FI82204C - POLYMER DISPERSION PROCEDURE FOR PIGMENT. - Google Patents

Description

1 82204

Water-soluble polymeric dispersants for pigments

Divided by application 841990 5

It is well known that water-soluble polymers with a low molecular weight, and in particular polymers containing acidic groups which may be partially or completely neutralized, are valuable pigment dispersants (including grinding aids). Polymers generally have a molecular weight of 1000-10,000 (weight average molecular weight, Mw). However, the polymer always consists of a mixture of molecules of different molecular weights, depending on the number of monomer units in each molecule. Virtually every commercial polymer is a mixture of molecules with very wide chain lengths. For example, a polymer with Mw = 5000 generally contains significant amounts of molecules with molecular weights less than 1000 and more than 20 6000. The extent to which a private product is composed of molecules that fall into a particular range of chain lengths is determined by its polydispersity. The polydispersity (PD) of a product is the weight average molecular weight (mw) divided by the number average molecular weight (Mn). If PD = 1, then the polymer consists exclusively of molecules of equal chain length. In practice, the PD is always much higher, usually above 2.

GB Patent No. 1,414,964 discloses some vinyl acetate copolymers for dispersing chalk. Example 2 shows polymers having a number average molecular weight of 1200-2300 and polymer fractional precipitation is said to give fractions having a number average molecular weight between 150-4000. The slightly different process conditions mentioned in Example 3 are said to give a narrower molecular weight distribution and fractions between 960-3000. Variation in molecular weight in each fraction is not mentioned. The patent does not disclose the use or properties of any of these fractions, but attempts to show that the polymer of Example 3 (a mixture of fractions with average molecular weights between 960-3000) has better properties than the product of Example 2 and this is due to " on the optimizing effect of the molecular weight distribution ". Since the polymers of Examples 2 and 3 can be divided into polymer fractions having such a wide molecular weight range, it is clear that the polymers of both Examples 2 and 3 have high polydispersity values, possibly of the order of 2. It is impossible to predict which polydispersity values would be as this may vary with the fractionation method, but was possibly more than 1.7. The patent publication does not suggest any use of the polymer fractions for any purpose.

Products that are commercially successful as dispersants include polyacrylic acid and copolymers of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS). One widely used polyacrylic acid is the applicant's product Dispex N40 (Dispex is a trademark). The products sold by the applicant generally have polydispersity values above 1.8 and most commercially available products have polydispersity values above 2. The applicant has considered that it is uneconomic and not necessary to aim for lower polydispersity values, and although the applicant sometimes to prepare batches of poly-30 with a polydispersity of slightly less than 1.8, these batches will always be mixed with batches with a higher polydispersity during storage.

It is standard practice to prepare water-soluble acidic polymers, such as polyacrylic acid, by solution polymerization 3 82204, in which case the solvent may be a mixture of water and an organic liquid such as isopropanol. The polymerization product is a solution of the polymer and, to some extent, the oligomer and the unreacted monomer.

In one method used by the applicant, the undesired low molecular weight components, i.e. to remove oligomers and monomer, additional sodium hydroxide is added to the solution to neutralize all acidic groups, and the mixture is partitioned into 10 upper isopropanol fractions containing undesired low molecular weight components and a lower aqueous fraction containing the desired polymer. This fractionation has only been considered as a means of separating the useful polymer from the undesired by-products. The useful polymer is a mixture of molecules of different molecular weights and the above PD values are the values of the purified polymer.

A specific method for separating undesired by-products is described in EP 20 46 573. This publication states that an aqueous solution of polyacrylic acid can be neutralized and that the neutralized polymer can then be treated in the usual manner with polar solvents such as methanol, ethanol, isanol, propanol, propanol. and tetrahydrofuran-25. In the examples, 80 g of fully neutralized sodium polyacrylate are fractionated in solution using 500 g of water with 400 g of methanol or 40 g of isopropanol. In each case, the lighter organic phase is discarded. This then appears to be a conventional fractionation-30 method for removing oligomers and the product would then be a standard mixture of molecular weights. If the starting polymer blend is conventional, the Mw, Mn and PD values of the extracted polymer are also conventional, e.g., a PD greater than 1.8.

Certain sulfide-, sulfone-, or sulfoxide-terminated oligomers are disclosed in U.S. Patent 3,759,860 4,820,204 for use as emulsifiers, for example. It does not suggest that they should be used to disperse pigments. It is said that their PD can be less than 2 "and often as low as 1.4-1.5". With the exception of this statement, the applicant is not aware that anyone has ever proposed the commercial use of low polydispersity water soluble polymers for any purpose and in particular there has been no suggestion that they could be used to disperse the pigment. Because it is once much easier to make polymers with high PDs than low PDs, the opportunity to try to make them on a commercial scale may never have occurred to anyone before, as it has never been known before that they have no special value.

Insoluble, low molecular weight polymers with low polydispersity values are described in EP 68,887, but since such polymers are insoluble, they cannot be used as pigment dispersants.

It has now been found that low molecular weight water-soluble polymeric dispersants have greatly improved pigment dispersing properties if the polymer has a much narrower molecular weight range than previously used, i.e. if the PD is significantly reduced to 1.8 and below conventional values. which is less than 1.5. Thus, it has been found that for each specific purpose, optimal results are obtained if the polymer consists of molecules with chain lengths in a very limited range. The presence of molecules with other chain lengths is detrimental for two reasons. First, and most importantly, these other molecules counteract the beneficial effects of the preferred molecules, possibly because some other detrimental or opposing mechanism contributes to the gain. Another reason may be that these other molecules dilute the polymer to less than the theoretical maximum preferred molecules.

The invention relates to a water-soluble polymeric or dispersant in the form of a water-soluble salt of a polymer, which polymer is formed from one or more ethylenically unsaturated monomers and contains acidic groups selected from carboxyl-10 and sulfonic acid groups. The dispersant is characterized in that the polymer has a polydispersity value of less than 1.5 and a molecular weight, determined as the sodium salt, of about 1500-6000.

According to the invention, the polydispersity values are less than 1.5, often between 1.05 and 1.45 and most preferably between 1.1 and 1.4.

The preparation of dispersants (polymers) according to the invention is described in FI patent 77674.

The invention also relates to the dispersion of a pigment in a water-20 second medium containing a pigment dispersant which is a water-soluble polymer formed from one or more ethylenically unsaturated monomers and containing acidic groups selected from carboxylic and sulfonic acid groups, or an aqueous solution salt having a polydispersity of less than 1.5 and a molecular weight, determined as the sodium salt, of about 1500-6000. The molecular weight is preferably up to about 5,000. Particularly preferred products are those with a PD of less than 1.4 and an Mw of less than 4000.

In general, the PD is between 1.0 and 1.45 and in particular between 1.1 and 1.4. The best results are obtained when the PD is less than 1.4 and preferably less than 1.35. Although it is desirable that said value be as close to 1 as possible, it is generally suitable that its value be greater than 1.25.

Preferably, the polymer is acrylic acid or a copolymer thereof with 6,820,204 AMPS (2-acrylamido-2-methylpropanesulfonic acid). Each acidic polymer may be present in the form of a partial or complete salt which it forms with an alkali metal, often sodium, or ammonia or an amine or some other cation which gives a water-soluble salt. For example, the polymer may be a copolymer of acrylic acid with a salt of AMPS, or it may be a complete salt of an acrylic acid-AMPS copolymer or a partial or complete salt of acrylic acid. All molecular weights presented herein are determined as the total sodium salt.

Although these are preferred polymers, other water-soluble polymers may be used in the invention, which are generally polymers obtained by polymerizing an ethylenically unsaturated monomer containing acidic groups, either alone or with another ethylenically unsaturated monomeric material. Instead of monomers, oligomers formed from the corresponding monomers can be used. Acidic groups are generally carboxylic acid or sulfonic acid groups. The monomers are often acrylic monomers, and in this case the preferred acidic monomers are one or more methacrylic acid or in particular acrylic acid or 2-acrylamino-2-methylpropanesulfonic acid, although many other acidic monomers or polymers which can be polymerized can be used, for example Any comonomers that can be copolymerized in such amounts with the acidic monomer or monomers to form a water-soluble polymer can be used and include monomers such as acrylamide, acrylonitrile and acrylic esters. Generally, at least 50% by weight and often at least 80% by weight of the monomer from which the polymer is formed are acidic monomers. The polymer is generally a linear polymer.

As mentioned above, some low molecular weight vinyl acetate copolymers of about 7,820,204 are mentioned in GB Patent No. 1,414,964 for dispersing pigments and as dispersants for grinding used in grinding. In practice, however, these have not acquired any significant commercial significance and the dispersants commonly used are small molecule polyacrylic acids, which generally have a PD of more than 1.8.

In one aspect of the invention, the aqueous pigment dispersion comprises as a dispersant a polyacrylic acid having a PD of less than 1.5 and an Mw in the range of about 1500 to about 3300, preferably in the range of about 1500-3000 and most preferably in the range of about 1800 to about 2200, with best results generally , which are about 2000. The PD is preferably between 1.05-1.4, most preferably between 1.1-1.3 or 1.35.

These dispersions can be conveniently prepared simply by mixing the pigment particles with water in the presence of a dispersant, the amounts of pigment and dispersant being conventional. For example, the amount of dispersant is often 0.05 to 0.3% by weight of dry polymer based on the dry pigment. The amount of pigment is often between 10 and 90% by weight of the dispersion, most preferably 50 or 60 or up to 80% by weight. Pigment va-25 Iita taking into account the intended use of the dispersion. The dispersion is often used in the paper industry, for example for coating paper, and suitable pigments include kaolin, talc, titanium dioxide and precipitated calcium carbonate. The particle size is generally between 0.5 and 100, preferably between 30 and 50.

It is well known that it is particularly problematic to prepare concentrated pigment dispersions by grinding calcium carbonate in water, especially if very fine particle sizes are required. This is described in Applicant EP-35 108,842 and in the literature to which β 82204 is referred. It describes how sodium polyacrylate with an Mw of 2800 is commonly used, but improved results can be obtained using acrylic acid-AMPS copolymers, with the best result (lowest vis-5 cosity) being found at an Mw of about 5700.

According to another aspect of the invention, an aqueous dispersion of calcium carbonate is prepared by grinding calcium carbonate in water in the presence of a dispersant which is a polyacrylic acid having a PD of less than 1.5 and a Mw of between about 2500-4500, most preferably between 3300-3900.

According to a third aspect of the invention, an aqueous dispersion of calcium carbonate is prepared by grinding calcium carbonate in water in the presence of a dispersant which is a copolymer of acrylic acid and AMPS having a PD of less than 1.5 and a Mw of between about 1500 and about 3500, most preferably between about 2250 and about 2750.

The preferred molecular weight of the homopolymer is about 3600 and the preferred molecular weight of the copolymer is about 2500. The weight ratios of acrylic acid: AMPS are preferably 99: 1-50: 50.

When grinding calcium carbonate according to either of these aspects of the invention, the PD is preferably between 1.05 and 1.4, most preferably between 1.1 and 1.3.

The grinding is preferably sand grinding and the final particle size of the calcium carbonate (marble) is preferably substantially less than 2. The amount of pigment in the obtained dispersion is preferably more than 70% and most preferably more than 75%. For further explanations on suitable grinding methods, particle sizes and concentrations, reference is made to European Patent Publication 108,842 and the literature cited therein.

U.S. Patent No. 3,840,487 and GB Patent No. 1,505,555 describe various aqueous dispersion paints containing a pigment and a low molecular weight polymeric dispersant. These publications

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9,820,204 polymeric dispersants. Specific polyacrylic acid ester copolymers are used for the purposes described in these publications, but in many cases the polyacrylic acid homopolymer, and in particular Dispex N40 5, is generally considered to be completely satisfactory. Applicant believes that better results will be obtained using the novel polymer defined herein. In particular, according to a fourth aspect of the invention, the aqueous dispersion paint comprises a pigment, a paint binder and a pigment dispersant, wherein the dispersant is a polyacrylic acid having a PD of less than 1.5 and a Mw of between about 1500 and about 6000. The PD is preferably between 1.05 and 1.4 , most preferably 1.1-1.3 and the molecular weight is preferably between 1500-4500. The aqueous medium for the paint may be water or a mixture of water and a polar solvent, usually glycol.

The pigment is generally titanium dioxide, kaolin or calcium carbonate and generally has a particle size of between 0.1 and 50, preferably between 0.2 and 25. The amount of pigment in the paint may be conventional, typically 5-50 wt% to 20% by weight of the paint. Binders and other components in the paint may be conventional, such as those described in U.S. Patent 3,840,487 and GB 1,505,555.

The low polydispersity-25 values required in the invention can be achieved by various methods. For example, the polymer can be prepared by a conventional polymerization method, e.g., solution polymerization, to obtain a product having a high polydispersity value (typically 2 or higher) and then subjected to careful fractional precipitation to obtain fractions with a polydispersity of less than 1. 5. In order to carry out the fractional precipitation, the conditions must be such as to achieve this low polydispersity and must therefore include the addition of an isothermal distrust agent, as described in "Polymer Fractionation" in paragraph B1 of this work.

Another way to obtain the desired polymer is to perform its synthesis under conditions that lead to its formation. For example, the polymer can be prepared by performing solution polymerization in the presence of isopropanol as a chain regulator. The process must be carried out under very homogeneous and closely controlled conditions, constantly controlling the monomer and imitator feeds and the constant temperature. If the polydispersity of the obtained product 10 is above the desired value, it must be discarded or treated so as to reduce its polydispersity.

Yet another way to obtain the desired polymer is to prepare an insoluble acrylate polymer having active PD and Mw values, for example as shown in Example 49 of EP 68 887, and then hydrolyze the acrylate to the free acid, for example by reacting it. with sodium hydroxide at 85 ° C for 6 hours or as long as necessary to effect hydrolysis ai-20, the reaction generally being carried out in the presence of methanol as diluent.

The preferred method is the method described above. This can be done for all of the above polymers.

One very common solvent used in solution polymerizations is isopropanol, and mixtures of water and isopropanol are suitable for the purposes of the invention.

In the following examples, Example 1 shows how the desired PD and Mw values can be obtained for the polymer by careful polymerization, and Examples 2 and 3 illustrate how the polymer can be obtained by partial neutralization and fractionation.

Example 1 A 700 cm3 resin pan equipped with a thermometer, a stirrer and an external

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11 82204 la heating system, three different mixtures are continuously passed over a period of 6 hours.

Feed 1 comprised: 340 g of crystalline acrylic acid in 226 g of water Feed 5: 10.5 g (100 butt) of hydrogen peroxide and 57.1 g of water

Feed 3: 28 g of thioglycolic acid and 38.6 g of water

The contents of the pan were maintained at reflux temperature throughout the addition and then for an additional hour before cooling. The percentage of unpolymerized arylic acid was determined by gas-liquid chromatography and appeared to be 0.3% of the amount added. The remaining polymer was completely neutralized by the addition of 46.6% sodium hydroxide and the final product was diluted to give a 40% (w / w) solution of sodium polyacrylate.

The viscosity of the product at 25% was 12.1 cS (suspended level viscometer, No. 2, 25 ° C). GPC analysis showed the following values: Mw = 1740, Mn = 1321, polydispersity = 1.32.

Example 2

A 23% by weight solution of polyacrylic acid in a mixture containing equal parts by weight of isopropanol and hydrogen was prepared by polymerizing acrylic acid in a conventional manner using ammonium persulfate as initiator. Samples of the products were extracted while other samples were neutralized by the addition of various amounts of sodium hydroxide added as a 46% by weight aqueous solution. After the addition of sodium hydroxide, all the samples were allowed to stand long enough for the aqueous phase to separate from the organic phase (possibly containing some water), and these phases were then separated in the usual way. Both phases were then completely neutralized with sodium hydroxide and the remaining alcohol was removed by distillation. The yield of polymer in each rubber phase was recorded. Example 3

A 20% solution (50/50 w / w) of polyacrylic acid having an Mw of 3131 and a PD (polydis-5) of 1.677 in an isopropanol / water mixture was neutralized with various base compounds and the two layers were separated in each case. The amount and molecular weight of the polymer in each layer were determined. The results are shown in Table 1.

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table 1

Aqueous layer Organic layer

Base Neutralization Poly- Mw P.D. poly- Mw P.D.

5% seas seas%%

NaOH 25 75.2 3833 1.30 24.8 1452 1.402 NH4OH 25 55.6 4025 1.30 44.4 1689 1.34

LiOH 25 no difference

LiOH 50 50.2 3957 1.427 49.8 1783 1.44 10 KOH 25 63.5 3649 1.56 36.5 1402 1.49

NaOH 15 20.6 3976 1.49 79.4 2027 1.63

NaOH 50 95.7 3688 1.51 4.3 very ma tala

NaOH 75 99.3 3376 1.53 0.7 very low

Example 4

The products obtained in Example 2 were adjusted to 40% active solids and compared as marble grinding aids to those described in Example 11 of GB Patent No. 1,414,964. The results are shown in Table 2.

Table 2 25 Yield percentage

Neutra- Organic- Water- Water layer loose layer grinding coefficient% __ ros___ 30 0 * 0.25 10 87.2 12.8 15 79.3 20.7 25 23.1 76.9 1.94 50 4.0 96 .0 2.33 35 75 0.7 99.3 1.22 100 0.5 99.5 0.37 i4 82204 * This product is prepared in the usual way by carrying out complete neutralization and then removing the organic phase by distillation, in which case sesas has no fractionation at all.

5 In the described experiment, a grinding index value of about 0.5 is generally satisfactory because it shows commercially acceptable properties to prevent gelation of the marble dispersion.

The table shows that after complete neutralization 10, the polymer is almost completely in the aqueous phase, but that a considerable amount of polymer migrates to the organic phase at lower degrees of neutralization. It is also very significant that the grinding index is greatly improved when the amount of polymer present in the organic phase from the aqueous phase is very low. For example, with a degree of neutralization of 50%, the amount of polymer in the organic phase is low, but the grinding index is about five times that which would be considered commercially acceptable. At higher degrees of neutralization, only a small amount of well-20 polymer enters the organic phase.

Example 5

The polymer was prepared by a conventional polymerization method to a 23% solution of acrylic acid using equal amounts of isopropanol and water, and after polymerization, neutralization with 25% aqueous sodium hydroxide was performed. This caused the reaction mixture to split into two phases. These were separated and the polymer in such a phase was recovered after removal of the isopropanol by distillation. Samples 30 were completely neutralized with sodium hydroxide solution and adjusted to 40% active sodium polyacrylate.

The unfractionated reference polymer was also prepared from the original non-neutralized polymer in an isopropano-35 I / water mixture by removing isopropanol by distillation and completely neutralizing is 82204 with sodium hydroxide and adjusting to 40% based on active sodium polyacrylate.

The products were evaluated as dispersants for titanium-5 oxide, 97% of which were smaller than 2 μm in size, with a sludge solids content of 75% (w / w) by determining the viscosity (cP) of the slurry using 0.6, 0.8 and 1% dry polymer. calculated on the dry pigment. The results are shown in tau-10 lock 3.

Table 3

Mw Mn PD Viscosity-Neutrality (cP) of sludge 15 creation% 0.6 0.8 1.0 100 Comparison 3161 2019 1,565 - 1320 1380 25 Aqueous phase 4236 2795 1,515 - - 2600 25 Organic 1795 1367 1,314 3500 700 340

Example 6 Samples of narrow polydispersed sodium polyacrylates with decreasing molecular weight were evaluated as a dispersant for kaolin at a solids content of 64% (w / w) of slurry 25 at pH 6.8-7.0 and at various dosages as in Example 4. The results are shown in Table 4.

30 35 ie 82204

Table 4

Mw Μη PD Slurry viscosity (cP) 0.10 0.12 0.14 0.16 0.18 0.20 0.22 5543 4817 1.15 - - 1567 359 290 321 376 5 4876 3907 1.25 - 1483 643 285 296 330 405 4447 3592 1.24 - 933 423 255 269 312 361 4053 3273 1.24 - 703 263 225 249 284 - 3202 2709 1.18 - 234 198 211 233 271 - 2144 1901 1.13 533 183 190 207 231 266 - 10 1065 834 1.28 637 226 198 201 213 230 -

The results show that the most effective sodium polyacrylates for kaolin dispersants are those with a molecular weight between 1000 and 3000. A recommended Mole-15 wedge weight is about 2000.

Example 7

Samples of sodium polyacrylate having the same molecular weight but different polydispersity were evaluated as marble grinding aids. The results are flavored as a grinding factor using the same test method as described in Example 11 of GB Patent 1,414,964. The results are shown in Table 5.

Table 5 25

milling

Mw PD factor 3225 1.38 2.54 3105 1.52 1.23 30 3229 1.63 0.29 3358 2.01 <0.20

The results show that the efficiency of sodium polyacrylate in grinding marble is dependent on the polydisperse. The lower the polydispersity, the more effective the product.

Il 82204

Example 8

It can be shown that the ratio between the molecular weight and the polydispersity of the sodium AMPS / sodium acrylate (20/80 w / w) copolymer has a decisive effect when the product is evaluated as a marble grinding aid according to Example 11 of GB Patent No. 1,414,964. The relationship between these parameters is shown in Table 6.

10 Table 6

Mw PD Grinding factor

Low molecular weight, ai- 2351 1.26 2.6 15 hain polydispersity 2630 1.34 2.4

Low molecular weight, 2834 1.6 0.38 kea polydispersity 3151 1.55 0.44

High molecular weight, low 7193 1.40 0.67 polydispersity 4625 1.47 0.81 20 High molecular weight, high 6265 1.64 0.64 polydispersity 5836 1.53 0.77

The results show that low molecular weight polymers with narrow polydispersity are the most effective marble grinding aids. The optimal molecular weight is between 1500 and 3500 and preference is given to polymers with a molecular weight of 2500.

Example 9

Sodium polyacrylate samples having the same molecular weight but varying polydispersity were evaluated in the same manner as in Example 4 for calcium carbonate precipitated as dispersants at a sludge solids content of 70% (w / w). The results are shown in Table 7.

35 is 82204

Table 7

Brookfield viscosity

Mw PD 0.150 0.175 0.200 0.225 0.250 0.275 0.300 5 3226 1.38 117 114 130 177 114 107 104 3105 1.52 120 134 167 178 164 130 127

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Claims (17)

A water-soluble polymeric or dispersant in the form of a water-soluble salt of a polymer, which polymer is formed from one or more ethylenically unsaturated monomers and contains acidic groups selected from carboxylic and sulfonic acid groups, characterized in that the polydispersity of the polymer is less than 1.5 and has a molecular weight, as determined by the sodium salt, of about 1500-6000. Dispersant according to Claim 1, characterized in that it has a polydispersity of 1.05 to 1.4. Dispersant according to Claim 1 or 2, characterized in that it is a polyacrylic acid or a copolymer of acrylic acid and 2-acrylamido-2-methylpropanesulphonic acid. Dispersant according to Claim 1, 2 or 3, characterized in that it is prepared by a process comprising aqueous polymerization of one or more monomers in the presence of isopropanol as a chain regulator. 5. A pigment dispersion in an aqueous medium containing a dispersant for a pigment and wherein the dispersant is a water-soluble polymer or a water-soluble salt thereof formed from one or more ethylenically unsaturated monomers and containing acidic groups selected therefrom, carboxyl groups and carboxyl groups. that the polymer has a polydispersity value of less than 1.5 and a molecular weight, determined as the sodium salt, of about 1500-6000. Dispersion according to Claim 5, characterized in that the polymer used as dispersant has a polydispersity of 1.05 to 1.4 and a molecular weight of less than 4000. 82204 Dispersion according to Claim 5, characterized in that the polymer used as dispersant is polyacrylic acid or a copolymer of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid or a water-soluble salt thereof. Dispersion according to Claim 5 or 6, characterized in that the polymer used as dispersant is polyacrylic acid or a water-soluble salt thereof having a molecular weight, defined as the sodium salt, of less than about 3300. Dispersion according to Claim 8, characterized in that the polymer used as dispersant has a molecular weight of 1800 to 2200. Dispersion according to Claim 8 or 9, characterized in that kaolin, talc, titanium dioxide or precipitated calcium carbonate is used as pigment and has a particle size of 1 to 50 μm. Dispersion according to Claim 10, characterized in that it is a paper coating dispersion in which the concentration of pigment is 50 to 80% by weight. Dispersion according to Claim 5 or 6, characterized in that it is prepared by grinding calcium carbonate in water containing a polymer and wherein the polymer is (a) polyacrylic acid or a water-soluble salt thereof having a molecular weight, determined as the sodium salt, of about 2500 to 4500 , or (b) a copolymer of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid or a salt thereof having a molecular weight, determined as the sodium salt, of about 1500-3500. Dispersion according to Claim 12, characterized in that the amount of calcium carbonate in the dispersion is more than 70% by weight and its particle size is essentially less than 2 μm. Dispersion according to Claim 12 or 13, characterized in that the polymer used as dispersant is polyacrylic acid or a water-soluble salt thereof and has a molecular weight of about 3300 to 3900. Dispersion according to Claim 12 or 13, characterized in that the polymer used as dispersant is a copolymer of non-acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid in a weight ratio of 99: 1 to 50:50 and has a molecular weight of approximately 2250 to 2750. Aqueous dispersion according to Claim 5 or 6, characterized in that it is a dispersion paint which contains a binder for the paint and in which the polymer used as dispersant is polycrylic acid having a molecular weight of about 1500 to 5000, determined as the sodium salt. Dispersion according to Claim 16, characterized in that titanium dioxide, kaolin or calcium carbonate is used as pigment and has a particle size of 0.2 to 25 μm. 22 82204