Key performance and application considerations of fluororubber (FKM) O-ring seals

How do FKM O-Rings perform in extreme conditions?

In the modern industrial field, the operating environment of equipment is becoming increasingly complex, and extreme working conditions have put forward stringent requirements on sealing components. With its unique molecular structure and chemical properties, FKM O-rings have excellent performance in high temperature resistance, chemical resistance, aging resistance, etc., making them an ideal choice for sealing under extreme working conditions.

The high temperature resistance of FKM O-ring seals is particularly outstanding. Its main chain is composed of carbon-carbon bonds and fluorine-carbon bonds. The fluorine-carbon bond has an extremely high bond energy of up to 485kJ/mol, which is much higher than the carbon-hydrogen bond (413kJ/mol). This enables FKM to work stably for a long time in a high temperature environment of 200℃-250℃. Some specially formulated FKM can even withstand instantaneous high temperatures of 300℃. In scenarios such as automobile engine sealing and sealing of high-temperature parts of petrochemical pipelines, FKM O-rings can effectively prevent medium leakage with their high temperature resistance and ensure continuous and stable operation of equipment.

Resistance to chemical corrosion is another core advantage of FKM O-rings. The strong electronegativity of fluorine atoms forms a highly stable electron cloud shielding layer on the surface of the FKM molecular chain, greatly reducing the possibility of the molecular chain reacting with chemical media. Therefore, FKM O-rings have good tolerance to most organic solvents, inorganic acids, and strong oxidants. For example, in strong oxidizing acid environments such as concentrated sulfuric acid and concentrated nitric acid, as well as organic solvent environments such as gasoline and diesel, FKM O-rings can still maintain good sealing performance and physical and mechanical properties. However, it should be noted that FKM has poor tolerance to polar solvents such as amines, ketones, and esters, and careful evaluation is required when using it in these media environments.

In terms of aging resistance, FKM O-rings also perform well. Whether it is thermal oxidative aging, ozone aging or ultraviolet aging, FKM shows strong resistance. During the thermal oxidative aging process, the stability of the FKM molecular chain effectively slows down the rate of oxidative degradation; its molecular structure has a natural resistance to ozone and can be used for a long time in a high-concentration ozone environment without cracking; at the same time, FKM has a weak ability to absorb ultraviolet rays, and when used in outdoor environments, its aging rate is significantly lower than many other rubber materials.

How to correctly select the hardness and material formula of FKM O-rings?

The hardness and material formula of FKM O-rings directly affect their sealing performance and service life. Correct selection is the key to ensuring the sealing effect.

Hardness is one of the important performance indicators of FKM O-rings, usually expressed in Shore A, with a common range of 60-90 Shore A. FKM O-rings with lower hardness (such as 60-70 Shore A) have good flexibility and compression deformation recovery ability, and are suitable for working conditions with high surface roughness or large sealing gaps. They can better fill small defects on the sealing surface and form an effective seal. However, low-hardness O-rings are prone to extrusion deformation under high-pressure environments, resulting in seal failure. FKM O-rings with high hardness (80-90 Shore A) have higher anti-extrusion capabilities and are suitable for high-pressure sealing scenarios, but their flexibility is relatively poor and require a higher degree of fit on the sealing surface.

The fluorine content in the material formula is the core factor affecting the performance of FKM O-rings. The higher the fluorine content, the stronger the chemical resistance and high temperature resistance of FKM, but it will also lead to increased material hardness, increased processing difficulty and higher costs. Generally speaking, medium fluorine rubber with a fluorine content of 66% - 71% strikes a good balance between chemical resistance, physical and mechanical properties and cost, and is suitable for most conventional industrial sealing scenarios; while high fluorine rubber with a fluorine content of more than 75%, although the chemical resistance and high temperature resistance are further improved, is expensive and is mainly used in aerospace, semiconductors and other fields with extremely high performance requirements.

The curing system also has an important influence on the performance of FKM O-rings. Commonly used curing systems include peroxide curing system, amine curing system and phenolic resin curing system. FKM O-rings cured by peroxide curing system have excellent high temperature resistance and compression permanent deformation performance, and the vulcanized rubber has high purity, which is suitable for industries with high hygiene requirements such as food and medicine; the amine curing system has a fast curing speed, and the vulcanized rubber has high tensile strength, but the high temperature resistance is relatively poor; the phenolic resin curing system can give FKM O-rings good chemical resistance and temperature resistance, and is widely used in the petrochemical field.

Where is the applicable boundary between FKM and other elastomer seals?

In the selection of sealing ring materials, FKM and elastomers such as NBR, HNBR, and FFKM each have their own advantages and disadvantages. Clarifying their applicable boundaries will help to make a reasonable selection.

Nitrile rubber (NBR) is one of the most widely used rubber sealing materials. Its biggest advantage is that it has good tolerance to mineral oil, animal and vegetable oils, and it is low-priced and has excellent processing performance. The operating temperature range of NBR is generally -40℃ - 120℃. It is suitable for scenes such as automotive fuel systems and hydraulic systems that have high requirements for oil resistance but relatively mild temperature and chemical medium environments. However, NBR's temperature resistance, chemical resistance and aging resistance are far inferior to FKM, and it will age and fail quickly in high temperature and strong chemical medium environments.

Hydrogenated nitrile rubber (HNBR) is a hydrogenated product of NBR. By hydrogenating the double bonds in the NBR molecular chain, its high temperature resistance, aging resistance and chemical resistance are significantly improved. The operating temperature range of HNBR can reach -35℃ - 150℃. In some medium temperature and chemical medium environments, its performance is close to FKM, but the price is relatively low. However, the performance of HNBR in strong oxidizing media and high temperature environments is still not comparable to FKM. It is suitable for working conditions such as automotive engine peripheral seals and industrial gearbox seals.

Perfluoroelastomer (FFKM) is the rubber material with the highest fluorine content. It has better high temperature and chemical resistance than FKM. It can work for a long time at a high temperature of 327°C and can withstand almost all chemical media. However, FFKM is expensive, difficult to process, and has poor low-temperature performance. Therefore, it is mainly used in special fields such as semiconductor manufacturing and chemical reactor sealing, which have extremely high requirements for sealing performance and do not consider costs. In contrast, FKM has found a better balance between performance and cost, and is suitable for conventional sealing needs in most industrial fields.

What technical points should be paid attention to when installing and using FKM O-rings?

Correct installation and use are the key to maximize the sealing performance of FKM O-rings and extend their service life. Attention should be paid to technical points such as surface roughness, compression rate design and failure mode.

The surface roughness of the sealing surface has a significant impact on the sealing effect of the FKM O-ring. Generally speaking, the surface roughness Ra value of the sealing surface should be controlled between 0.8 and 3.2μm. A surface that is too rough will scratch the surface of the O-ring and form a leakage channel; a surface that is too smooth will not be conducive to the fit between the O-ring and the sealing surface, and interface leakage is prone to occur. In addition, the processing accuracy of the sealing surface must also be strictly controlled to avoid dimensional deviations that lead to improper installation of the O-ring.

The compression rate design of FKM O-rings is directly related to the sealing effect and service life. If the compression rate is too high, the aging and wear of the O-rings will be accelerated, shortening the service life; if the compression rate is too low, an effective seal cannot be formed. Generally, the compression rate of FKM O-rings is recommended to be controlled at 15% - 25% for static sealing, and the compression rate can be appropriately reduced to 10% - 15% for dynamic sealing. At the same time, the influence of factors such as working temperature and medium pressure on the compression rate must also be considered. In a high temperature environment, the material will undergo thermal expansion, and the compression rate should be appropriately reduced; in a high pressure environment, the compression rate needs to be appropriately increased to prevent extrusion deformation.

Understanding the failure modes of FKM O-rings can help prevent failures in advance. Common failure modes include extrusion failure, wear failure, aging failure, and chemical corrosion failure. Extrusion failure mainly occurs in high-pressure environments. When the sealing gap is too large, the O-ring will be squeezed into the gap and damaged. This can be avoided by selecting O-rings with appropriate hardness and controlling the sealing gap. Wear failure is mostly caused by friction during dynamic sealing. Wear can be reduced by optimizing the sealing structure and using lubricating media. Aging failure and chemical corrosion failure are closely related to the working environment. It is necessary to select the appropriate material formula according to the actual working conditions and regularly check and replace the O-rings.