Acrylic rubber (ACM) is a heat- and oil-resistant elastomer widely specified for automotive and industrial sealing systems exposed to aggressive lubricants, fuels, and chemicals. A rigorous corrosion and chemical resistance evaluation, based on standardized immersion testing, is essential to verify volume stability, mechanical retention, and long-term sealing reliability. This article covers the key properties of ACM, sample preparation, testing items, evaluation methods, and typical industrial applications.
Technical Overview
Acrylic rubber (ACM) is a synthetic elastomer based on alkyl acrylate monomers, typically ethyl acrylate or butyl acrylate, copolymerized with a reactive cure-site monomer that enables sulfur-free vulcanization. Its saturated polymer backbone, free from double bonds, gives ACM an exceptional balance of heat resistance and resistance to petroleum-based fluids, allowing continuous service temperatures of 150 to 175 °C and intermittent exposure up to approximately 200 °C.
- Excellent resistance to mineral oils, synthetic lubricants, gear oils, and automatic transmission fluids (ATF)
- Outstanding heat-aging resistance, far superior to NBR and comparable to many higher-cost specialty elastomers
- Strong resistance to ozone, UV radiation, and weathering due to the fully saturated backbone
- Known limitations: reduced resistance to hot water, steam, glycol-based coolants, strong acids, alkalis, and aromatic or oxygenated fuels
Because ACM seals and gaskets operate in dynamic, chemically aggressive environments, a systematic corrosion and chemical resistance evaluation is critical. Fluid attack can cause swelling, softening, extraction of plasticizers, embrittlement, or reversion, any of which may compromise seal integrity and lead to leakage or premature component failure.
Testing Samples
Raw Polymer and Laboratory Compounds
Evaluation typically begins with verification of the base polymer, followed by preparation of standard compounds containing carbon black or mineral fillers, processing aids, and a suitable vulcanization package such as soap-sulfur, amine, or donor-based cure systems. Standardized mixing and molding procedures, such as those described in ASTM D3182, ensure specimen consistency and reproducibility.
Molded Standard Specimens
Immersion and mechanical testing use molded plaques approximately 2 mm thick, from which dumbbell specimens are die-cut for tensile testing per ASTM D412. Flat discs or buttons are prepared for Shore A hardness measurement per ASTM D2240, and cylindrical pellets are molded for compression set testing per ASTM D395.
Finished Components
O-rings, radial shaft seals, valve stem seals, gaskets, and hose sections taken from actual production lots are tested to validate material performance in real part geometry. O-ring fluid resistance is commonly assessed per ASTM D1414, which accounts for the influence of part size and molding conditions on swelling behavior.
Testing Items
The core objective of the evaluation is to quantify physical and mechanical property changes following fluid exposure. The principal measured parameters include:
- Volume change (swell or shrinkage), the primary indicator of fluid compatibility
- Mass change, reflecting fluid absorption or leaching of soluble components
- Tensile strength retention before and after immersion
- Elongation at break retention, sensitive to chain scission or over-cure effects
- Hardness change (Shore A), indicating plasticization or embrittlement
- Compression set after fluid aging, critical for long-term sealing force retention
- Visual assessment of surface cracking, blistering, tackiness, or degradation
- Optional analytical characterization, such as FTIR spectroscopy or thermogravimetric analysis, to identify chemical degradation mechanisms and extractables
Testing Methods
Fluid Immersion Testing
The core method is immersion testing per ASTM D471 or its international equivalent ISO 1817. Specimens are fully immersed in the test fluid in sealed containers at controlled temperatures, typically 70 °C, 100 °C, 125 °C, or 150 °C, for standard durations of 22 hours, 70 hours, 168 hours, or extended aging periods of 336 to 1000 hours for qualification programs.
Test Media Selection
A representative range of fluids is selected to simulate both reference and real-world service conditions:
- ASTM No. 1 reference oil (low-swell aniline point oil) and IRM 903 oil (high-swell oil) for benchmarking oil resistance
- ASTM reference fuels A, B, and C to assess fuel resistance across increasing aromatic content
- Service fluids, including engine oils, ATF, hypoid gear oils, power steering fluids, brake fluids, and glycol-based coolants
- Chemical reagents such as dilute acids, alkalis, and salt solutions for environmental corrosion screening
Post-Exposure Measurement and Criteria
After immersion, specimens are removed, blotted to remove surface fluid, and immediately weighed and measured for volume, typically by hydrostatic (Archimedes) weighing. Tensile and hardness tests are then performed within the time window specified by the standard to avoid drying artifacts. Typical acceptance criteria for oil-resistant ACM compounds include volume change within roughly plus or minus 10 percent in engine oils, tensile strength retention above 70 percent, and compression set below 25 percent after extended heat and fluid aging, although exact limits depend on the end-use specification.
Applications
ACM is the material of choice for numerous demanding sealing and fluid-handling applications where heat and oil resistance are required but full fluoroelastomer performance is not economically justified:
- Automotive seals: crankshaft seals, valve stem seals, pinion seals, and transmission shaft seals
- O-rings and gaskets for engine, transmission, and axle systems
- Hose and tube components exposed to hot lubricants and air
- Diaphragms and gaskets in industrial fluid-handling equipment
- Wire and cable jacketing where heat resistance and weatherability are required
FAQ
Question: How does acrylic rubber compare with NBR and FKM in chemical resistance?
Answer: ACM offers significantly better heat and ozone resistance than NBR with comparable oil resistance, but it is inferior to FKM in resistance to fuels, aggressive chemicals, and hot water. ACM occupies a cost-effective middle ground for hot-oil environments up to approximately 175 °C.
Question: Which media most commonly degrade ACM?
Answer: Hot water, steam, glycol coolants, aromatic fuels, ketones, esters, strong acids, and alkalis are the most aggressive media. Standard ACM grades hydrolyze in hot aqueous environments, although special ethylene-acrylate (AEM) copolymers offer improved water resistance.
Question: What volume change is generally acceptable in immersion testing?
Answer: For lubricating oils and ATF, volume swell of plus or minus 10 percent or less is typically specified, while tensile retention of 70 percent or higher and minimal hardness change indicate good fluid compatibility. Requirements vary by industry specification and part function.