Caulks, sealants and adhesives

Walk into the aisle of any building supply store, and you’ll see a wall of tubes of “caulk.” They range in price from 99 cents to more than $7 each. Why so many choices? And why the big price range? Well, first, it’s not all caulk. You are looking at selections in three categories of products: adhesives, sealants and caulks, and each is engineered for distinctly different tasks. Plus, some are greener than others, if you know what to look for.

Adhesives: The term adhesive is the broadest product category, and it actually includes subcategories of construction adhesives, sealants or caulks. What makes these products different? Construction adhesives demonstrate very little movement capability when bonding. 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 you want when sealing joints between materials that expand and contract with heat and cold, moisture and dryness. Caulks are positioned between these extremes, with some movement capability and some adhesive capability.

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 water or air rushes in. So sealants are really adhesives 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 percent and contract 25 percent. Thus, in this instance, the total dynamic movement capability would be 50 percent. Class-50 (best in class) can expand 50 percent and contract 50 percent for a total dynamic movement capability of 100 percent.

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 a static gap filler. Caulks actually are adhesives that have been chemically modified to offer some adhesion and some elasticity, just not as much as sealants. Where you don’t need a sealant’s elasticity, such as with interior trim, you can pay less for a caulk.

Five Types

Adhesives, 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 get 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, 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 carbon dioxide. They can be green choices too, if they don’t contain high levels of solvent additives, which often enhances the product’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 few years.

4 Silicones typically have a good modulus of elasticity (it can be deformed by force and completely recover) and thus are more elastic than 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 typically are not paintable. A “paintable silicone” product has been modified with non-silicone 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; many are solvent-free and, therefore, very green options. QR

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