It’s a sticky problem that Forest Products Laboratory (FPL) researcher Juliet Tang in collaboration with faculty member Hyungsuk Lim and graduate students from the Department of Sustainable Bioproducts, Mississippi State University, find themselves researching.
The team is brainstorming innovative ways to make the building material of the future—mass timber—more versatile. But in order to do that, they have to find an adhesive and a preservative, two substances that tend to be uncooperative together when used on timber, that will work concurrently for optimal bond strength and durability.
Cross-laminated timber (CLT) is a widely established building material used throughout Europe and that is making its way into American construction. It is made by stacking kiln-dried lumber in alternating directions with an odd number of layers (usually three to seven). Construction adhesive is used to bond the boards together and then the stack is pressed in order to make solid panels.
CLT is an incredibly strong material that can be used in place of concrete, steel, and masonry in many construction applications. But what makes CLT such a compelling new material is its environmental friendliness.
“CLT is an emerging mass timber product for low- to mid-rise construction that can help cities meet their carbon neutral target dates,” explained Tang. “It has a low carbon footprint during the construction phase, and it acts as a carbon sink, keeping the carbon tied up for the entire life of the building.”
And because of its sustainable benefits, Tang and her co-collaborators wanted to expand the use of CLT from interior construction, where its primarily used, to exterior structural applications. They imagine CLT that could one day be used where wood is exposed to rainwater and is built either above or in contact with the ground, such as for balconies and ground-contact walls.
When wood is used in areas that expose it to weather, decay fungi, and wood-attacking insects, it needs to be treated with chemical preservatives for lasting protection.
But for Tang and her team, the crux is that wood preservatives can negatively affect adhesive bonding ability by physically and chemically blocking “surfaces where the intermolecular forces of bonding develop.” Preservatives can also reduce wettability of wood, which is important because adhesives need high rates of wettability in order to penetrate, spread, and cure.
Wood adhesives and preservatives basically butt heads when they meet on wood.
The seeming impossibility of two opposing substances didn’t deter Tang and her tenacious co-collaborators. In fact, they rose to the challenge. Tang explained her favorite part of the research was “working with Dr. Lim and his graduate students with the common goal of finding solutions to all the problems as they cropped up.”
And what they found is that treating CLT with wood preservative for exterior applications is possible.
However, there is still work to be done. Simulated weathering in a laboratory setting will help determine if their experimental CLT will meet rigorous performance standards, particularly in the areas of sheer strength and delamination.
Tang is already mapping steps into the future for her research.
“Our next immediate step is to see how well the treated CLT withstands biological attack from mold, decay fungi, and subterranean termites in both laboratory and field tests. We are also interested in examining how prolonged flooding affects treated CLT wall systems, both structurally and in terms of biological durability,” Tang stated.
With Tang and her indomitable team, CLT will have a bright future as the emerging, versatile building material for a greener planet.
To find out more about the amazing advancements our scientists are making, visit the Forest Products Laboratory at: https://www.fpl.fs.fed.us/