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Old June 26, 2013, 12:03 PM   #18
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 21,024
BigJimP

Desiccants work by adsorption, a molecular adhesion to the desiccant surface, not absorption, where the water is merely filling the volume between strands or particles, as in capillary action of a towel. This means water does not as easily come out of a desiccant the way it does from a towel by simple drying at room temperature. If you let it get saturated at it's maximum capacity temperature, then some portion will come out if the temperature changes, but not nearly as much as it took up. The solution to that is simple: just use an excess of the desiccant so it never reaches its full capacity, then you won't have moisture released as the temperature outside the bag changes.

Because of the molecular surface adhesion, a desiccant will not fully release its moisture until a minimum required amount of heat energy is put in. This normally requires baking in an oven. There is often more than one molecular adhesion level involved, so you may find you can drive one percentage of water off at one temperature, but have to get to a higher temperature to get then next portion released. For the smectite clays (Montmorillonite, Bentonite) used in mil-spec desiccant, it will not normally start releasing adsorbed moisture until you heat it to about 120°F, and you have to get it really hot (1600°F) to completely get moisture out. Silica gel won't start releasing much adsorbed moisture until you heat it to about 220°F, so it's better from that standpoint. You normally are instructed to heat desiccant pouches to 245°F for 12-24 hours to drive enough moisture out to reuse them. That low temperature and long time are because of not wanting to melt the Tyvek bags the stuff usually is packaged in. But if you put some of the clay out loose on a baking sheet you can heat it at 550°F in a home oven for an hour and to do that. Bag it in paper when it's cool enough not to burn the paper, and there you have a homemade desiccant pouch.

The bottom line is, if you put a desiccant in the plastic pouch with your ammo, it will take up water by adsorption up to its capacity at the temperature and relative humidity, and will hold that, so you don't need to worry about trapping moisture inside the plastic. It will gradually pick up moisture from cardboard or anything else inside. The clay will swell visibly if it gets to capacity, and gels with color indicator of that state can be had. However, unless you have the special mil-spec treated cardboard or have acid-free cardboard stock, the advice to avoid cardboard is good.

Note that no plastic has a zero water vapor transmission rate (WVTR) specification. At some time in the future enough water eventually will saturate the desiccant. With luck that will take years. If you also put it in an ammo can, it should easily last a long time like that, as WVTR through the rubber seal will be much lower than a high surface area plastic bag.
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