Cotton cars to bring down costs of composites
Built in East Germany, the Trabant 601 was notorious for its many faults – not the least of which was a body made out of Duroplast, a hard plastic made of cotton waste and phenol resins that led those in the West to describe the car as being made of cardboard. However, it now looks as if the Trabant is getting the last laugh as scientists look at ways of making cars out of cotton and other botanical fibers formed into a new class of hybrid composites.
New emission, safety, and mileage standards in Europe and North America call for vehicles that are ever stronger and lighter, which means that steel and aluminum are giving way to carbon composite materials. Basically, such carbon fiber-reinforced plastics (CFRP) are made up of carbon fibers reinforced by resins, which provides strength and durability, and by tweaking the materials put in, engineers can alter the composite to fit particular applications.
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These composites are light, strong, durable, and have proven their worth in everything from F1 racers to aircraft to surgical prostheses, but using them is a trade off. High-tech synthetic carbon composites may be marvelous materials, but they're also expensive and difficult to fabricate. Alternatives, such as glass fiber, can bring down the cost, but they tend to be heavier and not quite as strong.
The Application Center for Wood Fiber Research of the Fraunhofer Institute for Wood Research, the Wilhelm-Klauditz-Institut WKI in Braunschweig is looking into a more natural alternative to CFRPs in natural botanical fiber composites made out of flax, hemp, cotton, or wood. As the Trabant, with its Duroplast cotton composite, shows, the basic idea isn't new, but how Fraunhofer is applying it is.
The botanical composites don't seem like a very good choice at first. They aren't anywhere near as strong or durable as carbon composites, but they are as cheap as glass composites and lighter than glass. In addition, the botanicals burn cleanly without residues.
The clever bit is to take a bio-based textile and carbon fibers and combine them. The idea is not for the botanical fibers to replace carbon, but to supplement them. For example, the strength and durability of a composite panel doesn't need to be the same across the whole unit. Instead, carbon composites can be used in areas that are subject to high strength and wear, while the botanical composites cover other areas. Blending the two together therefore results in a cheaper, greener hybrid composite that can do the job of pure carbon composites.
Once explained, this hybrid composite seems fairly simple, but creating it isn't as straightforward. According to Fraunhofer, botanical fibers are usually made for use in textiles, which means they're treated so they'll run smoothly through spinners, looms, and other textile machines. However, composite engineers want fibers that are treated so they interact with the resins in a manner similar to roughening a wall so it will tightly grip the plaster. In the case of composites, properly treating the fibers can increase a material's durability by 50 percent. Such treatments are routine in carbon fibers, but Fraunhofer says that how to handle botanical fibers is still an unknown.
In addition to this, the Fraunhofer team is also studying how to manufacture the hybrid composites on an industrial scale, how to recycle them, and how to recover the materials in the panels.