Distinction Between Organic Sealants and Silicone Sealants
Among sealants, there are two broad categories of sealant chemistries:

• Organic sealants use a chemical polymer backbone consisting of carbon-based or organic polymers
(-C-C-O-C-C-). Examples of organic sealants include single and multi-component polyurethane sealants, polysulfide, acrylic and silyl terminated polyether or more commonly MS or modified silicones. These sealants do not contain silicone polymer.

• Inorganic sealants are the other type of sealant. An inorganic sealant does not have a carbon based or organic backbone, thus is a non-carbon-based polymer (-Si-O-Si-O-Si-). Silicone is an example of an inorganic sealant because it uses a polymer which links silicon and oxygen atoms. This is called siloxane
or silicone.

This photo shows the negligible impact of weathering on a silicone sealant (left) versus the degraded impact on an organic polyurethane sealant (right) after 3000 hours of accelerated weathering, which is equivalent to 3 to 6 years of actual outdoor weathering. Photo courtesy of Dow Corning Corporation

Why is sealant chemistry important? It comes down to how the chemistry of each type of sealant performs on a building when it is exposed to UV rays from the sun. In the presence of UV light, a carbon based polymer or organic sealant will change properties and degrade over time. The Si-O bond of a silicone polymer, on the other hand, is stronger than the rays of the sun and will not degrade over time. There is not enough energy in ultraviolet (UV) light to degrade the Si-O bond of a silicone sealant.

Polyurethane sealants are one kind of commonly used organic sealants. They typically exhibit desirable initial properties in terms of adhesion, modulus, and movement and are readily available. However their durability over time is limited or even weak in that they degrade noticeably in the presence of UV light found in sunlight. This degradation may take several forms. In most cases, the sealant gets stiffer, reducing it's modulus as it ages. Since joint movement continues to occur in the building however, the sealant commonly fails cohesively, or tears within itself. As the polyurethane continues to harden, the sealant may also put more stress on the substrate material, causing adhesive failure of the sealant, or substrate delamination or spalling. The surface of the polyurethane sealant can also chalk, craze or crack. These surface cracks can propagate through the sealant and cause it to fail. A form of polyurethane sealant failure common in the 1990s is called reversion. In this instance, the polyurethane sealant initially cures and displays normal sealant properties; but after exposure to UV light, the polymer reverts to an uncured sealant state. The material appears to turn into chewing gum in the joint and no longer performs as a flexible sealant.

Drawing Coordination

Typical "hourglass" shaped joint showing material substrate on either side filled first with a non-adhering backer rod then with sealant that fills the joint cavity with a slightly recessed front. Note that the sealant adheres only to the substrate material. Courtesy of Dow Corning Corporation

Joints in building materials that require sealant installation are typically shown on drawing elevations, plans, and details. It is important, therefore, that the joint design is properly thought through and that sealant details are shown on the drawings. Basic guidelines for the design of weatherseals are reviewed in such places as ASTM C-1193 and other industry documents. The most common and best performing joint design uses an hourglass-shaped sealant profile. This joint shape is effective because the sealant depth of half of the joint width reduces stress on the sealant. The hourglass shape allows for maximum contact with the building materials along the sides of the joint. This reduces stress on both the sealant and the building material. In all cases, there should be at least ¼-inch contact on each joint surface in a moving joint. And the sealant should be installed to allow adhesion to only the two sides of the joint and not the third side, namely the back of the joint. Backer rod or bond breaker tape can be used to prevent such three-sided adhesion.

Other common joints include: fillet joints, where two perpendicular joints abut each other; bridge joints, which are typically used for restoration over failed weatherseals; and double weatherseals, commonly used in modern commercial construction.

Specifying Silicone Sealants

Using the basic background information above, many architects are choosing to specify sealants predominantly or even exclusively made from silicone. Using the standard "three part" format, such a specification will need to address the following information.

Part 1 - General Requirements and Standards
Putting the specification section in context, the points below focus the sealant work of a construction contract on high quality, long term performance.

• Coordination with other trades. Sealants are used to fill the gaps between other materials. It is worth noting and cross− referencing the specification sections that address those other building materials such as concrete, aluminum window frames, masonry, Exterior Insulation and Finish Systems (EIFS), etc.

• LEED and green building coordination. Sealants need to be made from low amounts of or no Volatile Organic Compounds (VOC) in order to comply with LEED requirements for indoor air quality. In addition, a properly sealed building helps to assure that the projected energy performance is actually achieved.

• Quality Assurance. Material and manufacturing quality assurance is based on manufacturing information and warranties. Installation is based on the skill and experience of installers. It is appropriate
to specify requirements for both the material and the installer, therefore, to be sure that a quality job is produced.

• Testing standards and references. The relevant standards come from ASTM and SWRI. (See the on-line portion of this article for more information.)

• Pre-construction compatibility and adhesion testing (also may include mock-ups). Due to the critical weatherization nature of sealants and their long term requirements, it is more than appropriate to specify testing of the sealants before they are installed in the building. Testing should be done on joints of the actual building materials being used and witnessed by the specifier.

• Submittals. Product information should indicate full compliance with all specification requirements. Substitutions or use of "or equal" organic sealant products should be disallowed since they are not equal in terms of long term durability.

• Warranty. Manufacturers' and installers' warranties are reasonable to specify for at least 20 years.