Alternative gelling agents for the gelcasting of ceramic foams

In recent years, a new class of ceramic foams with porosity levels up to 90% was developed as a result of the association of the gelcasting process to the aeration of a suspension containing foaming agents. The gelation of foamed suspensions results from the in situ polymerization of water-soluble monomers. Nearly spherical pores and highly dense struts characterize the structure of these foams, which results in unprecedented properties, such as high permeability, low thermal conductivity, high specific surface area and high mechanical strength. The main drawbacks of this process are the inhibition of the polymerization in the presence of oxygen and the toxicity of the monomers. This work investigates two harmless alternative gelling agents that do not require atmosphere control to set foamed suspensions. The first route consists in the crosslinking of a previously dissolved polyvinyl-alcohol with the addition of an organotitanate. The other approach assessed the use of gelatin as a setting agent. This polymer gels the suspension due to changes in the structure of polypeptidic chains induced by temperature reduction. Gelatin-based systems presented higher storage modulus (G′) than the systems with crosslinked polyvinyl-alcohol chains. This characteristic prevented the formation of cracks during drying of gelatin-based samples, which constituted a limitation of PVAl-based systems. The results point out gelatin as a promising gelling agent to produce ceramic foams without the disadvantages of monomeric systems, such as the toxicity and the necessity of atmosphere control.