Transverse swelling determination is a key test for evaluating the oil resistance of fluoroelastomer (FKM) seals, O-rings, and molded rubber components. This technical article explains the test principle, applicable standards such as ASTM D471, ASTM D1414, ISO 1817, and GB/T 1690, sample requirements, measurement procedures, calculation formulas, and typical acceptance criteria. It serves as a practical reference for materials engineers, quality inspectors, and sourcing professionals in the rubber and sealing industry.
Technical Overview
Fluoroelastomers (FKM, commonly known by the generic family designation fluorocarbon rubber) are high-performance synthetic rubbers based on a fluorinated polymer backbone. Their strong carbon-fluorine bonds give them outstanding resistance to heat, mineral oils, fuels, hydraulic fluids, and many aggressive chemicals, which is why they are the material of choice for demanding sealing applications in automotive, aerospace, petrochemical, and industrial equipment.
However, oil resistance is never absolute. When a rubber component is in contact with an oil, fuel, or service fluid, liquid molecules diffuse into the polymer network and cause swelling, while plasticizers and other soluble ingredients may diffuse out and cause shrinkage. Transverse swelling, also called lateral or cross-sectional swelling, describes the dimensional change measured perpendicular to the principal axis of a specimen. For an O-ring, this is the change of the cord (cross-section) diameter; for a flat test piece, it is the change of thickness.
For sealing elements, transverse swell is an engineering-critical parameter: it directly determines groove fill, sealing interference, contact stress, and the risk of extrusion or nibbling. Excessive swelling softens the seal, accelerates compression set and stress relaxation, and can cause mechanical failure in service. Conversely, negative swell (shrinkage) from plasticizer extraction can open leakage paths. Quantifying transverse swelling under simulated service liquids and temperatures enables material selection, compound qualification, batch release testing, and root-cause failure analysis.
Compared with other oil-resistant rubbers such as NBR, a well-formulated FKM compound typically exhibits very low swell in mineral oils and standard reference fuels. Many specifications limit volume swell to 10 percent or less, and demanding automotive or aerospace specifications often require 5 percent or less after immersion in a reference oil such as IRM 903 for 70 hours at 150 degrees Celsius.
Testing Samples
O-Rings and Sealing Cord
O-rings in standard sizes, for example AS568 or ISO 3601 series, are the most representative specimens because the cord diameter can be measured directly as the transverse dimension. The cord diameter is measured at several evenly spaced positions around the circumference, rotating the ring to average out ovality and parting-line effects, in accordance with the dimensional measurement practice described in ASTM D1414.
Standard Rectangular Test Pieces
For compound evaluation, rectangular strips are die-cut or slit from vulcanized sheets, with a typical size of 25 mm by 50 mm by 2 mm as described in ASTM D471 and ISO 1817. For these specimens the thickness direction serves as the transverse dimension, and thickness is measured at several marked points before and after immersion.
Specimens Taken from Finished Parts
Sections cut from production components, such as gaskets, valve stem seals, or custom-molded parts, may also be tested. Specimens should be of uniform thickness, preferably at least 2 mm, free from surface defects, flash, or fabric layers, and large enough to permit accurate dimensional measurement.
Sample Preparation and Conditioning
- Cure state: samples must be fully vulcanized and aged sufficiently after molding, commonly 16 hours minimum, before testing.
- Cleaning: specimens are cleaned with a suitable volatile solvent that does not swell the rubber, then dried.
- Conditioning: specimens are conditioned per ISO 23529 or ASTM D618, typically at 23 plus or minus 2 degrees Celsius and 50 plus or minus 5 percent relative humidity for at least 16 hours.
- Quantity: a minimum of three specimens per condition is recommended so that a median value can be reported.
Testing Items
A comprehensive oil-resistance evaluation at accredited third-party laboratories typically includes the following measurements on FKM specimens before and after immersion:
- Transverse dimensional change, that is, the change in cord diameter or thickness, reported as a percentage.
- Volume change, determined gravimetrically by the Archimedes (buoyancy) method, reported as a percentage.
- Mass change, the increase or loss in weight expressed as a percentage of the original mass.
- Hardness change, measured in Shore A or IRHD micro hardness per ISO 48, ISO 7619, or ASTM D2240.
- Change in tensile strength and elongation at break per ISO 37 or ASTM D412.
- Visual examination for surface cracking, blistering, tackiness, discoloration, or disintegration after exposure.
Testing Methods
Applicable Standards and Reference Liquids
The core standards for immersion testing are ASTM D471 and ISO 1817, which cover the effect of liquids on vulcanized or thermoplastic rubbers. GB/T 1690 is the corresponding Chinese national standard with broadly equivalent methodology. For O-rings, ASTM D1414 provides the specific dimensional measurement techniques and is used together with ASTM D471, while ISO 3601 quality requirements reference immersion testing per ISO 1817.
Standard reference liquids defined by ASTM include IRM 901, IRM 902, and IRM 903 oils of increasing aniline point and swelling aggressiveness, together with ASTM Reference Fuels A, B, C, and D, which are defined blends of isooctane, toluene, and other hydrocarbons. Tests may also be performed in the actual service medium, such as engine oil, automatic transmission fluid, gear oil, hydraulic fluid, brake fluid, or a specific aviation fuel, when qualification against real-world conditions is required.
Test Conditions
- Temperature: common immersion temperatures are room temperature at 23 degrees Celsius for fuels, and 100, 125, 150, or 175 degrees Celsius for oils, chosen to reflect or accelerate service conditions.
- Duration: standard exposure periods are 22 hours, 70 hours, and 168 hours; extended exposures of 1000 hours or longer are used for heat and fluid aging studies.
- Temperature control: the oven or immersion bath must maintain the set point within plus or minus 2 degrees Celsius.
- Replenishment: for long-duration tests at high temperature, the test liquid is refreshed periodically, and closed or reflux systems are used for volatile fuels.
Measurement Procedure
- Measure the initial transverse dimension d1 of each conditioned specimen using a low-force micrometer, dial gauge with radiused anvils, or a non-contact optical or laser measuring system, and mark the measurement positions.
- If volume change is required, weigh each specimen in air (m1) and in water (m2) on an analytical balance with a density determination kit.
- Immerse the specimens completely in the test liquid, ensuring free exposure on all surfaces and a liquid volume of at least 15 times the total specimen volume, typically a minimum of 100 mL per specimen.
- Maintain the vessel at the specified temperature for the specified time in a ventilated oven or temperature-controlled bath; use sealed, pressure-rated containers for volatile fuels.
- On removal, immediately and lightly blot the surface liquid with lint-free cloth or filter paper within about 30 seconds, taking care not to squeeze liquid out of the swollen rubber.
- Measure the final transverse dimension d2 within 60 seconds of removal, because volatile test liquids evaporate quickly and the measured dimension can decrease with delay.
- If volume change is required, reweigh the specimen in air (m3) and in water (m4) within the same short time window.
- Examine each specimen visually and record any cracking, blistering, tackiness, or degradation.
Calculation Formulas
Transverse swelling percentage equals (d2 minus d1) divided by d1, multiplied by 100. A positive value indicates swelling and a negative value indicates shrinkage.
Volume change percentage by the Archimedes method equals the quantity (m3 minus m4 minus m1 plus m2) divided by (m1 minus m2), multiplied by 100, where m1 and m2 are the masses in air and in water before immersion and m3 and m4 are the corresponding values after immersion. Mass change percentage equals (m3 minus m1) divided by m1, multiplied by 100.
The median of at least three specimens is reported together with the test liquid, temperature, duration, and any visual observations. Reproducibility depends heavily on consistent blotting technique and rapid, repeatable measurement timing.
Equipment and Safety
Typical equipment includes a low-contact-force micrometer or video measuring machine, a Shore A durometer or IRHD micro hardness tester, an analytical balance readable to 0.001 g or better, a laboratory oven or liquid immersion bath, and borosilicate or stainless-steel immersion vessels. Hot oils cause burns and reference fuels are highly flammable and toxic, so tests must be conducted in fume hoods with appropriate personal protective equipment and explosion-proof ovens where flammable liquids are involved.
Applications
- Automotive: fuel injector and fuel line O-rings, crankshaft seals, valve stem seals, turbocharger gaskets, and transmission and axle seals exposed to engine oils, fuels, and lubricants at elevated temperatures.
- Aerospace: seals for jet fuel systems, lubrication circuits, and hydraulic components requiring qualification under aggressive fluid and temperature combinations.
- Oil and gas: downhole sealing elements, packer seals, and valve components exposed to crude oil, drilling fluids, and amine corrosion inhibitors.
- Chemical processing: gaskets, diaphragms, and pump seals in contact with solvents, acids, and hot hydrocarbon streams.
- Quality control and R&D: compound development and grade comparison, incoming material inspection, batch release testing, supplier qualification, shelf-life and long-term aging studies, and failure analysis of returned parts.
FAQ
Question: What is the difference between transverse swelling and volume swelling?
Answer: Volume swelling is the total volumetric increase of the specimen measured gravimetrically, while transverse swelling is the linear dimensional change in the direction perpendicular to the specimen axis, such as O-ring cord diameter or sheet thickness. For seals, the transverse value is directly usable in groove fill and interference calculations, which is why many specifications require it in addition to or instead of volume change.
Question: What swelling level is acceptable for fluoroelastomer?
Answer: Acceptance depends on the specification and fluid. As a guideline, general-purpose FKM compounds are often required to show no more than 10 percent volume swell in IRM 903 after 70 hours at 150 degrees Celsius, and many automotive and aerospace specifications limit swell to 5 percent or less. Low-temperature or peroxide-cured FKM grades may swell slightly more in certain fuels and should be verified case by case.
Question: Why must the final dimension be measured within one minute of removal from the liquid?
Answer: Immersion liquids, especially volatile fuels, evaporate rapidly from the specimen surface after removal, so the apparent size and mass decrease with time. Standardized timing, blotting technique, and measurement sequence are therefore essential for reproducible results.
Question: Which standard should be used for O-ring testing?
Answer: ASTM D1414 governs the physical measurement of O-rings and is applied together with ASTM D471 for the immersion procedure. When the ISO system is specified, ISO 1817 is used for the immersion method and ISO 3601 defines related quality requirements. In China, GB/T 1690 is the equivalent national method.
Question: Can the actual service oil be used as the test liquid?
Answer: Yes. Reference oils and fuels such as IRM 903 and Reference Fuel C provide standardized comparison between compounds, while testing in the actual service fluid, including any additive packages, gives the most realistic prediction of field performance and is recommended for final qualification.
Question: What does a negative swelling value mean?
Answer: A negative value indicates net shrinkage, usually caused by extraction of plasticizers, processing oils, or other soluble ingredients from the rubber into the test liquid. Shrinkage can be as harmful as swelling because it reduces seal interference and can create leakage paths.
Question: Why does FKM resist oil better than NBR?
Answer: The fluorinated backbone of FKM is chemically inert toward hydrocarbons, so oil and fuel molecules penetrate the network far more slowly than they do in NBR. As a result, FKM typically shows swell values several times lower than NBR in the same hot oil, while also tolerating much higher continuous service temperatures, generally up to about 200 degrees Celsius.