Fluorosilicone rubber was first developed by the American DowCorning Company and the Air Force in 1956 and was used in the aviation field. Subsequently, the former Soviet Union, Germany, Japan and other countries developed a series of products, which gradually improved the performance and quality of fluorosilicone rubber. . In my country, in 1966, the Chinese Academy of Sciences and Shanghai Organic Fluorine Research Institute collaborated to produce fluorosilicone rubber equivalent to the American LS-420, and successfully developed SF series fluorosilicone rubber materials with excellent performance.

1 Classification and properties of fluorosilicone rubber

There are many types of fluorosilicone rubber. The fluorosilicone rubber that has been mass-produced is mainly a polymer with γ-trifluoropropylmethylsiloxane as the structural monomer. At the same time, FEM26, FEM2802, and SKTFT-50 have also been developed. Copolymer fluorosilicone rubber is represented by copolymer fluorosilicone rubber. Fluorosilicone rubber can be divided into free radical type (vulcanized with peroxide), condensation type and addition type according to the vulcanization mechanism; according to the vulcanization temperature, it can be divided into high temperature vulcanization type and room temperature vulcanization type. Room temperature vulcanization type. Type is divided into single-component type and two-component type.

Fluorosilicone rubber combines the high and low temperature resistance of silicone rubber, excellent electrical properties and resilience, and the oil resistance, solvent resistance and saturated steam resistance of fluorine rubber. It is currently the only fuel medium that can be used in -68~230℃ The elastomer used, but similar to silicone rubber, fluorosilicone rubber has poor strength and low surface energy. It is a material that is difficult to bond and has defects such as difficulty in processing. In addition, with the introduction of trifluoropropyl, it causes vulcanization It is difficult, so developing appropriate formulas and processing techniques to improve the performance of the rubber plays a vital role in the application of fluorosilicone rubber.

2. Research progress of domestic fluorosilicone rubber

2.1 Research on formula

The compounding system of fluorosilicone rubber is not much different from that of general silicone rubber. The preparation of fluorosilicone rubber is mainly composed of fluorosilicone raw rubber, cross-linking agents, catalysts, reinforcing agents and additives mixed under certain conditions. After molding and vulcanization under certain conditions, fluorosilicone rubber products can be obtained.

2.1.1 Raw rubber grades and selection

Fluorosilicone rubber is mainly composed of polymethyltrifluoropropylsiloxane as a monomer. The condensation type is generally terminated with hydroxyl groups, while the addition type is mixed with a certain amount of unsaturated olefins. The room temperature vulcanization type fluorosilicone rubber has a smaller relative molecular mass, between 1×104~8×104, while the high temperature type fluorosilicone rubber The vulcanized type can reach 4×105~8×105. Currently, the most commonly used foreign fluorosilicone raw rubber brands include LS63 of DowCorning in the United States, SE3810 series of Japan's Shin-Etsu, etc.

2.1.2 Selection of vulcanization cross-linking and catalytic systems

In the rubber industry, the reagent used to cross-link raw rubber is rubber vulcanizing agent. At present, vulcanizing agents mainly include organic peroxides, organic silicones, inorganic oxides, high-energy rays, etc., which are often substances containing multiple active functional groups.

When vulcanizing at room temperature, the commonly used vulcanizing agents are silicates (such as ethyl orthosilicate) and titanates (such as n-butyl titanate). At the same time, organic tin, organic titanium, and platinum complexes are used. Acts as a catalyst to regulate vulcanization speed. Among them, organotin dibutyltin dilaurate is the most widely used catalyst. It should be pointed out that the real catalytic effect in the cross-linking reaction is not dibutyltin dilaurate itself, but its hydrolysis product dibutyltin dihydroxytin. Experiments show that the use of composite catalysts has a "synergistic effect" phenomenon. For example, when dibutyltin dilaurate is used in combination with titanium compounds or triethylamine (TA), the catalytic effect is significantly higher than that of ordinary catalysts. However, when using this type of catalyst, if catalysts such as butyltin remain in the rubber or impurities such as moisture are introduced, degradation, softening, and even liquefaction will occur in a closed environment even below 200°C. Therefore, fluorosilicone used in high-temperature environments The use of such catalysts should be minimized during the vulcanization process of materials. Cai Baolian et al. used three catalytic systems such as HI/I2 (molar ratio of 1/1), p-toluenesulfonic acid, and phosphate to prepare a one-component room temperature vulcanized fluorosilicone rubber adhesive that meets the requirements. Xie Zemin, Peng Wenqing and others used polysilazane KH-CL as a cross-linking agent to achieve non-catalytic cross-linking, avoiding the degradation caused by organotin catalysts at high temperatures. At the same time, excess polysilazane cross-linking agent can eliminate the system The silicone hydroxyl group and moisture in the rubber can inhibit the degradation caused by silicone hydroxyl group and moisture, and greatly improve the thermal stability of the rubber compound. Wu Songhua and others also used silicon nitrogen oligomers as vulcanizing agents to make the vulcanization reaction by-products NH3 and H2 volatiles, thus improving the heat resistance stability of fluorosilicone rubber sealants in a closed state, and the heat resistance of fluorosilicone rubber The properties increase as the amount of silicon-nitrogen oligomer increases.

When using high-temperature vulcanization, the most commonly used and most effective organic peroxides include benzoyl peroxide (BP), di-tert-butyl peroxide (DTBP), 2,5-dimethyl-2,5- Di-tert-butylhexyl peroxide (vulcanizing agent bis 2,5) and dicumyl peroxide (DCP), etc. The amount of peroxide used is affected by various factors such as the type and amount of raw rubber, filler type and amount, processing technology, etc. As long as the required degree of cross-linking can be achieved, the vulcanizing agent should be used as little as possible. Among peroxide vulcanizing agents, the vulcanizing agent double 2,5 has a moderate decomposition temperature and is suitable for use with carbon black. The copolymerized fluorosilicone rubber prepared by Mi Zhian using 0.8 parts of double 2,5 as the vulcanizing agent has relatively good properties. Zheng Hua et al. reported that the rubber prepared by double 2,5+DCP as vulcanizing agent has excellent aging resistance and compression set resistance, and has moderate oil resistance. In addition to peroxide, metallic platinum is also a commonly used high-temperature vulcanizing agent. Platinum-cured rubber has a particularly fast vulcanization rate and is not inhibited by oxygen. Unlike high-temperature dialkyl peroxides, very thin rubber edges can be fully vulcanized when platinum catalyst vulcanized rubber is used.

2.1.3 Selection of reinforcing agents

Unreinforced fluorosilicone rubber has very low strength and has no practical value. It is extremely important to add appropriate reinforcing agents to improve the performance of fluorosilicone rubber and extend the service life of the product. At present, the most commonly used reinforcing agent is fumed silica, and the molecular main chain structural unit of fluorosilicone rubber - Si-O-. Both have the same silicon-oxygen skeleton. SiO2 particles are filled into the gaps of the cross-linked network of fluorosilicone. When they come into contact, they have a strong interaction, thus playing a reinforcing effect. The reinforced rubber has high mechanical strength and good electrical properties of vulcanized rubber, and can be used together with other reinforcing agents or weak reinforcing agents to prepare rubber with different performance properties. However, because it is prone to structural phenomena during storage and use, it must undergo post-processing such as secondary vulcanization, and because there are active hydroxyl groups on the surface of silica black, it will have an adverse effect on the heat resistance of the rubber. Wang Chunhua and others used multiple Various types of white carbon black are used together, and the bonding strength of the bonding system is successfully improved by adjusting the amount of carbon black. During mixing, a certain amount of hydroxyfluorosilicone oil is added to pretreat the silica, which consumes part of the active hydroxyl groups on the surface of the silica, causing partial passivation of the surface and reducing the impact of silica on the "structure" of the fluorosilicone rubber. The performance is greatly improved and the quality of the rubber compound is improved. In addition, using carbon black instead of white carbon black to reinforce fluorosilicone rubber, such as using N-990 carbon black and spray carbon black imported from Canada, can also have a good reinforcing effect. Zheng Hua and others proved that using carbon black as a reinforcing agent not only simplifies the production process (eliminating the post-processing process) and reduces production costs, but also the various properties of the vulcanized rubber fully meet the index requirements; Wu Weili also reported that during the vulcanization of fluorine rubber, In addition to being a reinforcing filler, carbon black can also play a role as a hardness modifier; however, it should be noted that carbon black will greatly reduce the storage stability and effectiveness of some peroxides; when using platinized systems, carbon black Black will also inhibit the vulcanization reaction.

2.1.4 Develop new coupling agents

Coupling agents were first used in glass fibers to serve as a bridge at the interface between inorganic materials and organic materials. Although the coupling agent is used in a small amount in the adhesive, it has an important impact on the bonding strength. At this stage, silane coupling agents are used in the bonding of fluorosilicone rubber and metal, which can achieve good coupling between fluorosilicone rubber and inorganic substances or metals. Silane coupling agents are divided into aminosilane, peroxysilane, and stacked silane. Nitrogen silane and diazosilane coupling agents, etc. Among them, peroxysilane vinyl tert-butylperoxysilane (VTPS) has the most superior performance. It has wide applicability and is an ideal, efficient and broad-spectrum chemical. Tackifier, known as "universal tackifier". Su Zhengtao and others reported that VTPS is a good tackifier for high-temperature vulcanization bonding of silicone rubber (VMQ or FVMQ). Adding VTPS can improve the shear strength of the bonding between silicone rubber and metal and can withstand solution corrosion. Guan Jing's experiments showed that silane coupling agents A151 and VTPS can form a bridge between fluorosilicone rubber FSR and stainless steel sheets, playing a viscosity-increasing effect, but A151 is not stable enough, while VTPS is better; Zheng Shijian found that when fluorosilicone rubber and stainless steel The adhesive has the best bonding effect when the relative dosage of VTPS is 1:1 (mass ratio); in addition to VTPS, other silane coupling agents such as methyltert-butylperoxysilane (MATS), vinyltriacetoxy Silane, KH-550, KH-560, etc. also have certain applications in fluorosilicone rubber products.

2.1.5 Additives

2.1.5.1 Anti-aging and heat-resistant additives

The main method to improve the heat resistance of fluorosilicone rubber is to change the main chain structure, and add silicone hydroxyl scavengers, heat-resistant additives, etc. For different cross-linking systems, different methods should be used. For addition type and peroxide cross-linked rubber, a relatively simple and economical way to improve heat resistance is to add metal oxides, such as γ-Fe2O3, SnO2, active MgO, CaO, etc., which can inhibit the side effects of fluorosilicone rubber molecular chains. The chain organic functional group (trifluoropropyl) is oxidized and neutralizes the hydrofluoric acid that may be produced due to the decomposition of trifluoropropyl at high temperatures in the fluorosilicone rubber, thereby providing thermal stabilization and thus greatly improving the heat resistance of the rubber. . Su Zhengtao, Zheng Junping and others reported that SnO2 has a synergistic effect on Fe2O3, and the effect of adding Fe2O3 and SnO2 composite oxide is better than that of Fe2O3 or SnO2 alone. SnO2, Fe2O3 and iron-tin oxide complexes can prevent thermal oxidation and inhibit the decomposition of side chain groups of rubber molecules. Among them, iron-tin oxide complexes have the strongest effect. Su Zhengtao and others also studied that cerium dioxide as a heat-resistant additive can greatly improve the heat resistance of fluorosilicone rubber; Zheng Junping and others discussed the heat-resistant mechanism of transition metal oxides and proved that iron-tin composite oxides have a variety of properties. Anti-oxidation mechanism, thereby exerting a synergistic effect and improving the heat resistance of the rubber compound.

2.1.5.2 Other additives

Because the fluorosilicone rubber compound reinforced with gas phase silica will harden during storage, the plasticity value will decrease, and the processing performance will gradually lose. In order to prevent and weaken this "structural" tendency, the general method is to add a certain amount of structure control agent. Structural control agents are usually low-molecular organic silicon compounds containing hydroxyl or boron atoms. Commonly used ones include diphenylsilanediol, methylphenyldiethoxysilane, tetramethylethylenedioxydimethylsilane, Low molecular weight hydroxyfluorosilicone oil and silazane, etc.

In order to solve the problem of excessive viscosity of fluorosilicone rubber, a certain amount of plasticizer is often added to the rubber compound. The most commonly used plasticizer is stearic acid. However, since it cannot withstand high temperatures and easily interacts with peroxide, the added amount cannot be excessive. many. Zheng Hua and others tried adding a very small amount of stearic acid as a release agent, and the mold sticking phenomenon rarely occurred when the mold was released. In addition, as needed, a certain amount of colorants, such as iron oxide, cadmium and chromium compounds, carbon black, etc., can be added to the fluorosilicone rubber without affecting the performance.

Low compression permanent deformation fluorosilicone rubber