Sustainable Structural Material for Plastic Substitute

Broken by waves, sunlight, and marine animals, a single plastic bag can be broken down into 1.75 million microscopic fragments, which are called microplastics. (Image: via   pixabay  /  CC0 1.0)
Broken by waves, sunlight, and marine animals, a single plastic bag can be broken down into 1.75 million microscopic fragments, which are called microplastics. (Image: via pixabay / CC0 1.0)

Plastic is a kind of widely used artificial material. The invention of plastic gave us a lightweight, strong, and inexpensive material to use, but it also brought us the plastic apocalypse. Much of the unrecycled plastic waste ends up in the ocean, Earth’s last sink. Broken by waves, sunlight, and marine animals, a single plastic bag can be broken down into 1.75 million microscopic fragments, which are called microplastics.

Those microplastics might finally end up in your blood and body through the fish you eat or the water you drink. During the long-term evolution of most plants on Earth, cellulose-based materials have been developed as their own structural support materials. Cellulose in plants mainly exists in the form of cellulose nanofibers (CNF), which have excellent mechanical and thermal properties. CNF, which can be derived from plants or produced by bacteria, is one of the most abundant all-green resources on Earth.

CNF is an ideal nanoscale building block for constructing macroscopic high-performance materials, as it has higher strength (2 GPa) and modulus (138 GPa) than Kevlar and steel and a lower thermal expansion coefficient (0.1 ppm K-1) than silica glass. Based on this bio-based and biodegradable building block, the construction of sustainable and high-performance structural materials will greatly promote the replacement of plastic and help us avoid the plastic apocalypse.

The cellulose nanofiber-derived bulk CNFP structural material and its characterization. (a) Photograph of large-sized CNFP with a volume of 320 × 220 × 27 mm3. (b) The robust 3D nanofiber network of CNFP. Numerous CNFs are intertwined with each other and combined together by hydrogen bonds. (c) Parts with different shapes of CNFP produced by a milling machine. (d) Ashby diagram of thermal expansion versus specific strength for CNFP compared with typical polymers, metals, and ceramics. (e) Ashby diagram of thermal expansion versus specific impact toughness for CNFP compared with typical polymers, metals, and ceramics. Copyright 2020, American Association for the Advancement of Science. (Image: Shu-Hong Yu)

The cellulose nanofiber-derived bulk CNFP structural material and its characterization. (a) Photograph of large-sized CNFP with a volume of 320 × 220 × 27 mm³. (b) The robust 3D nanofiber network of CNFP. Numerous CNFs are intertwined with each other and combined together by hydrogen bonds. (c) Parts with different shapes of CNFP produced by a milling machine. (d) Ashby diagram of thermal expansion versus specific strength for CNFP compared with typical polymers, metals, and ceramics. (e) Ashby diagram of thermal expansion versus specific impact toughness for CNFP compared with typical polymers, metals, and ceramics. Copyright 2020, American Association for the Advancement of Science. (Image: Shu-Hong Yu)

Nowadays, a team lead by Prof. Shu-Hong Yu from the University of Science and Technology of China (USTC) reports a high-performance sustainable structural material called cellulose nanofiber plate (CNFP) (Fig. 1a and c) that is constructed from bio-based CNF (Fig. 1b) and is ready to replace the plastic in many fields. This CNFP has a high specific strength (~198 MPa/(Mg m³)), which is 4 times higher than that of steel and higher than that of traditional plastic and aluminum alloy.

In addition, CNFP has higher specific impact toughness (~67 kJ m-2/(Mg m³)) than aluminum alloy and only half of its density (1.35 g cm³). Unlike plastic or other polymer-based materials, CNFP exhibits excellent resistance to extreme temperatures and thermal shocks. The thermal expansion coefficient of CNFP is lower than 5 ppm K-1 from -120°C to 150°C, which is close to ceramic materials and much lower than typical polymers and metals. Moreover, after 10 times of rapid thermal shock between 120°C in a bake oven and -196 °C in liquid nitrogen, CNFP retains its strength. These results show its outstanding thermal dimensional stability, which allows CNFP to have great potential as a structural material under extreme temperatures and alternate cooling and heating.

Owing to its wide range of raw materials and bio-assisted synthesis process, CNFP is a low-cost material, costing only $0.5/kg, which is lower than most of plastic. With its low density, outstanding strength and toughness, and great thermal dimensional stability, all of these properties of CNFP surpass those of traditional metals, ceramics, and polymers (Fig. 1d and e), making it a high-performance and environmentally friendly alternative for engineering requirements, especially for aerospace applications.

CNFP not only has the power to replace plastic and save us from drowning in them, but it also has great potential as the next generation of sustainable and lightweight structural materials.

Provided by: University of Science and Technology of China [Note: Materials may be edited for content and length.]

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