Carbon fibers have a tremendous promise for a new form of ultralight, ultrastrong fibers and even more. Some are made from solid carbon whiskers, twisted and spun into long fibers or just used as fillers in polymers and we are familiar with them where lightness and strength are both needed at once, such as in bicycle frames or tennis racquets.
Another kind of carbon fiber is not solid but is made of little tubes of graphite, all combined together into tubes of hexagons. We call them nanotubes due to their tiny diameters. We know from diamond that carbon to carbon bonds can be incredibly strong. These do not have a diamond structure but they do have carbon-carbon bonds. They are fiber like because they are much longer than wide. The question is how to combine, or spin, these short fibers into long ones that could become strings. A team of scientists has just found the answer.
In the journal Science (Jan 11, 2013, vol. 339, p. 182) the article is called Strong, Light, Multifunctional Fibers of Carbon Nanotubes with Ultrahigh Conductivity. The key is to dissolve the nanotubes in CHLOROSULFONIC ACID (ClSO3H) the only known solvent. Then the solution is forced through tiny holes (called spinnerets) and when the mixture emerges, the chlorosulfonic acid is quickly removed and the nanotubes condense or coagulate into continuous fibers. This is the same method used by nylon or rayon manufacturers and by spiders spinning silk (it is the chlorosulfonic acid which is unique). A large number of spinnerets operate simultaneously and the very thin fibers are then caught and continually twisted together, then wound up on a drum. A picture of the drum shows hundreds of meters of a carbon rope on the drum. As the title indicates, one of the unique features of this rope is that it conducts electricity as well as metals while being as strong as steel but much lighter. A very unique combination.
In years to come, as we learn more, we will have more to say in these pages about the way that these ropes or fibers are used, worn down and discarded (if they are). As they wear, they will shed nanotubes into the environment and this will be problematical and is already a source of concern. But today we are talking about the solvent.
Chlorosulfonic acid is nasty stuff. Like a family of chlorinated sulfur acids, it breaks down instantly by reacting with water creating hydrochloric and sulfuric acids. If you are breathing it, this reaction happens in your lungs or eyes and you cough irrepressibly and eventually would die. In the carbon spinning process, as the mixture of chlorosulfonic acid and nanotubes exits the spinnerets and contacts a large bath of water, the chlorosulfonic acid disappears by reaction with the water, thus precipitating the carbon nanotubes as desired. The result is a mixture of hydrochloric and sulfuric acids in water with perhaps a few carbon nanotubes floating around. Chlorosulfonic acid can be purchased in bulk for about $200 per ton, a fairly cheap chemical. The solution is reported to be roughly 5% by weight of carbon in acid. Thus for 100 pounds of carbon filament, about one ton of chlorosulfonic acid is reacted with water. My question is: what happens to the acid solution?
Right now this is a laboratory demonstration. But someday it will be a large scale chemical process. What is the normal design of a reaction like this in industrial chemistry.
In the nineteenth century, there would have been no hesitation. The plant would have been located on a river and the bulk acid byproduct would have been continually dumped into the river, killing all living things for tens of miles, depending on the flow. No one with political clout would have complained, chalking it up to progress.
In the 20th century, laws were passed making the wholesale pollution of a river this way unacceptable. So what would be the outcome? Since problems must never be solved at their source, but must always be dealt with by post-pollution treatment, the acid solution would be collected in tanks, treated with sodium hydroxide until neutral and then discharged into the river. Thus additional sodium hydroxide would be brought in for additional costs thus adding to the GDP and the investment cost. The resulting salts, sodium chloride and sodium sulfate would be decreed to be benign to the benthic life of the river and an EPA permit to discharge the neutral salts would be issued. At least this is my expectation.
Lately, a different approach is much in use. It was realized that the core objection to river pollution is not the pollution itself but the fact that people – third parties – can see the pollution, they can see the outflows and the results of any excursions. So the better way is to find a home for the effluent that will not be seen by anyone for a while yet, until it is too late. These effluents are injected through deep wells, many miles down into the earth. Even treatment is unnecessary when no one complains. Sometimes there are salt caverns or sulfur mines to fill or more often no one knows where the stuff goes. Sort of like fracking, but deeper, it just makes a home and stays there.
Both of these disposal options will be justified with economic arguments. It’s cheap. It’s affordable. The price of the product will support the costs. Don’t bother us.
For thirty years my business was to find immediate homes for byproducts like this acid mixture. There are many uses for acid mixtures. A lot of acid is used in cleaning steel for galvanizing for example. A low grade use, that I don’t recommend, is to pair up with a company that has an alkaline effluent and use the two effluents to neutralize each other so that neither one has to buy an additional neutralizer. An example would be a refurbisher of silicon solar cells or microchip disks. Always I would search for the best solution of all. Process the mixture back to its original composition, chlorosulfonic acid in this case, and reuse it beneficially. That might require locating the plant near a plant that makes sulfuric acid. Most chlorosulfonic acid comes from Asia. Could the carbon filaments be made in Asia or could a law require that the carbon filament plant incorporate a chlorosulfonic acid plant within it? This would be bitterly opposed by industry on the basis of cost and effort. A chlorosulfonic acid plant should be fairly large, not just an add on to a single user. But what is the assumption that would underlie all of the discussion? Costs matter, the health of the planet does not. The planet is given to us to degrade any way we want, so long as a profit is in the offing.
I am not recommending any one solution here but only presenting the problems of chemical design that arise in a newly growing industry. In twenty years, these issues will have been passed through their design phase and some kind of “solution” will have been found. If the past is any signal post, the dumbest ideas will triumph. The earth will continue to groan and the planet will be some iota further destroyed. It will all be seen as the price of progress.