Kinetic Studies of Effects of Heat on PET Plastics: TGA, DTA, XRD and DSC Analysis

Abstract

In this work, the effects of heat on plastic waste (PET plastic bottles) were investigated. During thermogravimetric analysis, the kinetics of the PET pyrolysis was studied using a combination of model-fitting and model-free methods. About 12mg of the grinded PET sample was weighed into the ceramic crucible and then heated from temperature of 30 °C to 950 °C at two heating rates of 10°C/min and 20°C /min.  Thermogravimetric analysis indicated that the decomposition of plastic waste in a Nitrogen atmosphere can be divided into three stages: the minor loss stage (100°C), the major loss stage (450–520°C) and the stable loss stage (500–900°C). The corresponding weight loss at two different heating rates of 10°C/min and 20°C /min were determined to of 70% and 77.7% respectively. Based on R2 (regression coefficient) values, heating rate of 20oC/min gave the values (0.9279) for the model free method employed (KAS model). When compared to the model fitting, 0.75th order of reaction gave the closest  R2 value (0.9643) which also translated to a very close Activation energies of 264 KJ/mol and 255 KJ/mol respectively. All in all, it can be deduced that the activation energy of PET pyrolysis falls within 250 – 280 KJ/mol based on this study. Also, X-Ray Diffraction and Differential Scanning Calorimetry analysis were carried out. The narrow and high nature of the XRD peaks depicted that the sample is crystalline. And lastly, DSC analysis investigated the heat flowing in and out of the PET sample. The exodermal (where cold re-crystallization occurred) and endothermal (where melting occur) peaks were recorded as 165oC and 138oC respectively. It is recommended that other model-free and model-fitting methods should be invested in further research in order to generate a wide range of data for kinetics of PET.