Assessment On Mixing Properties Of Triple Screw Extruder And Research On Its Shear-Extensional Alternating Flow Mechanism

Extensive researches show the advantage of mixing equipment with extensional flow effect.Compared with conventional polymer processing machines,which dominated by shear effect,it can significantly improve the properties of high viscosity ratio bi-phase system.Due to the similarities and differences on structure between triangle-arranged triple screw extruder(TTSE)and twin screw extruder(TSE),HDPE/PP system with different viscosity ratios was chosen to reveal the shear and extensional flow mechanism in TTSE,and to evaluate its compounding properties.Two types of extruders were divided into different zones.The shear and extensional effects in different zones were calculated and their relationship with processing conditions(screw speed and output)and geometric parameters(screw lead and ratio of outer diameter to root diameter)were analyzed.The same parameter and processing condition with experiment were selected to evaluate the shear and extensional effects of TSE and TTSE.And the relationship between parameters of equipment,flow effect and mixing properties of blends was established.Considering the differences on structure of TTSE and TSE,shear and extensional effect levels in different zones and number of zones,we propose the concept of flow influencing range,which provides theory support for evaluation of mixing properties of TTSE.The specific central zone owned by TTSE was simplified reasonably.We propose a 'circumferential shear' and 'axial extension' theoretical model to analyze the central zone of TTSE and evaluate the relationship between shear,extensional effects and geometric parameters,processing conditions of this zone.The common feature on flow effects in TTSE and TSE is that high shear effect lies in clearance zone and intermeshing zone,but its flow influencing range is relatively narrow.The average shear rate in screw channel zone is much lower than that of high shear rate region.But extensional effect was found in this zone,and its influencing range was extremely wide.The circumferential shear rate in central zone of TTSE is lower than that of screw channel zone.And the axial extensional rate is close to the level in screw channel zone.The research shows that the average shear rate in different zones of TSE is a little higher than that of TTSE,but influencing range is smaller than the latter.And the extensional effect in screw channel zone has similar features.A'converging-diverging' alternating changing flow channel is formed in the central zone of TTSE.It's the main reason that causing the occurrence of fiber-shaped dispersive phase in the high viscosity ratio(more than 4)blends.Compared to TSE,the impact strength and bending strength of the blend prepared by TTSE improve 20%and 17%,respectively.