Detailed comparison of fluororubber seals and nitrile rubber seals

In the industrial field, the selection of sealing rings is crucial. Fluororubber (FKM) and nitrile rubber (NBR) are common sealing materials, each with different chemical compositions and physical properties, suitable for different working conditions. In order to help engineers and technicians better understand the advantages and disadvantages of the two, this article will comprehensively compare fluororubber and nitrile rubber sealing rings from the aspects of chemical composition, physical properties, mechanical properties, and application fields.

1. Chemical composition and molecular structure

Fluororubber (FKM):

Fluororubber is a type of fluorine-containing synthetic rubber, usually copolymerized with vinylidene fluoride (VDF), hexafluoropropylene (HFP) and other fluoroolefin monomers. Fluororubber contains a high proportion of fluorine atoms, and the fluorine content is usually between 66% and 70%. The C-F bond (bond energy of about 485 kJ/mol) of fluororubber is very stable, which gives it excellent chemical corrosion resistance and thermal stability. The higher the fluorine content, the stronger the chemical resistance and high temperature resistance of the material, but the flexibility and low temperature performance will be reduced accordingly.

Nitrile rubber (NBR):

Nitrile rubber is copolymerized by butadiene (BD) and acrylonitrile (ACN). The content of acrylonitrile is usually between 18% and 50%. The higher the content, the better its oil resistance, but the poorer its low temperature flexibility. The presence of polar cyano (-CN) in the molecular chain of nitrile rubber makes it highly oil-resistant, especially to mineral oil and petroleum-based oils.

2. Comparison of physical properties

Heat resistance:

Fluororubber (FKM): It has a wide temperature resistance range, usually between -20°C and 200°C. Some special types of fluororubber can even work at high temperatures above 250°C, which is suitable for sealing applications in high temperature environments.

Nitrile rubber (NBR): The temperature resistance range is -40°C to 120°C. Its heat resistance is not as good as fluororubber, and it is suitable for medium temperature environments. NBR with high acrylonitrile content has better heat resistance, but it is not recommended for use under extreme high temperature conditions.

Chemical resistance:

Fluororubber (FKM): Fluororubber has excellent resistance to a variety of chemicals, especially in acids, alkalis, oils, fuels and other media. FKM has good tolerance to most polar solvents (such as ketones and esters), but is sensitive to high concentrations of alcohol solvents and some amines.

Nitrile rubber (NBR): NBR performs well in mineral oils, petroleum-based oils and lipid media, but has poor tolerance to some polar organic solvents (such as ketones and aromatic compounds). Strong acids and strong alkalis can also corrode NBR.

Weather resistance:

Fluororubber (FKM): It has strong resistance to aging factors in the natural environment such as ozone, ultraviolet rays, and oxygen, and is very suitable for long-term exposure to outdoor use.

Nitrile rubber (NBR): It has poor weather resistance and ozone resistance, and is easily aged under the action of ultraviolet rays and ozone, so it is usually only used in indoor or protected applications.

3. Mechanical properties comparison

Wear resistance:

Fluororubber (FKM): Due to its harder structure, fluororubber has relatively poor wear resistance and is not suitable for frequent friction or heavy-load friction applications.

Nitrile rubber (NBR): Nitrile rubber has excellent wear resistance and mechanical fatigue resistance, and is widely used in seals that require friction resistance, such as oil seals, O-rings, etc.

Elasticity and flexibility:

Fluororubber (FKM): It can still maintain good elasticity under high temperature conditions, but will gradually harden and even lose flexibility under low temperature conditions. Such materials may become brittle at extremely low temperatures (below -20°C).

Nitrile rubber (NBR): Nitrile rubber has excellent elasticity under medium and low temperature environments, especially more flexible at low temperatures, and is suitable for occasions that require low-temperature flexibility.

Compression set resistance:

Fluororubber (FKM): It has good compression set resistance under high temperature and high pressure conditions, and can still maintain the sealing effect after long-term use.

Nitrile rubber (NBR): At room temperature, NBR has good resistance to permanent compression deformation, but it is prone to compression deformation when used for a long time in a high temperature environment.

4. Comparison of application fields

Fluororubber (FKM):

Due to its excellent chemical resistance and high temperature resistance, fluororubber is widely used in harsh working conditions, such as:

Engine seals in the aerospace field

Seals in petrochemical equipment

High-temperature parts in the automotive industry, such as engine seals

Corrosion-resistant equipment in the pharmaceutical and food industries

Nitrile rubber (NBR):

Nitrile rubber is mainly used in the following fields due to its excellent oil resistance and low cost:

Oil seals, O-rings, and hoses in the automotive industry

Seals and gaskets in mechanical equipment

Sealing systems for lubricating oil and fuel oil pipelines

Seals in hydraulic systems

5. Economic cost analysis

Fluororubber (FKM) has a high cost due to its complex production process and expensive raw materials, and is usually used in high-performance and high-demand applications. Nitrile rubber (NBR) is widely used in large-scale industrial production due to its low production cost. For occasions that do not require particularly high temperature and chemical resistance, nitrile rubber is a more economical choice.

6. Summary and suggestions

Fluororubber (FKM) and nitrile rubber (NBR) each have their own advantages and limitations. The following factors should be considered when making a specific choice:

Fluororubber (FKM): Suitable for environments with high temperature, high pressure, and highly corrosive chemical media, especially in aerospace, petrochemical, automotive engines and other fields with extremely high sealing requirements.

Nitrile rubber (NBR): Suitable for environments with medium temperature and medium chemical corrosion, especially for occasions with high oil resistance requirements, such as the automotive industry and general mechanical equipment.

The choice between the two should be weighed according to the working environment, performance requirements and budget. If the working environment temperature is high or the chemical medium is complex, fluororubber will be a better choice; in applications that are not demanding and cost-sensitive, nitrile rubber will provide good cost performance.

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