User:Baltimore1993

An Introduction to Desiccants
A desiccant is a drying agent. It adsorbs and holds particles of water to itself. It depends. The answer to the size of desiccant needed depends on the size of the air space to be desiccated, the nature of the material in the package, the moisture barrier surrounding the package, the type of desiccant being used, the desired shelf life, the atmosphere conditions where the package is sealed, the type of seal on the package, etc. When we receive a call from someone looking for a desiccant, we will ask a number of questions to determine this information so we can calculate which size desiccant will work best in a particular application.

There are two major forms of desiccant : Silica Gel and Molecular Sieve.

Silica Gel
Silica gel is silicon dioxide (SiO2). It is a naturally occurring mineral that is purified and processed into either granular or beaded form. As a desiccant, it has an average pore size of 24 angstroms and has a strong affinity for moisture molecules. The silica gel will pull in moisture at temperatures up to 220°F (105°C). As temperature goes above 100°F, the rate of moisture pick-up will slow down but the silica gel will still work. Silica gel performs best at room temperatures (70° to 90°F) and high humidity (60 to 90% Rh) and will drop the relative humidity in a container down to around 40% Rh. In the United States, silica gel is commonly used in food and pharmaceutical applications as only silica gel has been approved by the FDA for direct contact with these items. 

Indicating Silica Gel
Indicating silica gel is a silica gel bead or granule that has been washed with a concentration of cobalt chloride ( a heavy metal salt). The cobalt chloride is a deep blue colour when it is dry and turns from blue to purple to pink as it becomes saturated with moisture. Typically, the colour changes as the desiccant goes past 8% moisture levels (by weight) and indicates it is time to replace the desiccant. The most typical use for an indicating silica gel is for a moisture sensitive product that will be inspected regularly as it gives a quick visual indication of how well it is doing. Because of the addition of cobalt chloride, indicating silica gel should not be used in contact with products for consumption such as food or pharmaceuticals.

Molecular Sieve
Molecular sieve is a synthetic desiccant that has a very strong affinity for moisture molecules. As part of the manufacturing process, the pore size on the molecular sieve particles can be controlled. The most commonly used pore size is 4 angstroms (4A) although 3 angstroms (3A), 5 angstroms (5A) and 10 angstroms (13X) are available. Because the pore size is controllable, molecular sieve can be used for picking up specific gases as well as moisture. Molecular sieve can hold moisture to temperatures well past 450°F (230°C). Because of its high affinity for moisture, molecular sieve is able to bring the relative humidity in packages down as low as 10% RH.