The new adhesives available today are considered high tech, and require that instructions be followed or failure will result. The first consideration should be what flooring product is to be installed and which adhesive is required by the manufacturer of the product. Most adhesive applied wood flooring involves either engineered products or parquet patterns. Some manufacturers of the thin, less than 1/2-inch thick, solid linear products list adhesive application as a method for installing the product; check with the manufacturer. NOFMA does not recommend adhesive application of the unidirectional thicker solid wood products.
The following is a generic list of the adhesives most commonly available for application. The newer types are: pressure sensitive adhesives, which includes those containing rubber, acetate, and acrylic blends; acrylic, modified acrylics, acrylic urethanes; moisture curing urethanes; and cross linking resin blends. Older types are the PVA emulsions and asphalt-based adhesives. The pressure sensitive adhesives have a high water component, and thus require "flash" time or time for the excess water to evaporate before the adhesive is ready for the flooring.
If an occasional piece becomes loose, it is possible to weight the area and re-stick the flooring. The acrylic, modified acrylic, and urethane acrylic types of adhesives are generally considered wet lay adhesives, meaning they require the installer to place the flooring into the adhesive without a delay or flash time. These adhesives also have a small water component, and thus products sensitive to water absorption, particularly thin factory finished solid flooring, may show the effects by cupping. Since the water component is small, evaporation rates can affect the performance of the adhesive. The environment is critical; low humidity can set the adhesive too fast; conversely high humidity can extend the working window. Too much activity, such as too many swirls when troweling, can also affect the evaporation rate and thus the working time. Most engineered products can be installed over these adhesives, but always check with the manufacturer.
Moisture curing urethanes have no water component and only a small amount of solvent. They are a good choice for the thinner, less than 1/2-inch thick, solid materials and factory finished materials, which can be affected by the upper range of acceptable moisture levels. Application of a continuous film skim coat before the actual adhesive application may be considered a moisture retarder; check with the adhesive manufacturer. The urethanes generally require some flash time if "green grab" strength is desired. They are sensitive to environmental moisture, and will set faster in high humidity and more slowly with low humidity. The adhesive has an elastic factor which allows the wood products some natural seasonal movement without repositioning and associated gaps. Improper solvents for clean-up can leave residue that may not be removed from the flooring, particularly the factory finished products. PVA adhesives have been around for many years. They have a very high water component, and when set, are very strong. The PVAs are a traditional choice for unfinished thin slat paper-faced parquets. Since water is required to remove the paper, water equally affects both the face and back and cupping is not a problem.
Since the individual slat is small,l the expansion from water absorption is also small; in addition, the product is filled during finishing effectively eliminating the related gaps. This is a wet lay product that requires 100 percent adhesive-to product-coverage. The asphalt mastics, commonly called "cold stick" or "cut back" adhesives have been around the longest of the adhesives. The have mineral spirits as a vehicular component and thus are flammable. Generally considered a weak adhesive, they are very adequate to install the 3/4-inch thick parquets. Parquet systems with an asphalt adhesive may still be encountered that are performing very well and are more than 50 years old. They are only used for installing tongue and groove flooring.
If the flooring is not tongue and groove, then oil-based finish can contact the adhesive and bleed back at the joints, which is almost impossible to stop. These mastics, when applied, must flash off to reach their grab strength, which can be up to eight hours. Unique among the adhesives, they can also be reactivated by misting with mineral spirits. Since they are high in VOCs, they are not available in some areas and have restricted use in others. The asphalts are listed as the adhesive component in vapor retarding systems, which use two layers of #15 felt or polyethylene adhered to a slab.
Job Site Conditions for Adhesive Application
The subfloor or slab should be flat; most adhesives will tolerate a gradual variation of up to 3/16-inch in 10 feet or 1/8-inch in 6 feet. Do not use adhesive as a leveling compound. The moisture content of the substrate should be checked and should not be excessive. Will slabs with a high pH affect the adhesive? For many manufacturers, a pH greater than 10 will affect adhesive performance. Since pH is product specific, check with the manufacturer. Is the slab surface sound enough to perform with the adhesive? Lightweight concrete may fail if the bond of the mastic is stronger than its compressive strength. The adhesive may not bond if the slab surface is dusty or fractures easily. Is the environment correct, i.e. near the occupied condition? High humidity and cool temperatures can create surface conditions close to dew point. Condensation and or a high moisture content of the slab surface can adversely affect the adhesive. Is the job site too cold or to hot? Environmental temperatures directly affect the performance and work-ability of the adhesive by affecting the flash time, open time, working time, and set up and or cure time. The recommended jobsite conditions should all be listed on the instructions of the adhesive’s container.
Trowel Configuration and Use
Depth, spacing, and shape of trowel notches directly impact the performance of the adhesive. They determine the rate of spread, the rate of solvent evaporation, and the amount of adhesive-to-product contact. The spreading technique, angle of the trowel, normally at a 45- degree angle, also affects the spread rate; a flatter angle decreases spread and the height of ridges and a vertical angle may spread too much (Photo 1). The spreading pattern, excessive combing and swirling, can over expose the adhesive to air and allow too much solvent evaporation, shortening the working time and or setting the adhesive before product placement (Photo 2).
Other Adhesive Issues
Should the installer work on or off the flooring? Is it recommended to roll the flooring after installation? These can affect the transfer rate both positively and adversely; follow instructions. And finally, there is the clean-up. It is important to consider what specifically to use for clean-up that will remove adhesive residue and not impact the appearance of the flooring. This is very important with factory finished products.
Following manufacturer's instructions and recommendations will ensure that the products perform properly, and when problems do arise, the support of the manufacturer.