Rheological characterization of polymeric materials

Seppo Syrjälä, Johanna Aho, Heikki Parviainen

Research output: Book/ReportReportProfessional

Abstract

Reliable characterization of the rheological behaviour of polymer melts is of significant practical importance. It is well known that the rheological properties of polymers depend strongly on the underlying molecular structure such as molecular mass, molecular mass distribution and degree of long-chain branching. The data from the rheological measurements can therefore provide the link between molecular structure, processability, and end-use properties of polymers. Accurate rheological data are also needed for the numerical simulations of polymer processing flows. In industrial practice, some kinds of fillers are frequently added into polymers. Most frequently used fillers are inorganic in nature; examples include calcium carbonate, mica, talc, kaolin, wollastonite, glass bead and glass fibre. One of the original aims of introducing fillers was simply to reduce the cost of the polymers by using inexpensive fillers. At present, however, fillers are generally employed to improve the properties of polymer products. It is obvious that the addition of particulate fillers into polymers changes the rheology of the material, which in turn influences the processing behaviour. The presence of fillers can complicate the measurement and modelling of the flow behaviour of the material due to the onset of phenomena not necessarily seen in the unfilled material, for example wall slip and yield behaviour. Even though polymer melts are known to exhibit both viscous and elastic response, the key rheological property in most situations is the shear viscosity and its dependence on shear rate and temperature and possibly on pressure. The effect of pressure on the viscosity of polymer melts is quite often completely neglected, which obviously does not always reflect the reality. The assumption of pressure-independent viscosity can be justified for most extrusion operations, whereas in the case of injection moulding this assumption is much more questionable. Especially in thin-wall injection moulding applications pressures in excess of 100 MPa frequently occur, which can cause a significant increase in the polymer viscosity. For example, in the simulations of the injection moulding process the omission of these pressure effects may lead to largely inaccurate predictions of pressure and other variables. Instruments that are generally used to characterize the rheological properties of polymeric materials are the capillary rheometer and the rotational rheometer. The former is primarily intended for the measurement of shear viscosity, while most of the latter types of equipments are capable of yielding both the viscous and elastic properties of the material. As is well known, several corrections have often to be applied to the measured raw data, if the true rheological values are desired. This is especially the case in capillary rheometry; for details see [1-5]. This report covers the following topics, which constitute the essence of the research activity in this project directed to the rheological characterization of polymeric materials (some other more industrially-oriented subjects were also considered, but the results are not included in this report): - Bagley and Rabinowitch corrections in capillary rheometry - Slip on the capillary wall with a highly filled thermoplastic elastomer - Flow instability in the capillary flow of a HDPE melt - Pressure dependence of viscosity of polymer melts - Yield stress phenomenon with filled polymers - Effect of molecular mass distribution on the polymer viscosity - Cox-Merz rule.
Original languageEnglish
Place of PublicationEspoo
PublisherVTT Technical Research Centre of Finland
ISBN (Electronic) 978-951-38-7201-4
ISBN (Print) 978-951-38-7200-7
Publication statusPublished - 2008
MoE publication typeD4 Published development or research report or study

Publication series

NameVTT Tiedotteita - Research Notes
PublisherVTT
No.2428
ISSN (Print)1235-0605
ISSN (Electronic)1455-0865

Fingerprint

Polymers
Fillers
Polymer melts
Viscosity
Molecular mass
Injection molding
Shear viscosity
Rheometers
Molecular structure
Talc
Capillary flow
Kaolin
Filled polymers
Thermoplastic elastomers
Pressure effects
Calcium Carbonate
Polyethylene
Processing
Rheology
Shear deformation

Cite this

Syrjälä, S., Aho, J., & Parviainen, H. (2008). Rheological characterization of polymeric materials. Espoo: VTT Technical Research Centre of Finland. VTT Tiedotteita - Meddelanden - Research Notes, No. 2428
Syrjälä, Seppo ; Aho, Johanna ; Parviainen, Heikki. / Rheological characterization of polymeric materials. Espoo : VTT Technical Research Centre of Finland, 2008. (VTT Tiedotteita - Meddelanden - Research Notes; No. 2428).
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Syrjälä, S, Aho, J & Parviainen, H 2008, Rheological characterization of polymeric materials. VTT Tiedotteita - Meddelanden - Research Notes, no. 2428, VTT Technical Research Centre of Finland, Espoo.

Rheological characterization of polymeric materials. / Syrjälä, Seppo; Aho, Johanna; Parviainen, Heikki.

Espoo : VTT Technical Research Centre of Finland, 2008. (VTT Tiedotteita - Meddelanden - Research Notes; No. 2428).

Research output: Book/ReportReportProfessional

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N2 - Reliable characterization of the rheological behaviour of polymer melts is of significant practical importance. It is well known that the rheological properties of polymers depend strongly on the underlying molecular structure such as molecular mass, molecular mass distribution and degree of long-chain branching. The data from the rheological measurements can therefore provide the link between molecular structure, processability, and end-use properties of polymers. Accurate rheological data are also needed for the numerical simulations of polymer processing flows. In industrial practice, some kinds of fillers are frequently added into polymers. Most frequently used fillers are inorganic in nature; examples include calcium carbonate, mica, talc, kaolin, wollastonite, glass bead and glass fibre. One of the original aims of introducing fillers was simply to reduce the cost of the polymers by using inexpensive fillers. At present, however, fillers are generally employed to improve the properties of polymer products. It is obvious that the addition of particulate fillers into polymers changes the rheology of the material, which in turn influences the processing behaviour. The presence of fillers can complicate the measurement and modelling of the flow behaviour of the material due to the onset of phenomena not necessarily seen in the unfilled material, for example wall slip and yield behaviour. Even though polymer melts are known to exhibit both viscous and elastic response, the key rheological property in most situations is the shear viscosity and its dependence on shear rate and temperature and possibly on pressure. The effect of pressure on the viscosity of polymer melts is quite often completely neglected, which obviously does not always reflect the reality. The assumption of pressure-independent viscosity can be justified for most extrusion operations, whereas in the case of injection moulding this assumption is much more questionable. Especially in thin-wall injection moulding applications pressures in excess of 100 MPa frequently occur, which can cause a significant increase in the polymer viscosity. For example, in the simulations of the injection moulding process the omission of these pressure effects may lead to largely inaccurate predictions of pressure and other variables. Instruments that are generally used to characterize the rheological properties of polymeric materials are the capillary rheometer and the rotational rheometer. The former is primarily intended for the measurement of shear viscosity, while most of the latter types of equipments are capable of yielding both the viscous and elastic properties of the material. As is well known, several corrections have often to be applied to the measured raw data, if the true rheological values are desired. This is especially the case in capillary rheometry; for details see [1-5]. This report covers the following topics, which constitute the essence of the research activity in this project directed to the rheological characterization of polymeric materials (some other more industrially-oriented subjects were also considered, but the results are not included in this report): - Bagley and Rabinowitch corrections in capillary rheometry - Slip on the capillary wall with a highly filled thermoplastic elastomer - Flow instability in the capillary flow of a HDPE melt - Pressure dependence of viscosity of polymer melts - Yield stress phenomenon with filled polymers - Effect of molecular mass distribution on the polymer viscosity - Cox-Merz rule.

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M3 - Report

SN - 978-951-38-7200-7

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Syrjälä S, Aho J, Parviainen H. Rheological characterization of polymeric materials. Espoo: VTT Technical Research Centre of Finland, 2008. (VTT Tiedotteita - Meddelanden - Research Notes; No. 2428).