Abstract
The effects of temperature, pressure and shear stress on the viscosity of simplified automotive lubricants—polymer-thickened base oil solutions—were investigated. Various polymers—with different molecular weights and conformations (comb, linear and star)—were used at low concentration (1.2 % w/w) in a hydrocracked mineral base oil: a poly(alkylmethacrylate), an olefin copolymer and a poly(isoprene-styrene hydrogenated). Their rheological behavior was studied and modeled with a Vogel-Tamman and Fulcher equation, a modified Williams-Landel-Ferry-Yasutomi relationship and a Carreau-Yasuda-like formula. Then, the Einstein’s law was used to rapidly and simply determine the hydrodynamic radii of polymers as a function of temperature and pressure. Calculations from Flory equations, intrinsic viscosities and direct measurements confirmed the relevance of this methodology. Finally, molecular considerations allowed a good understanding of the rheological response of polymer solutions.
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Abbreviations
- a, n :
-
Parameters for the Carreau-Yasuda model
- C 1, C 2, A 1, A 2, B 1, B 2 :
-
WLF constants
- c :
-
Concentration in weight of active polymer (g cm−3)
- c*:
-
Critical concentration (g cm−3)
- D F :
-
Fragility parameter
- F :
-
Dimensionless relative thermal expansion of the free volume
- G :
-
Shear modulus (kPa)
- h :
-
Film thickness (nm)
- k :
-
Liquid thermal conductivity (W m−1 K−1)
- l :
-
Mean monomer size (cm)
- M :
-
Mass of one molecule (g)
- m0 :
-
Molecular mass of one monomer (g mol−1)
- M n :
-
Number average molecular mass (g mol−1)
- M w :
-
Weight average molecular mass (g mol−1)
- N :
-
Number of monomers (N = M w/m0)
- N a :
-
Avogadro number (mol−1)
- Na :
-
Nahme-Griffith number
- N experiment :
-
Number of experimental data
- N parameter :
-
Number of parameters in the model
- n :
-
Refractive index
- PDI:
-
Polydispersity index
- p :
-
Pressure (Pa)
- R g :
-
Radius of gyration (nm)
- R h :
-
Hydrodynamic radius (nm)
- Relative root mean square error:
-
\(\sqrt {\frac{{\sum {\left( {\frac{{\eta_{\text{experiment}} - \eta_{\text{model}} }}{{\eta_{\text{experiment}} }}} \right)^{2} } }}{{N_{\text{experiment}} - N_{\text{parameters}} }}}\)
- T :
-
Temperature (°C)
- \(T_{\infty }\) :
-
Vogel temperature at which the viscosity diverges (°C)
- T g(0):
-
Glass transition temperature at atmospheric pressure (°C)
- T g(p):
-
Glass transition temperature depending on pressure (°C)
- α :
-
Local pressure–viscosity coefficient (GPa−1)
- α*:
-
Reciprocal asymptotic isoviscous pressure coefficient (GPa−1)
- β :
-
Temperature–viscosity coefficient (K−1)
- Φ :
-
Volume fraction
- Φ 0 :
-
Universal Flory constant (mol−1)
- [η]:
-
Intrinsic viscosity (L g−1)
- η :
-
Viscosity (Pa s)
- η experiment :
-
Experimental viscosity (Pa s)
- η model :
-
Modeled viscosity (Pa s)
- η red :
-
Reduced viscosity (L g−1)
- η r :
-
Relative viscosity (cm3 g−1)
- \(\eta_{0}\) :
-
Low shear viscosity (Pa s)
- \(\eta_{\infty }\) :
-
Viscosity extrapolated to infinite temperature (Pa s)
- \(\eta_{g}\) :
-
Viscosity at the glass transition (Pa s)
- τ :
-
Shear stress (Pa)
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Acknowledgments
The authors are deeply grateful to Total for its financial and technical support. We would like to thank Dr. J. Eckelt and Prof. Dr. B.A. Wolf from WEE-Solve (Auf der Burg 6, D-55130 Mainz, Germany) for having performed LCST measurements. We also acknowledge Prof. C. Chassenieux, Prof. J.-F. Tassin and D. Chaveroux from the laboratory Polymères, Colloïdes, Interfaces (UMR Université du Maine CNRS-6120, 72085 Le Mans Cedex 9, France) for having kindly provided chromatography and light scattering results.
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Mary, C., Philippon, D., Lafarge, L. et al. New Insight into the Relationship Between Molecular Effects and the Rheological Behavior of Polymer-Thickened Lubricants Under High Pressure. Tribol Lett 52, 357–369 (2013). https://doi.org/10.1007/s11249-013-0214-y
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DOI: https://doi.org/10.1007/s11249-013-0214-y