High-temperature radical polymerization of methyl methacrylate in a continuous pilot scale process

The present PhD thesis deals with the high temperature polymerization of methyl methacrylate in a continuous pilot scale process. The major aim is to investigate the feasibility of a polymerization process for the production of PMMA molding compound at temperatures in the range from 140 °C to 170 °C. Increasing the process temperature has the advantage of decreasing molecular weight and viscosity of the reaction mixture, thus allowing to reduce the addition of chain transfer agent and to increase the polymer content in the reactor. At the same time, the reaction rates are higher and the devolatilization is facilitated compared to low conversion polymerizations. Altogether, it leads to an improved space time yield of the process. However, increasing the process temperature also has an important impact on both, polymerization kinetics and polymer properties. The first two parts of this work are, therefore, dedicated to the self-initiation respectively the high temperature gel effect observed for the polymerization of MMA at the given temperature range. The self-initiation of MMA is mostly caused by polymeric peroxides that form from physically dissolved oxygen and the monomer, itself. The formation, decomposition and constitution of these peroxides are intensively studied and a formal kinetic is proposed for the formation and decomposition reaction. The polymerization of MMA is subject to a rather strong auto-acceleration, called gel effect, the intensity of which depends on process conditions and solvent content. There are several models proposed in the specialized literature to describe this phenomenon by modifying the termination rate constant as a function of conversion and temperature. The second part of this study contains the evaluation of these models with regards to their applicability to high temperature MMA polymerization as well as the development of a new variant of an existing model, which correctly describes the gel effect in the temperature range of interest as a function of polymer content, temperature and molecular weight. The advantage of this new variant is that it includes all other factors influencing the gel effect, i.e. chain transfer agent, initiator load, comonomer and solvent content, and that it is suitable for the description of batch and continuous processes. A complete kinetic model for the description of the high temperature copolymerization of MMA and MA, containing the results from the first two parts of this work, is established within the software package PREDICI® and validated by means of several series of batch polymerizations. In the third part of this work, a complete pilot plant installation for the continuous polymerization of MMA is designed and constructed in order to study the impact of increasing the reaction temperature on process properties and product quality under conditions similar to those of an industrial-scale polymerization. The pilot plant is based on a combination of recycle loop and consecutive tube reactor, equipped with SULZER SMXL® / SMX® static mixing technology. Furthermore, it is equipped with a static one-step flash devolatilization and a pelletizer for polymer granulation. At the same time, a refined method for inline conversion monitoring by speed of sound measurement is developed and tested in the pilot plant. By means of this technique it is possible to follow the dynamic behavior of the reactor and to measure directly the monomer conversion without taking a sample. The results of several pilot plant polymerizations carried out under different conditions are presented and the impact of temperature, comonomer and chain transfer agent on the thermal stability of the product is analyzed. From these results, the r-parameters for the copolymerization of MMA and MA at 160 °C as well as the chain transfer constant for n-dodecanethiol at 140 °C are determined. Finally, the pilot plant experiments are used to validate the kinetic model established beforehand in PREDICI® for the continuous copolymerization.

Dark curing kinetics of UV printing inks by real-time ATR-FTIR spectroscopy

Eric Jacques Nouzille

In UV-curable printing technology, an ink or a varnish is exposed to the UV-light source within a fraction of a second. Subsequent polymerization therefore proceeds in the dark, referred to as dark curing. Because dark curing can be critical and contributes to a large part of the overall conversion and to the final properties of the printed ink, the aim of this work was to get a better understanding of the dark curing process. Because an ink is a complex mixture of components, simpler systems representative of free radical and cationic chemistry were studied. Special attention was given to free radical polymerization with a detailed kinetic modeling. For cationic systems, the study was more application oriented. Attenuated-Total-Reflection Fourier-Transform Infrared (ATR-FTIR) spectroscopy was found to be the method of choice to follow in real-time the progress of the reaction under UV irradiation and in the dark. This technique allowed the study of curing conditions close to those encountered in industry. Dark curing of free radical was examined in systems consisting of a monomer (multifunctional acrylate) and a photoinitiator. The reactivities of different monomers were investigated and it was found that the chemistry of the spacer between the acrylate functional groups had an influence on the extent of dark curing. For example, TPGDA (tripropylene glycol diacrylate) led to higher overall conversion than HDDA (hexanediol diacrylate) although both are diacrylate monomers. Several explanations were put forward, such as the effect of dissolved oxygen, primary cyclization and initiation efficiency. The efficiency of photoinitiators (PIs) was examined and different behaviours were highlighted. For example, it was observed that DMPA (dimethoxyphenyl acetophenone) has a better quantum yield for α-cleavage compared to HCPK (1-hydroxycyclohexylphenyl ketone), whereas HCPK is more efficient in initiating the reaction, resulting in a better overall conversion in the dark. It is suggested that the two primary radicals resulting from the cleavage of the PI behave differently for DMPA and HCPK. One radical effectively initiates the reaction and the second is mainly involved in the termination process. These two different behaviours were observed under specific experimental conditions: (i) by covering the sample with a quartz plate and taking into account dissolved molecular oxygen and (ii) by curing under inert conditions with nitrogen. Photo-crosslinking polymerization reactions are strongly influenced by diffusion effects due to the formation of an infinite network. Therefore, it was proposed that a redistribution of reactivity within the system occurs, such as the diffusion of unreacted photoinitiators towards concentrations of unreacted monomers. This diffusion process has two positive effects: (i) for the same UV dose, but different intensities, a higher extent of conversion was reached with longer irradiation times under the condition investigated; (ii) multiple exposures led to a higher overall conversion than for a single exposure, for a fixed overall UV dose. Other parameters were explored, such as temperature and diffusing oxygen from air. With temperature, different trends were observed depending on the photoinitiator and the start of dark curing, either starting before the maximum rate of polymerization (Rp,max), or after Rp,max. The development of a kinetic model to describe dark curing of free radical polymerization was proposed. Existing models based on physical parameters, such as those incorporating diffusion-controlled phenomena and free-volume are relatively complex and require extensive information on the system studied. Therefore, a semi empirical approach was chosen in order to study the variation of fitting parameters when changing curing parameters, such as the photoinitiator concentration and type, the irradiation time, the UV light intensity and the temperature. The model was derived from kinetic data and it was assumed that rate coefficients are time-dependent and that termination, which was considered to be a first-order process, is propagation dependent. The kinetic equation has the form of a stretched exponential function and incorporates four fitting parameters. The model was evaluated for a large number of experimental data and very good fits were obtained with a random distribution of residuals. The exponent β was found to be sensitive to the termination process and reflected different reactivities in the dark as in the case of DMPA and HCPK. Dark curing of cationic systems continues until complete monomer conversion but at a slower rate. Thus systems with high reaction rates, both during UV irradiation and during the first seconds of the dark period, are of most importance for UV-curable printing technology. The most widely used diepoxide monomer in cationic ink formulation is 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (DECC). In order to improve its reactivity during the first seconds of dark curing, the influence of several parameters was examined, such as the addition of an oxetane co-monomer (TMPO), the influence of relative humidity, UV dose, light intensity, temperature, and film thickness. The enhanced reactivity of DECC with TMPO was confirmed. During the cationic curing process, two reaction mechanisms compete; an activated chain end (ACE) mechanism and an activated monomer (AM) mechanism. It was observed that these two processes depend on the presence of hydroxyl containing compounds such as TMPO monomer, polyol compounds, and, most important, water. When the concentration of these components is increased the AM mechanism is enhanced, leading to higher conversion but at a slower curing rate, and a degradation of the mechanical properties. The presence of water is unavoidable and its concentration in the ink varies in presence of hydrophilic compounds, such as polyol. Furthermore, the water content increases with increasing relative humidity in air. It was shown that the reactivity of the system decreased as the percent of relative humidity increased. Moreover, although a higher overall conversion is achieved, the crosslink density of the film was severely reduced. This lower crosslink density was the result of the predominance of chain transfer reactions, leading to more pendant and short chains which were hydroxyl terminated. Moreover, the extent of conversion was thickness dependent since less water diffuses into the deeper layers occurred in thicker films.

EPFL2007

Novel structured adsorber and oxidation catalysts for the removal of VOCs in low concentration

Kim Martin Nikolajsen

One possibility for the removal of volatile organic compounds (VOCs) is catalytic oxidation. For the removal of VOCs in low concentration (≲ 3000 ppmv for propane) it is not possible to sustain the required oxidation temperature over the catalyst without external heating. An approach to overcome this problem is to concentrate the VOC by adsorption, in a "two-step adsorption-incineration" process. This is an unsteady state process in which the VOC laden effluent gas is cleaned by passing it through an adsorber. Once the adsorber is saturated the VOC is desorbed by heating and purging. The concentrated VOC from the desorber can then be oxidized over the catalyst, producing enough heat to sustain the oxidation temperature. Conventional adsorbents have some drawbacks such as tailing during desorption due to the large particle size, high pressure drop and hot-spot formation. The objective of this work was to develop, characterize and test the adsorbent and catalyst, with low resistance to mass transfer resulting from thin films of active material, for the two-step adsorption-incineration process. Both the adsorbent and the oxidation catalysts were supported on sintered metal fibers (SMF). This is a novel, structured support offering several advantages over conventional randomly packed beds: Low pressure drop, due to the high porosity of the SMF, which is important where large flow rates and/or high quantities of gas must be treated. High thermal conductivity, due to the metallic nature, leading to smaller temperature gradients in the fixed-bed. A very high geometric surface area, due to the small fiber size. On the order of 200,000 m2/m3 with a fiber diameter of 20 µm. Furthermore, a mathematical model was developed to show the feasibility of the coupling of the two steps in the process. The adsorber was made from a thin, homogeneous film of MFI-type zeolite covering the fibers of the SMF. The film was grown by the seed-film method by hydrothermal treatment. A 3 µm film was obtained with 10 wt.% zeolite on the fiber after 24 h synthesis at 125 °C. This material had a specific surface area of 30 m2/g. The adsorption equilibrium of propane was described well by the Langmuir isotherm and the heat of adsorption was found to be ΔH0ads = - 43.1 kJ/mol. Isothermal breakthrough curves were measured as a function of temperature and film thickness. A mathematical model, comprised of a tanks-in-series model with a linear driving force (LDF) mass transfer description, successfully described the breakthrough behavior. The overall mass transfer was found to be solely due to diffusion in the zeolite film. However, the thin films show very low resistance to mass transfer leading to low internal concentration gradients and efficient utilization of the adsorbent. The pressure drop was measured and compared to that of a randomly packed fixed-bed of spheres. The equivalent diameter for a constant volumetric flow rate and superficial cross sectional area was dp = 180 µm, for a fiber diameter of df = 26 µm at 10 wt.% zeolite loading. The oxidation catalysts were all supported on SMF and cobalt oxide was used as the active phase. Three different groups of catalysts were developed. The support was modified by coating the fibers either with a thin film of fine powder (alumina or catalyst powder) or with a MFI-type zeolite film identical to the adsorbent. The third group was made from cobalt oxide impregnated directly on the oxidized fibers. The latter type of catalyst was found to be the most active during the screening experiments. The base support composition was varied (stainless steel (SS), FeCrAlloy and inconel) to investigate the support effect on the kinetics. The stainless steel and FeCrAlloy supports had similar activities, surpassing that of the inconel supported catalyst, which might be attributed to a Co3O4 surface enriched with iron from the support. Between the 1.1 wt.%Co3O4/SMFSS and 1.5 wt.%Co3O4/SMFFeCrAlloy catalyst the FeCrAlloy based had the highest normalized activity at low temperature (< 310 °C), low propane mole fraction (0.13 %) and high oxygen mole fraction (16.7 %). The chemical kinetics were determined from a novel and efficient experimental design. The apparent activation energy for this catalyst was found to be 87.5 ± 2.6 kJ/mol, with reaction orders in propane of 0.38 ± 0.04 and in oxygen of 0.30 ± 0.08. The catalysts were tested up to 350 °C. Mass transfer limitations were absent and the kinetics was described well with the power rate law as compared to the Mars-van Krevelen model. Catalysts based on Co3O4, supported directly on oxidized SMF, are very active catalysts, simple to prepare, withstand deactivation due to hot-spot formation and show great potential in comparison to the industrial reference catalyst (copper and manganese oxide supported on alumina). The adiabatic desorption process was investigated by mathematical modeling. Simply purging the adsorption bed with the hot exhaust gas from the reactor cannot result in the concentration of the VOC due to the high heat capacity of the fixed-bed. Hence, it will be necessary to use either a high heat carrier like steam, with the adverse effects of drying and separation of water and VOC, or an approach in which the desorber is heated prior to purging the bed, changing the adsorption equilibrium in favor of high gas concentrations. For the latter method, the minimum theoretical propane mole fraction for an autothermal process was found to be 350 ppmv to sustain a catalytic incinerator at 250 °C. Finally, it can be concluded that for the first time zeolite films on SMF were used as structured adsorbents, leading to very low internal mass transfer and increased process efficiency. Furthermore, the advantages of the SMF can be exploited in a catalytic oxidizer, which can be combined with the adsorber to annihilate VOCs in low concentrations. A novel method for the deposition of fine powder catalyst on SMF was also developed for the first time.

EPFL2007

Novel structured adsorber and oxidation catalysts for the removal of VOCs in low concentration

Kim Martin Nikolajsen

EPFL2007

Dark curing kinetics of UV printing inks by real-time ATR-FTIR spectroscopy

Eric Jacques Nouzille

EPFL2007