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Match the Tube to the Task: The Difference Between Adhesives, Sealants, and Caulks.

2/3/2014 4:06:01 PM
Article by John Wagner

Adhesives, sealants, and caulks are engineered for distinctly different tasks. (That explains why one costs $.99 and the next $6.99.) And some are greener than others. Do you know the difference?

Adhesives: We all know that adhesives bond two surfaces together. The term “adhesive” is the broadest product category that includes subcategories of construction adhesives, sealants, or caulks. What makes these products different? Construction adhesives demonstrate very little “movement capability.” Little movement capability is what you want when gluing down wood floors, for instance, or when affixing granite countertops to a wood base. Sealants on the other hand exhibit the highest movement capability, which is what you want when sealing joints between materials that expand and contract with heat and cold. Caulks are positioned in-between these extremes, with some movement capability and some adhesive capability. Let’s look closer.

Sealant: When sealant is applied to substrates that expand and contract (wood, metal, glass, stone, plastic), the sealant must move along with it. Otherwise the bond is broken, and in rush water or air. So, sealants are really adhesives that are chemically modified to allow movement. The tech term for this is “dynamic movement capability.” The capability is expressed on the label as “Class.” A “Class-25” sealant can expand 25% and contract 25%. Thus the total dynamic movement capability would be 50% in this instance. Class-50 (best in class) can expand 50% and contract 50%, with a total dynamic movement capability of 100%.

Caulks: This same Class rating may be used on caulk labels, too. And those that don’t feature a Class distinction typically are not engineered for anything beyond use as a “gap filler,” so buyer beware. Caulks are adhesives that have been chemically modified to offer some adhesion and some elasticity, just not as much elasticity as sealants. Where you don’t need a sealant’s elasticity (e.g. interior trim), you can pay less for a caulk.

Five Types

Sealants, and caulks fall into these general chemical “technology platforms.”

  1. Water-based products tend to have good adhesion and good movement capability. Most are acrylic latex, a kind of plastic or synthetic polymer chemically dispersed in water. There is no basis for the myth that water-based products are necessarily low-performers. But they can’t tolerate getting wet when freshly applied. These products tend to be the greenest choices, because they often contain low or no solvents, and hence fewer volatile organic compounds (VOCs) which can cause smog and respiratory problems. (The EPA, CARB, or SCAQMD VOC limits are the standards most manufacturers cite in their labels when they claim they are green.)
  2. Polyurethane products grab tenaciously to nearly anything, much as they do to your skin and clothes. Polyurethanes are highly paintable. They don’t dry, they cure, like concrete. For pure polyurethane products, nothing flashes off as they cure, except CO2. They can be green choices too, if they don’t contain high levels of solvent additives, which often enhance the sealant’s ability to “bite” into a substrate. In general, many green polyurethane sealants are in the VOC range of water-borne sealants, so look for acceptance by LEED, Green Globes, or NAHB.
  3. Solvent-based products contain rubber dissolved in petroleum derivatives like xylene and benzene. When solvents flash off, you get VOCs in the air. These are your least-green choices. In fact, look for these products to be phased out entirely in the next decade.
  4. Silicones. Silicones typically have a lower modulus of elasticity (it can be deformed by force and completely recover) and thus are more elastic than most polyurethanes. Silicones do not have the great adhesion of urethane, but silicones place less stress on the bond line (where adhesive meets substrate) and therefore may be the optimal option for your construction project. However, silicones are typically not paintable. A paintable silicone product has been modified with filler that accepts paint. (The fumes you smell are from a solvent chemically identical to vinegar.)
  5. Hybrids offer the best of silicone with the best of polyurethane: Good elasticity, good gripping power, and paintability. Hybrids also cure rather than dry; man are solvent free, and therefore very green options.
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