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.