Aluminium oxide dispersion

ES 2 363 793 T3

DESCRIPTION

Aluminum oxide dispersion

The invention relates to a stable aqueous aluminum oxide dispersion having a high fill factor, a process for the production thereof and the use thereof.

From the European patent application document EP-A-1258458, an aqueous dispersion of aluminum oxide produced by pyrogenic means is known, with a specific surface area according to BET of 100 +/- 15 m²/ g. The pH of the dispersion can be varied in a range between 2 and 8 by adding acids or bases, the content of aluminum oxide can be approximately 30 +/- 20% by weight. From the international patent application document WO 03/035552, an aqueous dispersion of aluminum oxide produced by pyrogenic means with a specific surface area according to BET of more than 115 μm is known.²/ g and a Sears number of more than 8 ml / 2g.

A disadvantage with such dispersions is their low stability against sedimentation and reagglomeration. These dispersions reach their maximum stability with an aluminum oxide content of less than 30% by weight, in which, however, markedly acidic pH values ​​of <5 must be present.

In certain fields of use, for example in the production of varnishes, this region of pH values ​​is generally undesirable, since recipes are often normalized in a region of pH values ​​located around the neutral point. However, in this region of pH values ​​a dispersion of ALOs is no longer stable, since the zeta potential is low (isoelectric point at a pH of about 10).

Furthermore, other particles dispersed in aqueous paint and varnish recipes often have a negative surface charge. Such negatively charged particles would immediately coagulate with the still positively charged aluminum oxide particles and render the dispersion unusable.

However, aluminum oxide has interesting properties for applications in the paint and varnish sector, such as high hardness and high refractive index, compared to silicon dioxide.

High fillers with good workability, ie low viscosity, of the dispersion are particularly desirable in the field of scratch resistant varnishes.

In the field of polishing, in particular chemical-mechanical polishing, aluminum oxide is used as an abrasive material. The fact that these particles do not leave scratches on the surface to be polished is of particular importance here. This can be caused, for example, by a few coarse particles in a dispersion.

There was therefore the problem of providing a dispersion which, in a range of pH values ​​from weakly acidic to weakly basic, has a high stability, a high content of solids, a good workability in the form of a low viscosity, and is free of coarse particles. Furthermore, there was the problem of providing a process for the production of this dispersion.

An object of the invention is an aluminum oxide dispersion which is stable in a range of pH values ​​from 5 to 9 and has an aluminum oxide content of at least 40% by weight, which is to be obtained by dispersing one or various aluminum oxide powders with a specific surface area of ​​5 to 200 m²/ g in an aqueous phase, in which one or more hydroxycarboxylic acids at least dibasic that are dissolved in the dispersion, and at least one salt of a di- (alkali metal) hydrogen phosphate and / or of an alkali metal dihydrogen phosphate, in each case in a mutually independent manner, in an amount of 0.3 - 3 x 10^-6 mol / m² specific surface area of ​​aluminum oxide.

The term "stable" in the sense of the invention should be understood to mean stability against sedimentation and reagglomeration for a period of time of 1 month, generally at least 6 months.

Preferably, the aluminum oxide dispersion according to the invention may contain an aluminum oxide powder of pyrogenic origin. Here the concept of "pyrogenic" should be understood to mean that this aluminum oxide powder is obtained by converting a suitable starting material in a flame. Pyrogenic processes include flame oxidation and flame hydrolysis. For large-scale industrial production of aluminum oxide, the flame hydrolysis of aluminum chloride in a hydrogen-oxygen flame is mainly used. As a general rule, the aluminum oxide particles produced in this way are in the form of conglomerate primary particles, wherein the primary particles are free of pores and carry hydroxyl groups on their surface. In the conversion of aluminum chloride to aluminum oxide, acid 2 is formed

ES 2 363 793 T3 hydrochloride as a by-product, and this adheres to the aluminum oxide particles. Usually a major part of the hydrochloric acid is removed from the particles by steam treatment. An aluminum oxide powder in a 4 percent dispersion in water then generally exhibits a pH value of 3 to 5. Some suitable aluminum oxide powders can be AEROXIDE® Alu C, AEROXIDE® Alu 65, AEROXIDE® Alu 130, all of them from Degussa AG, SpectrAl® 100 Fumed Alumina, SpectrAl® 51 Fumed Alumina, SpectrAl® 81 Fumed Alumina, all them from Cabot Corp ..

Furthermore, it may be advantageous that the aluminum oxide particles in the dispersion according to the invention have a mean diameter of the conglomerates of less than 100 nm.

Furthermore, it may be advantageous if the dispersion according to the invention is free of particles with a diameter of more than 1 pm.

Preferably, the content of the aluminum oxide can be 40 to 60% by weight. With a high content of solids, the dispersion according to the invention exhibits a low viscosity in this range, as well as a high stability.

Furthermore, the pH of the dispersion according to the invention may preferably be 6 to 8. In this range, the dispersion exhibits a low viscosity at the same time as a high stability of the dispersion.

The zeta potential of the dispersion according to the invention is preferably less than -15 mV. A zeta potential in the range of -25 to -40 mV is particularly preferable. The zeta potential is a measure of the surface charge of the particles, which can be displaced by substances that are deposited on the surface. The zeta potential should be understood to mean the potential along the shear plane within the electrochemical double layer of electrolyte and aluminum oxide particles in the dispersion. An important quantity in connection with the zeta potential is the isoelectric point (IEP). The IEP indicates the pH value at which the zeta potential is zero. With aluminum oxide, the IEP is at a pH of about 9 to 10. The greater the difference between the pH of the dispersion and the IEP, the more stable the dispersion. The zeta potential can be determined, for example, by measuring the colloid vibration current (CVI) of the dispersion or by determining the electrophoretic mobility.

The at least dibasic hydroxycarboxylic acids which are added to the aqueous phase, and which are present dissolved in the dispersion, can preferably be citric acid or tartaric acid.

A further object of the invention is a process for the production of the dispersion according to the invention in which

- one or more hydroxycarboxylic acids at least dibasic, which are present dissolved in the dispersion, and at least one salt of a di- (alkali metal) phosphate and / or an alkali metal dihydrogen phosphate are first placed in water and in a quantity of 0.3 - 3 x 10^-6 mol / m² specific surface area,,

- the aluminum oxide particles, corresponding to the desired quantity in the dispersion, are added all at once, in portions or continuously,

- and are dispersed by an energy input of more than 1,000 KJ / m³.

Appropriate dispersing units can be: planetary kneaders, rotor and stator machines, a stirred ball mill or a roll mill.

A method in which the scattering is performed first with an energy input of less than 1,000 kJ / m has been found to be particularly suitable.³With the formation of a pre-dispersion, the pre-dispersion is divided into at least two partial streams, these partial streams are arranged inside a high-energy mill under a pressure of at least 500 bar, they are released through a nozzle and are allowed to collide with each other in a reaction chamber filled with a gas or with a liquid, and the high energy grinding is optionally repeated once or several times.

A further object of the invention is the use of the dispersion for coating glass, ceramic and metal surfaces and for the production of varnishes.

Examples

Analytical processes:

ES 2 363 793 T3

Viscosity is determined with an MCR300 instrument having a Parr-Physica Co. CC27 measurement system, with which measurements are made at shear rates of 0.01 to 1000 s.^-1 and 23 ° C. Viscosity values ​​at 10 s and 100 s' are reported.

The zeta potential and isoelectric point are determined with an instrument such as DT-1200, from Dispersion Technology Inc., by the CVI procedure. Titration is carried out with a mixture of KOH and HNO3.

The mean particle diameter d50 of the aluminum oxide particles in the dispersion is determined by laser diffraction. The Horiba LA-910 instrument (Horiba, Ltd., Japan) is used. The volume-weighted median value from the peak analysis is reported.

The specific surface area is determined as in DIN 66131.

Examples

Example 1 (according to the invention): 34.7 kg of deionized water is placed in a stainless steel batch container with a capacity of 60 l. Next, 7.0 kg of aErOXIDE® Alu 65 (BET 65 m²/ g), from Degussa Co., by means of the suction tube of the Ystral Conti-TDS 3 apparatus (stator slits: 4 mm ring and 1 mm ring, gap between rotor and stator approximately 1 mm) under conditions shear. In addition, 13.3 kg of a solution of 1.80 kg of anhydrous citric acid, 1.49 kg of disodium hydrogen phosphate dihydrate and 10 kg of water are added and another 65.0 kg of AEROXIDE® are sucked into it. Alu 65. After suction is complete, the suction connector is closed and shearing is continued at 3,000 RPM (revolutions per minute) for an additional 10 minutes. After grinding, 108 g of Acticide® MV, from THOR Co., are added as a preservative. This pre-dispersion is passed in two passes through the Sugino Ultimaizer HJP-25050 high-energy mill at a pressure of 2,500 bar and diamond nozzles with a diameter of 0.3 mm, and in this way it is intensively crushed from additional mode.

The pH value, measured directly after trituration, is 6.0. After about 48 hours, a stable pH value of 7.7 is reached. The solids content of the dispersion is 60% by weight. Figure 1 shows the viscosity in mPas as a function of the shear rate in s^-1. Figure 2 shows the appearance of the aluminum oxide particles as a function of their size in nm. The mean particle size d₅₀ is 84 nm. The zeta potential of the dispersion is -28 mV at a pH of 7.7. The dispersion does not show any signs of gelling even after 6 months.

Example 2 (according to the invention): 41.1 kg of deionized water is placed in a stainless steel batch container with a capacity of 60 l. Next, 5.8 kg of aErOXIDE® Alu C (BET = 100 m²/ g), from Degussa Co., by means of the suction tube of the Ystral Conti-TDS 3 apparatus (stator slits: 4 mm crown and 1 mm crown; gap between rotor and stator approximately 1 mm) under conditions shear. In addition, 9.80 kg of a solution of 1.70 kg of anhydrous citric acid, 1.42 kg of disodium hydrogen phosphate dihydrate and 6.70 kg of water are added and another 28.2 kg of AEROXIDE® are sucked into it. Alu C. After the suction is complete, the suction connector is closed and shearing is continued at 3,000 RPM for an additional 10 minutes. After grinding, 77 g of Acticide® MV, from THOR Co., are added as a preservative agent.This pre-dispersion is passed in two passes through the Sugino Ultimaizer HJP-25050 high-energy mill at a pressure of 2,500 bar and of 0.3 mm diamond nozzles, and is further intensively ground in this way.

The pH value, measured directly after grinding, is 5.8. After about 48 hours, a stable pH value of 7.5 is reached. The solids content of the dispersion is 40% by weight. Figure 3 shows the zeta potential in mV as a function of the pH value in the range 3.5 to 7.5. Figure 4 shows the zeta potential in mV as a function of the pH value in the range of 7 to 10.5. The mean diameter of the particles in the dispersion is 86 nm. Viscosity is approximately 26 mPas at a shear rate of 10 s^-1 and approximately 24 mPas at 100 s^-1. The dispersion does not show any signs of gelling even after 6 months.

Example 3 (Comparison example): 61.0 kg of deionized water is placed in a stainless steel batch container with a capacity of 60 l. Next, 26.6 kg of AEROXIDE® Alu C are sucked into it, through the suction tube of the Ystral Conti-TDS 3 device (stator slits: 4 mm crown and 1 mm crown; gap between rotor and the stator about 1 mm) under shear conditions. In addition, 0.89 kg of a 50 percent aqueous solution of acetic acid is added. After the suction is complete, the suction connector is closed and shearing is continued at 3,000 RPM for an additional 10 minutes. After grinding, 79 g of Acticide® MV (from THOR Co) are added as a preservative. This pre-dispersion is passed in two passes through the Sugino Ultimaizer HJP-25050 high energy mill at

ES 2 363 793 T3 a pressure of 2,500 bar and diamond nozzles with a diameter of 0.3 mm and is thus additionally intensively crushed.

The pH value, measured directly after trituration, is 4.1 and is adjusted to 4.0 with 133 g of a 50 percent aqueous solution of acetic acid. The solids content of the dispersion is 30% by weight. The zeta potential shows positive values ​​in the range of pH values ​​claimed. The mean diameter of the d50 particles is 86 nm. Viscosity is approximately 7 mPas at a shear rate of 10 s^-1 and approximately 7 mPas at 100 s^-1. The dispersion does not show any signs of gelling even after 6 months.

Example 4 (comparison example): 52.0 kg of deionized water and 1.19 kg of citric acid monohydrate are placed in a stainless steel batch container with a capacity of 60 l, and adjusted to a pH of 5, 6 with a 25 percent solution of sodium hydroxide (2.04 kg). Next, 25.5 kg of AEROXIDE® Alu C from Degussa Co. are sucked into it, through the suction tube of the Ystral Conti-TDS 3 apparatus (stator slits: 4 mm crown and 1 mm crown; gap between rotor and stator about 1 mm) under shear conditions. After the suction is complete, the suction connector is closed and shearing is continued at 3,000 RPM for an additional 10 minutes. After trituration, 85 g of Acticide® MV, (from THOR Co.) are added as a preservative. This pre-dispersion is passed in two passes through the Sugino Ultimaizer HJP-25050 high-energy mill at a pressure of 2,500 bar and diamond nozzles with a diameter of 0.3 mm, and in this way it is intensively crushed from additional mode.

The pH value is adjusted to 7.5 with 110 g of a 25 percent aqueous sodium hydroxide solution. Even after 48 hours, the pH was unchanged. The Al2O3 content of the dispersion is 31.5% by weight. The mean size of the d50 particles is 89 nm. Viscosity is approximately 1,245 mPas at a shear rate of 10 s^-1 and approximately 243 mPas at 100 s^-1. The dispersion gels after a few days.