Kaolin/poly(acrylic acid) composites as precursors for porous kaolin ceramics

Kaolin–polymer hydrogel composites were synthesized via redox-initiated polymerization of acrylic acid (AA) in a concentrated aqueous dispersion of kaolin, using N,N′-methylene bisacrylamide as a crosslinker. The mechanical and thermal properties of the composites were investigated by systematic variation of the monomer feed composition. From rheological measurements, it was found that kaolin had a negative effect on the polymerization rate, which decreased the stiffness of the green bodies prepared at room temperature with increasing kaolin content. The gelation time was varied by changing the concentration of the redox initiator. On drying at 105 °C, the crosslinking continued to a higher degree. Larger kaolin contents yielded to stiffer composites, as determined from uniaxial compression measurements. The compressive strength increases with the AA concentration, up to AA concentrations > 25 mass% (relative to kaolin). The equilibrium swelling ratio of the fully-cured kaolin/poly(acrylic acid) composites in water increased as expected with increasing AA content. These particular composite systems formed porous kaolin ceramics with porosity > 60% after burning off the organic materials.