New composites from invasive species

Materials World magazine
1 Jul 2018

A new composite merging wood pulp and dried tunicates seeks to emulate the mechanical properties of natural composites. Khai Trung Le reports.

Derivatives from two surplus materials – wood pulp and dried tunicate, a marine invertebrate considered an invasive species in the USA and parts of Europe – have been used to form a new composite material via the Bouligand structure, a layered and rotated microstructure that is resistant to cracking.

The researchers say the composite is flexible, sustainable, nontoxic, and UV light-reflective, and could see application in food packaging, building construction, and vehicular design. ‘The right product, if developed, could be used in everything from aerospace composites to packaging that would keep food fresh,’ said Jeff Gilman, Composites Project Team Leader at the National Institute of Standards and Technology (NIST), USA.

Wood does not have a natural Bouligand structure, whereby the force of an impact is guided by nano sized, stairwell-like detours, deflecting the energy of the force, but, Gilman said, ‘The idea of making useful products out of wood pulp has intrigued a lot of people in many different industries.’ 

The gold standard

By removing lignin and amorphous cellulose from the wood pulp, the remnant solution would dry to form a Bouligand structure, although the films were brittle and weak. The NIST team began exploring the combination of the short wood-derived nanocellulose rods within the film with longer crystalline rods, including the inner structure of the tunicate. Adding tunicates cause the nanocrystals to twist in a different way, reportedly accelerating the structure formation in the wood pulp, as well as forming a tighter, denser pattern that makes the composite materials UV-resistant. Johan Foster, Associate Professor at Virginia Tech University, USA, said, ‘Tunicates have stuck out as the gold standard for their physical properties.’ Foster helped collect tunicates for the team from a dock in western France. 

Although they are plentiful, they are expensive to process, and Bharath Natarajan, lead author of the paper, Binary Cellulose Nanocrystal Blends for Bioinspired Damage Tolerant Photonic Films, published in Advanced Functional Materials, managed to observe the exact combination point of greatest toughness. He said, ‘If you put a little tunicate into the wood pulp composite, it makes it a little stiffer, doesn’t break as quickly, and becomes a little more flexible.

‘Put in 10% and it’s twice as strong. If your mixture is 30% tunicate and 70% wood pulp, the resulting composite is 15–20 times tougher. But after that, you don’t really see an improvement in strength, and there is a reduction in toughness.’

Useful delicacy

Tunicates lack natural predators, and their populations have grown to super-abundant numbers. This can lead to outcompeting native fish populations, reducing healthy plankton, fouling productive shellfish beds, and interfering with boat engines and fishing equipment. Harvesting tunicates has been previously proposed, with the centre of the tunicate considered an edible delicacy. With the NIST team’s research, there would be a use for the otherwise discarded shell