Making Plastic as Strong as Steel

University of Michigan researchers have developed a nanoinfused polymer that is as strong as steel but as thin as plastic wrap

Courtesy of the University of Michigan

Could a seemingly simple clear plastic bag—the kind that you load your fruits and vegetables into at the supermarket—actually be as strong as steel? It could if it was made from a new composite plastic that blends the strength of nanoparticles with the pliancy of a water-soluble polymer.

Although it is no secret that nanotubes, nanosheets and nanorods are incredibly strong when combined in small numbers, larger materials made out of these microscopic building blocks cannot utilize much of that strength because the links between them are weak. But University of Michigan at Ann Arbor researchers report in Science that they have found a way to scale the strength of the nanomaterials to larger materials by transferring stress between nanosheets and a nanoscale polymer resembling white glue. Visually, it looks like a brick wall, where clay nanosheet "bricks" are held together by water-soluble polyvinyl alcohol "mortar." The result, according to the researchers, is a composite plastic that is light and transparent but as strong steel.

"If you take the nanoscale materials individually, say one carbon tube or one clay sheet, their mechanical properties will be astonishing," says U.M. engineering professor Nicholas Kotov, a co-author of the study. Simply combining a large volume of clay, nanosize platelets into one continuous block, however, results in a brittle chalklike material riddled with cracks.

Researchers created a strip of clear material as thick as a sheet of plastic wrap by using a robotic arm to uniformly blend many millions of square clay platelets 100 nanometers on each side and one nanometer thick (one nanometer equals 3.94 x 10-8 inch) with the same polymer used in Elmer's glue. The robo-arm crafted this new material by dipping a piece of glass about the size of a stick of gum alternately into the gluelike polymer solution and then into a liquid that was a dispersion of clay nanosheets. The end result—consisting of 300 layers of the blended nanomaterials and polymer—was modeled after mother-of-pearl found in the lining of mussel and oyster shells.

"The material is an exemplary structure where we have achieved nearly ideal transfer of the nanoscale mechanical properties to the macroscale," says Paul Podsiadlo, a doctoral candidate in U.M's College of Engineering who assisted with the research. "If we can further achieve the same with these other nanomaterials then we will be able to make lightweight composites which will be exceeding the properties of steel by far."

The bricks-and-mortar structure allowed the layers to form cooperative hydrogen bonds, which gives rise to what Kotov called "the Velcro effect"—one of the reasons the material is so strong. Such bonds, if broken, can reform easily in a new place. Kotov is developing methods to apply the composite in the development of microelectromechanical systems (MEMS) and devices, as well as microfluidics devices for actuation and valve manufacturing. In addition to military uses, improving the ductility of the researchers' nanoinfused plastics could aid in the development of dent and scratch-resistant cars and windshields.

Now that the researchers have created a composite exhibiting resistance to deformation (stiffness) and resistance to load (strength), they are working to improve the composite's ability to dissipate energy, thus improving its toughness, says U.M. mechanical engineering professor Ellen Arruda, another of the study's co-authors. "We want the material to have the ability to absorb the energy of a projectile," she says.

The impetus for the research was a $1.2-million grant awarded last year by the U.S. Defense Department, which was interested in developing more effective armor for the Air Force's unmanned aerial craft as well as for vehicles and body armor for other branches of the armed forces.

The cost of this composite is difficult to estimate, Kotov says. The components are inexpensive and the process does not require large energy expenditures, but it is by no means a fast process. Cost will depend largely on how efficiently processes are developed to create nanoinfused composites and whether these composites need to be produced in high volumes. For highly specialized technologies such as MEMS and microfluidics devices, cost would not be as great an issue as it would in creating large sheets of armor.

The development of these composites is also expected to take less of a toll on the environment, because this superstrong polymer does not require the high temperatures or great energy expenditures required to make steel.

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