Bridge the gap

Applying sealants to gaps and joints takes more than a strong wrist and a steady hand. Successful gap-filling starts with selecting the right sealant for the job, continues with proper surface preparation and application, then finishes with proper sealant curing, say sealant experts.

Select the proper sealant
“The first step is identifying whether you need a sealant or an adhesive. If you are filling a gap between two pieces of substrate and expect the sealant to be flexible, you need a sealant,” says Steve Sicree, senior chemist at OSI Sealants. “If you want the substrates and the joint to resist movement, you need to use an adhesive.”

Sicree says that contractors and specifying engineers can err by selecting the wrong type of sealant or expecting too much from it. “When selecting a sealant, look at the joint width and the amount of movement you expect. Sealants are rated on the width of the bead and the percent of flexibility it can offer at that width. You must consider the amount of expansion and contraction the joint will have at extreme temperature ranges,” he says.

Sealants used in commercial construction may often be specified by the architect or engineer, however, the contractor often has final say on brand and type. Generally, commercial construction-grade sealants must meet ASTM C-920 or Federal specification TT-S-00230. These standards set the minimum performance requirements for sealants used in commercial construction, says Ray Heck, construction market manager for GE Sealants.

Sealant selection is very much performance-based. The experts recommend you answer the following questions to select the best sealant for the job:

1. What is the substrate?
2. Inside or outside application?
3. How much joint movement is needed?
4. Above or below ground?
5. What is the ambient and surface temperature at application?
6. Does the sealant need to be paintable or match the substrate?

Contractors often choose sealants based on what they have used in the past or what is the lowest-cost product for the job, says Heck. New formulations are being introduced, so it’s a good idea to review what’s available before placing your order.

Generally, sealants are a member of one of the following types of chemical families. Like any family, each has strong points, weak points and idiosyncrasies that can make it a very good — or very poor — match for the job at hand.

Polyurethanes: These reactive- or chemical-curing compounds require solvent cleanup but have excellent flexibility and good to excellent durability. Some must be applied to a primed surface, but in general they adhere well to a variety of surfaces. They shrink little while curing and can withstand traffic once cured. Even with heavy traffic, the surface may degrade but the compound will still maintain an effective seal. They tend to be more expensive than other sealants.

Solvent-based sealants (as well as some water-based sealants) release volatile organic compounds (VOC) during the curing process, which has been cited as a source of air pollution. California will likely restrict the use of products with VOCs by the end of 2002; some experts predict tougher VOC rules will follow on the federal level.

In general, polyurethanes should be applied at temperatures above freezing. Once cured, polyurethanes have limited paintability and may react with alkyd- or oil-based paints.

Silicones: Silicones are also reactive-curing compounds that gun easily. Their jelly-like consistency makes them hard to tool for a nice finish, but they cure quickly and will cure at below-freezing temperatures. Once cured, they are translucent, very flexible, extremely durable (however, they tear easily) and resist ultra-violet light degradation. They maintain their properties over a wide temperature range. They may be tinted but can’t be painted. They are hard to remove completely, especially on porous surfaces, which can make repairs and future painting difficult. New silicone will not even stick to old silicone, so proper surface preparation is the key to successful silicone sealant repairs.

Modified silicone polymers are the closest compounds to a “one-size-fits-all” sealant. They adhere to vinyl and glass better than acrylics, yet can be painted. Unlike other silicones, they are not translucent; they are white or have been colored to match the substrate. While not widely used in the United States, they are used extensively in other parts of the world due to their wide application range and durability.

Acrylics: These are the most common types of sealants and come in a variety of formulations that affect cost, durability and adhesion. They are easy to gun and because they are water-based, they clean up easily. Typically, the higher the cost, the greater the performance you can expect.

Acrylics can not be applied in wet or freezing conditions or at temperatures over 100 F. All acrylics are paintable, and no primer is needed before application. Many come in colors to match the substrate, eliminating the need for painting.

Vulcanized acrylics and urethane-fortified acrylics are at the high end of the acrylic family in cost and performance. They can be used where a silicone or polyurethane sealant may be used.

Alkyds and rubbers: These are solvent-based sealants that can save the day under adverse conditions, such as wet substrate. The sealant properties come from the rubber or alkyd base that solidifies as the solvent evaporates from the sealant. They are difficult to gun and tool, and need solvent cleanup. Although relatively low in cost, their performance varies greatly in flexibility and durability.

Application tips
Even the best sealant can’t do its job if it applied to a substrate that’s covered with oil, dirt or other types of contaminants, says Eric Muench, sealant marketing manager at the SIKA Group. “Concrete should be sanded, blasted, water-blasted or wire-brushed; other base materials may only require washing. The substrate should be clean and dry.”

Joint size is also critical. The sealant should not be applied when the joint has expanded or contracted considerably because of temperature. “Look at the percent of movement the sealant can take and compare it to the joint size,” suggests Sicree. “For example, if you have a 1/2" joint and the sealant has a 25 percent movement tolerance, the joint must be within 1/8" of its normal width when the sealant is applied to assure it seals.”

If the joint is greater than 1/2" deep, Muench says it’s important to use a backer rod in the joint. “Use foam material so the gap is no greater than 1/2" deep."

The backing material maintains the optimal sealant thickness and prevents the sealant from making a three-point bond which can lead to sealant cracks, Muench says.

Muench suggests applying a sealant when the area is not in direct sunlight. He recommends cutting the tube’s tip slightly smaller than the width of the gap. “Run a continuous bead whenever possible. Keep the bead slightly ahead of the tip to eliminate air pockets or overlap that can leave an imperfect joint,” he says.

Once the sealant is in place, it must be tooled to get the proper shape and to work the sealant against the sidewalls of the joint for proper adhesion. “Don’t use a solvent when tooling. Use a dry blade. The tooling blade should be the same width or slightly narrower than the gap width,” Muench says.

Because silicones and polyurethanes are more difficult to tool, Heck recommends using masking tape on either side of the joint if aesthetics are extremely important.

Take time to cure
It’s not a good idea to have painters follow too closely behind the sealant applicators. The curing process depends on the sealant and the ambient temperature. Check the label for proper cure times before painting. “If you have any questions about the sealant, don’t hesitate to call the technical services number for help,” says Sicree. Or, your sealant distributor can also help you find the answer to your questions.

Published in the September/October, 2002 issue of Contractor Tools and Supplies magazine.

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