Everybody loves carbon fiber. It is increasingly coming into use in high-performance applications such as racing boats, aerospace, and structural reinforcement of buildings and infrastructure. It’s even starting to appear in architectural applications.
Everybody loves it, but we would venture to guess that few know what it is, or where it comes from. Today, we pause a moment to find out.
To be clear, the hard, shiny, high-performance material that most people refer to as “carbon fiber” is actually the carbon fiber reinforced polymer composite (CFRP). It’s a matrix of plastic infused into a mat or fabric of carbon fibers.
Carbon fibers are what they sound like: long strands of mostly carbon atoms. To be more specific, they is carbon crystals aligned along their long axis, so they form a long, thin structure.
These fibers have fantastic physical properties: high tensile strength, flexural strength, and stiffness, all of which come into play in CFRP. (The boasts about the strength of carbon fiber seem so over-the-top, they always remind me of Groucho Marx’s line in Monkey Business, describing himself as “a man who’s licked my weight in wild caterpillers!”)
Glass fiber, the most commonly used composite reinforcement, is largely silicon dioxide (aka silica), the most abundant compound in the earth’s crust. Glass fibers are formed by melting glass and forcing it through a fine nozzle to produce long, thin strands.
Carbon fiber production is much more complicated. Carbon fiber is a refinement of a plastic, usually in several steps that gradually eliminate more and more of the non-carbon materials. The exact processes are trade secrets closely guarded by the major carbon fiber manufacturers, but the broad concepts are public knowledge.
A “precursor” material is required, usually rayon or polyarcylonitrile (PAN). The first production of something like the carbon fibers we know today was accomplished in 1958 by Roger Bacon of Union Carbide Corporation, using rayon. Strands of rayon were heated and created a fiber that was about 20% carbon, and not very strong. In 1960, Dr. Richard Millington of H.I. Thompson Fiberglas developed a rayon-based process that had 99% carbon. In the early 1960’s, Dr. Akio Shindo of Japan developed a process using PAN as a precursor, initially yielding 55% carbon fibers.
Today, 90% of carbon fiber production uses polyacrylonitrile (PAN) as precursor.So what is PAN? It is made from acrylonitrile (AN). AN is made from propene (aka propylene, methyl ethylene), which does occur in nature, but is usually made from fossil fuels: petroleum, natural gas, or sometimes coal.
About 10% of todays carbon fiber production uses rayon as precursor. Rayon is derived from cellulose fibers, usually from wood pulp.
The process of trasnforming precursor into carbon fiber has several steps. First, the precursor is “spun” into fibers. In the PAN process, this means mixing PAN with a plastic (such as methyl acrylate or methyl methacrylate), heating, and then forming into fibers by forcing it through a fine nozzle. Then it is stretched to the desired diameter, which helps realign the molecules.
Next, it is stabilized by heating it in air to about 300ºC, which causes it to rearrange its atomic bonding.
Then, it is heated to high temperatures in a non-oxygen environment. This process is described rather dramatically on Zoltek’s website:
Without oxygen, the fiber cannot burn. Instead, the high temperature causes the atoms in the fiber to vibrate violently until most of the non-carbon atoms are expelled. This process is called carbonization and leaves a fiber composed of long, tightly inter-locked chains of carbon atoms with only a few non-carbon atoms remaining.
The resultant carbon fibers don’t adhere well to matrix materials such as epoxy, so they are oxidized to create a better bonding surface.
Bundles of thousands of fibers are wound as a kind of thread (tow) and can be woven into fabrics (roving). These are be basic forms of carbon fiber reinforcement in CFRP.
This process is expensive. The upfront equipment investment is upwards of $25 million. Moreover, the PAN raw material is fairly costly, too. Not surprisingly, carbon fiber manufacturers are very focused on trying to improve the value proposition of their product, either by increasing performance or reducing the cost.
Meanwhile, carbon fiber and its FRP composite have become so chic that the term no longer just applies to the material. Goods made of simple, unreinforced plastics that resemble CFRP are sold as “carbon fiber” referring to it as a ‘color.’
Consider, however, that in the long run, carbon fiber may be a material of the moment. If fossil fuel depletion continues (as it almost certainly will), petroleum-based plastics including PAN may become ridiculously expensive. Rayon is derived from a renewable source, wood pulp, but the use of wood is already outstripping its renewal planet-wide. Rayon, too, may become very costly. Other precursors could be developed, perhaps from more rapidly renewable sources. Or carbon fiber could just become a thing of the past.
From that perspective, it makes the need for reversible resins all the more obvious, so that carbon fiber composites could be recycled to harvest their fibers and reuse them.
It also makes you want to take a second look at glass fiber, which, as previously mentioned, is made from the most abundant compound in the earth’s crust.