Butadiene-styrene latices during polymerization

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. 

Information on particle growth during either a seeded polymerization or during the growth stage of an un-seeded polymerization at different degrees of conversion also could enhance the understanding of the kinetics. In earlier work (4,5) the rate of polymerization, for polystyrene latexes primarily, has been related to the latex particle diameter and the total number of particles in the reactor. It would be useful to obtain kinetic data and develop the kinetic relationships for styrene (S)-butadiene (B) latexes. 

Aqueous dispersions of poly(vinyl acetate) and vinyl acetate-ethylene copolymers, homo- and copolymers of acrylic monomers, and styrene-butadiene copolymers are the most important types of polymer latexes today. Applications include paints, coatings, adhesives, paper manufacturing, leather manufacturing, textiles and other industries. In addition to emulsion polymerization, other aqueous free-radical polymerizations are applied on a large scale. In suspension polymerization a water-irnrniscible olefinic monomer is also polymerized. However, by contrast to emulsion polymerization a monomer-soluble initiator is employed, and usually no surfactant is added. Polymerization occurs in the monomer droplets, with kinetics similar to bulk polymerization. The particles obtained are much larger (>15 pm) than in emulsion polymerization, and they do not form stable latexes but precipitate during polymerization (Scheme 7.2). 

Styrene—Butadiene Rubber (SBR). This is the most important synthetic mbber and represents more than half of all synthetic mbber production (Table 3) (see Styrene-butadiene rubber). It is a copolymer of 1,3-butadiene, CH2=CH—CH=CH2, and styrene, CgH5CH=CH2, and is a descendant of the original Buna S first produced in Germany during the 1930s. The polymerization is carried out in an emulsion system where a mixture of the two monomers is mixed with a soap solution containing the necessary catalysts (initiators). The final product is an emulsion of the copolymer, ie, a fluid latex (see Latex technology). 

Emulsion polymerization is the most important process for production of elastic polymers based on butadiene. Copolymers of butadiene with styrene and acrylonitrile have attained particular significance. Polymerized 2-chlorobutadiene is known as chloroprene rubber. Emulsion polymerization provides the advantage of running a low viscosity during the entire time of polymerization. Hence the temperature can easily be controlled. The polymerizate is formed as a latex similar to natural rubber latex. In this way the production of mixed lattices is relieved. The temperature of polymerization is usually 50°C. Low-temperature polymerization is carried out by the help of redox systems at a temperature of 5°C. This kind of polymerization leads to a higher amount of desired trans-1,4 structures instead of cis-1,4 structures. Chloroprene rubber from poly-2-chlorbutadiene is equally formed by emulsion polymerization. Chloroprene polymerizes considerably more rapidly than butadiene and isoprene. Especially in low-temperature polymerization emulsifiers must show good solubility and... 

Two latex samples (LS-10 and LS-11) were prepared by switching the order of monomer addition between a styrene mix (S/AA 98/2) and a high butadiene mix (S/B/AA 28/70/2) at the stage ratio of 50/50. The polymerization temperature was 80°C during the continuous addition of monomer mixes and then raised to 100°C for one-hour cook-down. 

Polymerization is carried out at 50°C and is allowed to continue for about 12 hours until conversion reaches 72%. Reaction is terminated at this point by the addition of hydroquinone in order to minimize the formation of cross-linked material. Unreacted monomers are removed, butadiene by flash-stripping under reduced pressure and styrene by steam-stripping. An antioxidant (e.g., A -phenyl-2-naphthylamine) is added to protect the rubber during drying and subsequent storage. The latex is then coagulated by the addition of a sodium chloride-sulphuric acid solution. The coarse crumb is washed with hot water and finally dried for about 2 hours at 80°C. 

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