Schematic of NFC-graphite composite (top) and comparison with other high-strength structural materials (bottom).
NFC-graphite dispersion with a high solid content of 20 wt%. (a) Schematic of how NFC nanofibers attach to and disperse graphite flakes via hydrogen bonding. After cast drying, the composite is composed of NFC fibers and multilayered graphite flakes. (b) AFM image of a graphite flake exfoliated by NFC. The NFC fibers are closely associated with the surface of the graphite flake. (c) The solid content of the resulting graphite-NFC slurry (1:1 mass ratio) is 4-5 times higher than that of NFC and graphene-NFC solutions.
Production of the NFC-graphite slurry and the composite. (a) Raw materials: graphite and wood chips from which NFC is obtained. Graphite is dispersed and exfoliated in NFC suspensions under sonication. (b) Large-volume graphite-NFC slurries. (c) Fabrication process of the graphite-NFC composite. (d) Large-scale (120?cm?×?30?cm) graphite-NFC composite sheet.Plastic waste is a serious concern, with a plethora of discarded nonbiodegradable items filling up landfill and accumulating in oceans. Reducing, reusing, and recycling initiatives are only having a limited degree of success, so a strong, cheap, biodegradable replacement for petroleum-based plastics is highly desirable.
Now researchers from the University of Maryland College Park, Rice University, and University of California Merced have come up with a viable alternative to plastic in the form of a composite made from a mixture of graphite and cellulose extracted from wood pulp.
“Our work aims to offer a long-sought solution to a high-performance, low-cost, and fully degradable structural material as a potential replacement for petroleum-based plastics and metal-based structural materials,” explains Teng Li, who led the effort with Liangbing Hu. “We report, for the first time, a completely new strategy for the hybridization of one-dimensional cellulose with two-dimensional graphite via dispersion to manufacture a bulk composite.”
The new composite uses secondary bonds, in this case hydrogen bonds, between flakes of graphite and nanofibrillated cellulose (NFC) to create an extremely strong material. The graphite and cellulose form a bricks-and-mortar-like arrangement similar to that seen in strong natural materials like nacre.
Making the composite is straightforward: graphite flakes are simply dispersed in NFC at room temperature in water to create a thick slurry. This slurry can then be printed in large areas and dried to produce a material with remarkable physical properties.
In standard tensile, fracture toughness, scratch, and ballistic tests, the material demonstrated remarkable results that rival steel, aluminum alloys, polyethylene composites, and even carbon fibers. The researchers recorded tensile strength up to 1 GPa, toughness up to 30 MJ/m3, and a specific strength of 794 MPa/g cm-3 thanks to the low density of both graphite and cellulose.
“Our material is not only stronger than many steels but also six times lighter than steel, yielding a specific strength higher than any existing metal or alloy (including titanium alloys),” says Hu. “These promising mechanical properties are 5-10 times better than commonly used plastics as well.”
The room temperature, solvent-free approach is easily scalable and has a much lower environmental footprint than the manufacturing processes of other plastic or metal-based structural materials. Moreover, the composite is completely degradable in water at higher temperatures. Conversely, the researchers show that the composite can be coated to resist the effects of water and humidity during use.
Li and Hu are confident that the mechanical properties of the graphite-cellulose composite can be increased further, while reducing costs, to make it an ideal replacement for existing non-biodegradable materials.
“[This composite] could enable a paradigm shift from petroleum-based plastics to a revolutionary material with a drastically reduced carbon footprint,” they say.
Zhou et al., Materials Today (2019), https://doi.org/10.1016/j.mattod.2019.03.016