Method for improving rheological properties of an aqueous pigment slurry and a dispersion agent

Abstract

The invention relates to a method for improving rheological properties of an aqueous pigment slurry. In the method dispersion agent is added to an aqueous phase of a slurry comprising pigment particles or to the aqueous phase into which the pigment particles are to be added. The dispersion agent comprises styrene acrylate copolymer. The invention relates also to dispersion agent for a pigment slurry comprising a first component comprising styrene acrylate copolymer and a second component comprising a conventional dispersion agent, such as a straight-chain polyacrylate.

Claims

1. A method for preparing an aqueous .[.pigment.]. slurry .Iadd.of pigment particles .Iaddend.with improved rheological properties, where the pigment is selected from calcium carbonate, kaolin, calcinated kaolin, talc, titanium dioxide, gypsum, chalk, satine white, barium sulfate, sodium aluminum silicate, aluminum hydroxide or any of their mixture, the method comprising adding a dispersion agent in .Iadd.an .Iaddend.amount of 0.1-3.5 parts per 100 parts of pigment to an aqueous phase of a slurry comprising pigment particles or to the aqueous phase into which the pigment particles are to be added, .Iadd.wherein .Iaddend.the dispersion agent .[.comprising.]. .Iadd.comprises .Iaddend.styrene acrylate copolymer which is obtained by polymerizing styrene and acrylate monomers in presence of starch and which has a mean particle size in the aqueous dispersion in the range of 20-150 nm.

2. The method according to claim 1, wherein the dispersion agent comprising styrene acrylate copolymer is added during the final stages of wet grinding of a pigment when the pigment particles have already obtained the desired particle size.

3. The method according to claim 1, further comprising adding a conventional dispersion agent to the aqueous phase.

4. The method according to claim 3, wherein the conventional dispersion agent is a straight-chain polyacrylate.

5. The method according to claim 1, wherein the dispersion agent comprising the styrene acrylate copolymer is used in the form of an aqueous polymer dispersion having a solids content of 10-50%.

Description

EXAMPLE

(1) 100 parts of dried undispersed PCC (precipitated calcium carbonate, 95%<1 μm) is added into water with 0.55 pph (parts per 100) conventional Na-polyacrylate dispersant, Colloid 220 (trademark) by Kemira, to the solid content of the slurry 68%. The pH is adjusted to the value 9.9 or 9.5 (sample 4) using 10% sodium hydroxide. The slurry is mixed 10 minutes at ambient temperature in Diaf-mixer at 3000 rpm, after which 1 pph of dispersion agent according to the present invention is added for formulations 2 to 4. The mixing of the slurry is continued for further 5 minutes. From the prepared slurries viscosities are measured using Brookfield viscometer, type DV-II, with speeds 100 and 50 rpm and using spindle 3. The sample size was 500 g of dry pigment, solid content of the slurry ca 68%. High shear viscosity at shear blocking is measured using Hercules Hi-Shear DV-10 rotational viscometer. The test procedure of evaluating the viscosity of slurry pigments is as outlined by TAPPI (Technical Association of the Pulp and Paper Industry) test method T-648 (valid in September 2008). Results are shown in table 1

(2) Dispersion agent in the samples is as follows:

(3) Reference: pigment slurry with 0.55 pph Na-polyacrylate

(4) Sample 2: pigment slurry with 0.55 pph Na-polyacrylate+1 pph styrene acrylate copolymer

(5) Sample 3: pigment slurry with 0.55 pph Na-polyacrylate+1 pph 90/10 styrene acrylate copolymer/Na-polyacrylate

(6) Sample 4: pigment slurry with 0.55 pph Na-polyacrylate+1 pph Na-polyacrylate

(7) In other words, samples 2 and 3 are according to the present invention, in reference sample and sample 4 employ conventional dispersion agent in different amounts.

(8) TABLE-US-00001 TABLE 1 Results of viscosity measurements Measured Property Unit Reference Sample 2 Sample 3 Sample 4 Viscosity, mPas 115 103 124 131 Brookfield 100 rpm Viscosity, mPas 132 141 164 173 Brookfield 50 rpm High shear viscosity mPas 95 85 63 101 at shear blocking Solids content % 68.2 68.1 68.1 68.3 pH 9.9 9.9 9.9 9.5 Temperature ° C. 24 24 24 24 Density g/cm.sup.3 1.84 1.84 1.84 1.84

(9) From the results it can be seen that when a dispersion agent according to the present invention comprising styrene acrylate copolymer is used, the high shear viscosity at shear blocking decreases compared to slurries with conventional Na-polyacrylate dispersant.

(10) Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.