Synthetic rubber emulsion polymerization

Latex originally meant the sap of the rubber plant and is a dispersion of particulate rubber. Emulsion polymerization produces a similar dispersion of synthetic rubber or polymers and was rapidly developed to obtain a substitute for natural rubber during World War II. Therefore the product of emulsion polymerization was first called polymer latex, but is now known simply as latex. Sometimes the product of emulsion polymerization is called polymer emulsion. But this terminology is incorrect for latices of solid polymer particles, because emulsion indicates liquid-in-liquid dispersion (1). 

Solubilization plays an important role in emulsion polymerization of unsaturated hydrocarbons, leading to the production of latexes, which are aqueous dispersions of synthetic rubber. The polymerization process primarily occurs in micelles containing solubilized hydrocarbon, rather than in the hydrocarbon emulsion droplets. Emulsion polymerization usually yields spherical particles of uniform size [22],... 

Two general features of emulsion polymerization reaction systems for the production of carboxylated rubber latexes are of special interest The first b that the polymerization usually takes place under acidic conditions (c. pH 3-4) and not under the alkaline conditions which are usual for the production of non-functionalized synthetic rubber latexes. Polymerization is carried out under acidic conditions in order to encourage the carboxylic acid monomer to become copolymerized into the molecxiles of rubber being prcxluced. If the reaction is carried oui under alkaline conditions, then the carboxylic acid monomer is present mainly as a carboxylate salt which partitions strongly in favour of the aqueous phase If it polymerizes at all under these conditions, polymerization occurs mainly ir the aqueous phase of the reaction system, and the polymer molecxiles in whicl it becomes incorporated are far more hydrophilic than are the majority of the polymer molecxiles, which are produced in the latex particles. The requiremen... 

Poly(butadiene- (9-acrylonitrile) [9008-18-3] NBR (64), is another commercially significant random copolymer. This mbber is manufactured by free-radical emulsion polymerization. Important producers include Copolymer Rubber and Chemical (Nysyn), B. F. Goodrich (Hycar), Goodyear (Chemigum), and Uninoyal (Paracdl). The total U.S. production of nitrile mbber (NBR) in 1990 was 95.6 t (65). The most important property of NBR mbber is its oil resistance. It is used in oil well parts, fuels, oil, and solvents (64) (see Elastomers, synthetic— nitrile rubber). 

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

Polychloroprene is the oldest synthetic rubber. It is produced by the polymerization of 2-chloro-1,3-butadiene in a water emulsion with potassium sulfate as a catalyst ... 

There are some applications for a-sulfo fatty acid esters in the production and processing of synthetic materials or natural rubber. Emulsifiers are needed for the emulsion polymerization, antistatic agents improve the properties of polymers, and wetting agents are needed as parting components for elastomers. 

The rubber may be natural, in which case the latex is produced by the rubber tree. Latex of the main synthetic rubbers is produced by the technique of emulsion polymerisation. The term latex has been broadened in recent years and a general definition is now a stable dispersion of a polymeric substance in an aqueous medium . Latices may be classified as natural (from trees and plants), synthetic (by emulsion polymerisation) and artificial (by dispersion of the solid polymer in an aqueous medium). They may also be classified according to the chemical nature of the polymer, e.g., SBR, nitrile, polychloroprene, etc. 

Latex. A milk-like fluid containing small particles of natural or synthetic rubber suspended in water. Synthetic latexes are made by carrying out a polymerization step in aqueous medium (an emulsion polymerization). Water-base paints are made from polyvinyl acetate latex created in this way. 

The production of synthetic rubbers by solution polymerization processes is a stepwise operation very similar in many aspects to production by emulsion polymerization. There are distinct differences in the two technologies, however. For solution polymerization, the monomers must be extremely pure and the solvent should be completely anhydrous. In contrast to emulsion polymerization, where the monomer conversion is taken to approximately 60%, solution polymerization systems are polymerized to conversion levels typically in excess of 90%. The polymerization reaction is also more rapid, usually being completed in 1 to 2 hours. 

Over 5.5 billion pounds of synthetic rubber is produced annually in the United States. The principle elastomer is the copolymer of butadiene (75%) and styrene (25) (SBR) produced at an annual rate of over 1 million tons by the emulsion polymerization of butadiene and styrene. The copolymer of butadiene and acrylonitrile (Buna-H, NBR) is also produced by the emulsion process at an annual rate of about 200 million pounds. Likewise, neoprene is produced by the emulsion polymerization of chloroprene at an annual rate of over 125,000 t. Butyl rubber is produced by the low-temperature cationic copolymerization of isobutylene (90%) and isoprene (10%) at an annual rate of about 150,000 t. Polybutadiene, polyisoprene, and EPDM are produced by the anionic polymerization of about 600,000, 100,000, and 350,000 t, respectively. Many other elastomers are also produced. 

Emulsion polymerization was first employed during World War II for producing synthetic rubbers from 1,3-butadiene and styrene. This was the start of the synthetic rubber industry in the United States. It was a dramatic development because the Japanese naval forces threatened access to the southeast Asian natural-rubber (NR) sources, which were necessary for the war effort. Synthetic mbber has advanced significantly from the first days of balloon tires, which had a useful life of 5000 mi to present-day tires, which are good for 40,000 mi or more. Emulsion polymerization is presently the predominant process for the commercial polymerizations of vinyl acetate, chloroprene, various acrylate copolymerizations, and copolymerizations of butadiene with styrene and acrylonitrile. It is also used for methacrylates, vinyl chloride, acrylamide, and some fluorinated ethylenes. 

Redox polymerizations are usually carried out in aqueous solution, suspension, or emulsion rarely in organic solvents. Their special importance lies in the fact that they proceed at relatively low temperatures with high rates and with the formation of high molecular weight polymers. Furthermore, transfer and branching reactions are relatively unimportant. The first large-scale commercial application of redox polymerization was the production of synthetic rubber from butadiene and styrene (SBR1500) at temperatures below 5 °C (see Example 3-44). 

Emulsion Polymerization. When the U.S. supply of natural rubber from the Far East was cut off in World War II, the emulsion polymerization process was developed to produce synthetic mbber. In this complex process, the oiganic monomer is emulsified with soap in an aqueous continuous phase. Because of the much smaller (<0.1 /xm) dispersed partides than in suspension polymerization and the stabilizing action of the soap, a proper emulsion is stable, so agitation is not as critical. In classical emulsion polymerization, a water-soluble initiator is used. This, together with the small particle size, gives rise to very different kinetics (6,21—23). 

Soap - [SOAP] (Vol 22) -centrifugal separation of [SEPARATION - CENTRIFUGAL SEPARATION] (Vol21) -disinfectant and antiseptic m (DISINFECTANTS AND ANTISEPTICS] (Vol 8) -m emulsion polymerization [STYRENE-BUTADIENE RUBBER] (Vol 22) -nut oils m [NUTS] (Vol 17) -potassium hydroxide mmfg of [POTASSIUM COMPOUNDS] (Vol 19) -sampling standards for [SAMPLING] (Vol 21) -as synthetic surfactant [SURFACTANTS] (Vol 23)... 

Polymerization in emulsion under normal pressure and in the temperature range from —20 C to 60°C uses a fine emulsion of oil-soluble monomers in water and initiates the reaction with a system of water-soluble catalysts. This method is probably the most important of all, because it is used in very large scale in die copolyiuerization of butadiene and styrene and in the polymerization of many other monomers, such as chloroprene and vinyl chloride, to produce latices of the various synthetic rubbers. 

In the late 1920s Bayer Company began studies of the emulsion polymerization process of polybutadiene for producing synthetic rubber. Incorporation of styrene as a comonomer produced a supenor polymer compared to polybutadiene. The product, Buna S, was the precursor of the single largest-volume polymer produced in the 1990s, emulsion styrcne-buladieiie rubber (ESBR). 

Continuous emulsion polymerization processes are presently employed for large scale production of synthetic rubber latexes. Owing to the recent growth of the market for polymers in latex form, this process is becoming more and more important also in the production of a number of other synthetic latexes, and hence, the necessity of the knowledge of continuous emulsion polymerization kinetics has recently increased. Nevertheless/ the study of continuous emulsion polymerization kinetics hasf to datef received comparatively scant attention in contrast to batch kinetics/ and very little published work is available at present/ especially as to the reactor optimization of continuous emulsion polymerization processes. For the theoretical optimization of continuous emulsion polymerization reactors/ it is desirable to understand the kinetics of emulsion polymerization as deeply and quantitatively as possible. 

The formation of coagulum is observed in all types of emulsion polymers (i) synthetic rubber latexes such as butadiene-styrene, acrylonitrile-butadiene, and butadiene-styrene-vinyl pyridine copolymers as well as polybutadiene, polychloroprene, and polyisoprene (ii) coatings latexes such as styrene-butadiene, acrylate ester, vinyl acetate, vinyl chloride, and ethylene copolymers (iii) plastisol resins such as polyvinyl chloride (iv) specialty latexes such as polyethylene, polytetrafluoroethylene, and other fluorinated polymers (v) inverse latexes of polyacrylamide and other water-soluble polymers prepared by inverse emulsion polymerization. There are no major latex classes produced by emulsion polymerization that are completely free of coagulum formation during or after polymerization. 

Polymer colloids involve dispersions containing polymer particles having sizes greater than about 1 nm. If dispersed in aqueous solution, such a polymer dispersion is called a latex. These are usually synthetic polymer particles formed by free radical polymerization [784], Many kinds of polymerization systems exist, involving almost all of the possible kinds of colloidal dispersion, including emulsion polymerization, hence the more general term heterophase polymerization is sometimes used. Several reviews are available [785-789]. Emulsion polymerization provides a convenient means of controlling the polymerization of monomers and is used to make, for example, synthetic rubber which is mostly a co-polymer of butadiene and styrene. 

A condition of flow in which all elements of a fluid passing a certain point follow the same path, or streamline there is no turbulence. Also referred to as streamline flow . A dispersion (suspension or emulsion) of polymer in water. Latex rubber, a heavily cross-linked polymer solid, is produced either by coagulating natural latex or by synthetic means through emulsion polymerization. Example Latex paint is a latex containing pigments and filling additives. 

As discussed in Chapter 4, emulsion polymerization received a significant boost in the United States during the Second World War. When Japan overran countries that supplied natural rubber to the West, a crash program to manufacture synthetic rubber was initiated in the United States and Canada. The product was called Government Rubber-Styrene (GR-S), and was produced by the emulsion polymerization of butadiene and styrene. The fundamental recipe for GR-S is still used as a teaching tool for those learning the art and science of emulsion polymerization. 

Butadiene-Styrene Rubber occurs as a synthetic liquid latex or solid rubber produced by the emulsion polymerization of butadiene and styrene, using fatty acid soaps as emulsifiers, and a suitable catalyst, molecular weight regulator (if required), and shortstop. It also occurs as a solid rubber produced by the solution copolymerization of butadiene and styrene in a hexane solution, using butyl lithium as a catalyst. Solvents and volatiles are removed by processing with hot water or by drum drying. 

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