FFKM (Perfluoro Rubber)




When a molded rubber part has to withstand exposure to a highly aggressive chemical environment or perform well in high temperatures, FKM is your right choice. However, for some applications, even FKM reaches its limits and cannot be deployed due to exceptionally aggressive chemicals or extremely high operating temperatures. In these special cases, engineers and users turn to FFKM. This material that incorporates special perfluorinated monomers, in other words, entirely hydrogen-free monomers, has been developed to resist virtually all media, and withstand very high temperature stresses. It is known to many by the tradename Kalrez marketed by DuPont de Nemours.

In this blog post, we will be looking in some detail at the properties of FFKM and discuss how, even for FFKM, there can be important differences in its thermal and chemical performance.

General information about FFKM

FFKM is an abbreviation for perfluorinated rubber. Another abbreviation commonly seen, often in Europe, is FFPM. In industry, the name Kalrez is routinely used as a synonym for FFKM. Kalrez is the trade name of DuPont de Nemours, under which the company markets its FFKM products. DuPont de Nemours primarily sells finished parts, whereas compounds of FFKM can only be purchased from other producers like Daikin Industries of Japan. 

The media and temperature resistance of FFKM is broadly like that of PTFE (polytetrafluoroethylene, also known under the trade name Teflon). Due to its high crystallinity (meaning it is unyielding and inflexible) PTFE belongs to the thermoplastics, but has properties that would justify its classification as a duroplastic (you can find out more about the difference between elastomers, thermoplastics and duroplastics here).    

Essentially, by incorporating monomers, an elastomer can be produced with resistance properties almost the same as those of PTFE. When partially fluorinated monomers are used, then you get fluoroelastomers (FKM). However, if completely fluorinated, that is, hydrogen-free monomers are used, the result is a perfluoroelastomer FFKM.

Cross-linking agents for FFKM compounds can be diamines, bisphenol or peroxides. Triazine cross-linking is also an option. The type of cross-linking significantly impacts the thermal and chemical resistance of an FFKM compound. This is something that must be taken into account when selecting the correct FFKM compound.  It makes good sense to work closely with the manufacturer regarding the application specifications for the material.

Mechanical properties of FFKM

FFKM is not generally deployed because it has excellent mechanical properties, but because it has exceptional media and thermal performance. 

FFKM is nonetheless an exceptionally reliable material that can be used in applications over an extended period of operation.

As a rule, FFKM compounds are offered with a Shore A hardness of 60 to 90.

As FFKM is used mainly for O-rings or seals, its compression set is important. The compression set of a typical FFKM compound with a Shore A hardness of about 70 is between 15 percent and 40 percent (at 70h and 200 °C).

Elongation at break can be estimated at between 125 and 250 percent. Tensile strength is generally between 10 and 20 MPa.

Thermal properties of FFKM

One of the great strengths of FFKM is its excellent heat performance. FFKM finished parts can be subjected to a temperature of up to +325 °C.

It is no surprise therefore, that FFKM is also used as a material for O-rings and seals, because it also boasts very good shape-retaining recovery, or compression set, at high temperatures. High temperatures can corrode rubber components and irreversibly damage the cross-linking structure. For example, if rubber seals are deployed in conditions above their maximum continuous service temperature, they can become hard, brittle and lose recovery and sealing efficiency. FFKM produces stable results even at extremely high temperatures.  

However, one of the main disadvantages of FFKM is its minimal resistance to cold. The low temperature flexibility (Tg) of Kalrez generally is given as -8 °C by DuPont de Nemours. This is a realistic figure for the majority of the FFKM compounds. There are also compounds approved for use in low temperatures of down to -30 °C. However, these compounds do not possess the thermal stability of the other FFKM compounds. The standard FFKM temperature range is therefore -8 °C to +325 °C.

Media resistance of FFKM

Regular FKM is known for its excellent media performance. But in some respects, it is outperformed by FFKM which offers the potential for universal application. 

FFKM is known as a material resistant to nearly all chemicals. This includes amines, esters, ketones and fuming HNO3. Unlike standard FKM, it is even resistant to fire-resistant hydraulic fluids.

Exposed to oil, FFKM is stable at even extreme temperatures and does not swell (see image 1).

There are however media that are a problem for FFKM. FFKM is not resistant to alkali metals and perfluorinated hydrogen.

Heat and Oil Resistance of FFKM (ASTM 2000)

Figure 1: Heat restistance and oil resistance of FFKM in comparison with other rubber materials according to ASTM D2000 (see an "Viton® - Von DuPont Performance Elastomers"-Broschüre, 2006) 

Disadvantages of FFKM

Despite its universal chemical resistance and excellent temperature resistance, FFKM is a material that of all the elastomers is relatively rarely used.

A major reason, no doubt, is its high price. A kilogram of FFKM costs several thousand euros and, as such, is about 100 times more expensive than a conventional FKM compound. However, there are several application areas in which the use of FFKM can also make economic sense (see below).

A further issue is that FFKM is difficult to work with, so making it into parts is not easy.

The use of FFKM is also limited by its minimal cold resistance. Temperatures below -8 °C can mean the malfunction of parts made from FFKM.

Application areas of FFKM

In general, FFKM is utilized in conditions of extreme high temperature, where other elastomers would fail, or where universal chemical resistance is required. This can, for example, be in oil production, for analytical instruments, or wipers and seals for printers.

FFKM is used primarily to manufacture O-rings and seals. For more complex molded components, the challenging workability of FFKM must be considered.

Although a FFKM finished part is far more expensive than the finished part of any other rubber material, such as FKM, the use of FFKM can still make good financial sense.  If you look at the total costs for a molded rubber part over its entire lifetime, and not just the cost of the rubber molded part itself, it is possible that the reliability of the material means maintenance intervals are significantly extended and downtime can be reduced. Potentially, the overall lifetime of a plant can be extended. When making a decision on FFKM, in addition to the costs for the molded rubber part, it is worth looking at installation costs, the costs for downtime and maintenance.

Overview of FFKM properties

In conclusion, here is a recap of the properties of perfluoro rubber FFKM.

Remember, this is only a general guide and not to be used for your ultimate selection of materials. The individual properties of blends can be positively and negatively influenced by targeted formulation and as such may be different from the information presented here.

The rating ranges from ☆☆☆☆☆ (very poor) to ★★★★★ (very good).

 Mechanical Properties  
 Hardness range:  55 Shore A to 95 Shore A
 Tear strength:     ★★★☆☆
 Elongation at break:       ★★★☆☆
 Tensile strength:  ★★☆☆☆
 Compression set at high temperatures:  ★★★★☆
 Compression set at low temperatures:   ★☆☆☆☆
 Rebound resilience:  ☆☆☆☆☆
 Abrasion resistance:  ★☆☆☆☆
 Thermal properties  
 Low-temperature flexibility  ★☆☆☆☆
 High-temperature resistance  ★★★★★
 (Chemical) resistance  
 Gasoline:  ★★★★★
 Mineral oil (at 100° C):     ★★★★★
 Acids:      ★★★★★
 Alkalis:    ★★★★★
 Water (at 100° C):  ★★★★★
 Weathering and ozone:  ★★★★★
 UV/light:  ★★★★★

For more details about properties or chemical resistance, or if you have a query about a particular application, please do not hesitate to contact us.  

If you have a question about this blog post or would like us to discuss a particular aspect of elastomers in an upcoming blog, please email us on info@hepako.de   

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