Studies On The Applications Of Supercritical Carbon Dioxide In Foaming And Processing Of Polyolefin Blends

Driven by the global advocacy of green development,traditional modes of process and production are facing challenges of transformation and upgradation.Developing advanced processing technologies that can reduce energy consumption and emission of harmful pollutants has been called worldwide for an economic,environmental-friendly and sustainable path to a better future.Over the past few decades,scCO₂ has attracted considerable interest owing to its excellent physical properties,and hence it has been widely applied in extraction separation,chemical reaction and biological engineering,etc.However,there are still some challenges in both basic research and practical application,such as how to apply the scCO₂ to traditional polymer processing,as well as how to enable the scCO₂ maximum limit to display its potency.Therefore,in order to effectively combine the scCO₂ with the polymer extrusion processing technology,this thesis carried out a series of studies,which include the following parts:1.Utilization of scCO₂ in preparation of polymer blend foamsThe PP/PS blend foams with a bimodal cell structure were prepared by using scCO₂ in continuous extrusion.By comparison of the cell structure of blend foams before and after extraction,it was found that the large cells were mainly formed in PS phase,while the small ones were mostly formed in PP phase.A possible mechanism was proposed to explain the formation of the bimodal cell structure.It was believed that a prominent difference in cell nucleation in the two phases of the PP/PS blends was required for the formation of bimodal cell structure.The proposed mechanism was confirmed by further experimental results,which showed that the bimodal cell structure could be changed by altering the nucleation ability in PP phase and PS phase through changing experimental conditions,such as the content of scCO₂,the pressure drop and the screw speed,or through applying a heterogeneous nucleating agent.The influence of phase separation in annealing process on the foaming properties of PS/PMMA blends was investigated.It was found that with the phase separation proceeding,the average domain size of the dispersed phase was increased,while the area of the interfaces was decreased.The effect of phase separation on the foaming behavior was more obvious at a higher content of the dispersed phase.With the change of the blending ratio,the cell morphologies of the PS/PMMA blend foams were entirely different.This was attributed to that PMMA had a higher affinity for CO₂,and hence the great pressure difference of CO₂ between the two phases could cause the CO₂ to diffuse from PMMA phase to PS phase during foaming process.2.Utilization of scCO₂ in polymer blendingDuring melt extrusion process,the scCO₂ was applied to control the phase morphologies of PP/PS and PP/POE blends.The addition of scCO₂ could decrease the viscosities of the polymer melts and change the viscosity ratio of the polymer blends.The average domain size of the dispersed phase was smallest when the viscosity ratio of the two phases was close to one.The results showed that the injection of scCO₂ obviously decreased the size of the dispersed phase,and hence improved the mechanical properties and foaming properties of the PP/PS blends.The scCO₂ also played an important role in controlling phase morphologies of PP/POE blends,and the improved phase morphologies favored better mechanical properties.The PP/POE blends prepared with or without scCO₂ presented brittle-tough transition at various temperatures.3.Utilization of scCO₂ in polymer reactive extrusionA novel strategy was designed for the preparation of a long-chain branched polypropylene?LCBPP?with improved foamability via reactive extrusion in the presence of scCO₂.Fourier transform infrared spectroscopy and high temperature GPC confirmed that TMPTA was grafted onto PP chains,and the presence of scCO₂ promoted the grafting and branching reactions,and hindered polymer degradation.A possible mechanism was proposed to explain the effect of scCO₂ on the branching reactions.In addition,rheological behavior of pure PP and modified PP samples was studied to investigate the effect of long chain branching of PP on the melt viscosity and strength,and foaming behavior was studied to confirm the subsequent effect on its foamability.It was found that the long chain branching increased the melt viscosity and strength of modified PP samples,which favored the foamability,and that the foaming windows were expanded in the presence of scCO₂.During reactive extrusion process,the scCO₂ was applied to in situ compatibilize the PP/PS blends.FTIR was used to confirm the formation of PP-b-PS and/or PP-g-PS copolymers,and the content of which was increased with the increased contents of scCO₂.The mechanical properties of PP/PS blends were found to be closely related to the phase morphologies,that better dispersion and stronger interfacial adhesion favored enhanced mechanical properties.