Homogenization, monomer emulsions

Alduncin et al. [28] studied the effect of a series of initiators with different water solubiUty (lauroyl peroxide, benzoyl peroxide, and 2,2 -azobisisobutyro-nitrile) on styrene emulsion polymerization. It was concluded that, among these oil-soluble initiators, only lauroyl peroxide with the lowest water solubility (2 X 10 g per 100 g water) is hydrophobic enough to stabilize the homogenized monomer emulsion against the degradation of monomer droplets by... 

The initial polymerizations were conducted with both homogeneous and emulsion conditions. In the homogeneous reactions, a mixture of metha-nolrdichloroethane was used to dissolve the sulfated monomer, and the initiator was introduced as a solution in dichloroethane. These reactions did not proceed to completion, even upon heating. During the reaction, the formation of a precipitate was observed, which could be attributed to the growing polymer chain. Although the monomer is soluble in methanol as a triethylammonium salt, the... 

Figure 13. Electron micrograph of monomer emulsion of Figure 12 with OPB + hexadecanol (HD) immediately after homogenization...

Abstract. Polymerization in direct mini-emulsions is a relatively new polymerization technique which allows the preparation of submicron latex particles within the range 100large population of submicron monomer droplets in water (termed the mini-emulsion) by intensive shear force with the aid of an adequate emulsifier and coemulsifier (or hydrophobe). These stable, homogenized monomer droplets have an extremely large surface area and, therefore, can compete effectively with the monomer-swollen micelles, if present, for the oligomeric radicals gen-... 

The presence of a small amount of polymer (PSt or PMMA) inside the homogenized monomer droplets reduces the sensitivity of Np to changes in [initiator]51 and the decrease of the dependency is more pronounced for the miniemulsion polymerization with PMMA (Table 2) [19, 79, 93]. Under these circumstances, the added polymer increases the lifetime of monomer droplets and the probability of monomer droplet nucleation. The ratio Np>f/Nm>i was found to be very close to 1 for the mini-emulsion polymerization with PMMA, but it is much above 1 for the run with PSt. The interaction between the polymer particle surface and emulsifier increases with increasing hydrophobicity of polymer and, thence, PSt should promote the formation of more stable monomer droplets in the preparation of the mini-emulsion. However, the reverse seems to be true for the highly diluted polymer particles with the predissolved PSt. PSt mainly locates in the monomer droplet core, whereas the more hydrophilic PMMA tends to diffuse closer to the droplet surface layer and interact with emulsifier therein. Thus, the stronger interaction of PMMA with the droplet surface in comparison with PSt makes PMMA a more efficient hydrophobe. 

An emulsion is a dispersion of one liquid in another where each liquid is immiscible, or poorly miscible in the other [1]. Emulsions exhibit all classical behaviors of metastable colloids Brownian motion, reversible phase transitions as a result of droplet interactions that may be strongly nradified and irreversible transitions that generally involve their destruction. They are obtained by shearing two immiscible fluids to the fragmentation of one phase into the other. From diluted to highly concentrated, emulsions exhibit very different internal dynamics and mechanical properties. Emulsifiers are usually added to oU/water mixture to enhance the formation of stable monomer emulsions. The molecules of emulsifier adsorb to the surface of oil droplets during homogenization and provide a protective membrane... 

Figure 5.1. Rate of Ostwald ripening for emulsions as a funotion of the solubility of the constituent in water. The constituents of the oil phase include />alkanes (n = 9-16) [19] and some common monomers. St, BA, and MMA represent styrene, n-butyl acrylate, and methyl methacrylate, respectively. The data of the solubility of monomers in water were used to estimate the Ostwald ripening rate of the homogenized monomer droplets via the extrapolation method.

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). 

Vinyhdene chloride copolymerizes randomly with methyl acrylate and nearly so with other acrylates. Very severe composition drift occurs, however, in copolymerizations with vinyl chloride or methacrylates. Several methods have been developed to produce homogeneous copolymers regardless of the reactivity ratio (43). These methods are appHcable mainly to emulsion and suspension processes where adequate stirring can be maintained. Copolymerization rates of VDC with small amounts of a second monomer are normally lower than its rate of homopolymerization. The kinetics of the copolymerization of VDC and VC have been studied (45—48). 

The rate of termination reaction is slower than that observed in the homogenous bulk or solution polymerization since the limited number of free radicals exists in the polymerization loci having a reasonably small volume (i.e., monomer swollen forming latex particle). Higher degree of polymerizations can be achieved in an emulsion system relative to the homogenous polymerization due to the existence of this limitation. 

This paper presents the physical mechanism and the structure of a comprehensive dynamic Emulsion Polymerization Model (EPM). EPM combines the theory of coagulative nucleation of homogeneously nucleated precursors with detailed species material and energy balances to calculate the time evolution of the concentration, size, and colloidal characteristics of latex particles, the monomer conversions, the copolymer composition, and molecular weight in an emulsion system. The capabilities of EPM are demonstrated by comparisons of its predictions with experimental data from the literature covering styrene and styrene/methyl methacrylate polymerizations. EPM can successfully simulate continuous and batch reactors over a wide range of initiator and added surfactant concentrations. 

Vinyl ethers constitute a third class of monomers which have been cationically polymerized in C02. While fluorinated vinyl ether monomers such as those described in Sect. 2.1.2 can be polymerized homogeneously in C02 because of the high solubility of the resulting amorphous fluoropolymers, the polymerization of hydrocarbon vinyl ethers in C02 results in the formation of C02-insoluble polymers which precipitate from the reaction medium. The work in this area reported to date in the literature includes precipitation polymerizations and does not yet include the use of stabilizing moieties such as those described in the earlier sections on dispersion and emulsion polymerizations (Sect. 3). 

Compositionally uniform copolymers of tributyltin methacrylate (TBTM) and methyl methacrylate (MMA) are produced in a free running batch process by virtue of the monomer reactivity ratios for this combination of monomers (r (TBTM) = 0.96, r (MMA) = 1.0 at 80°C). Compositional ly homogeneous terpolymers were synthesised by keeping constant the instantaneous ratio of the three monomers in the reactor through the addition of the more reactive monomer (or monomers) at an appropriate rate. This procedure has been used by Guyot et al 6 in the preparation of butadiene-acrylonitrile emulsion copolymers and by Johnson et al (7) in the solution copolymerisation of styrene with methyl acrylate. 

Dispersion polymerization involves an initially homogeneous system of monomer, organic solvent, initiator, and particle stabilizer (usually uncharged polymers such as poly(A-vinyl-pyrrolidinone) and hydroxypropyl cellulose). The system becomes heterogeneous on polymerization because the polymer is insoluble in the solvent. Polymer particles are stabilized by adsorption of the particle stabilizer [Yasuda et al., 2001], Polymerization proceeds in the polymer particles as they absorb monomer from the continuous phase. Dispersion polymerization usually yields polymer particles with sizes in between those obtained by emulsion and suspension polymerizations—about 1-10 pm in diameter. For the larger particle sizes, the reaction characteristics are the same as in suspension polymerization. For the smallest particle sizes, suspension polymerization may exhibit the compartmentalized kinetics of emulsion polymerization. 

Polymerization reactions in aqueous medium can be carried out in homogeneous solution if the monomers and the polymers are soluble in water as in the case of acrylamide or methacrylic acid (see Examples 3-5,3-9, and 3-35). Since most of the monomers are only sparingly soluble in water, suspension or emulsion techniques have to be applied in these cases. 

Suspension polymerization may be the most important particle-forming polymerization from an industrial viewpoint. The system is very simple, composed of monomer, initiator, stabilizer, and medium (water in most cases). The monomer droplets with dissolving initiator are dispersed in water and the stabilizer exists at the interface. But suspension polymerization is regarded as a kind of homogeneous polymerization because the polymerization occurs only in monomer droplets and water does not affect the polymerization. Water contributes only to dividing the polymerization locus into small droplets and absorbing the heat evolved by polymerization. On the contrary, in emulsion polymerization, which is another type of polymerization performed in water and as practically important as suspension polymerization, water affects the polymerization significantly. In this section, emulsion polymerization is first discussed, and then some modified emulsion polymerizations such as soap-free emulsion polymerization and micro and mini emulsion polymerizations are described. 

In homogeneous copolymerization, the instantaneous composition of copolymer is decided only by monomer reactivity ratio. On the contrary, in emulsion copolymerization, the copolymer composition depends not only on the monomer reactivity ratio but also on the distribution of monomers between oil (polymer-monomer particles) and aqueous phases (18). 

---