Modelling of free radical polymerisation of ethylene using difunctional initiators

Abstract

Low-density polyethylene (LDPE), commonly produced in high-pressure free radical polymerisation processes, is very important for the manufacture of engineering and commodity plastics. However, the thermodynamic conditions of the process hinder ethylene from going to full conversion. Other than recycling the product, one way of improving the monomer conversion is to initiate the polymerisation with difunctional organic peroxides. But, the kinetic decomposition of multi-/difunctional peroxides is still a controversial issue. The present study proposes a kinetic model based on a postulated reaction mechanism for free radical ethylene polymerisation initiated by difunctional initiators. The model describes the rates of initiation, propagation and the population balance equations in an isothermal autoclave reactor operated at a constant pressure of 1700 bars and a temperature range of 110–300°C. The simulation confirmed the trends of experimental data collected from literature for one monofunctional and two difunctional initiators. Due to the dual functionality of difunctional peroxide, the ethylene conversion obtained was about twice as much as that obtained with monofunctional peroxide, for only a half amount of the initial initiator concentration. The model also gives good prediction of the number average molecular weights data of the polymer and predicts reasonable values of the polydispersity index.