Fluorosilicone rubber is a polymer with a main chain formed by silicon-oxygen bonds and trifluoroalkyl groups on the side chains. Its polymerization method is similar to that of dimethyl silicone rubber, and it is manufactured by the ring-opening polymerization of cyclic siloxane. Fluorosilicone rubber is not as heat-resistant and chemical-resistant as other fluororubbers, but because its main chain contains silicon-oxygen bonds and its side groups contain fluorine atoms, it has the advantages of both, so it can be used in a very wide temperature range of -60℃-200℃, and has excellent resistance to fuel oil and low swelling to methanol, but its disadvantage is that its physical and mechanical properties are worse than other fluororubbers.

In summary, the introduction of SI-O bonds, double bonds in the main chain or ether bonds in the side chains of the molecules can improve the flexibility of the macromolecules, thereby improving the cold resistance of fluororubbers while retaining the original characteristics of fluororubbers, but due to its high price, its application and promotion are greatly limited. Therefore, while modifying the molecular chain to prepare new fluororubbers, it is also necessary to improve the cold resistance of the original series of fluororubbers with lower prices.

Rubber combination

Using fluororubber with some general rubber and special rubber can improve the shortcomings of fluororubber such as low elasticity, poor cold resistance and high price, and obtain materials with excellent performance and low cost. Due to the structural characteristics of fluororubber itself, it is difficult to choose a co-vulcanization system, and the unique characteristics of fluororubber will be partially reduced after the combination. Therefore, the research on the combination at home and abroad is not extensive. Among them, the combination systems that can improve the cold resistance of fluororubber are mainly the following.

Fluororubber/fluoroether rubber combination

In order to improve the low-temperature performance of fluororubber, foreign countries have synthesized fluoroether rubber in the early 1970s. Its low-temperature performance is significantly improved compared with fluororubber. For example, the TG of VITONGLT fluoroether rubber is about 14℃ lower than that of fluororubber F246, but its price is higher. Beijing Institute of Aeronautical Materials has developed fluororubber FX-13 through rubber combination technology and a small amount of fluoroether rubber, which improves the cold resistance of fluororubber. Its brittle temperature can reach -45℃, and other physical and mechanical properties are excellent. The combined use of fluoroether rubber not only effectively improves the cold resistance of fluororubber, but also greatly reduces its price compared to fluoroether rubber.

Fluororubber/EPDM combined use

Both EPDM and EPDM can be combined with fluororubber for modification, and the modification of tetrafluoroethylene rubber has been studied more. Compared with tetrafluoroethylene rubber, EPDM also has many excellent properties such as extremely high chemical stability, heat resistance, weather resistance, water resistance, water vapor resistance, chemical resistance, etc., but its typical characteristics are good resistance to polar media and excellent low temperature performance. Tetrafluoroethylene rubber is equivalent to the copolymer of fluorine substitution of ethylene molecules and propylene, so the similarity of the two in composition structure provides a compatibility basis for the combination of the two. According to the information, the combination of tetrafluoroethylene rubber and EPDM or EPDM can improve the cold resistance and elasticity of tetrafluoroethylene rubber, improve the processing performance and reduce the cost of materials. However, due to the great difference in polarity between the two, the key to the successful combination is to reasonably solve the co-vulcanization of the blend.

Fluororubber/cis-isoprene rubber combination

Cis-1,4-polyisoprene and fluororubber of different structures can be used to produce rubber seals. In this type of combination rubber, the combination rubber can show different properties as the combination ratio of fluororubber to isoprene rubber changes. When the fluororubber content is less than 50 parts, isoprene rubber is the continuous phase, and the combination rubber has good cold resistance, but its oil resistance is poor. This shortcoming can be improved by adopting a suitable vulcanization method (such as dynamic vulcanization method). The dynamic vulcanization method can reasonably control the oil resistance and cold resistance of the combination rubber.

Fluororubber/silicone rubber combination

Silicone rubber refers to a type of linear polymer with a main chain of -SI-O- bonds and a side group of organic groups (mainly methyl). It is a polymer elastomer with both inorganic and organic properties.

Due to the particularity of its molecular structure, compared with other rubbers, silicone rubber has excellent heat resistance, elasticity, cold resistance, excellent demoulding, electrical properties, air permeability, thermal conductivity, waterproofness and good temperature stability. By using silicone rubber and fluororubber together, a rubber with both characteristics can be obtained, which can significantly improve the shortcomings of fluororubber's poor cold resistance and elasticity. For example, the JSR-JENIXF series of fluororubber/silicone rubber developed by Japan Synthetic Rubber Company has excellent heat resistance, cold resistance, oil resistance, water resistance and steam resistance, and the price is relatively low.

Adding a suitable plasticizer

Plasticizer is the compounding agent that has the greatest impact on cold resistance in the formulation design of cold-resistant rubber products, except for raw rubber. Studies have shown that adding plasticizers can improve the flexibility of rubber molecular chains, thereby improving the cold resistance of vulcanized rubber. Generally, adding a plasticizer with a lower freezing point can reduce the glass transition temperature of the vulcanized rubber; conversely, if the freezing point temperature of the plasticizer is higher, the glass transition temperature of the vulcanized rubber can be increased. From the perspective of improving the cold resistance of vulcanized rubber, the selection of plasticizers must fully estimate the impact of plasticizers on the glass transition temperature.

The type and amount of plasticizers are determined by the type of rubber and the cold resistance index of the product. Adding appropriate plasticizers to polar rubber with poor cold resistance reduces the force between polar rubber molecules through the interaction between the polar groups of the plasticizer and the polar groups in the polar rubber molecular chain, making the molecular chain segments easy to move, thus obtaining better low-temperature performance. However, for fluororubber, due to its highly stable molecular structure, general plasticizers have no effect on it, and the plasticizer will evaporate during the two-stage vulcanization process at 200-250°C. Therefore, the types of plasticizers that can be used in fluororubber are extremely limited: one is to use low relative molecular weight fluorinated olefin copolymers, that is, fluorine wax, as plasticizers for fluororubber. For example, in the system of isoprene rubber and fluororubber, if a pre-prepared low molecular plasticizer ΦNA (made by mechanical degradation of isoprene rubber and fluororubber in a reactor with a special structure) is added, the cold resistance of the combined rubber can be significantly improved. Another is to use fluorinated acid esters, such as fluorinated adipate, as plasticizers for fluororubber. This plasticizer has good affinity with fluororubber because of the fluorine in its molecular structure, so it can improve the dispersion of compounding agents and the processing performance of rubber. However, its biggest advantage is that it can significantly improve the cold resistance of fluororubber without significantly affecting the physical and mechanical properties of vulcanized rubber, so that fluororubber products can be used at a temperature of -50°C.

Choose a suitable vulcanization system

The vulcanization system used for fluororubber can be roughly divided into three categories: amines, phenols and organic peroxides. According to data reports, for the F246 series fluororubber produced by San Ai Fu Company, when a single vulcanizing agent is used as the vulcanization system, whether amines, phenols or peroxide vulcanizing agents are used, the effect on the low-temperature performance of fluororubber is not very obvious, and the difference is not large. The effect of different fluororubber varieties with different vulcanization systems and different vulcanization methods on the cold resistance of fluororubber needs further study.

Choose a suitable filler

Generally speaking, the addition of fillers will not significantly change the glass transition temperature of rubber. The effect of fillers on the cold resistance of rubber depends on the structure formed after the interaction between fillers and rubber. Different physical adsorption bonds and strong chemical adsorption bonds will be formed between activated carbon black particles and rubber molecules, and an adsorption layer (interface layer) of raw rubber will be formed on the surface of carbon black particles. The performance of this interface layer is very close to that of glassy raw rubber, which restricts the movement of molecular chains, increases the glass transition temperature of adsorbed raw rubber, and hinders the change of chain segment configuration. Therefore, it is not possible to expect to improve the cold resistance of fluororubber by adding fillers.

Research on the cold resistance of existing fluororubber varieties and the development of new fluororubbers with excellent cold resistance are of great significance to broaden the application field of fluororubber and promote the development of domestic sealing industry.