Manufacturing Bits: Feb. 15

Strong plastics; hard plant composites; tough adhesives.


Strong plastics
The Massachusetts Institute of Technology has developed a new material that is stronger than steel but is light as plastic.

The new material, which can be made in large quantities, involves a two-dimensional polymer that self-assembles into sheets. The material’s Young modulus—or a measure of how much force it takes to deform a material—is between four and six times greater than bulletproof glass. The material has two times the yield strength but only one-sixth the density of steel, according to MIT.

The material could be used as a lightweight, durable coating for car parts or smartphones. It could be used as a building material for bridges or other structures.

A polymer is a material that contains multiple repeating units called monomers. Using injection molding, polymers can be used to form three-dimensional objects, such as water bottles and other products.

Conventional polymers resemble one-dimensional, spaghetti-like chains, according to MIT. For years, researchers have been looking into the development of polymers into two-dimensional sheets. In theory, these polymers could enable strong and lightweight materials.

So far, the industry believed that it was impossible to develop these sheets. Recently, though, researchers from MIT have devised a new polymerization process, enabling the development of a two-dimensional sheet called a polyaramide.

This material requires a new monomer building block. Researchers devised a new compound called melamine, which contains a ring of carbon and nitrogen atoms. These building blocks grow in two dimensions, forming disks. “These disks stack on top of each other, held together by hydrogen bonds between the layers, which make the structure very stable and strong,” according to MIT.

Once the material is produced, researchers coat the surfaces with films of the material, which they call 2DPA-1. “We don’t usually think of plastics as being something that you could use to support a building, but with this material, you can enable new things,” said Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering at MIT.

“Instead of making a spaghetti-like molecule, we can make a sheet-like molecular plane, where we get molecules to hook themselves together in two dimensions. This mechanism happens spontaneously in solution, and after we synthesize the material, we can easily spin-coat thin films that are extraordinarily strong,” Strano said.

“This could allow us to create ultra-thin coatings that can completely prevent water or gases from getting through,” Strano said. “This kind of barrier coating could be used to protect metal in cars and other vehicles, or steel structures.”

The research was funded by the Center for Enhanced Nanofluidic Transport (CENT) an Energy Frontier Research Center sponsored by the U.S. Department of Energy Office of Science, and the Army Research Laboratory.

Hard plant composites
In a separate development, MIT has developed a new plant-derived composite that is tough as bone and hard as aluminum.

The composite could pave the way towards a range of new products. For example, the material might one day be used to make cellulose-based dental implants. It could be used to make plastic products that are stronger, tougher, and more sustainable, according to MIT.

MIT developed the composite from cellulose nanocrystals and some synthetic polymers. Cellulose makes up the primary cell wall of green plants, algae and other organisms.

For its technology, MIT used wood fibers. A single wood cell wall is constructed from fibers of cellulose, according to MIT. Within each fiber are cellulose nanocrystals (CNCs), which are chains of polymers.

MIT developed a composite with a high fraction of CNCs, which will enable strong structures. Researchers mixed a solution of synthetic polymers with CNC powder.

This in turn formed a gel. Using a 3D printer, the gel was poured into a mold. Researchers used an ultrasonic probe to break the clumps of cellulose in the gel.

“By creating composites with CNCs at high loading, we can give polymer-based materials mechanical properties they never had before,” says A. John Hart, professor of mechanical engineering at MIT. “If we can replace some petroleum-based plastic with naturally-derived cellulose, that’s arguably better for the planet as well.”

Abhinav Rao, a researcher at MIT, added: “We basically deconstructed wood, and reconstructed it. We took the best components of wood, which is cellulose nanocrystals, and reconstructed them to achieve a new composite material.”

This research was supported, in part, by the Proctor and Gamble Corporation, and by the National Defense Science and Engineering Graduate Fellowship.

Tough adhesives
Using a polymer chemistry, the Department of Energy’s Oak Ridge National Laboratory has transformed a common household plastic into a strong and reusable adhesive.

With the technology, researchers enabled the development of a new class of tough adhesives. The technology can handle heavy loads, tolerate extreme stress and heat, and reversibly bond to various surfaces including glass, aluminum and steel.

It is targeted for various applications, such as aerospace, automotive and construction. “There are benefits to industry and the environment to save resources and reduce waste. By design, this adhesive allows you to make repairs or correct costly mistakes and can be reprocessed for new uses in very challenging applications,” said Tomonori Saito, a scientist at Oak Ridge.

Fig. 1: Researchers at Oak Ridge National Laboratory created a reusable adhesive with “exceptional strength and toughness.” Source: Carlos Jones/ORNL, U.S. Dept. of Energy

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