Flow diagram of the ESBR production process

Above a critical concentration, surfactant molecules form aggregates called micelles. One example of this phenomenon is a solution of the potassium or sodium salts of fatty or rosin acids. These are usually known as soaps. The addition of monomers which are insoluble in water, such as styrene or butadiene, to fte stirred soap solution results in droplets of monomer stabilised by soap molecules being formed. These droplets are approximately 1000 nm in diameter. 

The latex particle is stabilised by soap molecules adsorbed on the surface. As the particle grows, more soap is required and this is taken from inactivated micelles. At between 10 and 20 % of conversion from monomer to pol mier, no micelles are left, as the soap concentration has dropped below the critical micelle concentration. At about 60 % conversion, the monomer droplets disappear. 

Polymerisation is terminated before total conversion in order to avoid unwanted effects such as long chain branching and the formation of gel. Termination is effected by the addition of chemical substances known as a short stop, which immediately kills all free radicals. 

About ten years ago, all ESBR plants modified their processes to eliminate die presence of volatile nitrosamines. These potentially carcinogenic chemicals were present at concentrations in parts per billion. The changes made eliminated the use of sodiiun nitrite and one component of the short stop system, sodium dimethyl dithiocarbamate. 

The molecular weight of the pol mier molecules is regulated by the addition of a chain transfer agent or modifier. This has the effect of terminating one growing molecule and initiating another. The more modifier that is added, the lower the molecular weight of the final product. 

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Figure 7.3 Flow diagram of the ESBR production process...

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