Rheological Properties of Different Film Blowing Polyethylene Samples Under Shear and Elongational Flow

Abstract

Summary: The rheological behavior of polyethylenes is mainly dominated by the molecular weight, the molecular weight distribution and by the type, the amount and the distribution of the chain branches. In this work a linear metallocene catalyzed polyethylene (m‐PE), a branched metallocene catalyzed polyethylene (m‐bPE), a conventional linear low density polyethylene (LLDPE) and a low density polyethylene (LDPE) have been investigated in order to compare their rheological behavior in shear and in elongational flow. The four samples have similar melt flow index and in particular a value typical of film blowing grade. The melt viscosity has been studied both in shear and in isothermal and non‐isothermal elongational flow. The most important features of the results are that in shear flow the m‐PE sample shows less pronounced non Newtonian behavior while in the elongational flow the behavior of m‐PE is very similar to that of the linear low density polyethylene: the narrower molecular weight distribution and the better homogeneity of the branching distribution are reasonably responsible for this behavior. Of course the most pronounced non‐linear behavior is shown, as expected, by the LDPE sample and by the branched metallocene sample. This similar behavior has to be attributed to the presence of branching. Similar comments hold in non‐isothermal elongational flow; the LDPE sample shows the highest values of the melt strength and the other two samples show very similar values. As for the breaking stretching ratio the opposite is true for LDPE while m‐PE and LLDPE show higher values. The transient isothermal elongational viscosity curves show that the branched samples show a strain hardening effect, while LLDPE and m‐PE samples present a linear behavior.Dimensionless flow curves of different polyethylene samples.magnified imageDimensionless flow curves of different polyethylene samples.

Investigation of the molecular weight of polyethylene using rheological techniques

“…It is important to elucidate these aspects of molecular structure of polyolefins both in the production of polymers and their end‐use properties. It is well known that besides the dependence of the rheological behavior on external parameters such as shear rate and temperature, the rheological properties of polymer melts are strongly affected by the molecular structure 1–3. Since the rheological testing has its own advantage compared with gel permeation chromatography (GPC), temperature rising elution fractionation (TREF), 13 C‐NMR, etc, it is necessary to get some insight into the relationship between rheological properties and molecular characteristics of polyolefins 4…”

A decentralized and fault‐tolerant Desktop Grid system for distributed applications

“…In the family of polyethylenes, metallocene linear low‐density polyethylene (mLLDPE) presents superior physical and mechanical properties as compared to polyethylenes with the same average molecular mass produced using conventional catalysts. However, due to its molecular characteristics (narrow distribution of molecular mass with short chain branching and homogeneous distribution of the chain branching), the processability of mLLDPE is poor,3–6 so a high melt pressure and a high motor load during the extrusion are required. Consequently the market penetration of mLLDPE has been much slower than predicted earlier.…”

“…Some papers dealing with mLLDPE/LDPE blends have been published, mostly reporting on miscibility, rheological, thermal and mechanical properties 5,6,8,11–21. In general, for a given molecular mass, all the quoted papers stated that in mLLDPE/LDPE blends the long chain branching is the primary factor controlling the rheological and processing behavior of polyethylenes.…”

Structure and Morphology Development in Films of mLLDPE/LDPE Blends During Blowing

“…However, due to its molecular characteristics (narrow distribution of molecular mass with short chain branching and homogeneous distribution of the chain branching), the processability of mLLDPE is poor, [3][4][5][6] so a high melt pressure and a high motor load during the extrusion are required. Consequently the market penetration of mLLDPE has been much slower than predicted earlier.…”

“…[17] Some papers dealing with mLLDPE/LDPE blends have been published, mostly reporting on miscibility, rheological, thermal and mechanical properties. [5,6,8,[11][12][13][14][15][16][17][18][19][20][21] In general, for a given molecular mass, all the quoted papers stated that in mLLDPE/LDPE blends the long chain branching is the primary factor controlling the rheological and processing behavior of polyethylenes. It is shown that the addition of small amounts of high molecular mass distribution PE to a narrow molecular weight distribution mLLDPE promotes higher recoverable shear or elasticity.…”