Kinetic analysis and ballistic pyrolysis of carbon fiber reinforced phenolic composites using Joule heating

Pyrolysis of high-temperature composite materials is of fundamental interest. While it is straightforward to characterize pyrolysis at heating rates of less than 5 °C/min, rates exceeding 50 °C/min rates are difficult to assess experimentally. Here we investigate the pyrolysis of carbon fiber reinforced phenolic matrix composites via direct Joule heating, which enables extreme heating rates. First, the kinetics of the phenolic pyrolysis reaction was studied using an isoconversional analysis and fit to an nth order reaction model. The apparent kinetic parameters were then used to predict post-pyrolysis carbon mass at ballistic heating rates and compared to experimental results. Prior to the ballistic heating, the composites were preheated to 100, 300, and 500°C, which were the approximate onset temperatures of the three observed decomposition reactions that occur during pyrolysis. Ballistic Joule pyrolysis of the composites at heating rates of 4,000-8,000 °C/min was then performed and the resulting pyrolysis mass retention was recorded. The samples preheated to 500 °C prior to ballistic pyrolysis were found to have the highest mass retention, as well as matching the carbon mass of conventionally pyrolyzed composites (2-10 °C/min). These results suggest that rapid heating leads to increased volatilization of phenol groups, and that there is a pathway dependence associated with the applied thermal cycles, causing increased mass loss during ballistic phenolic pyrolysis relative to moderate heating rates. This dependence shows that the kinetic analysis performed in this paper can be utilized to optimize the cycle for increased char yields as well as cycle time reductions.