By Joe Hostler and Eric Wymer
In order to have a successful wood flooring installation, it helps to understand the types of wood flooring adhesives and how they work. This requires an understanding of both chemistry and bonding; here's a short primer.
Essentially there are three types of adhesive chemistries: modified silicone polymers, polyurethanes, and acrylics. Each has its place, and in order to understand how they work, you must first understand this: An adhesive can create a chemical or mechanical bond. A chemical bond is when molecule A combines with molecule B to form a new molecule, C. A mechanical bond consists of molecules interlocking but not chemically changing. Both types of bonds can be quite strong.
Modified silicone polymers (or MS+), which are relatively new, are considered hybrids. They create a mechanical bond with wood and thus penetrate into the cell structure of the wood. However, should adhesive end up on top of the finished wood, the outcome is much different. The MS+ will not bond well to this coated surface and will clean off easily. This gives MS+ adhesives excellent long-term cleanability.
With concrete, MS+ products form a chemical bond. MS+ polymers are unaffected by water when cured. They are also typically zero-VOC and very ecologically friendly. In my opinion, they are the future of wood flooring adhesives (full disclosure: we sell all three adhesive types).
Polyurethanes combine with the lignin (complex chemical compounds found in wood that form parts of the cell structure) to create a chemical bond. Polyurethanes create a mechanical bond with the concrete. They are enormously strong and form an elastic bond that has allowed the industry to bond ¾-inch solid wood to concrete. These adhesives are unaffected by water when cured. The biggest drawback is that a polyurethane adhesive will form an incredibly strong chemical bond with the polyurethane finish on top of the wood flooring. This makes trying to clean that spot on top of the floor you didn't notice during installation an absolute nightmare, so they must be cleaned when wet. Polyurethanes contain diisocyanates, are low-VOC and generally contain solvents.
Acrylic adhesives usually consist of polyvinyl acetate emulsions. (They are also sometimes called "latex," which is a term for natural and synthetic rubber resin materials, so the correct term is "acrylic.") Acrylic adhesives are highly filled polymer dispersions in water. The typical curing occurs when the water leaves the dispersion with the help of solvents. The polymers coalesce, or fuse, creating a particle entanglement or matrix. Acrylic adhesives are highly susceptible to moisture and usually require a flashing off period prior to floor installation. They clean off easily with water because of this, but will fail easily in a wet environment. When installed properly, they are an effective, proven option for engineered floors. Unlike the other two adhesives, which are waterproof at full cure, acrylic adhesives are always susceptible to dilution from water or water vapor. On the plus side, acrylics have a rebonding capacity: Contractors can weight a section of floor and have it bond after the rest of the floor was installed. Acrylic adhesives generally are low-VOC and contain solvents.
You don't need to be a chemist to understand how adhesives work, and understanding them is fundamental to any successful installation. One thing is clear: Demands for innovation and ecologically friendly or green adhesives will continue to drive the industry.
What to Use
Here's a quick guide from the authors for which adhesives are usually recommended for which types of wood flooring products:
¾-inch solids: Polyurethanes
¾-inch shorts: Polyurethanes, MS+ Polymers
5/16-inch solids: Polyurethanes, MS+ Polymers
Engineered plank and strip: Polyurethanes, MS+ Polymers, Acrylics
Parquet: Polyurethanes, MS+ Polymers, Acrylics