Precipitated silica with low surface hydroxyl concentration and preparation method and application thereof

Abstract

The invention relates to the technical field of silicon dioxide preparation, in particular to precipitated silicon dioxide with low surface hydroxyl concentration and a preparation method and application thereof. The preparation method comprises the following steps: adding water and a first sodium silicate solution into a container, heating to 55-75 DEG C, stirring, dropwise adding a dilute sulfuric acid solution, and controlling the pH value of a reaction endpoint to be 9.5-11.0; heating to 75-95 DEG C, aging for 30-120 minutes, dropwise adding a second sodium silicate solution and a dilute sulphuric acid solution into the container, controlling the pH to be 9.5-11.0, and after a preset amount of the second sodium silicate solution is dropwise added, adding cold water into the container to continue aging; after aging is completed, dropwise adding a dilute sulfuric acid solution into the container, and controlling the pH at the reaction endpoint to be 3.0-5.5 to obtain slurry; and carrying out filter pressing, washing, spray drying and crushing on the slurry to obtain the precipitated silicon dioxide with low surface hydroxyl concentration. By accurately controlling the flow rate of dilute sulfuric acid, the reaction temperature and the pH value in the reaction process, the surface hydroxyl concentration of precipitated silica can be greatly reduced.

Description

Technical Field

[0001] This invention relates to the field of silica preparation technology, and in particular to a precipitation method for silica with low surface hydroxyl concentration, its preparation method, and its application. Background Technology

[0002] Precipitation silica is an industrially used inorganic material produced through acid-base neutralization reactions. Silicon-containing substances such as water glass and sulfuric acid can react under suitable reaction conditions (temperature, concentration, etc.) to obtain silica with different properties.

[0003] Silicone rubber raw rubber is a straight-chain organosiloxane. The chains are bonded together by weak interactions such as van der Waals forces and entanglement. Its strength is not high, so fillers such as silica are needed to reinforce the raw rubber through van der Waals forces and hydrogen bonds.

[0004] Silica can be further classified into fumed silica and precipitated silica based on its production method. The surface hydroxyl content of silica is the most important functional group in its application to silicone rubber, affecting its reinforcing and other related properties. Ideally, the silanol content on the silica surface is 2-5 silanol groups per square nanometer (depending on the testing method), which is related to the silica crystal lattice configuration. However, due to lattice defects on the surface and within the particles, as well as incomplete condensation of silanol groups during the drying process, the number of silanol groups on the silica surface may exceed 5 per square nanometer. Compared to precipitated silica, fumed silica is almost entirely composed of isolated silanol groups, resulting in a silanol content on its surface that is closer to the ideal state. In contrast, precipitated silica has a large number of incompletely condensed hydroxyl groups and homologous silanol groups on its surface, leading to a silanol concentration that may be 3-4 times higher than that of fumed silica. Different contents and types of hydroxyl groups adsorbed on the surface of silica will affect the adsorption of polar molecules on the silica surface and further affect the reactivity of silica with other molecules.

[0005] In summary, the surface hydroxyl concentration of precipitated silica currently on the market is generally high, leading to severe water absorption during silicone rubber applications and affecting its various properties. Therefore, only fumed silica or modified precipitated silica can be used, with fumed silica being prohibitively expensive. The common modification method involves hydrophobic modification of silica to reduce its surface hydroxyl content; however, both in-situ and post-treatment modifications have drawbacks such as incomplete modification and inability to be kneaded at high temperatures, limiting its widespread application in the silicone rubber field. Therefore, whether the surface hydroxyl content can be controlled through the reaction process needs to be considered and studied. Summary of the Invention

[0006] The main objective of this invention is to provide a precipitated silica with low surface hydroxyl concentration, its preparation method, and its application, aiming to improve the technical problems of incomplete modification and inability to be kneaded at high temperatures when using hydrophobic modification in existing precipitated silica.

[0007] To achieve the above objectives, this invention proposes a method for preparing silica with low surface hydroxyl concentration by precipitation, comprising the following steps:

[0008] S1. Add water and sodium silicate solution to the container, heat to 55-75℃ and start stirring, then add dilute sulfuric acid solution dropwise, controlling the pH at the reaction endpoint to be 9.5-11.0;

[0009] S2. Heat to 75-95℃ and age for 30-120 minutes. Then, add sodium disilicate solution and dilute sulfuric acid solution dropwise to the container simultaneously, controlling the pH at 9.5-11.0. After the preset amount of sodium disilicate solution has been added, add cold water to the container to continue aging.

[0010] S3. After aging, continue to add dilute sulfuric acid solution to the container, and control the pH at the reaction endpoint between 3.0 and 5.5 to obtain the slurry;

[0011] S4. The slurry is subjected to pressure filtration, washing, spray drying and crushing to obtain precipitated silica with low surface hydroxyl concentration.

[0012] Preferably, the preparation of the first sodium silicate solution includes the following steps: dissolving sodium silicate in water to obtain a sodium silicate solution, adding an ion treatment agent to the sodium silicate solution, allowing it to stand, filtering, and then obtaining a purified first sodium silicate solution;

[0013] The preparation of the second sodium silicate solution includes the following steps: adding sodium hydroxide solution to the first sodium silicate solution and stirring thoroughly, adjusting to a modulus of 2.0-2.5 and a concentration of 1.0-1.2 mol / L to obtain the second sodium silicate solution.

[0014] Preferably, the dropping rate of the dilute sulfuric acid solution in step S1 is 0.5-2 m / s. 3 / h; the dropping rate of the dilute sulfuric acid solution in step S2 is 2-8m / h; 3 / h; the dropping rate of the dilute sulfuric acid solution in step S3 is 1-4m / h. 3 / h.

[0015] Preferably, the amount of water added in step S1 is 22m. 3 The amount of sodium silicate solution added is 1-5 mg. 3 .

[0016] Preferably, the preset amount of the second sodium silicate solution in step S2 is 7-11 ml. 3The dripping speed is 6-8m. 3 / h.

[0017] Preferably, the amount of cold water added in step S2 is 20-30m³. 3 .

[0018] Preferably, the concentration of the dilute sulfuric acid solution in step S1 is 1-4 mol / L.

[0019] Preferably, the stirring speed in step S1 is 30-40 Hz.

[0020] In addition, this invention also proposes a precipitated silica with low surface hydroxyl concentration, prepared by the above-described method for preparing precipitated silica with low surface hydroxyl concentration, wherein the surface hydroxyl content of the precipitated silica with low surface hydroxyl concentration is ≤2.5 hydroxyl groups / nm. 2 BET is 160-220m 2 / g, DBP oil absorption value is 210-235ml / 100g.

[0021] In addition, the present invention also proposes the application of the above-mentioned low surface hydroxyl concentration precipitated silica in silicone rubber.

[0022] Compared with the prior art, the precipitation method for producing silica with low surface hydroxyl concentration and its preparation method of the present invention have the following beneficial effects:

[0023] (1) Without adding surface treatment agents, this invention can greatly reduce the surface hydroxyl concentration of precipitated silica by precisely controlling the flow rate of dilute sulfuric acid, the reaction temperature and the pH value of the reaction process at different reaction stages of silica formation, so that it can basically reach the surface hydroxyl concentration level of gaseous silica; at the same time, it can also adjust the pore structure of silica and reduce a large number of micropore structures.

[0024] (2) In addition, the present invention also controls the impurity content of the raw material water glass by selecting low-impurity solid sodium silicate and adding it to an immediate processing machine, thereby reducing the lattice defects caused by impurities during the reaction process. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0026] Furthermore, the technical solutions of the various embodiments can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0027] A method for preparing precipitated silica with low surface hydroxyl concentration includes the following steps:

[0028] S1. Add 22m to the container 3 Water (the amount of water added can be more or less, such as adjusting to 24m) 3 or 20m 3 (Adjusted according to actual needs) and 1-5m 3 The first sodium silicate solution is heated to 55-75℃, then stirred at a speed of 30-40 Hz, followed by stirring at a speed of 0.5-2 m. 3 Add dilute sulfuric acid solution dropwise at a rate of / h, controlling the pH at the reaction endpoint between 9.5 and 11.0;

[0029] S2. After heating to 75-95℃ and aging for 30-120 minutes, simultaneously add sodium disilicate solution and dilute sulfuric acid solution dropwise into the container. The sodium disilicate solution is added at a rate of 6-8 minutes per minute. 3 The dropping rate of dilute sulfuric acid solution is 2-8 m / h. 3 / h, control the pH at 9.5-11.0, and after the preset amount of sodium silicate solution has been added, add 20-30ml to the container. 3 Continue aging in cold water (room temperature water) for 20-30 minutes; the preset volume of sodium disilicate solution is 7-11 ml. 3 ;

[0030] S3. After aging is complete, continue to add water to the container at a rate of 1-4 ml. 3 Dilute sulfuric acid solution is added dropwise at a rate of / h, and the pH at the reaction endpoint is controlled between 3.0 and 5.5 to obtain a slurry;

[0031] S4. The slurry is subjected to pressure filtration, washing, spray drying and crushing to obtain precipitated silica with low surface hydroxyl concentration.

[0032] The preparation of the first sodium silicate solution includes the following steps: selecting low-impurity solid sodium silicate (e.g., purity > 99%) and process water, adding them to a sealed container, dissolving them under high temperature and pressure to obtain a sodium silicate solution, adding an ion treatment agent (e.g., CaCl2 and Ca(NO3)2 can be used to remove impurity ions from the sodium silicate solution), allowing it to stand, filtering, and obtaining a purified first sodium silicate solution with a modulus of 3.45-3.55 and a concentration of 1.0-2.0 mol / L;

[0033] The preparation of the second sodium silicate solution includes the following steps: adding sodium hydroxide solution to the first sodium silicate solution and stirring thoroughly, adjusting to a modulus of 2.0-2.5 and a concentration of 1.0-1.2 mol / L to obtain the second sodium silicate solution.

[0034] Dilute sulfuric acid can be prepared from concentrated sulfuric acid, or a dilute sulfuric acid solution with a concentration of 1-4 mol / L can be used directly.

[0035] This invention controls lattice defects and the structure of silica (white carbon black) by adjusting reaction parameters such as the amount, dropping rate, and concentration of each raw material, and strictly controlling the temperature and pH range at each reaction stage. This reduces the surface hydroxyl concentration of silica, effectively controlling its reactivity and water absorption. The precipitated silica with low surface hydroxyl concentration obtained by the above process has a surface hydroxyl content ≤2.5 hydroxyl groups / nm. 2 BET is 160-220m 2 With an oil absorption value of 210-235 ml / 100 g and DBP, it can be a cheap and effective solution for the severe water absorption and agglomeration of precipitated silica in silicone rubber applications caused by excessively high hydroxyl content on the surface of silica.

[0036] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0037] There are many methods for testing the surface hydroxyl concentration of silica, commonly including infrared spectroscopy, thermogravimetric analysis, isotope substitution, and titration. Infrared spectroscopy, thermogravimetric analysis, and isotope substitution are not suitable for rapid and convenient testing of the surface hydroxyl concentration of silica due to expensive equipment and complex operation. Sodium hydroxide titration, on the other hand, is widely used for rapid testing of the surface hydroxyl concentration of silica because of its simple equipment and easy operation. Since the surface hydroxyl concentrations obtained by different methods will vary, comparisons can only be made horizontally. All surface hydroxyl concentration tests mentioned in this patent are based on the sodium hydroxide titration method.

[0038] Example 1

[0039] A method for preparing precipitated silica with low surface hydroxyl concentration includes the following steps:

[0040] (1) Select low-impurity solid sodium silicate and process water and add them to a closed container. After dissolving, a sodium silicate solution is obtained. Add an ion treatment agent to the sodium silicate solution, let it stand, and filter to obtain the first sodium silicate solution after purification.

[0041] (2) Add sodium hydroxide solution to the first sodium silicate solution, stir thoroughly, adjust the water glass modulus to 2.5 and the concentration to 1.0 mol / l to obtain the second sodium silicate solution.

[0042] (3) Prepare a dilute sulfuric acid solution with a concentration of 2 mol / L from concentrated sulfuric acid.

[0043] (4) Add 22m to the reaction vessel 3 The process water is then added, followed by 1 ml of the first sodium silicate solution obtained in step (1). 3 Heat to 65℃, turn on the stirrer, and press 0.8m 3 The dilute sulfuric acid solution prepared in step (3) was added dropwise at a flow rate of / h, and the pH at the reaction endpoint was controlled at 10.0.

[0044] (5) Turn on the heating device to raise the temperature of the reaction vessel to 85°C. After aging for 60 minutes, continue to add the sodium disilicate solution obtained in step (2) and the dilute sulfuric acid solution obtained in step (3) dropwise into the reaction vessel simultaneously. The flow rate of the sodium disilicate solution is 6 m / s. 3 / h, the flow rate of dilute sulfuric acid is 2.8m 3 / h, process pH is controlled at 10.0, and the second sodium silicate solution is preset to a volume of 7m³. 3 After the addition is complete, add 20ml of cold water to the reaction vessel. 3 Let it age for 20 minutes.

[0045] (6) After aging is complete, continue to add 2m of fuel to the reaction vessel. 3 Dilute sulfuric acid solution was added dropwise at a rate of / h to obtain a slurry when the final pH was controlled at 4.5.

[0046] (7) The slurry from step (6) is subjected to pressure filtration, washing, spray drying and crushing to obtain precipitated silica with low surface hydroxyl concentration.

[0047] Example 2

[0048] A method for preparing precipitated silica with low surface hydroxyl concentration includes the following steps:

[0049] (1) Select low-impurity solid sodium silicate and process water and add them to a closed container. After dissolving, a sodium silicate solution is obtained. Add an ion treatment agent to the sodium silicate solution, let it stand, and filter to obtain the first sodium silicate solution after purification.

[0050] (2) Add sodium hydroxide solution to the first sodium silicate solution, stir thoroughly, adjust the water glass modulus to 2.5 and the concentration to 1.1 mol / L to obtain the second sodium silicate solution.

[0051] (3) Prepare a dilute sulfuric acid solution with a concentration of 2.5 mol / L by dissolving concentrated sulfuric acid.

[0052] (4) Add 22m to the reaction vessel 3 The process water is then added, followed by 3 ml of the first sodium silicate solution obtained in step (1). 3 Heat to 65℃, turn on the stirrer, and press 1.8m 3 The dilute sulfuric acid solution prepared in step (3) was added dropwise at a flow rate of / h, and the pH of the reaction endpoint was controlled at 9.5.

[0053] (5) Turn on the heating device to raise the temperature of the reaction vessel to 85°C. After aging for 60 minutes, continue to add the sodium disilicate solution obtained in step (2) and the dilute sulfuric acid solution obtained in step (3) dropwise into the reaction vessel simultaneously. The flow rate of the sodium disilicate solution is 7 m / s. 3 / h, the flow rate of dilute sulfuric acid is 2.8m 3 / h, the process pH is controlled at 9.5, and the preset volume of the second sodium silicate solution is 9m. 3 After the addition is complete, add 25ml of cold water to the reaction vessel. 3 Let it age for 20 minutes.

[0054] (6) After aging is complete, continue to add fuel to the reaction vessel at a rate of 2.5m. 3 Dilute sulfuric acid solution was added dropwise at a rate of / h to obtain a slurry when the final pH was controlled at 3.0.

[0055] (7) The slurry from step (6) is subjected to pressure filtration, washing, spray drying and crushing to obtain precipitated silica with low surface hydroxyl concentration.

[0056] Example 3

[0057] A method for preparing precipitated silica with low surface hydroxyl concentration includes the following steps:

[0058] (1) Select low-impurity solid sodium silicate and process water and add them to a closed container. After dissolving, a sodium silicate solution is obtained. Add an ion treatment agent to the sodium silicate solution, let it stand, and filter to obtain the first sodium silicate solution after purification.

[0059] (2) Add sodium hydroxide solution to the first sodium silicate solution, stir thoroughly, adjust the water glass modulus to 2.4 and the concentration to 1.2 mol / L to obtain the second sodium silicate solution.

[0060] (3) Prepare a dilute sulfuric acid solution with a concentration of 4 mol / L by dissolving concentrated sulfuric acid.

[0061] (4) Add 22m to the reaction vessel 3 The process water is then added, followed by 5 ml of the first sodium silicate solution obtained in step (1). 3 Heat to 55℃, turn on the stirrer, and press 2.0m 3The dilute sulfuric acid solution prepared in step (3) was added dropwise at a flow rate of / h, and the pH of the reaction endpoint was controlled at 11.

[0062] (5) Turn on the heating device to raise the temperature of the reaction vessel to 75°C. After aging for 120 minutes, continue to add the sodium disilicate solution obtained in step (2) and the dilute sulfuric acid solution obtained in step (3) dropwise into the reaction vessel simultaneously. The flow rate of the sodium disilicate solution is 8 m / s. 3 The sulfuric acid flow rate is 2.2 m / h. 3 / h, process pH is controlled at 11, and the preset volume of the second sodium silicate solution is 11m. 3 After the addition is complete, add 30ml of cold water to the reaction vessel. 3 Let it age for 20 minutes.

[0063] (6) After aging is complete, continue to add 1.0 m³ of fuel to the reaction vessel. 3 Dilute sulfuric acid solution was added dropwise at a rate of / h to obtain a slurry when the final pH was controlled at 5.5.

[0064] (7) The slurry from step (6) is subjected to pressure filtration, washing, spray drying and crushing to obtain precipitated silica with low surface hydroxyl concentration.

[0065] Example 4

[0066] A method for preparing precipitated silica with low surface hydroxyl concentration includes the following steps:

[0067] (1) Select low-impurity solid sodium silicate and process water and add them to a closed container. After dissolving, a sodium silicate solution is obtained. Add an ion treatment agent to the sodium silicate solution, let it stand, and filter to obtain the first sodium silicate solution after purification.

[0068] (2) Add sodium hydroxide solution to the first sodium silicate solution, stir thoroughly, adjust the water glass modulus to 2.0 and the concentration to 1.0 mol / l to obtain the second sodium silicate solution.

[0069] (3) Prepare a dilute sulfuric acid solution with a concentration of 1 mol / L from concentrated sulfuric acid.

[0070] (4) Add 22m to the reaction vessel 3 The process water is then added, followed by the first sodium silicate solution (1) prepared in step (1) in 1m. 3 Heat to 75℃, turn on the stirrer, and press 2.0m 3 The dilute sulfuric acid solution prepared in step (3) is added dropwise at a flow rate of / h, and the pH at the reaction endpoint is controlled at 10.

[0071] (5) Turn on the heating device to raise the temperature of the reaction vessel to 95°C. After aging for 30 minutes, continue to add the sodium disilicate solution obtained in step (2) and the dilute sulfuric acid solution obtained in step (3) dropwise into the reaction vessel simultaneously. The flow rate of the sodium disilicate solution is 6 m / s. 3 The flow rate of dilute sulfuric acid is 5.6 m / h. 3 / h, process pH is controlled at 10, and the preset volume of the second sodium silicate solution is 7m. 3 After the addition is complete, add 20ml of cold water to the reaction vessel. 3 Let it age for 20 minutes.

[0072] (6) After aging is complete, continue to add 2.0m of fuel to the reaction vessel. 3 Dilute sulfuric acid solution was added dropwise at a rate of / h to obtain a slurry when the final pH was controlled at 4.5.

[0073] (7) The slurry from step (6) is subjected to pressure filtration, washing, spray drying and crushing to obtain precipitated silica with low surface hydroxyl concentration.

[0074] Comparative Example 1

[0075] In this comparative example, all preparation steps and parameters are the same as in Example 1, except that the heating temperature in step (4) is adjusted to 50°C and the reaction vessel temperature in step (5) is adjusted to 70°C.

[0076] Comparative Example 2

[0077] In this comparative example, all preparation steps and parameters are the same as in Example 1, except that the flow rate of dilute sulfuric acid in step (4) is adjusted to 3.0 m. 3 / h.

[0078] Comparative Example 3

[0079] In this comparative example, all preparation steps and parameters are the same as in Example 1, except that the pH at the reaction endpoint in step (4) is adjusted to 8.5, and the pH during the process in step (5) is adjusted to 8.5.

[0080] Comparative Example 4

[0081] In this comparative example, all preparation steps and parameters are the same as in Example 1, except that the dropping rate of dilute sulfuric acid in step (6) is adjusted to 5.0 m. 3 / h.

[0082] The performance of the precipitated silica prepared in Examples 1-4 and Comparative Examples 1-4, as well as the commercially available unmodified fumed silica prepared in the fumed silica method, was tested as follows. The specific test results are shown in Table 1 below.

[0083] Table 1 Performance test results of precipitated silica products

[0084] Group <![CDATA[BET specific surface area (m 2 / g)]]> Oil absorption value (ml / 100g) <![CDATA[Surface hydroxyl content (number / nm 2 )]]> Example 1 200 230 1.8 Example 2 190 220 2.2 Example 3 160 210 2.5 Example 4 190 228 2.0 Comparative Example 1 220 230 2.8 Comparative Example 2 210 230 2.8 Comparative Example 3 165 200 3.5 Comparative Example 4 230 228 3.0 gaseous silicon 200 210 1.77

[0085] Note: The test methods are as follows: the BET specific surface area of ​​silica is determined according to GB / T 10722; the oil absorption value of silica is determined according to HG / T 3072; and the surface hydroxyl content of silica is determined according to T / FSI 049-2020.

[0086] As can be seen from the test results of Examples 1-4 in Table 1, the BET specific surface area of ​​the precipitated silica with low surface hydroxyl concentration provided by the present invention is 160-220 m². 2 / g, oil absorption value 210-235ml / 100g, surface hydroxyl content ≤2.5 hydroxyl groups / nm 2 This indicates that the precipitated silica with low surface hydroxyl concentration prepared by the method of the present invention has a moderate specific surface area and oil absorption value, while the surface hydroxyl content is significantly lower than that of conventional precipitated silica. Among them, the silica prepared in Example 1 exhibits excellent properties in all aspects, especially with the lowest surface hydroxyl content, making it the optimal embodiment of the present invention, approaching the performance of unmodified fumed silica (1.77 hydroxyl / nm). 2 ).

[0087] The test results of Comparative Examples 1-4 show that adjusting the reaction temperature, the flow rate of dilute sulfuric acid at each stage, and the pH value in the preparation process of this scheme all have a significant impact on the BET specific surface area and surface hydroxyl content of precipitated silica. Some changes in conditions (such as pH value / dropping rate of dilute sulfuric acid in step (6)) also affect the oil absorption value of precipitated silica.

[0088] In addition, this scheme also tests the water absorption of the precipitation method prepared by Examples 1-4 and Comparative Examples 1-4, as well as commercially available unmodified fumed silica. Since there are no relevant national or industry standards for silica, this experiment was designed with reference to the water absorption testing standards of other hygroscopic materials. The test method is as follows: The sample to be tested was dried in an oven at 160℃ for 1 day. The dried sample was then placed in a constant temperature and humidity chamber at 25℃ and 50% humidity for 2 hours, 4 hours, and 8 hours. The mass of the sample after drying and after different placement times was measured. The water absorption rate of the sample was expressed as the percentage of the mass difference compared to the initial mass. The test results are shown in Table 2 below.

[0089] Table 2. Water absorption test results of the precipitation-based silica products with low surface hydroxyl concentration according to the present invention.

[0090] Group 2h water absorption rate % 4h water absorption rate % 8h water absorption rate % Example 1 3.08 4.12 5.58 Example 2 3.58 5.06 6.11 Example 3 4.17 5.33 6.52 Example 4 3.66 5.18 6.00 Comparative Example 1 4.98 6.02 8.76 Comparative Example 2 4.56 7.31 7.69 Comparative Example 3 8.42 9.55 12.06 Comparative Example 4 7.71 8.98 10.58 gaseous silicon 3.12 3.79 4.94

[0091] As can be seen from the test results of each group in Table 2, the water absorption rate of the precipitated silica with low surface hydroxyl concentration provided by the present invention is close to that of fumed silica. When the reaction temperature, the flow rate of dilute sulfuric acid at each stage or the pH value of the preparation process of this scheme are adjusted, the water absorption rate of the precipitated silica increases significantly.

[0092] In addition, this scheme also conducts silicon powder application tests on the precipitation method and commercially available unmodified fumed silica prepared in Examples 1-4 and Comparative Examples 1-4. The specific test method is as follows: the above-mentioned silica is prepared into silicone rubber. The silicone rubber formula is as follows: 100 parts raw rubber, 75 parts silica sample, and 6 parts hydroxyl silicone oil. After the silicone rubber is kneaded in a kneader and vacuumed, 0.5% of a bis(2,5)-5-vinyl chloride agent is added for vulcanization to prepare vulcanized sheets for subsequent testing. Specifically, haze / transmittance tests are performed: a TH-100 haze meter is used to measure the haze and transmittance of vulcanized sheets of the same thickness after they are freshly prepared and placed in a constant temperature and humidity chamber for a certain period of time. The results are shown in Tables 3 and 4 below.

[0093] Table 3. Haze test results of silicone rubber at different time periods

[0094] Group initial 1d 7d Example 1 17 20 22 Example 2 20 23 25 Example 3 18 21 26 Example 4 15 22 23 Comparative Example 1 20 25 35 Comparative Example 2 18 30 38 Comparative Example 3 21 31 45 Comparative Example 4 17 21 36 gaseous silicon 15 16 16

[0095] Table 4. Light transmittance test results of silicone rubber at different time periods

[0096]

[0097]

[0098] As shown in Tables 3 and 4, the haze and transmittance of fumed silica (fumed silica) did not change over time. However, after adjusting the reaction temperature, dilute sulfuric acid flow rate, or pH value in the preparation process of this scheme (i.e., using conventional precipitated silica preparation parameters), the haze increased significantly and the transmittance decreased markedly over time due to the excessively high surface hydroxyl content. In contrast, the precipitated silica of this invention with low surface hydroxyl content showed a significantly slower rate of increase in haze and decrease in transmittance over time. In Example 1, after being placed in a constant temperature and humidity chamber for 7 days, the haze was 22 and the transmittance was 82, which is close to that of unmodified fumed silica.

[0099] Example 5

[0100] In this embodiment, all preparation steps and parameters are the same as in Example 1, except that the reaction temperature is different, as detailed in the table below:

[0101]

[0102]

[0103] The precipitated silica prepared in Example 5 was subjected to performance testing, and the specific test results are shown in the table below:

[0104] Group <![CDATA[BET specific surface area (m 2 / g)]]> Oil absorption value (ml / 100g) <![CDATA[Surface hydroxyl content (number / nm 2 )]]> Example 1 200 230 1.8 Example 5-1 218 228 2.5 Example 5-2 215 230 2.4 Example 5-3 211 232 2.3 Example 5-4 206 231 2.0 Example 5-5 215 231 2.2 Examples 5-6 211 233 2.0 Examples 5-7 206 232 1.9 Examples 5-8 203 228 1.8 Examples 5-9 211 227 2.2 Examples 5-10 208 230 2.0 Examples 5-11 203 225 1.9 Examples 5-12 198 232 1.9 Examples 5-13 208 228 2.3 Examples 5-14 204 231 2.1 Examples 5-15 200 229 2.0 Examples 5-16 195 233 2.0

[0105] As shown in the test data in the table above, adjusting the reaction temperature mainly affects the BET specific surface area and surface hydroxyl content of precipitated silica with low surface hydroxyl concentration. In this scheme, the preferred temperature in step S1 / step (4) is 60-70℃, and the preferred temperature in step S2 / step (5) is 80-95℃. The lower the surface hydroxyl content of precipitated silica with low surface hydroxyl concentration obtained within this range, the better.

[0106] Example 6

[0107] In this embodiment, all preparation steps and parameters are the same as in Example 1. The difference lies in the pH at each stage, as detailed in the table below:

[0108]

[0109] The precipitated silica prepared in Example 6 was subjected to performance testing, and the specific test results are shown in the table below:

[0110] Group <![CDATA[BET specific surface area (m 2 / g)]]> Oil absorption value (ml / 100g) <![CDATA[Surface hydroxyl content (number / nm 2 )]]> Example 1 200 230 1.8 Example 6-1 185 222 2.5 Example 6-2 209 233 2.0 Example 6-3 215 235 2.3

[0111] As shown in the test data in the table above, the preferred reaction endpoint pH value in step S1 / step (4) of this scheme is 10-10.5, and the preferred reaction process pH value in step S2 / step (5) is 10-10.5. At this time, the BET specific surface area of ​​the precipitated silica with low surface hydroxyl concentration is moderate, the surface hydroxyl content is low, and the oil absorption value is moderate.

[0112] Example 7

[0113] In this embodiment, all preparation steps and parameters are the same as in Example 1. The difference is that the flow rate of dilute sulfuric acid in step (4) is different, as detailed in the table below:

[0114]

[0115]

[0116] The precipitated silica prepared in Example 7 was subjected to performance testing, and the specific test results are shown in the table below:

[0117] Group <![CDATA[BET specific surface area (m 2 / g)]]> Oil absorption value (ml / 100g) <![CDATA[Surface hydroxyl content (number / nm 2 )]]> Example 1 200 230 1.8 Example 7-1 180 228 1.6 Example 7-2 205 231 1.8 Example 7-3 208 233 2.0 Example 7-4 211 233 2.3

[0118] As shown in the test data in the table above, the preferred flow rate of dilute sulfuric acid in step S1 / step (4) of this scheme is 0.5-1.5 m. 3 / h, at this time, as the flow rate of dilute sulfuric acid in step S1 / step (4) decreases, the surface hydroxyl content continues to decrease. However, factors such as time cost need to be considered. At the same time, considering the influence of surface hydroxyl on the reinforcement of silicon powder in silicone rubber, the preferred dilute sulfuric acid flow rate in this scheme is 0.5-1.5m. 3 / h.

[0119] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A process for the preparation of a precipitated silica having a low surface hydroxyl concentration, characterized in that, The method comprises the following steps: S1. adding water and a first sodium silicate solution into a container, heating to 55-75℃, then starting stirring, and then adding a dilute sulfuric acid solution dropwise, controlling the pH at the reaction endpoint to be 9.5-11.0; S2. increasing the temperature to 75-95℃, aging for 30-120 min, then adding a second sodium silicate solution and a dilute sulfuric acid solution dropwise into the container simultaneously, controlling the pH to be 9.5-11.0, and then adding cold water into the container to continue aging after the preset amount of the second sodium silicate solution is added dropwise; S3. after aging, continuing to add the dilute sulfuric acid solution dropwise into the container, controlling the pH at the reaction endpoint to be 3.0-5.5 to obtain a slurry; S4. the slurry is subjected to pressure filtration, washing, spray drying and crushing to obtain the precipitated silica with low surface hydroxyl concentration.

2. The method of producing a precipitated silica having a low surface hydroxyl group concentration according to claim 1, characterized by, The preparation of the first sodium silicate solution comprises the following steps: dissolving sodium silicate in water to obtain a sodium silicate solution, adding an ion treatment agent into the sodium silicate solution, and then standing, filtering to obtain the purified first sodium silicate solution; The preparation of the second sodium silicate solution comprises the following steps: adding a sodium hydroxide solution into the first sodium silicate solution, stirring sufficiently, adjusting the modulus to be 2.0-2.5 and the concentration to be 1.0-1.2 mol / L to obtain the second sodium silicate solution.

3. The process for preparing a precipitated silica having a low surface hydroxyl concentration according to claim 1, characterized in that, The dropping speed of the dilute sulfuric acid solution in step S1 is 0.5-2 m 3 / h; the dropping speed of the dilute sulfuric acid solution in step S2 is 2-8 m 3 / h; and the dropping speed of the dilute sulfuric acid solution in step S3 is 1-4 m 3 / h.

4. The method of producing a precipitated silica having a low surface hydroxyl group concentration according to claim 1, characterized by, The amount of the first sodium silicate solution added in step S1 is 1-5 m 3 .

5. The process for preparing a precipitated silica having a low surface hydroxyl concentration according to claim 1, characterized in that, The second sodium silicate solution in step S2 is preset to be 7-11 m 3 / h, and the dropping speed is 6-8 m 3 / h.

6. The process for preparing a precipitated silica having a low surface hydroxyl concentration according to claim 1, characterized in that, The amount of cold water added in step S2 is 20-30 m 3 .

7. The process for preparing a precipitated silica having a low surface hydroxyl concentration according to claim 1, characterized in that, The concentration of the dilute sulfuric acid solution in step S1 is 1-4 mol / L.

8. The method of producing a precipitated silica having a low surface hydroxyl group concentration according to claim 1, characterized by, The stirring speed in step S1 is 30-40 HZ.

9. A precipitated silica of low surface hydroxyl concentration, characterized in that, a precipitated silica of low surface hydroxyl concentration having a surface hydroxyl content of < 2.5 / nm, prepared by the process of any of claims 1 to 8 2 , a BET of 160-220 m 2 / g, and a DBP oil absorption of 210-235 ml / 100g.

10. The use of the precipitated silica with low surface hydroxyl concentration according to claim 9 in silicone rubber.

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