Preparation of pigments

Abstract

The present invention relates to an aqueous calcium carbonate containing composition, to methods for producing such aqueous calcium carbonate containing compositions and to the use of the aqueous calcium carbonate containing composition in paper, paper coating, plastics and/or paint applications and as filler in paper.

Claims

1. An aqueous calcium carbonate containing composition comprising a) a calcium carbonate containing material in an amount of at least 10 wt.-%, based on the total dry weight of the composition, b) at least one anionically charged comb polymer having a specific charge of −5 to −500 C/g at pH 8, c) optionally at least one binding agent in an amount of at least 2.5 wt.-%, based on the total dry weight of the composition, and d) at least one salt of a divalent or trivalent cation in an amount of between 1 and 20 wt.-%, based on the total dry weight of the composition, whereby at least 95 wt.-% of the total amount of salt is dissolved within the composition, and wherein the composition has a Brookfield viscosity of below 2500 mPa.Math.s when measured 1 minute after stirring at a rotational speed of 100 rpm at a temperature of 25° C.

2. The aqueous calcium carbonate containing composition of claim 1, wherein the at least one anionically charged comb polymer has an average molecular weight M.sub.W in the range between 10,000 and 10,000,000 g/mol.

3. The aqueous calcium carbonate containing composition of claim 1, wherein the at least one anionically charged comb polymer has an intrinsic viscosity in the range of 5 to 500 ml/g.

4. The aqueous calcium carbonate containing composition of claim 1, wherein the at least one anionically charged comb polymer has a specific charge from −10 C/g to −500 C/g at pH 8.

5. The aqueous calcium carbonate containing composition of claim 1, wherein the at least one anionically charged comb polymer comprises structural units of formula (I) ##STR00003## wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5 and R.sup.6 are independently selected from hydrogen or alkyl groups, having 1 to 40 carbon atoms, X is a negatively charged functional group, Y represents a functional linkage group, which is independently selected from the group consisting of ether, ester, urethane and amide groups, Z is a positively charged functional group, R.sup.7 and R.sup.8 are independently selected from hydrogen or alkyl groups having 1 to 4 carbon atoms, R.sup.9 is selected from hydrogen or an alkyl group having 1 to 40 carbon atoms, a, b, c and d are integers such that 5≦(a+b+c).Math.d≦150, and n, m and o are selected such that the anionically charged polymer has a specific charge from −5 C/g to −500 C/g at pH 8.

6. The aqueous calcium carbonate containing composition of claim 1, wherein the at least one anionically charged comb polymer comprises a) 5 wt.-% to 40 wt.-%, based on the total amount of monomers, of (meth)acrylic acid, b) 60 wt.-% to 95 wt.-%, based on the total amount of monomers, of at least one monomer of Formula (II)
R-(OE).sub.α-(OP).sub.β-R′   (II) wherein R is a polymerizable functional group selected from methacrylate or methacryl-urethane, OE and OP designates ethylene oxide and propylene oxide, respectively, α and β are integers each having a value from 0 to 150 and at least one of α or β has a value of greater than 0, R′ represents hydrogen or an alkyl group having 1 to 4 carbon atoms.

7. The aqueous calcium carbonate containing composition of claim 1, wherein the at least one anionically charged comb polymer comprises a) 5 wt.-% to 20 wt.-%, based on the total amount of monomers, of (meth)acrylic acid, b) 80 wt.-% to 95 wt.-%, based on the total amount of monomers, of at least one monomer of Formula (II)
R-(OE).sub.α-(OP).sub.β-R′   (II) wherein R is a polymerizable functional group selected from methacrylate or methacryl-urethane, OE and OP designates ethylene oxide and propylene oxide, respectively, α and β are integers each having a value from 0 to 150 and at least one of α or β has a value of greater than 0, R′ represents hydrogen or an alkyl group having 1 to 4 carbon atoms.

8. The aqueous calcium carbonate containing composition of claim 1, wherein the composition comprises the at least one anionically charged comb polymer in an amount from 0.01 to 10 wt.-%, based on the total dry weight of the composition.

9. The aqueous calcium carbonate containing composition of claim 1, wherein the composition comprises the at least one anionically charged comb polymer in an amount from 0.15 to 1.75 wt.-%, based on the total dry weight of the composition.

10. The aqueous calcium carbonate containing composition of claim 1, wherein the calcium carbonate containing material is a ground calcium carbonate, a precipitated calcium carbonate, a modified calcium carbonate or a mixture thereof.

11. The aqueous calcium carbonate containing composition of claim 1, wherein the calcium carbonate containing material has a weight median particle size d.sub.50 from 0.1 to 100 μm.

12. The aqueous calcium carbonate containing composition of claim 1, wherein the calcium carbonate containing material has a weight median particle size d.sub.50 from 0.4 to 3.0 μm.

13. The aqueous calcium carbonate containing composition of claim 1, wherein the composition comprises the calcium carbonate containing material in an amount from 10 to 70 wt.-%, based on the total dry weight of the composition.

14. The aqueous calcium carbonate containing composition of claim 1, wherein the binding agent is selected from the group consisting of polyvinyl alcohol, polyvinyl acetate, starch, proteins, casein, cellulose, cellulosic derivatives, ethylhydroxylethyl cellulose and/or carboxymethyl cellulose, and any mixture thereof.

15. The aqueous calcium carbonate containing composition of claim 1, wherein the binding agent is preferably the binding agent is selected from the group consisting of polyvinyl alcohol, polyvinyl acetate, and mixtures thereof.

16. The aqueous calcium carbonate containing composition of claim 1, wherein the composition comprises the at least one binding agent in an amount between 2.5 and 20 wt.-%, based on the total dry weight of the composition.

17. The aqueous calcium carbonate containing composition of claim 1, wherein the at least one salt of a divalent or trivalent cation is selected from the group consisting of a chloride salt of a divalent or trivalent cation, a bromide salt of a divalent or trivalent cation, a sulfate salt of a divalent or trivalent cation, and any mixture thereof.

18. The aqueous calcium carbonate containing composition of claim 1, wherein the at least one salt of a divalent or trivalent cation is a chloride salt of a divalent or trivalent cation selected from the group consisting of calcium chloride, magnesium chloride, strontium chloride, zinc chloride, manganese chloride, and any mixture thereof.

19. The aqueous calcium carbonate containing composition of claim 1, wherein the composition comprises the at least one salt of a divalent or trivalent cation in an amount of between 3 and 17 wt.-%, based on the total dry weight of the composition, whereby at least 95 wt.-% of the total amount of salt is dissolved within the composition.

20. The aqueous calcium carbonate containing composition of claim 1, wherein the composition comprises one or more further additives, fixation aids, or cationic homopolymers based on monomer units of diallyl dialkyl ammonium salts or polyethylene imines.

21. The aqueous calcium carbonate containing composition of claim 1, wherein the composition has a solids content from 10 to 70 wt.-%, based on the total dry weight of the composition.

22. The aqueous calcium carbonate containing composition of claim 1, wherein the Brookfield viscosity of the composition is from 50 to 2500 mPa.Math.s when measured 1 minute after stirring at a rotational speed of 100 rpm at a temperature of 25° C.

23. The aqueous calcium carbonate containing composition of claim 1, wherein the Brookfield viscosity of the composition is from 25 to 2000 mPa.Math.s when measured 1 minute after stirring at a rotational speed of 100 rpm at a temperature of 25° C.

24. A method for producing an aqueous calcium carbonate containing composition as defined in claim 1, comprising the steps of a) providing water, b) providing the calcium carbonate containing material, c) providing the at least one anionically charged comb polymer having a specific charge of −5 to −500 C/g at pH 8, d) optionally providing the at least one binding agent in an amount of at least 2.5 wt.-%, based on the total dry weight of the composition, e) providing the at least one salt of a divalent or trivalent cation in an amount of between 1 and 20 wt.-%, based on the total dry weight of the composition, f) contacting the calcium carbonate containing material of step b) with the water of step a), g) contacting the calcium carbonate containing material of step b) with the at least one anionically charged comb polymer of step c) before and/or during and/or after step f), h) optionally contacting the calcium carbonate containing material of step b) with the at least one binding agent of step d) before and/or during and/or after step f) and/or before and/or during and/or after step g), and i) contacting the calcium carbonate containing material of step b) with the at least one salt of a divalent or trivalent cation of step e) before or after step g).

25. The method of claim 24, wherein the composition has a solids content from 10 to 70 wt.-%, based on the total dry weight of the composition.

26. The method of claim 24, wherein the Brookfield viscosity of the composition is from 50 to 2500 mPa.Math.s when measured 1 minute after stirring at a rotational speed of 100 rpm at a temperature of 25° C.

27. The method of claim 24, wherein the Brookfield viscosity of the composition is from 25 to 2000 mPa.Math.s when measured 1 minute after stirring at a rotational speed of 100 rpm at a temperature of 25° C.

28. A method for producing an aqueous calcium carbonate containing composition as defined in claim 1, comprising the steps of a) providing water, b) providing the calcium carbonate containing material, c) providing the at least one anionically charged comb polymer having a specific charge of −5 to −500 C/g at pH 8, d) optionally providing the at least one binding agent in an amount of at least 2.5 wt.-%, based on the total dry weight of the composition, e) providing the at least one salt of a divalent or trivalent cation in an amount of between 1 and 20 wt.-%, based on the total dry weight of the composition, f) combining the water of step a), the calcium carbonate containing material of step b) and the at least one anionically charged comb polymer of step c) in any order to form a suspension, g) dispersing and/or grinding the suspension obtained in step f), h) optionally contacting the calcium carbonate suspension obtained in step g) with the at least one binding agent of step d), i) contacting the calcium carbonate suspension obtained in step g) with the at least one salt of a divalent or trivalent cation of step e) after step g) or contacting the calcium carbonate suspension of step h) with the at least one salt of a divalent or trivalent cation of step e) after step h).

29. A method for producing an aqueous calcium carbonate containing composition as defined in claim 1, comprising the steps of a) providing water, b) providing the calcium carbonate containing material, c) providing the at least one anionically charged comb polymer having a specific charge of −5 to −500 C/g at pH 8, d) providing the at least one salt of a divalent or trivalent cation in an amount between 10 to 50 wt.-%, based on the total dry weight of the at least one anionically charged comb polymer of step c), e) optionally providing the at least one binding agent in an amount of at least 2.5 wt.-%, based on the total dry weight of the composition, f) combining the at least one anionically charged comb polymer of step c) and the at least one salt of a divalent or trivalent cation of step d), g) contacting the calcium carbonate containing material of step b) with the water of step a), h) contacting the calcium carbonate containing material of step b) with the at least one anionically charged comb polymer of step f) before and/or during and/or after step g), i) optionally contacting the calcium carbonate containing material of step b) with the at least one binding agent of step e) before and/or during and/or after step g) and/or before and/or during and/or after step h).

30. A method for producing an aqueous calcium carbonate containing composition as defined in claim 1, comprising the steps of a) providing water, b) providing the calcium carbonate containing material, c) providing the at least one anionically charged comb polymer having a specific charge of −5 to −500 C/g at pH 8, d) providing the at least one salt of a divalent or trivalent cation in an amount between 10 to 50 wt.-%, based on the total dry weight of the at least one anionically charged comb polymer of step c), e) optionally providing the at least one binding agent in an amount of at least 2.5 wt.-%, based on the total dry weight of the composition, f) combining the at least one anionically charged comb polymer of step c) and the at least one salt of a divalent or trivalent cation of step d), g) combining the water of step a), the calcium carbonate containing material of step b) and the at least one anionically charged comb polymer of step f) in any order to form a suspension, h) dispersing and/or grinding the suspension obtained in step g), i) optionally contacting the calcium carbonate suspension of step h) with the at least one binding agent of step e).

31. A paper coating, plastic and/or paint comprising the aqueous calcium carbonate containing composition of claim 1.

32. A paper filler comprising the aqueous calcium carbonate containing composition of claim 1.

Description

DESCRIPTION OF FIGURES

(1) FIG. 1 outlines the optical density of black of a coated paper product prepared from the inventive aqueous calcium carbonate containing composition.

(2) FIG. 2 outlines the optical density of colour (CMY) of a coated paper product prepared from the inventive aqueous calcium carbonate containing composition.

(3) FIG. 3 outlines the mottling for black ink of a coated paper product prepared from the inventive aqueous calcium carbonate containing composition.

(4) FIG. 4 outlines the mottling for colour (blue) ink of a coated paper product prepared from the inventive aqueous calcium carbonate containing composition.

EXAMPLES

(5) 1. Measurement Methods

(6) pH Measurement

(7) The pH is measured at 25° C. using a Mettler Toledo Seven Easy pH meter and a Mettler Toledo InLab® Expert Pro pH electrode. A three point calibration (according to the segment method) of the instrument is first made using commercially available buffer solutions having pH values of 4, 7 and 10 at 20° C. (from Aldrich). The reported pH values are the endpoint values detected by the instrument (the endpoint is when the measured signal differs by less than 0.1 mV from the average over the last 6 seconds).

(8) Brookfield Viscosity

(9) The Brookfield viscosity was measured after 1 minute of stirring by the use of a RVT model Brookfield™ viscometer at a temperature of 25° C., and a rotation speed of 100 rpm (revolutions per minute) with the appropriate disc spindle from N° 1 to 5.

(10) Particle Size Distribution (Mass % Particles with a Diameter<X) and Weight Median Grain Diameter (d.sub.50) of Particulate Material

(11) Weight median grain diameter and grain diameter mass distribution of a particulate material were determined via the sedimentation method, i.e. an analysis of sedimentation behaviour in a gravimetric field. The measurement was made with a Sedigraph™ 5120.

(12) The method and the instrument are known to the skilled person and are commonly used to determine grain size of fillers and pigments. The measurement is carried out in an aqueous solution of 0.1% by weight of Na.sub.4P.sub.2O.sub.7. The samples were dispersed using a high speed stirrer and ultrasonic.

(13) Weight Solids (Wt. %) of a Material in Suspension

(14) The weight solids were determined by dividing the weight of the solid material by the total weight of the aqueous suspension. The weight solids content was determined at 160° C. using a Moisture Analyser MJ 33, Mettler Toledo.

(15) Specific Surface (BET) Measurement

(16) The specific surface area (in m.sup.2/g) of the mineral filler was determined using the BET method, which is well known to the skilled man (ISO 9277:1995). The total surface area (in m.sup.2) of the mineral filler was then obtained by multiplication of the specific surface area and the mass (in g) of the mineral filler. The method and the instrument are known to the skilled person and are commonly used to determine specific surface of fillers and pigments.

(17) Specific Charge (C/g)

(18) The cationic polymer demand that is necessary to achieve a charge value of zero was measured using the Mettler DL 77 titrator and the Mütec PCD-02 detector by means of the cationic titration method. The cationic reagent was N/200 (0.005 N) methyl glycol chitosan (chitosan), and the anionic reagent was N/400 (0.0025 N) K-polyvinyl-sulfate (KPVS), both sold by WAKO Chemicals GmbH.

(19) If necessary, the sample was adjusted to pH 8.0+/−0.1 with NaOH (0.1M) prior to measurement.

(20) Since experience showed that the first titration is not correct, 10 ml water were first prepared in the detector followed by the addition of 0.5 ml KPVS. Afterwards, titration with chitosan was made until it is back to shortly after the equivalence point. Subsequently, the measurements were started. Between 0.5 and 2.0 ml of 0.005 molar reagent were used up during the titration to obtain reproducible values.

(21) To avoid rapid sedimentation, the sample was drawn under stirring, by means of a tared syringe. The content of the syringe was then rinsed into the sample vessel by means of distilled water. Afterwards, the detector was filled with distilled water up to the lower edge and the piston was inserted carefully. Subsequently, the cationic titration solution was put on the memotitrator and the top of the burette was fixed at the detector ensuring that it does not come into contact with the detector or the liquid. After each titration, the development of the titration was verified with the aid of the titration curve.

(22) Calculation of the Electrochemical Charge:

(23) $\mathrm{Charge}\left[\mu \mathrm{Val}\text{/}g\right]=\frac{V.Math.c.Math.z.Math.t}{E.Math.F}.Math.K$
wherein K=+1000 V: Consumption chitosan [ml] c: Concentration chitosan [mol/l] t: Titer factor chitosan E: Weight-in quantity [g] F: Mass fraction solids [g/g] z: Valence (equivalence number)

(24) The obtained charge value of μVal/g was converted into C/g by multiplication with the Faraday constant as follows:
[C/g]=[μVal/g].Math.0.096485
Intrinsic Viscosity

(25) The intrinsic viscosity was determined by a Schott AVS 350 system. The samples were dissolved in an aqueous 6 wt.-% NaCl solution, adjusted to pH 10 using NaOH. Measurements were performed at 25° C. with a capillary type 0a and corrected using the Hagenbach correction.

(26) Average Molecular Weight (M.sub.W)

(27) The average molecular weight was determined by size exclusion chromatography (SEC) also referred to as “gel permeation chromatography” (GPC). A device for liquid chromatography from WATERS™ equipped with two detectors was used. The first detector combined static dynamic light scattering at an angle of 90° and viscosity measurement with a VISCOTEK™ MALVERN™ viscometer and the second detector was a refractometric concentration detector from WATERS™. The liquid chromatography apparatus was equipped with an isocratic pump (WATER 515), an oven and size exclusion columns. The columns were a GUARD COLUMN ULTRAHYDROGEL WATERS™ precolumn with a length of 6 cm and an internal diameter of 40 mm, a ULTRAHYDROGEL WATERS™ column with a length of 30 cm and an internal diameter 7.8 mm, and a ULTRAHYDROGEL 120 ANGSTROM WATERS™ column with a length of 30 cm and an internal diameter 7.8 mm. The detecting system consisted of a RI WATERS™ 410 refractometric detector and of a dual 270 DUAL DETECTOR MALVERN™ detector for viscometry and light scattering at an angle of 90°.

(28) The oven was heated to 55° C. and the refractometer was heated to 45° C. The flow of the isocratic pump was set to 0.8 mL/min and the liquid eluent was an aqueous phase containing 1% KNO3.

(29) The chromatography apparatus was calibrated by a PEO 19k PolyCAL™ MALVERN™ single standard.

(30) The average molecular weight was measured by diluting the polymerization solution to 0.9% by dry-weight with the SEC eluent (1% KNO.sub.3 solution), and afterwards filtering the solution through 0.2 μm. 100 μL of the filtered solution were injected into the chromatography apparatus.

(31) Mottling

(32) The mottling was determined using a PaPEye software solution with internal test procedure developed by Omya AG.

(33) Optical Density

(34) The optical density is a dimension for the thickness of the colour layer above the substrate. Optical density values are calculated based on the spectral measurement, therefore slight differences to the measurement with a densitometer may occur. The calculation is made according to the DIN Norm 16536-2.

(35) Surface Strength and Rub Resistance

(36) Surface strength and rub resistance against black paper was determined using a Quartant-rub tester according to the following method: the coated paper is applied against a black tinted “Folia” drawing paper from Max Bringmann KG (Germany) under a weight of 600 g and the coated paper is rotated against the black paper.

(37) 2. Examples

Example 1

(38) This example refers to the addition of different anionically charged comb polymers to a commercially available calcium carbonate which is dispersed by a sodium polyacrylate allowing to achieve the compatibility with a salt of a divalent or trivalent cation.

(39) This is achieved by mixing water with the commercially available calcium carbonate suspension Omyajet® 5020 from Omya such that the obtained suspension has a content of calcium carbonate of about 50 wt.-%, based on the total weight of the suspension.

(40) The commercial product Omyajet® 5020 refers to an aqueous suspension comprising calcium carbonate which is anionically dispersed.

(41) Test 1

(42) This test corresponds to a reference.

(43) To 100 parts per weight of calcium carbonate (d/d), based on the total dry weight of calcium carbonate in the suspension, 10 parts per weight of calcium chloride (d/d), based on the total dry weight of calcium carbonate in the suspension, are directly added by moderate agitation.

(44) The obtained slurry shows a severe increase in Brookfield viscosity to a value of about 7000 mPa.Math.s at 25° C. and 100 rpm.

(45) A suspension having such viscosity is not suitable for all further handling steps or storage of the suspension.

(46) For the following tests 2 to 12, an anionically charged comb polymer is first added to the calcium carbonate suspension followed by 10 parts per weight of calcium chloride (d/d), based on the total dry weight of calcium carbonate in the suspension. For each test, 3 test samples were prepared corresponding to 3 different dosages of the anionically charged comb polymer, i.e. the anionically charged comb polymer was added in an amount of 0.8 wt.-%, 1 wt.-% and 1.5 wt.-%, based on the total dry weight of calcium carbonate, respectively.

(47) Test 2

(48) This test represents the prior art.

(49) A polymeric additive was used representing a homopolymer of acrylic acid totally neutralised with sodium ions and having an average molecular weight M.sub.W of about 6500 g/mol.

(50) The specific charge of this homopolymer of acrylic acid is −931 C/g measured at pH 8.

(51) Due to the development of an agglomerated material in the suspension, the measurement of the Brookfield viscosity was impossible for the obtained suspension.

(52) Test 3

(53) This test represents the prior art.

(54) A polymeric additive was used representing a homopolymer of acrylic acid of which 100% of the carboxylic acid groups are neutralised with sodium ions and having an average molecular weight M.sub.W of about 6500 g/mol.

(55) The specific charge of this homopolymer of acrylic acid is −931 C/g measured at pH 8.

(56) The measurement of the Brookfield viscosity was impossible for the obtained suspension.

(57) Test 4

(58) This test represents the invention.

(59) The anionically charged comb polymer is a copolymer comprising a) 12.8 wt.-%, based on the total amount of monomers, of acrylic acid, b) 87.2 wt.-%, based on the total amount of monomers, of a monomer of Formula (II) in which R is a methacrylate functional group, R′ is hydrogen, β=48, α=16.

(60) The average molecular weight M.sub.W is about 45000 g/mol and about 100% of the carboxylic acid groups are neutralised with sodium ions.

(61) The specific charge of the anionically charged comb polymer is −500 C/g measured at pH 8.

(62) Test 5

(63) This test represents the invention.

(64) The anionically charged comb polymer is a copolymer comprising a) 12.8 wt.-%, based on the total amount of monomers, of acrylic acid, b) 87.2 wt.-%, based on the total amount of monomers, of a monomer of Formula (II) in which R is a methacrylate functional group, R′ is hydrogen, β=48, α=16.

(65) The average molecular weight M.sub.W is about 130000 g/mol and about 100% of the carboxylic acid groups are neutralised with sodium ions.

(66) The specific charge of the anionically charged comb polymer is −500 C/g measured at pH 8.

(67) Test 6

(68) This test represents the invention.

(69) The anionically charged comb polymer is a copolymer comprising a) 7.4 wt.-%, based on the total amount of monomers, of acrylic acid, b) 92.6 wt.-%, based on the total amount of monomers, of a monomer of Formula (II) in which R is a methacrylate functional group, R′ is hydrogen, β=48, α=16.

(70) The average molecular weight M.sub.W is about 130000 g/mol and about 100% of the carboxylic acid groups are neutralised with sodium ions.

(71) The specific charge of the anionically charged comb polymer is −500 C/g measured at pH 8.

(72) Test 7

(73) This test represents the invention.

(74) The anionically charged comb polymer is a copolymer comprising a) 2.9 wt.-%, based on the total amount of monomers, of acrylic acid, 19.8 wt.-%, based on the total amount of monomers, of methacrylic acid b) 77.3 wt.-%, based on the total amount of monomers, of a monomer of Formula (II) in which R is a methacrylate functional group, R′ is hydrogen, β=48, α=16.

(75) The average molecular weight M.sub.W is about 39000 g/mol and about 100% of the carboxylic acid groups are neutralised with sodium ions.

(76) The specific charge of the anionically charged comb polymer is −500 C/g measured at pH 8.

(77) Test 8

(78) This test represents the invention.

(79) The anionically charged comb polymer is a copolymer comprising a) 12.5 wt.-%, based on the total amount of monomers, of methacrylic acid b) 87.5 wt.-%, based on the total amount of monomers, of a monomer of Formula (II) in which R is a methacrylate functional group, R′ is hydrogen, β=48, α=16.

(80) The average molecular weight M.sub.W is about 74000 g/mol and about 100% of the carboxylic acid groups are neutralised with sodium ions.

(81) The specific charge of the anionically charged comb polymer is −117 C/g measured at pH 6.9 and −125 C/g measured at pH 8.

(82) Test 9

(83) This test represents the invention.

(84) The anionically charged comb polymer is a copolymer comprising a) 6 wt.-%, based on the total amount of monomers, of acrylic acid, 1.8 wt.-%, based on the total amount of monomers, of methacrylic acid b) 92.2 wt.-%, based on the total amount of monomers, of a monomer of Formula (II) in which R is a methacrylate functional group, R′ is a methyl group, β=0, α=113.

(85) The average molecular weight M.sub.W is about 32500 g/mol and about 100% of the carboxylic acid groups are neutralised with sodium ions.

(86) The specific charge of the anionically charged comb polymer is −500 C/g measured at pH 8.

(87) Test 10

(88) This test represents the invention.

(89) The anionically charged comb polymer is a copolymer comprising a) 6 wt.-%, based on the total amount of monomers, of acrylic acid, 1.8 wt.-%, based on the total amount of monomers, of methacrylic acid b) 92.2 wt.-%, based on the total amount of monomers, of a monomer of Formula (II) in which R is a methacrylate functional group, R′ is a methyl group, β=0, α=113.

(90) The average molecular weight M.sub.W is about 5000000 g/mol and about 100% of the carboxylic acid groups are neutralised with sodium ions.

(91) The specific charge of the anionically charged comb polymer is −500 C/g measured at pH 8.

(92) Test 11

(93) This test represents the invention.

(94) The anionically charged comb polymer is a copolymer comprising a) 8 wt.-%, based on the total amount of monomers, of acrylic acid, 2.5 wt.-%, based on the total amount of monomers, of methacrylic acid, b) 89.5 wt.-%, based on the total amount of monomers, of a monomer of Formula (II) in which R is a methacrylate functional group, R′ is a methyl group, β=0, α=113.

(95) The average molecular weight M.sub.W is about 1800000 g/mol and about 50% of the carboxylic acid groups are neutralised with sodium ions, the other ones stay acidic.

(96) The specific charge of the anionically charged comb polymer is −500 C/g measured at pH 8.

(97) Test 12

(98) This test represents the invention.

(99) The anionically charged comb polymer is a copolymer comprising a) 8 wt.-%, based on the total amount of monomers, of acrylic acid, 2.5 wt.-%, based on the total amount of monomers, of methacrylic acid, b) 89.5 wt.-%, based on the total amount of monomers, of a monomer of Formula (II) in which R is a methacrylate functional group, R′ is a methyl group, β=0, α=113.

(100) The average molecular weight M.sub.W is about 3000000 g/mol and about 100% of the carboxylic acid groups are neutralised with sodium ions, the other ones stay acidic.

(101) The specific charge of the anionically charged comb polymer is −500 C/g measured at pH 8.

(102) Results

(103) For tests 4 to 12, suspensions were obtained without the development of agglomerations (differing from the reference) and the Brookfield viscosity of said suspensions was determined at 25° C. and 100 rpm; these measurements were carried out on each of the three anionically charged comb polymer dosages. The results can be gathered from Table 1.

(104) TABLE-US-00001 REFerence dosage (wt.-%)* Test INvention 0 0.8 1 1.5 1 REF 7000 — — — 4 IN — 1140 760 645 5 IN — 1800 1230 1150 6 IN — 2820 2880 1480 7 IN — 1375 1090 825 8 IN — 2010 1765 1980 9 IN — 1220 810 650 10 IN — 2290 1200 1065 11 IN — 2960 2040 1370 12 IN — 3290 2100 1275 *dosage (wt.-%): refers to the wt.-% of the anionically charged comb polymer based on the total dry weight of calcium carbonate.

(105) From the results obtained it can be gathered that only the inventive Examples comprising the at least one anionically charged comb polymer advantageously reduces the viscosity of the suspension comprising a salt of a divalent or trivalent cation. These suspensions, which are also stable and susceptible to manipulation, allow the preparation of coating compositions for use in inkjet digital printing applications.

Example 2

(106) This example refers to the addition of different anionically charged comb polymers to a commercially available calcium carbonate which is dispersed by a sodium polyacrylate allowing to achieve the compatibility with a salt of a divalent or trivalent cation.

(107) This is achieved by methods well known to the skilled person, by dispersing the commercially available calcium carbonate Hydrocarb® 90 from Omya in the presence of a dispersing agent in water such that the obtained aqueous suspension has a content of calcium carbonate of about 60 wt.-% of calcium carbonate, based on the total weight of the suspension.

(108) The dispersing agent represents a homopolymer of acrylic acid totally neutralised with sodium ions and having an average molecular weight M.sub.W of about 6500 g/mol.

(109) Test 13

(110) This test represents a reference.

(111) To 100 parts per weight of calcium carbonate (d/d), based on the total dry weight of calcium carbonate in the suspension, 10 parts per weight of calcium chloride (d/d), based on the total dry weight of calcium carbonate in the suspension, are directly added by moderate agitation.

(112) The obtained slurry shows a severe increase in Brookfield viscosity to a value of about 2230 mPa.Math.s at 25° C. and 100 rpm.

(113) For the following tests 14 and 15, a polymeric additive or an anionically charged comb polymer is first added to the calcium carbonate suspension followed by 10 parts per weight of calcium chloride (d/d), based on the total dry weight of calcium carbonate in the suspension.

(114) Test 14

(115) This test represents the prior art.

(116) 0.8 wt.-%, based on the total amount of calcium carbonate, of a polymeric additive is used representing a homopolymer of acrylic acid of which 100% of the carboxylic acid groups are neutralised with sodium ions and having an average molecular weight M.sub.W of about 6500 g/mol.

(117) The Brookfield viscosity of the obtained suspension measured at 25° C. and 100 rpm severely increased compared to the viscosity of the reference to a value of about 4260 mPa.Math.s. This clearly demonstrates the inefficiency of the tested polymeric additive.

(118) Test 15

(119) This test represents the invention.

(120) 0.8 wt.-%, based on the total amount of calcium carbonate, of an anionically charged comb polymer being a copolymer comprising a) 12.8 wt.-%, based on the total amount of monomers, of acrylic acid, b) 87.2 wt.-%, based on the total amount of monomers, of a monomer of Formula (II) in which R is a methacrylate functional group, R′ is hydrogen, β=48, α=16.

(121) The average molecular weight M.sub.W is about 45000 g/mol and about 100% of the carboxylic acid groups are neutralised with sodium ions.

(122) The specific charge of the anionically charged comb polymer is −500 C/g measured at pH 8.

(123) The Brookfield viscosity at 25° C. and 100 rpm of the suspension is clearly reduced in comparison to the viscosity measured for the reference to a value of about 450 mPa.Math.s. This clearly demonstrates the compatibility of the tested anionically charged comb polymer with an aqueous suspension comprising an anionically dispersed calcium carbonate by adding a salt of a divalent or trivalent cation.

(124) This inventive suspension may advantageously be directly used as coating composition for use in inkjet digital printing applications.

Example 3

(125) This example demonstrates the development of the printing quality, e.g. optical density and mottle, of the aqueous calcium carbonate containing composition used as coating colour formulation by adding a divalent or trivalent salt.

(126) 1. Materials

(127) Comb Polymer A (Inventive):

(128) The anionically charged comb polymer is a copolymer comprising a) 12.5 wt.-%, based on the total amount of monomers, of methacrylic acid b) 87.5 wt.-%, based on the total amount of monomers, of a monomer of Formula (II) in which R is a methacrylate functional group, R′ is hydrogen, β=48, α=16.

(129) The average molecular weight M.sub.W is about 74000 g/mol and about 100% of the carboxylic acid groups are neutralised with sodium ions.

(130) The specific charge of the anionically charged comb polymer is −117 C/g measured at pH 6.9 and −125 C/g measured at pH 8.

(131) The intrinsic viscosity is 24 mL/g.

(132) Comb Polymer B (Inventive):

(133) The anionically charged comb polymer is a copolymer comprising a) 2.9 wt.-%, based on the total amount of monomers, of acrylic acid, 19.8 wt.-%, based on the total amount of monomers, of methacrylic acid b) 77.3 wt.-%, based on the total amount of monomers, of a monomer of Formula (II) in which R is a methacrylate functional group, R′ is hydrogen, β=48, α=16.

(134) The average molecular weight M.sub.W is about 39000 g/mol and about 100% of the carboxylic acid groups are neutralised with sodium ions.

(135) The specific charge of the anionically charged comb polymer is −500 C/g measured at pH 8.

(136) The intrinsic viscosity is 22 mL/g.

(137) Calcium Carbonate A:

(138) A calcium carbonate containing material is obtained by first autogeneously dry grinding 10 to 300 mm natural calcium carbonate rocks of Norwegian origin to a fineness corresponding to a d.sub.50 value of between 42 to 48 μm, subsequent wet grinding this dry-ground product to a fineness corresponding to a d.sub.50 value of about 8 μm, and then grinding in presence of 5400 ppm of the comb polymer A at 30 to 35° C. in water in a 1.4-liter vertical attritor mill (Dynomill MultiLab) by using zirconium oxide/zirconium silicate grinding beads (0.6-1.0 mm) at a weight solids content of about 66 wt.-%, based on the total weight of the slurry, until a d.sub.50 value of 0.9 μm and a d.sub.90 value of 2.1 μm was reached.

(139) Calcium Carbonate B:

(140) A calcium carbonate containing material is obtained by first autogeneously dry grinding 10 to 300 mm natural calcium carbonate rocks of Norwegian origin to a fineness corresponding to a d.sub.50 value of between 42 to 48 μm, subsequent wet grinding this dry-ground product to a fineness corresponding to a d.sub.50 value of about 8 μm, and then grinding in presence of 7100 ppm of the comb polymer B at 30 to 35° C. in water in a 1.4-liter vertical attritor mill (Dynomill MultiLab) by using zirconium oxide/zirconium silicate grinding beads (0.6-1.0 mm) at a weight solids content of about 74 wt.-%, based on the total weight of the slurry, until a d.sub.50 value of 0.76 μm and a d.sub.90 value of 2.0 μm was reached.

(141) Calcium Carbonate C:

(142) It is the commercially available aragonitic PCC Omyaprime® HO40-GO 72% from Omya.

(143) Calcium Carbonate D:

(144) It is a MCC from Omya.

(145) Calcium Carbonate E:

(146) It is the commercially available PCC Omyajet® C4440-GO 38% from Omya.

(147) Calcium Carbonate F:

(148) It is the commercially available GCC Hydrocarb® 90-ME 78% from Omya.

(149) Calcium Carbonate G:

(150) It is the commercially available PCC Omyajet® B5260-GO 25% from Omya.

(151) Calcium Chloride:

(152) available from Sigma-Aldrich, Switzerland.

(153) Binding Agents

(154) Polyvinyl alcohol, available from CCP (Taiwan) as PVA BF-04.

(155) Polyvinyl acetate, available from Wacker Chemie AG as Vinnacoat LL 4444.

(156) Starch, available from Cargill, Switzerland as C*Film 07311.

(157) Further Additives

(158) Poly(DADMAC), available from BASF, Germany as Catiofast BP

(159) Application

(160) Composition were applied as 10 g/m.sup.2 coating on Biberist Inkjet, 80 g/m.sup.2, by using a rod coater

(161) Printer

(162) HP Officejet Pro8000 desktop printer with pigment based inks

(163) Zweckform 2585 was used as reference

(164) 2. Examples and Results

(165) This example demonstrates the optical and mechanical properties of a paper product coated with an aqueous calcium carbonate containing composition. The details regarding the compositions and the respective Brookfield viscosities (determined at 20° C. and 100 rpm) can be taken from Table 2.

(166) TABLE-US-00002 1 2 3 4 5 6 7 [parts] [parts] [parts] [parts] [parts] [parts] [parts] Calcium carbonate B 60 60 60 Calcium carbonate C 75 75 75 Calcium carbonate D 40 40 40 40 Calcium carbonate E 25 Calcium carbonate F 60 Calcium carbonate G 25 25 Comb polymer A 2 Comb polymer B 0.2 0.2 0.2 0.6 0.6 Polyvinyl alcohol 5 5 5 5 5 5 5 Starch 2 2 2 2 2 2 2 Poly(DADMAC) 5 5 5 5 5 5 Calcium chloride 10 10 10 10 10 color start Solids content start 54.3 56.5 53.3 52.7 56.4 56.6 56.2 [wt.-%] Viscosity at 100 rpm 6000 1600 690 880 2300 1200 3020 [mPa * s] color end Solids content end 37.5 35.6 35.3 35.5 35.5 36.1 35.0 [wt.-%] Viscosity at 100 rpm 660 65 67 70 72 50 90 [mPa * s]

(167) The effect of the aqueous calcium carbonate containing compositions on the optical density of black and colour of a coated paper product prepared therefrom is outlined in FIGS. 1 and 2. From FIGS. 1 and 2 it can be concluded that the addition of a salt of a divalent or trivalent cation is highly beneficial for colour inks. It can be further gathered that for black ink no salt of a divalent or trivalent cation is required. However, it has to be assumed that the inventive aqueous calcium carbonate containing composition impart positive effects on the optical and mechanical properties of paper end products coated with such composition.

(168) The biggest issue with the print quality when coated with prior art compositions is the mottling, especially with color inks. The effect of the aqueous calcium carbonate containing compositions on mottling for black and colour ink of a coated paper product prepared therefrom is outlined in FIGS. 3 and 4. From FIGS. 3 and 4 it can be clearly gathered that the addition of a salt of a divalent or trivalent cation significantly improves the mottling and all trial points for the salt of a divalent or trivalent cation are on an acceptable level. Thus, it has to be assumed that the inventive aqueous calcium carbonate containing composition impart positive effects on the optical and mechanical properties of paper end products coated with such composition.