Delayed curvatures in porcelain tiles : analysis and measurement of incluencing factors

Abstract

Göller Verlag GmbH Glazed and unglazed porcelain tiles exhibit a phenomenon known as 'delayed curvatures', which consist of the change in tile curvature after the tiles leave the kiln. This phenomenon becomes more problematic as tile size increases. In this study, the variation of curvature in time has been quantified in industrial glazed porcelain tiles. It was observed that the curvature usually displays an evolution in one direction (customarily in a concave direction or vessel shape) which, after a certain time has elapsed, reverses this trend. The kinetics of this process has been parameterised, assuming there are two simultaneous, opposing mechanisms, with different kinetics. Theoretical analyses indicate that only two factors can produce delayed curvatures: residual stresses and expansion of the tile body. In both cases, additional circumstances need to occur for delayed curvatures to appear; thus, for example, the presence of residual stresses is not synonymous with delayed curvatures, but an additional mechanism is needed that allows progressive release of these stresses, a mechanism known as creep. In addition, the condition that the stress profile is not symmetrical with respect to the centre plane of the tile also needs to be obeyed. In regard to the expansion of the bodies, special conditions must also occur for these to cause delayed curvatures. In particular, it is necessary for the expansion at the tile fair face and at the rib face to be different. Uniform expansion would cause a slight dimensional change, but not a delayed curvature, not even in the presence of glaze. The measurement of the factors that influence delayed curvatures is complex because different techniques from those typically used in characterising ceramic tiles are required. Procedures have been fine-tuned to measure the different factors that give rise to delayed curvatures, and the influence of certain variables on these factors has been studied