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Le recyclage des élastomères est devenu important pour des raisons environnementales et financières. Une forme de recyclage consiste à dévulcaniser l’élastomère afin de le réintroduire comme matière première. Il s'agit d'un processus délicat car l'objectif est de rompre le réseau polymère en brisant les ponts soufre sans endommager les chaînes polymères. Ce travail vise à étudier un procédé de dévulcanisation appelé "High Shear Mixing" (HSM). Une machine a été conçue, construite et instrumentée. Dans le procédé HSM, l’élastomère est cisaillé entre deux cônes ayant des géométries spécifiques. Pendant ce cisaillement, la matière est auto-chauffée, un système de refroidissement limite l’augmentation de température. Les objectifs de l’étude étaient de mieux comprendre le procédé et les processus physiques, de déterminer un paramètre procédé pour suivre l’évolution de la matière au cours du traitement et d’optimiser le procédé. Notre analyse a mis en évidence 2 paramètres: la température de l’élastomère et l'énergie consommée au cours du procédé. Cette énergie peut être corrélée au degré de dévulcanisation qui est mesuré par des analyses physico-chimiques. L’activation des surfaces d’élastomère créées pendant le traitement a été mis en évidence. Un modèle physique de l'évolution du réseau polymère est proposé. Pour valider l'efficacité du recyclage, l'effet de l'ajout de différentes fractions de la matière traitée dans la même formulation de matière première a été testé. Une méthode Taguchi a été utilisée pour concevoir les formulations et les facteurs HSM à tester. Les mélanges reformulés ont été vulcanisés et leurs propriétés ont été analysées. / The recycling of rubber in the industry has become highly important due to environmental and financial reasons. A recycling approach is to devulcanize the rubber in order to reintroduce it as a raw material. This is a challenging process since the objective is to rupture the rubber network by breaking sulfur bridges without damaging the polymer chains. This work aims to study a devulcanization process known as “High Shear Mixing” (HSM). A machine was designed, built and instrumented with the purpose of studying the different phenomena occurring during the devulcanization process. During the treatment, the rubber is sheared between two cones with special geometries. During this shearing the treated rubber is self-heated, a cooling system prevents the rubber degradation due to excessive heat. The objectives were to better understand the process and the physics behind, to determine a process parameter to follow the rubber evolution and to optimize the treatment. Our analysis highlighted 2 parameters: the rubber temperature and the specific mechanical energy consumed during the process. This energy can be correlated to the degree of devulcanization of the rubber which is measured by means of physicochemical analyses. An optimal state of surface activation on the treated rubber was also described. A physical model of the rubber network evolution along the HSM treatment is proposed. To validate the recycling efficiency, the effect of adding different fractions of treated rubber in the same raw material formulation was tested. A Taguchi method was used to design formulations and HSM factors to be tested. Reformulated mixtures were vulcanized and rubber properties were analyzed.

The preparation and initiation of movement has previously been described using a neural accumulation model; this model involves an increase of neural activation in the motor cortex (M1) from baseline to a subthreshold level following a warning signal, which is maintained until presentation of an imperative stimulus (IS). Activity then increases until reaching movement initiation threshold. This model predicts that variability in activation during preparation may influence reaction time (RT) and its variability. The purpose of this thesis project was to determine whether differences in RT/variability of RT during the completion of tasks with varying levels of complexity may be attributable to differences in neural excitability in M1. To test this prediction, transcranial magnetic stimulation (TMS) delivered concurrently with an IS was used to determine neural excitability for movements with different accuracy demands. It was hypothesized that higher accuracy demands would result in lowered amplitude and/or greater variability of neural activation, and consequently slower/more variable RT. Fifteen healthy participants completed a simple RT task involving a targeted wrist extension movement under three different accuracy conditions (easy, moderate, difficult). TMS was delivered concurrently with the IS on 50% of trials during each condition. While pilot testing showed RT differences between accuracy conditions (Appendix A), the data presented here failed to detect significant differences in RT latency (F(2, 28) = .074, p = .929) or variability (F(1.432, 20.053) = .633, p = .538) between conditions . Similarly, no difference in MEP amplitude was observed between difficulty conditions (F(2, 28) = 2.439, p = .106). However, a subset of participants (n = 7) did show significant RT increases between easy and hard conditions (t(6) = 2.531, p = .045), but this subset still failed to show differences in MEP amplitude (t(6) = 1.157, p = .291) or variability (t(6) = 1.545, p = .173), suggesting that preparatory levels at the IS may be similar for movements involving both high and low accuracy demands.

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This research work focused on studying the emission spectroscopy produced from the high energy ion-molecule collision processes in mass spectrometry. The collision experiments are described and divided into 4 chapters (Chapter 3, 4, 5, 6).N2O+● is an ion of atmospheric importance. In chapter 3 the investigation of the collision between high translational energy (4-8 keV range) N2O+● ions and Helium target gas in mass spectrometry using collision induced emission (CIE) spectroscopy is described.In chapter 4, the collision-induced emission (CIE) spectra from 4-8 keV collisions between projectile He+● ions and CO2 target gas (He+●/CO2) were obtained. In Chapter 5, to probe the validity of this hypothesis, CIE experiments were carried out to observe the photon emissions from keV collisions of a selection of projectile ions with O2 target gas. By studying the resulting CIE spectra, a second potential mechanism came to light, one that involves the nearly isoenergetic O2+. A → X state transition. In chapter 6, neutral hydroxymethylene and formaldehyde were generated by charge exchange neutralization of their respective ionic counterparts and then were reionized and detected as recovery signals in neutralization-reionization mass spectrometry in the modified VG-ZAB mass spectrometer.

A dissertation submitted to the Faculty of Science University of the Witwatersrand, Johannesburg, for the Degree of Master of Science. / The method of activation analysis is extended to the analysis of very large samples (up to 1kg) of ores and minerals. The use of such large samples has two advantages: first, the sensitivity of the method is greatly increased, and secondly, inhomogeneity of distribution and hence sampling problems are greatly reduced. Samples are used without any kind of treatment and the method is non - destructive. There are also disadvantages associated with the use of large samples. Absorption effects (both of neutron and y - rays) become important and as a result the calibration curves are influenced by the bulk density and composition of the samples. In certain cases, the calibration curves may also be non - linear. These effects have been studied in detail for the determination of gold, through the reaction 198Au(n,y)l98Au. After irradiating various samples of known concentrations of gold, calibration curves for gold in gold ore and carbon samples have been established. The effect of y -ray and neutron absorption has been studied in carbon samples using either thermal or epithermal neutrons and the optimum sample thickness has been established. In addition, the curve for the optimum decay time has been obtained for gold in ore samples. Based on the experimental results, a theoretical model has been applied to study the variation of the statistical error with the various factors involv~d, i.e. irradiation, decay, counting times, source strength and sample size. It has been also used to analyse theoretically the results for the optimum thickness and the variation of the background under the 411. 8 keV peak of Au-198. Further work has been undertaken to investigate whether the method can be used to determine elements from the platinum group which are much less active than the gold. Gamma ray peaks from Ir - 192, Au - 199, Pt - 199 Rh - 104m and Os-193 could be identified and possibly from Pd-109. Further work is necessary to study each element separately. In the case of gold, very satisfactory results were obtained. It is too early to state that it provides a replacement for the fire - essay method for gold, but the potential definitely exists. / Andrew Chakane 2021

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