KR20040082850A - A method for preparing polydimethylsiloxane having low viscosity - Google Patents

Abstract

PURPOSE: A preparation method of polydimethylsiloxane having a low viscosity is provided to improve the efficiency of a cracking process by minimizing the amount of dimethylsilicone loss. CONSTITUTION: The preparation method of polydimethylsiloxane having a low viscosity comprises: a step of cracking a hydrolyzed siloxane containing methyl silicate(CH3SiO3) in the presence of an alkali catalyst to form a cyclic polydimethylsiloxane, distilling the cyclic polydimethylsiloxane to obtain residues in liquid phase, adding thereto 1-3pts.wt of an organic solvent with stirring to dissolve the residues; a step of adding thereto 1-2pts.wt of water, based on the amount of the organic solvent which contains the dissolved residues, to extract and remove the alkali catalyst and alkali salts of the methyl silicate; and a step of repeating the extraction process until the pH value of water becomes 7 and distilling the organic solvent to recover a siloxane in the form of dimethylsilicone((CH3)2SiO).

Description

A method for preparing polydimethylsiloxane having low viscosity

The present invention relates to a method for preparing low viscosity polydimethylsiloxane, and more particularly, to a cyclic poly by cracking a siloxane hydrolyzate containing methyl silicate (CH ₃ SiO ₃ , hereinafter referred to as “T unit”) in the presence of an alkali catalyst. Dimethyl siloxane is prepared, and the remaining liquid residue after distillation is dissolved by adding an organic solvent, and the alkali salt of the alkali catalyst and methyl silicate (hereinafter referred to as "T-salt") dissolved in the organic solvent is water. The low point that increased the efficiency of the cracking process by minimizing the amount of dimethylsilicone (hereinafter referred to as "D unit") by recovering the siloxane of dimethylsilicone ((CH ₃ ) ₂ SiO) by distilling the extracted organic solvent. The present invention relates to a method for producing polydimethylsiloxane.

Organosiloxane polymers are commercially prepared by the following two basic methods. First, there is a method of preparing a siloxane polymer by condensation of silanol groups in a sock end using a monomer or oligomer having a silanol group in the sock end, and secondly, an equilibration reaction that causes decomposition and recombination of the siloxane bond under a catalyst. There is a method of manufacturing. Here, equilibration means that the ratio of the linear siloxane polymer and the cyclic siloxane polymer is constant, and in such an equilibration reaction, the siloxane bond is continuously decomposed and recombined until the ratio of the linear siloxane polymer and the cyclic siloxane polymer reaches an equilibrium state. do. Due to the characteristics of the above equilibration reaction, it is possible to use a molecular weight modifier such as hexamethyldisiloxane, and when the equilibrium reaction is carried out by adding a compound such as the end group of the polymer, It is possible to control the molecular weight and viscosity by the amount.

In general, distillation and hydrolysis of dimethyldichlorosilane in the methylchlorosilane mixture synthesized by the direct method yields a hydrolyzate containing 50 to 80% by weight of the linear siloxane polymer and 20 to 50% by weight of the cyclic siloxane polymer. This may be cracked to obtain a cyclic siloxane which is a raw material of the silicone polymer. The distilled dimethyldichlorosilane contained about 0.05 to 1% of methyltrichlorosilane constituting the T unit on the silicone polymer, and the complete removal of the methyltrichlorosilane is a problem of the distillation stage, so there is no commercial economy. It is preferable to remove by a cracking process.

In the cracking process, a basic catalyst such as potassium hydroxide or sodium hydroxide is added to the hydrolyzate containing T unit, and the temperature is raised to 120 ° C. or higher, and reduced to less than 100 torr, followed by distillation to separate only the cyclic siloxane having a relatively low boiling point. It is a process to make. Here, the T unit reacts with potassium hydroxide used as a catalyst during the reaction to form a T-salt, which has a high boiling point and thus remains cyclic siloxane having no T unit through the cracking process. The siloxane can be used to prepare a siloxane polymer without a T unit.

The cracking process as described above can be largely divided into a continuous method and a batch method. First, the continuous process continuously proceeds by continuously distilling the cyclic siloxane and continuously adding alkali catalysts such as siloxane hydrolyzate and potassium hydroxide to the amount of the cyclic siloxane to be distilled, in which case the T unit remains in the reactor. The concentration will continue to increase. Therefore, since the reaction increases for a certain time, the viscosity increases as the T unit accumulates and continuous operation is impossible. Therefore, it is necessary to periodically remove the reactants and then proceed with the reaction again. The batch method is a method of distilling cyclic polydimethylsiloxane by adding a batch of siloxane hydrolyzate and an alkali catalyst such as potassium hydroxide to the reactor and then cracking it.

The cracking reaction described above cannot be carried out continuously in any manner, such as continuous or batchwise, and the residues remaining in the reactor periodically or each time must be removed according to the above method. However, the residues removed include cyclic and linear siloxanes, alkali catalysts, T units, and the like, and the siloxanes contained in the residues to be disposed of reduce the yield of the overall cracking process. .

Accordingly, research on the treatment of residues to improve the yield of the cracking process is continuously made, and known techniques include the following.

US Patent No. 2,860,152 discloses a method for producing cyclic diorganosiloxane by cracking reaction using an inert solvent having a boiling point of 250 ° C. or more and more than 20% by weight and using an alkali catalyst at a temperature and pressure at which the solvent used does not boil. It is described. At this time, the solvent plays a role to move the equilibrium state of the linear and cyclic siloxane toward the cyclic siloxane, and the amount of the solvent is required more than 20% by weight in order to prevent gelation, but the reaction will be slow when using too much solvent And the solvent and the siloxane cannot be separated from the residue at which the reaction is terminated. That is, the patent does not clearly state how to effectively treat residues.

U.S. Patent No. 4,111,973 describes a method for increasing the yield and purity of fluoroalkyl cyclotrisiloxane by cracking diorganopolysiloxane using an appropriate amount of a cracking catalyst and a saturated alcohol having 14 to 30 carbon atoms. It is described that high yields of siloxanes of 80% can be obtained by using saturated alcohols having such long chains. However, the method described above relates to fluoroalkyl cyclotrisiloxane, which has a problem of very low yield.

Accordingly, the inventors of the present invention have made efforts to solve the above problems, and as a result of distilling the cyclic polydimethylsiloxane prepared by cracking a siloxane hydrolyzate containing T units in the presence of an alkali catalyst, By adding an organic solvent having a specific boiling point to dissolve the alkali catalyst and T-salt in the residue, and then remove it by water extraction, it was found that the siloxane of D unit can be recovered when the organic solvent is distilled off. The invention has been completed.

Accordingly, an object of the present invention is to provide a method for preparing a low viscosity polydimethylsiloxane in which the amount of D units lost as a residue is increased to increase the efficiency of the cracking process.

The present invention distills off a cyclic polydimethylsiloxane formed by cracking a siloxane hydrolyzate containing methyl silicate (CH ₃ SiO ₃ ) in the presence of an alkali catalyst, and then, in the remaining liquid phase, 1 to 3 parts by weight relative to the residue. Adding an organic solvent and then mixing and stirring to dissolve the residue; Extracting and removing an alkali salt of the alkali catalyst and methyl silicate dissolved in the organic solvent by adding water in an amount of 1-2 weight ratio of the amount of the organic solvent in which the residue is dissolved; And repeating the extraction process until the pH of water reaches 7 and then distilling the organic solvent to recover siloxane of dimethylsilicone ((CH ₃ ) ₂ SiO). It is characterized by.

The present invention will be described as follows.

The present invention provides a cyclic polydimethylsiloxane by cracking a siloxane hydrolyzate including a T unit by a continuous method or a batch method in the presence of an alkali catalyst, and remaining the liquid phase remaining in the reactor after distilling and recovering the cyclic polydimethylsiloxane. The amount of D units lost by dissolving water by adding an organic solvent and then removing the alkali catalyst and T-salt dissolved in the organic solvent by water extraction several times and then distilling the organic solvent to recover D units of siloxane. It relates to a method for producing a low viscosity polydimethylsiloxane that minimizes the increase in the efficiency of the cracking process.

First, the cracking process is a reaction in which a hydrolyzate and potassium hydroxide produced in the hydrolysis of dichlorosilane are introduced into a reactor equipped with a distillation apparatus to obtain a cyclic polydimethylsiloxane through a cracking reaction. The hydrolyzate used is obtained by adding an excess of water to dimethyldichlorosilane, hydrolyzing and neutralizing. In general, the hydrolyzate is composed of a linear polydimethylsiloxane having about 50 to 70% of its ends terminated with a silanol group and a cyclic polydimethylsiloxane of about 30 to 50%. Linear polydimethylsiloxane and cyclic polydimethylsiloxane can be represented by the following formula (1) and (2).

HO [(CH ₃ ) ₂ SiO] _n H

Here n is an integer of 10-100.

[(CH ₃ ) ₂ SiO] m

Here m is an integer of 3-12.

The dimethyldichlorosilane hydrolyte as described above may exhibit a wide variety of physical properties according to hydrolysis conditions, and generally has a viscosity of about 5 to 50 cPs and methyltrichlorosilane contained as an impurity in the dimethyldichlorosilane distillation process in the hydrolyzate. By T, about 1,000 to 10,000 ppm of T units are included.

Since the T unit acts as a factor for increasing the viscosity, when the hydrolyzate containing a lot of T units is used as a raw material in the cracking reaction, the viscosity of the reactant is increased, and thus the cyclic poly is distilled due to a decrease in heat transfer and mixing efficiency. The amount of dimethylsiloxane is reduced, and in subsequent reactions the stirrer may be stopped by overload. Therefore, it is important to reduce the amount of methyltrichlorosilane as much as possible in the distillation of dimethyldichlorosilane.

In the cracking reaction, 45% potassium hydroxide aqueous solution is often used as an alkali catalyst. In the case of a batch reaction, a solid potassium hydroxide having a high concentration of potassium hydroxide is used, but in a continuous reaction, a high temperature and a low pressure are maintained. This is because it is difficult to add potassium hydroxide in the middle of the reaction because potassium hydroxide must be added to the reactor in which the concentration of solid potassium hydroxide and water is less than 45%.

First, the batch cracking reaction and the continuous cracking reaction will be described in detail as follows.

The batch cracking reaction is carried out by adding a hydrolyzate and an alkali catalyst to a reactor equipped with a heat exchanger capable of condensing cyclic polydimethylsiloxane and a vacuum pump, and then reducing the temperature to 130 to 150 ° C and lowering the pressure to 20 to 50 torr. Dimethyl siloxane is distilled off. As an alkali catalyst, potassium hydroxide is used in an amount of about 0.2 to 2 parts by weight based on 100 parts by weight of hydrolyzate. When the amount of the catalyst is small, the viscosity is high during the cracking reaction, and the reaction efficiency is low. Falls. The viscosity of the reactant increases rapidly at about 60 to 100 ° C. at the initial temperature rise. The silanol group in the hydrolyzate causes a condensation reaction in the range of 60 to 100 ° C. in the presence of a catalyst. In this condition, a siloxane bond is not boiled, so that a high molecular weight polymer is produced by the condensation reaction, thereby rapidly increasing the viscosity. . However, if the reaction temperature is 130 ℃ or more, the reaction of the siloxane bonds are accompanied by the reaction viscosity is lowered, so the reaction temperature is at least 130 ℃, the temperature at which the reaction of boiling siloxane bonds early in the initial reaction It is important to raise it. When the reaction temperature and pressure reach the conditions where the cyclic polydimethylsiloxane is distilled off, the water contained in the hydrolyzate and the water contained in the 45% potassium hydroxide catalyst are distilled first, which can be separated by layer separation to obtain only the cyclic polydimethylsiloxane. have. As the reaction proceeds, the reactant decreases continuously, in which there is a certain amount of unreacted alkali catalyst, T-salt (e.g. potassium T salt) and cyclic and linear siloxanes of D unit. As the amount of catalyst acting as a terminator is relatively high, the viscosity is continually reduced, but when most of the siloxane is removed, the catalyst remains in the solid phase and a small amount of siloxane, resulting in a solid residue.

In the continuous cracking reaction, 1 to 3 parts by weight of potassium hydroxide is added to the reactor as 100 parts by weight of hydrolyzate and an alkali catalyst, and the reaction is started by raising the temperature and lowering the pressure. The initial reaction starts in the same way as in the batch, and when the reaction is stable, the hydrolyzate is continuously added in proportion to the amount of the distilled cyclic polydimethylsiloxane. As the reaction proceeds, T units are accumulated and the accumulated T units are Since potassium hydroxide reacts with potassium hydroxide in the reactants to form weakly reactive T-salts, the amount of catalyst decreases gradually as the reaction proceeds, and thus potassium hydroxide, which is a catalyst, must be continuously added in the middle of the reaction. The amount of potassium hydroxide added is proportional to the amount of T units in the hydrolyzate, and 1 to 2 times potassium hydroxide is added to the T units. When the amount of potassium hydroxide is small, the viscosity of the reactant increases, and when it is high, the continuous reaction cycle is shortened due to the amount of potassium hydroxide accumulated. The potassium hydroxide can be added continuously or periodically, and there is no problem in the reaction regardless of which method is used. As the reaction proceeds continuously, the viscosity of the reactant gradually increases and the amount of distillation gradually decreases, and may vary depending on the conditions and operating conditions of the hydrolyzed water. Generally, the T unit is 5 parts by weight based on 100 parts by weight of the reaction contents. If accumulate enough, the cracking reaction is impossible due to too high viscosity, so stop adding water and continue the cracking reaction to reduce the amount of remaining siloxane as much as possible. If the reaction cannot proceed further, add water in the same way as the batch reaction. After washing, the next reaction starts.

Comparing batch and continuous cracking reactions, since the amount of potassium hydroxide and T-salt accumulates more than the continuous reaction in a batch, the amount of residue in the solid phase is formed more than in the case of a batch reaction, so the continuous cracking reaction is more efficient. And in yield. That is, in the case of the continuous type, when 1,000 ppm of T unit is contained in the hydrolyzate, 5,000 parts by weight of hydrolyzate is required to accumulate 5 parts by weight of T unit with respect to 100 parts by weight of the reaction contents. In the case of 50 times, the amount is to be performed, and even in the case of residues, in the case of the batch reaction, the amount is generated more than the continuous reaction as a whole. Generally, the amount of residue is about 25 parts by weight based on 100 parts by weight of the reaction contents after the continuous cracking reaction. In the residue, siloxane of dimethylsilicon ((CH ₃ ) ₂ SiO), which is useful, is included and discarded. When cracking 10,000 ton of hydrolyzate having 1,000 ppm of T unit, the amount of residue is about 50 ton. Here, about 30 tons is a recoverable D unit.

That is, regardless of a batch reaction or a continuous reaction, conventionally, an excess of water was added to the formed residues to wash away, and then the next reaction was started. The siloxanes of dimethylsilicone ((CH ₃ ) ₂ SiO) are also treated as waste with water, which results in a reduction in the siloxane yield in the overall cracking process.

As the dimethyldichlorosilane hydrolyzate used in the present invention, a hydrolyzate obtained by hydrolyzing dimethyldichlorosilane including 1,000 ppm of methyltrichlorosilane was used.

The present invention can be equally applied in batch and continuous reactions. The residue treatment method in the continuous reaction lowers the level if the progress of the reaction becomes difficult due to the increase in viscosity after the continuous reaction in order to eliminate the loss of the D unit contained in the residue. The reaction is stopped before the residue reaches the solid phase. do.

That is, the cyclic polydimethylsiloxane formed by cracking the siloxane hydrolyzate including the T unit in the presence of an alkali catalyst is distilled off, and then the organic solvent is added to the remaining liquid phase to dissolve the residue.

The organic solvent to be used must be compatible with the siloxane and the boiling point must be higher than the next water extraction temperature, but if it is too high, it requires a large amount of heat to remove the solvent, thereby reducing economic efficiency. The organic solvent is selected from benzene, toluene, xylene and the like, and is most preferable when toluene is used. The organic solvent is used in an amount of 1 to 3 parts by weight based on the amount of the residue. When the amount of the organic solvent is less than 1 part by weight, the residue is not completely dissolved in the organic solvent, and the amount of the organic solvent is greater than 3 parts by weight. In this case, a large amount of heat is required to remove the organic solvent, which is inefficient.

The residue is dissolved in an amount of 1 to 2 parts by weight of the amount of the organic solvent dissolved therein to extract and remove the alkali catalyst and the T-salt dissolved in the organic solvent.

The residue containing the organic solvent is washed with water several times to remove the T-salt formed by combining alkali hydroxides of potassium hydroxide and methyl silicate used as alkali catalyst. The amount of water used for water extraction is 1 to 2 weight ratio of the amount of the organic solvent including the residue, and after adding the amount of water used, the mixture is stirred, mixed and separated to remove the water layer, and then the amount of water used is added and stirred. After mixing, repeat the method of layer separation and extract until the pH of the water separated by the layer reaches 7. The water used for the layer-removed extraction is stored in a large storage container, and recovered by separating the trace amount of organic solvent and siloxane of D unit contained therein.

In order to effectively remove the alkali catalyst and T-salt dissolved in the organic solvent by water extraction, the water extraction temperature is more preferably in the range of 60 to 100 ° C. At this time, if the temperature of the water extraction is lower than 60 ℃, the efficiency of water extraction is lowered, and if the temperature exceeds 100 ℃, the used water is boiled, it is impossible to extract the alkali catalyst and T- salt from the organic solvent.

As described above, the organic solvent from which the alkali catalyst and the T-salt are removed by water extraction removes the impurities and leaves cyclic and linear siloxanes of the D unit. The organic solvent may be removed by distillation at a high temperature. Thus only siloxanes can be obtained. The distilled organic solvent contains a trace amount of cyclic polydimethylsiloxane, where the distilled organic solvent is reused, and the trace amount of cyclic polydimethylsiloxane can be recovered. The recovered siloxane is composed of a linear polydimethylsiloxane and a cyclic polydimethylsiloxane having a silanol group at the terminal and no T unit, and are subjected to a cracking process to be converted into a cyclic polydimethylsiloxane. The cyclic polydimethylsiloxane obtained at this time can be represented by following formula (2).

[(CH ₃ ) ₂ SiO] _m

Where m is an integer ranging from 3 to 12.

The composition of the cyclic polydimethylsiloxane to be distilled may change little by little depending on the temperature and pressure conditions of the reactants.However, the composition of D ₄ (Octamethylcyclosiloxane) whose m is 4 is 50% or more and D ₃ , D ₅ , D ₆ Accounts for most of the remainder, and m is 7 or more cyclic polydimethylsiloxanes present in trace amounts.

As described above, according to the method of the present invention, it is possible to increase the amount of D units recovered by a simple water extraction process that is not complicated, and to maximize the yield of the cracking process, thereby minimizing the amount of D units siloxane discarded. Efficient

The present invention will be described in detail with reference to Examples, but the present invention is not limited by the following Examples.

Reference Example: Preparation of Polydimethylsiloxane by Continuous Cracking Reaction

A 60L jacketed reactor equipped with a stirrer, process viscometer, thermometer, pressure gauge, water inlet, 45% potassium hydroxide inlet and a pipe through which the cyclic polydimethylsiloxane is distilled out is used. The reservoir and the vacuum pump are connected.

20 kg of hydrolyzate and 889 g (2% KOH) of 45% aqueous potassium hydroxide solution were added to the reactor as described above, and the temperature of the reactor was raised to 140 ° C., and the pressure was gradually lowered to 30 torr. Water contained in the cyclic polydimethylsiloxane to be distilled at the beginning of the reaction was discarded after being separated in a layer separation tank, and the cracking reaction was performed while maintaining the temperature at 140 ° C. and the pressure at 30 torr. The cyclic polydimethylsiloxane to be distilled was stored in a storage tank, and when the cracking reaction was stabilized, the hydrolyzate was continuously added in accordance with the amount of the cyclic polydimethylsiloxane to be distilled. The viscosity of the reactants increased rapidly during the increase of the temperature at the start of the reaction, and then gradually stabilized as the temperature reached 130 ° C. and the cracking reaction proceeded gradually. 40 kg of hydrolyzate was added thereto, 53 g of 45% potassium hydroxide was added as an alkali catalyst, and thereafter, 53 g of 45% potassium hydroxide was added every time 40 kg of hydrolyzate was added. Viscosity temporarily decreased as the catalyst was added and gradually increased as the reaction proceeded. When 1000 kg of the hydrolyzate was cracked, the reaction was difficult to progress due to the high viscosity of the reactant. At this time, the level of the reactant was lowered while stopping the addition of the hydrolyzate and continuing cracking.

Example

The cracking reaction was stopped when the residue produced in the reference example was about 10 kg, and 10 kg of the residue was added to the 50L jacket reactor equipped with a stirrer containing 20 kg of toluene at 70 ° C. and stirred and mixed.

30 kg of distilled water was added thereto, stirred for 1 hour while maintaining the temperature at 80 ° C., and the layers were separated to remove the water layer. The water extraction was repeated five times so that the pH of the water layer was 7 (neutral). It was. The water used for the water extraction was collected and stored in a storage tank, and the total amount of the toluene and siloxane contained therein was separated to recover the whole amount. The water-extracted reactant was mixed with recovered toluene and siloxane, and after filtering, the temperature was raised to 170 ° C. to remove toluene to obtain 7 kg of D-unit siloxane, a residue from which toluene was removed. Reused.

Comparative example

As in the above example, after cracking 1000 kg of the hydrolyzate in the reference example, the supply of the hydrolyzate was stopped and the cracking was continued to lower the level of the reactant, and when the residue changed to a gel state, the temperature was lowered to 90 ° C. to introduce water. Wash and discard, with 5 kg of gel discarded.

As described above, according to the present invention, in the cracking reaction method applied to remove the T unit containing methylchlorosiloxane to prepare the low viscosity siloxane, the D unit included in the residue which was generally discarded entirely in the solid phase Most of the siloxane is dissolved by dissolving the residue using an organic solvent having specific physical characteristics and then extracting and removing impurities such as T units, used alkali catalysts and alkali salts of methyl silicate in the cracking reaction as water at a constant temperature. By recovering the siloxane of the D unit, the used organic solvent is distilled and reusable to improve the yield of the cracking process, it is possible to produce a low viscosity siloxane in a simpler and more efficient way.

Claims (3)

After distilling the cyclic polydimethylsiloxane formed by cracking the siloxane hydrolyzate containing methyl silicate (CH ₃ SiO ₃ ) in the presence of an alkali catalyst, an organic solvent in a weight ratio of 1 to 3 based on the residue was added to the remaining liquid residue. Adding and stirring to dissolve the residue; Extracting and removing an alkali salt of the alkali catalyst and methyl silicate dissolved in the organic solvent by adding water in an amount of 1 to 2 parts by weight of the amount of the organic solvent in which the residue is dissolved; And repeating the extraction process until the pH of water reaches 7 and then distilling the organic solvent to recover siloxane of dimethylsilicone ((CH ₃ ) ₂ SiO). Method for producing a low viscosity polydimethylsiloxane, characterized in that consisting of. The method of claim 1, wherein the organic solvent is selected from benzene, toluene, and xylene. The method for producing a low viscosity polydimethylsiloxane according to claim 1, wherein the extraction with water is in the range of 60 to 100 ° C.