Latex dispersion stability

Figure 9 The schematical representation of dispersion polymerization process, (a) initially homogeneous dispersion medium (b) particle formation and stabilizer adsorption onto the nucleated macroradicals (c) capturing of radicals generated in the continuous medium by the forming particles and monomer diffusion to the forming particles (d) polymerization within the monomer swollen latex particles, (e) latex particle stabilized by steric stabilizer and graft copolymer molecules (f) list of symbols.

Storage stability of latex dispersion at various temperatures... 

The p.c.s. measurements were carried out using a Malvern multibit correlator and spectrometer together with a mode stabilized Coherent Krypton-ion laser. The resulting time correlation functions were analysed using a non-linear least squares procedure on a PDP11 computer. The latex dispersions were first diluted to approximately 0.02% solids after which polymer solution of the required concentration was added. 

Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

Composite core-shell type microspheres were prepared by in situ heterogeneous polymerization on monodispersed seed latex particles suspended in an aqueous magnetite dispersion stabilized with sodium oleate (58). 

Particle electrophoresis studies have proved to be useful in the investigation of model systems (e.g. silver halide sols and polystyrene latex dispersions) and practical situations (e.g. clay suspensions, water purification, paper-making and detergency) where colloid stability is involved. In estimating the double-layer repulsive forces between particles, it is usually assumed that /rd is the operative potential and that tf/d and (calculated from electrophoretic mobilities) are identical. 

C is clearly an important quantity for a latex dispersion since it essentially represents the electrolyte concentration at which complete loss of stability occurs. It may be obtained experimentally by a variety of different methods (14,17, 18,19). It should be noted, however, that since coagulation is a kinetic phenomena time enters as a variable and consequently the various methods may yield somewhat different numerical results. This effect is illustrated by results obtained for the coagulation of polytetrafluoroethylene (PTFE) latices with sodium chloride as a function of pH (19). From Figure 4 it can be seen that different results are obtained according to whether the system was examined after 2 h or 24 h. As expected the results indicate that the state of aggregation is more advanced after 24 h and consequently systems at a lower electrolyte concentration have coagulated. Care must therefore be taken when comparing values... 

In the methodology developed by us [24], the incompatibility of the two polymers was exploited in a positive way. The composites were obtained using a two-step method. In the first step, hydrophilic (hydrophobic) polymer latex particles were prepared using the concentrated emulsion method. The monomer-precursor of the continuous phase of the composite or water, when that monomer was hydrophilic, was selected as the continuous phase of the emulsion. In the second step, the emulsion whose dispersed phase was polymerized was dispersed in the continuous-phase monomer of the composite or its solution in water when the monomer was hydrophilic, after a suitable initiator was introduced in the continuous phase. The submicrometer size hydrophilic (hydrophobic) latexes were thus dispersed in the hydrophobic (hydrophilic) continuous phase without the addition of a dispersant. The experimental observations indicated that the above colloidal dispersions remained stable. The stability is due to both the dispersant introduced in the first step and the presence of the films of the continuous phase of the concentrated emulsion around the latex particles. These films consist of either the monomer-precursor of the continuous phase of the composite or water when the monomer-precursor is hydrophilic. This ensured the compatibility of the particles with the continuous phase. The preparation of poly(styrenesulfonic acid) salt latexes dispersed in cross-linked polystyrene matrices as well as of polystyrene latexes dispersed in crosslinked polyacrylamide matrices is described below. The two-step method is compared to the single-step ones based on concentrated emulsions or microemulsions. 

This article has reviewed latex processing. The polymers used, synthesis of particles, major uses, and reasons for loss of dispersion stability have been outlined. The mechanism of latex film formation has been described, and the different properties resulting from different film forming conditions in latex explored. 

Modicol . [Henkel/Functional Prods.] Ethoxylated ester coagulant wetting agent dispersant stabilizer, thickener, protective colloid for latex and resin emulsions. 

Nekal . [Rhone-Poulenc Surf.] Sulfonates or sulfosuccinates emulsifier, wetting agent, dispersant, stabilizer, foamer for textiles, paints, pesticides, dyes, latex emulsions, emulsion polymerization, industrial use. 

The useful property of sodium CMC is its ability to increase the viscosity of mixtures to which it is added. At 25 C, water has a viscosity of slightly less than 1 centipoise. A 1 per cent solution of high-viscosity sodium CMC at the same temperature has a viscosity approximately 2,O00 times as great. Thus it finds application as a thickener in textile printing pastes, latex dispersions, lubricants, and the Ukc. As a stabilizer for emulsions ai(d suspensions, sodium CMC is an important ingredient in synthetic detergents to prevent redeposition of soil and is helpful in creams, lotions, tooth pastes, and in many types of oH-in-water emulsions. Its... 

Depletion layer effects occur in associative thickener formulations when the latex is larger in size ( 500 nm) and not highly stabilized with surface (hydroxyethyl)cellulose fragments. Syneresis is also observed in simple aqueous solutions and in latex dispersions when the hydrophobicity of the associative thickener is high. 

The dispersion stability can be increased due to thickeners added, e.g. ethoxylated urethanes, which are able to form associates with the film forming latex [201]. At the same time, the dispersion contains a certain amount of surfactants these is free surfactant present in the latex, and added surfactant to disperse pigments and fillers. Stability is achieved by varying interaction with the two dispersed phases - latex particles associated with the thickener, and pigment particles. 

The use of antifoams is of special importance for the preparation of water-based paints [202]. Although foam problems also occur in textile and paper industries, there are some special features for paints. First, foam is formed in machines with high and medium shear rates, such as high-speed mills. The presence of a considerable foam volume inhibits the process and considerably reduces the useful load volume of the machine. Besides, foam inhibits the operation of the filling equipment. Problems also occur when paints are applied to a surface, especially using effective sprays, dipping methods and foam-curtain devices. The main reason of foam formation are surfactants used to stabilize aqueous latex dispersions. Thus, nonionic surfactants, instead of anionics, are preferred as they form less foam of low stability. 

Figure 5.2 presents a similar plot for a poly(methyl methacrylate) latex sterically stabilized in n-heptane by poly(12-hydroxystearic acid). In this instance, however, the reduction in the solvency of the dispersion medium for the stabilizing moieties was achieved by adding a miscible nonsolvent (specifically ethanol) to the dispersion medium (Napper, 1968a). Flocculation was again accompanied by an abrupt increase in turbidity when a certain volume fraction of ethanol was added to the ra-heptane. In this instance, it was possible to observe the slow flocculation of the latex particles (i.e. flocculation apparently in the presence of a small repulsive potential energy barrier at a rate slower than that predicted by Smoluchowski, 1917). It is, however, usually diflicult to detect such slow flocculation because of the sharpness of the transition from stability to flocculation for stericaUy stabilized dispersions. 

Comparison of theory with experiment. It will be shown in Section 13.3.2.1 that the flat plate potentials can be used to calculate the osmotic disjoining pressures in concentrated monodisperse sterically stabilized dispersions. Evans and Napper (1977) have compared the theoretical predictions using the above equations with those measured by Homola and Robertson (1976) for polystyrene latex particles stabilized by poly(oxyethylene) of molecular weight ca 2 000 in aqueous dispersion media. The elastic repulsion in the interpenetrational-plus-compressional domain was estimated from the following expression for the constant segment density model... 

Experimental evidence for elastic steric stabilization There is a paucity of experimental studies of elastic steric stabilization. Smitham and Napper (1976a,b) have shown that it is possible to prepare polystyrene latex particles stabilized by poIy(oxyethylene) and dispersed in molten poly(oxyethylene). These experiments suggested that the maximum particle size that could be elastically stabilized was dependent upon the molecular weight of the stabilizing moieties, as would be expected intuitively. Everett and Stageman (1978a) have also reported the elastic stabilization of poly(methyl methacrylate) particles stabilized by poly(dimethylsiloxane) in liquid poly(dimethylsiloxane). 

This diagram is able to explain some puzzling observations disclosed by Cowell and Vincent (1982). They report clear differences between the stability behaviour of latex dispersions depending upon whether the particles are naked or pre-coated by terminally anchored chains. The addition of free polymer to the sterically stabilized particles resulted in the transition sequence stability- instability stability. This is precisely what would be predicted from Fig. 17.20 if bridging flocculation is absent (as it must be if the free polymer and stabilizing moieties are identical in chemical composition). 

The presence of surfactants, besides altering the latex particle surface, can also interact with the water-soluble polymer. For instance, poly(ethylene oxide) homopolymer and block copolymers interact with sodium dodecyl sulfate surfactant [109], and hence alter the latex viscosity behaviour [110]. Other water-soluble polymers are also capable of interacting with specifle surfactants [111]. When pigmented latex dispersions are thickened with associative thickeners one must consider the interactions with some of the pigment stabilizers [112] and other additives, like coalescing aids [113]. 

A thorough understanding of the reaction kinetics of VAc and its comonomers is important to fully comprehend and enhance the polymer chemist s chances to make a VAc latex tailored for a specific end-use. To this end recent advancements in initiator types, new concepts in undostanding of molar mass development and copolymerization kinetics are reviewed. Particle size, size distribution and the morphology of latexes are also presented. Hnally, film foimaticHi, dispersion stabilization and advances in high-performance VAc copolymer latexes employing branched esters are discussed. 

Models for the drying process are based upon measurements of the rate of water loss from latex dispersions. Almost all the etqreriments on which the traditional models were based involve dispersions stabilized by surfactants. Water is lost initially at a constant rate, and then the rate slows down until water loss is complete. There have been several very important recent contributions to the topic of how latex dispersions dry and the relative timing of particle contact and deiformation. As a consequence of the new information now avaUable, all of the traditional models of the drying process have to be revised. 

Van Tent [ 12] reported a series of experiments in which he monitored the turbidity (UV-visible transmission) of latex dispersions and weight loss simultaneously. These experiments are examined here in some detail. Van Tail s latex samples had a veiy narrow size distribution and were stabilized (at 48 wt% solids) by the presence of 1 wt% sodium dodecylbenzene sulfonate, based iqum latex. Thin films (e.g. 30-40 pm) of dispersion were placed on a horizontal glass disc. The disk was mounted on the pan of a balance, and the interrogafing beam monitored a spot 0.3 mm in diameter in the centre of the disc. Because the temperature in the room could be controlled, the authors were able to compare the drying process for the same dispersion at tenqiaatures below, near and above the MFT. 

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