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  • Writer's pictureNed Patton

A Little About the Strings

Carbon fiber is what most of us refer to when we talk about Advanced Composites. Like I said when I introduced glues, carbon fiber is mostly found in carbon / epoxy composites or in carbon / PEEK composites if a thermoplastic glue is used to bind the fibers together. There were about 150,000 tons of carbon fiber produced in 2021, and that market is expected to grow very rapidly due to pent up demand because of the pandemic.

The pic to the left is PAN fiber – all that fluffy chopped up stuff. PAN fiber is the most commonly used precursor for carbon fiber, and it is estimated that 280,000 tons of PAN fiber is made each year just for making carbon fiber. And that market is growing, not only for use to make carbon fiber, but also for use of PAN fiber itself. PAN is a very commonly used fiber in the high performance textile industry for things like tent fabric. It is also used with some modifications and additions in the clothing textile industry. If you have an acrylic sweater or acrylic socks, they are made largely from PAN fiber that has been co-polymerized with another organic precursor to make a fiber that works well for fabrics and will take a dye very easily.

The pic to the right here is a bundle of glass fibers that are ready to be made into a fiberglass composite. Glass fiber is usually sold as a tow or fiber bundle like what you see in the picture, or in a chopped strand mat, or in a woven product called a woven roving. The market for glass fiber in 2021 was on the order of 6 million tons produced. This is primarily because of all of the fibers used to make composites, glass fiber far outstrips all other fibers combined in total tons produced and used. Everyone has something made of fiberglass in their house or car or boat or sporting goods or tools – the list is endless. Fiberglass is everywhere.

The yellow spools of fiber in this pic are Kevlar being spooled into most probably a weaving machine to make Kevlar fabric. Most folks have heard of Kevlar, but if you have not, it is the most common aramid fiber made. DuPont developed Kevlar and is still the number one producer of it worldwide. There are also producers in Asia (Teijin in Japan is one of the major ones). And there is about 20,000 tons of it produced each year. Kevlar is used for bullet-proof vests, and for several applications where other fibers are too heavy or don’t have the capacity to do what Kevlar will do. One use of note is that Kevlar stays flexible down to very nearly absolute zero, and it has extremely low thermal conductivity, so it is used in particle physics experiments as a thermal standoff between superconducting magnets which have to be kept at temperatures very near absolute zero in order to function the way the particle physicists want them to.

And finally, the rest of the pics in this post are Spectra fiber – including the white spools of fiber, the wispy thread-like fibers, and finally some Spectra rope. Spectra is an ultrahigh molecular weight polyethylene fiber. Yes, polyethylene, the stuff of plastic grocery bags, Ziploc bags, the sheeting you put down on the floor when you paint your walls, etc. As it turns out if you can make a very long chain of polyethylene and pull it into a fiber, you have something that is stronger than steel and floats on the water.

Honeywell makes about 7500 to 8500 tons of this stuff a year, so it is in lots of products.

Well, that’s at least an introduction to the strings that are used in composites. In a future post I am going to delve more into the properties of these strings that make them particularly well suited to making composites. I will also make the distinction between what a dietician means when they use the word fiber and what a composites engineer means when they use the word fiber. They are very different fibers by and large, but there is some cross over that is happening now where so called “natural fibers” have started to be used in composites.


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