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Rheological behaviour of nano-composites based on polyamide 6 under shear and elongational flow at high strain rates

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Abstract

In this work, the rheological behaviour of high molecular mass polyamide 6 (PA6)/organo-montmorillonite nano-composites, obtained via melt blending, was investigated under shear and extensional flow. Capillary rheometry was used for the measurement of high shear rate steady state shear viscosity and die entrance pressure losses; further, by the application of a converging flow method (Cogswell model) to these experimental results, elongational viscosity data were indirectly calculated. The extensional behaviour was directly investigated by means of melt spinning experiments, and data of apparent elongational viscosity were determined. The results evidenced that the presence of the organo-clay in filled PA6 melts modifies the rheological behaviour of the material, with respect to the unfilled polymer, in dependence on the type of flow experienced by the fluid. In shear flow, the nano-composites showed a slightly lower viscosity than neat PA6, whereas in elongation, they appeared much more viscous, in dependence on the organo-clay content.

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Abbreviations

a :

stretching acceleration (mm/s2)

a T :

shift factor for the application of the time-temperature superposition principle

d :

diameter of the die (mm)

D :

diameter of the barrel (mm)

DR:

draw ratio, defined as the ratio of the stretching velocity (\({v}_{s}\)) to the die extrusion velocity (\({v}_{0}\))

E 0 :

viscous flow activation energy (kJ/mol)

F :

force exhibited by the polymer filament in resisting extension during a melt spinning test (cN)

GR:

gearbox rating of the extruder used in compounding operation (N·m)

k :

consistency in the apparent shear viscosity power-law equation (\(\eta_{\rm sh,a} = k\cdot {\dot{\gamma}_{\rm app}^{n-1}}\)), (Pa·sn)

l :

length of the capillary (mm)

L :

length of the spinline (mm)

n :

exponent in the apparent shear viscosity power-law equation (\(\eta_{\rm sh,a} = k\cdot {\dot{\gamma}_{\rm app}^{n-1}}\))

\({{\kern2pt}\dot{{\kern-2pt}M}}\) :

feed rate in compounding operation (kg/h)

N :

screw speed of the extruder in compounding operation (RPM)

Q :

volume flow rate (mm3/s)

R :

gas constant, R = 8.315 ·10 − 3 kJ/(mol·K)

T :

temperature (°C)

T 0 :

reference temperature for the construction of the master curve of η sh,app (°C)

\({v}_{\rm s}\) :

stretching velocity (at the stretching wheels) (\({v}_{\rm s} ={v}_0 +a\cdot t\)), (mm/s)

\({v}_{0}\) :

extrusion velocity in the capillary (\({v}_0 =\frac{Q}{\frac{\pi}{4}\cdot d^2}\)), (mm/s)

\({\dot{\gamma}}\) :

(true) shear rate at the die wall determined by the application of the Rabinowitsch–Weissenberg method (s − 1)

\({\dot{\gamma}_{\rm app}}\) :

apparent shear rate at the die wall (\({\dot{\gamma}_{\rm app}} = \frac{32\cdot Q}{\pi \cdot d^3}\)) (s − 1)

\(\mathit{\Delta} \eta_{\rm el,r}\) :

relative extensional viscosity increase for a polymer suspension with respect to the unfilled polymer, for a fixed elongational strain rate

\(\mathit{\Delta} P_{\rm ent}\) :

pressure losses localised at the die entrance (Pa)

ε dr :

elongational strain experienced by a polymer filament during the drawing process in a melt spinning experiment

\({{\kern1pt}\dot{{\kern-1pt}\varepsilon}}\) :

elongational strain rate (s − 1)

η el :

elongational viscosity (Pa·s)

η el,app :

apparent elongational viscosity determined from melt spinning experiments (Pa·s)

η sh :

(true) shear viscosity, defined as the ratio of the true shear stress (τ), corrected for entrance pressure drops (\(\mathit{\Delta} P_{\rm ent}\)), to the true shear rate (\({\dot{\gamma}}\)), (Pa·s)

η sh,a :

apparent shear viscosity, defined as the ratio of the true shear stress (τ), corrected for entrance pressure drops (\(\mathit{\Delta} P_{\rm ent}\)), to the apparent shear rate (\({\dot{\gamma}_{\rm app}}\)), (Pa·s)

η sh,app :

apparent shear viscosity, defined as the ratio of the apparent shear stress (τ app), uncorrected for entrance pressure drops (\(\mathit{\Delta} P_{\rm ent}\)), to the apparent shear rate (\({\dot{\gamma}_{\rm app}}\)), (Pa·s)

τ :

(true) shear stress at the die wall, corrected for entrance pressure drops (\(\mathit{\Delta} P_{\rm ent}\)), (Pa)

τ app :

apparent shear stress at the die wall, uncorrected for entrance pressure drops (\(\mathit{\Delta} P_{\rm ent}\)), (Pa)

%T :

torque fraction used in compounding operation (%)

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Acknowledgements

The authors are grateful to Dr. M. Grosso and Ing. M. Bronzoni of Ceast SpA (Pianezza, Torino, Italy) for the capillary rheometer kindly placed at their disposal and to Dr. A. Filippi and Dr. C. Armanni of Radici Novacips SpA (Chignolo d’Isola, Bergamo, Italy) for the materials kindly supplied.

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Authors and Affiliations

  1. Dipartimento di Chimica e Fisica per l’Ingegneria e per i Materiali, Università degli Studi di Brescia, Via Valotti 9, Brescia, 25123, Italy

    Francesco Baldi, Anika Franceschini, Fabio Bignotti & Theonis Riccò

  2. Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, Milano, 20133, Italy

    Giuseppe Tieghi

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  1. Francesco Baldi
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  2. Anika Franceschini
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Correspondence to Francesco Baldi.

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Baldi, F., Franceschini, A., Bignotti, F. et al. Rheological behaviour of nano-composites based on polyamide 6 under shear and elongational flow at high strain rates. Rheol Acta 48, 73–88 (2009). https://doi.org/10.1007/s00397-008-0315-y

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