Properties of raw glazes - The impact of composition, firing and functional coatings
Kronberg, Thomas (2020-09-18)
18.09.2020
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Publikationens permanenta adress är
https://urn.fi/URN:ISBN:978-952-12-3979-3
https://urn.fi/URN:ISBN:978-952-12-3979-3
Abstrakt
Raw glazes, i.e. glazes mixed from oxides and minerals, are cost-effective alternatives to fritted compositions for ceramics fired at a high peak temperature (> 1150 °C). Raw glazes are mainly used for sanitary ware and porcelain, but also e.g. for dense floor tiles. Typically, many of the raw materials come from local mining plants and thus are cost-efficient alternatives.
New demands like replacing raw materials to more environmentally friendly ones, using lower top firing temperatures, or going to shorter firing cycles all require an adjustments of the glaze composition. For achieving a high-quality surface, a thorough understanding of the influence of different raw materials on the sintering, the melting as well as on the crystallization behaviour of the glaze is essential. The phase composition of the fired raw glaze depends on the kinetics of the various raw material reactions during the heating and soaking periods at the top firing temperature as well as on the phase changes taking place during the cooling period.
Although ceramics are considered to provide easy-to-clean surfaces, considerable effort and money are spent on cleaning the soiled surfaces. Exposure to chemical agents or mechanical wear can alter the surface structure and thereby diminish the cleanability during the life cycle. Functional surfaces have been developed to increase the anti-soiling and cleanability properties of traditionally glazed surfaces.
These new requirements and changes have led to the need for a better understanding of the melting behaviour and surface modification to achieve surfaces with desired appearance and performance.
In this work, the melting behaviour was estimated from sintering curves and characteristic sample points measured with hot-stage microscopy (HSM). Altogether seventy-five compositions were mixed from seven different raw materials (Series I). All the compositions were also applied on raw tiles and fired in an industrial kiln for fast-firing (50 min) to verify the melting behaviour and characteristics of the final glaze (gloss, surface hardness and phase composition). The goal was to explore whether the typical sample shapes and sintering curves measured with HSM can be used to describe the melting behaviour of raw glazes. The temperatures describing the sintering and melting of the glazes were mathematically modelled as functions of the raw material composition. For each point, a model of the compositional dependence was established. A correlation between the gloss of the fast-fired glazes and the melting characteristics at higher temperatures of the HSM measurement could be found. Thus, the melting behaviour obtained by HSM can also be used to predict the melting behaviour in the industrial fast-firing kiln when optimizing glossy glazes.
The effect of soaking time on phase composition and topography was estimated from tiles fired in a laboratory kiln with different soaking times. The same tiles were also fired in an industrial fast-fired kiln. Fifteen experimental glaze compositions were used in this study (Series II). The goal was to establish the changes in phase composition and topography of raw glazes within the compositional field for tiles and sanitary ware when firing cycle varied from fast to traditional. The results showed that the phase composition and topography changed with the firing cycle. In short firing, the surface morphology depended on initial raw material reactions and melt formation. At prolonged firing, the surface morphology correlated with the total oxide composition and the equilibrium phase condition at high temperatures. The results also indicated that the soaking time of raw glazes can be shortened if the total glaze composition is adjusted to provide desired surface properties.
The results from the study of the melting behaviour showed that dolomite and wollastonite in the recipe enhanced the melting behaviour of the glaze at around 1200 °C. However, when the amount of the alkaline earths CaO and MgO increased, a matte surface appearance was obtained. The effect of adding these alkaline earths through dolomite and wollastonite on the melting behaviour and surface properties was explored. Twenty-five experimental glaze compositions (Series III) were used in this study. Both the total content and the ratio between wollastonite and dolomite were found tocontrolthe melting behaviour. The fusibility was increased with wollastonite, and thus, the CaO content in the glaze. Also, MgO introduced as dolomite increased the fusibility of the glazes. In fast-firing, the most pronounced effect was found when the MgO content was around 2 wt%. The main crystalline phases in the Series III glazes were wollastonite and diopside. The chemical durability of the glazes containing wollastonite crystals at the surface was poor in acidic solutions. The wollastonite crystals were rapidly attacked and reacted through incongruent dissolution of calcium and leaving a silica-rich remnant at the surface. The diopside crystals showed excellent durability in all acidic to alkaline the solutions. However, the amorphous phase in the glazes with high contents of alkaline earths was heavily attacked in the acidic solutions. Although the overall acid durability of the wollastonite-containing glazes was low, the excellent properties of the diopside crystals are encouraging for further studies of manufacture of diopside-based matted raw glazes, especially in a modified composition range with higher silica contents to ensure better durability.
Increasing demands for improved surface properties, such as easier cleanability, have led to applying also functional coatings on traditional glaze surfaces. However, the functionality, as well as the mechanical and chemical durability of these coatings are not yet fully understood or controlled. In this study, traditional white sanitary ware glazes were coated with commercial fluoropolymers and experimental hybrid sol-gel functional coatings. The cleanability and the resistance to mechanical and chemical wear of the coated surfaces were measured. The results showed that the additional functional coatings improved the cleanability of glazes. However, the increased cleanability by the additional functional films was lost for the surfaces, which were partially or totally destroyed by mechanical or chemical wear.
The results obtained in this study give valuable additional information and new tools to design glaze compositions with desired surface characteristics for different firing schedules. The models can be utilized when designing new glaze compositions for changed requirements or when comparing the melting behaviour of different raw glazes. Also, the calculated trends can directly be adapted to practical glaze firing processes. The results can also be applied to adjust the glaze composition for given firing cycles in order to improve the chemical durability and to achieve the desired microstructure through controlled surface composition. The results can also be used to estimate the criteria for tailoring additional functional coatings and glaze surfaces for a long-term effect on cleanability in everyday environments.
New demands like replacing raw materials to more environmentally friendly ones, using lower top firing temperatures, or going to shorter firing cycles all require an adjustments of the glaze composition. For achieving a high-quality surface, a thorough understanding of the influence of different raw materials on the sintering, the melting as well as on the crystallization behaviour of the glaze is essential. The phase composition of the fired raw glaze depends on the kinetics of the various raw material reactions during the heating and soaking periods at the top firing temperature as well as on the phase changes taking place during the cooling period.
Although ceramics are considered to provide easy-to-clean surfaces, considerable effort and money are spent on cleaning the soiled surfaces. Exposure to chemical agents or mechanical wear can alter the surface structure and thereby diminish the cleanability during the life cycle. Functional surfaces have been developed to increase the anti-soiling and cleanability properties of traditionally glazed surfaces.
These new requirements and changes have led to the need for a better understanding of the melting behaviour and surface modification to achieve surfaces with desired appearance and performance.
In this work, the melting behaviour was estimated from sintering curves and characteristic sample points measured with hot-stage microscopy (HSM). Altogether seventy-five compositions were mixed from seven different raw materials (Series I). All the compositions were also applied on raw tiles and fired in an industrial kiln for fast-firing (50 min) to verify the melting behaviour and characteristics of the final glaze (gloss, surface hardness and phase composition). The goal was to explore whether the typical sample shapes and sintering curves measured with HSM can be used to describe the melting behaviour of raw glazes. The temperatures describing the sintering and melting of the glazes were mathematically modelled as functions of the raw material composition. For each point, a model of the compositional dependence was established. A correlation between the gloss of the fast-fired glazes and the melting characteristics at higher temperatures of the HSM measurement could be found. Thus, the melting behaviour obtained by HSM can also be used to predict the melting behaviour in the industrial fast-firing kiln when optimizing glossy glazes.
The effect of soaking time on phase composition and topography was estimated from tiles fired in a laboratory kiln with different soaking times. The same tiles were also fired in an industrial fast-fired kiln. Fifteen experimental glaze compositions were used in this study (Series II). The goal was to establish the changes in phase composition and topography of raw glazes within the compositional field for tiles and sanitary ware when firing cycle varied from fast to traditional. The results showed that the phase composition and topography changed with the firing cycle. In short firing, the surface morphology depended on initial raw material reactions and melt formation. At prolonged firing, the surface morphology correlated with the total oxide composition and the equilibrium phase condition at high temperatures. The results also indicated that the soaking time of raw glazes can be shortened if the total glaze composition is adjusted to provide desired surface properties.
The results from the study of the melting behaviour showed that dolomite and wollastonite in the recipe enhanced the melting behaviour of the glaze at around 1200 °C. However, when the amount of the alkaline earths CaO and MgO increased, a matte surface appearance was obtained. The effect of adding these alkaline earths through dolomite and wollastonite on the melting behaviour and surface properties was explored. Twenty-five experimental glaze compositions (Series III) were used in this study. Both the total content and the ratio between wollastonite and dolomite were found tocontrolthe melting behaviour. The fusibility was increased with wollastonite, and thus, the CaO content in the glaze. Also, MgO introduced as dolomite increased the fusibility of the glazes. In fast-firing, the most pronounced effect was found when the MgO content was around 2 wt%. The main crystalline phases in the Series III glazes were wollastonite and diopside. The chemical durability of the glazes containing wollastonite crystals at the surface was poor in acidic solutions. The wollastonite crystals were rapidly attacked and reacted through incongruent dissolution of calcium and leaving a silica-rich remnant at the surface. The diopside crystals showed excellent durability in all acidic to alkaline the solutions. However, the amorphous phase in the glazes with high contents of alkaline earths was heavily attacked in the acidic solutions. Although the overall acid durability of the wollastonite-containing glazes was low, the excellent properties of the diopside crystals are encouraging for further studies of manufacture of diopside-based matted raw glazes, especially in a modified composition range with higher silica contents to ensure better durability.
Increasing demands for improved surface properties, such as easier cleanability, have led to applying also functional coatings on traditional glaze surfaces. However, the functionality, as well as the mechanical and chemical durability of these coatings are not yet fully understood or controlled. In this study, traditional white sanitary ware glazes were coated with commercial fluoropolymers and experimental hybrid sol-gel functional coatings. The cleanability and the resistance to mechanical and chemical wear of the coated surfaces were measured. The results showed that the additional functional coatings improved the cleanability of glazes. However, the increased cleanability by the additional functional films was lost for the surfaces, which were partially or totally destroyed by mechanical or chemical wear.
The results obtained in this study give valuable additional information and new tools to design glaze compositions with desired surface characteristics for different firing schedules. The models can be utilized when designing new glaze compositions for changed requirements or when comparing the melting behaviour of different raw glazes. Also, the calculated trends can directly be adapted to practical glaze firing processes. The results can also be applied to adjust the glaze composition for given firing cycles in order to improve the chemical durability and to achieve the desired microstructure through controlled surface composition. The results can also be used to estimate the criteria for tailoring additional functional coatings and glaze surfaces for a long-term effect on cleanability in everyday environments.