Soon, skyscrapers could be made of wood
US researchers find a simple way to make wood stronger than steel. Andrew Masterson reports.
Wood has been employed as a building material for thousands of years, but in today’s world of office towers and massive industrial complexes its usefulness is distinctly limited. Now, however, US scientists have demonstrated a treatment method that could see it being used instead of steel girders in major building projects.
Abundant and cheap, wood has long been a favoured choice as framework for houses. Beyond a certain size, however, it breaks or warps under pressure, limiting its use. It is also porous, and can expand and therefore weaken in humid conditions.
On the plus side, wood is a renewable resource that costs comparatively little and requires much less energy input than stronger materials – such as steel and alloys – routinely used in the making of big structures.
Recognising this, scientists and engineers have explored several avenues for strengthening wood. These include treating it with steam, heat, ammonia or mechanical processes such as putting under pressure through a roller.
All these methods, however, fall short of producing a sturdy construction material. Compressing it, for instance, reported a team led by Swedish architectural engineer Kristiina Laine in 2016, increased surface hardness but also left it prone to deformation.
The US team, led by Jianwei Song from the University of Maryland and reporting in the journal Nature, has created a two-step process that produces dense wood with “a specific strength higher than that of most structural metals and alloys”, making it a lightweight and low-cost alternative for building projects.
The first part of the process involves boiling it in a mixture of sodium hydroxide (NaOH) and sodium sulphite (Na2SO3) in order to partially remove two substances that play important structural roles in the formation of plant cells: lignin and hemicellulose.
After this, the wood is put through a hot-rolling process, which causes what’s left of the cell walls to collapse. The result, the scientists say, is a very dense form, characterised by tightly aligned cellulose nanofibres.
The process works with any species of tree.
As well as uses in the building industry, the scientists flag another potential application for their super-dense material – in the military.
Layered sheets of the material, they report, show promise “for low-cost armour and ballistic energy absorption”.