Here at Ruckus Composites, we have been cooking up some exciting new bicycle science, but it’s not yet ready to serve. Never fear, we will dish out some tasty Instagram morsels as our work progresses. In the meantime, we will be offering a tasting menu of information about carbon fiber, epoxy, and thermoplastics to help prepare you to fully appreciate the forthcoming main course. As you will see, heat plays a vital role in production and/or use of all of these materials.
How do you like your carbon served?
Carbon. In the form of coal, it evokes childhood misdeeds and the parental falsehood that Santa is omniscient. In the form of graphite, it can elicit memories of No 2 pencils, scantrons, and test anxiety. But carbon in fiber form conjures up joy and elation in the shape of some pretty nifty bicycles. The carbon content of coal ranges from 47-97% while both graphite and carbon fiber are almost 100% carbon. While graphite and carbon fiber share the same chemical make up, it is the molecular structure that makes the difference between dread and the top of your wishlist.
How does carbon fiber differ from graphite?
Carbon fiber compared to a human hair
The carbon atoms in graphite are arranged into sheets that are loosely held together by Van der Waal’s forces, yielding a material that is soft and slippery. In carbon fiber, the carbon atoms form crystals that are aligned parallel to the long axis of fibers. This produces a material that has a very high strength to weight ratio, with carbon fiber being five times stronger than steel and twice as stiff.
Carbon fibers are only 5 to 10 micrometers in diameter. To put this into perspective, human hair ranges from around 17 to 180 micrometers in diameter. It’s amazing to think that such strength is found in a structure with a diameter smaller than the 200 micrometer resolving power of the unaided human eye.
How is this small by mighty material manufactured?
It’s amusing to envision a super hightech version of a playdough spaghetti maker that converts slippery sheets of graphite into carbon fiber. However, there are two problems with this vision: it’s ridiculous and graphite is not the raw material for carbon fiber production. Polyacrylonitrile (PAN) is the most commonly used precursor, accounting for about 90% of carbon fiber produced. PAN is a synthetic polymer resin composed of repeating molecular units. The units that repeat are formed by three carbon atoms, three hydrogen atoms, and one nitrogen atom (C3H3N).
It’s hard to say exactly how the carbon fiber in your bicycle was manufactured because production is a notoriously difficult process, causing manufacturers to be tight lipped when it comes to details. However, there are some common threads in fiber production. PAN is spun into filament yarns and subjected to a series of heat treatments at increasing temperatures that drive off non-carbon atoms and strengthen the material by altering the molecular structure. A hypothetical process is described below along with changes in the molecular structure from PAN to carbon fiber.
Stretching and Thermosetting
PAN from a spool is first stretched and then thermoset at around 230 ℃ in atmospheric air to increase its strength. Energy in the form of heat causes a rearrangement of the bonds in the polymer such that the carbon-nitrogen triple bond is converted to a double bond and a new single bond is formed between the nitrogen and a carbon atom of the neighboring unit. The polymer units are now complete rings. This process is also known as cyclization. Why not skip right to the high temperature phase? The stabilization that occurs during thermosetting transforms PAN chains into structures that can withstand the high-temperatures required for carbonization without decomposing.
Spools of PAN precursor material. Photo: Photo: Oakridge National Laboratory
Carbonization
In the next step, carbonization, the heat is cranked up to 1000 ℃. The thermoset polymer is heated in air, causing carbon atoms to part ways with their bonded hydrogen buddies. The hydrogen is emitted in the form of a gas and polymer ladders are formed.
PAN being fed into an oxidization oven with oxidized material in the background. Photo: Oakridge National Laboratory.
Graphitization
Things really heat up during graphitization. The 2000 °C temperature makes thermosetting seem like the pimento of the Scoville Scale as compared to the ghost pepper of graphitization. Graphitization takes place in the presence of an inert gas such as argon, and nitrogen gas is given off as the polymer ladders join together to form sheets. The vast majority of the crystal is now composed of carbon, with nitrogen occurring only at the sheet edges.
Surface Treatment and Sizing
After graphitizing, the fibers have a surface that does not bond well with other materials. Chemical and mechanical bonding properties of the fibers are enhanced by oxidizing the surface. Oxidation can be achieved by subjecting the fibers to either gasse–such as air, carbon dioxide, or ozone–or liquids such as sodium hypochlorite or nitric acid.
Following surface treatment, the fibers are sized with a material such as epoxy to protect them from damage during winding onto spools and during subsequent weaving.
Energy is Expensive
Graphite is cheap–just think about how many pencils you could buy without breaking the bank. Carbon fiber bikes, on the other hand, are quite a bit spendier. Why are carbon bikes so pricey? Energy consumption.
Traditional oxidization during carbon fiber production is done in large, energy hungry ovens, and the oxygen reacting with the precursor material is in molecular form. This process accounts for 18% of the cost of carbon fiber manufacturing and it takes 80-120 minutes. Oak Ridge National Laboratory in Tennessee runs a Carbon Fiber Technology Facility where work is in process to innovate lower-cost precursor materials and conversion technologies. Conversion technologies under investigation include use of plasma and microwaves. In microwave assisted plasma ovens, the plasma creates highly reactive species from the air that react much faster with the PAN than molecular oxygen. A plasma oven reduces processing time to 20-30 minutes and uses 75% less energy, reducing the cost of manufacturing by 20%. Plasma carbonizing has the added benefit of producing fibers with more surface roughness which may enhance mechanical interlocking between fiber and resin in carbon fiber reinforced composites.
What’s Up Next? The Carbon Fiber Dream Team
I’m sure we’re all familiar with the cringe-worthy phrase “teamwork makes the dream work.” Carbon fiber has incredible tensile strength, but it is also very brittle, so in order to truly shine it needs to find a partner to form a composite. Composites excel at teamwork because they combine two or more different materials to create a product that has stronger combined physical properties than those of the individual parts. Epoxy, a thermoset resin, is one of the few materials that can adhere to carbon fiber. But epoxy brings more to the table than just stickiness: it is very strong, resistant to corrosive materials and impact, and highly adaptable.
Next week we will take a look at what gives epoxy its seemingly superhero properties.