While most types of polymer adhesives require some amount of active participation from the end user (mixing, heating, etc.), moisture cure adhesives offer an interesting, passive cure alternative. This blog post is dedicated to explaining this particular type of cure mechanism through the scope of the two most common examples, polyurethanes and silicones.
Other cure mechanisms:
- Cationic UV Cure Epoxy
- Heat Activated Adhesive Systems
- Free Radical Polymerization Of Polymer Adhesives
Moisture Cure Polyurethanes
First lets understand a traditional polyurethane reaction. In traditional two-part polyurethane systems, isocyanate terminated molecules are mixed with hydroxy (hydrogen bonded to oxygen) terminated molecules and the two are allowed to react (usually with help from heat or a catalyst). In this addition reaction, the isocyanate group will essentially attach directly to the oxygen in the hydroxy group, replacing the hydrogen. Assuming we combine these molecules in the correct ratio, the final reaction product will be a solid polymer material.
A moisture cure reaction is actually very similar. Water has the molecular formula H20, which means water has two hydrogen bonded to a single oxygen molecule. Just like in the reaction described above, each of these hydrogens is a potential reaction site for an isocyanate group. In this way, two isocyanate terminated molecules can basically be bridged together by a single water molecule and as they bridge together they will form a polymer structure. What makes moisture cure systems so useful, however, is that this reaction can proceed from water vapor in the air. As a result, no mixing is required. Simply apply the one-part liquid system and let the reaction proceed.
It should be noted, however, that moisture cure systems have their limitations. Because they rely on airborne water vapor in order to cure, they won't full cure in sections too deep. Curing sections deeper than a 10mm will result in an incomplete cure in the section farthest from the surface.
Condensation Cure Silicones
The condensation polymerization of silicones is also facilitated by airborne water vapor, however, the actual reaction mechanism and reaction products are slightly different. The biggest difference by far is that these types of condensation reactions can have nasty reaction by-products (acetic acid, ketoxime, methanol, acetone). These by-products can damage or impair electronics or other types of sensitive bonded components. For this reason, condensation cure silicones are generally not used in electronic applications.
However, similar to the polyurethane reaction described above, condensation cure silicones do not require mixing and can be applied as a one-component material. These systems are also limited by depth and will have an incomplete cure in sections deeper than 10mm.