Polymers and elastomers · Workhorse
Viton (FKM)
FKMvalues describe: Viton A
Why it wins
Best outgassing of the common elastomers, bakeable to about 150 to 200 C, cheap and forgiving: the seal of everything opened often.
Why not the alternative
Buna-N and EPDM outgas plasticizers; metal seals cost 10x and are single-use.
Watch out
Permeation floors systems around 1e-8 mbar; radiation hardens it.
Properties
The values below are candidate: compiled from the sources named, not yet individually validated. Provisional provenance: Elastomer supplier data and vacuum seal practice; O'Hanlon, A User's Guide to Vacuum Technology; NASA outgassing database.
Wet cleaning
| Recipe | lint-free wipe with isopropanol, air dry, then vacuum bake 150 °C before first use |
|---|---|
| Forbidden | acetone and ketones: the compound swells; prolonged ultrasonics; any grease to help it seat |
| Limit | swelling, and the fact that a wiped O-ring is as clean as an O-ring gets |
Vacuum and outgassing
| Outgassing, unbaked (10 h) | 5e-7 mbar·L/s/cm² |
|---|---|
| Outgassing, baked | 1e-8 mbar·L/s/cm² |
| Vapour pressure | plasticiser and cure residues evolve on the first bake |
Temperature
| Bake, assembled | 200 °C |
|---|---|
| Vacuum degas | 200 °C |
| Metallurgical limit | continuous 200 °C, excursion 250 °C |
Thermal
| CTE | 160 ppm/K |
|---|---|
| Thermal conductivity | 0.2 W/m·K |
| Specific heat | 1400 J/kg·K |
| Emissivity | 0.9 |
Mechanical
| Strength | 10–14 tens MPa |
|---|---|
| Tensile | 10–14 MPa |
| Elongation | 200–300 % |
| Young's modulus | 0.01 GPa |
| Hardness | 75–90 Shore A |
| Density | 1.85 g/cm3 |
Electrical and magnetic
| Relative permeability | 1.0 |
|---|---|
| Resistivity | >1e12 Ω·cm |
Engineering
| Corrosion | good; attacked by amines, ketones, hot water |
|---|---|
| Joining | compressed in a groove |
| Process notes | permeates helium and water; compression set decides seal life |
| Availability and cost | commodity, every size |
Outgassing
1 sourced measurement for this card, 1 declared absence, 1 prescribed condition or published threshold. A rate means nothing without the pumping time it was read at, so each one states it or declares it missing. The whole base, filters included, is at /tools/outgassing/.
Signature: 1e-6 unbaked to no baked reading baked.
| Material as published | Species | State | Pumping time | Rate |
|---|---|---|---|---|
| Viton A (fresh) | total | unbaked | 1 h | 1.52e-6mbar·L/s/cm² |
- 3M/Dyneon Fluorel FC2180, processed per LIGO-E970130-A. Two figures are published for this fluoroelastomer, 8.8e-13 unbaked and 1.9e-13 baked, in torr-liter/s/cm2, but the species column could not be resolved.Row I9 of Table 1. Same column alignment problem as row A14. The pair is worth reading for the ratio it shows, a factor of about 4.6 gained by baking, but the species is not written without certainty.
Prescribed, not measured
- procedure, Fluoroelastomer molded castings, Fluorel and Viton, manufactured per E970130. Bake 182 C, 48 h, vacuum.
Sources: S-LIGO-E960050-V13, S-LIGO-E960022-V24, S-ELSEY-1975-II. Full citations and conditions at the base and in /data/outgassing.json.
Grades
| FKM A (Viton A) | general vacuum O-rings |
|---|---|
| FKM GLT | low temperature sealing, down to about minus 40 C |
| Baked FKM | pre-baked cords and O-rings for lowest elastomer outgassing |
Sources · 2
- O Hanlon, A Users Guide to Vacuum Technology
- NASA outgassing database