Determining the exact cause of an adhesive-related complaint is a never-ending struggle. The construction and concrete industries insist that the cause of most bonding and moisture complaints is due to the fact that many adhesives are now water-based as the result of formulation changes made to decrease their content of volatile organic compounds (VOCs). The standards for allowable VOC levels in adhesives were established in the 1990 Montreal Protocol.

The real fact of the matter is that these reformulated adhesives, once cured, actually perform better than their solvent–based counterparts of the past. In fact, some of the adhesives that were around prior to 1990 are still used today. And the problems associated with the old adhesives are the same for new adhesives.

The installer’s adhesive-related responsibilities

Selection.The installer must be sure to choose the proper adhesive for both the substrate and flooring material. In many cases, even though the manufacturer specifies the suitable adhesive, the installer or flooring contractor uses another manufacturer’s adhesive formulation. This practice can create considerable consternation if a problem arises.

Flooring manufacturers do not want their products installed with someone else’s adhesive. The flooring manufacturer that produces adhesives, or has adhesives private labeled for their products, will not warrant their adhesive for use with a different supplier’s flooring material. The reason is simple: they don’t want to assume the liability (even though their adhesive may perform adequately with another manufacturer’s product). So, whoever makes the decision to use an adhesive that is not specified must be sure that the product will be warranted for that use by the adhesive manufacturer — or accept liability for his decision.

Mixing. Imagine what happens to a bucket of adhesive that travels hundreds of miles on a truck through varying temperature extremes, and is subjected to constant vibration while in transport. Then, ask an installer how many times he or she has gotten down to the bottom of the bucket of adhesive and found a hard layer of substance that could not be spread.

All adhesives must be thoroughly mixed. The mixing process blends the solids and liquids within the adhesive that have started to separate during transport and storage. Some formulations, over time and agitation, allow the solids in the mixed adhesive to settle to the bottom of the bucket. When an installer finds unspreadable solids at the bottom of the adhesive bucket he’s been using for an installation, what does that say about the quality of the adhesive that was applied prior to his discovery?

Trowel notch selection. The trowel notch represents the most important variable in the adhesive application. The trowel is really little more than a device that regulates the amount of adhesive that’s applied to the substrate surface. Yet a trowel is the single most misused tool in the installer’s toolbox. To properly determine which trowel to use, the installer first must consider the substrate’s condition. For instance, is it porous or non-porous? Is the backing of the floor covering material absorptive or non-absorptive? The next factor to consider is the adhesive manufacturer’s recommendations.

The shape of the notch (square, “U” or “V”), along with its width, depth and spacing, plus the angle at which the trowel is held, all play critical roles in the amount of adhesive film that’s left behind. The square notch allows for application of the most adhesive, while the V notch allows the least.

The amounts of adhesive allowed to pass through the notches, whatever their shape, is further regulated by the angle at which the installer holds the trowel. Changes in the width of the spacing also determine the amount of adhesive applied by working in concert with the notch to allow for uniform coverage and transfer to the back of the material.

For example, if the spacing width is changed from 1/16 to 3/32 of an inch, approximately 20% less adhesive will be applied to the substrate. Use of a trowel with notches spaced too far apart is the leading cause of trowel notch show-through in both resilient sheet vinyl and VCT installations. Adhesive application. As I indicated above, one of the variables that governs the amount of adhesive spread onto the substrate is the angle at which the installer holds the trowel. Under normal circumstances, the installer must maintain an angle of approximately 60º while spreading the adhesive. However, some situations may require the installer to change the trowel angle to fine-tune the amount of adhesive he spreads.

To place more adhesive in a specific area, such along the wall to combat sheet vinyl’s roll-curl tendencies, the installer often will stand his trowel up on edge to allow more adhesive to flow through the notches. In areas where the installer wants a little less adhesive, he will lay the trowel down in flatter orientation to the substrate to allow less adhesive through the notches.

In addition, he must plan his spread to allow for open time at its most efficient rate. For example, if the installation involves multiple pieces of material, the adhesive spread should start at the piece of material to be installed first and then be worked away from the material, or the direction that the material is to be installed. This may sound trivial, but you would be surprised how many installers do not realize the adhesive first reaches its open time in the area where they started the spread.

Open time. Open time of the adhesive is determined by four things: porosity of the substrate, temperature, humidity and the amount of adhesive applied. The goal is to allow the adhesive to develop body so that when the resilient material covers it, the adhesive will not migrate up into seams or be pushed around by the roller. This represents one of the most difficult times for the professional installer. The open time seems to take forever, but it is imperative that adequate open time be allowed for the adhesive to develop. Open time has been properly observed when the material is placed into the adhesive just before it starts to skin.

Once it’s placed into the adhesive, roll the flooring material with a properly weighted roller. Remove the material and look at the adhesive. If you’ve allowed sufficient open time, the adhesive bead will be flattened out — but not to the point where the notches are completely gone. Though the adhesive notches will nearly touch each other, they will remain clearly defined.

When the substrate is extremely dry or absorptive, which would make the adhesive dry too quickly, the substrate can be slightly dampened by misting it with a spray bottle of water. Remember: make the substrate damp, not wet.

Rolling the material into the adhesive. The question is: how much weight is required to apply the correct amount of pressure to the surface of the material? This is a key consideration, because that pressure is what forces the floor covering into the adhesive to allow transfer of the adhesive to the back of the material and flatten the adhesive to a uniform film.

Adhesives require a constant straight-line static load to apply a uniform pressure to the adhesive. With each individual adhesive, you may encounter different requirements for roller weight. Using a roller that’s too light allows the adhesive film to be too thick, which enhances the possibility of indentations and irregular distribution of the adhesive. A roller that is too heavy will tend to move the adhesive around, which creates roller marks, tracking and undulation in the material’s surface.

Installers who use brooms, towels and wall rollers in place of a properly weighted roller do nothing to ensure a good bond or uniformity in the adhesive film. This typically leads to bubbling, indentations, tracking, and undulations that show through the finished floor.

Adhesive failures. I’ve often been invited to inspect installations where the adhesive is reported to have failed. During my many years in this industry, I’ve seen very few true adhesive failures. However, I have observed adhesives that fail due to external causes. Examples include:

  • Drying too fast. When adhesives are dried too quickly, the processing oils are taken out of the adhesive. This allows them to become brittle and, on a short-term basis, lose their bond.

  • Moisture conditions. Moisture plays a huge role in adhesive failures. Moisture prevent curing of an adhesive, and it can decompose the adhesive after it has cured. If a moisture vapor emissions condition is present in a concrete slab, it will manifest itself in about two days. But an adhesive’s cure time can be five to 10 days, depending upon substrate porosity.

  • Alkali attack. Alkaline salts (“hydrated lime,” “caustic soda” and “lye,” in layman’s terms) attack adhesives, resulting in decomposition to the point where the adhesive disappears entirely.

  • Freezing. Some latex adhesives that freeze after they’ve cured can be broken down by the effects of the cold temperatures and lose their bond. Freeze-thaw stable adhesives experience a change each time they are frozen.

  • Flooding. Moisture, from leaks or topical exposures, can migrate beneath a flooring material to degrade the adhesive.

  • Improper maintenance. Floor strippers and highly alkaline floor cleaners are responsible for many adhesive-related failures.

  • Adhesive removers. Residue from adhesive removers that were improperly used or cleaned up are sometimes left behind in the substrate. Eventually, these residues can rise to the surface of the substrate where they attack and destroy the adhesive. Adhesive removers don’t make any distinctions between old adhesives and new.

  • Residuals on the substrate. Residues on the substrate — left behind by sealers, curing compounds, waxes, silicones, solvents, and oil-based products — can break adhesive bonds or destroy the adhesive altogether.

  • Improper adhesive selection. The use of an incompatible adhesive, such as a latex adhesive on a vinyl-backed material, can be an invitation to installation failure. Such circumstances can result in “plasticizer migration,” which transforms the adhesive into a gummy, slimy substance.

  • Storage. Adhesives stored in environments subject to temperature extremes can be damage in ways that are not visibly apparent at the time of use.

  • Solvent in the adhesive attacking resilient materials. The vapors emanating from adhesives with a high-solvent content can migrate up into the floor covering material and cause severe swelling in the vinyl and its surface. An example: “flex” adhesives used over non-porous surfaces.

Fortunately, only rarely does an adhesive fail. When you encounter a truly defective adhesive, it usually cannot be spread. And if you have a true adhesive failure, it is not selective in the areas where it fails — typically, it fails throughout the installation. Bond failures in selective areas are not adhesive failures. Be aware of this distinction.