Abstract
The time-dependent viscoelastic behaviors of a low-density polyethylene melt (LDPE) in a triangular- and trapezoidal-loop shear experiment reported previously are described here by an integral-type Rivlin–Sawyers (RS) constitutive equation. The linear viscoelasticity of the melt was obtained through a dynamic frequency sweep experiment at a small strain and fitted by a relaxation spectrum. The nonlinear viscoelasticity was characterized by viscosity. All the experimental viscoelastic behaviors of the melt can be divided into two types in terms of the predictions of the RS model: (1) predictable time-dependent viscoelastic behaviors at low shear rates or during short-term shear, and (2) unpredictable shear weakening behavior occurring at the high shear rate of 3–5 s−1 during long-term shear with the characteristic time interval of about 40–100 s. The influence of experimental error caused possibly by inhomogeneous samples on the viscoelasticity of the melt was analyzed, and the large relative error in the experiment is about 10–30%.
Highlights
There are many flow phenomena in the plastic and rubber industry [1], e.g., profile extrusion, film casting, and molding, and the shear viscosity of polymer melts is a fundamental parameter governing these flows since shear viscosity is related to the flow loss or pressure drop in flows
This study aims to extensively understand the transient shear viscoelastic property of a polymer melt, other viscoelasticity, such as extensional, is of importance in practice
Five types of shear viscoelastic properties are usually included in these publications, which are: (1) linear viscoelastic property, i.e., frequency sweep at small strain, (2) steady shear viscosity, (3) steady first normal stress difference (N1), (4) shear stress growth in step rate experiment, and (5) N1 growth in step rate experiment
Summary
There are many flow phenomena in the plastic and rubber industry [1], e.g., profile extrusion, film casting, and molding, and the shear viscosity of polymer melts is a fundamental parameter governing these flows since shear viscosity is related to the flow loss or pressure drop in flows. This study aims to extensively understand the transient shear viscoelastic property of a polymer melt, other viscoelasticity, such as extensional, is of importance in practice. The shear viscoelastic properties of variety of commercial or industry-grade polymer melts have been published, e.g., low-density polyethylene (LDPE) [2,3,4,5,6,7,8], linear low-density polyethylene (LLDPE) [9,10], high-density polyethylene (HDPE) [4,5,10,11,12], polypropylene (PP) [13,14,15], polystyrene (PS) [4,16], and polyamide 6 (PA6) [17]. The published works above—and other similar and unlisted studies not discussed here—raise the understanding of the viscoelastic properties of industrial polymer melts. The applications of the reported experimental viscoelastic properties of polymer melt that are seen in publications are mainly based on two aspects. One aspect is to examine the theoretical model of viscoelastic property of polymer melt [3,5,6,7,8,9,10,11,13,15,16,17,18,19,20,21,22], and the other is to simulate the flow in polymer processing numerically according to the published viscoelastic experimental data [23,24,25,26,27,28]
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