Mechanisms and kinetics of thermal decomposition of plastics from isothermal and dynamic measurements

The kinetics of decomposition of plastics are of interest from different po ints of view, i.e. evolution of harmful substances during fires or waste in cineration, recovering of chemical raw materials from plastic refuses and d esigning of recycling procedures. To measure the formal kinetic parameters of the degradation of polymers isothermal and dynamic methods are applied i n this work. Dynamic measurements are performed by combined thermogravimetr y mass spectrometry (TG-MS), the isothermal measurements are carried out wi th a new closed loop-type reactor. To evaluate consistent kinetic data from isothermal and dynamic measurements, the energy balance for the sample in dynamic measurements has to be considered to obtain the true sample tempera ture and heating rate. Subject of this investigation is the exploitation of dynamic and isothermal methods for measuring and interpreting the kinetics of thermal decomposition of plastics. Results for commodity plastics polye thylene and poly(vinyl chloride) (PVC) are presented. The combined applicat ion of TG-RIS, isothermal experiments in the closed loop-type reactor and D SC leads to new results for the decomposition kinetics of PVC. The dehydroc hlorination mechanism at moderate temperature can be distinguished in an en dothermal and exothermal part. The benzene formation is identified as a sec ond order reaction. A great advantage of the isothermal method is, that cha nges in the mechanisms are detectable, i.e. changes in the apparent order o f the reaction and the apparent activation energy. From that, new mechanist ic aspects of the decomposition kinetics of polyethylene were obtained. (C) 1999 Elsevier Science B.V. All rights reserved.