Stronger than Kevlar, light as a tee-shirt, and cheap all over

Image courtesy of Marilyn Minus.

Image courtesy of Marilyn Minus. Click for larger view.

Forty years ago, Dupont Company revolutionized protective gear when they introduced Kevlar, a fiber made of super-strong, rigid polymer molecules belonging to a small class called aramids. Since then, improvements to strong textile fibers have been incremental.

That’s because most flexible polymers are inherently flimsy. When you look at their micro-structures it’s easy to see why: They look like piles of entangled spaghetti strands. This leads to weak performance, says Northeastern University mechanical engineering professor Marilyn Minus, who is taking advantage of another scientific revolution to change this behavior: carbon nanotechnology.

Several strong fiber already exists.  For example, carbon fibers are all over the place these days, from golf clubs to Formula One racecars. They were used in the new Boeing 787 to decrease its total weight by 60 percent — one of several reasons why the aircraft is so energy efficient.

Traditionally, carbon fibers are made by “carbonizing” a polymer called poly-acrylonitrile, or PAN. First, the polymer is spun into a fiber and then it is heated to very high temperatures.  This causes the polymer molecules to  to be converted into a homogenous carbon structure, causing the material to become a stiff solid.

Some research groups are designing new fibers that are made with 100 percent carbon nanotubes, which are among the strongest materials out there. But they’re extremely expensive. Minus’ goal is to design a composite fiber that is twice as strong as current commercial materials, but cheaper.

To do so, she’s adding small amounts of nanotubes to the polymer fibers. The tubes, she says, act as needle-like skates allowing the long, flexible polymer chains to slide into a more ordered conformation. Now the spaghetti strands aren’t jumbled in a messy pile, but are neatly aligned, one strand evenly stacked atop the next. The alignment affords much stronger properties, says Minus.

She’s playing around with several different types of polymers and nanomaterials and varying the concentrations of each. Ultimately she hopes to have a library of sorts, with a variety of materials designed for a variety of applications. Also, because she’s using textile grade polymers with only a small percentage of nanomaterials, the prices of her fibers may not be much higher than a silk shirt.

And speaking of silk, Minus has already developed fibers that are stronger than spider silk — one of the strongest natural materials around. At the same time, her fibers are pushing the limits of Zylon, the strongest synthetic material currently available. Still at the beginning of her research, Minus believes there is still a lot  room for improvement.