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, but excludes 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.
The United States FDA has not approved molecular sieve for direct contact with consumable items, although in Europe molecular sieve is used with pharmaceuticals. Being man-made rather than naturally occurring, molecular sieve is slightly higher in cost per unit, but due to its extremely large range of adsorptive capabilities, it might often be the best value, especially in areas of low relative humidity.
Lack of government approval for the use of molecular sieves in food and drug packaging has limited its more widespread use. Independent testing suggest that molecular sieves meet government requirements. Presumably, however, the industry has been unwilling to fund the expensive testing required for government approval.