Capillary method, casting

A common method to slip-cast ceramic membranes is to start with a colloidal suspension or polymeric solution as described in the previous section. This is called a slip . The porous support system is dipped in the slip and the dispersion medium (in most cases water or alcohol-water mixtures) is forced into the pores of the support by a pressure drop (APJ created by capillary action of the microporous support. At the interface the solid particles are retained and concentrated at the entrance of pores to form a gel layer as in the case of sol-gel processes. It is important that formation of the gel layer starts... 

The procedure for micromolding in capillaries (MIMIC) for fabricating the microstructure of polymeric materials has been reported [155-157], Vacuum-assisted MIMIC was later reported. These methods are essentially casting a soft curable polymer against a mold [158],... 

A major difference between the two methods is the process by which the liquid is removed from the slurry. In slip casting, the liquid is removed by capillary suction provided by a porous mold. In contrast, the liquid is removed by evaporation in tape casting. In most tape casting processes, the liquid is nonaqueous because the higher volatility allows faster evaporation. 

To get around these problems a method called pressure sUp casting (or, more simply, pressure casting) has been developed. Instead of plaster, the moulds are made of polymeric materials while porous, these have larger pores that do not generate the same capillary forces but require an externally applied pressure to drive the filtration process. 

With capillary casting, the same mould can be turned several times per day, so reducing capital investment with respect to bench and battery casting. The moulds are still made from inexpensive materials, and the skill level required is again lower than with bench casting. Excellent recovery can be achieved with this method. 

With capillary casting, the demands placed on ball clay and kaolin producers are the same as with traditional bench casting. Additionally, faster casting and firmer casts are also required. The methods used to address these needs can be defined as similar to those of battery casting. Custom slurry products seem to offer the potential to improve performance in this area. 

Slip casting is a low-pressure filtration method where capillary suction provides the driving force (of the order of 0.1-0.2 MPa) for liquid removal and formation of a cast layer at the mould surface. The casting rate is controlled by the resistance to flow by the cast layer and the mould. Usually, the mould resistance is negligible and the increase in the cast layer thickness, Z, with time, t, can be written as follows ... 

Routine methods of chemical analysis can be applied to water-based coatings. For example, the nature of the binder can be determined by infrared spectroscopy. Water-resistant plates, such as KRS5 or zinc selenide, must be used if capillary films are cast from the aqueous dispersion. Dried films can be analyzed by surface-sensitive techniques such as attenuated total reflection (ATR) or photoacoustic spectroscopy (PAS). Both these techniques require the use of Fomrier transform infrared (FT IR) instruments to obtain spectra in a reasonable time. 

Slip casting method It is a fabrication method. In this process low-viscosity slurry of ceramic powder is poured into a porous plaster mould. The porous nature of the plaster mould draws water fix)m the slurry that eontacts the mould. This process continues as the capillary section of the mould continues to draw water through the increased ceramic solids at the mould wall. After establishing a sufficiently thick wall at the mould wall, the remaining liquid is drained, dried and when the cast is sufficiently dried, the mould is removed. 

---