A high tap density sheet silver powder for high solid content low temperature curing silver paste and a preparation method thereof

Abstract

The application discloses a high-tapped-density flaky silver powder for high-solid-content low-temperature curing silver paste and a preparation method thereof, and can be used in the preparation of low-temperature high-thermal-conductivity LED conductive silver paste, high-solid-content low-temperature HJT silver paste and other low-temperature silver pastes requiring high solid content, and can replace the use of other foreign silver powders. The high-tapped-density flaky silver powder for high-solid-content low-temperature curing silver paste has concentrated flaky powder particle size distribution, and the tapped density reaches 5.5 or above, so that the market demand is met, and the blank of the high-tapped-density flaky silver powder required by the high-solid-content low-temperature curing silver paste in China is filled.

Description

Technical Field

[0001] This invention belongs to the technical field of high tap density flake silver powder used in low-temperature curing silver paste, specifically relating to a high tap density flake silver powder for high solids content low-temperature curing silver paste and its preparation method. Background Technology

[0002] With the development of the domestic low-temperature curing silver paste application industry, many low-temperature curing silver pastes have embarked on the path of localization. For example, low-temperature keyboard paste, low-temperature touch screen paste, low-temperature LED conductive silver paste, low-temperature crystal oscillator conductive adhesive, and low-temperature sintering encapsulation paste are gradually replacing foreign low-temperature silver pastes with domestic low-temperature silver pastes. Moreover, domestic low-temperature silver pastes are constantly improving their related technical levels.

[0003] Many domestic companies dedicated to the development and production of low-temperature silver paste have emerged. The development of domestic low-temperature silver paste technology has also driven the development and progress of related domestic low-temperature silver powder industries. For example, in the field of low-temperature keyboard silver paste, various conductive low-temperature silver powders with better performance have been developed, replacing the use of foreign silver powders and improving the overall comprehensive strength of domestic low-temperature keyboard silver paste. Summary of the Invention

[0004] To address the common problems of low tap density in domestically produced high-tap-density sheet silver powders (generally less than 5.3), leading to high oil absorption, high viscosity and dryness in the subsequent production of high-solids-content low-temperature curing silver pastes, resulting in various performance issues, this invention provides a high-tap-density sheet silver powder for high-solids-content low-temperature curing silver pastes and its preparation method. The resulting high-tap-density sheet silver powder can be used in the preparation of low-temperature high thermal conductivity LED conductive silver pastes, high-solids-content low-temperature HJT silver pastes, and other low-temperature silver pastes requiring high solids content, replacing other imported silver powders. The high-tap-density sheet silver powder obtained by this invention has a concentrated particle size distribution and a tap density exceeding 5.5, meeting market demand and filling the gap in the domestic market for high-tap-density sheet silver powders required for high-solids-content low-temperature curing silver pastes.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] A method for preparing high-tap-density sheet silver powder for high-solids-content low-temperature curing silver paste includes the following steps:

[0007] 1) In reactor A, silver nitrate and deionized water are mixed and stirred at a mass ratio of 1:(5-10) to dissolve the mixture. The temperature is controlled at 25-30℃ and the time is maintained at 5-10 min.

[0008] 2) Add deionized water, vitamin C, and dispersant to reactor B. The mass ratio of vitamin C to silver nitrate is 1:(1-1.5), the amount of dispersant is 3-10‰ of the mass of silver nitrate, and the mass ratio of silver nitrate to deionized water is 1:5. Stir and dissolve in the reactor, control the temperature at 25-30℃, and the time is 5-10 min.

[0009] The dispersant is selected from hydroxyacrylic acid modified polyether with a molecular weight of 20,000-50,000;

[0010] 3) The solution in reactor A is slowly and uniformly added to reactor B while continuously stirring. The addition process is controlled within 15-20 minutes. After all the solution is added, stirring is continued for another 10-15 minutes to rapidly separate the silver powder and the solution. The wet silver powder is washed with deionized water until the conductivity reaches 20. Then, alcohol with a mass concentration of 99% or higher is added for dehydration and washing until the conductivity of the alcohol-mixed wet silver powder reaches below 10. The particle size distribution of the obtained wet silver powder is D10 0.3-0.6, D50 0.8-1.2, and D90 1.5-1.9.

[0011] 4) Add the wet silver powder obtained in the previous step to a ball mill jar, then add zirconium balls with a diameter of 2 mm to the ball mill jar, with a mass ratio of silver nitrate to zirconium balls of 1:4; then add alcohol with a mass concentration of 99% or higher, with a mass ratio of silver nitrate to alcohol of 1:(0.5-0.8), and then add oleic acid equivalent to 1-2% of the mass of silver nitrate. Ball mill at 40-60 r / min for 5-10 h.

[0012] 5) Then, the silver powder, alcohol solution, and zirconium balls are separated using a 100-mesh sieve. The silver powder is then further separated from the alcohol solution by vacuum filtration. After drying at a constant temperature of 60℃ for 12 hours, a high tap density flake powder for high solid content low-temperature curing silver paste is finally obtained. The particle size distribution of this silver powder is D10 1.5-3.0, D50 3.5-5.5, D90 6.0-6.5, loose density 2.5-3.5, and tap density 5.5-5.7.

[0013] In this invention:

[0014] The dispersant mentioned in step 2) is hydroxyacrylic acid modified polyether. The functions of this dispersant are: ① to concentrate the particle size of the generated silver powder, so that the silver powder particles are separated in the solution without forming hard agglomerates; ② at the same time, the use of a high molecular weight dispersant can also increase the viscosity of the aqueous solution, increase the suspension performance of the silver powder particles, enhance the dispersion performance of the silver powder particles, and control the molecular weight of 20,000-50,000 so that the silver powder particles produced by the reaction are within the target range and achieve the expected experimental results; ③ at the same time, this dispersant is highly soluble in water and can be easily washed away, making the subsequent powder washing process simple and rapid.

[0015] The rapid separation described in step 3) involves using a vacuum filter to quickly separate the silver powder from the deionized water. The final two washes are performed with 10-30% alcohol by mass. Since alcohol has a dehydrating and rapid evaporation effect, the silver powder can be dried quickly afterward.

[0016] Step 3) continues until the conductivity of the alcohol-mixed wet silver powder reaches below 10, wherein the residual moisture in the alcohol-mixed wet silver powder is negligible.

[0017] This invention also relates to a high-tap-density sheet silver powder for high-solids-content low-temperature curing silver paste, obtained by the above-mentioned preparation method of high-tap-density sheet silver powder for high-solids-content low-temperature curing silver paste. The silver powder has a particle size distribution of D10 of 1.5-3.0, D50 of 3.5-5.5, D90 of 6.0-6.5, a loose density of 2.5-3.5, and a tap density of 5.5-5.7. This silver powder is suitable for preparing low-temperature high thermal conductivity LED conductive silver paste, high-solids-content low-temperature HJT silver paste, and other low-temperature silver pastes requiring high solids content.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] 1. The method for preparing high tap density flake silver powder for high solids content low-temperature curing silver paste described in this invention is simple in process. No difficult-to-clean dispersants (such as PVP, gelatin, arabic resin, etc.) are added during the synthesis stage. Moreover, the ball milling process is also a conventional method, which is low in cost and easy to implement. Therefore, the flake silver powder obtained has stable quality in large-scale production and performs excellently when applied to downstream high solids content low-temperature curing pastes.

[0020] 2. The high tap density flake silver powder for high solids content low-temperature curing silver paste described in this invention has a high tap density of 5.5 or higher, which is higher than the tap density of existing flake silver powders in China. The high solids content low-temperature curing paste prepared using the flake silver powder obtained by this invention has better printing performance, flow performance and construction performance than the low-temperature flake powder produced by existing domestic technology. Attached Figure Description

[0021] Figure 1 This is a SEM image of a high tap density sheet silver powder prepared in Example 1 of the present invention for use in high solids content low-temperature curing silver paste. Detailed Implementation

[0022] The present invention is further described in detail below through embodiments, but these embodiments should not be considered as limitations on the present invention. Unless otherwise specified, the raw materials and equipment used in the embodiments of this application were purchased commercially, and the methods in the embodiments, unless otherwise specified, are conventional methods in the art.

[0023] Example 1:

[0024] A method for preparing high-tap-density sheet silver powder for high-solids-content low-temperature curing silver paste includes the following steps:

[0025] 1) In reactor A, mix and dissolve 12 kg of silver nitrate and 80 L of deionized water, controlling the water temperature at 27-29℃ for 5 min;

[0026] 2) Add 10 kg of vitamin C to 60 L of deionized water in reactor B, and simultaneously add 90 g of 3 W molecular weight hydroxyl acrylic acid modified polyether (Zhejiang Weifa New Material Technology Co., Ltd.). Stir the solution and control the water temperature at 27-29℃ for 5 min.

[0027] 3) The solution in reactor A is slowly and uniformly added to reactor B while continuously stirring. The addition process is controlled within 15 minutes. After all the solution is added, stirring is continued for another 10 minutes. Then, the silver powder and solution are rapidly separated. The wet silver powder is washed with deionized water until the conductivity reaches 20. Then, alcohol with a mass concentration of 99% or higher is added for dehydration and cleaning. The conductivity of the alcohol-mixed wet silver powder reaches 10. The residual moisture in the wet silver powder is negligible. The particle size distribution of the obtained wet silver powder is D10 = 0.35, D50 = 1.0, and D90 = 1.8.

[0028] 4) Add the wet silver powder obtained in the previous step to a ball mill jar, then add 48 kg of zirconium balls with a diameter of 2 mm to the ball mill jar, then add 6 kg of alcohol with a mass concentration of 99% or higher, and then add oleic acid equivalent to 120 g of silver nitrate. Ball mill at 50 r / min for 8 h.

[0029] 5) Then, the silver powder, alcohol solution and zirconium balls are separated by a 100-mesh sieve. The silver powder is then separated from the alcohol solution by vacuum filtration. After drying at a constant temperature of 60 degrees for 12 hours, high tap density flake powder A for high solid content low temperature curing silver paste is finally obtained. The particle size distribution of this silver powder is D10 is 1.5, D50 is 4.0, D90 is 6.0, loose density is 3.5 and tap density is 5.5.

[0030] Figure 1 This is an SEM image of a high tap density sheet silver powder prepared in Example 1 for use in high solids content low-temperature curing silver paste.

[0031] Example 2:

[0032] A method for preparing high-tap-density sheet silver powder for high-solids-content low-temperature curing silver paste includes the following steps:

[0033] 1) In reactor A, mix and dissolve 12 kg of silver nitrate and 60 L of deionized water, controlling the water temperature at 28-30℃ for 8 min;

[0034] 2) In reactor B, add 12 kg of vitamin C to 60 L of deionized water, and simultaneously add 5‰ of the mass of silver nitrate and 5W molecular weight hydroxyl acrylic acid modified polyether (Zhejiang Weifa New Material Technology Co., Ltd.). Stir the solution, control the water temperature at 28-30℃, and the time is 8 min.

[0035] 3) The solution in reactor A is slowly and uniformly added to reactor B while continuously stirring. The addition process is controlled within 20 minutes. After all the solution is added, stirring is continued for another 12 minutes to quickly separate the silver powder and the solution. The wet silver powder is washed with deionized water until the conductivity reaches 20. Then, alcohol with a mass concentration of 99% or higher is added for dehydration and washing. The conductivity of the alcohol-mixed wet silver powder reaches 9.9. The residual moisture in the wet silver powder is negligible. The particle size distribution of the obtained wet silver powder is D10 is 0.30, D50 is 0.8, and D90 is 1.5.

[0036] 4) Add the wet silver powder obtained in the previous step to a ball mill jar, then add 48 kg of zirconium balls with a diameter of 2 mm to the ball mill jar, then add 7.2 kg of alcohol with a mass concentration of 99% or higher, and then add oleic acid equivalent to 240 g of silver nitrate. Ball mill at 40 r / min for 10 h.

[0037] 5) Then, the silver powder, alcohol solution, and zirconium balls are separated using a 100-mesh sieve. The silver powder is then separated from the alcohol solution by vacuum filtration. After drying at a constant temperature of 60 degrees Celsius for 12 hours, high tap density flake powder A for high solid content low-temperature curing silver paste is finally obtained. The particle size distribution of this silver powder is D10 is 2.0, D50 is 3.0, D90 is 6.2, loose density is 2.5, and tap density is 5.6.

[0038] Example 3:

[0039] A method for preparing high-tap-density sheet silver powder for high-solids-content low-temperature curing silver paste includes the following steps:

[0040] 1) In reactor A, mix and dissolve 12 kg of silver nitrate and 120 L of deionized water, controlling the water temperature at 25-27℃ for 10 min;

[0041] 2) In reactor B, add 18 kg of vitamin C to 60 L of deionized water, and simultaneously add 10‰ of the mass of silver nitrate of 2W molecular weight hydroxyl acrylic acid modified polyether (Zhejiang Weifa New Material Technology Co., Ltd.). Stir the solution and control the water temperature at 25-27℃ for 10 min.

[0042] 3) The solution in reactor A is slowly and uniformly added to reactor B while continuously stirring. The addition process is controlled within 18 minutes. After all the solution is added, stirring is continued for another 15 minutes. Then, the silver powder and solution are rapidly separated. The wet silver powder is washed with deionized water until the conductivity reaches 20. Then, alcohol with a mass concentration of 99% or higher is added for dehydration and washing. The conductivity of the alcohol-mixed wet silver powder reaches 9.8. The residual moisture in the wet silver powder is negligible. The particle size distribution of the obtained wet silver powder is D10 0.60, D50 1.2, and D90 1.9.

[0043] 4) Add the wet silver powder obtained in the previous step to a ball mill jar, then add 48 kg of zirconium balls with a diameter of 2 mm to the ball mill jar, then add 9.6 kg of alcohol with a mass concentration of 99% or higher, and then add oleic acid equivalent to 180 g of silver nitrate. Ball mill at 60 r / min for 5 h.

[0044] 5) Then, the silver powder, alcohol solution, and zirconium balls are separated using a 100-mesh sieve. The silver powder is then separated from the alcohol solution by vacuum filtration. After drying at a constant temperature of 60 degrees Celsius for 12 hours, a high tap density flake powder A for high solid content low-temperature curing silver paste is finally obtained. The particle size distribution of this silver powder is D10 of 3.0, D50 of 5.5, D90 of 6.5, loose density of 3.0, and tap density of 5.7.

[0045] Comparative Example 1:

[0046] The difference between Comparative Example 1 and the Example is that the amount of dispersant hydroxyacrylic acid modified polyether added in step 2) is reduced, while the rest is the same as in Example 1;

[0047] Specifically, the steps include the following:

[0048] 1) In reactor A, mix and dissolve 12 kg of silver nitrate and 80 L of deionized water, controlling the water temperature at 27-29℃ for 5 min;

[0049] 2) In reactor B, add 10 kg of vitamin C to 60 L of deionized water, and at the same time add 1 g of 3 W molecular weight hydroxyl acrylic acid modified polyether (Zhejiang Weifa New Material Technology Co., Ltd.). Stir the solution and control the water temperature at 27-29℃ for 5 min.

[0050] 3) The solution in reactor A is slowly and uniformly added to reactor B while continuously stirring. The addition process is controlled within 15 minutes. After all the solution is added, stirring is continued for another 10 minutes. Then, the silver powder and solution are rapidly separated. The wet silver powder is washed with deionized water until the conductivity reaches about 20. Then, alcohol with a concentration of 99% or higher is added for dehydration and cleaning. The conductivity of the alcohol mixed with the wet silver powder is below 10. The residual water in the wet silver powder is negligible. The particle size distribution of the obtained wet silver powder is D10 is 1.5, D50 is 2.5, and D90 is 4.9.

[0051] 4) Add the wet silver powder obtained in the previous step to a ball mill jar, then add 48 kg of zirconium balls with a diameter of 2 mm to the ball mill jar, then add 6 kg of alcohol with a concentration of 99% or higher, and then add oleic acid equivalent to 120 g of silver nitrate. Mill at 50 revolutions per minute for 8 hours.

[0052] 5) Then, the silver powder, alcohol solution, and zirconium balls are separated using a 100-mesh sieve. The silver powder is then further separated from the alcohol solution by vacuum filtration. After drying at a constant temperature of 60 degrees Celsius for 12 hours, high tap density flake powder B for high solid content low-temperature curing silver paste is finally obtained. The particle size distribution of this silver powder is D10 is 2.0, D50 is 3.2, D90 is 6.3, loose density is 2.1, and tap density is 3.7.

[0053] Comparative Example 2:

[0054] The difference between Comparative Example 2 and the Example 1 lies in the steps or ingredients: in step 2), the dispersant is replaced with another type (PVP-K30), otherwise it is the same as Example 1;

[0055] Specifically, the steps include the following:

[0056] 1) In reactor A, mix and dissolve 12 kg of silver nitrate and 80 L of deionized water, controlling the water temperature at 27-29℃ for 5 min;

[0057] 2) Add 10 kg of vitamin C to 60 L of deionized water in reactor B, and add 90 g of dispersant PVP-K30 (Shanghai Vitamin C). Stir the solution and control the water temperature at 27-29℃ for 5 min.

[0058] 3) The solution in reactor A is slowly and uniformly added to reactor B while continuously stirring. The addition process is controlled within 15 minutes. After all the solution is added, stirring is continued for another 10 minutes. Then, the silver powder and solution are rapidly separated. The wet silver powder is washed with deionized water until the conductivity reaches about 20. Then, alcohol with a concentration of 99% or higher is added for dehydration and cleaning. The conductivity of the alcohol-mixed wet silver powder is below 10. The residual water in the wet silver powder is negligible. The particle size distribution of the obtained wet silver powder is D10 = 0.12, D50 = 0.9, and D90 = 3.5.

[0059] 4) Add the wet silver powder obtained in the previous step to a ball mill jar, then add 48 kg of zirconium balls with a diameter of 2 mm to the ball mill jar, then add 6 kg of alcohol with a concentration of 99% or higher, and then add oleic acid equivalent to 120 g of silver nitrate. Mill at 50 revolutions per minute for 8 hours.

[0060] 5) Then, the silver powder, alcohol solution and zirconium balls are separated by a 100-mesh sieve. The silver powder is then separated from the alcohol solution by vacuum filtration. After drying at a constant temperature of 60 degrees for 12 hours, high tap density flake powder C for high solid content low temperature curing silver paste is finally obtained. The particle size distribution of this silver powder is D10 is 0.8, D50 is 3.5, D90 is 7.5, loose density is 1.8 and tap density is 4.3.

[0061] Comparative Example 3:

[0062] The difference between Comparative Example 3 and the Example 1 lies in the following steps or ingredients: Step 4) The ball milling process is changed, while the rest is the same as in Example 1;

[0063] Specifically, the steps include the following:

[0064] 1) In reactor A, mix and dissolve 12 kg of silver nitrate and 80 L of deionized water, controlling the water temperature at 27-29℃ for 5 min;

[0065] 2) Add 10 kg of vitamin C to 60 L of deionized water in reactor B, and simultaneously add 90 g of 3 W molecular weight hydroxyl acrylic acid modified polyether (Zhejiang Weifa New Material Technology Co., Ltd.). Stir the solution and control the water temperature at 27-29℃ for 5 min.

[0066] 3) The solution in reactor A is slowly and evenly added to reactor B while continuously stirring. The addition process is controlled within 15 minutes. After all the solution is added, stirring is continued for another 10 minutes. Then, the silver powder and solution are quickly separated. The wet silver powder is washed with deionized water until the conductivity reaches about 20. Then, alcohol with a concentration of 99% or higher is added for dehydration and cleaning. The conductivity of the alcohol mixed with the wet silver powder is below 10. The residual water in the wet silver powder is negligible. The particle size distribution of the obtained wet silver powder is D10 0.35, D50 1.0, and D90 1.8.

[0067] 4) Add the wet silver powder obtained in the previous step to a ball mill jar, then add 60 kg of zirconium balls with a diameter of 5 mm to the ball mill jar, then add 6 kg of alcohol with a concentration of 99% or higher, and then add oleic acid equivalent to 120 g of silver nitrate. Mill at 80 revolutions per minute for 24 hours.

[0068] 5) Then, the silver powder, alcohol solution, and zirconium balls are separated using a 100-mesh sieve. The silver powder is then further separated from the alcohol solution by vacuum filtration. After drying at a constant temperature of 60 degrees Celsius for 12 hours, a high tap density flake powder D for high solid content low-temperature curing silver paste is finally obtained. The particle size distribution of this silver powder is D10 = 2.5, D50 = 6.0, D90 = 10.0, loose density = 1.2, and tap density = 2.5.

[0069] Results and Discussion:

[0070] 1. As can be seen from the above results, the overall effect of the embodiment is very good. The silver powder particle size distribution is very concentrated, and the tap density reaches more than 5.5 (5.5-5.7), which can meet the requirements of high solid content low temperature curing silver paste.

[0071] 2. As can be seen from the comparison between the examples and Comparative Example 1, if the amount of dispersant hydroxyacrylic acid modified polyether is insufficient, it will have an adverse effect on the silver powder particles produced in the subsequent reaction, resulting in the final flake powder product not reaching the ideal state in terms of particle size and compaction.

[0072] 3. As can be seen from the comparison between the examples and Comparative Example 2, if the dispersant is replaced with other types, such as PVP-K30 in Comparative Example 2, it will also have an adverse effect on the silver powder particles produced in the subsequent reaction, resulting in the final flake powder product not reaching the ideal state in terms of particle size and compaction.

[0073] 4. As can be seen from the comparison between the examples and Comparative Example 3, if the ball milling process is changed, the final product will not achieve the desired effect.

[0074] The above embodiments of the present invention are merely examples for clearly illustrating the present invention and are not intended to limit the implementation of the present invention. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively describe all possible implementations here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A process for the production of high tap density sheet silver powder for high solids low temperature cure silver paste, characterized by: It comprises the following steps: 1) mixing and stirring silver nitrate and deionized water in a mass ratio of 1:(5-10) in a reaction kettle A, controlling the temperature at 25-30℃, and maintaining the time for 5-10 min; 2) adding deionized water, vitamin C and dispersant in a reaction kettle B, wherein the mass ratio of the added vitamin C to silver nitrate is 1:(1-1.5), the added amount of the dispersant is 3-10‰ of the mass of silver nitrate, the mass ratio of silver nitrate to deionized water is 1:5, stirring and dissolving in the reaction kettle, controlling the temperature at 25-30℃, and maintaining the time for 5-10 min; The dispersant is selected from hydroxypropylene acid modified polyether with a molecular weight of 20-50 thousand; 3) slowly adding the solution in the reaction kettle A into the reaction kettle B at a constant speed while continuously stirring, controlling the adding process at 15-20 min, continuing to stir for 10-15 min after all the solution is added to rapidly separate the silver powder and the solution, and washing the wet silver powder with deionized water until the conductivity of the alcohol mixed wet silver powder reaches 10 μS / cm or less, obtaining the wet silver powder with a particle size distribution of D10 being 0.3-0.6 μm, D50 being 0.8-1.2 μm, and D90 being 1.5-1.9 μm; 4) adding the wet silver powder separated in the above step into a ball milling tank, adding zirconium balls with a diameter of 2 mm in the ball milling tank, controlling the mass ratio of silver nitrate to zirconium balls at 1:4, then adding alcohol with a mass concentration of 99% or more, controlling the mass ratio of silver nitrate to alcohol at 1:(0.5-0.8), then adding oleic acid equivalent to 1-2% of the mass of silver nitrate, and carrying out ball milling treatment at 40-60 r / min for 5-10 h; 5) Then the silver powder, alcohol solution and zirconium ball are separated by 100 mesh screen, the silver powder is continuously separated from the alcohol solution by suction filtration, and finally the high tap density flaky powder for high solid content low temperature curing silver paste is obtained by drying at 60°C for 12h, the particle size distribution of the silver powder is D10 of 1.5-3.0μm, D50 of 3.5-5.5μm, D90 of 6.0-6.5μm, loose density of 2.5-3.5g / cm 3 , and tap density of 5.5-5.7g / cm 3 .

2. A process for the production of high tap density sheet silver powder for high solids low temperature cure silver paste as claimed in claim 1, wherein: In step 3), the conductivity of the alcohol mixed wet silver powder reaches 10 μS / cm or less, wherein the residual water in the alcohol mixed wet silver powder is negligible.

3. A high tap density sheet silver powder for high solids low temperature cure silver paste, characterized by: The silver powder has a particle size distribution of D10 of 1.5-3.0 μm, D50 of 3.5-5.5 μm, D90 of 6.0-6.5 μm, loose density of 2.5-3.5 g / cm 3 , and tap density of 5.5-5.7 g / cm 3 .

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