Functions of glycerol in the tape-casting process: Revealing with AFM colloid probe measurements

The purpose of the present study is to reveal functions of organic additives utilized in tape casting, a key technique for ceramic manufacturing. Interaction forces between alumina surfaces were measured using colloid probe atomic force microscopy (AFM) in dilute aqueous solutions containing both or one of polyvinyl alcohol (PVA) and glycerol. PVA and glycerol are typical additives for tape casting and are used as a binder and a plasticizer, respectively. In the PVA solution, repulsive forces due to steric hindrance were observed at separation distance about 10 nm. The glycerol solution exhibited a combination of attractive and repulsive forces, with a strong attraction at around 20 nm, attributed to the depletion effect caused by striking hydration ability of glycerol. The mixed solution of PVA and glycerol showed an increased repulsion distance of approximately 30 nm, suggesting that glycerol interacts with PVA molecules, altering their molecular conformation and expanding their gyration radius. The results indicate that addition of glycerol not only imparts mechanical flexibility to the green sheets but also stabilizes the alumina slurry. The empirically selected and optimized organic additives could fulfill multiple functions to improve the ceramic forming processes. As is presented in the study, the AFM colloid probe measurements would contribute to the understanding of how organic additives work and the development of more sophisticated ceramic processing.