Coagulum formation

Continuous polymerization in a staged series of reactors is a variation of this process (82). In one example, a mixture of chloroprene, 2,3-dichloro-l,3-butadiene, dodecyl mercaptan, and phenothiazine (15 ppm) is fed to the first of a cascade of 7 reactors together with a water solution containing disproportionated potassium abietate, potassium hydroxide, and formamidine sulfinic acid catalyst. Residence time in each reactor is 25 min at 45°C for a total conversion of 66%. Potassium ion is used in place of sodium to minimize coagulum formation. In other examples, it was judged best to feed catalyst to each reactor in the cascade (83). 

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. 

In contrast to other polymers proposed for agglomeration, it is unimportant whether or not the base latex has a fairly uniform particle size in order to avoid coagulum formation (34). With base latexes containing large amounts (e.g., 40-50 wt %) of large particles (> 1200 A), no coagulum at all was obtained after agglomeration. 

Method of Mixing PEO and Latex. Again in contrast to other polymers (10, 34), no effect of mixing method on either agglomeration rate or coagulum formation was detectable in this system. Whether or not it has noticeable effect on particle size distribution remains to be elucidated by evaluating electron micrographs. 

In conclusion we can say that the inisurfs known today have different chemical structures and consequently different properties. Experimental data are available showing that emulsion polymerization is posssible using inisurfs without any additional emulsifiers, thus reducing the electrolyte content in the latex serum as well as foam formation. From a more technical point of view problems existing today concern the low initiator efficiencies as well as the fact that the solid content of the latexes is restricted to approximately 40% without coagulum formation. 

The effects of agitation rate, feed time, initiation rate, and latex viscosity on coagulum formation in the indnstrial emulsion polymerisation of styrene and bntyl acrylate were studied, and the results were analysed by the use of computational fluid dynamics to simulate the reactor flow pattern. Large stagnant regions occurred close to the agitator shaft and reactor wall, a situation which was worsened by towards the end of the process, when the agitator shaft was covered completely by the contents of the reactor. 22 refs. 

Eslami et a/. [73,74] studied ATRP of 2-ethylhexyl methacrylate in emulsion system, using the EBiB as an initiator and CuBr/4,4 -di(5-nonyl)-2,2 -bipyridl (dNbpy) as the catalyst. Five surfactants were studied and the Tween 80 and Brij 98 showed the most promising results. They found that the colloidal stability was a major issue while attempting to control the polymerizations. In addition, the coagulum formation was observed and the operating windows... 

Increasing the agitation speed to increase the internal heat-transfer coefficient is not an option because a high impeller speed can lead to coagulum formation. 

---