All in the Desiccant
Choosing the Right Desiccant Can Make or Break an IG System
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.
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
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.
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).
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.
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.
“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.
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.
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.
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