As well as being light and strong, carbon fibre products are also very difficult to break down or recycle, and disposing of them has become of increasing concern. While thermoplastics can be melted and easily re-used, most composites used in planes are thermosets. These types of plastics are cured and can't easily be undone and returned to their original materials.
To recycle them, researchers have tried grinding them down mechanically or breaking them down with very high temperatures or harsh chemicals to recover the expensive carbon fibre. Often, however, the carbon fibre is damaged in the process. The caustic chemicals used are hazardous and difficult to dispose of. They also destroy the matrix resin materials in the composites, creating a messy mixture of chemicals and an additional waste problem.
In their project, Jinwen Zhang, a professor in the School of Mechanical and Materials Engineering, and his team developed a chemical recycling method that used mild acids as catalysts in liquid ethanol at a relatively low temperature to break down the thermosets. The combination of chemicals proved effective. To break down cured materials effectively, the researchers raised the temperature of the material so that the catalyst-containing liquid can penetrate the composite and break down the complex structure. Prof Zhang used ethanol to make the resins expand and zinc chloride to break down critical carbon-nitrogen bonds.
“It is critical to develop efficient catalytic systems that are capable of permeating into the cured resins and breaking down the chemical bonds of cured resins,” said Prof Zhang.
The researchers preserved the carbon fibres as well as the resin material in a useful form that could be easily re-used. They have filed for a patent and are working to commercialise their methods.