Polymer processing conversion operations

The type of initiator used affects the molecular weight and conversion limits in a reactor of fixed size and the molecular weight distribution of the material produced at a given conversion level. The initiator type also dictates the amount of initiator which is necessary to yield a given conversion to polymer, the operating temperature range of the reactor and the sensitivity of the reactor to an unstable condition. Clearly, the initiator is the most important reaction parameter in the polymer process. 

Kiparissides, et al. (8) developed mathematical models of two levels of sophistication for the vinyl acetate system a comprehensive model that solved for the age distribution function of polymer particles and a simplified model which solved a series of differential equations assuming discrete periods of particle nucleation. In practice, the simplified model adequately describes the physical process in that particle generation generally occurs in discrete intervals of time and these generation periods are short in duration when compared with operation time of the system. The simplified model is expanded here for a series of m reactors. The total property balances for number of particles, polymer volume, conversion, and area of particles, are written as ... 

A polymer/monomer (polymer/repeat-unit or polymer/macrocycle) switch may become of practical importance where a polymer decorated with certain groups has specific size-dependent properties that the monomeric units do not have. The modulation of the conversion between polymeric and monomeric (or macrocyclic) states would also result in the modulation of these properties. Moreover, such size switches, represented by polymerization/depolymerization processes that operate under the control of external events, are examples of environmentally-friendly recyclable polymers (reduction of waste treatment). As well, if the polymer has low solubility and the polymer/monomer switch can work in spite of this, then it becomes possible to reversibly generate a precipitating (solid) polymeric material from a liquid solution of monomer. 

The sputter coating of polymers (misrepresentation of the real phenomenon) is another important demonstration of how important the CAP mechanism is not only in plasma polymerization but also in other plasma processes. Thus, inadvertent plasma polymerization can take place in an attempted etching process conversely, etching can occur in an elfort to deposit a polymer by plasma polymerization. Therefore, a thorough understanding of the CAP principle seems to be important for the successful operation of any plasma process. 

As reactor temperature increases, the heat load from heating the feed becomes significant. At very high operation temperatures, the process can operate at net endothermic and require heat addition. The energy balance of the system is such that at low temperatures, the CSTR operates in exothermic mode, that is, net heat must be removed from the process jacket or sometimes internal coils. At elevated process conditions, the system becomes net endothermic as a result of the considerable heat required to raise the feed to reactor temperature. The total heat of polymerization is dependent on both the energetics of the system and the conversion of monomer to polymer. This in turn is defined by the kinetics of the system [50, 91, 92],... 

In an RD process, two operations are coupled and run simultaneously. One operation is chemical synthesis and the other is separation by distillation, enabling a shift in composition to drive equilibrium-limited reactions to higher conversion. In this chapter we summarize some of our published papers and extend the work to other applications of polymer carrier composite materials and components. 

Free-radical polymerization is a highly exothermic process and reactor temperature control is an important issue for both polymer quality and operation safety. At the temperatures used in commercial practice, most radical polymerizations are irreversible. However, because it is an exothermic reaction, at sufficiently high temperatures the reaction becomes reversible and complete conversion cannot be achieved. Methyl methacrylate is a major monomer that suffers from this problem, with an equilibrium concentration of 0.139 mol at 110°C [12[. 

Extrusion is a polymer conversion operation in which a solid thermoplastic material is melted or softened, forced through an orifice (die) of the desired cross section, and cooled. The process is used for compounding plastics and for the production of tubes, pipes, sheet, film, wire coating, and profiles. All extrusion lines include a melt pump called an extruder, but other equipment is specific to the particular process. Although there are many types of extruders, - the most... 

In this seetion the principal fabrication processes pertaining to polyethylene are outlined. End use products are discussed in terms of how the attributes of various polyethylene resins suit them to particular conversion processes and ap-plieations. It is not intended to provide a detailed discussion of the theory or operation of proeessing equipment. Those wishing to inquire further into these aspeets of polymer processing should consult the bibliography at the end of the ehapter. 

Achieving steady-state operation in a continuous tank reactor system can be difficult. Particle nucleation phenomena and the decrease in termination rate caused by high viscosity within the particles (gel effect) can contribute to significant reactor instabilities. Variation in the level of inhibitors in the feed streams can also cause reactor control problems. Conversion oscillations have been observed with many different monomers. These oscillations often result from a limit cycle behavior of the particle nucleation mechanism. Such oscillations are difficult to tolerate in commercial systems. They can cause uneven heat loads and significant transients in free emulsifier concentration thus potentially causing flocculation and the formation of wall polymer. This problem may be one of the most difficult to handle in the development of commercial continuous processes. 

The engineering analysis and design of these operations addresses questions which are different than those addressed in connection with the shaping operations. This is illustrated in Fig. 1 which is a flow sheet, cited by Nichols and Kheradi (1982), for the continuous conversion of latex in the manufacture of acrylonitrile-butadiene-styrene (ABS). In this process three of the nonshaping operations are shown (1) a chemical reaction (coagulation) (2) a liquid-liquid extraction operation which involves a molten polymer and water and (3) a vapor-liquid stripping operation which involves the removal of a volatile component from the molten polymer. The analysis and design around the devolatilization section, for example, would deal with such questions as how the exit concentration of... 

---