Novel Coagulation Method for Direct Coagulation Casting of Aqueous Alumina Slurries Prepared Using a Poly(Acrylate) Dispersant

Abstract

Coagulation of concentrated aqueous alumina slurries prepared using an ammonium poly(acrylate) dispersant by MgO has been studied for direct coagulation casting (DCC). A small amount of MgO (0.2 wt% of alumina) increased the viscosity of the concentrated alumina slurry with time and finally transformed it into a stiff gel. The mechanism of coagulation is proposed such that the time‐delayed in situ generation of Mg2+ ions from the sparingly soluble MgO forms Mg–poly(acrylate) with the unadsorbed ammonium poly(acrylate) molecules in solution that shift the poly(acrylate) adsorption equilibrium toward the left by depleting the poly(acrylate) molecules adsorbed on the alumina particle surface. This leads to insufficient dispersant coverage on the particle surface and coagulation of the slurry. DCC using MgO is possible only if the slurry is prepared at a dispersant concentration higher than that required for optimum dispersion as the slurries prepared at the optimum dispersant concentration underwent premature coagulation. The gelation time could be tailored within 20 min to a few hours by maintaining the temperature in the range of 70°–30°C. The wet coagulated bodies prepared from 50 vol% alumina slurry showed a compressive strength of nearly 0.05 MPa.

Processing of Alumina and Corresponding Composites

“…This solid body retains the structure of the stable suspension. This transformation can be done by manipulation of the electrical characteristics of the particles (the electrical double layer) or by changing the pH or the additive concentration, as occurs in the process of direct coagulation (Prabhakaran, Raghunath, & Melkeri, 2008). Another possibility is the addition of chemical species that polymerize under controlled conditions.…”

Magnesia induced coagulation of aqueous PZT powder suspensions for direct coagulation casting

“…_ 6 ) T D $ F I G ] The mechanism of coagulation of alumina suspensions by MgO is reported in our previous publication [19]. In the case of alumina suspension, at higher ammonium poly(acrylate) concentration, viscosity remains more or less constant until majority of the un-adsorbed ammonium poly(yacrylate) present in the dispersion medium react with the Mg 2+ ions produced from MgO [19]. Thereafter, the viscosity of the suspension increased with time and finally forms a gel due to desorption of ammonium poly(acrylate) from particle surface.…”

“…_ 8 ) T D $ F I G ] [ ( F i g . _ 9 ) T D $ F I G ] PZT powder suspensions from that of alumina are attributed to strong adsorption of ammonium poly(acrylate) on PZT particle surface compared to weak and highly reversible adsorption on alumina surface [19]. It appears that a fraction of the carboxylate groups in the dispersant forms complex with lead atoms present in the particle surface and the remaining extend in to the medium to provide electrosteric stabilization.…”

“…Recently we have reported a novel coagulation method for direct coagulation casting of alumina suspensions prepared using ammonium poly(acrylate) dispersant using MgO as coagulating agent [19][20][21][22][23]. In this, Mg 2+ generated from the sparingly soluble MgO reacts with the un-adsorbed ammonium poly(acrylate) in the dispersion medium and form precipitate of Mg-poly(acrylate).…”

Biomimetic PVPA in vitro model for estimation of the intestinal drug permeability using fasted and fed state simulated intestinal fluids

“…This apparent discrepancy in reduced permeability in FeB and FeSSIF, which is unexpected for a class III drug, may possibly be explained by the pH differences between the fasted and fed state media. Maleic acid, which is a component in both FaSSIF and FeSSIF, has pKa values of 1.9 for the first deprotonation and 6.1 for the second deprotonation (Prabhakaran et al, 2008). The pKa value of 6.1 gives a variation of the -1 and -2 charge states of maleic acid between fed and fasted states, which might have high impact on salt formation between maleic acid and the basic nadolol (Roche, 2007).…”