Intensification of polyethylene devolatilization in twin-screw extruder with additives

Aiming at the problem of difficult to remove high-carbon alpha-olefin in polyethylene, the influences of operation conditions and devolatilization additives on the devolatilization of LLDPE/1-octene system in an exhaust twin-screw extruder are studied by headspace gas chromatography. The results showed that the devolatilization efficiency increases with the devolatilization temperature and vacuum. With the screw rotational speed, the content of liquid additives and the gas velocity of gas additives increasing, the devolatilization efficiency first increases and then decreases. The devolatilization difficulty of different grades of polyethylene varies due to the difference of the viscosities. When the volatile content is high, the effect of devolatilization additives is anhydrous ethanol > surfactant > water > nitrogen. When the volatile content is low, the effect of nitrogen on devolatilization is obviously better than anhydrous ethanol and water. Finally, a novel devolatilization process is proposed, that is, an evacuation system (vacuum degree 0.08-0.1 MPa) is set up in each stage of the devolatilization section, the anhydrous ethanol is introduced in the first stage, and high-purity nitrogen is introduced in the second, third, and fourth stages, which can ultimately reduce the VOC content of LLDPE products to less than the target value of 50 ppm. Highlights The devolatilization process is used to remove high carbon alpha-olefins from LLDPE. Each additive has an optimal injection value. Ethanol is the most effective in the foam-dominated devolatilization stage. Nitrogen is the most effective in the diffusion-dominated devolatilization stage. A new additive-enhanced twin-screw extruder devolatilization process is proposed.