Permeation

Definition and Distinction from Leakage

Permeation is the slow mass transport of molecules through the material itself — for example through an elastomer or a plastic. In sealing technology, this means: even when a seal is correctly designed and installed, a medium can pass “through the material” to the other side over time. Permeation therefore occurs in particular where a seal is mechanically tight (no gap, no installation error) but is not completely substance-tight in the sense of “zero substance passage”.

Leakage, by contrast, describes a flow through an existing passage — usually a gap or a defect. Such paths arise, for example, through insufficient surface pressure, surface roughness, deformation, damage, or installation errors. In practice, “tight” is often understood as “no visible loss”. With gases, this is problematic, because permeation in many applications practically never fully disappears. This becomes particularly noticeable in vacuum systems: an O-ring can mechanically seal a volume, yet air diffuses through the elastomer over time and creates a measurable gas load in the vacuum.

Brief Comparison: Leakage vs. Permeation

Mechanism and Parameters of Permeation

Permeation in a polymer typically proceeds in three steps. First, molecules are taken up at the high-pressure side (adsorption/absorption: attachment at the surface and penetration into the material). After this, they migrate inside the material by diffusion — that is, by random molecular motion along a concentration or partial pressure gradient. Finally, the molecules are released again on the low-pressure side (desorption).

What is usually decisive is the partial pressure difference of a substance. In gas mixtures, therefore, not only the total pressure is relevant but the fraction of the gas under consideration. In seals, permeation is also strongly influenced by geometry: a larger effective area increases the passage, while a larger wall thickness extends the diffusion path and reduces it.

Permeation Rate vs. Permeability (Material Parameter)

In practice, two quantities are often confused. The permeation rate describes how much substance passes per unit time through a specific component under specific conditions. It therefore depends on area, thickness, temperature, and pressure conditions. Permeability (also called the permeation coefficient), by contrast, is a material parameter for a specific medium under defined test conditions. As a result, it allows material comparisons, but it remains meaningful only when boundary conditions such as temperature, gas type, and pressure are cleanly stated.

Influencing Factors: Medium, Material, Temperature, Pressure Differential, Wall Thickness

Which substances permeate depends strongly on molecular size and on the interaction with the polymer. Small molecules such as hydrogen or helium pass through many elastomers comparatively quickly. CO₂ can also permeate noticeably, depending on the material, because it dissolves in the polymer to some extent. This solubility is a separate effect: a gas can diffuse slowly but dissolve in large amounts, or vice versa.

The material determines permeation through its polymer structure, cross-linking degree, and fillers. Higher cross-linking can reduce the mobility of the polymer chains and thereby hinder diffusion. Fillers and barrier components can extend the path for molecules or change solubility. As a result, elastomers and plastics often differ noticeably under identical conditions.

Temperature in many cases raises chain mobility in the polymer. As a result, diffusion speed — and therefore permeation — frequently increases noticeably. The driving force also acts directly: the larger the partial pressure difference, the larger the substance flow typically is. Wall thickness acts as a design lever, because the diffusion path becomes longer. In sealing technology, however, this is often limited by installation space, friction, and deformability.

Consequences in Seals and Testing/Standards

In pneumatic, vacuum, or gas-loaded systems, permeation frequently shows up as a slow pressure loss or as a pressure rise on the low-pressure side. This is not “leakage” in the sense of an assembly mishap, but a material-related transport. In separated volumes, this can lead to gas migration — for example when a medium passes from a high-pressure chamber into a reference chamber. In addition, gas can be dissolved in elastomers and thereby change the material state.

A practice-relevant special case is damage from rapid pressure changes. When an elastomer is under high-pressure gas, gas can permeate into the material and accumulate there. During rapid depressurization, the dissolved gas expands, generates internal stresses, and can cause cracks or blisters. This damage pattern is known under the terms Rapid Gas Decompression (RGD) or Explosive Decompression (ED).

Rapid Gas Decompression (RGD/ED) as a Practice-Relevant Damage Pattern

RGD/ED occurs particularly at high gas pressures and rapid depressurization. The critical step is not the gas uptake itself, but the too-fast depressurization, during which the gas in the material has no time to escape in a controlled way. As a result, local overpressures arise within the material that can lead to microcracks or even visible spalling. For seal design, this means: medium, pressure cycle profile, and suitable material approvals must be considered together.

Testing and Parameters (GTR, Permeability Coefficient), Including Standards

For documentation, the Gas Transmission Rate (GTR, gas passage per unit time and area) and the gas permeability coefficient are frequently stated. Both values are only meaningfully comparable when the medium, temperature, pressure conditions, and specimen thickness are defined.

Relevant standards in this context are:

• ISO 2782: determination of gas transmission rate and gas permeability coefficient for rubber/elastomers.

• ISO 15105: determination of the gas permeability of plastics (depending on the procedure and test arrangement).

For critical applications, it pays off to assess permeation data and pressure cycle loading together; specialized material and sealing consultation can be sensible in this context.