How to Choose a Sealant That Works

This is part of an ongoing series. Read all the Sticky Business posts here.

Photo: DAP

When selecting a sealant, these properties are typically the most important:

• movement tolerance (rated by Class per ASTM C920, with 25 meaning joint movement of 25% of the linear width dimension of the sealant bead)
• substrate compatibility
• workability, particularly based on temperature
• paintability and its converse—substrate staining
• relative cost
• service life
• material constituency and hazardous content

There are seven basic types of liquid sealants, largely based on their chemistries and subsequent strengths and limitations. Each sealant’s suitability to an application is based primarily on its performance properties, the properties of the substrates, and cost.

Latex

Latex sealants are water-based, easy to tool, easy to clean up, paintable, and relatively less expensive than other types of sealants. Some premium latex sealants may be appropriate for exterior use (appropriate service life) and are rated for movement in classes 12½ and 25. Latex sealants may be best suited to interior finish applications.

Acrylic

Acrylic sealants are also paintable but are solvent-based and more difficult to tool. They are used more in commercial and exterior applications than latex and have very limited movement capacity (Class 7½). Acrylic sealants tend to be used in commercial construction in low-movement joints. Their cost tends to be in the low to moderate range.

Butyl

Butyl sealants are solvent-based, synthetic rubber materials demonstrating strong adhesion to a wide variety of substrates. They have excellent weathering characteristics but tend to be stringy and difficult to apply. They generally have limited movement accommodation (Class 7½ ). Butyl sealants are sometimes used in curtainwall systems where adhesion to rubber materials is required. The cost of butyl sealants tends toward the moderate range.

The next group are sometimes called “high-performance” sealants and are most often used in commercial building assemblies.

Polysulfide

Polysulfide sealants are particularly water- and chemical-resistant but do not tolerate much cyclic movement for a high-performance sealant (Class 12½ –25). Their use in buildings is most common in swimming pools and other locations where submersion must be tolerated. Polysulfide sealants often require a primer. They tend to be relatively expensive.

Silicone

Silicone sealants are used in a wide variety of building applications because of strong performance characteristics: UV resistance, temperature resistance, highest movement capability (Class 50–100), generally longer service life, and continued flexibility over time. Silicone sealants can have a strong odor and take considerable time to fully cure. They can be used structurally in glass assemblies. Cost for silicone sealants is in the high range. Pure silicone sealants are not paintable.

Polyurethane

Polyurethane sealants are tough—even abrasion-resistant. Unlike silicone sealants, they can be painted. They have excellent adhesion and good movement capability (Class 12½, 25, and 50). They can be stiff and more difficult to apply and tool than silicone and cannot be used in structural glass assemblies. As one of the “high-performance” sealants (including polysulfides and silicones), polyurethane caulks are relatively expensive.

“Hybrids” – MS Polymers

Hybrids are relative newcomers to the sealant world; they have chains (silyl) that modify both silicone and polyurethane sealants (MS stands for silyl-modified), combining some of the strengths of each. Their chemical profiles are better because they are solvent- and isocyanate-free (more on this topic in the last blog of the series).

So which one should I use?

As with most building materials, the answer is, “It depends.” The information above and in our BuildingGreen Approved product guide should help a lot to match the best liquid sealant to the substrate, the application, and your budget.

But what about the other two properties: service life and material constituency? You will have to come back for more on these. The next post is about sealant service-life prediction, and the series will wrap up with material constituency considerations for both sealants and tapes.