Volume 6 Issue 2 March 2005
THE CUTTING EDGE
Today’s IG Sealant Technology:
Combining Economy with Performance
by Jim Plavecsky
In order to build an insulating glass (IG) unit that will withstand the effects of weather and the stresses that accompany various wind load conditions, temperatures and barometric pressure changes, we must use both an adhesive and a sealant. The function of the adhesive is to hold the spacer in place between the two or more lites of glass in such a manner that the structural integrity of the IG system is maintained. The function of the sealant is to retard the permeation rate of moisture vapor across the bond line interface and into the inside cavity of the unit. The rate at which moisture vapor is able to penetrate into the space between the lites is inversely proportional to the life expectancy of the IG unit. Moisture vapor that does find its way inside the IGU is subsequently absorbed by the desiccant or molecular sieve that is contained within the spacer system to prolong the clarity of the view through the unit. Once this desiccant is completely saturated with moisture, the IG unit will fog, resulting in what is termed an IG unit failure.
Single-seal IG systems use one sealant/adhesive that is called upon to perform both tasks. This is not accomplished without some degree of compromise. Based on my experience, which includes eight years of work as a polymer chemist, I have found that the types of polymers typically used to formulate high-strength adhesives are not the best in terms of providing the highest degree of protection against moisture vapor infiltration. Hot-melt butyl is outstanding in terms of its ability to inhibit the permeation of moisture vapor, but as a thermoplastic sealant, it offers a lower degree of structural adhesion than does polyurethane or polysulfide sealants, which are very strong, chemically crosslinked polymer systems called thermosets. These thermoset sealants offer a high degree of adhesion to glass and spacer and can withstand greater degrees of repeated stress under both very high and very low temperatures. The compromise here is that the structurally superior thermosets also allow a significantly higher degree of moisture vapor penetration compared to the non-structural butyl sealants. Additionally, the thermoset sealants must be crosslinked or cured chemically. This adds labor, time and cost to the overall manufacturing process.
Silicone is an example of a thermoset sealant that offers structurally superior adhesion at both the highest and lowest operating temperatures and under the highest degree of ultraviolet (UV) radiation exposure. However, it cannot be used by itself in an IG unit because it allows an unacceptably high rate of moisture vapor penetration.
Adding a Partner
For this reason, we see silicone taking on the role of what is referred to as a secondary sealant in what is referred to commonly as a dual-seal design. Silicone is coupled with what is called a primary sealant that is typically formulated from a polyisobutylene (PIB) polymer. The PIB material is extremely resistant to moisture vapor penetration, but it lacks structural properties as an adhesive.
Now assuming that workmanship is all top-notch, dual-seal designs generally are considered to be superior to single-seal designs in terms of longevity and durability. Accelerated weather testing supports this. However, when it comes to economy, the single-seal designs are the more common practice, especially in the new construction market where builders care much more about price and usually offer only a one-year warranty on a new home. Thus, single-seal systems are more popular than dual-seal designs by about three to one.
However, innovative material suppliers are coming to the rescue. For example, one company combines a structural adhesive with a thermoset foam spacer that is packaged and applied at the same time. Also, the major sealant suppliers have been busy formulating new sealants that combine polymers having greater resistance to moisture vapor infiltration than conventional two-part secondary sealants with other polymers capable of crosslinking during a single-step application process. The benefit here is a single-step process that results in a sealant system with very good moisture vapor barrier properties combined with a higher degree of structural performance than would normally be expected in a single-seal design. These sealants, which are referred to as Dual Seal Equivalent or DSE sealants, were designed to help bridge the gap between the economic single-seal systems and the higher performance dual-seal designs.
Whatever type of sealant system is used, it must be combined with superior workmanship practices in order to offer customers the highest degree of performance at each given price point.
Jim Plavecsky is the owner of Windowtech Sales Inc., a sales and consulting firm specializing in the fenestration industry based in Columbus, Ohio.
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