Re: Diffusion through PVC foil (0.2 mm)  

Posted by Composite Agency on January 17, 2004 at 18:13:50:

In Reply to: Diffusion through PVC foil (0.2 mm) posted by Kasper M. Paasch on January 15, 2004 at 13:03:51:

Dear Kasper Paasch,

Based on experience the following diffusion parameters can be estimated for normal pure PVC (without plasticizers) and water in
atmospheric conditions and at 25 degrees Celsius:

Water density: 10^6 gram/m3
Water solubility in PVC: 0,006 volume fraction liquid (=0,6%) -> very low volume fraction: case 1 diffusion (Fick's law)
Water diffusion coefficient: 2 E-12 m2/s (amorphous polymer)

Permeability is: [10^6 gram/m3] x [0,006] x [2 E-12 m2/s ] =1  E-8 gram / ms in the stationary state.

Based on a wall of thickness of 0.2 mm, stationary state is reached after 4 hours.
The amount of water that diffuses through in the stationary state is [Permeability
Figure] divided by [wall thickness], resulting in 0,2 gram /m2 hour = 2 kg / m2 year.

Based on the information above the following can be stated:

The figure you got of 16 kg / m2 year would indeed by high for 'pure' PVC, like it
is used for building pipelines, however the PVC you got may be plasticized with a
plasticizer that has relative strong affinity for water that may cause swelling, giving
to a larger permeability. However when you compare your figure with the figure
for a polyester resin and water (as it is used for building boats) - 5 kg/ m2 year - the
performance of your Polyvinylchloride bag is not so bad - of course depending on your application.

Mind that the air humidity plays a role in the total amount that diffuses through. If
You got water on one side and air with relative humidity of e.g. 70% on the other side,
The total through put from the liquid side - in stationary state, and taking some  shortcuts -
would be 16 kg / m2 year x (1-70%) = 5 kg /m2 year.

Best regards,
Diffusion in Polymers

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