Devolatilization polymer melts

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. 

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

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. 

Fig. 8.10 Residual styrene concentration in PS extruded at 225°C. The open symbols refer to experiments without ultrasound, while the filled ones refer to experiments where ultrasound radiation was applied. The parameter is the absolute pressure in the chamber. Triangles 150 mmHg squares 50 mmHg, and circles 12 mmHg. [Reprinted by permission from A. Tukachinsky, Z. Tadmor, and Y. Talmon, Ultrasound-enhanced Devolatilization in Polymer Melt, AIChE J., 39, 359 (1993).]...

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). 

R. J. Albalak, Z. Tadmor, and Y. Talmon, Polymer Melt Devolatilization Mechanisms, J. Non-Newtonian Fluid Mech., 36, 1313-1320 (1990). 

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). 

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. 

The devolatilizer (7) is held under a very high vacuum to remove unreacted monomer and solvent from the polymer melt. The monomer is distilled in the styrene recovery unit (8) and recycled back to the prepolymizer. The polymer melt is then pumped through a die head (9) to form strands, a waterbath (10) to cool the strands, a pelletizer (11) to form pellets and is screened to remove large pellets and fines. The resultant product is air-conveyed to bulk storage and packaging facilities. 

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. 

Hash devolatilization is a simple and effective method to remove the majority of solvent and unreacted monomers from the polymer solution. Product from the reactor is charged to a flash vessel and throttled to vacuum conditions whereby the volatile solvent and monomers are recovered and condensed. In the process, the polymer melt cools, sometimes considerably, due to the evaporation of volatiles. The polymer product is pumped from the bottom of the flash vessel with a gear pump or other suitable pump for viscous materials. Critical to operation of the flash devolatilization unit is prevention of air back into the unit that reduces stripping ability and potentially allows oxygen into the unit that can discolor products or pose a safety hazard if low autoignition temperature solvents are used. Often one flash devolatilization unit is insufficient to reduce the residual material to a sufficient level and thus additional units can be added in series [61]. In each vessel, the equilibrium concentration of volatile material in the polymer melt, is a function of the pressure and temperature the flash unit operates at, with consideration for the polymer solvent interaction effects described by the Hory-Huggins equation. Flash devolatilization units, while simple to operate, may be prone to foam development as the superheated volatiles rapidly escape from the polymer melt. Viscous polymers or polymers with mixed functionalities... 

An alternative to a flash devolatilization unit is the oil heated thin film or WFE. In this equipment, the molten polymer/solvent solution is throttled to the WFE comprising a rotating set of blades that draws the melt into a thin film. In this manner, very good heat transfer from the oil heated surface is obtained and the thin film minimizes diffusion distances and allows rapid mass transfer of volatiles out of the melt. Both vertical and horizontal WFE units are in commercial production and are effective for small-to-medium-sized plants with moderate viscosity melts. Larger units require very large motors to strip viscous resins. Like flash devolatilization units, bubble formation and collapse are essential to effective mass transport of solvent from the polymer melt. 

Diffusion coefficients are very much temperature dependent. When the polymer is below the melting point, diffusion generally occurs at an extremely low rate. The polymer, therefore, should be above the melting point to increase the rate of diffusion and with it the devolatilization efficiency. Even when the polymer is in the molten state, the diffusion coefficients can often be increased substantially by increasing the temperature of the polymer melt [11]. Further, when the polymer is in the molten state, surface renewal is possible. This greatly enhances the devolatilization process. The extent of surface renewal is a strong function of the screw... 

If the flight pitch is constant and the polymer melt viscosity can be described by Eq. 8.78, the maximum diehead pressure for effective devolatilization can be written as [2] ... 

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