As a very important member of traditional high-performance materials, silicone materials have many varieties and are widely used. They have excellent properties such as high and low temperature resistance, weather aging resistance, water resistance, electrical insulation, and biocompatibility. Fluorosilicone polymer materials are obtained by introducing fluorine-containing groups into the molecular structure of polysiloxane, which not only retains the excellent performance of silicone materials, but also has the characteristics of organic solvent resistance of fluororubber. The application field has a certain market scale and is widely used in aerospace, transportation, chemical industry, medical and health care, construction and electronic appliances and other industrial fields. According to reports, the global annual output of fluorosilicone rubber in 2020 is about 10 kt, and the market size is about 214 million US dollars. In addition, the compound annual growth rate of the global fluorosilicone rubber market is as high as 7.4%, and it is estimated that by 2026, the global market size will increase to 328 million US dollars.
1 Composition and classification of fluorosilicone polymer materials
1.1 Composition of fluorosilicone polymer materials
1.1.1 Raw rubber
The raw rubber used in fluorosilicone polymer materials is usually fluoropolysiloxane with active end groups, which can be cross-linked by capping with reactive groups. The capped active groups are mainly hydroxyl and vinyl groups, and there are also raw rubber for fluorosilicone capped by other groups such as alkoxy groups. As a new type of fluorine-containing structure, perfluoroalkyl polyether has great advantages in heat resistance and flexibility, and has gradually been introduced into fluorosilicone raw rubber.
1.1.2 Cross-linking agent
The crosslinking agent usually uses silane or siloxane containing 2 or more silicon functional groups, such as methyltrimethoxysilane, ethyl orthosilicate, and the like. A single cross-linking agent molecule reacts and cross-links with multiple raw rubber molecular end groups by virtue of its own multifunctional groups, thereby forming a cross-linked network. Different types of fluorosilicon polymer materials use different crosslinking agents, so the characteristics of different types of fluorosilicon polymer materials are also different.
1.1.3 Crosslinking catalyst
Cross-linking catalysts are mainly used to accelerate the vulcanization process of fluorosilicone polymer materials and improve the process performance and construction performance of materials. The crosslinking catalysts used in condensation fluorosilicone polymer materials mainly include organic tin carboxylate and its chelate, titanate and its chelate, etc., among which the catalytic effect of the former is better (for example, the catalyst has two lauryl dibutyltin acid, stannous octoate, etc.). The catalysts used in addition-type fluorosilicon polymer materials are mainly Group VIII transition metal compounds and their complexes, such as platinum, palladium, nickel, and the like. Among them, the platinum catalyst exhibits excellent catalytic activity (such as the catalyst has chloroplatinic acid, etc.).
Filler is a series of substances added according to the material use environment and engineering application-related requirements, mainly to improve material-related properties (such as mechanical properties, aging resistance, etc.), adjust material process performance and construction performance (such as cross-linking Speed, difficulty of construction, etc.). Depending on the requirements, the fillers used are also different. In the process of preparing fluorosilicon polymer materials, white carbon black, titanium dioxide, etc. can be used as reinforcing fillers, and iron oxide red can be used as heat-resistant fillers.
1.2 Classification of fluorosilicone polymer materials
1.2.1 Classification by vulcanization process
According to the vulcanization process, fluorosilicon polymer materials are divided into two types: condensation fluorosilicon polymer materials and addition fluorosilicon polymer materials.
The main polymerization method of condensation-type fluorosilicone polymer materials is to achieve polymerization by removing small molecules through condensation reaction between raw rubber and vulcanizing agent with special reactive functional group end groups, as shown in Figure 1
Reactive functional group terminal groups include hydroxyl groups, amine groups, carboxyl groups, silyl groups, ketoxime groups, isocyanate groups, silicate groups, and the like. Such fluorosilicon polymer materials often use organotin (usually dibutyltin dilaurate, etc.) or titanate as vulcanization catalysts. According to the different types of molecules removed by the condensation reaction, condensation fluorosilicon polymer materials can be divided into dehydrogenated fluorosilicon polymer materials, dealcoholized fluorosilicon polymer materials, deacidified fluorosilicon polymer materials, and deketone-based fluorosilicon polymer materials. /Oxime type fluorosilicon polymer materials, deamination type fluorosilicon polymer materials, etc. This kind of fluorosilicone polymer material has strong vulcanization activity and good process performance, and can be vulcanized well at room temperature. After vulcanization, it can be used for a long time in the environment of -55-180 ℃, and can even be used in the environment of 230 ℃ for a short time. At the same time, it has extremely strong resistance to hydrocarbon fuels, water and other aging properties, and has good bonding performance to base materials such as anodized aluminum alloy and stainless steel.
During the vulcanization process of addition-type fluorosilicone polymer materials, polyene (commonly polyvinyl)-terminated fluorosilicone raw rubber adds to the vulcanizing agent containing silicon-hydrogen bonds, thereby forming a network structure and crosslinking , as shown in Figure 2 .
Commonly used catalysts are platinum catalysts (such as chloroplatinic acid H2ClPt·6H2O). Considering that the catalyst activity is too strong, it is necessary to avoid premature vulcanization due to high activity during storage. Addition-type one-component fluorosilicon sealants are mostly vulcanized at low temperature and high temperature. Therefore, the addition-type two-component fluorosilicone sealant is researched and developed for room temperature vulcanization. This type of sealant has strong high and low temperature resistance, and can be used for a long time in the environment of -55 ~ 180 ℃ after vulcanization; at the same time, it has good electrical properties and excellent oil resistance (mainly reflected in the small mass loss and volume expansion after being soaked by fuel oil) ). Compared with condensation-type fluorosilicone sealants, addition-type fluorosilicone sealants are capable of deep vulcanization and will not degrade when vulcanized in a closed environment.
1.2.2 Classification by component
According to the number of components of each component of the sealant before vulcanization, fluorosilicon polymer materials can be divided into single-component fluorosilicon polymer materials and multi-component fluorosilicon polymer materials.
Single-component fluorosilicon polymer materials refer to fluorosilicon polymer materials in which each main component of the material is packaged and stored in one container. The components of this type of fluorosilicon polymer material are usually stored under specific inert conditions, and will not react with each other (or the reaction speed is very slow); and under appropriate conditions (such as special reaction temperature, relatively high humidity, etc.) , each component vulcanizes rapidly and forms a cross-linked network. The multi-component fluorosilicon polymer material is stored separately according to the needs of the process. During construction, the components of the fluorosilicone polymer material are uniformly mixed according to the specific construction environment and construction methods, and then cross-linked. One-component fluorosilicone polymer material has simple and convenient construction process, but it is difficult to store (it is difficult to find a suitable environment to suppress the premature vulcanization of the sealant). Multi-component fluorosilicon polymer materials are easy to store and maintain reactivity, but they put forward some requirements on construction conditions and methods.
1.2.3 Classification by Fluorine Content
Depending on the type of structure of the organosilicon monomer involved in the polymerization (such as whether the branched chain contains fluorine, etc.), the fluorine content of the prepared fluorosilicon polymer material is usually different. This in turn enables the classification of products according to the proportion of fluorine-containing monomers in the raw rubber monomers of fluorosilicon polymer materials. In common fluorosilicon polymer materials, the proportion of fluorine-containing groups is usually 30%, 50%, 100%, etc. There are also related studies on fluorosilicon polymer materials obtained by mixing monomers in other proportions.
The content of fluorine groups will directly affect the polarity of fluorosilicon polymer materials, thereby affecting its vulcanization characteristics, assembly process, and performance. When the content of fluorine-containing groups is large, its solvent tolerance is relatively stronger, but due to the large steric hindrance effect brought by side-chain fluorine-containing groups, its vulcanization activity is relatively low, and higher vulcanization is required. temperature or a longer curing time to achieve the desired degree of curing.
2 The development history of fluorosilicone polymer materials
Fluorosilicone polymer materials were first developed by Dow Corning (Dow Corning) Company with the support of the U.S. Air Force Department in the 1950s based on the research and development of dimethyl silicone rubber that had been basically formed previously.
In 1951, Pierce et al. used heptafluoropentanol as the initial reactant, dehydrated and dried under the action of phosphorus pentoxide to obtain olefins, and added hydrogen bromide at 120-140 °C to obtain brominated heptafluoro Alkanes; and then react with silicon tetrachloride or ethyl silicate under the action of Grignard reagent to prepare a variety of fluorine-containing organic compounds, which laid the foundation for the subsequent birth of fluorosilicon polymer materials. In 1971, Pierce published their further research results in JACS. They synthesized polymethyltrifluoropropylsiloxane (PMTFPS) using the ring-opening polymerization of methyltrifluoropropylcyclotrisiloxane (D3F). This is the earliest synthesized organic fluorosilicon polymer with trifluoropropyl instead of methyl. In addition, they also studied the effect of reaction conditions and the type of end-capping agent on the molecular weight and end group of the product. The comprehensive test results show that the vulcanized fluorosilicone rubber is suitable for a wide temperature range and has excellent oil and solvent resistance. The performance of the fluorosilicone sealant prepared on this basis meets the requirements of the working environment of aviation aircraft, so further application research was carried out, and it was finally commercialized.
Due to the long research time and the strong driving effect of the development of the aviation industry, foreign countries are temporarily in the leading position in the research and development of new sealants. The famous research and product development institutions include PRC-Desoto in the United States (formerly known as PPG Aeropace, which belongs to the American PPG industry. ), Minnesota Mining and Manufacturing (3M), Dow Corning (a subsidiary of Dow Chemical in the United States), ACTECH, Flamemaster, Chemetall in Germany, ShinEtsu in Japan, etc. . Many institutions in China have also conducted some research on this, such as Shanghai Sanaifu (formerly Shanghai Institute of Organic Fluorine), Shanghai Organic Institute, Beijing University of Chemical Technology, Beijing Institute of Aeronautical Materials, etc. . However, due to the relatively late start of domestic research in this area and relatively limited literature available for research and study, there is a gap between the research and development of the domestic aviation sealant industry and related products and foreign countries.
It was only in the 1960s that my country made some progress in the research of fluorosilicon polymer materials. At that time, the Institute of Chemistry of the Chinese Academy of Sciences led the research on the synthesis of fluorosilicone rubber; and then cooperated with the Shanghai Institute of Organic Fluorine Materials at that time, and successfully prepared fluorosilicone rubber in 1966 with properties equivalent to related products of Dow Corning in the United States.
In addition, the development of related industries in my country is relatively slow, and the demand for various sealants is relatively low; in addition, there are few types of foreign products available for research, and the technology has just started, and the performance of self-made products is unstable. High, leading to domestic research and development of fluorosilicon polymer materials into a quagmire.
Since the 21st century, important industries represented by the aviation industry have developed rapidly, and the domestic demand for new materials has become increasingly strong, which has spurred the rapid development of related industries in our country. In order to meet the needs of supporting materials in the aviation industry, domestic practitioners have analyzed the literature on fluorosilicon polymer materials at home and abroad, combined with some of their own work and research to improve the preparation technology of fluorosilicon polymer materials, and have made some progress . At present, compared with the advanced level of foreign countries, my country’s fluorosilicon industry still has defects such as fewer types, poor performance, and preference for low-end applications.
3 Research direction of fluorosilicon polymer materials
3.1 Improve vulcanization process
Affected by product structure, physical and chemical properties, etc., although fluorosilicon polymer materials have various excellent performance parameters, there are certain problems in their preparation and vulcanization process. As mentioned above, the problem of vulcanization activity of fluorosilicone polymer materials with high fluorine content is a typical representative. In addition, other fluorosilicone polymer materials have similar problems. How to better solve some technical problems in the vulcanization process of fluorosilicone polymer materials under the premise of ensuring performance has become an important research direction.
Some scholars have designed a new synthesis method of fluorosilicone polymer materials. They used the dehydrogenation condensation capping reaction of silicon hydroxyl and silicon hydrogen, as shown in Figure 3; the original hydroxyl-terminated liquid fluorosilicone rubber was alkoxy-capped (mainly methoxy, ethoxy), so that Liquid fluorosilicone rubber endcapped by trimethoxyl, triethoxyl, etc. was obtained. The experimental results show that three kinds of room temperature vulcanization (RTV) trimethoxy-terminated fluorosilicone rubbers with different molecular weights have high reactivity in the room temperature vulcanization. Under the premise, it only takes half the time of general liquid fluorosilicone sealants to reach the specified degree of vulcanization, which shows that the introduction of alkoxy groups effectively reduces the shielding effect of the highly polar trifluoropropyl group on the vulcanization reaction. . Using SEM to analyze the microscopic morphology of the liquid nitrogen brittle section, it was observed that the section phase of the material was relatively uniform, and there were no obvious particles or pits, showing good cohesion. In addition, fluorosilicone polymer materials also exhibit good mechanical properties, heat resistance and aging resistance, the results are shown in Table 1.
A new heat-resistant fluorosilicone elastomer material was designed and synthesized by adding a small amount of reduced graphene oxide (rGO) to the system and using 2,5-dimethyl-2,5-bis(tert-butyl ) for cross-linking with hexane peroxide, and the desired product was successfully prepared. The test results show that the elongation at break of the fluorosilicone elastomer material after the introduction of rGO decreases slightly, but it can still maintain stable mechanical properties after being treated at 230 °C for 72 h, and the results of the thermogravimetric test also show that the introduction of rGO The Td of the fluorosilicone elastomer material after rGO is significantly higher than that of the common fluorosilicone elastomer material. The mechanism by which a small amount of rGO can improve the thermal oxidation resistance of materials may be that rGO scavenge the free radicals generated by the side group cracking of materials, thereby blocking the autoxidative degradation pathway.
3.2 Improve existing performance
With the expansion of our country’s material research and application fields, the performance requirements for fluorosilicone polymer materials have also gradually increased. For example, the harsh high-altitude environment and the advancement of fuel systems have put forward higher requirements for the use temperature range of fluorosilicon sealants. At the same time, it is also hoped that fluorosilicon polymer materials can have better tolerance in increasingly rich working environments. Aging resistance. In addition, the mechanical performance index is also a performance that must be considered for fluorosilicon polymer materials.
A fluorosilicone sealant resistant to 15# aviation hydraulic oil with wide temperature range was prepared. They used homopolyfluorosilicone rubber and modified copolyfluorosilicone rubber for mechanical blending to achieve the effect of vulcanization, and observed the impact of this variable on the physical properties of the rubber by controlling the ratio of the two. The research results prove that the amount of modified copolyfluorosilicone rubber has little effect on the mechanical properties of the rubber compound (such as hardness, tensile strength, elongation at break and permanent deformation at break, etc.), but this variable has little effect on the The impact on oil resistance is more obvious, and the rubber with a low proportion of modified copolyfluorosilicone rubber has less change in mechanical properties after soaking in hydraulic oil, showing better oil resistance. At the same time, the rubber compound with a high proportion of modified copolyfluorosilicone rubber has more advantages in brittle temperature, low-temperature compression cold resistance coefficient, and low-temperature retraction performance, and shows better high and low temperature resistance.
The effect of high vinyl silicone oil on the properties of fluorosilicone polymers was studied. They added high-vinyl silicone oil to the fluorosilicone polymer material to allow it to participate in the vulcanization process, and observed the effect of its dosage on the properties of the sealant material. The test results show that the high vinyl silicone oil significantly increases the number of reaction sites inside the polymer, and the overall crosslink density increases, thereby increasing the hardness and reducing the elongation at break. At the same time, since the high vinyl silicone oil significantly reduces the polarity of the fluorosilicone sealant material, the material’s resistance to polar solvents (such as phosphate ester hydraulic oil) has been significantly improved. Although there is little change in the heat resistance of the material, its low temperature resistance has been significantly improved.
A new construction strategy of fluorine-silicon composites was designed and constructed to obtain fluorine-silicon composites with higher mechanical strength. Utilizing the thermodynamic migration of nano-SiO2 premixed in the silicone rubber (SR) phase, it reaches the interface between fluororubber (FKM) and SR and the FKM phase, assisting the interface between the two to generate chain entanglement and form a core-shell structure, improving the interface Compatibility, thereby improving the mechanical properties of the product. After testing, the maximum tensile strength of the fluorosilicon composite material obtained by this method is 7.6 MPa, and the elongation at break reaches 165%, which is significantly higher than that of the fluorosilicon composite material obtained by other methods.
3.3 Develop new features
Today, the working environment and tasks performed by fluorosilicon polymer materials are becoming more and more abundant, and different work needs require fluorosilicon polymer materials with different properties. Therefore, many researchers try to endow aviation sealants with new properties through some means. For example, equip the sealing position of aerospace vehicles with stealth requirements with fluorosilicon sealants with wave-absorbing functions, and use sealants with good conductivity for the sealing positions of some microelectronic components with sealing requirements.
The dynamic properties of fluorosilicone polymers were studied. They used a dynamic testing machine to observe and record the dynamic characteristics of fluorosilicone polymer materials under different conditions of temperature, frequency, and amplitude under low-frequency and large-deformation conditions, and characterized them by loss angle and dynamic stiffness. The test results show that due to the influence of the polarity of the trifluoropropyl side group, the oil resistance of the fluorosilicon sealant is improved compared with the silicone sealant, and its damping performance is also improved to a certain extent.
By introducing graphene nanosheets (GNPs) and barium titanate nanoparticles (BT) into the fluorosilicone rubber system, the dielectric and mechanical properties of the fluorosilicone rubber system were improved. Graphene and barium titanate particles were processed by co-milling to obtain GNPs and BT, which were subsequently introduced into the fluorosilicone rubber system. The test results of dielectric properties show that the dielectric constant of the prepared material at 100 Hz is 14.54, and the dielectric loss tangent is 0.016. The dielectric loss tangent of the material can be kept at 10-2 by controlling the number of nanoparticles and its magnitude below. In addition, the mechanical properties of the material have also been improved, with a tensile strength of 1.9 MPa and an elongation at break of 332.12%. This work provides a simple new idea for dielectric elastomer materials, which has broad development prospects in the field of electronics.
4 Prospects of fluorosilicone polymer materials
4.1 Introduction of special functional groups
The main structure of the main chain of the fluorosilicon polymer material is relatively single (silicon-oxygen bond Si—O), which also limits the further improvement of the material performance to some extent. Therefore, the introduction of other functional groups can improve the performance of the material. The molecular structure can be adjusted to further improve the performance of the material. For example, introducing a polysesquioxane structure (POSS) structure into a fluorosilicon polymer material can significantly improve the high temperature resistance of the molecule.
4.2 Research and application of chemical reaction fillers
The preparation of fluorosilicon polymer materials mostly uses physical fillers (such as calcium carbonate, white carbon black, etc.). Physical fillers are often difficult to disperse evenly and agglomerate, which affects material properties. By chemically modifying the surface of commonly used physical fillers, that is, introducing groups that can react with other components (such as vinyl, mercapto, etc.) The uniform distribution of the filler is conducive to the improvement of the overall performance of the material.
4.3 Application in combination with other materials
On the whole, the application products of fluorosilicon polymer materials on the market are mainly divided into the following categories: fluorosilicon elastomers, fluorosilicon defoamers, fluorosilicon adhesives and sealants, fluorosilicon coatings, etc., which are basically single Appropriate introduction of some other material systems is conducive to the complementarity of the properties of materials, thereby improving the overall performance. For example, a coating material with high anti-icing performance can be obtained by mixing nano-polytetrafluoroethylene, epoxy resin and fluorosilicon polymer materials.
This paper summarizes the composition classification of fluorosilicon polymer materials and the current main research directions. On the whole, fluorosilicon polymer materials are rich in types and have a wide range of application scenarios. Formula design can be carried out based on performance requirements. It has good designability and engineering applicability, and has a promising development prospect.