RU2766905C1 - Finely dispersed precipitated aluminum hydroxide and method for its production - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to production of highly dispersed precipitated aluminum hydroxide used as fire retardant and filler. Highly dispersed precipitated aluminum hydroxide is presented in the form of rounded particles in an amount of not less than 90 %, note here that D50 particle size makes not over 10 mcm while D99/D10 particle size makes 7–12. When producing highly dispersed precipitated aluminum hydroxide, alkaline aluminate and neutralizing solutions are mixed. Neutralizing solution used is an aqueous solution containing bicarbonates and carbonates of alkali metals. Neutralization is carried out until residual content of Na₂O_ku in the mixed solution is 7–15 g/l. Pulp formed after mixing the solutions is stirred for at least 30–240 minutes under the effect of a liquid phase shear rate of 10–210 s^-1, after which the precipitate is thickened, filtered and washed. Obtained aluminum hydroxide can be modified by at least one surface-treating agent and/or a binding silane agent.

EFFECT: considerable reduction of polymer melt viscosity and filler abrasiveness, minimized chalking of compound surfaces.

6 cl, 4 dwg, 3 tbl, 8 ex

Description

Technical field

The invention relates to the production of highly dispersed precipitated aluminum hydroxide used as a flame retardant and filler.

State of the art

A method is known for producing finely dispersed aluminum trihydroxide (more commonly referred to simply as hydroxide) aluminum (patent US 9038931, publ. 02/23/2012), in which preferably wet aluminum trihydroxide with an average size of 50-150 microns is fed into the drying-grinding apparatus and subjected to abrasion in a stream of hot air . Preferred grinding equipment are rotor-stator systems with peripheral (linear speed) from 20 to 200 m/s. Among the disadvantages of this method, one can note a relatively large size D50: 6-10 μm and a wider particle size distribution. Also, during grinding, a partial mechanical activation of the surface occurs, which leads to an increase in the specific surface area and partial dehydration of the product, which negatively affects its processing and flame retardant properties. The resulting particles are highly abrasive and, without additional processing, are prone to reverse agglomeration and enlargement.

Known methods (patent application US 5500480, publ. 03/19/1996, patent application US 4421876, publ. 12/20/1983, patent application US 5100641, publ. seeding or adding pre-crushed Bayer trihydroxide to a caustic solution containing no aluminum, as a result of which, after long exposure and decomposition, Al (OH) ₃ crystals are formed with a non-porous surface and an average particle size of 5 - 25 μm or less, depending on the amount and size initial seed.

Similar methods are also known, for example, according to patent EP 2032506, publ. 03/11/2009, in which finely dispersed aluminum trihydroxide with an average particle size D50 of about 2.4 μm is obtained in this way, after which a dispersant is added to the wet precipitate after separation, filtration and washing and subjected to grinding and deagglomeration, for example, in a bead mill, and dried, e.g. in a spray dryer. The final product may have an average size of 1.4-1.9 µm and a specific surface area of the order of 5-7 m ² /g.

The well-known patent states that these methods can produce round particles of fine aluminum hydroxide with a smooth surface and improved rheology, for example, melt flow characteristics, although, in fact, crystals with a size of less than 5 μm, especially obtained in combination with a milling and deagglomeration step, are characterized by tabular hexagonal shape with a large number of sharp corners. So, in Fig. 1 is a photograph of particles of industrial samples of aluminum trihydroxide.

Closest to the claimed technical solution is a method for producing highly dispersed aluminum hydroxide (patent RU 2355638, publ. 05/20/2009), including mixing alkaline aluminate and neutralizing solutions, characterized in that an aqueous solution with a density of 1100-1300 kg/m is used as a neutralizing solution ³ containing bicarbonates and carbonates of alkali metals, and neutralization is carried out to a residual content of Na ₂ O _ku in a mixed solution of 1-15 g/l. The prototype method makes it possible to obtain aluminum hydroxide microparticles in a given range of average reduced particle diameters, however, the resulting powders are characterized by a relatively wide particle size distribution, an irregular acute-angled particle shape with increased roughness and the presence of large agglomerates, which lead to increased viscosity of polymer melts and worsen consumer properties of the final product.

Disclosure of invention

The technical objective of the claimed invention is to increase the bulk density and fluidity of the resulting highly dispersed precipitated aluminum hydroxide, allowing to improve its consumer qualities, namely, to ensure a significant reduction in the viscosity of polymer melts, the abrasiveness of the filler, and the chalking of compound surfaces.

The technical problem is achieved by the fact that the highly dispersed precipitated aluminum hydroxide obtained by the claimed method, according to scanning electron microscopy, contains particles of round shape (ellipsoid shape) in an amount of at least 90%, while the size of particles with a particle size of D50 is not more than 10 microns, and the particles themselves are highly uniform and have a narrow size distribution, with particle size ratios D99/D10 ranging from 7-12.

The smaller this value, the smaller the size spread of powder particles, i.e. its higher homogeneity is obtained, the D99/D10 ratio less than 7 is technologically difficult to achieve, and the D99/D10 ratio greater than 12 indicates the presence of large particles or agglomerates in the sample, which can subsequently degrade the quality of the polymer, reduce its tensile strength, and also the absence of small particles. particles that increase the specific surface of the filler and increase the viscosity of the polymer melt.

The technical problem is also achieved due to the fact that in the method of obtaining highly dispersed precipitated aluminum hydroxide, including mixing alkaline aluminate and neutralizing solutions, while an aqueous solution containing bicarbonates and alkali metal carbonates is used as a neutralizing solution, neutralization is carried out to a residual content of Na ₂ O _ku in a mixed solution of 7-15 g / l, according to the claimed invention, the pulp formed after mixing the solutions is stirred in the reactor for at least 30-240 minutes under the influence of shear rates of the liquid phase of 10-210 sec ^-1 , after which the precipitate is thickened, filtered and washed. Further, the resulting aluminum hydroxide can be modified with at least one surface treatment agent and/or silane coupling agent.

As a surface-treating agent, compounds of the group of saturated and unsaturated fatty acids or their alkali metal salts and/or mixtures thereof can be used.

As a silane agent, groups of organofunctional alkoxysilanes of monomeric and oligomeric forms can be used.

The treatment of aluminum hydroxide with an agent and/or a silane binder is carried out in an amount of 0.1-3.0 wt. % based on said aluminum hydroxide.

In FIG. Figure 1 shows the integral dependence of the residence time of particles in the reactor on the shear rates of the melt liquid.

It has been established that in the prototype method with prolonged (at least 30 minutes from the moment of mixing) intensive mixing of the pulp at high shear rates of the liquid phase in the specified range (Fig. 1) during the sol-gel transition during the neutralization of the aluminate solution with soda-bicarbonate granulometric the composition of the product is leveled, the particle size distribution becomes narrower (more evenly dispersed), and the particles themselves acquire a characteristic rounded shape.

Depending on the intensity of mixing (a characteristic informative parameter of which is the shear rate of the liquid phase) in the specified range of changes in the shear rate, it is possible to obtain particles with different length-width ratios - more elongated ellipsoidal or more spherical.

With more intensive mixing, mechanical grinding of particles begins with secondary agglomeration and enlargement, as a result of which the average particle size increases due to the contribution of agglomerates, the shape of the particles becomes splintered, and the specific surface area of the resulting powders increases significantly. With less intense mixing, the shear stresses imparted by the mixing device are not sufficient to change the shape of the particles. The particle size distribution of the resulting product is leveled, the ratio of particles with a size of D99/D10 decreases to 7-12, but the particles are in the form of prisms. The introduction of stirring after the stage of gel maturation (after 15 minutes or more from the moment of neutralization) leads to grinding of already formed particles in the form of an ellipsoid, but the surface of the particles is not smooth, and a large number of fragments are present in the samples, which leads to an undesirable increase in the specific surface of the product.

The sol-gel process is a method of obtaining materials from solutions, in which gel formation is recorded at one of the stages of the process. In the case of obtaining finely dispersed aluminum hydroxide, the primary particles of the sol (nanosized particles) form a stable gel in the first seconds after neutralization, and intensive mixing contributes to the destruction of the gel particles, its dehydration and subsequent enlargement in the form of agglomerates of a characteristic shape to a target particle size of 1–5 μm.

The above process increases the dispersion, flowability of the aluminum hydroxide powder obtained after drying, its bulk density, which in turn contributes to uniform distribution in the polymer matrix during subsequent compounding, and, as a result, improves the physico-mechanical and flame retardant properties of polymer products of reduced flammability, in turn First of all, cable insulation.

After separating, filtering, washing and drying by any known method, the resulting aluminum hydroxide can be modified with one or more surface treating agents and/or silane coupling agents.

Surface modification can improve processing properties during the processing of aluminum hydroxide powder into a polymer compound by product consumers, as well as improve the wettability of the filler surface with polymer, ensure the formation of a chemical bond between the particle surface and the polymer, which subsequently improves the physical and mechanical properties of the final product, for example, tensile strength and elongation at break.

As surface-treating agents, compounds of the group of saturated and unsaturated fatty acids or their alkali metal salts and/or mixtures thereof can be used.

Compounds of the group of organofunctional alkoxysilanes (monomeric and oligomeric forms), including alkylalkoxysilanes, aminosilanes, etc., can be used as silane coupling agents.

It is preferable to use agents for surface treatment in an amount of from 0.1 to 2.0 wt. % based on aluminum hydroxide, while surface treatment can be carried out by any known method: dry or wet.

Below is one of the possible ways to obtain highly dispersed precipitated aluminum hydroxide with a rounded particle shape.

Implementation of the invention

Example 1

The process of obtaining particles of aluminum hydroxide rounded shape is carried out using synthetic solutions: aluminate content, g/l: Al ₂ O ₃ 70; Na ₂ O _ku 66.2 and soda-bicarbonate with a content of Na ₂ O _total 87.3 g / l and HCO ₃ ^- 74 g / l. The aluminate and neutralizing solutions were taken in a volume ratio that ensured the final content of Na ₂ O _ky = 12 g/l, heated to 75°C, and mixed in a 2-liter laboratory reactor with a paddle stirrer, providing high intensity mixing of the solution with a shear rate of the liquid phase of the order 210 s ^-1 . The resulting pulp was kept under constant stirring for 240 minutes, after which the precipitate was thickened, filtered and washed. After washing, the aluminum hydroxide crystals were dried in an oven.

The results of this experiment, as well as all subsequent ones, are presented below in tables 1-3 for comparison.

Example 2

The process of obtaining rounded aluminum hydroxide particles is carried out in a batch reactor equipped with a paddle-type stirrer and dividers, with a working volume of 200 dm ³ . The experiments were carried out using an aluminate solution of one of the alumina plants operating on the technology of sintering with subsequent hydrochemical processing of sinter, the following composition, g/l: Al ₂ O ₃ 75; Na ₂ O _ku 70.6; Na ₂ O _total 73.1 and a soda-bicarbonate solution obtained by bubbling off-gases from sintering furnaces through a soda solution containing Na ₂ O _total 89 g/l and HCO ₃ ^- 91.8 g/l.

Hot technological solutions were fed into the reactor: aluminate and soda-bicarbonate in a ratio that provided the residual content of caustic alkali after neutralization of Na ₂ O _ku = 10 g/l. The resulting slurry was kept under constant stirring for 45 minutes with a shear rate of the melt liquid of at least 10 s ^-1 , after which the obtained aluminum hydroxide crystals were thickened, filtered, washed and dried.

Example 3

The synthesis was carried out analogously to example 2, in the resulting pulp of aluminum hydroxide after 60 minutes of exposure with stirring was added a modifier based on monomeric vinyltriethoxysilane in the amount of 0.1 wt. % of the calculated yield of Al(OH) ₃ . The pulp was kept under moderate agitation for another 30 minutes, after which the resulting product was thickened, filtered, washed and dried.

Example 4

The synthesis was carried out analogously to example 2. After 3 stages of washing the production aluminum hydroxide, an emulsion of stearic acid, consisting of 48 wt.% stearic acid, 48 wt. % water and 4% sodium stearate in the amount of 2.0% of the calculated yield of Al(OH) ₃ . The suspension of fully hydrophobized aluminum hydroxide was stirred for 5 minutes and filtered. Next, the resulting product was dried.

Example 5

The synthesis was carried out analogously to example 2, in the resulting pulp of aluminum hydroxide after 90 minutes of exposure with stirring was added a modifier based on oligomeric vinyltriethoxysilane in the amount of 0.3 wt. % of the calculated yield of Al(OH) ₃ . The pulp was kept under stirring for another 30 minutes, after which the resulting product was thickened, filtered, washed, and dried.

Example 6

The process of obtaining particles of aluminum hydroxide is carried out using synthetic solutions: aluminate content, g/l: Al ₂ O ₃ 65; Na ₂ O _ku 60 and soda-bicarbonate with the content of Na ₂ O _total 85 g/l and HCO ₃ ^- 79 g/l. The aluminate and neutralizing solutions were taken in a volume ratio that provided the final content of Na ₂ O _ky = 8 g/l, heated to 70°C, and mixed in a 20-liter laboratory reactor with a low-speed frame stirrer, which ensured the intensity of mixing of the solution at a shear rate of the liquid phase 7 with ^-1 . The resulting pulp was kept under stirring for 60 minutes, after which the precipitate was thickened, filtered and washed. After washing, the aluminum hydroxide crystals were dried in an oven.

Example 7

The process of obtaining particles of aluminum hydroxide is carried out using synthetic solutions: aluminate with content, g/l: Al ₂ O ₃ 64.5; Na ₂ O _ku 62.1 and soda-bicarbonate with a content of Na ₂ O _total 85 g / l and HCO ₃ ^- 72.3 g / l. The aluminate and neutralizing solutions were taken in a volume ratio that ensured the final content of Na ₂ O _ky = 10 g/l, heated to 75°C, and mixed in a 2-liter laboratory reactor with a milling stirrer providing high intensity mixing of the solution at a shear rate of the melt liquid 214 with ^-1 . The resulting pulp was kept under stirring for 60 minutes, after which the precipitate was thickened, filtered and washed. After washing, the aluminum hydroxide crystals were dried in an oven.

Example 8

The process of obtaining particles of aluminum hydroxide is carried out using synthetic solutions: aluminate with content, g/l: Al ₂ O ₃ 67,2; Na ₂ O _ku 64.6 and soda-bicarbonate with a content of Na ₂ O _total 84.3 g / l and HCO ₃ ^- 73 g / l. The aluminate and neutralizing solutions were taken in a volume ratio that provided the final content of Na ₂ O _ky = 9 g/l, heated to 70°C, and mixed in a 2-liter laboratory reactor with a paddle stirrer providing high intensity mixing of the solution with a shear rate of the liquid phase of 200 with ^-1 , and stirring was included after 15 minutes from the start of the reaction. After that, the pulp was kept under stirring for 60 minutes, after which the precipitate was thickened, filtered and washed. After washing, the aluminum hydroxide crystals were dried in an oven.

The characteristics of the products obtained according to example 1, 2, 6-9 are shown in Table 1. Micrographs of the particles are shown in Figs. 2, where a) - obtained by technologies US 5500480, EP 2032506 and their analogues, b) - by technologies US 4421876, US 5100641 and their analogues, and Fig. 3 (a, b, c), while in Fig. 2 shows photographs of samples obtained by known technologies, in Fig. 3 (a) shows a photo of the samples obtained by the prototype method, in Fig. 3 (b, c) photo of samples obtained by the claimed method. In FIG. 4 (a, b, c) shows a photo of scanning electron microscopy (SEM) of samples according to examples 6-8.

Figure 2(a) shows a commercially available product on the market - finely dispersed precipitated aluminum trihydroxide, obtained using the technologies of US 5500480, EP 2032506 and their analogues. The filler particles have a tabular hexagonal shape characteristic of gibbsite. The particle size in the field of view ranges from 0.5 to 5 µm with a predominant size of the order of 2.0-2.5 µm. In FIG. 2(b) shows a product commercially available on the market - finely dispersed precipitated aluminum trihydroxide, obtained using the technologies of US 4421876, US 5100641 and their analogues. The particles also have a tabular hexagonal shape, but smaller size, in the field of view there is a greater number of agglomerates and intergrown particles, the predominant size of single particles is about 1-1.5 microns.

According to the granulometric analysis presented in Table 1, samples (a) and (b) of Fig. 2 have the following fractional composition and specific surface measured by the BET method:

Table 1

Sample Granulometric
composition*, µm Specific surface, m ² /g D10 D50 D90 D99 but 0.82 1.91 3.59 4.49 3.2 b 0.78 1.63 3.38 7.2 4.5

In FIG. 3(a) shows SEM photographs of a sample of fine precipitated aluminum trihydroxide obtained by the prototype method (sample 3 in table 2). Aluminum hydroxide particles have the form of lamellar agglomerates with a rough surface or smooth elongated hexagonal prisms and/or their fragments. The particle size in the field of view ranges from 0.3-0.5 µm to 3-4 µm. Due to the developed rough surface, the sample has a high specific surface area - 6.3 m ² /g, low bulk density - 250 kg/m ³ , is prone to hanging in the hopper and dusting.

In FIG. 3 (b) and 3 (c) show samples obtained by the claimed method, and differ in size, the sample in Fig. 3(c) is surface modified with vinyltriethoxysilane (samples 4 and 5 in Table 2). Particles of highly dispersed aluminum hydroxide are strong ellipsoidal agglomerates with a smooth surface. The samples have a reduced specific surface area, high bulk density, good flowability and are less prone to dusting.

In FIG. 4(a) shows a sample obtained with insufficient agitation during the synthesis (sample 6 in Table 2). The particles have a uniform granulometric composition, but do not have the shape of an ellipsoid. In FIG. 4(b) shows a sample obtained with excessive mixing intensity. The micrograph shows a lot of splintered and lamellar particles, which tend to collect into large rounded agglomerates up to 100 µm in size. In FIG. 4(c) shows a sample subjected to agitation after gel maturation. Some of the particles, through mechanical grinding, begin to acquire the shape of an ellipsoid, but do not have a smooth surface, and a large number of small fragments are also recorded in the field of view.

table 2

Sample Granulometric
composition*, µm Specific surface, m ² /g Mass fraction of residue on a sieve 45 microns**, % Bulk density, kg / m ³ D10 D50 D90 D99 D99/D10 Prototype method one 0.65 1.91 14.88 101.3 155.85 4.5 6.3 308.1 2 0.8 2.0 8.78 82.79 103.49 3.7 4.7 336.4 3 0.733 1.62 6.55 56.62 77.24 6.3 3.1 250.9 According to the claimed method one 0.79 1.67 3.6 7.14 9.04 4.6 Less than 0.1 389.0 2 0.75 1.57 3.3 8.56 11.41 5.0 Less than 0.1 367.2 3 0.89 2.02 3.86 7.99 8.98 3.1 Less than 0.1 408.5 4 1.21 2.66 4.95 9.11 7.53 2.5 Less than 0.1 452.3 five 0.994 1.88 3.91 8.21 8.26 3.5 Less than 0.1 412.3 Out of range 6 0.79 1.62 3.52 9.23 11.68 4.5 Less than 0.1 356.4 7 4.97 29.95 74.18 168.5 33.9 21.6 27.9 689.1 8 0.81 1.94 3.54 14.78 18.25 5.1 0.3 343.2

*determined by laser diffraction on a Microtrac S3500 instrument. For completely and partially hydrophobic samples 4 and 5, the size composition was determined by the dry method.

** The residue on the sieve was checked by laser diffraction on a Microtrac S3500 device and by the sieve method according to GOST 21119.4-75.

For samples of highly dispersed aluminum hydroxide with an average particle size of 1.8-2.0 μm, tests were carried out on the synthesis of compounds based on EVA (ethylene vinyl acetate) with a filler content of aluminum hydroxide of 50%. At the same time, the flow index of the polymer melt with filler (FFR) and the physical and mechanical characteristics of the final compound were evaluated: relative elongation at break and strength. As can be seen from table 3, the samples according to the proposed method are significantly superior to the samples according to the prototype method in all respects. In terms of elongation at break and melt flow, samples according to the prototype method in the standard recipe do not allow reaching the permissible threshold values regulated by the compound manufacturers.

Table 3

Indicators Requirements Sample according to the prototype method, unmodified Sample according to the claimed method Unmodified Modified with vinylsilanes at a dosage of 0.2% Strength, MPa At least 10.5 10.8 11.0 11.2 12.3 Elongation at break, % 150 135.2 176 155 158 PTR (160/21.6) At least 5 3.3 5.3 7.6 7.1 Density, g / cm ³ 1.51 1.52 1.51 1.53 1.52

The given examples show that the proposed production method makes it possible to obtain highly dispersed precipitated aluminum hydroxide of improved quality: with a controlled granulometric composition, its high uniformity, with a reduced specific surface area with an equal average particle size relative to the prototype method, and the almost complete absence of large agglomerates and residue on a sieve (45 µm). The round shape of the particles provides a higher bulk density of the new product, improved rheology (fluidity) in the form of a dry powder, and further - rapid and uniform distribution in a viscous polymer matrix, which leads to an improvement in the physical and mechanical properties of the compound.

Claims (6)

1. Highly dispersed precipitated aluminum hydroxide, characterized in that it contains round particles in an amount of at least 90%, while the particle size D50 is not more than 10 microns, and the particle size ratio D99 / D10 is 7-12. 2. The method of obtaining highly dispersed precipitated aluminum hydroxide according to claim 1, including mixing alkaline aluminate and neutralizing solutions, while an aqueous solution containing bicarbonates and alkali metal carbonates is used as a neutralizing solution, neutralization is carried out to a residual content of Na ₂ O _ku in a mixed solution 7-15 g/l, while the pulp formed after mixing the solutions is stirred for at least 30-240 min under the influence of a liquid phase shear rate equal to 10-210 sec ^-1 , after which the precipitate is thickened, filtered and washed. 3. Method according to claim 2, characterized in that the obtained aluminum hydroxide is modified with at least one surface treatment agent and/or silane coupling agent. 4. The method according to claim 3, characterized in that compounds of the group of saturated and unsaturated fatty acids or their alkali metal salts and/or mixtures thereof are used as a surface treatment agent. 5. The method according to p. 3, characterized in that groups of organofunctional alkoxysilanes of monomeric and oligomeric forms are used as a silane agent. 6. The method according to p. 3, characterized in that the treatment of aluminum hydroxide with an agent and/or a silane coupling agent is carried out in an amount of 0.1-3.0 wt.% based on said aluminum hydroxide.

Images