Everything IG September 2020

July 27th, 2021 by Nathan Hobbs

Measuring Up: When it Comes to IG Failures, Knowing is Preventing

By Dave Cooper

This is my first article for a fenestration magazine. But I hope to be a longtime contributor with a common theme: insulating glass (IG).

I’ve been involved with IG, vacuum insulating glass (VIG) and fenestration in general for decades, working in various capacities for a few different component manufacturers (glass, sealants and spacers). I’ve also had long term involvement and leadership roles with various fenestration organizations. This has allowed me to possess a deep technical and market knowledge base for the nuances of fabricating components into reliable IG products.

For this article, I’ve chosen to focus on IG edge strength. There are many failure modes, but the major and most prevalent culprits can be broken down as follows: workmanship, seal breaches, glass breakage, abnormal gas loss and/or moisture ingress, and chemical incompatibilities. Failure of the edge sealant under load leads to seal breaches. The question is: What causes these failures under load?

Tracking the Culprit

Secondary sealant failure (which also applies to systems without applied secondary seals, but where the spacer itself acts as a secondary seal) can occur under more extreme loads. Practices to minimize workmanship issues start with cleanliness—including clean materials (e.g. glass) and clean environments. Beyond those, we’re left with the possibility for inadequate sealant strength when the edge of the IG is under tensile or shear load.

What does this mean? There are two major causes of failure. The first involves a change in the volume of trapped gas within the sealed IG (usually positive, resulting from higher elevations than the point of manufacture, hot temperatures, etc.) which deflects the glass outward and imparts a tensile or “pulling
apart” force.

The second cause is shear stress, where one lite of glass is moving parallel to the other lite in plane. This may be caused by a large temperature difference between the two lites of glass or when one lite is unsupported. This failure mode can occur in sealed IG with larger cavity widths and/or high aspect ratios that are transported through or located at higher elevations.

Fortunately, there is a way to gauge the possibility for failure. A calculation may be performed to understand the strength of the sealant for a given IG construction. A computer simulation program is available that can also perform the calculation, though several assumptions about sealants are built in and several input parameters must be known. But those parameters are well understood for silicone secondary sealant as supported by ASTM C1184 Specification for Structural Silicone Sealants and ASTM C1249 Guide for Secondary Seal of IGU for SSG Applications.

For any other sealant or spacer system, the manufacturer may have the data or physical testing of the sealant to glass may be needed. The good news is that, for any IG system, the ultimate tensile strength can be measured with a tensile tester. This is compared to the anticipated loads at point of location. Noting that a safety factor is always involved, the IG or window fabricator can then assess the risk of IG sealant breach due to failure under load and make necessary adjustments to the edge sealant specification. In this case, knowing is preventing.

Dave Cooper is a consultant and president of DJ Cooper Consulting LLC.

To view the laid-in version of this article in our digital edition, CLICK HERE.

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