What is polyurethane sealant?

Written by andy pasquesi
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Used on wood, masonry, concrete and steel, polyurethane sealant waterproofs joints and surfaces while also adding protection against scuffing. While the original polyurethane sealants were hydrocarbon-based, a new generation of aqueous, latex-based polyurethane products are now available in stores.

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Chemistry

"Polyurethane" is actually an umbrella term used to describe any polymer linked together by a carbamate group—"N-(C=O)-O"—between each monomer unit. These carbamate linkages are formed by reacting an isocyanate (i.e., an organic molecule with multiple N=C=O functional groups) and a polylol (i.e., a monomer with two or more hydroxyl groups) in the presence of a catalyst. Water molecules (H2O) are formed as a byproduct of this reaction as well. The specific properties (and subsequent applications) of a particular sealant arise from the type(s) of isocyanate and polylol compounds used. In fact, many sealant formulas use a mixture of different isocyanates.

One-Component vs. Two-Component

Polyurethane sealant is sold both in ready-to-use, single-component formulas or two-component formulas where the user must mix two products manually to start the polymerisation reaction. Most popular for residential use, single-component polyurethane sealant comes in packaging especially designed for caulking guns. Applied in a bead along joints or spread across small surface areas with a paintbrush, this type of sealant begins to cure (i.e., form an internal matrix of urethane polymers) as soon as it is exposed to air. The rate of curing usually depends on the amount of catalyst (or inhibitor) in the formula. Meanwhile, two-component polyurethane sealants are typically used by professional builders, carpenters or experienced hobbyists. Unlike single-component products, these sealants begin to cure only when mixed together. This allows users to apply the first component over large areas without worrying about accidentally disturbing the ongoing curing process. Rather, curing doesn't begin until users add the second component on top of it.

Waterproof

As an organic, rubberlike compound, polyurethane naturally repels liquid water. Similarly, polyurethane sealant effectively resists corrosion from both acidic and alkaline environments. At the microscopic level, polyurethane's nonporous structure also acts as a barrier against water vapour, i.e., humidity.

Durability

In general, polyurethane demonstrates far more elasticity than other sealant materials. This allows the solid polyurethane to distribute the force from a collision throughout its entire mass, which lowers the net force at the site of impact. Accordingly, polyurethane tends to retain fewer permanent scuffs or marks. Elasticity also allows polyurethane to better withstand seasonal temperature changes. Like most sealants, polyurethane expands when its temperature rises and shrinks when its temperature decreases. However, because polyurethane is so elastic, the tension from repeated expansion and shrinkage doesn't cause it to crack.

Adhesion

Polyurethane sealant will adhere to almost any building material, so long as the surface has first been thoroughly cleaned of dust, sealant, loose cement, paint, water and any other solvents. However, for certain metals or sealant formulas, you may need to first apply a primer onto the surface. In this case, be sure to use only the primer specified on the sealant's printed instructions.

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