I came across an addendum to Ellern's "Military and Civilian Pyrotechnics", describing water vapor transmission through several plastic films. FEP (fluorocarbon, not cheap) is by far the best, with polyethylene film being about ten times worse---but it's still better than the rest, with polypropylene being a fairly close third. The two columns are from two different sources. Note that they're talking about films. Thicker-walled plastic containers will be much less permeable, though some moisture will migrate through eventually.
[Context: this appeared in the section on Matches, which is a fascinating discussion on the history, chemistry, and production. The point made in the addendum is that matches---other than windproof or camping-style matches---that have been stored in plastic bags, even heat-sealed, may not strike if even a little moisture has migrated through the plastic.]
FWIW I've stored chemicals in silvery anti-static bags for quite some time without significant lumping. The bags I used appear to be made from PET/PETE which is, I think, less permeable than most of the others.
Bottom line: if you really want to keep moisture out: plastic bag (for convenient dispensing) inside a metal can such as a paint can. A mayo jar with many holes punched, and filled with silica gel ("crystal cat litter") goes in the bag or in the bucket.
[Context: this appeared in the section on Matches, which is a fascinating discussion on the history, chemistry, and production. The point made in the addendum is that matches---other than windproof or camping-style matches---that have been stored in plastic bags, even heat-sealed, may not strike if even a little moisture has migrated through the plastic.]
FWIW I've stored chemicals in silvery anti-static bags for quite some time without significant lumping. The bags I used appear to be made from PET/PETE which is, I think, less permeable than most of the others.
Bottom line: if you really want to keep moisture out: plastic bag (for convenient dispensing) inside a metal can such as a paint can. A mayo jar with many holes punched, and filled with silica gel ("crystal cat litter") goes in the bag or in the bucket.
