Nanomaterials in bicycles???? Part 2

In Tuesday’s post, I told you about the impressive properties of carbon nanotubes (CNTs) and how they can increase strength and reduce weight in bicycle frames compared to traditional designs. I also told you about the high cost and difficult handling of CNTs that seems to have stymied more widespread adoption of the nanomaterial in bikes.

While CNTs have been coasting, a related carbon material called graphene has pulled onto the scene. Graphene is a single sheet of carbon atoms arranged in a hexagonal honeycomb—basically an unwrapped carbon nanotube. You might be shocked to hear this, but every elementary school student in the world has made a form of graphene! That’s because graphene is a single layer of the oh-so-common graphite found in pencils:

Margy-graphene — Pencil “lead” is made out of graphite. Graphene is a single sheet of graphite! (images by Juliancolton (left), Mattman723 (middle) and Ququ from ru (right))

Though some theoretical knowledge of graphene existed as early as 1947, a means of isolating single sheets of graphene wasn’t discovered until 2004 when researchers at the University of Manchester in the UK lifted atomic layers of carbon off of graphite using clear adhesive tape (those researchers won the 2010 Nobel Prize in Physics!).¹ This easy method of making graphene opened the gates to studying the material.

Similar to CNTs, graphene is more than 100 times stronger than steel* and is 1.6 times more electrically conductive and 10 times more heat conductive than copper. Graphene has the added advantages that it is 2 times less dense and has a surface area about 5 times that of CNTs, all while maintaining a flexible sheet configuration.² The flexibility and high surface area of graphene are particularly appealing because they might make it easier to incorporate graphene sheets into other materials (like the resin binder used to make a carbon fiber bike frame) without the clumping problems of CNTs.³

Another potentially useful property of graphene is that it transmits heat efficiently. Good heat conductivity relies on long, continuously connected threads of material. CNTs cut off at around ~1000 micrometers, so unless a lot of CNTs could be linked together they aren’t terribly useful for conducting heat on the large scale. A sheet of graphene, on the other hand, can theoretically be as large as the crystal of graphite it is pulled off of – millimeters or even centimeters long!

Why is thermal conductivity important for bikes? The brakes of a bike rely on friction between the brake pad and the rim of the wheel.

Margy-brakes — Bike brakes squeeze the sides of the tire, using friction to slow it down. (photo by Jeff Archer)

During sustained braking, such as when riding down the side of a mountain, heat can build up quickly and result in the walls of the tire deforming and potentially blowing out. Not good! Ideally, graphene incorporated into the braking surface of the wheel would conduct heat away from the tire and prevent it from bursting.

Margy-blowout — A blown out tire, maybe from heat building up! (photo by Thirteen of Clubs)

Unlike carbon nanotubes, graphene didn’t wait 16 years from its discovery to jump into cycling-related products. In 2015 alone, Catlike helmets⁴ and cycling shoes⁵ and Vittoria tires and wheels (as you saw in the video at the end of part 1)⁶ all included graphene… But can you really believe the marketing? Catlike advertises the incorporation of “graphene nanofibers” into the internal skeletons of helmets and treads of shoes for improved strength and durability. Vittoria markets the use of “Graphene Plus” in their wheels for heat dissipation and in their tires for puncture resistance.

Margy-helmet — Is there really graphene in these??? (photos by Glory Cycles, left and right)

While all these claims match up with our expectations of the properties of graphene, it’s rather unlikely that either of these companies are using true graphene. Womp womp womp. How do I know that? For Catlike’s “graphene nanofibers,” familiarity with science jargon is helpful. Nanofibers made entirely of carbon can come in a multitude of structures including stacked layers of cones, cups, plates, or in single or multilayered tubes. A carbon nanofiber in the shape of a single-layered tube can be unrolled into a single sheet of graphene, which is probably where Catlike is getting the graphene part of their “graphene nanofibers.” But those tubes of carbon nanofibers are better known as carbon nanotubes, as we talked about in Part 1 of this series. It appears that Catlike has given a new name to a known material!

This is a video from the Vittoria website explaining their use of graphene in their tire and wheels.

Vittoria’s website offers a lot of interesting and accurate information about graphene. With a bit of digging, you can find that Vittoria and their graphene supplier Directa Plus do specify that the materials being incorporated into tires and wheels are 2-8 atomic layers thick.⁷ Recalling that graphene is by definition only one atomic layer of carbon, these multilayer materials are technically not graphene but very small crystals of graphite, sometimes called graphite nanoplatelets.

Margy-nanofibers — Carbon nanofibers come in all sorts of shapes! One of them is a tube shape, called a carbon nanotube.⁸ A graphite nanoplatelet, such as the one on the right, is distinct from graphene. (image left by Margaret Robinson; image right adapted from Michael Ströck)

Though each sheet within a graphite nanoplatelet likely has similar properties to graphene, the whole nanoplatelet does not. Mechanical robustness, for instance, is vastly different between the two materials. The layers of graphite are loosely connected to one another; so loose that they are peeled apart when (for example) a pencil is simply dragged across paper. So maybe Vittoria isn’t getting as much strength out of their nanomaterial additive as they possibly could, but that is not something I am qualified to assess. I do grumble at the inaccurate name and implication that they are utilizing the superlative properties of single layered graphene; however, I can imagine that the term “graphene” has much more public recognition than “graphite nanoplatelet.”

Now you might be disappointed to hear about these graphene misnomers, but let’s take a moment to appreciate that these are still nanomaterials. They have distinct properties as a result of their size, large surface area compared to volume, and reactivity that can be leveraged in many applications. Every step in the journey of turning academic research into useful products can be a positive one—these uses of carbon nanotubes and graphite nanoplatelets can hopefully guide us closer to the next great application. Maybe that will be a Tour-de-France-winning bike. And maybe my friend Mimi Hang will ride it.

Margy-Mimi2 — Mimi riding a bike! (photo by Margaret Robinson)

Mimi’s bike riding lesson last summer only lasted 30 minutes—partially because the bikeshare was going to charge more for a longer rental, but mostly because she was zipping along the bike path without the need for a balancing hand. I’m happy to have shared my love of riding bikes with Mimi and to have shared some information about nanomaterials in the cycling industry with you!

*A theoretical layer of steel 0.335 nanometers thick was used as a comparison²

EDUCATIONAL RESOURCES

Nanoscale Informal Science Education Network: Exploring Materials – Graphene (ages 4+)
Engineers in the Classroom: Nanotechnology Revolution: Graphene (high school)
UCSB Materials Research Laboratory: Isolate Graphene from Graphite activity using pencil, paper, sticky tape, & tweezers!

REFERENCES