Advancing Densified Wood Technology through Oxygen Delignification
Forest Products Laboratory researchers and collaborators developed a sustainable and commercially scalable technology (US Patent 12,011,854, 2024) for delignifying dimensional wood using oxygen at low temperatures with subsequent densification to produce super strong wood from low-quality wood. This research removes a key barrier for commercialization of super strong densified wood as an alternative to metals and creates a new market for wood.
Side-by-side comparison of pristine wood (left), oxygen-delignified wood (center), and oxygen-delignified wood that has been densified (right). The top row (I) shows delignified wood after 2 hours of alkali impregnation and the bottom row (II) shows delignified wood after 7 days of alkali impregnation.
Wood has long been used as a structural material and is preferred by some designers as a substitute for ore- or mineral-based materials. Some structural applications of concrete and steel, however, cannot currently be replaced by wood due to wood’s limited strength. Recent research has demonstrated that wood can be substantially transformed by removing lignin (“delignifying”) and densifying wood to make it much stronger. Effective delignification and densification of wood is required for super strong wood to be commercially viable, and most methods of delignification do not keep the physical structure of wood intact and have many practical barriers in delignification chemical recovery.
Researchers from the Forest Products Laboratory and their collaborators at North Carolina State University and the University of Wisconsin-Madison have demonstrated that oxygen delignification can be applied to solid, dimensional wood to create super strong, densified wood by integrating the wood densification concept and technology developed at University of Maryland College Park and currently practiced at InventWood, Inc. This densified wood is strong enough to replace steel in some applications and has a lower environmental impact. Using oxygen delignification at lower temperatures and densifying wood at lower pressures for shorter time periods in the process of making super strong wood overcomes significant barriers to commercial success.
Key Findings:
- Subsequent wood densification of the oxygen-delignified wood at a low pressure of 2 megapascals (MPa) for only 20 minutes nearly doubled wood density and substantially improved wood mechanical strength. With bending modules of rupture over 220 MPa and Brinell hardness over 80 MPa, this densified wood material is suitable for many structural applications that are currently not possible using pristine wood.
- Life cycle assessment showed that densified wood made using oxygen delignified wood has substantial benefits in terms of environmental impact even compared to metals with high recycled content.
Diagram of the process by which pristine wood undergoes oxygen delignification and then is densified.
This method of oxygen delignification begins by impregnating wood with an alkali solution, such as potassium hydroxide, sodium hydroxide, sodium carbonate, or urea. Then, wood is delignified using oxygen at 0.75 MPa and 110 degrees Celsius for one hour. This new method uses a lower alkali loading than previous methods and a lower temperature, which saves resources and energy and reduces carbohydrate degradation.
The oxygen delignified wood is then pressed at relatively low pressure (2 MPa) and 150 degrees Celsius for a relatively short time (20 minutes) to densify it. Using a lower pressure and shorter time for densification reduces the cost of this process.
Outcomes
- Densification changed cell wall structure and increased wood density.
- Increasing wood density to 0.9 g/cm3 or more with 20 percent delignification led to a 254 percent increase in Brinell hardness. Increasing wood density to 1.11 g/cm3 led to a 691 percent increase in Brinell hardness.
- Densifying wood samples with less than 15 percent delignification nearly doubled the modulus of rupture (i.e., bending strength) compared to pristine wood. Densifying samples with 20 percent or more delignification tripled the modulus of rupture.
- Densifying wood increased the bending modulus of elasticity.
People
-
Person
Junyong Zhu, PhD
Research General Engineerhttps://research.fs.usda.gov/about/people/junyong.zhu -
Person
Roland L. Gleisner
Engineering Technicianhttps://research.fs.usda.gov/about/people/roland.gleisner -
Person
Carl J. Houtman, PhD
Chemical Engineerhttps://research.fs.usda.gov/about/people/carl.houtman -
Person
Peter Kitin, PhD
Forest Product Technologist (Microscopy)https://research.fs.usda.gov/about/people/peter.kitin -
Person
Sara Fishwild
Lead Forest Products Technologisthttps://research.fs.usda.gov/about/people/sara.j.fishwild -
Person
Marshall Begel
General Engineerhttps://research.fs.usda.gov/about/people/marshall.begel -
Person
Hoon Kim, PhD
Research Chemisthttps://research.fs.usda.gov/about/people/hoon.kim -
North Carolina State University
Naycari Forfora
-
North Carolina State University
Rolands Gonzalez
-
University of Wisconsin-Madison
Xiaoxue Zhang
-
University of Wisconsin-Madison
Xuejun Pan
-
Yale University
Liangbing Hu
-
InventWood, Inc.
Jiaqi Dai