PROCESS FOR PREPARING AN AQUEOUS SUSPENSION COMPRISING A GROUND CALCIUM CARBONATE-COMPRISING MATERIAL

Abstract

The present invention relates to a process for preparing an aqueous suspension comprising a ground calcium carbonate-comprising material, the use of a homopolymer or a copolymer in such a process for preparing an aqueous suspension comprising a ground calcium carbonate-comprising material, an aqueous suspension comprising a ground calcium carbonate-comprising material obtained by the process, and the use of an aqueous suspension comprising a ground calcium carbonate-comprising material in paper applications such as paper filling and paper coating applications, in coating applications, in paints, in adhesives, in sealants, in concrete, in agriculture applications, in food applications, in cosmetic applications, or in pharmaceutical applications.

Claims

1. A process for preparing an aqueous suspension comprising a ground calcium carbonate-comprising material in a solids content of at least 70 wt.-%, based on the total weight of the suspension, the process comprising the steps of a) providing water, b) providing at least one calcium carbonate-comprising material, c) providing at least one grinding aid polymer selected from the group consisting of a homopolymer (P1) prepared from the polymerisation reaction of acrylic acid or methacrylic acid or salts thereof, the homopolymer (P1) having an average molecular weight (M.sub.W) as measured by gel permeation chromatography (GPC) from 3 000 to 5 000 g/mol, and a polydispersity index (PI) from 1.5 to 2.9 as measured by gel permeation chromatography (GPC), and carboxylic acid functions that are totally or partially neutralized by Na.sup.+ in an amount of 70 mol-% and by Na.sup.+, K.sup.+, Li.sup.+ or combinations thereof in an additional amount from 10 to 30 mol-%; a copolymer (P2) prepared from the polymerization reaction of at least one compound selected from acrylic acid, methacrylic acid, salts thereof and combinations thereof, and at least one non-ionic compound selected from hydroxyethylacrylate, hydroxypropylacrylate, hydroxyethylmethacrylate, hydroxypropylmethacrylate, C.sub.1-C.sub.5 esters of acrylic acid, C.sub.1-C.sub.5 esters of methacrylic acid, and combinations thereof;  the copolymer (P2) having an average molecular weight (M.sub.W) as measured by gel permeation chromatography (GPC) from 4 000 to 20 000 g/mol, and a polydispersity index (PI) from 1.5 to 4.0 as measured by gel permeation chromatography (GPC), and carboxylic acid functions that are totally or partially neutralized by Na.sup.+ in an amount of 70 mol-% and by Na.sup.+, K.sup.+, Li.sup.+ or combinations thereof in an additional amount from 10 to 30 mol-%; and mixtures thereof, d) forming an aqueous suspension by mixing the water of step a), the at least one calcium carbonate-comprising material of step b) and the at least one polymer of step c), e) wet grinding the at least one calcium carbonate-comprising material in the aqueous suspension of step d) in the presence of the at least one polymer in at least one wet grinding step to obtain a ground calcium carbonate-comprising material, and f) up-concentrating or partially dewatering the aqueous suspension comprising the ground calcium carbonate-comprising material obtained in step e) to achieve a higher solids content than that of step e) and the solids content achieved in step f) is at least 70 wt.-%, based on the total weight of the suspension.

2. The process according to claim 1, wherein the aqueous suspension obtained in step f) has a higher solids content than that of step e) and the solids content achieved in step f) is from 70 to 80 wt.-%.

3. The process according to claim 1, wherein the Brookfield viscosity of the aqueous suspension comprising a ground calcium carbonate-comprising material in a solids content of at least 70 wt.-%, based on the total weight of the suspension, is less than 1500 mPa.Math.s, as measured by the use of a RVT model Brookfield™ viscometer at a rotation speed of 100 rpm (revolutions per minute) with the appropriate disc spindle 2, 3 or 4 and a temperature of 25° C.

4. The process according to claim 1, wherein the at least one calcium carbonate-comprising material in the aqueous suspension obtained in step e) and/or f) has a weight median particle size d.sub.50 from 0.1 to 3.0 μm, determined by the sedimentation method.

5. The process according to claim 1, wherein the aqueous suspension of step d) and/or e) has a solids content from 40 to 74 wt.-%, based on the total weight of the aqueous suspension.

6. The process according to claim 1, wherein the carboxylic acid functions of the homopolymer (P1) or the carboxylic acid functions of the copolymer (P2) are totally neutralized; or neutralized in an amount of at least 85 mol-%; or solely neutralized by Na.sup.+.

7. The process according to claim 1, wherein the homopolymer (P1) has an average molecular weight (M.sub.W) as measured by gel permeation chromatography (GPC) from 3 100 to 4 900 g/mol; or the copolymer (P2) has an average molecular weight (M.sub.W) as measured by gel permeation chromatography (GPC) from 4 000 to 18 000 g/mol.

8. The process according to claim 1, wherein the at least one polymer is present in the aqueous suspension during wet grinding step e) in an amount ranging from 0.05 to 0.5 wt.-%, based on the total dry weight of the calcium carbonate-comprising material.

9. The process according to claim 1, wherein process step f) is carried out by mechanical up-concentration, by thermal up-concentration or by combining mechanical and thermal up-concentration, optionally in combination with vacuum.

10. The process according to claim 1, wherein the aqueous suspension obtained in step f) has a higher solids content than that of step e) and the solids content achieved in step f) is from 78 to 80 wt.-%.

11. An aqueous suspension comprising a ground calcium carbonate-comprising material in a solids content of at least 70 wt.-%, based on the total weight of the suspension, obtained by a process according to claim 1.

12. An aqueous suspension comprising water, a ground calcium carbonate-comprising material in a solids content of at least 70 wt.-%, based on the total weight of the suspension, and at least one grinding aid polymer selected from the group consisting of a homopolymer (P1) prepared from the polymerisation reaction of acrylic acid or methacrylic acid or salts thereof, the homopolymer (P1) having an average molecular weight (M.sub.W) as measured by gel permeation chromatography (GPC) from 3 000 to 5 000 g/mol, and a polydispersity index (PI) from 1.5 to 2.9 as measured by gel permeation chromatography (GPC), and carboxylic acid functions that are totally or partially neutralized by Na.sup.+ in an amount of 70 mol-% and by Na.sup.+, K.sup.+, Li.sup.+ or combinations thereof in an additional amount of from 10 to 30 mol-%; a copolymer (P2) prepared from the polymerisation reaction of at least one compound selected from acrylic acid, methacrylic acid, salts thereof and combinations thereof, and at least one non-ionic compound selected from hydroxyethylacrylate, hydroxypropylacrylate, hydroxyethylmethacrylate, hydroxypropylmethacrylate, C.sub.1-C.sub.5 esters of acrylic acid, C.sub.1-C.sub.5 esters of methacrylic acid, and combinations thereof;  the copolymer (P2) having an average molecular weight (M.sub.W) as measured by gel permeation chromatography (GPC) from 4 000 to 20 000 g/mol, and a polydispersity index (PI) from 1.5 to 4.0 as measured by gel permeation chromatography (GPC), and carboxylic acid functions that are totally or partially neutralized by Na.sup.+ in an amount of 70 mol-%, and by Na.sup.+, K.sup.+, Li.sup.+ or combinations thereof in an additional amount of from 10 to 30 mol-%; and mixtures thereof.

13. The aqueous suspension obtained by a process according to claim 1, wherein the aqueous suspension is suitable for use in paper applications, in paper filling, in paper coating applications, in coating applications, in paints, in adhesives, in sealants, in concrete, in agriculture applications, in food applications, in cosmetic applications, or in pharmaceutical applications.

14. The process according to claim 1, wherein the Brookfield viscosity of the aqueous suspension comprising a ground calcium carbonate-comprising material in a solids content of at least 70 wt.-%, based on the total weight of the suspension, is less than 500 mPa.Math.s, as measured by the use of a RVT model Brookfield™ viscometer at a rotation speed of 100 rpm (revolutions per minute) with the appropriate disc spindle 2, 3 or 4 and a temperature of 25° C.

15. The process according claim 1, wherein the at least one calcium carbonate-comprising material in the aqueous suspension obtained in step e) and/or f) has a weight median particle size d.sub.50 from 0.6 to 2.5 μm, determined by the sedimentation method.

16. The process according to claim 1, wherein the aqueous suspension of step d) and/or e) has a solids content from 50 to 70 wt.-%, based on the total weight of the aqueous suspension.

17. The process according to claim 1, wherein the carboxylic acid functions of the homopolymer (P1) or the carboxylic acid functions of the copolymer (P2) are neutralized in an amount of at least 90 mol-%.

18. The process according to claim 1, wherein the carboxylic acid functions of the homopolymer (P1) or the carboxylic acid functions of the copolymer (P2) are solely neutralized by Na.sup.+ in an amount of 100 mol-%.

19. The process according to claim 1, wherein the homopolymer (P1) has an average molecular weight (M.sub.W) as measured by gel permeation chromatography (GPC) from 3 200 to 4 800 g/mol.

20. The process according to claim 1, wherein the copolymer (P2) has an average molecular weight (M.sub.W) as measured by gel permeation chromatography (GPC) from 10 000 to 16 000 g/mol.

21. The process according to claim 1, wherein the at least one polymer is present in the aqueous suspension during wet grinding step e) in an amount ranging from 0.12 to 0.2 wt.-%, based on the total dry weight of the calcium carbonate-comprising material.

22. The aqueous suspension obtained by a process according to claim 11, wherein the aqueous suspension is suitable for use in paper applications, in paper filling, in paper coating applications, in coating applications, in paints, in adhesives, in sealants, in concrete, in agriculture applications, in food applications, in cosmetic applications, or in pharmaceutical applications.

23. An article made using the aqueous suspension obtained by a process according to claim 1, wherein the article is a paper, a paper filling, a paper coating, a coating, a paint, an adhesive, a sealant, a concrete, a fertilizer, a food, a cosmetic, or a pharmaceutical.

24. An article made using the aqueous suspension according to claim 11, wherein the article is a paper, a paper filling, a paper coating, a coating, a paint, an adhesive, a sealant, a concrete, a fertilizer, a food, a cosmetic, or a pharmaceutical.

Description

EXAMPLES

A. Measurement Methods

[0272] The following measurement methods are used to evaluate the parameters given in the examples and claims.

pH Measurement

[0273] Any pH value was measured at 25° C. (+/−1° 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 was first made using commercially available buffer solutions having pH values of 4, 7 and 10 at 25° C. (from Aldrich). The reported pH values were the endpoint values detected by the instrument (signal differs by less than 0.1 mV from the average over the last 6 seconds).

Conductivity Measurement

[0274] Conductivity of a suspension was measured at 25° C. (+/−1° C.) using Mettler Toledo Seven Multi instrumentation equipped with the corresponding Mettler Toledo conductivity expansion unit and a Mettler Toledo InLab 731 conductivity probe, directly following stirring the suspension at 1500 rpm using a Pendraulik tooth disc stirrer. The instrument was first calibrated in the relevant conductivity range using commercially available conductivity calibration solutions from Mettler Toledo. The influence of temperature on conductivity was automatically corrected by the linear correction mode. Measured conductivities were reported for the reference temperature of 20° C. The reported conductivity values were the endpoint values detected by the instrument (the endpoint is when the measured conductivity differs by less than 0.4% from the average over the last 6 seconds).

Particle Size Distribution and Weight Median Grain Diameter

[0275] Particle size distribution (mass % particles with a diameter <X) and weight median grain diameter (d.sub.50) of particulate materials were determined via the sedimentation method, i.e. an analysis of sedimentation behaviour in a gravimetric field. The measurement was made with a Sedigraph™ 5100 at 25° C. (+/−1° C.). The method and the instrument are known to the skilled person and are commonly used to determine grain size of fillers and minerals. The measurement was 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.

Viscosity Measurement

[0276] Brookfield viscosity was measured after 1 minute (if no other indication) of stirring by the use of a RVT model Brookfield™ viscometer at a rotation speed of 100 rpm (revolutions per minute) with the appropriate disc spindle 2, 3 or 4. Without further indication the viscosity was measured at 25° C. (+/−1° C.).

Weight Solids (% by Weight) of a Material in Suspension

[0277] Weight solids was determined by dividing the weight of the solid material by the total weight of the aqueous suspension. The weight of the solid material was determined by weighing the solid material obtained by evaporating the aqueous phase of suspension and drying the obtained material to a constant weight.

Average Molecular Weight (Mw) and Polydispersity Index (PI)

[0278] A test portion of the polymer solution corresponding to 90 mg of dry matter was introduced into a 10 ml flask. Mobile phase, with an additional 0.04 wt. % of dimethylformamide, was added, until a total mass of 10 g was reached. The composition of this mobile phase at pH 9 was as follows: NaHCO.sub.3: 0.05 mol/l, NaNO.sub.3: 0.1 mol/l, triethanolamine: 0.02 mol/l, 0.03 wt. % of NaN.sub.3.

[0279] The gel permeation chromatography (GPC) equipment was equipped with an isocratic pump of the Waters™ 515 type, the flow rate was set to 0.8 ml/min., a Waters™ 717+ sample changer, a kiln containing a precolumn of the “Guard Column Ultrahydrogel Waters™” type which was 6 cm in length and had an internal diameter of 40 mm, followed by a linear column of the “Ultrahydrogel Waters™” type which was 30 cm in length and had an internal diameter of 7.8 mm.

[0280] Detection was accomplished by means of a Waters™ 410 type differential refractometer. The kiln was heated to a temperature of 60° C. and the refractometer was heated to a temperature of 45° C. The GPC equipment was calibrated with a series of powders of sodium polyacrylate standards supplied and certified by Polymer Standard Service or American Polymers Standards Corporation. (maximum (M.sub.P) molecular weight of between 900 and 2.25.Math.10.sup.6 g/mol and a polydispersity index of between 1.4 and 1.8).

[0281] The calibration graph was of the linear type and took account of the correction obtained using the flow rate marker (dimethylformamide).

[0282] Acquisition and processing of the chromatogram were accomplished through use of the PSS WinGPC Scientific v. 4.02 application. The chromatogram obtained was incorporated in the area corresponding to molecular weights higher than 200 g/mol.

B. Materials Used

Calcium Carbonate-Comprising Material A

[0283] Natural CaCO.sub.3 marble from Italy, Avenza, having a d.sub.50 value of 50 μm, a d.sub.50 value of 10 μm, and a d.sub.20 value of 2 m.

Grinding Aid Agents

[0284] The at least one grinding aid polymer used as grinding aid agents are set out in the following table 1.

TABLE-US-00001 TABLE 1 Grinding aid polymers used Grinding aid Mw PI Neutralization polymer Composition [g/mol] (Mw/Mn) [mol %] A (comparative) Homopolymer of 6 000 2.6 70% Na.sup.+, acrylic acid 30% Ca.sup.2+ B (inventive) Homopolymer of 3 600 2.0 100% Na.sup.+ acrylic acid C (inventive) Homopolymer of 3 770 2.0 90% Na.sup.+; acrylic acid 10% carboxylic acid functions D (comparative) Homopolymer of 6 000 2.6 100% NH.sub.4.sup.+ acrylic acid E (inventive) Acrylic acid/ 14 110  3.8 100% Na.sup.+ hydroxypropyl acrylate copolymer (74/26 molar) F (inventive) Acrylic acid/ ethyl 4 925 2.2 100% Na.sup.+ acrylate copolymer (88/12 molar) G (comparative) Acrylic acid/ 2 000 7.5 100% NH.sub.4.sup.+ hydroxypropyl acrylate copolymer (74/26 molar)

C. Test Results

2. Trials 1 to 8

Preparation of Pigment Particles Suspension

[0285] An aqueous suspension having solids content of 71 wt.-% (+/−1 wt.-%), based on the total weight of the suspension, was prepared by mixing tap water with 1 500 ppm of the respective grinding aid polymer as set out in tables 2 and 3 and the calcium carbonate-comprising material A using a Ystral mixer (Dispermix, Ystral GmbH, Germany). Subsequently, the obtained mixture was wet ground in a 200-litre vertical attritor mill using zircon silicate beads of 0.6 to 1.0 mm diameter. The slurry temperature at the mill inlet was 20° C. and at the outlet between 50 and 70° C. The mill parameters where adjusted in order to reach a particle size distribution of at least 45%<2 μm.

[0286] The results are summarized in tables 2 and 3 below.

TABLE-US-00002 TABLE 2 Wet grinding of a calcium carbonate-comprising material suspension with various homopolymers (P1) as grinding aid polymers Grinding aid PSD Grinding polymer Slurry Slurry % % aid quantity solids Viscosity <2 <1 d.sub.50 Trial polymer [ppm] [wt.-%] [mPa .Math. s] μm μm [μm] pH 1 (comparative) A 1 500 71.7 237 46.7 27.0 2.2 9.0 2 (inventive) B 1 500 71.7 123 46.2 26.5 2.2 9.5 3 (inventive) C 1 500 71.8  88 46.0 27.0 2.3 9.3 4 (comparative) D 1 500 71.0 Viscosity N/A N/A N/A N/A too high, not able to grind

TABLE-US-00003 TABLE 3 Wet grinding of a calcium carbonate-comprising material suspension with various copolymers (P2) as grinding aid polymers Grinding aid Grinding polymer Slurry Slurry PSD aid quantity solids Viscosity % % d.sub.50 Trial polymer [ppm] [wt.-%] [mPa .Math. s] <2 μm <1 μm [μm] pH 5 (inventive) E 1 500 71.5 310 45.6 27.1 2.3 9.3 6 (inventive) F 1 500 71.4 102 47.1 28.3 2.2 9.4 7 (comparative) A 1 500 71.5 237 46.7 27.0 2.2 9.0 8 (comparative) G 1 500 71.5 >1 000 N/A N/A N/A N/A (not possible to grind)

[0287] In a subsequent stage, the suspensions set out in tables 2 and 3 were up-concentrated using a thermal pilot evaporator from EPCON™ brand. The evaporator was operated at a suspension temperature of 9500. Samples were taken at different solids concentration and the viscosity was measured after cooling down of the suspension to 2500. The target was to identify the solids content corresponding to a viscosity (at 2500) of approximately 500 mPa.Math.s. The results are shown in tables 4 and 5 below.

TABLE-US-00004 TABLE 4 thermal up-concentration - viscosity at different solids concentrations Trial Solids [wt.-%] Viscosity [mPa .Math. s] at 25° C. 2A (inventive) 74.6 320 76.8 780 3A (inventive) 74.9 143 77.2 245 78.8 510

TABLE-US-00005 TABLE 5 thermal up-concentration - viscosity at different solids concentrations Trial Solids [wt.-%] Viscosity [mPa .Math. s] at 25° C. 5A (inventive) 74.9   537 77 1 170 77.8 1 416 6A (inventive) 76.3   324 77.7   558  7A (comparative) 76.9 3 112

[0288] From tables 4 and 5, it can be gathered that the viscosity of the aqueous suspensions comprising a ground calcium carbonate-comprising material in a solids content of at least 70 wt.-%, based on the total weight of the suspension, prepared by the inventive process, i.e. by using the at least one grinding aid polymer, is lower compared to a comparative aqueous suspension prepared by a process using the same amount of a grinding aid agent of the prior art. In addition, the grinding aid polymers used according to the invention allow such result where a comparative polymer (see trials 4 and 8; comparatives in tables 2 and 3) even did not allow grinding the mineral material.