TWI Frequently asked questions
by Sam Rostami
Although water uptake is used as an indicator of the moisture sensitivity of polymers, permeability is a more reliable variable to use. To determine a polymer's permeability, its diffusion coefficient and its solubility coefficient are required. Both these parameters are temperature and pressure dependent.
Diffusion coefficient
Water uptake is usually measured gravimetrically as a function of immersion time and/or water temperature. When the amount of water absorbed has reached a maximum, a suitable mathematical model can be fitted to data of weight gained as a function of time. From this, the diffusion coefficient can be extracted, which corresponds to the velocity of water molecules transported into the polymer.
Solubility coefficient
The solubility coefficient corresponds to the amount of water dissolved in a polymer.
Permeability is the product of the diffusion coefficient and the solubility coefficient. Typical water permeability values for a number of base polymers are shown in Fig.1.
Fig.1. Typical water vapour permeability [g/(day.m 2)] through 25micron thick films at 38°C/90% RH.
Further information
Is it possible to predict the service life of a plastic component? For more information please contact
TWI's Polymers Group
