Devolatilization vacuum staging

Two-stage and multiple-stage vented extruders are commonly used to remove volatile components from molten resin streams prior to downstream processing. The vent ports can be open to the atmosphere, or they can be attached to elaborate vacuum systems. For very specialized systems, stripping agents such as water, nitrogen, and carbon dioxide can be added upstream of the vent, mixed into the resin stream, and then devolatilized in the vent area. This technique can be employed to remove difficult components or components at a higher rate from the resin. 

The melt polymerization process involves the base-catalyzed transesterification reaction of BPA with diphenyl carbonate (Fig. 8). A small amount (less than 0.01% molar) of basic catalyst such as Na, Li, K, or tetralkylammonium hydroxide or carbonate is used during the initial stages of the reaction. The reaction is performed under vacuum at 180-300°C. At later stages of the reaction, the temperature and the vacuum are increased (less than ImmHg) to remove phenol and drive the product to high molecular weight. Subsequently, the polymer becomes very viscous, and special devices, such as devolatilizing extruders, are required to ensure complete removal of phenol. 

Numerous variations of the interfacial process have been published. The reactions can be carried out in batch in stirred tank reactors or continuously in series of CSTRs and tubular reactors. Intensive mixing with dispersion and redispersion is required throughout the reaction stages. After the reaction is complete, the brine phase is separated and the polymer solution washed to remove residual amine and base. Several processes for devolatilization are in use, including solventless precipitation, steam precipitation, spray drying, falling-strand devolatilization, and vacuum extrusion in devolatilizing extruders. 

Fig. 29. TWin-screw extrusion compounding line for the preparation of mineral-filled thermoplastics showing pre-mixing, compounding and pelletizing stages [155] (a) Filler/poly-mer/additive feedstock, (b) high-speed pre-mixer, (e) dosing screw, (d) twin-screw compounder, (e) vacuum devolatilization, (f) water-cooled die-face cutter, (g) start-up diverter, (h) de-watering chute, (i) pellet dryer, (k) bagging...

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