High-concentration silica sol stabilization method based on surface site shielding

Abstract

The invention provides a high-concentration silica sol stabilization method based on surface site shielding, and relates to the technical field of colloidal chemistry and material scientificity, and the high-concentration silica sol stabilization method comprises the three steps of initial silicon solution preparation, surface site shielding treatment and concentration and stabilization verification. According to the present invention, the efficient stabilization of the high-concentration silica sol is achieved, specifically, the active silicon hydroxyl site on the silica sol particle surface can be precisely targeted, the condensation reaction between the particles is blocked through the dual effects of the adsorption and the covalent condensation of the screening agent molecule, and the core problem of easy gelation at the high concentration is completely overcome; according to the preparation method, silica sol is dispersed stably during long-term storage, the effect obviously exceeds that of a traditional strong base inhibition method, a single silane modification method or an organic dispersing agent method, the shelf life of the product is effectively prolonged, a trace and efficient bifunctional silane combination is adopted as a screening agent, the dosage is far lower than that of a traditional modification means, and the industrial production cost is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of colloid chemistry and material science technology, and particularly relates to a high-concentration silica sol stabilization method based on surface site shielding. BACKGROUND

[0002] As a colloidal system formed by dispersing nano-silica particles in water, silica sol occupies an important position in the fields of precision casting, coating, ceramics, electronic packaging, etc. due to its unique nanostructure, high specific surface area and good chemical inertness. However, the stability of silica sol is directly related to its application performance: when the silica concentration is low (usually less than 20%), the electrostatic repulsion between particles can be achieved by adjusting the pH to alkaline (pH 9-10) or adding a small amount of electrolyte; but when the concentration is increased to more than 30%, the distance between particles is significantly reduced, and the probability of dehydration condensation of surface silanol (-SiOH) is greatly increased, which easily leads to particle agglomeration or even gelation, resulting in product scrap or shortened storage period, which becomes a bottleneck restricting the industrial application of high-concentration silica sol.

[0003] To solve the stabilization problem of high-concentration silica sol, the existing technology mainly adopts three strategies: the first is the "strong alkali inhibition method", which adjusts the pH of the system to more than 12 by using sodium hydroxide and other strong alkalis to inhibit the dissociation and condensation of silanol in a high-pH environment. However, strong alkalis not only easily corrode production equipment, but also introduce metal ion impurities (such as Na + ), affecting downstream applications (such as causing surface defects in castings in precision casting), and the storage period is still short (usually not more than 3 months) at high concentrations. The second is the "single silane modification method", which uses methyltrimethoxysilane and other single silanes to coat the surface of the particles, and the silanol formed by the hydrolysis of the silane is condensed with the surface of the particles to achieve shielding. However, due to the lack of directional adsorption groups, the shielding agent is unevenly distributed on the surface of the particles, which easily leads to local over-crosslinking or insufficient shielding, resulting in limited stabilization effect (the storage period is usually not more than 4 months), and the amount of single silane used is large (3%-5% of the mass of silica), which is high in cost. The third is the "organic dispersant method", which adds polyvinyl alcohol, polyethylene glycol and other high molecular dispersants to enhance the repulsion between particles through steric hindrance. However, high molecules are easily detached and ineffective at high temperatures or during long-term storage, and the residual organic impurities will contaminate the silica sol, limiting its application in high-purity required fields (such as electronic-grade ceramics).

[0004] Therefore, the above methods still have problems such as insufficient stability, high cost, and residual impurities, which are difficult to meet the comprehensive needs of long storage period, low impurity, and low cost for industrial production of high-concentration silica sol. Therefore, the present application proposes a high-concentration silica sol stabilization method based on surface site shielding to solve the problems existing in the prior art. SUMMARY

[0005] In view of the above problems, the purpose of the present application is to provide a high-concentration silica sol stabilization method based on surface site shielding. The present application realizes efficient stabilization of high-concentration silica sol by a combination of a bifunctional small-molecule silane combined shielding agent and stepwise pH regulation, and can solve the problems in the prior art.

[0006] To achieve the purpose of the present application, the present application realizes the following technical solution: a high-concentration silica sol stabilization method based on surface site shielding, comprising the following steps:

[0007] Step one: prepare a silicon source and water, mix them, and then hydrolyze and condense under alkaline conditions to obtain an initial silica sol, wherein the mass fraction of silicon dioxide in the initial silica sol is 25% to 40%, the pH is 8.5 to 10.0, and the particle size is 15 to 50 nm;

[0008] Step two: add a surface shielding agent to the initial silica sol, adjust the pH to 5.0 to 7.0, allow the surface shielding agent molecules to be adsorbed on the surface active sites of the silica sol particles through the first functional group, and then react at 30 to 50℃ for 1 to 2 hours to allow the second functional group of the surface shielding agent to hydrolyze and condense with the surface silicon hydroxyl groups to form a covalent bond, complete the site shielding, and after the reaction is completed, adjust the pH to 8.0 to 9.0, and let it stand for 12 to 24 hours to obtain a treated product;

[0009] Step three: remove the water in the treated product by ultrafiltration, and make the mass fraction of silicon dioxide in the treated product reach 35% to 50%, thereby obtaining a to-be-verified silica sol, and then perform stabilization verification, when the absolute value of the Zeta potential of the to-be-verified silica sol is ≥ 30 mV or no obvious gelation is precipitated after 30 days of accelerated aging at 60℃, it is a stabilized high-concentration silica sol.

[0010] Further improvement lies in that in step one, the silicon source is a mixture of tetraethyl orthosilicate and sodium silicate, and the mass ratio of tetraethyl orthosilicate to sodium silicate is 1:1 to 3:1.

[0011] Further improvement lies in that in step one, the alkaline condition is realized by adding an alkaline adjusting agent, wherein the alkaline adjusting agent is ammonia water or sodium hydroxide, and the specific parameters for hydrolysis and condensation under alkaline conditions are: temperature 40 to 60℃, stirring speed 200 to 400 rpm, and reaction time 2 to 4 hours.

[0012] Further improvement lies in that in step two, the surface shielding agent is a combination of 3-aminopropyltriethoxysilane and methyltrimethoxysilane, the molar ratio is 1:2 to 1:4, and the total amount is 0.5% to 2.0% of the mass of silicon dioxide.

[0013] Further improvement lies in that in the step two, the acid used for adjusting pH to 5.0-7.0 is dilute hydrochloric acid or dilute acetic acid, and the base used for adjusting pH to 8.0-9.0 is dilute ammonia.

[0014] Further improvement lies in that in the step two, the standing and aging is carried out in a closed container, and the aging environment is 25-30℃ and relative humidity ≤60%.

[0015] Further improvement lies in that in the step three, the molecular weight cut-off of the ultrafiltration method is 1000-3000Da, the operating pressure is 0.1-0.3MPa, and the temperature is controlled to ≤40℃ during the concentration process.

[0016] Further improvement lies in that in the step three, the verification of the stabilization further comprises determining the viscosity of the silica sol, and the viscosity after the stabilization is 10-20mPa·s.

[0017] The present application has the following beneficial effects: the present application adopts a dual-functional small-molecule silane combination shielding agent and a step-by-step pH regulation strategy, realizes efficient stabilization of high-concentration silica sol, and specifically can precisely target the active silicon hydroxyl sites on the surface of silica sol particles, blocks the condensation reaction between particles through the adsorption and covalent condensation of shielding agent molecules, completely overcomes the core problem of easy gelation at high concentration, makes the silica sol remain dispersed and stable during long-term storage, significantly surpasses the effects of traditional strong alkali inhibition, single silane modification or organic dispersant method, and effectively prolongs the shelf life of the product.

[0018] In addition, a trace amount of efficient dual-functional silane combination is used as a shielding agent, the amount used is much lower than that of traditional modification methods, and the cost and particle size control precision are balanced by compounding the silicon source, further reducing the raw material and process cost of industrial production. The step-by-step pH regulation not only guarantees the hydrolysis efficiency of the shielding agent but also restores the electrostatic repulsion on the surface of the particles, avoids the risk of strong alkali corrosion, and adapts to the existing production line modification under mild concentration conditions, improving the technical landing feasibility. As a result, the electrostatic repulsion between the silica sol particles after stabilization is enhanced, the viscosity is moderate and the flowability is good, and there is no residual metal ion or high molecular impurities, the product has high purity, and lays a good foundation for downstream applications. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a step flow schematic diagram of the present application. DETAILED DESCRIPTION

[0020] In order to deepen the understanding of the present application, the present application will be further described in combination with examples, and the present examples are only used to explain the present application and do not constitute a limitation on the protection scope of the present application.

[0021] Example One

[0022] According to Figure 1As shown, the present embodiment proposes a high-concentration silica sol stabilization method based on surface site shielding, including the following steps:

[0023] Step one, preparation of initial silica solution

[0024] Prepare 50 g of tetraethyl orthosilicate, 50 g of sodium silicate (water glass, modulus 3.3) (mass ratio of tetraethyl orthosilicate to sodium silicate 1:1), and mix with 200 g of deionized water, add ammonia water (25 wt%) to adjust the system pH to 8.5, hydrolysis and polycondensation under the conditions of temperature 40℃ and stirring speed 200 rpm for 2h, to obtain the initial silica sol. The initial silica sol has a silica mass fraction of 25%, a pH of 8.5, a particle size of 15 nm (laser particle size instrument D50 value), and a transparent appearance without precipitation;

[0025] Step two, surface site shielding treatment

[0026] Add a surface shielding agent to the initial silica sol. The surface shielding agent is a combination of 3-aminopropyltriethoxysilane and methyltrimethoxysilane, with a molar ratio of 1:4, and the total amount is 2.0% of the mass of silica (based on the initial silica mass of 40g, the total amount is 0.8g, of which 3-aminopropyltriethoxysilane is 0.13g and methyltrimethoxysilane is 0.67g, corresponding to a shielding agent silicon atom to silica silicon atom molar ratio of about 0.003:1). First, adjust the system pH to 7.0 with 0.2 mol / L dilute acetic acid, and react at a temperature of 50℃ and a stirring speed of 500 rpm for 2h; after the reaction, adjust the pH to 9.0 with 0.2 mol / L dilute ammonia water, and let it stand and age in a sealed container at 30℃ and a relative humidity of 55% for 24h to obtain the treated product;

[0027] Step three, concentration and stabilization verification

[0028] Remove the water from the treated product using ultrafiltration with a molecular weight cutoff of 1000 Da, an operating pressure of 0.1 MPa, and a temperature of 35℃, to obtain a final silica mass fraction of 35%, and obtain the silica sol to be verified. Perform stabilization verification, and the verification results are as follows: Zeta potential -42 mV (absolute value ≥ 30 mV), viscosity at 25℃ of 12 Pa·s, no gel precipitation after 30 days of accelerated aging at 60℃, and stable and effective.

[0029] Example two

[0030] According to Figure 1 As shown, the present embodiment proposes a high-concentration silica sol stabilization method based on surface site shielding, including the following steps:

[0031] Step one, preparation of initial silica solution

[0032] Prepare 75g of tetraethyl orthosilicate and 25g of sodium silicate (water glass, modulus 3.3) (tetraethyl orthosilicate to sodium silicate mass ratio 3:1), and mix with 150g of deionized water; add sodium hydroxide solution to adjust the pH of the system to 10.0, and hydrolyze and condense at 60℃ and 400rpm for 4h to obtain the initial silica sol. This initial silica sol has a silica mass fraction of 40%, a pH of 10.0, a particle size of 40nm, and is transparent without precipitation.

[0033] Step 2: Surface Site Shielding Treatment

[0034] A surface shielding agent, a combination of 3-aminopropyltriethoxysilane and methyltrimethoxysilane in a molar ratio of 1:4, was added to the initial silica sol. The total amount was 2.0% of the silica mass (based on an initial silica mass of 40g, the total amount was 0.8g, including 0.13g of 3-aminopropyltriethoxysilane and 0.67g of methyltrimethoxysilane, corresponding to a molar ratio of silicon atoms in the shielding agent to silicon atoms in the silica of approximately 0.003:1). The pH of the system was first adjusted to 7.0 with 0.2mol / L dilute acetic acid, and the reaction was carried out at 50℃ and 500rpm for 2h. After the reaction, the pH was adjusted to 9.0 with 0.2mol / L dilute ammonia, and the mixture was aged in a sealed container at 30℃ and 55% relative humidity for 24h to obtain the treated product.

[0035] Step 3: Concentration and Stabilization Verification

[0036] Moisture was removed from the treated material using ultrafiltration. The ultrafiltration molecular weight cutoff was 3000 Da, the operating pressure was 0.3 MPa, and the temperature was 38℃, resulting in a final silica mass fraction of 50%, yielding the silica sol to be validated. Stabilization validation was performed, and the results showed: a Zeta potential of -38 mV (absolute value ≥30 mV), a viscosity of 18 Pa·s at 25℃, and no gel precipitation after 30 days of accelerated aging at 60℃, indicating effective stabilization.

[0037] Example 3

[0038] according to Figure 1 As shown, this embodiment proposes a high-concentration silica sol stabilization method based on surface site shielding, including the following steps:

[0039] Step 1: Preparation of initial silicon solution

[0040] Preparation of tetraethyl orthosilicate 67 g, sodium silicate (water glass, modulus 3.3) 33 g (mass ratio of tetraethyl orthosilicate to sodium silicate 2:1), and deionized water 180 g were mixed; ammonia water was added to adjust the pH of the system to 9.0; hydrolysis and polycondensation were carried out at a temperature of 50°C and a stirring speed of 300 rpm for 3 h to prepare an initial silica sol. The initial silica sol had a silica mass fraction of 30%, a pH of 9.0, a particle size of 25 nm, and a transparent appearance without precipitation;

[0041] Step two, surface site shielding treatment

[0042] A surface shielding agent was added to the initial silica sol. The surface shielding agent was a combination of 3-aminopropyltriethoxysilane and methyltrimethoxysilane at a molar ratio of 1:3 (preferably), and the total amount was 1.0% of the mass of silica (based on 30 g of initial silica, the total amount was 0.3 g, of which 3-aminopropyltriethoxysilane was 0.06 g and methyltrimethoxysilane was 0.24 g, corresponding to a shielding agent silicon atom to silica silicon atom molar ratio of about 0.0025:1). First, 0.1 mol / L dilute hydrochloric acid was used to adjust the pH of the system to 6.5, and the reaction was carried out at a temperature of 40°C and a stirring speed of 400 rpm for 1.5 h. After the reaction, 0.1 mol / L dilute ammonia water was used to adjust the pH to 8.5, and the treated product was obtained by standing and aging in a sealed container at 28°C and a relative humidity of 50% for 18 h.

[0043] Step three, concentration and stability verification

[0044] The treated product was dehydrated by ultrafiltration with a molecular weight cutoff of 2000 daltons, an operating pressure of 0.2 megapascals, and a temperature of 36°C to obtain a final silica mass fraction of 42.5%. The verification results were as follows: Zeta potential -45 mV (absolute value ≥ 30 mV), 25°C viscosity 15 mPa·s, no gel precipitation after 30 days of accelerated aging at 60°C, and the stability effect was optimal.

[0045] In combination with Example One, Example Two, and Example Three, Comparative Example One, Comparative Example Two, and Comparative Example Three were set up for effect comparison, wherein:

[0046] Comparative Example One was a strong alkali method. The initial silica sol (silica mass fraction 28%) of Example One was directly adjusted to a pH of 12.0 (strong alkali environment) with sodium hydroxide (NaOH), without adding any shielding agent, and was concentrated to a silica mass fraction of 38%.

[0047] Comparative Example Two was a single silane modification method. In Step Two, only methyltrimethoxysilane (MTMS, 1.7 g) was used as a shielding agent (single functional group), and the other parameters were the same as those in Example One.

[0048] The comparative example three is an organic dispersant method, polyvinyl alcohol (PVA) is added to the initial silica sol (silica mass fraction 30%) as an organic dispersant, the amount is 8.0% of the mass of silica, and after stirring and mixing uniformly, it is concentrated to a silica mass fraction of 30%.

[0049] As shown in Table 1 below, it is the stabilization effect of each scheme in 60℃ accelerated aging for 30 days (equivalent room temperature storage period (25℃) based on Arrhenius formula calculation):

[0050] Further, the example two is compared with the comparative example one, the comparative example two and the comparative example three, the comparison index is Zeta potential (mV), viscosity (25℃, mPa·s), additive amount and impurities, and the comparison results are shown in Table 2 below:

[0051] It can be found from the above Table 1 and Table 2 that the storage period of the present application is more than 6 months (room temperature) under 30% to 50% silica concentration, which is more than doubled compared with the prior art (≤4 months); in terms of cost control, the shielding agent amount is only 0.5% to 2.0%, in terms of process friendliness, there is no strong alkali corrosion risk and no impurity residue, which is suitable for industrialization. The key parameters (Zeta potential absolute value ≥ 30 mV, viscosity 10-20 mPa·s) are better, and the comprehensive performance is suitable for high-end fields such as precision casting and coating.

[0052] The above shows and describes the basic principles, main features and advantages of the present application. It should be understood by those skilled in the art that the present application is not limited by the above examples, and the above examples and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the framework and scope of application of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for stabilizing high concentration silica sols based on surface site shielding, characterized by: The method comprises the following steps: Step 1: preparing a silicon source and water, mixing them, and then hydrolyzing and polycondensing under alkaline conditions to obtain an initial silica sol, wherein the mass fraction of silica in the initial silica sol is 25-40%, the pH is 8.5-10.0, and the particle size is 15-50 nm; Step 2: adding a surface shielding agent to the initial silica sol, adjusting the pH to 5.0-7.0, allowing the molecules of the surface shielding agent to be adsorbed on the surface active sites of the silica sol particles through the first functional group, and then reacting at 30-50℃ for 1-2 h to allow the second functional group of the surface shielding agent to hydrolyze and condense with the surface silicon hydroxyl groups to form covalent bonds, complete site shielding, and after the reaction is completed, adjusting the pH to 8.0-9.0 and standing for aging for 12-24 h to obtain a treated product; Step 3: removing the water in the treated product by using an ultrafiltration method, and allowing the mass fraction of silica in the treated product to reach 35-50% to obtain a silica sol to be verified, and then performing a stabilization verification, when the absolute value of the Zeta potential of the silica sol to be verified is ≥ 30 mV or no obvious gelation is precipitated after 30 days of accelerated aging at 60℃, the silica sol is a stabilized high-concentration silica sol.

2. The method for stabilizing high concentration silica sol based on surface site shielding according to claim 1, characterized in that: In step 1, the silicon source is a mixture of tetraethyl orthosilicate and sodium silicate, and the mass ratio of tetraethyl orthosilicate to sodium silicate is 1:1-3:

1.

3. The method for stabilizing high concentration silica sol based on surface site shielding according to claim 1, characterized in that: In step 1, the alkaline condition is achieved by adding an alkaline adjusting agent, wherein the alkaline adjusting agent is ammonia water or sodium hydroxide, and the specific parameters of the hydrolysis and polycondensation under the alkaline condition are as follows: the temperature is 40-60℃, the stirring speed is 200-400 rpm, and the reaction time is 2-4 h.

4. The method for stabilizing high concentration silica sol based on surface site shielding according to claim 1, characterized by: In step 2, the surface shielding agent is a combination of 3-aminopropyltriethoxysilane and methyltrimethoxysilane, the molar ratio is 1:2-1:4, and the total amount is 0.5%-2.0% of the mass of silica.

5. The method for stabilizing high concentration silica sol based on surface site shielding according to claim 1, characterized by: In step 2, the acid used for adjusting the pH to 5.0-7.0 is dilute hydrochloric acid or dilute acetic acid, and the base used for adjusting the pH to 8.0-9.0 is dilute ammonia water.

6. The method for stabilizing high concentration silica sol based on surface site shielding according to claim 1, characterized in that: In step 2, the standing for aging is performed in a sealed container, and the aging environment is 25-30℃ and a relative humidity of ≤ 60%.

7. The method for stabilizing high concentration silica sol based on surface site shielding according to claim 1, characterized by: In step 3, the ultrafiltration method has a molecular weight cut-off of 1000-3000 Da, and the operating pressure is 0.1-0.3 MPa, and the temperature is controlled to ≤ 40℃ during the concentration process.

8. The method for stabilizing high concentration silica sol based on surface site shielding according to claim 1, characterized by: In step 3, the stabilization verification also includes measuring the viscosity of the silica sol, and the viscosity after stabilization is 10-20 mPa·s.

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