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.
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 color when it is dry and turns from blue to purple to pink as it becomes saturated with moisture. Typically, the color 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.
The Dri-Box canister is packed with an indicating silica gel that is regenerable. Instead of replacing the desiccant inside, heating the canister will reverse the adsorbing action and allow the material to be reused.
Montmorillonite clay is a naturally occurring adsorbent created by the controlled drying of magnesium aluminum silicate of the sub-bentonite type. This clay will successfully regenerate for repeated use at very low temperatures without substantial deterioration or swelling. However, this property causes clay to give up moisture readily back into the container as temperatures rise.
Clay is a good basic desiccant that works satisfactorily below 120°F (approximately 50°C). Above 120° F, there is a possibility that the clay will give up moisture rather than pulling it in, so anticipated storage and transportation conditions should be considered. The upside to clay is that it is normally the least expensive desiccant per pound.
Clay is highly effective within normal temperature and relative humidity ranges. Its appearance is that of small gray pellets. Care should be taken to be sure that any low level impurities in the clay are not incompatible with the packaged product.
Molecular sieve is the best desiccant based on technical performance characteristics. Its ability to adsorb moisture, in this case water vapor, is so pronounced that it can remove trapped H20 molecules from a fully saturated silica gel bead, which in turn changes the silica gel back to its original Cobalt blue color.
Molecular sieves are synthetic porous crystalline aluminosilicates which have been engineered to have a very strong affinity for specifically sized molecules. The definitive feature of the molecular sieve structure, as compared to other desiccant medias, is the uniformity of the pore size openings.
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 distinctive feature allows for the selection of a molecular sieve product which can adsorb water vapor yet exclude most other molecules such as volatile organic compounds (VOCs) which may or may not be present in the package.
For example: 3A molecular sieve's structure, which if you will remember has a 3 angstrom pore opening, allows water vapor adsorption but excludes most hydrocarbons. 3A is good for ammonia (NH3), water vapor (H2O) and polar liquids. 4A molecular sieve has a slightly higher water vapor capacity but adsorbs molecules as large as butane. 4A is good for water vapor (H2O); carbon dioxide (CO2); sulfur dioxide (SO2); hydrogen sulfide (H2S); ethylene (C2H4); ethane (C2H6); propene (C3H6) and ethanol (C2H6O). 5A molecular sieve adsorbs normal (linear) hydrocarbons to n-C4H10, alcohols to C4HgOH, mercaptans to C4HgSH. 5 angstrom molecular sieve will not adsorb iso compounds or rings greater than C4. 10A molecular sieve (13X) adsorbs di-n-butylamine (not tri-n-butylamine) and is useful for drying HPMA.
The selective adsorption characteristics of molecular sieves can be useful when it is necessary to dry a package or environment without removing other desirable compounds within the system. Molecular sieve can trap water vapor to temperatures well past 225°C in some cases, and due to its high affinity for water vapor, molecular sieve is able to bring the relative humidity (RH) in environments down to as low as 1% RH.
In Europe molecular sieve is used in pharmaceutical applications. In the United States it is often the desiccant of choice for pharmaceuticals and bio-chemicals and/or reagents. Although molecular sieve is sligthtly higher in cost per unit due to its extremely large range of adsorptive capabilities and high capacity at low relative humiddity it is often the best value.
Lack of government approval for the molecular sieves in food and drug packaging has unfortunately limited its more widespread use. Independent testing suggests that molecular sieves meet or exceed government requirements. Presumably, however, the industry has been unwilling to fund the expensive and time consuming testing that is required for government approval.
Calcium oxide is calcinated or recalcinated lime having a moisture adsorptive capacity of not less than 28.5% by weight. The distinguishing feature of calcium oxide (also known as quicklime) is that it will adsorb a much greater amount of water vapor at a very low relative humidity than other materials. It is most effective where a low critical relative humidity is necessary, and were there is a high concentration of water vapor present.
Calcium oxide removes water from a package very slowly, often taking days to reach its maximum capacity. As calcium oxide adsorbs moisture, it swells. Proper desiccant packaging is required for effective use. For these reasons, its use has been limited to primarily the packaging of dehydrated foods.
Calcium sulfate (better known commercially as DrieriteT) is created by the controlled dehydration of gypsum. It is a general purpose desiccant geared mainly toward laboratory use. It is chemically stable, non-disintegrating, nontoxic, non-corrosive and does not release its adsorbed water easily when exposed to higher ambient temperatures.
The low cost of calcium sulfate must be weighed against its equally low adsorptive capacity; it adsorbs only up to 10% of its weight in water vapor. Calcium sulfate also has regeneration characteristics that tend to limit its useful life. Although available, it is not normally sold in package form.
Other adsorbents are available for specialized functions. For example, activated alumina is a porous desiccant which performs very similarly to silica gel, providing somewhat lower moisture capacity at low temperatures, but slightly improved capacity at higher temperatures. It should be noted that some of these alternative desiccant products have a specialized function.
Activated alumina is extremely effective for drying gases. Activated carbon has been used extensively for many years as an adsorbent of odors and toxic gases - it has long been used in military gas masks.
Others, ranging from metal salts to phosphorus compounds, have specific strengths that would be impossible to address individually. Often it is left up to the desiccant supplier to answer specific questions.