Molecular modelling, the future of lubrication research and development

Modeling Molecular Structure to Tribological Performance

SAE World CongressDetroit, MichiganApril 2016

Chad Chichester, Aleksandra Nevskaya,

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Outline of Presentation

• Current Siloxane Primer• Molecular Structure to Rheology to Tribology• Results, Model-Guided Design• Future technologies

2014 Dow Corning . All rights reserved

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Siloxanes: Functionalized groups in organo-silicon chemistry with Si–O–Si backbone

• Strong Bonds: Si-O (460 kJ/mol) vs. C-C (348 kJ/mol)• Long Bonds: Si-O (0.164 nm) vs. C-C (0.153 nm)• Flexible Bonds: Si-O-Si (143º) vs. C-C-C (110º) • Low Steric Hindrance: Unencumbered Oxygen• Low Glass Transition Temperature (148 K)• High Oxidative Stability PDMS (573 K) to PPMS (649 K)• Thermal-Viscous Stability: PDMS (15 kJ/mol) vs. PAO

(30 kJ/mol)• Permanent Shear Stability: Very Low Monomeric

Friction

CH3

SiH3C

CH3

OSi

OCH3

CH3

SiCH3

CH3

CH3n


= Si = O = C = H

• Viscosity range 0.65 to 1.000.000 cSt• Very High Viscosity Indices• Excellent thermal & oxidative stability• Excellent low temperature flow ability • Low volatility (even low viscosities)• Good plastic and rubber compatibility • High chemical resistance• Insoluble in water

Three Primary Types of Silicone Polymers

• “Standard” SiliconesDimethy

l Silicone

• Additional thermal and oxidation stability

Phenyl-Methyl Silicone

• Excellent chemical resistant• Better load carrying capacity

and wear resistance

Fluoro Silicone

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H3C –Si – O – Si – O — Si – O – CH3

CH3 CH3

n

CH3

CH3CH3CH3

H3C –Si – O – Si – O — Si – O – CH3

CH3 CH3

n

CH3

CH3C6H5CH3

H3C –Si – O – Si – O — Si – O – CH3

CH3 CH3

n

CH3

CH3

CH2CH3 CH2

CF3

comparison

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SRVok load (N)

4-ball scar (mm)

400N/1hr

DSC, OOT

Visc -35°C

Visc40°C

Visc100°C VI

PAO-6 350 0.822 202 3424 26 5.6 163PDMS 300 SEVERE 286 234 35 16 466PDMS, Formulated 975 1.378 221 141 22 9.7 473Dimethyl+hexyl 2000 0.879 204 x 28 8.3 298PFPE-Y Branched 2000 0.727 500 54450 94 15.1 169Trifluoropropyl 550 1.182 246 36386 159 29.6 228PFPE-Z Linear 600 1.485 500 3591 160 46.3 332

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Film FrictionElastohydrodynamic Non-Conformal contact Hamrock-Dowson

Film thickness depends on:• Entrainment Speed• Contact Load as function of

Applied load Poison ratio of materials Young’s modulus of materials

• Geometry as function of Surface roughness of materials Diameter of moving parts

• Lubricant’s Viscosity as function of its Monomeric friction Radius of gyration Activation energy (temperature

dependence) Pressure viscosity index (pressure

dependence)


Conventional Film Thickness Modeling Equation Example

Applying model to siloxanes assumes behavior similar to mineral oil.

Idea…Rather than repeat empirical approach…Develop molecular structure approach

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Model-Based Siloxane Structure DesignFrom Molecule to Tribology

Molecular properties

Rheological properties Tribological parameters

Tribological Properties

• Molecular structure• Molecular mass

distribution

• Volume structure andVolume Pressure Temperature• Viscosity structure andViscosity Pressure Temperature• Pressure viscosity index

• Geometry• Poisson ratio• Young Modulus• Load• Speed

• Asperity friction• Film friction


Siloxane Structures

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Si O Si OCH3

H3CCH3

CH3

SiCH3

CH3

O Si CH3

CH3

CH3m n

DP

J

QL


ZJLQv ,,,0 ZJLQ ,,,0

MPaPKT1.0298

MPaPKT1.0298

Molecular Structure – Specific Volume

Packing Factor

Wv

PTv ,Equation of State

Molecular Structure - Viscosity

PT ,

Viscosity (T,P)

Shear Viscosity s

00v

cXF

Viscosity Variation

Molecular to Rheological

v η

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The Stribeck Curve is divided into three primary regions Boundary Lubrication Mixed Lubrication Full Film Lubrication

22qBqA RR

h

3

10

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0 1 3

Asperity Friction

Film Friction

Total Friction Total

FilmAsp

Friction Components


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Rheology to Tribology

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Structural OptimizationThe optimization of molecular structure can be approached as follows

Feedback to Molecular Structure DesignVariation of Input

Parameters

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Siloxane StructuresPolydimethylsiloxane: PDMS

Polycyclohexylsiloxane: PCMS

Polyphenylmethylsiloxane: PPMS

CH3

SiH3C

CH3

OSi

OCH3

CH3

SiCH3

CH3

CH3nMw

increasePDMS

PPMS

PCMS

phenyl branch

cyclohexyl branch

dimethyl branch

NEW


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Siloxane StructuresCopolymers and pendant branches

PAMS

PPAMS

Polyphenylhexylsiloxane: PPAMS

Polyalkylmethylsiloxane: PAMS

Polytrifluoropropylsiloxane: PFMS

alkyl branch

phenylalkyl branch

ALLNEW

NEW


PA100: 100% Phenylalkyl 0% PDMSPA30: 30% Phenylalkyl 70% PDMS

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Graphic User Interface for SiloxanesSiloxane Species and

Structure

Interface Geometry and conditions

Rheological Properties

Tribological Performance

Si O Si OCH3

H3CCH3

CH3

SiCH3

CH3

O Si CH3

CH3

CH3m n

DP

J

QL


J

L

Q DP

Film Thickness and Friction


Newtonian and Non-Newtonian Siloxanes


PPMS 90 (90% Phenyl) A100-12 (100% Dodecyl)

Ring branches (PPMS and PCMS) show nearly Newtonian nature High aryl and cycloalkyl content causes high pressure-viscosity PPMS 90: High monomeric friction allows a relatively low molecular mass (Mw=1990

g/mol) to build viscosity, so shear thinning is low

Linear branches may exhibit temporary shear-thinning Low to high alkyl branch length causes low pressure-viscosity coeff. A100-12: Low monomeric friction requires a relatively high molecular mass

(Mw=29900 g/mol) to build viscosity, so shear thinning is high

EHD Friction Increases With Cyclic Branch Content


PPMS:Phenyl-methyl PCMS:Cyclohexyl-methyl

Friction at 398 K and Σ=0.5

Cyclohexyl siloxanes have greater thermal stability of performanceFriction at 303 K and Σ=0.5

Siloxane Is Adaptable To Diverse Applications


CH3

SiH3C

CH3

OSi

OCH3

CH3

SiCH3

CH3

CH3n

PCMS:Cyclohexyl-methyl

PDMS:Dimethyl

PAMS:Alkyl-methyl

Traction Fluids Energy-Efficient Lubricants

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Boundary Friction and WearLubricants were tested using a ball-on-disk machine to determine the effects of molecular structure on boundary friction and wear


Comparison with Existing Technologies


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Future Research Opportunities


Si O Si OCH3

H3CCH3

CH3

SiCH3

CH3

O Si CH3

CH3

CH3m n

DP

J

QL

Other types of chemical structures

Mixtures Functional groups Additives

Selected fluids from this work exhibit the following properties: Additive compatibility Miscibility with Hydrocarbons Special High Temperature

Summary

• Silicones stand out synthetic fluids for their exceptional thermal stability and temperature-viscosity indices

• Newly synthesized siloxanes are examples of the tribological potential beyond currently use of silicones– Temporary shear thinning Polyalkylmethyl siloxanes– High traction Polycyclohexylmethylsiloxanes– High temperature Ph/F co-polymers

• Flexible structure of new Ph/F copolymers allows design of fluids with high termal stability and improved wear resistance

• Their additive acceptance offers additional possibilities for lubricant formulation

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Summary

• Silicones stand out synthetic fluids for their exceptional thermal stability and temperature-viscosity indices

• Newly developed silicone fluids offers enhanced lubricity in combination with high temperature resistance

• Additionally additive acceptance offers new possibilities for lubricant formulation

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Engineering

Molecular modelling, the future of lubrication research and development