plastic film permeability

[tony1234]

Does anyone know if there is a method to calculate permeability of plastic film (ex. beach ball material)?
Also, are there any test rigs available for testing samples of different materials?

 

[btrueblood]

I've not heard of a method to calculate permeability from "first principles". The problem involves too many variables including how the polymer was processed and how large the resulting pores in the material are.

http://www.matweb.com

has a lot of property data for polymers, including water vapor permeability and oxygen gas permeability. Data for oxygen will probably be pretty close to a value you would find for "air".

A fairly standard test rig for water vapor permeability is to stretch a piece of film over a dry cup, then chill the cup below the dewpoint of a humid atmosphere. Wait 24 hours and then measure the weight gain of the cup and film after drying all external moisture. Gas permeability is similar, but measures pressure rise in an evacuated cup, with a known constant pressure of the gas of interest on the other side. Data are typically normalized to (grams of moisture or moles of whatever gained)/(pressure difference across film)/(area of film exposed to gas or moisture)/(film thickness)/(days). There may be an ASTM standard for this, but I've not come across it.

If you ever come across data regarding helium gas permeability of polymers, please post back here and let me know!

 

[btrueblood]

Whoops. Soon as I said that, I went and looked on global.ihs.com; input search term "permeability" -

ASTM D 1434
Revision: 82 Chg: W/ REAP Date: 00/00/03
STANDARD TEST METHOD FOR DETERMINING GAS PERMEABILITY CHARACTERISTICS OF PLASTIC FILM AND SHEETING

Price is $42 US, or $38 to download in pdf

There are some applicable British Standards (BS) as well.

 

[tony1234]

thanks, can't believe I didn't find this one! I only found perm. of fabrics testing. It didn't come up on ASTM's site when I searched permeability.
thanks for the matweb.com tip also. I'll try it out.
Is there a test std for the cup method so I can research it?
We had a helium test performed on some pressurized components, but it was for leak testing purposes. They used a 'sniffer' to locate helium loss. It was on a 10^-6 scale for plastic film ~.015" thick.
thanks again.