Going the Distance: What to Know About Ensuring Sealant Durability

By Jordan Scott

Sealants play a major role in maintaining the effectiveness of glazing systems, which is why durability is vital to ensuring a maximum service life for these systems on a building. Several factors can impact a sealant’s lifespan, including its chemical composition, joint design and substrate preparation.

Sealant Type

The durability of a sealant can depend upon the technology used. There are several different types of sealants, including hybrids, polyurethanes and silicones. These fall into organic and inorganic categories, with silicone being inorganic, meaning it’s not sensitive to UV-radiation.

“By its chemical nature, it has high durability and resistance,” says Florian Doebbel, business development manager of façades for Sika of Lakewood, N.J. “Organic sealants have a limited lifespan.”

While organic sealants have advantages when used with the appropriate materials, Doebbel says that silicone is often preferred for glass applications.

“A material with limited resistance or sensitivity to UV degradation on the surface would lose adhesion,” he explains, adding that 50-year-old structural glazing projects with silicone sealants are still performing well today.

Kevin Zuege, senior director of tech service and marketing for the Industrial Products Group at Tremco Inc. of Beachwood, Ohio, also describes silicones as the gold standard among sealant types for durability and resistance to aging, with hybrids right behind them.

Jon Kimberlain, senior technical services and development scientist with Dow, based in Midland, Mich., says that silicones are similar to glass when it comes to preventing UV degradation.

“Materials such as urethanes or polyethers that are based on carbon can be susceptible to damage. Silicones are similar molecularly to glass. When sunlight hits glass it typically passes through or reflects,” he says. “Carbon-based materials such as urethanes and polyethers are like us. When sun hits us it accumulates and, over time, will damage our skin.”

For a sealant to qualify to be used for structural glazing applications it has to be tested at a variety of temperatures and other accelerated aging tests such as water immersion and UV radiation. If a sealant passes these tests he said a company can ensure that it would have at least a 25- to 30-year surface life.

In addition to qualifying the sealant material itself, there are also project-related tests which involve the sealant in combination with the glass substrate and framing system.

“It’s important to make sure the material will perform appropriately on a certain surface,” says Doebbel.

System Design

A glazing system’s joint design also plays a factor in its service life. Doebbel says the joint needs to be protected and not exposed to a wet environment, because stagnating water can corrode the sealant, coating or even the substrate over a long period of time. An engineer should also verify the geometry of the joints to ensure the thickness can accommodate the movements of the application, which can include sway, creep, settling and windloads.

“Is the joint bite big enough to take over loads acting on the system with an appropriate level of safety?” asks Doebbel.

Kimberlain explains that most weather seals have a rated movement tested to ASTM C719 – 14(2019), Standard Test Method for Adhesion and Cohesion of Elastomeric Joint Sealants Under Cyclic Movement (Hockman Cycle).

“The sealant needs to be designed in a way so that when that windload transfers from the glass to the frame, it’s not going to overwhelm the tensile property of the sealant,” he says.

Kimberlain recommends that a structural engineer be involved in the design of the joint to ensure allowable stress is factored in.

Zuege explains that many common joint designs call for a 4:1 ratio, which would allow for +/-25% movement.

“If there will be a ¼-inch of movement then the joint needs to be four times larger than the movement itself, which would lead to a 1-inch joint. Some really high performance materials require +/-50% movement,” he says, adding that the introduction of stretchier sealants has allowed for narrower joints and the use of 3:1 to 2:1 joint design since they can expand and contract without degrading.

Application Preparation

User error is another reason for sealant failures in the field. It’s important for an applicator to be educated on the use of quality control measures defined by the sealant manufacturer. Kimberlain describes workmanship as a critical aspect of sealant durability.

Doebbel points out one example of human error could be a two-part sealant not mixed with the correct ratio which could compromise the sealant. Another example is the application of an incorrect primer or not applying the sealant with the proper amount of depth.

The substrate also needs to be cleaned properly so the sealant doesn’t attach to dirt instead of the substrate. Kimberlain explains that how quickly a dirty sealant bond fails can depend upon the environment.

“In Arizona, where there are not a lot of major weather events, you might not detect it for some time but in Miami where there’s high humidity you might detect it quickly because the sealant is being exposed to different loads,” he says.

Zuege says that first it’s important to wipe the substrate’s surface with a clean, dry cotton cloth to remove dust or dry film from the product. He also emphasizes that substrate temperatures matter as porous material absorbs moisture. If it’s below freezing, the sealant could be adhering to the frozen water instead of the substrate itself.

Knowledge of joint design and proper sealant application can prevent most infield failures, according to Zuege, who says that if there are issues they usually become noticeable after the sealant is exposed to seasonal highs and lows.

“The best way for an applicator to ensure durability is to get in touch with the manufacturer about the project application and use the experience and knowledge the manufacturer has on products and sealants. That’s the best way to get a durable application and reduce any risk of failure,” adds Doebbel.

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