Selecting the Right Desiccant
Adsorbent Selection Process
Packaging engineers face a confusing array of variables when selecting moisture adsorbents partly because moisture control is a multifaceted challenge. There are four sources of water contamination in a closed container or package: the water vapor in the air inside the package; the water vapor adsorbed by the materials inside the package; the water vapor on the walls of the package; and the permeation of water vapor into the package.
Historically, desiccants are chosen by application-testing, commonly known as trial and error. Formal testing can become quite costly in time and money, and the purpose of this article is to provide basic information about currently available desiccants and their properties. It will help the packaging engineer to make better informed selections, and reduce the number of variables that must be addressed in desiccant-testing.
These variables can be classified into two main groups: those pertaining to the product, its package and its environment; and the various properties of commercially available desiccants.
Through first-hand testing and published research, the packaging engineer determines the conditions surrounding optimum product preservation and performance. In attempting to meet and maintain these conditions, one often encounters moisture and its accompanying hazards: corrosion, rust, mold, mildew, fungus, swelling and other undesirable factors affecting product integrity. Where can one turn for answers?
Based upon its wartime experience in the development of food and drug drying agents, the US Department of Defense developed specifications addressing the elimination of corrosion and mildew by adsorbing the moisture from the air of an enclosed space. In November 1963, the DOD released MIL-D-3464C, covering the use of bagged desiccants for packaging and static dehumidification. Three years later, MIL-D-3464D served to update the original specification, creating 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. In 1973, the DOD followed with specifications for cleaning, drying, preserving, and the packaging of items, equipment and materials for protection against corrosion, mechanical and physical damage and other forms of deterioration. MIL-D-3464D and MIL-P-116G have long been the only objective source for packaging engineers. The strength of these specs lies in determining 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 from, 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 saturated air. For example, at zero °C, the air can hold no more than 4.84g/m3 of water vapor; at 40°C the air can hold no more than 50.7g/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 packaged 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 (Si02 · H20)
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 400/0 of its weight in water. Silica gel is extremely efficient at temperatures below 770F (2500) (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 US government approval for use in food and drug packaging.
Molecular Sieve (Synthetic Zeolite - Na12AIO3SiO2 12X H20)
Molecular sieve contains a uniform network of crystalline pores and empty adsorptive cavities, which give it an internal adsorptive surface area of 700 to 800 sq. m per g (1/2 the total volume of the crystals). 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 manmade 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 28.50/0 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 100/0 of its weight in water vapor (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-3464D 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 3g of water vapor at 200/o relative humidity and at least 6g of water at 40% relative humidity at 770F (250C). 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 sq. ft.) or 0.011 x A (in sq. in.) where
A = the area of the barrier in sq. in. or sq. ft.
For rigid containers: units of desiccant required = K x V, where K = 0.161 (in gal.) or 0.0007 (in Cu. in.) or 1.2 (in Cu. ft.) and V = the volume within the barrier (in gal., Cu. in. or cu. ft.).
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 size of the container in sq. ft. or sq. in. of the container walls if it is flexible; in gal., cu. ft. or cu. in., if it is rigid.
3) Determine the number of units required using Table 3 (chart I) 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 watervapor transmission rate is often adversely affected.
The search for a solution has led to the development of 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.
When Tyvek is sealed with thermoplastic lamination (requiring heat and pressure), it will lose part of its permeability. Multiform Desiccants has developed a process that will heat-seal the bag by joining Tyvek directly to Tyvek with no adverse effect on the adsorption rate.
Engineers consulted in the preparation of this article expressed a common desire to have the ability to mold the desiccant bag into any shape while retaining its permeability. Reshaping the bag will allow the packaging engineer to position the desiccant anywhere within the product container or casing.
A recent development from Multiform Desiccants involves the use of a felt cover stock, which can be thermoformed into nearly any shape without losing its permeability. This makes it possible for the packaging engineer to adapt the form to almost any application.
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 gases. Activated carbon has been used extensively for many years as an adsorbent of odors and toxic gases - it has long been used in the 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.