Process for the preparation of flocculated filler particles

Abstract

The present invention concerns a process for the preparation of flocculated filler particles, wherein at least two aqueous suspensions of at least one filler material and at least one flocculating additive are combined.

Claims

1. A process for the preparation of flocculated filler particles comprising: providing at least one first aqueous suspension S1 in a first vessel that comprises at least one flocculating additive A and at least one first filler material; providing at least one second aqueous suspension S2 in a second vessel that comprises at least one flocculating additive B and at least one second filler material, wherein the at least one flocculating additive B is different from the at least one flocculating additive A, wherein more than 99 wt.-% of the at least one first filler material in the first aqueous suspension S1 and the at least one second filler in the second aqueous suspension S2 are the same; and combining the at least one first aqueous suspension S1 with the at least one second aqueous suspension S2 simultaneously in a third vessel, under conditions effective to form a mixture, the mixture comprising an aqueous suspension SM of flocculated filler particles, wherein the aqueous suspension SM of flocculated filler particles fine fractions has a mono-modal particle size distribution, and wherein the content of the at least one flocculating additive A in the first aqueous suspension S1 is from 0.5 to 10 wt.-%, based on the total weight of the first aqueous suspension S1, and/or the content of the at least one flocculating additive B in the second aqueous suspension S2 is from 0.1 to 10 wt.-%, based on the total weight of the second aqueous suspension S2.

2. The process according to claim 1, wherein a Brookfield viscosity of the first aqueous suspension S1 and/or the second aqueous suspension S2 and/or the aqueous suspension SM is less than 5,000 mPa.Math.s at 25° C.

3. The process according to claim 1, wherein a Brookfield viscosity of the first aqueous suspension S1 and/or the second aqueous suspension S2 and/or the aqueous suspension SM is less than 1,000 mPa.Math.s at 25° C.

4. The process according to claim 1, wherein a Brookfield viscosity of the first aqueous suspension S1 and/or the second aqueous suspension S2 and/or the aqueous suspension SM is between 10 and 200 mPa.Math.s at 25° C.

5. The process according to claim 1, wherein the at least one flocculating additive A is a cationic polymer selected from the group consisting of cationic starch, polyamines, polyethyleneimines, polyacrylamides, cationic amine amide, epichlorohydrin resins, polydiallyldimethylammonium chloride, cationic guar, and any mixture thereof.

6. The process according to claim 1, wherein the at least one flocculating additive A is a cationic starch.

7. The process according to claim 1, wherein the at least one flocculating additive B is an anionic carboxymethyl cellulose.

8. The process according to claim 7, wherein the at least one flocculating additive A is a cationic starch.

9. The process according to claim 8, wherein the at least one filler material is precipitated calcium carbonate.

10. The process according to claim 1, wherein the aqueous suspension SM is sheared during and/or after the simultaneous combination of the at least two aqueous suspensions.

11. The process according to claim 1, wherein the aqueous suspension SM is sheared in at least two steps at different shear rates.

12. The process according to claim 1, wherein the aqueous suspension SM is sheared in at least two steps at different shear rates, wherein the first shear rate is lower than the second shear rate.

13. The process according to claim 1, wherein the at least one first filler material and/or the at least one second filler material is selected from the group consisting of a calcium carbonate-comprising material, ground calcium carbonate, precipitated calcium carbonate, modified calcium carbonate, talc, clay, dolomite, marble, titanium dioxide, kaolin, silica, alumina, mica, aluminium trihydrate, magnesium hydroxide, plastic pigments, a hybrid material comprising an organic filler and an inorganic chemical composition, and any mixture thereof.

14. The process according to claim 1, wherein the at least one first filler material and/or the at least one second filler material is precipitated calcium carbonate.

15. The process according to claim 1, wherein the content of the at least one flocculating additive A in the first aqueous suspension S1 is from 3 to 5 wt.-%, based on the total weight of the first aqueous suspension S1, and/or the content of the at least one flocculating additive B in the second aqueous suspension S2 is from 0.2 to 0.8 wt.-%, based on the total weight of the second aqueous suspension S2.

16. The process according to claim 1, wherein the content of the at least one first filler material in the first aqueous suspension S1 is from 15 to 65 wt.-%, based on the total weight of the first aqueous suspension S1, and the content of the at least one second filler material in the second aqueous suspension S2 is from 15 to 65 wt.-%, based on the total weight of the second aqueous suspension S2.

17. The process according to claim 1, wherein the aqueous suspension SM has a solids content of from 1 to 75 wt.-%, based on the total weight of the aqueous suspension SM.

18. The process according to claim 1, wherein the aqueous suspension SM has a solids content of from 2 to 60 wt.-%, based on the total weight of the aqueous suspension SM.

19. The process according to claim 1, wherein the aqueous suspension SM has a solids content of from 5 to 35 wt.-%, based on the total weight of the aqueous suspension SM.

20. The process according to claim 1, wherein the aqueous suspension SM of flocculated filler particles has a volume defined particle size polydispersity expressed as full width at half maximum height (FWHM) of less than 40 μm.

21. The process according to claim 1, wherein the aqueous suspension SM of flocculated filler particles has a volume defined particle size polydispersity expressed as full width at half maximum height (FWHM) of less than 20 μm.

22. The process according to claim 1, wherein the aqueous suspension SM of flocculated filler particles has a volume defined particle size polydispersity expressed as full width at half maximum height (FWHM) of less than 10 μm.

23. The process according to claim 1, wherein the content of the at least one first filler material in the first aqueous suspension S1 is from 1 to 85 wt.-%, based on the total weight of the first aqueous suspension S1, and the content of the at least one second filler material in the second aqueous suspension S2 is from 1 to 85 wt.-%, based on the total weight of the second aqueous suspension S2.

24. The process according to claim 1, wherein the at least one flocculating additive B is an anionic polymer selected from the group consisting of carboxymethyl cellulose, anionic starch, anionic guar, anionic xanthan gum, and any mixture thereof.

25. The process according to claim 1, wherein the mass ratio of the at least one first filler material in the first aqueous suspension and the at least one second filler material in the second aqueous suspension S1:S2 is from 1:100 to 100:1.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows an example of a batch process of the present invention.

(2) FIG. 2 shows the particle size distribution of filler particles before and after addition of flocculating additive A and B, respectively.

(3) FIG. 3 shows the particle size distribution of comparative and inventive flocculated filler particles after shearing.

(4) FIG. 4 shows the elastic modulus of hand sheets prepared from non-flocculated and flocculated filler particle suspensions obtained according to a prior art process and the process of the present invention.

(5) FIG. 5 shows the bending stiffness of hand sheets prepared from non-flocculated and flocculated filler suspensions obtained according to a prior art process and the process of the present invention.

(6) FIG. 6 illustrates floc desaggregation under shear using a static mixer.

EXPERIMENTS

1. Measuring Methods

(7) In the following the measurement methods implemented in the examples are described.

(8) Particle Size Distribution (PSD) of the Employed Filler Materials Before Step a)

(9) The particle size distribution of the employed filler materials, i.e. before step a) of the process of the present invention, was measured using a Malvern Mastersizer 2000 Laser Diffraction System (Malvern Instruments Plc., Great Britain) using the Fraunhofer light scattering approximation. The method and instrument are known to the skilled person are commonly used to determine particle sizes of fillers and other particulate materials.

(10) 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 in the presence of ultrasonics.

(11) Particle Size Distribution (PSD) of Filler Particles in Suspension S1, S2 and SM

(12) The particle size distribution of the filler particles in suspensions S1, S2, and SM was measuring using a Malvern Mastersizer 2000 Laser Diffraction System (Malvern Instruments Plc., Great Britain) using the Fraunhofer light scattering approximation. The method and instrument are known to the skilled person are commonly used to determine particle sizes of fillers and other particulate materials.

(13) The measurement was carried out in an aqueous solution comprising 0.1 wt.-% Na.sub.4P.sub.2O.sub.7. The samples were dispersed without a high speed stirrer and in the absence of ultrasonics.

(14) Filler Content

(15) 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.

(16) Brookfield Viscosity

(17) 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. For a viscosity range between 200 and 800 mPa.Math.s the spindle number 3 was used, for a viscosity range between 400 and 1 600 mPa.Math.s the spindle number 4 was used, and for a viscosity range between 800 and 3 200 mPa.Math.s the spindle number 5 was used.

(18) Mechanical Strength Properties

(19) The mechanical strength properties were characterized after drying of the handsheets. The elastic modulus was determined according to ISO 1924-2:2008 and the bending stiffness according to ISO 5629:1983.

(20) Full Width at Half Maximum Height (FWHM)

(21) The particle size distribution data were displayed in an xy scatter diagram and the data were arranged accordingly in x and y columns whereas the size data were put in the x-column and the frequency data was arranged in they column. The maximum height (y.sub.m) was determined by sorting the particle size distribution curve by the frequency data points. The respective x-value was then defined as the peak position at the maximum height (x.sub.m). By dividing the maximum peak height by 2, the half maximal height was obtained (y.sub.m0.5).

(22) The four data points that have the closest y-value compared to the half maximum height value were defined, whereas, compared to the data point of the half maximal height at the position of the maximal height P.sub.HM2 (x.sub.m/y.sub.m0.5):

(23) P.sub.IL was the data point having the nearest lower x- and the nearest lower y-value (x.sub.1L/y.sub.1L). P.sub.1H was the data point having the nearest lower x- and the nearest higher y-value (x.sub.1H/y.sub.1H). P.sub.2L was the data point having the nearest higher x- and the nearest lower y-value (x.sub.2L/y.sub.2L). P.sub.2H was the data point having the nearest higher x- and the nearest higher y-value (x.sub.2H/y.sub.2H)

(24) The linearly interpolated x-positions (x.sub.1I and x.sub.2I) of the data points having the y-value of the half of maximal height value (y.sub.m0.5) were calculated as follows:

(25) $x_{1I}=x_{1L}+\left(\frac{y_{m0.5}-y_{1L}}{y_{1H}-y_{1L}}\right)\left(x_{1H}-x_{1L}\right)$$x_{2I}=x_{2H}+\left(\frac{y_{2H}-y_{m0.5}}{y_{2L}-y_{2H}}\right)\left(x_{2L}-x_{2H}\right)$$\mathrm{and}$$\mathrm{FHMW}=x_{2I}-x_{1I}.$

2. Materials

(26) Filler material (PCC): undispersed, scalenohedral precipitated calcium carbonate (d.sub.50=4.3 μm, measured with Malvern Mastersizer 2000), commercially available from Omya AG, Switzerland.

(27) Flocculating additive 1 (FA1): Carboxymethyl cellulose (Finnfix 10, M.sub.W=60 000 g/mol, degree of substitution=0.8), commercially available from CP Kelko, USA.

(28) Flocculating additive 2 (FA2): Starch powder (C*Bond HR 35845), commercially available from Cargill, USA.

3. Examples

Example 1—Preparation of Flocculated Filler Particles (Comparative Example)

(29) A solution of FA1 was prepared by adding FA1 into tap water at a temperature of 23° C. under stirring. Stirring was continued for 60 minutes until FA1 had completely dissolved. FA1 was added in such an amount that a solution with a FA1 content of 4 wt.-%, based on the total weight of the FA1 solution, was obtained.

(30) A solution of FA2 was prepared by adding FA2 into deionized water and heating the mixture for 30 minutes at 100° C. FA2 was added in such an amount that a solution with a FA2 content of 1 wt.-%, based on the total weight of the FA2 solution, was obtained. The FA2 solution was cooled down to room temperature using a water bath and the amount of water lost by evaporation was added to readjust the solution to a FA2 content of 1 wt.-%.

(31) An aqueous suspension was prepared by adding the amount of the FA1 solution indicated in Table 1 below to 2 kg of a slurry of PCC having a solids content of 15 wt.-%, based on the total weight of the slurry. Subsequently, the amount of the FA2 solution indicated in Table 1 below was added to the aqueous suspension of PCC and FA1, whereby flocculated filler particles formed. After complete addition the suspension was stirred for 5 minutes at a shear rate of 50 s.sup.−1, and then subjected for 15 minutes to a Megatron treatment at a shear rate of 40 000 s.sup.−1 (Megatron MT 5000 with MTO 5000 Q working chamber, Kinematica AG, Luzern CH, circulation mode, 14 000 rpm).

Example 2—Preparation of Flocculated Filler Particles

(32) A solution of FA1 was prepared by adding FA1 into tap water at a temperature of 23° C. under stirring. Stirring was continued for 60 minutes until FA1 had completely dissolved. FA1 was added in such an amount that a solution with a FA1 content of 4 wt.-%, based on the total weight of the FA1 solution, was obtained.

(33) A solution of FA2 was prepared by adding FA2 into deionized water and heating the mixture for 30 minutes at 100° C. FA2 was added in such an amount that a solution with a FA2 content of 1 wt.-%, based on the total weight of the FA2 solution, was obtained. The FA2 solution was cooled down to room temperature using a water bath and the amount of water lost by evaporation was added to readjust the solution to a FA2 content of 1 wt.-%.

(34) A first aqueous suspension S1 was prepared by adding the amount of the FA1 solution indicated in Table 1 below to 1 kg of a slurry of PCC having a solids content of 15 wt.-%, based on the total weight of the slurry.

(35) A second aqueous suspension S2 was prepared by adding the amount of the FA2 solution indicated in Table 1 below to 1 kg of a slurry of PCC having a solids content of 15 wt.-%, based on the total weight of the slurry. During the addition of the FA2 solution flocculated filler particles formed.

(36) The first aqueous suspension S1 and the second aqueous suspension S2 were combined at room temperature in a separate vessel by pouring both suspensions simultaneously into the vessel under stirring. After complete addition the resulting mixture was stirred for another 10 minutes at a shear rate of 50 s.sup.−1. Then the slurry was subjected for 15 minutes to a Megatron treatment for desaggregation of the flocs at a shear rate of 40 000 s.sup.−1 (Megatron MT 5000 with MTO 5000 Q working chamber, Kinematica AG, Luzern CH, circulation mode, 14 000 rpm).

(37) TABLE-US-00001 TABLE 1 Composition of the aqueous filler suspensions. Amount FA1 Amount FA2 Brookfield [pph, based [pph, based viscosity Example on dry filler] on dry filler] [mPa .Math. s] 1 0.5 4 <1 000 2.sup.a 1 (in S1) 8 (in S2) <1 000 (in both S1 and S2) 2.sup.b 0.5 4 <1000 (in SM) .sup.aBefore mixing; .sup.bafter mixing.

(38) FIG. 2 shows the particle size distributions of an aqeuous suspension of PCC having a solids content of 15 wt.-%, and the separate first and second aqueous suspensions S1 and S2 prepared according to Example 2. It can be gathered from said figure that the flocculation that occurred in the second aqueous suspension S2 during the addition of flocculating additive FA2 resulted in an increased size of the particles. The particle size of PCC in the first aqueous suspension S1 did not change at all after addition of the flocculating additive FA1.

(39) FIG. 3 shows the flocculated filler particle suspension obtained according to Example 1 and the flocculated filler particle suspension obtained according to Example 2. As can be seen in FIG. 2, the PCC contains a fine fraction around 0.7 μm. FIG. 3 shows that this fraction was reduced, but is still present in comparative Example 1. However, in the inventive Example 2, this fine fraction was not present anymore. Furthermore, the fine fractions observed for suspension S2 of Example 2 in FIG. 2, have also disappeared after combining the first and the second aqeuous suspensions S1 and S2 simultaneously. Thus, Example 2 confirms that the process of the present invention allows to manufacture flocculated fillers without fine fractions having a mono-modal particle size distribution.

(40) The peak position, peak height, half peak height, and FWHM values of the main peak of the particles size distributions of the flocculated filler particle suspension obtained according to Example 1 (comparative) and the flocculated filler particle suspension obtained according to Example 2 (inventive) are given in Table 2 below.

(41) TABLE-US-00002 TABLE 2 Peak position, peak height, half peak height and FWHM values of particle size distributions of flocculated fillers of Example 1 and Example 2. Example 1 Example 2 Peak position at max. height [μm] 10.0 8.7 Height [%] 10.4 13.2 Half height [%] 5.2 6.6 FWHM [μm] 12.8 9.0

Example 3—Preparation and Testing of Handsheets

(42) 60 g (dry) pulp were diluted in 10 dm.sup.3 tap water, and then the filler to be tested was added in an amount so as to obtain the overall filler content based on the final paper weight. The suspension was stirred for 30 minutes. 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 80 g/m.sup.2 were formed using the Rapid-Kothen hand sheet former. Each sheet was dried using the Rapid-Kothen drier. The composition of the handsheets is given in Table 3 below.

(43) TABLE-US-00003 TABLE 3 Composition of handsheets. Flocculated Flocculated filler of filler of Pulp PCC Example 1 Example 2 Sample [wt.-%] [wt.-%] [wt.-%] [wt.-%] 1 (comparative) 80 20 — — 2 (comparative) 75 — 25 — 3 75 — — 25

(44) The mechanical properties of the prepared handsheets were tested by determining the elastic modulus and the bending stiffness. The results are shown in FIGS. 4 and 5.

(45) As can be gathered from FIG. 4 comparative sample 2 showed a reduced elastic modulus compared to comparative sample 1. The inventive sample 3 containing the flocculated filler suspension prepared according the Example 2, however, showed even a superior elastic modulus compared to sample 1.

(46) FIG. 5 reveals that the hand sheet containing a flocculated filler according to comparative Example 1 (sample 2) has a reduced bending stiffness compared to the handsheet containing PCC as filler (sample 1). The hand sheet containing the flocculated filler suspension prepared according the Example 2, however, showed an improved bending stiffness compared to sample 1.