Polymer composition, temperature control

As shown in Fig. 19, polymer adheres to one of the two counter-rotating rolls passing through an adjustable nip gap which creates intense shear. The rolls are temperature controlled and may run at differential speeds to influence further shear intensity. A degree of lateral cross-mixing must also be imposed to ensure overall compositional uniformity of the batch. A rolling bank of polymer located above the rolls provides some additional mixing capability. 

The state of the art for many polymer processes is a simple time-temperature recipe for the process equipment. This is often a satisfactory solution for processes in which the process equipment temperature and the part temperature are nearly identical. Temperature control for pultrusion of thin cross-sections, for instance, can be satisfactorily based on the die and barrel temperatures. In processes where there is a major difference between the setpoint and the local conditions, it is now becoming more common to implement supervisory controllers which account for these differences. The autoclave curing of composites, for instance, may have large temperature lags between the autoclave and the part as is shown in Figures 15.3... 

Many polymer reactions, for example, are highly exothermic, so the temperature control concepts outlined in this book must be applied. At the same time, controlling just the temperature in a polymer reactor may not adequately satisfy the economic objectives of the plant, since many of the desired polymer product properties (molecular weight, composition, etc.) are created within the polymerization reactor. These key properties must be controlled using other process parameters (i.e. vessel pressure in a polycondensation reactor or chain transfer agent composition in a free-radical polymerization reactor). 

Some polymerization reactions are highly exothermic, so the problems of temperature control, which are the major emphasis of this book, are important in these systems. However, beyond the issue of temperature control, polymer reactors must produce a product with the desired properties. The final polymer product properties, such as viscosity, molecular weight distribution, particle size, and composition, are important for consistent performance of the polymer. These properties depend on more than just temperature and few can be measured online.12... 

Char compositions can be controlled quite precisely through the proper choice of polymer composition, pyrolysis temperature, and atmosphere, as well as through an improved understanding of the mechanisms of polymer pyrolysis. 

The polymerization of vinyl monomers is an exothermic reaction in which a considerable amount of heat is released, about 75.3 kj/ mol. In both the catalyst-heat and y-radiation processes the heat released during polymerization is the same for a given amount of monomer. The rate at which the heat is released is controlled by the rate at which the free-radical initiating species is supplied and at which the chains are growing. The vazo and peroxide catalysts are temperature dependent consequently, the rate of decomposition, and thus the supply of free radicals, increases rapidly with an increase in temperature. Because wood is an insulator due to its cellular structure, heat flow into and out of the wood-monomer-polymer material is restricted. In the catalyst-heat process, heat must be introduced into the wood-monomer to start the polymerization, but once the exothermic reaction begins the heat flow is reversed. The temperature of the wood-monomer-polymer composite increases rapidly, because the heat flow out of the wood is much slower than the heat generation. Figure 3 illustrates the heat-transfer process (3J). 

Temperature control policies have also been suggested to diminish the composition spread in batch reactors (92, 94). However, the low sensitivity of the reactivity ratios to temperature (Figure 7), the poor heat transfer characteristics of reacting polymer mixtures (slow response times) and the considerable excursions in temperature (and, therefore, molecular weights) required to maintain adequate uniformity in composition make the application of these temperature control policies unrealistic. 

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