A low molecular weight dimethicone and a method for preparing the same

Abstract

The application relates to the technical field of organic silicon, and provides a low-molecular-weight dimethyl silicone oil and a preparation method thereof. The device used in the preparation method comprises a preheater, a static mixer, a first fixed-bed reactor, a second fixed-bed reactor, a filter, a first rectifying tower and a second rectifying tower which are sequentially communicated; the preheater is provided with a dimethyl siloxane mixed ring body inlet and an end-capping agent inlet through pipelines. According to the preparation method, the raw materials and end-capping agents, adjusting agents and the like are preheated and mixed in the high-efficiency static mixer, then enter the reaction from the bottom of the fixed-bed reactor provided with a solid catalyst at a certain flow rate, and the outlet of the reactor is provided with a filtering device and a flow regulating valve to control the residence time. In order to make the reaction complete, two or more reactors are connected in series, and the conversion rate of the low-molecular-weight dimethyl silicone oil can reach 94% after the reaction is completed.

Description

Technical Field

[0001] This application relates to the field of organosilicon technology, and in particular to a low molecular weight dimethyl silicone oil and its preparation method. Background Technology

[0002] Low molecular weight polysiloxane, also known as dimethyl silicone oil (viscosity ≤ 100 mm). 2 With an average molecular weight ≤10,000 g / mol, dimethyl silicone oil is a linear polymer with a main chain composed of repeating Si-O (silicon-oxygen) bonds, each silicon atom bonded to two methyl groups (-CH3). Its chemical formula is typically represented as [CH3]3SiO{[CH3]2SiO}nSi[CH3]3; "low molecular weight" refers to the low degree of polymerization (n value), usually between tens and hundreds. This results in a wide viscosity range, from a water-like liquid to a very viscous semi-fluid. Dimethyl silicone oil is renowned for its excellent chemical stability, hydrophobicity, lubricity, silky feel, and physiological inertness. It plays an indispensable role in everything from our daily necessities (washing and care products, cosmetics) to industrial production and high-tech fields, making it a highly valuable "universal" material in modern chemical industry.

[0003] Currently, the industrial production methods for dimethyl silicone oil in China all involve batch reaction operation using a batch reactor. First, dimethylsiloxane mixed cyclic compounds (DMC) (D3 / D4 / D5) and dimethylsilyl end-capping agent (MM) are added to a dehydration reactor in a specific ratio (based on the required dimethyl silicone oil viscosity). Water is removed under reduced pressure, nitrogen purging, and heating to 50°C before being transferred to a polymerization reactor. Sulfuric acid is added as a catalyst, and D4 is used as a polymerization modifier. The reaction is carried out under nitrogen purging and reduced pressure, slowly heated to 88-95°C for 4 hours. After the reaction, the mixture enters a separator to remove residual sulfuric acid. Then, alkali is added to neutralize the residual acid in the reactants, followed by water washing to remove acid. Finally, the mixture enters a distillation unit and is heated to 200°C to evaporate the low-boiling-point compounds D3, D4, and D5. After cooling, it is filtered and decolorized to obtain colorless and transparent dimethyl silicone oil. This production process involves many steps, requires high-end equipment, involves significant investment, has a long production cycle, low efficiency, unstable product quality, high energy consumption, and generates serious waste problems. Therefore, there is an urgent need to develop a low-molecular-weight dimethyl silicone oil preparation method that is short in process, has few steps, is simple to operate, has low energy consumption, stable product quality, and low production cost. Summary of the Invention

[0004] In view of the above-mentioned shortcomings in the prior art, the purpose of this application is to provide a low molecular weight dimethyl silicone oil and its preparation method.

[0005] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows: In a first aspect, embodiments of this application provide a method for preparing low molecular weight dimethyl silicone oil. The apparatus used in the preparation method includes a preheater, a static mixer, a first fixed-bed reactor, a second fixed-bed reactor, a filter, a first distillation column, and a second distillation column connected in sequence. The preheater is provided with a dimethylsiloxane mixed ring inlet and a capping agent inlet via pipelines. The top of the second distillation column is provided with a first circulation pipeline, on which a condenser is provided and connected to the preheater via the capping agent inlet. The bottom of the second distillation column is provided with a second circulation pipeline and connected to the preheater via the dimethylsiloxane mixed ring inlet. The bottom of the first distillation column is provided with a dimethyl silicone oil outlet. The preparation method includes the following steps: Dimethylsiloxane mixed cyclic compounds and end-capping agents are preheated in the preheater through the dimethylsiloxane mixed cyclic compound inlet and the end-capping agent inlet, respectively. After preheating, the mixture is mixed in a static mixer and then fed into the first and second fixed-bed reactors. The mixture undergoes polycondensation in the first and second fixed-bed reactors. After the polycondensation reaction, the reactants pass through a filter and enter the first distillation column. The reactants undergo a first distillation process in the first distillation column. After the first distillation process, low-molecular-weight dimethyl silicone oil is collected through the dimethyl silicone oil outlet, and the remaining material enters the second distillation column. The remaining material undergoes a second distillation process in the second distillation column, and then passes through a condenser and a second circulation pipeline before entering the preheater to participate in the reaction again.

[0006] In an alternative embodiment, the apparatus further includes a viscometer, through which the viscosity of the reactants exiting the second fixed-bed reactor is detected in the operating method.

[0007] In an optional embodiment, the apparatus further includes a first outlet regulating valve and a second outlet regulating valve. In the operation method, if the viscosity value detected by the viscometer reaches the target viscosity, the first outlet regulating valve is opened and the second outlet regulating valve is closed, allowing the reactants to pass through the filter and enter the first distillation column; if the viscosity value detected by the viscometer does not reach the target viscosity, the second outlet regulating valve is opened and the first outlet regulating valve is closed, allowing the reactants to return to the first fixed-bed reactor for reaction again.

[0008] In an optional embodiment, the dimethylsiloxane mixed cyclic compound comprises at least one or a combination of hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecylcyclohexasiloxane, tetradecylcycloheptasiloxane, and hexadecylcyclooctasiloxane; and / or the end-capping agent comprises a dimethylsilyl end-capping agent; and / or the catalyst comprises a strongly acidic cation exchange resin, a solid superacid solid titanium dioxide (TiO2 / SO4). 2- Zirconium-titanium alloy ZrO2-TiO2 / SO4 2-Titanium-iron alloy TiO2-Fe2O3 / SO4 2- Zirconium iron alloy ZrO2-Fe2O3 / SO4 2- At least one or a combination thereof.

[0009] In one alternative implementation, the preheater operates at a temperature of 60-80°C; and / or the first distillation column operates at a temperature of 60-80°C; and / or the second distillation column operates at a temperature of 60-80°C.

[0010] In an alternative embodiment, the preheater is also provided with a regulator inlet via a pipeline; the regulator enters the preheater through the regulator inlet.

[0011] In one alternative embodiment, the modifier comprises at least one or a combination of hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, and dodecylcyclohexasiloxane.

[0012] In an optional embodiment, the apparatus further includes pumps I, II, and III, wherein pump I feeds the mixture flowing out of the static mixer and / or the second outlet regulating valve into the first fixed-bed reactor; pump II feeds the reactants flowing out of the filter into the first distillation column; and pump III is a vacuum pump used to evacuate the first and second distillation columns to a negative pressure state.

[0013] In one alternative implementation, the operating pressure of the first distillation column is -90 to -100 kPa; and / or the operating pressure of the second distillation column is -90 to -100 kPa.

[0014] Secondly, this application provides a low molecular weight dimethyl silicone oil, which is prepared by any of the preparation methods described above.

[0015] The beneficial effects of this application include at least the following: (1) The preparation method of this application uses a solid strong acid catalyst as the catalyst. After the raw materials, end-capping agent, regulator, etc. are preheated and mixed in a high-efficiency static mixer, they enter the reaction from the bottom of a fixed bed reactor containing solid catalyst at a certain flow rate. A filter device and a flow regulating valve are set at the reactor outlet to control the residence time. In order to make the reaction complete, two or more reactors can be connected in series. After the reaction is completed, the conversion rate of low molecular weight dimethyl silicone oil can reach 94%. (2) By using two fixed-bed reactors in series, the residence path of the material in the catalytic reaction zone can be effectively extended, and the reaction process can be completed in stages, thereby improving the overall controllability and completion of the reaction. Compared with a single large-volume reactor, the series connection of two fixed beds is more conducive to the gradual advancement of the reaction and reduces the temperature fluctuation and uneven reaction caused by the excessive load of a single bed. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the apparatus for preparing low molecular weight dimethyl silicone oil according to an embodiment of this application; Explanation of reference numerals in the attached drawings: 1-Preheater, 2-Static mixer, 3-First fixed bed reactor, 4-Second fixed bed reactor, 5-Viscometer, 6-First outlet regulating valve, 7-Second outlet regulating valve, 8-Filter, 9-First distillation column, 10-Second distillation column, 11-Condenser, 12-Pump I, 13-Pump II, 14-Pump III. Detailed Implementation

[0018] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are only for explaining this application, but the implementation of this application is not limited thereto.

[0019] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this application pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; the amounts of experimental reagents used are, unless otherwise specified, the amounts used in conventional experimental operations; and the experimental methods used are, unless otherwise specified, conventional methods.

[0020] Currently, the industrial production of dimethyl silicone oil in China generally adopts a batch operation mode using a batch reactor. The process flow is as follows: DMC and MM are mixed according to the target product viscosity requirements and added to a dehydration reactor. Under reduced pressure, nitrogen purging, and heating conditions, the moisture in the raw materials is removed. The material is then transferred to a polymerization reactor. During the polymerization stage, a catalyst and regulator are added, and the polymerization reaction proceeds under continuous nitrogen purging and reduced pressure. After the reaction, an alkaline solution is added to the reaction product to neutralize any residual acidic substances, followed by a water washing process to ensure the system is neutral. The purified material enters a distillation unit, where low-boiling-point components are evaporated at high temperature. Finally, the material is cooled, filtered, and decolorized to obtain a colorless and transparent dimethyl silicone oil product. However, the above operation suffers from several problems: complex procedures, high equipment requirements and large initial investment; a long production process leading to low efficiency and high energy consumption; fluctuating product quality; and significant wastewater and waste acid generation issues.

[0021] In a first aspect, embodiments of this application provide a method for preparing low molecular weight dimethyl silicone oil. The apparatus used in the preparation method includes a preheater 1, a static mixer 2, a first fixed-bed reactor 3, a second fixed-bed reactor 4, a filter 8, a first distillation column 9, and a second distillation column 10 connected in sequence. The preheater 1 is provided with a dimethylsiloxane mixed ring inlet and a capping agent inlet via pipelines. The top of the second distillation column 10 is provided with a first circulation pipeline, on which a condenser 11 is provided and connected to the preheater 1 via the capping agent inlet. The bottom of the second distillation column 10 is provided with a second circulation pipeline and connected to the preheater 1 via the dimethylsiloxane mixed ring inlet. The bottom of the first distillation column 9 is provided with a dimethyl silicone oil outlet. The preparation method includes the following steps: Dimethylsiloxane mixed cyclic compounds and end-capping agents enter preheater 1 through the dimethylsiloxane mixed cyclic compound inlet and end-capping agent inlet, respectively, for preheating. After preheating, the mixture enters static mixer 2 for mixing. The resulting mixture then enters the first fixed-bed reactor 3 and the second fixed-bed reactor 4. The mixture undergoes polycondensation in the first fixed-bed reactor 3 and the second fixed-bed reactor 4. After the polycondensation reaction, the reactants pass through filter 8 and enter the first distillation column 9. The reactants undergo a first distillation process in the first distillation column 9. After the first distillation process, low molecular weight dimethyl silicone oil is collected through the dimethyl silicone oil outlet, and the remaining material enters the second distillation column 10. The remaining material undergoes a second distillation process in the second distillation column 10, and after passing through condenser 11 and the second circulation pipeline, it enters preheater 1 again to participate in the reaction.

[0022] Preferably, the preparation method of this application uses a solid strong acid catalyst. The raw material DMC, along with end-capping agents and regulators such as octamethylcyclotetrasiloxane (D4), is metered at a certain flow rate and pumped into a high-efficiency static mixer 2 for preheating and mixing. This mixture then enters the reaction mixture from the bottom of a fixed-bed reactor containing the solid catalyst at a certain flow rate. A filter and flow control valve are installed at the reactor outlet to control the residence time. To ensure complete reaction, two or more reactors can be connected in series. After the reaction, dimethyl silicone oil with a conversion rate of up to 94% enters an intermediate tank and is then pumped at a certain flow rate into a molecular distillation apparatus to remove low-volatility components and unreacted monomers. Unreacted monomers are collected and returned to the reactor for reuse. Finally, purified and stable dimethyl silicone oil is obtained in the bottom of the distillation column. This process allows for continuous feeding and continuous production.

[0023] Preferably, a preheater 1 is first installed in the apparatus to ensure that DMC and the end-capping agent reach a suitable temperature for the polycondensation reaction before entering the reaction section. Preheating not only increases the material temperature but, more importantly, ensures good flowability and reactivity of the material before it enters the subsequent reactor, preventing slow reaction start-up, excessive local temperature differences, or uneven reaction caused by cold material directly entering the fixed bed. Since the reusable material separated by the subsequent second distillation column 10 is also returned to the preheater 1, the preheater 1 effectively serves to thermally integrate the recycled material with the fresh raw material, resulting in a more consistent material state entering the reaction section, smoother reactor operation, less subsequent reheating burden, and better conditions for the recycled material to re-participate in the reaction.

[0024] Preferably, a static mixer 2 is installed after the preheater 1. This is because while fixed-bed reactors are suitable for continuous polycondensation reactions, the composition of the materials entering the bed must be as uniform as possible. If the raw materials are not sufficiently mixed before entering the fixed bed, localized areas of excessively high end-capping agent concentration or mixed ring concentration may occur within the bed, leading to uneven local reaction rates and affecting the molecular weight distribution and viscosity stability of the final product. The static mixer 2 does not rely on mechanical stirring but uses internal components to cut, divide, and reorganize the material stream, enabling continuous and uniform mixing of the preheated material within a shorter flow path. With this setup, the mixing function is completed before entering the reactor, allowing the fixed-bed reactor to focus on the reaction itself without having to handle mixing. Therefore, its direct technical effect is to improve bed utilization, reduce localized over-reaction or under-reaction phenomena, and further improve the consistency of product viscosity and quality.

[0025] Furthermore, the dimethylsiloxane mixed ring includes at least one or a combination of hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecylcyclohexasiloxane, tetradecylcycloheptasiloxane, and hexadecylcyclooctasiloxane, allowing for more flexible raw material sources and easier control of the reaction system. Dimethylsilyl end-capping agents are preferred. If the temperature of preheater 1 is too low, preheating will be insufficient, resulting in poor material flowability and reaction preparation, and making it difficult to remove light components during subsequent distillation. If the temperature is too high, energy consumption will increase, and the product may be subjected to excessive heat history, affecting quality stability. Therefore, the operating temperature of preheater 1 should ideally be within the range of 60~80℃.

[0026] In some embodiments of this application, the preheater 1 is further provided with a regulator inlet via a pipeline; the regulator enters the preheater 1 through the regulator inlet.

[0027] Preferably, the regulator is a material that needs to be preheated along with DMC and MM at the front end of the reaction. An additional regulator inlet is provided on the preheater 1, allowing the regulator to be heated and homogenized together with the mixed rings and end-capping agent before entering the fixed-bed reactor. This avoids localized concentration differences and uneven reaction caused by subsequent addition, and facilitates precise control of the final product within the target low viscosity range. The regulator preferably includes at least one or a combination of hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, and dodecylcyclohexasiloxane.

[0028] Preferably, a first fixed-bed reactor 3 and a second fixed-bed reactor 4 are set after the static mixer 2 because the generation of low molecular weight dimethyl silicone oil is not instantaneous, but a continuous process including ring opening, chain growth, and equilibration. The reaction principle is as follows: Where n≥3. Using two fixed-bed reactors in series effectively extends the residence path of materials within the catalytic reaction zone, allowing the reaction process to be completed in stages, thereby improving the overall controllability and completeness of the reaction. Compared to a single large-volume reactor, dual fixed-bed reactors in series are more conducive to progressively advancing the reaction, reducing temperature fluctuations and uneven reaction caused by excessive load on a single bed. Furthermore, by coordinating with circulation control logic, residence time and discharge rhythm can be adjusted based on whether the product viscosity meets requirements, thus better controlling the product within the target low molecular weight range.

[0029] Furthermore, this application employs a fixed-bed reactor packed with a solid catalyst, instead of a traditional liquid sulfuric acid catalytic system. The catalyst preferably comprises a strongly acidic cation exchange resin, a solid superacid titanium dioxide (TiO2 / SO4), or other similar catalysts. 2- Zirconium-titanium alloy ZrO2-TiO2 / SO4 2-Titanium-iron alloy TiO2-Fe2O3 / SO4 2- Zirconium iron alloy ZrO2-Fe2O3 / SO4 2- At least one or a combination of the following. While traditional liquid acid catalytic processes can achieve the reaction, a series of post-treatment steps such as acid separation, neutralization, and water washing are required after the reaction. This not only results in a long process but also generates large amounts of waste acid and wastewater, and requires equipment with high corrosion resistance. This application fixes the solid catalyst within the reactor bed, preventing the catalyst from flowing out with the reactants. This fundamentally avoids the problem of subsequent catalyst separation in liquid acid processes. After the reaction, no complex neutralization and water washing operations are needed, reducing waste emissions, shortening the process flow, and lowering the risk of equipment corrosion and product acid residue.

[0030] Preferably, although the catalyst in the fixed bed should not theoretically flow out in large quantities with the material flow, a small amount of fine catalyst powder, mechanically worn particles, or other impurities may still be carried into subsequent processes. If these solid impurities directly enter the distillation system, they may cause contamination of heat exchange surfaces, column blockage, vacuum fluctuations, or even a decrease in product cleanliness. Therefore, by setting up filter 8, potentially carried-out solid particles can be intercepted in advance, thereby effectively protecting the subsequent distillation column and the entire vacuum separation system.

[0031] Preferably, a first distillation column 9 and a second distillation column 10 are provided after the filter 8. Since low molecular weight dimethyl silicone oil is a relatively heavy component compared to the end-capping agent MM and some unreacted mixed rings, the target product can be preferentially separated from the reaction mixture through the first distillation column 9 and continuously collected from the bottom of the column. This allows the target product to leave the subsequent complex circulation separation system as early as possible, reducing its extra residence time under high temperature or vacuum conditions, which is more conducive to maintaining product quality stability. The remaining material in the first distillation column 9 continues to enter the second distillation column 10. This remaining material mainly contains the end-capping agent MM, which still has utilization value, and unreacted or re-reactable mixed rings. Through further separation in the second distillation column 10, these components can be drawn from the top and bottom of the column according to their different volatility and then refluxed to the preheater 1 to participate in the reaction again, thereby improving the raw material utilization rate, reducing waste, and reducing the consumption of fresh raw materials. The operating temperature of the first distillation column 9 is 60~80℃, and the operating temperature of the second distillation column 10 is 60~80℃. This ensures that the conditions are suitable for separation, utilizes the residual heat of the reaction, reduces the reheating load, and minimizes the impact of high temperature on product stability.

[0032] Preferably, the second distillation column 10 is equipped with a circulation pipeline at the top, and a condenser 11 is installed on the circulation pipeline. This is because the MM exiting from the top of the second distillation column 10 is a relatively light volatile component, which is usually discharged in the form of gas or rich gas phase. If it is directly refluxed without passing through the condenser 11, it will be detrimental to stable delivery and metering, and will also easily cause temperature disturbances to the upstream system. Therefore, it needs to be condensed into liquid by the condenser 11 first, and then sent back to the preheater 1 to be reheated and mixed with fresh material. The second circulation pipeline at the bottom of the second distillation column 10 and returning to the preheater 1 is for the purpose of recovering unreacted mixed cyclic compounds. The bottom material is mainly hexamethylcyclotrisiloxane (D3), D4, decamethylcyclopentasiloxane (D5), and other dimethylsiloxane mixed cyclic compounds that can participate in the condensation reaction again. It is not waste material, but a valuable reaction raw material. If this part of the material is discharged directly, it will not only reduce the overall yield, but also increase the production cost. After being sent back to preheater 1 through the second circulation pipeline, these rings can be reheated, homogenized by static mixer 2, and then enter the fixed bed reactor to participate in a new round of reaction.

[0033] Preferably, both the MM obtained from the top of the second distillation column 10 and the mixed ring obtained from the bottom are returned to the preheater 1. Since the preheater 1 is immediately followed by the static mixer 2, once the reflux returns to the preheater 1, it can first achieve temperature uniformity with the fresh material, then achieve composition homogenization, and finally enter the fixed-bed reactor in a more stable state. If the reflux is directly fed into the fixed-bed reactor, its temperature, composition, and flow rate may fluctuate significantly, easily causing changes in local reaction conditions within the bed and affecting the stability of the entire system.

[0034] In some embodiments of this application, the apparatus further includes a viscometer 5, in which the viscosity of the reactants flowing out of the second fixed-bed reactor 4 is detected by the viscometer 5.

[0035] Preferably, the dimethylsiloxane mixed rings and the end-capping agent undergo a continuous polycondensation and equilibration reaction in the fixed bed. As the reaction proceeds, the chain segments gradually increase in length, and the average molecular weight of the system changes. This change in average molecular weight directly reflects the change in system viscosity; a lower degree of polymerization results in a low-viscosity liquid, while a further increase in the degree of polymerization leads to greater viscosity. Therefore, simply estimating the completion of the reaction based on time or flow rate is insufficient, as bed catalytic activity, fluctuations in feed composition, and changes in the reflux stream ratio all affect the final degree of reaction. Directly detecting the viscosity of the effluent from the outlet of the second fixed-bed reactor 4 provides a more accurate reflection of whether the material has reached the predetermined degree of polymerization. Therefore, by installing a viscometer 5, it is possible to accurately determine whether the reaction has reached the target, control the reaction endpoint in a timely manner, stabilize the product molecular weight and viscosity, reduce the entry of substandard materials into the subsequent rectification section, and improve product consistency.

[0036] Furthermore, the device also includes a first outlet regulating valve 6 and a second outlet regulating valve 7. In the operation method, if the viscosity value detected by the viscometer 5 reaches the target viscosity, the first outlet regulating valve 6 is opened and the second outlet regulating valve 7 is closed, so that the reactants enter the first distillation column 9 after passing through the filter 8; if the viscosity value detected by the viscometer 5 does not reach the target viscosity, the second outlet regulating valve 7 is opened and the first outlet regulating valve 6 is closed, so that the reactants return to the first fixed bed reactor 3 to react again.

[0037] Preferably, by setting two outlet regulating valves, different flow directions can be selected based on two different detection results: the reaction has reached the target or it has not. When the viscometer 5 detects that the viscosity of the reactant flowing out of the second fixed-bed reactor 4 has reached the target value, it indicates that it has basically fallen into the molecular weight and viscosity range of the target low molecular weight dimethyl silicone oil. If it is still allowed to continue to return to the reactor for circulation at this time, there is a risk of further condensation, leading to an increase in the molecular weight and viscosity of the product. To avoid this over-reaction phenomenon, the first outlet regulating valve 6 needs to be opened while the second outlet regulating valve 7 is closed, so that the material leaves the reaction circulation system, passes through the filter 8 to remove any possible entrained solid impurities, and then enters the first distillation column 9 for subsequent separation and purification. The principle here is very clear: since the product has reached the target state, it should be switched from the reaction section to the separation section in a timely manner to avoid it deviating from the target due to continued catalyst action. When the viscometer 5 detects that the viscosity value has not yet reached the target value, it indicates that the stream flowing out of the second fixed-bed reactor 4 has not yet completed a sufficient degree of polycondensation and equilibration reaction. If the first outlet regulating valve 6 is opened directly at this time to send it to the first distillation column 9, the unreacted material will enter the separation section prematurely. As a result, not only will the actual production of the target low molecular weight dimethyl silicone oil be insufficient, but the processing burden of unreacted rings and end-capping agents in the subsequent distillation system will also increase, reducing the stability and efficiency of the entire process.

[0038] In some embodiments of this application, the apparatus further includes pump I12, pump II13, and pump III14. In the operation method, pump I12 sends the mixture flowing out of the static mixer 2 and / or the second outlet regulating valve 7 into the first fixed bed reactor 3; pump II13 sends the reactants flowing out of the filter 8 into the first distillation column 9; pump III14 is a vacuum pump, which is used to draw the first distillation column 9 and the second distillation column 10 to a negative pressure state.

[0039] Preferably, pump I12 is used to ensure that the mixture flow has sufficient and stable conveying power in the continuous fixed bed system, which can overcome the pressure drop of the fixed bed and allow the reactants to enter the first fixed bed reactor 3 stably, avoiding discontinuous feeding, unstable bed flow, or even local flow deviation due to insufficient pressure. After pump I12 provides a stable flow rate, the effective residence time in the fixed bed reactor can be more easily controlled, and the product viscosity and molecular weight can be more easily kept stable. Pump II13 ensures a continuous and stable feed to the first distillation column 9 before the reaction-completed stream enters the vacuum distillation system. This is crucial for continuous molecular distillation or continuous distillation, as the separation efficiency of the column largely depends on the stability of the feed rate. After filtration, the stream is then fed into the column by pump II13, which avoids unstable feed, fluctuations in the column level, and decreased separation efficiency caused by insufficient gravity flow or natural flow. The feed to the first distillation column 9 can utilize the waste heat of the reaction, thereby saving reheating energy consumption. This indicates that only with the continuous delivery of pump II13 can the hot reaction stream enter the distillation column in a timely and continuous manner, fully utilizing the energy-saving effect of waste heat utilization and reducing reheating.

[0040] Furthermore, pump III14 is a vacuum pump, whose main function is to provide a negative pressure environment for the first distillation column 9 and the second distillation column 10, reducing the vaporization temperature of low-boiling components such as MM in the system. This allows them to be effectively distilled off at a relatively low reboiler temperature, eliminating the need to raise the temperature to 200°C to remove low-boiling substances as in traditional batch processes. Under negative pressure, the total system pressure decreases, and the boiling point of the light components decreases accordingly. Therefore, MM and unreacted mixed rings can be vaporized and separated at a relatively mild temperature of 60-80°C, significantly reducing the temperature required for devolatilization and distillation, thereby reducing energy consumption. It also shortens the residence time of materials at high temperatures, preventing product quality fluctuations due to prolonged heating. The operating pressures of both the first distillation column 9 and the second distillation column 10 are within the range of -90 to -100 kPa, which is conducive to forming a stable and continuous material migration and volatile matter transfer relationship between the two stages of separation. This allows the light components distilled off the first column to smoothly enter the second column, where MM and mixed rings are further separated and recovered, thus achieving efficient operation of the entire closed-loop circulation system.

[0041] In some embodiments of this application, after the low molecular weight dimethyl silicone oil is collected from the dimethyl silicone oil outlet, the preparation method further includes filtration and devolatilization. Although this application uses a fixed-bed solid catalyst route, which avoids complex post-processing such as extensive acid separation, neutralization, and water washing compared to the traditional sulfuric acid process, in continuous industrial operation, the reaction stream may still carry a small amount of catalyst fine powder, mechanical wear particles, or other impurities after passing through the fixed-bed reactor. Filtration helps remove solid impurities, ensuring the cleanliness and appearance of the final product; devolatilization further removes unreacted monomers, improving product purity and stability.

[0042] Secondly, this application provides a low molecular weight dimethyl silicone oil, which is prepared using any of the preparation methods described above. Therefore, it possesses all the beneficial effects of the preparation methods for low molecular weight dimethyl silicone oil, which will not be elaborated further here. Example 1

[0043] Combination Figure 1 The apparatus provided in this embodiment offers a method for preparing low molecular weight dimethyl silicone oil, wherein the end-capping agent is MM, the DMC is D3, the regulator is D3, and the catalyst is a strong acid cation exchange resin. D3 and MM enter the preheater 1 through the dimethylsiloxane mixed cyclic inlet and the end-capping agent inlet, respectively, for preheating. The regulator D3 enters the preheater 1 through the regulator inlet and is preheated together with D3 and MM. The operating temperature of the preheater 1 is 60℃. After preheating, the mixture enters the static mixer 2 for mixing and then enters the first fixed bed reactor 3 and the second fixed bed reactor 4. The mixture undergoes a polycondensation reaction in the first fixed-bed reactor 3 and the second fixed-bed reactor 4. After the polycondensation reaction is completed, the viscosity is measured by the viscometer 5. Once the target viscosity is reached, the first outlet regulating valve is opened and the second outlet regulating valve 7 is closed, allowing the reactants to pass through the filter 8 and enter the first distillation column 9. The reactants undergo a first distillation process in the first distillation column 9 at a temperature of 60°C and a pressure of -95 kPa. After the first distillation process, the low molecular weight dimethyl silicone oil is collected through the dimethyl silicone oil outlet and then subjected to filtration and devolatilization treatment. The remaining material enters the second distillation column 10. The remaining material undergoes a second distillation process in the second distillation column 10 at a temperature of 60°C and a pressure of -95 kPa. MM enters the preheater 1 after passing through the condenser 11 and participates in the reaction again. The mixed ring enters the preheater 1 through the second circulation pipeline and participates in the reaction again. The conversion rate of the obtained low molecular weight dimethyl silicone oil was 94.3%, and the viscosity of the low molecular weight dimethyl silicone oil was 25 mm. 2 / s, with an average molecular weight of 1220 g / mol. Example 2

[0044] This embodiment provides a method for preparing low molecular weight dimethyl silicone oil, wherein the end-capping agent is MM, the DMC is D4, the regulator is D4, and the catalyst is a zirconium-titanium alloy ZrO2-TiO2 / SO4. 2- ; D4 and MM enter the preheater 1 through the dimethylsiloxane mixed cyclic inlet and the end-capping agent inlet, respectively, for preheating. The regulator D4 enters the preheater 1 through the regulator inlet and is preheated together with D4 and MM. The operating temperature of the preheater 1 is 80℃. After preheating, the mixture enters the static mixer 2 for mixing and then enters the first fixed bed reactor 3 and the second fixed bed reactor 4. The mixture undergoes a polycondensation reaction in the first fixed-bed reactor 3 and the second fixed-bed reactor 4. After the polycondensation reaction is completed, the viscosity is measured by a viscometer 5. Once the target viscosity is reached, the off-site outlet regulating valve is opened and the second outlet regulating valve 7 is closed, allowing the reactants to pass through the filter 8 and enter the first distillation column 9. The reactants undergo a first distillation process in the first distillation column 9 at a temperature of 80°C and a pressure of -100 kPa. After the first distillation process, the low molecular weight dimethyl silicone oil is collected through the dimethyl silicone oil outlet and then subjected to filtration and devolatilization treatment. The remaining material enters the second distillation column 10. The remaining material undergoes a second distillation process in the second distillation column 10 at a temperature of 80°C and a pressure of -100 kPa. MM passes through the condenser 11 and then enters the preheater 1 to participate in the reaction again. The mixed ring enters the preheater 1 through the second circulation pipeline to participate in the reaction again. The conversion rate of the obtained low molecular weight dimethyl silicone oil was 96.1%, and the viscosity of the low molecular weight dimethyl silicone oil was 30 mm. 2 / s, with an average molecular weight of 1650 g / mol. Example 3

[0045] This embodiment provides a method for preparing low molecular weight dimethyl silicone oil, wherein the end-capping agent is MM, the DMC is D5, the regulator is D4, and the catalyst is a titanium-iron alloy TiO2-Fe2O3 / SO4. 2- ; D5 and MM enter the preheater 1 through the dimethylsiloxane mixed cyclic inlet and the end-capping agent inlet, respectively, for preheating. Regulator D4 enters the preheater 1 through the regulator inlet and is preheated together with D5 and MM. The operating temperature of the preheater 1 is 70°C. After preheating, the mixture enters the static mixer 2 for mixing and then enters the first fixed bed reactor 3 and the second fixed bed reactor 4. The mixture undergoes a polycondensation reaction in the first fixed-bed reactor 3 and the second fixed-bed reactor 4. After the polycondensation reaction is completed, the viscosity is measured by a viscometer 5. Once the target viscosity is reached, the off-site outlet regulating valve is opened and the second outlet regulating valve 7 is closed, allowing the reactants to pass through the filter 8 and enter the first distillation column 9. The reactants undergo a first distillation process in the first distillation column 9 at a temperature of 70°C and a pressure of -98 kPa. After the first distillation process, the low molecular weight dimethyl silicone oil is collected through the dimethyl silicone oil outlet and then subjected to filtration and devolatilization treatment. The remaining material enters the second distillation column 10. The remaining material undergoes a second distillation process in the second distillation column 10 at a temperature of 70°C and a pressure of -98 kPa. MM enters the preheater 1 after passing through the condenser 11 and participates in the reaction again. The mixed ring enters the preheater 1 through the second circulation pipeline and participates in the reaction again. The conversion rate of the obtained low molecular weight dimethyl silicone oil was 95.4%, and the viscosity of the low molecular weight dimethyl silicone oil was 28 mm. 2 / s, with an average molecular weight of 1580 g / mol. Example 4

[0046] This embodiment provides a method for preparing low molecular weight dimethyl silicone oil, wherein the end-capping agent is MM, DMC consists of D6 and D7, the regulator is D5, and the catalyst is a zirconium-iron alloy ZrO2-Fe2O3 / SO4. 2- ; D6, D7, and MM enter preheater 1 through the dimethylsiloxane mixed cyclic inlet and the end-capping agent inlet, respectively, for preheating. Conditioner D4 enters preheater 1 through the conditioner inlet and is preheated together with D6, D7, and MM. The operating temperature of preheater 1 is 75°C. After preheating, the mixture enters static mixer 2 for mixing and then enters the first fixed-bed reactor 3 and the second fixed-bed reactor 4. The mixture undergoes a polycondensation reaction in the first fixed-bed reactor 3 and the second fixed-bed reactor 4. After the polycondensation reaction is completed, the viscosity is measured by a viscometer 5. Once the target viscosity is reached, the off-site outlet regulating valve is opened and the second outlet regulating valve 7 is closed, allowing the reactants to pass through the filter 8 and enter the first distillation column 9. The reactants undergo a first distillation process in the first distillation column 9 at a temperature of 75°C and a pressure of -96 kPa. After the first distillation process, the low molecular weight dimethyl silicone oil is collected through the dimethyl silicone oil outlet and then subjected to filtration and devolatilization treatment. The remaining material enters the second distillation column 10. The remaining material undergoes a second distillation process in the second distillation column 10 at a temperature of 75°C and a pressure of -96 kPa. MM enters the preheater 1 after passing through the condenser 11 and participates in the reaction again. The mixed ring enters the preheater 1 through the second circulation pipeline and participates in the reaction again. The conversion rate of the obtained low molecular weight dimethyl silicone oil was 94.8%, and the viscosity of the low molecular weight dimethyl silicone oil was 40 mm. 2 / s, with an average molecular weight of 2075 g / mol. Example 5

[0047] This embodiment provides a method for preparing low molecular weight dimethyl silicone oil, wherein the end-capping agent is MM, the DMC is D8, the regulator is D4, and the catalyst is a strong acid cation exchange resin. D8 and MM enter the preheater 1 through the dimethylsiloxane mixed ring inlet and the end-capping agent inlet, respectively, for preheating. The regulator D4 enters the preheater 1 through the regulator inlet and is preheated together with D8 and MM. The operating temperature of the preheater 1 is 65℃. After preheating, the mixture enters the static mixer 2 for mixing and then enters the first fixed bed reactor 3 and the second fixed bed reactor 4. The mixture undergoes a polycondensation reaction in the first fixed-bed reactor 3 and the second fixed-bed reactor 4. After the polycondensation reaction is completed, the viscosity is measured by a viscometer 5. Once the target viscosity is reached, the off-site outlet regulating valve is opened and the second outlet regulating valve 7 is closed, allowing the reactants to pass through the filter 8 and enter the first distillation column 9. The reactants undergo a first distillation process in the first distillation column 9 at a temperature of 65°C and a pressure of -97 kPa. After the first distillation process, the low molecular weight dimethyl silicone oil is collected through the dimethyl silicone oil outlet and then subjected to filtration and devolatilization treatment. The remaining material enters the second distillation column 10. After the remaining material undergoes a second distillation process in the second distillation column 10, the temperature is 65℃ and the pressure is -97kPa. MM enters the preheater 1 after passing through the condenser 11 to participate in the reaction again. The mixed ring enters the preheater 1 through the second circulation pipeline to participate in the reaction again. The conversion rate of the obtained low molecular weight dimethyl silicone oil was 96.2%, and the viscosity of the low molecular weight dimethyl silicone oil was 45 mm. 2 / s, with an average molecular weight of 2300g / mol.

[0048] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A method for preparing low molecular weight dimethyl silicone oil, characterized in that, The apparatus used in the preparation method includes a preheater (1), a static mixer (2), a first fixed-bed reactor (3), a second fixed-bed reactor (4), a filter (8), a first distillation column (9), and a second distillation column (10) connected in sequence. The preheater (1) is provided with a dimethylsiloxane mixed cyclic inlet and a sealing agent inlet through a pipeline; The second distillation column (10) is provided with a first circulation pipeline at the top, and a condenser (11) is provided on the first circulation pipeline and connected to the preheater (1) through the end-sealing agent inlet; The second distillation column (10) is provided with a second circulation pipeline at the bottom, and is connected to the preheater (1) through the inlet of the dimethylsiloxane mixed ring; The first distillation column (9) is provided with a dimethyl silicone oil outlet at the bottom; The preparation method includes the following steps: The dimethylsiloxane mixed cyclic compound and the end-capping agent enter the preheater (1) through the dimethylsiloxane mixed cyclic compound inlet and the end-capping agent inlet, respectively, for preheating. After preheating, the mixture enters the static mixer (2) for mixing. The resulting mixture then enters the first fixed-bed reactor (3) and the second fixed-bed reactor (4). The mixture undergoes a polycondensation reaction in the first fixed-bed reactor (3) and the second fixed-bed reactor (4), and the reactants after the polycondensation reaction are completed pass through the filter (8) and enter the first distillation column (9). The reactants undergo a first distillation process in the first distillation column (9). After the first distillation process is completed, the low molecular weight dimethyl silicone oil is collected through the dimethyl silicone oil outlet, and the remaining material enters the second distillation column (10). After undergoing a second distillation process in the second distillation column (10), the remaining material passes through the condenser (11) and the second circulation pipeline and then enters the preheater (1) to participate in the reaction again.

2. The preparation method according to claim 1, characterized in that, The device also includes a viscometer (5), in which the viscosity of the reactants flowing out of the second fixed-bed reactor (4) is detected by the viscometer (5).

3. The preparation method according to claim 2, characterized in that, The device further includes a first outlet regulating valve (6) and a second outlet regulating valve (7), in the operating method, If the viscosity value detected by the viscometer (5) reaches the target viscosity, the first outlet regulating valve (6) is opened and the second outlet regulating valve (7) is closed, so that the reactants enter the first distillation column (9) after passing through the filter (8). If the viscosity value detected by the viscometer (5) does not reach the target viscosity, the second outlet regulating valve (7) is opened and the first outlet regulating valve (6) is closed, so that the reactants are returned to the first fixed bed reactor (3) to react again.

4. The preparation method according to claim 1, characterized in that, The dimethylsiloxane mixed cyclic compound comprises at least one or a combination of hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecylcyclohexasiloxane, tetradecylcycloheptasiloxane, and hexamethylcyclooctasiloxane; and / or The end-capping agent includes a dimethylsilyl end-capping agent; and / or The catalyst includes a strongly acidic cation exchange resin and solid superacid titanium dioxide (TiO2 / SO4). 2- Zirconium-titanium alloy ZrO2-TiO2 / SO4 2- Titanium-iron alloy TiO2-Fe2O3 / SO4 2- Zirconium iron alloy ZrO2-Fe2O3 / SO4 2- At least one or a combination thereof.

5. The preparation method according to claim 1, characterized in that, The operating temperature of the preheater (1) is 60~80℃; and / or The operating temperature of the first distillation column (9) is 60~80℃; and / or The operating temperature of the second distillation column (10) is 60~80℃.

6. The preparation method according to claim 1, characterized in that, The preheater (1) is also provided with a regulator inlet via a pipeline; The regulator enters the preheater (1) through the regulator inlet.

7. The preparation method according to claim 6, characterized in that, The regulator includes at least one or a combination of hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane and dodecylcyclohexasiloxane.

8. The preparation method according to claim 1, characterized in that, The device also includes pump I (12), pump II (13) and pump III (14), in the operation method, The pump I (12) delivers the mixture flowing out of the static mixer (2) and / or the second outlet regulating valve (7) into the first fixed bed reactor (3); The pump II (13) delivers the reactants flowing out of the filter (8) into the first distillation column (9); Pump III (14) is a vacuum pump used to draw the first distillation column (9) and the second distillation column (10) to a negative pressure state.

9. The preparation method according to claim 8, characterized in that, The operating pressure of the first distillation column (9) is -90~-100 kPa; and / or The operating pressure of the second distillation column (10) is -90~-100kPa.

10. A low molecular weight dimethyl silicone oil, characterized in that, The low molecular weight dimethyl silicone oil is prepared by the preparation method described in any one of claims 1-9.

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