Desiccants are compounds or agents, such as Montmorillonite Clay or Silica Gel, used in facilitating low humidity environments by absorbing moisture content from the air. Typically used in transport, storage or maintenance of materials and products, desiccants are used to keep everything from gym shoes to military munitions dry.
There are four sources of water contamination in a closed container or package which desiccants are designed to combat:
The U.S. Department of Defense (DoD) has developed specifications to address issues of moisture saturation, the most commonly used being MIL-D-3464C and MIL-P-116. Having long been the only objective resource for packaging, DoD specifications create a uniform standard of comparison in a wide variety of areas. Unfortunately, these specifications fail to deal with more precise variables such as product environment, the packaging of the product itself, and the type of desiccant most suitable for any refined need. For more specialized situations, such as electronic equipment or otherwise sensitive products and components - or if you are unsure as to alternative uses for desiccant - it is probably best to consult an authority.
Determine the conditions of maximum product integrity; the size and type of container used and the actual conditions (temperature and relative humidity).
Know the environment: In order to select an appropriate desiccant, it is important to know the conditions surrounding the shipment and storage of the product; the extremes of temperature and relative humidity to which the product will be exposed and the average duration of such exposures. The most useful combined measure of temperature and relative humidity is the dew point.
Dew point is the temperature at which the water vapor content of the air exceeds saturation and the excess water is squeezed out, forming dew or condensation. The dew point varies with the amount of water vapor in the air. It is low with dry air, and high with moist air. For example, at 32°F (0°C), the air can hold up to 4.84 g/m3 of water vapor; at 104°F (40°C) the air can hold up to 50.7 g/m3 of water vapor (Table 1). An effective desiccant will adsorb the water vapor in the air, lowering the relative humidity to the point where water cannot condense.
Know the packaging: The container in which the product will be packaged, shipped, and stored is vital in determining how much of a particular desiccant is needed and in what packaging form. Determine the size of the container based on the flexibility of the container’s wall structure.
Start by comparing the properties and capabilities of different desiccants.
Figures 1 and 2 illustrate the adsorption rate (how quickly water vapor is adsorbed inside containers) and the adsorption capacity (how much water vapor is adsorbed to reach equilibrium at various relative humidity readings) of five common desiccants. These are: montmorillonite clay, silica gel, molecular sieve (synthetic zeolite), calcium sulfate and calcium oxide.
Table 2 shows adsorptive tendencies of some common desiccants, including effectiveness at elevated temperatures and extreme water vapor concentrations. An engineer may refer to these tables to supplement the following brief description of the principal commercially available desiccants.
MIL-STD-2073-1 details the generally accepted method for determining the amount (in desiccant units) of bagged desiccant required based on the size and the type of a container. A unit of desiccant is defined in MIL-D-3464E as, “the amount of desiccant that will absorb at least 3g of water vapor at 20% relative humidity and at least 6g of water vapor at 40% relative humidity at 77°F (25°C)." Table 3 provides a convenient reference to help determine how many units will be required based on the following formula:
For flexible containers: units of desiccant required = 1.6 x A (in ft2) or 0.011 x A (in2) where A = the area of the barrier in ft2 or in2.
For rigid containers: units of desiccant required = K x V, where K = 0.161 (in gal.) or 0.0007 (in3) or 1.2 (ft3) and V = the volume within the barrier (in gal., in3or ft3).
Calculation: To determine the amount of desiccant required:
In calculating the number of desiccant units required, dunnage (interior packing, cushioning, blocking, and bracing materials) must be considered.
Montmorillonite clay is a naturally occurring adsorbent created by the controlled drying of magnesium aluminum silicate of the subbentonite type. This clay will successfully regenerate for repeated use at very low temperatures without substantial deterioration or swelling. However, this property causes clay to desorb moisture readily back into the container as temperatures rise. Clay is inexpensive and highly effective within normal temperature and relative humidity ranges (Table 2).
Perhaps the most commonly used desiccant, silica gel, is an amorphous form for silica manufactured from sodium silicate and sulfuric acid. Its interconnected pores form a vast surface area that will attract and hold water by adsorption and capillary condensation, allowing silica gel to adsorb about 40% of its weight in water. Silica gel is extremely efficient at temperatures below 77°F (25°C) (see Figures 1 and 2), but will lose its adsorption capacity as temperatures begin to rise, much like clay (Table 2). Much of silica gel's popularity is due to its noncorrosive and non-toxic nature; some grades have received U.S. government approval for use in food and drug packaging.
Molecular sieve contains a uniform network of crystalline pores and empty adsorption cavities, which give it an internal adsorptive surface area of 700 to 800 m2. Because of its uniform structure, molecular sieve will not desorb moisture into the package as readily as silica gel or clay as temperatures rise. Being synthetic rather than naturally occurring, molecular sieve is higher in cost per unit, but due to its extremely large range of adsorptive capabilities, it might often be the best value.
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 (Table 2). It is most effective where a low critical relative humidity is necessary, and where there is a high concentration of water vapor present. Calcium oxide is used mainly in the packaging of dehydrated foods.
Calcium sulfate (better known commercially as Drierite®) is an inexpensive alternative available in suitable packaging forms. Calcium sulfate is created by the controlled dehydration of gypsum, acting as a general-purpose desiccant geared mainly toward laboratory use. It is chemically stable, non-disintegrating, non-toxic, non-corrosive, and does not release its adsorbed water 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 (Figure 2). Calcium sulfate also has regeneration characteristics that tend to limit its useful life. Although available, it is not normally sold in package form.
Cover Stock: An important factor in the efficiency of any desiccant is the bag material (cover stock). The cover stock must allow the desiccant to do its job without harming the product. This means maintaining an acceptable adsorption rate and conforming to the product's dusting requirements.
One common cover stock is a spunbonded, high-density, polyethylene material known commercially as Tyvek®. Created by DuPont, Tyvek® resembles a waxy paper with good whiteness and exceptional strength, maintaining its size and shape with changes in humidity. Tyvek® will not allow dust to be released into the container, is resistant to staining, mold, and mildew growth, and will not reduce adsorption rate. However, because of its special properties, Tyvek® is more expensive than conventional cover stock materials.
It should be noted that some desiccants have a specialized function. For example, activated alumina (a very porous material) is extremely effective for drying compressed gases. Activated carbon has been used extensively for many years as an adsorbent of odors and toxic gasses - 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. Consult a supplier for answers to specific questions.
AGM Container Controls, Inc. has an entire division dedicated to desiccants and their functionality. AGM also offers a moisture analysis service which takes into account a wide range of variables in order to prescribe desiccant combinations for maintaining targeted humidity level goals over any length of time for highly sensitive equipment. If at any time you have questions or concerns, please contact our Specialists at 520-881-2130 or via email and we will be happy to assist you.
AGM Container Controls, Inc.
3526 E. Fort Lowell Rd.
Tucson, AZ 85716