It's all in the Desiccant

May - June 2003

It’s All in the Desiccant
Choosing the Right Desiccant Can Make or Break an IG System

by Tom Dangieri

Since the advent of the dual-lite insulating glass (IG) industry, one of the cornerstones of a well-made IG unit has been the desiccant system. The main role of a desiccant is to remove all traces of moisture and chemical vapors that may be present during the construction of the window unit. The second task is to maintain a low dew point over the warranted life of the unit and beyond. If a desiccant system meets both these criteria, a well-constructed window should provide many years of trouble-free performance even in the harshest of climates.

One of the original products in our company’s desiccant portfolio was a type 3A molecular sieve that is mixed with a clay binder and formed as a bead. The 3A molecular sieve has a vast three-dimensional internal network of active sites that have a special affinity to attract and physically bond water molecules in the crystal structure at low concentration levels (dew points). The precise pore openings in this product allow only the water to penetrate into the internal cavity.

Changing Demands
As IG construction methods advanced, the demands on the desiccant system grew more complex. The ability to remove larger organic molecules that may outgas from sealant systems, paints and muntins between the lites became necessary to prevent formation of a chemical fog. This fog became easier to detect as construction with low-E glass grew.

The construction of a dual or triple-lite IG system cannot avoid trapping air between the lites which contain an amount of moisture equal to the relative humidity of that air at the time of construction. A 30- by 36-inch standard residential window unit with a half-inch air space, assembled at sea level on a 70° F day in 80 percent relative humidity air will trap the equivalent of 0.1 gram of water in the vapor space. During the initial dry down of the window less than one gram of type 3A desiccant capacity would be needed to achieve a 40° F dew point. If the unit is argon filled even more desiccant capacity is available to protect the window from fogging after placement in the building. (Figure 1 shows the available capacity of some desiccants at several dew point conditions. The desiccants are fully activated).

Figure 1: The above chart shows the available capacity of some desiccants at several dew point conditions. The desiccants are fully activated.

The amount of desiccant capacity required in a window system should depend on moisture vapor transmission through and around the sealant system, temperature extremes expected in the window environment and the length of the warranty claim. The relative humidity surrounding the unit will always be greater than the space between the lites and will penetrate the space over time.

For the industry, determining the amount of water adsorbed in the desiccant before it is sealed in the unit is important since it affects the total water capacity expected. Keeping moisture away from the desiccant during storage and handling are critical to maintaining the high adsorptive capacity. Adsorption is a physical phenomenon, so the material never changes characteristics visually. However, as water is adsorbed, heat is released. In scientific language this is called an exotherm. Our company used this unique behavior to develop a quick, reliable quality control method to determine the level of water adsorbed on the desiccant before it is sealed inside the IG unit.

The “Heat Rise Test” provides a simple, stepwise, reasonably accurate and easily understood determination of the remaining water capacity of desiccant. By measuring the heat evolved when water is added to the molecular sieve and consulting a chart, a manufacturer can prevent faulty made units from leaving the factory because the desiccant was compromised.

Some companies may provide more detailed data for IG manufacturers through user-friendly programs. In ours, window manufacturers can choose any window dimensions, dual- or triple-lite construction, desiccant used, inert gas fill options, the altitude of manufacture and installation, several glass parameters and environmental conditions in the field. Once the data is entered, the program will calculate differential pressure, deflection, stress and breakage probabilities of the window system, expected U-value and raw material cost of the assembled unit.

Desiccants Evolve
As automated manufacturing equipment became more prevalent in insulating glass, desiccants evolved to meet the changing needs. A variety of bead sizes are now available to accommodate the trimline spacer widths. The beads are also made more resistant to dusting and chipping that may occur when handled in harsher, pneumatic conveying systems.

The development of open channel spacers, foam silicone rubber, butyl spacers and other warm-edge technologies required the molecular sieve desiccants to be supplied in powder form. The desiccant is blended into the organic matrix before the window is constructed. The materials still deliver the drying capacity for water and organic molecules the manufacturers have come to expect over the years. (Figure 2 shows some desiccant/ sealant systems used in window construction today). As this segment of the industry has grown, we have had to construct a dedicated manufacturing line to produce these powders in sufficient quantities to meet the demand.

Figure 2: The above graphic shows some desiccant/ sealant systems used in window construction today.

The future of the insulating glass business is limited only by imagination and human ingenuity. Innovative components in the IG system and faster assembly techniques present fresh challenges and demands to the system’s desiccant component.

Tom Dangieri is a senior application specialist in the adsorbents and specialty products division of UOP LLC in Des Plaines, Ill.