Packaging engineers face a confusing array of variables when selecting moisture adsorbents, partly because moisture control is a multi-faceted challenge. There are four sources of water contamination in a closed container or package: the water vapor in the air inside the package; the moisture contained in the materials inside the package; the moisture in or on the walls of the package; and the entry of moisture into the package due to permeation or leakage.
Historically, desiccants are chosen by application-testing, commonly known as trial and error. However, testing can become quite costly in time and money. The purpose of this article is to provide basic information about currently available desiccants and their properties and to suggest some guidelines for matching desiccant to applications. It will help the packaging engineer to make better informed desiccant selections, and reduce the number of variables that must be addressed in desiccant testing.
The U.S. Department of Defense has developed specifications to address the elimination of corrosion and mildew by adsorbing the moisture from the air of an enclosed space. The most commonly used specification is MIL-D-3464C, which covers the use of bagged desiccants for packaging and static dehumidification. This document creates a uniform standard of comparison in a wide variety of areas: adsorption capacity and rate, dusting characteristics of the package, strength and corrosiveness of the package, and particle size of the desiccant.
The DOD has also published the MIL-P-116 specification for the cleaning, drying, preserving, and packaging of items, equipment and materials for protection against corrosion, mechanical and physical damage, and other forms of deterioration.
MIL-D-3464 and MIL-P-116 have long been the only objective resources for packaging engineers. The strength of these specifications is that they establish a uniform unit of drying capacity, enabling one to compare desiccant effectiveness on a common scale. The specifications, however, fail to deal specifically with variables, such as product environment, packaging of the product itself, and the type of desiccant suitable for a specific need. Also, other important factors are defined but not applied: desiccant packaging form, cover stock, adsorption rate, and adsorption capacity.
These specifications seem only to compare the conformance of the desiccant selected to Defense Department standards. As a result, the packaging engineer may be at a loss to choose with confidence which particular desiccant is best for each application. Thus, the engineer must move outside the limited scope of military specifications and into the real world of moisture-proof packaging: the product’s environment and package.
Temperature, relative humidity, and other considerations constitute the product’s environment, which must be controlled to match the conditions of optimum product preservation and performance. Before selecting the correct desiccant, the packaging engineer must 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.
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. Before the adsorbent selection process itself, the packaging engineer must determine the size of the container based on the flexibility of the container’s wall structure.
Adsorbent Selection Process
The engineer has, to this point, determined the following: conditions of maximum product integrity; size and type of container used; actual conditions (temperature and relative humidity).
By comparing the properties and capabilities of each desiccant product, the engineer can identify the correct desiccant and make a clear choice.
Figures 1 and 2 illustrate the adsorption rate (how quickly the desiccant adsorbs the water vapor inside the package) and the adsorption capacity (how much water vapor is adsorbed to reach equilibrium at various relative humidity readings) of five common desiccant products. These are: montmorillonite clay, silica gel, molecular sieve (synthetic zeolite), calcium sulfate and calcium oxide.
Table 2 shows adsorptive tendencies of each desiccant, including effectiveness at elevated temperatures and extreme water vapor concentrations. The engineer can refer to these tables to supplement the following brief description of the principal commercially available desiccants.
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).
Silica Gel (SiO2 * H2O)
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 (Synthetic Zeolite, also known as aluminosilicate)
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. Lack of government approval, has limited a more widespread use of molecular sieve, presumably due to the industry's unwillingness to fund an expensive government study. Independent testing suggests that molecular sieve does meet all government requirements.
Calcium Oxide (CaO)
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 (CaSO4)
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.
MIL-D-3464E details the generally accepted method for determining the amount of bagged desiccant required, based on the size and the type of the container and the basic unit of desiccant as defined in the MIL spec. A unit of desiccant is defined as "the amount of desiccant that will adsorb at least 3 g of water vapor at 20% relative humidity and at least 6 g of water at 40% relative humidity at 77°F (25°C). Table 3 provides a convenient reference to help determine how many units will be required. It is based on the following formula:
For flexible containers: units of desiccant required = 1.6 x A (in ft2) of 0.001 x A (in2) where A = the area of the barrier in2 or ft2.
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., in3 or ft3).
Calculation: To determine the amount of desiccant required:
(1) Identify the type of container. Is it a flexible barrier type (foil or poly bag), or is it a rigid type (drum or pail)?
(2) Calculate the surface area of the container walls in ft 2 or in 2 if it is flexible; or the volume of the container in gal., ft3 or in3, if it is rigid.
(3) Determine the number of units required using Table 3 (chart 1) for flexible containers, (chart 2) for rigid containers.
(4) Select the type of desiccant that meets your needs according to Figures 1 and 2.
In calculating the number of desiccant units required, dunnage (interior packing, cushioning, blocking, and bracing materials) must be considered.
Cover Stock: An important factor in the efficiency of the selected desiccant is the bag material (cover stock) of the desiccant. 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.
The selected desiccant's adsorption rate is greatly affected by the water vapor transmission rate of its cover stock. This is the measure of the gain or loss of water vapor through the package of the bagged desiccant.
By their nature, certain products require a very non-dusting desiccant bag to maintain their integrity. While dealing with dusting requirements, however, the packaging engineer encounters another problem: in preventing the release of dust into the container, the water vapor transmission rate is often adversely affected.
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. It will not allow dust to be released into the container, is resistant to staining, mold, and mildew growth, and will not reduce the adsorption rate of the desiccant it holds. Because of its special properties, Tyvek is more expensive than conventional desiccant package materials.
It should be noted that some desiccant products have a specialized function. For example, activated alumina (a very porous desiccant) 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. Often it is left up to the desiccant supplier to answer the packaging engineer's specific questions.
If you have any questions or would like more information, please call one of our Desiccant Specialists at 520-881-2130. You can also email our product specialists using the Contact Us form.