Paper grocery bags that can hold a few liters of milk without tearing? That may not be too far off thanks to the development of a "nanopaper" that is tougher than cast iron. The material--made from nanosized whiskers of cellulose--is also lighter than conventional paper and could provide sturdy scaffolds for growing replacement tissues and organs.
Conventional paper is made from cellulose, a crystalline polymer of glucose that's the primary component of plant cell walls. At the nanoscale level, cellulose can be extremely strong, with individual fibers capable of withstanding more stress than glass fibers or steel wire. But paper processing generates relatively large cellulose microfibers riddled with defects that can break apart under stress. That leaves most commercial paper with a tensile strength that tops out at about 30 megapascals (MPa), says Lars Berglund, a lightweight structures engineering expert at the Royal Institute of Technology in Stockholm, Sweden.
To toughen paper up, Berglund and his colleagues kept the cellulose fibers small. They did this by breaking down wood pulp in water with a combination of enzymes and mechanically beating it further. The result: defect-free nanofibers about 1000 times smaller than typical cellulose fibers. As a final step, the researchers treated their nanofibers with carboxymethanol, which coated the fibers in carboxyl groups. These groups readily form hydrogen bonds that helped the fibers make tight contacts with one another, further strengthening the material. The final result--published in the current issue of Biomacromolecules--was a paper with a tensile strength of 214 MPa, far above the 130 MPa of cast iron and the previous record of 103 MPa for a high-strength paper. In addition to improving paper products directly, the new cellulose nanofibers could help create reinforced plastic composites cheaper than those reinforced by carbon fibers, the researchers say.
The new nanopaper is "quite interesting," says Mike Wolcott, a materials scientist and cellulose fiber expert at Washington State University in Pullman. In addition to making paper stronger, the nanopaper has large pores between the fibers, which should also make it easier and cheaper to dry, thus reducing the cost of any final product, he says. And because cellulose is the most abundant organic compound on the planet, nanopaper has the potential to be cheaper than more-exotic, expensive-to-produce nanomaterials such as carbon nanotubes, says John Simonsen, a physical chemist and nanocrystalline cellulose expert at Oregon State University in Corvallis.