Volume 9, Issue 6 - June 2008
Keeping Argon in the Unit
Taking Measures to Ensure Proper Workmanship
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
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
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
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.