Aqueous solutions of alkaline silicates with a general composition formula:
M may represent the alkali ions Li+,K+ or Na+.
Within the group of silicate binder solutions, it is particularly the sodium and partly also the potassiumwater glass solutions (s. Water glass) that have become important. The adhesion and binding effect of these solutions has been used in foundry engineering for decades.
The almost unlimited raw material resources and their favorable environmental behavior are two factors that have increased the effort for overcoming any existing disadvantaged. The various types of polymer silicates that have been verified, their crosslinking structures, and the polymerization mechanism greatly differ from the properties of organic binder systems. This explains the relatively diminished strength characteristics of water glass-bonded molding materials.
With the chemical hardening processes (s. Hardening) used in practice, the overall bonding properties are only utilized by 10 to 20 %. Only with physical, and partly also with chemical dehydration is it possible to achieve specific strength values that come close to those of organic binder systems.
For mold material binders, water glass solutions are preferably used; the addition of sodium hydroxide solution achieves a molar ratio SiO2 : Na2O of 2.5 to 2.8 (s. a. Water glassmodulus) and this ratio may be increased even more by evaporation of water in the solid matter concentration.
Subsequent decanting or filtration serves the purpose of separation of impurities (alkaline-earth, iron, and aluminum composites).
Deviations in the mixture charge, as well as with regard to melting and dissolution conditions may have a significant effect on the water glass quality. In addition to viscosity, this also affects formation of different structures of silicate ions. In the classification process, water glass solutions can be produced by direct transformation of silicic acid by means of sodium hydroxide solution. The starting substances are diatomaceous earth or finely ground quartz sand. In general, this requires higher temperatures and extended reaction periods.
For mold material binders alkaline solutions with a high SiO2 concentration and preferably low alkali content are generally desirable. The type and proportion of the alkaline substances used determine the SiO2 maximum concentration at which the solution is still stable.
With regard to the hardening principle, processes such as split-off or chemical bonding of water as well as the occurring gel-type structure characterize the mold material properties. As a general principle, the hardening method is based on sol-gel-transformation through dehydration of water from the binder solution.