Self-binding pigment hybrid

Abstract

The present invention relates to a process for preparing self-binding pigment particles comprising the following steps: a) providing an aqueous mineral pigment material suspension; b) providing at least one polymeric binder, wherein the binder comprises at least one modified polysaccharide having a degree of carboxylation in the range of 0.4 to 2.0 and having an intrinsic viscosity in the range of >300 to 500 ml/g, c) mixing the binder of step b) with the aqueous mineral pigment material suspension of step a) and adjusting the solids content of the obtained suspension so that it is >45 to 95 wt.-%, preferably from 45 to 80 wt.-%, based on the total weight of the suspension, and d) grinding the aqueous mineral material suspension of step c).

Claims

1. A process for preparing self-binding pigment particles comprising the following steps: a) providing an aqueous mineral pigment material suspension, wherein the mineral pigment comprises calcium carbonate; b) providing at least one modified polysaccharide binder having a degree of carboxylation in the range of 0.4 to 2.0 and having an intrinsic viscosity in the range of >300 to 500 ml/g; c) mixing the binder of step b) with the aqueous mineral pigment material suspension of step a) so that the binder is present in the suspension in an amount from 0.1 to 10.0 wt.-% and the resulting suspension has a solids content of 45 to 95 wt.-%, based on the total weight of the suspension; and d) grinding the resulting suspension of step c) to obtain a suspension of self-binding pigment particles, wherein the grinding is carried out until the fraction of self-binding pigment particles having a particle size of less than 1 m is greater than 60 wt.-%, based on the total weight of the pigment particles.

2. The process according to claim 1, wherein the at least one modified polysaccharide is carboxymethylcellulose.

3. The process according to claim 1, wherein in step c) the binder is present in the suspension in an amount from 0.2 to 5 wt.-%, based on the total weight of the suspension.

4. The process according to claim 1, wherein in step c) the binder is present in the suspension in an amount from 0.25 to 3.0 wt.-%, based on the total weight of the suspension.

5. The process according to claim 1, wherein the binder is in form of a solution or a dry material.

6. The process according to claim 1, wherein the binder is in form of an aqueous solution having a binder concentration from 0.5 to 50 wt.-%, based on the total weight of the solution.

7. The process according to claim 1, wherein the binder is in form of an aqueous solution having a binder concentration from 1 to 40 wt.-%, based on the total weight of the solution.

8. The process according to claim 1, wherein the binder is in form of an aqueous solution having a binder concentration from 3 to 20 wt.-%, based on the total weight of the solution.

9. The process according to claim 1, wherein the binder is in form of an aqueous solution having a binder concentration from 4 to 10 wt.-%, based on the total weight of the solution.

10. The process according to claim 1, wherein the binder is composed of a mixture of two or more types of modified polysaccharide.

11. The process according to claim 1, wherein the solids content of the resulting suspension in step c) is from 45 to 80 wt.-%, based on the total weight of the suspension.

12. The process according to claim 1, wherein the solids content of the resulting in step c) is from 45 to 60 wt.-%, based on the total weight of the suspension.

13. The process according to claim 1, wherein the solids content of the resulting suspension in step c) is from 48 to 58 wt.-%, based on the total weight of the suspension.

14. The process according to claim 1, wherein the solids content of the resulting suspension in step c) is from 50 to 55 wt.-%, based on the total weight of the suspension.

15. The process according to claim 1, wherein the carboxylic groups of the at least one modified polysaccharide are at least partly neutralized by adding to the aqueous mineral pigment material suspension prior or during the grinding step d) one or more polyvalent cations, wherein the polyvalent cations are selected from the group consisting of Sr.sup.2+, Ca.sup.2+ and Mg.sup.2+.

16. The process according to claim 1, wherein the carboxylic groups of the at least one modified polysaccharide are at least partly neutralized by adding to the aqueous mineral pigment material suspension prior or during the grinding step d) one or more polyvalent cations, wherein the polyvalent cations are Ca.sup.2+, added in form of Ca(OH).sub.2 in suspension and/or solution.

17. The process according to claim 1, wherein the carboxylic groups of the at least one modified polysaccharide are at least partly neutralized by adding to the aqueous mineral pigment material suspension prior or during the grinding step d) one or more polyvalent cations, in situ formed, by adding a compound selected from the group consisting of H.sub.3PO.sub.4, Na.sub.2HPO.sub.4, and CaHPO.sub.4.

18. The process according to claim 1, wherein the carboxylic groups of the at least one modified polysaccharide are at least partly neutralized by adding to the aqueous mineral pigment material suspension prior or during the grinding step d) one or more monovalent cations, wherein the monovalent cations are selected from the group consisting of Li.sup.+, Na.sup.+ and K.sup.+.

19. The process according to claim 1, wherein the carboxylic groups of the at least one modified polysaccharide are at least partly neutralized by adding to the aqueous mineral pigment material suspension prior or during the grinding step d) a combination of one or more polyvalent cations and one or more monovalent cations, wherein the polyvalent cations are selected from the group consisting of Sr.sup.2+, Ca.sup.2+ and Mg.sup.2+, and wherein the monovalent cations are selected from the group consisting of Li.sup.+, Na.sup.+ and K.sup.+.

20. The process according to claim 1, wherein the carboxylic groups of the at least one modified polysaccharide are at least partly neutralized by adding to the aqueous mineral pigment material suspension prior or during the grinding step d) a combination of one or more polyvalent cations and one or more monovalent cations, wherein the polyvalent cations are Ca.sup.2+ added in form of Ca(OH).sub.2 in suspension and/or solution, and wherein the monovalent cations are selected from the group consisting of Li.sup.+, Na.sup.+ and K.sup.+.

21. The process according to claim 1, wherein the grinding step d) is carried out until the fraction of self-binding pigment particles having a particle size of less than 1 m is greater than 75 wt.-%, based on the total weight of the pigment particles.

22. The process according to claim 1, wherein the grinding step d) is carried out until the fraction of self-binding pigment particles having a particle size of less than 1 m is greater than 85 wt.-%, based on the total weight of the pigment particles.

23. The process according to claim 1, wherein before or during or after steps c) and/or d) a dispersing agent is added.

24. The process according to claim 1, wherein the mineral pigment material is calcium carbonate, or calcium carbonate and one or more of dolomite, magnesium, clay, talc, kaolin, aluminium hydroxide, mica, titanium dioxide, synthetic fibers and natural fibers.

25. The process according to claim 1, wherein the mineral pigment is selected from the group consisting of a ground natural calcium carbonate, a precipitated calcium carbonate, a modified calcium carbonate, and any mixture thereof.

26. The process according to claim 1, wherein the degree of carboxylation of the at least one modified polysaccharide is in the range of 0.5 to 1.8.

27. The process according to claim 1, wherein the degree of carboxylation of the at least one modified polysaccharide is in the range of 0.6 to 1.0.

28. The process according to claim 1, wherein the degree of carboxylation of the at least one modified polysaccharide is in the range of 0.7 to 0.9.

29. The process according to claim 1, wherein the intrinsic viscosity of the at least one modified polysaccharide is in the range of 320 to 450 ml/g.

30. The process according to claim 1, wherein the intrinsic viscosity of the at least one modified polysaccharide is in the range of 330 to 550 ml/g.

31. The process according to claim 1, wherein the at least one modified polysaccharide has a degree of carboxylation of 0.4 to 1.

32. The process according to claim 1, wherein the at least one modified polysaccharide has a degree of carboxylation of 0.4 to 1, and an intrinsic viscosity in the range of 320 to 450 ml/g.

33. The process according to claim 1, wherein the at least one modified polysaccharide has a degree of carboxylation of 0.4 to 1, and an intrinsic viscosity in the range of 330 to 350 ml/g.

34. The process according to claim 1, wherein the grinding step d) is carried out at a temperature from 25 to 110 C.

35. The process according to claim 1, wherein the grinding step d) is carried out at a temperature from 35 to 75 C.

36. The process according to claim 1, wherein the grinding step d) is carried out in batch or continuously.

37. A self-binding pigment particle suspension obtained by the process of claim 1.

38. Paper, plastic, paint, concrete or agriculture product comprising the self-binding pigment particle suspension of claim 37.

39. Paper comprising the self-binding pigment particle suspension of claim 37.

40. An agriculture product comprising the self-binding pigment particle suspension of claim 37.

Description

EXAMPLES

(1) Methods and Materials

(2) In the following, materials and measurement methods implemented in the examples are described.

(3) Brookfield Viscosity

(4) The Brookfield viscosity of the self-binding pigment particles suspension was measured after one hour of production and after one minute of stirring at room temperature at 100 rpm by the use of a Brookfield viscometer type RVT equipped with an appropriate spindle.

(5) Particle Size

(6) The particle distribution of the self-binding pigment particles was measured using a Sedigraph 5120 from the company Micromeritics, USA. 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 was carried out in an aqueous solution comprising 0.1 wt.-% Na.sub.4P.sub.2O.sub.7. The samples were dispersed using a high speed stirrer and supersonics.

(7) Solids Content of an Aqueous Suspension

(8) The suspension solids content (also known as dry weight) was determined using a Moisture Analyser HR73 from the company Mettler-Toledo, Switzerland, with the following settings: temperature of 120 C., automatic switch off 3, standard drying, 5 to 20 g of suspension.

(9) Intrinsic Viscosity

(10) The intrinsic viscosity was determined by a Schott AVS 370 system. The samples were dissolved in a 0.2 M NaCl solution, and subsequently, the pH was adjusted to 10 with NaOH. Measurements were performed at 25 C. with a capillary type 0a and corrected using the Hagenbach correction.

(11) Degree of Carboxylation

(12) The degree of carboxylation was determined by conductometric titration according to Katz et al. The determination of strong and weak acidic groups in sulfite pulps (Svensk Paperstidn., 1984, 6, pp. 48-53).

(13) Whiteness (R457) and Yellowness Index Measurement

(14) Whiteness and yellowness index was determined according to norm TAPPI T452/ISO 247. Glossiness was determined according to DIN 54 502/TAPPI 75.

(15) PPS Roughness ISO 8791-4

(16) The geometric form of a paper surface is defined as deviation from the ideal dead level. The more the surface approaches the ideal level, the smoother the paper is. The measuring method (PPS) is based on the measurement of the air leakage between the paper surface and an even measuring head. In case of the PPS roughness, the depth of the pores is measured by a defined circle. The higher the measured value is, the rougher the paper surface is.

(17) 1. Testing of the Self-binding Properties of the Pigment Particles Obtained by the Present Invention

(18) Tablet Crushing Test

(19) This test is a measure for the self-binding power of a pigment. It is a measure for the force needed to crush tablets that were formed from the self-binding pigment slurries.

(20) To demonstrate the suitability for the self-binding character of the pigmentary particles thus obtained, tablets were formulated using a membrane filtration process. In this regard, an apparatus of the high-pressure filter press type was used, manufactured from a hollow steel tube. The said tube is closed at the top by a lid and contains the filtration membrane at the bottom.

(21) A volume of 80 ml of the suspension obtained in Examples 1 to 5 was introduced into the tube of the high-pressure filter press. A constant pressure of 15 bar was then applied, which enables the water to be eliminated via the membrane filter, until a tablet of 20 mm thickness is obtained.

(22) The obtained tablets were then dried at a temperature of 60 C. for 2 days.

(23) The device and method used are described in detail in the document entitled Modified calcium carbonate coatings with rapid absorption and extensive liquid uptake capacity (Colloids and Surfaces A, 236 (1-3), 2003, pp. 91-102).

(24) The quasi-cylindrical solid tablets of pigmentary particles were then ground using a disk mill (Jean Wirtz, Phoenix 4000) in the form of disk-shaped samples having a diameter of 2.0-2.1 cm and a thickness of 0.6-0.7 cm. This procedure is described in the document entitled Fluid transport into porous coating structures: some novel findings (Tappi Journal, 83 (5), 2000, pp. 77-78).

(25) The samples thus obtained underwent a crush resistance test on a Zwick-Roell tension machine with a WN158988 control unit, using a rod/flat system (with a hemispherical end). The force of the cell was 20 kN.

(26) The samples were crushed at a speed of 3 mm/min.sup.1 over a length of 10 mm. The maximum force needed to crush the tablet was recorded. The results for Examples 1 to 5 are listed in Table 1

Example 1

Comparative Example

(27) The pigment particles of this example are commercially available as Hydrocarb 90-ME from Omya. The product is in the form of a slurry of a natural CaCO.sub.3 and has a solid content of 78.0 wt.-%.

(28) The particle size distribution of the mineral pigment material, measured on a Sedigraph 5120, was as follows: 90 wt.-% was smaller than 2 m, 65 wt.-% was smaller than 1 m, and 15 wt.-% was smaller than 0.2 m.

(29) The tablet crushing test gave a maximum Force F.sub.max of 256 N.

Example 2

Inventive Example

(30) A natural CaCO.sub.3 from Norway having a fineness corresponding to a d.sub.50 value of 42 to 48 m was employed as mineral pigment material, and a carboxymethylcellulose (commercially available from Sigma Aldrich, No. 419273) was used as the polymeric binder.

(31) The intrinsic viscosity of the carboxymethylcellulose (CMC) was 327 ml/g, and the degree of substitution was 0.7.

(32) The natural CaCO.sub.3 was used in the form of a filter cake having a solids content of 70.0 wt.-%. From this filter cake a slurry with a solid content of 50.0 wt-% was prepared by adding 2.0 wt.-% of a 4.7% solution of the above carboxymethylcellulose.

(33) The wet grinding of the slurry was done in tap water (15 dH) in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 0.6 litres in a recirculation mode, using zirconium silicate beads of 0.6 to 1.0 mm diameter, until a d.sub.50 value of 0.6 m was achieved.

(34) The particle size distribution of the mineral pigment material, measured on a Sedigraph 5120, was as follows: 95 wt.-% was smaller than 2 m, 75 wt.-% was smaller than 1 m, and 13 wt.-% was smaller than 0.2 m.

(35) The tablet crushing test gave a maximum Force F.sub.max of 1 300 N.

Example 3

Inventive Example

(36) A natural CaCO.sub.3 from Norway having a fineness corresponding to a d.sub.50 value of 42 to 48 m was employed as mineral pigment material, and a carboxymethylcellulose (commercially available from Sigma Aldrich, No. 419281) was used as the polymeric binder.

(37) The intrinsic viscosity of the carboxymethylcellulose (CMC) was 460 ml/g, and the degree of substitution was 1.2.

(38) The natural CaCO.sub.3 was used in the form of a filter cake having a solids content of 70.0 wt.-%. From this filter cake a slurry with a solid content of 45.0 wt-% was prepared by adding 2.0 wt.-% of a 2.5% solution of the above carboxymethylcellulose.

(39) The wet grinding of the slurry was done in tap water (15 dH) in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 0.6 litres in a recirculation mode, using zirconium silicate beads of 0.6 to 1.0 mm diameter, until a d.sub.50 value of 0.6 m was achieved.

(40) The particle size distribution of the mineral pigment material, measured on a Sedigraph 5120, was as follows: 95 wt.-% was smaller than 2 m, 75 wt.-% was smaller than 1 m, and 13 wt.-% was smaller than 0.2 m.

(41) The tablet crushing test gave a maximum Force F.sub.max of 970 N.

Example 4

Inventive Example

(42) A natural CaCO.sub.3 from Norway having a fineness corresponding to a d.sub.50 value of 42 to 48 m was employed as mineral pigment material, and a carboxymethylcellulose (commercially available from Sigma Aldrich, No. 419311) was used as the polymeric binder.

(43) The intrinsic viscosity of the carboxymethylcellulose (CMC) was 460 ml/g, and the degree of substitution was 0.7.

(44) The natural CaCO.sub.3 was used in the form of a filter cake having a solids content of 70.0 wt.-%. From this filter cake a slurry with a solid content of 45.0 wt-% was prepared by adding 2.0 wt.-% of a 2.5% solution of the above carboxymethylcellulose.

(45) The wet grinding of the slurry was done in tap water (15 dH) in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 0.6 litres in a recirculation mode, using zirconium silicate beads of 0.6 to 1.0 mm diameter, until a d.sub.50 value of 0.6 m was achieved.

(46) The particle size distribution of the mineral pigment material, measured on a Sedigraph 5120, was as follows: 95 wt.-% was smaller than 2 m, 75 wt.-% was smaller than 1 m, and 13 wt.-% was smaller than 0.2 m.

(47) The tablet crushing test gave a maximum Force F.sub.max of 2 663 N.

(48) TABLE-US-00001 TABLE 1 Tablet crushing test results Example Comment F.sub.max [N] 1 Comparative Co-grinding with 256 Polyacrylate 2 Inventive Co-grinding with CMC 1 300 (327 ml/g; 0.7) 3 Inventive Co-grinding with CMC 970 (460 ml/g; 1.2) 4 Inventive Co-grinding with CMC 2 663 (460 ml/g; 0.7)

(49) From the results listed in Table 1 it is clearly evident that the pigment particles obtained by the present invention show a much better self-binding power compared to pigment particles that have been prepared by the same process, however, with the use of another polymeric binder, namely a polyacrylate binder.

(50) 2. Testing of Mechanical Strength Properties of Paper Containing the Self-binding Pigment According to the Present Invention

(51) Handsheet Preparation and Evaluation

(52) The handsheet study and the consequent testing of the mechanical strength properties of the paper is a measure for the ability of the self-binding pigment to bind to other surfaces like cellulosic fibers.

(53) Thermo mechanical pulp (TMP) 85% and Pine Kraft pulp (15%) refined to 27 SR (Schopper-Riegler) were used for the handsheet study. The blend of the thermo mechanical pulp and the pine kraft pulp had 80 SR. 60 g (dry) pulp blend were diluted in 10 dm.sup.3 tap water, and then the filler was added. The filler was a blend of 25% Intramax 50 (Clay, commercially available from Imerys International Ltd, UK) and 75% of the products described in examples 5, 6 or 7. The suspension was stirred for 30 minutes. Subsequently 0.06% (based on dry weight) of a polyacrylamide (Polymin 1530, commercially available from BASF, Ludwigshafen, Germany) was added as a retention aid and sheets of 52 g/m.sup.2 were formed using the Rapid-Kthen hand sheet former. Each sheet was dried using the Rapid-Kthen drier. The filler content in the handsheets was determined by burning a quarter of a dry handsheet in a muffle furnace heated to 570 C. After the burning was completed, the residue was transferred in a desiccator and allowed to cool down. When room temperature was reached, the weight of the residue was measured and the mass was related to the initially measured weight of the dry quarter hand sheet. The filler content in the examples 6-8 was 41-42%.

(54) The sheets were calendered with a Voith calender to 0.95-1.05 PPS roughness.

(55) The mechanical strength properties of the handsheets were characterized by the tensile index and internal bond according to ISO 1924-2 and SCAN-P80:98/TAPPI T541, respectively, after drying of the handsheets. The results for the mechanical strength properties of the tested papers are listed in Table 2.

Example 5

Comparative Example

(56) The pigment particles used in this example are commercially available as Hydrocarb HO-ME from Omya. The product is in the form of a slurry of a natural CaCO.sub.3 and has a solid content of 65.0 wt.-%.

(57) The particle size distribution of the mineral pigment material, measured on a Sedigraph 5120, was as follows: 85 wt.-% was smaller than 2 m, 60 wt.-% was smaller than 1 m, and 20 wt.-% was smaller than 0.2 m.

(58) The measured tensile index of the handsheet prepared with the pigment particles obtained in this example was 17 Nm/g, and the measured internal bond was 560 kPa.

Example 6

Comparative Example

(59) A natural CaCO.sub.3 from Norway having a fineness corresponding to a d.sub.50 value of 42 to 48 m was employed as mineral pigment material, and a carboxymethylcellulose (CMC) as polymeric binder. The intrinsic viscosity of the carboxymethylcellulose (CMC) was 179 ml/g, and the degree of substitution was 1.2.

(60) Preparation of Carboxymethylcellulose (CMC) Binder

(61) 214 g commercially available CMC (from ACROS Organics) having an M.sub.w of 250 000 g/mol, a carboxylation degree of 1.2, and an intrinsic viscosity of 774 ml/g, was dissolved in 2 460 ml water and stirred for 12 h at room temperature. Subsequently, the solution was heated to 80 C., and 800 l of a 30 wt.-% H.sub.2O.sub.2 solution were added dropwise. After 5 h, 60 l of said H.sub.2O.sub.2 solution were added dropwise. Thereafter, 2 times another 60 l of said H.sub.2O.sub.2 solution were added dropwise in 1.5 h intervals. Finally, the solution was stirred for another 1.5 h at 80 C. The obtained CMC binder had an intrinsic viscosity of 179 ml/g and a pH of 7.

(62) A slurry with a solid content of 60 wt.-% was prepared by adding 2 wt.-% of the prepared CMC binder in form of a 9.9 wt.-% solution in water to the mineral pigment material suspension.

(63) The wet grinding of the slurry was done in tap water (15 dH) in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 0.6 litres in a recirculation mode, using zirconium silicate beads of 0.6 to 1.0 mm diameter, until a d.sub.50 value of 0.8 m was achieved.

(64) The particle size distribution of the mineral pigment material, measured on a Sedigraph 5120, was as follows: 90 wt.-% was smaller than 2 m, 65 wt.-% was smaller than 1 m, and 20 wt.-% was smaller than 0.2 m.

(65) The measured tensile index of the handsheet prepared with the pigment particles obtained in this example was 23 Nm/g, and the measured internal bond was 610 kPa.

Example 7

Inventive Example

(66) A natural CaCO.sub.3 from Norway having a fineness corresponding to a d.sub.50 value of 42 to 48 m was employed as mineral pigment material, and a carboxymethylcellulose (CMC) (commercially available from Sigma Aldrich, No. 419273) was used as the polymeric binder.

(67) The intrinsic viscosity of the CMC was 327 ml/g, and the degree of substitution was 0.7.

(68) The natural CaCO.sub.3 was used in the form of a filter cake having a solids content of 70.0 wt.-%. From this filter cake a slurry with a solid content of 50.0 wt-% was prepared by adding 2.0 wt.-% of a 4.7% solution of the above carboxymethylcellulose.

(69) The wet grinding of the slurry was done in tap water (15 dH) in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 0.6 litres in a recirculation mode, using zircon silicate beads of 0.6 to 1.0 mm diameter, until a d.sub.50 value of 0.6 m was achieved.

(70) The particle size distribution of the mineral pigment material, measured on a Sedigraph 5120, was as follows: 95 wt.-% was smaller than 2 m, 75 wt.-% was smaller than 1 m, and 13 wt.-% was smaller than 0.2 m.

(71) The measured tensile index of the handsheet prepared with the pigment particles obtained in this example was 25 Nm/g, and the measured internal bond was 630 kPa.

(72) TABLE-US-00002 TABLE 2 Mechanical strength properties for the tested papers Comment: Polymeric binder (intrinsic viscosity; Tensile degree of index Internal Example carboxylation) [Nm/g] bond [kPa] 6 Comparative Co-grinding with 17 560 Polyacrylate 7 Comparative Co-grinding with 23 610 CMC (179 ml/g; 1.2) 8 Inventive Co-grinding with 25 630 CMC (327 ml/g; 0.7)

(73) It is clearly apparent from Table 2 that the self-binding pigments prepared by the inventive process show an improved ability to bind to other surfaces like cellulosic fibers. This is demonstrated by the tensile index of inventive example 8 (25 Nm/g) which is higher than the tensile indices of comparative examples 6 (17 Nm/g) and 7 (23 Nm/g). In addition, the internal bond of the inventive example 8 (630 kPa) is also higher than the internal bond of comparative examples 6 (560 kPa) and 7 (610 kPa).

(74) 3. Grinding Efficiency

(75) The following examples are intended to demonstrate the grinding efficiency of the inventive process of the present invention.

Example 8

Comparative Example

(76) A natural CaCO.sub.3 from Norway having a fineness corresponding to a d.sub.50 value of 42 to 48 m was employed as mineral pigment material, and a carboxymethylcellulose (CMC) (commercially available from CP Kelco, Atlanta, USA under the tradename Finnfix 5) as polymeric binder. The intrinsic viscosity of the carboxymethylcellulose (CMC) was 118 ml/g, and the degree of substitution was 0.7.

(77) The natural CaCO.sub.3 was used in the form of a filter cake having a solids content of 70.0 wt.-%. From this filter cake a slurry with a solid content of 50.0 wt-% was prepared by adding 2.0 wt.-% of a 4.7% solution of the above carboxymethylcellulose.

(78) The wet grinding of the slurry was done in tap water (15 dH) in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 0.6 litres in a recirculation mode, using zirconium silicate beads of 0.6 to 1.0 mm diameter. The recirculation was carried out six times.

(79) The particle size distribution of the particles obtained after passing the mill six times, measured on a Sedigraph 5120, was as follows: 97 wt.-% were smaller than 2 m, 68 wt.-% were smaller than 1 m, and the d.sub.50 was 0.8 m.

Example 9

Inventive Example

(80) A natural CaCO.sub.3 from Norway having a fineness corresponding to a d.sub.50 value of 42 to 48 m was employed as mineral pigment material, and a carboxymethylcellulose (commercially available from Sigma Aldrich, No. 419273) was used as the polymeric binder.

(81) The intrinsic viscosity of the carboxymethylcellulose (CMC) was 327 ml/g, and the degree of substitution was 0.7.

(82) The natural CaCO.sub.3 was used in the form of a filter cake having a solids content of 70.0 wt.-%. From this filter cake a slurry with a solid content of 50.0 wt-% was prepared by adding 2.0 wt.-% of a 4.7% solution of the above carboxymethylcellulose.

(83) The wet grinding of the slurry was done in tap water (15 dH) in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 0.6 litres in a recirculation mode, using zirconium silicate beads of 0.6 to 1.0 mm diameter. The recirculation was carried out six times.

(84) The particle size distribution of the particles obtained after passing the mill six times, measured on a Sedigraph 5120, was as follows: 98 wt.-% were smaller than 2 m, 87 wt.-% were smaller than 1 m, and the d.sub.50 was 0.5 m.

Example 10

Inventive Example

(85) A natural CaCO.sub.3 from Norway having a fineness corresponding to a d.sub.50 value of 42 to 48 m was employed as mineral pigment material, and a carboxymethylcellulose (commercially available from Sigma Aldrich, No. 419311) was used as the polymeric binder.

(86) The intrinsic viscosity of the carboxymethylcellulose (CMC) was 460 ml/g, and the degree of substitution was 0.7.

(87) The natural CaCO.sub.3 was used in the form of a filter cake having a solids content of 70.0 wt.-%. From this filter cake a slurry with a solid content of 45.0 wt-% was prepared by adding 2.0 wt.-% of a 2.5% solution of the above carboxymethylcellulose.

(88) The wet grinding of the slurry was done in tap water (15 dH) in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 0.6 litres in a recirculation mode, using zirconium silicate beads of 0.6 to 1.0 mm diameter. The recirculation was carried out six times.

(89) The particle size distribution of the particles obtained after passing the mill six times, measured on a Sedigraph 5120, was as follows: 99 wt.-% were smaller than 2 m, 85 wt.-% were smaller than 1 m, and the d.sub.50 was 0.5 m.

(90) From the results obtained by this testing it is evident that the inventive process provides a much higher fraction of particles finer than 1 m (namely 87 wt.-% and 85 wt.-%, respectively) compared to the comparative example (68 wt.-%). This is also confirmed by a d.sub.50 of 0.5 m for the inventive examples, compared to the d.sub.50 of 0.8 m for the comparative example.