Coated pigments for coloring PVC

Abstract

Coated pigments for plastics, as well as processes for production of the pigments, the use thereof for colouring polyvinyl chloride (PVC), processes for colouring PVC, PVC coloured with such coated pigments, and also plastics products including such coated pigments, may include inorganic pigments coated with of at least one of hydroxides of magnesium, hydroxides of calcium, oxides of magnesium, and oxides of calcium.

Claims

1. A coated pigment comprising: inorganic pigment selected from the group consisting of iron oxides, iron oxide hydroxides, zinc ferrites, magnesium ferrites, manganese ferrites, and mixtures thereof; and at least one of hydroxides and oxides of at least one of magnesium and calcium bonded directly to the surface of the core of the inorganic pigment to form coated pigment; wherein, based on the total weight of the coated pigment, the coated pigment comprises: 3.5% to 30% by weight of magnesium and 0.001% to 0.1% by weight of calcium, or 5.5% to 30% by weight of calcium.

2. The coated pigment according to claim 1, wherein the coated pigment comprises: a core consisting of the inorganic pigment; and a coating surrounding at least a portion of the core and consisting of the hydroxides and/or oxides of magnesium and/or calcium.

3. The coated pigment according to claim 1, wherein the inorganic pigment is haematite, and the coated pigment has an isoelectric point of <7.

4. The coated pigment according claim 1, wherein the inorganic pigment is selected from the group consisting of iron oxides, iron oxide hydroxides, zinc ferrites, magnesium ferrites, manganese ferrites, and mixtures thereof, excluding haematite, and the coated pigment has an isoelectric point of 2 to 12.

5. The coated pigment according to claim 1, wherein the coated pigment comprises 3.5% to 25% by weight of magnesium or 5.5% to 25% by weight of calcium, based on the total weight of the coated pigment.

6. The coated pigment according to claim 1, wherein the coated pigment comprises 3.5% to 25% by weight of magnesium based on the total weight of the coated pigment.

7. The coated pigment according to claim 1, wherein: the at least one of hydroxides and oxides of at least one of magnesium and calcium comprises hydroxides of at least one of magnesium and calcium, and the at least one of magnesium and calcium comprise 5% to 25% by weight of the coated pigment; the inorganic pigment forms a core of the coated pigment, the hydroxides and oxides define a surrounding layer around the core, and a surface of the core of the inorganic pigment is bonded directly to the surrounding layer; and wherein, when the inorganic pigment is haematite, the coated pigment has an isoelectric point of <6.8; or when the inorganic pigment is iron oxides other than haemetite, iron oxide hydroxides, zinc ferrites, magnesium ferrites, manganese ferrites, or mixtures thereof, the coated pigment has an isoelectric point of 2 to 12.

8. The coated pigment according to claim 1, wherein the coated pigment comprises 5.5% to 25% by weight, of calcium, based on the total weight of the coated pigment.

9. A plastics product comprising at least one polymer and at least one coated pigment according to claim 1, wherein at least 50% by weight of the polymer is formed from monomer vinyl chloride.

10. The plastics product according to claim 9, wherein at least 80% by weight of the polymer is formed from the monomer vinyl chloride.

11. The plastics product according to claim 9, wherein the polymer is polyvinyl chloride.

12. The plastics product according to claim 11, wherein the polyvinyl chloride is pulverulent polyvinyl chloride.

13. The plastics product according to claim 11, wherein the polyvinyl chloride is rigid polyvinyl chloride.

14. The plastics product according to claim 9, comprising 10% to 90% by weight of the at least one coated pigment.

15. The plastics product according to claim 9, comprising 20% to 70% by weight of the at least one coated pigment.

16. The plastics product according to claim 9, comprising 0.1% to 10% by weight of the at least one coated pigment.

17. The plastics product according to claim 9, comprising 0.5% to 5% by weight of the at least one coated pigment.

18. The plastics product according to claim 9, comprising 0% to 15% by weight of plasticizer, based on the amount of vinyl chloride present in the plastics product.

19. The plastics product according to claim 9, comprising 0% to 12% by weight of plasticizer, based on the amount of vinyl chloride present in the plastics product.

20. A process for producing the coated pigment according to claim 1, the process comprising precipitating the coating onto the inorganic pigment by adding: at least one of magnesium salts and calcium salts; and a precipitant selected from the group of alkali metal hydroxides, alkaline earth metal hydroxides, and mixtures thereof, to an aqueous suspension of the at least one inorganic pigment.

21. The process for producing the coated pigment according to claim 20, wherein the process comprises one of: a) precipitating the coating onto the inorganic pigment by adding the at least one of magnesium salts and calcium salts to the aqueous suspension of the at least one inorganic pigment, followed by adding the precipitant, or b) precipitating the coating onto the inorganic pigment by adding the precipitant to the aqueous suspension of the at least one inorganic pigment, followed by adding the at least one of magnesium salts and calcium salts to the suspension.

22. The process for producing coated pigment according to claim 20, wherein the at least one of magnesium salts and calcium salts, and the precipitant are added in dissolved form.

23. The process for producing coated pigment according to claim 20, additionally comprising one, two, three or four of the steps of: i) isolating the coated pigment, ii) washing the coated pigment, iii) drying the coated pigment, and iv) comminuting the coated pigment.

24. The process for producing coated pigment according to claim 20, wherein the process does not include a calcination step.

25. A process for producing the plastics products according to claim 11, the process comprising at least one of kneading or extruding the polyvinyl chloride with the at least one coated pigment.

26. A process for producing the plastics product according to claim 12, the process comprising mixing the at least one coated pigment and the pulverulent polyvinyl chloride at a temperature just below the melting point of the polyvinyl chloride.

27. A product comprising at least one plastics product according to claim 9.

28. The product according to claim 27, wherein the product is one of window profiles, pipes, floor coverings, insulation material or roofing membranes.

Description

EXAMPLES AND METHODS

(1) I. Description of the Measurement and Test Methods Used

(2) The results of the measurements for the examples are summarized in Table 1.

(3) I.1 Zeta Potential (Isoelectric Point, IEP)

(4) The zeta potential was determined to ISO 13099-2 on a Zetasizer Nano Z from Malvern Instruments GmbH (having a 633 nm HeNe laser).

(5) I.2 DIN pH

(6) The DIN pH of pigments was measured to DIN ISO 787-9.

(7) I.3 Mg and Ca Determination

(8) The magnesium and calcium content of the pigments was measured via optical emission spectrometry after excitation in an inductively coupled plasma (ICP-OES: inductively coupled plasma-optical emission spectrometry) as the content of elements.

(9) I.4 Specific BET Surface Area

(10) The specific BET surface area was measured to DIN ISO 9277.

(11) I.5 Testing of PVC Stability by Means of a Mathis LTE-T Oven from Werner Mathis AG (Mathis Oven Stability)

(12) The test strips required for the testing are produced on a mixing roll mill (Servitec Polymix 150L). For this purpose, 50 g of rigid PVC compound (SorVyl DB 6668 Natur 3/03 PVC Compound from Polymer-Chemic GmbH) and 2 g of the pigment to be tested (4% by weight) are weighed in a polyethylene cup and mixed intimately with a spatula. The mixture is applied to the mixing roll mill and rolled at 175 C. and a friction of 1:1.2 to give a milled sheet of thickness 200 m. With constant turning, the pigment is dispersed for 10 min.

(13) The milled sheet thus produced is removed from the roller. A guillotine is used to prepare the strips of length 50 cm and width 2 cm required for the subsequent test in the Mathis oven, and they are clamped in the material carriers of the Mathis oven.

(14) In the Mathis oven thermal test (LTE-T thermal tester, test cabinet for testing the thermal stability of plastics from Werner Mathis AG. Oberhasli, Zurich, Switzerland), PVC samples are exposed to thermal stress for different periods of time, defined by the discrete advance of a sledge out of the oven. The temperature during the test is 190 C. The material carrier containing the PVC test specimens is moved 75 mm further out of the oven every 5 minutes (oven setting: interval 5 minutes, advance 75 mm), with rapid cooling of the part moved out of the oven to room temperature. As a result, there are directly successive zones each of length 75 mm in the test specimen, each of which has been subjected to thermal stress for 5 minutes longer than the previous zone. The PVC breakdown sets in at different times for different pigments, and causes the test strip to become darker to black from a particular zone. To assess the pigment suitability in terms of the stabilization of PVC, two times are reported in each case, e.g. 35/45, these values stating the number of minutes (min.). The first time, 35 min. in this example, is the residence time in minutes for the corresponding test specimen zone in the Mathis oven where there is still no visible discolouration of the corresponding test specimen zone. The second time, 45 min. in this example, is the residence time in minutes for the corresponding test specimen zone in the Mathis oven where the discolouration of the corresponding test specimen zone is at its maximum for the first time. In general, the corresponding test specimen zones have turned black after the maximum discolouration.

(15) I.6 Testing of PVC Stability by Means of Thermo Haske Rheomix 600p Kneader (Kneader Stability)

(16) A pulverulent PVC mixture consisting of 50% by weight of Vestolit B 7021 Ultra+50% by weight of SorVyl DB 6668 Natur 3/03 is mixed homogeneously with the pulverulent pigment sample to be tested (4% by weight based on 100% PVC composition).

(17) The pigmented PVC mixture is introduced manually into the recording kneader (Thermo Haake Rheomix 600p with R6 roll rotors) which has been preheated to 190 C. and is equipped with a rotary rheometer. The recording program (PolyLab Monitor) is started and records the torque on the roll rotors and the temperature of the sample against time. The pigmented PVC mixture is kneaded at 190 C. and 50 rpm. In order to determine the suitability of the pigment with regard to the stabilization of PVC, the maximum torque on the time axis in minutes at which the highest product temperature was measured is reported. After this maximum torque, the torque declines significantly, since the PVC breaks down after this time and the viscosity of the kneaded product decreases as a result. These values are compared with the corresponding value for the same, albeit uncoated, pigment powder. The longer the period of time until said maximum, the higher the stability of the pigment-coloured PVC.

(18) II: Examples

(19) II.1 Properties of the Inorganic Pigments and Plastics Used

(20) Bayferrox 110 pigment powder from LANXESS Deutschland GmbH: haematite (red iron oxide, -Fe.sub.2O.sub.3) having a BET surface area to DIN ISO 9277 of 13-16 m.sup.2/g.

(21) Bayferrox 330 paste: magnetite (black iron(II,III) oxide, Fe.sub.3O.sub.4) in paste form from LANXESS Deutschland GmbH, from which Bayferrox 330 powder is produced by drying and grinding. The pulverulent pigment has a BET surface area to DIN ISO 9277 of 9-17 m.sup.2/g. Alternatively, it is also possible to mix the pulverulent Bayferrox 330 pigment with water to give a slurry having the appropriate pigment concentration.

(22) Bayferrox 420 paste: goethite (yellow iron oxide hydroxide, -FeOOH) in paste form from LANXESS Deutschland GmbH, from which Bayferrox 420 powder is produced by drying and grinding. The pulverulent pigment has a BET surface area to DIN ISO 9277 of 11-15 m.sup.2/g. Alternatively, it is also possible to mix the pulverulent Bayferrox 420 pigment with water to give a slurry having the appropriate pigment concentration (see examples).

(23) Bayferrox 920 paste: goethite, -FeOOH paste from LANXESS Deutschland GmbH, from which Bayferrox 920 powder is produced by drying and grinding. The pulverulent pigment has a BET surface area to DIN ISO 9277 of 11-15 m.sup.2/g. Alternatively, it is also possible to mix the pulverulent Bayferrox 920 pigment with water to give a slurry having the appropriate pigment concentration (see examples).

(24) Bayferrox 943 paste: lepidocrocite (iron oxide hydroxide, -FeOOH) in paste form from LANXESS Deutschland GmbH, from which Bayferrox 943 powder is produced by drying and grinding. The pulverulent pigment has a BET surface area to DIN ISO 9277 of 16-19 m.sup.2/g. Alternatively, it is also possible to mix the pulverulent Bayferrox 943 pigment with water to give a slurry having the appropriate pigment concentration.

(25) Bayrerrox 3950 pigment powder from LANXESS Deutschland GmbH: zinc ferrite (ZnFe.sub.2O.sub.4) having a BET surface area to DIN ISO 9277 of 5-7 m.sup.2/g.

(26) Bayferrox 645T pigment powder from LANXESS Deutschland GmbH: non-stoichiometric manganese ferrite (having MnO content <9% by weight) having a BET surface area to DIN ISO 9277 of 8-13 m.sup.2/g.

(27) SorVyl DB 6668 Natur 3/03: Rigid PVC compound from Polymerchemie (powder form, stabilized with Ca/Zn, where the content of bis(pentane-2,4-dionato)calcium is less than 1% by weight, with softening point >120 C., flashpoint >190 C., ignition temperature >300 C., density to DIN EN ISO 1183-1 method A, of 1.39 g/cm.sup.3, bulk density to DIN EN ISO 60 of 0.54 g/ml, thermal stability to DIN EN 60811-3-2 of greater than/equal to 25 min.).

(28) VESTOLIT B 7021 Ultra: Mikro-S-PVC homopolymer from Vestolit (powder form, K value to DIN EN ISO 1628-2 of 70, viscosity number to DIN EN ISO 1628-2 of 125 cm.sup.3/g, bulk density to DIN EN ISO 60 of 0.3 g/cm.sup.3, sieve analysisresidue on 0.063 mm sieve to DIN EN ISO 1624 of <1%, water content according to K. Fischer DIN 53 715 of 0.3%, pH of the aqueous extract to DIN EN ISO 1264 of 8, 1.5/s paste viscosity of 1.8 Pa s, 451s paste viscosity of 2.2 Pa s).

(29) II.2 Inventive Examples and Comparative Examples

Example 1

(30) To 5 dm.sup.3 of aqueous suspension of Bayferrox 110 (9.39 mol of -Fe.sub.2O.sub.3) having a pH of about 4.5 were added, at room temperature while stirring, 574 ml of an MgSO.sub.4 solution (2.02 mol/dm.sup.3). Subsequently, 0.25 mol of NaOH per mole of -Fe.sub.2O.sub.3 was added dropwise as a solution while stirring within 30 min. (310 ml with concentration 7.65 mol/dm.sup.3). The suspension was stirred for a further 60 min. The pH of the suspension was 12.

Example 2

(31) To 5 dm.sup.3 of aqueous suspension of Bayferrox 110 (9.39 mol of -Fe.sub.2O.sub.3) having a pH of about 4.5 were added, at room temperature while stirring, 1148 ml of an MgSO.sub.4 solution (2.02 mol/dm.sup.3 as MgO). Subsequently, 0.51 mol of NaOH per mole of -Fe.sub.2O.sub.3 was added dropwise while stirring within 60 min. (625 ml with concentration 7.65 mol/dm.sup.3). The suspension was stirred for a further 60 min. The pH of the suspension was 12.

Example 3

(32) To 5 dm.sup.3 of aqueous suspension of Bayferrox 110 (9.39 mol of -Fe.sub.2O.sub.3) having a pH of about 4.5 were added, at room temperature while stirring, 162 ml of an MgSO.sub.4 solution (2.02 mol/dm.sup.3 as MgO). Subsequently, 0.068 mol of NaOH per mole of -Fe.sub.2O.sub.3 was added dropwise while stirring within 30 min. (84 ml with concentration 7.65 mol/dm.sup.3). The suspension was stirred for a further 60 min. The pH of the suspension was 12.

Example 4

(33) To 2.13 dm.sup.3 of aqueous suspension of Bayferrox 110 (4.01 mol of -Fe.sub.2O.sub.3) having a pH of about 4.5 were added, at room temperature while stirring, 5517 ml of an MgSO.sub.4 solution (1.15 mol/dm.sup.3 as MgO). Subsequently, 3.18 mol of NaOH per mole of -Fe.sub.2O.sub.3 were added dropwise while stirring within 60 min. (with the aid of 7.65 mol/dm.sup.3 solution). The suspension was stirred for a further 60 min. The pH of the suspension was 12.

Example 5

Comparative Example

(34) To 5 dm.sup.3 of aqueous suspension of Bayferrox 110 (9.39 mol of -Fe.sub.2O.sub.3) having a pH of about 4.5 was added dropwise NaOH solution (0.06 mol, 7.65 mol/dm) while stirring within 30 min. The suspension was stirred for a further 60 min. The pH of the suspension was about 10.

Example 6

Comparative Example

(35) To 5 dm.sup.3 of aqueous suspension of Bayferrox 110 (9.39 mol of -Fe.sub.2O.sub.3) having a pH of about 4.5 was added dropwise NaOH solution (0.18 mol, 7.65 mol/dm.sup.3) while stirring within 30 min. The suspension was stirred for a further 60 min. The pH of the suspension was 12.

(36) The products from Examples 1 to 6, after the reaction had ended, were filtered through a suction filter, washed until the conductivity of the filtrate was below 300 S/cm, dried to constant weight in a drying cabinet at 180 C. and ground in a Bauermeister mill (cross-beater mill with a sieve insert, mesh size 1 mm).

Example 7

(37) To 7.7 kg of Bayferrox 420 paste having pigment content 19.4% by weight and having a pH of 6-7 were added, at 60 C. while stirring, 1.12 mol of MgO from an MgSO.sub.3 solution (1.15 mol/dm.sup.3 as MgO). Subsequently, NaOH solution (7.7 mol/dm.sup.3) was added dropwise while stirring within 15 min., until the pH had reached about 10.

Example 8

(38) To 7.7 kg of Bayferrox 420 paste having pigment content 19.4% by weight and having a pH of 6-7 were added, at 60 C. while stirring, 2.24 mol of MgO from an MgSO.sub.4 solution (1.15 mol/dm.sup.3 as MgO). Subsequently, NaOH solution (7.7 mol/dm.sup.3) was added dropwise while stirring within 15 min., until the pH had reached about 10.

(39) The products from Examples 7 and 8, after the reaction had ended, were filtered through a suction filter, washed until the conductivity of the filtrate was below 300 S/cm, dried to constant weight in a drying cabinet at 120 C. and ground in a Bauermeister mill (cross-beater mill with a sieve insert, mesh size 1 mm).

Example 9

(40) To 3.29 kg of Bayferrox 920 paste having 130.8 g/dm.sup.3 of pigment and having a pH of 5-6 were added, at 50 C. while stirring, 1.65 mol of NaOH as a solution (7.87 mol/dm.sup.3). Subsequently, 0.82 mol of Mg from an MgSO.sub.4 solution (2.05 mol/dm.sup.3 as MgO) was added dropwise over the course of 15 min. The pH of the suspension was about 10.

Example 10

(41) To 3.29 kg of Bayferrox 920 paste having 130.8 g/dm.sup.3 of pigment and having a pH of 5-6 was added, at 50 C. while stirring, 0.82 mol of Mg from an MgSO.sub.4 solution (2.05 mol/dm.sup.3 as MgO). Subsequently, 1.65 mol of NaOH (7.87 mol/dm.sup.3) were added dropwise while stirring. The pH of the suspension was about 10.

(42) The products from Examples 9 and 10, after the reaction had ended, were filtered through a suction filter, washed until the conductivity of the filtrate was below 500 S/cm, dried to constant weight in a drying cabinet at 80 C. and ground in a cross-beater mill.

(43) The properties of products 1 to 10 can be found in Tables 1 and 2. Examples 1 to 10 have a calcium content of 0.001% to 0.05% by weight, based on the total weight of the pigment.

Example 11

(44) To 4.7 dm.sup.3 of aqueous suspension of Bayferrox 330 paste (6.48 mol of Fe.sub.3O.sub.4) having a pH of 5.8 were added, at 60 C. while stirring, 866 ml of an MgSO.sub.4 solution (2.58 mol/dm).

(45) Subsequently, 0.86 mol of NaOH per mole of Fe.sub.3O.sub.4 was added dropwise as a solution while stirring within 15 min. (379 g with concentration 7.9 mol/dm.sup.3). The suspension was stirred for a further 60 min. The pH of the suspension was 11.

Example 12

(46) To 2.85 dm.sup.3 of aqueous suspension of Bayferrox 330 paste (3.24 mol of Fe.sub.3O.sub.4) having a pH of 5.8 were added, at 60 C. while stirring, 2856 ml of an MgSO.sub.4 solution (2.58 mol/din). Subsequently, 5.74 mol of NaOH per mole of Fe.sub.3O.sub.4 were added dropwise as a solution while stirring within 15 min. (3000 g with concentration 7.9 mol/dm.sup.3). The suspension was stirred for a further 60 min. The pH of the suspension was 11.

(47) The products from Examples 11 to 12, after the reaction had ended, were filtered through a suction filter, washed until the conductivity of the filtrate was below 300 S/cm, dried to constant weight in a drying cabinet at 95 C. and ground in a Bauermeister mill (cross-beater mill with a sieve insert, mesh size 1 mm).

Example 13

(48) To 5 dm.sup.3 of aqueous suspension of Bayferrox 3950 (6.22 mol of ZnFe.sub.2O.sub.4) having a pH of about 7 were added, at 60 C. while stirring, 1442 ml of an MgSO.sub.4 solution (2.58 mol/din).

(49) Subsequently, 1.5 mol of NaOH per mole of ZnFe.sub.2O.sub.4 were added dropwise as a solution while stirring within 15 min. (1500 g with concentration 7.9 mol/dm.sup.3). The suspension was stirred for a further 60 min. The pH of the suspension was >11.

Example 14

(50) To 2.5 dm.sup.3 of aqueous suspension of Bayferrox@3950 (3.11 mol of ZnFe.sub.2O.sub.4) having a pH of about 7 were added, at 60 C. while stirring, 1443 ml of an MgSO.sub.4 solution (2.58 mol/dm.sup.3).

(51) Subsequently, 3 mol of NaOH per mole of ZnFe.sub.2O.sub.4 were added dropwise as a solution while stirring within 15 min. (1500 g with concentration 7.9 mol/dm.sup.3). The suspension was stirred for a further 60 min. The pH of the suspension was >11.

(52) The products from Examples 13 to 14, after the reaction had ended, were filtered through a suction filter, washed until the conductivity of the filtrate was below 300 S/cm, dried to constant weight in a drying cabinet at 120 C. and ground in a Bauermeister mill (cross-beater mill with a sieve insert, mesh size 1 mm).

Example 15

(53) To 5 dm.sup.3 of aqueous suspension of Bayferrox 645T (1500 g) having a pH of about 6 were added, at 60 C. while stirring, 395 ml of an MgSO.sub.4 solution (2.83 mol/dm.sup.3).

(54) Subsequently, 2.24 mol of NaOH were added dropwise as a solution while stirring within 15 min. (362 g with concentration 7.9 mol/dm.sup.3). The suspension was stirred for a further 60 min. The pH of the suspension was >11.

Example 16

(55) To 5 dm.sup.3 of aqueous suspension of Bayferrox 645T (1500 g) having a pH of about 6 were added, at 60 C. while stirring, 1974 ml of an MgSO.sub.4 solution (2.83 mol/dm.sup.3).

(56) Subsequently, 13.95 mol of NaOH were added dropwise as a solution while stirring within 15 min. (2250 g with concentration 7.9 mol/dm.sup.3). The suspension was stirred for a further 60 min. The pH of the suspension was >11.

(57) The products from Examples 15 to 16, after the reaction had ended, were filtered through a suction filter, washed until the conductivity of the filtrate was below 300 S/cm, dried to constant weight in a drying cabinet at 120 C. and ground in a Bauermeister mill (cross-beater mill with a sieve insert, mesh size 1 mm).

Example 17

(58) To 8.01 dm.sup.3 of aqueous suspension of Bayferrox 943 paste (16.88 mol of FeOOH) having a pH of about 6 were added, at room temperature while stirring, 900 ml of an MgSO.sub.4 solution (2.48 mol/dm.sup.3).

(59) Subsequently, 0.26 mol of NaOH per mole of FeOOH was added dropwise as a solution while stirring within 15 min. (724 g with concentration 7.9 mol/dm.sup.3). The suspension was stirred for a further 60 min. The pH of the suspension was >11.

Example 18

(60) To 8.01 dm.sup.3 of aqueous suspension of Bayferrox 943 paste (16.88 mol of FeOOH) having a pH of about 6 were added, at room temperature while stirring, 5455 ml of an MgSO.sub.4 solution (2.73 mol/dm.sup.3).

(61) Subsequently, 1.76 mol of NaOH per mole of FeOOH were added dropwise as a solution while stirring within 15 min. (4800 g with concentration 7.9 mol/dm.sup.3). The suspension was stirred for a further 60 min. The pH of the suspension was >13.

(62) The products from Examples 17 to 18, after the reaction had ended, were filtered through a suction filter, washed until the conductivity of the filtrate was below 300 S/cm, dried to constant weight in a drying cabinet at 120 C. and ground in a Bauermeister mill (cross-beater mill with a sieve insert, mesh size 1 mm).

Example 19

(63) To 3.3 dm.sup.3 of aqueous suspension of Bayferrox 110 (6.26 mol of Fe.sub.2O.sub.3) having a pH of about 4.5 were added, at 60 C. while stirring, 6476 ml of an MgSO.sub.4 solution (2.6 mol/dm.sup.3).

(64) Subsequently, 5.39 mol of NaOH per mole of Fe.sub.2O.sub.3 were added dropwise as a solution while stirring within 15 min. (5440 g with concentration 7.9 mol/dm.sup.3). The suspension was stirred for a further 60 min. The pH of the suspension was >13.

(65) The product from Example 19, after the reaction had ended, was filtered through a suction filter, washed until the conductivity of the filtrate was below 300 S/cm, dried to constant weight in a drying cabinet at 180 C. and ground in a Bauermeister mill (cross-beater mill with a sieve insert, mesh size 1 mm).

(66) The properties of products 11 to 19 can be found in Table 2. Examples 11 to 19 have a calcium content of 0.001% to 0.1% by weight, based on the total weight of the pigment.

(67) TABLE-US-00001 TABLE 1 Kneader Mg content BET surface area stability Mathis oven Example (% by wt.) IEP DIN pH (m.sup.2/g) (min.) stability (min.) .sup.c) Bayferrox 110 .sup.a) 0.0055 2.8 5.3 14.8 12 40/50 1 1.6 3.0 9.9 30.9 14 50/>50 2 2.2 3.1 9.6 33.4 16 50/>50 3 0.3 3.6 9.2 19.1 13 40/50 4 16.9 4.3 10.2 86.3 17 50/>50 5 not determined 2.2 6.6 14.2 12 40/50 6 not determined 2.3 8.9 13.6 12 40/50 Bayferrox 420 .sup.b) 0.0051 8.1 6.9 12.5 11 30/40 7 1.6 5.8 10.1 14.9 15 35/45 8 2.9 5.9 10.1 16.8 16 35/45 Bayferrox 920 .sup.c) 0.007 7.1 4.9 13.5 11 30/40 9 4.7 9.2 10.2 20.2 17 40/50 10 4.9 9.2 10.2 18.5 16 40/50 .sup.a) uncoated pigment as direct comparison with Examples 1 to 6, calcium content: 0.0121% by weight .sup.b) uncoated pigment as direct comparison with Examples 7 to 8, calcium content: 0.0236% by weight .sup.c) uncoated pigment as direct comparison with Examples 9 to 10, calcium content: less than 0.0300% by weight d): for evaluation see test methods

(68) TABLE-US-00002 TABLE 2 Kneader Mg content BET surface area stability Mathis oven Example (% by wt.) IEP DIN pH (m.sup.2/g) (min.) stability (min.).sup.f) Bayferrox 330 .sup.a) 0.2 5.4 8.8 15.9 12 not determined 11 3.3 6.5 10.1 28.4 16 not determined 12 16 >9.8 10.2 84.1 18 not determined Bayferrox 3950 .sup.b) 0.01 6.1 8.0 5.4 12 not determined 13 5.8 8.3 10.4 24.1 18 not determined 14 9.0 9.2 10.4 28.1 19 not determined Bayferrox 645T .sup.c) 0.04 2.8 6.6 8.8 13 not determined 15 1.8 3.1 10.1 18.7 15 not determined 16 11.5 3.1 10.2 60.2 18 not determined Bayferrox 943 .sup.d) <0.01 8.6 4.1 17.2 12 not determined 17 3.9 8.6 10.1 23.3 15 not determined 18 17 10.7 10.1 49.2 16 not determined 19.sup.e) 23 3.6 10.3 100.0 19 50/>50 .sup.a) uncoated pigment as direct comparison with Examples 11 to 12, calcium content: less than 0.099% by weight .sup.b) uncoated pigment as direct comparison with Examples 13 to 14, calcium content: less than 0.0320% by weight .sup.c) uncoated pigment as direct comparison with Examples 15 to 16, calcium content: less than 0.0400% by weight .sup.d) uncoated pigment as direct comparison with Examples 17 to 18, calcium content: less than 0.0360% by weight .sup.e)for uncoated pigment as direct comparison see Bayferrox 110 from Examples 1-6 .sup.f)for evaluation see test methods