Screw degassing zones

Suitable screw configuration and screw speed ensure compensation of heat loss, particularly after flash and also after the initial forward venting. The vacuums in the individual degassing zones are designed to avoid excessive foaming of the polymer solution or even crumbs in the degassing opening. 

Huneault and Li described compatible blends of TPS-PLA made by free-radical grafting of maleic anhydride (MA) on the PLA backbone and then by reacting the modified PLA with the starch macromolecules in a twin-screw extruder provided with a degassing zone to eliminate water and free unreacted MA at both the PLA grafting and TPS-PLA blending steps. MA-grafted PLA showed a much finer dispersed phase, in the 1- 3 pm range, and exhibited a dramatic improvement in mechanical properties [69]. 

FIGURE 8.33 Degassing in a single-screw extruder. Zone 1 consists of a typical plasticating screw section zone 2 is a deep channel section with a vacuum port zone 3 is a metering section. The channels in zone 2 are partially full to facilitate degassing. 

The polymer or the polymer solution cools down because the vaporization energy required for the vaporization process is taken from the in-fed polymer solution. The required ZSK size depends on the gas quantity to be removed, the gas velocity, the rheological behavior of the polymer melt, and the thermodynamic behavior of the degassed components, the screw speed, and the throughput. Mechanical energy has to be introduced in the processing zone... 

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