Silicone rubber (Q) is an elemental organic elastomer obtained by hydrolysis and polycondensation of various dichlorosilanes. Its main chain is mainly composed of Si-O (silicon-oxygen bond) units and a linear polymer with monovalent organic groups as side groups. The molecular chain has both inorganic and organic properties.
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Types of Silicone Rubber
These rubbers can generally be classified by the vulcanization method and chemical structure.
According to vulcanization temperature and use characteristics, it is divided into two categories: high-temperature vulcanization or thermal vulcanization (HTV) and room temperature vulcanization (RTV). The former is a high molecular weight semi-solid rubber, and the processing technology of molding and vulcanization is similar to that of ordinary rubber. The latter is a low molecular weight liquid glue with active end groups or side groups, which can be vulcanized and formed at room temperature. It can also be divided into two-component RTV (referred to as RTV-2) and single-component RTV (referred to as RTV-1).
Classification by chemical structure is based on the difference in the introduction of organic pendant groups into silicone rubber. The introduction of pendant groups can significantly improve its physical and mechanical properties, temperature resistance and processability. There are mainly dimethyl silicone rubber (MQ), methyl vinyl silicone rubber (MVQ), methyl phenyl vinyl silicone rubber (PVMQ) and trifluoropropyl methyl vinyl silicone rubber (FMVQ). Among them, the amount of methyl vinyl silicone rubber is larger. Q is the code name of polysiloxane rubber, M is methyl, V is vinyl, P is phenyl, N is nitrile ethyl, and F is alkyl.
It is usually sold in three forms: pure rubber, binder with some fillers, and rubber with most or all of the compounding ingredients. When the rubber is sold in the form of mixed rubber, it is divided into: ① general type (general strength type); ② high strength type; ③ high-temperature type; ④ low-temperature type; ⑤ low compression permanent deformation type; ⑥ electric wire Cable type; ⑦ oil and solvent resistant type; ⑧ flame retardant type; ⑨ conductive type; ⑩ heat shrinkable type, etc.
The main varieties of this hot vulcanized rubber are as follows.
(1) Dimethyl silicone rubber (MQ)
Dimethyl silicone rubber is referred to as methyl silicone rubber. It is the oldest variety of silicone rubber and can maintain good elasticity in the temperature range of -60~250 °C. Due to its low vulcanization activity, it has the disadvantages of poor process performance, easy foaming of thick-walled products during two-stage vulcanization, and large compression deformation at high temperature. replace.
(2) Methyl vinyl silicone rubber (MVQ or VMQ)
Methyl vinyl silicone rubber, referred to as vinyl silicone rubber is a copolymer of dimethylsiloxane and a small amount of methyl vinyl siloxane. The introduction of a small amount of unsaturated vinyl makes the vulcanization process and finished product properties, especially heat aging resistance and high-temperature compression deformation resistance, greatly improved. The content of methyl vinyl siloxane units has a great influence on the vulcanization effect and the heat resistance of the vulcanizate. If the content is too small, the effect will not be significant. If the content is too large [up to 0.5% (molar fraction)], the resistance of the vulcanizate will be reduced. hot sex. In the production of silicone rubber, methyl vinyl silicone rubber has the largest output, the most widely used, and the most varieties and brands.
Properties of Silicone Rubber
The main performance characteristics of the rubber are as follows.
- Excellent high and low-temperature resistance
Working temperature range -100~350℃.
- Excellent ozone aging resistance, light aging resistance, and weather aging resistance
This rubber has no change in performance when placed outdoors in a free state for several years.
- Excellent electrical insulation properties
The electrical insulating properties of silicone change little when exposed to moisture, frequency changes, or temperature increases, 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 no carbon black is used as a filler, so it is not easy to burn during arc discharge, so it is very reliable to use it in high-voltage applications. Its corona resistance and arc resistance are extremely good, the corona resistance life is 1000 times that of PTFE, and the arc resistance life is 20 times that of FKM.
- Special surface properties and physiological inertia
It has low hygroscopicity, and its water absorption rate is only about 1% when immersed in water for a long time, the physical and mechanical properties do not decrease, and the mildew resistance is good; it is odorless, non-toxic, has no adverse effect on the human body, and has a slight reaction with the body tissue. Physiological inertness and physiological aging.
- High air permeability
Compared with other polymer materials, it has extremely superior air permeability, and the permeability of nitrogen, oxygen, and air at room temperature is 30 to 40 times higher than that of natural rubber. In addition, it also has the selective performance of gas permeability, that is, the permeability of different gases (such as oxygen, nitrogen and carbon dioxide, etc.) is quite different. For example, the permeability of oxygen is about 1 times that of nitrogen and the permeability of carbon dioxide. Transient is about 5 times that of oxygen.
Compression Set of Silicone Rubber
The tensile strength and elongation at the break of silicone vulcanized rubber have little relationship with the tensile speed, while the tear strength is strongly dependent on the tensile speed. In the hardness range of 30 to 80 (Shore A), the residual deformation (permanent deformation) of the vulcanizate after tension or compression is very low. The elastic properties of vulcanizates depend on the formulation of the compound, especially in relation to hardness. The elastic properties in the hardness range of 50-60 (Shore A) are better, and the change with temperature is also small.
An important property is low-temperature flexibility. Silicone vulcanized rubber will not become hard and brittle even when the temperature is lowered to –55°C. Low-temperature performance is also related to hardness. Usually, the hardness range is better when it is in the range of 50 to 60 (Shore A). The low-temperature performance of VMQ rubber cannot be improved by the use of plasticizers, which is different from NBR; the same low-temperature performance cannot be improved by sacrificing heat resistance, which is also different from many other rubbers. Therefore, VMQ rubber products can be used both at low temperature and high temperature, and the performance changes little.
Application of Silicone Rubber
This rubber can be used in auto parts, electronic parts, aerospace sealing products, adhesive joints in the construction industry, sealing rings for household appliances, medical artificial organs, urinary catheters, etc. It has also achieved good benefits in textile high-temperature equipment and equipment with high concentrations of alkali, sodium hypochlorite, and hydrogen peroxide.