Types of Dessicants

Various substances are used as desiccants, each with different properties and limitations. All desiccant types accomplish their jobs by a process known as adsorption, in which moisture forms a film on the substance’s surface, but does not permeate it.

Montmorillonite Clay is a naturally-occurring form of magnesium aluminum silicate that is capable of adsorbing large amounts of moisture, and can be used repeatedly without deterioration. It is usually the cheapest desiccant type available.

Clay only performs well at temperatures below 50 C. At higher temperatures, it tends to give up moisture instead of adsorbing it. In applications where the desiccant must be used or stored at high temperatures, it is necessary to use some other desiccant.

Silica gel is the only desiccant type that is approved by the FDA for use with products to be consumed internally, so it is generally used in packaging for food and pharmaceutical products. It will perform well up to about 105 C. As temperature rises, the adsorption rate slows but remains high enough to be effective. The ideal environment for silica gel performance is about 70 to 90 degrees F and about 60% to 90% humidity.

Both silica gel and clay have variable pore sizes, and can be used to adsorb substances other than water.

Molecular sieves are more precise desiccant types. These are crystalline substances that can be manufactured with pores of a specific size, allowing them to adsorb certain substances while allowing others to pass. They perform well at temperatures above 230 C, and have a very high affinity for moisture. They have not yet been approved for use with consumable items in the US, but are being used with pharmaceutical products in Europe.

Calcium oxide, also called quicklime, is a form of lime that can adsorb very large amounts of water. It is used in situations where there is a large amount of moisture present, and low humidity is desired. Its disadvantage is that it works slowly, sometimes taking days to remove all of the moisture. It also swells as it adsorbs, making it undesirable for some applications.

Calcium sulfate is a product made from gypsum that is mainly used in laboratories. It is inexpensive and does not lose its moisture at high temperatures. It has a relatively low adsorption rate, adsorbing only about 10% of its weight in water. It is available, but is not normally sold in package form.

Read more: Desiccants and the environment