Emulsion Polymerisation Processes

Emulsion polymerisations are heterogeneous polymerisation processes, which unlike bulk or solution polymerisations, are not uniform in composition throughout the reaction medium, but rather contain polymerising colloidal particles dispersed within an otherwise inert aqueous environment. There are three 

---

In the late 1920s Bayer Company began reevaluating the emulsion polymerisation process of polybutadiene as an improvement over their Buna technology, which was based on sodium as a catalyst. Incorporation of styrene (qv) as a comonomer produced a superior polymer compared to polybutadiene. The product Buna S was the precursor of the single largest-volume polymer produced in the 1990s, emulsion styrene—butadiene mbber... 

Poly(vinyl chloride) is commercially available in the form of aqueous colloidal dispersions (latices). They are the uncoagulated products of emulsion polymerisation process and are used to coat or impregnate textiles and paper. The individual particles are somewhat less than 1 p,m in diameter. The latex may be coagulated by concentrated acids, polyvalent cations and by dehydration with water-miscible liquids. 

Since poly(vinyl acetate) is usually used in an emulsion form, the emulsion polymerisation process is commonly used. In a typical system, approximately equal quantities of vinyl acetate and water are stirred together in the presence of a suitable colloid-emulsifier system, such as poly(vinyl alcohol) and sodium lauryl sulphate, and a water-soluble initiator such as potassium persulphate. 

Polymerisation of a monomer or mixture of monomers which have been emulsified with soap in water. SBR, neoprene and nitrile rubbers are manufactured by the emulsion polymerisation process. 

A larger elementary particle size can be achieved by seeding the initial emulsion system. A PVC polymer latex is introduced and the particles of the new polymer grow on the seed. A continuous emulsion polymerisation process is also used. 

VCM and an emulsifier. These components are circulated through a mixing pump (homogeniser) which causes the mixture to disperse into very fine droplets, before being placed in the autoclave. The droplets are coated with surfactant which stabilises them during the reaction. Initiation and polymerisation occur within the droplets. After polymerisation, the autoclave contains a stable dispersion of fine particles of PVC in water. Thereafter the subsequent operations for obtaining the final product are similar to the emulsion polymerisation process. 

The emulsion polymerisation process (after EVC International NV with permission)... 

The particles are heterogeneous by definition. As with singlepolymer particles, heterogeneities in density can arise as a result of the mechanism of particle formation. Usually a polymer has a different density to the liquid monomer from which it is derived, and in most emulsion polymerisation processes to produce reasonably concentrated dispersions, propagation is dominated by arrival at the particle surface of oligomeric radicals which can lead to non-homogeneous shrinking within the particle. 

Vautz, W. Mauntz, W. Engell, S. Baumbach, J.I., Monitoring of emulsion polymerisation processes using ion mobility spectrometry—a pilot study, Macromol. React. Eng. 2009, 3(2-3), 85-90. 

Recovery of solid polymer can be obtained by coagulation of the latex. As the size of the particles is submicronic, recovery by filtration without precipitation is not possible. Separation without precipitation cannot be obtained by usual centrifugation but requires ultracentrifugation. The emulsion polymerisation process is very well adapted for production of large amounts of polymeric colloids used in the paint industry by polymerisation of acrylic and methacrylic monomers, i.e. acrylic paints. Similarly, poly(alkylcyanoacrylate) nanoparticles can be obtained by such a process. 

This article will provide a general overview of the emulsion polymerisation process and explain how the resulting latexes are used in industrial applications. An introduction to the basic concepts of emulsion polymers will be given, followed by a description of the various production processes and characterisation methods. The classes of emulsion polymers will be surveyed, and the commercial technologies and potential future uses discussed. A number of comprehensive texts on emulsion polymers are available for more in-depth study (60, 89, 94,95, 364, a.l-a.ll). 

Monomers are of principal interest in emulsion polymerisation, and must be chosen based on the performance requirements of the intended application. Cost is another critical factor in the selection of an appropriate monomer. The monomer cannot be completely miscible with the water phase (otherwise it would be a dispersion polymerisation), nor can it be completely insoluble (or polymerisation by conventional emulsion polymerisation could not proceed). Most monomers are sparingly soluble in water and fall within these guidelines. Typical monomers used in emulsion polymerisation processes are the styrenics (149, 353), acrylics (141), methacrylics (309), vinyl acetate (164), vinyl chloride (363), acrylonitrile (152), butadiene (307), ethylene (114), as well as various speciahty (100) and functional monomers (332). 

The size, stmcture, and composition of latex particles depend not only upon the components used in the emulsion recipe, but also upon how the components were combined and polymerised. The emulsion polymerisation process can be modified by controlling the feed strategy to produce latex particles with specific features. Each feed strategy has its advantages and disadvantages, and the optimal one must be selected based upon individual circumstances. The major feed strategies are listed next and are illustrated in Figure 3. 

In the continuous emulsion polymerisation process (295), a monomer emulsion is fed at a constant rate to... 

Since emulsion polymerisation is generally a sensitive process, care must be taken to ensure the consistency of the final product from batch to batch or during a single continuous operation. Furthermore, emulsion polymers are products by process , meaning that the manner in which the polymerisation is carried out is perhaps more important than the raw materials in determining the form of the final product. For this reason, it is desirable to monitor the emulsion polymerisation process on-line (although in practice, many commercial producers measure properties after... 

Acrylonitrile is also commonly found in impact modifiers, such as the acrylonitrile-butadiene-styrene (ABS) type, produced by emulsion polymerisation. Polybutadiene seed latex particles are grafted onto styrene and acrylonitrile in a seeded emulsion polymerisation process. As the styrene-acrylonitrile copolymer shell forms, polybutadiene domains are spontaneously separated within. The resulting impact modifier particles are subsequently compounded with polystyrene to product high impact polystyrene (HIPS). The impact modification properties of the latex particles may be optimised through varying the butadiene content, the particle size and structure, and the shell molecular weight. A basic formulation for an ABS impact modifier is given in Table 6. 

Water is present as a consequence of the emulsion polymerisation process used in preparing the latex, and provides liquidity and spreadability so that it may be distributed evenly. Water-based paints are much less harmful to the environment than solvent-based paints, and this is what drives the trend towards the use of water-based paints. The performance of oil-based paints is generally better because the solvent acts as a plasticiser (197), and aids in the film formahon process. The performance of the water-based paints can approach that of the solvent-based paints through careful formulation of the latex. Paints are usually formulated to contain 60 to 70 volume percent water. 

Emulsion polymer latexes are essential components in many of the commercial applications encountered daily. The emulsion polymerisation process is unique and involves many complex and interrelated chemical and physical phenomena. Emulsion polymerisation is a flexible process by which a wide range of practical materials can be made, and in each case, the process is tailored to optimise the performance properties of the final product. Although the emulsion polymers field is mature, there still exist many new possibilities for polymer colloids. 

Monodispersed PS latex colloids with aldehyde and acetal functionalities were synthesised by a two-step emulsion polymerisation process. The samples were chosen as the polymeric support to carry out covalent coupling with the antibody IgG anti-C reactive protein. 14 refs. 

A review is presented of topics discussed at the 2nd International Symposium on Free Radical Polymerisation, held in Santa Margherita Figure, Italy, on 26th-31st May 1996. These included methods for the control of molecular weight and molecular structure, control of particle size and particle size distribution in emulsion polymerisation processes, and results of experimental and theoretical studies of polymerisation mechanisms and kinetics. 

Heterogeneous latices were prepared by a two-stage seeded emulsion polymerisation process at 80C using potassium persulphate as initiator and sodium dodecyl sulphate as emulsifier. Styrene-methacrylic acid (MAA) copolymer latices containing varying amounts of MAA were used as seeds. The second stage polymerisation was performed either as a seeded batch process or as a seeded... 

The packing efficiency of a monodisperse latex may be increased by the addition of one or more latices of smaller particle size. Hence certain blends of PVC latices can be used to produce plastisols with low viscosity over a wide range of shear rates (5). However it is more usual to produce multi-disperse particle size distributions at the polymerisation stage in a seeded emulsion polymerisation process. 

Figure 2 Predicted and Pilot Plant data for the particle size distributions in a seeded PVC emulsion polymerisation process Reproduced from K W Min and H I Gostin, Ind Eng Chem Prod Res Dev 18 (4), (1979), 272-278 by permission of the publishers, American Chemical Society (C).

Redox systems are used to particular effect in emulsion polymerisation processes where the lower temperatures involved preclude the use of peroxides which form radicals at higher temperatures. They, are also useful where gaseous monomers (e.g. vinyl chloride, ethylene) are involved. 

Multi-component Redox systems which generate both free radicals and radical ions are often employed in emulsion polymerisation processes ... 

---