1. Brief Overview
Fluororubber refers to a polymer elastomer containing fluorine atoms on carbon atoms in the main chain or side chains, resulting in exceptional properties. The material discussed here is Fluoro-26 or Fluoro-246 (also known as Viton-type fluororubber), which are binary and ternary copolymers of vinylidene fluoride and hexafluoropropylene (the third monomer is tetrafluoroethylene). Fluororubber accounts for over 90% of global fluororubber consumption. Since the commissioning of a pilot plant at DuPont in the United States in 1956. and the completion of an 1.800-ton/year production facility in 1958. fluororubber has experienced rapid growth. In the mid-to-late 1960s, annual growth rates reached 20%-30%, followed by 10% in the 1970s and a continued growth rate of 7%-8% into the 1980s. This trend has persisted. Fluororubber is widely used in the production of specialized sealing products. Reportedly, 50% of fluororubber in the United States is used for rubber seals; in Japan, the proportion is even higher, reaching 80%.
The rapid development of Viton type fluororubber is mainly due to its best comprehensive performance, including good mechanical strength, good thermal stability, excellent medium resistance, convenient processing and production technology, and low cost. Therefore, it occupies an absolute dominant position in fluoroelastomers. It has been widely used in aerospace, aviation, transportation, petroleum, machinery, metallurgy, chemical industry and other industrial sectors, and has achieved good economic and social benefits in various fields.
2. Main properties of fluororubber
(1) Mechanical properties under normal conditions
The general matching strength of type 26 fluororubber is 10-20Mpa; elongation is 150-300%; tear strength is between 20-40KN/m, but its elasticity is poor.
The friction coefficient of fluororubber (0.8) is smaller than that of nitrile rubber (0.9~1.5).
(2) High temperature resistance
The high temperature resistance of fluororubber and silicone rubber is the best among the existing rubbers. F26-41 fluororubber can operate for long periods at 200-250°C and even for short periods at 300°C. F246 has slightly better heat resistance than F26.
Heat Resistance of Fluororubber
Test Temperature (°C) Time (hours)
204°C 10.000+ hours 232°C 3.000 hours 260°C 1.000 hours
288°C 240 hours 316°C 48 hours
In terms of aging resistance, fluororubber and silicone rubber outperform other rubber types.
Heat Aging Resistance of Various Rubbers
Ultimate Temperature Limits of Rubber Types (°C)
Fluororubber: 320°C Silicone Rubber: 320°C Nitrile Rubber: 180°C Natural Rubber: 130°C
*Rubbers with a tensile strength ≥7 MPa and an elongation ≥100% after aging for 24-26 hours at this concentration are considered to be working.
The tensile strength and hardness of fluororubber decrease significantly with increasing temperature. The characteristics of the change in tensile strength and hardness are: below 150℃, they decrease rapidly with increasing temperature; between 150 and 260℃, they decrease slowly with increasing temperature.
Performance changes of fluororubber at different temperatures
Performance 24℃ 150℃ 204℃
Tensile strength Mpa 16.83.42.0
Elongation 100% 3301 2080
Hardness (Shore A) 756563
Based on the above situation, fluororubber seals used under high temperature conditions should avoid the effect of non-assembly stress to avoid early damage at high temperature.
(3) Corrosion resistance
Fluororubber has excellent corrosion resistance. It has excellent stability to organic liquids, different fuel oils and lubricating oils. It has good corrosion resistance to most inorganic acids, hydrocarbons, benzene and toluene. It is only not resistant to low molecular weight esters, ethers, ketones and some amine compounds.
(4) Resistance to hot water and superheated steam
The stability of rubber to hot water depends not only on the base material but also on the combination of the rubber compound. For fluororubber, peroxide-cured fluororubber is superior to rubber compounds with amine and phenol curing systems. It should be said that the resistance of fluororubber to hot water and superheated steam is average, and it is not as good as EPDM. After soaking in superheated water at 180℃×24h, the volume change does not exceed 10%, and the physical properties do not change much.
(5) Compression set performance
Fluororubber is used in high-temperature sealing, and compression set is its key performance. The wide application of Viton-type fluororubber is inseparable from the improvement of its compression set.
(6) Low temperature resistance
The low temperature resistance of fluororubber is average, and the limit temperature at which it can maintain elasticity is -15~-20℃. As the temperature decreases, its tensile strength increases, and it appears strong and tough at low temperatures. When measuring a 2mm thick standard specimen, its brittle temperature is around -30°C; when the thickness is 1.87mm, it is -45°C; when the thickness is 0.63mm, it is -53°C; when the thickness is 0.25mm, it is -69°C. Generally, the operating temperature of fluororubber can be slightly lower than the brittle temperature. For example, the US military standard MIL-25879D stipulates that the operating temperature is -40~205°C. Foreign countries limit the temperature limit for the use of fluororubber in aircraft engines to -35°C.
(7) Weathering and ozone resistance
The performance of VitonA after 10 years of natural storage is still satisfactory. In air with an ozone concentration of 0.01%, there is no obvious cracking after 45 days of exposure. Its weathering and ozone resistance are satisfactory.
(8) Electrical properties
The electrical insulation performance of fluororubber is not very good and is only suitable for use under low frequency and low voltage. Temperature has a great influence on its electrical properties. When the temperature rises from 24°C to 184°C, its insulation resistance drops by 35.000 times.
(9) Gas permeability
Fluororubber has a relatively high gas solubility, a relatively low diffusion rate, and a very low overall gas permeability. The addition of fillers to fluororubber fills the gaps within the rubber, thereby reducing the gas permeability of the vulcanized rubber, which is very beneficial for vacuum sealing. If properly matched, fluororubber can achieve a 10-7Pa vacuum seal.
(10) Flame resistance
Fluororubber can burn when exposed to fire and extinguish itself when removed from the fire.
