A method for preparing a modified silica sol in an acidic system

Abstract

The application discloses a preparation method of modified silica sol stable in an acid system. Under the action of a zwitterionic polymer, a silane coupling agent treated by an amino acid is directly grafted on the surface of silica particles, so that the grafting rate of the silane coupling agent on the surface of small-particle-size silica particles in an acid system is effectively improved; meanwhile, the silane coupling agent treated by the amino acid can reduce the agglomeration between the silica particles, and the acid stability of the silica particles is improved. The heat-expandable microspheres prepared by using the method have a small outflow rate and good stability.

Description

Technical Field

[0001] This invention belongs to the field of microsphere preparation, specifically relating to a method for preparing modified silica sol. Background Technology

[0002] Thermally expandable thermoplastic microspheres are a type of foamed material that softens upon heating and expands into spheres using a foaming agent inside. This material has significant application prospects in industries such as automotive interior and exterior trim, building materials, and home appliances. A typical preparation process for thermally expandable thermoplastic microspheres involves internal polymerization of oil droplets in an emulsion system. To ensure proper reaction, a Pickering emulsion of silica is generally used to improve the stability of the oil droplets. Therefore, a prerequisite for stable microsphere preparation is improving the stability of the silica dispersion.

[0003] The stability of silica dispersions depends on many factors, including particle size, solid content, pH value, and surface hydrophilicity / hydrophobicity. Stabilizing small-sized silica particles in acidic systems has always been a challenge because smaller particle sizes result in more silanol groups on the particle surface, leading to higher surface energy and a greater tendency for particle aggregation. Furthermore, silica particles have an isoelectric point close to 0 in acidic conditions, making them even less stable. Currently, the most effective method for preparing small-particle-size, high-solid-content, and long-term stable acidic silica sols is surface modification.

[0004] The exploration of modified silica sol has never ceased. Patent CN112094509A proposes using a relatively dense hydrated layer formed on the surface of silica particles by small-molecule organic acids, combined with a silane coupling agent. This results in the silica surface being coated with a large number of organic groups, reducing the further aggregation of silanols, dimers, and polymers generated by the hydrolysis of tetraethyl orthosilicate. However, the results show that the viscosity of the silica sol increased after 12 months of storage, indicating that this process is unsuitable for more demanding applications. Patent CN115703931 proposes modifying colloidal silica under acidic conditions using a hydrolyzed silane coupling agent. This effectively inhibits the formation of flocculents, precipitates, or gelation during or after the addition of the silane coupling agent. However, this method results in larger silica particle sizes, and the particles re-aggregate after standing, making it unsuitable for high-solids systems. Some studies have used silane coupling agents and sterically hindered additives or zwitterionic compounds to graft onto silica particles. This method produces stable silica particles with easily controllable modification levels. For example, patent CN113713625 proposes modifying silica particles with silane coupling agents and zwitterions, drying the powder particles, and then polymerizing the zwitterionic compound. However, this method requires multiple drying processes, making it difficult to guarantee particle uniformity and stability. Furthermore, the process is cumbersome, time-consuming, and unsuitable for industrial application.

[0005] Therefore, the preparation of modified silica sol still requires continuous exploration to improve the stability and uniformity of silica particles, simplify the process flow, and meet the high requirements in specific fields. Summary of the Invention

[0006] To address the aforementioned problems, this invention innovatively proposes a method for preparing stable acidic silica sol, achieving stable storage of silica sol under acidic conditions. This invention prepares modified silica sol with no change in particle size after one year of storage at room temperature. Furthermore, the thermal expansion microspheres prepared using the silica sol of this invention produce less filter residue and exhibit greater stability.

[0007] Another object of the present invention is to provide such a modified silica sol.

[0008] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0009] A method for preparing modified silica sol in an acidic system, comprising the following steps:

[0010] 1) Pre-hydrolysis of silane coupling agent: A certain amount of amino acids, inorganic acids or organic acids are added to water to obtain an aqueous solution of amino acids;

[0011] The silane coupling agent was uniformly dissolved in ethanol, added to an aqueous solution of amino acids, and then subjected to a low-temperature water bath to obtain solution A.

[0012] 2) Preparation of acidic silica sol: Add inorganic or organic acid to water, stir evenly, and then add silica sol dropwise to obtain solution B;

[0013] 3) Modification of acidic silica sol: Take solution A and zwitterionic compound, stir them evenly, and then drop them into solution B. After water bathing at a certain temperature for a certain period of time, the modified silica sol is obtained.

[0014] In one specific implementation, the water added in steps 1 and 2) is preferably high-purity water, and the mass of water in step 1) is 70-90 parts, preferably 75-80 parts.

[0015] In one specific implementation, in step 1), the amino acid is one or more of glycine, alanine, valine, leucine, isoleucine, methionine, proline, tryptophan, serine, tyrosine, cysteine, phenylalanine, asparagine, glutamyl, threonine, L-aspartic acid, glutamic acid, lysine, arginine, and histidine, preferably glutamic acid or L-aspartic acid.

[0016] In one specific implementation, in steps 1 and 2), the inorganic or organic acid is selected from one or more of nitric acid, hydrochloric acid, phosphoric acid, sulfuric acid, acetic acid, citric acid, or perchloric acid; the pH of the aqueous solution of amino acids in step 1) is controlled at 3-4, and the pH is controlled at 2-3 after adding silica sol in step 2).

[0017] In one specific implementation, in step 1), the silane coupling agent may be one or more of γ-aminopropyltriethoxysilane (KH550), γ-glycidoxypropyltrimethoxysilane (KH560), γ-methacryloyloxypropyltrimethoxysilane (KH570), and mercaptopropyltrimethoxysilane (KH580);

[0018] The amount of silane coupling agent added is 5-10 parts, and the amount of ethanol added is 10-20 parts in order to quickly dissolve the silane coupling agent; in step 1), the molar ratio of amino acid to silane coupling agent is 1:1 to 3:1.

[0019] In one specific implementation, in step 1), the temperature of the low-temperature water bath is 5-20°C; the water bath duration is 0.5-3 hours, preferably 0.5-1 hour.

[0020] In one specific implementation, in step 2), the water mass is 10-20 parts. The amount of silica sol added is at least 75 parts, preferably 75-80 parts, and its source is not limited to silica sol prepared by sol-gel method, ion exchange method or silica powder hydrolysis, and its solid content is not less than 40 wt%, for example 40-60%.

[0021] In one specific implementation, the dropping rate of the silica sol in step 2) does not exceed 10 ml / min, preferably 3-5 ml / min; and the stirring time after the silica sol is dropped does not exceed 30 min.

[0022] In one specific implementation, in step 3), the amount of liquid A added is 0.3-3 parts, preferably 0.9-2.4 parts.

[0023] In one specific implementation, in step 3), the zwitterionic compound can be one or more of polymethyl methacrylate carboxybetaine (PCBMA), polymethyl methacrylate sulfobetaine (PSBMA), and polyphosphoric acid betaine (PPC); the molar ratio of the amount added to the silane coupling agent in the added solution A is 3:1 to 6:1.

[0024] In one specific implementation, in step 3), the dropping rate of solution A and the zwitterionic compound does not exceed 5 ml / min, preferably 0.5-3 ml / min.

[0025] In one specific implementation, in step 3), the water bath temperature is 30-70℃, preferably 40-60℃, and the water bath duration is 2-12h, preferably 3-9h.

[0026] Another object of the present invention is to provide such a modified silica sol.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1) This invention modifies the silane coupling agent by using amino acids after hydrolysis, so that the molecular structure of the silane coupling agent has strong hydrophilicity and hydrophobicity, which hinders the aggregation of silica particles.

[0029] 2) This invention uses zwitterionic compounds to directly graft silica under acidic conditions. The method is simple and convenient to operate, providing a reliable preparation method for acidic silica sol. Attached Figure Description

[0030] Figure 1 These are the modified silica sol particles prepared in Example 1 of this invention. Detailed Implementation

[0031] To better understand the technical solution of the present invention, the following embodiments will further illustrate the method provided by the present invention. However, the present invention is not limited to the listed embodiments, but should also include any other known modifications within the scope of the claims of the present invention.

[0032] The main raw materials used in the following examples and comparative examples are shown in Table 1 below:

[0033] Table 1 Main Raw Materials

[0034]

[0035]

[0036] Detection method:

[0037] The secondary particle size of the modified silica sol particles was measured using dynamic light scattering (DLS) and a Malvern laser particle size analyzer.

[0038] The morphology of the modified silica sol particles was observed using TEM.

[0039] Example 1

[0040] Add 7g of glutamic acid and 10 vol% hydrochloric acid to 75g of water and stir until well mixed. The pH is 3.25.

[0041] 10g KH550 was dissolved in 15.8g ethanol and then poured into an aqueous solution of glutamic acid. The solution was placed in a low-temperature constant temperature bath at 5℃ and stirred at a rate of 400r / min for 30min to obtain solution A.

[0042] 80g of silica sol (40wt%) was pumped into 15g of water containing nitric acid at a rate of 3ml / min using a peristaltic pump. The pH was 2.1. The mixture was stirred for 10min to obtain solution B.

[0043] Take 1.2g of solution A, add 8g of water and 0.8g of PSBMA, and add to solution B at a rate of 0.5ml / min. Place in a water bath at 60℃ and stir for 3h to obtain modified acidic silica sol.

[0044] Example 2

[0045] Add 9g of L-aspartic acid and 10 vol% nitric acid to 80g of water and stir until homogeneous; the pH is 3.89.

[0046] Dissolve 8g of KH560 in 19.7g of ethanol and pour the solution into an aqueous solution of L-aspartic acid. Place the solution in a low-temperature constant temperature bath at 10℃ and stir at a rate of 300r / min for 45min to obtain solution A.

[0047] 75g of silica sol (40wt%) was pumped into 12g of water containing hydrochloric acid at a rate of 4ml / min using a peristaltic pump. The pH was 2.3. The mixture was stirred for 20min to obtain solution B.

[0048] Take 1.6g of solution A, add 10g of water and 0.56g of PCBMA, and add it to solution B at a rate of 1ml / min. Place the mixture in a water bath at 40℃ and stir for 9h to obtain modified acidic silica sol.

[0049] Example 3

[0050] 78g of water was mixed with 9.8g of leucine and 10 vol% sulfuric acid, and the pH was 3.67.

[0051] Dissolve 7g KH570 in 17g ethanol and pour it into an aqueous solution of glutamic acid. Place the solution in a low-temperature constant temperature bath at 15℃ and stir at a rate of 400r / min for 60min to obtain solution A.

[0052] 77g of silica sol (40wt%) was pumped into 14g of water containing sulfuric acid at a rate of 5ml / min using a peristaltic pump. The pH was 2.71. The mixture was stirred for 30min to obtain solution B.

[0053] Take 2.4g of solution A, add 17.7g of water and 0.72g of PPC, and add it to solution B at a rate of 3ml / min. Place the mixture in a water bath at 40℃ and stir for 10h to obtain modified acidic silica sol.

[0054] Example 4

[0055] 77g of water was mixed with 9g of serine and 10 vol% phosphoric acid, and the pH was 3.68.

[0056] Dissolve 8.1g KH580 in 16.8g ethanol and pour it into an aqueous solution of serine. Place the solution in a low-temperature constant temperature bath at 10℃ and stir at a rate of 500r / min for 120min to obtain solution A.

[0057] 79g of silica sol (40wt%) was pumped into 10g of water containing 10vol% phosphoric acid at a rate of 3ml / min using a peristaltic pump. The pH was 2.47. The mixture was stirred for 25min to obtain solution B.

[0058] Take 1.8g of solution A, add 11g of water and 0.42g of PCBMA, and add to solution B at a rate of 2ml / min. Place in a water bath at 60℃ and stir for 5h to obtain modified silica sol.

[0059] Example 5

[0060] Add 14.5g of lysine and 10 vol% acetic acid to 80g of water and stir until homogeneous; the pH is 3.17.

[0061] 9.6g KH580 was dissolved in 16.1g ethanol and then poured into an aqueous solution of lysine. The solution was placed in a low-temperature constant temperature bath at 13℃ and stirred at a rate of 400r / min for 150min to obtain solution A.

[0062] 80g of silica sol (40wt%) was pumped into 14.6g of water containing 10vol% acetic acid at a rate of 3ml / min using a peristaltic pump. The pH was 2.99. The mixture was stirred for 20min to obtain solution B.

[0063] Take 1.9g of solution A, add 7g of water and 0.81g of PCBMA, and add it to solution B at a rate of 1ml / min. Place the mixture in a water bath at 45℃ and stir for 5h to obtain modified silica sol.

[0064] Comparative Example 1

[0065] Add 10 vol% nitric acid to 80g of water and control the pH to 3.89;

[0066] Dissolve 8g KH560 in 19.7g ethanol, then pour into nitric acid aqueous solution, place in a low temperature constant temperature bath at 10℃ and stir at a rate of 300r / min for 45min to obtain solution A.

[0067] 75g of silica sol (40wt%) was pumped into 12g of water containing 10vol% hydrochloric acid at a rate of 4ml / min using a peristaltic pump. The pH was 2.3. The mixture was stirred for 20min to obtain solution B.

[0068] Take 1.5g of solution A, add 10g of water and 0.56g of PCBMA, and add it to solution B at a rate of 1ml / min. Place the mixture in a water bath at 40℃ and stir for 9h to obtain modified silica sol.

[0069] Comparative Example 2

[0070] Add 9g of L-aspartic acid and 10 vol% nitric acid to 80g of water and stir until homogeneous; the pH is 3.89.

[0071] Dissolve 8g of KH560 in 19.7g of ethanol and pour the solution into an aqueous solution of L-aspartic acid. Place the solution in a low-temperature constant temperature bath at 10℃ and stir at a rate of 300r / min for 45min to obtain solution A.

[0072] 75g of silica sol (40wt%) was pumped into 12g of water containing 10vol% hydrochloric acid at a rate of 4ml / min using a peristaltic pump. The pH was 2.3. The mixture was stirred for 20min to obtain solution B.

[0073] Take 1.6g of solution A and add it to 10.9g of water. Add the solution to solution B at a rate of 1ml / min and place it in a water bath at 40℃ and stir for 9h to obtain modified acidic silica sol.

[0074] Comparative Example 3

[0075] After adding 10 vol% nitric acid to 80 g of water and stirring until homogeneous, the pH is 3.89.

[0076] Dissolve 8g of KH560 in 19.7g of ethanol and pour the solution into a nitric acid aqueous solution. Place the solution in a low-temperature constant temperature bath at 10℃ and stir at a rate of 300r / min for 45min to obtain solution A.

[0077] 75g of silica sol (40wt%) was pumped into 12g of water containing 10vol% hydrochloric acid at a rate of 4ml / min using a peristaltic pump. The pH was 2.3. The mixture was stirred for 20min to obtain solution B.

[0078] Take 1.6g of solution A and add it to 10.9g of water. Add the solution to solution B at a rate of 1ml / min and place it in a water bath at 40℃ and stir for 9h to obtain modified acidic silica sol.

[0079] To evaluate the stability of the modified silica sol, the silica sol was placed in an oven at 60°C for aging observation (according to industry experience, the aging effect of an oven at 60°C is equivalent to 15 times the aging time at room temperature). The stability of the acidic silica sol was characterized by periodically observing the particle size change using the dynamic light scattering method. The results are shown in Table 2.

[0080] Table 2. Changes in silica sol particle size

[0081]

[0082] Evaluation of the application of modified silica sol in the preparation of thermoplastic expandable microspheres:

[0083] The implementation method was based on the embodiment of patent CN115197519B, and the stability of modified silica particles was evaluated by assessing the amount of filter residue obtained by passing the microsphere slurry through a 100-mesh filter.

[0084] The specific process is as follows:

[0085] S1: Weigh 110g of water, 6.5g of potassium chloride, and 5g of colloidal silica, and mix them to prepare mixture A.

[0086] S2: Weigh 10g acrylonitrile, 2g methacrylonitrile, 8g methyl acrylate, 5g methyl methacrylate, 0.25g diallyl phthalate, 1.5g foaming agent isobutane, and 0.375g polymerization initiator dilauryl peroxide, and mix them to prepare mixture B.

[0087] S3: Add mixture A and mixture B into sealed containers and stir at 1200 rpm for 25 min to form an emulsion.

[0088] S4: The resulting emulsion was polymerized at 62℃ and 500rpm for 20h. After drying in an oven at 60℃, expandable microspheres were obtained. The mass of the residue remaining on the sieve after passing the microspheres through a 100-mesh sieve was measured, and the results are shown in the table below:

[0089] Example Filter residue amount / % Example 1 0.09 Example 2 0.12 Example 3 0.09 Example 4 0.10 Example 5 0.08 Comparative Example 1 24.8 Comparative Example 2 25.6 Comparative Example 3 27.9

[0090] In the examples, stable acidic silica sols with a solid content of approximately 30 wt% were prepared by adding different types of amino acids and zwitterionic polymers. After aging in a 60°C oven for one month (equivalent to 15 months at room temperature based on industry experience), the particle size remained almost unchanged. The resulting thermoplastic microspheres exhibited very low residue, indicating their ability to stably coat the oil droplet phase. Comparison of Example 2 with Comparative Examples 1-3 reveals that amino acids and zwitterionic polymers are key to the stability of the acidic silica sol system.

[0091] Although the present invention has been described in detail through the preferred embodiments described above, it should be understood that the above description should not be considered as limiting the present invention. Those skilled in the art will understand that modifications or adjustments can be made to the present invention based on the teachings of this specification. These modifications or adjustments should also be limited by the claims of the present invention.

Claims

1. A method for preparing a modified silica sol in an acidic system, comprising the following steps: 1) pre-hydrolysis of silane coupling agent: adding a certain amount of amino acid, inorganic acid and / or organic acid into water to obtain an aqueous solution of amino acid; the amino acid is one or more of glycine, alanine, valine, leucine, isoleucine, methionine, proline, tryptophan, serine, tyrosine, cysteine, phenylalanine, aspartic acid, glutamic acid, threonine, L-aspartic acid, glutamic acid, lysine, arginine and histidine; dissolving the silane coupling agent in ethanol, mixing with the aqueous solution of amino acid, and obtaining A liquid by low-temperature water bath; 2) Preparation of acidic silica sol: Inorganic acid and / or organic acid is added to water, and after stirring uniformly, silica sol is slowly added thereto, obtaining B liquid; 3) modification of acidic silica sol: stirring A liquid and zwitterionic compound uniformly, then slowly adding them together into B liquid, and obtaining modified silica sol after water bath; the zwitterionic compound is one or more of polymethylacrylic acid carboxybetaine, polymethylacrylic acid sulfobetaine and polyphosphobetaine.

2. The method of claim 1, wherein, The amino acid in step 1) is glutamic acid and / or L-aspartic acid.

3. The method of claim 1, wherein, The silane coupling agent in step 1) is one or more of γ-aminopropyl triethoxysilane, γ-glycidyl ether propyl trimethoxysilane, γ-methacryloyloxy propyl trimethoxysilane and mercaptopropyl trimethoxysilane.

4. The method of claim 3, wherein, In step 1), the molar ratio of the amount of amino acid to the amount of silane coupling agent is 1:1-3:

1.

5. The method of claim 1, wherein, In step 1), the inorganic acid or organic acid is one or more of nitric acid, hydrochloric acid, phosphoric acid, sulfuric acid, acetic acid, citric acid or perchloric acid.

6. The method of claim 5, wherein, The pH of the aqueous solution of amino acid in step 1) is 3-4.

7. The method according to any one of claims 1 to 4, characterized in that, In step 1), the temperature of low-temperature water bath is 5-20℃; the water bath duration is 0.5-3h.

8. The method of claim 1, wherein, In step 2), the inorganic acid or organic acid is one or more of nitric acid, hydrochloric acid, phosphoric acid, sulfuric acid, acetic acid, citric acid or perchloric acid, and the amount of addition is such that the pH after adding silica sol is 2-3.

9. The method of claim 1, wherein, In step 2), the adding speed of silica sol is not more than 10ml / min; the stirring duration after adding silica sol is not more than 30min; The solid content of silica sol is not less than 40wt%.

10. The method of claim 9, wherein, In step 2), the adding speed of silica sol is 3-5ml / min.

11. The method of claim 1, wherein, In step 3), the molar ratio of the amount of addition of zwitterionic compound to the amount of silane coupling agent in A liquid is 3:1-6:

1.

12. The method of claim 1 or 11, wherein, In step 3), the adding speed of A liquid and zwitterionic compound is not more than 5ml / min; In step 3), the water bath temperature is 30-70℃, and the water bath duration is 2-12h.

13. The method of claim 12, wherein, In step 3), the adding speed of A liquid and zwitterionic compound is 0.5-3ml / min; In step 3), the water bath temperature is 40-60℃, and the water bath duration is 3-9h.

14. The method according to any one of claims 1 to 6, characterized in that, The amount of components in each step is as follows: the mass of water in step 1) is 70-90 parts; In step 1), the amount of addition of silane coupling agent is 5-10 parts, and the amount of addition of ethanol is 10-20 parts; In step 2), the mass of water is 10-20 parts, and the amount of silica sol is 75-80 parts; In step 3), the amount of addition of A liquid is 0.3-3 parts.

15. The method of claim 14, wherein, The amount of components in each step is as follows: the mass of water in step 1) is 75-80 parts; In step 3), the amount of addition of A liquid is 0.9-2.4 parts.

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