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<p>The aim of this project was to compare the shear bond strengths of three light cure orthodontic adhesive resins with three different stainless steel molar brackets.</p>

Submitted in fulfillment of the Degree of Master in Technology: Dental Technology in the Department of Dental Services Faculty of Health Sciences, Durban University of Technology, 2008. / Globally in the field of Dental Technology, polymethyl methacrylate (PMMA) resin continues to be the popular material for the fabrication of denture bases in removable prosthodontics. However, the mechanical strength of the denture base is a concern due to fractures occurring intra-orally or when accidentally dropped. The objective of this in vitro investigation was therefore to evaluate and compare the flexural strength and microstructure of stick® glass fibre and wire mesh reinforced PMMA resin after thermocycling. The selection of the materials used in this study was based primarily on their popularity and availability in South Africa. These materials were selected to ensure that the results of this study would have further implicational value in the commercial dental industry when published. This investigation was conducted by means of fabricating a total of 90 PMMA resin specimens and divided in three groups consisting of 30 specimens each. Sample groups 1 and 2 were reinforced with stick® glass fibres and wire mesh respectively. The un-reinforced sample group was the control. All 90 specimens were thermocycled in water at temperatures between 5˚C and 55˚C for 2100 cycles. The flexural strength of each specimen was tested using a universal testing machine and the microstructure of the fractured surfaces was then analysed using scanning electron microscopes (SEM). SPSS version 15.0 was used for data analysis. A p-value of <0.05 was considered as statistically significant. Data were analysed using parametric and non-parametric statistical methods. Statistically significant differences in flexural strength existed between the three sample groups (p<0.001) with the stick® glass fibre and wire mesh sample groups being significantly superior to the control. Furthermore there was a significant association between fracture modes and sample groups. Microscopic analysis revealed the presence of voids. Statistically, in terms of microstructure (% of voids present), a significant difference existed between all sample groups. With regards to surface texture of the compression and tension sides of the test specimens, significant differences existed between the three sample groups. Furthermore microscopic analysis revealed partial impregnation and distribution of the fibres to the PMMA resin matrix and un-bonding between the wire mesh and PMMA resin matrix. Statistically, the Mann-Whitney test was conducted to compare flexural strength between sample groups with and without voids. The flexural strength was higher in sample groups with voids than those without. This is an important finding from the clinical perspective because, in some structures of dentures, toughness is a desired property. Nevertheless in order to find the long-term data especially on clinical behaviour of these new fibre reinforcement systems, more studies should be conducted.

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Epoxy resins are widely used as coatings, encapsulations, structural composites, castings, and adhesives in a number of electrical applications. Recently, novel uncatalyzed co-anhydride cured epoxy formulations that exhibit a high performance property profile, have been introduced. The objective of this thesis was to perform a comprehensive material characterization of these new resin/hardener combinations, which are potentially used as electrical insulation material in medium and high voltage engineering. The thermal cure behaviour of commercial tetraglycidyl-diamino-diphenylmethane (TGDDM) and a co-anhydride mixture consisting of maleic anhydride (MA) and hexahydrophthalic anhydride (HHPA) was extensively studied. Different analytical real-time methods, such as FT-Raman spectroscopy, differential scanning calorimetry, and chemo-rheological methods were applied to investigate the principal polymerization mechanism and the related curing kinetics. It was demonstrated that kinetic parameters that were based on isothermal measurements provided consistent and reliable results. On the other hand the general limitations of different dynamic methods for kinetic parameter calculations were outlined and discussed. Temperature modulated differential scanning calorimetry provided a powerful technique to confirm a TGDDM/MA and TGDDM/HHPA sub-network structure of the co-anhydride cured epoxy. A generalized time-temperature-transformation-diagram was developed in order to predict the complex material transformations (e.g. gelation and vitrification) occurring during the entire isothermal curing process. In the selected system, the mechanical deformation and fracture behaviour as a function of temperature, strain rate, inorganic filler fraction, particle size, and filler/matrix-adhesion were thoroughly studied by using compression-, tension- and double torsion fracture-tests. The potential of hyperbranched polymers (HBPs) as low viscosity toughening modifiers for highly crosslinked anhydride-cured epoxy networks was experimentally evaluated. The effects of the HBP molecular structure, in particular the specific shell chemistry, on thermo-mechanical properties, final morphology, and blend concentration, were assessed. For the neat investigated epoxy-system the most efficient toughening modifier was obtained for a molecular HBP-design that provided a pseudo-homogeneous blend morphology. Thus, by using suitable HBPs in a concentration of 20% w/w, the fracture toughness, expressed by the critical stress intensity factor (Klc) of 0.58 MPam(to the power of)0.5, was increased by over 50% to 0.88 MPam(to the power of)0.5. The corresponding Young's modulus and glass transition temperature were only affected to a limited extent by the addition of the HBP-additive. The toughest epoxy blend (critical stress intensity factor of about 1.6 MPam(to the power of)0.5) was achieved by the incorporation of 60% w/w inorganic silica particles. The application of hybrid concepts by utilizing synergistic toughening mechanisms (HBP and silica), revealed only moderate benefits within the investigated highly crosslinked materials. As the examined epoxy/anhydride formulations are generally considered for high temperature applications, it was essential to determine their long-term thermooxidative ageing performance. The long-term thermo-oxidative ageing behaviour has been investigated by means of thermo-gravimetric analysis (TGA), dynamic mechanical analysis (DMA) and vibrational infrared spectroscopy (FT-IR) methods with special emphasis on fundamental understanding of the ageing mechanism. Effects of the thermal ageing on the characteristic viscoelastic and flexural behaviour, weight-loss and oxidation susceptibility of the examined epoxy networks were assessed and discussed, thus providing an understanding of the principal material endurance properties. The thermo-mechanical behaviour and the related structural changes with thermal ageing were examined by Cole-Cole plots in combination with a molecular theory previously developed by Perez. It was demonstrated that this new methodology provides a connection between conversion, glass transition temperature, and mechanical relaxation data and allows a fundamental molecular interpretation with respect to physical and chemical ageing phenomena. A variety of thermo-gravimetric experiments were carried out in order to determine and model the specific weight-loss profile as a function of anhydride nature and ageing temperature. The influence of different inorganic fillers on the thermooxidative response was systematically studied. Different models were applied to extract meaningful kinetic parameters in order to describe the thermo-oxidative degradation and to facilitate an extrapolation of weight-loss data outside the experimental time- and temperature-scale. FT-IR micro ATR-spectroscopy was used to identify, localize and quantify the complex oxidation behaviour. A thermo-oxidative degradation mechanism, that involved predominantly radical oxidation processes and C-N as well as C-O chain scissions, was proposed to account for the experimental observations. Specific oxidation-front profiles were constructed to describe the heterogeneous oxidation processes. Finally, the comprehensive material characterization of these novel co-anhydridecured amino-glycidyl resins in terms of curing mechanism and kinetic, deformation and fracture behaviour, and thermo-oxidative ageing performance, allows the assessment of the potential of these materials for demanding applications in electrical and electronic industries.

This thesis explores the behaviour of metal cyanide complexes under oxidative acid conditions in ion exchange systems, with the objective of developing an ion exchange based process for the treatment of gold cyanidation tailings. The novel cyanide detoxification process developed from this study employs strong base ion exchange resins to extract cyanide from tailings. Variations in the stability of cyanide complexes are exploited to concentrate, recover, or destroy cyanide species loaded on the resin, through the use of an oxidative acid eluent containing H2O2 and H2SO4. This eluent removes all base metal cyanide complexes from strong base resins, while regenerating the resin. The spent eluent, containing the base metals recovered from the tailings, can be used as a source of such base metals. Copper can be recovered separately from other base metals if necessary. Low levels of precious metals present in the tailings are accumulated on the resin as the ion exchange bed is cycled between loading and elution stages. They can be recovered economically, so as to offset the cost of the tailings detoxification. Cyanide is initially concentrated as an alkaline solution, which can be detoxified within the process or recovered for recycling. This process was successfully tested at pilot scale by treating approximately 14,000 m3 of cyanide contaminated tailings solution, over 14 loading/elution cycles on a standard strong base ion exchange resin bed. This treatment reduced the total cyanide concentration of the contaminated solution from approximately 50 mg/L to an average of 1.5 mg/L. The reagent cost was approximately ADD 0.50 per m3 of treated liquor. When the resin was repeatedly loaded with mixed metal cyanide species and eluted with the oxidative acid eluent, a gradual deterioration of the ion exchange resin performance was noted. The reduction of net operating capacity of the columns due to resin deterioration was in the order of 1-3% per loading/elution cycle. The oxidation of resin catalysed by copper, the precipitation of metal hexacyanoferrates on the resin and the oxidation of Au(CN)2- to Au(CN)4- were identified as possible factors giving rise to the reduction of resin loading capacity.