Desiccants: Technical Data and Terminology


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How Are "Adsorption" And "Absorption" Different?

How Are Adsorption And Absorption Different?
How Do Desiccants Work?
How Are Desiccants Rated?
How Does Moisture Cause Damage To A Product?
What Is "Saturation" And "Equilibrium Capacity"?

You may have run across some terms used in the description of the Moisture Control System's operation that are unclear to you. You may for instance wonder at the term "adsorption" and think it a typo where we meant to write"absorption". In actuality these terms represent two very different scientific concepts. For the answers to this and other questions on desiccant operation, read on.

Absorption is when a substance is chemically integrated into another. When you drink a glass of water, you are absorbing it, as the water becomes part of you.

Adsorption is when one substance is being held inside another by physical bonds. If you spill a glass of water on your shirt, it is adsorbed as the fibers will hold the water until heat dries out the shirt.

Most desiccants don't chemically combine with water or the other substances they're present to protect against. They capture them through adsorption, and stow them away where they can't cause harm to the product.

How Do Desiccants Work?

It is important to understand how the desiccant does its job of protecting the product. Most porous adsorbents, such as silica gel, activated clay or molecular sieves rely upon physical adsorption rather than chemical adsorption to accomplish their function.

Physical adsorption involves relatively weak intermolecular forces (van der Waals forces and electrostatic interactions) between the moisture and surface of the desiccant.

Chemisorbents, such as calcium oxide, involve an actual chemical bond. Physical adsorption of moisture is typically exothermic. The strength of the adsorptive bonds can thus be measured by the heat of adsorption. The higher the heat of adsorption for moisture on the desiccant, the stronger the bonding and the less easily that moisture can be subsequently removed.

In a porous desiccant such as the silica gel used by Dri-Box, water is removed from the airspace by:

1) multi-layer adsorption, which is the attraction of thin layers of water molecules to the surface of the desiccant. Because the desiccant is very porous, the surface area is high and significant amounts of water can be attracted and adsorbed;
2) by capillary condensation in which the smaller pores become filled with water.
Capillary condensation occurs because the saturation water vapor pressure in a small pore is reduced by the effect of surface tension.

How Are Desiccants Rated?

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. The 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 firsthand 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, MID-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 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 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 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.

For more information, see the entries and charts on our selection page.

How Does Moisture Cause Damage To A Product?

Moisture trapped within a product package or leaking into it during storage and shipping can cause many harmful effects. The stability of many pharmaceutical formulations and diagnostic reagents, as well as the maintenance of their physical product integrity, is often closely tied to the moisture conditions of the package environment.

In unsterile or poorly sterilized situations, moisture can promote the growth of mold, mildew and fungus. Products using some polymers are prone to swelling in high humidity conditions as intermolecular bonding between polymer chains can be weakened by the presence of water. In some cases water can become an integral part of the bulk crystal structure of a product through the formation of hydrates. If a solid is very water soluble, such as a sugar coating, and the right conditions exist, dissolution into the sorbed layer can trigger irreversible water uptake and subsequent deliquescence.

In the case of electronics, the presence of moisture can cause not only corrosion but also can lead to "short" circuits, wherein the electricity powering the device reroutes through the water instead of its assigned conductive path. These will not necessarily lead to cinematic instance of panels blowing open and sparks flying. In fact, they will usually be much more subtle and untraceable by making the device's operation unreliable. Sometimes it works... sometimes it doesn't. At the very least a situation like this is frustrating; at worst, life-threatening.

What Is "Saturation" And "Equilibrium Capacity"?

Although technically they are different situations, for most practical purposes these two terms cover the point at which a desiccant no longer adsorbs moisture.

Saturation is when the desiccant is full and even if there were moisture molecules to pick up, the desiccant could not adsorb them.

Equilibrium capacity is when the desiccant has pulled so much moisture out of the air that the air retains a stronger hold on the moisture molecules than the desiccant can exert. At equilibrium capacity, adding more desiccant will not bring the relative humidity any lower.

 


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