From: BretCahill@aol.com   
      
   > > Actually I'm looking for a thin flexible elastic film that has low   
   permeability to gas.   
   > >   
   > > Mylar is very impermeable but polyethylene terephthalate or BoPET has a   
   yield strain of only 5%.   
   > >   
   > > In contrast butyl rubber is an order of magnitude stretchier than Mylar   
   but gas diffuses through butyl rubber an order of magnitude faster than Mylar,   
   2 orders if the Mylar is aluminized.   
   > >   
   > > Put other polymers and elastomers on the permeability v elasticity curve   
   and the general trend is stretchy = permeability, however, there are a few   
   outliers.   
   > >   
   > > One of interest is the old Saran cling wrap, polyvinylidene chloride:    
   With an ultimate yield strain of 20% it is 4X stretchier than Mylar yet has a   
   permeability an order of magnitude lower than Mylar.   
   > >   
   > >   
   > What about the question, how the permeability   
   > changes with stretching ( not elasticity ) ?   
   >   
   > As I suppose the permeability is measured at rest state,   
   > but the stretching may significantly impact permeability.   
      
   It certainly does.   
      
      
   Bret Cahill   
      
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