Kinetics And Modelling Of Melt/Solid Polycondensation Of Poly (L-lactic Acid)

As an important biobased and biodegradable polymer, poly(L-lactic acid) (PLLA) has found many end-uses in biomaterials, fibers, disposable commodities and package materials because of its good mechanical properties and excellent biodegradability and biocompatibility. Melt/solid state polycondensation (MP/SSP) is a cost effective route for synthesis of high molecular weight poly(L-lactic acid) (PLLA). But the reaction rates in its four stages need to be enhanced greatly and the reaction times to be shortened largely before the MP/SSP technology can be industrialized. To promote the industrialization of the MP/SSP technology of PLLA, we aimed in the thesis to intensify the melt polycondensation, to study the kinetics of the melt polycondensation, to develop kinetic modeling of melt/solid state polymerization and to simulate the melt/solid state polymerization process.First of all, a new catalyst addition policy, i.e., adding TSA at the dehydration stage and SnCl2·2H2O at the MP stage, and more appropriate temperature and pressure programs(dehydration:140℃/3000 Pa; oligomerization:160℃/300 Pa) were presented and applied in the MP process of LLA, which appeared very effective for speeding up the dehydration and oligomerization stages as well as depressing racemization in the whole MP process. Also, the kinetics that Xn increased linearly in two steps, slowly in the first step and fast in the second step, was observed and the critical Xn where the change appeared was about 35. So, we found an interesting strong dependence of the Mw of final PLLA product on the Xn of the oligomer. When the Xn of the oligomer was smaller than the critical Xn, the larger the Xn of the oligomer was, the faster the reaction rate was.Second, we built the kinetics model of melt polymerization of L-lactic acid including poly/depolymerization equilibrium and ring/chain equilibrium, and obtained kinetic parameters through open and close experiments. The activation energy of esterification in the first step is 99.8 kJ/mol, in the second step is 127 kJ/mol and the activation energy of lactide form reaction is 213 kJ/mol. So the reaction rate of lactide form reaction increased fast as temperature raised. Simulation results showed that, if all lactide are taken out of the system, both Xn and yield would decrease heavily, so the distillation to return lactide is very important in the process of melt polycondensation of PLLA. Additionally, the coefficient of diffusion of water affect the reaction rate, and the critical diffusion coefficient exists. At 180℃, the critical value is 103 mol·L-1·h-1. If the coefficient of diffusion of water is larger than the critical value, the water can be removed quickly enough and almost not affect the reaction rate.Finally, we built the model of solid polycondensation of PLLA and analyzed the effect of different conditions, such as particle radius, temperatures and degree of crystallinity, on solid polycondensation. Results showed that, small diameter, large coefficient of diffusion, high degree of crystallinity and high temperature benefit the solid polycondensation. And in a certain coefficient of diffusion, there is a critical diameter which can make reaction rate independent on the water diffusion. In terms of degree of crystallinity, large crystallininty can promote the esterification. In terms of temperature, although higher temperature, higher reaction rate, but if the temperature is higher than the melt temperature, the polycondensation rate will decrease because of the agglomeration of particles, so the best temperature is the highest one which can not melt the crystal of PLLA. Base on the increase of melt temperature as Xn increases, increasing temperature step by step will be a good strategy to enhance solid polycondensation efficiency. Finally, the reaction rate of solid polymerization is independent on the coefficient of diffusion of lactide.