Initiator emulsion

Wheieas the BPO—DMA ledox system works well for curing of unsaturated polyester blends, it is not a very effective system for initiating vinyl monomer polymerizations, and therefore it generally is not used in such appHcations (34). However, combinations of amines (eg, DMA) and acyl sulfonyl peroxides (eg, ACSP) are very effective initiator systems at 0°C for high conversion suspension polymerizations of vinyl chloride (35). BPO has also been used in combination with ferrous ammonium sulfate to initiate emulsion polymerizations of vinyl monomers via a redox reaction (36). 

Many random copolymers have found commercial use as elastomers and plastics. For example, SBR (62), poly(butadiene- (9-styrene) [9003-55-8] has become the largest volume synthetic mbber. It can be prepared ia emulsion by use of free-radical initiators, such as K2S20g or Fe /ROOH (eq. 18), or in solution by use of alkyl lithium initiators. Emulsion SBR copolymers are produced under trade names by such companies as American Synthetic Rubber (ASPC), Armtek, B. F. Goodrich (Ameripool), and Goodyear (PHoflex) solution SBR is manufactured by Firestone (Stereon). The total U.S. production of SBR in 1990 was 581,000 t (63). 

The butadiene-acrylonitrile rubbers were first prepared about 1930 about five years after the initial development of free-radical-initiated emulsion polymerisation. Commercial production commenced in Germany in 1937, with the product being known as Buna N. By the late 1980s there were about 350 grades marketed by some 20 producers and by the early 1990s world production was of the order of 250000 tonnes per annum, thus classifying it as a major special purpose rubber. 

Fig. 7. Time-conversion curves of thermally initiated emulsion polymerization of 1,4-DVB at 0.1 (I) 0.65 (II) and 0.85 (III) M SDS concentrations. Polymerization temperature = 90 °C water/monomer volume ratio = 12.5. [Reproduced from Ref.84 with permission,Hiithig Wepf Publ., Zug, Switzerland].

Chemical degradation of phospholipids results in the production of lyso-compounds but also free fatty acids (Figure 9.4). The pH of a heated emulsion will tend to fall as more fatty acids are produced and the hydrolysis rate slows down until a pH of around 4 is reached, at which point it starts to increase again. Accordingly, the initial emulsion is adjusted to pH 9 by the addition of small quantities of sodium hydroxide, and this has the effect of prolonging the shelf life of the product. 

Once the droplet concentration of the diluted emulsion has been measured, it is possible to determine the droplet concentration of the initial emulsion = DVyV)> where 4> is... 

The sizes of the droplets in the initial emulsion significantly affect the size and properties of the microspheres (Table 6)36). As the mixing rate used to produce the emulsions is increased, the average size of the droplets decreases whereas the microsphere size increases. Very dispersed emulsions seem to be more likely to coalesce, thus yielding larger droplets and hence larger microspheres after the heat treatment. This method can produce spherical unicellular particles with diameters of200-400 pm, densities of 260-700 kg/m3, and space factors of up to 59% 35 36). 

Free-Radical Polymerization in Emulsion. In suspension polymerization, the particle size is fixed by the size of the monomer droplet which contains the initiator. Emulsion polymerization differs from suspension polymerization in that the initiator is dissolved in the aqueous phase and the polymer particle grows during polymerization. Free radicals are generated in the water and diffuse to the monomer-water interface. The length of the polymer chain formed, or equivalently the molecular weight, depends on the rate of free radical arrival and termination. S. Katz,... 

Nonionic polyethoxylated surfactants exemplified by Triton X-405, tend to chain transfer in redox-initiated emulsion polymerizations and become chemically bound to the polymer. 

A larger elementary particle size can be achieved by seeding the initial emulsion system. A PVC polymer latex is introduced and the particles of the new polymer grow on the seed. A continuous emulsion polymerisation process is also used. 

A third class is phase inversion. Here, emulsions are made by starting with an emulsion in which the ultimate continuous phase is the dispersed phase and vice versa. Then by adding more and more dispersed phase, one can induce the emulsion to suddenly invert (catastrophic inversion). Alternatively, one can choose the surfactant system such that, for example, by a temperature change, the surfactant system changes from favouring the initial emulsion to favouring an inverted emulsion. This is called transitional phase inversion. 

It is suggested that the chemically initiated emulsion polymerization of vinyl chloride can be represented by curve 3 in Figure 11 (m > 1). Desorption plays such a dominant role with this monomer that TT rarely exceeds 1/2. This is true not only because the desorption constant, kd, is high but also because with vinyl chloride and other comparatively water soluble monomers the number of latex particles formed per unit of surfactant is very high. 

Figure 11. Solutions of the Smith-Ewart recursion equation for the case of no aqueom propagation or termination. Dotted line m = 0 (Smith-Ewart Case II). Curve 1 (m = 10 ) depicts typical styrene-like polymerization. Curve 2(m = 0.01) depicts radiation initiated emulsion polymerization of vinyl chloride. Curve 3 (m > 1.0) depicts chemically initiated emulsion polymerization of vinyl chloride.

As expected, the pH of the latex produced by the reaction correlates closely with the Initial pH of the reaction system. However, rather unexpectedly It was found (see Figure 4) that the final pH Is always about two units lower than the Initial pH. Similar effects are observed In persulphate - Initiated emulsion polymerisations, where the reduction in pH Is attributed to the formation of blsulphate Ions by reaction between water molecules and sulphate radical Ions formed by decomposition of the persulphate. No such ready explanation Is available In this Instance. 

Since the product here contains ionic and nonionic groups, it will be an anionic surfactant. Such materials, which are always formed in persulfate-initiated emulsion polymerizations, have been termed in situ surfactants. Their nature has not been studied extensively. In one study, of the polymerization of a 64 36 (w/w) methyl methacrylate butyl acrylate copolymer in the presence of a chain transfer... 

In general, for most investigations, y radiation has been used because of its high degree of penetration and the ccanparative ense of estimating dose-depth characteristics and because radical fluxes comparable to those used with chemical initiation can easily be achieved- There have also been a few, comparatively brief, studies using electron accelerators to initiate emulsion polymerization in emulsion. These have mainly been conducted in Japanese laboratories. 

The digestion of triacylglycerols in adult nonruminant mammals has been described as initiated in the mouth by hngual lipase released in the sahva at the base of the tongue (52). Up to 6% of the fatty acids are hydrolyzed and initiate emulsion formation in the stomach. The digesta (called chyme at this location) is released from the stomach slowly into the duodenum to ensure complete mixing with the bile salts and emulsification. Lipolysis occurs by association of pancreatic lipase and co-lipase at the surface of the bile salt-stabihzed emulsion. Amphipathic molecules (fatty acids, sn-2 monoacylglycerols, and lysolecithins) are produced and associate with the bile salts to form water-soluble micelles from which absorption occurs. 

Initiation Rate in y-lNDUCEo Emulsion Polymerization. In all our foregoing discussions on the kinetics of the y-initiated emulsion polymerization with constant dose rate we have assumed that the initiation rate or the rate of formation of radicals in the system remains constant throughout the reaction. 

Aqueous solutions of proteins such as albumin can be emulsified in an oil and induced to form microspheres, either by crosslinking the protein molecules with glutaraldehyde or other agents or by coagulating the protein by heating. Incorporation of a dmg within the initial protein solution results in dmg-laden microspheres which are biodegradable. The particle size of the microspheres (generally 0.2-300 jum diameter) is determined by the size distribution of the initial emulsion. 

The effect of oil viscosity on initial emulsion viscosity is not clear from these experiments. The St. Lina crude is about six times as viscous as the California crude. The apparent viscosity of the lower viscosity St. Lina Crude emulsion (2 x 10 moles NaOH/gram oil) is less than 50% greater than the lowest viscosity moderately stable California crude emulsion (4.0 x 10 NaOH). The average particle size of the St. Lina emulsion is 7 microns while that of the Shell crude emulsion is about 3 microns (see Figure 8). Since particle sizes, particle size distributions and types of oil are different, no conclusions can be drain about the influence of oil viscosity. There is, however one fact which should be emphasized, namely that viscosities 600 times lower than that of the crude were observed for 60% St. Lina crude emulsions. 

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