Refining method of polydimethylsiloxane

Abstract

The invention discloses a refining method of polydimethylsiloxane. The refining method comprises the following steps: fully mixing crude polydimethylsiloxane with a heavy metal composite adsorbent to form a suspension system; treating for 1-3 hours in a low-frequency-high-frequency alternating ultrasonic field and a critical temperature condition; tiny impurities are removed and peculiar smell is eliminated through primary filtering of the primary filter; and carrying out secondary filtration by using a polytetrafluoroethylene filter membrane to obtain the daily chemical grade polydimethylsiloxane. Compared with the prior art, the method disclosed by the invention has the beneficial effects that heavy metals such as lead, arsenic and cadmium are efficiently removed by combining a heavy metal composite adsorbent under the conditions of a low-frequency-high-frequency alternating ultrasonic field and a critical temperature, and tiny impurities, harmful ions and peculiar smell in a polydimethylsiloxane product are removed through primary filtration and secondary filtration; therefore, the quality meets the daily chemical grade requirement, and large-scale cleaning and replacement of existing equipment pipelines can be avoided.

Description

Technical Field

[0001] This invention relates to the field of polydimethylsiloxane refining technology, and more particularly to a method for refining polydimethylsiloxane. Background Technology

[0002] As a high-performance synthetic polymer, polydimethylsiloxane exhibits numerous advantages and wide applicability in the application of daily chemical products, making it an ideal additive for many products. Its excellent lubricity and film-forming properties allow it to form a thin, breathable protective film on the skin's surface, preventing moisture loss and effectively blocking external pollutants. Furthermore, its low surface tension makes it easy to spread on the skin, providing a silky smooth feel. It also possesses good antioxidant and weather-resistant properties, helping to extend the shelf life and service life of daily chemical products. In conclusion, polydimethylsiloxane plays an irreplaceable role in the daily chemical industry due to its superior performance and broad application prospects.

[0003] Compared to general applications, polydimethylsiloxane has higher requirements when used as a raw material in daily chemical products. According to the relevant standards published in the "Cosmetic Safety Technical Specifications" (2015 edition), the limits for heavy metals lead, arsenic, and cadmium are 10 mg / kg, 2 mg / kg, and 5 mg / kg, respectively. In addition, high-end personal care and cosmetic products have higher requirements for the transparency of siloxane products (turbidity ≤ 1 NTU).

[0004] Traditional polydimethylsiloxanes are mainly produced by acid-base catalysis using dimethylsiloxane mixed cyclic compounds (DMC). Impurities and odors are difficult to remove. While resin-catalyzed polydimethylsiloxanes show some improvement over traditional methods, the heavy metal ions and other impurities carried by equipment and pipelines still do not meet the requirements of the daily chemical industry. Patent CN111072971A can reduce the cyclic content in siloxanes through staged atmospheric distillation, but it does not improve the removal of heavy metal ions. Patent CN114805814A utilizes ion exchange resins and ion exchange membranes to treat harmful metal ions and some non-metal ions in organosilicon materials, and is applied in the semiconductor field. However, its ability to remove heavy metals restricted in daily chemical products from polysiloxane products is limited, and it does not improve transparency or common odors, thus limiting its application in the daily chemical industry. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a refining method for polydimethylsiloxane, which effectively removes heavy metals such as lead, arsenic, and cadmium, ensuring that the appearance, transparency, and odor meet the requirements for daily chemical grade products, thus avoiding large-scale cleaning and replacement of existing equipment and pipelines.

[0006] To achieve this technical objective, the present invention adopts the following solution: This invention provides a method for purifying polydimethylsiloxane, comprising the following steps: S1. Crude polydimethylsiloxane and heavy metal composite adsorbent are thoroughly mixed to form a suspension system; S2. Treat the suspension system after step S1 under alternating low-frequency and high-frequency ultrasonic fields and critical temperature conditions for 1 to 3 hours. S3. The material processed in step S2 is initially filtered through a filter to remove minute impurities and eliminate odors. S4. The material processed in step S3 is filtered a second time through a polytetrafluoroethylene filter membrane to obtain chemical-grade polydimethylsiloxane.

[0007] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a method for refining polydimethylsiloxane. First, crude polydimethylsiloxane is subjected to alternating low-frequency and high-frequency ultrasonic fields and critical temperature conditions, combined with a heavy metal composite adsorbent to achieve efficient removal of heavy metals such as lead, arsenic, and cadmium. Then, through primary and secondary filtration, minute impurities, harmful ions, and odors in the polydimethylsiloxane product are removed, so that its quality meets the requirements for daily chemical grade, thus avoiding large-scale cleaning and replacement of existing equipment and pipelines.

[0008] Furthermore, the viscosity of the crude polydimethylsiloxane in step S1 is 100~1000 cSt.

[0009] Further, the heavy metal composite adsorbent in step S1 includes: 50-70 wt% mercapto-modified mesoporous silica with a pore size of 5-10 nm and a mercapto loading of ≥2.5 mmol / g; 20-30 wt% of bentonite modified with polyethyleneimine (PEI), where the molecular weight of PEI is 800-2000 Da and the modification amount is 10-15 wt%. The activated carbon supported by nano-zero valent iron (nZVI) is 10~20 wt%, with an iron loading of 5~8 wt% and a particle size of 50~100 nm.

[0010] A ternary composite system of mercapto-SiO2 / PEI-bentonite / nZVI-activated carbon is adopted to remove heavy metals through chemical chelation, electrostatic adsorption and reduction reaction, improving efficiency by more than 30%.

[0011] Furthermore, mercapto-modified mesoporous silica is prepared via the following steps: (a) Mesoporous silica and 3-mercaptopropyltrimethoxysilane were refluxed in an ethanol solution and reacted at room temperature for 6-8 hours. (b) The product after the reaction was washed with acetone using a Soxhlet extractor and then dried under vacuum to obtain mercapto-modified mesoporous silica.

[0012] Furthermore, in step S1, the mass ratio of the heavy metal composite adsorbent to the crude polydimethylsiloxane is 1:20 to 1:50, and the heavy metal adsorbent after adsorption is recovered by centrifugation or filtration, and can be recycled ≥5 times after washing with ethanol.

[0013] Furthermore, in step S2, the low frequency of the alternating low-frequency and high-frequency ultrasonic field is 20~40 kHz, the high frequency is 80~120 kHz, and the alternation period is 5~10 minutes; the ultrasonic power density is 100~300 W / L, and pulsed stirring is applied during ultrasonic treatment with a pulse frequency of 1~2 Hz and an amplitude of 5~10 cm.

[0014] The combination of alternating low-frequency and high-frequency ultrasonic fields with pulsed stirring enhances the surface mass transfer and diffusion of the heavy metal composite adsorbent, thereby improving the removal of heavy metals from crude polydimethylsiloxane.

[0015] Furthermore, the critical temperature in step S2 is 50~70℃. Heating can promote mass transfer and diffusion, but excessively high temperatures can damage the material. At this critical temperature, the heavy metal composite adsorbent reaches its optimal adsorption state.

[0016] Furthermore, the filter material in the primary filter in step S3 consists of activated carbon and diatomaceous earth, with a mass ratio of activated carbon to diatomaceous earth of 1:2 to 1:5. Activated carbon, utilizing its excellent adsorption properties, is mainly used to adsorb other impurities, harmful ions, and eliminate odors, while diatomaceous earth is used to remove minute impurities.

[0017] Furthermore, in step S4, the pore size of the polytetrafluoroethylene filter membrane is 0.2~0.5 μm.

[0018] Furthermore, the chemical-grade polydimethylsiloxane obtained in step S4 meets the following requirements: lead <10mg / kg, arsenic <2mg / kg, cadmium <5mg / kg; viscosity at 25°C is 100~1000 cSt, turbidity ≤1 NTU, and even further, turbidity <0.3NTU. Detailed Implementation

[0019] To fully understand the purpose, features and effects of the present invention, the present invention will be described in detail through the following specific embodiments, but the present invention is not limited thereto.

[0020] This invention provides a method for purifying polydimethylsiloxane, comprising the following steps: S1. Crude polydimethylsiloxane (viscosity 100~1000 cSt) is thoroughly mixed with heavy metal composite adsorbent to form a suspension system.

[0021] The heavy metal composite adsorbent includes: 50-70 wt% mercapto-modified mesoporous silica with a pore size of 5-10 nm and a mercapto loading of ≥2.5 mmol / g; 20-30 wt% of bentonite modified with polyethyleneimine (PEI), where the molecular weight of PEI is 800-2000 Da and the modification amount is 10-15 wt%. The activated carbon supported by nano-zero valent iron (nZVI) is 10~20 wt%, with an iron loading of 5~8 wt% and a particle size of 50~100 nm.

[0022] Thiol-modified mesoporous silica is prepared by the following steps: (a) Mesoporous silica and 3-mercaptopropyltrimethoxysilane were refluxed in an ethanol solution and reacted at room temperature (25°C) for 6-8 hours. (b) The product after the reaction was washed with acetone using a Soxhlet extractor and then dried under vacuum to obtain mercapto-modified mesoporous silica.

[0023] The mass ratio of the heavy metal composite adsorbent to crude polydimethylsiloxane is 1:20 to 1:50, and the heavy metal adsorbent is recovered by centrifugation or filtration after adsorption, and can be recycled ≥5 times after washing with ethanol.

[0024] S2. The suspension system after step S1 is treated under alternating low-frequency and high-frequency ultrasonic fields (low frequency 20~40 kHz, high frequency 80~120 kHz, alternation period 5~10 minutes) and critical temperature (50~70℃) for 1~3 hours.

[0025] The ultrasonic power density is 100~300 W / L, and pulsed stirring is applied during ultrasonic treatment with a pulse frequency of 1~2 Hz and an amplitude of 5~10 cm.

[0026] S3. The material processed in step S2 is initially filtered through a filter to remove minute impurities and eliminate odors; the filter material is composed of activated carbon and diatomaceous earth, with the mass ratio of activated carbon to diatomaceous earth being 1:2 to 1:5.

[0027] S4. The material processed in step S3 is filtered twice through a 0.2~0.5 μm polytetrafluoroethylene filter membrane to obtain chemical-grade polydimethylsiloxane.

[0028] The obtained chemical-grade polydimethylsiloxane meets the following requirements: lead <10mg / kg, arsenic <2mg / kg, cadmium <5mg / kg; viscosity at 25℃ is 100~1000 cSt, and turbidity ≤1 NTU. Example 1

[0029] (1) Using a mass ratio of 5.5:1, dimethylsiloxane mixed cyclic molecule (DMC) and 10cSt polydimethylsiloxane with caps, the reaction was carried out at 60°C for 4 hours with sulfonic acid type acidic resin catalysis, followed by multi-stage vacuum distillation at 150~180°C and -0.1MPa to obtain crude polydimethylsiloxane. (2) Mesoporous silica and 3-mercaptopropyltrimethoxysilane were refluxed in ethanol solution and reacted at room temperature (25°C) for 6 hours to prepare mercapto-modified mesoporous silica. (3) A heavy metal composite adsorbent was prepared by mixing 50 wt% mercapto-modified mesoporous silica, 30 wt% polyethyleneimine (PEI) modified bentonite, and 20 wt% nano-zero valent iron (nZVI) supported activated carbon. (4) The heavy metal composite adsorbent and crude polydimethylsiloxane are thoroughly mixed to form a suspension system with a mass ratio of 1:50. (5) The suspension system was treated for 3 hours under alternating low-frequency and high-frequency ultrasonic fields (low frequency 20 kHz, high frequency 80 kHz, alternation period 10 minutes) and critical temperature 65℃. The ultrasonic power density was 200 W / L. During the treatment, pulse stirring was applied with a pulse frequency of 2 Hz and an amplitude of 10 cm. (6) After initial filtration by a filter, tiny impurities are removed and odors are eliminated; (7) After the first filtration, the polydimethylsiloxane was filtered again through a 0.5 μm polytetrafluoroethylene filter membrane to obtain the refined polydimethylsiloxane. Example 2

[0030] (1) Using a 7.8:1 mass ratio of dimethylsiloxane mixed cyclic compound (DMC) and 10cSt polydimethylsiloxane with a cap, the reaction was carried out at 105°C with tetramethylammonium hydroxide alkaline gel catalysis for 4 h, followed by catalyst breaking at 130~150°C for 3~4 h, and then multi-stage vacuum distillation at 150~180°C and -0.1MPa to obtain crude polydimethylsiloxane; (2) Mesoporous silica and 3-mercaptopropyltrimethoxysilane were refluxed in ethanol solution and reacted at room temperature (25°C) for 7 hours to prepare mercapto-modified mesoporous silica. (3) A heavy metal composite adsorbent was prepared by mixing 70 wt% mercapto-modified mesoporous silica, 20 wt% polyethyleneimine (PEI) modified bentonite, and 10 wt% nano-zero-valent iron (nZVI) supported activated carbon. (4) The heavy metal composite adsorbent and crude polydimethylsiloxane are thoroughly mixed to form a suspension system with a mass ratio of 1:50. (5) The suspension system was treated for 3 hours under alternating low-frequency and high-frequency ultrasonic fields (low frequency 30 kHz, high frequency 100 kHz, alternation period 10 minutes) and critical temperature 65℃. The ultrasonic power density was 200 W / L. During the treatment, pulse stirring was applied with a pulse frequency of 2 Hz and an amplitude of 10 cm. (6) After initial filtration by a filter, tiny impurities are removed and odors are eliminated; (7) After the first filtration, the polydimethylsiloxane was filtered again through a 0.5 μm polytetrafluoroethylene filter membrane to obtain the refined polydimethylsiloxane. Example 3

[0031] (1) Crude polydimethylsiloxane was prepared by using a dimethylsiloxane mixed cyclic compound (DMC) with a mass ratio of 12.6:1 and a capping agent of 10cSt polydimethylsiloxane, under the same conditions as in Example 2. (2) Mesoporous silica and 3-mercaptopropyltrimethoxysilane were refluxed in ethanol solution and reacted at room temperature (25°C) for 8 hours to prepare mercapto-modified mesoporous silica. (3) A heavy metal composite adsorbent was prepared by mixing 60 wt% mercapto-modified mesoporous silica, 25 wt% polyethyleneimine (PEI) modified bentonite, and 15 wt% nano-zero-valent iron (nZVI) supported activated carbon. (4) The heavy metal composite adsorbent and crude polydimethylsiloxane are thoroughly mixed to form a suspension system with a mass ratio of 1:40. (5) The suspension system was treated for 3 hours under alternating low-frequency and high-frequency ultrasonic fields (low frequency 40 kHz, high frequency 120 kHz, alternation period 10 minutes) and critical temperature 65℃. The ultrasonic power density was 200 W / L. During the treatment, pulse stirring was applied with a pulse frequency of 2 Hz and an amplitude of 10 cm. (6) After initial filtration by a filter, tiny impurities are removed and odors are eliminated; (7) After the first filtration, the polydimethylsiloxane was filtered again through a 0.5 μm polytetrafluoroethylene filter membrane to obtain the refined polydimethylsiloxane. Example 4

[0032] The method in this embodiment is basically the same as that in embodiment 1, except that: (5) The suspension system was treated for 3 hours under alternating low-frequency and high-frequency ultrasonic fields (low frequency 30 kHz, high frequency 100 kHz, alternation period 10 minutes) and critical temperature 65℃. The ultrasonic power density was 200 W / L. During the treatment, pulse stirring was applied with a pulse frequency of 2 Hz and an amplitude of 10 cm. Example 5

[0033] The method in this embodiment is basically the same as that in embodiment 1, except that: (5) The suspension system was treated for 3 hours under alternating low-frequency and high-frequency ultrasonic fields (low frequency 40 kHz, high frequency 120 kHz, alternation period 10 minutes) and critical temperature 65℃. The ultrasonic power density was 200 W / L. During the treatment, pulse stirring was applied with a pulse frequency of 2 Hz and an amplitude of 10 cm. Example 6

[0034] The method in this embodiment is basically the same as that in embodiment 1, except that: (5) The suspension system was treated for 3 hours under alternating low-frequency and high-frequency ultrasonic fields (low frequency 20 kHz, high frequency 80 kHz, alternation period 8 minutes) and critical temperature 65℃. The ultrasonic power density was 200 W / L. During the treatment, pulse stirring was applied with a pulse frequency of 2 Hz and an amplitude of 10 cm. Example 7

[0035] The method in this embodiment is basically the same as that in embodiment 1, except that: (5) The suspension system was treated for 3 hours under alternating low-frequency and high-frequency ultrasonic fields (low frequency 20 kHz, high frequency 80 kHz, alternation period 5 minutes) and critical temperature 65℃. The ultrasonic power density was 200 W / L. During the treatment, pulse stirring was applied with a pulse frequency of 2 Hz and an amplitude of 10 cm. Example 8

[0036] The method in this embodiment is basically the same as that in embodiment 1, except that: (4) The heavy metal composite adsorbent and crude polydimethylsiloxane are thoroughly mixed to form a suspension system with a mass ratio of 1:40. Example 9

[0037] The method in this embodiment is basically the same as that in embodiment 1, except that: (4) The heavy metal composite adsorbent and crude polydimethylsiloxane are thoroughly mixed to form a suspension system with a mass ratio of 1:30. Example 10

[0038] The method in this embodiment is basically the same as that in embodiment 1, except that: (5) The suspension system was treated for 3 hours under alternating low-frequency and high-frequency ultrasonic fields (low frequency 20 kHz, high frequency 80 kHz, alternation period 10 minutes) and critical temperature 50℃. The ultrasonic power density was 200 W / L. During the treatment, pulse stirring was applied with a pulse frequency of 2 Hz and an amplitude of 10 cm. Example 11

[0039] The method in this embodiment is basically the same as that in embodiment 1, except that: (5) The suspension system was treated for 3 hours under alternating low-frequency and high-frequency ultrasonic fields (low frequency 20 kHz, high frequency 80 kHz, alternation period 10 minutes) and critical temperature 70℃. The ultrasonic power density was 200 W / L. During the treatment, pulse stirring was applied with a pulse frequency of 2 Hz and an amplitude of 10 cm. Comparative Example 1

[0040] (1) Using a 7.8:1 mass ratio of dimethylsiloxane mixed cyclic compound (DMC) and 10cSt polydimethylsiloxane with a cap, the reaction was carried out at 105°C with tetramethylammonium hydroxide alkaline gel catalysis for 4 h, followed by catalyst breaking at 130~150°C for 3~4 h, and then multi-stage vacuum distillation at 150~180°C and -0.1MPa to obtain crude polydimethylsiloxane; (2) The crude polydimethylsiloxane is filtered through a polypropylene filter cloth to remove impurities. Comparative Example 2

[0041] Compared to Example 1, the only difference is: (5) The suspension system was treated for 3 hours under a 100 kHz ultrasonic field and a critical temperature of 65℃. The ultrasonic power density was 200 W / L. During the treatment, pulse stirring was applied with a pulse frequency of 2 Hz and an amplitude of 10 cm. Comparative Example 3

[0042] Compared to Example 1, the only difference is: (5) It was treated for 3 hours under alternating low-frequency and high-frequency ultrasonic fields (low frequency 20 kHz, high frequency 80 kHz, alternation period 10 minutes) and critical temperature 65℃, with an ultrasonic power density of 200 W / L. Comparative Example 4

[0043] Compared to Example 1, the only difference is: (4) The mass ratio of heavy metal composite adsorbent to crude polydimethylsiloxane is 1:70. Comparative Example 5

[0044] Compared to Example 1, the only difference is: (3) A heavy metal composite adsorbent was prepared by mixing 71.4 wt% mercapto-modified mesoporous silica and 28.6 wt% nano-zero-valent iron (nZVI) supported activated carbon. Comparative Example 6

[0045] Compared to Example 1, the only difference is: (3) A heavy metal composite adsorbent was prepared by mixing 62.5 wt% mercapto-modified mesoporous silica and 37.5 wt% polyethyleneimine (PEI) modified bentonite. Comparative Example 7

[0046] Compared to Example 1, the only difference is: (3) A heavy metal composite adsorbent was prepared by mixing 60 wt% polyethyleneimine (PEI) modified bentonite and 40 wt% nano-zero valent iron (nZVI) supported activated carbon. Comparative Example 8

[0047] Compared to Example 1, the only difference is: (3) The heavy metal adsorbent is mercapto-modified mesoporous silica.

[0048] The polydimethylsiloxanes obtained in the above examples and comparative examples were tested, and the results are shown in Table 1. The standards for the three heavy metal contents in Table 1 refer to the "Cosmetic Safety Technical Specifications"; the standards for appearance and viscosity refer to the industry standard: HG / T 2366-2015 "Dimethyl Silicone Oil"; and the requirements for odor and turbidity refer to those given by manufacturers in the daily chemical industry.

[0049] Table 1. Detection results of polydimethylsiloxane

[0050] Compared with Example 2, Comparative Example 1 did not undergo a refining process. The test results showed that the heavy metal content did not meet the relevant standards, the turbidity was too high, and the residual trimethylamine and other substances in the catalyst decomposition process were not completely removed, resulting in an off-odor in the product. Combined with Examples 1 and 3, it is shown that the method of the present invention has a good refining effect on crude polydimethylsiloxane of conventional viscosity used in the daily chemical industry.

[0051] Compared with Example 1, Examples 4 and 5 increased the frequency of ultrasound, and Examples 6 and 7 accelerated the high-low frequency conversion cycle compared with Example 1. The test results show that as the ultrasound frequency increases or the high-low frequency conversion cycle accelerates, the heavy metal content in polydimethylsiloxane generally decreases. Both methods promote the heavy metal mass transfer and diffusion process by increasing the ultrasound energy density and accelerating the periodic change of ultrasound. Comparative Example 2 fixed the ultrasound frequency, and its promoting effect on the mass transfer process was weaker than that of the high-low frequency conversion.

[0052] Compared with Example 1, Comparative Example 3 removed pulse stirring during the ultrasonic process, resulting in insufficient contact between the heavy metal composite adsorbent and the crude polydimethylsiloxane, which affected the purification effect.

[0053] Compared with Example 1, Comparative Examples 4, 8, and 9 changed the proportion of heavy metal composite agent in the refining process. When the proportion was too small, the contact area between the heavy metal adsorbent and the crude polydimethylsiloxane was small, and the removal effect of heavy metals may be affected due to the adsorption reaching saturation. However, as the proportion of heavy metal composite agent increased, the contact area with the crude polydimethylsiloxane increased, and the removal effect of heavy metals improved.

[0054] Compared with Example 1, Comparative Examples 5-8 changed the composition of the heavy metal composite adsorbent. The results showed that the composite adsorbent composed of two materials or the single thiol-modified mesoporous silica had insufficient removal capacity for heavy metals. Since heavy metals exist in free and compound forms, the effect of a single treatment method is limited. The removal of heavy metals by the three components in the composite adsorbent is not a simple additive process, but rather a synergistic effect of "structure-chemistry-mass transfer" formed through the chelation of thiol-modified mesoporous silica, the active reduction supported by nano-zero-valent iron (nZVI), and the electrostatic adsorption of bentonite modified with polyethyleneimine (PEI). This helps to improve the mass transfer limitation and activity decay of single materials in the high viscosity and non-polar environment of silicone oil, and achieves integrated removal of heavy metals through "pretreatment-capture-fixation".

[0055] Compared with Example 1, Examples 10 and 11 changed the critical temperature in the purification process. The increase in temperature accelerated the movement of molecules and enhanced the activity of the heavy metal composite adsorbent groups. Therefore, a certain range of increase in the critical temperature helps to remove heavy metals.

[0056] Finally, it should be noted that the above-listed embodiments are merely preferred embodiments of the present invention. Of course, those skilled in the art can make modifications and variations to the present invention. If such modifications and variations fall within the scope of the claims of the present invention and their equivalents, they should be considered as being within the protection scope of the present invention.

Claims

1. A method for purifying polydimethylsiloxane, characterized in that, Includes the following steps: S1. Crude polydimethylsiloxane and heavy metal composite adsorbent are thoroughly mixed to form a suspension system; S2. Treat the suspension system after step S1 under alternating low-frequency and high-frequency ultrasonic fields and critical temperature conditions for 1 to 3 hours. S3. The material processed in step S2 is initially filtered through a filter to remove minute impurities and eliminate odors. S4. The material processed in step S3 is filtered a second time through a polytetrafluoroethylene filter membrane to obtain chemical-grade polydimethylsiloxane.

2. The method for purifying polydimethylsiloxane according to claim 1, characterized in that, The viscosity of the crude polydimethylsiloxane in step S1 is 100~1000 cSt.

3. The method for purifying polydimethylsiloxane according to claim 1, characterized in that, The heavy metal composite adsorbent in step S1 includes: 50-70 wt% mercapto-modified mesoporous silica with a pore size of 5-10 nm and a mercapto loading of ≥2.5 mmol / g; 20-30 wt% of polyethyleneimine-modified bentonite, with polyethyleneimine having a molecular weight of 800-2000 Da and a modification amount of 10-15 wt%; Nano-sized zero-valent iron-supported activated carbon, 10-20 wt%, with an iron loading of 5-8 wt% and a particle size of 50-100 nm.

4. The method for purifying polydimethylsiloxane according to claim 3, characterized in that, Thiol-modified mesoporous silica is prepared by the following steps: (a) Mesoporous silica and 3-mercaptopropyltrimethoxysilane were refluxed in an ethanol solution and reacted at room temperature for 6-8 hours. (b) The product after the reaction was washed with acetone using a Soxhlet extractor and then dried under vacuum to obtain mercapto-modified mesoporous silica.

5. The method for purifying polydimethylsiloxane according to claim 1, characterized in that, In step S1, the mass ratio of the heavy metal composite adsorbent to crude polydimethylsiloxane is 1:20 to 1:50, and the heavy metal adsorbent is recovered by centrifugation or filtration after adsorption, and can be recycled ≥5 times after washing with ethanol.

6. The method for purifying polydimethylsiloxane according to claim 1, characterized in that, In step S2, the low-frequency alternating ultrasonic field has a frequency of 20~40 kHz and a high-frequency frequency of 80~120 kHz, with an alternation period of 5~10 minutes; the ultrasonic power density is 100~300 W / L; and pulsed stirring is applied during ultrasonic treatment with a pulse frequency of 1~2 Hz and an amplitude of 5~10 cm.

7. The method for purifying polydimethylsiloxane according to claim 1, characterized in that, The critical temperature in step S2 is 50~70℃.

8. The method for purifying polydimethylsiloxane according to claim 1, characterized in that, The filter material in the primary filter in step S3 is composed of activated carbon and diatomaceous earth, with a mass ratio of activated carbon to diatomaceous earth of 1:2 to 1:

5.

9. The method for purifying polydimethylsiloxane according to claim 1, characterized in that, In step S4, the pore size of the polytetrafluoroethylene filter membrane is 0.2~0.5 μm.

10. The method for purifying polydimethylsiloxane according to claim 1, characterized in that, The chemical-grade polydimethylsiloxane obtained in step S4 meets the following requirements: lead <10mg / kg, arsenic <2mg / kg, cadmium <5mg / kg; viscosity at 25℃ is 100~1000 cSt, and turbidity ≤1 NTU.

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