Processing reaction viscosity

Styrene-butadiene rubber, or E-SBR as it is known in manufacturing circles, was first developed in the 1930s. Known as Buna S, the compound was prepared by I.G. Farbenindustrie in Germany. Manufacturing styrene-butadiene rubber was through an emulsion polymerization process which produced a material that had a low reaction viscosity, yet had all the attributes of natural rubber. 

A specially designed thin-film machine can be used to process very viscous, non-Newtonian materials. The apparatus can also be used to remove solvents from polymers and polycondensation processes having viscosities exceeding 10,000 poises. The Luwa thin-film machine has a small clearance between the heated wall and rotor blade. This clearance results in high shear gradients and considerably reduces apparent viscosity. The increased turbulence and improved surface renewal that ensue improve reaction velocities and aid the required forced product flow on the walls of the apparatus. 

Finally, in the reaction injection-molding (RIM) process, low-viscosity reacting monomers or prepolymers are intimately mixed just before being injected into a hot cavity, where they react further and solidify. The RIM process, then, is a variation of the casting process, where highly reactive liquid systems are injected quickly, rather than being allowed to flow by gravity, into complex shape cavities, where they quickly react and solidify. 

The emulsion polymerization process has several advantages. It is normally carried out under mild reaction conditions that are tolerant to water in the absence of oxygen. The process is relatively resistant to impurities and amenable to using a range of functionalized and nonfunctionalized monomers. Additional benefits include the fact that emulsion polymerization gives high solid contents with low reaction viscosity and is a cost-effective process. The physical state of the emulsion (colloidal) system makes it easy to control the process. Thermal and viscosity problems are much less significant than in bulk polymerization. 

In reactive polymerization it is important to control extrusion conditions (extruder profiles, temperature profiles, residence times) as well as having a good understanding of the polymerization kinetics, thermodynamics and properties (i.e. reaction rates, conversion times, heats of polymerization, ceiling termperatures of the polymerization reactions, viscosity (M ) changes). All of the processes discussed in Section 1.2 are brought to bear in the environment of the extruder. 

S. Glasstone, K.J. Laidler and H. Eyring, The theory of rate processes the kinetics of chemical reactions, viscosity, dilfusion and electrochemical phenomena, McGraw-HiU, New York, 1941. 

Although the name Henry Eyring had come to my attention many times before the autumn of 1947, it was only then that my intimate association with him began. It lasted for six inspiring years. Before that time he had already won fame in his application of reaction-rate theory to a myriad of problems, including chemical reactions, biological processes, and viscosity. 

In the introduction to this section it was already referred to the massive change in properties of the reaction mixture with proceeding reaction time as an extraordinary characteristic of polymerization processes. The viscosity is the most significant property. This substance property has an influence on, or interacts with, so many parameters included in the balance equations and other factors that a graphical presentation seems to be the oidy appropriate form to explain this (c.f. Figure 4-63). 

In concluding this part, the emulsification process is influenced by various parameters, such as the volume phase ratio, the viscosity of both phases, the mutual solubility of both phases, the kind and concentration of additives, the power input, the stirrer as well as the vessel geometry, the diminution energy and the thermodynamic changes during the emulsification process (reactions, temperature etc.). 

The polymer slurry circulates through the reactor tubes, and boiling liquid ethylene in the reactor jackets keeps the reactor contents cold. The butyl polymerization process is complex because of the combination of low temperature operation, polymer slurry formation, and difficulty in directly measuring polymer quality. To achieve high molecular weight, the reaction temperature must be kept low to reduce the amount of chain transfer. In the slurry process, the viscosity... 

S. Glasston, K.J. Laidler, and H. Eyring (1940), Theory of Rate Processes The Kinetics of Chemical Reactions, Viscosity, Diffusion, and Electrochemical Pheno-mena, McGraw-Hill, New York. 

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