Taking Measures to Ensure Proper Workmanship
By Jim Plavecsk

Why put argon in an insulating glass (IG) unit when the laws of nature tell us that these molecules will try to escape? 

At two pennies a liter, the cost of gas filling with argon is modest while it results in a nice jump in thermal performance. High-speed gas fill machines are quite common these days, filling units at rates as high as 90 liters per minute. This means we can fill IG units very efficiently, translating to a significant value for the consumer. 

The differences in composition of a 95-percent argon gas fill inside a unit versus the chemical composition of air outside a unit are shown in the accompanying table (see box below). The third column shows the ratio of the concentration of the higher side versus the lower side for each gas. For nitrogen, oxygen and carbon dioxide, the outside concentration is higher than the inside concentration. When it comes to argon, the inside concentration is higher than the outside concentration. The ratio relates to the driving force for each particular gas to permeate to the other side. Larger concentration differences (greater ratios) correlate to a greater drive for equilibrium to be achieved. As you can see, the ratio for argon from inside to outside is three to five times what it is for the other gasses in the system. Therefore, argon tries to get out faster than the other gases try to get inside. The same materials and workmanship principles that surround the task of keeping moisture vapor from entering the IG unit also apply to the job of keeping argon from escaping the unit. However, the forgiveness factor does not apply.

Desiccant Plays Major Role
The forgiveness factor refers to the role that molecular sieve or desiccant plays to prevent an IG unit from fogging as moisture vapor enters the unit over time. The same principles of equilibrium outlined here also apply to moisture vapor. The inside of the unit contains very dry air, which has a very low concentration of moisture vapor. The outside of the unit has a much higher concentration of moisture vapor. However, as moisture vapor enters the unit, molecular sieve or desiccant absorbs it, thereby preventing it from accumulating as a fog within the IG unit. So, workmanship errors manifested as small voids or flaws can go for years before resulting in a failure since the drying agent is “buying time” or providing “forgiveness.” 

But when argon escapes, there is no forgiveness. It creates more problems. Since argon moves out faster than air moves in, the result becomes a negative pressure differential from inside to outside the unit. A pressure drop inside the unit results in the inward bowing of the glass (see illustration at left). The IG unit now becomes a concave lens, and visible distortion is the result. The glass can touch the internal grids resulting in a thermal short circuit. The inward flexing of the glass in the middle causes a corresponding outward flexing of the glass at the edges where the edge seal is trying to hold everything together. This places a high degree of stress upon the bond between the glass-sealant interface as well as the sealant-spacer interface. The result can be adhesion loss between glass and sealant or between sealant and spacer. Premature failure can result. The moral of this story is that when it comes to gas filling, top-quality workmanship is a must. 

Take Preventative Measures
What can be done to prevent all of this? There are three basic things that need to be addressed: 

Choose a sealant with a very low argon permeation rate. In general, silicone sealants have relatively high permeation rates, so in dual-seal systems utilizing silicone as a secondary sealant, proper application of the primary seal (PIB) is critical. I often see manufacturers stacking units so they can apply sealant more easily to each side of the stack. The weight of multiple units can squeeze the PIB out of place on the bottom three or four units, resulting in higher permeation rates. Once the gas makes it through the PIB, it will move easily through the silicone. In residential applications, where UV requirements are not a top priority, then polyurethane can be used as it has a lower argon permeation rate. Lower yet are polysulfide sealants. The best selection as far as gas permeability is concerned are the butyls. There are also single-seal butyl-based sealants available that continue to cure after application, and these offer the lowest gas permeability rates while still providing structural properties. 

Choose a sealant and spacer system that is strong, yet flexible and resilient. An IG unit is a dynamic system. Barometric pressure changes, windloads, and temperature changes all lead to pressure differentials outside versus inside the unit. The unit is expanding constantly and contracting. This puts stress on the sealant-spacer interface as well as the sealant glass interface. Flexible spacer systems can help alleviate this stress by moving with the glass as opposed to more rigid systems. Stress management helps lengthen the life span of the adhesion system allowing the gas sealing capability of the system to last longer. Flexibility also can vary with temperature so it is worth taking a look at the low and high temperature properties of the spacer and sealants involved. Some of these become stiff at extremely low temperatures or can become too soft and creep at higher temperature extremes. The best ones remain flexible at the lowest subzero temperatures while remaining strong and dimensionally stable at high temperature extremes.

Pay greater attention to sealant adhesion and application techniques. No matter how good of a sealant you use, it will not stick well to dirty glass. So, make sure that your washer has a clean air filter and that the water temperature is in line with your glass supplier’s recommendation. Also, make sure that two-part sealants are being mixed properly (correct ratio and good dispersion) or that, in the case of one-part sealants, make sure that the application temperature is in the correct range. Pay very close attention to application techniques. Even small voids or cold joints (see photo at left) can lead to very high rates of argon loss. As the IG unit expands and contracts on a daily basis, these workmanship flaws can grow in size allowing even more gas to escape.

Gas Retention Standards
Gas retention standards now have become part of the Insulating Glass Manufacturers Alliance (IGMA) and Insulating Glass Manufacturers Association of Canada (IGMAC) certification programs and are voluntary for the Insulating Glass Certification Council (IGCC). The National Fenestration Rating Council (NFRC) soon will make IG durability a requirement as well, and this also will include a gas retention requirement. The standard involves a minimum initial fill requirement of 90 percent (average of six units) with a retained average of 80 percent minimum after undergoing the ASTM D2190 durability test. Hundreds of window manufacturers currently are utilizing the NFRC and Energy Star® labels as part of their marketing and sales programs soon will be required to pass these tests, proving that their window systems contain the required amount of gas both initially and after accelerated weather cycling. For many, this surely is going to be a wake-up call. Injecting the proper amount of gas into the unit is only half the battle. Keeping it in there will prove to be the bigger challenge. 

Jim Plavecsky is a regional sales manager for Edgetech IG. He also is the owner of Windowtech Sales Inc., a Columbus, Ohio-based sales and consulting firm that specializes in the door and window industry. He can be reached at Jim.Plavecsky@edgetechig.com.

DWM
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