Part 2 - Silicone Sealant Products
Typically, the deciding factor on which type of silicone sealant product to specify is based on the application, i.e., what types of materials are being sealed and what type of use.

New Construction Applications
• General weatherseal applications. For sealing common building materials, (concrete products, metals, etc.) general purpose sealants with the following characteristics are typical:

• Ultra-low-modulus (+100 percent expansion /-50 percent contraction) joint sealant best for porous substrates and high joint movement locations.

• Medium modulus, (±50 percent expansion and contraction) general weatherseal sealant.

• Higher modulus, lower cost Sealant (±25 percent expansion and contraction) for less critical general weatherseal applications.

• Concrete sealant (±50 percent expansion and contraction) - a primerless product providing adhesion to most porous substrates, specifically concrete joints.

• Primers. There are many types of primers including film-forming primers and chemical treatment primers. The correct primer must be used appropriate to the application. Refer to the sealant manufacturer's guidelines for compatibility with both the sealant and building material types.

• Sensitive substrate applications. The single greatest negative voiced against silicone sealants is that they can stain a building. Staining is caused by non-reacted plasticizers or fluids in the sealant that can leach out of the sealant after it cures. Every sealant formulation, whether silicone or organic, is different. Any poorly formulated sealant can stain. Therefore, if staining is a concern, require that the sealant manufacturer test its sealant on an actual sample of your project's materials in accordance with ASTM
C-1248. You should ask for the results and a non-staining warranty from the manufacturer since silicone sealants that are specifically formulated for sensitive substrates have been introduced to the market. These sealants have no non-reacted ingredients and have been found to be non-staining on even the most porous marbles. Additionally, some sealants generate a surface during cure that imparts a smooth matte finish that helps minimize dirt pick-up by the sealant. This reduced dirt pick-up surface minimizes streaking on adjacent substrates, such as metal panel systems, and keeps the building looking cleaner, longer.

• Exterior Insulation and Finish Systems (EIFS) applications. Silicone sealants have been broadly accepted for use with EIFS because silicone offers long-term durability, high movement capability and low modulus, particularly in cold temperatures. These features should be clearly specified and coordinated with the EIFS specification.

• Silicone surface coating applications. In addition to joint sealing, a broad weather seal coating over a surface is sometimes called for. A silicone elastomeric coating can be used on porous substrates such as poured or precast concrete, stucco, EIFS and brick. Silicone elastomeric coating is a water-based emulsion of silicone polymer. This product is similar to other acrylic elastomeric emulsions with the exception that the polymer is silicone instead of acrylic. Like acrylic elastomeric coatings, silicone elastomeric coating is 50 percent solids by weight and can be easily applied by roller, brush or spray. The coating is applied to a dry film thickness of 10 mils. Being water-based, the coating is VOC compliant. A silicone elastomeric coating, like a silicone sealant, offers long-term silicone flexibility. Since silicone is also inherently very permeable, water vapor is allowed to easily permeate the coating, allowing the building to breathe, while the silicone provides a tight seal against liquid water.

The world's first four-sided silicone structural glazing project. The metal ornamentation is for decorative purposes only, and does not provide structural support. Courtesy of Dow Corning Corporation

 

Finally, silicone is also inherently fungus and mildew resistant which is particularly useful in high-humidity environments.

• Parking structure applications. Silicone sealants designed especially for parking structures are available for use in expansion joints, control joints, cove beads and vertical joints. These joints experience high UV exposure and high movement. Silicone sealants perform best under these conditions. Silicone sealants are available in two-part fast-cure formulations for dynamic joints on parking decks. For other joints, one-part
self-leveling or non-sag sealants may be appropriate.

• Non-structural glazing applications. Glass and glazing sealing such as glass-to-glass butt joints, silicone cap beads from the glass to the window frame, and internal seals in the window system are often overlooked but are often very critical to weather sealing. Many glazing systems provide for a window drainage system that can effectively control water when it enters the window system. The key is to be sure that such water control systems don't allow water or air infiltration to move into other parts of the building. Silicone is the best product to provide long-term window drainage system performance and guard against unwanted infiltration.

• Structural glazing applications. In structural glazing, silicone plays a different role and is used primarily for its strength as opposed to its weatherseal properties. In structural glazing, the sealant acts an adhesive, adhering the glass to the framing structure of the building. The sealant must be strong enough to allow windload stresses on the glass to be transferred to the structure but it must also be flexible to allow for differential thermal movement between the glass and metal and the sealant must have a long life. For these reasons, silicone sealants are the only sealants that can be used in structural glazing. It is common, then to specify such structural glazing sealant either directly with the glazing specification or in a separate section devoted to the specific needs of this type of system.

• Protective glazing applications. As a result of events like hurricanes, protective, laminated glazing has grown in popularity. New code requirements often dictate that glass openings remain intact during such events. To meet these requirements, a silicone sealant is often used to retain the laminated glass during missile impact and cyclic wind loads on the windows. Numerous window systems that use silicone sealants and laminated glass have successfully met these requirements. Further, the use of silicone sealant to anchor a protective film or laminated glass into a framed opening can help prevent flying glass debris from entering or falling from a building and injuring people. Similar to structural glazing applications, the specification for such sealant may be with the glazing or in a separate specialty section.