Wood competes with metals

Materials World magazine
,
1 Apr 2018

A new process turns any wood into a material competitive with titanium, researchers claim. Ellis Davies investigates.

Developed by researchers at the University of Maryland, USA, a treatment makes wood 12 times stronger than it would be naturally, meaning it could become a competitor for steel and titanium alloys and be comparable to carbon fibre – but cheaper. While the densified wood is stronger than steel, it is also six to seven times lighter, leading to a strength (over density) higher than almost all metals and alloys.

Many methods have previously been developed to pre-treat wood for densification, such as steam, heat, ammonia, and cold rolling treating processes. Liangbing Hu, Associate Professor at the University of Maryland, told Materials World, ‘All these existing methods only soften the wood structure without changing the chemical composition of wood, resulting in incomplete densification – 60% reduction as maximum.

‘We developed a densification technique that combines chemical modification – partially removing lignin and hemicellulose – and hot pressing.’ The lignin is removed using a boiling process in an aqueous mixture of sodium hydroxide and sodium sulphate. 

The resulting densified wood features a highly dense – up to 80% thickness reduction – and laminated structure with intertwined cell walls that are fully collapsed, without pores. 

Lignin plays an important role in getting the high mechanical performance. If wood is hot-pressed without partial lignin or hemicellulose removal, it will only be modestly densified, leaving many pores between collapsed cell walls. 

Hot-pressing the completely delignified wood will cause structure collapse – both lead to unsatisfactory mechanics. ‘By contrast, our invented partial delignification process, followed by pressing, can fully densify the structure without collapse,’ said Hu. 

The team chose three hardwoods – basswood, oak, and poplar – and two softwoods – western red cedar and eastern white pine. 

The results show that the two-step processing approach is universally effective for all types of wood, and can enhance strength and toughness simultaneously.

Treated wood was tested for tensile, bending, and compressive properties, while the team also tested the new wood material against natural wood by shooting them with bullet-like projectiles. They blew through the natural wood, but stopped partway in the treated wood. Researchers did not carry out any testing with steel, but compared the mechanical properties of it with those of the treated wood. 

Thinking of the environment

Aside from the economical advantages, the treated wood could have environmental benefits. Metals are considered non-renewable, and are associated with high greenhouse gas emissions during their extraction and synthesis processes. The team believes that using wood instead would be more economical and environmentally friendly, as it would create less greenhouse gases and doesn’t require high cost processing. Soft woods, such as pine or balsa, are fast growing, providing a safe supply. Hu said, ‘The advantages of densified wood are its high mechanical performance, low-cost, abundant resource, and light weight.’

On a similar trend, the researchers are investigating potential integration into applications such as lightweight vehicles, wind turbines, and energy efficient buildings, extending the environmental claim. These applications do, however, require the material to meet the complete performance matrix, which the team is now working on.