Permeation is the process of molecular penetration of gases, vapours or fluids through the material membrane of a solid.
Permeation rates are inversely proportional to the subsurface concentration. These are, in turn, affected by the thickness of the membrane, the intrinsic permeability and mass diffusivity of the material. The permeability of a given substance increases with temperature but is unaffected by pressure.
How Permeation Works
Permeation occurs when molecules of the permeant are diffused through a membrane or interface. As the permeant is diffused through the interface it moves from a high to a low concentration. The permeant molecules can either be absorbed or desorbed at the interface according to the process of sorption.
A thin membrane has a constant permeation flux which is only governed by the charging crossing the entry face. However, with thicker membranes the diffusion flux is inversely proportional to the concentration of the subsurface.
The process can occur through most materials, including ceramics, polymers, and metals. However, metals have a lower permeability than ceramics or polymers due to their porosity and crystal structure. However, conversely, polymers have a high rate of permeability. Materials can also be semipermeable, whereby only those molecules or ions with particular properties are able to diffuse across the membrane.
Permeation in metals causes embrittlement and blistering, while permeation through coatings occurs as a result of the diffusion and solubility of water or another fluid which can, in turn, cause corrosion of the substrate.
In addition to the characteristics of the permeant component and the material being permeated, the level of permeability is affected by the temperature of the interaction between the two.
Measuring Permeation
There are a variety of different ways in which the permeation of a material can be measured. These methods quantify the permeability of a substance through a specific material.
Sensors are used to measure permeation in metals. These sensors establish the permeation current density in conjunction with weight loss or gain in the substrate through a wet-dry cycle experiment. The permeability coefficient is directly proportional to the diffusing substance’s concentration and inversely proportional to the solubility coefficient.
Any gas or liquid can be used to measure the permeation of films and membranes, with one method using a central module which is separated by a test film. With this method, a testing gas is fed up one side of the cell and a sweep gas carries the permeated gas to the detector.
Another method, intermittent contact, takes a sample of the test chemical and placing it on the surface of the material whose permeability is being measured. After adding or removing specific amounts of the test chemical, the material is analysed to assess the concentration of the test chemical through the structure. When assessed alongside the amount of time the test chemical was in contact with the material it is possible to determine the cumulative permeation of the test chemical.
Permeation is modelled by equations such as Fick's laws of diffusion, and can be measured using a minipermeameter.
Permeability as a result of diffusion is measured in SI units of mol/(m･s･Pa), although Barrers are also frequently used.
Applications for Permeation
Permeation has a variety of uses in both industry and everyday life. It is used in gas separation as well as for determining the effects of hydrogen in high strength steel for the petroleum and petrochemical industries. Hydrogen permeation is associated with corrosion, electrochemical processes, surface reaction and cathodic protection. Cracks can be caused by the accumulation of hydrogen in the bulk of an alloy as well as areas of high stress and trapping. These cracks compromise the integrity of a structure.
Thermoplastic and thermoset pipes for transporting water under high pressure can also be subject to permeation, whereby they can be considered to have failed if there is detectable permeation of water through the pipe wall.
Permeation is also important for insulating material, such as those used in submarine cables, since preventing water vapour permeation is vital to protect the conductor from corrosion.
Permeation resistance is also important for protective clothing, for example when it comes to understanding the chemical protective properties and breakthrough times for clothing to be worn in industrial situations, such as in the nuclear industry.
Other areas where permeation is to be avoided includes tyres, where the air pressure needs to be maintained for as long as possible. While permeation will occur naturally over time, the better the material the lower the rate of gas escaping from the tyre is.
In other cases, selective permeation is important, such as with packaging. Where some packages will require hermetic seals others may require selective permeation, so understanding permeation and the requirements of your packaging is important.
The medical industry is another area where permeation is important, for example, in the field of drug delivery. Polymer-based drug patches contain a chemical reservoir, allowing the chemical to be transferred to the body by contact and permeation through the polymer membrane according to the concentration gradient. Since the chemical reservoir is over loaded the drug follows the burst and lag mechanism, whereby there is a high transfer rate when the patch makes contact with the skin, but settles to a constant rate as the concentration gradient is established. This is vital for drug delivery as used in the Ocusert System. However, in other instances, permeation must be kept at a minimum in cases such as pharmaceuticals for injection, where the material prevents the other substances entering the pharmaceutical product or from stopping the product from evaporating. In instances such as these the chemical-containing ampoules are often made from glass or synthetic materials with very low permeability.