Sorption, Diffusion and Permeation

Small amounts of water or organic substances can act as softening agents in plastics and cause long-lasting changes in the structure of the material. To record moisture absorption and sorption of organic substances in polymers, sorption and permeation measurements are available. Sorption isotherms, permeation rates and diffusion coefficients can be determined at different levels of humidity, temperature and also in vapors of organic solvents.

The knowledge of sorption properties (e.g. swelling) and permeation as a material property assists the development of different products such as:

Plastic parts for vehicle construction (e.g. the effects of fuels and oils)
Composite materials for lightweight construction
Varnishes, coatings and adhesives
Plastics for construction (for example foils, geotextiles)
Materials for solar cells or fuel cells
Packaging for foods or pharmaceuticals

Among others, tests and validatons according to following standards are possible

DIN EN ISO 7783
ASTM E96

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Sorption Scales

The sorption balances of the model VTI-SA+ from TA Instruments allow the determination of diffusion and sorption coefficients of water and vapors of organic liquids (for example fuels, organochlorides/chlorinated hydrocarbons and solvents).

The calculation of diffusion coefficients usually takes place on the basis of equations that describe transportation processes of solvents in the polymer. A standardized determination according to ISO 62 or ASTM D570 is also possible.

The operating temperature range lies between 5°C and 140°C. The relative humidity (RH) can be adjusted between 0 %rH and 98%rH at a temperature up to 55°C.

Permeation Measurement Cells

For the determination of permeation rates, automated sorption balances and permeation cells are available. The determination of the change in mass takes place by gravimetric measurements.

Many vapors and liquids can serve as permeates:

Water
Water vapor
Organic solvents (Ethanol, Benzene etc.)
Vapors of organic solvents
Oils, fats, wetting agents

Application Example

Permeation Experiment

The term permeation refers to the transport of a substance through a solid. Permeation measurements provide permeation rates. These permeation rates indicate how much of a substance wanders through the solid over time. In materials free of defects, permeation takes place over molecular mechanisms and is characterized by the sorption and diffusion coefficients. Since small amounts of water or organic substances can change the material structure, permeation properties determine the range of application and service life time of plastics under the influence of media. For product development, choice of material and a reliable service lifetime forecast of plastics under the influence of media, conclusive material data, and understanding of permeation mechanisms are required.

With special permeation cells, which can be adjusted to the particular sample requirements, the transportation of substances through a thin polymer samples is recorded gravimetrically. This is also feasible for plastics that generally have a small diffusion coefficient, such as epoxy resins (see upper figure). Besides conducting the permeation examinations we also support our customers in sample production and preparation.

Sorption Experiment

Sorbtion experiments allow the determination of the absorption capacity of a material, for example, toward solvents or water vapor (see upper figure). From the time dependency of the mass change, the diffusion coefficient can be determined.

Sorption isotherms describe the solubility in dependence of partial pressure and humidity (lower figure). From the form of the isotherms, molecular ad- and absorption mechanisms can be deducted.

These material data can be determined for different temperatures and humidities, so that the specific operation conditions of the polymers can be taken into account.

Publications:

Ingo Alig, Harald Oehler, Maike Bargmann und Daniel Kunkel, Sorptionsmessungen und dynamisch-mechanische Analyse zur Vorhersage der Wirkung von Feuchte und organischen Substanzen auf die Kunststoffalterung, in Würzburger Tage 2014 (proceedings, ISBN 878-3-940184-11-5), Wolfgang Kunze (Hrsg.), page 49-160.