Detailed mechanistic modeling of polymer degradation: Application to polystyrene, polypropylene, and polystyrene/polypropylene mixtures

The push to recycle waste plastics has increased dramatically in recent years as environmental concerns over landfill capacity grow. Resource recovery involves converting waste polymers to valuable chemicals, and this recycling method seems to hold significant promise. The thermal technique of pyrolysis is an attractive resource recovery strategy due to its simplicity and ability to handle mixed plastic waste streams. One obstacle to successfully implementing a resource recovery process utilizing pyrolysis is the lack of a comprehensive understanding of the complex underlying reaction pathways. Because of the high temperatures used, the mixed feedstocks employed, and the diversity of the underlying free radical reactions, pyrolysis of polymeric waste typically affords a very complex product distribution. Therefore, studies of viable resource recovery strategies can be best initiated with well-defined systems of single components. Once the individual components are well understood, the complexity of the study can be increased, and mixed plastic waste can be addressed.;The purpose of this research was to develop the necessary framework for the mechanistic modeling of the decomposition of polymer mixtures during pyrolysis. This research has concentrated on polystyrene (PS) and polypropylene (PP), two voluminous components of mixed plastic waste. First, the modeling of the pyrolysis of PS and PP individually was tackled, followed by the binary degradation of PS/PP mixtures. Model predictions were in excellent agreement with experimental data collected in our laboratory and in the literature for the degradation of PS and PP alone and in binary mixtures. The binary model was able to mimic the enhancement of nearly a factor of four in the PP degradation rate that was observed experimentally during the pyrolysis of 50/50 wt% PS/PP mixtures at 350°C and 380°C. The binary model was then used to determine how to enhance synergistic interactions between polystyrene and polypropylene during their binary pyrolysis by manipulating the morphology of the binary polymer melt. The degree of initial mixing was varied using solid-state shear pulverization (SSSP), and using this technique to premix the binary system resulted in a 25% increase in the enhancement in the polypropylene degradation rate compared to no premixing.