Rheological properties and melt strength of LDPE during coextrusion process

Abstract

Rheological properties and mechanical strength of coextruded low‐density polyethyelene (LDPE) were studied in this work. The influences of die temperature, shear rate at skin and core layers, and number of extrusion passes at the core layer were of interest. The experimental results suggested that the viscosity and swelling behavior of LDPE coextrudate were more dependent on the shear rate of skin layer as compared with that of core layer. Increasing die temperature resulted in decreases in viscosity and die swell ratio. The reductions in viscosity and die swell ratio of the coextrudate for the reprocessing numbers 1 and 2 at the core layer arose from the disentanglement of long chain branching of LDPE molecules, while the increase of viscosity and die swell ratio for the reprocessing numbers 3 and 4 mainly involved an occurrence of crosslinking structure. For mechanical strength results, the changes in elongational stress for different shear rates, die temperature, and number of extrusion passes were in line well with the rheological measurements. That was, the higher the stored energies in polymer melts due to higher shear rates, the greater the amount of force required to conquer the elastic resistance. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Improvement of Poly(ethylene terephthalate) Melt-Foamability by Long-Chain Branching with the Combination of Pyromellitic Dianhydride and Triglycidyl Isocyanurate

“…20 Frequency functions in shear and elongational rheology are the two main methods to characterize viscoelasticity. 21,22 Strain-hardening behavior in elongational flow is an important component of viscoelasticity. Thus, in the previous work, the tensilehardening behaviors of polymers were used to describe the melt foamability.…”

“…Extensive work has demonstrated that the polymer foamability is closely related to its viscoelasticity . Frequency functions in shear and elongational rheology are the two main methods to characterize viscoelasticity. , Strain-hardening behavior in elongational flow is an important component of viscoelasticity. Thus, in the previous work, the tensile-hardening behaviors of polymers were used to describe the melt foamability. − This hypothesis assumes that the apparent tensile-hardening behavior prevents the bubbles from cracking and thus corresponds to good foamability.…”

Mechanical fatigue of recycled and virgin high‐/low‐density polyethylene

“…Stabilizing additives, such as peroxides, can enhance the tetra‐ and tri‐functional branches in the polyethylene chains 24 and crosslinks 25 . On the other hand, low‐density polyethylene (LDPE) is already a highly branched polymer, so the extrusion process can result in further branching or crosslinking reactions generating more end groups and decreasing the mechanical properties 7,26–29 . Radical attacks can increase the scission rate in an oxidative environment, 30,31 while macroradicals can lead to higher branching reaction in inert or reducing environments 15,32–34 …”

“…25 On the other hand, low-density polyethylene (LDPE) is already a highly branched polymer, so the extrusion process can result in further branching or crosslinking reactions generating more end groups and decreasing the mechanical properties. 7,[26][27][28][29] Radical attacks can increase the scission rate in an oxidative environment, 30,31 while macroradicals can lead to higher branching reaction in inert or reducing environments. 15,[32][33][34] A high amount of research was performed on the mechanical degradation of recycled polymers such as PE with similar physical properties.…”

Three‐dimensional isothermal simulation of PP melt flow in laminating‐multiplying elements based on the finite element method

“…Until now, most studies concerning layer interface deformation concentrate on traditional coextrusion processing, which shows relatively simple flow behavior . Experimental and numerical analysis methods were widely used to study these problems in the first 20 years of development; however, these methods were gradually replaced by the finite element method (FEM), which is a powerful tool for easily and quickly studying the rheology of polymer melts in a mold.…”