Latex system, feeding

M ass Process. In the mass (or bulk) (83) ABS process the polymerization is conducted in a monomer medium rather than in water. This process usually consists of a series of two or more continuous reactors. The mbber used in this process is most commonly a solution-polymerized linear polybutadiene (or copolymer containing sytrene), although some mass processes utilize emulsion-polymerized ABS with a high mbber content for the mbber component (84). If a linear mbber is used, a solution of the mbber in the monomers is prepared for feeding to the reactor system. If emulsion ABS is used as the source of mbber, a dispersion of the ABS in the monomers is usually prepared after the water has been removed from the ABS latex. 

The latexes were prepared using a conventional semi-batch emulsion polymerization system modified for power-feed by the addition of a second monomer tank. Polymerization temperatures ranged from 30-85°C using either redox or thermal initiators. Samples were taken periodically during the polymerization and analyzed to determine residual monomer in order to assure a "starved-feed" condition. As used in this study this is a condition in which monomer feed rate and polymerization rate are identical and residual monomer levels are less than 5%. 

Synthesis. A series of latexes was prepared by semicontinuous emulsion polymerization of methyl methacrylate. A dialkyl ester of sodium sulfosuccinic acid surfactant yielded the narrow particle size distribution required. An ammonium persulfate/sodium metabisulfate/ferrous sulfate initiator system was used. The initiator was fed over the polymerization time, allowing better control of the polymerization rate. For the smaller size latexes (200 to 450 nm), a seed latex was prepared in situ by polymerizing 10% of the monomer in the presence of the ammonium persulfate. Particle size was adjusted by varying the level of surfactant during the heel reaction. As the exotherm of this reaction subsided, the monomer and the sodium metabisulfate/ferrous sulfate feeds were started and continued over approximately one hour. The... 

Emulsion polymerization reactions are sometimes carried out with small seed particles formed in another reaction system. A number of advantages can he derived from using seed particles. In a batch reactor seed latex can he helpful hi controlling particle concentration, polymerization rate, particle morphology, and particle size characteristics. In a CSTR the use of a feed stream containing seed particles can also help to prevent conversion and/or surface tension oscillations, which are caused by particle formation phenomena, This factor will be discussed in more detail later in this chapter. 

If the emulsifier feed and initiation system are formulated to yield a long nucleatlon period, a relatively broad PSD latex can be produced. In extreme cases of delayed-emulslfier feed, multiple nucleatlon periods will result, and even broader PSDs can be produced. Broad distributions can be an asset when high-solids, low-visibility products are desired. 

Continuous Reactors. A variety of continuous reactor systems are used commercially, but the most common are comprised of a number of stirred-tank reactors (CSTR) connected in series. Operation normally Involves pumping all ingredients into the first CSTR and removing the partially converted latex from the final reactor. Intermediate feed streams can also be employed. Detailed reviews... 

The methods used for introducing feed streams into continuous reactors can be quite important. All ingredients are charged and mixed before the latex is formed in most batch reactor processes. The major purposes of mixing after the reaction begins are to facilitate heat removal through the cooling surface and to maintain mass transfer from the monomer phase to the polymer particles. With a CSTR reaction system, however, the feed streams are added to partially converted latexes, and other factors need to be considered. 

Large irregular-shaped objects that are impossible to coat by dipping can be protected by spraying. All the liquid vinyls can be handled in this manner. Low-viscosity systems such as the solution vinyls and latexes can be handled easily on conventional suction feed equipment. Because of the acidic nature of vinyl latexes, the materials of construction of the spray equipment will have to receive special consideration. 

If the feed is preconditioned properly, the UF flux is often quite stable. One thousand hours of continuous operation between cleanings is common. When flux decay does occur, detergent washing is usually sufficient to restore flux. In some cases, polymer solvents may be required. Proper selection of a solvent resistant membrane and/or solvents which will dissolve the latex but not affect the membrane is crucial. For PVC latex, the solvents of choice are methyl isobutyl ketone (MIBK) and methyl ethyl ketone (MEK). Styrene butadiene rubber will swell in MIBK, MEK or toluene. Polyvinyl acetate will dissolve in the low MW alcohols such as propyl alcohol. Generally, the membranes are first washed with water, then detergent, followed by another water flush. The system is then drained of all water since it will affect polymer solubility in MEK. Finally, a solvent rinse is employed. If the module is tubular, sponge balls will enhance cleaning. 

With the objective of promoting polymer formation at the surface of Ti02 pigments and prevent secondary nucleation, Haga et al. used a diazoic amidinium initiator previously anchored on the mineral surface [213], whereas Janssen used redox initiators [208]. Although real benefit was taken from the nature of the initiator, in particular in the case of hydrophilic monomers like MMA, there was still a competition between the formation of surface polymer and free latex particles in these systems. In both cases, better results were obtained when the monomer was introduced under starved-feed conditions, which enabled a significant decrease in the extent of secondary nucleation. 

Uses Corrosion/scale inhibitor used in various aq. systems (boiler feed water, recirculated cooling water) over wide temp, range dispersant for dyes, pigments, latex paints food pkg. adhesives, paper/paperboard Features Inhibits scale deposits on equip, surfs., promotes efficient heat transfer, decreases equip, damage Reguiatory FDA 21CFR 175.105,176.180 Properties Pale yel. clear liq. m.w. 15,000 dens. 9.2 Ib/gal vise. 19-182 cps f.p. 28 F pH 9.5-10.5 (10% aq.) surf. tens. 64 dynes/cm (0.1%) 24-26% total solids... 

Wu and Zhao studied LIPN systems by a two-stage emulsion polymerization technique (Wu and Zhao 1995). A latex seed (polymer 1) was synthesized first in a semicontinuous emulsion polymerization, swollen by the second-stage monomer or monomer mixture (forming polymer 2), and followed by polymerization to form IPN materials. Six kinds of monomers were used acrylonitrile (AN), vinyl acetate (VAc), n-butyl acrylate (liBA), methyl methacrylate (MMA), ethyl methacrylate (EMA), and ethyl acrylate (EA). The effect of composition, cross-linking level, feeding sequence of polymer 1 and polymer 2 on the IPN miscibility, and... 

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