1. Classification and characteristics of silicone rubber
  1.1 Classification
  Silicone rubber can be divided into three categories: thermal vulcanization type, room temperature vulcanization type and addition reaction type according to its vulcanization mechanism.
  1.2 Features
  (1) High and low temperature resistance
   Among all rubbers, silicone rubber has the widest working temperature range (-100～350℃). For example, properly formulated vinyl silicone rubber or low-phenyl silicone rubber can still maintain elasticity after thousands of hours at 250°C or hundreds of hours of hot air aging at 300°C; It can still maintain elasticity after hours of hot air aging. Its glass transition temperature is -140°C, and its vulcanizate is still elastic at a temperature of -70 to 100°C. When silicone rubber is used for heat-resistant coating on the inner wall of rocket nozzle, it can withstand instantaneous high temperature of thousands of degrees.
  (2) Resistance to ozone aging, oxygen aging, light aging and weathering
  Silicone rubber vulcanizate has no significant change in performance after being exposed to the outdoors for several years in a free state.
  (3) Electrical insulation properties
  The electrical insulation performance of silicone rubber vulcanizate changes little when it is exposed to moisture, frequency changes or temperature rises, and the silicon dioxide formed after burning is still an insulator. In addition, there are few carbon atoms in the molecular structure of silicone rubber, and carbon black is not used as a filler, so it is not easy to scorch during arc discharge, and it is very reliable in high-voltage applications. It has excellent corona resistance and arc resistance. The life of corona resistance is 1000 times that of polytetrafluoroethylene, and the life of arc resistance is 20 times that of fluororubber.
  (4) Special surface properties and physiological inertia
The surface energy of silicone rubber is smaller than that of most organic materials, and it has low hygroscopicity. The water absorption rate is only about 1% after being immersed in water for a long time. effect. Silicone rubber is odorless, non-toxic, has no adverse effects on the human body, reacts slightly with body tissues, and has excellent physiological inertia and physiological aging properties.
     (5) High air permeability
Compared with other polymer materials, silicone rubber has good gas permeability, and the permeation rate of nitrogen, oxygen and air at room temperature is 30 to 40 times higher than that of NR; it is selective for gas permeation, such as oxygen permeation for carbon dioxide. about 5 times.
  (6) Biomedical properties
  The characteristics of the molecular structure of silicone rubber make it have excellent biomedical properties, which has been proved by a large number of successful applications in animal and human experiments.

2. Heat vulcanized silicone rubber
  Heat-vulcanized silicone rubber refers to silicone rubber with a high relative molecular weight (400,000 to 600,000). The organic peroxide is used as the vulcanizing agent, and the organic peroxide is decomposed by heating to generate free radicals, which form crosslinks with the organic side groups of the rubber, thereby obtaining vulcanized rubber.
     2.1 Varieties and characteristics
  Heat-vulcanized silicone rubber is the earliest type of rubber, and it has been developed into many varieties. According to the chemical composition, it is divided into the following seven types:
  (1) Dimethyl silicone rubber
  Dimethyl silicone rubber (polydimethylsiloxanerubber) referred to as methyl silicone rubber, is the oldest variety of silicone rubber.
  It can maintain good elasticity in the temperature range of -60～250℃. Due to the disadvantages of low vulcanization activity, poor process performance, easy foaming of thick-walled products during two-stage vulcanization, and large high-temperature compression deformation, it has been replaced by methyl vinyl silicone rubber except for a small amount of fabric coating.
  (2) Methyl vinyl silicone rubber
  Methylvinylpolysiloxanerubber (methylvinylpolysiloxanerubber) is referred to as vinyl silicone rubber, which is copolymerized from dimethyl siloxane and a small amount of vinyl siloxane, and the vinyl mole fraction is generally 0.001-0.003. The introduction of a small amount of unsaturated vinyl can greatly improve the vulcanization process and product performance, especially the heat aging resistance and high temperature compression deformation resistance. In the production of silicone rubber, methyl vinyl silicone rubber has the largest output, the widest application, and the most varieties and grades. Silicone rubber, low compression set silicone rubber, conductive silicone rubber, thermally conductive silicone rubber, and non-two-stage vulcanized silicone rubber, granular silicone rubber, etc.) are also processed and matched on the basis of it.
  (3) Methyl vinyl phenyl silicone rubber
    Methylvinylphenylpolysiloxanerubber (methylvinylphenylpolysiloxanerubber) is referred to as phenyl silicone rubber, which is made by introducing diphenylsiloxane chains (or methylphenylsiloxane chains) into the molecular chain of vinyl silicone rubber. into. When the phenyl mole fraction is 0.05 to 0.10, it is collectively referred to as low phenyl silicone rubber. At this time, the hardening temperature of the rubber drops to the lowest value (-115°C), which makes it have the best low temperature resistance. The following is still elastic. With the increase of phenyl mole fraction, the rigidity of molecular chain also increases, and its crystallization temperature rises instead.
   When the phenyl mole fraction is 0.15 to 0.25, it is collectively referred to as medium phenyl silicone rubber, which has the characteristics of flame resistance. When the phenyl mole fraction is above 0.130, it is collectively referred to as high phenyl silicone rubber, which has excellent radiation resistance. Phenyl silicone rubber is used in occasions that require low temperature resistance, ablation resistance, high energy radiation resistance, and heat insulation. The production and application of medium phenyl and high phenyl silicone rubbers are limited due to processing difficulties and poor physical properties.
  (4) Methyl vinyl trifluoropropyl silicone rubber
Methylvinyltrifluoropropylpolysiloxanerubber (methylvinylrtrifluoropropylpolysiloxanerubber) referred to as fluorosilicone rubber (fluorosilicorubber), is the introduction of fluoroalkyl (generally trifluoropropyl) in the molecular chain of vinyl silicone rubber, which has excellent oil resistance, Solvent resistance. For example, for aliphatic, aromatic and chlorinated hydrocarbon solvents, various petroleum-based fuel oils, lubricating oils, hydraulic oils, and some synthetic oils, the operating temperature range is -50 to 250 ° C, at room temperature and high temperature Good stability.
  (5) Phenylene silicone rubber and phenylene ether-based silicone rubber
Phenylene silicone rubber and phenylene ether silicone rubber (phenylenepolysiloxanerubberandphenylenesiliconerubber) are new varieties of silicone rubber containing phenylene or phenylene ether-based chain links in the molecular chain, which are developed to meet the requirements of nuclear power devices and navigation technology. Its main characteristics are high tensile strength, γ-ray resistance, and high temperature resistance (above 300°C), but its cold resistance is not as good as low-phenyl silicone rubber.
  (6) Nitrile silicone rubber
Nitrile silicone rubber (nitrilsiliconerubber) is mainly an elastomer containing methyl-β-nitrile ethyl siloxane chain or methyl-γ-nitrile propyl siloxane chain link in the molecular chain. Its main characteristics are similar to those of fluorine Silicone rubber is similar in that it is oil resistant, solvent resistant and has good low temperature resistance. However, due to the existence of factors that cause hydrolysis of nitrile groups under polymerization conditions, the reproducibility of raw rubber is poor, and its application and development are limited.
   (7) Silicon boron rubber
Boron silicon rubber (boronsiliconerubber) is a new type of silicone rubber containing decaborane cage structure in the molecular main chain, which has a high degree of heat aging resistance, can work for a long time at 400 ° C, and can continue to work at 420 ~ 480 ° C Work for several hours, while still maintaining elasticity at -54°C. It is suitable as a sealing material in high-speed aircraft and spacecraft. At the end of the 1960s, the United States had a series of commercial grades of silicon boron rubber, but it was rarely reported after the 1970s. The main reason may be that the process performance of the rubber material and the elasticity of the vulcanized rubber are very poor, and the synthesis of carbon boron is very complicated and toxic. Big and expensive.
  Heat-vulcanized silicone rubber is sold in the form of raw rubber or vulcanized rubber. It is generally formulated into rubber materials with various characteristics for users to choose. According to different characteristics, they can be divided into the following categories:
  1) Universal type (general strength type). Composed of vinyl silicone rubber and reinforcing agent, the physical properties of vulcanized rubber are medium, and it is a type of rubber with the largest amount and the strongest versatility.
  2) High strength type. Vinyl silicone rubber or low-phenyl silicone rubber is used, and fumed silica or modified silica with a large specific surface area is used as a reinforcing agent, and by adding suitable processing aids and special additives, etc. Improvement measures are taken to improve the cross-linking structure and improve the tear strength.
  3) High temperature resistant type. Using vinyl silicone rubber or low phenyl silicone rubber, the type of reinforcing agent and heat-resistant additives can be properly selected to produce silicone rubber that can withstand high temperatures of 300-350 °C.
  4) Low temperature type. Using low phenyl silicone rubber, the brittle temperature reaches -120°C, and it is still elastic at -90°C.
  5) Low compression set type. Vinyl silicone rubber is mainly used, and vinyl-specific organic peroxides are used as vulcanizing agents. When the compression ratio is 30%, the permanent deformation after compression at 150°C for 24-72 hours is 7.0%-15% (ordinary silicone rubber 20% to 30%).
  6) Wire and cable type. Vinyl silicone rubber is mainly used, and fumed white carbon black with good electrical insulation performance is selected as the reinforcing agent, which has good extrusion process performance.
  7) Oil-resistant and solvent-resistant type. Ammonia silicone rubber is mainly used, which is generally divided into two categories: general-purpose and high-strength.
  8) Flame retardant type. It is made of vinyl silicone rubber and added with halogen or platinum compound as flame retardant, which has good flame resistance.
  9) Conductive type. Vinyl silicone rubber is used, acetylene carbon black or metal powder is used as filler, and high-temperature vulcanization or addition vulcanization is selected to obtain silicone rubber with a volume resistivity of 2.0-100Ω·cm.
  10) heat shrinkable type. Adding thermoplastic materials with a certain melting temperature or softening temperature to vinyl silicone rubber, the heat shrinkage rate of the silicone rubber compound can reach 35% to 50%.
  Do not use two-stage vulcanization type. It adopts vinyl silicone rubber with high vinyl mass fraction, and it is made by controlling the pH value of raw rubber and compounding ingredients, and adding special additives.
  11) Sponge type. Adding organic foaming agents such as nitroso compounds, azo and diazo compounds to vinyl silicone rubber can make a sponge with uniform foaming.
   In addition, there are thermal conductive silicone rubber, fluorescent silicone rubber and medical grade rubber compound for sale abroad.

  With the continuous development of the use of silicone rubber, the varieties and grades of rubber materials are increasing day by day, too many grades will cause confusion in production, storage, transportation and sales. Some factories have correspondingly divided multiple varieties into several typical base rubbers and several characteristic additives (including pigments, vulcanizing agents, etc.) for sale. Users can mix them according to certain formulas and mixing techniques according to their needs to obtain the final product.
  This method not only makes the varieties simple and clear, but also has large production batches, stable quality, reduced costs, and improved competitiveness.
  

        2.2 Coordination

  The compounding ingredients of heat-vulcanized silicone rubber mainly include reinforcing agent, vulcanizing agent and some special additives. Generally, only 5-6 components are needed to form a practical formula. Formulation design should consider the following points:
(1) Silicone rubber is a raw rubber with high saturation, and generally cannot be vulcanized with sulfur. Organic peroxides should be used as vulcanizing agents. Therefore, the rubber compound must not contain active substances that can interact with peroxide decomposition products (such as sulfur method carbon black, certain organic accelerators and anti-aging agents, etc.), otherwise it will affect vulcanization.
  (2) Silicone rubber products are generally used at high temperatures, and their compounding agents should remain stable at high temperatures. Inorganic oxides are usually used as reinforcing agents.
  (3) Silicone rubber is easy to cause cleavage and rearrangement of siloxane bonds under the action of chemical reagents such as trace acids or alkalis, resulting in a decrease in the heat resistance of silicone rubber. Therefore, when selecting the compounding agent, its acidity and alkalinity and the acidity of the peroxide decomposition product must be considered, so as not to affect the performance of the vulcanizate.
       2.2.1 Selection of raw rubber

Vinyl silicone rubber can be used for silicone rubber products with general temperature requirements (-70-250°C); when the temperature requirements for products are high (-90-300°C), low-phenyl silicone rubber can be used ; When the product requires high and low temperature resistance and resistance to fuel or solvent, fluorosilicone rubber should be used.
  2.2.2 Vulcanizing agent and vulcanization mechanism
  (1) Vulcanizing agent
  The vulcanizing agents used for thermally vulcanized silicone rubber mainly include organic peroxides, aliphatic azo compounds, inorganic compounds and high-energy rays, among which organic peroxides are the most commonly used. This is because organic peroxides are generally stable at room temperature, but can rapidly decompose at higher vulcanization temperatures to generate free radicals, thereby crosslinking silicone rubber. Common vulcanizing agents for silicone rubber are shown in Table 3.
These peroxides can be divided into two categories according to their activity: one is general-purpose type, which has high activity and can vulcanize all kinds of silicone rubber; the other is vinyl-specific type, because of its low activity, It can only vulcanize vinyl-containing silicone rubber.
  The amount of peroxide is affected by various factors such as the type of raw rubber, the type and amount of filler, and the processing technology. As long as the required cross-linking degree can be achieved, the vulcanizing agent should be used as little as possible. For vinyl silicone rubber (vinyl mole fraction is 0.0015) rubber compound for molded products, the usual dosage ranges of various peroxides are as follows (based on 100 parts of raw rubber): BP 0.5～1 part; DCBP 1～2 parts ; DTBP 1~2 parts; DCP 0.5~1 part; DBPMH 0.5~1 part; TBPB 0.5~1 part.
  As the mass fraction of vinyl increases, the amount of peroxide should decrease. The amount of peroxide in glue, extruded product glue and adhesive glue should be larger than that in molding glue. In some occasions, two kinds of peroxides are used together, which can reduce the amount of vulcanizing agent, and can appropriately reduce the vulcanization temperature and improve the vulcanization effect.
  (2) Vulcanization mechanism
  When silicone rubber is vulcanized with peroxide, the crosslinking of peroxide to silicone rubber is carried out by free radical reaction between two activated methyl or vinyl groups. The cross-linking of dimethyl silicone rubber is carried out according to the following reaction formula:
  The cross-linking of vinyl silicone rubber is carried out according to the following reaction formula:
  Silicone rubber can be vulcanized by high-energy rays in addition to the above-mentioned peroxide vulcanization. Radiation vulcanization is also carried out according to the free radical mechanism. When the vinyl molar fraction in the raw rubber is high (0.01) or when it is used in combination with other rubbers, it can also be vulcanized with sulfur, but the performance is extremely poor.
2.2.3 Reinforcing agents and related mechanisms
  The strength of unreinforced silicone rubber vulcanizate is very low, only about 0.3MPa, and has no practical use value. Adding appropriate reinforcing agent can make the strength of silicone rubber vulcanizate reach 3.9-9.8MPa, which is extremely important to improve the performance of silicone rubber and prolong the service life of products. The choice of silicone rubber reinforcing filler should take into account the high temperature use of silicone rubber and the adverse effects of peroxide vulcanization (especially with acidic and alkaline substances) on silicone rubber.
Reinforcing fillers for silicone rubber can be divided into reinforcing fillers and weak reinforcing fillers according to their reinforcing effects. The former has a particle size of 10-50nm and a specific surface area of 70-400m2·g-1 , the reinforcing effect is better; the particle size of the latter is 300-1000nm, the specific surface area is below 30m2·g-1, and the reinforcing effect is poor.
  (1) Reinforcing filler
  The reinforcing filler of silicone rubber mainly refers to synthetic silica, also known as white carbon black. Silica can be divided into fumed silica and precipitated silica. Fumed silica is one of the most commonly used reinforcing agents for silicone rubber. The vulcanized rubber reinforced by it has high mechanical strength and good electrical properties, and can be used together with other reinforcing agents or weak reinforcing agents. Prepare rubber materials with different usage requirements.
Compared with the silicone rubber compound reinforced with fumed silica, the mechanical strength of the compound reinforced with precipitated silica is slightly lower, and the dielectric properties (especially the dielectric properties after being exposed to moisture) are poor, but The thermal aging resistance is better, and the cost of the compound rubber is low. When the requirements for the mechanical strength of the product are not high, precipitated silica can be used alone or in combination with fumed silica.
  Using treated white carbon black as a reinforcing agent, the mechanical strength of the rubber compound is high, the performance of the mixing and re-milling process is good, and the transparency of the vulcanized rubber is also good, so it is widely used in medical products. In addition, this kind of glue has good adhesion and excellent solubility, and can be used for adhesion and making glue.
  (2) Weak reinforcing filler
Weak reinforcing fillers can also be called inert fillers, which have little reinforcing effect on silicone rubber. They are generally not used alone in silicone rubber, but are used together with white carbon black to adjust the hardness of silicone rubber and improve the hardness of rubber. The process performance and the oil resistance and solvent resistance of the vulcanized rubber reduce the cost of the rubber. Commonly used weak reinforcing agents include diatomaceous earth, quartz powder, zinc oxide, ferric oxide, titanium dioxide, zirconium silicate and calcium carbonate.
  Amount and physical properties of commonly used reinforcing agents for silicone rubber.
  2.2.4 Additives
  (1) Structure control agent
  The silicone rubber compound reinforced with fumed silica will harden during storage, the plastic value will decrease, and the processing performance will be gradually lost. This phenomenon is called "structuring" effect. The compounding agent added to prevent and weaken this "structure" tendency is called "structure control agent". Structure control agents are usually low-molecular-weight organosilicon compounds containing hydroxyl or boron atoms, commonly used are diphenylsilanediol, methylphenyldiethoxysilane, tetramethylethylenedioxydimethylsilane, Low molecular hydroxy silicone oil and silazane, etc.
  (2) Heat-resistant additive
Adding some metal oxides or their salts and organic compounds of certain elements can greatly improve the hot air aging performance of silicone rubber, the most commonly used of which is ferric oxide, the general dosage is 3 to 5 parts; others such as manganese, Metal oxides such as zinc, nickel, and copper have similar effects. Adding a small amount (less than 1 part) of sprayed carbon black can also improve heat resistance. The effect of these additives can only be shown by hot air aging in the temperature range of 250-300 °C.
  (3) Colorant
  Silicone rubber commonly used colorants are as follows:
  Iron oxide (Fe2O3) Red
  Cadmium Yellow (cadmium dioxide) Yellow
  Chrome Green (Chromium Trioxide) Green
  Charcoal black Black
  Titanium dioxide (titanium dioxide) white
  ultramarine blue blue
  (4)Others
  When preparing silicone rubber sponge products, foaming agents must be added. Commonly used foaming agents for silicone rubber include N, N2 dinitrosopentamethylenetetramine and azodicarbonamide. Adding a small amount (generally less than 1 part) of tetrafluoroethylene powder to the rubber compound can improve the calendering process performance and film-forming property of the rubber compound, and increase the tear strength of the vulcanized rubber. Borate esters and boron-containing compounds make silicone rubber vulcanizates self-adhesive. When using fumed silica with a large specific surface area for reinforcement, add a small amount (3 to 5 parts, vinyl mass fraction is generally about 0.10) of high vinyl silicone oil. After the rubber is vulcanized, the tear resistance can be improved to 30～50kN·m-1. The formulations and physical properties of commonly used vinyl silicone rubber and fluororubber are shown in Table 5.

  2.3 Processing
Silicone rubber can be processed with ordinary rubber processing equipment, but attention should be paid to: ①The processing process should be kept clean, and no other rubber, oil or impurities should be mixed, otherwise it will affect the vulcanization and performance of silicone rubber; ②Silicone rubber products need to be processed in an oven. Second stage vulcanization with hot air for a longer time to improve the performance of vulcanizate.
  2.3.1 Mixing
  Silicone rubber raw rubber is relatively soft and has a certain degree of plasticity. It can be directly mixed with an open mixer or an internal mixer without plastication. There are several mixing methods:
  (1) Kneading machine mixing
  The roller speed ratio of the double-roll mill is preferably (1.2~1.4):1, with the fast roller behind, the higher speed ratio leads to faster mixing, and the lower speed ratio can make the film smooth. The roller must have cooling water, and the mixing temperature should be below 40°C to prevent scorching or volatilization loss of vulcanizing agent. When mixing, the roller distance is small (1-5mm) at the beginning, and then gradually increased. Feeding and operation sequence: raw rubber (wrapped roll) - reinforcing filler - structure control agent - heat-resistant additive - coloring agent - thin pass 5 times - blanking, oven heat treatment - remilling - vulcanizing agent - thin pass - parking Overnight - re-refinement - film out. The rubber can also not be heat-treated in an oven. After adding heat-resistant additives, add a vulcanizing agent and thin it out, park it overnight for re-milling, and then park it for several days for re-milling to produce tablets for use. The mixing time is 20-40 minutes.
  Silicone rubber is covered with slow rolls (front rolls) when it is added to the rubber mixer, and fast rolls (back rolls) are quickly wrapped during mixing. It must be able to operate on both sides during rubber mixing. Because the silicone rubber material is relatively soft, it can be operated with an ordinary putty knife during mixing. When it is thin, it cannot be pulled off like ordinary rubber, but it can be scraped off with a steel, nylon or wear-resistant plastic scraper.
   In order to facilitate cleaning and prevent lubricating oil from leaking into the rubber, movable rubber plates should be used. Fumed silica is easy to fly and is harmful to human body, so corresponding protective measures should be taken. If powdered peroxide is used directly during mixing, explosion-proof measures must be taken, and it is best to use paste peroxide.
  (2) Internal mixer mixing
  When mixing with a laboratory 2L internal mixer, the mixing time is 6 to 16 minutes. There is no special difficulty in mixing. When the loading coefficient is 0.74, the mixing can also be carried out normally with the Φ160mm open mill. The discharge temperature is related to the type of reinforcing filler. When weakly reinforcing filler and precipitated silica are used, the discharge temperature is below 50°C; when fumed silica is used, the discharge temperature is 70°C about.
  (3) Parking and re-milling of rubber
   After the silicone rubber compound is mixed, the parking time should not be less than 24 hours, so that various compounding agents (especially structure control agents) can fully work with the raw rubber. After parking, the rubber material becomes hard and the plastic value decreases, so it must be re-milled before use. The open mill is used for re-milling, and the roller distance is relatively large (3-5mm) at the beginning. At this time, the rubber material is hard and the surface is wrinkled, and it is wrapped on the front roller. With the prolongation of the re-milling time, the rubber material gradually becomes soft; slowly reduce the roll distance (0.25 ~ 0.5mm), and the rubber material is quickly wrapped on the rear roller. After the rubber material becomes soft and the surface is smooth and flat, the material can be unloaded and produced. Insufficient re-smelting will cause wrinkles on the film surface; excessive re-smelting will make the rubber material sticky and stick to the roller. The remelting temperature is generally controlled at room temperature.
  2.3.2 Extrusion
    Silicone rubber is generally relatively soft, with good extrusion effect and easy operation. It can extrude products of various shapes and sizes, and its processing equipment and tools are basically similar to ordinary rubber.
  The extruder generally uses a Φ30 or Φ65mm single-thread screw, and the length-to-diameter ratio is (10～12):1. Keep the temperature as low as possible during extrusion, preferably no more than 40°C, so cooling water must pass through the barrel and screw. For products with high quality requirements, an 80-140-mesh filter screen can be installed near the machine head to remove impurities in the rubber compound and improve extrusion quality.
  Silicone rubber extruded semi-finished products are soft and easy to deform, so they must be vulcanized immediately. The most commonly used method is hot air continuous vulcanization; the wire and cable industry usually uses high-pressure steam for continuous vulcanization. If continuous vulcanization is not possible after extrusion, in order to prevent deformation, it should be picked up by discs, drums or conveyor belts immediately after extrusion, and isolated with talcum powder to avoid mutual adhesion. If it is found that the rubber material is too soft to be extruded, 3 to 5 parts of fumed silica can be mixed into the rubber material.
For the rubber compound used for extrusion, the amount of vulcanizing agent should be increased appropriately compared with that of molded products. The extrusion speed of silicone rubber is lower than that of other rubbers. When it is required to achieve the same extrusion speed as other rubbers, a higher vulcanizing agent should be used. screw speed.
  2.3.3 Calendering
  Calender generally adopts vertical three-roll calender. When used to produce film, the middle roller is fixed, the speed ratio of the middle roller is faster than that of the upper roller, and the speed ratio is (1.1～1.4): 1, and the speed of the lower roller is the same as that of the middle roller. When the calender is running, the temperature of the upper roll is 50°C, the middle roll should be kept at room temperature, and the lower roll should be cooled with cooling water. The calendering speed is generally 60-300cm·min-1, and it should not be too fast. First, adjust the roller distance (middle and lower rollers) at a low speed to ensure a certain calendering thickness, and then increase to normal speed for continuous operation.
  When the three-roll calender is used for silicone rubber lamination and wiping, the fabric replaces the pad (polyester film) and passes between the middle and bottom rolls. The three-roll calender is only suitable for one-sided rubber coating, and the four-roll calender should be used for long-term production.
The rubber used for calendering must correctly control the degree of re-milling. It is best not to fully re-mill on the rubber mixer, in order to obtain enough re-milling during the calendering process, so as to prevent the rubber from being damaged due to excessive re-milling during the calendering process. sticky roll. The formula of the rubber compound also has a certain influence on the calendering, and the calendering process performance of the rubber compound with the reinforcing filler is better.
  2.3.4 Gluing
  Glue coating means that the silicone rubber glue is evenly distributed on the fabric by soaking or squeegeeing to improve the strength and flexibility of film products, make the fabric moisture-resistant, and manufacture high-temperature-resistant electrical insulation materials.
  (1) Mucilage preparation
  The vulcanizing agent of the silicone rubber compound used for making mortar is mostly dibenzoyl peroxide (BP), and the dosage is slightly larger than that of general model products. If the reinforcing filler is fumed silica, the dosage should not exceed 40 parts, and the dosage of the structure control agent should be appropriately increased, and volatile solvents such as toluene or xylene should be used.
   After fully remilling, the mixed rubber is thinly sliced, then cut into small pieces, soaked in a solvent overnight, and stirred with a mixer or mixer to make a glue with a solid mass fraction of 0.15 to 0.25. The glue should be stored in an environment below 40°C.
  (2) Fabric pretreatment
  The underlying fabric used for silicone rubber coating is generally glass cloth, nylon and polyester. Among them, glass cloth is widely used because of its good heat resistance, high strength and low hygroscopicity. During the drawing process, the film of glass fiber is coated with paraffin lubricant (accounting for 0.2% to 0.5% of the fabric mass), which is volatile at vulcanization temperature and affects the combination of rubber and fabric, so it must be dewaxed before glue coating. The heat deformation of nylon and polyester is large, which affects the combination of rubber and fabric. Therefore, heat setting must be carried out before glue coating, that is, the fabric is subjected to short-term heat treatment under a certain draft. The treatment temperature of nylon is 170-175 ° C. , Polyester treatment temperature is 215 ~ 220 ℃.
  (3) Glue
  After the fabric is pretreated, it needs to be treated with surface adhesive, and then glue can be applied. The adhesive is a solution consisting of alkoxysilane, borate ester, vulcanizing agent and solvent (ethyl acetate or ethanol).
  After the surface treatment of the fabric with the adhesive, the glue can be evenly coated on the fabric with a glue applicator, and then dried and vulcanized. Vulcanization is generally carried out in two stages: the temperature of the first stage is 120-130°C, and the temperature of the second stage is 230°C. The thickness of the coating can be controlled by changing the viscosity of the glue or adjusting the speed at which the fabric passes through the gluing tank.
2.3.5 Bonding
Silicone rubber can be bonded to many materials, including metals, plastics, ceramics, fibers, silicone rubber itself and some other rubbers. Using an adhesive that can be vulcanized simultaneously with the silicone rubber itself can achieve the best adhesion between the silicone rubber and the coating. combine.
   Adhesives are generally used for bonding between silicone rubber vulcanizates. Examples of common adhesive formulations are as follows: vinyl silicone rubber   100; fumed silica   35; ferric oxide   5; n-butyl borate   3; paste vulcanizing agent DCBP   3.
  2.3.6 Vulcanization
The silicone rubber vulcanization process is not completed at one time, but is carried out in two stages. The rubber material is heated and shaped under pressure (such as molded vulcanization, vulcanization tank direct steam vulcanization, etc.) or under normal pressure (such as hot air continuous vulcanization). It is called one-stage vulcanization (or setting vulcanization); it is vulcanized at high temperature in an oven to further stabilize the physical properties of the vulcanized rubber, which is called two-stage vulcanization (or post-vulcanization).
  (1) One stage vulcanization
  ①Model products are vulcanized. Flat vulcanization, transfer molding vulcanization and injection vulcanization are available. The curing conditions are shown in Table 6.
  When silicone rubber products are vulcanized, generally do not use a release agent, and should be loaded, molded, and pressurized quickly, otherwise it is easy to scorch, especially the rubber containing vulcanizing agents BP and DCBP. Transfer molding vulcanization is a widely used process for processing silicone rubber materials. Compared with flat vulcanization with a single hole in each mold, it has the advantage of short processing cycle and can vulcanize complex rubber parts especially with inserts and pins. . Compared with injection pressure vulcanization, the equipment cost is lower.
  Injection vulcanization molded products can increase labor productivity, reduce labor intensity, and at the same time reduce the amount of peroxide, improve the tear resistance of the product, and improve the compression set performance, but the shrinkage rate of the product is relatively large.
  ②Vulcanization of extruded products. Methods such as steam pressurized vulcanization, hot air continuous vulcanization, liquid vulcanization tank continuous vulcanization, drum vulcanization and radiation vulcanization can be used. The first three methods are more commonly used.
  (2) Two-stage vulcanization
  After a period of vulcanization of silicone rubber products, some low-molecular substances exist in the vulcanized rubber, which affects the performance of the product. For example, the rubber compound using a general-purpose vulcanizing agent (such as vulcanizing agent BP or DCBP), after a period of vulcanization, the amount of acidic substances decomposed by the vulcanizing agent will increase the pore size of the sponge pores and reduce the density; increase the vulcanizing agent. The amount used will reduce the pore size of the sponge pores, increase the density, and produce thicker pore walls. In addition, the vulcanization temperature also has a great influence on the porosity of the sponge.
  2.3.7 Foaming
   Add foaming agent to the silicone rubber compound, and then heat and vulcanize under pressure to make the rubber foam, and the silicone rubber sponge can be obtained. But must pay attention to the following issues.
  (1) The foaming agent whose decomposition products do not affect the heat resistance of silicone rubber should be selected. Generally, organic foaming agents such as foaming agent BN, urea, etc. are used, and their decomposition products are removed in the two-stage vulcanization.
  (2) Properly control the amount of vulcanizing agent and foaming agent so that the foaming speed matches the vulcanization speed. Increasing the amount of foaming agent will increase the pore size of the sponge pores and reduce the density; increasing the amount of vulcanizing agent will reduce the pore size of the sponge pores, increase the density, and produce thicker pore walls. In addition, the vulcanization temperature also has a great influence on the porosity of the sponge.
  (3) Appropriate use of vulcanizing agent can better control the porosity and density of the sponge. Usually the combination of vulcanizing agent DBPMH and BP or TBPB and DCBP is better.
  (4) No. 2 fumed silica or No. 2 fumed silica and precipitated silica can be used as a reinforcing agent. The plasticity value of the rubber should be strictly controlled. If the plasticity value is too large, it is easy to cause excessive expansion when the pores are formed, forming a rough open-pore structure, and even many pores are broken; if the plasticity value is too small, the pores will be insufficient and the product will be hard. It is easier to control the plasticity value of the rubber with weak reinforcing filler, and the remilled rubber is best used on the same day.
  (5) The blowing agent should be evenly dispersed in the rubber compound. Generally, the foaming agent particles are easy to agglomerate and difficult to disperse. The ratio of raw rubber/foaming agent masterbatch (1:1) can be made first, and then mixed to improve the dispersion effect.
  (6) When using the rubber compound of the molding process, care should be taken to remove the air bubbles in the rubber compound to prevent damage to the sponge structure.
  The rubber material used for molding sponge products should be cut according to the mold specifications after being kneaded and produced, and the surface should be coated with a release agent for vulcanization in the mold. Talc powder is generally used as a release agent, and white carbon black can also be used.
There are two methods for the vulcanization of silicone rubber sponge model products: one is a one-step method, that is, the rubber material is made into a sponge of a certain shape and size in the mold once, and the second-stage vulcanization no longer makes holes; the other is a two-step method. The method is to vulcanize the rubber material in the mold for a short time to make it initially porosity and just form a layer of surface, and then put it in an oven to porosity into a certain shape and size. For sponge sheets, the vulcanization time of the former is usually 15-20 minutes, and the vulcanization time of the latter is generally within 5 minutes.



3. Room temperature vulcanized silicone rubber

  Room temperature vulcanized (also known as condensation vulcanized) silicone rubber (room temperature vulcanized silicon rubber), refers to the silicone rubber that can be vulcanized at room temperature without heating. Its molecular structure is characterized by active functional groups such as hydroxyl or acetoxy at both ends of the main chain of the molecule. Under certain conditions, these functional groups undergo condensation reactions to form a cross-linked structure and become an elastomer.
  3.1 Species and characteristics
  RTV silicone rubber has a low relative molecular weight and is usually a viscous liquid. According to its vulcanization mechanism and application process, it can be divided into one-component RTV silicone rubber and two-component RTV silicone rubber.
  3.1.1 One-component RTV silicone rubber
One-component room temperature vulcanized silicone rubber is mixed with hydroxyl-terminated low-molecular-weight silicone rubber and reinforcing agent, dried and dehydrated, and then added with a cross-linking agent (containing a hydrolyzable polyfunctional siloxane). At this time, mixing The glue has become a polymer containing multi-functional end groups. It is packaged in a closed container and is in contact with moisture in the air when it is extruded, so that the functional groups in the rubber are hydrolyzed to form unstable hydroxyl groups, and then condensed and cross-linked to form an elastomer.
  One-component RTV silicone rubber can be divided into deacidification type and non-deacidification type according to the type of crosslinking agent. The former is widely used, and the crosslinking agent used is acetoxysiloxane (such as methyltriacetoxysilane or methoxytriacetoxysilane), which releases acetic acid as a by-product during the vulcanization process, which is corrosive to metals. effect. There are many types of non-deacidification condensation vulcanization types, among which there is a dealcoholization condensation vulcanization type that uses alkoxy (such as methyltriethoxysilane) as a crosslinking agent. This reaction only depends on the action of moisture in the air, and the vulcanization is slow. Alkyl titanate vulcanization accelerators need to be added, and alcohols are released during vulcanization, which has no corrosive effect and is most suitable for electrical insulation products. In addition, there are deamination condensation vulcanization using silazane as a crosslinking agent (organic amine is released during vulcanization, which has an odor and is corrosive to copper) and deoxime vulcanization using acetone oxime and butanone oxime as a crosslinking agent, Deamidation vulcanization and deketone vulcanization type with fast vulcanization speed, etc.
According to the modulus of the product, it can be divided into low modulus (deamidation type), medium modulus (suitable for building sealant) and high modulus (dealcoholization type); according to the practical performance of the product, it can be divided into general type and special type. There are two types of products, and the special types include flame-retardant, surface-coatable, mildew-proof and pollution-resistant.
  One-component room temperature vulcanized silicone rubber has good adhesion to various materials (such as metal, glass, ceramics, etc.), and is particularly convenient to use. Generally, no weighing, stirring, and defoaming operations are required. Vulcanization starts from the surface and proceeds gradually to the depth. One-component room temperature vulcanizing silicone rubber is mainly used as an adhesive and as a sealing gap material in the construction industry.
  3.1.2 Two-component RTV silicone rubber
The vulcanization of this type of rubber is carried out by the hydroxyl group of the raw rubber under the action of a catalyst (organotin salt, such as dibutyltin dilaurate, stannous octoate, etc.) It can be divided into dealcohol condensation vulcanization, dehydrogenation condensation vulcanization, dehydration condensation vulcanization and dehydroxylamine condensation vulcanization, among which the dealcoholization type is the most common. Two-component room temperature vulcanized silicone rubber is usually mixed with raw rubber, filler and crosslinking agent as one component, raw rubber, filler and catalyst as another component, and the two components are mixed by metering when used. The vulcanization time of the two-component mainly depends on the amount of catalyst used, the larger the amount, the faster the vulcanization. In addition, the higher the ambient temperature, the faster the vulcanization; there is no internal stress, no shrinkage, no expansion during vulcanization, and the condensation reaction occurs simultaneously inside and on the surface during vulcanization, so there is no difficulty in deep vulcanization of thick products. It has no adhesion to other materials, and it needs to use surface treatment agent as primer when it is bonded to other materials. Two-component room temperature vulcanized silicone rubber can be used as molding and potting materials.
   Room temperature vulcanizing silicone rubber can be made into different viscosities according to the application requirements, generally there are fluid grade, medium consistency grade and consistency grade. Fluid-grade rubber is fluid, suitable for pouring and spray gun operation; if lower viscosity rubber is required (when pouring into narrow gaps), methyltriethoxysilane or its oligomers can be infiltrated into the rubber, and can also be used Dilute with methyl silicone oil 201. The viscosity of the medium-consistency glue is just enough to flow fully without dripping down completely, and a product with a smooth surface can be obtained, which is suitable for gluing and dipping. Consistency grade rubber has a putty-like consistency, which can be operated by hand, scraper or caulking knife, and can also be applied to various fabrics by calendering.
In recent years, room temperature vulcanized silicone rubber has been very active in improving research. With the expansion of application areas, there have been high-adhesion, high-strength, high-elongation, low-modulus, flame-retardant, oil-resistant, and fast-curing types. New varieties.
  3.2 Coordination
3.2.1 Vulcanizing agent
One-component room temperature vulcanized silicone rubber mainly relies on moisture in the air for crosslinking reaction, and the rubber material should be stored in airtight storage before use; in two-component room temperature vulcanized silicone rubber (except addition reaction system), silicone rubber containing terminal hydroxyl Commonly used vulcanizing agents are silicate (such as ethyl orthosilicate) and titanate (such as n-butyl titanate), etc.; catalysts mainly use organotin salts, such as dibutyltin dilaurate, stannous octoate, etc. Wait. Adjusting the amount of vulcanizing agent and catalyst can change the vulcanization rate. The amount of vulcanizing agent is generally 1 to 10 parts; the amount of catalyst is generally 0.5 to 5 parts. Tests have shown that the catalyst dibutyltin dilaurate has a corrosive effect on copper. Dibutyltin oxide [(C4H9)2SnO] or dioctyltin oxide [(C8H17)2SnO] and ethyl orthosilicate [Si(OC2H5)4] are used to corrode copper. The reflux product was used as the vulcanization system, and the vulcanized rubber was stored in contact with copper for 1 year without any corrosion.
  3.2.2 Reinforcing filler
   Room temperature vulcanized silicone rubber must also be added with white carbon black as a reinforcing agent, otherwise the strength will be lower than that of heat vulcanized type. Its matching method is the same as that of heat vulcanization type.
  3.3 Processing
  3.3.1 One-component RTV silicone rubber
  One-component RTV silicone rubber must be stored in an airtight container isolated from water and air, and can generally be used within a few months. There is no need to add a catalyst when using it, just squeeze the rubber out of the airtight container to contact the air, so it is very convenient to use; it can be processed for a short time by molding, extrusion or other methods, and then exposed to the air for a certain period of time. The vulcanized material becomes an elastomer.
  One-component RTV silicone rubber is an adhesive.
   When used for bonding, no surface treatment agent is required, that is, it has good bonding performance to glass, ceramics, metal, wood, plastic and vulcanized silicone rubber. Because this kind of rubber is vulcanized from the surface in contact with moisture in the air at room temperature, and then gradually vulcanized inward through the diffusion of moisture. It takes a long time for the internal vulcanization of products that are too thick, so there is a certain limit to the thickness of the product (or the depth of the seal). The thickness should generally not exceed 10mm.
  Multiple construction methods can be used if it is required to exceed 10mm.
  The humidity of the air has a decisive influence on the vulcanization speed, the greater the humidity, the faster the vulcanization. When the climate is relatively dry and the humidity is low, water can be sprayed to increase the moisture in the air to achieve the actual required vulcanization speed.
  3.3.2 Two-component RTV silicone rubber
   Two-component room temperature vulcanized silicone rubber should be stored in a cool and dry place, away from direct sunlight. If the storage time exceeds 4 months, it should be inspected, and the performance can not be continued before use.
  (1) Addition of catalyst
Add the catalyst to the liquid or medium-consistency room temperature vulcanized silicone rubber compound, and stir it by hand to disperse it. After mixing evenly, put the compound in a closed container to vacuum, and maintain it at 0.67-2.67kPa for 3-5 minutes. to remove air bubbles. When thick grade rubber is used, the catalyst can be mixed into the rubber compound by rubber mixer, kneader or pulper. Catalyst can be measured by weighing method or volumetric method. Since the amount of catalyst is generally only 0.5 to 5 parts, attention should be paid to mixing evenly. Room temperature vulcanized silicone rubber will be gradually crosslinked and cured after being mixed with the catalyst, so it should be prepared according to the required amount. If there is any remaining, it can be stored in a low temperature place (such as in the refrigerator) to prolong the use time.
  (2) Coating of fabrics
   Room temperature vulcanized silicone rubber can be added with catalyst according to the following method, and coated on various fabrics by gluing or calendering, and it does not need to be diluted with solvent to make glue.
The method of adding the catalyst during gluing is as follows: ①adding to the sizing material before gluing; ②adding to the other side of the rubberized fabric, allowing the catalyst to permeate through the cloth layer to cross-link the rubber; ③adding to the fabric to be coated before gluing. glued surface. The first method limits the operating time to be within the pot life of the rubber, otherwise it will be cured and cannot be used; the operating time of the latter two methods is not limited by the pot life of the rubber.
  (3) Dissolution of glue
  Common aromatic solvents, such as toluene or xylene, can be used to dissolve the rubber to prepare room temperature vulcanized dimethyl silicone rubber glue.
  This glue can be dip-coated on the fabric by the universal dipping method.
  (4) Bonding
   Room temperature vulcanized silicone rubber can be made into adhesives to bond various materials. When used for the bonding between various vulcanized silicone rubbers and metals or non-metals (such as glass, FRP, polyethylene, polyester, etc.), the adhesive is prepared by two groups of A and B. Group A is paste-like room temperature vulcanized silicone rubber containing an appropriate amount of reinforcing filler, a small amount of titanium dioxide and iron oxide. ) and a catalyst (dibutyltin dilaurate, etc.). Mix the two components thoroughly and evenly according to the mass ratio of 9:1 before use. The active period of the adhesive is 40min (20°C, 65% relative humidity). If you want to prolong the active period, you can reduce the amount of catalyst, but the amount should not be less than 1 part, otherwise the adhesion will be deteriorated. Excessive amount of catalyst will reduce the heat resistance of vulcanizate.
  The bonding process is completed at room temperature with or without pressure. The surface of the adherend should remove dirt and clean it with acetone or toluene; then apply a layer of surface treatment agent on the metal or non-metal surface, and dry it at room temperature for 1 to 2 hours (the specific time depends on the temperature and humidity at that time. After setting), the adhesive can be applied for bonding. The surface treated with surface treatment agent will not affect the adhesion effect when glue is applied within 1 week.
  (5) Vulcanization
  The vulcanization of two-component RTV silicone rubber is achieved by adding a liquid catalyst. The curing time varies with the amount of vulcanizing agent and catalyst, from ten minutes to 24 hours; raising or lowering the temperature can also shorten or prolong the curing time.
Room temperature vulcanized silicone rubber products generally do not need to be vulcanized in an oven, but since a small amount of volatile substances (such as ethanol) will be produced during the vulcanization process, the volatile substances are not easy to escape when the thick product is vulcanized. Secondary vulcanization method, that is, pouring or filling each time with a thickness of 10-15mm, and placing it for 30 minutes after losing fluidity, and then continuing to pour or fill. If the service temperature of thick products is higher than 150°C, it is best to heat-treat at 100°C after room temperature vulcanization to drive off volatile substances and improve the heat resistance of the product.
4. Addition vulcanized silicone rubber
Addition vulcanization silicone rubber refers to the addition reaction of dimethylsiloxane with a vinyl end group with a functionality of 2 and a multifunctional hydrogen-containing silane under the catalysis of a platinum compound, resulting in chain growth. A silicone rubber that is cross-linked with chains. Raw rubber is generally liquid, with a degree of polymerization of more than 1000, and is usually called liquid silicone rubber (LSR).
  4.1 Features
  In addition to maintaining many typical characteristics of silicone rubber, liquid silicone rubber products have the following characteristics:
  (1) Clean and stable
Liquid silicone rubber does not contain solvents and moisture, and has no pollution to the environment; the rubber is supplied in two components, all of which have been filtered and exhausted; the two-component mixture can be stored at normal room temperature for more than 24 hours, and it can even be stored for 2 days when cooled. Above, no further cleaning is required.
  (2) The process is simple and fast
  ①The two-component rubber is mixed at 1:1, and the batching process is simple; ②For molded products, from batching to finished product, in principle, it can be said to be one step, which simplifies the process; ③The vulcanization speed is fast. For example, for molded products made of porous molds (about 7g rubber per hole), the vulcanization cycle is 20-30s, which is about 1/12-1/20 of ordinary rubber (vulcanization cycle 4-10min); ④ unless The product is required to have extremely low compression set resistance, and post-vulcanization is generally not required; ⑤The shrinkage rate is low, generally below a few per thousand; ⑥The coloring process of the product is simple; ⑦In general, the finished product does not need to be trimmed.
  (3) Energy saving
   According to estimates from DowCorning, due to the simplification of the process and the change of the vulcanization method, the energy consumption can be reduced by about 75%.
  (4) Low cost
  The processing and production of liquid silicone rubber generally requires only one person. According to DowCorning's calculation, each piece of molded product of about 6g can reduce the cost by 42% due to the reduction of manpower, not including the above two factors of energy consumption reduction and production space reduction.
  (5) Less fixed investment
  No matter molded or extruded products, light machinery can replace heavy machinery, and the workshop area can be greatly reduced.
  4.2 Fitting and processing
  4.2.1 Coordination
  The compounding of liquid silicone rubber is very simple. It is composed of raw rubber, filler, crosslinking agent, catalyst, reaction inhibitor and necessary additives. The following is a brief description.
   (1) Raw rubber. Polysiloxane containing vinyl end groups with a functionality of 2 (or more than 2) is used, and the degree of polymerization is more than 1000. The structural formula is:
  (2) Filler. The filler is mainly fumed white carbon black, and trimethyl-terminated polysiloxane is used as a surface treatment agent. The addition of surface-treated fumed silica, in addition to reinforcement, can also increase the viscosity. The increase in viscosity is relatively stable and less affected by time.
  (3) Cross-linking agent. The crosslinking agent is actually the main component of the other component of the two-component liquid silicone rubber, which is composed of polysiloxane with a hydrogen terminal functionality of at least 2, which undergoes an addition reaction with the vinyl group to form Cross-linked structure, and the rubber compound is cured. Its dosage should not be too large, otherwise its heat resistance will be reduced.
  (4) Catalyst. The catalyst is mainly a complex of organoplatinum, and the newer development is to introduce a coordination compound of low molecular weight polysiloxane containing vinyl, and its dosage is extremely small.
  (5) Reaction inhibitor. Reaction inhibitors are used to adjust the active period and prolong storage stability. It is generally an alkyne compound, and compounds containing amines, tin, phosphorus, etc. can also be used.
  (6) Other additives. Other additives include colorants, release agents, and the like.
  The various compounding agents mentioned above, including raw rubber, are supplied in two components. When used, the two components are mixed and vulcanized under certain conditions. This is the same as the preparation of condensation vulcanized silicone rubber.
  4.2.2 Processing
  (1) Measuring fit
  Although appropriate reaction inhibitors have been added to the compounded rubber, if the storage conditions are not proper, it may still cause part of the rubber to vulcanize at room temperature. Therefore, the rubber compound is usually divided into two compounding components. One contains a catalyst and the other contains a crosslinking agent, with a ratio of 1:1.
  (2) Vulcanization
  The vulcanization reaction of liquid silicone rubber belongs to the addition type, and the reaction formula is as follows:
  A part of the vinyl group in the side chain may undergo a cross-linking reaction similar to that of vinyl silicone rubber.
   Liquid silicone rubber can be vulcanized at a high speed at high temperature without causing scorch. The biggest characteristic of liquid silicone rubber vulcanization is high temperature and fast. According to the data of DowCorning, for a porous mold with a 7g part, in continuous operation, the entire mold entry, vulcanization and mold removal time only takes 20-30s, which is 1/10-1/20 of the so-called rapid vulcanization of general rubber. .
  (3) Injection molding
  The injection molding of liquid silicone rubber is different from ordinary silicone rubber and plastic. Compared with other rubber injection molding, liquid silicone rubber does not need to be plasticized before injection molding, the viscosity is much lower, and the vulcanization is extremely fast. Compared with plastics, the viscosity of liquid silicone rubber is similar to the "melt" viscosity of plastics, but it is thermoset rather than thermoplastic.
  From the process point of view, liquid silicone rubber is mainly used in injection pressure, extrusion and coating. The main extruded products are wires and cables; the coated products are silicon rubber cloth with various materials as the backing or the film reinforced with textiles; the injection pressure is various model products. Due to its good fluidity and high strength, it is more suitable for making molds and pouring antique artworks. Because there is no overflow of by-products such as cross-linking agents during vulcanization, the purity of raw rubber is very high and the production process is clean and hygienic. Liquid silicone rubber is especially suitable for the manufacture of medical products with high requirements.
5. Regeneration of silicone rubber
  Silicone rubber has many excellent properties, but the price is high, so the recycling of waste rubber has economic significance for saving raw materials and reducing production costs.
  Silicone rubber regeneration process includes selection, cutting, cracking, refining and batching. That is, the waste rubber is first cleaned to remove garbage and impurities, then selected and classified, cut into small pieces, and sent to cracking. There are many cracking methods, including mechanical rolling cracking, direct steam thermal cracking, dry thermal cracking and chemical cracking. Among them, the former two are more commonly used and simple, which are mainly introduced as follows.
  (1) Mechanical rolling cracking
  Mechanical rolling cracking can be carried out on ordinary rolling mills. The waste rubber that is about to be cut into small pieces is put on the rubber mixing machine for rolling, and the roll distance should be larger at the beginning, and the roll distance should be reduced after the rubber material is covered with rolls. When it is tied finely, it can be thinned. After gradually forming a continuous strip shape, it can be stored in the next piece. The disadvantage of this method is that the waste rubber using fumed white carbon black is not easy to crush, and the rolling time is long. In order to shorten the rolling time and improve the rolling regeneration efficiency and reclaimed rubber quality, precipitated silica can be added appropriately (about 10%) during the rolling process to crush and mix different soft and hard rubber materials to shorten the regeneration time.
  (2) Direct steam pyrolysis
  Direct steam thermal cracking is to put small pieces of waste rubber into a steam vulcanization tank with a pressure of about 0.5MPa (temperature 150-160°C) for 3-4 hours. If the hardness of waste rubber is high, the processing time can be extended appropriately, but it should not be too long, so as to avoid excessive degradation of waste rubber and deterioration of performance. The heat-treated waste rubber is fully crushed on the rubber mixing machine, and then thinly passed out after being wrapped with rolls. Generally, a small amount of alcohol can be added during crushing to increase mechanical friction and improve rolling efficiency. In case of sticky rollers, some white carbon black or other inorganic fillers can be added. The reclaimed rubber obtained by this method has uniform quality and high plasticity. The disadvantage is that the processing time is long and it is difficult to remelt after storage. If a structure control agent, such as diphenylsilanediol or hydroxyl silicone oil, is added to the compound, it is easier to remill.
  The performance of reclaimed rubber is generally worse than that of raw rubber, with lower tensile strength, elongation at break, tear strength and compression set resistance, and higher hardness. Its tensile strength is generally 2.9 to 3.8
  MPa, the elongation at break is 100%～150%. With the increase of filler content, the hardness increases and the elongation at break decreases. When reclaimed rubber is used alone, in addition to adding reinforcing agent white carbon black, weak reinforcing fillers (such as titanium dioxide or zinc oxide 50-100 parts) can also be added to reduce costs. An appropriate amount of vulcanizing agent (about 1 times higher than that of ordinary raw rubber) should be added to keep the vulcanized rubber at a certain hardness. When the raw rubber is mixed with 30%-50% reclaimed rubber, the physical properties of the vulcanized rubber do not change much (only the tensile strength and elongation at break are slightly lower), and it can withstand high temperature aging at 250-300 °C. The highest blending amount of reclaimed rubber can reach 70% to 80%. Reclaimed rubber can be used alone to make rubber sheets, and should not be used alone as extruded products. If the reclaimed rubber is filtered and used together with raw rubber, it can be used to make extruded products.

6. Application
Silicone rubber has unique comprehensive properties, especially the biocompatibility of silicone rubber is a key characteristic. It has been successfully used in occasions where other rubbers are ineffective, solved many technical problems, and met the needs of modern industry and society. various needs of daily life. Like other rubbers, silicone rubber can be processed into various profiles, rubber hoses, rubber strips, rubber rollers, rubber cloths and other products; room temperature vulcanized silicone rubber can be used for large-scale construction on site and is easy to use.
  (1) Automobile industry
  The application of silicone rubber in the automobile industry is growing rapidly. Silicone rubber (especially silicone rubber with various characteristics) can resist the erosion of fuel and lubricating oil, improve the performance of various parts of automobiles, and reduce maintenance costs. It can be used in automobile ignition wires, spark plug protection covers, heating and radiator hoses, muffler linings, battery connectors, and fuel pumps made of fluorosilicone rubber. With the development of vehicle electronics and electrification, room temperature vulcanized silicone rubber is widely used in potting materials for electronic parts, electrical assemblies, windshields, seals around the car body, and adhesive sealants for reflectors.
  (2) Electronics and electrical industry
  Electronic and electrical industries are the earliest fields where silicone rubber is used as an insulating material and has a large demand. Silicone rubber is mainly used in TV anode covers, high voltage protective covers, high voltage lead wires, refrigerator defroster wires, power or signal transmission wires and cables, etc. Insulators made of silicone rubber will replace ceramic products and are widely used in power transmission lines, especially ultra-high voltage lines. Conductive silicone rubber is used in electrical contacts and liquid crystal display contacts of electronic computers, telephones and other instruments. Wires and cables made of flame-retardant and radiation-resistant silicone rubber are widely used in atomic power stations. Silicone rubber heating sheets and heating belts are used to control the working temperature of various precision instruments and oil pipelines, and are used as heating blankets for physiotherapy heat compresses in medical treatment. Room temperature vulcanizing silicone rubber can be used as a potting material for waterproof, moisture-proof and shock-proof.
  Silicone rubber has been widely used in pump seals of dishwashers and washing machines due to its heat-resistant detergent properties. Silicone rubber is ideal for use as gaskets on appliances such as coffee pots, electric fryers and steam irons. The ear and head pads of the stereo earphones are made of silicone rubber, which can eliminate external noise and is soft and comfortable.
  (3) Aerospace industry
  Silicone rubber is an indispensable high-performance material in the aerospace industry. It can withstand the ultra-cold space and the scorching heat of returning to the atmosphere, prolong the life of aircraft parts, reduce maintenance costs, and reduce accidents. Mainly used for aircraft body cavity seals, electrical connectors, sealed switches, dustproof and waterproof covers, gasket gaskets, "O" rings for jet engines and hydraulic devices, oxygen masks, regulating diaphragms, hot air ducts and Radar wireless shock absorbers, etc. Burn-resistant silicone rubber is suitable for rocket fuel valves, power source cables and rocket silo cover coatings, so as not to be burned by rocket jets. Room temperature vulcanized silicone rubber can be used as the airtight seal of the body, window frame seal and shockproof, moisture-proof potting material.
  (4) Construction Industry
  Silicone rubber has good weather resistance and workability, and has been widely used as an adhesive sealant in the construction industry, surpassing other types of sealants. In recent years, a low-modulus high-elongation two-component sealant has been developed, which is used for large components such as concrete prefabricated parts and curtain walls with large joint movement. RTV silicone rubber is also used for sealing joints of asbestos cement boards, bathroom tile joints and toilet utensils. With the decline of rubber prices in the future, the scope of application will be further expanded, such as replacing asphalt and neoprene in the application of road joints. High-temperature vulcanized silicone rubber sponge strips are used as sealing strips for doors and windows of buildings.
  (5) Medical field
Silicone rubber has good biocompatibility, small reaction to the body, stable performance, low hemagglutination, can withstand high temperature and high pressure for multiple cooking, and can be processed into products of various shapes, such as films, sponges, and air bags. One of the most widely used materials in medicine.
  (6)Other fields
  In addition to the various applications mentioned above, the application of silicone rubber in the fields of textiles, printing, machinery, plastics, chemistry, papermaking, food and cosmetics has also increased significantly. Typical products are rollers made of silicone rubber for heat setting of fabrics, plastics, embossing, calendering and fusing in copiers. Air-conditioning windows made of silicone rubber films have been used to store fruits and vegetables. In addition, silicone rubber has also achieved good results as a sealing material in textile high-temperature equipment and equipment with high concentrations of alkali, sodium hypochlorite and hydrogen peroxide.

7. New progress in silicone rubber research
  Silicone rubber has excellent high and low temperature resistance, ozone resistance, oxygen resistance, light resistance and weathering resistance, excellent electrical insulation properties, special surface properties, physiological inertia and high air permeability, and has a wide range of applications. However, the tensile strength and tear strength of silicone rubber are low, the acid and alkali resistance is poor, and the processing performance is also poor when manufacturing complex products. .
  (1) By using the copolymerization (blending) of silicone and other monomers or polymers, a new polymer (blend) is obtained. For example, the block copolymer of silicone and polycarbonate can be used as a selective breathable membrane; the characteristics of the blend of silicone and ethylene-propylene rubber are between silicone rubber and ethylene-propylene rubber; blending silicone rubber with EVA, The prepared blend has excellent physical properties, electrical properties, high-temperature aging resistance and thermal shrinkage, and can be endowed with excellent flame-retardant properties through proper matching. Using high temperature resistant silicone rubber and high tensile strength PMMA to manufacture interpenetrating polymer grids, the strength of silicone rubber and the heat resistance of PMMA are improved, and polydimethylsiloxane (PDMS)/polystyrene is synthesized (PS) interpenetrating polymer network, which improves the mechanical and elastic properties of the silicone network.
  (2) By improving the matching technology, develop new varieties with high strength, high tear resistance, low compression deformation, etc., as well as high-temperature resistance, ultra-low temperature resistance, flame-retardant, heat-conducting and heat-shrinkable silicone rubber.
  Pure silicone rubber does not have the mechanical strength required for most applications and therefore requires reinforcement. The traditional reinforcement method is to add reinforcing fillers before the silicone rubber is cured, and then perform mixing, vulcanization and other processes. However, it is time-consuming, energy-consuming, pollutes the environment, and it is difficult to control the degree of dispersion and aggregation of fillers in the rubber compound. Studies have shown that fillers such as precipitated silica, precipitated titanium dioxide and ferrite, zeolite and alumina have a good reinforcing effect on silicone rubber.
(3) In terms of improving processing performance, silicone rubber without secondary vulcanization, granular silicone rubber (also known as powder rubber) and catalytic addition between vinyl-containing polysiloxane and hydrogen-containing siloxane have been developed. In response, a liquid injection molding system for silicone rubber was developed. Using the addition type two-component system, a putty-type mold-making silicone rubber with good thixotropy, excellent construction performance and convenient use is developed.