Reseach On Elongational Rheological Behavior Of Polyethylene Melts Flow In Contraction Channel

Elongational flow with high strain rate of polymer melt flow in a constrtion channel iscommon in polymer processing process. Resently, the polymer plasticization and transportmethod based on volume elongational deformation make elongational flow to be the domin inmelt transporting process. And many special viscoelastic behaviors can happen at the channelentrance because of the significant non-Newtonian properties (Trouton numberT r>>3) andviscoelastic properties of the polymer melts.These behaviors are affected not only by thefactors including channel geometry parameters, flow rate and temperature etc., but essentiallyalso by the molecular chain structure and nature of the polymer. So it’s scientific andenginering valuable for study on the elongational rheological behaviors of different polymermelts flow in the contraction channel.In this paper, a multi-mode double convected Pom-Pom model, which bases on thebranched molecule structure and pipe model, was chosen as the constitutive equation used inthe theoretical simulation. The affect of the molecular structure parameters on DCPP modeland the polymer melt viscosity properties in simple shear and single planar elongational flowconducted from DCPP model were discussed. The results show that the visicositycharacteristics of the LDPE and HDPE applied in this work could acquire a good consistencybetween measured data and multi-mode DCPP model prediction.LDPE、LLDPE and HDPE were applied in this work. By using GPC, DSC and dynamicrheological tests, it’s found that the chain structures of thee thee Pes were very different inspite of their same repeat units. The LDPE’s chain is long-chain branched,the LLDPE’s isshort-chain branched and molecular weight distribution is narrow, the HDPE is linear chainstructure and most wide molecular weigh distribution; The three PEs have differentviscoelasticity, the melt elasticity of LDPE is the highest,then HDPE and LLDPE.A measure system was developed. With this system，the melt flow rate can be preciselycontroled and the entrance flow pattens can be recorded by high speed video camera..and fourflow pressures can be acquried. Particle Image Velocimetry was used to measure particlemotion velocity precisely. The pressure datas were used to caculate the shear and elongationalviscosity properties.The power law relation between shear viscosity and shear strain rate of three PEs incapillary and slit channel were caculated, and then the relation between elongational viscosityand elongational strain rate in capillary and slit channel by using Binding method. Althoughthe shear viscosities in capillary and slit were almost the same, the elongational viscosities were not. It’s not suitable for measuring shear or elongational viscosity when the slit widthexceeds2mm.This work studied the elongational rheological behavior of three PEs flow in the abruptcontracted slit, and discussed the mechanism of different entrance effect with different chainstructure materials. The result indicated that the material elasticity had significant i influenceon the entrance flow characteristics.For LDPE melt which has obvious elasticity compared tothe other two PEs, the flow accelerated and elongated more earlier in the upsteram, theelongational strain rate is higher and the flow emerged obvious velocity overshoot after itentried the entrance.By contrast the lowest elastic LLDPE flow emerged elongating pointclose to the entrance,and no velocity overshoot; HDPE’s flow emerged velocity overshoot butweaker then LDPE’s because HDPE’s elasticity was lower than LDPE’s. The simulationresults using DCPP model showed the velocity overshoot characteristics and overshootextent’s difference between LDPE and HDPE, simulation result and experimental result havea good consistency. The elongation increases with constraction rate and volume flow rateincreases, and melt flow velocity of center symmetry has a near linear relation with width ofdie and extrusion flow rate. Melt elasticity also affect the entrance pressure drop, entrancepressure drop increases obviously with elasticity increases, and more sensitive to constractionrate and volume flow rate. Viscoelastic effect of slit entrance is obviously related to molecularchain structure, only the flow of long-chain branched LDPE has obvious corner votex and lipvortex., and the streamline and corner votex shows instability when the flow rate exceedssome value, But there were no corner votex in the flow of linear chain HDPE and short-chainbranched LLDPE and the streamlines were stable. These diffrent phenomenas are coursedessentially by the diffrent steric bulk of the molecule chian at entrance, for linear chain, it canbe oriented along the streamline direction by elongating so little steric bulk can happen and itflow stablely, for long-chain branched molecules, however, the backbone can hardly beoriented by elongating,during the entrance flow period when flow rate increasing, thus leadsto partial steric bulk.so the streamline oscillation and corner votex patten became enlarged.