Determination of volume change in acrylic rubber (ACM) is a critical laboratory procedure for evaluating how this heat- and oil-resistant elastomer swells or shrinks after exposure to fuels, lubricants, and service fluids. This article explains the testing principles, sample requirements, measurement methods, applicable standards, and typical applications of ACM volume change determination in seals, gaskets, and automotive components.
Technical Overview
Acrylic rubber (ACM) is a specialty synthetic elastomer based on alkyl acrylate monomers, valued for its exceptional resistance to heat, ozone, oxidation, and petroleum-based oils at service temperatures up to approximately 150–175°C. Because ACM compounds are used predominantly in dynamic and static sealing applications, their behavior upon contact with liquids is a decisive performance characteristic. Volume change determination measures the percentage of swelling or shrinkage that occurs when a cured ACM specimen is immersed in a reference fluid or an actual service medium under controlled temperature and time conditions.
Excessive swelling can cause seal extrusion, increased friction, loss of mechanical strength, and dimensional interference, while excessive shrinkage can lead to leakage due to loss of sealing force. Volume change testing therefore provides a direct, quantitative indicator of fluid compatibility and long-term sealing reliability. The principal international standards governing this test include ASTM D471, ISO 1817, and GB/T 1690, all of which define immersion procedures, specimen geometry, and the calculation of volume change percentage.
Testing Samples
Standard Test Coupons
The preferred specimen is a flat, cured coupon cut from a compression-molded or calendar-cured sheet, typically with dimensions of 25 mm × 50 mm × 2 mm or circular discs of approximately 30 mm diameter and 2 mm thickness. Specimens must be fully cured, free from porosity, blisters, and surface contamination, and conditioned for at least 16 hours at standard laboratory temperature (23 ± 2°C) before initial measurements are taken.
O-Rings and Finished Seals
Complete O-rings, shaft seals, or gaskets may be tested when evaluating actual production parts. The specimen mass in air and in water is determined on the intact component, and the same calculation methods apply. Testing finished parts captures the influence of post-curing, surface treatment, and molding conditions on fluid resistance.
Cut Sections from Components
When a full part cannot be immersed, representative sections may be cut from thick-walled components such as bushings or molded housings. Cut surfaces should be smooth, and the specimen geometry should be documented because edge effects and diffusion path length influence the rate of fluid uptake.
Testing Items
A comprehensive ACM fluid resistance evaluation typically includes the following measured parameters:
- Volume change percentage after immersion (the primary result, expressed as ΔV %)
- Mass change percentage (weight gain or loss, often measured in parallel with volume change)
- Dimensional changes in length, width, and thickness
- Change in hardness (Shore A) before and after immersion
- Change in tensile strength and elongation at break (optional, using dumbbell specimens)
- Visual assessment of the specimen surface, including cracking, tackiness, or disintegration
Common immersion media include ASTM reference oils (IRM 901, IRM 902, IRM 903), ASTM reference fuels (Fuel A, B, and C), automatic transmission fluid, engine lubricating oil, hydraulic fluid, gear oil, and coolants. The test fluid is selected to simulate the intended service environment of the ACM component.
Testing Methods
Specimen Conditioning and Initial Volume Measurement
After conditioning, the initial volume V1 of the specimen is determined gravimetrically using Archimedes' principle. The specimen is weighed in air (m1) and then fully immersed in distilled water containing a small quantity of wetting agent and weighed again (m2). The immersed weight is corrected for the density of the liquid and the mass of the suspension wire. The initial volume is calculated as:
V1 = (m1 − m2) / ρwater, where ρwater is the density of the immersion liquid at test temperature.
Immersion Exposure
The specimen is fully submerged in the test fluid at a specified temperature for a defined duration. Typical test conditions for ACM include immersion in IRM 903 oil or transmission fluid at 150°C for 70 hours or 168 hours, with extended exposures of 500 or 1000 hours used for qualification programs. The fluid volume must be at least 15 times the specimen volume, and the fluid should be refreshed for long-duration tests to prevent depletion of aggressive constituents.
Post-Immersion Measurement
Upon removal from the fluid, the specimen is quickly blotted with lint-free cloth or filter paper to remove surface liquid without squeezing out absorbed fluid. The specimen is then reweighed in air and in water following the same Archimedes procedure to obtain the final volume V2. All post-immersion measurements should be completed within 30 minutes of removal to minimize errors from evaporation or continued diffusion. If the test fluid is miscible with or attacks the water used for measurement, an alternative non-interacting liquid of known density is used.
Calculation of Volume Change
The volume change percentage is calculated as follows:
ΔV (%) = (V2 − V1) / V1 × 100
A positive value indicates swelling due to fluid absorption, while a negative value indicates shrinkage, which may result from extraction of plasticizers or soluble compounding ingredients. Where direct volumetric measurement is impractical, volume change may be estimated from mass change and density data, though the Archimedes method remains the reference procedure.
Influencing Factors
- Test temperature: higher temperatures accelerate both diffusion and chemical interaction between fluid and polymer
- Immersion duration: volume change approaches equilibrium asymptotically; short tests may understate equilibrium swelling
- Fluid polarity and aromatic content: ACM resists aliphatic oils well but swells more in highly aromatic or polar media
- Compound formulation: filler loading, plasticizer type, and cure state significantly affect swelling behavior
- Specimen thickness: thicker specimens require longer immersion to reach equilibrium fluid uptake
Applications
Volume change determination is applied throughout the ACM product lifecycle, from material development to quality assurance:
- Automotive sealing systems: crankshaft seals, valve stem seals, transmission shaft seals, and cylinder head gaskets exposed to hot engine oil and ATF
- Powertrain and driveline components: seals for automatic and manual transmissions, differentials, and torque converters
- Hose and tubing qualification: ACM-based oil cooler and transmission cooler hoses
- Material certification: verifying that rubber compounds meet automotive OEM specifications, which commonly limit oil swell to within ±10% after hot oil immersion
- Comparative compound development: benchmarking new ACM formulations against competitive elastomers such as FKM, AEM, and NBR
- Failure analysis: diagnosing seal leakage attributed to excessive swell, shrinkage, or fluid incompatibility in service
FAQ
What is an acceptable volume change for acrylic rubber in oil?
Most automotive and industrial specifications require ACM volume change within roughly −5% to +15% after immersion in reference oil at elevated temperature, with many OEM limits set at ±10% or tighter. The acceptance criterion depends on the sealing design, operating temperature, and service fluid, so the relevant specification should always be consulted.
Which standards govern ACM volume change testing?
The primary standards are ASTM D471 (Rubber Property—Effect of Liquids), ISO 1817 (Rubber—Determination of resistance to liquids), and GB/T 1690, which is the Chinese national method substantially harmonized with ISO 1817. These standards specify immersion procedures, reference fluids, measurement techniques, and reporting requirements.
Why does ACM exhibit low swell in hot oil compared to NBR?
The saturated polar backbone of polyacrylate rubber has low thermodynamic affinity for non-polar hydrocarbon oils, so diffusion is limited and equilibrium swelling remains small even at high temperature. NBR, by contrast, degrades and hardens at temperatures where ACM remains stable, making ACM the preferred choice for continuous service above 120°C in oil.
How long does a typical test take?
Standard screening tests use 22-hour or 70-hour immersion at the specified temperature, while qualification testing commonly requires 168 hours or longer. Including conditioning, initial measurement, immersion, and post-immersion measurement, a routine 70-hour test is typically completed within one week.
Can finished parts be tested instead of standard coupons?
Yes. Complete O-rings, gaskets, and seals can be immersed and measured using the same Archimedes procedure. Testing finished parts is often preferred for lot acceptance because it evaluates the actual product as manufactured, including any effects of molding and post-cure.