Slip Optimization by Use of Additives Explained with an Example of a Conventional Sanitaryware Body

摘要

In the sanitaryware industry, the quality of ceramics is influenced not only by typical glaze defects, such as for example crawling and running of the glaze, but mainly by topographic irregularities in the body. During the shaping process, there are only a few possibilities to increase the output. One of these potentials to increase the efficiency for instance is the reduction of loss and scrap material, which depends considerably on deformations, dimples, convexities/bellies and cracks. The origin of these defects will be examined in general as follows. The first part of the paper serves to determine their occurrence resulting from the variety of their designations used in the individual factories followed by their description in general. Subsequently, a theoretical model is presented, which tries to explain the different defects by means of a conventional slip casting example. It assumes that all these defects occur in the body during firing due to local shrinkage differences, when the material compensates for intrinsic structure tensions during its viscoplastic condition. On the other hand, the partially varying linear shrinkage during firing results from deviating structures in the body and the anisotropic properties of the clay particles. The well-known electrostatic interaction forces and the differently charged surfaces and edges of the clay minerals play a particular role during the formation of structures. The model describes how the structural differences will appear at the area of the above mentioned defects and creates a correlation with the body formation/casting, which fixes/"solidifies" the arrangement of the particles. It will prove conclusively that the structural differences are limited locally and that they are the main reason for the topographic surface defects described above. If these structures finally lead to a defect, depends considerably on the slip's tendency to build these structures, and at which speed and intensity. Indirectly, its viscoelastic character reflects this property. Modern digital rheometers with air bearings allow us to characterize excellently these processes in the slip at rest by oscillating measurements. Examples will show how such rheological measurements can reveal the slip properties, which favour the formation of such defects. The final part of the paper treats the development of new additives serving to control the formation of structures in the slip and to prevent defects in a targeted manner. Finally, sanitaryware factories are invited to participate actively in this development work.