Volume 8, Issue 6 - June 2007
Preventing the Great Escape
With energy conversation and ecological thinking becoming ever-important, consumer interest in energy saving technology is at an all-time high. Vinyl, wood and thermally-broken aluminum frames have taken over. Low-E coatings are more popular than ever. Warm-edge technology is a given. However, gas-filling, although very popular, remains somewhat controversial. Several lawsuits have caught the attention of the industry (see sidebar) and these have led to a heightened concern among manufacturers with respect to gas-fill rates and long-term gas retention. Homeowners want to be reassured that their windows are gas-filled to the proper level. And once that is accomplished, what about gas retention? The minute an insulating glass (IG) unit is gas-filled, the gas starts trying to escape. It is inevitable that it eventually will. The question is “How do we make this a very long time?”
Many fabricators seal a unit leaving one corner open or create a separate opening through which a filling/sniffling wand is placed. Argon or krypton flows into the unit through the filling wand while the sniffling port sucks air from the cavity and analyzes for oxygen. The absence of oxygen indicates the unit is full and shuts off the flow of argon or krypton. Once the unit is filled, the area where entry was made needs to be sealed with the application of a gas impermeable sealant. This is a critical workmanship step. Hot sealant is applied against sealant that has already cooled, and care must be taken to make sure that this is a high integrity patch. If not, when the unit expands and contracts, a fissure could develop resulting in rapid gas loss. For this reason, some fabricators have invested in gas-filling presses which seal the unit inside of a cavity while being flooded with argon. The advantage of this method is that the argon is trapped within the unit as it is sealed so that no entry gap is required. There are no concerns with how well the entry gap has been sealed because there is no entry gap.
This is especially important because when argon permeates through the IG sealing system, it does so faster that air permeates from the outside to within the unit. Each gas behaves independently from the others and permeates through a polymeric sealant at its own rate. Scientists use the laws of entropy to describe these types of movements. Argon is present in air naturally at a concentration of slightly under one percent. So when we pack a large umber of argon gas particles into a confined space, we are adding order to the world. Entropy change has often been defined as a change to a more disordered state at a molecular level. Gas wants to migrate from areas of high concentration to areas of lower concentration, and the greater the disparity in concentration, the faster the gas movement. For each gas, this rate is related to the difference in the amount of that gas on each side of the barrier. Air is a mixture containing 78-percent nitrogen, less than 21-percent oxygen, and less than 1-percent argon. If an IG unit is filled with 95-percent argon, the ratios of each gas from the high concentration side to the low concentration side are shown in the table. A larger ratio indicates a greater disparity, and, therefore, a greater driving force for gas migration. As you can see, the driving force for argon to return to its natural state is the greatest compared to the other gases. Therefore, it will move faster from inside to outside while the other gases are entering the IG unit at a slower pace.
Strains and Stresses
IG and window fabricators should be very concerned with the type of spacer-sealant combination used in the IG unit as well as the workmanship practices in place during unit fabrication. Fabricators of IG units exhibiting high rates of gas permeability and gas loss have much more to lose than the gas itself. The end result could result in high warranty expense, lawsuits, negative publicity and loss of reputation. These are all big reasons to prevent the great escape!
Jim Plavecsky is the owner of Windowtech Sales Inc., a sales and consulting firm specializing in the door and window industry based in Columbus, Ohio.