Richard JagelsThis naturally occurring flattened and “densified” dawn redwood (Metasequoia) fossil was collected by the author from an Eocene-era site on Axel Heiberg Island at 50 degrees north latitude in Nunavut, Canada.
After World War II, the world was inundated with an explosion of new technologies that had been developed as part of the war effort. New materials such as fiberglass-reinforced, petroleum-derived plastics began to compete with natural products, including wood. In response, the U.S. Department of Agriculture’s Forest Products Laboratory (FPL) in Madison, Wisconsin, experimented with methods to modify wood in ways to compete with the new synthetic materials.
In March 1956, FPL chemists R.M. Seborg, M.A. Millett, and A.J. Stamm published report No. 1580, titled, “Heat-Stabilized Compressed Wood (Staypak).” While Staypak was never commercially produced, other compressed, densified wood products made from chips, shavings, or fibers soon appeared—among them particleboard, medium-density fiberboard (MDF), and high-density fiberboard (HDF), and all are now ubiquitous in the market. Even earlier, in the 1930s, German engineers produced a compressed-wood product called Lignostone. Patterning on that model, the FPL crew produced Compreg, which was intended for structural products such as aircraft propellers, connector plates, concrete forms, and other applications requiring high strength. Compreg is produced by cross-layering veneers slathered with phenol-formaldehyde resin and subjecting the panels to heat and high pressure. Think of it as compressed plywood.
While these compressed-wood products found some niche markets, the imagined high expectations were never realized, mostly because of their high cost. For example, the advantages of Compreg over greased plywood for concrete forms was not convincing in the marketplace when the price difference was factored in.
Step to the Present
As nonrenewable petrochemicals and metals have become more expensive, or less desired due to their climate harms, researchers in recent years have returned to renewable wood as a possible alternative—if modified in some way. This brings us to an email from Matthew B. Marsh, who wrote, “I just finished reading a very interesting paper by Dafang Huang et al., https://bit.ly/WB309ModifiedWood, who have produced a self-densified, high-strength wood without the use of hot pressing. I have considered using hot-compressed densified wood in a couple of boat designs that require extremely high strength-to-weight ratios. The problem, as you know, is that mechanically hot-compressed wood always ends up highly anisotropic, with one very weak axis…and presses capable of applying 3+ MPa to large pieces for days at a time are not cheap.
“Huang et al.’s process seems to have turned their basswood samples into something stronger than 5083 aluminum in most respects, without a hot press and yielding similar enhanced properties in both transverse axes. It would also appear, to my eye, that—while in the intermediate swollen or partly treated state with the cellulose fibrils unbound from the lignin—their wood must be very amenable to tight-radius bending, which would then become locked in as it dries and densifies. The reduction in lignin ratio and the collapse of the lumen pores would also suggest potentially higher rot-resistance.
“Apart from the obvious difficulty of needing to deal with 2.5 mol/L sodium hydroxide and N,N-dimethylacetamide, I am curious if this process might eventually become practical for wooden boats. The idea of being able to make bent-to-form frames and beams of easily workable wood, yet stronger than their dimensional equivalents in extruded aluminum and at half the weight of aluminum, is rather appealing.”
The self-densified wood that Matthew Marsh finds appealing is just one of several potential modified wood products that have been developed in recent years—products with names such as Superstrong, Bio-Strong, Self-densified, Superwood, and MettleWood, to name a few. All have the goal of improving strength-to-weight ratios. Most involve partial delignification combined with densification.
Various strategies have been devised to partially digest lignin in the initial stages of wood densification. Traditional pulping chemicals such as sodium hydroxide and sodium sulphate are often employed, but one process (Bio-Strong) exploits a white-rot fungus to partially degrade lignin. And while Defang Huang’s lab adds the organic solvent N,N-dimethylacetamide, other research groups add different chemicals such as amino-functionalized polyglycerol phosphate and disodium octa borate tetrahydrate for fire, insect, and decay resistance.
Other kinds of wood modification currently being explored are: (1) thermo-mechanical modification (TMD) using heat and pressure, alone, to improve mechanical properties; (2) impregnation with nanomaterials such as nanocrystalline iron oxyhydroxide to improve cell-wall strength; (3) impregnation with an aqueous solution of biochar followed by thermomechanical treatment to enhance mechanical strength and water resistance.
Help from Nature?
The Bio-Strong wood process employing fungi to partially degrade lignin makes the claim that 85 percent of the original mass is retained—which is higher than acid-delignified super woods—and requires “little solvent or energy beyond the press cycle” (Lu et al., Adv. II, eady 0183, 2025). The authors say the product “was inspired by ancient, buried wood, a naturally formed material after wood endures in microbial-rich and high-pressure environments for thousands of years.” I have extracted fossilized wood of this kind in the high Arctic, as shown in the accompanying photograph. Geologists call it lignite or “brown coal,” a substance that could become bituminous or anthracite coal with several million more years of increasing pressure. Denser than wood, fossilized wood may be stronger in compression, but unlike wood fresh from the tree, it is quite brittle (see Wood Technology, WB No. 139).
Commercial Prospects
Most of the new “super” woods are still in the experimental or prototype production stages. How many of these will ever reach the market will likely depend on how much start-up and promotion cash is pumped into the technology. Right now, wealthy investors seem to be dumping all their money into artificial intelligence, not wood products for the future. Even if any of these new modified-wood products reach the marketing stage, they may experience the same fate as Staypak and Compreg, falling by the wayside or finding a tiny niche market. I am skeptical that construction-sized dimensional planks or sheet products will be cost-effective enough to replace current materials, but I may be proved wrong. The most optimistic scenario, in my view, would need to combine very cheap wood sources with a low-energy manufacturing process. The self-densified product that Marsh cited might be a reasonable candidate.
Soon we will have a test case. The company Invent Wood has built an 88,971-sq-ft manufacturing plant in Frederick, Maryland, where they will manufacture a product they’re calling MettleWood. The company has received a $20 million grant from the U.S. Department of Energy and claims that MettleWood is 80 percent lighter and 50 percent stronger than steel; is cheaper to manufacture; is sustainably sourced; and is highly resistant to moisture, infestation, and fire. Production was scheduled to begin in 2025, but I don’t know if that goal has been met. Stay tuned. 
Dr. Richard Jagels is an emeritus professor of forest biology at the University of Maine, Orono. Please send correspondence to Dr. Jagels by mail to the care of WoodenBoat, or via e-mail to Senior Editor Tom Jackson, tom@woodenboat.com.
References:
- Chen et al., DOI: 10.1021/acsnano.8b06409.
- Koo et al., DOI: 10.1021/acsomega.5a05203.
- Lu et al., DOI: 10:1126/sciadv.adv0183.
- Song et al., DOI: 10.1038/nature25476.