Polymerization, dispersion continued

AF CHEMICALS CORF. ALKASPERSE Polymeric Dispersants (Continued) ... 

Fig. 1 Schematic representation of the film formation process for an aqueous polymeric dispersion (A) atomization of the polymeric dispersion (B) deposition of the polymeric dispersion on the substrate surface (C) packing of the polymer spheres with water filling the void spaces (D) formation of continuous polymeric film.

Polymeric surfactant Continuous phase Disperse polymer... 

The nonreactive NAD are structures without polymerisable double bonds, but have a remarkable stabilisation effect on polymeric dispersions in polyether media. The main characteristic of such kinds of nonreactive NAD is to have an organic segment with high affinity for the carbocatenary vinylic polymer chemically linked to a high molecular weight polyether chain, which has a high affinity for the continuous liquid polyether matrix. 

As water-based paints become much more widespread in use, then the challenges to achieving a direct match to solvent based paints become more acute. For example, automotive coloured base coats in Europe are mainly water based whilst repair paints are still mainly solvent based. A key target for water-based auto paints is to achieve the same colour properties as solvent-based and in a number of areas this still represents a challenge to the industry. This paper discusses three areas where the continuing development of polymeric dispersants can help to meet the high performance challenge. 

A microemulsion polymerization method [62,63] was also reported to produce magnetic polypyrrole nanocomposites filled with 7-Fc203. The nanoparticles were dispersed in the oil phase. FeCla was used as an oxidizing agent. Sodium dodecylbenzenesulfonic acid (SDBA) and butanol were used as the surfactant and cosurfactant, respectively. FeCl3 (0.97 g) was dissolved in a mixture of 15 mol deionized water, SDBA (6 g), and butanol (1.6 ml). A specific amount of 7-Fc203 suspended nanoparticle solution was added to the above solution for dispersion. Pyrrole was added for nanocomposite polymer fabrication in the microemulsion system. The polymerization was continued for 24 hours and quenched by acetone. 

Dispersion polymerization is a technique which permits the preparation of polymer particles in the range 1-15 microns in a rapid process with high convosions. Li a dispersion polymerization, the continuous phase is chosen to be a solvent for the monomer to be polymerized, and a non-solvent for the resultant polymer. A steric stabilizer is used to produce a colltxdally stable dispersion (see Section 3.6). In the absence of this stabilize, the polymerization produces macroscopic particles of polymer of an uncontrolled size, and is thra known as precipitation polymerization [1]. 

It may be that the polymer is insoluble in the monomer-solvent mixture from which it is formed. Polypropylene and PVC are two examples where the polymer has very limited solubility in the monomer. As polymerization proceeds, the polymer will precipitate from the reacting mass to form a dispersed phase of polymer swollen with the monomer-solvent mixture. This is called a slurry polymerization. (Phase inversion can occur at high conversions to give a bulk polymerization.) A typical slurry polymerization is autorefrigerated. The heat of polymerization causes the reacting mass to boil it is condensed and returned to the reactor. The gas-phase processes for polyethylene and polypropylene are conceptually similar to slurry polymerizations. The continuous phase is now a gas and the dispersed phase is a fluidized solid, but the heat of polymerization is still removed through the low-viscosity, continuous phase. 

Heterophase polymerization systems can be defined as two-phase systems in which the resulting polymer and/or starting monomer are in the form of a fine dispersion in an immiscible liquid medium defined as the polymerization medium , continuous phase , or outer phase . Even if oil-in-water (o/w) systems are greatly preferred on an industrial scale, water-in-oil (w/o) systems may also be envisaged for specific purposes. Heterogeneous polymerization processes can be classified as suspension, dispersion, precipitation, emulsion, or miniemulsion techniques according to interdependent criteria which are the initial state of the polymerization mixture, the kinetics of polymerization, the mechanism of particle formation and the size and shape of the final polymer particles (Fig. 4.2) [18]. 

In a classical heterogeneous dispersion polymerization, the continuous phase is organic in nature, althou in some instances water has been used as a component of the continuous phase to increase polarity. Research investigations have focused on the composition of the dispersion m um, reaction kinetics, the structure and influence of the stabilizer polymer, particle size, molecular weight and molecular weight distribution (4, S). Dispersions of poly(methyl methacrylate) (8) and poly(styrene) (9) are widely studied and among Ae best characterized systems. Recently, dispersion polymerizations conducted in supercritical carbon dioxide have also been reported (9-12). 

Figure 13.2. Dispersion index as a function of monomer conversion for polymerizations with continuous initiation and bimolecular termination (e.g., free radical polymerizations) and no chain transfer, for three reactor types. The initial ratio of propagation and initiation rates is 10. For higher values of this ratio the differences between the curves increase.

Emulsion polymerization also has the advantages of good heat transfer and low viscosity, which follow from the presence of the aqueous phase. The resulting aqueous dispersion of polymer is called a latex. The polymer can be subsequently separated from the aqueous portion of the latex or the latter can be used directly in eventual appUcations. For example, in coatings applications-such as paints, paper coatings, floor pohshes-soft polymer particles coalesce into a continuous film with the evaporation of water after the latex has been applied to the substrate. 

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. 

In order to maintain a definite contact area, soHd supports for the solvent membrane can be introduced (85). Those typically consist of hydrophobic polymeric films having pore sizes between 0.02 and 1 p.m. Figure 9c illustrates a hoUow fiber membrane where the feed solution flows around the fiber, the solvent—extractant phase is supported on the fiber wall, and the strip solution flows within the fiber. Supported membranes can also be used in conventional extraction where the supported phase is continuously fed and removed. This technique is known as dispersion-free solvent extraction (86,87). The level of research interest in membrane extraction is reflected by the fact that the 1990 International Solvent Extraction Conference (20) featured over 50 papers on this area, mainly as appHed to metals extraction. Pilot-scale studies of treatment of metal waste streams by Hquid membrane extraction have been reported (88). The developments in membrane technology have been reviewed (89). Despite the research interest and potential, membranes have yet to be appHed at an industrial production scale (90). 

The inverse emulsion form is made by emulsifying an aqueous monomer solution in a light hydrocarbon oil to form an oil-continuous emulsion stabilized by a surfactant system (21). This is polymerized to form an emulsion of aqueous polymer particle ranging in size from 1.0 to about 10 pm dispersed in oil. By addition of appropriate surfactants, the emulsion is made self-inverting, which means that when it is added to water with agitation, the oil is emulsified and the polymer goes into solution in a few minutes. Alternatively, a surfactant can be added to the water before addition of the inverse polymer emulsion (see Emulsions). 

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