(5c) The Influence of Particle Size Distribution on the Performance of Ceramic Particulate Suspensions

When preparing ceramic glaze suspensions it is necessary to guarantee that the suspension has got the required properties, several parameters having to be rigorously controlled: particle size and solids concentration, composition of the suspension, ionic charge of the liquid, etc. In this study we will focus our attention on the influence of particle size distribution on three important properties of glaze suspensions: rheological behaviour, opacity and reflectance characteristics and tendency to dissolution. Glaze suspensions are complex mixtures of different particulate materials, essentially the frit and kaolin. The frits can have different compositions, but are always a solid mixture of oxides of several metals, depending on the final properties required for the glaze (hardness, optical properties, etc.). Thus, as mentioned before, in such a complex mixture, the interactions between the particles are also very intricate and are strongly affected by the size of the particles. In this paper we will present results for two glaze formulations prepared with two different commercial frits of distinct composition. The glazes were prepared in a laboratory ball mill and, for each formulation, the grinding time was adjusted in order to obtain suspensions with different particle sizes (at least three grinding times). Particle size distribution was evaluated by LDS. The rheological characterization was performed in a Brookfield rotating viscometer. Opacity of the final glaze was measured with a colorimeter. And, finally, the dissolution of the glaze was evaluated by measuring, with atomic absorption spectrometry, the cationic content of the suspending medium. Our effort was, nevertheless, directed mainly to the control of the rheology of the glaze suspension, since the preparation of ceramic glaze suspensions requires that adequate flow behaviour is reached. On the other hand, the rheological behaviour is strongly dependent on particle size and also on changes in the interactions between the particles, caused by modifications in the cationic content of the suspending medium, as a result of frit dissolution. Additionally, the rate of cations release is also influenced by the size of the particles. In this paper we will correlate the rheology of the glaze with particle size but, in addition, we will also show how particle size influences the ageing of the glaze, that is deterioration of its rheological behaviour with time. The influence of particle size on glaze opacity has also been checked, since texture is an essential quality parameter in glaze production and opacity has to remain within certain limits. Furthermore, it is obvious that an adequate control of particle size in glaze production can contribute to energy savings, since grinding is one of the most inefficient operations in industry, as far as energy is concerned. Thus, being able to control grinding time and avoid over grinding, without affecting product quality, will be most important for the ceramic industry.