Latex, viscosity

Rheology. Flow properties of latices are important during processing and in many latex appHcations such as dipped goods, paint, inks (qv), and fabric coatings. For dilute, nonionic latices, the relative latex viscosity is a power—law expansion of the particle volume fraction. The terms in the expansion account for flow around the particles and particle—particle interactions. For ionic latices, electrostatic contributions to the flow around the diffuse double layer and enhanced particle—particle interactions must be considered (92). A relative viscosity relationship for concentrated latices was first presented in 1972 (93). A review of empirical relative viscosity models is available (92). In practice, latex viscosity measurements are carried out with rotational viscometers (see Rpleologicalmeasurement). 

The Third Stage. The beginning of the third stage is characterized by gradual acceleration of AA polymerization, accompanied with an increase in the latex viscosity as shown in Figure 1. 

Disappearance of St droplets leads to a decrease in the concentration of St in the aqueous phase as well as in the particles. The decrease in the concentration of St in the aqueous phase causes growing radicals to keep their hydrophilicity for a long time. The radicals can react with many AA molecules before they diffuse into the particles. Some radicals would not lose their solubility in water even after they propagated to be polymer molecules and such kind of molecules would cause an increase in the latex viscosity. Judging from the dependence of the latex viscosity on the reaction time, most of the polymer dissolved in water is considered to form during the third stage. To reduce the... 

The essential ingredients in an emulsion polymerization are the water, monomer, surfactant, and free-radical source. Other ingredients are frequently added for a variety of reasons. Stabilizers, which are usually water-soluble high polymers or carbohydrate gums, are employed to control latex viscosity and freeze-thaw stability of products that are used in the latex form. 

Other reactor design considerations may be necessary in special cases. Monomer mass transfer, not normally a problem, can he important if the monomer- aqueous phase interface is small. This is more likely in systems involving gaseous monomers in which the large surface area of the monomer emulsion is not present. In such cases special attention must he paid to gas dispersion and transport. Giher factors that can have a significant effect on reactor design include latex viscosity, heat transfer rates, reaction pressure, and control mechanisms. 

Technical Application. The experimental finding that with PEO, in contrast to other polymers, many factors have little or no effect on agglomeration is unquestionably the main reason for the great versatility that the agglomeration process offers in the manufacture of SBR latexes. It is noteworthy that the agglomeration result is the same irrespective of whether agglomeration is carried out in a 1-1 beaker or in a 200-m3 reactor, even if base latex viscosity is extremely high as the result of short-time polymerization techniques. 

A series of latex copolymers were prepared using a typical emulsion polymerization recipe and procedure only the monomer composition was varied. The control composition (80/20 vinyl acetate/butyl acrylate) is similar to that used for interior latex paint. Table V lists the compositions and properties of the latexes. Percent solids, pH, and particle size are similar for all the latexes. Viscosity varies somewhat, but is within limits for this type of latex. The only unreacted monomer detected was the vinyl acetate. Thus, the incorporation of VEC into the emulsion polymerization via the monomer mixture did not affect the latex synthesis. The Tg and minimum film formation temperature (MFFT) of the latexes increase with increasing VEC content, which is expected based on the previous results. 

Increasing the concentration of surfactant and Latex viscosity increases particle size... 

Increasing the temperature of polymerization Increases latex viscosity reduces particle size... 

Increasing the potential solids contraction of the Increases latex viscosity... 

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]. 

The control and reproducibility of particle size and particle size distribution is important to the quality of acrylic and styrene-acrylic latex products. Particle size has large effects on latex viscosity and the rheology of formulated products and may also exert subtle effects on the end-use peiformaiKe properties. The particle size is controlled primarily by the choice and amount of surfactant, or by the use of seed latexes. A recoit article [32] addresses the use of surfactants to control particle size in semi-continuous acrylic polymmzations. Many surfactants are reconunended by surfactant manuhicturras for the preparation of acrylic and styrene-acrylic latexes [33]. Sodium lauryl sulfate, sodium dodecylbenzene sulfonate, sulfosuccinates and the aUtylphonl ethoxylates ate typical. The patent literature contmns many discussions of the use of single [34] or multiple [3S] polymer seed latexes to control particle size. 

S/2EHA/ MAA SLS/HD, Alipal CO-436/HD Ultra- sonification K2S2O8 80 55 Semi-continuous miniemulsion feed fewer particles and lower latex viscosity compared to conventional latexes [13] Masa et al. 1993... 

Because a latex polymer is insoluble in water, the factors affecting the viscosity of latexes differ significantly from polymers dissolved in solvents. Since the aqueous continuous phase interacts with the polymer only at the surface of the latex particle, the molecular stracture of the polymer does not have the effect that it would if the polymer were dissolved in the liquid. The molar mass and polymer backbone flexibility, in particular, have no direct effect on the viscosity of the latex. The principal latex parameter that influences latex viscosity is of course... 

Figure 13A Limiting parameters for latex viscosity. At extremely low concentrations, the contribution of the individual particles is reflected in [ij], the intrinsic viscosity. Viscosity increases r idly as (p approaches the maximum concentration of the latex particles...

Effect of irradiation on latex viscosity and HHA conversion of irradiated a nixture of NR latex and MMA ( 30 phr). 

Particularly thin articles (e.g., rubber gloves) are produced by dipcoating. In this case, the mold negative is dipped into a latex (a dispersion) or a paste for as long and/or as often as is necessary to obtain the desired thickness. The latex viscosity should be less than 12 Pa s the flow limit as low as possible. Latices of natural rubber poly(chloroprene), and silicones, as well as PVC pastes, are processed in this way. 

Latexes are typically low in viscosity since they are water-based dispersions. However, in some cases, it may be necessary to increase the viscosity of the final latex product. For example, the viscosity of latex paints should be such that they flow evenly on a substrate, but do not run off. Latex viscosities may be controlled by the addition of a viscosity modifier (thickener) to the aqueous phase (371). Polyethylene oxide (PEO), hydroxyethyl cellulose (HEC), and various associative thickeners (such as HEUR (91, 281)) or HASE (130, 282) thickeners) are often used as viscosity modifiers. 

Usually, the viscosity of a latex is not appropriate for its intended end use, and must be increased by the incorporation of a water-soluble viscosity modifier. Particle-particle interaction in latexes strongly affects viscosity behaviour (147). At higher solids contents, the viscosity rapidly inaeases. Alternatively, smaller particles (at the same solids content) have less distance between particles, and as a result, a higher latex viscosity. Latex viscosity is also a function of the particle size distribution, such that high solids latexes may be produced as polydisperse or bimodal distribution latexes, in which smaller particles fill the interstices between the larger ones. The formation of... 

The effects of agitation rate, feed time, initiation rate, and latex viscosity on coagulum formation in the indnstrial emulsion polymerisation of styrene and bntyl acrylate were studied, and the results were analysed by the use of computational fluid dynamics to simulate the reactor flow pattern. Large stagnant regions occurred close to the agitator shaft and reactor wall, a situation which was worsened by towards the end of the process, when the agitator shaft was covered completely by the contents of the reactor. 22 refs. 

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