Free-radical dispersion polymerization of methyl methacrylate in supercritical carbon dioxide: a parametric analysis with reaction calorimetry

Raphaël Barbey, Sophie Fortini, Charalampos Mantelis, Thierry Meyer
2007
Article

Résumé

In this study, the application of reaction calorimetry, an efficient tool to obtain kinetic and safety data, is presented on the free radical dispersion polymn. of MMA in scCO2. The effect of three major parameters with respect to the reaction evolution are studied, namely of stirrer type and stirring speed, the monomer and CO2 concn. and the stabilizer concn. The effective formation of the dispersion as well as the balance between the polymer formed in the continuous phase or inside the particles are key features of this anal.

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Reaction calorimetry in supercritical fluids

Sophie Fortini

This thesis is devoted to the study of the dispersion polymerization of methyl methacrylate in supercritical carbon dioxide (scCO2), using a poly(dimethylsiloxane) macromonomer (PDMS macromonomer) as stabilizer. Supercritical fluids (SCF) and SCF mixtures are characterized by a temperature and a pressure above their critical point(s), which is the last point on the vaporization line of a pure component. This means that these fluids operate from moderate to high pressure. They can be used in various processes ranging from extractions, nanoparticle formation for controlled drug release, chemical reactions and polymer processing. Nowadays, the best candidate for SCF processing is carbon dioxide. The fundamental motivation of using scCO2 as a solvent is based on its potential to replace harmful chemical organic volatile compounds (VOCs) in order to develop more sustainable and environmentally friendly chemical processes. At this point, the crucial role of CO2 in the development of the so-called "green chemistry" comes on the stage. CO2 is a natural abundant compound with low toxicity exhibiting no inflammability. This last property is very advantageous considering the cost investments spent by the chemical industry to control the safety of the chemical processes using highly flammable compounds like VOC solvents. As expected, environmental arguments are not sufficient to motivate the development of new chemical process routes. Therefore, additional arguments to use SCFs have to be found, and they do exist. As supercritical fluids are compressible fluids they can exhibit liquid-like and gas-like properties, which can be tuned easily by varying the operating conditions, like pressure and temperature. This fundamental behavior of SCF is their main asset and demonstrates their superiority to develop more flexible processes. The polymer industry is one of the industries that uses the largest volumes of organic solvents and sometimes halogenated ones, well known to destroy the ozone layer. The use of scCO2 gives to chemists and engineers the opportunity to develop more sustainable polymer processes, considering the numerous chemical and physical advantages of carbon dioxide. The processing of scCO2 for polymer production is no more than fifteen years-old. This means that a certain quantity of knowledge has been acquired but still a lot of unknowns hinder their promotion at industrial level. This work is inserted in this context and finds there its main motivations. This thesis is composed of two different but intrinsically connected approaches of the dispersion polymerization of the methyl methacrylate (MMA) in scCO2. A part of this thesis is devoted to the development of techniques allowing the on-line monitoring of polymerizations in scCO2 at "larger" scale, conducted from an engineering approach. The intrinsically connected part is devoted to the understanding of the fundamental phenomena that govern the dispersion polymerization of MMA in scCO2, its kinetic features and the product characteristics of the polymer produced, being conducted from a chemical approach of the subject. Up to now, most of the studies dealing with polymerization reactions in scCO2 are realized in small autoclaves between 2 and 60 ml allowing pertinent fundamental analysis but with poor similarities with an industrial reactor. A keystone of this work is the development of a supercritical reaction calorimeter composed of a 1.3 liters high pressure reactor allowing the kinetic study of the dispersion polymerization, which in turn gives a direct insight into the parameters that control the dispersion polymerization stability, efficiency and mechanism. Based on an adapted heat balance, the calorimeter can give the profile of monomer conversion (thermal conversion). Furthermore, the volume of the reactor allows inserting on-line sensors inside the reactor. This possibility led to the development of an ultrasonic sensor to monitor the polymerization process. The combination between the calorimetric information and the sensor signal shows the potential of these sensors to monitor polymerization reactions in scCO2. By measuring the speed of sound evolution during the course of the polymerization, it is possible to calculate the composition of the medium and thus evaluate the monomer conversion. The analysis of the effects of temperature, stirring speed and impeller types demonstrate that the dispersion polymerization of MMA can be effective under a wide range of operating conditions using the PDMS macromonomer as stabilizer. The experiments point out that the stability of the dispersion polymerization and in turn the rate of polymerization, the polymerization loci1 and the polymer quality depend strongly on the stabilizer concentration but more fundamentally on its degree of solubility in the carbon dioxide. Phase behavior measurements demonstrate that the 5'000 g/mol PDMS macromonomer exhibits a relative good solubility in carbon dioxide and that this solubility can be greatly improved by the presence of the monomer in the mixture. In fact, the monomer acts as a cosolvent for the PDMS macromonomer improving the solvency of the scCO2 (polarity, density). This study demonstrates that the concentration of the monomer in the reacting medium is a key parameter to control the stability of the dispersion throughout the polymerization process. A dispersion polymerization is characterized by polymer-rich particles dispersed in a continuous phase, i.e. the CO2-rich continuous phase. The results give evidence of the fact that the polymerization can take place in both phases depending on the concentration of stabilizer and its solubility in the reacting medium. In this case, bimodal molecular weight distributions and intermediate rate of polymerization are observed. When only one reaction locus is active, monomodal molecular weight distributions are obtained. Furthermore, the locus of the polymerization influences directly the rate at which the polymer is produced and the degree of polymerization. In order to complete the study, a model has been developed demonstrating that diffusion limitations are operative in the dispersion polymerization of MMA in scCO2. A gel effect is present in the polymer-rich phase that leads to the increase of the molecular weight of the polymer produced, as observed also experimentally. Moreover, the presence of this gel effect occurring inside the polymer-rich particles explains the auto-acceleration of the polymerization rate as the conversion increases. The model demonstrates that in the case of an effective dispersion polymerization of the methyl methacrylate in scCO2 the main reaction loci are the polymer-rich particles. ______________________________ 1 A dispersion polymerization is composed of polymer-rich particles dispersed in the CO2-rich continuous phase. The polymerization locus is referred to the phase into which the polymerization can take place.

EPFL2006

Chargement

The PhD thesis deals with the optimization of the reaction calorimetry technique to monitor chemical reactions in supercritical fluids. The aim is to develop this thermal analysis technique to monitor the heat released by a chemical reaction in a high pressure reactor and consequently discuss upon the reaction evolution. The experimental equipment used was previously developed in the group of chemical and physical safety at the EPFL. Additionally, a mass flow meter was installed during this thesis to measure the quantity of supercritical fluid inserted in the reactor. Initially, the core of the technique, meaning the heat flow equation, was examined in a term-by-term analysis to adapt and optimize each term for supercritical reaction systems. For this analysis a model reaction was chosen, namely the free-radical dispersion polymerization of methyl methacrylate in supercritical carbon dioxide, and a set of conditions was set as a reference. The particularities linked to the supercritical nature of the solvent were taken into consideration and more precisely the fact that the solvent occupies the entire available reactor volume. As a result it was found that every reactor part has to be thermally controlled and that special caution has to be paid on the estimation of the overall heat transfer coefficient between the reaction mixture and the temperature regulating fluid running in the reactor jacket and on the estimation of the specific heat capacity of the reaction mixture. Furthermore, the injection phase of the additional reactants, according to the reaction protocol, was found to be subject to considerable calorimetric errors; therefore an optimized injection pattern was designed to minimize the undesired temperature oscillations during this phase. The second step of the thesis consists of the discussion on the reaction evolution based on the optimized results obtained. First a direct comparison is presented between the calorimetric results before and after the analysis to highlight the points were significant improvements were introduced, mainly in terms of accuracy and reproducibility of the results. Then the discussion focuses on the nucleation phase of the polymer particles, where the data show that the reaction takes place almost exclusively in the continuous phase. This conclusion is also found to be in very good agreement with the results of other experimental techniques. Further, the role of the pressure on the reaction evolution was examined through a series of experiments, based on a small reaction deceleration observed, and was found to have a drastic effect on the creation of stable dispersion conditions. A key parameter in this investigation was the partitioning of the solvent, the monomer and the produced polymer in the two reaction phases. The latter also helped in the formulation of an explanation for the measured pressure profiles. Finally, the reaction heat rate data of the previous tests were used to discuss some safety aspects of the reaction, primarily through a cooling system failure scenario. Once the results on the model reaction were sufficiently confident, different reaction systems were tested to demonstrate the robustness and to explore the limits of the equipment and of the technique. It is shown that the calorimeter succeeds in detecting very small amounts of heat; therefore can monitor much less exothermic reactions, like the dispersion polymerization of styrene in supercritical carbon dioxide. On the other hand, the applicability of the technique is limited for very exothermic chemical reactions, like the precipitation polymerization of acrylic acid in supercritical carbon dioxide. Additionally, non-polymerization reaction systems can also be monitored and this is demonstrated with the example of the esterification of acetic anhydride with methanol in supercritical carbon dioxide. Finally, some preliminary tests were carried out to investigate the possibility of working with the high pressure reactor in a continuous mode but the results show that the developed set-up is not suited for such an application. The last part of the thesis deals with the use of the reaction calorimeter to produce polymeric foams using supercritical carbon dioxide as the blowing agent. The capability to control the reactor temperature with high accuracy and measure precisely the temperature and the pressure were exploited in this case. The equipment was initially calibrated in terms of the depressurization profiles that can be achieved and consequently polymer foams were produced. The effects on the final foam morphology of the polymer type, the temperature, the pressure, the depressurization rate and of the calorimeter operation were studied and various trends were identified.

EPFL2008

Chargement

Dispersion polymerization of methyl methacrylate in supercritical carbon dioxide: control of molecular weight distribution by adjusting particle surface area

Charalampos Mantelis, Thierry Meyer

Expts. of Me methacrylate dispersion polymn. are carried out in a reaction calorimeter using PDMS-mMA (methacrylate-terminated polydimethylsiloxane) as surfactant. Different stabilizer concns. from 0 to 10 wt% with respect to monomer have been considered in order to control particle morphol. The anal. by SEM reveals a definite decrease of the total particle surface area at decreasing stabilizer concn. At the same time, the anal. of the polymer microstructure by gel permeation chromatog. shows a trend of the av. mol. wt. towards smaller values. In particular, a second mode at low mol. wts. has been obsd. leading to bimodal mol. wt. distributions. The exptl. results are compared with simulation results obtained through a detailed kinetic model developed in previous studies. The key role of the radical exchange between continuous and dispersed phases is confirmed.

2007

Chargement

Reaction calorimetry in supercritical fluids

Sophie Fortini

EPFL2006

EPFL2008