Thermal Hazard Study On Synthesis Of Polyimides

Polyimide（PI）as the new polymer materials is widely used in aviation,aerospace,microelectronics and other high-tech fields.However,the polymerization reaction and involved explosive polymerization are challenging from the view of thermal safety.In this paper,based on the two-step method adopted by enterprises,the thermal hazards assessment of PI synthesis through the automatic reaction calorimetry,simultaneous thermal analyzer,accelerating rate calorimeter and thermal dynamics software.The calorimetric characteristics of the polyamide acid（PAA）synthesis were performed by automatic reaction calorimeter,experimental device,gas chromatograph and rheometer.The results showed that the synthesis of PAA was exothermic reaction and the molar reaction enthalpy（Δ_rH_m）of low viscosity PAA（<7000 Pa·s）was 154.42 kJ/mol.The relationship curve between the dosing amount（x）of heat release（y）was:y=0.7229x-0.0646（R²=0.9971）.Furthermore,the synthesis of PAA was the autocatalytic second-order reversible reaction,Besides,explosion polymerization has a great relationship with dosing times and conversion rate of reactants,decreasing dosing times was easy to cause explosion polymerization.According to the calculation of polymerization kinetics,the reaction rate constants of explosion polymerization were greater than these of normal reaction.It was proposed that explosion polymerization can be effectively avoided by increasing the dosing times and controlling the rate constants of positive reaction and self-catalytic with less than0.0037 and 0.0192,respectively.Simultaneous thermal analyzer,accelerating rate calorimeter and AKTS software were used to study the calorimetric and decomposition characteristics of the PAA imidization process.The results showed that the imidization of PAA was an endothermic reaction,the absorbed heat was105.77 J/g.The involved reactants had good thermal stability and the average heat release of PI in the self-decomposition was-380.07 J/g.The apparent activation energy of PI ranged from 179.74 kJ/mol to 308.26kJ/mol,and the value of T_D24 was 494.3 ℃.The risk matrix method and the stoessel criticality diagram were used to evaluate the thermal safety of PI synthesis.The results demonstrated that the thermal runaway is mainly concentrated in the synthesis of PAA.The severity level of runaway was level 1,in which the possibility of the reaction runaway was infrequent;the risk level was class 1,which was acceptable risk.But the large exothermic in the reaction was easy to reach flash point of involved solvent,causing a high risk.It is suggested that the dosing scheme can be optimized to four times dosing in order to reduce the concentrated heat release of polymerization reaction,effectively ensuring the process safety and improving the production efficiency.