Ceramics-Silikáty 50, (4) 232 - 238 (2006)

Carrageenan, a polysaccharide biopolymer of the agaroid group, can be used as a gelling agent in ceramic shaping techniques. Its function is based on the ability to form elastic hydrogels from viscous aqueous solutions upon cooling. In this paper the performance of carrageenan systems is studied by oscillatory shear rheometry. It is shown that aqueous solutions with 2.2, 3.1 and 3.9 wt.% carrageenan undergo reversible gelation within a temperature range of approx. 5°C at gelation temperatures of 46, 51 and 56°C, respectively. The resulting storage moduli of the gels are approx. 5, 15 and 20 kPa, respectively, thus exhibiting a strong depedence on the carrageenan concentration. The rheology of zirconia suspensions, containing 67 wt.% zirconia (Tosoh TZ-3YE) and effective carrageenan concentrations of 0.7, 1.0 and 1.3 wt.% in the aqueous phase, is briefly summarized as well. In contrast to pure carrageenan solutions, which are purely viscous at elevated temperatures, the mixed suspensions (containing zirconia and carrageenan) are viscoelastic at 80°C and exhibit lower gelation temperatures on cooling (29, 33 and 37°C, respectively). The resulting storage moduli after cooling to room temperature are in the range 2-25 kPa, i.e. of the same order as for pure carrageenan hydrogels. This green rigidity guarantees easy demolding and further handling of the as-shaped ceramic green bodies.