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 their different vulcanization mechanisms.

1.2 Characteristics

(1) High and low temperature resistance

Among all rubbers, silicone rubber has the widest operating temperature range (-100~350℃). For example, properly formulated vinyl silicone rubber or low phenyl silicone rubber can still maintain elasticity after being aged for thousands of hours at 250°C or hundreds of hours at 300°C in hot air; low phenyl silicone rubber vulcanized rubber can be aged for tens of hours at 350°C. It can still maintain elasticity after being aged in hot air for 1 hour. Its glass transition temperature is -140℃, and its vulcanized rubber is still elastic at temperatures of -70~100℃. When silicone rubber is used as a heat-proof coating on the inner wall of a rocket nozzle, it can withstand instantaneous high temperatures of thousands of degrees.

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

There is no significant change in the performance of silicone rubber vulcanized rubber after being exposed outdoors for several years in a free state.

(3) Electrical insulation properties

The electrical insulation properties of silicone rubber vulcanized rubber change little when affected by moisture, frequency changes or temperature increases. The silicon dioxide generated after combustion is still an insulator. In addition, silicone rubber has few carbon atoms in its molecular structure and does not use carbon black as a filler. Therefore, it is not prone to scorch during arc discharge and is very reliable in high-voltage applications. Its corona resistance and arc resistance are excellent, its corona resistance life is 1,000 times that of polytetrafluoroethylene, and its arc resistance life is 20 times that of fluororubber.

(4) Special surface properties and physiological inertness

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% when immersed in water for a long time. The physical properties do not decrease, and it has good mildew resistance. It does not adhere to many materials and can be used for isolation. effect. Silicone rubber is odorless and non-toxic, has no adverse effects on the human body, reacts slightly with body tissues, and has excellent physiological inertness and physiological aging properties.

(5) High breathability

Compared with other polymer materials, silicone rubber has good air permeability. The permeability of nitrogen, oxygen and air at room temperature is 30 to 40 times higher than that of NR. It is selective for gas permeability, such as the permeability of carbon dioxide to oxygen. About 5 times.

(6) Biomedical performance

The characteristics of the molecular structure of silicone rubber give it excellent biomedical properties, which has been proven by the successful application of a large number of animal and human experiments.

2 Hot vulcanized silicone rubber

Thermal vulcanization silicone rubber refers to silicone rubber with a high relative molecular weight (400,000 to 600,000). Organic peroxide is used as the vulcanizing agent. After heating, the organic peroxide decomposes to generate free radicals, which cross-link with the organic side groups of the rubber to obtain vulcanized rubber.

2.1 Varieties and characteristics

Thermal vulcanized silicone rubber is the earliest type of rubber used. There are many varieties developed so far. According to different chemical compositions, 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 shortcomings of low vulcanization activity, poor process performance, easy foaming of thick-walled products during secondary vulcanization, and large compression deformation at high temperatures, it has been replaced by methyl vinyl silicone rubber except for a small amount of fabric coating.

(2) Methyl vinyl silicone rubber

Methyl vinyl silicone rubber (methylvinylpolysiloxanerubber), referred to as vinyl silicone rubber, is copolymerized by dimethylsiloxane and a small amount of vinyl siloxane. The vinyl mole fraction is generally 0.001~0.003. A small amount of unsaturated vinyl The introduction has greatly improved the vulcanization process and finished product properties, especially the heat aging resistance and high temperature compression deformation resistance. In the production of silicone rubber, methylvinyl silicone rubber has the largest output, the widest application, and the most varieties and brands. In addition to a large number of general-purpose rubber materials, various special-purpose silicone rubbers and silicone rubbers with processing characteristics (such as high-strength Silicone rubber, low compression permanent deformation silicone rubber, conductive silicone rubber, thermally conductive silicone rubber, non-two-stage vulcanized silicone rubber, granular silicone rubber, etc.) are also processed and matched based on it.

(3) Methyl vinyl phenyl silicone rubber

Methylvinylphenylpolysiloxanerubber (methylvinylphenylpolysiloxanerubber), referred to as phenyl silicone rubber, is made by introducing diphenylsiloxane links (or methylphenylsiloxane links) into the molecular chain of vinyl silicone rubber. Done. When the phenyl mole fraction is 0.05~0.10, it is collectively called low phenyl silicone rubber. At this time, the hardening temperature of the rubber drops to the lowest value (-115°C), giving it the best low-temperature resistance. At -100°C The following remains resilient. As the mole fraction of phenyl groups increases, the rigidity of the molecular chain also increases, and its crystallization temperature increases instead.

When the phenyl mole fraction is between 0.15 and 0.25, it is collectively called medium phenyl silicone rubber, which is flame-resistant. When the phenyl mole fraction is above 0.130, it is collectively called high phenyl silicone rubber and has excellent radiation resistance. Phenyl silicone rubber is used in situations requiring low temperature resistance, ablation resistance, high-energy radiation resistance, heat insulation, etc. Medium phenyl and high phenyl silicone rubber are difficult to process and have poor physical properties, so their production and application are subject to certain restrictions.

(4) Methyl vinyl trifluoropropyl silicone rubber

Methylvinyltrifluoropropylpolysiloxanerubber, referred to as fluorosilicorubber, is a fluoroalkyl group (usually trifluoropropyl) introduced into the molecular chain of vinyl silicone rubber. It 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 certain synthetic oils, the operating temperature range is -50~250°C, at normal and high temperatures. Better stability.

(5) Phenylene silicone rubber and phenylene ether silicone rubber

Phenylene silicone rubber and phenylene ether silicone rubber (phenylene polysiloxanerubber and phenylatylenes silicone rubber) are new varieties of silicone rubber containing phenylene or phenylene ether group links in the molecular chain. They were developed to meet the requirements of nuclear power devices and navigation technology. Its main characteristics are high tensile strength, gamma 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 silicone segments or methyl-γ-nitrile propyl silicone segments introduced into the molecular chain. Its main characteristics are similar to those of fluorine. Silicone rubber is similar in that it is oil and solvent resistant and has good low temperature resistance. However, due to factors that cause hydrolysis of nitrile groups under polymerization conditions, the repeatability of raw rubber is poor, and its application development is subject to certain limitations.

(7) Silicone boron rubber

Boron silicone rubber is a new type of silicone rubber containing a decaborane cage structure in the main chain of the molecule. It has a high degree of heat aging resistance and can work at 400°C for a long time and continuously at 420~480°C. Works for hours while remaining flexible at -54°C. Suitable for use as sealing material in high-speed aircraft and spacecraft. The United States had a series of silicone boron rubber product brands in the late 1960s, but few reports were made after the 1970s. The main reason may be that the process performance of the rubber and the elasticity of the vulcanized rubber are poor, and the synthesis of carbon boron is very complex and toxic. Large and expensive.

Hot vulcanized silicone rubber is sold in the form of raw rubber or vulcanized rubber. It is generally formulated into rubber materials with various properties for users to choose. According to different properties, it can be divided into the following categories:

(1) Universal type (general strength type). It is composed of vinyl silicone rubber and reinforcing agents. The physical properties of vulcanized rubber are medium, and it is the type of rubber with the largest dosage and the most versatility.

(2) High strength type. Vinyl silicone rubber or low phenyl silicone rubber is used, and gas phase silica with a larger specific surface area or modified silica is used as a reinforcing agent, and is comprehensively integrated by adding appropriate processing aids and special additives. Sexual coordination improvement measures, improved cross-linked structure, and increased tear strength.

(3) High temperature resistant type. Using vinyl silicone rubber or low phenyl silicone rubber, with appropriate selection of reinforcing agent types and heat-resistant additives, silicone rubber that can withstand high temperatures of 300~350℃ can be produced.

(4) Low temperature type. Made of low phenyl silicone rubber, the brittleness temperature reaches -120℃ and is still elastic at -90℃.

(5) Low compression permanent deformation type. It mainly uses vinyl silicone rubber and uses vinyl-specific organic peroxide as the vulcanizing agent. When the compression rate is 30%, the permanent deformation after compression at 150°C for 24 to 72 hours is 7.0% to 15% (ordinary silicone rubber is 20%~30%).

(6) Wire and cable type. It mainly uses vinyl silicone rubber and uses fumed silica with good electrical insulation properties as the reinforcing agent, which has good extrusion process performance.

(7) Oil-resistant and solvent-resistant. Ammonia silicone rubber is mainly used, and is generally divided into two categories: general-purpose and high-strength types.

(8) Flame retardant type. It is a rubber compound composed of vinyl silicone rubber and adding halogen-containing or platinum compounds as flame retardants, which has good flame resistance.

(9) Conductive type. Using vinyl silicone rubber, using acetylene carbon black or metal powder as filler, and choosing high-temperature vulcanization or addition vulcanization method, can the volume resistivity be 2.0~100Ω? cm of silicone rubber.

(10) Heat shrinkable type. When thermoplastic materials with a certain melting temperature or softening temperature are added to vinyl silicone rubber, the thermal shrinkage rate of the silicone rubber compound can reach 35% to 50%.

Second-stage vulcanization type is not required. It is made of vinyl silicone rubber with a higher vinyl mass fraction, by controlling the pH value of raw rubber and compounding agents, and adding special additives.

(11) Sponge type. Adding organic foaming agents such as nitroso compounds, azo and diazo compounds to vinyl silicone rubber can produce a sponge with uniform foaming.

In addition, there are thermally conductive silicone rubber, fluorescent silicone rubber and medical grade compound rubber for sale abroad.

With the continuous development of silicone rubber applications, the variety and brands of rubber compounds are increasing day by day. Too many brands will cause confusion in production, storage, transportation and sales. Some factories have divided multiple varieties into several typical basic glues and several characteristic additives (including pigments, vulcanizing agents, etc.) for sale accordingly. Users can obtain the final product by mixing them according to certain formulas and mixing techniques according to their needs.

This method not only makes the varieties simple and clear, but also enables large production batches, stable quality, reduced costs, and improved competitiveness.