Increased demand for hardwood floors, on the part of both residential and commercial customers, and the elimination of chlorinated solvents, has served as a wake-up call to providers of adhesive.
For many years in the installation community, flammable solvent-based adhesive was the product of choice because it provided the best features for a quality installation -- especially in its capacity to flash off and develop a quick grab. However, a safer system was developed that made use of non-flammable solvents.
This product rapidly replaced the flammable-solvent adhesives, and with good reason. I remember one job in which a flammable adhesive was used to install wood flooring in the brand new office of the president at a department store in Philadelphia. After finishing the job, the installer decided to light a cigarette and admire his work The result? No more office.
The new non-flammable solvent adhesives worked admirably. However, they did have disadvantages. They were not as fast setting as earlier-generation adhesives. In addition, they could be hazardous to the health of the installer and, indeed, created an environmental problem by contributing to the depletion of the atmosphere's protective ozone layer. In fact, due to its negative effect on the ozone layer, an international agreement was instituted to stop manufacturing these particular chemicals for our industry.
The question arose as to what would or could replace the non-flammable solvent product. The answer was urethanes, which had already been in use in the ceramic tile industry. Initially, wood floor installers were somewhat reluctant to accept urethanes because their use required a redefinition of installation skills. For one thing, urethanes did not have the quick-grab characteristics or "legginess" associated with solvent-based adhesives. In addition, urethanes, once cured, were difficult to clean from the surface of the wood.
Wood is hygroscopic, which means it is sensitive to moisture exposure. I've seen severe wood floor cupping in residences owned by people who travel for long periods of time and fail to take into account that the indoor humidity and temperature must be consistent, lest the floor suffer. Because a wood floor installation is a beautiful addition to most any home, it behooves the dealer/contractor to not only check the concrete slab for moisture content but also takes the time to take a look around the exterior of the home. Often, one will find an existing situation that allows water penetration from a lateral source (see photo 1).
In recent years, there have been attempts to replace old non-flammable solvents with new types of non-flammable solvents, but they've yet to garner overall acceptance among the installation community.
Aside from the 100% urethane and non-flammable solvent-based products, water-based adhesives that contain acrylic or styrene butadiene rubber have been developed. Some of these are suitable for installing wood flooring over poured gypsum, as are urethanes. Poured gypsum underlayments have always been a concern for the floor covering industry.
The main concern of which you should be aware is the fact that wet-set water-based adhesives are formulated to lose water. Although a certain amount is lost to evaporation, some of this water will be absorbed by the substrate and wood flooring. This absorption can produce end lift in the wood planks or strips. To compensate for this, a certain degree of "tackiness," or rebondability, has been built into the adhesive formulation. There are other water-based flooring adhesives, similar to the familiar cutback and thin-spread tile adhesives, that allow the flooring to be installed after the adhesive has reached a dry-tacky consistency.
The primary difference between 100% urethanes and other adhesives is that the former variety does not contain water that can be absorbed by the wood and thereby promote end lifting and cupping.
There's one more point I should add about wood floor adhesives and installation. In many areas, inexpensive vinyl is used as a moisture barrier between the hardwood and substrate. When used to install wood flooring, some solvent-based adhesives may cause plasticizer migration from the vinyl. This results in softening of the adhesive.
On the other hand, when water-based adhesives are applied over a vinyl moisture barrier, all of the water in the adhesive migrates into the wood floor, rather than going into both the wood and the substrate. The presence of the vinyl also slows the development of adhesive grab and tack which, in turn, can result in more-than-usual end lifting and cupping.
In any event, do not lay building paper underneath the wood flooring as a moisture barrier. If you do, any moisture penetration of the slab will either disbond the paper or, in some cases, attack the paper and asphalt -- which will create black stains that destroy the beauty of the finished floor (see photo 2).