Development of Chemical Processes for the Recycling of Carbon Fiber/Epoxy Composites
Carbon fiber/epoxy composites are a vital and heavily used component in a variety of industries, ranging from aerospace to sports. However, major difficulties may soon arise in regard to environmental considerations for their disposal. These composites are designed to be strong and resilient, and thus are difficult to break down or decompose. The only current option for carbon fiber/epoxy composite wastes is to leave them in landfills or plane graveyards. The high price of carbon fiber also makes finding ways to recover and reuse carbon fiber very attractive. The purpose of this dissertation is to investigate an inexpensive method to break down the epoxy in carbon fiber/epoxy composites and recover fibers while maintaining their mechanical strength.
A new chemical solvolysis process was utilized to oxidize and decompose the epoxy in carbon fiber-reinforced epoxy composites, allowing for carbon fiber recovery and recycling. This process involved heating the composite in a mixture of ethanol and hydrogen peroxide at elevated temperatures for 4 hours. This process results in significant epoxy removal with minimal fiber strength loss. Epoxy removal was confirmed with visual and mass analysis, scanning electron microscopy and electron dispersive spectroscopy, and thermogravimetric analysis. The liquid byproduct solution remaining was chemically analyzed using gas chromatography/mass spectroscopy and found to contain organic compounds that could be attributed to the degradation of epoxy by the solution.
A kinetic model was developed for the purpose of identifying optimal parameters for the reaction. Parameters considered included time, temperature, concentrations of the reactants, and the amount of surface area exposed to the oxidizing solution. Mass loss over time was studied for each reaction variable while keeping the other parameters static, creating a mass loss rate that could be plotted against each parameter. The fittings for these plots were used to identify the constants to be used in the model. The model was compared to experimental data and found to correlate well, with an average error below 8%.
The new chemical solvolysis process in this study was compared to using 98% pure sulfuric acid to dissolve the epoxy. The solvolysis process was found to remove epoxy at a higher rate, while keeping similar fiber strength when comparing samples processed for similar fiber recovery performance. These comparisons resulted in the conclusion that the solvolysis process performs better than dissolution by sulfuric acid.
A further modification to the solvolysis process involves the same mixture of ethanol and hydrogen peroxide with the addition of ferrous ions in an acidic environment. This process could potentially be done at lower temperatures and lower concentrations of hydrogen peroxide, and thus could allow for a reduction in costs when recovering fibers using this process. Initial experiments did confirm the ability to remove significant amounts of epoxy at much lower temperatures and lower concentrations of hydrogen peroxide.