SorbentSystems, the online presence of IMPAK Corporation of Los Angeles, California is a leader in providing complete packaging systems for products that are sensitive to specific aspects of their environment – whether it be humidity, ultraviolet light, oxygen, or trace gases.
How are "Adsorption" and "Absorption" Different?
You may have run across some terms used in the description of the Moisture Control System's operation that are unclear to you. You may for instance wonder at the term "adsorption" and think it a typo where we meant to write "absorption". In actuality these terms represent two very different scientific concepts. For the answers to this and other questions on desiccant operation, read on. Absorption is when a substance is chemically integrated into another. When you drink a glass of water, you are absorbing it, as the water becomes part of you. Adsorption is when one substance is being held inside another by physical bonds. If you spill a glass of water on your shirt, it is adsorbed as the fibers will hold the water until heat dries out the shirt. Most desiccants don't chemically combine with water or the other substances they're present to protect against. They merely capture them through adsorption, and stow them away where they can't cause harm to the product. Remember: the next time you wipe up a spill with a paper or cloth towel, you're adsorbing, not absorbing. Now there's some trivia to stump your friends with!
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 pickup 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. As with clay, silica gel, with its wide range of pore sizes, has the capability of adsorbing compounds other than water. The relative order of adsorbability is: water, ammonia, alcohols, aromatics, diolefins, olefins and paraffins. When the potential for multicomponent adsorption is present, expect the more strongly adsorbed compounds, such as water, to displace the more weakly held ones.
Molecular Sieve
Molecular sieves are porous crystalline aluminosilicates, a synthetic desiccant that has a very strong affinity for moisture molecules. The distinctive feature of the molecular sieve structure, as compared to the other desiccants, is the uniformity of the pore size openings in the crystal lattice structure. There is no pore size distribution with molecular sieves. 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. This feature allows the selection of a molecular sieve product which can adsorb water, yet exclude most other molecules, such as volatile organics, which might be present in the package. For example, Type 3A molecular sieve's structure, with a 3 angstrom pore opening, allows moisture adsorption, butexcludes most hydrocarbons. Type 4A molecular sieve has a slightly higher moisture capacity, but adsorbs molecules as large as butane. Type 13X molecular sieve has a different crystal structure from the types 3A and 4A, and has a pore opening of about 10 angstroms. This allows for the adsorption of a wide range of organic molecules as well as moisture. The selective adsorption characteristics of molecular sieves can be useful when it is necessary to dry a package without removing other desirable compounds from the system. Molecular sieve can hold moisture to temperatures well past 450°F (230°C), and because of its high affinity for moisture, molecular sieve is able to bring the relative humidity in packages down as low as 10% RH.