Strong, lightweight carbon fibers made from recycled plant matter could replace glass fibers in high-performance composites used in cars and airplanes, say UK researchers.
Carbon fibers have revolutionized the development of lightweight engineering composites in recent years, particularly in aviation and automotive manufacture. Boeing’s new 787 Dreamliner, for example, relies on lightweight composites to improve fuel use and its environmental credentials. But the carbon fiber used in composites is typically produced from polyacrylonitrile (PAN) in a process that generates toxic gases such as hydrogen cyanide as by-products. Now, with a resurgence of interest in more sustainable and environmentally friendly carbon fibers, Professor Steve Eichhorn and his colleagues at the Universities of Exeter and Manchester have successfully converted cellulose fibers into high strength carbon fibers [Lewandowska et al., Composites Science & Technology 116 (2015) 50, http://dx.doi.org/10.1016/j.compscitech.2015.05.009].
The researchers used a commercially available regenerated cellulose fiber, known as Cordenka™, as a starting point. Fibers were first pre-treated in a furnace heated to 200°C to stabilize the structure. Further heat treatment and graphitization at 2000°C produces carbon fibers that look rather like multiwalled carbon nanotubes with ordered graphitic-like crystallites. High-resolution transmission electron microscopy (TEM) reveals a microstructure of carbon layers arranged in a cylindrical shape, which the researchers believe arises from graphitization of the cellulose’s fibrous structure.
The carbon fibers have very good mechanical properties, says Eichhorn, with a Young’s modulus of over 70 GPa, strength of 1.5 GPa, and breaking strain of 2.2%. But not only did the researchers graphitize cellulose fibers, for the first time they also graphitized fabrics woven from Cordenka™. Lightweight woven carbon fiber sheets are particularly attractive to car manufacturers as they can be formed into panels and infused with resin to form aesthetically pleasing and hardwearing composites.
“We have shown that you can convert cellulose fibers, which are typically used for textiles, into high performance carbon fibers that could compete with glass [in composites], for use in car body parts,” Eichhorn told Materials Today. “We have even shown that this conversion could take place from a woven (textile) fabric form of the fibers, which is useful because it means you don’t have to weave brittle carbon fibers after they’ve been formed.”
Cellulose fibers are not only more sustainable and environmentally friendly, but could save time and cost in composite production, says Eichhorn. He is now looking at how to improve the mechanical properties of the fibers further and weave more complex structures. Ultimately, Eichhorn and his team plan to create some cellulose-based test composites for the automotive industry.
The work was completed with financial support from the Engineering and Physical Sciences Research Council through the EPSRC Centre for Innovative Manufacturing in Composites (CIMComp).