Professor Lars Berglund and researchers at Sweden's KTH Royal Institute of Technology started their process by chemically removing lignin from natural wood fibers – lignin is one component of wood cell walls that lend strength. (After cellulose, lignin is the second most abundant renewable carbon source on Earth.) What was left was a material that was "beautifully white," but still not transparent.
This actually isn't the first time we've seen wood turned into a transparent material, as nanofibrillated cellulose has been used to create items such as the substrate for wood-based computer chips. According to KTH, however, the new process should be particularly well-suited to large-scale applications and mass production.
For the lignin wood fibers to become transparent, the researchers mixed the material with prepolymerized methyl methacrylate (PMMA) which is a transparent plastic polymer. This altered the refractive index of the resulting mixture, turning it transparent. Depending on the intended application, the finished product can also be made more translucent, by fine-tuning the wood-to-PMMA ratio.
The thickness of the transparent wood is more than 10 times larger than transparent nanocellulose paper with similar transparency. The thick transparent plates could be used for walls and translucent solar cell windows. Different species of trees produce transparent wood of varying densities, which might be configured to suit specific applications.
The researchers are now looking at increasing the material's transparency, scaling up the manufacturing process, and using a wider variety of woods.
"It's attractive that the material comes from renewable sources," says Berglund. "It also offers excellent mechanical properties, including strength, toughness, low density and low thermal conductivity."