Study On Mechanism Of Interfacial Distortion Of Multilayer Co-extrusion

Co-extrusion molding technology is the manufacturing technology of polymer multilayer composite materials,how to control the layer thickness uniformity in composite multilayer polymer co-extrusion process is the main technological bottlenecks in the preparation of high performance multilayer composites.The viscous encapsulation phenomena induced Interfacial distortion and layer thickness reconstruction will seriously affects quality of high performance multilayer composite products in multilayer composite polymer co-extrusion molding process.To solve this technical bottleneck,it is possible to realize the precise control of the thickness uniformity of coextrusion process of polymer multilayer composite materials.The the theoretical key to solve this technology bottleneck is to study and establish the mechanism theory of viscous encapsulation formation.In this way,the formation mechanism of viscous encapsulation in multiphase multilayer viscoelastic melt coextrusion molding process was revealed.It has important theoretical and engineering value for solving the major technical bottlenecks in the preparation of high performance multilayer composites product.Therefore,the effects of viscoelasticity,rheological parameters and process parameters of polymer on viscous encapsulation in multiphase viscoelastic rectangular coextrusion flow forming process are systematically studied.Based on the study of the relevance theory of viscous encapsulation-secondary flow-second normal stress difference,an attempt is made to establish a universal and scientific theory of viscous encapsulation mechanism of multiphase multilayer viscoelastic melt flow,which will lay a scientific foundation for developing precise shape control technology of layer thickness uniformity in coextrusion process of polymer multilayer composite materialsFor the first time,a scientific assumption is proposed in which the viscous encapsulation of multiphase stratified viscoelastic melt flow is induced by the second flow driven by the second normal stress difference of melt near the interface.The scientific assumption is that the trend of viscous encapsulation is controlled by the secondary flow direction of the melt near the stratified interface,and the degree of viscous encapsulation is controlled by the secondary flow velocity magnitude of the melt near the stratified interface.It is found that when the low viscosity upper melt relaxation time is increased from 0.4s to 1.8s,the three-dimensional viscous encapsulation interface morphology of two layered coextrusion at first takes on positive direction viscous encapsulation phenomenon,but with the increase of the low viscosity upper melt relaxation time,viscous encapsulation interfacial morphology evolve into opposite direction viscous encapsulation phenomenon,and the degree of three-dimensional viscous encapsulation increases with the increase of the low viscosity upper melt relaxation time.Obviously,this research results is inconsistent with minimum viscous dissipation principle of viscous encapsulation in which low viscosity melt tending to encapsulate high viscosity melt,and violate interfacial normal stress balance principle of viscous encapsulation which suggests when both viscous and elastic stratification exist simultaneously,the viscous encapsulation is controled by melt viscidity.therefore existing two traditional mechanism theories of viscous encapsulation cannot explain the evoluting mechanism of this viscous encapsulation interfacial morphology.It is found that when the low viscosity upper melt material coefficient is increased from 0.2 to 0.8,the three-dimensional viscous encapsulation interface morphology of two layered coextrusion at first takes on positive direction viscous encapsulation phenomenon,but with the increase of the low viscosity upper melt material coefficient,viscous encapsulation interfacial morphology evolve into opposite direction viscous encapsulation phenomenon,and the degree of three-dimensional viscous encapsulation increases with the increase of the low viscosity upper melt material coefficient.Obviously,this research results is inconsistent with minimum viscous dissipation principle of viscous encapsulation in which low viscosity melt tending to encapsulate high viscosity melt,and violate interfacial normal stress balance principle of viscous encapsulation which suggests when viscous stratification exist,the viscous encapsulation is mainly governed by melt elasticity.With the increase of low viscosity upper melt inlet flow,the two layer co extrusion viscous encapsulation interfacial morphology embodies mainly opposite direction viscous encapsulation phenomenon in which the high viscosity lower melt tending to encapsulate the low viscosity upper melt,and the degree of three-dimensional viscous encapsulation increases with the increase of the low viscosity upper melt inlet flow.According to the scientific assumptions of viscous encapsulation mechanism proposed in this paper,the mechanism deducing results of the positive and opposite direction viscous encapsulation induced by varing of melt viscoelastic rheological parameters and process parameters are consistent with the simulation results of the viscous encapsulation interfacial morphology.According to the viscous encapsulation mechanism proposed in this paper,the secondary flow direction and flow strength predicted by the second normal stress difference of the melt near the layered interface agree well with the simulation results of the secondary flow.It is proved that the mechanism theory of viscous encapsulation in this paper is universal and reliable under different viscoelastic rheological parameters and process parameters condition.