|Re: relationship of diffusion/permeation coefficient versus relative humidity|
Posted by Diffusion-polymers.com on August 03, 2005 at 09:15:56:
In Reply to: relationship of diffusion/permeation coefficient versus relative humidity posted by Martin Tenpierik on August 02, 2005 at 17:11:14:
Dear Martin Tenpierik,
For cast PET, HDPE/LLDPE, the solubility of water is a linear function of water activity. For a water activity/humidity of ~100%, 1 bar, 25 degrees Celsius, the solubility of water in PET is around 0.8 volume percent and in HDPE/LLDPE between 0.1 and 0.3 volume percent. If, for example, the water activity in the described conditions is 50%, the water volume percentage in PET is ~0.4.
Dependent on the type of metal that is used in PET, the solubility may be higher and show another relationship with the pure water activity/humidity.
The diffusion coefficient for water in cast PET, HDPE,/LLDPE can be considered as a constant function with regard to activity. In the described conditions the diffusion coefficient of pure water in PET is 4E-13 m2/s and for HDPE/LLDPE ~2E-11 m2/s.
Conclusion: the change of the permeation figure as a function of pure water activity/humidity for the described cast polymers is driven by the linear change of the solubility. Do not hesitate to contact us for more information regarding diffusion, chemical resistance or mechanical properties of metallized composite materials,
: At Delft University of Technology, we are doing research on vacuum insulation panels. The panels we use consist of an open- celled core material (fumed silica) which is after evacuation tightly sealed into a high barrier film. This film is a bi- or trilaminate metallized polymer film of 97 mu (3 metallized PET layers and 1 HDPE or LLDPE layer). For estimating service lives of these panels, permeation models have been developed. Since however the permeance (or permeation coefficient for single-layered polymers) of these films depends on temperature and relative humidity, these relationships must be known. The temperature effect can be modelled according to an Arrhenius equation. Does anyone know a relationship between permeance or permeation coefficient (or diffusion coefficient) and relative humidity for thin metallized films.
: Kind regards,
: Martin Tenpierik