An organosilicon compound refers to a compound containing a silicon-carbon bond, and at least one organic group is bonded to a silicon atom through a silicon-carbon bond. For example, methylsilane CH3SiH3, dimethyldichlorosilane (C2H5) 2SiCl2, etc. are organosilicon compounds, while SiC, Si3N4, etc. belong to inorganic silicon compounds.
No organosilicon compounds have been found in nature so far, only silicate compounds have been found in animal feathers and grasses, but these substances do not contain silicon-carbon bonds (Si-C), but only silicon-oxygen Carbon bond (Si—O—C).
There are many kinds of organic silicon polymers, including polysiloxane, polysilane, polycarbosilane, polyazasilane, etc. Organopolysiloxane is one of the most important classes, and its structure can be expressed as follows:
Among them, R is an organic group (such as methyl, phenyl, etc.); n is the number of organic groups connected to the silicon atom (n = 1, 2, 3), and m is the degree of polymerization.
It is generally believed that silicone materials mainly refer to oligomers or polymers containing polysiloxane main chains. The reason why organopolysiloxanes are widely used is that they have unique properties unmatched by other polymer materials: such as high temperature resistance, low temperature resistance, moisture resistance, insulation, corrosion resistance, aging resistance and physiological inertia. Silicone polymer products are very diverse, including liquid (silicone oil), elastomer (silicone rubber), resin, emulsion, etc. They are widely used in aerospace, aviation, electrical, electronics, light industry, machinery, chemical industry, construction, agriculture, medicine, It has been widely used in daily life and other aspects.
Silicone material has both inorganic siloxane chains and organic groups in its composition, and is a typical semi-inorganic polymer. And it is this structural feature that makes it a very special polymer material, and has a series of excellent properties such as high temperature resistance, flame retardancy, electrical insulation, radiation resistance and physiological inertia that other materials cannot possess at the same time. In particular, the development history of the silicone industry is different from that of general-purpose synthetic materials. General-purpose synthetic materials are developed centered on raw material manufacturing technology, large-scale production technology and product processing; while organic silicon is developed centered on product development. In recent decades, the production process of silicone monomers has not changed much, while the focus of silicone technology is mainly on product applications, such as the introduction of organic groups, polymer structure and cross-linking technology. Silicone materials can be designed with various molecular structures to meet the requirements of various industries and occasions. When designing multi-purpose products, the following paths can be taken.
① Change the molecular structure of siloxane. For example, changing the molecular weight and molecular shape (linear, branched, cross-linking density), etc.
② Change the organic groups bound to silicon atoms. For example, alkyl (methyl, ethyl, long carbon chain), phenyl, vinyl, hydrogen, polyvinyl, arginine-containing alkyl, fluorine-containing alkyl, amino-containing alkyl, etc.
③Select different curing methods. For example, free radical curing, condensation reaction curing (including dealcoholization reaction, deketone reaction, dehydrogenation reaction, dehydration reaction, etc.), addition reaction curing, etc. The curing conditions may be heating curing, ultraviolet curing, radiation curing, and the like.
④Using organic resin modification (copolymerization or blending). Such as epoxy, polyester, poly, acrylate and other resins.
⑤Choose a variety of different fillers. For example, metal soap, silicon dioxide, carbon black, titanium dioxide, iron oxide, etc.
⑥ choose a variety of secondary processing technology. Such as emulsion, solution fat, rubber compound, adhesive tape, etc.
⑦ Using various polymerization techniques. For example, bulk polymerization, emulsion polymerization, block copolymerization, etc.
The key to polymer materials lies in the development of monomer technology. The silicone industry is characterized by centralized monomer production and decentralized product processing. Therefore, monomers play a role in the process of synthesizing silicone materials, and the production level of monomers directly reflects the development level of the silicone industry. There are thousands of silicone monomers, but there are not many monomers with industrial value. Industrial silicone monomers generally include four types: chlorosilane, cyclosiloxane, alkoxysilane and acyloxysilane, of which the first two are the most important.
Most of the silicone is made of polydimethylsiloxane prepared from dimethyldichlorosilane as the base polymer, and then other groups are introduced to process polymer products and products that meet various functional requirements in various forms. . At present, the direct method of synthesizing methylchlorosilane technology invented by American GE company Luoqiao is generally adopted at home and abroad, that is, the method of producing methylchlorosilane mixed monomer by reacting silicon powder and methyl chloride gas in the presence of a copper catalytic system. Raw materials are easy to obtain and large-scale continuous production is easy to achieve. It is the most successful and only industrialized production method for the synthesis of organosilicon monomers.
The mixed methyl monomers obtained through synthesis are separated by rectification to obtain dimethyldichlorosilane and other refined monomers. Dimethyldichlorosilane is hydrolyzed and cracked to produce dimethylsiloxane oligomers (DMC, D4), which are used as basic raw materials for further processing into various silicone polymerization products. Hydrogen chloride by-product of hydrolysis of methyl chlorosilane is recovered and synthesized into methyl chloride with methanol. The whole set of equipment includes silicon powder processing, methyl monomer synthesis and separation, hydrolysis, cracking and ring distillation, hydrogen chloride recovery and methyl chloride synthesis and other devices.
(1) Chlorosilane monomer
Silicone chlorine-containing basic monomers include methylchlorosilane (referred to as methyl monomer), phenylchlorosilane (referred to as phenyl monomer), methylphenylchlorosilane, methylvinylchlorosilane, vinyltrichlorosilane And fluorosilicone monomer, etc. Among them, methylchlorosilane is the most important, and its consumption accounts for more than 90% of the total amount of chlorine-containing monomers; followed by phenylchlorosilane.
1. Preparation process of methylchlorosilane
The first step of crude monomer synthesis: At present, the industrial production of methylchlorosilane monomer adopts the direct synthesis method, while the major organosilicon manufacturers in the world use the direct synthesis method of ebullating bed (fluidized bed) for production.
Fluidization is a technology that uses fluid (such as gas or liquid) to drive solid particles to move, so that solid particles have certain flow characteristics similar to fluid. The structural design of the cylindrical fluidized bed is simple, generally composed of upper and lower cylinders and internal heat exchange tube bundles, the gas distributor is located at the bottom of the lower cylinder, this kind of reactor is easy to manufacture, the utilization rate of silicon powder raw materials is high, and continuous reaction The time can be up to about a week.
The main production principle of monomer synthesis is: Silicon powder and catalyst copper powder are metered into the fluidized bed reactor, with superheated methyl chloride gas as the fluidized medium, under the conditions of 280~310℃ and 0.3~0.35MPa (G) React with methyl chloride to generate methylchlorosilane mixed monomer. Its main reaction formula is as follows:
The raw materials used in the direct synthesis of methylchlorosilane monomer are mainly metallurgical grade metal silicon and methyl chloride with a purity of more than 99%.
The reaction product and unreacted methyl chloride gas mixture and the silicon and copper fine powder carried out are separated by a cyclone separator to separate most of the silicon and copper fine powder, and are directly returned to the bed through the hopper or intermittently discharged into the fine powder storage tank, and alternately returned to the bed . After passing through the three-stage cyclone separator, the silicon and copper fine powder remaining in the synthesis gas are removed in the washing tower. The slurry in the reboiler of the washing tower is flashed and condensed to recover the high boilers in the slurry discharged from the system.
The gas passes through the crude monomer tower and the methyl chloride tower operated under pressure, and the methyl chloride is separated and returned to the fluidized bed reactor for recycling. Remove the heat of reaction. By-produce steam through the waste heat boiler and merge it into the steam pipe network to achieve the purpose of energy saving.
The reaction temperature of the direct method should not be too high, otherwise the yield of dimethyldichlorosilane will decrease and the content of polychlorosilane will increase. Properly increasing reaction pressure (4～5atm) is conducive to the generation of dimethyl dichlorosilane.
Introduction of by-products: In the process of industrial production of methylchlorosilane by the “direct method”, there are side reactions such as thermal decomposition, disproportionation, and moisture brought in from the hydrolysis raw material of chlorosilane, which makes the reaction product more complicated. In addition to the main product is dimethyldichlorosilane (abbreviated as dimethyl) or 5-15wt% methyltrichlorosilane (abbreviated as a), and 1-2% trimethylchlorosilane (abbreviated as trimethyl) And methylhydrogen dichlorosilane (referred to as a hydrogen-containing), 1-2wt% low boiling point mixture (abbreviated as low boiler) and 6-8wt% high boiling point mixture (abbreviated as high boiler), etc.
The above-mentioned by-products except trimethyl and monomethan contain hydrogen, and the market application of the remaining high boilers, low boilers and monomethan is extremely limited. Because these by-products are very easy to react with the moisture in the air to produce hydrogen chloride, they are very easy to react with the moisture in the air to produce hydrogen chloride corrosive gas during discharge and storage, causing ecological environmental pollution. Among them, high boiling substances and low boiling substances The composition of matter is more complicated.
The second step of monomer rectification: in order to further purify dimethyl dichlorosilane, fractional distillation must be carried out with a split layer of 150-200 theoretical plates. The crude methyl monomer can obtain dimethyl dichlorosilane with a purity of more than 99.95% after fractional distillation.
The crude monomer from the tank area is separated from high boilers (main components include Cl2CH3SiSi(CH3)2Cl, Cl2CH3SiSiCH3Cl2, etc.) through the de-higher tower (tower 130-175°C), and the top of the tower (70-120°C) is discharged into Get off the lower tower.
Take off the top of the tower to get the light components into the light fractionation tower. The mixture of the first monomer and the second monomer is obtained by taking off the tower tank.
The first product is obtained from the top of the binary tower, which is pumped to the first monomer storage tank in the tank area; the second product is obtained from the bottom of the binary tower, which is pumped to the second first monomer storage tank in the tank area.
The top of the light fractionation tower is extracted into the low-boiling first tower, and the bottom of the tower is extracted into the hydrogen-containing tower. A hydrogen-containing monomer is produced from the top of the hydrogen-containing tower, which is transported by a pump to a hydrogen-containing monomer storage tank in the tank area. The crude trimethylate is extracted from the hydrogen-containing tower kettle, which is pumped to the crude trimethylate storage tank in the tank area. The top of the low-boiling first tower is extracted into the second low-boiling tower, and the bottom of the tower is extracted back to the light fractionation tower. The low-boiling two-column overhead tank separately extracts low-boiling components of different components into the storage tanks in the tank farm.
The high-boiler column handles cracked products from the high-boiler cracking section. The high-boiler column removes the high-boiler cracking residue, and the overhead distillate enters the three-methyl column. The low boilers distilled from the top of the trimethyl tower are pumped to the low boiler storage tank. The liquid from the bottom of the top three towers is sent to the crude monomer storage tank as the raw material for rectification.
The third step is a monomer and its conversion: for methyltrichlorosilane, it contains three hydrolyzable groups, so it is very easy to see water and generate gas during storage. Yijia can be used to prepare a variety of silane crosslinking agents: for example, Yijia reacts with methanol or ethanol to form methyltrimethoxysilane and methyltriethoxysilane; reacts with acetic acid, vinegar, sodium acetate, etc. to produce methyltriethylsilane Phthaloxysilanes react with ketones to form corresponding methyltriketone fatty silanes. These alkoxysilanes and phthaloxysilanes are crosslinking agents for one-component room temperature vulcanized silicone rubber. Yijia can also be hydrolyzed to prepare methyl silicone resin, co-hydrolyzed and polycondensed with dimethyl, phenyltrichlorosilane, diphenyldichlorosilane to prepare methylphenyl silicone resin; used to prepare sodium methyl silicate, silicone resin Micropowder and other building waterproofing agents. However, on the one hand, the market demand for these products is limited, on the other hand, the product quality is unstable and the cost is high, so it is difficult to process and utilize Yijia in a large-scale manner. At the same time, most of them at home and abroad have realized industrialization or are used to produce fumed white carbon black for recycling. Another utilization method that has attracted more attention is the monomer conversion method, that is, the reaction of monomethyl and excess trimethyl to form dimethyl, and the comprehensive utilization of methyl chlorosilane by-product monomer to increase the overall yield.
The specific operation is that, in the monomer converter, under the action of a catalyst, monomer conversion occurs between the added monomethyl monomer and trimethyl monomer to obtain dimethyl monomer. The mixture obtained from the reaction is sent to the rectification section as the crude monomer raw material for further rectification. The product is a mixed monomer, which is sent back to the rectification section as a raw material to continue rectification to obtain a refined monomer product. The reaction in the monomer converter is as follows:
Crude trimethyl monomer and monomethyl monomer, monomer conversion occurs in the monomer converter (130-175 ° C), and the steam at the top of the reactor enters the reflux tower. The steam at the top of the reflux tower enters the product tank after being condensed by the condenser, part of the product flows back to the reflux tower, and part of it is sent to the crude monomer tank in the tank area as a monomer conversion product (as a raw material for monomer rectification). The liquid phase at the bottom of the reflux tower is refluxed to the monomer converter.
The fourth step is high boiling matter and its cracking: high boiling matter is a mixed liquid with caramel color, pungent smell and strong corrosiveness. The density at normal temperature and pressure is about 1.13g/cm3 and the boiling range is 80- Between 215°C. The composition of high boilers is closely related to the purity of silicon powder in the production of organosilicon monomers, the properties of catalysts, and reaction conditions. The composition and content of high boilers are different in different production processes or even in different batches of the same production process.
It can be seen that the high boiler contains a small amount of Cu, Al, Zn and silicon powder, and its main components are Cl2CH3SiSi(CH3)2Cl, Cl2CH3SiSiCH3Cl2. However, the above reports did not introduce an accurate analysis method for the composition and quantification of high boilers. The complexity of the composition of high boilers and their ease of hydrolysis also determine the difficulty of analysis.
The handling of large quantities of high boilers is problematic due to their complex composition and the inability to form them into useful organopolysiloxanes in a simple manner. For a long time, high-boilers have only been used as low-grade products such as waterproofing agents, which cannot be effectively utilized due to the small market demand for such products. Long-term storage will severely corrode the storage tank, and direct discharge will pollute the environment.
2. Varieties and Properties of Methylchlorosilane
Due to the special activity of silicon-chloride bonds, chlorosilanes can react with many functional groups to produce a series of organosilicon compounds, so they are widely used in the fields of silicone rubber, silicone resin and silicone oil. Although there are many types of chlorosilanes, not many of them can play a role in industrial silicon. The following table shows the most widely used chlorosilanes, and their importance gradually decreases from left to right.
3. Treatment of low boilers
Low boilers are a mixture of by-products with a boiling point below 40 degrees, and the composition is extremely complex.
Like high boilers, the composition of low boilers is also affected by the purity of silicon powder, the nature of the catalyst, and reaction conditions. The composition and content of low boilers are different in different production lines or even in different batches of the same production line.
It can be considered that the main components in low boilers are methylchlorosilane, hydrochlorosilane and a small amount of hydrocarbons. The most important components are (CH3) 4Si and (CH3) 2SiHCl.
Although its main components (CH3) 4Si and (CH3) 2SiHCl have high use value, they are difficult to use because of their close boiling points and separation. Foreign countries use low boilers to produce fumed silica, but the domestic technology is immature, so they can only be discharged directly.
The latest research has found that using aluminum chloride ( AlCl3 ) as a catalyst, under certain temperature and pressure conditions, using low boilers and hydrogen chloride to react, can successfully convert tetramethyl in low boilers into trimethyl, ( CH3)2SiHCl is converted into dimethylformamide, which has the largest demand in industry, which greatly improves the utilization rate of low boilers.
4 . Treatment of Monomethyl Hydrogen-Containing Monomer
Monomethyl hydrogen-containing monomer can be co-hydrolyzed to generate hydrogen-containing silicone oil and by-product hydrochloric acid when trimethylchlorosilane is used as a chain stopper. Its reaction formula is:
The monomethyl hydrogen-containing monomer from the Yijia hydrogen-containing monomer storage tank and the Sanjia from the Sanjia storage tank are completely mixed in the mixer in advance, and then enter the hydrolysis loop to react. The reaction mixture separates from the top of this loop into the hydrolysis stratifier. Pure water is fed to the loop as the water source.
Hydrogen-containing silicone oil is separated by hydrolysis layerer to separate hydrogen-containing silicone oil (top) and 20% hydrochloric acid (bottom). Fully wash the hydrogen-containing silicone oil in the hydrolysis stabilization tank. Separation is carried out through the first stratifier and the second stratifier, the hydrogen-containing silicone oil at the top enters the dehydration tower, and the acidic wastewater at the bottom is sent to the sewage treatment station for treatment. The dehydrated hydrogen-containing silicone oil enters the fixed-bed reactor.
(1) Cyclosiloxane (DMC)
1. Introduction to Cyclosiloxane
Necessity of intermediates: Generally, linear polysiloxanes can be synthesized by hydrolyzing difunctional chlorosilanes. However, if it is necessary to synthesize linear polysiloxane with a higher molecular weight (the degree of polymerization exceeds 1000), it is necessary to make high requirements on the purity of the monomers, and the content of Yijia must be much lower than 1/10,000, otherwise it will Branching or even cross-linking will occur, affecting product quality. Since the boiling points of methazine and methazine are relatively close, it is difficult to obtain high-purity dimethylformazine.
In order to overcome this problem in the industry, people synthesize linear polydimethylsiloxane through cyclic siloxane monomers. Among all the cyclic siloxane monomers, only cyclotrisiloxane (D3), cyclotetrasiloxane (D4) and cyclopentasiloxane (D5) have so far been of commercial value and are used in relatively large quantities. production and application. Among them, D3 and D4 are used as monomers to synthesize linear polysiloxane, and because D5 has good compatibility with most alcohols and other cosmetic solvents, and is odorless, non-toxic, non-irritating and clean and non-greasy, it has Good spreadability and spreadability, so it is used as a base oil for various personal care products. In addition, it replaces perchlorethylene as an environmentally friendly dry cleaning solvent.
2. Preparation process of cyclosiloxane
The first step dimethyl hydrolysis
Production principle: The dimethyl monomer undergoes hydrolysis reaction with water in an acidic environment, and finally produces a multi-component mixture composed of linear and cyclic siloxanes. The reaction equation is as follows:
The temperature needs to be controlled at 38-45°C, and the pressure should be controlled at 0.1-0.25MPaG.
The process route of continuous concentrated hydrochloric acid loop hydrolysis of dimethyl monomer is adopted, that is, the hydrolysis reaction of dimethyl monomer and hydrochloric acid generates hydrolyzate and releases HCI gas;
After being neutralized and washed with water, the hydrolyzate is sent to the siloxane cracking and rectification section and the 107 rubber section as intermediate raw materials, and the HCl gas is sent to the methyl chloride synthesis section and high boiling cracking section as intermediate raw materials after degreasing and mist removal.
This step consists of hydrolysis, first and second water washing, first and second alkali washing, third water washing, HCl degreasing and water removal, falling film absorption (and tail gas treatment, etc.).
The dimethylformamide and hydrochloric acid from the monomer tank area first enter the concentrated acid loop hydrolysis reaction system. The product of the hydrolysis reaction is a mixture of hydrolyzate and concentrated hydrochloric acid, the two are initially separated in the pre-separator, the acid-rich solution at the bottom flows to the hydrochloric acid circulation pump by gravity to form a loop, and the oil-rich phase at the top overflows to the phase separation further liquid-liquid phase separation. The oil containing a small amount of acid in the upper layer of the acid separator is dediluted and purified by the acid primary and secondary water washing systems. The concentrated acid in the lower layer of the acid separator flows to the concentrated hydrochloric acid tank by itself, and then is piped to the concentrated hydrochloric acid storage tank.
The oil is purified by an alkali neutralization system, which is divided into primary and secondary alkaline washing systems. The waste lye produced is sent to sewage treatment.
After alkali neutralization, the oil-alkali mixture in the upper layer of the fifth loop stratifier overflows to the three-stage water washing tank for oil-alkali separation, and the lower layer is lye, which is continuously returned to the alkali tank for recycling.
The hydrolyzate in the upper layer of the sixth loop stratifier overflows to the hydrolyzate storage tank, and then is pumped to the hydrolyzate tank area for use in the siloxane cracking and rectifying section and the 107 rubber section.
The second step epoxy silane cracking
Production principle: dimethyl hydrolyzate is a multi-component mixture composed of linear and cyclosiloxane. This device cracks and rearranges linear siloxane through catalytic cracking to generate D4-based cyclic mixture (DMC).
The reaction is a silicon-oxygen bond rearrangement reaction, which can be carried out quickly by heating to 135-165°C with steam and under the catalysis of KOH. The rearrangement reaction is a reversible reaction, so during the reaction, the generated rings must be continuously evaporated, so that the linear polymer can be completely transformed into rings.
The fourth step of ring distillation
Ring body distillation is to separate the cracked ring body by two-tower continuous process to separate D3, high ring, DMC and other products.
The liquid in the bottom of the tower enters the product tower, and DMC is obtained from the top of the product tower, and is pumped through the middle of the DMC to the finished product tank area for storage. The output from the tower kettle is high-ring, enters the high-ring storage tank, and is sent back to the dimethyl hydrolysis process.
The definition of organopolysiloxane is: a polymer compound with Si-O-Si bond as the main chain and organic groups directly connected to silicon atoms. Its general structure is as follows:
Where R is an organic group, such as methyl, phenyl, vinyl, etc., and n is the degree of polymerization.
Like ordinary polymer compounds, organopolysiloxanes can also be divided into linear, branched, and cross-linked types according to different chain structures.
The preparation methods of organopolysiloxane can be roughly divided into two categories: polycondensation reaction and ring-opening polymerization reaction.
Polycondensation reaction is a chemical reaction in which monomers with two or more functional groups react with each other to form high molecular compounds and small molecules at the same time. The ring-opening polymerization reaction is a kind of polyaddition reaction, that is, no small molecular by-products are released during the reaction, so the chemical composition of the polyaddition polymer is the same as that of the starting monomer.
Principle of polycondensation reaction: The hydrolysis and condensation of chlorosilane in industry is one of the main methods for preparing polysiloxane. The hydrolysis of chlorosilanes includes two reaction processes, first it hydrolyzes and generates silanols, and then dehydrates silanols or dehydrochlorides with chlorosilanes to polycondense siloxanes.
Dimethicone is the most commonly used chlorosilane monomer. The main product of its hydrolysis is a mixture of cyclic siloxane and hydroxyl-terminated linear polydimethylsiloxane, often called hydrolyzate. The chain length of the linear product and its yield ratio to the cyclic siloxane can be controlled by changing the reaction conditions.
The concentration of hydrochloric acid and its contact time with the product play a deterministic role. If the generated hydrochloric acid is quickly neutralized, or the reaction is carried out under pressure, a short-chain siloxane diol is formed, which can be used as a structure control agent in silicone rubber.
The composition of the hydrolyzed product can also be adjusted by the amount of water used and the order of addition. Under the condition of a large excess of water (adding chlorosilane to water), the products are mainly linear polysiloxanes and cyclic siloxanes with smaller molecular weights (the same principle as the hydrolysis of dimethyl in the previous section). When water is gradually added to the chlorosilane (reverse hydrolysis), the structure of the product will be different, and a linear polysiloxane with a larger molecular weight can be obtained.
Therefore, in order to completely convert dimethylformamide into hydroxyl-terminated linear polysiloxane, continuous hydrolysis is used in the industry. This is based on the principle that the silicon-oxygen bond will break and form an equilibrium system in the presence of a catalyst, allowing the cyclic oligosiloxane fractionated from the reaction system to react with dichlorosilane under acid-catalyzed conditions to form Dichloro-terminated linear polysiloxane.
Principle of ring-opening polymerization: The ring-opening polymerization of cyclic siloxane refers to the process in which the cyclic siloxane monomer obtained after dimethyl hydrolysis and cracking is broken and rearranged into linear siloxane under the action of a catalyst. Compared with the polycondensation reaction, the structure and molecular weight of the product can be well controlled by ring-opening polymerization, and compounds with higher molecular weight can be obtained. The ring-opening polymerization of cyclic siloxane is the most important and common method for the industrial preparation of linear polysiloxane at present, such as high-temperature glue and silicone oil, which are mostly synthesized by this method. Among the large number of cyclic siloxane monomers, octamethylcyclotetrasiloxane (D4) and hexamethylcyclotrisiloxane (D3) are the two most important monomers for the synthesis of linear polysiloxanes. body.
The polymerization process includes operations such as raw material metering, dehydration, batching, polymerization, coal breaking, low molecular weight removal, cooling, and discharging.
The moisture present in the raw materials will destroy the catalyst and affect the polymerization reaction. At the same time, the moisture is also an end-capping agent, producing hydroxyl-terminated polysiloxane molecules, thereby affecting product quality.
Under basic catalysis, cyclosiloxane can start ring-opening polymerization when the temperature is higher than 95 degrees, and the specific reaction temperature depends on the catalyst used. The most widely used catalysts in raw rubber synthesis are potassium siloxanolate and tetramethylammonium siloxanolate (alkali gum).
Silicone rubber only releases a small amount of heat during the polymerization process, so the polymerization temperature is mainly obtained by external heat supply.
After the cyclosiloxane is initiated, the reaction speed is very fast, forming a viscosity peak. Due to the poor heat transfer of polymers, mass transfer and heat transfer are very difficult. To achieve uniformity and uniform heat, it is necessary to rely on enhanced stirring.
After the polymerization reaction reaches equilibrium, the conversion rate of the raw material cyclosiloxane is only about 85%, and about 15% of the low-molecular rings exist in the glue, which needs to be removed by heating and breaking coal.