Pickering emulsions are widely applied as a tool for manufacturing porous ceramic components in the combination with three-dimensional modelling technologies. However, general processes suffer from slow heating for dewaxing and firing to prevent structural collapse due to thermal decomposition of highly-loaded binders. To overcome this problem, we have recently proposed a new series of water in oil (w/o) Pickering emulsion suspensions, which can be photo-cured with small amount of monomers through interparticle photo-cross-linking reactions. Briefly, SiO₂/toluene suspension stabilized by functional polyethyleneimine (PEI-OA), which PEI was partially complexed with oleic acid, are vigorously mixed with small amount of multifunctional acrylate (MA), photo-radical initiator and water. These suspensions were photo-curable by exothermic photo-radical polymerization of MA followed by Michael addition reaction between the polymerized MA and the free amine groups of PEI-OA on particles. The photocured material were found to have the pore structures originated from the aqueous phase, and rapidly heat-treatable for dewaxing and neck growth without any structural collapse. In this system, the OA content of PEI-OA adsorbed on particles is expected to change the contact angle with water in toluene and the number of active amine groups present on the particle surface, which serves as the reaction point during light irradiation. Herein, we aim to systematically clarify the effects of OA content (15-50 mol% to total EI units) on the microstructures, light-curing behaviors, and moldability of photocurable Pickering emulsions.
From the observation of water droplets in toluene solvent on the pressed compacts of PEI-OA modified SiO₂ particles, an increase in the contact angle with water in toluene was shown as the OA content increased, confirming the change in the particle surface properties. All Pickering emulsion suspensions with different OA contents were photo-curable by UV light irradiation, however, the changes in storage modulus after UV light irradiation was relatively small for the conditions with OA contents of 30 and 50 mol%. Further, while the photocured bodies having 15 mol% OA on PEI maintained the external morphologies upon drying, photocured bodies from other conditions possessed a severe structural collapse. We suspect that at higher OA contents, the number of active amine groups have reduced which induced the suppression in the strength of photocured bodies as well as the inter-particle crosslinking reaction. The control of OA content in PEI was found to be one of the major keys toward successful shaping of porous material.