Papers by Author: Leena Hupa

Abstract: The increasing use of biomass and waste derived fuels in combustion challenges the chemical durability of refractories. Durability of an alumina refractory was studied in a chemically aggressive environment. A mixture of potassium chloride and carbonate (molar ratio 1:9) was placed on the sample and heated at 700-1000°C in an electric laboratory furnace in air for one week. Cross-sections of the samples were studied by SEM-EDXA to determine penetration of potassium in the refractory. Potassium was found only in the silicate matrix phase of the alumina refractory. Penetration of potassium decreased steeply from the surface to 1 mm, after which the decrease was linear but varied with temperature. At 700 and 800°C the thickness of the matrix layer that had reacted with potassium was 3 mm, while the layer was thinner at 900 and 1000°C. At the higher temperatures a glassy layer consisting of K2O, Na2O, CaO and SiO2 formed on the refractory surface. At 900°C the thickness of the surface layer was of 10μm, while a 200μm layer was measured at 1000°C. The procedure used in this work can be used to develop a laboratory scale method to be used to study corrosion of refractories in biomass combustion devices.

Abstract: Glass and glazes are easy-to-clean surfaces often used in everyday environments where the surface needs to repel soils and deposits. In general, these surfaces have good chemical durability in everyday environments. However, the durability is rapidly degraded in solutions of high or low pH. This kind of surface corrosion has been found to diminish the cleanability. Surface topography has also a certain influence on the soil attachment and cleanability. Self-cleaning and easy-to-clean coatings have been employed to enhance the cleanability of surfaces. In this report surface properties of three coatings reported to enhance the cleanability of glass and glazed surfaces are summarized. The properties discussed are the surface appearance, roughness, wettability, soil attachment and soil removal. Also the chemical and mechanical durability of the coatings are discussed. The coatings studied were a commercial fluoropolymer film applied at room temperature, an experimental sol-gel derived TiO2 coating calcined at 500°C, and an experimental liquid flame sprayed TiO2-Ag coating applied on the substrates at 500-800°C. The advantages of the fluoropolymer coating are easy application and soil good soil repellence, but the coating has limited chemical and mechanical durability. The manufacture of the sol-gel TiO2 coating requires several processing steps. The coated surface showed excellent cleanability, and good chemical and mechanical durability. The liquid flame sprayed coating has potential to be applied online in the material manufacture. However, the processing parameters should be optimized in order to achieve desired improvements in the cleanability.

Abstract: In this work we summarize the most important findings of the influence of glass composition, sample shape and fluid circulation on in vitro behavior of bioactive glasses in the system Na2O-K2O-MgO-CaO-B2O3-P2O5-SiO2. The sample shapes included plates, particulates, powdered glasses, glass fibers and sintered cones with interconnected porosity. The in vitro bioactivity was measured as the changes observed in the immersion solution, SBF, and the formation of reaction layers on the samples at 4 to 168 h immersion. A lower surface area to volume ratio gave smaller changes on the ion concentrations and pH of the immersion solution but thicker reaction layers on the glass surfaces. In particulate systems with circulating fluid the pH gradients in SBF were lower but surface layers more even than in static systems. The influence of glass composition on reaction layer formation as suggested by glass plates correlated with the in vitro behavior of glass particulates larger than 250 µm, porous cones and fibers when using similar SA/V ratio.

Abstract: A method for measuring the early stage ion release of glasses was developed in order to gain information on leaching kinetics and chemical resistance of glasses in aqueous environments. A continuous flow-through-reactor was designed in which the aqueous solution is fed through a bed of glass particles and the dissolved ions continuously recorded with a sensitive on-line analysis system. Experimental parameters, such as solvent flow rate and temperature, could easily be adjusted according to the needs. The flow-through-reactor system was tested with powdered samples of float and lead glasses, E-glass and bioactive glasses 45S5 and 1-98, all of which showed very different chemical durability in aqueous environments. The reactor was connected to inductively coupled plasma optical emission spectrometry (ICP-OES) and concentrations of the dissolved ions were measured simultaneously on-line every 30-40 seconds. In this work the initial stages of ion release were measured during the first 15 minutes of the leaching experiments at 40°C and 80°C. The results were compared with standard water durability test of the glasses. The dissolution of the glasses according to both methods showed similar behaviour.

Abstract: Melts in the composition range of bioactive glasses have their liquidus temperatures within or close to the working range. Additionally, bioactive glasses show a strong tendency to crystallization due to their composition. Thus, many bioactive glass compositions are unsuitable for demanding working procedures such as fiber drawing. In this work we discuss the suitability of different methods for measuring liquidus temperature of bioactive glasses. Thermal analysis was used, and complemented by sintering curves as given by hot stage microscopy. Hot stage microscopy could be used to measure liquidus also when the thermal effect associated with liquidus was low and consequently the DTA was not suitable. Liquidus of some glasses was measured with optical microcopy for samples heat treated in a gradient furnace. The values indicating the crystallization of the samples during viscosity measurement with rotational viscosimeter were compared with the other measurements. The crystalline phases formed at liquidus were identified with XRD and SEM-EDXA. The measured liquidus temperatures were also tested by drawing fibers directly from molten glass and from preforms heated to typical fiber drawing viscosity values. For all compositions the fiber drawing viscosity values were below liquidus. Continuous fibers could be manufactured of compositions only with low rates of crystal nucleation and growth.

Abstract: In vitro reactions of bundles of fibers with diameters 20-500 μm and crushed glasses of fractions 500-800 μm were compared with the reactions of plates of the same bioactive glasses. The samples were immersed in simulated body fluid (SBF) for 2-7 days. After immersion the changes on the surfaces of the samples were observed by SEM/EDXA. Layer formation on the glass surface was found to vary with glass composition, sample shape and local condition of single particle/fiber. However, only some fibers or particles formed similar in vitro reaction layers as the plates. The product form did not change the in vitro bioactivity of particles or fibers exposed to the bulk immersion solution. When the glasses were used as fiber bundles or particle beds, the packing degree and the flow of body fluids within the system interfered with the reactivity. Also a clear correlation between in vivo layer formation in bone and in vitro of the glass plates could be found.

Abstract: Factors controlling the antibacterial ability of three bioactive glasses were studied by comparing the changes in the SBF during immersion of the glasses with their response to four microorganisms. After immersion of 100 mg/ml fine powder (<45μm) of the glasses in the SBF for 1, 2, 4, 8, 16, 27 and 48 hours, the immersion solutions were filtered and the pH in the bulk solution was measured. Ionic concentrations of Na, K, Ca, Mg, P and Si ions in the immersion solutions were determined by ICP-OES. The antibacterial activity of the glasses showed good correlation with the changes of the pH values in the SBF solutions. No correlation was found between the ionic concentration and the antibacterial ability of the three glasses against the four tested microorganisms. The results suggest that, the antimicrobial effect of the glass powder against the microorganisms tested is mostly dependent on the increase of the pH in the solution to values detrimental for the bacteria growth.

Abstract: Implants with long lasting bioactivity and mechanical sustainability would be of interest in several novel clinical applications. By processing bioactive glass fibers and biodegradable polymers into 3D structures, bone formation ability of glasses and flexibility of polymers can be combined. In order to achieve desired physiological response, reactivity of bioactive glass fibers must be specified. Bundles of fibers within the range of bioactivity were soaked in the simulated body fluid at stationary conditions for several time intervals after which the cross-sectional surfaces of the fibers were studied with SEM-EDXA. The reaction layers and precipitations formed on the fiber surfaces suggest that the fibers react according to three mechanisms depending on the glass composition. Fibers with a high in vitro bioactivity showed the formation of distinct and thick silica –rich and calcium phosphate –rich layers already at one day’s immersion. Fibers of medium bioactivity did not show any clear silica –rich layer but a formation of calcium phosphate precipitations or layers at one day’s immersion. Slow glasses showed sporadic calcium phosphate precipitation only after the longest immersion times. The results indicate that the medium and slow glasses are interesting alternatives for applications where a long term mechanical durability suggested by their slow reactivity in combination with their osteoconductive tendency is desired.

Abstract: The influence of wollastonite and dolomite on melting behaviour and crystallization was studied for 25 raw glazes. The melting behaviour was observed by hot-stage microscopy. The glazes were also fired industrially both in a fast-fired gas kiln for floor tiles (50 min) and in a traditional gas kiln for sanitaryware (25 hours). The surface composition and structure of the glazes were studied by SEM/EDXA. The crystalline phases in the glazes were identified by XRD. Main crystalline phases found in the glazes were wollastonite and diopside. Fusibility and surface structure on the raw glazes containing wollastonite and dolomite as raw materials depended both on the total content and the ratio of CaO and MgO in the glaze. Low content of alkaline earths gave incomplete fusion, while high content gave extensive crystallization. The results can be applied to tailor raw glaze compositions for a desired surface structure to be fired at a certain fusion temperature.

Abstract: The goal of this work was to establish the compatibility of mat glazes with functional films known to render the surfaces with self-cleaning or easy-to-clean properties. Glazes with wollastonite, pseudowollastonite, diopside and zircon as the main crystalline phases in the surfaces were coated with fluoropolymer as well as ceramic, sol-gel derived titania and zirconia films. The glazes were soaked in typical detergent solutions used in everyday life up to four days. The surface roughness was measured with confocal optical microscope and the surface was imaged and analyzed with SEM/EDXA. When applied on wollastonite and pseudowollastonite containing glazes the functional films readily reacted in water solutions by pitting of the surface in the vicinity of the crystals. The ceramic titania and zirconia films showed better chemical resistance on wollastonite –free glazes, while the fluoropolymer film corroded in the most alkaline environments. The results indicate that functional films could be used also on rough surfaces without markedly affecting the surface topography. However, the films should be applied only on glazes with an excellent chemical resistance.