A Forest of Invisible Trees

New York City skyline in sunset (credit: Raoyang – stock.adobe.com)

The urban jungle could one day be a forest of invisible trees.

The Forest Products Laboratory (FPL) is pushing the technological concepts of science fiction into reality. A scientist at FPL is helping progress the feasible development of an alternative transparent material to glass that is made from wood.

When picturing the densely populated cities of the world—London, New York, Shanghai, Tokyo, Mumbai—the mind’s eye might bring up images of endless behemoths towering with glinting plate glass. By far, glass is currently the most ubiquitous building material for windows. Gazing through our windows, the economic and ecological cost of this building material is not immediately perceptible.

Unless its winter—and the cold seeping through brings on a shiver.

Glass, especially single paned glass, has a high intrinsic thermal conductivity. Or in other words, it has a natural ability to transfer heat through it. In cold months, that equates to a heftier heating bill. And during the summer, homes heat up faster creating pricier cooling costs. Energy flows through glass as if all the windows have been left open.

Additionally, manufacturing current building glass comes with a heavy carbon footprint. Yearly manufacturing emissions are approximately measured at 25,000 metric tons.

An invisible forest sprouts—

Finding alternatives to glass has been an ongoing research exploration. However, transparent wood has emerged as one of the most promising, feasible materials of the future. Wood as a transparent material did pose some challenges—haze, durability, and thermal insulation for energy efficiency were previously barriers to a workable product.

Forest Products Laboratory scientist Junyong Zhu and his colleagues from the University of Maryland Department of Materials Science and Engineering, and Department of Mechanical Engineering, and the University of Colorado Department of Mechanical Engineering have developed a process for a transparent wood composite that simultaneously enhanced transparency, increased thermal insulation, and improved durability. They reported their findings in the Journal of Advanced Functional Materials in their paper, “A Clear, Strong, and Thermally Insulated Transparent Wood for Energy Efficient Windows.”

How are transparent trees even possible?

Wood from balsa, one of the lowest density trees in the world due to its fast growth rate, is treated to a room temperature bath of sodium hypochlorite (NaClO). NaClO is a strong oxidizing agent that can remove the compounds in wood responsible for light absorption. After a long oxidizing bath, the balsa wood structure becomes more porous and cell walls are significantly thinned. In essence, it is bleached and leached of nearly all visibility.

After its bath, the wood is then infiltrated with polyvinyl alcohol (PVA), a synthetic polymer, which creates a product with high optical transmittance and low haze. The transparency produced from the PVA addition is approximately 91%, which is comparable to today’s glass translucency standard.

Schematic illustration of the fabrication process of the transparent wood, which features various outstanding characteristics compared to glass windows. a) The large-scale naturally aligned wood is obtained by a scalable industry-adopted rotary cutting method. The transparent wood features optimized optical properties after delignifcation and PVA infltration. b) A radar chart compares the various properties of the transparent wood, natural wood, and glass when used as building materials. c) A schematic of the energy saving process while using the transparent wood in the exterior windows of a building in cold weather.

Moreover, the addition of the synthetic polymer has made for a far more durable, lighter, transparent material than glass. It can sustain strong impact primarily because of its naturally dense wood structure combined with cellulose and the energy absorbing polymer filler. This gives it higher ductility, the ability of a material to undergo significant stress before deformation. And unlike glass, when it does fail under stress it bends or splinters instead of sharply shattering.

Transparent wood also outperforms glass in thermal insulation. Its low thermal conductivity means that it transfers less heat and energy. It is estimated to be more than five times less thermally conductive than glass. As the world looks for ways to make a positive impact on climate change, that equates to real world economic and environmental benefits.

Economically, it won’t break the bank either. Transparent wood is made from a sustainable, renewable resource with low carbon emissions. And because of its compatibility with existing industrial processing equipment, it has the potential for an easy transition into fabrication.

With all the potential benefits transparent wood has to offer, an invisible forest could create our greenest planet yet.

To find out more about the amazing advancements our scientists are making, visit the Forest Products Laboratory at: https://www.fpl.fs.fed.us/