Researchers Toughen Wood Coatings

TUESDAY, JULY 20, 2021

Researchers of Aalto University (Espoo, Finland) are saying they have found a way to turn wood waste into a coating that resists abrasion, stains and sunlight better than traditional coatings. The team is reportedly studying lignin, a natural polymer in wood and other plant materials, to create what is being billed as a “safe, low-cost and high-performing coating for use in construction.”

Our new coating has great potential to protect wood. It's more water repellent than a lot of commercial coatings because it retains the natural structure of wood and its micro-scaled roughness,” said Alexander Henn, doctoral candidate at Aalto University, The School of Chemical Engineering.

“Since it's hydrophobic, the coating is also quite resistant to stains, while lignin’s inherent structure resists color changes from sunlight. It also does an excellent job of retaining wood’s breathability.”

Lignin is typically regarded as waste during the biorefinery process, but it has several beneficial properties, according to researchers.

The Research

In the team’s recent paper, published in Applied Materials & Interfaces, researchers argue that lignin is a natural polyphenol that can be used to cure epoxies, but it’s poor water solubility has impeded its use in coatings in the past.

To address this issue, then, the team used water-dispersible colloidal lignin particles (CLPs) and an epoxy compound, glycerol diglycidyl ether (GDE) to prepare multi-protective bio-based surface coatings.

According to researchers, with the GDE/CLP ratios of 0.65 and 0.52 g/g, the cured films became highly resistant to abrasion and heat and, when applied as a coating on wooden substrates, the coating enabled effective protection against water, stains and sunlight with very thin layers (less than half the weight of commercial coatings) while retaining the wood’s breathability.

The team says that optimal hydrophobicity was reached with a coat weight of 6.9 g(CLP)/m2, resulting in water contact angle values of up to 120 degrees.

“Due to their spherical shape and chemical structure, the CLPs acted as both a hardener and a particulate component in the coating, which removed the need for an underlying binding polymer matrix. Light interferometry measurements showed that while commercial polymeric film-forming coatings smoothened the substrate noticeably, the particulate morphology retained the substrate’s roughness in lightweight coatings, allowing for a high water contact angle,” the team says in its abstract.

“This work presents new strategies for lignin applications in durable particulate coatings and their advantages compared to both currently used synthetic and bio-based coatings.”

“Lignin as a coating material is actually very promising with its many benefits compared to the synthetic and bio-based coatings currently used. It has excellent anti-corrosion, anti-bacterial, anti-icing, and UV-shielding properties,” said Monika Österberg, Head of the Department of Bioproducts and Biosystems at Aalto University.

“Our future research will concentrate on developing characteristics like elasticity of the coating.”

Other Lignin Studies

The Aalto team isn’t the first to look at lignin. In 2018, engineers based out of the University of Maryland, College Park, developed a method involving compression that makes wood stronger than most titanium alloys by way of removing lignin.

Liangbing Hu, the leader of the research team, noted at the time that the treatment method made the wood “12 times stronger than natural wood and 10 times tougher.”

The process begins with the removal of the wood’s lignin, which is responsible for making the wood brown and rigid. The wood is then compressed at 150 degrees Fahrenheit, which packs the cellulose fibers closely together. As a result, any defects are crushed together. A coat of paint is also added, to extend the treatment process.

The research team discovered that when the fibers were pressed together, they could form strong hydrogen bonds. The wood was also five times thinner than it had been originally. To test the wood’s strength, the team shot bullet-like projectiles at it; a projectile went straight through the natural wood, but the treated wood stopped it halfway through.

“It is both strong and tough, which is a combination not usually found in nature,” said Teng Li, the co-leader of the team. “It is as strong as steel, but six times lighter. It takes 10 times more energy to fracture than natural wood. It can even be bent and molded at the beginning of the process.”

According to Hu, the treated wood could be used in cars, airplanes and buildings, in “any application where steel is used.”  Hu also noted that soft, more environmentally friendly woods, like pine or balsa, could replace woods like teak in furniture and buildings.


Tagged categories: Coating Materials; Coatings Technology; Coatings Technology; EMEA (Europe, Middle East and Africa); EU; Research and development; Wood coatings

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