María J. Barandiaran

VOC formation during monomer removal by post-polymerization

The formation of volatile organic compounds through secondary reactions during the post-polymerization of vinyl acetate containing latexes using organic hydroperoxides with ascorbic acid as redox initiator was investigated. It was found that such initiator systems are highly effective in reducing free vinyl acetate but organic by-products are produced. A reaction scheme consistent with the kinetic data is presented.

Kinetics of the emulsion copolymerization of styrene and butyl acrylate

The kinetics of the chemically initiated seeded emulsion copolymerization of styrene and butyl acrylate was investigated in experiments in which the diameter of the seed, the number of seed particles, and the concentration of initiator were widely varied. The experimental data were fitted with a model in which the rate coefficients for radical entry and exit were the adjustable parameters.

Comparison between conventional emulsion and miniemulsion polymerization of vinyl acetate in a continuous stirred tank reactor

The conventional emulsion and miniemulsion polymerization of vinyl acetate in a continuous stirred tank reactor (CSTR) were studied in an attempt to demonstrate the feasibility of miniemulsion polymerization as a strategy to eliminate the oscillatory behaviour of CSTR reactors, even in very unfavourable conditions. Reactions were carried out under a wide range of operating conditions. No oscillations in the time evolution of the conversion and particle number were observed in any of the miniemulsion polymerizations, while the opposite was found in most of the conventional emulsion polymerizations. Different dependencies of the emulsifier and initiator concentrations on the reaction rate and particle number were obtained for both emulsion and miniemulsion polymerizations, as a consequence of the different nucleation mechanisms. Further, almost the same average molecular weight was obtained for the emulsions and miniemulsions carried out under similar experimental conditions.

Use of a Routh-Russel Deformation Map to Achieve Film Formation of a Latex with a High Glass Transition Temperature

In the film formation of latex, particle deformation can occur by processes of wet sintering, dry sintering, or capillary action. When latex films dry nonuniformly and when particles deform and coalesce while the film is still wet, a detrimental skin layer will develop at the film surface. In their process model, Routh and Russel proposed that the operative particle deformation mechanism can be determined by the values of control parameters on a deformation map. Here, the film formation processes of three methyl methacrylate/butyl acrylate copolymer latexes with high glass transition temperatures (T(g)), ranging from 45 to 64 °C, have been studied when heated by infrared radiation. Adjusting the infrared (IR) power density enables the film temperature, polymer viscosity, and evaporation rate during latex film formation to be controlled precisely. Different polymer particle deformation mechanisms have been demonstrated for the same latex under a variety of film formation process conditions. When the temperature is too high, a skin layer develops. On the other hand, when the temperature is too low, particles deform by dry sintering, and the process requires extended time periods. The deduced mechanisms can be interpreted and explained by the Routh-Russel deformation maps. Film formation of hard (high T(g)) coatings is achieved without using coalescing aids that emit volatile organic compounds (VOCs), which is a significant technical achievement.

High solids content miniemulsion polymerization of vinyl acetate in a continuous stirred tank reactor

Abstract The dynamics of high solids content miniemulsion polymerization of vinyl acetate produced in a continuous stirred tank reactor (CSTR), under different emulsification processes and hydrophobes were studied. In order to gain more insight into the process, a previous kinetic study in a batch reactor was performed. In addition, the rheological behavior of latexes produced by miniemulsion was compared with those obtained by conventional emulsion. From the experiments made in batch mode, it was observed that poly(vinyl acetate) or polystyrene as the sole hydrophobes were able to create miniemulsions as far as the time elapsed between sonication and polymerization was small. The same behavior was observed in the CSTR, where the feasibility of those systems to eliminate the oscillatory performance of CSTR reactors, also at high solids content was demonstrated. The sonication itself or the sole presence of hydrophobe — without sonication — was not enough to avoid the oscillatory behavior. Further, it was shown that the viscosity of miniemulsions at the beginning of the CSTR process is notably lower than emulsions, thus preventing the initial mixing and heating problems.

Influence of the polymeric hydrophobe on the kinetics of vinyl acetate miniemulsion polymerization

Abstract The influence of polymeric hydrophobes on the stabilization and kinetics of the vinyl acetate miniemulsion polymerization was investigated by conducting reactions in batch mode of miniemulsions prepared with poly(vinyl acetate) or polystyrene as cosurfactants. The results were compared with a miniemulsion stabilized with hexadecane and with the conventional emulsion. The shelf life stability analysis showed that miniemulsions prepared with polymer as the unique cosurfactant were not stable. However, the appearance of the separated phases was not similar to the conventional emulsion, which was interpreted as the formation of small monomer swollen particles in the case of the miniemulsion. The kinetic analysis confirmed the different nucleation mechanism of these unstable miniemulsions compared with the conventional emulsion.

Effect of oxygen on emulsion polymerisation kinetics : a study by reaction calorimetry

Abstract The influence of oxygen on the kinetics of chemically initiated seeded emulsion homopolymerisation of styrene and the seeded emulsion copolymerisation of styrene/butyl acrylate was investigated by reaction calorimetry. A decrease of the inhibition period and an increase of the polymerisation rate have been observed when the system is purged with nitrogen. It has also been seen that oxygen has a great influence on the polymerisation kinetics. producing not only inhibition, but also decreasing the reaction rate. An increase of the dependence of the average number of radicals per particle with the initiator concentration with respect to theoretical predictions was observed in the presence of oxygen. The overall experimental results show that, after an inhibition period, there is a continuous flow of oxygen from the gas phase, which causes the retardation of the polymerisation rate.

Emulsion polymerization: particle growth kinetics

An attempt to develop a predictive and manageable mathematical model for particle growth in emulsion homopolymerization was carried out by fitting the time evolution of the conversion in the chemically initiated seeded emulsion polymerization of styrene carried out under a wide range of experimental conditions with models of different complexity. Model discrimination based on the best fitting of the experimental data was carried out. No advantage was gained by increasing the complexity of the mathematical model. The dependence of the radical entry and exit rate parameters on the particle size was used to elucidate between the different mechanisms proposed for these processes.

Evolving Stresses in Latex Films as a Function of Temperature

Latex films were dried on a flexible substrate, and the substrate deflection was monitored over time to give an averaged film stress-evolution profile. Films were dried at various temperatures below and above the minimum film-formation temperature of the latex dispersion. The effect of polymer rheology, which is a temperature-dependent parameter, on film formation, was investigated. The reliability of the Stoney model in predicting film stress from substrate curvature was also examined and compared to the Euler-Bernoulli model. It was shown that the linearized Stoney model was unsuitable for the larger measured stresses.

Determination of the Coalescence Temperature of Latexes by Environmental Scanning Electron Microscopy

A new methodology for quantitative characterization of the coalescence process of waterborne polymer dispersion (latex) particles by environmental scanning electron microscopy (ESEM) is proposed. The experimental setup has been developed to provide reproducible latex monolayer depositions, optimized contrast of the latex particles, and a reliable readout of the sample temperature. Quantification of the coalescence process under dry conditions has been performed by image processing based on evaluation of the image autocorrelation function. As a proof of concept the coalescence of two latexes with known and differing glass transition temperatures has been measured. It has been shown that a reproducibility of better than 1.5 °C can be obtained for the measurement of the coalescence temperature.