Clever chemical eco-coating creates fire-resistant wood

As a country vulnerable to severe conflagrations, Australia is familiar with the scale of damage that fire can cause to homes and other buildings.

Now, researchers from Texas A&M University have reported the development of a new environmentally friendly coating that makes wooden construction materials fire-resistant. The research was presented at the Spring meeting of the American Chemical Society (ACS) this week.

The fire-resistant coating could be used to protect construction materials such as wooden studs or strand board.

In lab tests, wood treated with the new coating released less heat and less smoke when burnt. A layer of char formed quickly on the timber’s surface, protecting the underlying structure. These features are likely to substantially reduce damage from fires and slow down their spread.

 “This type of treatment, which could be deposited via dipping, spraying or pressure treatment, could make homes much safer,” says Thomas Kolibaba, a postdoctoral researcher leading the development of the coating.

“The coating could reduce flame spread and smoke production, which could limit damage and give people more time to evacuate.”

Fire-resistant coating demonstration a piece of charred wood on the left and a piece of wood that has been almost entirely burnt away on the right
Wood treated with the coating (left) can survive fire much better than untreated wood (right). Credit: Ethan Iverson.

The new material belongs to a category known as a polyelectrolyte coatings, which have previously been applied to textiles.

Typically, this type of coating is constructed by dipping a material in a solution containing a polymer with lots of positive electrical charges, then in another solution containing a polymer with lots of negative electrical charges, and repeating this process several times.

The positive and negative charges in the different layers of the solutions are attracted to one another and form complexes on the surface of the material. This coating can stop or retard flames, protecting the material beneath.

Read more: Explainer: what is a polymer?

However, this approach isn’t suitable for wood. Since wood is so porous, it takes too long for the different solutions to soak in and deposit to form the coating. Furthermore, wood is already comparatively heavy, so adding lots of layers of coating can add up.

So Kolibaba had to take a different tack to make a coating suitable for protecting wood. Rather than alternating layers of positively and negatively charged polymer solutions, the new method uses a solution that contains a positively charged polymer called polyethyleniminine (PEI), a monomer called hydroxyethyl methacrylate phosphate (HMP), and a third molecule, the photoinitiator diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO).

The wood is dipped into the solution and then exposed to UV light for a few minutes. The UV radiation activates the TPO, which causes the HMP monomer subunits to join together to create a new, negatively charged polymer. The positive and negative polymers can then unite to form a protective coating without the need for multiple steps.

The coating created with the new technique is transparent and has a thickness of only a few micrometres, meaning that coated wood looks the same and is only slightly heavier.

The molecules in the coating are held together by covalent chemical bonds, which are stronger than the ionic bonds present in earlier polyelectrolyte coatings, so the team expects that the treatment will be water-resistant as well.

The researchers suggest it could be sprayed onto existing wooden structures using a backpack sprayer. According to senior researcher Jaime Grunlan, the coating could also be applied to other materials, such as textiles and polyurethane foam, with applications for clothing, furniture, and even in aviation.

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