Determination of Transverse Swelling of Fluoroelastomer (FKM/FPM) in Fuel Resistance Testing

Transverse swelling is a key performance indicator for fluoroelastomer (FKM/FPM) components such as fuel hoses, O-rings, and gaskets that operate in continuous contact with gasoline, diesel, and oxygenated fuel blends. This article explains how transverse expansion of fluoroelastomer is determined and evaluated in accordance with recognized standards such as ASTM D471, ISO 1817, and GB/T 1690, covering specimen preparation, immersion procedures, dimensional measurement, calculation methods, and typical acceptance criteria. Understanding this test enables engineers to select the correct fluoroelastomer compound and validate long-term sealing reliability in fuel-contact applications.

Technical Overview

Fluoroelastomer, classified as FKM under ASTM D1418 and FPM under ISO 1629, is a high-performance synthetic rubber based on fluorinated monomers such as vinylidene fluoride (VF2), hexafluoropropylene (HFP), and tetrafluoroethylene (TFE). Its high fluorine content, typically 66% to 70% by mass, gives the polymer exceptional resistance to hydrocarbon fuels, aromatic solvents, engine oils, and oxygenated fuel blends containing ethanol or methanol. As a result, FKM is the preferred sealing material in automotive fuel injection systems, fuel lines, fuel sender units, and aerospace fuel handling components.

When an elastomer is immersed in fuel, fluid molecules diffuse into the crosslinked polymer network and cause swelling. Swelling manifests as an increase in mass, volume, and linear dimensions. Transverse swelling, also called lateral expansion, refers to the dimensional increase measured perpendicular to the principal axis of the product, for example, the width of a strip specimen, the wall thickness of a hose inner tube, or the cross-section diameter of an O-ring. Because fuel resistance testing is used to qualify compounds for sealing applications, transverse swelling is a direct indicator of whether a seal will overfill its groove, lose compression force, extrude into clearances, or leak after prolonged fuel exposure.

The determination is typically performed by immersing conditioned specimens in a specified reference fuel at a controlled temperature for a defined duration, then measuring the change in transverse dimension relative to the original value. The method is governed internationally by ASTM D471 (Rubber Property—Effect of Liquids), ISO 1817 (Rubber, vulcanized or thermoplastic—Determination of effect of liquids), and in China by GB/T 1690. These standards define specimen geometry, liquid-to-specimen ratios, immersion conditions, measurement timing, and calculation formulas. Accredited third-party laboratories routinely perform this test for material qualification, incoming inspection, and failure analysis.

Testing Samples

Standard Strip Specimens from Molded Sheets

The most common specimen is a rectangular strip cut from a cured FKM compression-molded sheet, typically 50 mm in length, 25 mm in width, and 2 mm in thickness, in line with the preferred specimen dimensions of ASTM D471 and ISO 1817. Reference marks are applied at defined positions so that width (the transverse dimension) and thickness are re-measured at identical locations after immersion. Sheets are preferably molded in the cure state intended for production, since under-cured material will swell disproportionately and produce non-representative results.

Finished Sealing Components

O-rings, gaskets, diaphragms, and custom seals may be tested as finished articles. For O-rings, the transverse dimension is the cross-section diameter, measured with a calibrated optical projector, laser gauge, or low-pressure micrometer at several marked positions. Testing finished parts captures the combined influence of compound formulation, cure cycle, and post-curing, and is frequently required for part-level qualification in automotive and aerospace fuel systems.

Extruded Products and Hose Inner Tubes

For extruded tubing and fuel hose inner liners, short ring sections are cut perpendicular to the extrusion direction, and the transverse swelling is evaluated through changes in inner diameter, outer diameter, and wall thickness. Because extrusion and calendering can induce molecular orientation, swelling of extruded or calendered stock may differ between the machine direction and the transverse direction; measuring the transverse dimension specifically captures the direction most relevant to seal gap filling and hose fit.

Testing Items

A fuel resistance evaluation built around transverse swelling typically includes the following measured and calculated parameters:

• Transverse dimensional change (width or cross-section diameter change), expressed as a percentage of the original dimension
• Thickness change and length change, to characterize anisotropic swelling behavior
• Volume swell, determined gravimetrically or by the immersion (Archimedes) method
• Mass change, reflecting both fluid absorption and extraction of soluble constituents such as plasticizers and process aids
• Change in hardness (Shore A, per ASTM D2240 or ISO 48) after immersion
• Change in tensile strength and elongation at break (ASTM D412 or ISO 37) after immersion
• Non-volatile extractable content, determined by re-drying and re-weighing the specimen after immersion
• Recovery measurement, re-measuring dimensions after a defined drying or reconditioning period to separate reversible swelling from permanent deformation

Testing Methods

Specimen Conditioning and Initial Measurement

Specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 16 hours in accordance with ASTM D618 or ISO 291 before testing. Initial transverse dimensions are measured at three or more marked positions using a calibrated micrometer with flat anvils, a traveling microscope, or a non-contact optical system. For soft FKM grades, the measuring force must be minimized to avoid local deformation, and multiple readings are averaged to reduce measurement uncertainty.

Fuel Immersion Conditions

Specimens are fully immersed in the selected test fluid at a volume of at least 15 times the specimen volume, with the liquid refreshed for long-duration tests to prevent depletion of the most aggressive fuel fractions. The choice of reference fuel is critical and should reflect the service environment:

• Fuel A (isooctane), a reference for low-aromatic, low-swell fuels
• Fuel B (70:30 isooctane to toluene), representing conventional high-aromatic service fuel
• Fuel C (50:30:20 toluene, diisobutylene, and ethanol), the classic aggressive screening fuel for fuel-resistant elastomers
• Fuel D through Fuel K and ethanol-blend service fluids such as E10, E25, and E85, used to evaluate oxygenated fuel compatibility
• Standard ASTM No. 1, IRM 901, IRM 902, and IRM 903 oils when lubricating-oil resistance is also required

Common immersion temperatures are 23 °C, 40 °C, 60 °C, and 100 °C, with volatile fuels tested in sealed vessels or under reflux to maintain fluid composition. Standard test intervals are 22 h, 70 h, and 168 h, with extended durations of 336 h, 504 h, or 1000 h for qualification programs. Test temperature, fuel type, and duration are always reported together with the results.

Measurement of Transverse Swelling and Calculation

Immediately after removal, specimens are quickly blotted with lint-free cloth or filter paper to remove surface liquid without squeezing out absorbed fuel, and the transverse dimension is re-measured at the same marked positions, ideally within 30 to 60 seconds of removal. The transverse swelling is calculated as the percentage change relative to the original dimension:

Transverse swelling (%) = (T1 − T0) / T0 × 100, where T0 is the original transverse dimension and T1 is the transverse dimension after immersion. Positive values indicate expansion and negative values indicate shrinkage, which can result from plasticizer extraction in high-aromatic fuels. For comparison, linear swelling can be estimated from volume swell using the cube-root relationship; a 10% volume swell corresponds to approximately 3.2% linear expansion in an isotropically swelling compound.

Supplementary Measurements and Result Interpretation

Volume swell, mass change, and post-immersion mechanical properties are normally determined in parallel to give a complete picture of fuel resistance. Well-formulated FKM copolymers and terpolymers typically show volume swell below 10% in Fuel C, with corresponding transverse dimensional changes generally below 5%, whereas NBR compounds may swell 40% to 60% or more under the same conditions. After measurement, specimens may be dried at 40 °C or reconditioned for 24 to 48 hours and re-measured to distinguish reversible physical swelling from permanent degradation of the polymer network.

Quality Control and Test Report

Calibrated measuring instruments, controlled bath temperature within ± 2 °C, defined liquid-to-specimen ratios, and duplicate or triplicate specimens are essential for reliable results. The test report should identify the material and compound designation, specimen type, fuel or reference liquid, temperature and duration, individual and mean transverse swelling values, and any supplementary volume, mass, hardness, and tensile data, together with the test standard applied (ASTM D471, ISO 1817, or GB/T 1690).

Applications

Transverse swelling data for fluoroelastomer directly supports material selection and qualification across fuel-handling industries:

• Automotive fuel systems: fuel injector O-rings, fuel pump seals, fuel sender unit gaskets, quick-connectors, filler neck seals, and fuel hose inner tubes exposed to gasoline, diesel, and ethanol blends
• Aerospace: fuel tank seals, line couplings, and access panel gaskets exposed to jet fuel and high-aromatic aviation fuels
• Marine and small engine equipment: carburetor diaphragms, fuel valves, and primer bulbs in contact with blended and oxygenated fuels
• Oil and gas and chemical processing: seals in fuel additives, solvent transfer, and refinery service where aromatic resistance is required
• Material development: ranking FKM copolymer, terpolymer, and low-temperature FKM grades, verifying cure state, and detecting formulation changes or contamination
• Quality assurance: incoming compound inspection, supplier audits, and failure analysis of swollen or leaking seals, typically performed by accredited third-party laboratories

FAQ

What is transverse swelling in fuel resistance testing?

Transverse swelling is the percentage increase in the dimension of an elastomer specimen measured across, rather than along, its principal axis, such as strip width, hose wall thickness, or O-ring cross-section diameter, after immersion in a test fuel under controlled temperature and time.

Which standards govern the determination?

The core methods are ASTM D471, ISO 1817, and GB/T 1690, which specify specimen preparation, immersion liquids, temperatures, durations, measurement timing, and calculation. Compound classification is based on ASTM D1418 or ISO 1629, while hardness and tensile changes reference ASTM D2240, ISO 48, ASTM D412, and ISO 37.

What transverse swelling values are typical for FKM?

A well-cured FKM terpolymer in Fuel C at 23 °C to 40 °C for 70 h commonly shows volume swell in the range of 2% to 10%, corresponding to transverse dimensional change of roughly 1% to 4%. Many automotive specifications limit linear dimensional change to 5% or less and volume swell to 10% to 20%, depending on the seal design and groove fill allowance.

Why is the transverse direction specified rather than overall swelling?

Sealing performance depends on dimensional growth across the sealing interface. Excessive transverse expansion causes O-rings to overfill grooves and extrude, and hose liners to restrict flow or detach from fittings. Extruded and calendered products can also swell anisotropically, making the transverse dimension the more conservative and design-relevant measurement.

What causes abnormally high swelling in fluoroelastomer?

Frequent causes include under-cure or missing post-cure, low fluorine content for the selected grade, high levels of extractable plasticizers or process aids, fuel-attacking filler or cure-system residues, and testing in a more aggressive fuel than the compound was designed for, such as high-aromatic or high-ethanol blends against a copolymer grade.

Can finished parts be tested instead of molded test slabs?

Yes. O-rings, gaskets, and hose sections are routinely tested as finished articles, and this is often preferred for part qualification because it captures the effects of production curing, trimming, and post-curing in addition to the base compound behavior.

How is a shrinkage result interpreted?

Negative transverse change indicates net shrinkage, usually caused by extraction of soluble ingredients such as plasticizers outweighing fuel absorption. Persistent shrinkage can degrade sealing force just as swelling does, and it should trigger compound review and extractables analysis.

Where can this test be performed?

The determination can be carried out in-house with calibrated immersion baths, optical or micrometer measurement systems, and controlled conditioning facilities, or outsourced to accredited third-party laboratories that offer fuel immersion testing to ASTM D471, ISO 1817, and GB/T 1690 with traceable instrument calibration and documented quality systems.