Multistage extrusion

In 1977 Phillips Petroleum Co. developed blends of Z-N-LLDPE using C2+4 and C2+6 copolymers which were disclosed as suitable for pipe extrusion (Larsen 1982). The same year, Mobil Oil announced reactor blends of PE (copolymer of C2+4 and/or C2+6) for production of blown films that exhibited improved MD/TD tear balance. The latter materials were produced in a multistage, gas-phase, fluidized bed polymerization process with in situ blending. The resulted bimodal MWD resins had 0.35 to about 0.75 wt% of a higher-MW component (Ah et al. 1994). 

The corotating twin-screw extruders also offer the choice of multiple ports for liquid injection, venting, and downstream feeding purposes. Eor example, the twin-screw extruder can be readily adapted to combine both the polymer modification and polymer-polymer grafting steps of a reactive blending process into a singlepass, multistage reactive extrusion process. 

Muiti-vent devoiatiiization is used when iarge amounts of voiatiles need to be removed from the poiymer. In a well-designed system, as much as 15% of volatiles can be removed in one extrusion operation. Figure 8.45 shows a schematic of a multistage system used for devoiatiiization of moiten poiystyrene. 

More recently, reactive extrusion under an inert atmosphere was found to be very effective to minimize the side effects of composites prepared by in-situ reactive processing [14]. The economic feasibility of such processes are recently questioned due to the involvement of multistage... 

At present there are three types of profile dies used. orifice dies, multistage dies, and tapered profile dies. An example of an orifice die is shown in Figure 7.42. Basically the orifice die consists of a die base and a die plate in which the profile is formed. These dies are used for the extrusion of inexpensive profiles where dimensional accuracy is not necessary. Because of the abrupt change in cross-sectional area, there is usually a buildup of stagnant material behind the die plate and high extrusion rates are not possible. These dies are not commonly used for most thermoplastics but are restricted primarily to PVC and mbber. 

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