Process for the preparation of a low viscosity dimethicone

Abstract

This invention belongs to the field of materials technology, specifically relating to a method for preparing low-viscosity dimethyl silicone oil. Through an innovative three-stage dynamic temperature control and segmented replenishment process—"S1 high-temperature chain formation – S2 low-temperature end-capping – S3 medium-temperature equilibrium"—the limited water content in the system is used as a key control factor, guiding it to participate in specific reactions at different stages. In the S1 high-temperature stage, water molecules mainly participate in the hydrolysis and condensation reactions of cyclic siloxanes (such as D4) or linear siloxane oligomers, tending to generate hydroxyl-terminated or hydroxyl-containing low-polymerization-degree linear polydimethylsiloxane intermediates. Subsequently, in the S2 low-temperature stage, by introducing the end-capping agent hexamethyldisiloxane (MM) and maintaining its high concentration, the hydroxyl-terminated intermediate generated in the S1 stage undergoes a condensation end-capping reaction with MM, thereby efficiently converting it into low-viscosity dimethyl silicone oil with trimethylsiloxy (Me3SiO-) stable end-capping. With the optimized D4 / MM formulation ratio (1-1.5:1), the reaction equilibrium is continuously driven to shift towards the generation of 1cs, 1.5cs, and 2cs, and the proportion of the target product is significantly higher than that of the traditional isothermal process.

Description

Technical Field

[0001] This invention belongs to the field of materials technology, specifically relating to a method for preparing low-viscosity dimethyl silicone oil. Background Technology

[0002] Traditional volatile silicone oils are mainly cyclotetrameric dimethylsiloxane (D4), cyclopentameric dimethylsiloxane (D5), and cyclohexylsiloxane (D6), widely used in personal care, cosmetics, medical, and textile industries. They are compounds synthesized from dimethyldichlorosilane through hydrolysis and then purified by distillation. On May 17, 2024, the European Commission's amendment 2024 / 1328, published in the Official Journal of the European Union, imposed new restrictions on D4, D5, and D6 in Annex XVII of the REACH Regulation: from June 6, 2026, D4, D5, and D6 may not be placed on the market as substances themselves, as components of other substances, or as mixtures thereof, at a concentration equal to or greater than 0.1% by weight.

[0003] Ultra-low viscosity dimethyl silicone oil (especially 1-2 cs silicone oil) possesses excellent chemical stability, thermal stability, lubricity, and hydrophobicity. It has specific needs and promising applications in the electronics, cosmetics, and pharmaceutical industries. For example, in electronics, it can be used as a cleaning agent and heat dissipation agent for precision instruments; in cosmetics, it can improve the feel and spreadability of products; and in pharmaceuticals, it can be used as a drug carrier and can replace EU-restricted substances such as D4, D5, and D6.

[0004] However, traditional preparation methods have some problems in producing ultra-low viscosity dimethyl silicone oil.

[0005] In the traditional synthesis of dimethyl silicone oil, cyclic impurities such as octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecylcyclohexasiloxane (D6) are easily generated. These cyclic compounds have boiling points very close to those of linear siloxanes, making them difficult to separate using conventional methods and severely affecting the purity and performance of the product.

[0006] Furthermore, in traditional equilibrium reactions, catalysts can lead to cyclic formation. For example, in existing technologies for preparing ultra-low viscosity dimethyl silicone oils (1cs, 1.5cs, 2cs), 98% concentrated sulfuric acid is commonly used as a catalyst. While concentrated sulfuric acid is highly reactive, it readily introduces difficult-to-remove silanol groups (Si-OH) into the product molecular chain ends. These silanol groups slowly condense over time, leading to a wider molecular weight distribution, viscosity drift, and decreased storage stability. While using lower concentrations of sulfuric acid directly can reduce the formation of silanol groups, it introduces moisture, resulting in a significant decrease in reaction rate, an increase in side reactions, and a significant reduction in the yield of the target product. The presence of impurities in the raw materials can lead to incomplete reactions or the formation of byproducts, affecting product quality.

[0007] The problems of uncontrollable reaction, low product purity, and numerous byproducts severely restrict the production of low-viscosity dimethyl silicone oil. Therefore, it is necessary to develop a new preparation method to obtain high-quality ultra-low viscosity dimethyl silicone oil. Summary of the Invention

[0008] The main objective of this invention is to develop a method for preparing ultra-low viscosity dimethyl silicone oil to obtain high-quality ultra-low viscosity dimethyl silicone oil.

[0009] This invention provides a method for preparing ultra-low viscosity dimethyl silicone oil, using 70-80% sulfuric acid as a catalyst and octamethylcyclotetrasiloxane (D4) and hexamethyldisiloxane (MM) as raw materials, wherein the mass ratio of D4 to MM is 1-1.5:1; the specific preparation steps are as follows:

[0010] S1: Mix D4, 50%-70% by weight of MM with the catalyst and react at 70-85°C for 40-90 minutes;

[0011] S2: Cool the reaction system to 35-50℃, add 20%-30% by weight of MM, and continue the reaction for 60-120 minutes;

[0012] S3: Heat the reaction system to 55-65℃ and add 10%-20% by weight of MM, and continue the reaction for 30-60 minutes.

[0013] Furthermore, the amount of sulfuric acid used is 1.5% to 2.5% of the total mass of all D4 and all MM feed.

[0014] Furthermore, after the reaction in step S3 is completed, the following post-processing steps are also included:

[0015] (1) Add an alkaline substance to neutralize the reaction system until the system is neutral to remove the acidic catalyst, and then filter the salt produced by neutralization;

[0016] (2) The filtered mixture was subjected to vacuum distillation to remove low-boiling-point impurities and unreacted raw materials to obtain dimethyl silicone oil;

[0017] (3) Transfer the dimethyl silicone oil after vacuum distillation to a distillation column and distill it in sections to obtain 1cs, 1.5cs and 2cs dimethyl silicone oil.

[0018] Preferably, the alkaline substance is sodium carbonate or sodium bicarbonate. The pressure of the vacuum distillation is controlled at 1-5 kPa, and the temperature is 40-80°C.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] 1. Through an innovative three-stage dynamic temperature control and segmented replenishment process—"S1 high-temperature chain formation - S2 low-temperature end-capping - S3 medium-temperature equilibrium"—limited water content in the system is used as a key control factor, guiding it to participate in specific reactions at different stages. In the S1 high-temperature stage, water molecules mainly participate in the hydrolysis and condensation reactions of cyclic siloxanes (such as D4) or linear siloxane oligomers, tending to generate hydroxyl-terminated or hydroxyl-containing low-polymerization-degree linear polydimethylsiloxane intermediates. Subsequently, in the S2 low-temperature stage, by introducing the end-capping agent hexamethyldisiloxane (MM) and maintaining its high concentration, the hydroxyl-terminated intermediate generated in the S1 stage undergoes a condensation end-capping reaction with MM, thereby efficiently converting it into low-viscosity dimethyl silicone oil with trimethylsiloxy (Me3SiO-) stable end-capping. Combined with an optimized D4 / MM formulation ratio (1-1.5:1), the reaction equilibrium is continuously driven towards the generation of 1cs, 1.5cs, and 2cs, with the proportion of the target product significantly higher than in traditional isothermal processes.

[0021] 2. Using 70-80% sulfuric acid as a catalyst significantly reduces the generation of terminal silanol groups in the product from the source. The resulting product has a narrow molecular weight distribution and no viscosity drift during long-term storage, completely solving the stability risks of products from concentrated sulfuric acid processes.

[0022] 3. This invention overcomes the technical prejudice that "low-concentration sulfuric acid inevitably leads to low reaction efficiency." Through a precise three-stage temperature control design, the ring-opening polymerization rate is guaranteed in the high-temperature stage, and precise end-capping is achieved by utilizing thermodynamic trends in the low-temperature stage, enabling efficient conversion even in a 70% sulfuric acid system, thus achieving a balance between product quality and production efficiency. Detailed Implementation

[0023] The technical solutions of the present invention will be described in detail below with reference to embodiments, but this does not limit the present invention to the scope of the embodiments described. Experimental methods and techniques in the following embodiments that do not specify specific conditions are generally performed according to conventional conditions in the relevant art or according to the conditions recommended by the manufacturer.

[0024] Comparative Example 1

[0025] Prepare according to the following steps:

[0026] (1) Raw material preparation: Dehydrated octamethylcyclotetrasiloxane (D4) and hexamethyldisiloxane (MM) were selected as the main raw materials, with a mass ratio of D4 to MM of 1.3:1.

[0027] (2) Reaction mixing: Add all D4 and all MM to a reactor equipped with a stirrer, thermometer and reflux condenser. Under stirring conditions, slowly add 2% of the total mass of 98% concentrated sulfuric acid as a catalyst and react at 70°C for 3 hours.

[0028] (3) Post-processing: After the reaction is completed, sodium bicarbonate is added to neutralize the system until it is neutral, and the salt is removed by filtration. The filtrate is subjected to vacuum distillation at 3 kPa and 60 °C to remove low-boiling substances. Finally, the crude product is transferred to a distillation column and fractionally distilled to obtain 1cs, 1.5cs and 2cs products.

[0029] Comparative Example 2

[0030] Prepare according to the following steps:

[0031] (1) Raw material preparation: Dehydrated octamethylcyclotetrasiloxane (D4) and hexamethyldisiloxane (MM) were selected as the main raw materials, with a mass ratio of D4 to MM of 1.3:1.

[0032] (2) Reaction mixing: Add all D4 and all MM into the reactor. Under stirring conditions, slowly add 70% sulfuric acid, which accounts for 2% of the total mass of the materials, as a catalyst. React at a constant temperature of 70°C for 3 hours.

[0033] (3) Post-processing: After the reaction is completed, sodium bicarbonate is added to neutralize the system until it is neutral, and the salt is removed by filtration. The filtrate is subjected to vacuum distillation at 3 kPa and 60 °C to remove low-boiling substances. Finally, the crude product is transferred to a distillation column and fractionally distilled to obtain 1cs, 1.5cs and 2cs products.

[0034] Example 1

[0035] Prepare according to the following steps:

[0036] (1) Raw material preparation: Dehydrated octamethylcyclotetrasiloxane (D4) and hexamethyldisiloxane (MM) were selected as the main raw materials, and the total mass ratio of D4 to MM was 1.3:1.

[0037] (2) Catalyst preparation: Prepare sulfuric acid with a concentration of 70% as a catalyst, and use 2% of the total mass of D4 and MM.

[0038] (3) Three-stage complex reaction:

[0039] S1 (High-temperature chain formation stage): Mix all D4, the first part of MM which accounts for 60% of the total mass of MM, with the catalyst and react at 80°C for 60 minutes;

[0040] S2 (low-temperature capping stage): Cool the reaction system to 45°C, add the second part of MM accounting for 25% of the total mass of MM, and continue the reaction for 90 minutes;

[0041] S3 (Medium-temperature equilibrium stage): Heat the reaction system to 60°C, add the third part of MM, which accounts for 15% of the total mass of MM, and continue the reaction for 45 minutes.

[0042] (4) Post-processing: After the reaction is completed, sodium bicarbonate is added to neutralize the system until it is neutral, and the salt is removed by filtration. The filtrate is subjected to vacuum distillation at 3 kPa and 60 °C to remove low-boiling substances. Finally, the crude product is transferred to a distillation column and fractionally distilled to obtain 1cs, 1.5cs and 2cs products.

[0043] Example 2

[0044] Prepare according to the following steps:

[0045] (1) Raw material preparation: Dehydrated octamethylcyclotetrasiloxane (D4) and hexamethyldisiloxane (MM) are selected as the main raw materials, and the total mass ratio of D4 to MM is 1.0:1.

[0046] (2) Catalyst preparation: Prepare sulfuric acid with a concentration of 70% as a catalyst, and use 1.5% of the total mass of D4 and MM.

[0047] (3) Three-stage dynamic response:

[0048] S1: Mix all D4, the first part of MM which accounts for 50% of the total mass of MM, with the catalyst, and react at 85°C for 40 minutes;

[0049] S2: Cool the reaction system to 50°C, add the second part of MM, which accounts for 30% of the total mass of MM, and continue the reaction for 60 minutes;

[0050] S3: Heat the reaction system to 65°C, add the third part of MM, which accounts for 20% of the total mass of MM, and continue the reaction for 30 minutes.

[0051] (4) Post-processing: After the reaction is completed, sodium bicarbonate is added to neutralize the system until it is neutral, and the salt is removed by filtration. The filtrate is subjected to vacuum distillation at 3 kPa and 60 °C to remove low-boiling substances. Finally, the crude product is transferred to a distillation column and fractionally distilled to obtain 1cs, 1.5cs and 2cs products.

[0052] Example 3

[0053] Prepare according to the following steps:

[0054] (1) Raw material preparation: Dehydrated octamethylcyclotetrasiloxane (D4) and hexamethyldisiloxane (MM) are selected as the main raw materials, and the total mass ratio of D4 to MM is 1.5:1.

[0055] (2) Catalyst preparation: Prepare sulfuric acid with a concentration of 80% as a catalyst, and use 2.5% of the total mass of D4 and MM.

[0056] (3) Three-stage dynamic response:

[0057] S1: Mix all D4, the first part of MM which accounts for 70% of the total mass of MM, with the catalyst and react at 70°C for 90 minutes;

[0058] S2: Cool the reaction system to 35°C, add the second part of MM, which accounts for 20% of the total mass of MM, and continue the reaction for 120 minutes;

[0059] S3: Heat the reaction system to 55°C, add the third part of MM, which accounts for 10% of the total mass of MM, and continue the reaction for 60 minutes.

[0060] (4) Post-processing: After the reaction is completed, sodium bicarbonate is added to neutralize the system until it is neutral, and the salt is removed by filtration. The filtrate is subjected to vacuum distillation at 3 kPa and 60 °C to remove low-boiling substances. Finally, the crude product is transferred to a distillation column and fractionally distilled to obtain 1cs, 1.5cs and 2cs products.

[0061] In the above embodiments and comparative examples, the products were evaluated using the following methods:

[0062] Product distribution determination: The content ratios of 1cs, 1.5cs, 2cs and other components in the reaction products were determined by gas chromatography (GC).

[0063] Silyl hydroxyl content detection: The presence of silyl hydroxyl groups in the product is determined by infrared spectroscopy (IR) or nuclear magnetic resonance.

[0064] Product purity testing: The purity of each component after distillation was determined by gas chromatography area normalization method.

[0065] Comparison of test results

[0066] Table 1. Comparison of reaction product distribution between each example and the comparative example (after vacuum distillation, before rectification).

[0067] 1. CS content (%) 8.2 5.1 15.6 18.3 13.5 1.5 CS content (%) 12.5 7.8 21.2 24.5 19.8 2CS content (%) 18.3 11.4 28.7 30.1 26.4 Total percentage of target product (%) 39.0 24.3 65.5 72.9 59.7 Silyl hydroxyl group (Si-OH) Detected Not detected Not detected Not detected Not detected

[0068] Table 2 Comparison of product purity after distillation in each example and the comparative example

[0069] 1cs purity (%) 99.1 99.0 99.5 99.6 99.4 1.5cs purity (%) 99.0 98.9 99.4 99.5 99.3 2cs purity (%) 98.8 98.7 99.3 99.4 99.1

[0070] Results Analysis

[0071] 1. Comparative analysis of the proportion of target products

[0072] As can be seen from the data in Table 1:

[0073] In Comparative Example 1 (conventional concentrated sulfuric acid + isothermal process), the total proportion of the target products (1cs, 1.5cs, 2cs) was 39.0%, which is relatively low. This is because the isothermal reaction tends the system to a broad equilibrium state, making it impossible to selectively drive towards the short-chain products.

[0074] The total yield of the target product in Comparative Example 2 (70% sulfuric acid + isothermal process) was only 24.3%, significantly lower than that in Comparative Example 1. Replacing concentrated sulfuric acid with 70% sulfuric acid resulted in an increase in side reactions due to the introduction of water, which made the reaction equilibrium unfavorable for the formation of short-chain products, leading to a significant decrease in the yield of the target product.

[0075] The total proportions of the target products in Examples 1-3 (the method of this invention) reached 65.5%, 72.9%, and 59.7%, respectively, which are significantly higher than those in the two comparative examples. This fully demonstrates that the three-stage dynamic temperature control and segmented replenishment process of this invention successfully overcomes the disadvantages of the 70% sulfuric acid system under isothermal conditions. Instead, through precise temperature control, it efficiently drives the reaction equilibrium towards the formation of 1cs, 1.5cs, and 2cs.

[0076] 2. Comparative analysis of silanol groups

[0077] Comparative Example 1 used 98% concentrated sulfuric acid, and silanol groups were detected in the product, which is the root cause of the product's instability during long-term storage.

[0078] Comparative Example 2 and all examples used 70% or 80% sulfuric acid, and no silanol groups were detected in the products. This indicates that the present invention retains its core advantage of "not producing silanol groups" while using a lower concentration of sulfuric acid, fundamentally ensuring the long-term storage stability of the product.

[0079] 3. Comparative analysis of product purity

[0080] As can be seen from the data in Table 2, the purity of each component after distillation in Examples 1-3 of this invention reached over 99.3%, which is superior to the two comparative examples. This indicates that the method of this invention not only increases the proportion of the target product, but also has a more concentrated molecular weight distribution and a more regular end-group structure, making it easier to obtain ultra-high purity single-component products through distillation.

[0081] The comparative experimental data above clearly demonstrate that this invention, through its innovative three-stage dynamic temperature control and segmented replenishment process of "high-temperature chain formation - low-temperature end-capping - medium-temperature balancing," successfully solved the technical problem that "although sulfuric acid at a concentration of about 70% can avoid silanol groups, the reaction efficiency is low." While maintaining the absence of silanol groups and high stability of the product, the total proportion of the target products of 1cs, 1.5cs, and 2cs has been increased from less than 40% in the traditional process to more than 60%, achieving a balance between product quality and production efficiency.

[0082] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a low-viscosity dimethyl silicone oil, characterized in that, Using 70-80% sulfuric acid as a catalyst, and octamethylcyclotetrasiloxane (D4) and hexamethyldisiloxane (MM) as raw materials, with a total mass ratio of D4 to MM of 1-1.5:1, the specific preparation steps are as follows: S1: Mix D4, 50%-70% by weight of MM with the catalyst and react at 70-85°C for 40-90 minutes; S2: Cool the reaction system to 35-50℃, add 20%-30% by weight of MM, and continue the reaction for 60-120 minutes; S3: Heat the reaction system to 55-65℃ and add 10%-20% by weight of MM, and continue the reaction for 30-60 minutes.

2. The preparation method according to claim 1, characterized in that, The amount of sulfuric acid used is 1.5% to 2.5% of the total mass of D4 and MM.

3. The preparation method according to claim 1 or 2, characterized in that, After the deep equilibrium stage reaction is completed, the following steps are also included: (1) After the reaction is complete, add an alkaline substance to neutralize the reaction system until the system is neutral to remove the acid catalyst and then filter the neutralized salt. (2) The filtered mixture is subjected to vacuum distillation to remove low-boiling-point impurities and unreacted raw materials to obtain dimethyl silicone oil; (3) Transfer the dimethyl silicone oil after vacuum distillation to a distillation column and distill it in sections to obtain 1cs, 1.5cs and 2cs dimethyl silicone oil.

4. The preparation method according to claim 3, characterized in that, The alkaline substance is sodium carbonate or sodium bicarbonate.

5. The preparation method according to claim 3, characterized in that, The pressure of the vacuum distillation is controlled at 1-5 kPa and the temperature is 40-80℃.