Sealing the Deal
by Brooke Smith | 1 May 2024 7:00 am
In metal construction, caulks and sealants serve the crucial function of sealing gaps and cracks. Their role as a shield against air, water, moisture, gas, noise, dust, and smoke is vital, despite typically constituting less than 1 percent of a building’s total cost. This small investment is key to ensuring the building’s resistance to water and air penetration. Therefore, choosing the right type based on their specific properties and intended applications is essential for maintaining the integrity of the building’s exterior.
The role of caulks and sealants
Caulks are commonly used to fill gaps that undergo minimal expansion or contraction, often as a preparatory step before painting. They are characterized by their rigid and inflexible nature. Within metal construction, caulks find their application in sealing spaces between drywall, windows, trim, or casework on the interior, prior to the application of paint.
The term caulk originates from boat building and is often used by manufacturers to describe acrylic materials intended for applications involving little to no movement. Although these materials can be painted with either water- or solvent-based paints, they are not commonly used in metal construction. This is due to their tendency to shrink and crack over time. Manufacturers typically refer to their more flexible sealing products as “sealants.”
Sealants are crucial in metal construction, serving as a bonding agent between metal and various external materials. They create a protective barrier that prevents the ingress or egress of moisture, air, and airborne particles.
In situations involving the expansion, contraction, or movement of metal components, specially formulated sealants play a key role. These products, designed with flexible polymers, are essential in metal construction for tasks such as structural glazing and creating flexible, watertight seals at the seams between two metal substrates.
In metal construction, sealants find a broad spectrum of applications, encompassing everything from standing seams, end laps, and roof penetrations to curbs, expansion joints, transitions from roof to wall, steps and height variations on roofs, ridge expansions, gutter seams, and much more.
Metal panels present unique challenges for sealants due to a couple of key factors. First, metals undergo expansion and contraction with temperature changes, resulting in dynamic movement at the joints. Additionally, the longevity of common industry paints and surface coatings means that bonding effectively to some specialty coatings can be difficult, and maintaining a strong bond over time poses its own set of challenges.
Understanding silicone sealants
Silicone sealants excel in accommodating joint movement, adhering to ASTM C920 standards for Classes 25, 35, 50, and 100/50, with capabilities of up to 50 percent movement. They are highly effective in bonding with a wide range of common building materials, such as glass, aluminum, wood, steel, vinyl, and masonry. Nonetheless, for optimal adhesion on certain substrates, especially those that are cement-based, the application of a primer is advised.
Research conducted by major manufacturers on the long-term effectiveness of silicone sealants suggests they can maintain their performance for more than 20 years, demonstrating resistance to moisture, oxidation, high temperatures, and UV exposure. However, their use in high-traffic areas is not advised due to potential surface degradation from abrasion. The application temperature range for silicone sealants far exceeds that of other polymeric sealants, often ranging from as low as -29 to 60 C
(-20 to 140 F), and they also feature extensive service temperature ranges from -40 to 204 C (-40 to 400 F).
While most silicone sealants cannot be painted, there are modified versions that are designed to be paint-compatible. It is important to note that silicone sealants might leave stains on porous materials like concrete and certain types of natural stone. One of the advantages of silicone sealants is their straightforward application and cleanup process. These sealants are non-flammable and are usually composed entirely of solids, meaning they do not contain solvents, which allows them to comply with environmental regulations regarding volatile organic compounds (VOCs).
Pre-cured silicone sealants, also known as silicone membranes, are frequently used across a range of metal building applications due to their exceptional movement capability, exceeding 200 percent, and distinct characteristics. For instance, they are particularly effective in areas of high movement within metal buildings, including expansion joints, transitions from roof to wall, changes in roof height, connections between new and existing structures, ridges, and various repair scenarios. These pre-cured sealants are applied to metal and other materials with a distinct adhesive, eliminating the need for any fasteners.
Historically, there have been concerns regarding the compatibility of certain silicone sealants with metals like Galvalume and galvanized steel. These concerns were primarily associated with acetoxy (acid-based) cure products, which are now obsolete in metal construction applications. Instead, neutral cure silicone sealants have been employed in metal construction for many years, demonstrating no corrosion issues with these types of metals.
Exploring polyurethane sealants
While not as widely used as silicones, polyurethane sealants provide excellent joint movement capabilities and adhere well to a wide range of common building materials. In the metal building sector, polyurethanes typically come as one-component, moisture-cure sealants that are formulated to skin over and cure quickly. They are often specified due to their superior UV resistance and long-term durability, making them a premium option for various applications.
Builders are advised to choose a polyurethane sealant that offers at least 50 percent joint movement capability, which equates to +/-25 percent. The suitability of a polyurethane sealant can vary based on the type of surface it is applied to. Given the range of today’s specialized coatings, not all polyurethanes will bond the same to every surface. Some are capable of adhering to wet surfaces or even underwater, while others might need a primer or some form of pretreatment, depending on the substrate. High-quality polyurethanes are valued for their flexibility, with life expectancies that can exceed 20 years, especially when not exposed to severe conditions. They cure to form a tough, durable, and elastic consistency with excellent resistance to cutting and tearing. These sealants are available in various colors, and most types are paintable.
Metal building contractors often request polyurethanes due to their wide service temperature range of -40 to 93 C (-40 to 200 F) and their ability to elongate between 500 to 600 percent. Polyurethanes are offered in gun grade (cartridges) and can be either non-sag or self-leveling, making them versatile for various applications. They are also easy to shape for a visually appealing finish. However, it is important to note when polyurethanes are applied too close to neutral cure silicones, they can inhibit the polyurethanes’ curing process. This issue can be avoided by ensuring that either product is fully cured before the application of the other.
Polyurethanes generally show good compatibility with metal and masonry surfaces. However, it is advised not to use them in structural glazing applications that involve contact with glass. These sealants can be engineered to offer better than average UV resistance and might be suitable for painting. It is important to note that some polyurethane formulations contain solvents, so shrinkage resulting from solvent evaporation should be considered. Some health professionals often recommend the use of respirators during their application.
Introduction to solvent-based and butyl sealants
The most frequently encountered solvent-based synthetic rubber sealants are acrylic. These are usually applied in perimeter sealing or in applications where there is little joint movement. Special handling may be necessary due to their flammability and environmental impact. Solvent-based sealants are known for their durability and can be applied even at temperatures below freezing. While they can be painted, this is only advisable after they have fully cured, which takes between seven and 14 days. In their wet state, they are flammable, may be challenging to tool due to their short open times, and can emit a noticeable odor during application and curing.
Butyl sealants, known for their non-skinning and non-drying properties, are primarily used in standing seam roofs and the joints of insulated metal panels (IMPs). Their formulation allows them to remain soft and flexible, providing a positive seal for roof or wall panel joints. Ideal butyl sealants have a non-stringy consistency and allow for easy cutoff, facilitating a clean application process.
Butyl sealants are well suited for application into the female leg of standing seam roof panels and are compatible with various paints and coatings used by rollformers. This type of sealant does not cure in the same manner as typical pumpable sealants, allowing for flexibility, self-healing properties, and a lifespan that can equal that of the roof system itself. For application, the temperature should range between -12 and 49 C (10 and 120 F), while the service temperature span is quite broad, from -51 to 93 C (-60 to 200 F). Given its non-curing nature, this sealant cannot be painted, and is supplied in white or off-white. It is designed to be used alongside mechanical fasteners for effective installation.
Hybrid sealant considerations
Hybrid sealants, such as modified silicone (MS) (silyated polyether) and SPUR (silyated polyurethane), combine the best features of both silicone, like UV resistance, and polyurethane, such as paintability. While their application in metal construction is relatively limited, their usage is gradually increasing.
Typical hybrid sealants are designed to endure expansion and contraction of up to
+/-50 percent. The lifespan of these hybrids is generally quite long, and they excel in both appearance and UV resistance. They are often paintable, which can be useful for addressing any discoloration that might arise from long-term exposure to the elements. Additionally, these sealants are user-friendly in terms of application and shaping, and they emit low to no odor.
Achieving optimal results
When choosing caulks and sealants, it is important for contractors to assess all performance attributes to find the best balance between quality and cost. Consulting with the manufacturers about specific sealant requirements is essential to identify the most suitable product for each application, as no single product fits all scenarios. Further, the compatibility of the sealant with the intended surface may necessitate different curing methods.
Stick to success: Keeping metal panels sealed tight
Butyl tapes are commonly used for sealing the side and end laps of standard single-skin panels. These tapes are made from non-curing solids compound, creating a highly rubbery and tacky sealant that remains permanently flexible. Designed for easy handling and application, butyl tapes come in rolls or strips equipped with a removable release liner. They are soft and pliable, meant to be compressed between overlapping panels to ensure a tight seal. Exhibiting elongation capabilities greater than 1,000 percent, butyl tapes can last for more than 25 years and are compatible with all types of painted metals, including those coated with Galvalume, Zincalume, and Kynar. They are suitable for use in a wide range of temperatures, from -20 to 49 C (-5 to 120 F) for application and -40 to 93 C (-40 to 200 F) for service, making them versatile for all climates. Despite their excellent UV resistance, they are not typically meant to be exposed directly to sunlight. Some butyl tapes also contain antimicrobial additives to prevent mold and mildew growth.
With files from Tremco Inc., Novagard, Franklin International–Titebond Adhesives and Sealants, and ITW Performance Polymers.
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