Smooth Membrane Formation on Resorcinol-Formaldehyde Aerogel Balls Gelated Using a Basic Phase-Transfer Catalyst

Abstract

Low-density foam balls with a diameter of ~1 mm were produced from a density-matched emulsion consisting of a resorcinol-formaldehyde (RF) aqueous solution (W) and an exterior oil of carbontetrachloride/(mineral oil) (O). Phase-transfer catalysts such as an alkyl amine were dissolved in the exterior oil, following which the catalyst moved into the RF solution from the exterior oil. A gelation process was monitored by a complete gelation test. When the basic catalysts were used at room temperature as a phase-transfer catalyst, gelation occurred within 30 to 120 min, whereas when the acidic catalyst was used, gelation occurred within 20 to 30 min at room temperature. When ~0.39 wt% of triethylamine and tri(n-butyl)amine in the oil phase were used, complete gelation took place. A basic catalyst with a long alkyl chain such as dimethyl(n-hexyl)amine did not induce gelation. The gelated balls obtained using the basic catalyst with a short alkyl chain were dried by extraction using supercritical fluid CO2 and the solvent was replaced with 2-propanol to produce the foam structure. Except 0.39 wt% tri(n-butyl)amine, the basic catalysts yielded foam balls with higher densities of 173 to 184 mg/cm3 as compared to those obtained from a benzoic acid catalyst, namely, 158 mg/cm3. The density difference can be attributed to the inclusion of the basic catalyst in the RF solution. Scanning electron microscopy images revealed a surface membrane formation, which can be explained by local concentration at the W/O interface. The cell size of the bulk foam was observed to depend on the catalysts, and it was surmised that the cell sizes varied because of the different gelation rates. A smooth surface membrane tri(n-butyl)amine was used as a catalyst. The membrane obtained on using a basic phase-transfer catalyst was smoother than that obtained on using an acid catalyst. Such a smooth membrane is useful for coating the ablation layer of foam capsule targets.