The Study On Phase Equilibrium For MMA-SCCO₂-PMMA Ternary System

Polymethyl methacrylate（PMMA）is an amorphous thermoplastic polymer with high degree of transparency.With good comprehensive mechanical properties,it has a wide application background.In the stage of devolatilization during the production,there need to remove the small molecule volatile after the reaction,such as unreactive monomer,solvent,initiator,etc.As the monomer of PMMA,methyl methacrylate（MMA）is poisonous.Before being applied to food packaging and pharmaceutical industry,if residual in polymer do not reach production safety standards,it will result in disease to the body.The existence of volatile will affect performance of the polymer and its molecular weight.Conventionally,the volatile are removed by vacuum stripping,which is not only difficult to remove some high boiling substance,but also consuming large of energy.Furthermore,if the polymer in a state of high temperature for a long time,it will also cause the degradation,so that vacuum stripping can not completely reach the production requirements.Supercritical fluid assisted the devolatilization of polymer（SFDV）utilize supercritical carbon dioxide（SCCO₂）as an extractant to separate the residue from the substrate.Due to the supercritical carbon dioxide is safe and non-toxic,some problems appear in traditional separation method can be avoided.The research on the phase equilibrium of solute-supercritical fluid-polymer system is the theoretical basis of supercritical fluid removal technology,and the research on this aspect is still in its infancy.In this study,a supercritical fliud phase equilibrium appparatus based on a static method was used to investigate the interactions among MMA,SCCO₂,PMMA at 313.15-353.15 K over the pressure range of 8-20 MPa,including MMA solubility in the SCCO₂,CO₂ adsorption in the PMMA and the distribution coefficients of MMA between the polymer and the supercritical fluid phase.The experimental and modeling methods in phase equilibrium were analyzed and evaluated to provide fundamental data for the development of supercritical fluid assisted devolatilization of polymer technology.The experimental results show:（1）The solubility of MMA in SCCO₂ was 0.36×10^-317.32×10^-33 mol/mol.When the temperature was constant,with the pressure elevating,solubility date increases correspondingly.The effect of temperature on solubility was more complex.Solubility curve will appear a interval cross area at18²0MPa.When the pressure is less than the transition pressure,solubility decreases with the rising of temperature.Whem the pressure is higher than the transition pressure,the solubility curve will show the opposite trend.According to the calculation results of the revised model of Chrastil,the trend of model correlation calculation is consistent with the experimental results,and the average relative deviation is 5.24%,which can predict the change of MMA solubility more accurately.（2）The mass fraction of CO₂ absorped in PMMA was 0.98¹2.51%.When the temperature remains constant,the adsorption amount increased with the elevating of pressure,and eventually reached a saturation value.When the pressure remains constant,the sdsorption amount decreased with the increase of temperature.The correlation calculation of S-L equation is basically consistent with the experimental results,and the average relative deviation is 7.70%,which can accurately predict the adsorption of CO₂ in PMMA.（3）The distribution coefficient K_MMA is range of 2.462¹6.530.At the same temperature,K_MMAMA decreases with pressure first,then presents a slightly larger trend.Under the same pressure,K_MMA increased with the rising of temperature.Based on the correlation calculation results of S-L state equation,the calculated value of the model is basically in line with the experimental value,with the maximum deviation of17.99%and the average relative deviation of 5.89%.