I have been working to improve the gas retention of the idefx prototype. Previously an aluminized gas barrier was used. However it was found that the aluminized barrier tended to develop cracks over time as the envelope was used and stored. So I tried something more flexible that would not crack. This envelope is made of a composite of UHMPE fibers for tensile strength and ripstop ability, a synthetic elastomer for puncture resistance and interlayer adhesion, and a layer of PVOH for a gas barrier. While PVOH is not as strong a gas barrier as Aluminum, it is a flexible polymer that does not crack.
To test the durability of the envelope, it was turned inside out several times. This adds many folds , wrinkles, and creases to the envelope. When this same procedure was performed on the aluminized envelope, the leakage rate increase dramatically. The polymer based envelope performed better.
The "roughed up" envelope was tested for gas retention both with and without the layer of PVOH. The leakage rate is then expressed in terms of per surface area of the envelope, since leakage generally scales with surface area. At least when comparing envelope materials.
Without PVOH:
0.009 m3 of H2 was lost over 10 hours, correlating to a leakage rate of 0.00023 m3 of H2 per m2 of envelope per hour
With PVOH:
0.003 m3 of H2 was lost over 10 hours, correlating to a leakage rate of 0.000076 m3 of H2 per m2 of envelope per hour
So we can see the PVOH gas barrier does seem to be working. Overall the material is much better than the aluminized envelope after it has been abused a bit. However it is not magic. It is still a very thin material. It is 24 microns thick and 30 g/m2, a small fraction of the thickness you would see in typical outdoor blimps. So inevititably it does leak after it has been abused. It will probably be necessary to treat future envelopes with some care to ensure leakage does not become a big problem.
