An experimental investigation of the stability of ethylcellulose latex: correlation between zeta potential and sedimentation.
ABSTRACT This paper aims at explaining the experimental observations of the stability and redispersibility of an aqueous ethylcellulose latex through the electrokinetic characterization of the particles. The surface charge and the electrical double layer thickness play an essential role in the stability of the system, hence the need for a full characterization of the polymeric particles. The effect of both pH and ionic strength of the dispersion medium were investigated. It was found that at acid pH values the latex displays "delayed" or "hindered" sedimentation: in such conditions, the electrophoretic mobility and zeta potential are rather low, indicating a small electrokinetic charge on the particles. At alkaline pH, when the dissociation of ionizable surface groups must be complete, the zeta potential is high and negative. The electrostatic repulsion between polymer particles is responsible for the low sedimentation volume and poor redispersibility of the latex. The effect of NaCl and CaCl(2) concentration on both the zeta potential and stability of the latexes was also investigated: it was found that CaCl(2) has the greatest influence, yielding flocculated, easily re-dispersible systems when its concentration in the dispersion medium is high enough. There qualitative observations were ascertained by means of calculations of the potential energy of interaction between particles. In the case of NaCl solutions, a high and relatively wide potential energy barrier was predicted, that may prevent the particle aggregation. Above 5mM NaCl a shallow minimum in the potential energy curves must lead to the formation of aggregates. Similar results were found with CaCl(2) solutions, although in this case the secondary minima are deeper and appear at lower concentrations.
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ABSTRACT: To understand the impact of surface charge density on the redispersibility of spray dried powders, the redispersibility of styrene-butadiene (SB) powders prepared from latexes with various particle sizes and acid contents was investigated. The redispersibility of these powders showed a step function from fully redispersible to non-redispersible. The phase boundary between these two regimes is determined by the product of the latex particle size and acid content, which is strongly related to the surface charge density of the latex particle. Results indicate that to achieve fully redispersible polymer powder, a proper balance of latex particle size and surface charge is needed. For the class of SB latexes studied in this work, with a certain acid level, it is possible to predict the minimum latex particle size that enables redispersibility of its spray dried powder. This phenomenological observation is unique and it may help us to reveal the fundamentals which govern the redispersibility. To maintain the redispersibility of a spray dried powder, a colloidal stabilizer that adsorbs onto the surfaces of latex particles is required. When latexes are brought to close proximity, strong van der Waals forces may deplete the surface stabilizers, causing irreversible latex agglomeration at spray drying temperatures. A qualitative DLVO analysis was employed to study the impact of various colloidal interactions in this system, including van der Waals, electrostatic, and steric interactions. Zeta potential measurements of the mixture of SB latex and polyvinyl alcohol (PVOH, used as a colloidal stabilizer) suggest that the surface charge density plays a significant role on redispersibility, and strong electrostatic repulsive interactions are needed to overcome the attractive van der Waals interaction. Size exclusion chromatography of serum phase analysis indicated that PVOH adsorption onto latex particles did not change significantly at different acid levels, suggesting low impact of steric repulsion on redispersibility. Conclusions from this work provide additional means to control the performance of redispersible materials.Colloids and Surfaces A Physicochemical and Engineering Aspects 10/2014; 459:274–281. DOI:10.1016/j.colsurfa.2014.07.015 · 2.35 Impact Factor
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ABSTRACT: This study describes the low temperature synthesis of alpha-alumina from aluminum hydroxide hydrothermally synthesized using [Al(C2O4)(x)(OH)(y)] complexes, in which Al3+ chelated by bidentate oxalate at various molar ratios. Aluminum hydroxide hydrothermally synthesized at 200 degrees C without oxalic acid exhibited a monoclinic boehmite (AlOOH) structure with hexagonal shape. However, adding oxalic acid caused the morphology to change to a spear-shaped boehmite/gibbsite mixed structure. Thermal treatment of the bohemite typed aluminum hydroxide-0 hydrothermally synthesized without oxalic acid produced a rhombohedral alpha-phased alumina with a rice-shaped at above 1200 degrees C. However alpha-alumina was obtained at lower temperature by the bohemite/gibbsite mixture typed aluminum hydroxide-1, -2, and -3 hydrothermally synthesized with oxalic acid. In particular, the aluminum hydroxide-3 having more gibbsite portion hydrothermally synthesized using 3.0 mol of oxalic acid was easily changed to the egg-shaped rhombohedral alpha-alumina at temperatures lower than 900 degrees C. On increasing the oxalic acid concentration, the density of alpha-alumina increased to 1.359 g cm(-3).Chemical Engineering Journal 08/2013; 230:351-360. DOI:10.1016/j.cej.2013.06.099 · 4.06 Impact Factor
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ABSTRACT: The cationic poly(vinyl acetate-butyl acrylate-DMC) copolymer emulsions were successfully prepared using the emulsion polymerization technique with 2-methylacryloylxyethyl trimethyl ammonium chloride (DMC) as a functional cationic monomer, cetyltrimethylammonium bromide (CTAB) as the surfactant, and K2S2O8 as the initiator. The morphology, structure and composition, and thermal properties of the emulsions were characterized by TEM, FTIR and DSC, respectively. Also, the correlations between the particle size, zeta potential, viscosity, water absorption and antibacterial efficiency of prepared cationic emulsions and DMC cationic monomer content were established. The results show that the content of the DMC monomer could affect the properties of the emulsions and their films significantly. This offers an attractive opportunity for adjusting the particle size and zeta potential of cationic emulsions over a wide range by feeding DMC monomer in varying concentrations. Furthermore, the prepared emulsions present a high antibacterial efficiency of 98.2% even containing only 2.0 wt% DMC in its monomers, which results in a great potential application as a kind of antibacterial coating material in the future.Journal of Macromolecular Science Part A 01/2013; 50(2). DOI:10.1080/10601325.2013.742381 · 0.74 Impact Factor