FKM/FPM

Definition and Standards Codes (FKM vs. FPM)

FKM and FPM refer to the same material family: fluoroelastomers. These are fluorinated synthetic rubbers used in sealing technology as elastomer materials for demanding media and temperatures. Both abbreviations appear in datasheets, which often raises the question of whether two different materials are involved. Technically, the classification is identical — the difference lies in the standards system.

FKM is the abbreviation according to ASTM D1418. FPM is the abbreviation according to ISO/DIN 1629. Which designation appears in a datasheet therefore often depends on whether the document is written more in the ASTM or in the ISO/DIN context. For material selection, however, the abbreviation alone is rarely sufficient — what counts is the specific compound. A compound is the finished rubber mixture made up of base polymer, fillers, crosslinking system and additives, and it is the compound that defines the actual property profile.

What Does “Fluoroelastomer” Mean in Sealing Technology?

In sealing technology, FKM/FPM is regarded as a high-performance elastomer, because in many cases it offers high resistance to oils, fuels and many chemicals. For this reason, it is frequently used in O-rings, molded seals and other elastomer seals when the media are more aggressive or the temperature is significantly above what standard elastomers can reliably cover.

A practical rule of thumb: when a seal is in continuous contact with oil, fuel or chemically loaded fluids and an elevated temperature is also expected, FKM/FPM is often considered early in material selection.

Typical Properties Relevant for Seals

For seals, the decisive properties are those that keep the sealing contact stable over time. FKM/FPM is frequently chosen when temperature resistance and media resistance are needed at the same time. Many compounds are specified for service ranges of about 200–220 °C, although this range depends strongly on the formulation, the crosslinking system and the operating duration.

The same logic applies to chemical resistance: FKM/FPM frequently shows very good results against mineral oils and fuels as well as against many solvents and chemicals. Whether a specific fluid is actually suitable, however, is ultimately decided by the media list or the approval issued for the relevant compound, since two FKM/FPM grades can differ markedly.

Compression Set as a Selection Criterion

The compression set describes how much an elastomer remains permanently deformed after sustained compression. When a seal stays “flattened”, the restoring force decreases, and the contact pressure can drop over time. In static seals and especially at higher temperatures, this effect quickly becomes the dominant failure mechanism.

In datasheets, compression set is reported as a test value, often in line with ASTM D395 (the specific test method and conditions vary from datasheet to datasheet). For practice, the key point is to always read the value together with temperature, test duration and specimen geometry, because only then is a meaningful comparison possible.

Limits, Variants and Typical Trade-offs

FKM/FPM is not the best choice in every situation. A typical limit of many grades is cold flexibility. At low temperatures, standard FKM/FPM compounds can become stiffer, which complicates installation, dynamic sealing functions or response to small deformations.

In addition, trade-offs arise because properties can rarely be maximized at the same time. Formulation and crosslinking influence the behavior significantly. In practice, several boundary conditions therefore decide together: medium, temperature range including peaks, type of motion (static or dynamic) and the requirement on compression set. A material decision is therefore always a balancing act between tightness, service life and the boundary conditions of the application.

Low-Temperature FKM/FPM: When It Is Useful, and Which Compromises Are Involved

Low-temperature FKM/FPM is used when the seal must remain sufficiently elastic at low temperatures. This is relevant, for example, when systems start outdoors, when cold starts are frequent, or when a seal has only small deformation reserves during installation and operation.

These improved low-temperature properties are often achieved through formulation and crosslinking. Other indicators may shift in the process — for example the set behavior or the high-temperature stability. For this reason, “better in the cold” is in many cases a targeted optimization for one specific requirement that must then be checked against the remaining operating conditions.

Distinction from FFKM/FFPM and Practical Selection Notes

When FKM/FPM reaches its chemical or thermal limits, perfluorinated elastomers come into play: FFKM (ASTM) or FFPM (ISO/DIN). These materials have a higher degree of fluorination and typically offer even better chemical and temperature resistance. In many applications, however, they are considerably more expensive, which is why they usually only make sense once the boundary conditions truly demand it.

For sealing technology, the most important consequence is this: neither “FKM/FPM” nor “FFKM/FFPM” alone guarantees suitability. What counts are the compound data (datasheet, media resistance, test data) and the specific application.

Short Checklist: Which Information Must Be Available for Selection?

• Which medium is in contact (e.g. hydraulic fluid, fuel, solvent, additive packages)?
• Which temperature range actually applies, including temperature peaks and exposure duration?
• Which type of motion does the sealing position involve (static, oscillating, dynamic)?
• Which set requirement follows from the installation space and preload (compression set relevant)?
• Which datasheet applies to the specific compound, including test standards and conditions?

For critical media or limit temperatures, a brief consultation with material or sealing specialists is often advisable, because small differences in the compound can have major effects on tightness and service life.