acrylic latex (water-based)
The characteristics listed below are arranged in order of their importance to performance, with the most important first. If you work through the list from the top to the bottom, you'll rule out products that can't do the job more quickly.
Consistency. Joint sealants come in two consistencies: non-sag and pourable.
Vertical joints require non-sag sealants so the sealant will not run down out of the joint. (Non-sag is the term used in standards; gunnable is an equivalent term.)
Horizontal joints can use either non-sag or pourable sealants, but the pourable variety will yield better-looking results with less effort—the sealant is poured into the joint and levels itself under gravity.
Polyurethane sealants are usually available in both consistencies. Acrylic latex, butyl, solvent-based acrylic, and silicone are non-sag only.
Continuous immersion durability. In a fountain or other circumstance where the sealant must be in contact with water at all times, use only products that manufacturers state are suitable for continuous immersion or which are tested to ASTM C 1247 with the minimum values specified in ASTM C 920. Polysulfide sealants are among the few varieties that are suitable for continuous immersion.
Suitability for exterior exposure. The qualifications for exterior exposure consist of low-temperature resistance, UV and ozone resistance, and heat-aging resistance.
ASTM C 920 sealants are suitable for exterior exposure. Acrylic latex is not suitable for exterior exposure, unless it is not subject to rain or freezing temperatures. Silicone sealants are eminently suitable for exterior exposure from a durability point of view, but they tend to “pick up” atmospheric dust. Because rain can cause the dust to streak down the face of the building, some specifiers do not use silicone for exterior joints.
Suitability for traffic exposure. Sealants subject to vehicular and pedestrian traffic need to have a certain hardness to resist puncture and tearing. This is generally in conflict with movement capability (see below), which requires the sealant to be resilient. Elastomeric sealants tested to ASTM C 920 Use T (for traffic) are generally suitable for traffic applications, as are other sealants recommended by their manufacturers for this use: polyurethanes (pourable types) and specialty silicone (non-sag). Products that meet other standards written specifically for pavement use are also acceptable.
Mildew resistance. ASTM C 920 sealants are not tested for mildew resistance. For “bathtub calk,” specify a white silicone sealant specifically manufactured for mildew resistance.
Acceptable joint widths. Most elastomeric sealants are limited to joint widths between ¼ inch (6 mm) and 1¼ inch (32 mm). Some polyurethanes are designed for wider joints. The problem with narrower joints is that the absolute movement (especially compression) exceeds the capability of the sealant; some polyisobutylene sealants are designed for very narrow joints. Some narrow joints have no movement at all.
Movement capability. This is the gauge of how much extension and compression the sealant can withstand without either pulling away from the sides of the joint or failing in the body of the sealant. It is measured as a plus/minus percentage of the joint width at the time of installation, tested to ASTM C 719. This test also evaluates adhesion and cohesion as criteria for the movement limits. Movement capability over 7.5 percent rules out latex and butyl sealants. Movement capability over 25 percent is available but not universal in polyurethane and silicone sealants. Movement capability over 50 percent is rare.
Adhesion. Suitability for a specific substrate usually comes down to adhesion. Minimum adhesion is usually taken for granted, but there are variations—most of which cannot easily be quantified. Polyurethanes generally have the best adhesion, followed by silicones, then butyl and acrylic. Some substrates that may be problematic include masonry, stone, and vinyl. For instance, products tested to ASTM C 920 Use M (for masonry) have acceptable adhesion on the most common porous substrates, such as concrete, masonry, and stone.
Hardness. Primarily a measure of indentation resistance, hardness is mostly used to judge whether the sealer is suitable for traffic use; minimum and maximum values are specified in ASTM C 920.
Hardness is also a measure of vandal resistance, but the harder the material, the less movement capability it has. Epoxy joint sealants are used in detention occupancies, as they are hard enough to resist picking with fingernails and plastic spoons; however, they have much lower movement capability.
Porous substrate staining probability. Sealants can potentially be made of chemical compounds whose component materials might leach or migrate, especially into a porous substrate such as stone. This is a pass-fail judgment—the sealant either stains or it doesn't. Elastomeric products tested to ASTM C 920 meet minimum nonstaining requirements. Although this is mostly intended to eliminate oil-based, putty-type caulks, most manufacturers also recommend that sealants be tested with the actual stone to be used as stone porosity varies.
Volatile compound emission. Volatile organic compounds (VOCs) evaporate easily into the air under normal conditions; VOCs that are of concern are those that irritate or impede respiration or that damage the ozone layer. Solvents and refrigerants are the two primary VOCs that occur in construction and that are regulated by law and international convention.
Solvents occur in some sealants. VOC emissions from architectural joint sealants are regulated by states and regional air-management districts. In some cases, independent regional commissions develop rules that are adopted by states.
Cure type. All joint sealants cure (change from being of toothpaste consistency to being solid) except those that are intended never to cure for some specific reason.
In principle, we usually don't care how sealants cure, as long as they perform as intended, but sometimes the cure type is relevant because it affects other characteristics, notably VOC emission and installation friendliness:
Noncuring sealants are deliberately designed to never cure; they don't evaporate anything. They are also usually described as nondrying and nonskinning. They stay sticky and malleable and are usually intended for completely concealed moving joints and connections.
Sikaflex 15lm is a polyurethane sealant that offers the same low modulus and joint movement (+100/-50%) capabilities of a silicone sealant, while offering all of the benefits of a polyurethane, including primer-less adhesion to porous substrates and paintability with acrylic coatings. www.sikaindustry.com
Color. Color options vary by product, by manufacturer, and by quantity. Custom colors usually require a minimum-quantity order. An alternative to integral color is to paint the finished joint. This is only appropriate when the type of paint used will withstand the movement, and the manufacturer says the sealant may be painted.
Expected service life. Service life is usually used to represent the longevity of the material under the conditions it was designed for. There are no tests for service life; statements of expected service life are usually based on field experience and are seldom published by the manufacturers. If you ask, though, a manufacturer's technical rep will tell you the relative ranking of the company's own sealants and estimated life span. Silicones and polyurethanes typically last 20 years or more. Most other types last less—sometimes a lot less.
Relative cost. The least expensive sealant is acrylic-latex, followed by butyl and solvent-based acrylic, with polyurethanes and silicones the most expensive. If the higher movement capability or weather resistance of the latter are not required, the former are more cost-effective.
On the whole, however, it is usually false economy to scrimp on sealants, especially when you consider the cost of labor to remove failed sealant and install a replacement (and the effects of enduring leakage for an extended period before deciding to make the replacement).
Extreme and unusual exposures. Always verify suitability with the manufacturer. For example: