Costel D. Denson

Stripping Operations in Polymer Processing

Publisher Summary Polymer processing operations that do not involve shaping are that are concerned with the synthesis of high-molecular-weight polymers, or with affecting physicochemical changes in the nature of polymeric materials. This chapter discusses the developing stresses during flow and and describes how these, as well as solidification rates, influence the morphology and performance properties of the shaped structure. The engineering analysis and design of these operations are addressed. In the process of polymer processing, three of the “nonshaping” operations are shown: (1) a chemical reaction, that is, coagulation; (2) a liquid–liquid extraction operation that involves a molten polymer and water; and (3) a vapor–liquid stripping operation that involves the removal of a volatile component from the molten polymer. The analysis and design of any stripping operation would be relatively straightforward if the velocity and concentration profiles that obtain in the extraction unit are known. Solutions to the momentum and diffusion equations provide this information, but, for most cases of interest in the chemical process industries, solutions to these equations are difficult to obtain since the flow geometry is often not well defined and flow may be both tortuous and turbulent.

Imidization reaction parameters in inert molten polymers for micromixing tracer studies

Abstract A competitive-consecutive reaction involving a two-step imidization reaction in an inert molten polymer is proposed as a reaction tracer for studying micromixing in highly viscous liquids in polymer processing geometries. In the proposed tracer system, phthalic anhydride reacts with p-phenylene diamine in molten polyethylene at dilute concentrations and elevated temperatures where the reactants and intermediates are soluble in the molten polymer. Rate constants were measured using a reactive film procedure and are reported in this paper over a range of temperatures. The rate constant for the first step was found to be an order-of-magnitude larger than the second step, and both steps were shown to be essentially irreversible. These conditions are used to show that the proposed reaction is mixing-sensitive and can be used to measure micromixing through measurements of selectivity.

Polymer Processing Operations Other Than Shaping

Polymer processing operations which do not involve shaping are defined here as those which are concerned with the synthesis of high molecular weight polymers, and those which are concerned with affecting physico-chemical changes in the nature of polymeric materials. These operations, which are most often conducted upstream from the shaping operations (i.e., extrusion, injection molding, blow molding, tubular film blowing, fiber spinning), include by way of example: vapor-liquid stripping operations when the liquid phase is a molten polymer or polymer solution (devolatilization), liquid-liquid stripping operations where one liquid is a molten polymer or polymeric solution, gas absorption in molten polymers, polymerization and grafting reactions, mixing, pumping and pressurization, and filtration.

The Role of Rheology in Polymer Processing

Polymer processing operations have typically been classified in one of two ways, by process or by the type of unit building blocks involved. Classifying operations by process is reminiscent of the unit processes concept found in the chemical engineering literature and in this approach we think of operations along the lines of specific polymers and whether the process is one of extrusion, injection molding, compression molding, blow molding and so on. Classifying operations according to the unit building blocks involved is, in some respects, similar to the unit operation concept and here we would cite as examples, pressurization and pumping of molten polymers, devolatilization, heat transfer, and polymer filtration. In either case, the analysis and design of polymer processes requires a knowledge and understanding of both the transport modes which are operative in a given process, and the rheological behavior of a given polymer under the conditions at which the process is being conducted.