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Type, formula and application field of silicone rubber

* Source: * Author: admin * Publication time: 2021/09/16 13:59:43 * Browse: 18

Silicone rubber refers to the rubber whose main chain is composed of silicon and oxygen atoms alternately, and the silicon atoms are usually connected with two organic groups. Silicone rubber has good low temperature resistance and can still work at - 55 ℃. After introducing phenyl, it can reach - 73 ℃. The heat resistance of silicone rubber is also outstanding. It can work for a long time at 180 ℃, and it can withstand several weeks or more at a temperature slightly higher than 200 ℃. It still has elasticity, and can withstand high temperatures above 300 ℃ instantaneously. Silicone rubber has good air permeability, and the oxygen transmittance is the highest in synthetic polymers. As one of the most important forms of silicone, silicone rubber can maintain elasticity for a long time in a certain temperature range, has excellent electrical properties and chemical stability, and can play the role of water resistance, ozone resistance and aging resistance. It plays an extremely important role in industry and civil use.

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Classification of silicone rubber


Silicone rubber can be divided into thermal vulcanization type (high temperature vulcanized silica gel HTV) and room temperature vulcanization type (RTV) according to its vulcanization mechanism. At present, there are about 50 room temperature silicone rubber production enterprises in China, with a production capacity of 1.092 million tons / year and an output of about 607900 tons.



Silicone rubber properties


(1) High and low temperature resistance

 Among all rubbers, silicone rubber has the widest working temperature range (- 100 ~ 350 ℃). For example, properly matched vinyl silicone rubber or low phenyl silicone rubber can still maintain elasticity after aging in hot air at 250 ℃ for thousands of hours or 300 ℃ for hundreds of hours; The low phenyl silicone rubber vulcanizate can still maintain elasticity after aging in hot air at 350 ℃ for tens of hours. Its glass transition temperature is - 140 ℃, and its vulcanizate still has elasticity at - 70 ~ 100 ℃. When silicone rubber is used as heat-resistant coating on the inner wall of rocket nozzle, it can withstand thousands of degrees of instantaneous high temperature.


(2) Ozone aging resistance, oxygen aging resistance, light aging resistance and weather aging resistance

The properties of silicone rubber vulcanizate did not change significantly after being exposed to the sun for several years in a free state.


(3) Electrical insulation performance

The electrical insulation performance of silicone rubber vulcanizate changes little when affected with moisture, frequency change or temperature rise, and the silicon dioxide generated after combustion 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 filler, so it is not easy to burn during arc discharge, and it is very reliable in high-voltage occasions. Its corona resistance and arc resistance are excellent. Its corona resistance life is 1000 times that of PTFE and 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. It has low moisture absorption. The water absorption rate is only about 1% when immersed in water for a long time. It has no decline in physical properties and good mildew resistance. It does not bond with many materials and can play an isolation role. Silicone rubber is tasteless, non-toxic, has no adverse effect on human body, has slight reaction with body tissue, and has excellent physiological inertia and physiological aging.


(5) High air permeability

Compared with other polymer materials, silicone rubber has good air permeability, and its permeability to nitrogen, oxygen and air at room temperature is 30 ~ 40 times higher than Nr; It is selective for gas penetration, for example, its permeability to carbon dioxide is about 5 times that of oxygen.


(6) Biomedical properties

Silicone rubber has excellent biomedical properties due to its molecular structure, which has been proved by the successful application of a large number of animal and human experiments.



Hot vulcanized silicone rubber


1. Cooperate

The compounding agent of hot vulcanized silicone rubber mainly includes reinforcing agent, vulcanizing agent and some special additives. Generally, it only needs 5 ~ 6 components to form a practical formula.


The following points should be considered in the formulation design:

(1) Silicone rubber is a raw rubber with high saturation, which can not be vulcanized with sulfur. Organic peroxide should be used as vulcanizing agent. Therefore, the rubber shall not contain active substances that can interact with peroxide decomposition products (such as grooved carbon black, some organic accelerators and anti-aging agents), otherwise it will affect vulcanization.
(2) Silicone rubber products are generally used at high temperature, and its compounding agent should be stable at high temperature. Inorganic oxide is usually selected as reinforcing agent.
(3) Under the action of trace acid or alkali and other chemical reagents, silicone rubber is easy to cause the cracking and rearrangement of siloxane bonds, resulting in the reduction of heat resistance of silicone rubber. Therefore, when selecting the compounding agent, its acidity and alkalinity and the acidity of peroxide decomposition products must be considered to avoid affecting the properties of vulcanizate.



Typical formula of high temperature vulcanized silicone rubber

Raw materialmixture ratioMain roleother
Other raw rubber65%Endow the material with basic propertiesViscosity range 50-80wcs
White carbon black25%Change mechanical properties and flow propertiesViscosity range: 50-80wcs, mainly precipitated silica at the low end and gaseous silica at the high end
Hydroxyl silicone oil2%As a structural control agentHydroxyl content is mainly 8-9%
Dimethyl silicone oil2%Fine tune the hardness and viscosity of high temperature adhesiveMainly 500 and 1000 viscosity
Vulcanizing agent3%Form network crosslinking system/
other3%Mold release, anti-corrosion and other additives/
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2. Selection of raw rubber

 Vinyl silicone rubber can be used for silicone rubber products with general service temperature requirements (- 70 ~ 250 ℃); When the service temperature of the product is high (- 90 ~ 300 ℃), low phenyl silicone rubber can be used; When the products are required to be resistant to high and low temperature and fuel oil or solvent, fluorosilicone rubber shall be used.


3. Curing agent and curing mechanism

(1) Vulcanizing agent

 The curing agents used for thermal vulcanization of silicone rubber mainly include organic peroxides, aliphatic azo compounds, inorganic compounds and high-energy rays, among which the most commonly used is organic peroxides. This is because organic peroxides are generally stable at room temperature, but can decompose rapidly to produce free radicals at higher vulcanization temperature, resulting in crosslinking of silicone rubber.

These peroxides can be divided into two categories according to their activity: one is general-purpose, with high activity, which can vulcanize all kinds of silicone rubber; The other is vinyl special type, which can only vulcanize silicone rubber containing vinyl because of its low activity.


(2) Vulcanization mechanism

     When silicone rubber is vulcanized with peroxide, the crosslinking of silicone rubber by peroxide is carried out by free radical reaction between two activated methyl or vinyl groups.

      In addition to the above peroxide vulcanization, silicone rubber can also be vulcanized by high-energy radiation. Radiation vulcanization is also carried out according to the free radical mechanism. When the vinyl mole fraction in raw rubber is high (0.01) or used together with other rubber, sulfur vulcanization can also be used, but the performance is very poor.

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4. Reinforcing agent and related mechanism

      

The strength of unreinforced silicone rubber vulcanizate is very low, only about 0.3MPa, which has no practical value. The strength of silicone rubber vulcanizate can reach 3.9 ~ 9.8MPa by adding appropriate reinforcing agent, which is very important to improve the performance of silicone rubber and prolong the service life of products. The selection of silicone rubber reinforcing filler shall take into account the adverse effects of high-temperature use of silicone rubber and vulcanization with peroxide (especially acid-base substances) on silicone rubber.

The 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 ~ 400 m2 / g, which has a good reinforcing effect; The particle size of the latter is 300 ~ 1000nm, and the specific surface area is less than 30 ㎡ / g, so the reinforcement effect is poor.



(1) Reinforcing filler

The reinforcing filler of silicone rubber mainly refers to synthetic silica, also known as silica. White carbon black can be divided into gas phase white carbon black and precipitation white carbon black. Fumed silica is one of the most commonly used reinforcing agents for silicone rubber. The rubber reinforced by fumed silica has high mechanical strength and good electrical properties. It can be used together with other reinforcing agents or weak reinforcing agents to prepare rubber with different application requirements.

Compared with the silicone rubber compound reinforced by fumed silica, the compound reinforced by precipitated silica has slightly lower mechanical strength and poor dielectric properties (especially after moisture), but better heat aging resistance and low cost of compound. When the mechanical strength requirements of the products are not high, precipitated silica can be used alone or combined with gas-phase silica.

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5. Auxiliaries

(1) Structural control agent

The silicone rubber reinforced by fumed silica will harden, the plasticity value will decrease and gradually lose the processing performance during storage. This phenomenon is called "structural" effect. The compounding agent added to prevent and weaken this "structural" tendency is called "structural control agent". Structural control agents are usually low molecular silicone compounds containing hydroxyl or boron atoms. Commonly used are diphenylsilicon glycol, methylphenyl diethyloxysilane, tetramethylethylenedioxymethylsilane, low molecular hydroxyl silicone oil and silazane.

(2) Heat resistant additive

The hot air aging properties of silicone rubber can be greatly improved by adding some metal oxides or their salts and organic compounds of some elements. The most commonly used is ferric oxide, with a general dosage of 3 ~ 5 phr; Other metal oxides such as manganese, zinc, nickel and copper have similar effects. Adding a small amount (less than 1 parts) of spray carbon black can also enhance the heat resistance. Generally, the function of these additives can be shown only when hot air aging is carried out in the temperature range of 250 ~ 300 ℃.

(3) Colorant

Common colorants for silicone rubber are as follows: iron oxide (ferric oxide) red, cadmium yellow (cadmium dioxide) yellow, chromium green (chromium oxide) green, carbon black (black), titanium dioxide (titanium dioxide) white and ultramarine (blue).


6. Processing

     Silicone rubber can be processed by ordinary rubber processing equipment, but attention shall be paid to:

① The processing process shall be kept clean and shall not be mixed with other rubber, oil stain or impurities, otherwise the vulcanization and performance of silicone rubber will be affected;

② Silicone rubber products need to be vulcanized in hot air for a long time in the oven to improve the properties of vulcanizates.



7. Mixing

   Silicone rubber raw rubber is relatively soft and has certain plasticity. It can be directly mixed by open mixer or internal mixer without plastic refining. There are several mixing methods:

(1) Open mixer mixing

    The roller speed ratio of double roll open mill should be (1.2 ~ 1.4): 1. After the fast roll, the higher speed ratio leads to faster mixing, and the low speed ratio can make the film smooth. The roller must be filled with cooling water, and the mixing temperature should be below 40 ℃ to prevent coking or volatilization loss of vulcanizing agent. At the beginning of mixing, the roll distance is small (1 ~ 5mm), and then gradually increases.

Feeding and operation sequence: raw rubber (roll wrapping) - reinforcing filler - structure control agent - heat resistant additive - colorant - thin pass for 5 times - blanking, oven heat treatment - remanufacturing - vulcanizing agent - thin pass - parking overnight - remanufacturing - sheet discharge. The rubber can also not be subjected to oven heat treatment. After adding heat-resistant additives, add vulcanizing agent and then thin it. Store it overnight for refining, and then store it for several days to refine it for use. The mixing time is 20 ~ 40min.

(2) Internal mixer mixing

When mixing with laboratory 2L internal mixer, the mixing time is 6 ~ 16min. There is no special difficulty in mixing. When the loading coefficient is 0.74, the Φ The mixing of 160mm open mixer can also be carried out normally. The discharging temperature is related to the type of reinforcing filler. When weak reinforcing filler and precipitated silica are used, the discharging temperature is below 50 ℃; When fumed silica is used, the glue discharge temperature is about 70 ℃.


8. Extrude

Silicone rubber is generally soft, with good extrusion effect and easy operation. It can extrude products of different shapes and sizes. Its processing equipment and tools are basically similar to ordinary rubber. Extruder is generally used Φ 30 or Φ The length diameter ratio of 65mm single thread screw is (10 ~ 12): 1, and the effect is better. Low temperature shall be kept as far as possible during extrusion, preferably no more than 40 ℃, so the barrel and screw must be supplied with cooling water. For products with high quality requirements, 80 ~ 140 mesh filter screen can be installed near the head to remove impurities in the rubber and improve the extrusion quality.

Silicone rubber extruded semi-finished products are soft and easy to deform, so they must be vulcanized immediately. The most common method is hot air continuous vulcanization; High pressure steam continuous vulcanization is usually used in wire and cable industry. If continuous vulcanization is not possible after extrusion, in order to prevent deformation, it shall be picked up immediately with disc, drum or conveyor belt after extrusion, and isolated with talc powder to avoid mutual bonding. If it is found that the rubber is too soft for extrusion, 3 ~ 5 parts of fumed silica can be mixed into the rubber.


9. Calendering

Calender generally adopts vertical three roll calender. When used to produce film, the middle roller is fixed, the speed of the middle roller is faster than that of the upper roller, and the speed ratio is (1.1 ~ 1.4): 1. The speed of the lower roller is the same as that of the middle roller. When the calender starts, the temperature of the upper roll is 50 ℃, the middle roll shall be kept at room temperature, and the lower roll shall be cooled with cooling water. The calendering speed is generally 60 ~ 300cm? Min-1, which should not be too fast. First adjust the roll distance (middle and lower rolls) at a low speed to ensure a certain calendering thickness, and then increase it to the normal speed for continuous operation.

When the three roll calender is used for silicone rubber gluing and wiping, the fabric replaces the backing cloth (polyester film) to pass between the middle roll and the lower roll. Three roll calender is only applicable to single-sided glue coating, and four roll calender shall be used for long-term productio。


10. Gluing

      Gluing refers to the uniform distribution of silicone rubber slurry on the fabric by dipping or scraping, so as to improve the strength and flexibility of film products, make the fabric moisture resistant, and manufacture high-temperature electrical insulation materials.

(1) Slurry preparation

Benzoyl peroxide (BP) is mostly used as curing agent for silicone rubber compound used for mortar, and the dosage is slightly larger than that of general model products. If the reinforcing filler adopts fumed silica, the dosage shall not exceed 40 phr, and the dosage of structure control agent shall be appropriately increased. Volatile solvents such as toluene or xylene shall be used.

(2) Fabric pretreatment

The underlying fabric for silicone rubber coating generally adopts glass cloth, nylon and polyester. Glass cloth is widely used because of its good heat resistance, high strength and low moisture absorption. During the drawing process of glass fiber, the film is coated with paraffin lubricant (accounting for 0.2% ~ 0.5% of the fabric mass). It is easy to volatilize at the vulcanization temperature, which affects the combination of rubber and fabric. Dewaxing must be carried out before gluing. The thermal deformation of nylon and polyester is large, which affects the combination of rubber and fabric. Therefore, heat setting must be carried out before gluing, that is, the fabric shall be heat treated for a short time under certain drafting. The treatment temperature of nylon is 170 ~ 175 ℃ and that of polyester is 215 ~ 220 ℃.

(3) Gluing

After pretreatment, the fabric must be treated with surface adhesive before gluing. Adhesive is a solution composed of alkoxysilane, borate, vulcanizing agent and solvent (ethyl acetate or ethanol).


12. Bonding

Silicone rubber can be bonded with many materials, including metals, plastics, ceramics, fibers, silicone rubber itself and other rubbers. The best bonding between silicone rubber and coating can be achieved by using an adhesive that can be vulcanized at the same time with silicone rubber itself.

Adhesive is 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.



13. Vulcanization

The vulcanization process of silicone rubber is not completed at one time, but is carried out in two stages. The rubber is heated and shaped under pressure (such as molding vulcanization, direct steam vulcanization in vulcanizing tank, etc.) or under normal pressure (such as hot air continuous vulcanization), which is called one-stage vulcanization (or shaped vulcanization); Vulcanization at high temperature in the oven to further stabilize the physical properties of the vulcanizate is called secondary vulcanization (or post vulcanization).

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(1) Primary vulcanization

① Vulcanization of model products. Plate vulcanization, transfer molding vulcanization and injection vulcanization can be used. During vulcanization of silicone rubber products, release agent is generally not used, and loading, mold closing and pressurization shall be carried out quickly, otherwise it is easy to burn, especially the rubber containing vulcanizing agent BP and dCBP. Transfer molding vulcanization is a widely used process for processing silicone rubber compound. Compared with plate vulcanization with single hole per mold, it has the advantages of short processing cycle and can vulcanize complex rubber parts, especially those with inserts and pins. Compared with injection vulcanization, the equipment cost is lower. Injection vulcanization molded products can improve labor productivity and reduce labor intensity. At the same time, it can also reduce the amount of peroxide, improve the tear resistance and compression set performance of products, but the shrinkage of products is large.

② Vulcanization of extruded products. Steam pressure vulcanization, hot air continuous vulcanization, liquid vulcanization tank continuous vulcanization, drum vulcanization and radiation vulcanization can be adopted. The first three methods are commonly used.



(2) Two stage vulcanization

After a period of vulcanization of silicone rubber products, some low molecular substances exist in the vulcanizate, which affects the properties of the products. For example, for the compound with general curing agent (such as curing agent BP or dCBP), after a period of curing, the quality of acid decomposed by the curing agent will increase the porosity of sponge hole and reduce the density; Increasing the amount of curing agent will reduce the porosity of sponge hole, increase the density and produce thicker hole wall. In addition, the vulcanization temperature also has a great influence on the porosity of sponge.

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Silicone rubber application



Silicone rubber has unique comprehensive properties, especially the biocompatibility of silicone rubber is a key characteristic. It has been successfully used in other invalid occasions, solved many technical problems and met the needs of modern industry and daily life. Like other rubbers, silicone rubber can be processed into various profiles, hoses, strips, cots, tapes and other products; Room temperature vulcanized silicone rubber can be used in large area and is convenient to use.


(1) Automobile industry

The application of silicone rubber in automobile industry is growing rapidly. Silicone rubber (especially silicone rubber with various characteristics) can resist the erosion of fuel and lubricating oil, improve the service performance of automobile components and reduce maintenance costs. It can be used for automobile ignition wire, spark plug protective cover, hose for heating and radiator, muffler lining, battery connector and refueling pump made of fluorosilicone rubber. With the development of vehicle electronic electrification, RTV silicone rubber is widely used in potting materials of electronic parts and electrical assembly parts, windshield glass, sealing around vehicle body and bonding sealant of reflector.


(2) Electronic and electrical industry

Electronic and electrical industry is a field in which silicone rubber is used as insulating material first and in great demand. Silicone rubber is mainly used for TV anode cover, high-voltage protective cover, high-voltage outgoing line, refrigerator defroster wire, power or signal transmission wire and cable, etc. Insulators made of silicone rubber will replace ceramic products and be widely used in transmission lines, especially EHV lines. Conductive silicone rubber is used for electrical contact parts of electronic computers, telephones and other instruments and liquid crystal display contact parts. Wires and cables made of flame retardant and radiation resistant silicone rubber are widely used in atomic power stations. Silicone rubber heating sheet and heating belt are used to control the working temperature of various precision instruments and oil pipelines, and are used as heating blanket for physiotherapy hot compress in medical treatment. RTV silicone rubber can be used as potting material for waterproof, moisture-proof and shockproof.

Silicone rubber has been widely used in the sealing of dishwashers and washing machines because of its heat-resistant detergent performance. Silicone rubber is very suitable for gaskets on appliances such as coffee pots, electric fryers and steam irons. The ear and head pads of stereo headphones 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 supercooling of space and the burning heat of returning to the atmosphere, prolong the service life of aircraft parts, reduce maintenance costs and reduce accidents. It is mainly used for hole seals, electrical connectors, sealing switches, dust-proof and waterproof covers, gaskets, O-shaped sealing rings for jet engines and hydraulic devices, oxygen masks, control diaphragms, hot air ducts and radar wireless shock absorbers. Burning resistant silicone rubber is suitable for coating rocket fuel valves, power source cables and rocket launching well covers to avoid burning by rocket jet flow. RTV silicone rubber can be used as air tightness sealing, window frame sealing, shockproof and moisture-proof potting materials.


(4) Construction industry

Silicone rubber has good weather resistance and workability. As a bonding sealant, it has been widely used in the construction industry, surpassing other types of sealants. In recent years, a two-component sealant with low modulus and high elongation has been developed, which is used for large components such as concrete precast units and curtain walls with large joint movement. RTV silicone rubber is also used for sealing the joints of asbestos cement board, bathroom brick joints and toilet appliances. With the decline of rubber price in the future, the application range will be further expanded, such as replacing asphalt and neoprene in the application of highway joints. High temperature vulcanized silicone rubber sponge strip is used as sealing strip for doors and windows of buildings.


(5) Medical field

Silicone rubber has good biocompatibility, small reaction to the body, stable performance and low hemagglutination. It can withstand high temperature and high pressure cooking for many times, and can be processed into products of various shapes, such as film, sponge and air bag. It is the most widely used material in medicine at present.


epilogue

Silicone rubber has excellent resistance to high and low temperature, ozone, oxygen, light and weather aging, excellent electrical insulation, special surface properties, physiological inertia and high air permeability. It 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 of complex products is also poor. In recent years, countries all over the world have carried out a lot of improvement research work and achieved some results.

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