The role of surface microstructure on the resistance to stains of porcelain stoneware tiles

Porcelain stoneware tiles frequently undergo a polishing process, aimed at improving their aesthetical appearence, that brings about a consistent material removal, with formation of superficial defects and opening of closed pores. The consequent degradation of surface characteristics—and especially the increased sensitivity to stains—represent the main limit to the use of polished tiles in many indoor and outdoor applications. In order to better understand the role of microstructure on the resistance to stains, a phenomenological study of staining/cleaning operations (ISO 10545 parts 14 and 16) and a thoroughful physico-microstructural characterisation of tile working surfaces (SEM, open and closed porosity, rugosimetry, MIP) were carried out on twelve industrially manufactured and polished products. Diverse staining behaviours proved to be connected with different tile microstructures, being the surface roughness as well as the amount and shape of coarser pores the most influent variables. Through a statistical approach, an empirical predictional model of the amount of stain retained by the tile surface after mild washing with warm water was set up. It is based on roughness measurements (both Ra and Rt), estimation of macropores (i.e. 1–50 μm by MIP) and pore roundness (by image analysis of SEM photomicrographs).