REDUCING HYGROSCOPICITY OF A MINERAL MATERIAL

Abstract

The invention pertains to the field of materials based on calcium carbonate and in particular to the use thereof as a filler in polymeric plastics. The invention provides a method for reducing the hygroscopicity of a material (M) comprising calcium carbonate by treatment with at least one homopolymer grinding assistant (P) which is neutralized in a particular way. The invention pertains also to a method for producing said material (M), which is then of reduced hygroscopicity.

Claims

1. A method for reducing hygroscopicity of a material (M) comprising calcium carbonate, the method comprising: treating the material (M) with at least one homopolymer-based grinding aid (P) prepared by polymerisation reaction of a compound selected from the group consisting of acrylic acid, an acrylic acid salt, methacrylic acid and a methacrylic acid salt, wherein the at least one homopolymer (P) has a molecular mass M.sub.w (measured by SEC) ranging from 3,000 to 5,000 g/mol, a polymolecularity index (.sub.PI) ranging from 1.5 to 2.9, and carboxylic acid groups partially or totally neutralised by: 70 mol % of Na.sup.+ and from 10 to 30 mol % of at least one cation selected from the group consisting of Na.sup.+, K.sup.+, and Li.sup.+.

2. The method according to claim 1, comprising: A. preparing an aqueous suspension of the material (M) comprising: A1. water, A2. at least one material (M), A3. at least one homopolymer (P), B. wet grinding the material (M) in the aqueous suspension, and C. partially separating the water from the aqueous suspension comprising the ground material (M).

3. The method according to claim 2, also comprising: D. drying the aqueous suspension of the ground material (M) and optionally E. treating the ground and dried material (M) with at least one hydrophobising agent.

4. The method according to claim 2, wherein: the ground material (M) has a median particle diameter d.sub.50 by weight (determined by settling) of from 0.1 to 3 μm; the ground material (M) has a specific surface area, measured using the BET method in accordance with standard ISO 9277 (2010), ranging from 1 to 50 m.sup.2/g; or the material (M) is one selected from the group consisting of marble, limestone, chalk, dolomite, mixtures and combinations thereof or is of natural origin or synthetic origin mixtures thereof and combinations thereof.

5. The method according to claim 2, wherein the aqueous suspension in step (A) comprises from 40 to 78% by weight of material (M).

6. The method according to claim 2, wherein the aqueous suspension obtained after separation (C) comprises at least 74% by weight of ground material (M) relative to an amount by weight of suspension.

7. The method according to claim 1, wherein the carboxylic acid groups of the homopolymer (P) are partially neutralised.

8. The method according to claim 2, wherein the wet grinding (B) is carried out in the presence of from 0.05 to 0.5% by weight of homopolymer (P) relative to an amount by weight of material (M).

9. The method according to claim 2, wherein the partial separation (C) of the water from the aqueous suspension is carried out using at least one means selected from the group consisting of a mechanical means and a thermal means optionally under vacuum or optionally combined with a means of suction.

10. The method according to claim 3, wherein the ground material (M) is dried by at least one means selected from the group consisting of a spray-drying means, a lyophilisation means, a spraying means, a rotary kiln, a heating strip, a means of air-pulsed heating, a fluidised bed, a means of fluidised lyophilisation and a means of nozzle-drying.

11. The method according to claim 3, wherein the hydrophobising agent is selected from the group consisting of: a carboxylic acid; a succinic acid anhydride derivative mono-substituted by a straight, branched or cyclic C.sub.2-C.sub.30-alkyl group, and a reaction product of such a derivative; a mixture of phosphoric acid monoesters, phosphoric acid diesters or their reaction products; a polyhydrogenosiloxane, an inert silicone compound, and mixtures thereof; a C.sub.6-C.sub.14-aliphatic aldehyde.

12. The method according to claim 3, wherein the ground and dried material (M) comprises less than 0.5% by weight of moisture relative to a total amount of material (M).

13. The method according to claim 1, wherein the material (M) that has been ground, dried and treated, optionally with at least one hydrophobising agent, has a hygroscopicity less than or equal to 0.26 mg/m.sup.2.

Description

EXAMPLES

Methods for Measuring the Properties

Measurement of the Weight-Average Molecular Mass (Mw) and of the Polymolecularity (PI) of the Polymers According to the Invention and Comparative Polymers

[0059] According to the invention, the molecular weight of the copolymers is determined by Size Exclusion Chromatography (SEC), a.k.a. “Gel Permeation Chromatography” (GPC). This technique uses a Waters liquid chromatography apparatus equipped with a detector. This detector is a Waters refractive index detector. This liquid chromatography appartus is equipped with a size exclusion column in order to separate the various molecular weights of the copolymers studied. The liquid elution phase is an aqueous phase adjusted to pH 9.00 using IN sodium hydroxide containing 0.05 M of NaHCO.sub.3, 0.1 M of NaNO.sub.3, 0.02 M of triethanolamine and 0.03% of NaN.sub.3.

[0060] According to a first step, the copolymer solution is diluted to 0.9% by dry weight in the dissolution solvent of the SEC, which corresponds to the liquid elution phase of the SEC to which is added 0.04% of dimethyl formamide which acts as a flow marker or internal standard. Then it is filtered using a 0.2 μm filter. Then 100 μL are injected into the chromatography appartus (eluent: an aqueous phase adjusted to pH 9.00 by 1N sodium hydroxide containing 0.05 M of NaHCO.sub.3, 0.1 of M NaNO.sub.3, 0.02 M of triethanolamine and 0.03% of NaN.sub.3).

[0061] The liquid chromatography appartus has an isocratic pump (Waters 515) the flow rate of which is set to 0.8 mL/min. The chromatography appartus also comprises an oven which itself comprises the following system of columns in series: a Waters Ultrahydrogel Guard precolumn 6 cm long and 40 mm in inner diameter and a Waters Ultrahydrogel linear column 30 cm long and 7.8 mm in inner diameter. The detection system is comprised of a Waters 410 RI refractive index detector. The oven is heated to 60° C. and the refractometer is heated to 45° C.

[0062] The chromatography appartus is calibrated using powdered sodium polyacrylate standards of different molecular masses certified by the supplier: Polymer Standards Service or American Polymers Standards Corporation (molecular mass ranging from 900 to 2.25×10.sup.6 g/mol and polymolecularity index ranging from 1.4 to 1.8).

Measurement of the pH

[0063] The pH is measured at 25° C. using a Mettler Toledo Seven Easy pH meter and a Mettler Toledo InLab Expert Pro pH electrode. A three-point calibration (according to the segment method) of the appartus was first carried out using commercially-available buffer solutions with pH values of 4.7 and 10 at 25° C. (Aldrich). The pH values reported were the final values detected by the appartus (the signal differs by less than 0.1 mV from the average over the last 6 seconds).

Measurement of the Conductivity of the Suspensions of Mineral Material

[0064] The conductivity of the suspensions is measured at 25° C. with a Mettler Toledo Seven Multi appartus equipped with a corresponding Mettler Toledo extension and a Mettler Toledo InLab 731 conductivity probe, directly after preparation of the suspension under stirring at 1,500 rpm with a Pendraulik tooth disc stirrer. The appartus was calibrated within the appropriate conductivity range using commercial solutions (Mettler Toledo) for conductivity calibration. The influence of the temperature on the conductivity is automatically corrected by the linear correction mode. The conductivities measured are reported for the reference temperature of 20° C. The reported conductivity values are the end point values detected by the appartus, when the conductivity measured differs by less than 0.4% from the average conductivity for the previous 6 seconds.

Particle Size Distribution and Median Diameter by Weight of the Particles of Mineral Material

[0065] The particle size distribution (% by mass of particles with a diameter below a fixed value) and the median diameter by weight (d.sub.50) of the particles of material were determined using a known settling method. This is an analysis of the settling behaviour in a gravimetric field. The measurement was carried out at 25° C. using a Sedigraph 5100 appartus. The measurement was carried out in an aqueous solution at 0.1% by weight of Na.sub.4P.sub.2O.sub.7. The sample particles of mineral material are dispersed using a high-speed, ultrasound stirrer.

Measurement of the Viscosity of the Suspensions of Mineral Material

[0066] The Brookfield viscosity was measured at 25° C. and at a rotation speed of 100 rpm, using a Brookfield RVT viscometer equipped with an appropriate disk shaft 2, 3 or 4.

Measurement of the Specific Surface Area of the Ground Mineral Material

[0067] The specific surface area (m.sup.2/g) was determined using the BET method (with nitrogen as adsorbent gas) in accordance with standard ISO 9277 (2010). The total surface area (m.sup.2) of the filler material was then obtained by multiplying the surface area and the mass (g) of the corresponding sample.

Measurement of the Solids Content of the Suspensions of Mineral Material

[0068] The solids content (% by weight) is determined by dividing the mass of the solid material by the total mass of the aqueous suspension. The solids content of the solid material is determined by weighing the amount of solid material after evaporation of the aqueous phase of the suspension and then drying the material obtained until a constant mass is obtained.

Measurement of the Hygroscopicity of the Material (M) Obtained According to the Invention or the Comparative Material

[0069] After exposure to an atmosphere of 10% relative humidity for 2.5 hours and at a temperature of 23±2° C., the mass of a sample of material (M) obtained according to the invention is measured using a GraviTest 6300 appartus (Gintronic).

[0070] Then, the atmospheric humidity is increased to 85% relative humidity and the sample of material (M) is exposed to this atmosphere for 2.5 hours and at a temperature of 23±2° C. The mass of the material sample (M) is then measured again.

[0071] The mass change (mg/g of sample) is then calculated to determine the hygroscopicity of the material (M).

[0072] Similarly, the hygroscopicity of a comparative material (M) obtained using a comparative polymer is determined.

Measurement of the Moisture Content of the Materials (M) Obtained According to the Invention or the Comparative Materials

[0073] A sample of material (M) obtained according to the invention (10 g) is heated to 150° C. in an oven until its mass is constant for 20 minutes. The loss of mass is determined by gravimetry and is expressed in % by weight based on the initial mass of the sample. This mass change makes it possible to determine the moisture content of the sample.

[0074] Similarly, the mass change is determined for a comparative material (M) obtained using a comparative polymer.

Preparation of Homopolymers (P) According to the Invention and of Comparative Polymers

Polymer (P1) According to the Invention

[0075] The following are introduced into a synthesis reactor equipped with a mechanical stirring system and an oil bath heating system: [0076] 654 g of water, [0077] 8 g of sodium hypophosphite monohydrate.

[0078] The medium is heated to 95° C., then the following are added simultaneously and continuously, over 2 hours using 3 pumps: [0079] an aqueous solution consisting of 8.4 g of sodium persulphate and 150 g of water, [0080] an aqueous solution consisting of 71 g of sodium hypophosphite monohydrate and 150 g of water, [0081] 1,017 g of acrylic acid.

[0082] Heating is continued for 1 hour at 95° C.

[0083] The polymer is then neutralised to a pH of 8.4 using sodium hydroxide at 50% by weight in water. The polymer is diluted with water in order to obtain a concentration of 42% by dry weight.

[0084] A polymer is obtained with a molecular mass Mw of 3,630 g/mol and a polymolecularity index of 2.0.

Polymer (P2) According to the Invention

[0085] The following are introduced into a synthesis reactor equipped with a mechanical stirring system and an oil bath heating system: [0086] 654 g of water, [0087] 8 g of sodium hypophosphite monohydrate.

[0088] The medium is heated to 95° C., then the following are added simultaneously and continuously, over 2 hours using 3 pumps: [0089] an aqueous solution consisting of 8.4 g of sodium persulphate and 150 g of water, [0090] an aqueous solution consisting of 71 g of sodium hypophosphite monohydrate and 150 g of water, [0091] 1,017 g of acrylic acid.

[0092] Heating is continued for 1 hour at 95° C.

[0093] The polymer is then treated with sodium hydroxide at 50% by weight in water to neutralise 90 mol % of the carboxylic groups. A polymer solution is obtained with a pH of 5.8.

[0094] The polymer is diluted with water in order to obtain a concentration of 42% by dry weight.

[0095] A polymer is obtained with a molecular mass Mw of 3,630 g/mol and a polymolecularity index of 2.0.

Comparative Copolymer (CP1)

[0096] The following are introduced into a synthesis reactor equipped with a mechanical stirring system and an oil bath heating system: [0097] 241 g of water, [0098] 0.32 g of copper sulphate pentahydrate, [0099] 0.276 g of ferrous sulphate heptahydrate.

[0100] The medium is heated to 95° C., then the following are added simultaneously and continuously, over 2 hours: [0101] an aqueous solution of 3.5 g of DPTTC sodium salt (CAS number 86470-33-2) at 20.9% by weight in water, diluted in 31 g of water, [0102] 35.3 g of hydrogen peroxide 130 V diluted in 9.4 g of water and [0103] 279.9 g of acrylic acid diluted in 31 g of water.

[0104] Heating is continued for 1.5 hours at 95° C.

[0105] A polyacrylic acid solution is obtained with a molecular mass Mw of 6,000 g/mol and a polymolecularity index of 2.6.

[0106] The polyacrylic acid solution is treated with a solution of sodium hydroxide at 50% in water (212 g) and of Ca(OH).sub.2 (42.5g) and water, up to pH 8.7 and a concentration of 38% by dry weight.

Preparation, Treatment and Evaluation of the Properties of Materials (M) According to the Invention and of Comparative Materials

[0107] An aqueous suspension is prepared from a material (M) (natural marble from Avenza, Italy) whose measured particle size distribution is characterised by a d.sub.90 value of 50 μm, a d.sub.50 value of 10 μm and a d.sub.20 value of 2 μm. The solids content of the suspension is approximately 71% by weight.

[0108] Under stirring by means of an Ystral Dispermix mixer, a polymer according to the invention or a comparative polymer is added to the suspension in an amount by dry weight of 1,500 ppm.

[0109] Then, the material (M) present in the suspension is wet ground in the presence of the polymer introduced previously. The grinding is carried out in a 200-litre vertical attrition grinder using zirconium silicate beads with a diameter of from 0.6 to 1.0 mm. The temperature of the suspension on entering the grinder is 20° C. and from 50 to 70° C. when exiting. The grinder settings are adjusted to achieve a particle size distribution of at least 46% of particles less than 2 μm in size. The polymers used and the characteristics of the suspensions are shown in Table 1.

TABLE-US-00001 TABLE 1 Suspension S1 S2 CS1 Polymer P1 P2 CP1 Solid content (% by weight) 71.8 71.7 71.7 Viscosity (mPa.s) 123 88 237 Final particle size (% < 2 μm) 46.2 46.0 46.7 pH 9.5 9.3 9.0

[0110] It can be seen that at very high solids contents (greater than 71%), the viscosities of the suspensions according to the invention are much lower than the viscosity of the comparative suspension that does not comprise a particular polymer selected according to the invention.

[0111] Then, part of the water is separated from the suspensions at a suspension temperature of 95° C. using a thermally-controlled Epcon evaporator. The suspensions are allowed to cool and the solids content, the viscosity at 25° C. and the specific surface area of the ground material are measured. The results are shown in Table 2.

TABLE-US-00002 TABLE 2 Suspension S1 S2 CS1 Polymer P1 P2 CP1 Solid content (% by weight) 74.6 74.9 74.5 Viscosity (mPa.s) 320 143 700 Specific surface area (m.sup.2/g) 4.9 5.1 5.9

[0112] It can again be seen that for even higher solids contents (approximately 74.5%), the suspensions according to the invention have viscosities that are much lower than the viscosity of the comparative suspension that does not comprise a particular polymer selected according to the invention.

[0113] The suspensions are then spray-dried using a Niro (GEA) dryer set at a spraying speed of 16,680 rpm. The temperature of the burner is 400° C. The temperature of the drying tower is 130° C.

[0114] The ground and dried material (M) obtained is then treated with stearic acid as a hydrophobising agent in an amount of 1.2% by weight of stearic acid relative to the dry weight of the material (M), using a Somakon MP-LB mixer (Somakon Verfahrenstechnik). The material (M) is first stirred at 2,000 rpm and heated to 120° C., then the stearic acid is added over a period of 10 minutes while maintaining the heating and stirring. The hygroscopicity is then determined for the various materials (M) according to the invention and for the comparative materials. The results are shown in Table 3.

TABLE-US-00003 TABLE 3 Suspension S1 S2 CS1 Polymer P1 P2 CP1 Hygroscopicity (mg/m.sup.2) 0.24 0.25 0.27

[0115] It can be seen that the use of a particular polymer chosen according to the invention confers improved hygroscopicity on the material (M) after drying and treatment with a hydrophobising agent compared to a ground material (M) in the presence of a comparative polymer.