Acrylic rubber (ACM) permeability testing measures the rate at which gases and vapors pass through this heat- and oil-resistant elastomer, delivering critical performance data for automotive seals, gaskets, hoses, and fluid-handling components. This technical guide explains the testing standards, sample preparation, measurable parameters, laboratory methods, and typical applications of ACM gas permeability evaluation.
Technical Overview
Acrylic rubber (ACM, also designated ANM) is a synthetic elastomer based on alkyl acrylate monomers, typically ethyl acrylate or butyl acrylate, combined with a small amount of a curing-site monomer. The material is widely valued for continuous heat resistance up to approximately 150–175 °C, excellent resistance to mineral oils, lubricants, and ozone, and good aging stability, which makes it a preferred sealing material in automotive powertrain and industrial applications.
Gas permeability is a fundamental barrier property that determines how well an ACM component retains fluids and blocks unwanted gas migration. For shaft seals, valve stem seals, gaskets, and hose liners, permeation data directly influence evaporative emission compliance, refrigerant containment, pneumatic efficiency, and long-term sealing reliability. Because ACM parts frequently operate at elevated temperatures, permeability must be characterized not only at ambient conditions but across the full service temperature range.
Gas transport through ACM follows the classic solution–diffusion mechanism: gas molecules absorb on the high-pressure surface, dissolve into the polymer matrix, diffuse along a concentration gradient, and desorb at the low-pressure side. This behavior is described by Fick's law of diffusion and Henry's law of solubility, and the three key transport coefficients are related by the equation P = D × S, where P is the permeability coefficient, D is the diffusion coefficient, and S is the solubility coefficient. All three parameters increase with temperature in an Arrhenius-type relationship, which is why elevated-temperature testing is essential for realistic ACM qualification.
Testing Samples
Molded Sheets and Test Plaques
Standard specimens are flat cured plaques or slabs, typically 1–2 mm thick, compression-molded from the production compound using its specified cure cycle and post-cure conditions. Homogeneous, void-free sheets with uniform thickness are essential because pinholes or density variations produce falsely high transmission rates. Plaques are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 16 hours before testing.
Finished Product Specimens
When qualification of an actual component is required, specimens are cut or sliced from finished seals, gaskets, diaphragms, or hose liners. Curved parts are microtomed or die-cut into flat discs large enough to cover the permeation cell opening. The sample must remain representative of the cured state of the product, and damaged, cracked, or thermally degraded sections must be excluded.
Specimen Geometry and Preparation
Circular discs are prepared to match the permeation cell diameter, with the effective test area defined by a sealing gasket or metal mask. Thickness is measured at a minimum of five points to 0.01 mm precision, and the mean value is used in the calculations. Surfaces must be clean, dry, and free of release agents. Before measurement, specimens are typically evacuated briefly to remove dissolved air, and at least three replicate specimens are recommended to account for material variability.
Testing Items
A comprehensive ACM permeability evaluation normally covers the following parameters and conditions:
- Gas permeability coefficient (P), expressed in cm³·cm/(cm²·s·Pa) or in barrer, normalized for material thickness and test area
- Gas transmission rate (GTR) or oxygen transmission rate (OTR), expressed in cm³/(m²·24 h) under a stated pressure gradient
- Permeance (P/L), the thickness-normalized flux, in cm³/(m²·24 h·0.1 MPa)
- Diffusion coefficient (D), in cm²/s, describing the kinetic mobility of gas molecules in the elastomer
- Solubility coefficient (S), in cm³(STP)/(cm³·Pa), describing gas uptake in the rubber phase
- Time-lag and full permeation transient curves for mechanistic analysis
- Test gases including oxygen, nitrogen, air, carbon dioxide, helium, hydrogen, refrigerants, and fuel vapors such as ethanol-blend fuel (CE10)
- Temperature sweeps, for example at 23 °C, 40 °C, 70 °C, 100 °C, 125 °C, and 150 °C, used to derive apparent activation energies and service-life projections
Testing Methods
Differential-Pressure (Manometric) Method
The most widely applied technique for rubber is the differential-pressure method described in ISO 2782-1 and ASTM D1434. The specimen is clamped as a barrier between two sealed chambers. Both chambers are evacuated, and the test gas is then introduced into the upstream chamber at a controlled pressure. The gradual pressure rise in the downstream chamber is recorded with a high-sensitivity pressure transducer. The steady-state slope of pressure versus time yields the permeability coefficient, while the extrapolated time lag yields the diffusion coefficient through the relation D = L²/(6τ). The method is gas-nonspecific, works for essentially any pure dry gas, and supports testing from ambient temperature up to 150 °C or higher.
Constant-Pressure Volumetric Method
In the volumetric variant covered by ISO 2782-1, the downstream side is maintained at atmospheric pressure and the volume of permeated gas is measured with a calibrated capillary, burette, or moving-piston system. This approach is straightforward and robust, although it is generally less sensitive than the manometric method and is best suited to thicker specimens with higher transmission rates.
Coulometric (Sensor-Based) Method
For oxygen-specific measurements, ASTM D3985 and ISO 15105-2 describe a coulometric sensor method in which oxygen permeating through the specimen is swept by a nitrogen carrier gas to a fuel-cell detector that quantifies the oxygen transmission rate with very high sensitivity. This method is particularly useful for thin ACM films, coatings, and barrier laminate evaluations, and it avoids artifacts associated with water vapor or other gases.
Test Procedure and Data Evaluation
The specimen is mounted in the cell with an inert sealing gasket, the system is leak-checked with a blank or impermeable reference, and the cell is equilibrated at the target temperature. The test gas is applied at the specified pressure, and transmission is monitored until steady state is reached, typically defined as a variation of less than 5% over consecutive measurement intervals. Reported results include the permeability, diffusion, and solubility coefficients, the transmission rate, specimen thickness, test gas, temperature, pressure gradient, and the standard deviation across replicates.
Quality Assurance
Reliable permeability data depend on rigorous quality control: verified cell sealing to eliminate edge leakage, calibrated pressure transducers and temperature controllers, blank-run subtraction for outgassing contributions, duplicate or triplicate specimens, and periodic verification using certified reference films or interlaboratory comparisons.
Applications
Permeability test results for ACM directly support engineering decisions across multiple industries:
- Automotive sealing: crankshaft seals, valve stem seals, camshaft seals, and transmission seals, where blow-by gas and oil vapor permeation data validate emission and durability targets
- Low-permeation hose systems: turbocharger hoses, fuel vapor hoses, and filler neck components designed to meet stringent evaporative emission regulations such as LEV III, EPA, and CARB requirements
- Refrigeration and HVAC: shaft seals and gaskets exposed to refrigerants, where refrigerant permeation rates affect charge retention and system efficiency
- Industrial pneumatics: diaphragms, O-rings, and valve seats where gas loss through the elastomer influences pressure-holding performance
- Emerging energy applications: hydrogen storage, fuel cell, and battery pack sealing components that demand quantified barrier performance at elevated temperatures
- Material development: comparing filler loadings, nanoclay or carbon black reinforcement, plasticizer content, and cure systems to optimize the balance between barrier properties, compression set, and low-temperature flexibility
FAQ
Which standards govern acrylic rubber permeability testing?
The principal standards are ISO 2782-1 for rubber tested by differential-pressure methods, ASTM D1434 for gas permeability by the pressure and volumetric techniques, and ASTM D3985 or ISO 15105-2 for oxygen transmission by the coulometric sensor method. Related procedures in GB/T 1038 and JIS K 6276 are also referenced for specific markets.
What is the difference between permeability and transmission rate?
The transmission rate describes how much gas passes through a specific specimen of a given thickness per unit area and time, whereas the permeability coefficient is a material property normalized to unit thickness and unit pressure gradient. Transmission rate is used for component qualification; permeability is used for material comparison and simulation input.
What specimen thickness is recommended?
Most laboratories test ACM specimens between 0.5 mm and 2 mm. Thin specimens reach steady state quickly and suit high-sensitivity sensor methods, while thicker plaques better represent molded goods. Because permeability is thickness-normalized, results from different thicknesses should agree once properly evaluated.
Can testing be performed at elevated temperatures?
Yes. ACM permeability is commonly measured from 23 °C up to 150 °C to reflect engine-bay and powertrain service conditions. Arrhenius plots of the resulting coefficients allow extrapolation and long-term permeation projections.
How does ACM compare with other elastomers?
ACM offers moderate gas barrier properties with superior heat and oil resistance relative to NBR and much better oil resistance than EPDM. Fluoroelastomers (FKM) generally provide lower permeability, while silicone rubber (VMQ) is markedly more permeable. Carbon dioxide permeates through ACM significantly faster than oxygen, and oxygen faster than nitrogen, following typical elastomer selectivity behavior.
How long does a typical test take?
Thin specimens at elevated temperature typically reach steady state within a few hours, while thick specimens at low temperature may require 24 hours or longer. Testing is complete only after the transmission curve stabilizes, and the report must state the time allowed for equilibration.