Acrylic Rubber (ACM) Swelling Degree Testing: Standards, Methods, and Applications

Acrylic rubber (ACM) swelling degree testing measures the mass and volume change of polyacrylate elastomers after controlled immersion in oils, fuels, and service fluids, providing a direct indicator of fluid resistance and long-term sealing reliability. This technical guide explains the governing standards, specimen requirements, test procedures, calculation formulas, and acceptance criteria commonly applied to ACM materials in automotive and industrial applications.

Technical Overview

Acrylic rubber (ACM) is a specialty elastomer built from alkyl acrylate monomers, most commonly ethyl acrylate (EA) and butyl acrylate (BA), copolymerized with reactive cure-site monomers that enable crosslinking. Because of its saturated polymer backbone, ACM combines heat resistance up to approximately 150 to 175 degrees Celsius with excellent resistance to mineral oils, lubricants, ATF, and ozone, which makes it a preferred material for automotive shaft seals, valve stem seals, O-rings, and gaskets.

Swelling degree testing quantifies how much a crosslinked ACM vulcanizate expands when exposed to a liquid. Unlike uncrosslinked polymers, a vulcanized rubber cannot dissolve; instead, fluid molecules diffuse into the three-dimensional network and stretch the chain segments apart until the elastic retraction force balances the osmotic swelling force. The magnitude of this equilibrium swelling is governed by the solubility parameter match between the fluid and the rubber, the crosslink density of the compound, the fillers and plasticizers present, and the test temperature.

Because ACM is a polar polymer, it swells only moderately in non-polar hydrocarbon oils but swells significantly in polar media such as ester-based lubricants, ketones, water-glycol brake fluids, and alcohol-containing fuels. Swelling degree testing therefore serves as the primary screening method for confirming that a compound is compatible with its intended service environment.

• ASTM D471 – Standard Test Method for Rubber Property – Effect of Liquids
• ISO 1817 – Rubber, vulcanized or thermoplastic – Determination of the effect of liquids
• GB/T 1690 – Chinese national standard for the effect of liquids on vulcanized or thermoplastic rubber
• DIN ISO 1817 and related OEM specifications derived from these base standards

Testing Samples

Standard Test Coupons

Rectangular strips cut from cured ACM sheets, typically 25 mm by 50 mm with a thickness of 2 mm plus or minus 0.2 mm, are the most common specimens. Coupons are preferred for fundamental swelling studies because they provide a well-defined surface-to-volume ratio, which shortens diffusion paths and shortens the time required to approach equilibrium swelling.

Finished Sealing Articles

Complete products such as O-rings (for example, standard AS568 sizes), radial shaft seals, gaskets, and diaphragms may be immersed as received. Testing whole articles captures the influence of molding skin, post-cure state, and surface treatment, and it reflects real service geometry more closely than laboratory coupons.

Molded Test Slabs

When the test program also requires mechanical property retention, slabs are molded and die-cut into dumbbell specimens per ASTM D412 or ISO 37 so that tensile strength and elongation can be measured before and after fluid immersion on identical geometry.

Testing Items

A complete swelling evaluation of acrylic rubber normally reports the following parameters:

• Volume change percentage, the core swelling degree indicator
• Mass change percentage, including both fluid uptake and plasticizer or oil extraction losses
• Dimensional change in length, width, and thickness
• Hardness change in Shore A points, measured per ASTM D2240 or ISO 48
• Tensile strength and elongation at break retention, per ASTM D412 or ISO 37
• Extractable content and mass loss after drying, which reveals plasticizer depletion
• Optional apparent crosslink density derived from equilibrium swelling through the Flory-Rehner equation

Testing Methods

Specimen Conditioning and Initial Measurement

Specimens are conditioned in a standard laboratory atmosphere of 23 plus or minus 2 degrees Celsius and 50 plus or minus 5 percent relative humidity for at least 16 hours before testing. Initial mass in air is recorded to 1 mg precision. Initial volume is determined by the Archimedes method: the specimen is weighed in air and then in water, and the volume is calculated as the difference between the two readings divided by the density of water. Initial hardness, dimensions, and, where required, tensile properties are also recorded.

Immersion Conditions

Specimens are fully submerged in the test fluid at a volume ratio of at least 20 mL of fluid per gram of rubber, or 100 mL per specimen for whole articles, so that extracted matter does not saturate the liquid. Typical fluids and conditions for ACM include:

• IRM 903 reference oil at 150 degrees Celsius for 70 hours, the classic heat-plus-oil screening test for ACM
• ASTM No. 1 oil at 150 degrees Celsius for 70 hours, indicating resistance to low-aniline-point lubricants
• Automatic transmission fluid (ATF) or gear oil at 125 to 150 degrees Celsius for 70 to 168 hours
• ASTM Reference Fuel C or service fuels at 23 degrees Celsius for 22 to 70 hours, where ACM shows limited resistance
• DOT 4 or water-glycol brake fluid at 100 to 125 degrees Celsius, used to characterize ACM weakness in polar media
• Long-term aging sequences of 336, 500, or 1008 hours for service-life validation

Post-Immersion Measurement

Upon removal, the specimen surface liquid is quickly blotted with lint-free cloth or filter paper without squeezing the sample, and the mass in air is measured within 60 seconds of removal to prevent evaporative loss. The immersed (in-water) weight is then taken immediately using the same balance setup. For specimens that undergo subsequent tensile or hardness testing, a reconditioning period of 24 to 96 hours in the standard atmosphere, or drying to constant mass, is applied before final mechanical measurement.

Calculation of Results

The swelling degree is expressed as percentage volume change, calculated as follows:

Volume Change (%) = (V2 minus V1) divided by V1, multiplied by 100, where V1 is the initial volume and V2 is the volume after immersion.

Mass Change (%) = (m2 minus m1) divided by m1, multiplied by 100, where m1 is the initial mass in air and m2 is the post-immersion mass in air.

Hardness change and tensile property change are reported as the arithmetic difference and as percentage retention, respectively. For research purposes, the equilibrium volume fraction of rubber in the swollen network can be inserted into the Flory-Rehner equation to estimate the effective crosslink density of the ACM vulcanizate.

Acceptance Criteria

Typical automotive and industrial specifications for ACM oil seals require a volume change in IRM 903 oil at 150 degrees Celsius for 70 hours within 0 to plus 15 percent, with some specifications allowing a window of minus 5 to plus 25 percent. Negative volume change indicates extraction of plasticizers or process oils and is treated as a durability warning even when the sample shrinks. Hardness change is commonly limited to plus or minus 10 Shore A points, and tensile strength retention above 70 percent is frequently required after fluid aging.

Applications

Swelling degree testing is applied throughout the ACM value chain:

• Qualification of shaft seals, valve stem seals, transmission seals, and turbocharger gaskets against engine oils and ATFs
• Compound development, where filler loading, plasticizer type, and cure system are optimized to balance swell resistance with low-temperature flexibility
• Incoming material quality control and batch-to-batch consistency verification for rubber compounders
• Validation of ACM compatibility with new lubricant chemistries, including low-viscosity fuel-economy oils and EV thermal management fluids
• Failure analysis, where abnormal swelling or plasticizer loss confirms fluid incompatibility as the root cause of seal leakage

FAQ

What is the swelling degree of acrylic rubber?

The swelling degree is the percentage increase in volume (or mass) of a vulcanized ACM specimen after immersion in a defined fluid at a defined temperature and time. It measures how much service fluid the crosslinked network absorbs before elastic forces stop further expansion.

Which standards govern ACM swelling tests?

ASTM D471, ISO 1817, and GB/T 1690 are the principal standards. They define specimen geometry, immersion liquid volume ratios, post-immersion handling timing, and the calculation of mass change, volume change, and mechanical property retention.

What volume change is considered acceptable?

For ACM in mineral oils and ATFs at 150 degrees Celsius, most specifications accept 0 to plus 15 percent volume change. Applications with tighter seal interference, such as precision O-rings, may require plus 10 percent or less.

Why does acrylic rubber swell more in brake fluid than in engine oil?

ACM is a polar elastomer. Engine oil is largely non-polar hydrocarbon, so compatibility is favorable. Brake fluids and water-glycol media are polar, and their solubility parameters are close to that of ACM, driving much higher absorption. For brake fluid service, EPDM is generally selected instead.

How long does a standard test take?

A routine 70-hour immersion with conditioning and post-test measurement is typically completed within 4 to 5 working days. Long-duration validation programs of 500 or 1008 hours require several weeks plus laboratory scheduling.

Can a negative swelling result occur, and is it a problem?

Yes. A negative volume or mass change indicates that soluble ingredients, such as plasticizers or process oils, were extracted from the compound faster than the test fluid was absorbed. Persistent extraction hardens the seal and accelerates cracking, so shrinkage beyond specification limits is treated as a rejection criterion.