Emulsifier concentration

Most synthetic latices contain 5—10 wt % of nonelastomeric components, of which more than half is an emulsifier or mixture of emulsifiers. One reason for this relatively high emulsifier concentration as compared with natural latex is that emulsifier micelles containing solubiHzed monomer play a principle role in the polymerization process. A high emulsifier concentration is usually necessary to achieve a sufficiently rapid rate of polymerization. Secondly, a considerable fraction of the surface of the polymer particles must be covered by adsorbed soap or equivalent stabilizer to prevent flocculation... 

Propagation. The rate of emulsion polymerization has been found to depend on initiator, monomer, and emulsifier concentrations. In a system of vinyl acetate, sodium lauryl sulfate, and potassium persulfate, the following relationship for the rate of polymerization has been suggested (85) ... 

The Smith-Ewart theory predicts = K [I. The rate of polymerization of vinyl acetate is virtually independent of emulsifier concentration,... 

The introduction of Hquid crystals as stabilizing elements for emulsions occurred in 1969 when it was found that the sudden stabilization at emulsifier concentrations in excess of 2.5% of a water—% xylene emulsion by a commercial octa(ethylene glycol) nonylphenyl ether was due to the formation of a Hquid crystalline phase in the emulsion (26). Later investigations confirmed the strong stabilizing action of these stmctures (27). 

The final factor influencing the stabiHty of these three-phase emulsions is probably the most important one. Small changes in emulsifier concentration lead to drastic changes in the amounts of the three phases. As an example, consider the points A to C in Figure 16. At point A, with 2% emulsifier, 49% water, and 49% aqueous phase, 50% oil and 50% aqueous phase are the only phases present. At point B the emulsifier concentration has been increased to 4%. Now the oil phase constitutes 47% of the total and the aqueous phase is reduced to 29% the remaining 24% is a Hquid crystalline phase. The importance of these numbers is best perceived by a calculation of thickness of the protective layer of the emulsifier (point A) and of the Hquid crystal (point B). The added surfactant, which at 2% would add a protective film of only 0.07 p.m to emulsion droplets of 5 p.m if all of it were adsorbed, has now been transformed to 24% of a viscous phase. This phase would form a very viscous film 0.85 p.m thick. The protective coating is more than 10 times thicker than one from the surfactant alone because the thick viscous film contains only 7% emulsifier the rest is 75% water and 18% oil. At point C, the aqueous phase has now disappeared, and the entire emulsion consists of 42.3% oil and 57.5% Hquid crystalline phase. The stabilizing phase is now the principal part of the emulsion. 

Fig. 16. An increase of the emulsifier concentration in an emulsion with a Hquid crystal leads to a drastic increase of the amount of Hquid crystal. See text.

The kinetic mechanism of emulsion polymerization was developed by Smith and Ewart [10]. The quantitative treatment of this mechanism was made by using Har-kin s Micellar Theory [18,19]. By means of quantitative treatment, the researchers obtained an expression in which the particle number was expressed as a function of emulsifier concentration, initiation, and polymerization rates. This expression was derived for the systems including the monomers with low water solubility and partly solubilized within the micelles formed by emulsifiers having low critical micelle concentration (CMC) values [10]. 

The rate of an ideal emulsion polymerization is given by Eqn (4). In this expression [/] is the initiator concentration, [ ] is the emulsifier concentration, and [M] is the concentration of monomer within the forming latex particles. This value is constant for a long reaction period until all the monomer droplets disappear within the water phase. 

Based on the Smith-Ewart theory, the number of latex particles formed and the rate of polymerization in Interval II is proportional with the 0,6 power of the emulsifier concentration. This relation was also observed experimentally for the emulsion polymerization of styrene by Bartholomeet al. [51], Dunn and Al-Shahib [52] demonstrated that when the concentrations of the different emulsifiers were selected so that the micellar concentrations were equal, the same number of particles having the same size could be obtained by the same polymerization rates in Interval II in the existence of different emulsifiers [52], The number of micelles formed initially in the polymerization medium increases with the increasing emulsifier concentration. This leads to an increase in the total amount of monomer solubilized by micelles. However, the number of emulsifier molecules in one micelle is constant for a certain type of emulsifier and does not change with the emulsifier concentration. The monomer is distributed into more micelles and thus, the... 

Figure 6 The effect of emulsifier concentration on the variation of monomer conversion by the polymerization time in the emulsion polymerization of styrene. Styrene-water = 1/3 KPS = 1,65 mM reaction volume = 300 ml stirring rate = 250 rpm temperature = 70 C,...

Achieving steady-state operation in a continuous tank reactor system can be difficult. Particle nucleation phenomena and the decrease in termination rate caused by high viscosity within the particles (gel effect) can contribute to significant reactor instabilities. Variation in the level of inhibitors in the feed streams can also cause reactor control problems. Conversion oscillations have been observed with many different monomers. These oscillations often result from a limit cycle behavior of the particle nucleation mechanism. Such oscillations are difficult to tolerate in commercial systems. They can cause uneven heat loads and significant transients in free emulsifier concentration thus potentially causing flocculation and the formation of wall polymer. This problem may be one of the most difficult to handle in the development of commercial continuous processes. 

Figure 8, Monomer conversion vs, polymerization time in the helical tubular reactor effect of varying initiator and emulsifier concentrations...

Mice that were exposed dermally to residues of methyl parathion in emulsifiable concentrate on foliage, and were muzzled to prevent oral intake, developed inhibition of plasma cholinesterase and erythrocyte cholinesterase after two 10-hour exposures (Skinner and Kilgore 1982b). For the organophosphate pesticides tested in this study, cholinergic signs generally were seen in mice with cholinesterase inhibition >50% results for this end point were not broken down by pesticide. 

Methyl parathion is marketed as a technical grade solution (80% methyl parathion) or in emulsifiable concentrate, wettable powder, ultra-low volume (ULV) liquid, dustable powder, and encapsulated suspension forms (HSDB 1999). The technical grade solution contains 80% active ingredient, 16.7%... 

Methyl parathion may also be introduced into the air as a result of its volatilization from plant surfaces, and somewhat from soil, especially in the period just after application. Under simulated field conditions (20° C air velocity 1 meter/second relative air humidity 40-60%), an emulsifiable concentrate formulation of methyl parathion was applied to bare soil and bean plants. After 24 hours, the amounts of methyl parathion that had volatilized from bare soil and bean plants were 5 and 64% of the applied amount, respectively (Rudel 1997). 

Atmospheric concentrations of methyl parathion following application of the pesticide to tobacco fields were studied by Jackson and Lewis (1978). They found that levels of methyl parathion decreased rapidly following application of either the emulsifiable concentrate or the microencapsulated form. Air concentrations for the emulsifiable concentrate ranged from 7,408 ng/m immediately following application to 13 ng/m 9 days later. The corresponding measurements for the microencapsulated form were 3,783 and 16 ng/m. ... 

Elnulsifler Magg Balance. The overall emulsifier concentration in the system, Cgt. is constant however, it is distributed among the aqueous liiase (Cg ), the polymer particles and the monomer droplets intertaces (Cga) and the micellar aggregates (Cgm), according to the sinple balance ... 

OPs are often applied as sprays. Commonly, the formulations used for spraying are emulsifiable concentrates, where the OP is dissolved in an organic liquid that acts as a carrier. OPs are also used as seed dressings and as components of dips used to protect livestock against ectoparasites. Some highly toxic OPs have been incorporated into granular formulations for application to soil or to certain crops. 

Pyrethroids can also persist in sediments. In one study, alpha-cypermethrin was applied to a pond as an emulsifiable concentrate (Environmental Health Criteria 142). After 16 days of application, 5% of the applied dose was still present in sediment, falling to 3% after a further 17 days. This suggests a half-life of the order of 20-25 days—comparable in magnitude to half-lives measured in temperate soils. 

The receipt of the test substance should be documented upon arrival at the test site. The name of the product, manufacturer, active ingredient concentration, expiration date, storage location, storage requirements, lot or batch number, the amount received, the condition at receipt, and whether the material is an emulsifiable concentrate (EC), fiowable, powder or otherwise should be noted in the research notebook. In addition, one should note the purchase date, the shipment date, and the carrier of the product. 

The formulation of aldrin and dieldrin can be readily accomplished in normal fashion no difficulty has been encountered in incorporating these materials into dusts, wet-table powders, or emulsifiable concentrates. 

Note M/L = mixing/loading EC = emulsifiable concentrate 50 WP = 50% wettable power A = application. 

Note EC = emulsifiable concentrate WP = wettable powder DAT = days after treatment. 

Figure 1. Conversion-time histories in batch PVC emulsion polymerization for different initiator and emulsifier concentrations.

Lee, G.W.J. and Tadros, Th.F. (1982) Formation and stability of emulsions produced by dilution of emulsifiable concentrates. Part I. An investigation of the dispersion on dilution of emulsifiable concentrates containing cationic and non-ionic surfactants. Colloids Surf,... 

According to the classical Smith-Ewart mechanism [85], the number of particles, N, is related to the emulsifier concentration, E, by... 

Fig. 6. Amount of coagulum as a function of the emulsifier concentration in 1,4-DVB polymerization. Polymerization temperature = 50 °C,water/monomer volume ratio = 6.25 (0),and 12.5 ( ). [Reproduced from Ref. 79 with permission, Hiithig Wepf Publ., Zug, Switzerland].

As shown in Fig. 6, the amount of polymerized monomer droplets strongly depends on the emulsifier concentration. With increasing emulsifier concentration, the amount of monomer initially present in the monomer droplets decreases in favor of monomer solubilized in micelles. Concurrently the fraction of polymerized monomer droplets decreases and more microgels are formed. Above a certain emulsifier concentration which is about 0.8 mol/1 in thermal initiation, the monomer is completely solubilized prior to polymerization and no polymerized monomer droplets are formed. 

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