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Performance
In terms of their capabilities and performance as desiccants, these two materials have numerous
similarities. Both are highly adsorbent materials. This means that even at full water vapor capacity,
both remain dry and free-flowing, without any apparent change in size, shape or texture. And, both of
these desiccants can be “reactivated” by oven-drying them to remove retained water vapor between
uses.
They are also quite similar in terms of “unit” weight and relative adsorption performance. Desiccants
are measured in “units” according to military and industry specifications including MIL-D-3464 (U.S),
DIN 55473 (Germany) and AFNOR NF H00-321 (France). The MIL and DIN specifications define a
unit as the amount of desiccant that will, at a temperature of 25° C, adsorb 3 g of water vapor at 20%
relative humidity and 6 g of water vapor at 40% relative humidity. By this standard, a unit of bentonite
clay desiccant weighs about 33 grams, while a unit of silica gel weighs about 26 grams. Both will adsorb
water vapor equivalent to between 30 and 40 percent of their unit weight. (Note: the AFNOR NF
H00-321 specification defines a desiccant unit with 100 g moisture adsorption.)
Although the adsorption properties of bentonite clay and silica gel desiccants are comparable on a unit-
for-unit basis, tests show that bentonite clay offers higher adsorption capacity in the most common and
relevant package conditions. The adsorption capacity of bentonite is substantial at low humidity levels
and increases with increasing relative humidity. Silica gel behaves similarly, but becomes even more
adsorbent at higher levels of relative humidity. At these higher humidity levels, its very high rate of
adsorption can, at times, make silica gel more difficult to handle.
Figure 1: Comparing desiccant performance
Figure 1 compares the adsorption capacity of bentonite and silica gel at various relative humidity levels. In package
conditions below 30 percent relative humidity, bentonite adsorbs more humidity than the equivalent amount of silica gel.
Package conditions below 40% relative humidity are by far the most commonly required.
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