How Quality Assurance Prevents Structural Adhesive Failures

By Jordan Scott

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, 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.

Quality Assurance

A third-party can be hired by the building owner, architect or construction management firm to observe the quality assurance process. Horschig says that quality assurance data is then submitted to the sealant manufacturer, who will verify and, if acceptable, issue a multi-year structural silicone glazing warranty if all requirements have been met.

If a failure does happen in the manufacturing facility, the sealant manufacturer will do testing to find out if the preparation and priming recommendations were followed correctly, according to Kimberlain, who adds that there is a paper trail due to past quality control records that can give insight into what went wrong.

“There’s also a forensics portion here when we look to see what this failure might tell us. If whoever put [the sealant] on said they used the proper sealant and did everything properly but it still pealed cleanly away then we can do testing to see if the surface changed,” says Kimberlain. “This is hard to do because, analytically, it’s like looking for a needle in a haystack. If we redo the testing and get perfect adhesion every time then maybe someone had a bad day or the primer used was out of shelf life. Those are challenging forensic situations. We can’t put an ultrasound on the sealant and come up with a reason of why it failed … We have to develop an understanding of how the failure may have occurred.”

Doebbel says it’s important for the fabricator to ensure uniform product quality over the entire span of the project. This includes the bonding surfaces.

“Significant variation of quality and properties of the coating system applied to the aluminum frame could lead to a problem,” he says. Some structural glazing projects can take years to complete, meaning there could be changes to the substrate over time. Kimberlain recommends monitoring for variability of the substrate. He says Dow recommends that fabricators develop a robust quality process, especially when it comes to structural glazing. All of this comes back to companies having a solid quality assurance process in place.

“Companies that invest time into training programs and that have a focus on quality tend to have less probability for adhesion issues,” he says. “Training your applicators on how to use the two-cloth cleaning method, which is the industry-recommended practice for cleaning the surface, having them understand the different primers and completing the proper documentation are all key aspects.”

“Applicator training is important so that all of the operators involved in the pre-manufacturing and installation are aware of common standards, application guidelines and properties of the material to implement these procedures, which required daily and periodic tests,” adds Doebbel.

Installation Considerations

“The sealant must fully fill the glazing pocket where the structural silicone is to be applied. If the glazing fabricator does not fill that depth of the joint and does not tool the sealant, whether through a semi-automated process or by hand, they may not achieve the sealant ‘bite’ dimension that was approved in the shop drawings,” says Horschig.

For a sealant manufacturer to offer a structural glazing warranty on a structural silicone, the fabricator must deglaze a unit once it is fully cured to verify adhesion to the aluminum and glass substrates and that the depth of the sealant is consistent for the unitized unit, according to Horschig. He adds that the fabricator needs to wait until the unit is fully cured and adhesion has developed, noting that adhesion development is the last performance characteristic to develop.

“Due to construction schedules there’s a desire to move units as fast as possible to the jobsite … Although a sealant is cured, adhesion development continues to occur,” he says, adding that it’s important to verify with the sealant manufacturer the cure-adhesion development profile for the specific sealant for the specific project substrates involved.

Horschig encourages fabricators to deglaze units in accordance with the sealant manufacturer’s warranty requirements until they confirm acceptable cure and adhesion.

“Deglazing allows the fabricator to check the adhesion on the frame and glass as well as the mixing quality and hardness of the cured material as part of the original unit,” adds Doebbel. “We can compare if it complies with the shop drawings and verification report for structurally required joints. It’s a useful process control and a very important step to ensure the highest level of quality.”

Doebbel says that if a failure is observed in the field, deglazing is difficult to realize. The glazing contractor can perform a load test, which involves applying a load to a unit and measuring the deflection of the system. This test is done on several units to find any variation in deflection, which could be an indicator of a partial failure of the adhesive joint.

Another important thing for fabricators and glaziers to keep in mind is that no rubber accessory can come into contact with the structural silicone sealant that could cause it to fail. The sealant manufacturer must perform the compatibility test outlined by ASTM C1087 – Standard Test Method for Determining Compatibility of Liquid-Applied Sealants with Accessories Used in Structural Glazing Systems.

Verification Opportunities

There isn’t a standardized methodology certified by ASTM for the verification that adhesion is still intact on existing buildings. However, as it relates to the jobsite retrofit or re-glaze, Horschig suggests that the glazing contractor engage the glass fabricator, sealant manufacturer and/or independent glazing consultant if required to confirm the best practices for the repairs.

“[They] can use this opportunity to re-verify performance of sealant adhesion. Those are rare but are opportunities to take advantage and learn. In my almost 40 years I have rarely, if ever, been called out to an existing building to perform a deglaze,” he says. “If conducting any glazing system replacement, the glazing contractor should contact the sealant, glass and aluminum manufacturers to review the proper deglazing and re-glazing procedures so as not to damage the glazing components.”

Doebbel says that currently there are no established ways to monitor
the adhesive properties of a unit on a completed building without degrading the silicone.

“It’s much more difficult to make a proper investigation after completion when a building is already in service. That is a weak point of contemporary façade design,” he says. “Many applications are concealed and we rarely have access to the adhesive joints. It’s hard to inspect them without touching the façade system itself to create some access to the joint, especially while the building is occupied. The building owner wouldn’t like that too much.”

Like Horschig, he recommends taking advantage of opportunities such as those presented between change of ownership or after a natural disaster when other repairs are being made to review the structural glazing elements. Doebbel also suggests determining early in the design phase how to monitor the façade and how to access it to repair structural glazing elements in case problems are observed.

Pushing the Envelope

Larger lites of glass have led to more complexities when it comes to the structural silicone sealant. Because the glass is larger, the transfer of the windload to the sealant could require that more sealant be used. If more is needed, the fabricator needs to have the proper tools to apply a larger amount of sealant correctly to still get the proper bite dimension and performance. Horschig says a joint oversized insulating glass task group has been formed by the National Glass Association’s Fabricating Committee, in part, to answer questions about which engineering changes need to happen to ensure safety and successful adhesion development.

It’s not just larger glass lites and complex façade designs that the manufacturer and fabricator need to keep in mind, but new developments in coatings as well. Doebbel says that new materials and types of coatings can also change requirements and parameters for the bonding application.

“The glass and glazing industry is challenged to bring new innovations and develop new systems. All of that interacts with the performance requirements of the structural glazing system,” he says.

Jordan Scott is an assistant editor for USGlass magazine. She can be
reached at jscott@glass.com.

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