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

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.

Influence of the protein environment on spectroscopy and ultrafast dynamics of retinal in bacteriorhodopsin

Selma Schenkl

The environment is important for the exact course of a chemical reaction. As an example of the strong influence of the environment on the reaction, this work studies the isomerization reaction of the chromophore retinal in the binding pocket of the protein bacteriorhodopsin. Three selected distance scales with reference to the chromophore are addressed: The distant protein surface at a length scale of 3-5 nm, the fuzzy environment of the binding pocket (1-2 nm) and the immediate neighborhood represented by the amino acid tryptophan Trp86 (5 Å). First, static absorption and fluorescence spectroscopies are applied to three dimensionally crystallized bacteriorhodopsin. An especially adapted set-up was developed for each task. The analysis of data is carried out in comparison with the measured spectra of the native protein membrane in solution. While the absorption spectrum shows a broadening on the high-energy side, the fluorescence spectrum deviates in the general shape of that measured in solution. The synoptic analysis of all spectra reveals the existence of three spectroscopically distinct species in the crystal: BR490 (absorption maximum at 490 nm) with a relative contribution of 28%, BR570 (native spectrum of solution, 68%), and BR610 ("blue membrane", 4%). BR490 appears particularly in the additional fluorescence band at 550 nm and is assigned to the dehydrated species of bacteriorhodopsin. The presence of that particular species indicates the suppression of rehydration in a partially dehydrated crystal and shows that the three dimensional arrangement hinders water diffusion. A simple model, close domains are assumed to form, eventually suppressing free water diffusion in a crystal. The lack of water on the protein surface modifies the protonation states of the amino acids in the binding pocket and therefore the optical properties of the retinal. The chromophore actually reacts on the alteration of its distant environment upon incorporation in the protein crystal. The second distance scale - the fuzzy environment - is addressed by a time-integrated fluorescence spectroscopy on native bacteriorhodopsin as function of the excitation wavelength (430 nm to 650 nm). The optimized experimental set-up guaranteed that the fluorescence of only the excited state I460 contributed. The measured spectra show maxima at 740 nm, and exhibit a Stokes shift of 5000 cm-1, independent of the excitation wavelength. The similarity of the fluorescence excitation spectrum with the spectrum of absorbed photons excludes energy-dependent loss channels. However, the spectra show a growing asymmetric broadening on the high-energy side with decreasing excitation wavelength. The additional fluorescence originates from vibrational hot states, also supported by the decomposition by Gaussian functions. A comparison with the fluorescence spectrum of blue membranes shows that the low-energy part also arises from vibrational hot states. Consequently, the first fast intramolecular vibrational energy relaxation does not entirely distribute the access-energy to other modes. The remaining energy is found in a quasi-static occupation of vibrational modes. The subsequent second vibrational relaxation — probably not only involving the retinal but also the protein — takes place on a time scale similar to that of the isomerization. The fast isomerization reaction prevents dissipation of energy relaxation of the excited state of the retinal into the protein environment. On a third distance scale, femtosecond spectroscopy in the UV offers a direct view into the electrostatic interaction of the chromophore with its immediate neighborhood. For the first time, the retinal isomerization was studied within the spectral window of the tryptophan absorption from 265 nm to 310 nm. For this, a set-up adapted to UV wavelengths was realized, basing on non-linear optics. The developed single-shot detection system yields a sensitivity of 10-5 in optical density. In the transient absorption measurements, a time resolution of 90 fs was achieved. At short probe wavelengths (265 nm - 294 nm), the measured transients show first a fast reduction of the absorption which subsequently recovers on three time scales (380 fs, 3 ps, > 16 ps). With increasing probe wavelength an induced transient absorption becomes dominant (282 nm - 294 nm). It has a rise time of 280 fs, and decays with a time constant of 4.3 ps. With further increasing probe wavelength the absorption signal decays faster. In response to the complex signature of the transients, an iterative and model-independent transient-decomposition is introduced. A set of three basis elements were identified: one bleach, and two absorption transients. The two induced absorption transients are assigned to retinal. The dynamics of the excited state (I460) in the first retinal-transient is described by two time constants (280 fs and 1 ps). The fast time constant leads to the photoproduct, while the slower one leads back to the ground state, presumably by internal conversion. The 250 fs delayed onset of the second transient (photo-intermediate J) is connected to a wave packet motion in the excited state. This delayed onset of the transient was observed for the first time. It demands a rise time of the J photo-intermediate of only 280 fs. This contrasts common estimates of 500 fs given in literature. For the first time, the temporal behavior of the permanent dipole moment of retinal is observed experimentally. Its evolution is imprinted in the dominant bleach feature (265 nm - 294 nm), originating from tryptophan Trp86. In the excited state, the permanent dipole moment increases strongly within the first 250 fs, and decays exponentially afterwards, following the formation of the photoproduct. However, the related time constant of 380 fs is slightly greater than that of 280 fs of the photoproduct J. This discrepancy may indicate a remaining long-lived polarization of the binding pocket. In brief, by using the intrinsic tryptophan as a probe, the local evolution of the electric field during the isomerization reaction was investigated. Indeed the isomerization reaction is reflected in the immediate environment. The variety of optical spectroscopy provides methods especially adapted for studying the influences of the environment on a chemical reaction. The results achieved here highlight the importance of these influences.

EPFL2004

Mesoscopic thin films of hematite as photoanode for water photoelectrolysis

Alexis Joseph Duret

Due to the limiting amount of fossil fuel available and to the continuous growth of the world energy consumption, it becomes important to find alternative energy sources. Hydrogen produced by the photoelectrolysis of water is a perfect candidate as a clean energy source since it can be produced and used efficiently without generating any pollution. The water oxidation to oxygen is a slow reaction that requires an important overpotential in order to reach a significant current. This problem is crucial in water photoelectrolysis: a high overpotential means more solar cells in series in order to obtain high enough conversion efficiency. In order to lower the number of solar cells in series, a photoactive anode can be used. One potentially good material for a photoanode is hematite: it is cheap, stable and it has a bandgap small enough to absorb a part of the visible light. The objective of the present work is to develop and characterize highly photoactive semi transparent thin films of hematite that could be used efficiently to photo oxidize water. Three different deposition methods have been tested: Spray Pyrolysis (SP), Ultrasonic Spray Pyrolysis (USP) and Electrostatic Spray Deposition (ESD). The films obtained were characterized using spectroscopic (UV-Vis absorption, SEM and Raman spectroscopy) and electrochemical techniques (current potential curves in dark and under light, IPCE spectra, photopotential measurements and electrochemical impedance spectroscopy). The films made by ESD were not photoactive. The films made by SP were weakly photoactive and the films made by USP gave the highest photocurrent. After an optimization step, films with a photocurrent of 0.5 and 1.07mA/cm-2 at 1200mV vs RHE and under 1.5AM were produced by SP and USP respectively. Ti[IV] doping improved the photoresponse of the films deposited by SP. An inverse effect was observed for the films deposited by USP: the introduction of a dopant lowered the photocurrent and increased the dark current. The properties of layers made by USP were compared to those deposited by SP. Although both types of films show similar XRD, UV-visible and Raman spectra they differ greatly in their morphology and in their photoactivity. The mesoscopic α-Fe2O3 layers produced by USP consist mainly of 100 nm- sized platelets with a thickness of 5-10 nanometers. These nanosheets are oriented perpendicularly to the FTO support, their flat surface exposing (001) facets. The films made by SP consist of spherical hematite particles of 50-100nm in diameter densely packed. Open circuit photovoltage measurements indicated a lower surface state density for the films made by USP as compared to the films made by SP. This factor explained the much higher photoactivity of the USP compared to the SP deposited α-Fe2O3 films. Addition of hydrogen peroxide to the alkaline electrolyte further improves the photocurrent-voltage characteristics of films generated by USP and SP indicating hole transfer from the valence band of the semiconductor oxide to the adsorbed water to be the rate limiting kinetic step in the oxygen generation reaction. Electrochemical impedance spectroscopy (EIS) experiments have been performed on the most photoactive USP films. The construction of a frequency independent Mott-Schottky plot showed that the hematite flat band potential lies at 0.26V vs RHE in 1M NaOH and that the donor concentration is 5.6 1017cm-3. EIS allowed the identification of a deep donor level 0.3V above the flat band potential. This level explains the difference between the onset potential and the flat band potential. The electrochemical and photoelectrochemical behaviors of the most photoactive hematite thin films deposited by USP were found to be really close to those of single crystal hematite indicating that the influence on the photoactivity of the grain boundaries can be neglected.

EPFL2005

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

Philip Nising

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.

EPFL2006