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