Permeation

Permeation, in physics and engineering, is the penetration of a permeate (such as a liquid, gas, or vapor) through a solid, and is related to a material's intrinsic permeability. Permeability is tested by permeation measurement, for example by a minipermeameter.

Description

The permeate always migrates to the lower concentration in three steps:

1. Sorption (at the interface): Gases, vapour or dissolved chemicals or suspended substances are adsorbed at the surface of the solid.
2. Diffusion (through the solid): The permeate penetrates the solid material through pores or molecular gaps.
3. Desorption: The adsorbate leaves the solid as a gas.

Related terms

• Permeate: The substance permeating through the solid.
• Permeability: The grade of transmissibility of a solid, meaning how much penetrates in a specific time, dependent on the type of permeate, pressure, temperature, thickness of the solid and the area size.
• Semipermeability: Property of a material to be permeable only for some substances and not for some others.
• Permeation measurement: Method for the quantification of the permeability of a material for a specific substance.

History

Abbé Jean-Antoine Nollet (physicist, 1700–1770)

Nollet tried to seal wine containers with a pig's bladder and stored them under water. After a while the bladder bulged outwards. He noticed the high pressure that discharged after he pierced the bladder. Curious, he did the experiment the other way round: he filled the container with water and stored it in wine. The result was a bulging inwards of the bladder. His notes about this experiment are the first scientific mention of permeation (later it would be called semipermeability).

Thomas Graham (chemist, 1805–1869)

Graham experimentally proved the dependency of gas diffusion on molecular weight, which is now known as Graham's law.

Richard Barrer (1910–1996)

Barrer developed the modern "Barrer" measurement technique, and first used scientific methods for measuring permeation rates.

Permeation in everyday life

• Packaging: The permeability of the package (materials, seals, closures, etc) needs to be matched with the sensitivity of the package contents and the specified shelf life. Some packages must have nearly hermetic seals while other can (and sometimes must) be selectively permeable. Knowledge about the exact permeation rates is therefore essential.
• Tires: Air pressure in tires should decrease as slowly as possible. Therefore it is good to know which gas permeates least through the rubber wall.
• Insulating material: Water vapour permeation of insulating material is important as well as for submarine cables to protect the conductor from corrosion.
• fuel systems: To meet legal regulations, e.g. CARB (California Air Resource Board) for Low Emission Vehicles, it is essential to use barrier materials for fuel hoses and tanks.

Permeation measurement

Permeation measurement with sweep gas

Testing cell for pipes

The permeation of films and membranes can be measured with any gas or liquid. The method uses a central module which is separated by the test film: the testing gas is fed on the one side of the cell and the permeated gas is carried to the detector by a sweep gas. The diagram on the right shows a testing cell for films, normally made from metals like stainless steel. The photo shows a testing cell for pipes made from glass, similar to a Liebig condenser. The testing medium (liquid or gas) is situated in the inner white pipe and the permeate is collected in the space between the pipe and the glass wall. It is transported by a sweep gas (connected to the upper and lower joint) to an analysing device.

Simple approximation

The mass flow Q through a barrier layer can within certain pressure limits be linearized and approximated by:

with Q the mass flow, P the specific material permeability, A the surface area, Δp the pressure difference, and d the material thickness. In Europe the permeability unit is mostly cm³.mm/(m².Bar.day), so that the gas flow in cm³/day results when the area is given in m², the thickness in mm, and the pressure difference in Bar.

In American units, the permeability is often given in cm³.mil/(100 inch².Bar.day) which can be converted to the European unit by dividing by 2.54.

See also

• Moisture vapor transmission rate
• Oxygen transmission rate
• Carbon dioxide transmission rate
• Hermetic seal

Further reading

• Brody, A. L., and Marsh, K, S., "Encyclopedia of Packaging Technology", John Wiley & Sons, 1997, ISBN 0-471-06397-5
• Massey,L K, "Permeability Properties of Plastics and Elastomers", 2003, Andrew Publishing, ISBN 978-1-884207-97-6
• ASTM F1249 Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor
• ASTM E398 Standard Test Method for Water Vapor Transmission Rate of Sheet Materials Using Dynamic Relative Humidity Measurement
• ASTM F2298 Standard Test Methods for Water Vapor Diffusion Resistance and Air Flow Resistance of Clothing Materials Using the Dynamic Moisture Permeation Cell
• F2622 Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using Various Sensors