-diyl of 2 to 10 carbon atoms or cycloalkane-1,2-diyl of 5 to 10 carbon atoms and may have different meanings within any one polymer. A is preferably selected from alkane-1,2-diyl radicals, especially those of 2 to 4 carbon atoms, e.g., 1,2-ethanediyl, 1,2-propanediyl, 1,2-butanediyl, 1-methyl-1,2-propanediyl and 2-methyl-1,2-propanediyl. In one specific embodiment of the present invention, A is 1,2-propanediyl to a predominant extent, i.e., to an extent of at least 70 mol % and especially to an extent of at least 80 mol % or at least 90 mol %, based on all repeat units. In this case, the aliphatic polycarbonate is polypropylene carbonate. The proportion of carbonate repeat units of formula I in the polycarbonate is dependent on the reaction conditions as well as particularly the catalyst used. In the preferred polycarbonates, more than 80 mol % and preferably more than 90% of all repeat units are repeat units of formula I.
Aliphatic polycarbonates are generally prepared by reacting aliphatic oxiranes, i.e., alkylene oxides, having in general from 2 to 10 carbon atoms or cycloalkylene oxides having in general from 5 to 10 carbon atoms, with CO2 in the presence of one or more suitable catalysts, see for example Inoue, Makromol. Chem., Rapid Commun. 1, 775 (1980), Soga et al., Polymer Journal, 1981, 13, 407-10, U.S. Pat. No. 4,789,727 and U.S. Pat. No. 7,304,172. Suitable catalysts are in particular zinc and cobalt catalysts as described for example in the aforementioned references and especially in U.S. Pat. No. 4,789,727 and U.S. Pat. No. 7,304,172. Examples of suitable polyalkylene carbonates are the polyethylene carbonates known from EP-A 1264860, which are obtained by copolymerization of ethylene oxide and carbon dioxide in the presence of suitable catalysts, and especially polypropylene carbonate (see WO 2007/125039 for example), obtainable by copolymerization of propylene oxide and carbon dioxide in the presence of suitable catalysts. The polymer is also commercially available, for example from Empower Materials Inc. or Aldrich.
The number average molecular weight Mn of polyalkylene carbonates and especially of polypropylene carbonates is generally in the range from 5000 to 500 000 daltons and especially in the range from 10 000 to 250 000 daltons. Weight average molecular weight Mw is then typically in the range from 7000 to 5 000 000 daltons and especially in the range from 15 000 to 2 000 000 daltons. In one specific embodiment of the present invention, the number average molecular weight Mn of polypropylene carbonates is in the range from 50 000 to 100 000 daltons and specifically in the range from 70 000 to 90 000 daltons. Weight average molecular weight Mw is then typically in the range from 100 000 to 500 000 daltons and especially in the range from 150 000 to 400 000 daltons. The proportion of carbonate repeat units included in the total amount of carbonate and ether repeat units in the polymer is generally at least 80 mol %, especially 90 mol %. Polydispersity (ratio of weight average (MW) to number average (MN)) is generally between 1 and 80 and preferably between 2 and 10. The polypropylene carbonates used may comprise up to 1% of carbamate and urea groups.
Useful aliphatic polycarbonates also include chain-extended polyalkylene carbonates. Chain extenders used for polyalkylene carbonates are especially maleic anhydride, acetic anhydride, di- or polyisocyanates, di- or polyoxazolines or -oxazines or di- or polyepoxides. Examples of isocyanates are aromatic diisocyanates such as toluoylene 2,4-diisocyanate, toluoylene 2,6-diisocyanate, 2,2′-diphenylmethane diisocyanate, 2,4′-diphenylmethane diisocyanate, 4,4′-diphenylmethane diisocyanate, naphthylene 1,5-diisocyanate or xylylene diisocyanate and aliphatic diisocyanates such as especially 1,6-hexamethylene diisocyanate, isophorone diisocyanate or methylenebis(4-isocyanatocyclohexane). Particular preference is given to aliphatic diisocyanates and of these especially to isophorone diisocyanate and particularly 1,6-hexamethylene diisocyanate. Useful bisoxazolines include 2,2′-bis(2-oxazoline), bis(2-oxazolinyl)methane, 1,2-bis(2-oxazolinyl)ethane, 1,3-bis(2-oxazolinyl)propane or 1,4-bis(2-oxazolinyl)butane, especially 1,4-bis(2-oxazolinyl)benzene, 1,2-bis(2-oxazolinyl)benzene or 1,3-bis(2-oxazolinyl)benzene. Chain extenders are preferably used in amounts of 0.01% to 5%, more preferably 0.05% to 2% and even more preferably 0.08% to 1% by weight, based on the polycarbonate quantity. Chain-extended polyalkylene carbonates typically have a number average molecular weight Mn in the range from 30 000 to 500 000 daltons, preferably in the range from 35 000 to 250 000 daltons and more preferably in the range from 40 000 to 150 000 daltons.
Useful secondary dispersions include for example secondary dispersions based on polyesters. The polyesters are preferably thermoplastic polymers comprising a multiplicity of ester groups and/or carbonate groups in the polymer backbone and having an acid number of preferably not more than 10 mg of KOH/g, and that includes biodegradable polyesters. Unlike aqueous polymeric dispersions where polymer chains comprise a backbone constructed of carbon atoms, aqueous dispersions of polymers comprising a multiplicity of ester groups and/or carbonate groups in the polymer backbone are generally not obtainable via an emulsion polymerization process. On the contrary, it is usually necessary to prepare such polymers by way of a polycondensation and to convert them subsequently into an aqueous dispersion, i.e., into a so-called secondary dispersion. There are several ways of doing this in principle. First, the dissolved polymer in organic, preferably water-miscible solvent can be mixed with the aqueous dispersing medium and the organic solvent removed again. Polymers having a high acid number can in turn be emulsified in water by rendering the aqueous dispersing medium alkaline with a base in order thereby to deprotonate the carboxyl groups and thereby to further the self-emulsification of the polymer. Such a procedure is described in WO 98/12245 for example. Further ways to prepare polyester secondary dispersions are described in EP 1302502 A1, US 2005/058712, US 2002/0076639, U.S. Pat. No. 6,521,679 and WO 2011/117308.
The polyesters typically have a number average molecular weight MN in the range from 5000 to 1 000 000 daltons, especially in the range from 8000 to 800 000 daltons and specifically in the range from 10 000 to 500 000 daltons. The weight average molecular weight Mw of polyesters is generally in the range from 20 000 to 5 000 000 daltons, frequently in the range from 30 000 daltons to 4 000 000 daltons and especially in the range from 40 000 to 2 500 000 daltons. The polydispersity index MW/MN is generally at least 2 and frequently in the range from 3 to 20 and especially in the range from 5 to 15. Molecular weight and polydispersity index can be determined via gel permeation chromatography (GPC) as per DIN 55672-1 for example. The viscosity number of polyesters, which is indirect measure of the molecular weight, is typically in the range from 50 to 500 ml/g, frequently in the range from 80 to 300 ml/g and especially in the range from 100 to 250 ml/g (determined to EN ISO 1628-1 at 25° C. on a 0.5% by weight solution of polymer in 1:1 (w/w) o-dichlorobenzene/phenol.
Polyesters used may be amorphous or partly crystalline, branched or unbranched.
An aliphatic polyester is a polyester constructed exclusively from aliphatic monomers. An aliphatic copolyester is a polyester constructed exclusively from at least two and especially at least three aliphatic monomers, wherein the acid component and/or the alcohol component preferably comprises at least two mutually different monomers. An aliphatic-aromatic copolyester is a polyester constructed not only from aliphatic monomers but also from aromatic monomers, wherein the acid component preferably comprises at least one aliphatic acid and at least one aromatic acid.
Aliphatic polyesters and copolyesters are particularly polylactides, polycaprolactone, block copolymers of polylactide with poly(C2-C4 alkylene glycol), block copolymers of polycaprolactone with poly(C2-C4 alkylene glycol) and also the hereinbelow defined copolyesters which are constructed from at least one aliphatic or cycloaliphatic dicarboxylic acid or an ester-forming derivative thereof and at least one aliphatic or cycloaliphatic diol component and also optionally further components.
Copolyesters, especially aliphatic or aliphatic-aromatic copolyesters, constructed from at least one aliphatic or cycloaliphatic dicarboxylic acid or an ester-forming derivative thereof and at least one aliphatic or cycloaliphatic diol component and optionally one or more aromatic dicarboxylic acids or their ester-forming derivatives or mixtures thereof and also optionally further components are also suitable.
Useful aromatic dicarboxylic acids are generally aromatic dicarboxylic acids having 8 to 12 carbon atoms and preferably aromatic dicarboxylic acids having 8 carbon atoms. Examples are terephthalic acid, isophthalic acid, 2,6-naphthoic acid and 1,5-naphthoic acid and also ester-forming derivatives thereof. Especially the di-C1-C6-alkyl esters, e.g., dimethyl, diethyl, diethyl, di-n-propyl, diisopropyl, di-n-butyl, diisobutyl, di-t-butyl, di-n-pentyl, diisopentyl or di-n-hexyl esters, are suitable. The anhydrides of dicarboxylic acids a2 are similarly suitable ester-forming derivatives. In principle, however, it is also possible to use aromatic dicarboxylic acids having a larger number of carbon atoms, for example up to 20 carbon atoms. Aromatic dicarboxylic acids or ester-forming derivatives thereof can be used singly or as mixture of two or more thereof. Particular preference is given to using terephthalic acid or its ester-forming derivatives such as dimethyl terephthalate.
In general, the diols are selected among branched or linear alkanediols having 2 to 12 carbon atoms, preferably 4 to 8 or especially 6 carbon atoms, or cycloalkanediols having 5 to 10 carbon atoms. Examples of suitable alkanediols are ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,4-butanediol, 1,5-pentanediol, 2,4-dimethyl-2-ethylhexane-1,3-diol, 2,2-dimethyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-isobutyl-1,3-propanediol, 2,2,4-trimethyl-1,6-hexanediol, especially ethylene glycol, 1,3-propanediol, 1,4-butanediol and 2,2-dimethyl-1,3-propanediol (neopentylglycol); cyclopentanediol, 1,4-cyclohexanediol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol or 2,2,4,4-tetramethyl-1,3-cyclobutanediol. Mixtures of different alkanediols can also be used. In these copolyesters, diol component B is preferably selected among C2-C12 alkanediols and mixtures thereof. Preference is given to 1,3-propanediol and especially 1,4-butanediol.
Terephthalic acid and the aliphatic dicarboxylic acid can be used as free acid or as ester-forming derivatives. Useful ester-forming derivatives include especially the di-C1-C6-alkyl esters, e.g., dimethyl, diethyl, di-n-propyl, diisopropyl, di-n-butyl, diisobutyl, di-tert-butyl, di-n-pentyl, diisopentyl or di-n-hexyl esters. Anhydrides of dicarboxylic acids can likewise be used. The diol is preferably 1,4-butanediol.
Useful polymers for secondary dispersions also include hydrocarbon waxes, for example hydrocarbon waxes produced via free-radical polymerization in the high-pressure process or in the presence of organometallic catalysts in the low-pressure process, and having an average molar mass range of 2000-20 000 (mass average), especially polyethylene waxes.
Oxidation with air or oxygen gives more polar oxidized PE waxes (polyethylene wax oxidates) which are somewhat easier to convert with surfactants into aqueous wax dispersions.
Suitable secondary dispersions are obtainable either by precipitation of wax dissolved in hot solvent or vegetable oil, by controlled cooling under agitation (precipitated waxes, typical particle size 0.5 to 10 μm) or by emulsification of molten hot wax in water with subsequent cooling. The particle size of latter wax preparations is generally in the region around 100 nm.
Useful secondary dispersions include for example secondary dispersions based on polyethylene and are available as polyethylene wax emulsions under the designation Poligen®, e.g. Poligen® WE1 or Poligen® WE6 with a particle size of about 100 nm and a molecular weight between 2700 and 11 000. The dispersions have a solids content of about 35% coupled with viscosities <500 mPas (Brookfield).
The glass transition temperature of primary dispersion polymers is preferably in the region of below 50° C. and more preferably in the range from −30 to +30° C.
The average size of primary dispersion polymer particles is preferably below 200 nm and more preferably in the range from 80 to 160 nm.
Polymers useful as binders in the primary dispersion are obtainable as emulsion polymer via free-radically initiated emulsion polymerization from one or more ethylenically unsaturated, free-radically polymerizable monomers in the presence or absence of a chain transfer agent composition. The polymeric binders have a glass transition temperature Tg of less than 50° C. and preferably below 30° C. The glass transition temperature can be determined as differential scanning calorimetry midpoint temperature (ASTM D 3418-08).
Useful ethylenically unsaturated, free-radically polymerizable monomers may be selected from the group consisting of vinylaromatic compounds, conjugated aliphatic dienes, ethylenically unsaturated acids, ethylenically unsaturated carboxamides, ethylenically unsaturated carbonitriles, vinyl esters of saturated C₁to C₂₀carboxylic acids, esters of acrylic acid or methacrylic acid with monohydric C₁to C₂₀alcohols, allyl esters of saturated carboxylic acids, vinyl ethers, vinyl ketones, dialkyl esters of ethylenically unsaturated dicarboxylic acids, N-vinylpyrrolidone, N-vinylpyrrolidine, N-vinylformamide, N,N-dialkylaminoalkylacrylamides, N,N-dialkylaminoalkylmethacrylamides, N,N-dialkylaminoalkyl acrylates, N,N-dialkylaminoalkyl methacrylates, vinyl halides, aliphatic hydrocarbons having 2 to 8 carbon atoms and one or two double bonds, or mixtures thereof.
The emulsion polymer consists to an extent which is preferably at least 40% by weight, more preferably at least 60% by weight and even more preferably at least 80% by weight of so-called principal monomers. Principal monomers are selected from C1-C20 alkyl (meth)acrylates, vinyl esters of carboxylic acids comprising up to 20 carbon atoms, vinylaromatics having up to 20 carbon atoms, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of alcohols comprising 1 to 10 carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and one or two double bonds, or mixtures thereof. Examples include alkyl (meth)acrylates having a C1-C10 alkyl moiety, such as methyl methacrylate, methyl acrylate, n-butyl acrylate, ethyl acrylate and 2-ethylhexyl acrylate. Mixtures of alkyl (meth)acrylates are also suitable in particular. Vinyl esters of carboxylic acids having 1 to 20 carbon atoms include, for example, vinyl laurate, vinyl stearate, vinyl propionate, vinyl versatate and vinyl acetate. Useful vinylaromatic compounds include vinyltoluene, α-methylstyrene, p-methylstyrene, α-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene and preferably styrene. Examples of nitriles are acrylonitrile and methacrylonitrile. Vinyl halides are chlorine-, fluorine- or bromine-substituted ethylenically unsaturated compounds, preferably vinyl chloride and vinylidene chloride. Specific examples of vinyl ethers are vinyl methyl ether and vinyl isobutyl ether. Preference is given to vinyl ethers of alcohols comprising 1 to 4 carbon atoms. Specific examples of hydrocarbons having 2 to 8 carbon atoms and one or two olefinic double bonds are ethylene, propylene, butadiene, isoprene and chloroprene.
Preferred principal monomers are C1-C10 alkyl (meth)acrylates and mixtures thereof with vinylaromatics, more particularly styrene (also referred together as polyacrylate binders) or hydrocarbons having 2 double bonds, more particularly butadiene, or mixtures of such hydrocarbons with vinylaromatics, more particularly styrene (also referred together as polybutadiene binders). In the case of polybutadiene binders, the weight ratio of butadiene to vinylaromatics (more particularly styrene) can be for example between 10:90 to 90:10, more particularly 20:80 to 80:20. Polybutadiene binders are particularly preferred.
In addition to the principal monomers, the polymer may comprise further monomers, for example monomers having carboxylic acid, sulfonic acid or phosphonic acid groups. Preference is given to carboxylic acid groups. Specific examples are acrylic acid, methacrylic acid, itaconic acid, maleic acid or fumaric acid and aconitic acid. The level of ethylenically unsaturated acids in the emulsion polymer is generally below or equal to 10% by weight, for example from 0.1% to 10% by weight. Further monomers include, for example, hydroxyl-containing monomers, more particularly C1-C10 hydroxyalkyl (meth)acrylates, or amides such as (meth)acrylamide.
In one embodiment of the present invention the emulsion polymer is constructed from butadiene or mixtures of butadiene and styrene to an extent of at least 60% by weight or from C1 to C20 alkyl (meth)acrylates or mixtures of C1 to C20 alkyl (meth)acrylates and styrene to an extent of at least 60% by weight.
Preferred polymeric binders are

(a) copolymers from (a1) 19.8 to 80 parts by weight of at least one vinylaromatic compound, preferably styrene or methylstyrene, (a2) 19.8 to 80 parts by weight of at least one conjugated aliphatic diene, preferably butadiene, (a3) 0.1 to 10 parts by weight of at least one ethylenically unsaturated acid, preferably acrylic acid and/or methacrylic acid, and (a4) 0 to 20 parts by weight of at least one other monoethylenically unsaturated monomer, wherein the parts by weight of monomers (a1) to (a4) sum to 100;
(b) copolymers from (b1) 19.8 to 80 parts by weight of at least one vinylaromatic compound, preferably styrene or methylstyrene, (b2) 19.8 to 80 parts by weight of at least one acrylate monomer selected from C1 to C10 alkyl acrylates and C1 to C10 alkyl methacrylates, preferably methyl acrylate, ethyl acrylate, n-butyl acrylate, ethylhexyl acrylate, propylheptyl acrylate or their mixture, (b3) 0.1 to 10 parts by weight of at least one ethylenically unsaturated acid, preferably acrylic acid and/or methacrylic acid, and (a4) 0 to 20 parts by weight of at least one other monoethylenically unsaturated monomer, wherein the parts by weight of monomers (a1) to (a4) sum to 100;
(c) copolymers from vinyl acetate and at least one (meth)acrylate monomer selected from C1 to C10 alkyl acrylates and C1 to C10 alkyl methacrylates, and
(d) ethylene/vinyl acetate copolymers.

One embodiment of the present invention utilizes as monomers

(A1) 19.8 to 80 parts by weight, preferably 25 to 70 parts by weight, of at least one vinylaromatic compound,
(B1) 19.8 to 80 parts by weight, preferably 25 to 70 parts by weight, of at least one conjugated aliphatic diene,
(C1) 0.1 to 15 parts by weight of at least one ethylenically unsaturated acid, and
(D1) 0 to 20 parts by weight, preferably 0.1 to 15 parts by weight, of at least one further monoethylenically unsaturated monomer other than said monomers (A1) to (C1),
wherein the parts by weight of monomers (A1) to (D1) sum to 100.

One embodiment of the present invention utilizes as monomers

(A2) 19.8 to 80 parts by weight, preferably 25 to 70 parts by weight, of at least one vinylaromatic compound,
(B2) 19.8 to 80 parts by weight, preferably 25 to 70 parts by weight, of at least one monomer selected from C1 to C18 alkyl esters of acrylic acid and C1 to C18 alkyl esters of methacrylic acid,
(C2) 0.1 to 15 parts by weight of at least one ethylenically unsaturated acid, and
(D2) 0 to 20 parts by weight, preferably 0.1 to 15 parts by weight, of at least one further monoethylenically unsaturated monomer other than said monomers (A2) to (02),
wherein the parts by weight of monomers (A2) to (D2) sum to 100.

The monomers of group (A1)/(A2) are vinylaromatic compounds, for example styrene, α-methylstyrene and/or vinyltoluene and their mixture. Of this group of monomers, styrene is preferred. 100 parts by weight of total monomer mixtures used in the polymerization comprise for example from 19.8 to 80 parts by weight and preferably from 25 to 70 parts by weight of at least one monomer of group (A1)/(A2).
Examples of monomers of group (B1) are 1,3-butadiene, isoprene, 1,3-pentadiene, dimethyl 1,3-butadiene and cyclopentadiene. Of this group of monomers, 1,3-butadiene and/or isoprene are preferred. 100 parts by weight of monomer mixtures used altogether in the emulsion polymerization comprise for example from 19.8 to 80 parts by weight, preferably from 25 to 70 parts by weight and especially from 25 to 60 parts by weight of at least one monomer of group (B1).
Examples of monomers of group (C1)/(C2) are ethylenically unsaturated carboxylic acids, ethylenically unsaturated sulfonic acids and vinylphosphonic acids and salts thereof. Ethylenically unsaturated carboxylic acids used are preferably α,β-monoethylenically unsaturated mono- and dicarboxylic acids having 3 to 6 carbon atoms in the molecule. Examples thereof are acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, vinylacetic acid and vinyllactic acid. Useful ethylenically unsaturated sulfonic acids include for example vinylsulfonic acid, styrenesulfonic acid, acrylamidomethylpropanesulfonic acid, sulfopropyl acrylate and sulfopropyl methacrylate. Particular preference is given to acrylic acid and methacrylic acid, especially acrylic acid. The group (C1)/(C2) monomers comprising acid groups may be used in the polymerization as free acids and also after partial or complete neutralization with suitable bases. Aqueous sodium hydroxide solution, aqueous potassium hydroxide solution or ammonia is preferably used as neutralizing agent. 100 parts by weight of monomer mixtures used in the emulsion polymerization comprise for example from 0.1 to 15 parts by weight, preferably from 0.1 to 10 parts by weight or from 1 to 8 parts by weight of at least one monomer of group (C1)/(C2).
Useful monomers of group (B2) include esters of acrylic aid and methacrylic acid with monohydrate C₁to C₁₈alcohols such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-propyl acrylate, n-propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, pentyl acrylates, pentyl methacrylates, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate. 100 parts by weight of total monomer mixtures used in the polymerization comprise for example from 19.8 to 80 parts by weight and preferably from 25 to 70 parts by weight of at least one monomer of group (B2).
Monomers of group (D2) are other monoethylenically unsaturated compounds. Examples thereof are ethylenically unsaturated carboxamides such as more particularly acrylamide and meth-acrylamide, ethylenically unsaturated carbonitriles such as more particularly acrylonitrile and methacrylonitrile, vinyl esters of saturated C₁to C₁₈carboxylic acids, preferably vinyl acetate, allyl esters of saturated carboxylic acids, vinyl ethers, vinyl ketones, dialkyl esters of ethylenically unsaturated dicarboxylic acids, N-vinylpyrrolidone, N-vinylpyrrolidine, N-vinylformamide, N,N-dialkylaminoalkylacrylamides, N,N-dialkylaminoalkylmethacrylamides, N,N-dialkylaminoalkyl acrylates, N,N-dialkylaminoalkyl methacrylates, vinyl chloride and vinylidene chloride. Useful monomers of group (D1) include the monomers of group (D2) and also esters of acrylic acid and of methacrylic acid with monohydric C₁to C₁₈alcohols such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-propyl acrylate, n-propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, pentyl acrylates, pentyl methacrylates, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate. This group of monomers is optionally used to modify the polymers. 100 parts by weight of monomer mixtures used in the emulsion polymerization comprise for example from 0 to 20 parts by weight or from 0.1 to 15 parts by weight and especially from 0.5 to 10 parts by weight of at least one monomer of group (D1)/(D2).
In one embodiment of the present invention, the further monomers (D1) and (D2) are each used in amounts of 0.1-15 parts by weight; the vinylaromatic compound is selected from styrene, methylstyrene and their mixture; the conjugated aliphatic diene is selected from 1,3-butadiene, isoprene and their mixture; and the ethylenically unsaturated acid is selected from one or more compounds of the group consisting of acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, vinylacetic acid, vinyllactic acid, vinylsulfonic acid, styrenesulfonic acid, acrylamidomethylpropanesulfonic acid, sulfopropyl acrylate, sulfopropyl methacrylate, vinylphosphonic acid and salts thereof.
The emulsion polymerization typically uses initiators that form free radicals under the reaction conditions. Initiators are used for example in amounts up to 2% by weight, preferably at not less than 0.9% by weight, for example in the range from 1.0% to 1.5% by weight, based on the monomers to be polymerized. Suitable polymerization initiators include, for example, peroxides, hydroperoxides, hydrogen peroxide, sodium persulfate, potassium persulfate, redox catalysts and azo compounds such as 2,2-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2-azobis(2,4-dimethylvaleronitrile) and 2,2-azobis(2-amidinopropane) dihydrochloride. Examples of further suitable initiators are dibenzoyl peroxide, tert-butyl perpivalate, tert-butyl per-2-ethylhexanoate, di-tert-butyl peroxide, diamyl peroxide, dioctanoyl peroxide, didecanoyl peroxide, dilauroyl peroxide, bis(o-tolyl) peroxide, succinyl peroxide, tert-butyl peracetate, tert-butyl permaleate, tert-butyl perisobutyrate, tert-butyl perpivalate, tert-butyl peroctoate, tert-butyl perbenzoate, tert-butyl hydroperoxide, azobisisobutyronitrile, 2,2″-azobis(2-methylbutyronitrile), 2,2″-azobis(2,4-dimethylvaleronitrile) and 2,2″-azobis(N,N″-dimethyleneisobutyroamidine) dihydrochloride. Initiators are preferably selected from the group consisting of peroxodisulfates, peroxosulfates, azo initiators, organic peroxides, organic hydroperoxides and hydrogen peroxide. Particular preference is given to using water-soluble initiators, for example sodium persulfate, potassium persulfate, ammonium persulfate, sodium peroxodisulfate, potassium peroxodisulfate and/or ammonium peroxodisulfate. The polymerization can also be initiated by means of high-energy rays such as electron beams or irradiation with UV light.
The amount of chain transfer agents is for example in the range from 0.01% to 5% and preferably in the range from 0.1% to 1% by weight, based on the monomers used in the polymerizetion. The chain transfer agents are preferably added together with the monomers. However, they can also be partly or wholly present in the initial charge. They can also be added in stages at different times to the monomers.
To augment the dispersal of the monomers in the aqueous medium, the protective colloids and/or emulsifiers customarily used as dispersants can be used. A detailed description of suitable protective colloids is given in Houben-Weyl, Methoden der organischen Chemie, volume XIV/1, Makromolekulare Stoffe, Georg-Thieme-Verlag, Stuttgart, 1961, pages 411 to 420. Suitable emulsifiers include surface-active substances whose number average molecular weight is typically below 2000 g/mol or preferably below 1500 g/mol, while the number average molecular weight of the protective colloids is above 2000 g/mol, for example in the range from 2000 to 100 000 g/mol and more particularly in the range from 5000 to 50 000 g/mol. Suitable emulsifiers include, for example, ethoxylated C₈to C₃₆fatty alcohols having a degree of ethoxylation in the range from 3 to 50, ethoxylated mono-, di- and tri-C₄-C₁₂-alkylphenols having a degree of ethoxylation in the range from 3 to 50, alkali metal salts of dialkyl esters of sulfosuccinic acid, alkali metal and ammonium salts of C₈to C₁₂alkyl sulfates, alkali metal and ammonium salts of C₁₂to C₁₈alkylsulfonic acids and alkali metal and ammonium salts of C₉to C₁₈alkylarylsulfonic acids. Cation-active emulsifiers are, for example, compounds having at least one amino or ammonium group and at least one C₈to C₂₂alkyl group. When emulsifiers and/or protective colloids are used as auxiliaries to disperse the monomers, the amounts used thereof are for example in the range from 0.1% to 5% by weight, based on the monomers.
Useful protective colloids include for example degraded starch, especially maltodextrin. Useful starting starches for preparing the degraded starches include all native starches such as starches from maize (corn), wheat, oats, barley, rice, millet, potatoes, peas, tapioca, sorghum or sago. Also of interest are those natural starches which have a high amylopectin content such as wax maize starch and wax potato starch. The amylopectin content of these starches is above 90%, usually in the range from 95 to 100%. Starches modified chemically by etherification or esterification can also be used for preparing the polymer dispersions of the present invention. Such products are known and commercially available. They are prepared for example by esterification of native starch or degraded native starch with inorganic or organic acids, their anhydrides or chlorides. Of particular interest are phosphated and acetylated degraded starches. The most common method to etherify starches consists in treating starch with organic halogen compounds, epoxides or sulfates in aqueous alkaline solution. Known starch ethers are alkyl ethers, hydroxyalkyl ethers, carboxyalkyl ethers and allyl ethers. The reaction products of starches with 2,3-epoxypropyltrimethylammonium chloride are also useful. Particular preference is given to degraded native starches, more particularly native starches degraded to maltodextrin. Further suitable starches include cationically modified starches, i.e., starch compounds having amino groups or ammonium groups. The degraded starches have for example an intrinsic viscosity η_iof less than 0.07 dl/g or less than 0.05 dl/g. The intrinsic viscosity η_iof the degraded starches is preferably in the range from 0.02 to 0.06 dl/g. The intrinsic viscosity η_iis determined in accordance with DIN EN1628 at a temperature of 23° C.
In one embodiment of the present invention, the emulsion polymerization is carried out in the presence of seed particles. The initial charge then comprises polymer seed, especially a polystyrene seed, i.e., an aqueous dispersion of finely divided polymer, preferably polystyrene, having a particle diameter of 20 to 40 nm.
The emulsion polymerization takes place in an aqueous medium. The aqueous medium may comprise for example completely ion-free water or else mixtures of water and a miscible solvent such as methanol, ethanol or tetrahydrofuran. As soon as the particular polymerization temperature desired is reached or within the time span of 1 to 15 minutes, preferably 5 to 15 minutes after reaching the polymerization temperature, the metered addition of the monomers is commenced. They can be for example pumped into the reactor continuously within for example 60 minutes to 10 hours, usually within 2 to 4 hours. Preferably, the reaction mixture in the initial charge is heated to the requisite temperature at which the polymerization proceeds. These temperatures are for example from 80 to 130° C. and preferably from 85 to 120° C. The polymerization can also be carried out under superatmospheric pressure, for example at pressures up to 15 bar, e.g., at 2 to 10 bar. Monomer addition can take the form of a batch, continuous or staged operation.
After the polymerization has ended, further initiator may optionally be added to the reaction mixture and a postpolymerization performed at the same temperature as the main polymerization or else at a lower or higher temperature. To complete the polymerization reaction, it will in most cases suffice to stir the reaction mixture at the polymerization temperature for example 1 to 3 hours after addition of all the monomers. The pH in the polymerization can be for example in the range from 1 to 5. After polymerization, the pH is adjusted to a value of between 6 and 7 for example. An aqueous polymer dispersion is obtained whose dispersed particles have an average particle diameter of preferably 80 to 200 nm. The average particle diameter of the polymer particles can be determined by dynamic light scattering on a 0.005% to 0.01% by weight aqueous polymer dispersion at 23° C. by means of an Autosizer IIC from Malvern Instruments, England. The reported data are all based on the cumulant z-average diameter of the measured autocorrelation function as per ISO standard 13321.
The mixture of aqueous secondary dispersion and aqueous primary dispersion is used in the present invention for producing paper coating compositions. Paper coating compositions, in addition to water, generally comprise pigments, binders and optionally auxiliaries for setting the requisite rheological properties, for example thickeners. The pigments are typically dispersed in water. The paper coating composition comprises pigments in an amount of preferably at least 80% by weight, for example 80% to 95% by weight or 80% to 90% by weight, based on the total solids content. White pigments are contemplated in particular. Suitable pigments include, for example, metal salt pigments such as, for example, calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate and calcium carbonate, of which carbonate pigments, more particularly calcium carbonate, are preferred. The calcium carbonate may be natural ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), lime or chalk. Suitable calcium carbonate pigments are available for example as Covercarb® 60, Hydrocarb® 60 or Hydrocarb® 90 ME. Further suitable pigments include, for example, silicas, aluminas, aluminum hydrate, silicates, titanium dioxide, zinc oxide, kaolin, argillaceous earth, talc or silicon dioxide. Suitable further pigments are available for example as Capim® MP 50 (Clay), Hydragloss® 90 (Clay) or Talcum C10.
The paper coating composition comprises as binder the polymers present in the above-described primary and secondary dispersions. The most important functions of binders in paper coating compositions are to bind the pigments to the paper and the pigments to each other and to some extent fill voids between pigment particles. For every 100 parts by weight of pigments, the amount of organic binder used (in terms of binder solids, i.e. without water or other solvent liquid at 21° C., 1 bar) is for example in the range from 1 to 50 parts by weight, preferably in the range from 1 to 25 parts by weight or in the range from 5 to 20 parts by weight.
Optional further binders include natural-based binders, more particularly binders based on starch. A binder based on starch is in this context to be understood as referring to any native, modified or degraded starch. Native starches can consist of amylose, amylopectin or mixtures thereof. Modified starches may comprise oxidized starch, starch esters or starch ethers. Hydrolysis can be used to reduce the molecular weight of the starch (degraded starch). Possible degradation products include oligosaccharides or dextrins. Preferred starches are cereal starch, maize starch and potato starch. Particular preference is given to cereal starch and maize starch and very particular preference is given to cereal starch.
Paper coating compositions of the present invention may additionally comprise further addition and auxiliary substances, for example fillers, cobinders and thickeners to further optimize viscosity and water retention, optical brighteners, dispersants, surfactants, lubricants (e.g., calcium stearate and waxes), neutralizing agents (e.g., NaOH or ammonium hydroxide) for pH adjustment, defoamers, deaerators, preservatives (biocides for example), flow control agents, dyes (soluble dyes in particular), etc. Useful thickeners in addition to synthetic polymers (crosslinked polyacrylate for example) include particularly celluloses, preferably carboxymethylcellulose. Optical brighteners are, for example, fluorescent or phosphorescent dyes, particularly stilbenes.
The paper coating composition of the present invention preferably comprises an aqueous paper coating composition; water is present therein particularly due to the make-up form of the constituents (aqueous polymer dispersions, aqueous pigment slurries); the desired viscosity can be set by adding further water. Customary solids contents of paper coating compositions range from 30% to 70% by weight. The pH of the paper coating composition is preferably adjusted to values in the range from 6 to 10, more particularly in the range from 7 to 9.5.
One embodiment of the present invention relates to a paper coating composition wherein the polymers of the emulsion polymer and of the secondary dispersion are used in an amount of altogether 1 to 50 parts by weight, based on the total amount of pigments, and wherein the pigments are present in an amount of 80 to 95 parts by weight, based on total solids content.
The pigments are preferably selected from the group consisting of calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate, calcium carbonate, silicas, aluminas, aluminum hydrate, silicates, titanium dioxide, zinc oxide, kaolin, argillaceous earth, talc and silicon dioxide.
The paper coating composition preferably additionally comprises at least one auxiliary sub-stance selected from the group consisting of thickeners, further polymeric binders, cobinders, optical brighteners, fillers, flow control agents, dispersants, surfactants, lubricants, neutralizing agents, defoamers, deaerators, preservatives and dyes.
In a preferred paper coating composition the polymeric particles of the emulsion polymer have an average particle size in the range from 80 to 200 nm and the polymeric particles of the polymer of the secondary dispersion have an average particle size in the range from 100 to 400 nm.
The present invention also provides paper or card coated with a paper coating composition of the present invention and a process for coating paper or card, which comprises

- providing a paper coating composition according to the present invention; and
- applying the paper coating composition to at least one surface of paper or card.

The paper coating composition is preferably applied to uncoated base papers or uncoated card. The amount is generally in the range from 1 to 50 g, and preferably in the range from 5 to 30 g (in terms of solids, i.e., without water or other solvent liquid at 21° C., 1 bar) per square meter. Coating can be effected by means of customary methods of application, for example via size press, film press, blade coater, air brush, doctor blade, curtain coating or spray coater. Depending on the pigment system, the paper coating compositions of the present invention can be used for the basecoat and/or for the topcoat.
Paper coating compositions according to the present invention have good performance characteristics. They have a high binding force and are obtainable in a convenient and inexpensive manner. Papers coated with paper coating compositions are readily printable in the customary printing processes, such as relief printing, gravure, offset, digital, inkjet, flexographic, newsprint, letterpress, sublimation printing, laser printing, electrophotographic printing or a combination thereof.

EXAMPLES

Unless the context suggests otherwise, percentages are always by weight. A reported content is based on the content in aqueous solution or dispersion.
Primary dispersion P1:
Styronal® D 809 (50% aqueous polymer dispersion based on carboxylated styrene/butadiene copolymer); particle size: 160 nm, glass transition temperature 20° C.
Primary dispersion P2:
Acronal® S 728 (50% aqueous polymer dispersion based on carboxylated styrene/butyl acrylate copolymer); particle size: 175 nm, glass transition temperature 20° C.
Secondary dispersion S1:
Polypropylene carbonate dispersed in water
Mn ca. 80 000 g/mol
Average particle size by HDC: 390 nm
Secondary dispersion S2
Ecoflex® (polyester dispersion in water; aliphatic/aromatic copolyester based on terephthalic acid, adipic acid and 1,4-butanediol)
Secondary dispersion S3
Poligen® WE1 (about 35% strength aqueous polyethylene wax emulsion)

Paper Coating Composition:

The coating slip is prepared in a stirred assembly (Deliteur) into which the individual components were fed in succession. The pigments are added in pre-dispersed form (as a slurry). The other components are added after the pigments, the order corresponding to the order in the recited coating slip formulation. The final solids content is set by adding water.
Paper coating slip compositions were prepared using mixtures of dispersion P1 with dispersions S1 to S3 as a binder of the following composition:
70 parts by weight of Hydrocarb® 60 slurry (coarse calcium carbonate)
30 parts by weight of fine clay (Amazon 88)
0.3 part by weight of dispersant (Polysalz S; polyacrylic acid)
0.22 part by weight of Sterocoll® FD rheology modifier
9 parts by weight of binder (see Table 1)
pH set to about 9.0 with aqueous sodium hydroxide solution
Brookfield viscosity about 2000-4000 mPas
solids content: 65-66% by weight

TABLE 1

Binder mixing ratios (parts
by weight based on solids)

	Example	P1	P2	S1	S2	S3

1	2		1
2	1		1
3	1		2
4		2	1
5		1	1
6		1	2
7			1
8	2			1
9	1			1
10	1			2
11				1
12					1
13					1
14					2
15					1

The coating slip is applied to one side of a paper substrate using a pilot-scale coating machine. Coating layer add-on was 10 g/m².
The coated paper was tested for surface resistance using test methods known to a person skilled in the art. The following test methods were used:
IGT dry pick resistance
IGT wet pick resistance

Offset Test

The results are summarized in Table 2.

Measurement of Dry Pick Resistance Using IGT Tester (IGT Dry)

Strips were cut out of the in-test papers and printed using the IGT tester. The printing inks used are specialty test inks from Lorillieux, which transmit different tensile forces. The test strips are fed through the press at continuously increasing speed (maximum speed 200 cm/s). For evaluation, the point at which 10 picks have occurred on the paper surface after the start of printing is determined on the sample printing strip. The measure reported for dry pick resistance is the speed in cm/s present at this point during printing and also the test ink used. The higher this printing speed at the tenth pick point, the better the quality rating of the paper surface.

Measurement of Wet Pick Resistance Using IGT Tester (IGT Wet)

Strips were cut out of the in-test papers and printed using the IGT tester. The tester was set up such that the test strips are moistened with water before printing. The printing inks used are specialty test inks from Lorilleux (No. 3807), which transmit different tensile forces. The print is performed at a constant speed of 0.6 cm/s. Picks from the paper surface are visible as unprinted areas. To determine wet pick resistance, an ink densitometer is used to determine ink density as a % age of the full hue. The higher the reported ink density, the better the wet pick resistance.

Offset Test:

Samples having a size of 240×46 mm are cut out of the in-test papers in the longitudinal direction. An appropriate amount of printing ink is applied to the inking roll and left to run for 1 minute. A printing disk is then inserted and inked for 30 s. The printing speed is 1 m/s. A paper strip is brought back to the starting position on a printing test support with the printed paper strip. After a specified time interval, the printing process is started again without replacing the printing disk. This operation is repeated more than once. After each printing cycle, the pick on the printed side of the paper strip is assessed by visual inspection. The table reports the number of cycles before picking occurred for the first time. The higher the number of cycles up to the occurrence of picking, the better the suitability of the papers for offset printing.

TABLE 2

Measured binding force results

	Dry pick	Wet pick
	resistance	resistance	Offset
Example	[cm/s]	[%]	cycles

1	56	31	3.0
2	51	20	3.0
3	45	9	2.5
4	99	89	3.0
5	87	81	2.5
6	47	21	2.25
7	12	4	1.25
8	56	48	3.0
9	49	25	2.75
10	37	14	2.25
11	13	20	1.0
12	40	16	3.25
13	32	11	3.0
14	21	10	2.0
15	0	9	1.0

As the experimental results show, the non-inventive coating slips of examples 7, 11 and 15 featuring the straight secondary dispersions as binders have relatively poor values in respect of the important offset printing properties of dry pick resistance. Binder mixtures with primary dispersions display a disproportionately large increase in pigment binding capacity starting with a mixing ratio of just 1:2.

Claims

1. A process for improving the properties of a paper coating, the process comprising combining a paper composition with a binder comprising a mixture of:

(a) an aqueous secondary dispersion of at least one polymer selected from the group consisting of a polyalkylene carbonate, a polyester and a polyethylene; and

(b) an aqueous primary dispersion of at least one emulsion polymer having a glass transition temperature of below 50° C.,

to form an improved paper coating.

2. The process of claim 1,

wherein the emulsion polymer has a glass transition temperature in the range from −10 to +30° C.

3. The process of claim 1,

wherein the at least one polymer of the secondary dispersion has a glass transition temperature in the range from −50 to +50° C.

4. The process of claim 1,

wherein the weight ratio of the emulsion polymer to the polymer of the secondary dispersion is in the range from 1:2 to 2:1.

5. The process of claim 1, wherein the emulsion polymer is obtained by a free-radically initiated emulsion polymerization from one or more ethylenically unsaturated, free-radically polymerizable monomers selected from the group consisting of a vinylaromatic compound, a conjugated aliphatic diene, an ethylenically unsaturated acid, an ethylenically unsaturated carboxamide, a ethylenically unsaturated carbonitrile, a vinyl ester of an saturated C₁to C₂₀carboxylic acid, an ester of acrylic acid or methacrylic acid with a monohydric C₁to C₂₀alcohol, an allyl ester of a saturated carboxylic acid, a vinyl ether, a vinyl ketone, a dialkyl ester of an ethylenically unsaturated dicarboxylic acid, N-vinylpyrrolidone, N-vinylpyrrolidine, N-vinylformamide, an N,N-dialkylaminoalkylacrylamide, an N,N-dialkylaminoalkyl acrylate, an N,N-dialkylaminoalkyl methacrylate, a vinyl halide, an aliphatic hydrocarbon having 2 to 8 carbon atoms and one or two double bonds, and a mixture thereof.

6. The process of claim 1, wherein the emulsion polymer is selected from the group consisting of

(i) a copolymer from 19.8 to 80 parts by weight of at least one vinylaromatic compound, 19.8 to 80 parts by weight of at least one conjugated aliphatic diene, 0.1 to 10 parts by weight of at least one ethylenically unsaturated acid, and 0 to 20 parts by weight of at least one other monoethylenically unsaturated monomer, wherein the parts by weight of monomer sum to 100;

(ii) a copolymer from 19.8 to 80 parts by weight of at least one vinylaromatic compound, 19.8 to 80 parts by weight of at least one acrylate monomer selected from C1 to C10 alkyl acrylates and C1 to C10 alkyl methacrylates, or their mixture, 0.1 to 10 parts by weight of at least one ethylenically unsaturated acid, and 0 to 20 parts by weight of at least one other monoethylenically unsaturated monomer, wherein the parts by weight of monomer sum to 100;

(iii) a copolymer from vinyl acetate and at least one (meth)acrylate monomer selected from C1 to C10 alkyl acrylates and C1 to C10 alkyl methacrylates, and

(iv) an ethylene-vinyl acetate copolymer.

7. The process of claim 1, wherein the emulsion polymer is constructed from butadiene or mixtures of butadiene and styrene to an extent of at least 60% by weight or from C1 to C20 alkyl (meth)acrylates or mixtures of C1 to C20 alkyl (meth)acrylates and styrene to an extent of at least 60% by weight.

8. The process of claim 1, wherein the emulsion polymer is prepared from

(A1) 19.8 to 80 parts by weight of at least one vinylaromatic compound,

(B1) 19.8 to 80 parts by weight of at least one conjugated aliphatic diene,

(C1) 0.1 to 15 parts by weight of at least one ethylenically unsaturated acid, and

(D1) 0 to 20 parts by weight of at least one further monoethylenically unsaturated monomer other than said monomers (A1) to (C1);

or

(A2) 19.8 to 80 parts by weight of at least one vinylaromatic compound,

(B2) 19.8 to 80 parts by weight of at least one monomer selected from C1 to C18 alkyl esters of acrylic acid and C1 to C18 alkyl esters of methacrylic acid,

(C2) 0.1 to 15 parts by weight of at least one ethylenically unsaturated acid, and

(D2) 0 to 20 parts by weight of at least one further monoethylenically unsaturated monomer other than said monomers (A2) to (C2),

wherein the parts by weight of monomers (A1) to (D1) or (A2) to (D2) sum to 100 in either case.

9. The process of claim 8, wherein:

said monomers (D1) and (D2) are included in amounts of 0.1-15 parts by weight;

the vinylaromatic compound is selected from the group consisting of styrene, methylstyrene and their mixture;

the conjugated aliphatic diene is selected from the group consisting of 1,3-butadiene, isoprene and their mixture; and

the ethylenically unsaturated acid is selected from one or more compounds of the group consisting of acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, vinylacetic acid, vinyllactic acid, vinylsulfonic acid, styrenesulfonic acid, acrylamidomethylpropanesulfonic acid, sulfopropyl acrylate, sulfopropyl methacrylate, vinylphosphonic acid and salts thereof.

10. A paper coating composition, comprising

(a) an aqueous secondary dispersion of at least one polymer selected from the group consisting of a polyalkylene carbonate, a polyester, and a polyethylene,

(b) an aqueous primary dispersion of at least one emulsion polymer having a glass transition temperature of below 50° C., and preferably in the range from −10 to +30° C., and

(c) inorganic pigments.

11. The paper coating composition of claim 10, wherein the polymeric particles of the emulsion polymer have an average particle size in the range from 80 to 200 nm and the polymeric particles of the polymer of the secondary dispersion have an average particle size in the range from 100 to 400 nm.

12. The paper coating composition of claim 10, wherein the pigments are selected from the group consisting of calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate, calcium carbonate, silicas, aluminas, aluminum hydrate, silicates, titanium dioxide, zinc oxide, kaolin, argillaceous earth, talc and silicon dioxide, and wherein the paper coating slip optionally further comprises at least one additive selected from the group consisting of thickeners, further polymeric binders, co-binders, optical brighteners, fillers, flow control agents, dispersants, surfactants, lubricants, neutralizing agents, defoamers, deaerators, preservatives and dyes.

13. The paper coating composition according to claim 10, wherein:

the polymers of the emulsion polymer and of the secondary dispersion are present used in total in an amount of 1 to 50 parts by weight, based on the total amount of pigments, and

the pigments are present in an amount of 80 to 95 parts by weight, based on total solids content.

14. A paper or card coated with a paper coating composition of claim 10.