Parameter Optimization of the Steam Thermolysis: A Process to Recover Carbon Fibers from Polymer-Matrix Composites

Abstract

Life cycle assessment has confirmed the effectiveness of carbon fiber reinforced polymer-matrix composite (CFRP) materials’ application in CO₂ reduction as well as in weight reduction (JCMA in 2008). Thanks to these strong and ultimate lightweight structure materials, burdens imposed by vehicles on environment nowadays could be significantly relieved. However, speedy manufacturing and recycle technology have to be established to apply to mass-production of automobiles using CFRP components. It is estimated that 3,000 t of CFRP scrap being generated annually in Europe and the USA. Some 6,000–8,000 commercial planes are expected to reach end-of-life dismantlement by 2030 (McConnell in Reinf Plast 54(2):33–37, 2010). Historically, CFRP waste has been disposed of in landfills. Since 2004, most European countries have banned the landfill disposal of CFRP waste. Future EU regulations will be imposed on the recycling of end-of-life aircraft along the same lines as those for scrapped vehicles. Besides, high cost of the carbon fibers is likely to be one of the strongest drivers for recycling of CFRP waste if an economic and environmentally friendly technology can be developed to provide a potential sustainable resource of reusable fibers. Numerous recycling approaches have been proposed and investigated: microwave treatment (Lester et al. in Mater Res Bull 39(10):1549–1556, 2004), vacuum pyrolysis (Cunliffe et al. in J Anal Appl Pyrol 70(2):315–338, 2003), chemical solvolysis (Piñero-Hernanz et al. in J Supercrit Fluids 46(1):83–92, 2008; Goto in J Supercrit Fluids 47(3):500–507, 2009), as well as hydro-thermolysis (Piñero-Hernanz et al. in Compos A 39(3):454–461, 2008). Steam thermolysis, a combination of vacuum pyrolysis and mild gasification, has been studied in our laboratory. The process takes place under medium temperatures (until 600 °C) and atmospheric pressure. By using superheated steam as a soft oxidant, the polymer matrix can be converted into lower molecular weight hydrocarbons, CO and CO₂ without seriously reducing the mechanical properties of the carbon fiber reinforcement. The steam-thermolysis has been studied and confirmed efficient in treating epoxy based CFRP materials at bench scale. In order to optimize the process, an experimental design has been carried out by using Taguchi method at semi-industrial scale. Initially, an orthogonal array testing was applied to determine the potential influence of the operational parameters on the decomposition rate of the polymer matrix as wall as on the mechanical property of the reclaimed carbon fibers. Three operational parameters at two levels each (target temperature, isothermal dwell time and steam flow-rate) were identified and tested. During the test, some 100 g epoxy based CFRP scrap samples were loaded into a thermal reactor preheated till desired temperature. The steam is flushed with a flow of nitrogen throughout the test to avoid oxidative side reactions. Additional tests were also conducted to complete and validate the results.