Chemical Compatibility Guide for Elastomer Seals

Why Chemical Compatibility Matters

Selecting the wrong elastomer material for a chemical environment is one of the most common — and most costly — causes of seal failure in industrial applications. Chemical incompatibility can manifest as excessive swelling, hardening, softening, cracking, dissolution, or complete disintegration of the seal, leading to leaks, contamination, equipment damage, and potentially dangerous situations. This guide provides a systematic approach to matching elastomer materials with chemical environments.

Mechanisms of Chemical Attack

Understanding how chemicals interact with elastomers helps predict compatibility and explains why different materials react differently to the same fluid:

Absorption and Swelling

The most common form of chemical attack. Fluids that are chemically similar to the elastomer (like dissolves like principle) are absorbed into the polymer matrix, causing volume increase (swelling), reduced hardness, and loss of mechanical properties. An O-ring that swells excessively may extrude from its groove, while one that swells moderately may actually improve its seal — some designers intentionally allow 5-10% swell.

Extraction

Solvents can extract plasticizers, processing aids, and other additives from the rubber compound, causing shrinkage, hardening, and embrittlement. This is particularly problematic with compounds that rely heavily on plasticizers for flexibility.

Chemical Degradation

Strong oxidizing agents, concentrated acids, or aggressive chemicals can break the polymer chains themselves, destroying the material's structure. This is irreversible and typically manifests as softening, tackiness, or complete dissolution.

Material-Chemical Matrix

Petroleum Oils and Fuels

Best choices: NBR (standard choice), FKM (premium/high-temperature), HNBR (enhanced temperature range)

Avoid: EPDM, Natural Rubber, Silicone (VMQ)

For fuels containing high levels of biofuel blends (E85, B100), standard NBR may be insufficient — FKM or specialty NBR compounds are recommended.

Water and Steam

Best choices: EPDM (best overall), Silicone (moderate temperature), FFKM (aggressive conditions)

Avoid: Polyurethane (AU — hydrolysis), NBR (above 80°C)

Hot water and steam are particularly aggressive to many elastomers. EPDM with peroxide cure is the gold standard for steam service up to 175°C.

Acids

Dilute inorganic acids (HCl, H2SO4 <30%): EPDM, FKM, FFKM

Concentrated acids: FFKM, PTFE (as backup ring or encapsulated seal)

Hydrofluoric acid (HF): EPDM only — FKM is attacked by HF

Nitric acid: FKM, FFKM — avoid NBR and Natural Rubber

Bases and Amines

Dilute bases (NaOH, KOH <30%): EPDM (best), NBR (acceptable)

Amines and amine-containing fluids: EPDM, FFKM — FKM is attacked by amines

This is a critical distinction: FKM's excellent acid resistance does NOT extend to bases and amines.

Solvents

Ketones (acetone, MEK): EPDM, FFKM — FKM and NBR are both attacked

Chlorinated solvents: FKM Type B, FFKM

Aromatic solvents (toluene, xylene): FKM Type B, FFKM

Testing and Validation

Published compatibility charts should be used as a starting point, not a final answer. Variables like temperature, concentration, pressure, fluid mixtures, and the specific rubber compound formulation can all affect real-world performance. For critical applications:

1. Conduct immersion testing per ASTM D471 at the actual operating temperature
2. Measure volume change, hardness change, tensile strength change, and elongation change
3. Test for at least 70 hours, preferably 168 hours (7 days) or longer
4. If volume change exceeds ±25%, or hardness change exceeds ±15 points, the material is generally unsuitable

Quick Reference Rules

• Oil environment → NBR first, FKM for high temp
• Water/steam → EPDM
• Acids → FKM (except HF → EPDM)
• Bases/amines → EPDM (NOT FKM)
• Solvents → Check specific solvent type carefully
• Unknown or mixed chemicals → FFKM (most universal resistance)