I find this interesting, in most cases, it is the ether, alcohols and solvents evaporating that decreases the energy of the powder over time.
I have seen IMR take a quantity of the first batch ever made of IMR4064 from a container filled with powder that is covered with WATER.
They dry it out and test it against newer batches to see if the recipe stays the same.
If powders were hygroscopic, why would IMR store the powder in water?
I have never seen any real evidence that powders are hygroscopic, only anecdotal tales of such. None of the MSDS mention moisture other than the solvents used.
Yes, there is a moisture content %, but no where does it say this increases with exposure to air. It is the solvents degrading that causes powder to go bad, not moisture.
Anyway...battle on.
Cheers.