Papers by Author: A. Johnson

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Authors: S. Bandyopadhyay-Ghosh, Ian M. Reaney, A. Johnson, I.M. Brook, K. Hurrell-Gillingham, P.V. Hatton
Abstract: Novel fluorcanasite based glass-ceramics were produced by controlled two stage heattreatment of as-cast glasses. Castability of parent glasses was determined using a graduated spiral cast piece. Fluorcanasite glasses were also cast to form complex shapes using the lost wax casting technique. Gypsum and phosphate bonded investments were used to investigate their effect on the casting process, cast surface crystallinity and biocompatibility. The stoichiometric composition had the greatest castability but the other two modified compositions also had good castability. X-ray diffraction showed similar bulk crystallisation for each glass irrespective of the investment material. However, some differences in surface crystallisation in the presence of different investment materials were detected. Discs cast using gypsum bonded investment showed greater in vitro biocompatibility than equivalent discs cast using phosphate bonded investment under the conditions used. Gypsum and phosphate bonded investments could both be successfully used for the lost wax casting of these novel fluorcanasite glasses.
Authors: T. Nejatian, A. Johnson, R. Van Noort
Abstract: The fracture resistance of polymethyl methacrylate (PMMA) the most popular denture base materials, is not satisfactory. The aim of this study is to improve the mechanical properties of PMMA by adding filler particles. Different inorganic filler particles including micronised glass flakes, acryl silane micronised, glass flakes 350nm, TiO2 and ZrO2 with different ratios by weight were incorporated into heat-cure plain PMMA and processed with optimal condition [2.5:1 Powder/ monomer ratio, conventional packing method and water bath curing for 2 hours at 95°C] to produce 12mm diameter and 2mm thick discs. Plain PMMA without additives was prepared as a test control. Three types of mechanical tests, biaxial flexure, microindentation fracture toughness and Vickers hardness were carried out on the samples. It was found that incorporating the particles did not improve the biaxial flexural strength of the resin. However, TiO2, ZrO2 and micronised glass flakes increased the fracture toughness of the resin. The hardness of PMMA was improved by the incorporation of all the particles.
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