Carbon dioxide (CO2) is one of the most chemically aggressive service media for polymeric and elastomeric sealing elements, causing swelling, plasticization, and rapid gas decompression (RGD) damage in high-pressure systems. This article provides a technical analysis of how gaseous, liquid, and supercritical CO2 affects seal materials, and outlines the sample types, testing items, test methods, and standards used by accredited third-party laboratories to qualify seals for CO2 service.
Technical Overview
Carbon dioxide is widely encountered in modern industrial systems, including carbon capture and storage (CCS), enhanced oil recovery (EOR), carbonated beverage production, CO2 refrigeration (R744), biogas upgrading, and geothermal energy. Although CO2 is chemically inert compared with many process gases, its unique physical behavior makes it particularly challenging for sealing elements. Above the critical point of 31.1 °C and 7.38 MPa, CO2 enters the supercritical phase, combining gas-like diffusivity with liquid-like solvency, which allows it to permeate rapidly into elastomeric materials.
The principal damage mechanisms caused by CO2 exposure on seals include:
- Rapid gas decompression (RGD), also known as explosive decompression, in which dissolved CO2 expands during depressurization and forms internal blisters, cracks, and splits.
- Sorption-induced swelling and volume change, which alters seal compression, contact stress, and dimensional stability.
- Plasticization and extraction of plasticizers or processing additives, leading to changes in hardness, tensile strength, and elongation.
- Increased compression set and stress relaxation, reducing long-term sealing force.
- Liquid and supercritical CO2 acting as a strong solvent for certain elastomer compounds, causing accelerated chemical aging.
- Low-temperature stiffening effects in high-pressure CO2 environments, which can compromise sealing during thermal cycling.
Material selection is therefore critical. Fluoroelastomers (FKM), perfluoroelastomers (FFKM), HNBR, and specially formulated EPDM and PTFE-based compounds typically show better CO2 resistance than standard NBR or plasticized compounds, but each must be validated through qualification testing.
Testing Samples
Elastomeric O-Rings
O-rings are the most commonly tested seal geometry for CO2 service. Standard cross-sections are exposed to pressurized CO2 and subsequently examined for blistering, cracking, and dimensional change. Because O-rings are frequently used as static seals in valves, couplings, and pressure vessels in CCS and EOR systems, they are a primary focus of rapid gas decompression qualification programs.
Lip Seals, Rod Seals, and Piston Seals
Dynamic seals used in compressors, boosters, and injection equipment are evaluated for CO2 compatibility under combined pressure, temperature, and mechanical motion. Testing assesses wear, extrusion resistance, and the ability to maintain a lubricating film after plasticization by CO2.
Gaskets and Static Seals
Flat gaskets, valve seat inserts, and flange seals from elastomer-bonded or PTFE-based materials are tested for chemical compatibility, creep relaxation, and leak tightness after exposure to dense-phase CO2 and impurity streams such as H2S, NOx, or water-saturated CO2.
Material Plaque and Slab Samples
Standard cured plaques or slabs of the seal compound are tested alongside finished seals to determine fundamental material properties, including hardness, tensile strength, elongation at break, volume swell, and CO2 diffusion and permeation coefficients.
Testing Items
A comprehensive CO2 compatibility and qualification program for sealing elements typically includes the following test items:
- Hardness change (Shore A / IRHD) before and after CO2 exposure
- Tensile strength and elongation at break retention
- Volume swell and mass change after immersion in gaseous, liquid, or supercritical CO2
- Compression set under CO2 exposure conditions
- Rapid gas decompression (explosive decompression) resistance and crack rating
- CO2 permeability, solubility, and diffusion coefficient measurement
- Thermal aging behavior in CO2 atmosphere at elevated temperature
- Low-temperature flexibility, glass transition temperature, and TR-10 measurement
- Leak rate and functional pressure-cycling performance of assembled seals
- Resistance to CO2 with typical impurities (water, H2S, SOx, NOx, CH4, O2) representative of captured CO2 streams
Testing Methods
High-Pressure Autoclave Immersion and Aging
Seal samples are exposed to CO2 in high-pressure, high-temperature autoclaves per the general principles of ASTM D471 and ISO 1817, adapted for pressurized gas media. Exposure conditions are selected to simulate actual service, typically ranging from 5 MPa to over 50 MPa at temperatures from ambient to 200 °C, using gaseous, liquid, or supercritical CO2 as required by the application.
Rapid Gas Decompression Testing
RGD testing is performed according to recognized qualification frameworks such as NORSOK M-710 and ISO 23936-2, which are widely applied to non-metallic sealing materials in oil and gas and CO2 injection service. Samples are pressurized in CO2 to allow saturation, held for a defined soak period, and then decompressed at a controlled rate over multiple cycles. After each cycle, seals are sectioned and microscopically examined, and blistering, internal cracks, and splits are graded on a defined rating scale to determine pass or fail performance.
Physical and Mechanical Characterization
Hardness is measured per ASTM D2240 or ISO 48-4, tensile properties per ASTM D412 or ISO 37, and O-ring-specific properties per ASTM D1414 or ISO 3601. Compression set is determined per ASTM D395 or ISO 815-1. Volume change is measured by the buoyancy method (Archimedes principle) after CO2 exposure and outgassing. Low-temperature behavior is characterized by TR-10 recovery per ASTM D1329 and glass transition analysis by DSC or DMA.
Permeation and Diffusion Measurement
CO2 transport properties are quantified using manometric or volumetric permeation cells, providing permeability, diffusion, and solubility coefficients. These data support RGD risk modeling by estimating saturation time and the internal gas content that drives decompression damage.
Functional and Leak Performance Testing
Assembled seals are tested in representative hardware under pressure and thermal cycling to measure leak rate, pressure retention, and sealability after aging in CO2. Functional testing validates that material-level test results translate into reliable performance in real equipment.
Applications
CO2 seal compatibility testing and failure analysis support qualification and material selection across a broad range of industries, including:
- Carbon capture, utilization, and storage (CCUS) systems, compressors, and injection wells
- Enhanced oil recovery (EOR) using CO2 flooding
- Carbonated beverage dispensing, bottling, and food-grade CO2 handling equipment
- CO2 (R744) refrigeration systems, heat pumps, and transport refrigeration units
- Biogas upgrading and CO2 separation plants
- Geothermal and downhole tools exposed to CO2-rich formation fluids
- Fire suppression systems, gas cylinders, valves, and CO2 regulators
- Hydrogen and syngas process streams containing CO2 impurities
Test results from accredited third-party laboratories are used to support material qualification, supplier approval, failure investigations, fitness-for-service assessments, and regulatory documentation for these demanding applications.
FAQ
Why is CO2 more damaging to seals than other gases such as nitrogen?
CO2 has exceptionally high solubility in many elastomers, often several times higher than nitrogen or methane under the same conditions. During depressurization, the large quantity of dissolved gas expands violently, causing internal blistering and cracking, whereas a less soluble gas causes far less rapid gas decompression damage.
What is rapid gas decompression (RGD) failure and how can it be prevented?
RGD failure occurs when a seal saturated with high-pressure CO2 is depressurized quickly: the gas cannot diffuse out fast enough, expands within the elastomer matrix, and produces blisters, splits, and ruptures. It is prevented by selecting RGD-resistant compounds, controlling decompression rates, and qualifying materials through cyclic decompression testing per NORSOK M-710 or ISO 23936-2.
Does supercritical CO2 behave differently from gaseous CO2 toward seals?
Yes. Supercritical CO2 has liquid-like density and strong solvency, which increases swelling, plasticizer extraction, and chemical interaction with the polymer, in addition to the decompression risk common to all dense CO2 phases.
Which seal materials are generally recommended for CO2 service?
High-fluorine FKM, FFKM, HNBR, and dedicated CO2-resistant EPDM and PTFE compounds are commonly used, but no single material is universally suitable. Qualification testing under realistic pressure, temperature, and impurity conditions is essential before final material selection.
How long does CO2 seal qualification testing take?
Depending on the chosen standard and severity of the test conditions, RGD and aging programs typically require several weeks to a few months, since they involve multiple saturation and decompression cycles plus post-exposure mechanical characterization.