Particle size in emulsions

Under suitable conditions a reasonably stable latex may be formed (/). Although many of the factors relating to polymerization rate, molecular weight and particle size in emulsion have been studied [reference (134) is an example] the literature on homopolymers is by no means as extensive as that pertaining to copolymers. Attempts to prepare bead homopolymers have not been very successful. 

Particle Size in Emulsions When a solid drug is suspended in an emulsion, the liquid dosage form is known as a coarse dispersion. In addition, a colloidal dispersion has solid particles as small as 10nm-5pm and is considered a liquid between a true solution and a coarse dispersion [44],... 

The dependence of the risk parameters on process variables such as the concentrations of monomer, polymer, initiator or catalyst, solvent, water and particle size (in emulsion) and MWD are of paramount importance to establish the safe operation regions of polymerization reactors, and furthermore to develop optimal control strategies imder safe conditions. The maximum pressure, Pmax> and maximum temperature, Tmax achieved during the runaway depends on the process conditions (e.g., the higher the amount of monomer in the reactor and the process temperature, the higher Pmax and Tmax)- Also important is the rate at which the runaway reaches the maximum pressures and temperatures. This rate will provide an indication of the time that the operator/control system of the plant has to react in order to keep the polymerization imder safe conditions. 

Ultrasonic spectroscopy allows measuring a wide variety of liquid systems, from dilute to concentrated solutions, and can be used to monitor processes such as molecular structural changes, thermal transitions, chemical reactions, aggregation formation, crystallisation, etc. Attenuation measurements are used for particle sizing in emulsions and suspensions and for kinetics of fast chemical reactions. 

Raman has been used to follow monomer concentrations [2, 66-68]. However, it has been shown that Raman spectra could be affected by particle size in emulsion polymerization [69, 70]. Many of the foregoing referenced works, and others, combine IR and Raman measurements [71, 72]. Raman has not yet been applied to ACOMP although such application is expected soon. 

The control of the particle size in emulsion polymerization using closed-loop strategies is a very attractive yet challenging problem [1]. Difficulties associated with online measurement of the particle size together with the complex mechanisms involved in emulsion polymerization systems limit the options and make control implementation a formidable task. In many cases, conventional optimization strategies fail to ensure a consistent product quality with the result that industries rely on traditional recipes and experience. 

The foregoing indicates that the implementation of process optimization for the optimal control of emulsion polymerization processes is relying on technological advancement in the areas of mathematical process modeling, soft-sensing, and model-based control. This chapter illustrates some of the most successful approaches and their application to the control of the particle size in emulsion polymerization. 

There are two principal PVC resins for producing vinyl foams suspension resin and dispersion resin. The suspension resin is prepared by suspension polymerization with a relatively large particle size in the 30—250 p.m range and the dispersion resin is prepared by emulsion polymerization with a fine particle size in the 0.2—2 p.m range (245). The latter is used in the manufacture of vinyl plastisols which can be fused without the appHcation of pressure. In addition, plastisol blending resins, which are fine particle size suspension resins, can be used as a partial replacement for the dispersion resin in a plastisol system to reduce the resin costs. 

Because of this interaction, PVP has found appHcation in surfactant formulations, where it functions as a steric stabilizer for example to generate uniform particle-size polystyrene emulsions (110—112). In a variety of formulations, a surfactant s abiHty to emulsify is augmented by PVP s abiHty to stabilize coUoids stericaHy and to control rheology. 

An example of liquid/liquid mixing is emulsion polymerization, where droplet size can be the most important parameter influencing product quality. Particle size is determined by impeller tip speed. If coalescence is prevented and the system stability is satisfactory, this will determine the ultimate particle size. However, if the dispersion being produced in the mixer is used as an intermediate step to carry out a liquid/liquid extraction and the emulsion must be settled out again, a dynamic dispersion is produced. Maximum shear stress by the impeller then determines the average shear rate and the overall average particle size in the mixer. 

VS Koster, PFM Kuks, R Langer, H Talsma. Particle size in parenteral fat emulsions, what are the true limitations Int J Pharm 134 235-238, 1996. 

The other major type of catalytic reactor is a situation where the fluid and the catalyst are stirred instead of having the catalyst fixed in a bed. If the fluid is a liquid, we call this a slurry reactor, in which catalyst pellets or powder is held in a tank through which catalyst flows. The stirring must obviously be fast enough to mix the fluid and particles. To keep the particles from settling out, catalyst particle sizes in a slurry reactor must be sufficiently small. If the catalyst phase is another Hquid that is stirred to maintain high interfacial area for reaction at the interface, we call the reactor an emulsion reactor. These are shown in Figure 74. 

Acetonitrile was the most efficient of all dipolar aprotic solvent for the formation of monodisperse particles. Figure 1.2.6 shows the relation between solubility curves of metal alkoxide and particle size in octanol/acetonitrile solution. The solubility of metal alkoxide in octanol gradually decreased with increasing acetonitrile concentration. Metal alkoxide was precipitated to form an emulsion when the acetonitrile was added at more than 40 vol%. The particle size increased with increasing... 

Data in Table I show that emulsion capacity of peanut flour decreased with increasing flour or protein concentration while emulsion viscosity increased. This phenomenon was also demonstrated by McWatters and Holmes (2D. A decrease in flour particle size increased emulsion capacity and viscosity appreciably. Increasing the rate of mixing, however, decreased emulsion capacity but increased viscosity. Increased speeds produce greater shear rate, which decreases the size of the oil droplet thus, there is an increase in the surface area of the oil to be emulsified by the same amount of soluble protein (23, 24). 

For accurate measurements it is important to use an electrolyte solution that does not contain extraneous particulate matter and that does not promote droplet aggregation. In addition, it is important to use an aperture that is appropriate for the range of particle sizes in the sample being analyzed. Typically, the diameter of the emulsion droplets should be between 2% and 60% of the diameter of the aperture in order to obtain accurate measurements. If the particles cover a large particle size range, it may be necessary to use two or more tubes with different apertures, and overlap the results. 

Most ABS is made by emulsion polymerization. A polybutadiene or nitrile rubber latex is prepared, and styrene plus acrylonitrile are grafted upon the elastomer in emulsion. The effect of rubber particle size in ABS graft copolymer on physical properties is the subject Chapter 22 by C. F. Parsons and E. L. Suck. Methyl methacrylate was substituted for acrylonitrile in ABS by R. D. Deanin and co-workers. They found a better thermoprocessability, lighter color, and better ultraviolet light stability. 

The rubber particle size in the final product increases several fold if the prepolymerization is carried out in the presence of a dilute aqueous solution of an alkane sulfonate or polyvinyl alcohol in place of pure water. The addition of a surface-active agent converts the coarsely dispersed oil-water mixture—obtained as above in the presence of pure water—into an oil-in-water emulsion. In this case even prolonged stirring during prepolymerization does not decrease the rubber particle size appreciably in the final product. The stabilization of the droplets of the organic phase in water by the emulsifier obviously impedes or prevents agitation within the polymeric phase. Figure 1 shows the influence of these three prepolymerization methods (under otherwise equal reaction conditions) on the dispersion of rubber particles in polystyrene. 

The size of the monomer droplets plays the key role in determining the locus of particle nucleation in emulsion and miniemulsion polymerizations. The competitive position of monomer droplets for capture of free radicals during miniemulsion polymerization is enhanced by both the increase in total droplet surface area and the decrease in the available surfactant for micelle formation or stabilization of precursors in homogeneous nucleation. 

The performance of a fluidized bed combustor is strongly influenced by the fluid mechanics and heat transfer in the bed, consideration of which must be part of any attempt to realistically model bed performance. The fluid mechanics and heat transfer in an AFBC must, however, be distinguished from those in fluidized catalytic reactors such as fluidized catalytic crackers (FCCs) because the particle size in an AFBC, typically about 1 mm in diameter, is more than an order of magnitude larger than that utilized in FCC s, typically about 50 ym. The consequences of this difference in particle size is illustrated in Table 1. Particle Reynolds number in an FCC is much smaller than unity so that viscous forces dominate whereas for an AFBC the particle Reynolds number is of order unity and the effect of inertial forces become noticeable. Minimum velocity of fluidization (u ) in an FCC is so low that the bubble-rise velocity exceeds the gas velocity in the dense phase (umf/cmf) over a bed s depth the FCC s operate in the so-called fast bubble regime to be elaborated on later. By contrast- the bubble-rise velocity in an AFBC may be slower or faster than the gas-phase velocity in the emulsion... 

The cationic Surfmers produced much smaller particle sizes in the emulsion polymerization of styrene and styrene/butyl acrylate than the amphoterics (20-50 nm versus 100-300 nm). Some of the latter, however, conferred to the copolymer lattices stability to electrolytes and freeze-thaw [24]. Similar, but nonreactive surfactants produced from succinic anhydride gave similar stability but had much inferior water resistance [25]. 

---