PROCEDURE FOR DISPERSING TALC

Abstract: The talc is dispersed in the presence of at least one hydrophilic mineral substance. For hydrophilic mineral substance is used kaolin, calcium carbonate, gypsum or a mixture of these. For talc is used finely divided, powdery talc or granulated talc having average grain size 5 mu m at maximum. The quantity of talc is between 10-95% by weight of the suspension quantity.

Description: Procedure for dispersing talc The present invention concerns a procedure for dispersing talc. In particular, the present invention concerns a procedure with which granulated, finely divided talc can be advantageously dispersed to become a suspension which fulfills the requirements of paper coating paste, at the same time retaining the advantages of granulated talc regarding its handling, and in which standard blunging apparatus can be used. Granulating finely ground talc is a method commonly used in order to improve the processability of a bulk-type talc product. In the granulation process, the moistened talc is compressed with fairly great force into small non-dusting briquettes. However, the talc particles which have been pressed too close to each other and are mutually suitably oriented become combined by action of a van der Waals force like those in the original talc crystal. The energy threshold of talc is 4.1 kcal/mol (17.2 kJ/ ol) and the critical distance, 3 A (R.F. GIESE, JR: Interlayer Bonding in Talc and Pyrophyllite, 1974). In conventional granulation, and in storage silos, these values are often exceeded, with the consequence that upon granulation the particle size distribution of the suspended talc is coarser than before granulation. In paper coating paste, the pigment must be completely dispersed. Success in this respect implies that the aggregated particles are separated. It is in fact necessary, in suspending finely divided talc, to use especially powerful dispersing agents. The energy re¬ quirements in the coating talc suspending process are twice those in suspending coating kaolin (J-E, TEIRFOLK, INSKO p. 82-80 VI). The viscosity and rheological properties of the suspension to be used in paper coating paste must be stable. No agglomeration or sedimentation must take place, not even during prolonged storage. The object of the invention is to provide a procedure in which the use of expensive, high-power special mixers can be avoided. The aim of the invention is achieved by a method which is mainly characterized in that the talc is suspended in the presence of at least one hydrophilic mineral substance. The other characteristic features of the procedure of the invention are stated in claims 2-9. In the procedure of the invention, the talc may be either finely divided powdery talc or granulated talc. The original grain size of the granulated finely divided talc is 1-5 urn on the average. To the granulating water have been added 0.5-1.5% by weight, of the talc dry weight, e.g. of pH-regulating hydrophilizing and defoaming auxiliary substances, such as e.g. NaOH, polyalkylene glycol and/or its derivatives, and appropriate tensides. To the aqueous suspen¬ sion prepared of hydrophilic mineral material(s) or of mixtures of such, the above-mentioned auxiliary substances are added at 0.1-0.5% by weight, referred to the total quantity of pigments. In view of — maintaining the effectivity of the additives, the temperature must not rise above 50 C in the suspending operation. The hydrophilicity of a mineral is caused by the difference between the surface energy of said mineral and that of the water surface: the smaller this difference, the higher the hydrophilicity of the mineral. 2 The surface energy of normal water is 72 mJ/m " kaolin is 65 " " " " " calcium carbonate is 70 " " " " " gypsum is abt. 70 " " talc is 45 " The differences between the surface energies of the minerals are due to different crystal structures and chemical composition. Talc is by nature strongly hydrophobic. The other minerals mentioned are easily wettable. A talc/water suspension prepared without auxiliary substances is thixotropic, but it becomes dilataπt later as the air escapes from the suspension. The stability of viscosity is poor, and the rheological properties change with the slow wetting of the talc. The viscosity of a suspension with more than 60% dry matter content produced without additives is far too high in view of its use. For hydrophilic mineral substance, the contents of which in the suspension should be 5-90% of the weight of the ultimate suspension, one may use for instance kaolin, calcium carbonate or gypsum suited for paper coating. The effect of the mineral substance in dispersing finely divided, granulated talc is based on the following: - the refining effect with which the particles in the suspension scatter the talc agglomerates; - different surface energy: by interposing themselves in the inters¬ tices of talc particles, they decrease the hydrophobicity of the pigment and inhibit the reagglomeration of talc particles under pressure. In a suspension prepared as described, one obtains the original particle size distribution and a high dry matter content: 60-70%, advantageously 65-68%. Complete wetting of the pigments is achieved, and the handling properties of the suspension are good. The auxili- ary substances that are used are suited for making a coating paste, and the suspension is stable also in prolonged storage. Dispersing as taught by the invention can be carried out in standard blungers, and the processability advantages afforded by the talc granulation can be preserved. Example Finely divided, granulated talc, with average grain size 2.2 urn, and to the granulating water of which was added polyalkylene glycol (Pluriol PE 6400) at 0.8% of the dry weight of talc, was dispersed in a suspension made of SPS kaolin to the suspension water of which had been added 0.3 parts by weight of a dispersing agent (Polysalz S) , 0.02 parts by weight of sodium hydroxide and 0.02 parts by weight of defoaming agent (Nopco 8034), (all percentages referring to the total pigment weight) . The ultimate dry matter content of the suspension was 65%, thereof 50% talc and 50% kaolin. The suspension process took place in a Kady Mill apparatus. On dilution to 63% by weight, the sus¬ pension had a viscosity of 100 mPas/50 r/min. The viscosity and rheological properties did not change during storage for 24 hours. The suspension was used to coat LWC paper, with good results. Example 2 Finely divided, granulated talc with average grain size 2.2 um, and to the granulating water of which was added polyalkylene glycol (Pluriol PE 6400) at 1.2% of the dry weight of talc, was dispersed in a suspension prepared of SPS kaolin and to the suspension water of which had been added 0.3 parts by weight of a dispersing agent (Polysalz S) , 0.02 parts by weight of sodium hydroxide (all these referred to the total pigment weight) . The ultimate dry matter con¬ tent of the suspension was 60.0%, thereof 50% talc and 50% kaolin. The suspension process took place in the laboratory with a Diaf apparatus. The viscosity of the suspension was 75 mPas/20 r/min. The viscosity and rheological properties did not change in 24-hr storage. When merely finely divided talc was suspended to 60% suspension density, the required suspending time was doubled. The suspension had viscosity 80 mPas/20 r/min. The viscosity and the rheological properties changed substantially during storage for 24 hrs. The sus¬ pension was unstable. Example 3 Fine-grained granulated talc with 2.5 μm average grain size was dispersed in a suspension prepared of calcium carbonate with 1.7 urn average grain size, to the suspension water of which had been added, among others, 1.2 % polyalkylene glycol (Pluriol PE 6400) at 1.2% by weight, and sodium hydroxide at 0.02% by weight of the talc- dry weight. Dispersion was carried out in the laboratory with a Diaf apparatus. The ultimate dry matter content of the suspension was 60%. The grain size of the dry matter of the suspension with different carbonate/talc proportions was as follows: Talc Carbonate Average grain size Calculated Measured Difference 95% 5% 2.47 2.30 7,4% 90% 10% 2.42 2.20 10% 70% 30% 2.26 2.10 7,6% 50% 50% 2.10 1.90 10,5% The grain sizes were measured with a Sedigraph apparatus. In the experiments, nearly 10% smaller grain sizes were achieved than might be expected, calculating from the original grain size of the pigments. This is believed to be due to the scattering of the talc agglomerates. Example 4 Fine-grained granulated talc with average grain size 2.2 urn was dispersed with a Diaf apparatus in a suspension prepared of gypsum (CoCoat ) meant for paper coating, to the suspension water of which was added polyethylene glycol (Pluriol 6400) 0.8% by weight and sodium hydroxide 0.02% by weight, of the weight of talc. The ultimate solid matter content of the suspension was about 60%. With mixing proportions 5-30% gypsum 95-70% talc, it was easy to obtain a well dispersed stable suspension in which no sedimentation took place, nor any substantial change of viscosity, during storage for two days. Example 5 Finely divided granulated talc with average grain size 2.2 um and to the granulating water of which was added polyalkylene glycol (Pluriol Pe 6400) at 0.8% of the talc dry weight, was dispersed in a suspension prepared of SPS kaolin, to the suspension water of which had been added 0.3 parts by weight of dispersing agent (Polysalz S) , 0.02 parts by weight of sodium hydroxide (all referred to the total pigment weight) . The ultimate dry matter content of the suspension was about 64% and it contained 70-95% talc and 30-5% kaolin. Suspension was carried out in the laboratory with a Diaf apparatus. The viscosity and rheological properties did not change at storage for 24 hrs. The viscosity at different mixing proportions was: ale Kaolin Dry matter, % Viscosity, mPas/20 r/min 95 5 63.9 450 90 10 64.1 550 70 30 63.9 420 When only finely divided talc was suspended to 64% dry matter content, the viscosity of the suspension was about double the above-mentioned values.

Claims: Claims 1. A procedure for dispersing talc, characterized in that the talc is suspended in the presence of at least one hydrophilic mineral 5 substance. 2. Procedure according to claim 1, characterized in that for hydro¬ philic mineral substance is used kaolin. 10 3. Procedure according to claim 1, characterized in that for hydro¬ philic mineral substance is used calcium carbonate. 4. Procedure according to claim 1, characterized in that for hydro¬ philic mineral substance is used gypsum. 15 5. Procedure according to any one of claims 1-4, characterized in that for talc is used finely divided, powdery talc. 6. Procedure according to any one of claims 1-4, characterized in 20 that for talc is used granulated talc. 7. Procedure according to any one of claims 1-6, characterized in that the average grain size of -the talc is 5 zim at maximum. 25 8. Procedure according to any one of claims 1-7, characterized in that the quantity of talc is between 10-95% by weight, advantage¬ ously 10-60% by weight, of the quantity of suspension. 9. Procedure according to any one of claims 1-8, characterized in 30 that to the suspension and/or to the granulating water have been f added pH-regulating, hydrophilizing and defoaming auxiliary sub¬ stances such as e.g. NaOH, polyalkylene glycol and/or its deriva¬ tives and tensides. 35

Inventors: ESKELINEN EELIS, KAILA MIKKO, TYNI MATTI, ESKELINEN, EELIS, KAILA, MIKKO, TYNI, MATTI
Applicants: MYLLYKOSKI OY
Application year: 1985
Main classification: B01F3
IPC codes: D21H19, D21H21