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| Re: Diffusion through PVC foil (0.2 mm) |
Posted by Composite Agency on
In Reply to: Diffusion
through PVC foil (0.2 mm) posted by Kasper M. Paasch on January 15,
2004 at
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
- Re: Diffusion through PVC foil
(0.2 mm) Dave
