Conventional emulsion

Higher alkyl acrylates and alkyl-functional esters are important in copolymer products, in conventional emulsion appHcations for coatings and adhesives, and as reactants in radiation-cured coatings and inks. In general, they are produced in direct or transesterification batch processes (17,101,102) because of their relatively low volume. 

Preparation of uniform seed particles Soapless emulsion polymerization is usually preferred for the preparation of uniform seed particles since this technique provides emulsifier-free, larger, and highly uniform micropheres relative to those that can be obtained by the conventional emulsion recipes including emulsifiers and various additives. The size of uniform seed particles with the soapless emulsion procedure is in the range of 0.6-1.2 tm depending on the polymerization conditions [75,108]. 

Microemulsion and miniemulsion polymerization differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 inn)4" and there is no monomer droplet phase. All monomer is in solution or in the particle phase. Initiation takes place by the same process as conventional emulsion polymerization. 

Microemulsion and miniemulsion polymerization processes differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 ran)77 and there is no discrete monomer droplet phase. All monomer is in solution or in the particle phase. Initiation usually takes place by the same process as conventional emulsion polymerization. As particle sizes reduce, the probability of particle entry is lowered and so is the probability of radical-radical termination. This knowledge has been used to advantage in designing living polymerizations based on reversible chain transfer (e.g. RAFT, Section 9.5.2)." 2... 

Some of the issues associated with RAFT emulsion polymerization have been attributed to an effect of chain length-dependent termination.528 In conventional emulsion polymerization, most termination is between a long radical and a short radical. For RAFT polymerization at low conversion most chains are short thus the rate of termination is enhanced. Conversely, at high conversion most chains are long and the rate of termination is reduced. 

Fig. 5.52. Coating of a pigment during conventional emulsion polymerisation.

Complex formation takes place in an organic solvent or in a water/monomer mixture by reaction of the macroligand with a metal compound (e.g. a Cu(I)-ha-lide). It is supposed that the conditions in the reaction mixture are comparable to those in conventional emulsion polymerization, where monomer droplets stabilized by surfactant molecules coexist with monomer swollen micelles [64]. Reaction sites are presumably the hydrophobic core of the micelles and the monomer droplets as well. Initial results of the micellar-catalyzed ATRP of methyl methacry-... 

In the conventional emulsion polymerization, a hydrophobic monomer is emulsified in water and polymerization initiated with a water-soluble initiator. Emulson polymerization can also be carried out as an inverse emulsion polymerization [Poehlein, 1986]. Here, an aqueous solution of a hydrophilic monomer is emulsified in a nonpolar organic solvent such as xylene or paraffin and polymerization initiated with an oil-soluble initiator. The two types of emulsion polymerizations are referred to as oil-in-water (o/w) and water-in-oil (w/o) emulsions, respectively. Inverse emulsion polymerization is used in various commerical polymerizations and copolymerizations of acrylamide as well as other water-soluble monomers. The end use of the reverse latices often involves their addition to water at the point of application. The polymer dissolves readily in water, and the aqueous solution is used in applications such as secondary oil recovery and flocculation (clarification of wastewater, metal recovery). 

Nonionic surfactants such as sorbitan monooleate yield more stable emulsions than do ionic surfactants, However, the latices from inverse emulsion polymerizations are generally less stable than those from conventional emulsion polymerizations, and flocculation is a problem. 

Copolymer particles can also be prepared from HIPEs [159]. Thus, a HIPE dispersed phase consisting of styrene and methacrylic acid was polymerised to give copolymers. The surface concentration of carboxylic acid groups increased linearly with concentration of methacrylic acid in the feed. The small amount of water present in the concentrated emulsion, relative to conventional emulsion polymerisation, reduces the loss of methacrylic acid, which is highly water-soluble. 

In a conventional emulsion process, the conversion of the monomers to the copolymers reaches 97%. Unreacted monomers in general are problematic with respect to pollution. In particular, unreacted monomeric acrylonitrile (AN) is known to be highly toxic. It has been shown that the conversion can be increased by the addition of certain antioxidants (3). 

Polymerization reactions were principally conventional emulsion types, although solution, bulk, or suspension techniques may also be used. 

Several methodologies for preparation of monodisperse polymer particles are known [1]. Among them, dispersion polymerization in polar media has often been used because of the versatility and simplicity of the process. So far, the dispersion polymerizations and copolymerizations of hydrophobic classical monomers such as styrene (St), methyl methacrylate (MMA), etc., have been extensively investigated, in which the kinetic, molecular weight and colloidal parameters could be controlled by reaction conditions [6]. The preparation of monodisperse polymer particles in the range 1-20 pm is particularly challenging because it is just between the limits of particle size of conventional emulsion polymerization (100-700 nm) and suspension polymerization (20-1000 pm). 

Microemulsion polymerizations follow a different mechanism from the conventional emulsion polymerizations. The most probable locus of particle nucle-ation was suggested to be the microemulsion monomer droplets [27], although homogeneous nucleation was not completely ruled out. The particle generation rate in microemulsion polymerization is given by an expression similar to Eq. (21), which was used for the miniemulsion polymerization of styrene [28] ... 

Unlike micelles, an emulsion is a liquid system in which one liquid is dispersed in a second, immiscible liquid, usually in droplets, with emulsiLers added to stabilize the dispersed system. Conventional emulsions possess droplet diameters of more than 200 nm, and are therefore optically opaque or milky. Conventional emulsions are thermodynamically unstable, tending to reduce their total free energy by reducing the total area of the two-phase interface. In contrast, microemulsions with droplet diameters less than 100 nm are optically clear and thermodynamically stable. Unlike conventional emulsions that require the input of a substantial amount of energy, microemulsions are easy to prepare and form spontaneously on mixing, with little or no mechanical energy applied (Lawrence and Rees, 2000). 

In contrast to the conventional emulsions or macroemulsions described earlier are the disperse systems currently termeraiicroemulsions. The term was Lrst introduced by Schulman in 1959 to describe a visually transparent or translucent thermodynamically stable system, with much smaller droplet diameter (6-80 nm) than conventional emulsions. In addition to the aqueous phase, oily phase, and surfactant, they have a high proportion of a cosurfactant, such as an alkanol of 4-8 carbons or a nonionic surfactant. Whereas microemulsions have found applications in oral use (as described in the next chapter), parenteral use of microemulsions has been less common owing to toxicity concerns (e.g., hemolysis) arising from the high surfactant and cosolvent levels. In one example, microemulsions composed of PEG/ethanol/water/medium-chain triglycerides/Solutol HS15/soy phosphatidylcholine have been safely infused into rats at up to 0.5 mL/kg. On dilution into water, the microemulsion forms a o/w emulsion of 60-190 nm droplet size (Man Corswant et al., 1998). 

Physical stability. As indicated earlier, conventional emulsions are inherently unstable from a physical standpoint. Poor physical stability is ultimately exhibited by phase separation, which can be visually monitored. Certain properties of the emulsion will start to change long before this separation is visually apparent. An increase in particle size is particularly indicative of physical instability, since this monitors the coalescence or Locculation that is part of the process involved in ultimate phase separation. Increases in viscosity (due to Locculation) and changes in zeta potential (arising from a decrease in droplet surface area) are both indicative of poor physical stability. The presence of drug and cosolvents can potentially hasten the phase separation. 

Self-Emulsifying Systems Emulsion systems have the disadvantage of being physically unstable, and over time a separation between the oil and water phases of the emulsion will occur. The use of conventional emulsions is also less attractive due to poor precision of the taken dose and the relatively large volume that has to be administered. To overcome these limitations, self-emulsifying drug delivery systems (SEDDS) have been developed. The... 

The environmentally benign, nontoxic, and nonflammable fluids water and carbon dioxide (C02) are the two most abundant and inexpensive solvents on Earth. Water-in-C02 (w/c) or C02-in-water (c/w) dispersions in the form of microemulsions and emulsions offer new possibilities in waste minimization for the replacement of organic solvents in separations, reactions, and materials formation processes. Whereas the solvent strength of C02 is limited, these dispersions have the ability to function as a universal solvent medium by solubilizing high concentrations of polar, ionic, and nonpolar molecules within their dispersed and continuous phases. These emulsions may be phase-separated easily for product recovery (unlike the case for conventional emulsions) simply by depressurization. 

Research (Fontenot and Schork 1993a, b) indicates that miniemulsion polymerization can provide benefits over the current process technology of conventional emulsion polymerization. Among these are a process which is much more robust to contamination and operating errors, a more uniform copolymer composition when used for copolymerization, and a final product which is far more shear-stable than the product of conventional emulsion polymerization. 

Microemulsions are defined as isotropic, transparent, and thermodynamically stable (in contrast to conventional emulsions) mixtures of a hydrophobic phase (lipid), a hydrophilic phase (often water), a surfactant, and in many cases a co-surfactant. From a lipid formulation perspective, microemulsions are generally regarded as the ultimate extension of the decreased particle size/increased surface area mantra, because emulsion particle sizes are usually less than 50 nm. Microemulsions also have additional pharmaceutical advantages in terms of their solubilizing capacity [54, 55], thermodynamic stability, and capacity for stable, infinite dilution. 

Microemulsion polymerization, as the name implies, involves free-radical polymerization in extremely small size, microemulsified monomer droplets of about 1-10 nm diameter [792], The produced polymer particles tend to be small (10 nm) and have higher molar mass (106-107 g/mol) than can be obtained from conventional emulsion polymerization [792]. 

Develop conventional emulsion films for 3 minutes in each solution do not rinse in-between. Develop T-grain films for 4 minutes in each bath. Use continuous agitation in both baths. 

One major concern regarding the safety profile of ME systems intended for oral administration is the comparatively high amphiphile content. Both o/w and w/o ME systems are amphiphile-rich systems compared to conventional emulsions and would contain in the most conservative case up to 15-20% w/w surfactant-cosurfactant. This is further complicated by the limited models available to evaluate chronic toxicology in comparison to conventional oral dosage forms such as tablets [91]. 

Several studies have reported on the enhanced bioavailability of cutaneous drugs using o/w and w/o MEs compared to conventional emulsions, gels or solutions, mesophases, micellar and inverse micellar systems, and vesicles [93], Moreover, a diverse range of drug molecules such as ketoprofen, apomorphine, estradiol, lido-caine [94-97], indomethacin and diclofenac [98], prostaglandin Ei [99], aceclofenac... 

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