Chapter 2 – Product Characteristics

Joint sealants are usually given generic names by combining the name of their material or chemical composition with some major characteristics. The generic product types listed below are the most common, but there are also many unique chemical compositions made by only a few manufacturers. Note: Although there are many types of joint sealers, this review covers joint sealants only -- gunnable and self-leveling material of mastic consistency that sticks to each side of a joint.





Generic Sealant Chemical Compositions

The following sealant types are arranged in roughly descending order of cost -- roughly, because it is very difficult to obtain cost data from manufacturers. Key differentiating properties are listed for the generic group as a whole; individual proprietary products may differ. No generic type is suitable for continuous immersion in liquids unless specifically stated. ASTM C920 is the minimum standard unless otherwise indicated.

Silicone Rubber:  Elastomeric sealant suitable for exterior use; longest lasting; wide range of movement capabilities; one of a few types with the greatest movement capability (up to +100/-50%); fewer self-leveling or traffic grade options than polyurethane. Mostly single-part, moisture-curing; acid-curing type is incompatible with some glazing adhesives. Drawbacks include 1) a tendency for airborne dust to stick to the surface until washed off by rain, leaving streaks on the wall below, and 2) some tendency to stain porous substrates, particularly marble and other stones. Both drawbacks have been reduced in newer formulations -- look for "reduced dirt pick-up" and "non-staining".

Silyl-Terminated Polyether/Polyurethane:  Elastomeric sealant suitable for exterior use; newer formulations than silicones or polyurethanes, so life span has not been seriously tested; high movement capability; reportedly lower cost than 100% silicone. These are two chemical formulations and manufacturers' literature sometimes doesn't say which. They are also often referred to as "hybrid" without any mention of chemical composition. Mostly single-part, moisture-curing. They appear to have the "reduced dirt pick-up" and "non-staining" characteristics of newer silicones as described above.

Polyurethane/Urethane:  Elastomeric sealant suitable for exterior use; shorter life span than silicone due to lesser UV resistance; wide range of movement capabilities (up to +100/-50%); one of the few types with the greatest movement capability; both non-sag and self-leveling and traffic grade widely available. Both single-part, moisture-curing and multi-part, chemically-curing. Many products available for continuous immersion in liquids. The terms polyurethane and urethane mean the same thing. These don't typically have the drawbacks mentioned for silicone.

Polysulfide Rubber:  Elastomeric sealant suitable for exterior use; longest history of use; only medium movement capability; both nonsag and self-leveling with traffic grade available. Polysulfide sealants are less widely available because they don't have the higher movement capability of silicones or urethanes. However, polysulfides continue to be made because they typically have better resistance to chemicals or solvents and can withstand continuous immersion in liquids. 

Epoxy:  Weather resistant but its main use is for low movement joints that must be filled with a relatively hard material for durability, such as concrete slab control joints, and for tamper resistance. Two-part, nonsag and self-leveling, limited color range (one manufacturer refers to it as "non-aesthetic"). Good chemical resistance. High traffic resistance.

Polyurea:  Self-leveling, fast curing floor joint filler; low movement. Usually two-part, chemically-curing.

Butyl Rubber:  Solvent-based, non-elastomeric sealant suitable for exterior use; history pre-dates all elastomeric sealants; low movement capability, but better than most acrylic latex; nonsag type and non-traffic grade only. ASTM C1311 is a minimum standard. "Non-skinning" formulation is permanently tacky, for concealed applications only, and typically used where very low movement is expected. Besides low movement capability, the main drawback is that it's solvent-based, emitting a lot of VOC's during curing.

Acrylic Emulsion Latex:  Suitable for interior use; typically not recommended for exterior use or interior wet conditions; lowest movement capability; least expensive. ASTM C834 is a minimum standard. As a water-based product, this contains no VOC's as defined for VOC content and it will revert to paste form if wetted. It can be painted, which most other sealant types cannot. "Siliconized" formulations have slightly better movement capability and slightly better water resistance.

Sealant Properties

Expected Service Life:  Service life means 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. Silicones are reputed to last 20 years, polyurethanes less, and most other types much less.

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 do (except for non-curing applications), but sometimes the cure type is relevant because it affects other characteristics, notably VOC emission and compatibility with other materials.

  • Solvent release sealants cure by the evaporation of solvents -- usually referred to as VOCs (see VOC content) (limited to solvent-based butyl).
  • Water-based sealants cure by evaporation of water (limited to acrylic emulsion latex and its hybrids).
  • Chemically curing sealants cure by the combination of chemical compounds. There are two sub-types:
    • Multipart sealants combine two or more synthetic materials just before application.
    • Some single component sealants absorb moisture from the ambient air to combine chemically (usually referred to as "moisture curing").
  • Non-curing sealants are deliberately designed to never cure. They are also usually described as "non-drying" and "non-skinning." They stay tacky and malleable and are usually intended for completely concealed moving joints and connections.

VOC Content:  Volatile organic compounds 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 VOC's that occur in construction and are regulated by law and international convention. Solvents occur in some sealants (see Cure Types). VOC content in uncured sealants is regulated by the federal government, states, and regional air management districts. In some cases, independent regional commissions develop rules that are adopted by states (e.g. OTC).

VOC Emissions:  Unlike VOC content, emission measurements are made on the installed product, after curing.  For instance, LEED v4 includes credits for low-emitting products that require both low emissions and low VOC content. The low emissions testing is newer and seldom mentioned in product literature.

Consistency:  Sealants come in two consistencies: 'non-sag' and 'self-leveling' ('pourable'). Vertically oriented joints require non-sag sealants, so the sealant will not run down out of the joint. Horizontally oriented joints can use either non-sag or pourable sealants but pourable will yield better looking results with less effort. 'Non-sag' is the term used in ASTM C920 (Grade NS). 'Pourable' is the term used in ASTM C920 (Grade P) for self-leveling sealants -- the sealant is poured into the joint and levels itself under gravity. Polyurethane sealants are usually available in both consistencies; silicones have fewer self-leveling options; acrylic latex and butyl are non-sag only.

Suitability for Exterior Exposure:  This is a combination of low temperature resilience, UV and ozone resistance, and heat aging resistance. ASTM C920 and butyl sealants are suitable for exterior exposure. Acrylic latex is not suitable for exterior exposure, unless not subjected to rain or freezing temperatures.

Movement Capability:  This is the measurement 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. The test method, ASTM C719, also evaluates adhesion and cohesion as criteria for the movement limits. Movement capability over 7-1/2 percent rules out latex and some butyl sealants. Movement capability over 25 percent is available but not universal in polyurethane and silicone sealants. Movement capability over 50 percent is rare. There is generally an inverse relationship between movement capability and hardness. "Low modulus" means less stress at the bond line, not necessarily a softer sealant.

Adhesion:  Suitability for a specific substrate usually comes down to adhesion. Minimal adhesion is usually taken for granted but there are variations that cannot be quantified easily. There are so many variables that affect adhesion, commercial product manufacturers recommend that sealants be tested for adhesion with each type of substrate being sealed too, even if they have been tested on the ASTM C920 standard substrates. Qualifying for Use M represents acceptable adhesion to a mortar-like substrate, Use A to an aluminum substrate, and Use G to glass. Masonry and stone substrates are particularly problematic.

Suitability for Traffic Exposure: The hardness necessary to resist puncture and tearing due to grit under traffic is generally in conflict with movement capability and requires resilience. ASTM C920 includes designations for traffic exposure (Use T1 or T2), as tested according to ASTM C661. Manufacturers' recommendations are all that can be relied upon.

Hardness:  Hardness is a measure of indentation resistance. Both ASTM C920 and C1311 specify maximum hardness when tested according to ASTM C661, which requires use of the Shore A scale (0 to 100, softer to harder). For comparison, Wikipedia says rubber bands are Shore A 25, door seals are 55, automobile tire treads are 70, and hard skateboard wheels are 98.

Unfortunately, maximum hardness is not particularly relevant as a comparison characteristic -- unless one is trying to eliminate oil-based putty or acrylic latex. Since there is a roughly inverse relationship between hardness and flexibility, if you want flexibility you get whatever hardness is the result of that formulation.  Typical highly stretchy elastomeric sealants have hardness much lower than the maximum hardness values, so the values stated in the standards have no real meaning.

Designations for Use T (traffic) (T1 and T2) are given but without minimum or maximum values. If the manufacturer lists Use T, that constitutes their recommendation of the sealant for traffic surfaces. By review of product literature, Use T sealants have hardness in the range of 40, whereas non-T sealants with movement class of 25 or above usually have hardness of 25 or less. This is borne out by one manufacturer's statement that despite Use T noted for a product, it needs to be recessed into the joint (so it's not actually impacted by the traffic) and that product has hardness of much less than 40.

Hardness is also a measure of vandal- or tamper-resistance, but the harder the material, the less movement capability it has. Hardness of 75-85 seems to be recommended by manufacturers for tamper-resistance. Continuous Immersion Durability: If the sealant must be immersed in water, not just wetted and later dried, this criterion will rule out many sealants. Assume that a sealant is not suitable for continuous immersion unless you know otherwise. ASTM C 920 Use I (the letter I, not the numeral 1) qualifies products as suitable for continuous immersion in liquids. Also, if the manufacturer does not explicitly state that the product is suitable for continuous immersion it probably isn't. More suitable sealants are available than in the past but products appear to be limited to urethanes and polysulfides.

Mildew Resistance:  ASTM C920 sealants are not tested for mildew resistance. Most sealants are not mildew-resistant -- unless the manufacturer says so. Mildew-resistant sealants are available in white and clear, usually referred to as "bathtub" or "kitchen and bath" sealants. Silicone has long been available as mildew-resistant; newer varieties may be available.

Acceptable Joint Width:  Acceptable joint width is mostly a matter of practicality.  

  • Very narrow joints simply don't have enough room to insert a backer, or if that's possible, the absolute movement (especially compression) may exceed the capability of the sealant. If the very narrow joint is actually non-moving, a pourable sealant made for joints between components of windows and similar products can be used. This is usually referred to as a "seam sealer."
  • Very wide joints are more difficult to tool to the necessary shape. Very large diameter backer rods are available, but verify -- in this case backer rod design is not trivial. Backer rods must be compressed at installation, so that when the joint expands they don't fall out. For all these reasons, most manufacturers give a joint width range -- usually between 1/4 inch (6 mm) and 1-1/4 inch (32 mm). If the very wide joints vary because of construction tolerances, many different diameter backer rods may be needed, based on field measurements, complicating the entire installation process.

Porous Substrate Staining Probability:  Sealants can potentially be made of chemical compounds whose component materials may leach out or migrate, especially into a porous substrate. This is a 'pass-fail' judgment, it either stains or not. Elastomeric products tested to ASTM C920 meet minimum non-staining requirements, but that minimum clearly doesn't eliminate silicones that are known to stain some porous stones. ASTM C1248 is cited by silicone manufacturers who claim their products are "non-staining" to sensitive stones. In addition, all commercial product manufacturers recommend that sealants be tested with all the actual substrates to be sealed, for both staining and adhesion.

Color Availability:  Color options vary by product, by manufacturer, and by ordered quantity. Custom colors usually require a minimum quantity. 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. Typically sealants should not be painted, unless explicitly stated by the manufacturer.

Dust Pick-Up:  Silicone sealants tend to 'pick up' atmospheric dust - dust sticks to the surface and is subsequently washed off by rain (usually streaking down the face of the building) - this makes it unsuitable, by some specifiers, for exterior use.

Relative Cost:  Although butyl and acrylic latex sealants are lower performing products, they are still made because they are less expensive than higher performing products like polyurethanes and silicones. When the higher performance is not required, particularly high movement capability or unusual exposure conditions, lower cost sealants are more cost effective. Other than this consideration, it is usually false economy to scrimp on sealants.  The difference in life cycle cost of re-sealing the exterior enclosure could be significant. And if the sealant fails, the cost of labor to remove failed sealant and install a replacement could be greater than the initial installation. Also consider the effects of enduring water leakage for an extended period before deciding to make the replacement.


Identify these characteristics for the product described in this sample data sheet. Suggestion:  Print both this page and the sample data sheet. Circle the characteristics and values on the data sheet and mark with a code to match the property descriptions on this page.


1. Which of the following is not a sealant cure type?

  • Evaporation of solvent.
  • Evaporation of water.
  • Non-skinning.
  • Absorption of moisture.
  • Combining one or more chemicals.

2. Acrylic emulsion latex is a solvent release type sealant. True / False

3. Siliconized acrylic is which type of sealant:

  • Silyl-terminated polyether.
  • Silicone.
  • Urethane.
  • Water-based.

4. Arrange the following sealants in ascending order of cost (based on the information in this guide):

  • Polyurea.
  • Silyl-terminated polyether/polyurethane.
  • Urethane.
  • Butyl rubber.
  • Epoxy.
  • Polysulfide.
  • Silicone rubber.
  • Acrylic emulsion latex.

5. Why is silicone rubber considered to be longer lasting than urethane?

  • Its movement capability is greater.
  • It resists traffic damage better.
  • It resists UV degradation better.
  • It is moisture curing.
  • It resists physical damage better because it is softer.

6. What is the typical Shore A hardness of an elastomeric sealant that is recommended by its manufacturer for traffic applications?

  • 0
  • 20 to 25.
  • 40 to 45.
  • 75 to 85.
  • 100

Next -- Chapter 3 - Product Selection