Experimental And Simulation Study On Dissolution Of Supercritical CO₂ In Polymer Melt Under Dynamic Conditions

In recent years,the application of supercritical CO₂ in polymer processing,such as supercritical CO₂ as a physical foaming agent to produce microcellular foams,has attracted much attention.Among them,the key to the continuous molding process of microporous plastics is the formation of homogeneous system.As a result,the dissolved amount and dissolution rate of supercritical CO₂ in polymer melts under dynamic condition are particularly important.In this paper,the author takes polystyrene as an example to study in depth the dissolution process of supercritical CO₂ in polymer melts from both experimental and simulation aspects,that is to say,the diffusion process of interfacial absorbed CO₂ diffused into the melts and the CO₂ being accelerated to dissolve into the polymer melt under dynamic conditions.It not only develops the theory about the dissolution of supercritical CO₂ in polymer melts,but also provides a scientific basis for the structure design of mixing units and the drawing up of processing parameters.The main work and achievements are as follows:1.Experimental study on dissolutionThe experimental equipment for the dissolution of supercritical CO₂ in polymer melts based on volume-changing method was established,and the installation and debugging of the device were carried out.By changing the experimental parameters,under static conditions,the dissolution characteristics of supercritical CO₂ in PS melt under high temperature and high pressure were studied,and the influence of temperature,pressure and molecular weight on the dissolved amount and dissolution rate of supercritical CO₂ in polymer melt was analyzed.Combining adsorption dynamics and interfacial mass transfer(diffusion)theory to describe the dissolution mechanism of supercritical CO₂ in polymer melt under static conditions,which is the adsorption-diffusion mechanism,and calculate the dissolved amount of supercritical CO₂ in polymer melt and dissolution rate,and the projections for the solubility,the rationality of the dissolving mechanism is verified through the experimental data.2.Study on the diffusion coefficient of supercritical CO₂Under static conditions,experiments were conducted under different parameters to study the influence of pressure,temperature,and different molecular weights of PS on the diffusion coefficient of supercritical CO₂.Considering that the diffusion coefficient of supercritical CO₂ in PS melt is constantly changing,analyzing the influence trend of supercritical CO₂ dissolution on the diffusion coefficient can not only better understand the diffusion process of supercritical CO₂ in PS,but also accurately express the true value of the diffusion coefficient.3.Study on static condition simulationThe dissolution model under static condition was established,and the previously measured diffusion coefficient value was substituted into the simulation to obtain the variation of the dissolved amount and dissolution rate with time.The experimental values were compared with the simulated ones to verify the feasibility of the proposed method.Further analysis of the real-time relative errors and dissolution rate under different conditions demonstrated the accuracy of the simulation method,which laid a foundation for the numerical simulation of the dissolution process of supercritical CO₂in PS melt under dynamic conditions.4.Study on shear condition simulationEstablish the conical stirring dissolution model.Firstly,analyze the influence of shear on the amount of dissolution,solubility and dissolution rate under different temperature and pressure conditions through the simulation results.Secondly,through the distribution of the shear velocity flow field and the temperature field,the reasons for the faster dissolution under the shear conditions compared with the static conditions are revealed.5.Study on stirring condition simulationEstablish linear pin stirring and curved pin stirring and dissolution models respectively,and analyze the interaction between the rotation speed and the structure of the stirring elements and dissolved amount,solubility and dissolution rate of supercritical CO₂ under different temperature and pressure conditions according to the simulation results.Secondly,through the analysis of pressure and velocity flow fields,the dissolution behaviors of supercritical CO₂ under stirring conditions are revealed.