Formaldehyde’s designation as a human carcinogen has caught the attention of both consumers and workers in plants where engineered wood products are made. This heightened awareness makes it critical that architects and others in the building industry understand the facts about formaldehyde-based resin systems. There are some other details to consider first, however, about the fundamental characteristics of widely used manufactured wood products such as hardwood plywood, oriented strand board (OSB), particle board, and medium-density fiberboard (MDF), including their composition, assembly, and manufacturing processes.
Hardwood plywood used for furniture, cabinets, paneling, and engineered wood flooring is primarily made with oak, poplar, maple, and cherry veneers bonded by an adhesive. The outer layers surround a core made of veneer, particle board, or MDF. A multistep process turns logs into hardwood plywood. First, logs are carefully debarked without damaging the wood. They are then heated to facilitate cutting the veneers, which can be sliced thinner than 3 millimeters. The veneers are then dried at temperatures up to 400 F. When they reach their specified moisture content, the veneers are conveyed to a layup operation, where a thermosetting resin is spread on them. This laid-up assembly of veneers is then sent through a hot press, where the resin is pressed into a thin layer between each sheet of veneer and the thermoset resins are activated. Finally, the finished hardwood plywood panel is cut and sanded.
In contrast, OSB, particle board, and MDF are made from a variety of finely ground woods called “furnish.” Adhesive is sprayed on the furnish, which is compressed as it moves through a hot press to form the panel. These finished products are then cut and sanded.
These manufacturing processes are now receiving increased scrutiny because many of these wood products for building and furnishing homes and offices are made with adhesives that emit formaldehyde. In mid-2011, the U.S. government designated formaldehyde as a known human carcinogen, following the lead of other governments around the world.
Several factors determine the amount and rate of formaldehyde emissions from wood-bonding adhesives. Formaldehyde resins react with moisture in the air to release free formaldehyde. Emissions increase with higher temperature and humidity levels. Also, the higher the concentration of formaldehyde-based adhesive in a product, the higher the emission levels will be. Finally, a “scavenger”—most commonly urea—may be present in the adhesive system, which amplifies formaldehyde emissions.
Because adhesive providers have been diligent in reducing formaldehyde emissions from their products, a range of adhesive options are available to manufacturers. At one end of the spectrum are highly efficient and effective adhesives that add no formaldehyde to the environment. Among them, soy-based adhesives represent the latest resin technology, on top of being based on a renewable resource.
At the opposite end of the spectrum are widely used urea-formaldehyde adhesives, which release higher rates of formaldehyde than other adhesives and do not qualify for LEED points. Mid-spectrum are no-added-urea formaldehyde resins made with phenol-formaldehyde or melamine-formaldehyde. Unfortunately, the term “no-added-urea formaldehyde” can be easily misinterpreted to mean there is no formaldehyde of any kind in the adhesive when, in fact, these adhesives do contain and emit formaldehyde.
Knowing the origin and composition of common construction materials, including but not limited to engineered wood products, is a critical advantage to architects. Armed with the facts about the DNA of these products, they can ease client concerns about sustainability issues and make the most informed decisions possible about the products they specify from the standpoints of indoor air quality and sustainability. —Melinda Burn and Richard Hayes, AIA
Melinda Burn is global business director of sustainable building and construction at Ashland. Richard Hayes, AIA, is director of knowledge resources at the AIA.