Process for preparing self-binding pigment particles

Abstract

The present invention concerns a process for preparing self-binding pigment particles from an aqueous suspension of a calcium carbonate containing material, wherein an anionic binder and at least one cationic polymer are mixed with the suspension.

Claims

1. A process for preparing self-binding pigment particles comprising the following steps: a) providing a suspension comprising at least one calcium carbonate containing material, b) providing an anionic polymeric binder that comprises at least one modified polysaccharide, c) providing at least one cationic polymer selected from the group consisting of one or more polyamines, polyethyleneimines, polyacrylamides, cationic epichlorohydrin resins, polydiallyldimethylammonium chlorides, cationic starches, cationic guars, and any mixture thereof, d) mixing the suspension of step a) and the binder of step b), and e) grinding the mixed suspension of step d) so that the particle size of the calcium carbonate material is reduced and self-binding pigment particles are obtained, wherein the at least one cationic polymer of step c): i) is mixed in step d) with the suspension of step a) and the binder of step b), and/or ii) is mixed with the suspension obtained after grinding step e), and the obtained mixture is deagglomerated, wherein the binder of step b) is added in an amount from 0.001 to 20 wt.-% based on the total weight of the dry calcium carbonate containing material, and wherein the at least one cationic polymer of step c) is added in an amount from 0.001 to 20 wt.-% based on the total weight of the dry calcium carbonate containing material, and wherein the grinding step e) is carried out until the fraction of self-binding pigment particles having a particle size of less than 2 m is greater than 20 wt.-% based on the total weight of the pigment particles, as measured with a Mastersizer 2000.

2. The process of claim 1, wherein in step d) the suspension of step a) is, in a first step, mixed with the binder of step b), and then, in a second step, is mixed with the at least one cationic polymer of step c).

3. The process of claim 2, wherein in the first step the suspension of step a) is mixed with a first part the binder of step b), the obtained mixture is ground and then mixed with the remaining part of the binder of step b).

4. The process of claim 1, wherein in step d) the binder of step b) is, in a first step, mixed with the cationic polymer of step c), and then, in a second step, is mixed with the suspension of step a).

5. The process of claim 1, wherein in step d) the suspension of step a) is mixed with the binder of step b) and the cationic polymer of step c) in one step.

6. The process of claim 1, wherein the cationic polymer is added in an amount such that the charge density of the obtained self-binding pigment particles is lower compared to self-binding pigment particles not containing the cationic polymer.

7. The process of claim 1, wherein the cationic polymer is added in an amount such that the charge density of the obtained self-binding pigment particle is between 100 and 5 Eq/g.

8. The process of claim 1, wherein the cationic polymer is added in an amount such that the charge density of the obtained self-binding pigment particle is between 80 and 10 Eq/g.

9. The process of claim 1, wherein the cationic polymer is added in an amount such that the charge density of the obtained self-binding pigment particle is between 70 and 15 Eq/g.

10. The process of claim 1, wherein the at least one calcium carbonate containing material is selected from the group consisting of calcium carbonate, calcium carbonate containing minerals, mixed calcium carbonate based fillers, and any mixture thereof.

11. The process of claim 1, wherein the at least one calcium carbonate containing material is ground calcium carbonate.

12. The process of claim 1, wherein the at least one calcium carbonate containing material of step a) is provided in form of particles having a weight median particle diameter d.sub.50 value from 0.1 to 100 m.

13. The process of claim 1, wherein the at least one calcium carbonate containing material of step a) is provided in form of particles having a weight median particle diameter d.sub.50 value from 0.5 to 50 m.

14. The process of claim 1, wherein the at least one calcium carbonate containing material of step a) is provided in form of particles having a weight median particle diameter d.sub.50 value from 5.0 to 25 m.

15. The process of claim 1, wherein the at least one calcium carbonate containing material is provided in form of particles have a specific surface area of from 0.1 to 200 m.sup.2/g.

16. The process of claim 1, wherein the at least one calcium carbonate containing material is provided in form of particles have a specific surface area of from 1 to 25 m.sup.2/g.

17. The process of claim 1, wherein the at least one calcium carbonate containing material is provided in form of particles have a specific surface area of from 2 to 15 m.sup.2/g.

18. The process of claim 1, wherein the suspension of step a) has a solid content of at least 1 wt.-% based on the total weight of the suspension.

19. The process of claim 1, wherein the suspension of step a) has a solid content of from 1 to 90 wt.-% based on the total weight of the suspension.

20. The process of claim 1, wherein the suspension of step a) has a solid content of from 20 to 75 wt.-% based on the total weight of the suspension.

21. The process of claim 1, wherein the suspension of step a) has a solid content of from 45 to 65 wt.-% based on the total weight of the suspension.

22. The process of claim 1, wherein the at least one modified polysaccharide is a carboxymethyl derivate, a carboxymethyl hydroxypropyl derivate, and/or a carboxymethyl hydroxyethyl derivate of a polysaccharide.

23. The process of claim 1, wherein the at least one modified polysaccharide is selected from the group consisting of one or more carboxymethylcelluloses, anionic starches, anionic guars, and any mixture thereof.

24. The process of claim 1, wherein the at least one modified polysaccharide has a degree of substitution of hydroxyl groups from 0.4 to 2.0.

25. The process of claim 1, wherein the at least one modified polysaccharide has a degree of substitution of hydroxyl groups from 0.5 to 1.8.

26. The process of claim 1, wherein the at least one modified polysaccharide has a degree of substitution of hydroxyl groups from 0.6 to 1.6.

27. The process of claim 1, wherein the at least one modified polysaccharide has a degree of substitution of hydroxyl groups from 0.7 to 1.5.

28. The process of claim 1, wherein the binder of step b) is a carboxymethylcellulose having an intrinsic viscosity from 5 to 500 ml/g.

29. The process of claim 1, wherein the binder of step b) is a carboxymethylcellulose having an intrinsic viscosity from 10 to 400 ml/g.

30. The process of claim 1, wherein the binder of step b) is a carboxymethylcellulose having an intrinsic viscosity from 20 to 350 ml/g.

31. The process of claim 1, wherein the binder of step b) is in form of a solution or dry material.

32. The process of claim 1, wherein the binder of step b) is in form of a solution having a binder concentration from 1 to 70 wt.-% based on the total weight of the solution.

33. The process of claim 1, wherein the binder of step b) is in form of a solution having a binder concentration from 2 to 30 wt.-% based on the total weight of the solution.

34. The process of claim 1, wherein the binder of step b) is in form of a solution having a binder concentration from 3 to 15 wt.-% based on the total weight of the solution.

35. The process of claim 1, wherein the binder of step b) is in form of a solution having a binder concentration from 4 to 10 wt.-% based on the total weight of the solution.

36. The process of claim 1, wherein the binder of step b) is added in an amount from 0.005 to 15 wt.-% based on the total weight of the dry calcium carbonate containing material.

37. The process of claim 1, wherein the binder of step b) is added in an amount from 0.01 to 10 wt.-% based on the total weight of the dry calcium carbonate containing material.

38. The process of claim 1, wherein the binder of step b) is added in an amount from 0.05 to 5 wt.-% based on the total weight of the dry calcium carbonate containing material.

39. The process of claim 1, wherein the at least one cationic polymer of step c) is in form of a solution or dry material.

40. The process of claim 1, wherein the at least one cationic polymer of step c) is in form of an aqueous solution having a concentration from 1 to 70 wt.-% based on the total weight of the solution.

41. The process of claim 1, wherein the at least one cationic polymer of step c) is in form of an aqueous solution having a concentration from 2 to 55 wt.-% based on the total weight of the solution.

42. The process of claim 1, wherein the at least one cationic polymer of step c) is in form of an aqueous solution having a concentration from 5 to 50 wt.-% based on the total weight of the solution.

43. The process of claim 1, wherein the at least one cationic polymer of step c) is in form of an aqueous solution having a concentration from 30 to 50 wt.-% based on the total weight of the solution.

44. The process of claim 1, wherein the at least one cationic polymer of step c) is added in an amount from 0.005 to 15 wt.-% based on the total weight of the dry calcium carbonate containing material.

45. The process of claim 1, wherein the at least one cationic polymer of step c) is added in an amount from 0.01 to 10 wt.-% based on the total weight of the dry calcium carbonate containing material.

46. The process of claim 1, wherein the at least one cationic polymer of step c) is added in an amount from 0.05 to 5 wt.-% based on the total weight of the dry calcium carbonate containing material.

47. The process of claim 1, wherein the at least one cationic polymer of step c) is added in an amount from 0.5 to 2.5 wt.-% based on the total weight of the dry calcium carbonate containing material.

48. The process of claim 1, wherein the grinding step e) is carried out at a temperature from 5 to 110 C.

49. The process of claim 1, wherein the grinding step e) is carried out at a temperature from 10 to 100 C.

50. The process of claim 1, wherein the grinding step e) is carried out at a temperature from 15 to 80 C.

51. The process of claim 1, wherein the grinding step e) is carried out at a temperature from 20 to 25 C.

52. The process of claim 1, wherein the grinding step e) is carried out in batch or continuously.

53. The process of claim 1, wherein the process further comprises a step of concentrating the obtained suspension of self-binding pigment particles.

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

55. A paper, plastic, paint, coating, concrete or agriculture product comprising the self-binding pigment particle suspension of claim 54.

56. The paper, plastic, paint, coating, concrete or agriculture product of claim 55, wherein the self-binding pigment particle suspension is used in wet end processes of a paper machine, in a cigarette paper, board and/or coating application, or as a support for rotogravure and/or offset and/or ink jet printing and/or continuous ink jet printing and/or flexography and/or electrography and/or decoration surfaces, or the self-binding pigment particle suspension is used to reduce sun light and UV exposure of plant leaves.

57. The process of claim 1, wherein the grinding step e) is carried out until the fraction of self-binding pigment particles having a particle size of less than 2 m is greater than 40 wt.-% based on the total weight of the pigment particles, as measured with a Mastersizer 2000.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows the relative retention values obtained for paper suspensions comprising the pigment particles of Example 1, 3, and 11 as fillers, wherein the pulp was diluted in tap water.

(2) FIG. 2 shows the relative retention values obtained for paper suspensions comprising the pigment particles of Example 1, 3, and 11 as fillers, wherein the pulp was diluted in clear filtrate.

(3) FIG. 3 shows the breaking length of wood free, uncoated paper containing Hydrocarb HO ME-67% and the pigment particles of Example 4 as fillers in different amounts.

(4) FIG. 4 shows the tensile index of wood free, uncoated paper containing Hydrocarb HO ME-67% and the pigment particles of Example 4 as fillers in different amounts.

(5) FIG. 5 shows the tensile energy absorption of wood free, uncoated paper containing Hydrocarb HO ME-67% and the pigment particles of Example 4 as fillers in different amounts.

(6) FIG. 6 shows the tear growth work of wood free, uncoated paper containing Hydrocarb HO ME-67% and the pigment particles of Example 4 as fillers in different amounts.

(7) FIG. 7 shows the internal bond (z-direction) of wood free, uncoated paper containing Hydrocarb HO ME-67% and the pigment particles of Example 4 as fillers in different amounts.

(8) FIG. 8 shows the bending stiffness of wood free, uncoated paper containing Hydrocarb HO ME-67% and the pigment particles of Example 4 as fillers in different amounts.

(9) FIG. 9 shows the modulus of elasticity of wood free, uncoated paper containing Hydrocarb HO ME-67% and the pigment particles of Example 4 as fillers in different amounts.

(10) FIG. 10 shows the amount of retention aid required for paper suspensions containing Hydrocarb HO ME-67%, Hydrocarb HO ME-67% and a cationic polymer, the pigment particles of Example 1, the pigment particles of Example 3 as fillers.

(11) FIG. 11 shows the tensile index of supercalendered paper containing Hydrocarb HO ME-67%, Hydrocarb HO ME-67% and a cationic polymer, the pigment particles of Example 1, the pigment particles of Example 3 as fillers.

(12) FIG. 12 shows the tensile energy absorption of supercalendered paper containing Hydrocarb HO ME-67%, Hydrocarb HO ME-67% and a cationic polymer, the pigment particles of Example 1, the pigment particles of Example 3 as fillers.

(13) FIG. 13 shows the internal bond of supercalendered paper containing Hydrocarb HO ME-67%, Hydrocarb HO ME-67% and a cationic polymer, the pigment particles of Example 1, the pigment particles of Example 3 as fillers.

(14) FIG. 14 shows the amount of free polymer in aqueous suspensions of self-binding calcium carbonate particles versus the amount of the added cationic polymer.

(15) FIG. 15 shows the turbidity values obtained for paper suspensions containing Hydrocarb HO ME-67% or the pigment particles of Examples 15 to 20 as fillers.

(16) FIG. 16 shows the turbidity values obtained for paper suspensions containing Hydrocarb HO ME-67% or the pigment particles of Examples 21 to 25 as fillers.

(17) FIG. 17 shows the breaking length of wood free, uncoated paper containing Hydrocarb HO ME-67% with and without cationic polymer, the pigment particles of Example 27 with and without cationic polymer, and the pigment particles of Example 28 as fillers in different amounts.

(18) FIG. 18 shows the tensile index of wood free, uncoated paper containing Hydrocarb HO ME-67% with and without cationic polymer, the pigment particles of Example 27 with and without cationic polymer, and the pigment particles of Example 28 as fillers in different amounts.

(19) FIG. 19 shows the tensile energy absorption of wood free, uncoated paper containing Hydrocarb HO ME-67% with and without cationic polymer, the pigment particles of Example 27 with and without cationic polymer, and the pigment particles of Example 28 as fillers in different amounts.

(20) FIG. 20 shows the bending stiffness of wood free, uncoated paper containing Hydrocarb HO ME-67% with and without cationic polymer, the pigment particles of Example 27 with and without cationic polymer, and the pigment particles of Example 28 as fillers in different amounts.

(21) FIG. 21 shows the internal bond (z-direction) of wood free, uncoated paper containing Hydrocarb HO ME-67% with and without cationic polymer, the pigment particles of Example 27 with and without cationic polymer, and the pigment particles of Example 28 as fillers in different amounts.

EXAMPLES

1. Measurement Methods

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

(23) Brookfield Viscosity

(24) 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.

(25) Particle Size

(26) The particle distribution of the ground calcium carbonate particles was measured using a Sedigraph 5120 from the company Micromeritics, USA. The particle size distribution of the inventive self-binding pigment particles was measured using a Mastersizer 2000 from the company Malvern Instruments Ltd, England. The method and the instruments 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.

(27) Solids Content of an Aqueous Suspension

(28) 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.

(29) Intrinsic Viscosity

(30) 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.

(31) Polyelectrolyte Titration (PET)

(32) The polyelectrolyte content in the aqueous suspension was determined using a Memotitrator Mettler DL 55 equipped with a Phototrode DP 660 commercialised by Mettler-Toledo, Switzerland. The measurements of the poylelectrolyte content was carried out by weighing a sample of the calcium carbonate suspension into a titration vessel and diluting said sample with deionized water up to a volume of approximately 40 ml. Subsequently, 10 ml of 0.01 M cationic poly(N,N-dimethyl-3,5-dimethylene-piperidinium chloride) (PDDPC; obtained from ACROS Organics, Belgium) were slowly added under stirring into the titration vessel within 5 min and than the content of the vessel was stirred for another 20 min. Afterwards the suspension was filtered trough a 0.2 m mix-ester membrane filter ( 47 mm) and washed with 5 ml of deionized water. The thus obtained filtrate was diluted with 5 ml of phosphate buffer pH 7 (Riedel-de Han, Germany) and than 0.01 M of a potassium polyvinylsulfate (KPVS; obtained from SERVA Feinbiochemica, Heidelberg) solution was added slowly to the filtrate to titrate the excess of cationic reagent. The endpoint of titration was detected by a Phototrode DP660, which was adjusted to 1200 to 1400 mV in deionized water, prior to such measurement. The charge calculation was carried out according to the following evaluation:

(33) $Q_{\mathrm{atro}}=\frac{\left(\left(V_{\mathrm{PDDPC}}*t_{\mathrm{PDA}}\right)-V_{\mathrm{KPVS}}\right)*\left(-1000\right)}{E_P*\mathrm{Fk}}\left[\mathrm{Val}\text{/}g\right]$$w_{\mathrm{atro}}=-\frac{Q_{\mathrm{atro}}}{K_{\mathrm{DM}}*100}\left[\%\right]$
Calculation of the optimal sample weight:

(34) $E_p=\frac{60}{w_{\mathrm{DM}}*K_{\mathrm{DM}}*\mathrm{Fk}}$
Calculation of adapted sample weight for 4 ml consumption:

(35) $E_{4ml}=\frac{E_1*6}{\left(10-V_{\mathrm{KPVS},1}\right)}$
Abbreviations
E.sub.P=sample weight [g]
w.sub.DM=Dispersing agent content in [%]
K.sub.DM=Dispersing agent constant [Val/0.1 mg dispersing agent]
Fk=Solids content [%]
V.sub.PDDPC=Volume PDDPC [ml]
V.sub.KPVS=Volume KPVS [ml]
t.sub.PDDPC=Titer PDDPC
E.sub.DM=Dispersing agent weight [mg]
Q=Charge [Val/g]
w.sub.atro=Dispersing agent content atro [%]
E.sub.1=Sample weight of experiment to be optimised [g]
V.sub.KPVS,1=experimental consumption KPVS [ml] of experiment to be optimised
Loss on Ignition (LOI) Method and Free Polymer

(36) For the measurement of the loss on ignition, samples of the self-binding pigment material suspensions were dried in a microwave at approximately 200 W for about 75 min such that the samples had maximum moisture of about 0.5 wt.-%, based on the total weight of the particulate material. Subsequently, the dried samples were de-agglomerated by using a RETSCH ultra-centrifugal mill (type ZM) with 200 m screen and rotor having 24 teeth. 3 to 4 g of the obtained sample was weighed into a porcelain crucible and heated in a muffle oven at about 570 C. until constant mass. After cooling in a desiccator, the porcelain crucible was weighed with the obtained residue. The values given herein are the average of two measurements of independently prepared samples.

(37) The loss on ignition is an absolute measurement displayed in percent and calculated according to the following formula:

(38) $\mathrm{LOI}\left(\mathrm{slurry}\right)=\frac{100*\left(m_1-m_2\right)}{m_1}$
with
m.sub.1: mass of initial weight [g]
m.sub.2: mass after heating to about 570 C. in a muffle oven [g]

(39) An aliquot of the slurry was diluted with deionised water to a concentration of 10 wt.-%, based on the total weight of the slurry. The suspension was stirred for 5 minutes. The suspension was centrifuged with a lab centrifuge at 2600 g for 15 minutes. The upper water phase was poured and the sedimented cake was dried in a microwave at approximately 200 W for about 75 min such that the samples had a maximum moisture content of about 0.5 wt.-%, based on the total weight of the particulate material. Subsequently, the dried samples were de-agglomerated by using a RETSCH ultra-centrifugal mill (type ZM) with 200 m screen and rotor having 24 teeth. 3 to 4 g of the obtained sample was weighed into a porcelain crucible and heated in a muffle oven at about 570 C. until constant mass. After cooling in a desiccator, the porcelain crucible was weighed with the obtained residue. The values given herein are the average of two measurements of independently prepared samples.

(40) The loss on ignition is an absolute measurement displayed in percent and calculated according to the following formula:

(41) $\mathrm{LOI}\left(\mathrm{cake}\right)=\frac{100*\left(m_1-m_2\right)}{m_1}$
with
m.sub.1: mass of initial weight [g]
m.sub.2: mass after heating to about 570 C. in a muffle oven [g]

(42) The amount of free polymer can be calculated according to the following formula:

(43) $\mathrm{Free}\mathrm{polymer}\%=100-\frac{\mathrm{LOI}\left(\mathrm{cake}\right)-0.6\%}{\mathrm{LOI}\left(\mathrm{slurry}\right)-0.6\%}*100$

(44) The LOI of the original GCC was found to be 0.6% and this is taken into account by subtracting these 0.6%.

(45) Whiteness (R457) and Yellowness Index Measurement

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

(47) Turbidity

(48) The turbidity was measured with a Hach Lange 2100AN IS Laboratory Turbidimeter and the calibration was performed using StabCal turbidity standards (formazin standards) of <0.1, 20, 200, 1000, 4000, and 7500 NTU (Nephelometric Turbidity Units).

(49) Chemical Oxygen Demand

(50) Chemical oxygen demand (COD) was measured according to the Lange Method (ISO 15705), as described in the document issued by HACH LANGE LTD, entitled DOC042.52.20023.Nov08. Approximately, 2 ml of the liquid phase were added in a Lange CSB LCK 014 cuvette, covering a range between 1000 and 10000 mg/l and heated in the closed cuvette for two hours at 148 C. in a dry thermostat. This suspension was then analyzed according to the Lange Method.

2. Examples 1 to 12

Example 1

GCC with CMC (Comparative Example)

(51) A chemical free calcium carbonate from Norway, Molde, having a fineness corresponding to a d.sub.50 value of 0.8 m and a d.sub.98 value of 5.0 m was employed as ground calcium carbonate (GCC). The specific surface (BET) of the ground calcium carbonate was 7.5 m.sup.2/g and the charge density was 24.8 Eq/g. The GCC was provided in the form of a filter cake having a solids content of 70.0 wt.-%.

(52) The anionic polymeric binder was a carboxymethylcellulose (CMC) having a molecular weight of 90000 g/mol (No. 419273, commercially available from Sigma Aldrich, Germany). The intrinsic viscosity of the CMC was 327 ml/g, and the degree of substitution (degree of carboxylation) was 0.7. The CMC was used in form of an aqueous solution containing 6 wt.-% CMC, based on the total amount of the solution.

(53) A slurry with a solid content of 50.0 wt.-%, based on the total amount of the slurry, was prepared from the GCC filter cake by adding 2.0 wt.-% CMC, based on the total weight of the dry ground calcium carbonate.

(54) The obtained slurry had a Brookfield viscosity of 237 mPas. The particle size distribution of the pigment particles, measured on a Sedigraph 5120, had a fraction of 88 wt.-% smaller than 2 m, and 61 wt.-% smaller than 1 m.

(55) The obtained slurry was wet ground at room temperature. The wet grinding of slurry was done in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 600 cm.sup.3 at a speed of 2500 rpm and at a flow rate of 500 cm.sup.3/min. 480 cm.sup.3 (80%) of the grinding chamber was filled with grinding beads having a diameter of 0.6-1.0 mm (melt fused beads consisting of 68% baddeleyit and 32% amorphous silicate). The slurry was passed four times through the mill.

(56) The product obtained was analyzed with respect to particle size, specific surface (BET), brightness, electrochemical charge (PET), and LOI. Tables 2 and 3 summarize the properties of the obtained slurry and the pigment particles contained therein.

Examples 2 to 8

GCC with CMC and polyDADMAC (Inventive Examples)

(57) A chemical free calcium carbonate from Norway, Molde, having a fineness corresponding to a d.sub.50 value of 0.8 m and a d.sub.98 value of 5.0 m was employed as ground calcium carbonate (GCC). The specific surface (BET) of the ground calcium carbonate was 7.5 m.sup.2/g and the charge density was 24.8 Eq/g. The GCC was provided in the form of a filter cake having a solids content of 70.0 wt.-%.

(58) The anionic polymeric binder was a carboxymethylcellulose (CMC) having a molecular weight of 90 000 g/mol (No. 419273, commercially available from Sigma Aldrich, Germany). The intrinsic viscosity of the CMC was 327 ml/g, and the degree of substitution (degree of carboxylation) was 0.7. The CMC was used in form of an aqueous solution containing 6 wt.-% CMC, based on the total amount of the solution.

(59) As cationic polymer PolyDADMAC (Catiofast BP liquid, commercially available from BASF, Germany) was employed in form of a solution containing 50 wt.-% PolyDADMAC, based on the total weight of the solution.

(60) In a first step, a slurry with a solid content of 50.0 wt.-%, based on the total amount of the slurry, was prepared from the GCC filter cake by adding 2.0 wt.-% CMC, based on the total amount of the slurry. In a second step, polyDADMAC was added in different amounts, namely, in amounts of 0.50 wt.-% (Example 2), 1.00 wt.-% (Examples 3 and 5), 1.25 wt.-% (Example 6), 1.50 wt.-% (Example 7), 1.75 wt.-% (Example 8), and 2.00 wt.-% (Example 4), based on the total weight of the dry ground calcium carbonate (see also Table 1).

(61) Subsequently, the obtained slurry was wet ground at room temperature. The wet grinding of slurry was done in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 600 cm.sup.3 at a speed of 2500 rpm and at a flow rate of 500 cm.sup.3/min. 480 cm.sup.3 (80%) of the grinding chamber were filled with grinding beads having a diameter of 0.6-1.0 mm (melt fused beads consisting of 68% baddeleyit and 32% amorphous silicate). The slurry was passed four times through the mill.

(62) The products obtained were analyzed with respect to particle size, specific surface (BET), brightness, electrochemical charge (PET), and LOI. Tables 2 and 3 summarize the properties of the obtained slurries and the inventive self-binding pigment particles contained therein.

Examples 9 to 11

GCC with CMC and Cationic Starch (Inventive Examples)

(63) A chemical free calcium carbonate from Norway, Molde, having a fineness corresponding to a d.sub.50 value of 0.8 m and a d.sub.98 value of 5.0 m was employed as ground calcium carbonate (GCC). The specific surface (BET) of the ground calcium carbonate was 7.5 m.sup.2/g and the charge density was 24.8 Eq/g. The GCC was provided in the form of a filter cake having a solids content of 70.0 wt.-%.

(64) The anionic polymeric binder was a carboxymethylcellulose (CMC) having a molecular weight of 90000 g/mol (No. 419273, commercially available from Sigma Aldrich, Germany). The intrinsic viscosity of the CMC was 327 ml/g, and the degree of substitution (degree of carboxylation) was 0.7. The CMC was used in form of a dry powder.

(65) As cationic polymer, the cationic starch Cargill C*Bond (no. HR05947, commercially available from Cargill Deutschland GmbH, Germany) was employed in form of a dry powder.

(66) Premixed solutions of starch and CMC were prepared with different amounts and ratios of cationic starch and CMC, namely, 1.5 wt.-% CMC and 0.5 wt.-% starch (Example 9), 1.0 wt.-% CMC and 1.0 wt.-% starch (Example 10), and 0.5 wt.-% CMC and 1.5 wt.-% starch (Example 11), based on the total weight of the dry ground calcium carbonate (see also Table 1). Said premixed solutions were produced by dissolving the cationic starch in water and heating the solution up to 100 C. The starch solution was cooled down to room temperature, and the CMC powder was added. The solution was stirred at room temperature for 30 to 60 min until the CMC had dissolved.

(67) A slurry with a solid content of 50.0 wt.-%, based on the total amount of the slurry, was prepared from the GCC filter cake by adding a premixed solution of starch and CMC.

(68) Subsequently, the obtained slurry was wet ground at room temperature. The wet grinding of slurry was done in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 600 cm.sup.3 at a speed of 2500 rpm and at a flow rate of 500 cm.sup.3/min. 480 cm.sup.3 (80%) of the grinding chamber were filled with grinding beads having a diameter of 0.6-1.0 mm (melt fused beads consisting of 68% baddeleyit and 32% amorphous silicate). The slurry was passed four times through the mill.

(69) The products obtained were analyzed with respect to particle size, specific surface (BET), brightness, electrochemical charge (PET), and LOI. Tables 2 and 3 summarize the properties of the obtained slurries and the pigment particles contained therein.

Example 12

Chalk with CMC and polyDADMAC (Inventive Example)

(70) A chalk from France, Omey, having a fraction of particles finer than 2 m of 43% by weight, and a fraction of particles finer than 1 m of 18% by weight was employed as ground calcium carbonate (GCC). The GCC was provided in the form of a dry powder.

(71) The anionic polymeric binder was a carboxymethylcellulose (CMC), commercially available under the trade name Finnfix 10 from CP Kelko, USA. The intrinsic viscosity of the CMC was 135 ml/g. The CMC was used in form of an aqueous solution containing 3 wt.-% CMC, based on the total amount of the solution.

(72) As cationic polymer PolyDADMAC (Catio fast BP liquid, commercially available from BASF, Germany) was employed in form of a solution containing 50 wt.-% PolyDADMAC, based on the total weight of the solution.

(73) In a first step, to the above CMC solution consisting of 2020 g water and 64 g Finnfix 10, 3000 g of the chalk were added and the resulting mixture was stirred for 10 minutes. In a second step, polyDADMAC was added in an amount of 1.80 wt.-%, based on the total weight of the dry ground calcium carbonate (see also Table 1).

(74) Subsequently, the obtained slurry was wet ground at room temperature. The wet grinding of slurry was done in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 600 cm.sup.3 at a speed of 2500 rpm and at a flow rate of 500 cm.sup.3/min. 480 cm.sup.3 (80%) of the grinding chamber were filled with grinding beads having a diameter of 0.6-1.0 mm (melt fused beads consisting of 68% baddeleyit and 32% amorphous silicate). The slurry was passed four times through the mill.

(75) The products obtained were analyzed with respect to particle size, specific surface (BET), brightness, electrochemical charge (PET), and LOI. Tables 2 and 3 summarize the properties of the obtained slurries and the inventive self-binding pigment particles contained therein.

(76) TABLE-US-00001 TABLE 1 Amounts of anionic polymeric binder and cationic polymers used in Examples 1 to 12. PolyDADMAC Cationic starch Example CMC (wt.-%) (wt.-%) (wt.-%) 1 2.0 2 2.0 0.50 3 2.0 1.00 4 2.0 1.00 5 2.0 1.25 6 2.0 1.50 7 2.0 1.75 8 2.0 2.00 9 1.5 0.5 10 1.0 1.0 11 0.5 1.5 12 2.0 1.80

(77) TABLE-US-00002 TABLE 2 Particle size distributions of the obtained self- binding pigment particles of Examples 1 to 12. Particle size (Mastersizer 2000) d.sub.50 value d.sub.98 value Example <2 m (wt.-%) <1 m (wt.-%) (m) (m) 1 80 36 1.24 4 2 80 34 1.26 4 3 79 30 1.32 4 4 50 13 2.01 7 5 14 6 5.18 15 6 11 6 5.41 15 7 13 6 4.97 15 8 16 7 3.99 11 9 78 32 1.31 4 10 76 30 1.35 4 11 67 21 1.57 5 12 74 36 1.39 5

(78) TABLE-US-00003 TABLE 3 Particle size distributions of the obtained self-binding pigment particles of Examples 1 to 12. Examples 1 2 3 4 5 6 7 8 9 10 11 12 Specific 5.2 5.1 5.0 4.9 4.8 4.6 4.6 4.9 5.8 5.6 5.3 3.6 surface area (BET) (m.sup.2/g) Brightness 93.9 93.9 93.7 94.0 94.0 93.9 93.9 93.8 94.1 94.1 94.3 82.1 R457 (%) Charge 115 93 72 55 45 30 21 24 79 62 51 63 density (PET) (Eq/g) LOI 3.2 4.2 4.3 4.3 4.3 4.6 4.9 5.4 3.2 3.3 3.1 3.6 slurry (%) LOI 1.4 1.7 3.5 3.4 3.9 4.1 4.2 4.8 1.7 2.1 2.9 2.4 cake (%) Free 69 69 22 24 11 13 16 13 58 44 8 40 polymer in solution (%)

Results of Examples 1 to 12

(79) From the measured details, it can be gathered that the charge density of the self-binding pigment particles comprising the cationic polymer polyDADMAC is linearly reduced with the addition of the cationic polymer. A similar trend is observed for the self-binding pigment particles comprising Cargill C*Bond as cationic polymer.

(80) Furthermore, the results show that with increasing amount of cationic polymer, the amount of total polymer which is absorbed at the surface of the self-binding pigment particle increases and the amount of free polymer in the aqueous phase decreases, respectively (see Table 3, last two lines).

(81) The measured brightness R457 values measured for the produced self-binding pigment particles hardly differ from each other (see Table 3), which means that the presence of the cationic polymer on the surface of the self-binding does not have an impact on the optical properties of the pigment particles.

3. Example 13

Retention Studies

(82) The self-binding pigment particles of comparative Example 1 and inventive Examples 3 and 11 were tested as paper fillers in a dynamic retention study using the DFS 03 (BTG Mtek GmbH).

(83) Thermo mechanical pulp (TMP) fibers were used for the retention study. The pulp was either diluted in tap water which was treated with 0.057 g/l of a sodium polyacrylic acid homopolymer having a molecular weight of 3500 g/mol and a polydispersity D=2.5, or with clear filtrate, i.e. the water that was used in the paper mill for the dilution of the pulp. 6.00 g/l fibers were mixed with 6.00 g/l filler to obtain a pulp consistency of the pulp/filler mix of 12.0 g/l as in the headbox of a paper machine. The filler was added within 1 min stirring time by 10 s stirring at 800 rpm, 10 s stirring at 1200 rpm, 5 s stirring at 1000 rpm, and 5 s stirring at 800 rpm, wherein after 20 s Percol PBR 30 special powder (commercially available from BASF, Germany) in form of 0.2% solution was added as retention aid. After additional 30 s, the valve was opened. The tests were carried out with different concentrations of retention aid, namely, with 250 g/t, 500 g/t, 750 g/t, 1000 g/t, 1500 g/t, 2000 g/t, and 3000 g/t. The relative retention was determined by measuring the turbidity NTU of the clear filtrate and the sample filtrates over time:

(84) $\mathrm{Relative}\mathrm{retention}\left[\%\right]=\frac{T_0-T_t}{T_0}.Math.100\%$
wherein T.sub.0 is the turbidity of the clear filtrate and T.sub.t is the turbidity of the sample filtrate at the moment t.

(85) The results of the retention studies are shown in FIGS. 1 and 2, wherein FIG. 1 shows the results for pulp that was diluted in tap water and FIG. 2 shows the results for pulp that was diluted in clear filtrate. The retention values obtained for the inventive self-binding pigment particles of Example 3 and 11 are very good and significantly better than those of the comparative filler of Example 1.

4. Example 14

Testing of Mechanical Strength Properties of Wood Free, Uncoated Paper Containing the Inventive Pigment Particles as Filler

(86) Eucalyptus pulp (Jarilyptus) refined to 30 SR was used as pulp. The self-binding pigment particles of Example 4 were tested as paper fillers. In addition, Hydrocarb HO ME-67% was tested as comparative example. The pigment particle suspension were diluted with water to a concentration of 10 wt.-%, based on the total amount of the suspension, and deagglomerated with a high speed stirrer (Kinematica, Switzerland).

(87) 60 g (dry) pulp were diluted in 10 dm.sup.3 tap water, and then the pigment particle suspension to be tested was added in an amount so as to obtain the desired overall filler content based on the final paper weight. The obtained suspension was stirred for 30 min. Subsequently 0.06% (based on dry weight) of a polyacrylamide (Polymin 1530, commercially available from BASF, Germany) was added as a retention aid and sheets of 78 g/m.sup.2 were formed using the Rapid-K then hand sheet former. Each sheet was dried using the Rapid-K then drier.

(88) 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 was between 18 and 30%.

(89) The mechanical strength properties of the handsheets were characterized after drying of the handsheets by

(90) the breaking length according to ISO 1924-2,

(91) the tensile index according to ISO 1924-2,

(92) the tensile energy absorption according to ISO 1924-2,

(93) the tear growth work according to ISO 53115,

(94) the internal bond (z-direction) according to SCAN-P80:98/TAPPI T541,

(95) the bending stiffness according to ISO 53123-1, and

(96) the modulus of elasticity according to ISO 53123-1.

(97) FIGS. 3 to 9 show the mechanical properties of the tested handsheets. The data show that the use of the self-binding pigment particles of the present invention allows to increase the filler load from about 22% to about 28%, i.e. by about 6%, without affecting the mechanical strength of the paper. In case of the internal bond (FIG. 7), the effect is even more pronounced and an even higher filler content would be possible. A special highlight is the positive effect of the inventive self-binding pigment particles on the bending stiffness (FIG. 8) and the modulus of elasticity (FIG. 9) of the paper. In particular, a good bending stiffness is an important property in wood-free uncoated paper grades such as copy paper.

5. Example 15

Testing of Mechanical Strength Properties of Supercalendared (SC) Paper Containing the Inventive Pigment Particles as Filler

(98) 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. The self-binding pigment particles of Example 3 were tested as paper fillers. In addition, the particles of Example 1, Hydrocarb HO ME-67%, and Hydrocarb HO ME-67%, wherein 0.8 wt.-% cationic starch (Cargill C*Bond, no. HR05947, commercially available from Cargill Deutschland GmbH, Germany) were added to the fiber suspension, based on the total weight of the dry fibers, were tested as comparative examples. The pigment particle suspensions were diluted with water to a concentration of 10 wt.-%, based on the total amount of the suspension, and deagglomerated with a high speed stirrer (Kinematica, Switzerland).

(99) 60 g (dry) pulp blend were diluted in 10 dm.sup.3 tap water. In one comparative example, 0.8 wt.-% cationic starch (Cargill C*Bond, no. HR05947, commercially available from Cargill Deutschland GmbH, Germany), based on the total weight of dry fibers, were added to the fiber suspension and the suspension was stirred for 15 min. Then the pigment particle suspension was added in an amount so as to obtain the desired overall filler content based on the final paper weight. The suspension was stirred for 30 minutes, and subsequently the retention aid Percol PBR 30 special powder (commercially available from BASF, Germany) was added in the amounts given in Table 4 below, and sheets of 52 g/m.sup.2 were formed using the Rapid-K then hand sheet former. Each sheet was dried using the Rapid-K then drier. The sheets were calendared with a Voith calendar to 0.95-1.05 PPS roughness.

(100) TABLE-US-00004 TABLE 4 Amount of added retention aid Amount of added retention Amount of added retention aid at a filler content aid at a filler content 37% [wt.-%, based on 42% [wt.-%, based on Filler dry weight] dry weight] Example 1 0.13% 0.29% Example 3 0.036% 0.073%

(101) The filler content in the handsheets was determined by burning a quarter of the calendared 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 was 37 or 42%.

(102) The reduced anionic charge of the inventive self-binding pigment particles reduced the retention polymer demand in the handsheet making (see FIG. 10). The mechanical properties of the tested handsheets are shown in FIGS. 11 to 13. All handsheets with the inventive self-binding pigment particles showed increased sheet tensile and improved internal bond compared to Hydrocarb HO ME-67%, and HydrocarbHO ME-67% with addition of 0.8 wt.-% cationic starch, based on the total weight of dry fibers. In particular, the use of the pigment particles of Example 3 as filler lead to an increase in tensile index and internal bond of about 20% compared to the filler Hydrocarb HO ME-67% (see FIGS. 11 and 13).

6. Examples 16 to 21

(103) A chemical free calcium carbonate from Norway, Molde, having a fineness corresponding to a d.sub.50 value of 0.8 m and a d.sub.98 value of 5.0 m was employed as ground calcium carbonate (GCC). The specific surface (BET) of the ground calcium carbonate was 7.5 m.sup.2/g and the charge density was 24.8 Eq/g. The GCC was provided in the form of a filter cake having a solids content of 70.0 wt.-%.

(104) Different types of carboxymethylcellulose (CMC) having different molecular weights were employed as anionic polymeric binder (Finnfix 2 (lowest molecular weight), Finnfix 5, Finnfix 10, Finnfix 30, Finnfix 150, and Finnfix 300 (highest molecular weight), commercially available from CP Kelko, U.S.A.). The intrinsic viscosity of the CMC was from 90 to 300 ml/g, and the degree of substitution (degree of carboxylation) was 0.7. The CMC was used in form of an aqueous solution containing 3-6 wt.-% CMC, based on the total amount of the solution.

(105) As cationic polymer PolyDADMAC (Catiofast BP liquid, commercially available from BASF, Germany) was employed in form of a solution containing 50 wt.-% PolyDADMAC, based on the total weight of the solution.

(106) In a first step, a slurry with a solid content of 50.0 wt.-%, based on the total amount of the slurry, was prepared from the GCC filter cake by adding water and 2.0 wt.-% of the different types of CMC, based on the total amount of the dry GCC, namely Finnfix 2 (Example 16), Finnfix 5 (Example 17), Finnfix 10 (Example 18), Finnfix 30 (Example 19), Finnfix 150 (Example 20), and Finnfix 300 (Example 21) (see also Table 5 below). In a second step, polyDADMAC was added in an amount of 0.8 wt.-%, based on the total amount of the dry GCC.

(107) Subsequently, the obtained slurry was wet ground at room temperature. The wet grinding of slurry was done in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 600 cm.sup.3 at a speed of 2500 rpm and at a flow rate of 500 cm.sup.3/min. 480 cm.sup.3 (80%) of the grinding chamber was filled with grinding beads having a diameter of 0.6-1.0 mm (melt fused beads consisting of 68% baddeleyit and 32% amorphous silicate). If necessary the solid content was adjusted during grinding to avoid blocking of the mill. The grinding was carried out until more than 73 to 77% of the particles had a particle size of less than 1 m as determined by Sedigraph 5120.

(108) TABLE-US-00005 TABLE 5 Amounts and types of anionic polymeric binder and cationic polymers used in Examples 16 to 21. Example CMC type CMC (wt.-%) PolyDADMAC (wt.-%) 16 Finnfix 2 2.0 0.8 17 Finnfix 5 2.0 0.8 18 Finnfix 10 2.0 0.8 19 Finnfix 30 2.0 0.8 20 Finnfix 150 2.0 0.8 21 Finnfix 300 2.0 0.8
Results

(109) The products obtained were analyzed with respect to particle size, specific surface (BET), brightness, electrochemical charge (PET), and LOI. Tables 6 and 7 summarize the properties of the obtained slurries and the pigment particles contained therein.

(110) TABLE-US-00006 TABLE 6 Particle size distribution of the obtained self- binding pigment particles of Examples 16 to 21. Particle size (Mastersizer 2000) d.sub.50 value d.sub.98 value Example <2 m (wt.-%) <1 m (wt.-%) (m) (m) 16 59 8 1.8 4 17 50 7 2.0 5 18 70 17 1.6 4 19 68 22 1.6 5 20 75 30 1.4 4 21 70 28 1.4 6

(111) TABLE-US-00007 TABLE 7 Particle size distribution of the obtained self - binding pigment particles of Examples 16 to 21. Examples 16 17 18 19 20 21 Specific surface area (BET) 8.3 6.7 5.7 5.8 5.4 5.5 (m.sup.2/g) Charge density (PET) (Eq/g) 74 71 78 75 74 78 LOI slurry (%) 3.7 3.7 3.1 3.1 3.1 3.3 LOI cake (%) 3.0 2.8 2.5 2.6 2.4 2.5 Free polymer in solution (%) 23 29 24 20 23 30

(112) From the measured details, it can be gathered that the charge density of the self-binding pigment particles is between 70 and 80 Eq/g and only slightly varies depending on the molecular weight of the employed CMC. The free polymer in the solution was found to be below 30% for every one of Examples 16 to 21.

7. Examples 22 to 26

(113) A chemical free calcium carbonate from Norway, Molde, having a fineness corresponding to a d.sub.50 value of 0.8 m and a d.sub.98 value of 5.0 m was employed as ground calcium carbonate (GCC). The specific surface (BET) of the ground calcium carbonate was 7.5 m.sup.2/g and the charge density was 24.8 Eq/g. The GCC was provided in the form of a filter cake having a solids content of 70.0 wt.-%.

(114) The anionic polymeric binder was a carboxymethylcellulose (CMC) having an intrinsic viscosity of the CMC was 135 ml/g, and the degree of substitution (degree of carboxylation) was 0.7. The CMC was used in form of an aqueous solution containing 6 wt.-% CMC, based on the total amount of the solution.

(115) As cationic polymer PolyDADMAC (Catiofast BP liquid, commercially available from BASF, Germany) was employed in form of a solution containing 50 wt.-% PolyDADMAC, based on the total weight of the solution.

(116) In a first step, a slurry with a solid content of 50.0 wt.-%, based on the total amount of the slurry, was prepared from the GCC filter cake by adding water and 2.0 wt.-% of CMC, based on the total amount of the dry GCC. In a second step, polyDADMAC was added in different amounts, namely, in amounts of 0.4 wt.-% (Example 23), 0.6 wt.-% (Example 24), 1.0 wt.-% (Example 25), and 1.2 wt.-% (Example 26), based on the total amount of the dry GCC. In Example 21, no polyDADMAC was added.

(117) Subsequently, the obtained slurry was wet ground at room temperature. The wet grinding of slurry was done in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 600 cm.sup.3 at a speed of 2500 rpm and at a flow rate of 500 cm.sup.3/min. 480 cm.sup.3 (80%) of the grinding chamber was filled with grinding beads having a diameter of 0.6-1.0 mm (melt fused beads consisting of 68% baddeleyit and 32% amorphous silicate). If necessary the solid content was adjusted during grinding to avoid blocking of the mill. The grinding was carried out until more than 73 to 77% of the particles had a particle size of less than 1 m as determined by Sedigraph 5120.

(118) TABLE-US-00008 TABLE 8 Amounts and types of anionic polymeric binder and cationic polymers used in Examples 22 to 26. Example CMC (wt.-%) PolyDADMAC (wt.-%) 22 2.0 0.0 23 2.0 0.4 24 2.0 0.6 25 2.0 1.0 26 2.0 1.2
Results

(119) The products obtained were analyzed with respect to particle size, specific surface (BET), brightness, electrochemical charge (PET), and LOI. Tables 9 and 10 summarize the properties of the obtained slurries and the pigment particles contained therein.

(120) TABLE-US-00009 TABLE 9 Particle size distribution of the obtained self- binding pigment particles of Examples 22 to 26. Particle size (Mastersizer 2000) d.sub.50 value d.sub.98 value Example <2 m (wt.-%) <1 m (wt.-%) (m) (m) 22 88 45 1.1 4 23 78 34 1.3 5 24 73 29 1.4 5 25 39 3 2.3 6 26 22 6 3.3 6

(121) TABLE-US-00010 TABLE 10 Particle size distribution of the obtained self- binding pigment particles of Examples 22 to 26. Examples 22 23 24 25 26 Specific surface area (BET) (m.sup.2/g) 9.0 6.3 6.3 6.5 7.8 Charge density (PET) (Eq/g) 102 88 79 60 54 LOI slurry (%) 2.8 3.2 3.5 3.6 3.8 LOI cake (%) 0.9 1.5 2.1 3.0 3.3 Free polymer in solution (%) 86 65 48 19 16

(122) From the measured details, it can be gathered that the charge density of the self-binding pigment particles is linearly reduced with increasing amount of cationic polymer. As is shown in FIG. 14 the amount of free polymer in the solution is also reduced with increasing amount of cationic polymer and reaches a plateau of around 15 to 20% free polymer at an amount of cationic polymer of more than 0.6 wt.-%, based on the amount of dry GCC.

8. Example 27

Retention Studies

(123) The self-binding pigment particles of Examples 16 to 26 were tested as paper fillers in a dynamic retention study. In addition, Hydrocarb HO ME-67% was tested as comparative examples

(124) The fibers used for the retention study comprised 85% thermo mechanical pulp (TMP) and 15% pine kraft pulp. The pulp was either diluted in tap water which was treated with 0.057 g/l of a sodium polyacrylic acid homopolymer having a molecular weight of 3500 g/mol and a polydispersity D=2.5, or with clear filtrate, i.e. the water that was used in the paper mill for the dilution of the pulp. 5.5 g/l fibers were mixed with 5.5 g/l filler to obtain a pulp consistency of the pulp/filler mix of 11.0 g/l as in the headbox of the paper machine. The filler was added within 1 min stirring time by 10 s stirring at 800 rpm, 10 s stirring at 1200 rpm, 5 s stirring at 1000 rpm, and 5 s stirring at 800 rpm, wherein after 20 s Percol PBR 30 special powder (commercially available from BASF, Germany) in form of 0.2% solution was added as retention aid. After additional 30 s, the valve was opened. The tests were carried out with no retention aid and different concentrations of retention aid, namely, with 250 g/t, 500 g/t, 750 g/t, 1000 g/t, and 1500 g/t.

(125) The relative retention was determined by measuring the turbidity NTU of the clear filtrate and the sample filtrates over time:

(126) $\mathrm{Relative}\mathrm{retention}\left[\%\right]=\frac{T_0-T_t}{T_0}.Math.100\%$
wherein T.sub.0 is the turbidity of the clear filtrate and T.sub.t is the turbidity of the sample filtrate at the moment t.

(127) The results of the retention studies are shown in FIGS. 15 and 16. The retention values obtained for the self-binding pigment particles of Example 15 to 25 were in the most cased below those of the comparative filler Hydrocarb HO ME-67% but were still acceptable. It can be gathered from FIG. 15 that the retention values of the self-binding particles of Examples 16 to 21 differ depending on the molecular weight of the CMC; the lower the molecular weight of the CMC, the better the retention.

(128) FIG. 16 shows that the retention value of the self-binding particles of Examples 22 to 26 are increased with increasing amount of cationic polymer.

9. Examples 28 and 29

Example 28

GCC with CMC (Comparative Example)

(129) A chemical free calcium carbonate from Norway, Molde, having a fineness corresponding to a d.sub.50 value of 0.8 m and a d.sub.98 value of 5.0 m was employed as ground calcium carbonate (GCC). The specific surface (BET) of the ground calcium carbonate was 7.5 m.sup.2/g and the charge density was 24.8 Eq/g. The GCC was provided in the form of a filter cake having a solids content of 83.0 wt.-%.

(130) The anionic polymeric binder was a carboxymethylcellulose (CMC) having an intrinsic viscosity of the CMC was 135 ml/g, and the degree of substitution (degree of carboxylation) was 0.7. The CMC was used in form of an aqueous solution containing 6 wt.-% CMC, based on the total amount of the solution.

(131) A premixed solution of CMC was prepared by dissolving 31 g CMC in 1.4 kg water under vigorous stirring. After 30 min of stirring, the premixed CMC solution was added to 4.8 kg of the Hydrocarb HO-ME filter cake. The mixture was dispersed with a dissolver stirrer at 2000-4000 rpm.

(132) Subsequently, the obtained slurry was wet ground at room temperature. The wet grinding of slurry was done in a vertical attritor mill (Dynomill, Bachofen, Switzerland) having a volume of 600 cm.sup.3 at a speed of 2500 rpm and at a flow rate of 500 cm.sup.3/min. 480 cm.sup.3 (80%) of the grinding chamber was filled with grinding beads having a diameter of 0.6-1.0 mm (melt fused beads consisting of 68% baddeleyit and 32% amorphous silicate). The grinding was carried out until more than 60% of the particles had a particle size of less than 1 m as determined by Sedigraph 5120.

Example 29

GCC with CMC and Cationic Starch (Inventive Example)

(133) As cationic polymer, a cationic starch (Cargill C*Bond, no. HR05947, commercially available from Cargill Deutschland GmbH, Germany) was employed in form of a dry powder.

(134) A premixed solution of cationic starch was prepared by dissolving 16 g of the cationic starch in 2.25 kg water and heating the obtained suspension up to 95 C. for one hour. Subsequently, the cationic starch solution was mixed with the particles obtained in Example 28.

(135) The obtained slurry was deagglomerated by passing the slurry three times through a homogenizer (Megatron, Kinematica, Switzerland) at room temperature.

10. Example 30

Testing of Mechanical Strength Properties of Wood Free, Uncoated Paper Containing the Inventive Pigment Particles as Filler

(136) Eucalyptus pulp (Jarilyptus) refined to 30 SR was used as pulp. The slurry of the self-binding pigment particles obtained in inventive Example 29 was tested as paper filler. In addition, a slurry of Hydrocarb HO ME-67% having a solid content of 67 wt.-%, based on the total amount of the slurry, was tested as comparative example, as well as the slurry of the particles obtained in comparative Example 28. Furthermore, slurries of Hydrocarb HO ME-67% and Example 28 with cationic starch were prepared, wherein the amount of starch was 1.5 wt.-%, based on the total amount of dry GCC.

(137) 60 g (dry) pulp were diluted in 10 dm.sup.3 tap water, and then the pigment particle suspension to be tested was added in an amount so as to obtain the desired overall filler content based on the final paper weight. The obtained suspension was stirred for 30 min. Subsequently 0.06% (based on dry weight) of a polyacrylamide (Polymin 1530, commercially available from BASF, Germany) was added as a retention aid and sheets of 78 g/m.sup.2 were formed using the Rapid-K then hand sheet former. Each sheet was dried using the Rapid-K then drier.

(138) 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 was between 18 and 31%.

(139) The mechanical strength properties of the handsheets were characterized after drying of the handsheets by

(140) the breaking length according to ISO 1924-2,

(141) the tensile index according to ISO 1924-2,

(142) the tensile energy absorption according to ISO 1924-2,

(143) the tear growth work according to ISO 53115,

(144) the internal bond (z-direction) according to SCAN-P80:98/TAPPI T541,

(145) the bending stiffness according to ISO 53123-1, and

(146) the modulus of elasticity according to ISO 53123-1.

(147) FIGS. 17 to 21 show the mechanical properties of the tested handsheets. The handsheets without cationic starch, i.e. the handsheets containing Hydrocarb HO ME-67% or the particles of Example 28 as filler, show the lowest mechanical strength properties. The addition of cationic starch to the paper suspension can improve the mechanical strength slightly. However, the use of the inventive self-binding particles as filler hugely improves the mechanical strength properties of the tested handsheets at filler levels around 20%. The data also show that the use of the self-binding pigment particles of the present invention allows to increase the filler load from about 22% to about 27 to 28%, i.e. by about 5 to 6%, without affecting the mechanical strength of the paper. A special highlight is the positive effect of the inventive self-binding pigment particles on the bending stiffness (FIG. 20) of the paper. In particular, a good bending stiffness is an important property in wood-free uncoated paper grades such as copy paper.