Formaldehyde-Emitting Wood Bonding Adhesives

Formaldehyde-Emitting Wood Bonding Adhesives: Separating Fact from Fiction
By Dr. Melinda Burn and David Stutzman

Many manufactured wood products used to build and furnish schools, medical facilities and other commercial buildings and offices are made with formaldehyde-based adhesives. Thanks to their superior bonding properties, these wood-bonding adhesives are found in products such as construction-grade plywood, hardwood plywood, medium-density fiberboard (MDF), oriented strand board (OSB) and particleboard, as well as manufactured-wood furnishings, such as cabinets, desks, tables and engineered wood flooring.

However, the formaldehyde in these adhesives can be a concern, as well. That is because some of the formaldehyde in finished products made with these adhesives can be emitted into indoor environments. Since formaldehyde has been designated a human carcinogen by the U.S. government and other regulatory bodies around the world, concerns have been raised in the public and among public officials about the threat of these formaldehyde emissions to human health and the quality of indoor air, particularly when the formaldehyde is trapped indoors by increasingly tighter weatherproofing systems.

As a result of these heightened concerns, it is very important that specifiers separate the facts from fiction about formaldehyde-emitting adhesives in order to make informed decisions about the construction materials and manufactured-wood furnishings they specify.

Formaldehyde Emission Rates Vary
Several factors determine the amount and rate of formaldehyde emissions from wood-bonding adhesives.

• The nature of the bond within the formaldehyde adhesive system itself. The weaker the chemical bond, the more formaldehyde is released into the environment. Urea formaldehyde, one of the most commonly used materials, has the highest emission levels because its bond breaks down in high temperatures and very humid, moisture-generating conditions – a reaction called hydrolysis.
• The concentration of formaldehyde-based adhesive in the product itself. The higher the concentration, the more likely it will emit formaldehyde.
• Whether a “scavenger,” most commonly urea, has been added to the resin to bond with some of the free formaldehyde, so that it does not escape into the environment.
• Temperature and humidity. The higher the temperature and humidity, the more formaldehyde is released into the environment.
• Where the adhesive exists in the final product. With hardwood plywood and engineered wood flooring, for example, the top layer of the finished product is a wood veneer, which helps to seal the formaldehyde in the board and slow the rate of off-gassing.

Because the stakes have been raised, many adhesive manufacturers have found ways to reduce formaldehyde emissions from their products, although the fundamental chemistry has not changed significantly. As a result, there are more wood-bonding adhesive options on the market.

No Added Formaldehyde Adhesives
More and more manufacturers of engineered wood products are transitioning to no-added- formaldehyde (NAF) adhesives, which neither contain nor release any formaldehyde into the environment. Adhesives based on soy flour represent the newest industrial resin technology in the NAF category. They have gained significant market share in decorative plywood and engineered wood flooring and have been making in-roads into particleboard, MDF and OSB. Among their competitive advantages, soy-based adhesives meet performance specifications, can be substituted with relative ease in manufacturing processes and are safe to handle.

NAF resins also include isocyanates, which are extremely strong and highly resistant to water. They are widely used in OSB production and are gaining ground in other engineered wood products. However, the use of isocyanates in products such as plywood and engineered wood flooring is limited because manufacturers must install special environmental controls to protect workers from exposure to them and they are not compatible with some manufacturing processes.

Also in the NAF category are polyvinyl acetate (PVAc) adhesives, which are commonly used by fabricators assembling furniture or small components and are occasionally found in plywood or engineered wood flooring. They are easy to pump and handle, but require very fast manufacturing times, which can complicate production. Some PVAc adhesives contain small amounts of formaldehyde, so care must be taken when specifying them and then claiming they are formaldehyde free.

No-Added-Urea-Formaldehyde Adhesives
Occupying the middle ground with respect to formaldehyde emissions are resins made with either phenol-formaldehyde (PF) or melamine-formaldehyde (MF). They are categorized as “no-added urea-formaldehyde” (NAUF) adhesives, a term that can be confusing to the marketplace and easily misinterpreted to mean that there is no formaldehyde of any kind in the adhesive. In fact, NAUF adhesives do contain formaldehyde reacted with either phenol or melamine.

PF adhesives have a very low formaldehyde emission rate due to the resin’s resistance to hydrolysis. They also have certain disadvantages. They are more expensive than more widely-used urea- formaldehyde (UF) adhesives, have higher processing requirements to cure, are very dark in color and may not be suitable with some manufacturing equipment.

NAUF adhesives made with MF resins are typically found in interior grade products that require greater strength and improved moisture resistance, although they are less resistant to hydrolysis than PF resins. These adhesives also cost more than UF adhesives and may not be suitable for use with some manufacturing equipment.

Urea-Formaldehyde Adhesives
Urea-formaldehyde (UF) adhesives are the most widely used today. These low-cost adhesives are easy to use in a wide variety of conditions, cure at low temperatures, have excellent thermal properties and the cured resin does not impart any color change to the finished panels.

UF adhesives release formaldehyde into the indoor environment at rates higher than other adhesives because of their reduced resistance to moisture and humidity, especially when the ambient temperature rises. These conditions result in the hydrolysis of the UF bond, which increases the rate of formaldehyde emissions. As a result, UF adhesives are most often found in products used in building interiors, where there is less exposure to moisture.

UF adhesives that contain melamine as a co-reactant are often called melamine urea-formaldehyde (MUF) adhesives. The melamine provides improved strength and durability to the adhesive, while reducing the hydrolysis rate, which helps to lower formaldehyde emissions from the adhesive.

LEED Compliant Engineered Wood Products
Recent updates to LEED (LEED v4) changed how manufacturers can produce LEED-compliant engineered wood products, as well as the information they are required to convey about the adhesive used to make them. LEED v4 requires either that composite wood products meet the California Air Resources Board (CARB) emission testing requirements for ultra-low-emitting formaldehyde (ULEF) products or that these products be made with a no-added-formaldehyde (NAF) adhesive. Specifiers searching for a level of transparency and information on formaldehyde content in an adhesive will have to rely on the Material & Resource (MR) section for Building Product Disclosure and Optimization of LEED v4. The MR section requires that there be no Benchmark-1 hazards using the GreenScreen v1.2 Benchmark system. Formaldehyde would be identified as a Group 1 Human hazard by GreenScreen and any adhesive using formaldehyde would not qualify for this point.

Dr. Melinda Burn is global business director, Sustainable Building and Construction, Ashland Inc. She can be reached at David Stutzman is an architect and president of Conspectus, an independent specifications consulting firm that advises architects, designers, builders and others about building construction and building materials. He can be reached at