Skydrol (Phosphate Ester) Resistant O-Rings
Compatibility ratings for 7 O-ring materials in Skydrol (Phosphate Ester) service
Skydrol phosphate-ester hydraulic fluid is the aerospace special case: standard NBR, FKM and HNBR all rate D, while EPDM, PTFE and FFKM rate A. Never substitute standard FKM into Skydrol service.
Skydrol (Phosphate Ester) (Fluids) compatibility with the seven most common O-ring elastomers, rated on the standard A/B/C/D scale: A = excellent, B = good, C = fair, D = not recommended. This data also appears in our interactive chemical compatibility chart and is available machine-readable at /api/chemicals.json.
| Material | Rating | Guidance |
|---|---|---|
| NBR | D | not recommended — rapid degradation expected |
| FKM | D | not recommended — rapid degradation expected |
| EPDM | A | excellent — recommended for continuous service |
| VMQ | B | good — suitable for most applications |
| PTFE | A | excellent — recommended for continuous service |
| FFKM | A | excellent — recommended for continuous service |
| HNBR | D | not recommended — rapid degradation expected |
Why These Ratings
- FKM (D) — documented weaknesses include “Skydrol (phosphate ester hydraulic fluids).”
- EPDM (A) — documented strengths include “Skydrol.”
- PTFE (A) — documented strengths include “Virtually all chemicals.”
- FFKM (A) — documented strengths include “Virtually all chemicals except molten alkali metals.”
FluidsService: Concentration & Temperature Notes
This category is defined by chemistry families, not viscosity. Glycol-based fluids (brake fluid, antifreeze glycols) belong to EPDM — NBR and FKM both rate D in glycol brake fluid. Phosphate esters (Skydrol) defeat NBR, FKM and HNBR alike; only EPDM, PTFE and FFKM rate A. Never assume 'hydraulic fluid' describes a single chemistry.
Recommended
- EPDM O-rings (A)
- PTFE O-rings (A)
- FFKM O-rings (A)
Use With Caution
- VMQ O-rings (B)
Avoid
- NBR (D)
- FKM (D)
- HNBR (D)
Related Guides
Related Chemicals (Fluids)
Frequently Asked Questions
Is Skydrol (Phosphate Ester) compatible with FKM (Viton) O-rings?
FKM is rated D for Skydrol (Phosphate Ester): not recommended — rapid degradation expected. Ratings are general guidelines; concentration, temperature and exposure time all affect real-world compatibility, so validate critical applications with ASTM D471 immersion testing.
Is Skydrol (Phosphate Ester) compatible with NBR (nitrile) O-rings?
NBR is rated D for Skydrol (Phosphate Ester): not recommended — rapid degradation expected. NBR is the lowest-cost elastomer, so when it earns an A or B rating it is usually the economical default; when it rates C or D, do not substitute it for a higher-rated material to save cost.
Can I use PTFE O-rings with Skydrol (Phosphate Ester)?
PTFE is rated A for Skydrol (Phosphate Ester): excellent — recommended for continuous service. Remember that PTFE has no elastic memory — in demanding service it needs spring energization or an elastomer core (FEP/PFA encapsulation) to maintain sealing force.
What is the best O-ring material for Skydrol (Phosphate Ester)?
EPDM, PTFE, FFKM are rated A (excellent) for Skydrol (Phosphate Ester) in our compatibility data and are the recommended starting point for material selection. Final selection should also account for operating temperature, pressure and whether the seal is static or dynamic.
Can I use EPDM O-rings with Skydrol (Phosphate Ester)?
EPDM is rated A for Skydrol (Phosphate Ester): excellent — recommended for continuous service. Always confirm the specific fluid concentration and temperature before specifying EPDM for this service.
Need O-Rings for Skydrol (Phosphate Ester) Service?
Send us your fluid, concentration and temperature — our engineers will confirm the right compound. MOQ 1 piece.