Emulsions continued viscosity

The viscosity of the continuous phase affects the stability of the concentrated emulsion. The viscosity of the continuous phase can be modified either by adding thickeners or by increasing the surfactant concentration. For instance, the formation of a liquid-crystalline structure in the continuous phase when the surfactant concentration is sufficiently large can increase the stability of the emulsion. However, a too high viscosity of the continuous phase caused by a high surfactant concentration can hinder the formation of a concentrated emulsion because it generates resistance to the dispersion of the dispersed phase. 

It is frequently desirable to be able to describe emulsion viscosity in terms of the viscosity of the continuous phase tjq) and the amount of emulsified material. A very large number of equations have been advanced for estimating suspension (or emulsion, etc.) viscosities. Most of these are empirical extensions of Einstein s equation for a dilute suspension of spheres ... 

We also discuss the conditions under which emulsions can be considered as a continuous phase toward the added solids, where the prediction of the emulsion-solids viscosity is possible. 

Relative Viscosity In emulsions, the viscosity of the emulsion divided by the viscosity of the continuous phase = 7]/7]q). 

Emulsion viscosity is higher than either dispersed phase viscosity or continuous phase viscosity. As the dispersed volume fraction is increased, the emulsion viscosity is increased. For a W/O emulsion, the viscosity could be increased from 10s mPa s to 100s mPa s. For example, in the Daqing ZX block, the viscosity of a produced emulsion from an ASP well was 195 cP. After dehydration, the oil viscosity was 48 cP, which is still higher than the oil viscosity of 37 cP from a waterflood well. 

Considering melt flow of BC, it is usually assumed that the test temperature is UCST > T > T, where T stands for glass transition temperature of the continuous phase. However, at Tg < T < T g (T g is Tg of the dispersed phase) the system behaves as a crosslinked rubber with strong viscoelastic character. At UCST > T > T, the viscosity of BC is much greater than would be expected from its composition. The reason for this behavior is the need to deform the domain structure and puU filaments of one polymer through domains of the other. Viscosity increases with increase of the interaction parameter between the BC components in a similar way as an increase of the interfacial tension coefficient in concentrated emulsions causes viscosity to rise [Henderson and Williams, 1979]. 

Differences in emulsion viscosities are principally due to the different viscosities of the emulsion continuous phase (oil phases). Although for some emulsions clear phase separation was visible to the naked eye, viscosity values revealed no significant changes during storage, even at higher temperature, due to the dominance of the continuous phase viscosity on the bulk viscosity. 

For droplets with low viscosity (comparable to that of the medium) the transmission of tangential stress across the 0/W interface from the continuous phase to the dispersed phase causes liquid circulation in the droplets. Energy dissipation is less than that for hard spheres and the relative viscosity is lower than that predicted by the Einstein equation. For an emulsion with viscosity qj for the disperse phase and q for the continuous phase,... 

Eor air-assisted atomizer nozzles the interaction of the gas and liquid phases, gas-liquid ratio GLR, and total mass flow rate on the different levels of structure of SEs and DEs has been discussed. The effect of the disperse to continuous viscosity ratio A on the dispersion of emulsions in spray processing has been extensively explored, and a new dimensionless gas Weber number, Weg oropM related to the secondary emulsion drop diameter was defined. The resistance of the secondary emulsion droplets in two-phase nozzle spray processing (air assist) was given up to a critical value (Weg,DropA)c-... 

Emulsion polymerization also has the advantages of good heat transfer and low viscosity, which follow from the presence of the aqueous phase. The resulting aqueous dispersion of polymer is called a latex. The polymer can be subsequently separated from the aqueous portion of the latex or the latter can be used directly in eventual appUcations. For example, in coatings applications-such as paints, paper coatings, floor pohshes-soft polymer particles coalesce into a continuous film with the evaporation of water after the latex has been applied to the substrate. 

Microemulsion Polymerization. Polyacrylamide microemulsions are low viscosity, non settling, clear, thermodynamically stable water-in-od emulsions with particle sizes less than about 100 nm (98—100). They were developed to try to overcome the inherent settling problems of the larger particle size, conventional inverse emulsion polyacrylamides. To achieve the smaller microemulsion particle size, increased surfactant levels are required, making this system more expensive than inverse emulsions. Acrylamide microemulsions form spontaneously when the correct combinations and types of oils, surfactants, and aqueous monomer solutions are combined. Consequendy, no homogenization is required. Polymerization of acrylamide microemulsions is conducted similarly to conventional acrylamide inverse emulsions. To date, polyacrylamide microemulsions have not been commercialized, although work has continued in an effort to exploit the unique features of this technology (100). 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

Air-blown asphalts, more resistant to weather and changes ia temperature than the types mentioned previously are produced by batch and continuous methods. Air-blown asphalts, of diverse viscosities and flow properties with added fillers, polymers, solvents, and ia water emulsions, provide products for many appHcations ia the roofing industry. 

Viscosity Increase. The flocculation rate of an emulsion is iaversely proportional to the viscosity of the continuous phase and an iacrease of the viscosity from 1 mPa-s (=cP) (water at room temperature) to a value of 10 Pa-s (100 P) (waxy Hquid) reduces the flocculation rate by a factor of 10,000. Such a change would give a half-life of an unprotected emulsion of a few hours, which is of Httle practical use. 

To prepare stable emulsions ia this way gelation of the continuous medium is necessary. The appearance of a Hquid emulsion may be retained by choosing a polymer for the continuous phase, giving a thixotropic solution with short breakdown and buildup times. The polymers used for this purpose are natural gums (qv) or synthetic polymers. Clay particles also act as viscosity enhancers. The members of the bentonite family derived from... 

Many times solids are present in one or more phases of a solid-hquid system. They add a certain level of complexity in the process, especially if they tend to be a part of both phases, as they normally will do. Approximate methods need to be worked out to estimate the density of the emulsion and determine the overall velocity of the flow pattern so that proper evaluation of the suspension requirements can be made. In general, the solids will behave as though they were a fluid of a particular average density and viscosity and won t care much that there is a two-phase dispersion going on in the system. However, if solids are being dissolved or precipitated by participating in one phase and not the other, then they will be affected by which phase is dispersed or continuous, and the process will behave somewhat differently than if the solids migrate independently between the two phases within the process. 

Polymerization processes are characterized by extremes. Industrial products are mixtures with molecular weights of lO" to 10. In a particular polymerization of styrene the viscosity increased by a fac tor of lO " as conversion went from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1,800 K (3,240 R). Heat transfer coefficients in stirred tanks with high viscosities can be as low as 25 W/(m °C) (16.2 Btu/[h fH °F]). Reaction times for butadiene-styrene rubbers are 8 to 12 h polyethylene molecules continue to grow lor 30 min whereas ethyl acrylate in 20% emulsion reacts in less than 1 min, so monomer must be added gradually to keep the temperature within hmits. Initiators of the chain reactions have concentration of 10" g mol/L so they are highly sensitive to poisons and impurities. 

The use of the nomograph is as follows Find the intersecting point of the curves of continuous phase and dispersed phase viscosities on the binary field (left side of nomograph). A line is drawn from this point to the common scale volume fraction of dispersed phase and continuous phase liquids. The intersection of this line with the Viscosity of Emulsion scale gives the result. 

Example. The viscosity of the continuous phase liquid is 20. The viscosity of the dispersed phase liquid is 30. The volume fraction of the dispersed phase liquid is 0.3. The nomograph shows the emulsion viscosity to be 36.2. 

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