A novel woody composite that is as durable as bone and as hard as metal might pave the way for biodegradable polymers.
The strongest element of a tree is the walls of its tiny cells, not its trunk or widespread roots.
A single wood cell wall is made up of cellulose fibers, which are the most prevalent polymer in nature and the primary structural component of all plants and algae. Reinforcing cellulose nanocrystals, or CNCs, are chains of organic polymers organized in almost perfect crystal patterns inside each fiber. CNCs are stronger and stiffer than Kevlar at the nanoscale. CNCs might offer a path to stronger, more sustainable, organically produced polymers if the crystals could be worked into materials in large fractions.
An MIT team has developed a composite consisting mostly of cellulose nanocrystals with a little amount of synthetic polymer. The organic crystals make up 60 to 90% of the material, which is the largest percentage of CNCs yet obtained in a composite.
The cellulose-based composite was found to be stronger and tougher than certain forms of bone, as well as harder than ordinary aluminum alloys, according to the researchers. The material features a brick-and-mortar microstructure that mimics mollusc nacre, which is the hard inner shell lining.
The team came up with a recipe for a CNC-based composite that they could make with 3D printing and traditional casting. They utilized penny-sized bits of film to evaluate the material’s strength and hardness after printing and casting the composite. They then shaped the composite into the form of a tooth to demonstrate how it might be used to create stronger, harder, and more sustainable cellulose-based dental implants — and, for that matter, any plastic product.
“We can provide polymer-based materials mechanical capabilities they never have before by producing composites with CNCs at high loading,” says A. John Hart, professor of mechanical engineering. “It’s potentially better for the earth as well if we can replace some petroleum-based plastic with organically generated cellulose.”
Hart and his colleagues, which included Abhinav Rao PhD ’18, Thibaut Divoux, and Crystal Owens SM ’17, published their findings in the journal Cellulose today.
Gel ties
More than 10 billion tons of cellulose are generated each year from plant bark, wood, and leaves. The majority of this cellulose is used to make paper and textiles, with a small amount being powdered for use in food thickeners and cosmetics.
cellulose nanocrystals, which may be isolated from cellulose fibers by acid hydrolysis, have been studied by scientists in recent years. Natural reinforcements in polymer-based materials might be made from the very strong crystals. However, due to the crystals’ tendency to agglomerate and only weakly bind with polymer molecules, researchers have only been able to integrate small percentages of CNCs.
Hart and his colleagues wanted to create a composite that had a high percentage of CNCs and could be shaped into robust, long-lasting shapes. They began by combining a synthetic polymer solution with commercially available CNC powder. The scientists calculated the right amount of CNC and polymer to transform the solution into a gel that could be put into a mold to be cast or fed through the nozzle of a 3-D printer. They broke up any clumps of cellulose in the gel using an ultrasonic probe, making it more probable for the scattered cellulose to establish strong links with polymer molecules.
They used a 3-D printer to print some of the gel and poured the rest into a mold to be cast. The printed samples were then allowed to dry. The material reduced throughout the procedure, leaving behind a solid composite made mostly of cellulose nanocrystals.
Rao explains, “We simply disassembled wood and reassembled it.” “We reassembled the best components of wood, which are cellulose nanocrystals, to create a new composite material.”
Difficult cracks
When the researchers looked at the structure of the composite under a microscope, they saw that the cellulose grains settled into a brick-and-mortar pattern, similar to the architecture of nacre. This zig-zagging microstructure in nacre prevents a fracture from traveling straight through it. This was also the case with the novel cellulose composite discovered by the researchers.
They used instruments to create nano- and micro-scale fractures to assess the material’s resistance to cracks. They discovered that the arrangement of cellulose grains in the composite prevented fractures from breaking the material at numerous scales. The composite’s hardness and stiffness are at the border between normal plastics and metals due to its resistance to plastic deformation.
The team is working on strategies to reduce gel shrinkage when they dry in the future. When printing little things, shrinkage isn’t an issue, but anything larger might bend or shatter when the composite dries.
“You could keep scaling higher, maybe to the meter size,” Rao adds, if you could prevent shrinking. “Then, if we were to dream large, we could replace a major percentage of plastics with cellulose composites.”
The Proctor & Gamble Corporation, as well as the National Defense Science and Engineering Graduate Fellowship, contributed to this study.
