As we mentioned in Part I of our sealant blog series, the importance of sealant is often undervalued. It is a small, but crucial, part of the bigger picture when constructing a building’s exterior. In order to avoid sealant failure, start by selecting the appropriate sealant for the job. Karen Warseck, AIA and president of Buildings Diagnostics® Associates, wrote an insightful article about joint sealants and their role. Her approach to sealant selection is outlined below.
Understand the proper sealant terms
It is imperative to understand the descriptive terminology of sealants or the selection process will be difficult and most likely end in failure. What makes this so difficult? The main cause is manufacturers may use imprecise descriptions of their products. The terms “elongation”, “modulus,” and “performance” are used interchangeably, to the confusion of anyone attempting to evaluate the differences and similarities of the multitude of sealant products and formulations available.
Setting clear definitions of terms is the first step. For example, elongation defines the length to which the sealant can stretch and modulus defines the tensile strength of a sealant at a given amount of elongation. These terms must be clearly understood in order to correctly choose either a high-modulus or a low-modulus sealant.
Where to use a high or low modulus sealant?
High-modulus sealants are best when high strength is required and little movement is expected. These sealants are best used in situations where the strength of the adhesive is this highest priority. Low-modulus sealants have a propensity to be easily torn or punctured and are used where movement capability is a priority. An example would be weather seals on metal curtain-wall buildings where both high thermal and high wind movement capacity are required.
Choose your performance
The performance of the sealant correlates to joint movement and the sealant’s capacity to elongate and recover. There are three options when it comes to your sealant’s performance level: low, medium or high performance. In order to determine the appropriate performance level, be familiar with how much joint movement is expected as well as the capacity of the sealant to elongate and recover.
When a sealant is stretched, it may or may not return to its original shape, which is known as stress relaxation. Other sealants experience compression set, which means the sealant remains bulged out after being compressed and do not easily stretch out again. In both situations the sealant has developed a “memory” which will eventually cause joint failure. The sealant should be able to properly recover to its original form after any stretching or bulging.
Sealant and substrate compatibility
Avoid incompatibility between the sealant and the substrate. Improper sealant can cause staining or etching of the substrate and loss of sealant adhesion.
“Other symptoms of incompatibility problems include disintegration, discoloration or hardening of the sealant. When any chance of incompatibility is present, the architect should request that the manufacturer test the actual components to be used in the assembly to determine possible detrimental effects before construction begins,” Warseck said.
Suitable for the performance environment
The sealant must be appropriate for the environment in which it is expected to perform. Consider the environment characteristics of resistance to puncture and vandalism, ultraviolet resistance, chemical resistance, abrasion resistance, ability to withstand extreme weathering, ease of access for application and repairs, reaction to continued submersion and ability to resist dirt pickup due to slow cure or dust attraction.
Failure to consider these factors may result in adhesive failure, cohesive failure, craze cracking, hardening, disintegration, color changes, or other types of permanent failure.
Not all sealant brands are equal! Each project requires its own careful sealant consideration and application. Never assume that one formulation of sealant can substituted for another.
Stone Panels recommends to all of their clients is to have their sealant supplier conduct accelerated aging tests to determine if there might be bleeding of the sealant carrier into the stone, and the sealant supplier should also recommend a primer that won’t bleed into the stone. The customer should require a sealant warranty for bond to stone and also a warranty against staining from the sealant supplier.