Latex stabilization

Anotlier model system consists of polymetliylmetliacrylate (PMMA) latex, stabilized in organic solvents by a comb polymer, consisting of a PMMA backbone witli poly-12-hydroxystearic acid (PHSA) chains attached to it [10]. The PHSA chains fonn a steric stabilization layer at tire surface (see section C2.6.4). Such particles can approach tire hard-sphere model very well [111. 

Foam cement is a special class of lightweight cement. The gas content of foamed cement can be up to 75% by volume. The stability of the foam is achieved by the addition of surfactants, as shown in Table 10-9. A typical foamed cement composition is made from a hydraulic cement, an aqueous rubber latex in an amount up to 45% by weight of the hydraulic cement, a latex stabilizer, a defoaming agent, a gas, a foaming agent, and a foam stabilizer [359,362]. Foamed high-temperature applications are based on calcium phosphate cement [257]. 

It was observed that both the poly (vinyl acetate)(PVAc) and poly(methyl methacrylate) (PMMA) latexes stabilized by PIB and... 

When pressure is applied to the latex, the UCFT is seen to move to higher temperatures as indicated in Figure 2. It was found that the PMMA latex stabilized by PIB of Mn - 2 x lCrcould be fitted to the same curve as the PVAc latex stabilized by PIB of Mjj 4 x 10. A linear regression analysis of the UCFT as a function of applied pressure gave ... 

For some applications, eg, foam rubber, high solids (>60%) latices are required. In the direct process, the polymerization conditions are adjusted to favor the production of relatively large average particle-size latices by lowering the initial emulsifier and electrolyte concentration and the water level in the recipe, and by controlling the initiation step to produce fewer particles. Emulsifier and electrolyte are added in increments as the polymerization progresses to control latex stability. A latex of - 35-40 wt% solids is obtained and concentrated by evaporation to 60—65 wt % solids. 

A urethane latex composition, (I), was prepared by Kent et al. (1) and used as an in situ delivery agent for 4-hydroxyl-methyl benzoate, which was useful as an image stabilizer. Polyurea, (II), and polyurethane latex compositions were also prepared by the authors (2) containing pendant acid groups as latex stabilizers and as crosslinking sites. 

Earlier work (3) has shown that cleaned monodisperse polystyrene latexes stabilized with surface sulfate (and perhaps a few hydroxyl) groups an be used as model colloids. For example, the distribution of H ions in the electric double layer as determined by conductometric titration has been correlated with the particle diameter determined by ultracentrifugation (3). The conductometric titration gives two measures of the concentration of H+ ions the initial conductance of the latex and the amount of base required for neutralization. The number of H+ ions determined by conductance is always smaller than the number determined by titration. This difference is attributed to the distribution of the H+ ions in the electric double layer those closest to the particle surface contribute least to the overall conductance. This distribution is expressed as the apparent degree of dissociation a, which is defined as the ratio H+ ions... 

Thus the monodisperse polystyrene latex stabilized with strong-acid surface groups can be hydrolyzed to form a latex stabilized with the same number of nonionic hydroxyl groups, which in turn can be oxidized to form a latex stabilized with the same number of weak-acid carboxyl groups, thus offering model colloids with identical characteristics except for the type of chemically bound surface groups ---- strong-acid, weak-acid, non-... 

The failure of latex stability,and the resultant flocculation of the latex par tides, may cause the formation of coagulum that is recovered from the latex after polymerization as well as a buildup on the reactor surfaces. Moreover, the inherent instability of the latex may also cause flocculation during storage or transportation. 

The use of seed improves the latex stability, but phase separations still occur at low monomer conversions. 

Uses Ethylamine is a colorless, inflammable gas. A variety of manufacturing industries have been associated with this compound (e.g., dyestuff industry, pharmaceuticals, rubber latex stabilizers, oil refining). 

The simple maleate Surfmer (i.e. the neutralized hemi ester of a fatty alcohol) was used to prepare seeds of polystyrene latex which were grown with a shell of film-forming polymers. The reported incorporation yield was of the order of 75% [18]. The reported latex stability could be further improved by Surfmers in which the ester moiety was substituted for an amide moiety by reaction with a fatty amine. An overall improved stability and a reduced hydrolysis at high temperature were observed [19]. 

The kinetics of vinyl acetate emulsion polymerization in the presence of alkyl phenyl ethoxylate surfactants of various chain lengths indicate that part of the emulsion polymerization occurs in the aqueous phase and part in the particles (115). A study of the emulsion polymerization of vinyl acetate in the presence of sodium lauryl sulfate reveals that a water-soluble poly (vinyl acetate)—sodium dodecyl sulfate polyelectrolyte complex forms, and that latex stability, polymer hydrolysis, and molecular weight are controlled by this phenomenon (116). 

Pelton RH (1988) Polystyrene and polystyrene-butadiene latexes stabilized by poly(A-isopropylacrylamide). J Polym Sci 26 9-18... 

On the other hand, several reports have been published that point out that when a polymeric surfactant acting as an electrosteric stabilizer is used, the rate of radical entry into a polymer particle should decrease due to a diffusion barrier of the hairy layer built up by the polymeric surfactant adsorbed on the surface of the polymer particles [34-36]. Coen et al. [34] found that in the seeded emulsion polymerization of St using a PSt seed latex stabilized elec-trosterically by a copolymer of acrylic acid (AA) and St, the electrosteric stabilizer greatly reduced the radical entry rate p compared to the same seed latex... 

The ionic strength of the aqueous phase is lower because no inorganic salts are required. This should increase latex stability over persulfate catalyzed polymerizations. More and smaller particles will be stabilized yielding faster rates of absorption and particle termination. 

Latex stability measurement. Latex stability was measured by a high speed stirring test (4.-9). 

Latex stability. Effect of particle size and emulsifier level. Latex stability data for three latices with different particle size, are plotted in Figure 2. At a given emulsifier level, expressed as weight per cent of polymer, the stability increases with increasing particle size. The logarithm of the stability is a linear function of the emulsifier concentration (2 ) ... 

Similar results to those obtained here by the stability measurements have been reported by Roe and Brass (7.8) They studied polystyrene latex stabilized by potassium palmitate. The analysis supplied by these authors shows that the order of magnitude of the slope of the stability curves can be accounted for as an entropic effect of crowding of adsorbed molecules during an encounter between two particles. They pointed this out as a possible explanation as the amount of emulsifier adsorbed strongly affects the stability without altering the electrophoreti-cally derived double-layer potential. 

Bibeau and Matijevic studied the stability of a PVC latex by addition of electrolytes. They also found surface ion concentrations derived by electrophoresis to be poor indicators of latex stability. Their stability results were found to compare favorably with DLVO theory predictions, using the actual surface concentration of potential-determining species as the basis for interpretation. That means taking into account both fixed charges and adsorbed emulsifier. 

Latex stability will be determined by the combined effect of two factors the probability of collision between particles and the fraction of the encounters between particles which lead to permanent contact. Tha first factor, the collision frequency, will increase with increasing particle size and particle number. It will also increase with increasing shear rate. The influence of various test conditions on the second factor ought to be discussed on the basis of the DLVO theory of colloid stability. 

When copolymerizing VCM with vinyl esters it appears to be the combination of two competing effects which determines the latex stability. A stability increasing effect seems to arise from increasing the polarity of the polymer particle surface, and a stability decreasing effect from increasing the softness of the polymer particles by internal plasticization. 

Copolymerization with vinyl acetate has a strong effect on the nature of the surface of the polymer particles, but the plasticization effect is comparatively weak. With increasing content of vinyl acetate in the copolymer the latex stability will pass through a distinct maximum before decreasing below the stability level of the homopolymer. 

Latex stability. Effect of pH. The pH of the latices were adjusted to about b before doing any of the previously described stability measurements. The effect of pH on the stability is described in Figure 8. A sharp drop in stability occurs below pH 2. This is possibly due to deionization of the sulphate group of the emuisifier, indicating an electrostatic repulsion to have been operative. 

Latex stability. Effect of temperature. Usually no temperature control was imposed. The rise in temperature during the test was 1-2 °C. For purposes of temperature control the bottle containing the sample was provided with a water jacket through which water at specified temperature was circulated. The effect of temperature on the stability is described in Figure 9 To avoid confusion the experimental points are not indicated on the figure, except those obtained at room temperature. The temperature dependence was found to obey the Arrhenius equation. At any given emulsifier level a linear plot of the logarithm of the stability versus 1/T could be obtained. 

Latex stability. Effect of stirring speed and spindle disk diameter. Figures 10. and 11. show how sensitive the stability test is with regard to the speed of stirring and the diameter of spindle disk. 

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