Combining the strength of mother-of-pearl with the ready availability of wood waste could create a sturdy new building material, according to a team of researchers (ACS Nano 2020, DOI: 10.1021/acsnano.9b08647).
Nacre, or mother-of-pearl, coats the inside of mollusk shells. Its notable strength and hardness come from its unique structure, which consists of bricklike stacks of aragonite, a type of calcium carbonate, connected by layers of protein that act as a glue. Qingfeng Sun of Zhejiang A&F University, Huiqiao Li of Huazhong University of Science and Technology, and colleagues hoped they could mimic the microstructure of nacre to turn wood waste like sawdust, wood scraps, and thin branches into a building material much stronger than standard wooden composites such as particle board, which is made of wood chips and resin.
The researchers started by grinding wood waste and mixing it with calcium carbonate, sodium hydroxide, and sodium sulfite to make a powder of nanometer-scale particles. The team then added water to the resulting mix and heated it to 51 °C to encourage aragonite crystals to grow, creating mineralized scaffoldings of wood fibers that the researchers call lignocellulosic building blocks, equivalent to the aragonite layers in natural nacre.
Next, the researchers poured the mixture of building blocks and water into a mold and froze it. By controlling the speed and direction of the freezing, they forced ice crystals to form horizontally, starting at the bottom of the mold and working their way up. The ice forced the aragonite-and-lignocellulose building blocks into parallel layers similar to those found in nacre. The researchers freeze-dried the material to remove the ice, added polymethyl methacrylate (PMMA), then heated and compressed the substance. The PMMA polymerized around the platelets, acting as a binder like the protein layers in natural nacre, adding tensile strength and filling gaps so water cannot get into the final material.
The wooden nacre is similar in toughness to some copper alloys and almost as strong as natural nacre, but much lighter. Two people can stand on a thin board of wooden nacre without breaking it. “These qualities mean that our materials can absorb a large amount of energy and produce a certain amount of deformation without damage,” Sun says. Its waterproof qualities make it good for use in kitchens, bathrooms, and outdoors, he says.
“I think it will take a little while to scale up the synthesis to the level of manufacturing and to get it tested and qualified for use in construction,” says Jiaxing Huang, a materials scientist at Northwestern University. But it might already be useful for building furniture, he adds.
Both Huang and Shu-Hong Yu, who studies nanomaterials at the University of Science and Technology of China, worry that the freezing process could make the manufacture of wooden nacre expensive and hard to scale up. Sun agrees the process needs to be improved to meet manufacturing needs. He’d also like to look at refining the material to increase the strength and other material qualities to expand its range of applications.
This story was updated on Feb. 13, 2020, to correct the name of one of the study authors. It is Huiqiao Li, not Huiquaoi Li.