When carbon fibre was first trotted out in solid-rocket motor cases and tanks in the 1960s, it was poised to not only take on fibreglass, but also a whole host of other materials.
50 years later it's still an exotic material. Sure, Batman's got it in his suit, expensive cars feature smatterings of it in their dashboards and performance parts, but at $US10 a pound on the low end, it's still too pricy for wide-scale deployment. We've been using this stuff for deacdes. Where's our materials science Moore's Law to make this stuff cheap? Why is this stuff still so expensive?
Turns out that even half a century later, this stuff is still a major pain in the arse to make.
Before carbon fibre becomes carbon fibre, it starts as a base material — usually an organic polymer with carbon atoms binding together long strings of molecules called a polyacrylonitrile. It's a big word for a material similar to the acrylics in jumpers and carpets. But unlike floor and clothing acrylics, the kind that turns into a material stronger and lighter than steel has a heftier price tag. A $US3 per pound starting price may not sound exorbitant, but in its manufacturing, the number spikes.
See, to get the carbon part of carbon fibre, half of the starting material's acrylic needs to be kicked away. "The final product will cost double what you started with because half burns off", explains Bob Norris of Oak Ridge National Laboratory's polymer matrix composites group. "Before you even account for energy and equipment, the precursor in the final product is something around $US5 a pound."
That price — $US5 a pound — is also the magic number for getting carbon fibre into mainstream automotive applications. Seven bones will do, but five will make the biggest splash. So as it stands, the base material alone has already blown the budget.
There's more. Forcing the acrylic to shed its non-carbon atoms takes monstrous machines and a lot of heat. The first of two major processing steps is oxidisation stabilisation. Here fibres are continuously fed through 15-30m long ovens pumping out heat in the several-hundred-degrees Celsius range. The process takes hours, so it's a massive energy eater.
Then the material goes through a what's called carbonisation. Although the furnaces here are shorter and don't run for as long, they operate at much higher temperatures — we're taking around 1000 degrees Celsius for the initial step before and then another round of heating with even higher temperatures. That's a power bill you don't even want to think about.
And it doesn't end there. Manufacturers also have to deal with the acrylic that doesn't hold on during the heating process. Off gasses need to be treated so as not to poison the environment. It ain't cheap being green. "It's a lot of energy, a lot of real estate, and a lot of large equipment", says Norris. And that's just in the manufacturing of the individual fibres themselves.
Let's take a second to talk about where we are in the manufacturing process, and where we're trying to get. That awesome-looking, rock-hard, ultra-light, shiny panel with its visible weave is what you think of when you think of carbon fibre, right? Well, we've just made the strands; we've still got to arrange them into a lattice that takes advantage of the material's unidirectional strength and lacquer them together.
Nailing the woven product means making sure that all the strands are pulling their weight. "You have to be concerned that the fibres are all parallel and are all stretched evenly", explains Rob Klawonn, president of the carbon fibre manufacturer, Toho Tenax America. A wavy strand in a lattice will put extra stress of a straight fibre, and that straight one will end up breaking first. To compensate for the possibility of an imperfect weave, manufacturers might thread in 10 per cent more of the already expensive fibres than is necessary.
Alone, the strands aren't the strong stuff that manufacturers need. They're a reinforcer, like steel is in concrete. Right now carbon fibres work with a thermoset resin. Together they make a composite that can be manipulated to take a certain shape. The trouble is that once the resin has been shaped and cured in an autoclave, that shape cannot be modified without altering the structural integrity of the product. A small mistake means a lot of waste — and time. Thermoset parts take over an hour, which is a long time considering how fast the automotive industry stamps out body panels.
So carbon fibre doesn't just require one genius fix to get it into a lower price class, it requires an entire systems overhaul. As with anything offering a big financial reward, the industry is on it.
Those jumper-type acrylics, for instance, might be used in place of the ones manufacturers use now. "The equipment is less specialised, so that might cut the precursor cost by 20-30 per cent", says Norris. They're also checking out renewable carbon fibre starters like lignin, which comes from wood, instead of the current petroleum-based stuff.
Alternate conversion processes-namely swapping thermal for plasma heating — could lower costs as well. "It cuts the time down because you don't have to heat the entire furnace; you generate the plasma to surround the filaments", explains Norris.
Scientists haven't quite nailed the chemical process to get carbon fibre to work with thermoplastic resins quite yet, but once they do, Klawonn of Toho Tenax America predicts 60-70 per cent cut in cost in the conversion process. The big change is that thermoplastics are quick to set and can be melted and remelted, which limits waste when there's a mistake.
Change is on the horizon. Norris points out that carbon fibre has been installed in place of aluminium on newer commercial airliners like the Airbus A380. "They're moving more mainstream, but up until now it's always been in industries that can afford to pay for the performance." Let's just hope the cost caves before the industries that need it do.
Industrial oven image courtesy Harper International.