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Sticky Questions: Q&As on Essential Adhesive Know-How

Kim Wahlgren
Courtesy of Stauf USA LLC
Courtesy of Stauf USA LLC

When you're on a glue-down job, a lot more can go wrong than just getting your fingers or your jeans full of adhesive. It's a whole different animal from a typical nail-down job, and to boot, there's an overwhelming array of options for adhesives. Which type should you choose? And do you need just an adhesive, or one that's a combo product? In the following pages we have answers from top industry experts so you're better prepared the next time you find yourself in a ... sticky situation, so to speak.

BASICS

I've been running into trouble on my glue-down jobs. Are there basics I'm missing?

Here are some key steps you must consider when gluing down wood flooring:

Wood Flooring Manufacturer: The glue-down method should only be used if approved by the wood flooring manufacturer as an acceptable installation method for their wood flooring.

Surface Preparation: Is the concrete slab flat and free of imperfections (e.g., high or low spots)? Ignoring surface preparation will inhibit the adhesive from achieving a strong bond from the back of the wood to the slab, and in the long run cost you more time and money trying to get a floor to look good with a bad foundation. Correct any imperfections in the slab prior to installing your wood floor using an appropriate patching compound or self-leveling underlayment.

Moisture Management Systems: Since wood is hygroscopic, like a sponge, it gains or loses moisture from the air based on the conditions of the surrounding environment. Keeping this in mind, it is important to quantify the Moisture Vapor Emission Rate (MVER) in the slab using either the ASTM F1869 Calcium Chloride or ASTM F2170 in situ RH test methods. Ignoring unacceptable moisture levels in the concrete can lead to cupping, crowning, checking, etc. It may also break down the adhesive by exposing it to moisture, soluble salts and tannins from the wood and weaken the ability of some adhesives to bond and hold the floor in place. Once the MVER has been quantified, there are several ways to address the moisture in the slab:

1) Applying a 100 percent epoxy moisture system per ASTM F3010 "Standard Practice for Two-Component Resin Based Membrane-Forming Moisture Mitigation Systems for Use Under Resilient Floor Coverings" to ensure that the MVER remains within acceptable moisture levels through the life cycle of the wood flooring installation.

2) Using one of the new all-in-one wood adhesives. This is a great option to create a continuous moisture membrane while also adhering the wood floor to the substrate. When using these systems, it is imperative to not leave any substrate exposed or the product's ability to control the MVER will be compromised, causing failure. (See the "All-in-Ones" question.)

3) Applying a urethane moisture vapor membrane to the slab, allowing it to cure and then bonding the wood flooring with an approved adhesive. This allows visual inspection of the membrane so voids can be seen and addressed prior to flooring installation.


RELATED: Wood Floor Cupping: Why Does it Happen & What Can You Do?


Wood Adhesive Selection: Since wood is a natural product, much consideration should be given to the type of adhesive used, since not all adhesives are formulated the same. Most engineered wood floors can be installed using any of the three methods (nail-down, floating system or glue-down), but solid wood flooring can present more challenges, requiring an advanced adhesive like a urethane, modified silene or acrylic urethane that holds its trowel ridges well and has great initial grab, allowing for maximum contact between the adhesive and wood. Additional thought should also be given to the species of wood flooring being installed (is it an exotic or bamboo, for example) due to potential unique characteristics such as high tannin content or hard, impervious surfaces. Some might also require an advanced adhesive (like a urethane, modified silene or acrylic urethane) with strong initial grab.

Cris Bierschank is sustainability manager at Deerfield Beach, Fla.-based MAPEI Corp. Sam Biondo, MAPEI national technical presenter, also contributed to this answer.

TROWEL

One of the most common mistakes with glue-down jobs is using the wrong trowel. Trowels are designed for each adhesive to meter out the correct amount of adhesive in the correct pattern for that specific adhesive.One of the most common mistakes with glue-down jobs is using the wrong trowel. Trowels are designed for each adhesive to meter out the correct amount of adhesive in the correct pattern for that specific adhesive.

Does it really matter if I use the exact trowel recommended by the manufacturer?

Choosing and using the proper flooring trowel when applying your flooring adhesive on any given job is of vast importance. Simply employing the right or wrong trowel can determine the difference between installation success or failure—and with it, potentially thousands of dollars of added remediation expense. A thorough and comprehensive understanding of trowel requirements is essential to any glue-down flooring installation.

The most important reason for this is that trowel specifications or measurements determine how much adhesive is applied when gluing down a specific flooring type. Different flooring types require different amounts of adhesive application. Generally speaking, increasing gauge thickness of the wood flooring being installed usually requires a larger trowel specification (both size as well as notch profile) and subsequently more adhesive to ensure all necessary and required performance characteristics.

Applying an adequate amount of adhesive is important for several reasons. Insufficient adhesive application can promote substandard adhesion and/or final bond strength, and in many cases development of hollow spots throughout the installation. Applying too much adhesive can also adversely affect the flooring installation by preventing proper fit of planks as they are installed because adhesive gets captured in tongue and groove joints.

Some flooring adhesives are multifunctional and offer several performance attributes with one application. A good example of this is product technology offering moisture or sound control along with adhesion bond. It is extremely important to achieve the proper adhesive membrane with this type of flooring installation, and selecting the right trowel is critical to success.

Larry Scott is vice president of field technical services at DriTac Flooring Products in Clifton, N.J.

TYPES

I am not sure why I need one type over another. Shouldn't an adhesive just be an adhesive and stick stuff to the subfloor?

Well, one would think so, wouldn't they? The problem with the one-size-fits-all mentality is that it is impossible to do so. There are too many variables for hardwood, let alone vinyl, VCT and LVT. For the sake of this publication, let's discuss only hardwood.

Wood, as a general rule, doesn't care what brand it is when it comes to adhesive. It either will adhere to the subfloor with what an installer told it to or it won't. Not to oversimplify, but if wood had an absolute preference in adhesive, a lot of adhesive manufacturers wouldn't be around today. Don't over-think adhesive. Most adhesive works regardless of whose it is; the differences are in the installer's preference. Maybe he or she has had repeated success with it, is comfortable using it, been to that plant where it's made or thinks the guys who invented it are the adhesive gurus to end all gurus. For whatever reasons, the installer likes that adhesive.

I am going to put this into layman's terms, as that is what I am best at. There are a few differences that would make a difference in the way a specific product's characteristics are best utilized. Moisture-cured urethanes (MCU's) are exactly what the name says: moisture-cured, using relative humidity. The damper the climate, the faster they cure. These adhesives usually have relatively strong shear strengths and can be used in almost all circumstances. The drawbacks are that they make it very difficult to do board replacement, can etch the finish on factory-finished flooring and have a slow cure, so there can be board movement underfoot before they get a good set.


RELATED: Familiarize Yourself with Wood Floor Adhesive Basics


Then there are non-reactive adhesives. These are like what your grandfather used to use: solvents. They are great to work with and have extremely strong shear strengths. These work the opposite of the above MCUs: The more arid the climate, the faster they cure (for a lot of reasons that I care not to dive into). They set quickly, have an extremely strong bond and are probably the most friendly adhesives to work with. The problem is they are solvents, which can be a problem for some installers because it means an "odor" and usually VOCs, or flammability, or both.

Next there are the modified silane polymers, which are adhesives that usually have some pretty cool tricks up the sleeve. They are moisture-cured, they have good workability and they can mitigate moisture. Some manufacturers promote sound mitigation (STC IIC) and some take it a step further and say they aid in subfloor crack isolation. They clean up extremely easily because they do not contain isocyanate, which is found in regular MCU adhesives.

There are water-based adhesives that are sometimes referred to as acrylic urethane or latex-based adhesives or, most simply, just "water-based." These usually work well, but, like all, have advantages and disadvantages. Some advantages are that they are inexpensive, eco-friendly, easy to work with and clean easily. Disadvantages include the fact that water and wood could be a problem (not always), causing adverse effects of performance of wood floor, and they have lower shear strengths. There are others, but I am not trying to bash any one manufacturer—everyone has good and bad technology.

Basically, it all comes down to choice of the installer once you get past basic performance needs, e.g., moisture or sound control. Once you get past that, remember this about adhesive manufacturers: It's a lot like a beauty pageant. Who has the best dress (packaging), who speaks the most eloquently (marketing) and who has the most talent (performance). Other than that, we are all just contestants of the same gender.

David Ford is VP of sales and marketing at Arlington, Tenn.-based Stauf USA LLC.

GLUE-ASSIST

I want to glue in the end row on my jobs—it doesn't really matter what glue I use for that small of an area, does it?

Yes, it does. When installing solid nail-down wood flooring, it has become common practice to "glue-assist" when installing borders, medallions, starting rows and end rows, as well as wide-plank. For the sake of convenience, contractors often choose all-purpose construction adhesives for this important task. Those products work well when the purpose is to create a strong and rigid structural bond between two wood elements. That also means they aren't a good choice for wood flooring.

We need to keep in mind that wood floors are under a fairly constant state of shrink and swell due to factors like seasonal changes in the percentage of relative humidity within the home. If we use an adhesive with no elongation properties, the flooring can dislodge under shear stress. That's when using adhesives specifically designed for solid wood flooring installations becomes an important consideration, even just for glue-assist applications.

I learned this firsthand on a 1,000-square-foot installation of a training floor I had installed in the Boston area where I (like many of you) grabbed some "premium" construction adhesive from the box store for the perimeter and borders, thinking I was doing the conscientious thing. The space would go from about 20% RH in the winter to about 90% in the summer (just a tad out of wood's comfort zone, for sure). I ripped up those floors to replace them several years later. When pulling up boards that have been glued down, you generally expect that some of the substrate will remain attached to the board. But on these glue-assisted boards, the adhesive had sheared completely from the substrate. I also noticed that the adhesive was rather brittle and chalky, which contributed to its complete release.

Manufacturers of adhesives for solid wood flooring adhesive (whether urethane or silane) will generally publish both the shear strength of the product (expressed as psi) and the elongation rate (expressed as a percentage) to assist you in determining which product will provide the right balance of strength and flexibility. Without getting too folksy, I was once told that a good adhesive for a hardwood floor can be likened to a good parent. There are times when, as parents, we need to be rigid and firm with our children for their own protection, but there are just as many times we need to be flexible when confronting the ever-changing landscape our children must navigate. This illustrates nicely that a solid wood floor is going to experience a wide range of environments we may not anticipate during its life cycle. Therefore, it seems practical to use an adhesive that has been manufactured specifically for wood flooring installations on all glue-assist applications.

Dave Darche is sales and market manager—adhesives for Aurora, Colo.-based Bona US.

ALL-IN-ONES

I used an all-in-one adhesive and moisture protection system on a recent job, but it still ended up having a moisture problem. What did I do wrong?

Recently there has been a great deal of discussion about the ability of all-in-one adhesive and moisture vapor membranes to achieve adequate protection of the flooring. All-in-one adhesives must form a continuous membrane to be effective. Voids in the membrane could allow significant amounts of moisture through to the flooring and result in failures. As a result, coverage rates must never be stretched.


RELATED: Step by Step: How to Get a Concrete Slab Ready for Wood Floors


One of the most significant causes of voids in all-in-one membranes is the condition of the subfloor and the flooring. Nothing is more important than proper subfloor prep. Subfloors must be flat per NWFA standards to within 3⁄16 inch in 10 feet, and excessive roughness of the surface can influence the coverage rate. Using flooring boards that are warped will also impair an installer's ability to achieve a continuous membrane with no voids. Waste factor for the wood flooring may increase substantially from a typical 10 percent for nail-down or simple bonding. Every adhesive/membrane manufacturer has different requirements to achieve a proper membrane, so make sure you read the directions and follow them for every all-in-one product.

With all-in-one systems, the installation is faster, generally saving labor and material costs. Additionally, they are often less susceptible to future failures from cracks developing in the slab after installation because the adhesive membrane is thicker and more resilient than epoxy moisture vapor barrier coatings. The drawback is, as mentioned above, great care must be taken during subfloor prep and flooring installation to prevent voids in the membrane that could allow moisture through.

To reduce the inherent risks of not being able to see that proper coverage has been achieved, many installers prefer two-step systems, either epoxy or urethane. The advantage of a two-step process is that visual inspection is possible to catch and remedy any voids because the moisture barrier is applied prior to bonding the flooring. Photographs may also be taken before the flooring is installed. The disadvantages of the two-step process is that it increases the project timeline, requires additional labor to install, additional materials must be purchased (both the membrane/barrier and then the adhesive) and the flooring can't be installed until the membrane/barrier cures—typically requiring 24 hours and a second trip to the job site for the installers.

Eric Kurtz is market manager—hardwood and resilient installation systems at Bostik Inc., based in Wauwatosa, Wis.

Adhesive Terms 101

Fundamentals begin with a glossary of correct terminology. There are dozens of excellent books that are hundreds of pages about adhesives. At a university, an intro adhesives course will spend more than 20 hours on lectures, with an additional 60 to 80 hours of reading, homework and research. The following information is the tiniest tip of a mountain of information regarding adhesives. Maybe it would be helpful to pretend that you are learning a foreign language by memorizing basic vocab, like "Yes," "No," "Where is the bathroom?" "Steak medium rare, please," … you get the drift.

Glossary of Important Terms:

Adhesive: Liquid that converts to a solid and becomes an integral part of the assembly.

Adherend: Surface to be bonded.

Contact angle: A method of measuring the adherend's surface affinity for adhesion.

Surface tension: A method of expressing the cohesive attraction between molecules of a liquid.

Surface energy: A method of expressing the cohesive attraction between molecules of a solid.

Adhesive Strength: A measure of the integrity of the interface between the adhesive and the adherend.

Cohesive Strength: Measure of the integrity of the bulk adhesive or bulk adherend. Intermolecular forces of the individual materials.

Contact: Achievement of intimate molecular contact (see definition of that below) between the adhesive and adherend. For wetting to occur, adhesive must have an affinity attraction for the adherend.

Intimate Molecular Contact: The adhesive and adherend molecules are close enough to form primary chemical bonds.

Glossary of Adhesive Motions & Bond Formation:

Spread: Application of adhesive on the adherend surface.

Flow: Distribution of adhesive across the surface of the adherend.

Transfer: Force applied to the adhesive as it is spread and flows over the surface of the adherend.

Penetration: Movement of the adhesive into the microstructure of the adherend.

Wetting: Adhesive develops intimate molecular contact with the adherend.

Solidification: Adhesive transitions from a liquid to a solid or hardens.

Suggested Reading:

"Technology of Wood Bonding: Principles in Practice" by Alan A. Marra, 1992.

"Adhesive Bonding of Wood and Other Structural Materials" by Richard Frederick Blomquist, 1983.

"Handbook of Adhesion" by D.E. Packham, 2005.

"Adhesion and Adhesives: Science and Technology" by Anthony Kinloch, 1987.

"Understanding Wood: A Craftsman's Guide to Wood Technology" by Bruce Hoadley, 2000.

"Wood Handbook: Wood as an Engineering Material" by Forest Product Laboratory, 2013 (available for purchase or online for free).

"Understanding Physics" by Isaac Asimov, 1966.

Howard Brickman owns Brickman Consulting in Norwell, Mass.

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