Kinetic Analysis Of The Degradation Of Hdpe Plus Pp Polymer Mixtures

An in depth understanding of the kinetics of the decomposition of plastic mixtures is necessary to enhance chemical recycling approaches, in particular by pyrolysis. The fact that plastic waste is usually composed of mixtures of plastics constitutes a hindrance for secondary recycling but may not be a particular difficulty for tertiary recycling. In this study, an analysis of the thermal degradation of mixtures of high-density polyethylene (HDPE) and polypropylene (PP), in different proportions, was carried-out under pyrolysis conditions using thermogravimetric analysis (TGA) under nonisothermal conditions, at different heating rates. Both isoconversional and model-fitting approaches were used to estimate kinetic parameters from the experimental information. It was found that both the degradation of HDPE and PP, as well as their mixtures, can be described by a single-step degradation process but that the degradation process is influenced by the proportion between the two plastics. The HDPE/PP mixtures in which PP has a higher ratio than HDPE cause the degradation onset temperature for the mixture to be lower than either pure PP or pure HDPE. Also, it was found that the apparent activation energy estimated was lower for these mixtures in comparison with the pure polymers. This implies that there are extensive interactions between the plastics during the degradation procedure and, from an operational point of view, it may be actually easier to degrade plastic mixtures than the corresponding pure streams, a conclusion that is important in the context of tertiary recycling of plastic mixtures. Furthermore, it was observed that a compensation effect exists that encompasses all the kinetic data for the mixtures, regardless of the composition, indicating that, although significant changes occur in relation to the activation energy, the underlying degradation mechanism remains the same.