Complex façades and oversized glass are defying the limits of what many believed impossible thanks to advances in engineering and some creative thinking. Structural adhesives and sealants are among the key materials making these feats possible. It’s vital they function properly for the safety of those within the buildings, as well as passersby on the sidewalks below.
Adhesive and sealant manufacturers, fabricators and glazing contractors all play a major part in ensuring that the materials will not fail once installation is complete. This process requires diligent preparation and sample testing.
What Could Go Wrong?
Adhesive manufacturers all say the same thing: the number one cause of failure, which is usually caught before installation during the quality assurance process, is that full adhesion did not occur. Adhesion could fail to develop for several factors, such as a surface prepped incorrectly, incorrect primer application, incompatible components degrading the sealant, incorrect bite dimensions and more.
“The most common kinds of failure are caused by incorrect application or improper surface preparation of the bonding substrate,” says Florian Doebbel, business development manager of façades for Sika, based in Lakewood, N.J. “Whenever we discover these kinds of failures in a project, it usually happens during the application or installation before the building is handed over to the owner, during the construction phase, basically. There are standards in place to mitigate failures related to product performance and workmanship.”
Jon Kimberlain, technical services and development scientist with Dow, says that if a silicone was designed to hold a certain load but the load was underestimated, the strain could be too much. The adhesive could pull away from the substrate or break within itself.
However, this is unlikely due to rigorous design reviews and the capability of the sealants and structural adhesives. Structural silicone sealants are performing well above the ASTM C1184 requirement of 50 psi, according to David Horschig, senior glazing systems executive for Tremco Inc., based in Beachwood, Ohio. He says that many structural silicone sealants are consistently manufactured and tested to perform well above the industry-accepted tensile strength of 50 psi.
“As manufacturers, we can control the performance characteristics of the sealant. However, for the application, we can’t control the human or equipment-assisted factor,” he says. “The application concerns relate to surface preparation, priming, installation and the quality assurance verification processes. That is typically outside of the manufacturer’s control.”
Prepping the Surface
To ensure that a failure doesn’t occur, the sealant manufacturer, fabricator and contract glazier must all complete several quality assurance tests.
“Because of the high risk of structural silicone glazing there are well defined quality assurance procedures that must take place to ensure performance,” says Horschig.
He explains that in order for fabricators to understand the proper surface preparations necessary, they are requested by the sealant manufacturer to submit samples of glass and aluminum with any coatings or finishes that will be used on the specific project of record. The samples cannot be from the showroom and if a finish or coating changes they will need to be resubmitted. The sealant manufacturer will then run the ASTM C794 – Standard Test Method for Adhesion-in-Peel of Elastomeric Joint Sealants in its lab to determine whether the substrate requires a primer or not. Once the test is complete, the fabricator then follows the manufacturer’s sealant surface preparation recommendations for that sealant application.
“After that it’s up to the fabricator,” says Horschig, who adds that they will need to clean the surface properly using the two-cloth cleaning method. This involves applying an approved solvent with a lint-free cloth and then wiping it clean, leaving no residue.
“A lint-free surface is important. You don’t want fibers to contaminate the area. If the sealant application area isn’t cleaned properly there could be contaminates that would prevent the sealant from developing acceptable adhesion,” says Horschig.
If a primer is required, Horschig explains that it must be applied according to manufacturer instructions. These instructions often involve ensuring proper coverage and allowing the primer to cure completely before applying the sealant.
“If there isn’t proper coverage and some areas are left unprimed, it could create a weak link for sealant adhesion development, resulting in increased stress on the properly installed areas and, in some cases, causing the sealant under design windload conditions to exceed the industry and manufacturer’s allowable strength criteria,” he says.
This article originally appeared as part of the feature titled, “Without Fail.” To read the full article, which originally appeared in the October 2019 issue of USGlass magazine, click here.