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FKM rubber compounds

FKM advantages

  • Excellent temperature resistance
  • High chemical stability
  • Good weather, aging and oxygen stability
  • Excellent resistance in mineral oils and fats
  • Low gas permeability
  • Very good resistance in non-polar media
  • Temperature range from –40 °C to +225 °C

Introduction FKM

The original fluoroelastomer was developed by the DuPont Company in 1957 in response to high performance sealing needs in the aerospace industry. To provide even greater thermal stability and solvent resistance, tetrafluoroethylene (TFE) containing fluoroelastomer terpolymers were introduced in 1959. The latest FKM polymers have a much broader fluids resistance profile than standard fluoroelastomers, and are able to withstand strong bases and ketones as well as aromatic hydrocarbons, oils, acids, and steam. In 1975 a new fluoroelastomer family was introduced by Asahi Glass Co., FEPM, based on an alternating copolymer of TFE and propylene. It is marketed under the trade name AFLAS®.

FPM is the international abbreviation according to DIN/ISO, whereas FKM is the short form for the fluoroelastomer category according to the American standard ASTM. Viton® is the registered trade mark of DuPont Performance Elastomers, as is Tecnoflon® for Solvay Specialty Polymers, Dyneon™ for 3M Dyneon and DAI-EL™ for polymers supplied by Daikin. On this website we will use the abbreviation FKM.

Chemical Resistance and Stability

The chemical resistance and high temperature stability is due to the bulkiness of the fluorine atoms, shielding the polymer backbone and carbon-fluor bond from attack, and the high bonding energy of the carbon-fluor bond, as illustrated in the following figures.

FKM afbeelding 1

Figure 1. Bulkiness of fluorine atoms.

 

FKM afbeelding 2

Figure 2. Bonding energy (kJ/mol).

FKMs can normally be used at temperature up to 200 °C for prolonged periods of time. In automotive specifications often lifetimes of at least 5000 h are defined. See further “Heat resistance and thermal stability”.

Monomers for FKM

To exhibit elastomeric behavior, a polymer must be flexible and recover from substantial deformation. This requires the polymer to be substantially amorphous. Normally the polymer is cross-linked to form a three-dimensional network. The driving force for recovery upon deformation is the tendency of chain segments to return to their disordered state. Generally, fluorocarbon chains are relatively stiff compared to hydrocarbons, and therefore show rather slow relaxation and recovery from strain. Fluoroelastomers are made up of two or more different monomer units. Chains with monomers as VF2 (or VDF), TFE, and ethylene tend to crystallize if long enough. Therefore monomers with bulky side groups like HFP, PMVE and propylene are incorporated to produce an amorphous polymer. Figure 4 gives an overview of monomers typically used.

FKM afbeelding 3

 

Figure 3. Monomers for FKM production.

Fluoroelastomer Families

There are currently five (5) logical FKM elastomer categories defined differentiated only by trademarks. They might be classified as the following FKM types:

  • Type 1. Dipolymer (or copolymer) of hexafluoropropylene (HFP) and vinylidene fluoride (VF2/VDF). Copolymers are the standard type of FKMs showing a good overall performance. Their fluorine content typically ranges around 66 weight percent.
  • Type 2. Terpolymer of tetrafluoroethylene (TFE), hexafluoropropylene (HFP) and vinylidene fluoride (VF2/VDF). Terpolymers have a higher fluorine content compared to copolymers (typically between 68 and 69 weight percent fluorine), which results in better chemical and heat resistance. Compression set and low temperature flexibility may be affected negatively.
  • Type 3. Terpolymer of tetrafluoroethylene (TFE), a fluorinated vinyl ether (PMVE), and vinylidene fluoride (VF2/VDF). The addition of PMVE provides better low temperature flexibility compared to copolymers and terpolymers. Typically the fluorine content of type 3 FKMs ranges from 62 to 68 weight percent.
  • Type 4. Terpolymer of tetrafluoroethylene (TFE), propylene (P) and vinylidene fluoride (VF2/VDF). While base resistance is increased in type 4 FKMs, their swelling properties especially in hydrocarbons are worsened. Typically they have a fluorine content of about 67 weight percent.
  • Type 5. Pentapolymer of tetrafluoroethylene (TFE), hexafluoropropylene (HFP), ethylene (E), a fluorinated vinyl ether (PMVE) and vinylidene fluoride (VF2/VDF). Type 5 FKM is known for base resistance and high temperature hydrogen sulfide resistance.

Peroxide cured fluoroelastomers also contain a so-called cure-site monomer (CSM), a monomer that contains a site reactive towards free radicals. An example of such cure-site monomer is 4-bromo-3,3,4,4-tetrafluorobutene (BTFB).

Figure 5 gives an overview of the various monomers used for production of the DuPont FKM types. Instead of PMVE also MOVE (CF2=CF-O-CF2-O-CF3) is being used being used as a monomer.

FKM afbeelding 4

Figure 4. Overview of monomers used in various Viton types.

Crosslinking Theory

The process of curing (crosslinking) creates a three-dimensional structure that makes the elastomer suitable for long-term mechanical service under a sustained load (stress) or constant deformation (strain). The two services are the typical initial requirements for shock and vibration control or sealing. Invariably the crosslinks themselves are often the most vulnerable component of the cured elastomer.

By reviewing the crosslinking mechanism from both a physical (mechanical) perspective and its chemical structure, one gets first indications of long-term serviceability. In order to crosslink fluoroelastomers, there are two distinct halogen elimination reactions that are used to develop crosslinking sites in hydrofluorocarbon elastomers: 1. E2 Mechanism – the simultaneous departure of hydrogen and the adjacent fluorine initiated by a nucleophile (base). This is the logical route reaction for creating the crosslink site (a double bond in the backbone) for the vinylidene fluoride containing elastomers. 2. E1 Mechanism – normally operates without a base. The crosslink initiating mechanism is ionization provided by an electrophile (peroxide radical). The specific location is a cure-site monomer (CSM) having an iodine or bromine substitution that is readily abstracted by the peroxide radical. In the case of perfluoroelastomers, there are several suitable cure-site monomers having a reactive pendant group thus leaving the backbone intact, which in turn improves the long-term heat resistance.

After developing the crosslink site, the different chemical mechanisms for each cure route use the following different reactions:
a. Addition reaction – diamine crosslinks (old technology, currently hardly practised)
b. Aromatic nucleophillic substitution reaction – dihydroxy crosslinks (Bisphenol AF systems)
c. ENE reaction – triazine crosslinks
Keep in mind the first two reactions are ionic, and the third is a free radical reaction.

Diamine Cures

The original crosslinking chemistry used a blocked diamine, hexamethylene diamine carbamate (HMDAC). The very basic character of any amine initiates dehydrofluorination at a vinylidene fluoride (VF2) site, now followed by an amine addition (crosslink). The subsequent hydrogen fluoride (HF) molecule reacts with magnesium oxide (a compounding ingredient) rearranging to form magnesium fluoride and water. The water is loosely bound as magnesium hydroxide. Thus each crosslink is accompanied by one mole of water and one mole of carbon dioxide. This is essentially an equilibrium reaction until the water is driven off by an extended high temperature post cure. The amine cure system still enjoys some popularity as it enhances rubber-to-metal bonding and the hexamethylene crosslink is mechanically mobile offering interesting dynamic properties.

Dihydroxy Cures

The dihydroxy cure or Bispenol AF system was commercialized in the 1970’s and became an immediate success. The system offered dramatically improved compression-set resistance, excellent heat stability and greatly improved processing safety. Hydrolytic (water) stability was also improved. Again, the vinylide fluoride presence was necessary to develop a crosslinking site. For hydrocarbon service, it is the preferred cure system. However, Bisphenol cured FKMs are susceptible to attack by high temperature water and steam. Bisphenol curing is a condensation reaction in which water is a by-product. The interesting point to note is that on exposure to heat and water, this reaction can be reversed, thus ‘un-crosslinking’ the rubber. This reduction in cross-linking can also look almost the same as heat degradation, as the seal surface becomes brittle (due to the lower tensile strength). On flexing or compression, the surface starts to crack, resembling thermal degradation.

Triazine-Peroxide Cures

Triazine (peroxide) crosslinking occurs at specific sites available on the cure-site monomer (CSM). The cure-site monomer and curing mechanism was originally developed for an improved low-temperature fluoroelastomer based on vinylidene fluoride (VF2) and perfluoromethyl vinyl ether (PMVE). The very acidic VF2 and the ionic cure mechanisms (diamine and BPAF) create cleavage of the trifluoroalkoxy group on the PMVE resulting in backbone cleavage, and a perfluoromethanol by- product. The addition of the cure-site monomer allows an orderly crosslinking (electrophillic as opposed to nucleophillic) process whereby the triazine structure becomes the crosslink.

Post Cure

Most rubber compounds based on high performance elastomers require a “post-cure” after their normal press cure in order to achieve optimal cured physical properties, so do FKMs. The post-cure step is usually performed in an air-circulating oven (at ambient pressure) for 2 to 24 hours at 150-250 °C, depending on the compound.

Post-cure step is also used in order to eliminates residual volatiles formed during the vulcanization process that might disrupt or destroy the chemical crosslinks formed from curing and negatively affect the ultimate physical properties. Post curing will increase tensile strength, improve compression set, and lower elongation and thus may be required to meet demanding physical property specifications. For FKM cures, a post-cure is most critical for bisphenol cures

FKM Low-Temperature Properties

Low temperature flexibility refers to the temperature, at or below which an elastomer vulcanizate changes from an elastomeric, to a stiff, glassy state, at which point the vulcanizate is no longer flexible, and does not exhibit the ability to recover after being deformed.
Fluoroelastomers in general have low temperature properties dictated by two factors: the size of the fluorine atom and the substituent fluorocarbon molecules (trifluoro and trifluoroalkoxy groups) and the various intermolecular molecular forces that come into play due to fluorine’s high electronegativity.
Although the fluorine molecule is compact, a fully fluorine substituted carbon-carbon backbone (polytetrafluoroethylene) polymer is rod-like. To accommodate the larger size fluorine (as opposed to a hydrogen) the fluorines must be staggered giving the appearance of a slow spiral, a 180° turn for every 13-15 main chain carbons. This precondition eliminates any possibility of a rubbery state. The smaller hydrogen atom (ethylene, propylene, etc.), or a partially substituted alkyl (vinylidene fluoride) create room for chain (backbone) mobility.
The presence of a bulky branch group (methyl, trifluoromethyl or perfluoroalkoxy) causes the polymerization to create the “random walk” chain configuration that is necessary for a “rubbery” elastomer.
Several tests are useful for determining the lowest temperature at which fluoroelastomers retain their elastomeric properties:

  • Temperature of Retraction (TR-10)
  • Glass Transition Temperature (Tg)
  • Gehman Torsional Modulus
  • Clash-Berg Torsional Modulus

As all tests measure the same characteristic, the values obtained for all these techniques (on a given vulcanizate) typically will be essentially the same temperature, within 1-3 °C.

Table 1. Overview of some low temperature properties

FKM Copolymer FKM Terpolymer FEPM Copolymer FEPM Terpolymer
TR-10 (°C) -17 -14 +2 -11
Tg (°C, DSC) -15 -14 +3 -11
Brittle point (°C, ASTM D2137) -20 -35 -58 -34

Brittle point is not a measure of flexibility, but rather a measure of a vulcanizate’s resistance to impact. The brittle point does not correlate at all with the ability of an elastomer vulcanizate to maintain sealing capability at low temperatures. Fluoroelastomers that exhibit relatively poor low temperature flexibility, such as Viton GF, can be compounded to provide brittle point temperatures of below -40°C. High tensile strength is the most important characteristic for a vulcanizate, relative to maximizing brittle point performance, and has significantly greater influence on this test than the polymer composition.
Dynamic seal applications with FKM based products have been successful at -40°C, and in some cases, appropriately designed parts can still offer static sealing capabilities down to -60°C.
Polycomp is having special low temperature FKM elastomers in its portfolio for sealing at extremely low temperatures. Please contact us to provide you with the best solution in your low temperature application.

 

Heat Resistance, Thermal Stability

FKMs can normally be used at temperature up to 200 °C for prolonged periods of time. In automotive specifications often lifetimes of at least 5000 h are defined.
However, people often ask for higher service temperatures – this is possible but always comes with a shorter lifetime. Figure 3 clearly demonstrates that e.g. short (few hours) exposure to temperatures up to 300 °C will not immediately kill your seal or part, but continuous exposure will.

FKM afbeelding 5

Figure 5. Approximate number of hours at which typical vulcanizate of Viton® will retain 50% of its original elongation at break as a function of test temperature after ageing in air.

FKMs retain good performance in fluids at elevated temperatures.

FKM rubber Applications

The most common FKM product that is often cited in fluid resistance guides is the copolymer or type 1 FKM. While it is popular, it does not represent the best technology and enhanced properties available. Consider the comparison of a copolymer bisphenol cured FKM and a peroxide cured terpolymer FKM. Although the volume swell in oil is similar, there is a significant difference in steam resistance properties. In addition, the peroxide cured terpolymer will also be more resistant to alkaline environments and corrosion inhibitors. Bisphenol cured FKMs are susceptible to attack by high temperature water and steam

Biofuels

Biofuels – bioethanol, biobutanol and biodiesel – are becoming a significant reality in the fuels landscape. Biofuel growth worldwide is being driven by the need to reduce dependency on foreign oil by expanding and diversifying the domestic fuel supply, coupled with growing concerns about the effects of greenhouse gases (CO2) and the need to provide environmentally friendly oxygenates for gasoline to reduce ground level pollution. Prior to 2006, methyl tertiary butyl ether (MTBE) was used as an oxygenate in gasoline, but its use was discontinued, driven by US state bans due to water contamination concerns. Today MTBE is commonly replaced with ethanol.
The growing use of biofules is also posing new requirements regarding the seals used in systems being in contact with the biofuels. Some biofuels can be aggressive to the elastomers used in the refining, delivery and dispensing of the biofuels, as well as seals and hoses used in the automotive industry.
Relative to other fuel components, biodiesel is relatively unstable and subject to degradation and contamination due to reaction with oxygen and water. Biodiesel, or RME (methyl ester of rapeseed) is a methyl ester mixture made up of saturated and unsaturated C16 to C22 fatty acids. Technically, methyl esters of rapeseed are produced by chemical conversion of rapeseed oil (colza oil), using methanol.
Chemical conversion of refined rapeseed oil (colza oil) with methanol yields methyl esters of rapeseed in the form of a clear, thin, combustible, non water-soluble liquid that gives off a faint smell. It is used as a substitute for diesel fuel (Biodiesel) but methyl ester of rapeseed is also used as a solvent in the specialist industries (adhesives). The principal constituents of methyl ester of rapeseed are oleic (55-65%), linoleic (18-25%), linolenic (5-11%) and palmitic acids (5-8%). Other fatty acids account for <1% of the product.

The aggressive, contaminated fuel attacks hydrocarbon rubbers such as nitrile rubber, widely used in fuel handling hose, gaskets and seals.
Ethanol-containing fuels pose the challenge of permeation, particularly to nitrile rubbers. Excessive permeation increases volatile emissions and loses valuable fuel.

FKM afbeelding 6

 

Figure 6. Overview of swell of various rubbertypes in combination with biodiesel for 70 hours at 23°C.

Proper choice of the FKM raw gum, together with our compounding knowledge, result in FKM compounds that are fully compatible with biofuels. The table below gives an overview of tests performed with a number of Viton compounds, as well as their low temperature properties.

Table 2. Vvolume change of Viton compound in B20 RME and B20 wet* RME after 1008 hr at 125°C.

FKM Table 2

FKM afbeelding 7

Figure 7. The volume change of Viton compound in B20 RME and B20 wet* RME after 1008 hr at 125°C.

Blends of ethanol with gasoline motor fuels represent an increasing proportion of the fuel supply. In addition, 100% ethanol fuel is common in a few countries, notably Brazil. Fuels containing butanol are on the horizon.
Although nitrile rubber is resistant to the chemical attack by ethanol, it is highly permeable to it. FKM, however, has excellent resistance to both permeation and chemical attack by either pure ethanol or blends of ethanol with hydrocarbon fuel. The following figures give an impression on compatibility and permeation of FKM types with ethanol.

FKM afbeelding 8

Figure 8. Volume change in ethanol fuel blends, 1008 hr at 40 °C (weekly fuel change).

 

FKM afbeelding 9

Figure 9. Permeation of Fuel C/ethanol fuel blends at 40 °C.

Base Resistant FKM

High pH chemicals can attack on the hydrogen that is present on VF2-monomers, and particularly on the VF2 monomers that are adjacent to monomers of hexafluoropropylene (HFP). These are the most ‘active’ hydrogens, and the ones most easily abstracted by base attack. The loss of a hydrogen results in the expulsion of an adjacent fluorine, with the resulting formation of a site of unsaturation, which can undergo either chain cleavage, or cross-linking. The typical failure mode for fluoroelastomers attacked by bases is embrittlement that occurs by the formation of excessive crosslinking of the polymer, causing premature degradation.
Corrosion inhibitors, designed to protect metal components, introduce bases to counteract the long-term effects of acid build-up. They are typically amine-based, and are capable of extremely aggressive degradation of conventional fluoroelastomers. Likewise, chemicals used to clean or purge industrial and food processing equipment are often basic in nature.
The presence of aggressive basic chemicals, no matter where they are used, poses unique challenges for conventional fluoroelastomer materials. Base resistance is achieved in conventional fluorinated elastomers by two different structures, such as materials without VF2 and materials without HFP These materials, because of the complete absence of base sensitive sites, exhibit very good resistance to bases.
Indeed, the first category without VF2 appears to be an over designed solution since the materials involved are highly fluorinated; moreover, they contain large amounts of the very expensive PAVE so called “rubbery” monomers, especially perfluoromethylvinylether, thus leading also to very expensive materials.
In fluorinated elastomers that do not contain HFP, the role of “rubbery” monomer is played by propylene in place of PAVE. This creates less expensive materials, but with worse thermal resistance and higher swelling in hydrocarbons.
Various producers have developed special base resistant FKM types, such as DuPont with Viton Extreme TBR-S and ETP-600S, Solvay with Tecnoflon BR 9151 and 3M Dyneon with Dyneon BRE 7131X, 7132X and 7231X.

Viton Extreme

The Viton Extreme products combine the excellent thermal resistance of fluoroelastomers with resistance to chemicals that create high pH environments. This class of fluoroelastomer is designated as FEPM by ASTM D1418.
FEPM polymers exhibit excellent resistance to base attack but have had limited acceptance due to poor processing characteristics. DuPont’s Advanced Polymer Architecture (APA) technology expands the performance range with specialty Viton types like Viton Extreme. These products provide overall fluids resistance, base resistance and processing advantages versus some of the other existing TFE/Propylene polymers.

Viton Extreme TBR-S is a totally base resistant with a bisphenol cure. It provides:
• Inherent resistance to caustics/amines
• Good resistance to hydrocarbon oils, acids, and steam
• Good compression set resistance and lower volume swell for longer seal life and wear resistance
• Good processing versus other TFE/Propylene polymers

Viton Extreme ETP-600S is an upgrade to ETP-900 that significantly improves the processing and end-use performance, while maintaining the excellent fluid resistance of its predecessor. It provides:
• Excellent resistance to acid, hydrocarbon and low molecular weight esters, ketones and aldehydes
• Inherent resistance to base attack and volume changes in highly caustic solutions, amines and hot water
• Low-temperature flexibility (Tg –10°C)
• Improved compression set and physical properties for improved seal performance
• Improved mold flow, faster cure rates, and improvements in mold release and mold fouling for efficient manufacturing
Because Viton Extreme ETP-600S has very low swell in hydrocarbons, it is used in automotive oil seals and oil field applications, as well as in automotive and aircraft fuels. In complicated environments and chemically aggressive applications it often is the fluoroelastomer of choice.

FKM afbeelding 10

Figure 10. Impression of the performance of some FEPM elasotmers

FKM in Aerospace Applications

The first commercial use for Viton fluoroelastomer occurred in 1957 when the U. S. Air Force needed O-rings that had the ability to seal at high temperature in aggressive fluids, including jet fuels, engine lubricating oils and hydraulic fluids. Reliability of materials under extreme exposure conditions is a prime requisite for aerospace and aircraft service. In the air, absolute sealing integrity is essential. Today, the vast majority of the world’s commercial and military aircraft depend on the reliability and performance of FKM and FFKM based seals and parts.
FKM O-rings have a usable thermal range of -45°C to +275°C, and also resists the effects of thermal cycling, encountered in rapid ascent to and descent from the stratosphere. Moreover, FKMs have excellent abrasion resistance and the ability to seal against hard vacuum, as low as 133 nPa, absolute.
Seals fabricated with FKM are routinely used in commercial and military aircraft turbine engines, auxiliary power units and hydraulic actuators. They include:
• O-rings used in line fittings, connectors, valves, pumps and oil reservoirs
• Radial lip seals used in pumps
• T-seals
• Cap-seals
• Manifold gaskets
• Coated fabric covers for jet engine exhausts between flights
• Firewall seals
• Abrasion-resistant solution coating over braid-sheathed ignition cable
• Clips for jet engine wiring harnesses tire valve stem seals
• Siphon hose for hot engine lubricants

Prevailing trends in aircraft turbine engine applications are pushing current elastomeric seal materials to their limits. These trends include the continued drive towards more powerful, lighter weight engines, with accompanying reductions in noise, emissions and fuel consumption, as well as ongoing improvements in reliability, maintainability and longer intervals between engine overhauls.
This pushes engine thermodynamics to their limits, which manifests in higher operating and soakback temperatures. As a result, engine manufacturers are driven towards high temperature stabilized oils in order to achieve engine performance and life targets. Specialty types of Viton® (GLT) are being adopted, since they display outstanding resistance to these aggressive HTS-type oils.

 

FKM rubber in Automotive Applications

Fuel systems
Increasingly more stringent emission requirements result amongst others in higher temperatures in and around the engine, stricter norms on permeation of fuels and last but not least in more aggressive fuels and fuel additives being used (see also the chapter on biofuels). These oxygenate-rich gasolines exhibit higher volatility which can cause swelling, property deterioration and increased permeation through elastomeric materials.
FKM is the polymer of choice for fuel seals, head and intake manifold gaskets, quick-connect o-rings, fuel injection seals of all descriptions, caulks, and advanced fuel hose components.
Besides, warrantees are getting longer, requiring longer lifetimes of various seals and parts. FKM can often replace less thermally and chemically-stable materials.

FKM design advantages are e.g.:
• Application range -40 °C to 225 °C with intermittent exposure to 285 °C
• Resists hydrocarbons and sour gasoline
• Excellent dynamic properties
• Low permeation rates
• Solvent, acid and base resistant
• Low compression set

The graph below gives an impression fo various FKM types in relation to fuel resistance.

FKM afbeelding 11

 

Figure 11. Fuel resistance of various FKM types.

More information about FKM rubber compounds

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