Emulsifier rate dependence

However, the kinetics of PVC emulsion does not foUow the above theory. The rate shows the same increasing behavior with conversion as mass polymerization (94,95). [N depends on [3], but the relationship varies with the emulsifier type (96,97). However, the rate is nearly independent of [N (95). The average number of radicals per particle is low, 0.0005 to 0.1 (95). The high solubiUty of vinyl chloride in water, 0.6 wt %, accounts for a strong deviation from tme emulsion behavior. Also, PVC s insolubiUty in its own monomer accounts for such behavior as a rate dependence on conversion. 

Volume of the dispersed phase. Below a concentration of 50% aqueous phase and with 2-5% non-ionic emulsifier, emulsions behave as Newtonian fluids above 50% aqueous phase, emulsions become increasingly non-Newtonian (i.e., become shear rate-dependent and develop a yield value). 

The investigation of polymer rate dependence on Initiator concentration C. (with ionic strength of the solution equalized) and emulsifier concentration C (for various molecular structures of emulsifier) pennltted us tb establish that it can be described by the following equation ... 

The derived equation of the rate dependence on the emulsifier (CJ and initiator (Ci) concentrations during the constant rate period... 

The particle formation rate depends on the rale of production of free radicals and on the relative rates of the various mechanisms in which these free radicals participate. A high rate of particle formation in the presence of small amounts of free emulsifier will contribute to unstable cyclic behavior. 

Araki et at. (1967, 1969) carried out a more systematic study of the kinetics and other features of the y-iniliated emulsion polymerization of vinyl acetate using sodium lauryl sulfate as the emulsifier. This system had been thoroughly investigated with potassium persulfate as the initiator (Litt et cL. 1960,1970). Some post ei cts have been observed with vinyl acetate, particularly above 50% conversion (Friis, 1973 Sunardi, 1979). These effects had been used by Allen cr at. (1960,1962) for the possible synthesis of block and graft polymers and will be described later in this chapter. The half-life of the radicals in a vinyl acetate latex polymerization was determinad by Hummel et at. (1969) as 0.8 min at 53.8% conversion. Araki et fll. (1967, 1969) determined all the normal rate dependencies and included some seeded latex studies. Their results and those of other investigators are summarized in Table II together with those found with potassium persulfate initiation and those predicted by the Smith-Ewart Case 2 theory. The... 

Free FAs absorption rate depends on the type of fatty acid and intestine emulsifier environment. An important explanation of why palm oil does not "follow the Keys [138] and Hegsted [139] model is due to unsaturated fatty acids are in sn-2 position (> 58.25 % of oleic acid and > 18.41 % of linoleic acid) and a high proportion of PA is in xn-l and 3 positions (17-23 %) [23]. Therefore, as already mentioned in this text, fatty acids in sn-2 position are preferentially absorbed at bowel wall and, thereby, more bioavailable than the fatty acids in sn- and 3 positions [87]. On the assumption that all SFA localized at xn-1 and 3 positions in palm oil are preferentially absorbed, whilst saturated are faecal excreted as salts, therefore only 8 % of SFA localized at sn-2 position would be absorbed as consequence there is a less caloric intake and a lower serum TGA content. 

The Smith-Ewart theory predicts 7 p = 72 [7]° The rate of polymerization of vinyl acetate is virtually independent of emulsifier concentration, depending on the study, whereas the Smith-Ewart theory predicts the rate to be proportional to the 0.6th power of the emulsifier concentration for monomers such as styrene. This may be due to the high chain transfer to the vinyl acetate monomer. The... 

The rate at which a polymer adsorbs to an interface is one of the most important factors determining its efiScacy as an emulsifier [3,11,19]. The adsorption rate depends on the molecular characteristics of the polymer (e.g. size, flexibility, conformation, and interactions), the nature of the bulk liquid (e.g. viscosity, polarity), and the prevailing environmental conditions (e.g. temperature and fluid flow profile). It is often convenient to divide the adsorption process into two stages (i) movement of the emulsifier molecules from the bulk liquid to the vicinity of the interface, and (ii) attachment of the emulsifier molecules to the interface (Figure 5.9). In practice, emulsifier molecules are often in a dynamic equilibrium between... 

The emulsion polymerization of vinyl acetate (to homopolymers and copolymers) is industrially most important for the production of latex paints, adhesives, paper coatings, and textile finishes. It has been known that the emulsion polymerization kinetics of vinyl acetate differs from those of styrene or other less water-soluble monomers largely due to the greater water solubility of vinyl acetate (2.85% at 60°C versus 0.054% for styrene). For example, the emulsion polymerization of vinyl acetate does not follow the well-known Smith-Ewart kinetics and the polymerization exhibits a constant reaction rate even after the separate monomer phase disappears. The following observations have been reported for vinyl acetate emulsion polymerization [78] (a) The polymerization rate is approximately zero order with respect to monomer concentration at least from 20% to 85% Conversion (b) the polymerization rate depends on the particle concentration to about 0.2 power (c) the polymerization rate depends on the emulsifier concentration with a maximum of 0.25 power (d) the molecular weights are independent of all variables and mainly depend on the chain transfer to the monomer (e) in unseeded polymerization, the number of polymer particles is roughly independent of conversion after 30% conversion. 

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) ... 

Emulsion components enter the stratum corneum and other epidermal layers at different rates. Most of the water evaporates, and a residue of emulsifiers, Hpids, and other nonvolatile constituents remains on the skin. Some of these materials and other product ingredients may permeate the skin others remain on the surface. If the blend of nonvolatiles materially reduces the evaporative loss of water from the skin, known as the transepidermal water loss (TEWL), the film is identified as occlusive. AppHcation of a layer of petrolatum to normal skin can reduce the TEWL, which is normally about 4—8 g/(m h), by as much as 50 to 75% for several hours. The evaporated water is to a large extent trapped under the occlusive layer hydrating or moisturizing the dead cells of the stratum corneum. The flexibiHty of isolated stratum corneum is dependent on the presence of water dry stratum corneum is britde and difficult to stretch or bend. Thus, any increase in the water content of skin is beHeved to improve the skin quaHty. 

The choice of coagulant for breaking of the emulsion at the start of the finishing process is dependent on many factors. Salts such as calcium chloride, aluminum sulfate, and sodium chloride are often used. Frequentiy, pH and temperature must be controlled to ensure efficient coagulation. The objectives are to leave no uncoagulated latex, to produce a cmmb that can easily be dewatered, to avoid fines that could be lost, and to control the residual materials left in the product so that damage to properties is kept at a minimum. For example, if a significant amount of a hydrophilic emulsifier residue is left in the polymer, water resistance of final product suffers, and if the residue left is acidic in nature, it usually contributes to slow cure rate. 

The rate of polymerization with styrene-type monomers is directly proportional to the number of particles formed. In batch reactors most of the particles are nucleated early in the reaction and the number formed depends on the emulsifier available to stabilize these small particles. In a CSTR operating at steady-state the rate of nucleation of new particles depends on the concentration of free emulsifier, i.e. the emulsifier not adsorbed on other surfaces. Since the average particle size in a CSTR is larger than the average size at the end of the batch nucleation period, fewer particles are formed in a CSTR than if the same recipe were used in a batch reactor. Since rate is proportional to the number of particles for styrene-type monomers, the rate per unit volume in a CSTR will be less than the interval-two rate in a batch reactor. In fact, the maximum CSTR rate will be about 60 to 70 percent the batch rate for such monomers. Monomers for which the rate is not as strongly dependent on the number of particles will display less of a difference between batch and continuous reactors. Also, continuous reactors with a particle seed in the feed may be capable of higher rates. 

The rate of polymerization (at constant initiator concentration) depends on the number of micelles and therefore on the emulsifier concentration. The rate and degree of polymerization can be increased simultaneously. 

The self-emulsifying behaviour of a binary nonlonlc surfactant vegetable oil mixture has been shown to be dependant on both temperature and surfactant concentration. The quality of the resulting emulsions as assessed by particle size analysis showed that manipulation of these parameters can result In emulsion formulations of controlled droplet size and hence surface area. Such considerations are Important when the partition of lipophilic drugs Into aqueous phases and drug release rates are considered. 

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