Process hazard evaluation and thermal runaway prediction for styrene polymerization initiated by peroxide-azo composites

Styrene is an important monomer for synthetic resins, ion exchange resins and synthetic rubber. Styrene polymerization requires the use of initiators to increase the reaction rate, with composite initiators showing promise for increased reaction rates. However, increased reaction rates in polymerization, if uncontrolled, can lead to thermal runaway with disastrous consequences. Numerous runaway incidents have been documented, indicating inadequate awareness of the thermal hazards of polymerization reactions. This study focuses on determining via calorimetric techniques the thermal hazards of styrene polymerization using azodiisobutyronitrile (AIBN) and tert-Butyl peroxybenzoate (TBPB) composite initiators. Differential scanning calorimetry (DSC) is employed to investigate the thermal decomposition properties of composite initiators with varying composition. Non-isothermal experiments and adiabatic experiments are used to determine the thermal hazard parameters including initial exothermic temperature and heat release of styrene polymerization. The risk of secondary reactions is evaluated by reaction calorimetry (RC1e) and product thermogravimetric analysis (TGA). Key safety parameters of the exothermic reaction, such as the onset temperature, heat release, time to maximum rate under adiabatic condition as well as activation energy, are presented. The results show that the thermal hazard of the polymerization reaction is lowest when the ratio of AIBN to TPPB in the composite initiator is 1:1. In this scenario, the temperature reached by the uncontrolled reaction does not provoke the decomposition of the products, yet the runaway consequences are still unacceptable. This work provides extensive data as a reference for the process optimization of styrene polymerization from the perspective of safety.