(This article was originally posted on Inside the Forest Service.)
As great as engineered wood—such as plywood or particle board—is for a range of building and manufacturing uses, it has its limitations, especially in outdoor applications. One of the biggest limitations is not the wood, but the adhesive used to glue the wood veneers or particles together. In the United States, the most commonly used adhesive in outdoor engineered wood products is phenol-formaldehyde resin.
To accelerate development of new and improved wood adhesives for engineered wood products, Forest Service researchers Joseph Jakes, Chris Hunt, Nayomi Plaza, Dan Yelle, Chuck Frihart and Linda Lorenz, along with collaborators at Oregon State University, Argonne National Laboratory and Scion, a New Zealand research lab, are working to understand the optimal adhesive penetration into wood for specific products and applications. They want to know what controls performance at the bond line—where the adhesive meets the wood.
“The key to developing new and improved wood adhesives, especially for moisture durability, is for scientists to have a better understanding of how an adhesive penetrates the wood,” Hunt said. “We investigated both the flow of adhesive into microscopic voids in the wood’s structure and infiltration into the wood’s cell walls.”
How well a piece of plywood or product will perform in a real-life situation is tested by putting the product through several wet–dry cycles to mimic outdoor conditions. Wood swells when wet and contracts when dry. These fluctuations test the strength of the wood–adhesive bond. Changes to the wood are visible to the naked eye.
What’s happening at smaller length scales that cannot be seen with the naked eye requires advanced imaging tools such as X-ray computed tomography, X-ray fluorescence microscopy and small angle neutron scattering to study adhesive flow and infiltration, especially to see beneath the wood’s surface.
Although much additional work is needed, the new insights gained in this study will be useful in the development of improved models capable of predicting adhesive performance at the bond line. The researchers learned the pathways through which the adhesive flowed into the wood structure. They also found that the smaller adhesive molecules are more effective than larger molecules at infiltrating cell walls and minimizing the effects of moisture on the properties of the wood nearest the bondline.
Researchers also gained insight as to which interactions between the adhesive and nanoscale cell wall structures are likely the most important for creating moisture-durable wood adhesive bonds.
The researchers explain how adhesives moves through wood cellular structure and cell walls in a recently published research paper. The team, who work at the agency’s Forest Products Laboratory in Madison, Wisconsin, have made great progress in decoding the process by which adhesive moves through wood cell walls. “Since adhesives are a major cost in wood products, we expect this work to understand how adhesives work in wood will lead to better products at lower cost. Wood products are renewable, support rural communities and also remove CO2 from the atmosphere,” said Hunt. “Making them more competitive is good for business and for the environment.”