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Thermal insulation is extremely important in cityscapes today, keeping the heat in, in cold weather and keeping it out when it is hot outside. It is used in buildings, piping, refrigerators, and other applications. Commercial thermal insulation materials include glass wool, polystyrene foam or Styrofoam, cellulose, and mineral wool.
These materials are porous, with tiny air-filled compartments. The very low thermal conductivity of air along with it being trapped in small cells makes these materials very good at not allowing heat to pass through.
However, most of these materials are not environmentally friendly, are expensive, and usually end up in landfills. Styrofoam, for example, has been considered something of a villain recently for collecting in the Earth without ever degrading. Cellulose, the tough polymer that makes up the cell wall of plants and makes sturdy stems and trunks, has been used as thermal insulation for a long time.
Commercially, it is mainly made by milling waste newspaper. It has a thermal performance similar to glass wool and rock wool and is generally a low-cost insulation option.
Novel Cellulose-Based Insulator
Nanowood is a stripped-down version of wood, such that its constituent fibers are aligned in one direction. Tests show it has some of the best thermal insulation properties, with the added advantage of being biodegradable.
Liangbing Hu and colleagues at the University of Maryland in the US have developed a method that makes cellulose a super insulator, with a performance better than almost all other insulators.
Current methods for making nanocellulose-based products are typically “bottom-up” where the nanofibrils need to be reassembled after chemical or mechanical processes. The fibers are randomly oriented and have poor mechanical properties.
In contrast, to make nanowood, the researchers used a “top-down” approach. They subjected wood to a chemical treatment that removed all the lignin — a polymer glue that holds the cellulose in the wood together, and most of the hemicellulose, another part of the wood.
These materials make wood a good conductor of heat and removing them makes nanowood an excellent insulator. The material changed color to pure white, which will reflect the heat away, helping make it a good insulator.
Another crucial factor that makes nanowood such a good insulator is its structure. Most thermal insulation materials are isotropic; their properties are the same in all directions. But, the chemical treatment of wood leaves cellulose nanofibrils aligned in one direction.
The thermal conductivity perpendicular to the alignment direction (0.03 W/mK) is lower than that parallel to the fibrils (0.06 W/mK). Hence, heat spreads along the direction of alignment, preventing any local hotspots, and there is very little thermal conduction perpendicular to the direction of alignment. This makes nanowood a comparable or even slightly better insulator than Styrofoam.
The alignment of the fibers also makes nanowood extremely strong. It can withstand pressures of more than 50 times that of commercial cellulose foam and 30 times that of Styrofoam.
In addition to being mechanically strong, having one of the best insulation properties, and being light, cellulose is cheap and widely available. The authors estimate that it could be made for about $ 7.44/m2.
Nanowood is Sustainable and Biodegradable
Cellulose is the most abundant organic polymer. To make the production sustainable, the cellulose could be gathered from fast-growing tree species like balsa. The chemical treatment uses readily available chemicals like sodium hydroxide, sodium sulfite, and hydrogen peroxide, a chemical commonly used to bleach hair.
The researchers envision nanowood could be used in different applications that require thermal insulation. It can be rolled and folded easily when its thickness is less than 1 mm, making it suitable for wrapping around steam or chemical pipes. Other applications could include use in buildings, cars, and perhaps to protect fragile electronic components from heat.
The researchers are well on their way to commercialize the product through a spin-off company called Invent Wood.
Sources and Further Reading
- Li et al., “Anisotropic, lightweight, strong, and super thermally insulating nanowood with naturally aligned nanocellulose,” Sci. Adv. (2018), 4. http://advances.sciencemag.org/content/4/3/eaar3724
- Svagan et al., “Biomimetic Foams of High Mechanical Performance Based on Nanostructured Cell Walls Reinforced by Native Cellulose Nanofibrils,” Adv. Mater. (2008), 20, 1263. https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.200701215
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