New 'super-paper' is stronger than cast iron

Punching your way out of a paper bag could become a lot harder, thanks to the development of a new kind of paper that is stronger than cast iron.

The new paper could be used to reinforce conventional paper, produce extra-strong sticky tape or help create tough synthetic replacements for biological tissues, says Lars Berglund from the Swedish Royal Institute of Technology in Stockholm, Sweden.

Despite its great strength, Berglund's "nanopaper" is produced from a biological material found in conventional paper: cellulose. This long sugar molecule is a principal component of plant cell walls and is the most common organic compound on Earth.

Wood is typically about half cellulose, mixed with other structural compounds.

Support network

In plant cell walls individual cellulose molecules bind together to produce fibres around 20 nanometres in diameter, 5000 times thinner than a human hair. These fibres form tough networks that provide the cell walls with structural support.

"Cellulose nanofibres are the main reinforcement in all plant structures and are characterised by nanoscale dimensions, high strength and toughness," Berglund told New Scientist.

Cellulose is extracted from wood to make paper, is the basis of cellophane, and has also recently been used by materials scientists developing novel plastic materials. But they have used it only as a cheap filler material, ignoring its mechanical properties.

However, the mechanical processes used to pulp wood and process it into paper damage the individual cellulose fibres, greatly reducing their strength. So Berglund and colleagues have developed a gentler process that preserves the fibres' strength.

Tough as iron

The new method involves breaking down wood pulp with enzymes and then fragmenting it using a mechanical beater. The shear forces produced cause the cellulose to gently disintegrate into its component fibres.

The end result is undamaged cellulose fibres suspended in water. When the water is drained away Berglund found that the fibres join together into networks held by hydrogen bonds, forming flat sheets of "nanopaper".

Mechanical testing shows it has a tensile strength of 214 megapascals, making it stronger than cast iron (130 MPa) and almost as strong as structural steel (250 MPa).

Normal paper has a tensile strength less than 1 MPa. The tests used strips 40 millimetres long by 5mm wide and about 50 micrometres thick.

Dissipating stress

The secret to the nanopaper's performance is not only the strength of the undamaged cellulose fibres, but also they way they are arranged into networks. Although strongly bound together, they are still able to slip and slide over each other to dissipate strains and stresses.

The individual cellulose fibres are also much smaller than in conventional paper. "A regular paper network has fibres 30 micrometres in diameter, here we are at a scale three orders of magnitude smaller," says Berglund. "The material [has] very small defects compared with a conventional paper network."

"This [work] shows quite clearly the potential for cellulose nanofibres to provide a basis for reinforcement," says Stephen Eichhorn, a polymer scientist at the University of Manchester, UK.

Journal Reference: Biomacromolecules (DOI: 10.1021/bm800038n)

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