Devolatilization of Polymer Melts

A technique that is often employed in devolatilization of polymer solutions is flash devolatilization [34], in this technique, the polymer solution is delivered to a flash point under high pressure and at temperatures above the boiling point of the volatile component. The solution is then expanded through a nozzle large amounts of volatiles can thus be extracted rather quickly. The foamy liquid that results from this operation is often exposed to another devolatilization step to remove residual amounts of volatiles. This second step is generally a conventional melt film devolatilization, where the material in the film is continuously renewed to obtain an effective extraction of the volatiles. 

Stripping agents such as water are often added to the polymer to enhance the devolatilization process. The improvement is obtained by bubble formation, which substantially improves the devolatilization process. 

Consider a liquid polymer film with surface area A and depth H moving in direction X in plug flow with a volumetric flow rate V,. The film is losing a volatile solute by evaporation in the y direction at a rate of E. If the mass transport in the y direction occurs by molecular diffusion only, and if both dispersion in the x direction and changes in Vf due to loss of volatile are neglected, an expression for E can be developed. The exposure time of the film X, is defined as  

It can be shown [30] that if A,f/A,D 0.1, the film can be considered of infinite depth. In this case, the layer in which the concentration is varying is much thinner than the total thickness of the melt film H. The stage efficiency X for this situation can be expressed as  

The stage efficiency is the actual rate of evaporation divided by the maximum possible rate of evaporation  

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A. L. Yarin, D. Lastochkin, Y. Talmon, and Z. Tadmor Bubble Nucleation during Devolatilization of Polymer Melts, AIChE J., 45, 2590-2605 (1999). 

J.A. Biesenberger and G. Kessidis, Devolatilization of Polymer Melts in Single Screw Extruders, Polym. Eng. Sci., 22, 832 (1982). 

Several models for foam-based devolatilization are available. The fundamentals are reviewed by Lee [22]. Bubble nucleation during devolatilization of polymer melts has been explained by homogeneous nucleation [23, 24], heterogeneous nucleation [25-27], and a mixed-mode nucleation [28]. Yarin et al. [29] considered a secondary nucleation. 

Scanning Electron Microscopy Studies of Polymer Melt Devolatilization, 433... 

SCANNING ELECTRON MICROSCOPY STUDIES OF POLYMER MELT DEVOLATILIZATION... 

Fig. 8.14 PS-styrene sample extruded at 180°C into atmospheric pressure. The micrograph shows the smooth lateral surface and part of the cross section there is no evidence of huhhles. [Reprinted by permission from R. J. Albalak, Z. Tadmor, and Y. Talmon, Scanning Electron Microscopy Studies of Polymer Melt Devolatilization, AIChE J., 33, 808-818 (1987).]...

The preceding observations on the microscopic features of polymer melt devolatilization are not unique to the PS-styrene system, or to strand devolatilization. Similar, though somewhat less rich, features of blister-covered macrobubbles were observed with low-density polyethylene (PE), high-density PE and polypropylene (PP) systems (40,41). Furthermore, Tukachinsky et al. (11) discovered macrobubbles covered with microblisters in a 50-mm-diameter vented SSE, with PS showing more oblong shapes as a result of shearing. The onset of foaming with the application of vacuum was quicker with increased frequency of screw rotation, and the separation was more efficient. 

S. T. Lee and J. A. Biesenberger, Fundamental Study of Polymer Melt Devolatilization. IV Some Theories and Models for Foam-enhanced Devolatilization, Polym. Eng. Sci., 29, 782-790 (1989). 

Co being the initial interface volatile concentration. In the devolatilization of polymer solutions and polymer melts, the diffusion of the volatiles through the polymer is usually the rate-controlling part of the process [4]. Therefore, the volatile concentration at the interface, Cj, is in equilibrium with the concentration of the volatile in the gas phase. The equilibrium concentration is related to the partial pressure of the volatile in the gas phase, i, by means of Henry s law, Eq. (4), where H, the Henry s law constant, depends on the temperature, pressure, and nature of the volatile. 

Single-screw and double-screw extruders are normally used for polymer melts to accomplish the deaeration or devolatilization of residual volatiles. Devolatilization in an extruder is effected through formation of the venting zone inside the chamber by carefully designed upstream and downstream screw sections. 

Fig. 8.1 Schematic representation of the devolatilization process. The hatched area represents the polymer melt being devolatilized, which is almost always subject to laminar flow. The bubbles shown are created by the boiling mechanism and by entrapped vapors dragged into the flowing/ circulating melt by moving surfaces.

In this chapter, subsequent to an introduction to devolatilization equipment, we review the thermodynamics of polymer solution equilibrium, which determines the maximum amount of volatiles that can be separated under a given set of processing conditions the phenomena associated with diffusion and diffusivity of small molecules in polymeric melts, which affects the rate of mass transfer the phenomena and mechanisms involving devolatilization and their modeling and the detailed and complex morphologies within the growing bubbles created during devolatilization of melts. 

Devolatilization of Residual Toluene Residual toluene is continuously removed from a polymer melt stream of 454 kg/h at 230°C and 0.006 weight fraction of toluene, at a vacuum of 20 torr. The density of the polymer is 0.98 g/cm3, and the Florry-Huggins interaction parameter is yl2 = 0.43. (a) Calculate the equilibrium concentration, we- (b) If equilibrium is reached, that is, Wf = we, where uy is the final concentration, calculate the separation efficiency Fs = (wo — Wf)/wQ. (c) If the final concentration wy = 2we, calculate Fs- (d) Calculate for (c) the volumetric flow rate of the vacuum pump removing the volatiles. 

Fig. 9.52 Separation efficiency of a three-chamber co-rotating disk devolatilizer of 450°F PS melt containing 1500-3000 ppm styrene, fed at 42-lb/h into 0.54-in-wide chambers at 50-torr absolute pressure, as a function of disk speed and with flow rate as a parameter. Broken curves show calculated residence times. [Reprinted by permission from P. S. Mehta, L. N. Valsamis, and Z. Tadmor, Foam Devolatilization in a Multichannel Co-rotating Disk Processor, Polym. Process. Eng., 2, 103-128 (1984).]...

Devolatilization performance is usually measured against the equilibrium amount of volatile in the final polymer. The equilibrium level for the devolatilization conditions used can be calculated using a simplified Flory-Huggins equation for monomer activity in the polymer melt [6]. By equating the partial pressure of the monomer solution to the flash tank partial pressure, the following results ... 

The removal of residual volatile components from polymers is an operation of some importance in the plastics industry. A generalized, although somewhat idealized, model for continuous, wiped-film devolatilization of viscous polymer melts is presented which relates devolatilization capability to important geometry, < perating, and fluid property variables. The applicability and limitations of the model are analyzed experimentally. The data support many aspects of the theory, but also reveal certain deficiencies in the model which should be considered in designing for maximum efficiency. 

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