Delayed Curvature in Porcelain Tiles – Part 1
A significant percentage of porcelain tiles exhibit a phenomenon called “Delayed Curvature”. That includes a change in tile curvature after leaving the kiln. This phenomenon becomes problematic with increasing the size of the tile. Factors affecting delayed curvature can be categorized as direct and indirect factors. Direct factors include expansion of the body and release of residual stress.
The expansion of the porcelain tile body begins as the tile exits the industrial kiln. Expansion is very fast at first and reaches about 0.18% after 96 hours; however, the result depends on the composition used and the highest kiln temperature. If there is an overlap between the expansion of the top and the bottom of the tile, for example, a 0.1% difference in 410*410 mm tile expansion can cause a curvature of about 0.3 mm .
Even if the overall expansion of both surfaces is the same, differences in the expansion kinetics can cause changes in the tile curvature . Another direct factor affecting tile curvature are residual stresses in the tile that can be due to two reasons
1- Stresses caused by the rapid cooling of the tile in the industrial kiln, which causes thermal gradients in the tile .
2 – the stresses produced by the glazed body. Because the body is thicker than glaze and it has a relatively high elasticity .
There must be a mechanism to release the tile from these stresses, which is known as creep.
Among the factors that indirectly influence the delayed curvature are :
1 – Materials (Sprayer Powder, Particle Size, Mineralogy, glaze and chemical compounds)
2 – Process (kiln cycle, the highest kiln temperature, residence time in kiln) that this leads to changes in the ceramic tile (Microstructure, existence of different phases, modulus of elasticity, temperature expansion, environmental conditions, relative humidity and temperature).
Expansion of ceramic bodies due to moisture is a well-known feature. Moisture expansion of ceramic bodies is due to the physical and chemical absorption of water molecules at free capacities in the hydrated phases present in the samples removed from the kiln. For this reason, the expansion depends largely on the porous structure of the segment (which determines its access to water more or less) and the nature and amount of phases in the part removed from the kiln.
These properties are greatly influenced by the mineralogical composition , Particle size and firing schedule. Therefore, as the melting temperature of the body composition and the firing temperature or the residence time increases, the moisture expansion of these bodies due to porosity and the amount of hydrate phases reduced .
When the maximum kiln temperature is reached, the porcelain tile is composed of a large amount of liquid phase, quartz and residual albite, and sometimes mullite .
In this case, the piece is able to release any tension applied to it, as it is highly deformable. In the cooling phase, the residual stresses in the tiles, either due to a mismatch between the body layers and the glaze, or due to different shrinkage resulting from the higher cooling velocity in outer regions relative to the center of the piece. Therefore, the factors that determine the residual stresses of porcelain tiles are in principle the thermal expansion and modulus of elasticity of the body and the glaze relative thickness and the cooling rate of the tile. It has been observed that mismatches than thermal expansion of both layers increase the residual stress. In addition, as the cooling time get faster, the temperature gradient inside the tile, increase resulting in different contraction rates at the tile cross-section, which creates a stress profile within the tile that increases with cooling velocity increases .
In addition, when the cooling rate on both sides of the tile is not the same (common situation in the furnace rollers), the resulting stress profile is not symmetric.