Coming soon: Water-proof wood!

The lumber on the left has been treated using atomic layer deposition, resisting absorbing water even when submerged. The untreated lumber (R) readily soaks up water, causing a colour change within seconds. (Pic: Allison Carter, Georgia Tech).

Atomic layer deposition – which is already frequently used in manufacturing micro-electronics for computers and cell phones – is now being explored for new applications such as wood.

Like pressure treatments, the process is performed in an airtight chamber, but in this case the chamber is at low pressures to help the gas molecules permeate the entire wood structure.

Pressure treating, which involves putting lumber inside a pressurized watertight tank and forcing chemicals into the boards, has been used for more than a century to help stave off the fungus that causes in wet environments.

Now researchers at the Georgia Institute of Technology have developed a new method that could one day replace conventional pressure treating as a way to make lumber not only fungal-resistant but also nearly impervious to water.

The new method was recently reported in the journal Langmuir and involves applying a protective coating of metal oxide that is only a few atoms thick throughout the entire cellular structure of the wood.

Wood has pores that are about the width of a human hair or a little smaller. The researchers used these holes as their pathways for the gases to travel throughout the wood structure.

As the gas molecules travel down those pathways, they react with the pore’s surfaces to deposit a conformal, atomic-scale coating of metal oxide throughout the interior of the wood. The result is wood that sheds water off its surface and resists absorbing water even when submerged!

The researchers took finished pine 2x4s and cut them into 1-inch pieces. They then tested infusing the lumber with three different kinds of metal oxides: titanium oxide, aluminum oxide and zinc oxide.

With each, they compared the water absorption after holding the lumber under water for a period of time. Titanium oxide performed the best by helping the wood absorb the least amount of water.

This is likely because of how the precursor chemicals for titanium dioxide react less readily with the pore surfaces and, therefore, have an easier time penetrating deep within the pores of the wood. By comparison, untreated lumber absorbed three times as much water.

The same phenomena exist in atomic layer deposition processes used for micro-electronic devices. The same titanium oxide precursor chemistries are known to better penetrate and coat complex nanostructures in micro-electronics.

Yet another benefit of the new process: vapour-treated wood was far less thermally conductive compared to untreated wood.

“A lot of attention is paid in home building to insulating the cavities between the structural components of a home, but a massive amount of the thermal losses are caused by the wood studs themselves,” said Mr Shannon Yee, an associate professor in the George W. Woodruff School of Mechanical Engineering with expertise in thermal systems.

“Lumber treated with this new process can be up to 30% less conductive, which could translate to a savings of as much as 2 million BTUs of energy per dwelling per year,” he added.

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