I’m going to talk about sustainability again, only this time I want to focus on carbon fiber. Why do I talk about sustainability in composites so much? That is, again, a very good question.
The wonderful thing about composites is that they make things lighter weight. In the transportation sector, this corresponds to less fuel burned per mile. In the infrastructure sector this corresponds to faster and less expensive construction – even for large infrastructure projects.
However, the raw materials – fibers and resins – are mostly manufactured from precursors that are not always sustainable. This is especially true with carbon fibers, and all of the resin systems that are used commercially.
The making of carbon fibers starts with making a fiber out of a plastic material that is most commonly derived from petroleum. The highest tonnage production of carbon fiber is based on PAN (PolyAcryloNitrile), aka acrylic fiber. Once you have a petroleum-based PAN fiber, you have to cook it at very high temperatures to drive off everything that is not carbon. This entails running the PAN fiber through a furnace (oil or gas fired) heated to 500-600 degrees F in air and then in a vacuum to burn out everything that is not carbon.
And, of course, once you have made the carbon fiber and have incorporated it into whatever you are making, it is very difficult to get it back out and reuse it when the composite part containing the fiber comes to the end of its life.
Fortunately, there are people working on both ends of this problem. Some researchers at North Carolina State University are working on using lignin as a precursor for carbon fiber instead of PAN. Lignin is basically the structural part of the glue that holds the cellulose together in wood. The forest products industry – especially the paper making industry – extracts the sugars and the cellulose in wood, and what is left over is lignin. And, as it turns out, lignin is a very good precursor for making carbon fibers. It can be spun into a fiber in the same way that PAN and pitch (black tarry stuff left over when petroleum is refined) are used to make fibers.
Another precursor for carbon fiber comes from another source that I talked about previously – flax. The stalks of flax are rich in the components needed to make fibers – lignin and cellulose – and flax is very easy to grow, so it is an abundant natural precursor for carbon fiber.
The image on the left and the two above show how carbon fibers can be made using sustainable processes and sustainable precursors. This image to the left here is the use of microwave heating rather than a gas or oil fired furnace, making it emit almost zero CO2. The other two pics above are of carbon fiber made from naturally occurring plant fibers. So, all three of these pictures demonstrate that the process and the precursors for carbon fiber can be inherently sustainable.
The other side of this is what to do with the carbon fiber that can be recycled from composite parts that have reached the end of their life. That is what image on the right and the one below show. These images are more process oriented in that they depict the process for extracting carbon fiber from end of life composite parts.
This is called “Recycling by Design” and it involves all of the steps in carbon fiber production, use, and re-use (see the image to the left). Quite a bit of attention is paid to the surface of the fiber since that is where the epoxy resin sticks the fibers together to make a part. The researchers that put that graphic together use nanomaterials on the surface that make the carbon fiber inherently recyclable.
Finally, the image to the right is a concept frame for a BMW 7 series made from all recycled carbon fiber. If the automotive sector can get on board with this idea and make it work, that is a very good answer for the recycling of carbon fiber into new parts and structures that will last a very long time.
The bottom line that I am trying to get to here is that there is tremendous hope and some very smart and dedicated people who are going to make this work. At some point, carbon fiber at least will be completely carbon neutral, and could even be a carbon sink that may help us in some small way save this wonderful blue ball that all of us live on.
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