Volume 34, Number 2, February 1999

 

Desiccants 101

Keep Clear with Proper Desiccant Handling

by Todd Deschaine

Desiccant is an essential component in the fabrication of insulating glass windows, since it adsorbs moisture and solvent residues between the lites to keep the view perfectly clear. But if it is not handled properly, a desiccant can lose its adsorption capacity and will no longer be effective. The result can be a foggy window, leading to customer dissatisfaction and replacement costs. A dry environment between the lites of an insulating glass unit is essential to maintaining customer satisfaction.

What are desiccants and how do they work?

The desiccants used in the manufacture of insulating glass windows are synthetic zeolite molecular sieves. Zeolite is the "chemical" from which the desiccants are made; the term molecular sieve describes how they work. Zeolites are actually crystalline aluminosilicates, which, simply put, are microscopic crystals consisting of aluminum, silicon, and oxygen atoms linked together to form a honeycomb structure. Zeolites are manufactured in powder form. For use in the spacers of insulating glass windows, the powder is mixed with a clay binder and formed into small beads.

Properly manufactured zeolites have unit cells that are very uniform in structure and contain "pores"—openings in the molecules (see figure one). When zeolites come into contact with other chemicals, certain molecules get attached within the pores while others pass by. The zeolites act like a filter (or sieve) for specific molecules, thus the term "molecular sieve."

There are many different types of zeolites with different size and shape pores. The pores in the desiccant (Type 3A) zeolites are perfectly-sized and shaped to hold water molecules. Sometimes solvent vapors from the sealants used in the window fabrication process are present between the panes. In order to capture both water vapor and solvent vapor, a mixture of two types of zeolites (3A and 13X) is used.

The characteristic of zeolites that makes them such great desiccants is that they strongly attract water molecules. Even at very low relative humidity, a small amount of water vapor in the air will be attracted to and captured by the zeolite. This capacity for water is often referred to as its "activity."

Once water vapor is caught in the zeolite structure, it stays there. Each zeolite molecule has a finite capacity for water, so when the pores are filled, the zeolite can hold no more. This is why insulating glass windows must be sealed to prevent additional moisture from fogging between the panes. If the seal breaks, the constant exposure of the desiccant to moisture in the air will eventually use up its capacity for water, causing a loss of activity and cloudy glass.

How to handle desiccant

The same strong affinity for water that makes zeolites perfect desiccants means that care must be taken not to expose the desiccants to air. Even air that seems very dry to the average person still contains enough water vapor to be captured by the desiccant, and to use up its capacity.

Active desiccant is shipped from the manufacturing plant in sealed bags or drums to prevent exposure to air. As long as the bags or drums remain unopened, the desiccant will stay fully active for a year or more. All desiccant containers should be carefully inspected upon receipt to assure that there are no punctures or broken seals from the shipping process. Care should be taken to avoid puncturing containers during unloading, handling and storage.

During use, you should also minimize a desiccant’s exposure to air in order to maintain maximum activity. Partially used drums or bags should never be left open to the air. In just one hour, the top layer (as much as an inch deep) will be de-activated. Reseal bags by squeezing out the excess air, folding the open edge over several times and taping it tightly.

Always test previously opened containers of desiccant for activity before use. If the activity is too low, discard the top inch of desiccant in the container and try again. The longer the container is left open, the deeper the layer of de-activated desiccant, so keep skimming layers off and re-testing until you reach the level of active desiccant.

How to test for activity

Activity testing is a simple procedure you can perform right at your site (see figure two). The testing is based on a principle of simple heat rise. As the desiccant captures water molecules, it releases heat. If a small amount of desiccant is mixed with an equal volume of water, the heat released will cause a measurable rise in temperature.

You will need two small beakers or containers, a thermometer (32-212F or 0-100C), and a pen or pencil and paper to record your results. Desiccant manufacturers often provide a test kit that contains everything you need to conduct this test.

1. Put 40 milliliters of water in one beaker, and an equal volume of desiccant in the other.

2. Measure and record the temperature of the water. It should be at least 60F (15C).

3. Pour water into the desiccant and stir gently with the thermometer.

4. Watch the temperature rise and record the highest temperature the mixture reaches.

5. Subtract the initial temperature of the water from the final temperature of the mixture. If the temperature difference is 50F (28C) or greater, your desiccant is sufficiently active.

Todd Deschaine is a technical service specialist for molecular sieves at the PQ Corporation R&D Center in Conshohocken, PA.


USG

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