Wood is one of the oldest and best-known building materials in history, but that has not stopped researchers at the University of Maryland from exploring some new and particularly small applications for this arboreal product. Small, that is, in the sense of the wood’s microscopic structure.
Working with wood fibers at the nanoscale — at sizes approximately 10,000 times smaller than the diameter of a human hair — the University of Maryland researchers have developed nanocellulose-based products that are transparent, as strong as steel but six times lighter, and can help keep buildings cool, among various other benefits, explains Liangbing Hu, Ph.D., the Herbert Rabin Distinguished Professor in the University of Maryland’s Department of Materials Science and Engineering and the director of the university’s Center for Materials Innovation. Hu is also a co-founder of InventWood, a University of Maryland spinoff company working to develop markets for these new wood products.
Most recently, Hu and his colleagues published a research paper on a new technique to fabricate transparent wood, a product that often requires the use of an immersion bath of toxic chemicals to remove the light-absorbing polymer known as lignin, which is found in the cell wall of many plants. But the immersion method can impair the mechanical strength of the wood and generates a liquid waste that is difficult to recycle, according to the paper, “Solar-assisted fabrication of large-scale, patternable transparent wood,” which appeared in the Jan. 27 issue of the open-access scientific journal Science Advances.
Instead, Hu and his colleagues developed a technique that relies on a chemical brushing, rather than immersion, combined with an ultraviolet light illumination process to remove the light-absorbing chromophores of the lignin. This lignin-modified transparent wood “holds great potential in energy-efficient building applications,” among other uses, Hu and his co-authors concluded in the paper.
The team has also produced a nearly transparent wood product — akin to frosted glass — by replacing lignin with a clear epoxy.
Supported by an approximately $4 million grant from the U.S. Department of Energy, the University of Maryland researchers created a so-called super-wood product trademarked as MettleWood, which in tests proved strong enough to stop a speeding bullet. When the lignin is removed and the remaining material is compressed under high pressure, the result is a lightweight structural material with potential applications that range from automotive and aircraft components to bridge and building materials.
The University of Maryland research also produces a radiative cooling structural material that was tested at a farm in sun-drenched Arizona, where the product helped cool a structure by an average of 12 degrees Fahrenheit compared to natural wood, Hu says. As a Feb. 8 University of Maryland press release explained, the product is “pure white in the visual light spectrum, meaning that on building roofs, it doesn’t soak up the sun.” Paradoxically, it’s pure black in the invisible infrared spectrum, which helps it radiate heat back into outer space.
Hu and his colleagues have also researched techniques that convert wood to a squishy, bouncy material that might have applications as a shock absorber. They have also developed a process that could use wood inside a solar evaporator to help desalinate seawater.
The teams at the University of Maryland and InventWood have scaled the engineered wood from lab scale (typically about 10 by 20 cm) to pilot scale (1 by 3 ft), Hu notes. So he and his colleagues are actively seeking partners in government agencies and the manufacturing industry to create actual products to help “unlock the potential of this abundant material,” he explains.
The University of Maryland research works equally well with fibers from any type of tree, including faster-growing trees — such as poplars — that normally are not considered as desirable in terms of density and strength as slow-growing cedars or oaks, Hu notes. Other plants, including sugar cane and especially bamboo, generate strong, usable fibers, Hu adds.
Looking forward, Hu says the research can potentially replace some steel and concrete building materials with more sustainable, wood-based products. Moreover, since that will help make wood a more valuable commodity, he also hopes it will promote better forest management practices.