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FFKM rubber compounding

  • High stability in aggressive media
  • Highly stable in polar and non-polar media
  • High reliability for dynamic applications
  • High reliability for static applications
  • Temperature range from -30 °C to +325 °C
  • Commonly used in O-rings and gaskets

Introduction FFKM

The carbon-fluorine bond is the most stable single bond. The high bond dissociation energy of the C-F bond is the main reason for the thermal, oxidative and chemical stability of fluorinated polymers.Table 1 lists the typical bond dissociation energies of single bonds.

Table 1. Typical bond dissociation energies for aliphatic bonds (kJ/mol).

  • CH3-H 435 kJ/mol
  • CH3-F 451 kJ/mol
  • CH3-Cl 351 kJ/mol
  • CH3-Br 293 kJ/mol
  • CH3-I 234 kJ/mol

Homo-polytetrafluoroethylene (PTFE) contains all C-F substitution, which makes it one of the most chemically and thermally stable polymers. However, PTFE has very high crystallinity. As a result, it is a rigid thermoplastic, not an elastomer. Randomly incorporating other monomers into homopolymers breaks the regularity andforms amorphous polymers with a low glass transition temperature. Perfluoromethylvinylether(PMVE) and other perfluoro (alkoxy/alkyl) vinyl ethers are the monomers that are incorporated withTFE monomer in most commercial perfluoroelastomers. If other partially fluorinated monomers are incorporated, it forms fluoroelastomers.

So, fluorinated elastomers containeither a partially or fully fluorinated backbone. If the backbone is fully fluorinated, it is a perfluoroelastomer (FFKM); if it is partially fluorinated, it is a fluoroelastomer (FKM). The polymer backbone comprises many repeating monomerunits. Typical monomer units for these polymers are shown below.

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Crosslinking theory

In order for a material to be rubber-like, having a three-dimensional network structure isessential. For perfluoroelastomers, due to the inertness of the perfluorinated backbone, a cure sitemonomer (CSM), a monomer that contains a reactive site, is introduced. The CSM can then react to form a three-dimensional network. The most significant CSM for FFKMs are cyano-functionalvinyl ethers. The major advantages of using these CSMs are that they can be effectively cured using a variety of curatives. For example, curing with tetraphenyl tin or ammonia results in a triazine crosslink structure as shown in Figure 1.

Use of the di-functional curative diaminobisphenol AF resultsin condensation to form a benzoxazole crosslink as shown in Figure 2. These resulting networks have excellent thermal stability. Cyano-functional vinyl ethers can also be cured using organic peroxide with the resulting network having near universal chemical resistance. In addition, halogens are other common CSMs for FFKM which are readily crosslinked using peroxide-based systems. The major disadvantage of peroxide curing FFKM is the hydrocarbon nature of the crosslink, which is more susceptible to oxidative and thermal degradation.

figure1

Figure 1. Triazine Crosslink Structure.

figure2

 

Figure 2. Benzoxazole crosslink from diaminobisphenol AF

General properties

Perfluoroelastomers are ideal for demanding sealing applications when customers require high quality parts like O-rings. The unique properties of this material help maintain its seal integrity which can reduce maintenance, operating costs and improve safety:

  • Excellent thermal resistance
  • Temperature range from -30 °C to +325 °C
  • Outstanding chemical resistance
  • Highly stable in polar and non-polar media
  • Very low permeability
  • High reliability for static and dynamic applications
  • Commonly used to make O-rings and gaskets
  • Types with FDA approval
  • ED (explosive decompression) resistant compounds
  • Compounds with hardness between 45 and 90 ShA
  • Physical properties (typical data 70-75 ShA compound):
    • Compression set 15-40 % (on O-ring after 70h @ 200 °C)
    • Tensile strength 10-20 Mpa
    • Elongation at break 125-250 %
    • TR10 = -30 °C to 0 °C
  • FDA/USP Class VI type materials

FFKM compounds by Polycomp

As with all elastomers, FFKM is normally compounded with fillers, colorants, processing aids, and crosslinking systems.Typical fillers include carbon black, and mineral fillers such as silica.

Mechanical properties, chemical resistance, and thermal stability of the final material not only depend on the polymer but also heavily depend on the other ingredients contained in the compound formulation.

Polycomp always develops the optimal compound for the application under consideration. Therefore a selection of polymer and cure system is made based on:

  • Best chemical resistance
  • Best high temperature properties
  • Best low temperature properties
  • Lowest compression set
  • Lowest price

As an experienced and independent FFKM compounder, Polycomp develops and produces FFKM compounds based on polymers from all major producers like SolvaySolexis, 3M Dyneon, Daikin, and Lodestar.

High temperature applications

High heat and temperature spikes can degrade elastomeric seals causing their crosslinking structure to become irreversibly damaged. When elastomers are used above their maximum continuous service temperature limit over time, they can become hard and brittle, thereby losing their ability tomaintain effective sealing. The results can be unscheduled downtime, potential product loss or even worse, possible safety threats to the work environment.

Compression set and seal force retention are two properties that are commonly used to evaluate thesealing functionality of elastomers in high temperature applications. Figure 3 compares different materials after long-term (336 hours) compression set testing at elevated temperatures. Silicone (VMQ) shows a high compression set at 150°C while

FKM and FFKM maintained good compression set properties. At 200°C, compression set of FKM is increasing. Both FKM and VMQ completely lost their resilience after 336 hours at 250°C while FFKM maintained excellent recovery properties over theentire temperature range.

figure3

Figure 3. Long-Term Compression Set Test Data. Testing Performed per ASTM D395B & D1414 (AS568 K214 O-rings).

Outgassing

Elastomeric seals can degrade under high temperatures causing outgassing to occur, thereby contaminating the process environment. Hydrofluoricacid (HF) is one of the gases evolved when FFKM and FKM begin to degrade. It can be harmful toprocess equipment, especially to quartz and stainless steel components. Outgassing can also cause slow vacuum pump-down to occur. Figure 4 compares the outgassing performance of FFKM, FKM and VMQ.

figure4

Figure 4. Outgassing Comparison: FFKM versus FKM and VMQ. DuPont Performance Elastomers Proprietary Test Method (AS568 K214 O-rings).

 

More information about FFKM rubber compounding

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