O-ring swell and shrinkage in oils and solvents: acceptable limits and predicting material compatibility

O-ring swell and shrinkage are critical factors that determine seal longevity in chemically aggressive environments. An elastomer that swells intensely or shrinks in a given medium fails quickly — either through premature extrusion from the groove (excessive swell) or loss of sealing force (shrinkage). This guide explains the mechanics of these processes, the standards for measuring them (ISO 1817, ASTM D471), and how to select a material that remains stable in your specific medium.

What is O-ring swell and shrinkage?

An elastomer in a liquid behaves as a polymer network. Medium molecules diffuse into this network, increasing the material's volume (swell). Conversely, in a medium that extracts plasticisers from the elastomer, volume decreases (shrinkage). Both phenomena cause changes in properties — softening and hardness change during swell, but especially hardening and brittleness during shrinkage. In a groove with fixed depth, a swollen ring may exceed the groove capacity (over-compression), whilst a shrunken ring loses sealing force.

Standards: ISO 1817 and ASTM D471

Measurement of elastomer volume change in liquids is normalised in ISO 1817 (European standard) and ASTM D471 (US standard). Both standards mandate immersion of an O-ring in a test liquid for 24 hours at a specified temperature (typically 70 °C or 100 °C depending on application). Volume change is calculated as: Result = ((volume after immersion − original volume) / original volume) × 100 % A positive result indicates swell, a negative result indicates shrinkage. Hardness change (Shore A) and physical properties (tensile strength, tear) are similarly measured.

Swell spectrum and acceptable limits

The Parker O-Ring Handbook (aligned with Trelleborg and other manufacturers) defines volume change ranges between elastomers and media. However, ISO 1817 and ASTM D471 do not prescribe fixed numerical thresholds; engineers rely on empirical experience and manufacturer catalogues. In practice, these indicative categories are typically applied (based on Parker Handbook and industry practice): • Excellent: ≤ 5 % volume change — long-term service without reservation • Good: 5–15 % volume change — acceptable for service • Marginal: 15–30 % volume change — short-term or static applications only • Poor: > 30 % swell or rapid degradation — AVOID In practice: swell up to 15 % (good class) is acceptable for most applications if the groove is properly designed. Shrinkage is always worse than equivalent swell — shrinkage immediately causes loss of sealing force and leakage.

Why shrinkage causes immediate failure

Swell can be tolerated to some extent if the groove is shallow enough. But shrinkage has catastrophic effects on seal integrity: the ring loses its round cross-section and does not compress sufficiently against the groove to stay in place. The radial force the ring exerts on the mating surface drops sharply. The result is nearly instant leakage. For this reason, materials that shrink in a given medium are a poorer choice for reliability than those that swell modestly.

Per-material swell and shrinkage profile

Every elastomer behaves differently in different media. Below are typical profiles: NBR (Nitrile Butadiene Rubber) — Workhorse material in industry • Mineral oil (ISO VG 46): excellent (0–5 %) • Hydraulic oils HLP: good (5–15 %) • Unleaded gasoline: good (5–12 %) • Aromatic hydrocarbons (toluene, xylene): poor (>30 %) • Ketones (acetone): poor (>40 %) • Esters (phthalates): poor (20–40 %) FKM (Fluorocarbon, Viton®) — Premium for chemical duty • Mineral oil: excellent (≤5 %) • Aromatic hydrocarbons: good (5–15 %) • Fuels including biodiesel: good (5–15 %) • Hot oils (to 200 °C): good (5–10 %) • Mineral acids: good (5–15 %) • Esters: marginal (15–25 %, type-dependent) • Ketones: poor (>30 %) • Amines and solvents: poor (>40 %) EPDM (Ethylene Propylene Diene) — Water and steam • Water and distilled water: excellent (≤3 %) • Steam to 150 °C: excellent (≤5 %) • Brake fluid DOT 3/4 (glycol): good (5–12 %) • Polar solvents: good (5–15 %) • Mineral oils: shrinkage (−10 to −20 %) — AVOID • Fuels and hydrocarbons: shrinkage (−15 to −30 %) — CRITICAL VMQ (Silicone) — Wide range, biocompatible • Water and ozone-water: excellent (≤5 %) • Air and ozone: excellent (≤3 %) • Polar solvents: good (5–12 %) • Mineral oils: shrinkage (−10 to −15 %) — AVOID • Hot paraffinic oils: poor (>20 %) • Ketones and aromatics: poor (>30 %)

How to select a material for your medium

If you have an application with a known medium: 1. Find the material in the compatibility table (available in the chemical matrix on oringy.sk). Look for "good" or "excellent" class. 2. NEVER select a material in "poor" class without vendor consultation. High risk. 3. For marginal class (15–30 %), design the groove with LARGE MARGIN. Fill ratio must not exceed 85 %. Leave room for swell. 4. Never use a material that shrinks in the medium (even modest shrinkage = instant leakage). 5. For medium-plus-temperature combinations, always verify both conditions — temperature accelerates diffusion. Typical selections: • Hydraulics with mineral oil: NBR (economical, excellent compatibility) • Fuels and engine oils: FKM (stable to high temperature, no shrinkage) • Water and steam: EPDM (the only reasonable choice, does not shrink) • Chemical solvents and aggressive media: FFKM (last resort) • Biodegradable oils: HNBR or FKM (NBR shrinks)

How to design a groove accounting for swell

ISO 3601-2 assumes that an O-ring has a certain squeeze (compression) after fitting. For static applications, typical values are 15–30 % compression. If the ring swells 15 % in the medium, squeeze effectively vanishes — the ring merely drags loosely in the groove. Solution: • Fill ratio (proportion of ring volume in the groove) must not exceed 85 %. ISO 3601-2 recommends maximum 80–85 % fill to maintain margin. • If you know the material will swell ~15 % in your medium, design the groove ~10 % deeper to absorb that change without over-compression. • Monitor volume: groove volume = ((1.30 × CS) × depth) — approximate formula for a standard radial groove. Verify your calculation on the groove calculator at oringy.sk, then apply a 10–15 % safety margin on depth for high-swell media.

Reading the chemical compatibility table correctly

The chemical compatibility table on oringy.sk lists for each material–liquid pair: • The compatibility class (excellent / good / marginal / poor) • The recommended temperature window (typically narrower than the material's full range) • A note (e.g., "swell ~10 %", "marginal class — static only", "shrinkage − avoid") Do not skip the notes — they are there for good reason.

Practical examples

Example 1: Hydraulic cylinder with mineral oil HLP Material: NBR (standard) Compatibility: Excellent (0–5 % swell), temperature to 100 °C Groove: ISO 3601-2 standard, 80–85 % fill ratio — excellent Conclusion: Trouble-free service. Example 2: Fuel pump with gasoline (aromatic-content fuel) Material: EPDM (wrong choice) Compatibility: Shrinkage (−20 %), sudden leakage Conclusion: Failure within weeks. Fix: FKM or HNBR, good compatibility (5–12 % swell), temperature to 80 °C (fuels) Groove: ISO 3601-2 + 10 % depth margin Conclusion: Reliable 3–5 years. Example 3: Process cylinder with polyester oils (POE, silicone fluids) Material: VMQ (silicone) Compatibility: Marginal (15–20 % swell), temperature max 100 °C Groove: Minimum 75 % fill ratio, depth +12 % margin Conclusion: Possible, but monitor temperature carefully. Better: FKM Compatibility: Good (5–15 % swell) Conclusion: Safer choice.

Summary: Key rules

1. Swell up to 15 % (good class) is acceptable if the groove is well designed (fill ratio ≤ 85 %). 2. Shrinkage is ALWAYS worse and ALWAYS causes leakage — avoid it unconditionally. 3. For marginal swell (15–30 %), either design for static service only or select a better material. 4. Always check the compatibility table, not just the class but specific swell % figures. 5. Temperature accelerates swell and shrinkage — lower temperature range = less swell. 6. Design grooves with margin (fill ratio 80 % — 85 %, not at the edge). 7. When the medium or fluid supplier changes, always re-check compatibility — composition may shift.