MX2007001828A - Strontium carbonate dispersion and redispersible powder obtained therefrom - Google Patents

Abstract

The invention discloses dispersions of preferably modified strontium carbonate in organic liquids, e.g. in alcohols, ketones or, in particular, methylene chloride. The dispersions can be used for producing polymers, which have a reduced double refraction or no double refraction, and are thus suited for optical applications. The powder obtained by removing the organic liquid can be surprisingly converted into a dispersion with a low amount of energy.

Description

DISPERSION OF STRONTIUM CARBONATE AND REDIS ERSABLE POWDER OBTAINED FROM THE SAME FIELD OF THE INVENTION The invention relates to dispersions of strontium carbonate in organic liquids, a powder obtainable from dispersions and their use as additives in polymers with reduced or no birefringence. BACKGROUND OF THE INVENTION As described by A. Tagaya, H. Ohkita, M. Mukoh, R. Sakaguchi and Y. Koike in Science, vol. 301 (2003), pages 812 to 815, the optical polymers can be used for optical purposes as lenses or as functional films for liquid crystal displays. The corresponding polymers also exhibit a tendency to birefringence formation due to the orientation of the polymer chains during processing; With this an anisotropic material is formed. With this the optical properties worsen. As it is also said in the cited Science publication, different proposals have already been made to counteract birefringence. For example, the polymer mixture must undergo a copolymerization with a random orientation or through the introduction of anisotropic molecules lead to objects without birefringence. Especially the polymer mixture is a known method. However, it is difficult to mix sufficiently homogeneous polymers. Tagaya et al, proposes to counteract birefringence by introducing crystallized inorganic materials in the form of chips. Strontium carbonate was used, which is used in the form of crystals, which are chip-shaped and have a length of 200 nm and a thickness of 20 nm. These crystals are surface treated with bis- (dioctylpyrophosphate) oxyacetate titanate and in amounts of 0.3 to 1% by weight relative to the total weight, in a solution of poly [methyl methacrylate [MA) -benzyl methacrylate [ BzMA)] in tetrahydrofuran. The resulting mixture is spread in the form of films and the optical properties are tested. SUMMARY OF THE INVENTION The task of the present invention is to provide a manufactured strontium carbonate, which can be used for the use of optical polymers and can be easily mixed. This task is solved with dispersions according to the invention and the redispersible powder according to the invention containing these dispersions. One aspect of the invention encompasses strontium carbonate particles in the form of chips with a length of maximum 1000 nm, preferably maximum 500 nm, especially maximum 200 nm, dispersed in an organic liquid or water, where the dispersion of the particles in the organic liquid is carried out using a dispersant. Here the limits of at least 90% of all particles are applied, preferably at least 95%, especially in essential for all particles. The "splinter" concept means that the length of the particle is much greater than its thickness. Preferably the ratio of length to thickness is at least 2. The preferred continuous phase is organic liquids. Organic liquids, protic and aprotic, can be used as organic liquids. Liquids with protons that are so acidic that they react with strontium carbonate to form CO2 are less suitable. Very useful as organic liquids are linear or branched alcohols, in particular linear or branched ketones with from 1 to 6 carbon atoms, for example cyclic ketones with from 3 to 10 carbon atoms, such as cyclopentanone, hydrocarbons or mixtures of benzine hydrocarbons with limited boiling point and halogenated hydrocarbons, for example chlorinated hydrocarbons such as dichloromethane (methylene chloride). Esters of carboxylic acids, for example acids, can also be used carboxylic acids with, for example, in total 2 to 6 carbon atoms and alcohols with 1 to 4 carbon atoms. It is assumed that also linear aliphatic ethers or cyclic ethers with preferably up to 6 carbon atoms are also suitable as solvents. Strontium carbonate can be produced according to known methods for example of strontium hydroxide and CO2. Such a method is described in WO97 / 15530. An aqueous solution of strontium hydroxide is brought into contact with CO2 gas in a reactor act the thrust forces, tear and friction on the reaction mixture. Here, if desired, an anti-crystallization agent may be present during the precipitation. An unmodified strontium carbonate can certainly be used in the present invention, but preferably a strontium carbonate is used, which contains an anti-crystallization agent (crystallization inhibitor). It may be advantageous if at least a part of the crystallization inhibitor is deprotonated, for example when the crystallization inhibitor is used at least partially or completely as the alkali metal salt, for example as a sodium salt or as an ammonium salt. Of course, the inhibitor can also be used in the form of acids and a corresponding amount of base or bleach.

As the crystallization inhibitor, for example, a compound or a salt of the formula (I) with a carbon chain R and n substituents [A (0) OH] wherein R is an organic radical, which is hydrophobic and / or has structures, can be used. partial hydrophilic and in which R is a carbon chain of low molecular weight, oligomeric or polymer, optionally branched and / or cyclic, which optionally contains oxygen, nitrogen, phosphorus or sulfur as heteroatom, and / or is substituted by radicals , which are linked through oxygen, nitrogen, phosphorus or sulfur to the radical R and where AC, P (OH), OP (OH), S (0) or OS (0), and n is 1 to 10000. When is about monomeric or oligomeric compounds, preferably n is 1 to 5. The useful crystallization inhibitors of this type belong hydroxy-substituted carboxylic acid compounds. For example, mono- and dicarboxylic acids with 1 to 20 carbon atoms in the chain (calculated without the carbon atoms of the COO groups) can be used, such as, for example, citric acid, malic acid (2-hydroxy-4, dibutánico), dihydroxysuccinic acid and 2-hydroxylic acid. Very especially, the citric acid and polyacrylate as a crystallization inhibitor. Very useful are phosphonic acid compounds with an alkyl (or alkylene) radical with a chain length of 1 to 10 carbon atoms. Here compounds can be used which have two or more phosphonic acid radicals. They may additionally be substituted by hydroxy groups. For example, 1-hydroxyethylene diphosphonic acid, 1,1-diphosphonopropane-2,3-dicarboxylic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid can be used. These examples show that compounds having both phosphonic acid radicals and carboxylic acid radicals can be used. Also very useful are compounds containing 1 to 5 or even more nitrogen atoms, or several, for example up to 5 carboxylic or phosphonic acid radicals and optionally substituted by means of hydroxy groups. These include, for example, compounds with a basic structure of ethylenediamine or diethylenetriamine and substituents of carboxylic or phosphonic acid. Very useful compounds are, for example, diethylenetriamine pentakis- (methanphosphonic acid), succinic acid, diethylenetriaminepentaacetic acid, N- (2-hydroxyethyl) -ethylenediamine-N, N, N-triacetic acid. Polyamino acids are also very useful, polyaspartic acid example. Sulfur-substituted carboxylic acids with 1 to 20 carbon atoms (calculated without the C atoms of the COO group) and 1 or more COO groups, for example bis-2-ethyl ester of sulfosuccinic acid (dioctyl dioctylsuccinate), are very useful. ). It is of course also possible to use mixtures or additives with other additives such as phosphoric acid. As the crystallization inhibitor, for example, the substances usually used for this purpose can be contained, for example short chain or long chain polyacrylates, usually in the form of the sodium salt; polyester as polyglycol ether, ether sulfonate as lauryl ether sulfonate in the form of sodium salt; phthalic acid ester and its derivatives; polyglycerin ester; amines such as triethanolamine, and stearic acid ester, such as those mentioned in WO 01/92157. Very suitable are also the carboxylic acid salts mentioned in WO 97/15530 or its free acids, for example citric acid or its alkali or ammonium salts. Preferred crystallization inhibitors have at least one anionic group. Preferably the crystallization inhibitor as an anionic group has at least one sulfate group, at least one group sulfonate, at least two phosphate groups, at least two phosphonate groups, at least two carboxylate groups or at least one hydroxy group and at least one carboxylate group. The strontium carbonate used for the dispersions used according to the invention which according to an alternative does not contain any crystallization inhibitor according to a preferred alternative, a dispersing agent is subsequently ground in the present. This can, for example, be carried out in a pearl mill. The dispersing agent is further described below. The dispersing agent must avoid reagglomeration and stabilize the dispersion of strontium carbonate in the solvent. This occurs by means of electrostatic forces, which the dispersant exerts on the surface of the particles, for example by means of negatively charged substituents, which as a consequence of this prevents reagglomeration, or by means of the steric effects of the dispersant. Preferably the dispersant has "one or more anionic groups, which can be present by the reciprocal effect with the surface of the strontium carbonate." The preferred groups are the carboxylate groups, the phosphate groups, the groups phosphonate, the bisphosphonate groups, the sulfate group and the sulfonate group. Some of the aforementioned agents can be used as the dispersant, which in addition to the crystallization inhibiting effect also have a dispersing effect. When using these agents, the crystallization inhibitor and the dispersing agent can be identical. The appropriate means can be determined by means of manual tests. These agents with inhibitory effects of crystallization and dispersants have as a consequence that the strontium carbonate signs agglomerates good redispersables. If that agent is used with a crystallization inhibiting effect and simultaneously a dispersing effect, it can be added during precipitation and subsequent deagglomeration, for example in a bead mill as mentioned above. Usually different compounds with crystallization or dispersing inhibitor effect are used. Very advantageous is the deagglomerated strontium carbonate according to the invention which contains such a dispersing agent, which gives the strontium carbonate particles an electrostatic, spherical or electrostatic surface and etheric which reduces agglomeration and prevents reagglomeration. A deagglomerated strontium carbonate Especially preferred is characterized in that the dispersing agent contains carboxylate, phosphate, phosphonate, bisphosphonate, sulfate or sulfonate group, which can be present by the reciprocal effect with the strontium carbonate surface, and because it has one or more organic radicals R1, which have structures partial hydrophobic and / or hydrophilic. R1 is for example a carbon chain, for example of low molecular weight, oligomeric or polymeric, optionally branched and / or cyclic, optionally containing oxygen, nitrogen, phosphorus or sulfur as heteroatoms, and / or is substituted by radicals, which they are linked through oxygen, nitrogen, phosphorus or sulfur to the radical R1 and the carbon chain is optionally substituted by means of hydrophilic or hydrophobic radicals. An example for those substituted radicals are polyether groups. For example, polyether groups have from 3 to 50, preferably from 3 to 40, in particular from 3 to 30, alkyleneoxy groups. Preferably, the alkylenoxy groups are selected from the groups consisting of the methyleneoxy, ethyleneoxy, propyleneoxy and butyleneoxy groups. A strontium carbonate according to the invention preferably contains a dispersant, which has groups for coupling in or within the polymers. These groups can be groups that produce the chemically coupling, for example OH or NH or NH2 groups. The groups can be those that produce a physical coupling. The dispersing agent advantageously conforms to the solvent in which the strontium carbonate must be dispersed. The dispersant with rather hydrophobic properties is preferably used for the production of dispersions in non-polar or slightly polar solvents. An example of a dispersing agent, which is suitable for the production of strontium carbonate dispersions in non-polar or slightly polar solvents, is the phosphoric acid ester, which has side chains with polyether fractions of ethylene oxide units, by example in which an atom of the group P (0) can be substituted by means of an alkyl or alkylene radical with 3 to 10 carbon atoms and other oxygen atoms of the group (PO) can be substituted by a polyether function. Another acid oxygen atom of the group P (0) can be present by the interaction with the surface of strontium carbonate. Such dispersants can be obtained, for example, from BYK CHE IE under the designation Disperbyk® 102, 106 and 111. Polar to nonpolar solvents were also named earlier. Especially very useful are, for example, linear ketones such as methyl ethyl ketone, carboxylic acid ester with example in total 2 to 6 carbon atoms and alcohols with 1 to 4 carbon atoms, hydrocarbons or their mixtures such as benzine with limited boiling point (boiling point 21 to 55 ° C, 55 to 100 ° C and those with a boiling point higher than 100 ° C), solvent naphtha or halogenated hydrocarbons, especially methylene chloride. Other dispersants produce a good dispersion capacity of strontium carbonate in polar or protic solvents such as water, alcohols such as isopropanol or n-butanol or ketones such as acetone. A polymer containing anionic groups, which can be present by interacting with the strontium carbonate surface, for example the aforementioned groups and are substituted by means of polar groups for example by means of hydroxy or amino groups. Preferably they contain polyether groups, which are substituted at the end by means of hydroxyl groups. As a consequence of this substitution the strontium carbonate particles are extremely hydrophilized. That strontium carbonate according to the invention is easy to disperse and provide stable dispersions in polar solvents. With use, another deagglomeration can be obtained. Polar groups, especially hydroxy and amino groups, represent reactive groups which are suitable for coupling in resins corresponding, for example in epoxy resins. Especially good properties are presented by strontium carbonate, which is coated with a dispersing group, in which it has a plurality of polycarboxylate groups and a plurality of hydroxy groups as well as other substituents, which meet the requirements such as, for example, polyether groups. A most preferred group of dispersing agents are the polyether polycarboxylates substituted in the polyether groups by means of hydroxy groups. This type of strontium carbonate having a crystal growth inhibitor and especially a spherical dispersion agent which prevents reagglomeration, in particular a dispersant substituted by means of polar groups as described above, has a greater advantage in that it is preferred. it includes fine primary particles and in any case little agglomerated secondary particles, which are very useful because they are easily redispersible, for example they can be easily introduced into polymers and do not tend to reagomerate, since they continue to deagglomerate even during use. It has already been mentioned that preferably strontium carbonate is used with a crystallization inhibitor. When the sum of the crystallization inhibitor of strontium carbonate and the agent dispersant (this is calculated without the solvent) is 100% by weight, then the crystallization inhibitor and the dispersant preferably in an amount of 1 to 15% strontium carbonate represents the residue to complete 100%; preferably the strontium carbonate is contained in an amount of 20 to 80% by weight in the sum of crystallization inhibitor, dispersing agent and SrCC > 3. The dispersions according to the invention preferably contain 20 to 70% by weight of the strontium carbonate including the crystallization inhibitor and the dispersing agent and 30 to 80% by weight of a solvent or mixture of solvents. The dispersions may consist of strontium carbonate, dispersing agent and solvents as well as preferably the crystallization inhibitor or may also contain additives. DETAILED DESCRIPTION OF THE INVENTION The production of the dispersions according to the invention is described below. It is based on strontium carbonate which is in the form of crystals in the form of chips. It can, for example, be produced in the manner described in WO 97/15530. The procedure foresees that a solution of Sr (OH) 2 with a concentration of 0.1 to 0.75 mol / 1 of Sr (OH) 2 is mixed with carbon dioxide forming a reaction mixture, used per liter of solution approximately 2 to 30 1 of carbon dioxide gas, the reaction mixture is introduced through a continuously working mixing reactor in which on the reaction mixture act forces of tearing, pushing and friction of the tools that collide between if at a higher relative speed according to the rotor-stator principle, the strontium carbonate formed from the reaction mixture is separated and dried after passing through the reactor. The forces of tearing, pushing and friction in the reactor cause the carbon dioxide to be introduced into the strontium hydroxide solution in an extremely fine distributed form. Here the proportions in liters of the carbon dioxide gas are referred to normal conditions. Preferably during the precipitation a crystallization inhibitor is used as described above. The obtained strontium carbonate is then dispersed in an organic liquid (examples are mentioned above) or water using a dispersant. The dispersants are mentioned above. The dispersion is carried out for such a time until the strontium carbonate particles have a length of maximum 200 nm. The dispersion can be carried out in common apparatuses for grinding the already small particles. For example, pearl mills or Solvents with glass spheres or other hard materials. Premixing is possible, for example, in a dissolver (also without glass spheres). The grinding can be carried out for so long until the particles have the desired size, for example a length below 200 nm or up to below 150 nm. The dispersions described above are suitable for the introduction of strontium carbonate dispersed in plastic materials. Here it is possible for a part of the solvent to be separated from the dispersion originally produced. Then a concentrate of the strontium carbonate is obtained in the solvent, which by means of the addition of solvent can be re-diluted. Other possibilities of use are described below, in particular the production of strontium carbonate redispersible powder by means of solvent removal. The inventors in fact found that the dispersions according to the invention when separating the solvent (preferably organic) produce a strontium carbonate in powder form, which can be redispersed in solvents to form a dispersion, corresponding to the original dispersion, with regard to the finesse of the particles. Dispersion can occur with a comparatively low energy expenditure. Here in no way is it necessary to use the same solvent to redispersion, which was used for the production of the original dispersion, however this may be advantageous. It was unpredictable that after removing the solvent from the originally produced dispersion a powder could be obtained which with the same or with another solvent without a large energy expenditure again produced a fine dispersion of strontium carbonate comparable with the original dispersion. It is advantageous that the storage and transport of a readily redispersible solvent-free powder are naturally easier than the storage and transport of a dispersion. For the redispersion, the solvent is preferably selected, which has already been used for the production of the dispersion or a solvent having the comparable polarity. Manual tests can show if redispersing produces good results. It was mentioned before that the dispersant and the solvent must be determined with respect to each other. The strontium carbonate according to the invention as a redispersible powder or as a dispersion or redispersion is suitable for all purposes for which strontium carbonate or a dispersion thereof is used. In strontium carbonate in the dispersion or powder after redispersion, preferably at least 90% of all particles have a length less than 1000 nm, especially less than 500 nm, very especially preferred less than 300 nm, especially it is preferred to be less than 200 nm. It is especially suitable for producing dispersions which, for example, can be used as a plastic additive described above. The dispersion of the strontium carbonate in the selected solvent, preferably CH 2 Cl 2 or cyclopentanone, optionally occurs by means of dispersing the redispersible powder described above, is mixed with the precursor of the synthetic material or with the synthetic material itself. Here, a distribution that is as homogeneous as possible is desirable. When necessary the synthetic material or the precursor of the synthetic material is dissolved in a solvent or its viscosity is reduced. After work-up, the solvent is evaporated, if one is used, and when necessary, polymerization is started. Particularly advantageously, strontium carbonate is introduced into the aforementioned poly [methyl methacrylate [MMA) -co-benzylmethacrylate [BzMA]]. This is preferably dissolved in tetrahydrofuran. After introducing the dispersion the solvent evaporates. Synthetic materials or their precursors, which can be obtained using the carbonate dispersion of Strontium or redispersible powder are also object of the invention. The precursors are, for example, monomers which are then polymerized in the usual manner, or reagents which, under condensation, can form polymers. The following examples should further illustrate the invention without limiting its scope. Example 1: Production of strontium carbonate with crystalline particles in the form of chips The strontium carbonate is prepared as described in WO 97/15530 from a 10% solution of Sr (OH) 2 by means of reaction with CO2 During the precipitation, citric acid is used in an amount, such that in SrCC > 3 dry precipitate is contained about 1.5% by weight of citric acid. The particles produced are splinter-shaped. SrC03 has a BET surface area of 32 m2 / g. Example 2: Production of a strontium carbonate dispersion produced in Example 1 in methylene chloride Strontium carbonate is mixed with methylene chloride and Disperbyk® 102, a dispersing agent, obtainable from BYK and is a copolymer with acid groups based on a phosphoric acid ester with side chains with polyether fractions of epoxide units. The mixture contains 50% by weight of strontium carbonate and 10% by weight of the dispersing agent; the rest to form 100% by weight of methylene chloride is ethylene chloride. A sample of the mixture was examined before dispersion to determine the particle size; the result is presented in table 1 in the "Zero" row. The mixture was then dispersed in a dissolver with the aid of glass spheres. After 15 and in total 30 minutes the particle size of the samples was examined again. A treatment was also carried out in the dissolver (with the aid of glass spheres) of a mixture of SrC03 and methylene chloride in a weight ratio of 1.1, without dispersant and the resulting particle size is determined. The results are represented together with the zero test in table 1: In the case of the undispersed "zero" sample, 90% of all particles have a diameter of 39.6 μ a and less; only 10% of all the particles had a diameter of 2.61 μ ?? and less. In the case of the treated samples, the diameter is much smaller, 90% of all the particles have a diameter of 192 nm and less. Table 1 also shows that excellent grinding is achieved after 15 minutes. The subsequent treatment of the mixture in the dissolve no longer produces subsequent crushing. The treatment in the dissolver without dispersant produces on the contrary an enlargement of the size of the strontium carbonate particles. Example 3: Dispersion in a bead mill Example 2 was repeated after premixing in a glass-free dissolver in a bead mill, obtaining comparable results. Example 4: Production of a dispersion of strontium carbonate in cyclopentanone Example 2 was repeated, however the dispersing agent Melpers® 0030, which is a polyether polycarboxylate whose ether groups are used, is used. ends are substituted by hydroxy groups and therefore have a hydrophilic character. It can be obtained from the SKW firm. As a solvent, cyclopentanone was used. The dispersed material corresponds to that of Example 2. Example 5: Production of strontium carbonate in redispersible powder form from the methylene chloride dispersion The dispersion is produced as described in example 2 by means of the treatment in the dissolver and later in a pearl mill. The dispersion obtained is dried by removing the methylene chloride, obtaining the strontium carbonate in powder form, which contained the crystallization inhibitor mentioned in example 2 and the dispersing agent mentioned in example 2 (BYK 102). To verify if the powder is redispersible, the methylene chloride was added and redispersed in a dissolver, that is by means of a disk that rotates with a high number of revolution without glass spheres (it was considered unnecessary to carry out the dispersion for example in a pearl mill, because the powder was redispersed easily). It was found that the properties of the dispersion now produced only using the dissolver corresponded to those of the originally produced dispersion of strontium carbonate in methylene chloride. For comparison, SrCo3 was used, which had been precipitated with citric acid. It was immediately conducted to the dissolver and was treated to disperse without glass beads using BYK 102. The investigation showed that the dg0% value was 26.9 μp ?, the d50% value was 6.27 μp? and the value gave% was 194 nm. Example 6: Production of strontium carbonate in redispersible powder form from a cyclopentanone dispersion The dispersion of example 5 was dried by evaporating the solvent under reduced pressure. Strontium carbonate in powder form contains the crystallization inhibitor indicated in Example 2 as well as the dispersing agent given in Example 5. Also for this powder it was found that dispersed in cyclopentanone, it produces a dispersion whose properties correspond to the properties of the dispersion, of which the dust was produced. With this it has also been shown that this powder is redispersible.

Example 7: Production of a synthetic material As described in the aforementioned Tagaya publication, a solution of poly [methyl methacrylate [MA) -benzyl methacrylate [BzMA)] in tetrahydrofuran is produced. In this solution, a dispersion of the redispersible powder produced in Example 6 is produced, which is redispersed in cyclopentanone. Then the solvent evaporates. The amount of the dispersion is selected in such a way that the obtained synthetic material has a content of 0.5% by weight of SrCÜ3.

Claims (15)

1. NOVELTY OF THE INVENTION Having described the invention as above, the content of the following is claimed as property: CLAIMS 1. A dispersion containing particles of strontium carbonate in the form of chips with a maximum length of 1000 nm, preferably maximum 500 nm, in particular maximum 200 nm, determined with the laser reflection method, dispersed in an organic liquid or water, and the dispersion of the particles occurs in the organic liquid or in water using a dispersing agent and at least 90%, preferably at least 95% and especially all particles have the maximum indicated length.
2. 2. The dispersion according to claim 1, characterized in that the dispersion includes an organic liquid, and as organic liquid one or more protic and / or aprotic organic liquids are contained.
3. 3. The dispersion according to claim 2, characterized in that one or more alcohols, ketones are used as the organic liquid. linear such as methylbutyl ketone, cyclic ketones such as cyclopentanone, hydrocarbons or mixtures of aliphatic or aromatic hydrocarbons such as limited boiling benzene or solvent naphtha or halogenated hydrocarbons, for example chlorinated hydrocarbons such as dichloromethane (methylene chloride).
4. 4. The dispersion according to claim 1, characterized in that the strontium carbonate contains a crystallization inhibitor.
5. 5. The dispersion according to claim 1, characterized in that the strontium carbonate is obtained by means of the reaction of aqueous strontium hydroxide and CO2 in the presence of the crystallization inhibitor.
6. The dispersion according to claim 1, characterized in that it has a content of 20 to 70% by weight of strontium carbonate, including a crystallization inhibitor and a dispersing agent, and 30 to 80% by weight of the organic liquid or water .
7. 7. The process for producing a dispersion of claims 1 to 6 which includes strontium carbonate particles in the form of chips with a length of maximum 1000 nm, preferably maximum 500 nm, in particular maximum 200 nm, starting from strontium carbonate which is crystallized in the form of chips, in an organic liquid or water, where the dispersion of the particles is produced in an organic liquid or water used as an agent dispersant.
8. The method according to claim 7, characterized in that the dispersion is carried out by reducing the size of the secondary particles, for example in a bead mill.
9. 9. The process according to claim 7 or 8 characterized in that part of strontium carbonate that has been precipitated in the presence of a crystallization inhibitor.
10. The process according to claim 7, 8 or 9, characterized in that the dispersion is carried out until the strontium carbonate particles have an average particle size below 150 nm.
11. 11. A redispersible powder of strontium carbonate particles in the form of chips with a maximum length of 200 nm, characterized in that the particles are coated with a dispersing agent, and because the powder is obtained from a dispersion according to claim 1 when removing the organic liquid or water.
12. The redispersible powder according to claim 11, characterized in that the strontium carbonate contains a crystallization inhibitor.
13. 13. The redispersible powder according to claim 11 or 12, characterized in that it is obtained from a dispersion of the strontium carbonate in an organic liquid by evaporation of the solvent.
14. The use of the strontium carbonate dispersions according to one of claims 1 to 6 or redispersible powder according to one of claims 11 to 13 as an addition to the polymers in particular to counteract the birefringence properties of the material synthetic.
15. 15. A synthetic material with reduced birefringence characterized in that it is obtained using a dispersion of claims 1 to 6 or a redispersible powder of claims 12 to 14.