HIGH SOLIDS PCC WITH DEPOLYMERIZED CARBOXYLATED CELLULOSE

Abstract

The present invention relates to a process for producing an aqueous suspension of precipitated calcium carbonate, wherein a depolymerized carboxylated cellulose is added during lime slaking. Furthermore, the present invention relates to an aqueous suspension of calcium carbonate and a precipitated calcium carbonate obtained by said process as well as the use thereof.

Claims

1. A process for producing an aqueous suspension of precipitated calcium carbonate comprising the steps of: i) providing a calcium oxide containing material, ii) providing at least one depolymerized carboxylated cellulose having a molecular weight M.sub.w in the range from 10 000 to 40 000 g/mol, iii) preparing a milk of lime by mixing water, the calcium oxide containing material of step i), and the at least one depolymerized carboxylated cellulose of step ii) to obtain a milk of lime, wherein the calcium oxide containing material and the water are mixed in a weight ratio from 1:1 to 1:12, and iv) carbonating the milk of lime obtained in step iii) to form an aqueous suspension of precipitated calcium carbonate.

2. The process of claim 1, wherein step iii) comprises the steps of: a1) mixing the at least one depolymerized carboxylated cellulose of step ii) with water, and a2) adding the calcium oxide containing material of step i) to the mixture of step a1), or b1) mixing the calcium oxide containing material of step i), and the at least one depolymerized carboxylated cellulose of step ii), and b2) adding water to the mixture of step b1), or c) mixing the calcium oxide containing material of step i), the at least one depolymerized carboxylated cellulose of step ii) and water simultaneously.

3. The process of claim 1, wherein the process further comprises step v) of adding at least one slaking additive to process step iii), preferably the at least one slaking additive is selected from the group consisting of organic acids, organic acid salts, sugar alcohols, monosaccharides, disaccharides, polysaccharides, gluconates, phosphonates, lignosulfonates, and mixtures thereof

4. The process of claim 1, wherein the milk of lime obtained in step iii) has a Brookfield viscosity from 1 to 1000 mPa.Math.s at 25? C., more preferably from 5 and 800 mPa.Math.s at 25? C., and most preferably from 10 and 500 mPa.Math.s at 25? C., and/or the suspension of PCC obtained in step iv) has a Brookfield viscosity of less than or equal to 1600 mPa.Math.s at 25? C., more preferably less than or equal to 1500 mPa.Math.s at 25? C., and most preferably less than or equal to 1400 mPa.Math.s at 25? C.

5. The process of claim 1, wherein the suspension of PCC obtained in step iv) has a solids content of at least 10 wt.-%, preferably from 15 to 70 wt.-%, more preferably from 19 to 60 wt.-%, even more preferably from 21 to 50 wt.-%, and most preferably from 24 to 42 wt.-%, based on the total amount of the suspension.

6. The process of claim 1, wherein the depolymerized carboxylated cellulose has a polydispersity index from 2 to 10, preferably from 2 to 8, more preferably from 2.5 to 6, and most preferably from 3 to 5.

7. The process of claim 1, wherein the depolymerized carboxylated cellulose has degree of carboxylation from 0.2 to 2, preferably from 0.4 to 1.8, more preferably from 0.5 to 1.6, and most preferably from 0.6 to 1.4.

8. The process of claim 1, wherein the depolymerized carboxylated cellulose has molecular weight M.sub.w in the in the range from 13 000 to 35 000 g/mol, and preferably in the range from 13 000 to 25 000 g/mol.

9. The process of claim 1, wherein the depolymerized carboxylated cellulose is provided in form of a solution having a solids content from 10 to 60 wt.-%, based on the total weight of the solution, preferably from 25 to 45 wt.-%, more preferably from 30 to 40 wt.-%, and most preferably from 31 to 35 wt.-%, and/or is added in an amount from 0.001 to 5 wt.-%, based on the total weight of the calcium oxide containing material in the milk of lime, preferably from 0.01 to 2 wt.-%, more preferably from 0.05 to 1 wt.-%, and most preferably from 0.1 to 0.5 wt.-%.

10. The process of claim 1, wherein the depolymerized carboxylated cellulose is prepared by depolymerizing a high molecular weight carboxylated cellulose in a process comprising the following steps: I) providing a high molecular weight carboxylated cellulose having a molecular weight of more than 40 000 g/mol and a degree of carboxylation in the range from 0.2 to 2, II) providing a peroxide selected from hydrogen peroxide and/or an alkali metal salt thereof, III) mixing the high molecular weight carboxylated cellulose of step I) and/or the peroxide of step II) and water incrementally and in any order at a reaction temperature from 50 to 85? C., IV) maintaining the temperature of the mixture obtained from step III) until complete consumption of the peroxide, V) cooling the mixture to a temperature of below 50? C., and VI) optionally, neutralizing the obtained depolymerized carboxylated cellulose.

11. The process of claim 1, wherein the depolymerized carboxylated cellulose is a carboxymethyl derivate and/or carboxymethyl hydroxypropyl derivate and/or carboxymethyl hydroxyethyl derivate of cellulose, preferably the depolymerized carboxylated cellulose is depolymerized carboxymethylcellulose.

12. The process of claim 1, wherein the process further comprises step vi) of separating the precipitated calcium carbonate from the aqueous suspension obtained in step iv), and optionally step vii) of drying the separated precipitated calcium carbonate obtained in step vi).

13. The process of claim 1, wherein the process further comprises a step viii) of contacting at least a part of the surface of the obtained precipitated calcium carbonate with at least one hydrophobising agent after step iv) and/or after step vi), if present, and/or during and/or after step vii), if present, preferably the at least one hydrophobising agent is selected from the group consisting of an aliphatic carboxylic acid having a total amount of carbon atoms from C.sub.4 to C.sub.24 and/or reaction products thereof, a mono-substituted succinic anhydride consisting of succinic anhydride mono-substituted with a group selected from a linear, branched, aliphatic and cyclic group having a total amount of carbon atoms from at least C.sub.2 to C.sub.30 in the substituent and/or reaction products thereof, a phosphoric acid ester blend of one or more phosphoric acid mono-ester and/or reaction products thereof and one or more phosphoric acid di-ester and/or reaction products thereof, polyhydrogensiloxane and reaction products thereof, an inert silicone oil, preferably polydimethylsiloxane, and mixtures thereof.

14. An aqueous suspension of precipitated calcium carbonate obtainable by a process according to claim 1.

15. Precipitated calcium carbonate obtainable by a process according to claim 12.

16. A product comprising precipitated calcium carbonate according to claim 15, preferably the product is a paper, a paper product, an ink, a paint, a coating, a plastic, a polymer composition, an adhesive, a building product, a foodstuff, an agricultural product, a cosmetic product or a pharmaceutical product, and more preferably the precipitated calcium carbonate is a dried precipitated calcium carbonate and the product is a plastic or a polymer composition.

17. Use of an aqueous suspension of precipitated calcium carbonate according to claim 14 in paper, plastics, polymer compositions, paint, coatings, concrete, cosmetics, pharmaceutics and/or agriculture applications, wherein preferably a dried precipitated calcium carbonate, more preferably a dried powder of precipitated calcium carbonate, is used in plastics and/or polymer compositions.

Description

DESCRIPTION OF THE FIGURE

[0251] FIG. 1 is a sketch of a continuous slaking process.

EXAMPLES

1. Measurement Methods

[0252] In the following, measurement methods implemented in the examples are described.

[0253] Brookfield Viscosity

[0254] The Brookfield viscosity of the liquid coating compositions was measured after one hour of production and after one minute of stirring at 25? C.?1? C. at 100 rpm by the use of a Brookfield viscometer type RVT equipped with an appropriate disc spindle, for example spindle 2 to 5.

[0255] pH Value

[0256] The pH of a suspension or solution was measured at 25? C. using a Mettler Toledo Seven Easy pH meter and a Mettler Toledo InLab? Expert Pro pH electrode. A three point calibration (according to the segment method) of the instrument was first made using commercially available buffer solutions having pH values of 4, 7 and 10 at 20? C. (from Sigma-Aldrich Corp., USA). The reported pH values are the endpoint values detected by the instrument (the endpoint was when the measured signal differed by less than 0.1 mV from the average over the last 6 seconds).

[0257] Particle Size Distribution

[0258] The particle size distribution of the prepared PCC 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. For the measurement of dispersed samples, no further dispersing agents were added.

[0259] Solids Content of an Aqueous Suspension

[0260] The suspension solids content (also known as dry weight) was determined using a Moisture Analyser MJ33 from the company Mettler-Toledo, Switzerland, with the following settings: drying temperature of 160? C., automatic switch off if the mass does not change more than 1 mg over a period of 30 sec, standard drying of 5 to 20 g of suspension.

[0261] Specific Surface Area (SSA)

[0262] The specific surface area was measured via the BET method according to ISO 9277 using nitrogen, following conditioning of the sample by heating at 250? C. for a period of 30 minutes. Prior to such measurements, the sample was filtered within a Buchner funnel, rinsed with deionised water and dried overnight at 90 to 100? C. in an oven. Subsequently the dry cake was ground thoroughly in a mortar and the resulting powder placed in a moisture balance at 130? C. until a constant weight was reached.

[0263] Specific Carbonation Time

[0264] The monitoring of the conductivity, which slowly decreases during the carbonation reaction and rapidly decreases to a minimal level, thereby indicating the end of the reaction, was used to assess the time needed to perform the complete precipitation. The specific carbonation time (min/kg Ca(OH).sub.2) was determined by the following formula:

[00001]$\mathrm{Specific}.Math..Math.\mathrm{carbonation}.Math..Math.\mathrm{time}=\frac{{10}^5.Math.\mathrm{Tf}}{M.Math.{\mathrm{SC}}_{\mathrm{MoL}}}$

[0265] wherein: [0266] Tf (min) is the time needed to complete the carbonation of the milk of lime, as determined by monitoring the conductivity, [0267] M (g) is the weight of the milk of lime introduced into the carbonation reactor, and [0268] SC.sub.MoL (%) is the weight solids content of the milk of lime.

[0269] Charge measurementM?tek

[0270] The charge measurement was carried out using a M?tek PCD 03 device equipped with a Miitek PCD titrator.

[0271] About 1 g of the PCC suspension is weighed in the plastic measuring cell and is diluted with 20 mL of deionised water. Put the displacement piston on. While the piston oscillates in the cell, wait until the streaming current between the two electrodes stabilize.

[0272] The sign of the measured value shown on the display indicates whether the charge of the sample is positive (cationic) or negative (anionic). An oppositely charged polyelectrolyte of known charge density is added to the sample as a titrant (either sodium polyoxyethylene sulfate 0.001 N or pDADMAC 0.001 N). The titrant charges neutralize existing charges of the sample. Titration is discontinued as soon as the point of zero charge (0 mV) is reached.

[0273] Titrant consumption in mL forms the basis for further calculations. The specific charge quantity q [|?Val/g of sly] is calculated according to the following formula:

q=(V?c)/m

[0274] V: consumed titrant volume [l]

[0275] c: titrant concentration [?Val/l]

[0276] m: mass of the weighed slurry [g]

[0277] q: specific charge quantity [?Val/g of slurry]

[0278] Degree of Carboxylation

[0279] The degree of carboxylation was determined by conductometric titration according to

[0280] Katz et al. The determination of strong and weak acidic groups in sulfite pulps (Svensk Paperstidn., 1984, 6, pp. 48-53).

[0281] Molecular Weight M.sub.w and M.sub.n

[0282] The molecular weight M.sub.w of the carboxylated cellulose was determined by gel permeation chromatography (GPC) using a Waters? liquid chromatograph with a Waters? refractometric detector.

[0283] The mobile phase was a 1 N sodium hydroxide solution adjusted to pH 9 and containing 0.05 mol/l NaHCO.sub.3, 0.1 mol/l NaNO.sub.3, 0.02 mol/l triethanolamine, and 0.03 wt.-% of NaN.sub.3.

[0284] In a first step, the solution of carboxylated cellulose was diluted to a concentration of 0.9 wt.-%, based on the total weight of the solution, with a solvent which corresponded to the mobile phase, but additionally contained 0.04% dimethylformamide as flow rate marker or internal standard. Subsequently, the solution was filtered with a 0.2 ?m filter and 100 ?l of the filtered solution were injected into the GPC (mobile phase: 1 N sodium hydroxide solution adjusted to pH 9 and containing 0.05 mol/l NaHCO.sub.3, 0.1 mol/l NaNO.sub.3, 0.02 mol/l triethanolamine, and 0.03 wt.-% of NaN.sub.3).

[0285] The liquid chromatography apparatus contained an isocratic pump of the Waters? 515 type, the flow rate of which was set at 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. 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.

[0286] The liquid chromatography apparatus was calibrated with a series of certified sodium polyacrylate standards of differend molecular weights, supplied by Polymer Standard Service or American Standard Polymer Corporation.

[0287] The calibration graph is of the linear type and takes into account the correction obtained using the flow rate marker dimethylformamide. Acquisition and processing of the chromatogram were accomplished through the use of the PSS WinGPC Scientific v. 4.02 application. The chromatogram obtained was integrated in the area corresponding to molecular weights higher than 65 g/mol.

[0288] Polydispersity Index (DPI)

[0289] The polydispersity index of a polymer is the ratio of the mass-average molecular weight in weight M.sub.w to the number-average molecular weight M.sub.n. Both M.sub.w and M.sub.n were determined by gel permeation chromatography.

[0290] X-Ray Diffraction

[0291] The purity and morphology of the PCC samples was analysed with a D8 Advance powder diffractometer (Bruker Corporation, USA) obeying Bragg's law. This diffractometer consisted of a 2.2 kW X-ray tube (Cu), a sample holder, a goniometer, and a VANTEC-1 detector. Nickel-filtered Cu K.sub.? radiation was employed in all experiments (?K.sub.?-Cu=1.5406 ?). The profiles were chart recorded automatically using a scan speed of 0.7? per minute in 29 (XRD GV_7600). The measurement was carried out at angles from 5 to 70?.

[0292] The resulting powder diffraction pattern was classified by mineral content using the DIFFRAC.sup.suite software packages EVA and SEARCH, based on reference patterns of the ICDD PDF 2 database (XRD LTM_7603). Quantitative analysis of the diffraction data, i.e. the determination of amounts of different phases in a multi-phase sample, has been performed using the DIFFRAC.sup.suite software package TOPAS (XRD LTM_7604). This involved modelling the full diffraction pattern (Rietveld approach) such that the calculated pattern(s) duplicated the experimental one.

2. Polymer Additives and Slaking Additives

[0293] CMC: High molecular weight carboxymethylcellulose=250 000 g/mol, carboxylation degree=1.2), commercially available from Sigma-Aldrich, Switzerland, reference 419281. [0294] SA1: Sodium citrate, commercially available from Sigma-Aldrich, Switzerland. [0295] PA1: Depolymerized carboxymethylcellulose, produced according to Example 1. [0296] PA2: High molecular weight carboxymethylcellulose Blanose? (M.sub.w=395 000 g/mol, carboxylation degree=1.2), commercially available from Ashland Inc., USA. [0297] PA3: polyacrylic acid with the following formula,

##STR00003## [0298] wherein R.sub.1 is H, X is Na, and m=45; the M.sub.w being 4270 g/mol, and the polydispersity index being 2.3. The molecular weight M.sub.w and the polydispersity index were determined according to the corresponding method described in EP 14 166 751.9.

3. Examples

Example 1

Preparation of Depolymerized Carboxymethylcellulose

[0299] A one litre reactor was charged with 800 g distilled water and 0.017 g of catalyst FeSO.sub.4.7H.sub.2O. The reactor was heated to 80? C?2? C. Over a time period of 2 hours and 45 minutes, an aqueous hydrogen peroxide solution having a concentration of 35 wt.-% was continuously added at a rate of 189 mg/min, while CMC was added in portions of 25 g at a time every 15 minutes. After completed addition, the reaction mixture was maintained at 80? C. for 2 hours and 30 minutes until all hydrogen peroxide was consumed. Subsequently, the reaction mixture was cooled down to 70? C.

[0300] The pH of the obtained reaction mixture was 4.4. The reaction mixture was neutralized to pH 7.4 with an aqueous solution containing 10 wt.-% sodium hydroxide, based on the total amount of the solution.

[0301] The obtained depolymerized CMC had a M.sub.w of 13 310 g/mol, a polydispersity index of 4, and was in form of a solution having a concentration of 33.0 wt.-%, based on the total amount of the solution, and a Brookfield viscosity of 725 mPa.Math.s at 25? C. and 100 rpm.

Example 2

Preparation of PCC (Samples 1 to 5)

[0302] A milk of lime was prepared by mixing under mechanical stirring water with 0.1 wt.-%, based on the total amount of calcium oxide, dry sodium citrate (SA1) as slaking additive and 0.15 wt.-%, based on the total amount of calcium oxide, of the depolymerized CMC produced according to Example 1 (PA1) or one of the other polymer additives indicated in Table 1 below (PA2 or PA3), at an initial temperature between 40 and 41? C. Subsequently, calcium oxide (quicklime raw material) was added. The obtained mixture was stirred for 25 min and then sieved through a 200 ?m screen.

[0303] The obtained milk of lime was transferred into a stainless steel reactor, wherein the milk of lime was cooled down to 50? C. Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (26 vol-% CO.sub.2). During the carbonation step, the reaction mixture was stirred with a speed of 1400 rpm. The kinetic of the reaction was monitored by online pH and conductivity measurements.

[0304] In addition two comparative example were produced without depolymerized carboxymethyl cellulose or other polymer additives.

[0305] The characteristics of the prepared milks of lime and aqueous PCC suspensions are compiled in Table 1 below.

TABLE-US-00001 TABLE 1 Characteristics of the produced milks of lime and the obtained aqueous PCC suspensions of Example 2 (comp.: comparative, nm: not measured). Sam- Sam- Sam Sam- ple 1 ple 2 Sample 3 ple 4 ple 5 (comp.) (comp.) (inventive) (comp.) (comp.) Polymer additive PA1 PA2 PA3 Solids content of milk 25.1 16.2 28.1 28.1 29.5 of lime [wt.-%] Brookfield viscosity of too high 23 410 too high 329 milk of lime [mPa .Math. s] Carbonation time nm 44.2 47 nm 46.5 [min/kg Ca(OH).sub.2] Solids content of PCC nm 20.5 36.6 nm 37.6 suspension [wt.-%] d.sub.50 [?m] nm 1.6 1.5 nm 1.3 SSA [m.sup.2/g] nm 4.7 4.7 nm 5 pH nm nm 7.9 nm nm Brookfield viscosity of nm 20 597 nm 940 PCC suspension [mPa .Math. s] Mutek [?Val/g) nm 0.1 ?0.5 nm ?0.9

[0306] The results presented in Table 1 above confirm that a PCC suspension with a high solids content can be produced by using the process of the present invention (sample 3).

[0307] In contrast, it was not possible to produce a PCC suspension with a high solids content by carrying out the aforementioned process in the absence of a depolymerized cellulose (see comparative sample 1). The use of carboxymethylcellulose (PA2) instead of depolymerized carboxymethylcellulose (PA1) also resulted in a milk of lime having such a high Brookfield viscosity (above 1000 mPa.Math.s at 25? C.?1? C. at 100 rpm) that a further processing of the sample was impossible (see comparative sample 4).

Example 3

Preparation of PCC (Samples 6 and 7)

[0308] Inventive Sample 6

[0309] A milk of lime was prepared by mixing under mechanical stirring 91 water with 0.1 wt.-%, based on the total amount of calcium oxide, dry sodium citrate (SA1) as slaking additive and 0.15 wt.-%, based on the total amount of calcium oxide, of the depolymerized CMC produced according to Example 1 (PA1) at an initial temperature between 40 and 41? C. Subsequently, calcium oxide (quicklime raw material) was added, wherein the calcium oxide/water ratio was adjusted to 1:4.4-3.9 in order to obtain a milk of lime with a high solids content and an acceptable Brookfield viscosity of up to 350-400 mPa.Math.s. The obtained mixture was stirred for 25 min and then sieved through a 200 ?m screen.

[0310] 81 of the obtained milk of lime were transferred into a stainless steel reactor, wherein the milk of lime was cooled down to 50? C. Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (20 vol-% CO.sub.2) with a rate of 15 l/min. During the carbonation step, the reaction mixture was stirred with a speed of 750 rpm. The kinetic of the reaction was monitored by online pH and conductivity measurements.

[0311] Comparative Sample 7

[0312] A milk of lime was prepared by mixing under mechanical stirring 91 water with 0.1 wt.-%, based on the total amount of calcium oxide, dry sodium citrate (SA1) as slaking additive. Subsequently, calcium oxide (quicklime raw material) was added in an amount such that a solids content of 13.5 wt.-%, based the total amount of the milk of lime, was obtained. The obtained mixture was stirred for 25 min and then sieved through a 200 ?m screen.

[0313] 81 of the obtained milk of lime were transferred into a stainless steel reactor, wherein the milk of lime was cooled down to 50? C. Then the milk of lime was carbonated by introducing an air/CO.sub.2 mixture (20 vol-% CO.sub.2) with a rate of 15 l/min. During the carbonation step, the reaction mixture was stirred with a speed of 750 rpm. The kinetic of the reaction was monitored by online pH and conductivity measurements.

[0314] The obtained PCC suspension had a solids content of 17.5 wt.-%, based on the total amount of the suspension, and was mechanically up-concentrated to a solids content of 35.8 wt.-%.

[0315] The characteristics of the prepared milks of lime and aqueous PCC suspensions are compiled in Table 2 below.

TABLE-US-00002 TABLE 2 Characteristics of the produced milk of lime and the obtained aqueous PCC suspension of Example 3 (*after up-concentration). Sample 6 Sample 7 (inventive) (comparative) Solids content of milk of lime [wt.-%] 28.5 13.5 Brookfield viscosity of milk of lime 200 30 [mPa .Math. s] Solids content of PCC suspension [wt.-%] 33.6 17.5 (35.8*) d.sub.50 [?m] 1.95 2.37 SSA [m.sup.2/g] 5.7 6.1 pH 8.5 10 Brookfield viscosity of PCC suspension 285 270* [mPa .Math. s] morphology S-PCC S-PCC

[0316] As can be seen from the results shown in Table 2 above, a PCC suspension with a high solids content has been produced by using the process of the present invention (sample 6). Furthermore, the precipitated calcium carbonate (PCC) produced by the process of the present invention (sample 6) features a substantially lower average particle size, and thus, a higher fineness, compared to the PCC obtained by the prior art process (sample 7).