Emulsion polymerization

   Emulsion polymerization is a complex process in which the radical addition polymerization proceeds in a heterogeneous system. This process involves emulsification of the relatively hydrophobic monomer in water by an oil-in-water emulsifier, followed by the initiation reaction with either a water-soluble or an oil-soluble free radical initiator. At the end of the polymerization, a milky fluid called “latex”, “synthetic latex” or “polymer dispersion” is obtained. Latex is defined as “colloidal dispersion of polymer particles in an aqueous medium”. The polymer may be organic or inorganic. In general, latexes contain 40-60 % polymer solids and comprise a large population of polymer particles dispersed in the continuous aqueous phase (about 10¹⁵ particles per mL of latex). The particles are within the size range 10 nm to 1000 nm in a diameter and are generally spherical. A typical of particle is composed of 1-10000 macromolecules, and each macromolecule contains about 100–10⁶ monomer units.

  The earliest literature references to produce synthetic latex (the term first referred to the white, sticky sap of the rubber tree) are patents originated from Farbenfabriken Bayer in the years 1909 to 1912. These studies involved polymerization of dien monomers in the form of aqueous emulsions which are stabilized by gelatin, egg white (protein), starch, flour, and blood serum as protective colloids to produce something resembling natural rubber latex. Initiation of polymerization depended on aerial oxygen.   But these attempts and other similar studies that followed them were substantially different from what is known today as “emulsion polymerization”. In 1929, Dinsmore, who was working for The Goodyear Tire & R ubber Company, was the first to be granted a patent to produce a synthetic rubber in the presence of soap as emulsifying agen.     

  This was followed by addition of a free radical initiator (water- or monomer-soluble peroxides), which led to polymerization of the emulsified monomer. The reasons for regarding it as an “emulsion polymerization” were the addition of soap, presumably added as an emulsifying agent in the first instance, as well as of a protein-aceous protective colloid, and the assuming that polymerization took place in the emulsified monomer droplets. Later, the practice of emulsion polymerization grew rapidly and industrial-scale production started in the mid-1930. The major developments in emulsion polymerization took place around the Second World War as a result of the intensive collaborative efforts between academia, industry and government laboratories.

  During and after World War II, the production of many types of latex both in homopolymers and copolymers of different composition was achieved by using different monomers such as butadiene, styrene, acrylic esters, acrylonitrile and vinyl acetate. A wide variety of initiating systems were used. Conversions of the polymerization reactions were increased. Later, the works of that period were published in reports and books.  Otherwise very few papers on the subject were published in the scientific journals during the period 1910-1945, in comparison with the patents.

   From 1945 to the present, numerous books including the literature of emulsion polymerization, its mechanism, kinetics and formulation, and many other topics related to the emulsion polymerization have been published. In addition, many conferences on the emulsion polymerization have been organized by different institutes since 1966, and the proceedings/books of them have been published. There are also an excessive number of papers on the emulsion polymerization and related subjects in literature. The number of publications on this subject continues to increase steadily.  Looking at the historical development of the emulsion polymerization, it is seen that the trigger factor in this development was the necessity for synthetic rubber in the wartime. The production of styrene/butadiene rubber (SBR) satisfied this requirement. Today, millions of tons of synthetic latexes are produced by the emulsion polymerization process for use in wide variety of applications. In the synthetic latexes, the most important groups are styrene/butadiene copolymers, vinyl acetate homopolymers and copolymers, and polyacrylates. Other synthetic latexes contain copolymers of ethylene, styrene, vinyl esters, vinyl chloride, vinylidene chloride, acrylonitrile, cloroprene and polyurethane.

   Styrene/butadiene latexes account for 37% of the total waterborne synthetic latexes. They are widely used for tires and molded foam. They mostly consist of 70-75% butadiene by weight and 30-25% styrene by weight for use as general-purpose rubbers. Their carboxylated forms contain acrylic, methacrylic, maleic, fumaric or itaconic acid whose carboxylic groups provide stabilization of the polymer particles and a good interaction with fillers and pigments. They are used in carpet-backing and paper-coating applications.

  Styrene/butadiene ratios are commonly 50/50 and 60/40 (by weight). In these compositions, these copolymers are still rubbery at normal ambient temperatures, and the latex particles readily integrate to form coherent films as the latex dries. Styrene/butadiene copolymers become non-rubbery by increasing the styrene content in the copolymer composition, e.g., 85/15 and 90/10 by weight, and are used as organic stiffening and reinforcing fillers. When styrene is replaced by acrylonitrile, elastic and solvent resistant emulsion copolymers are obtained, which are used for dipping goods. Additionally, polychloroprene rubbers obtained by emulsion polymerization offer great resistance to chemicals and atmospheric ozone.

  Acrylic latexes include pure acrylics and styrene acrylics, which are about 30% of produced waterborne synthetic latexes. Acrylic monomers comprise the monomeric alkyl esters of acrylic acid and methacrylic acid, and also their derivatives. The most used acrylic monomers in the emulsion polymerizations are methyl-, ethyl-, butyl- and 2-ethylhexylacrylate, methyl methacrylate, and acrylic- and methacrylic-acid. Homopolymer latexes of these monomers are used as exterior or interior coatings, binder for leather, textiles and paper, and as adhesives, laminates, elastomers, plasticizer and floor polishes. These latexes are stable, have good pigment binding and durability. The copolymerizations of these esters with styrene in an enormous range of accessible copolymer compositions offer almost unlimited opportunities to choose for the glass transition temperature, the minimum film forming temperature, the hydrophilic/hydrophobic properties and morphology design. Vinyl chloride/vinylidene chloride monomers can be polymerized by emulsion polymerization. Poly(vinyl chloride) (E-PVC) product is mostly applied as the dried form. It is spray-dried and milled to form fine powders (“crumbs”) which is mixed with plasticizer form a plastisol, i.e., dispersion of poly(vinyl chloride) particles in liquid organic media.

  The plastisol is poured into molds to make rubber dolls, shower curtains, embossed wall coverings and many of other common objects. In packing materials, especially for food packaging, the films of poly(vinylidene chloride) latexes are used, which are highly impermeable for both, oxygen and water vapor.

  In both of the large volume and the small volume applications, this variety of emulsion polymers and the widespread use of them are caused by emulsion polymerization which offers many kinetic and technological advantages over other polymerization methods. The dispersion medium is water that provides inexpensive, nonflammable, nontoxic and relatively odorless systems. This polymerization has relative simplicity of the technological process. It is possible to produce high molecular weight polymer at a high reaction rates, and the viscosity of latex is independent of the molecular weight. Thus the producing of high solids content emulsions with low viscosity can be achieved in contrast to solutions of polymers. This method offers better temperature control during polymerization due to more rapid heat transfer in the low viscosity emulsion. There are possibilities of feeding the ingredients at any stage of reaction and the achievement of many copolymerizations that consist of different monomers in wide variety physical properties. The control of undesirable side reactions such as chain transfers, and the range and distribution of particle size can also be obtained. In addition, the dry form of emulsion polymers can be used in many applications as well as the use of latex itself (in wet form). For the formation of dry emulsion polymer, the polymer is isolated by coagulating the latex, filtering off the aqueous medium, and washing the derived crumb. The dried crumbs of polymers may be used as molding resins, or in some cases.