PLA/PHBH compounding by twin-screw extrusion: simulation-based evaluation of the processing parameters

In the last twenty years, plastic pollution has become a matter of great concern due to the massive use of polymers in disposable packaging applications. The most common materials employed in this field are polyolefins and polyethylene terephthalate (PET), synthesized from fossil resources. The final products are characterized by a low cost and good mechanical characteristics, but they result in a positive carbon footprint and require hundreds of years to be biodegraded. Then, many efforts have been made in order to produce large quantities of bio-based and biodegradable polymers that can be employed in the packaging industry. Unfortunately, the most common employed bioplastics, such as polylactic acid (PLA) and polyhydroxyalkanoates (PHA) show poor mechanical properties coupled with poor processability. An effective and economic solution could be the production of biopolymers-based blends in order to reciprocally compensate the drawbacks of each component. At industrial scale, blending is performed in twin-screw extruders when both the polymers are in the molten state, but the identification of the best processing conditions, such as the optimal screw speed or feeding rate, is usually done by a trial and error approach. It means that long times are required in order to find a suitable screw profile. A solution, whose feasibility for the case of a PLA-PHBH blend filled with Cloisite 5 is studied in this work, can be found in the use of softwares especially designed to simulate the extrusion process like Ludovic, that helps to find the optimal processing conditions. The present dissertation starts from a theoretical introduction of the topic with an overview on the thermodynamics of polymeric blends, followed by a section in which some considerations about the rheology are discussed. The mechanisms for coalescence and breakup of a polymeric particle inside a melt are studied and applied to the extrusion process thanks to a model proposed by Vergnes and Delamare. A further chapter is devoted to the description of the software Ludovic, starting from a general description of how it organizes the simulations, followed by a deepening on the principles used for calculation. In this chapter also the settings of the simulations and of the DoEs are reported. The DoE instrument is employed to evaluate the effectiveness of the software to correctly model the behaviour of the blend. In particular, the trends of Residence Time Distributions, temperature, local and total residence times, shear rate and viscosity are assessed employing three different screw profiles. Also the effect of the blend composition is considered. It has been concluded that the software correctly models the compounding process of both filled and not filled PLA/PHBH blends. The results of the simulations run at 400 rpm with a flow rate of 3 kg/h have been analysed in order to determine which one of the three profiles ensures the better performances in terms of final morphology. The screw profile identified as the best has then been employed to assess also the effect of the rotation speed. As an aid for the various evaluations, the model of Vergnes and Delamare has been employed to process the data returned by Ludovic. Finally, the various predictions have been compared with the SEM micrographs of a PLA/PHBH blend compounded with the same processing parameters employed for the simulations. A satisfying accordance between the computed results and the reality has been assessed.