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Sustainable insulation material is made from wood

By Prachi Patel April 4, 2018
nanowood
A schematic of the microstructure of nanowood shows that its aligned cellulose nanofibers give it anisotropic thermal properties that make it a good insulator since heat can flow along the fibers’ length but not across them. Credit: Science Advances

Researchers have found a way to convert wood into a light, strong material that could be a sustainable alternative to current heat-insulating substances. Dubbed nanowood, the new material works better than commercial insulators like silica aerogel and Styrofoam. It is also biodegradable, and should be low-cost and easy to make in large quantities, the researchers report in a recent issue of Science Advances.

Insulation materials prevent heat from flowing out of buildings in cold weather and seeping into buildings on hot days. They are typically made of polystyrene or polyurethane. Some, like rock wool and cellulose foam, are processed from natural materials. But it is costly and energy-intensive to produce all these materials, some of which work better than others. Plus, they are non-renewable and cannot easily be reused or recycled at the end of their lives.

To make the new eco-friendly insulating material, a team of scientists and engineers from the University of Maryland and the University of Colorado, Boulder started with a piece of American basswood cut along its grain. They soaked it in a boiling bath of sodium hydroxide and sodium sulfite, treated it with hydrogen peroxide, and then freeze-dried it.

The process strips out the lignin and hemicellulose, leaving behind a 91% porous white material, a skeleton-like scaffold of cellulose nanofibers lined up parallel to each other. The paper industry uses a similar process, except they mash the leftover cellulose instead of freeze-drying to keep its structure intact. The researchers chose basswood because it grows fast, but say any type of wood would work.

The nanowood’s microstructure is key to its anisotropic thermal properties, which change with direction, and make it an excellent insulator. Heat flows along the aligned cellulose nanofibers so it does not aggregate, but it cannot cross perpendicular to the fibers because of the air between them. Measurements using a laser to heat up the nanowood showed that its thermal conductivity is about 0.03 W/m.K perpendicular to the cellulose fibers and about 0.06 W/m.K along the fibers.

To test its insulating properties, the team heated nanowood, Styrofoam, and silica aerogel pieces of the same size with a radiative heat source that mimics the sun, while measuring temperature on the other side. Nanowood reflected 95% of thermal energy and was at least 10°C cooler on the backside than Styrofoam or silica aerogel. Mechanical tests done by crushing the nanowood in two directions showed that it was stronger along the fibers’ axes (20 MPa) than across their width (13 MPa) but was, overall, stronger than other insulation materials like silica and polymer aerogels, Styrofoam and wool.

Besides being hypoallergenic and biodegradable, the material should cost less than $7.44 per square meter to make. Plus, pieces less than 1 mm in thickness can be rolled and folded, making it suitable for insulating pipes.

In addition to insulation, the unusual combination of high strength and low thermal conductivity might also make it possible to use the nanowood as a structural component in buildings, says Lennart Bergstrom, professor of materials and environmental chemistry at the University of Stockholm in Sweden who was not involved in this study. However, its practical use might not be possible everywhere, and might require additional processing. “The intrinsic moisture sensitivity of cellulose may limit its use in warm and humid climates and the material has to be made fire-resistance to be acceptable for commercial use,” he says.

Read the article in Science Advances.