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Viscosity & Viscosity Modifiers

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Outline

• Viscosity
– Definition & Terminology
– Temperature Dependence
• Viscosity Modifiers
– Function
– Thickening Efficiency
– Shear-Thinning
– Types/Chemistry
• Pour Point Depressants
• SAE Viscosity Grades
• Appendix
– Viscosity measurement methods

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Viscosity

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Viscosity

• Dynamic viscosity is resistance to flow of a fluid

• Defined as shear stress divided by shear rate (how hard you push it
divided by how fast it slides)
– Units of dynamic viscosity:
• Pascal seconds (Pa-s)
• mPa-s = 1cP (CentiPoise)
• Dynamic viscosities are usually measured under high shear conditions:
– For example, the cone on plate or cylinder viscometer
• Kinematic viscosity is the dynamic viscosity divided by the fluid density.
– The physical principle of measurement is based on the rate at which a fluid
flows under gravity through a capillary tube.
– Usually measured under low shear conditions.
– Units of kinematic viscosity:
• mm2/s = CentiStoke (cSt)
• Famous scientists who contributed to viscosity fundamentals
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What is the optimal viscosity?

• Metal on metal contact leads to high energy losses and surface wear

• Oil film between metal surfaces reduces energy losses

– Oil provides less resistance to movement than metal

• Sufficient viscosity is needed to form the film

• Viscosity should be high enough to form the protective film, but low
enough not to give excessive energy losses within the fluid

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Stribeck curve

I II III Boundary

Mixed

Hydrodynamic

Friction
coefficient PCMO: 35-45% frictional losses in I
Additives that can help: Friction
Modifiers

PCMO: 55-65% frictional losses in II

and III
Additives that can help: Viscosity
modifiers, Friction Modifiers

HDD: 95% frictional losses in III

Viscosity x speed Introduction of thinner fluids is a big
Log
Load opportunity
Additives that can help: Viscosity
modifiers

Viscosity modifiers and friction modifiers can be used in a complementary

fashion in properly formulated engine oils to reduce friction because they
operate in different lubrication regimes, as noted in the Stribeck Curve.

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Molecular origins of viscosity

• Molecules in adjacent layers of oil interact, preventing layers from sliding

past each other
• The higher the interaction the higher is the resistance to the flow
(viscosity)
• Interacting Forces:

Van der Waals

Flow direction
Molecules interact with
each other, resist flow

Molecules interact with

surface, resist flow

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Gecko Toe Adhesion

Yu Tian, Noshir S. Pesika, Hongbo Zeng, Kenny Rosenberg, Boxin Zhao, Patricia McGuiggan, 

Kellar Autumn, Jacob N. Israelachvili. PNAS 103 (2006) 19320‐19325
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Viscosity of materials

Viscosity at room temp

Substance
(mPa-s or cP)
Ketchup 100,000
VM Concentrate 40,000
Molasses 8,000
Maple syrup 3,000
Motor oil (SAE 8 – SAE 40 grades) 25 - 350
Olive oil 80
Group III base oil 4 cSt 45
Mercury 2
Water 1
Gasoline 0.5
Acetone 0.3
Air 0.018

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Viscosity index

300

250
Kinematic Viscosity, mm 2/s

0 0VIVI
200

150 100 VI

100 Same at 100ºC

120
-20 VI
120 VI
50

0
30 40 50 60 70 80 90 100 110

Temperature, celsius
Viscosity Index (VI) defines the viscosity relationship with temperature.

• The Viscosity of low VI oils change significantly with temperature

• The Viscosity of high VI oils changes much less with temperature

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Viscosity modifiers

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Viscosity modifiers

• Viscosity modifiers (VM) are used to reduce the influence of temperature

on the viscosity of lubricants
– Also known as Viscosity Index Improvers (VII)

• Viscosity modifiers in crankcase lubricants are polymers

• VM’s are used in most engine oils and many transmission fluids

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Viscosity modifiers

• Polymeric Viscosity Modifier occupies large volume of solution

• Viscosity modifier increases viscosity proportionally to volume that
polymer occupies
• Volume of the majority of viscosity modifiers is almost independent of
temperature
PMAs, temperature sensitive OCPs
OCPs, Hydrogenated Styrene Dienes (HSD) and HSDs

Low Temperature High Low Temperature High

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Function of viscosity modifiers

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Function of viscosity modifiers

• Base oil viscosity has strong

temperature dependence:
• First described in 1920s and
now more precisely in ASTM
D341
Log (Viscosity)

Oil A
• Thinner base oils (Oil B)
SAE 30 provide good low temperature
properties, but cannot provide
Oil B
SAE 5W protection at high temperatures
• Thicker base oils (Oil A)
Temperature provide protection at high
temperatures, but have
insufficient pumpability at low
temperatures

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Function of viscosity modifiers

• VM adds viscosity to thinner oil

at both high and low
temperatures proportionally to
Oil B + VM = baseoil viscosity at the
SAE 5W-30
particular temperature
Log (Viscosity)

• VM added viscosity does not

Oil A
SAE 30 have strong dependence on
temperature
Oil B
SAE 5W • Reduces final oil temperature
dependence
Cold Starting Temperature Engine operation • Multigrade oils (SAE xW-xx)
provide engine protection at
both high and low temperatures
through use of viscosity
modifiers
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Function of viscosity modifiers - Demo

• Effect of Viscosity and VM Polymer on Friction – Stribeck Curve

I II III Boundary

Mixed

Hydrodynamic

Friction
coefficient Visc at
Room T
Substance (cP)
VM Conc. 40,000
Olive oil 80
4 cSt baseoil 45
Viscosity x speed
Water 1
Log
Load

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Thickening efficiency

• Thickening Efficiency or TE
is the amount of viscosity
increase per % polymer
• TE is highly dependent on
Molecular Weight and
chemistry
– Higher MW = Higher TE
– Less Branching = Higher TE
– Better match of VM polarity to
base oil polarity = Higher TE

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Shear-thinning: Temporary & Permanent Viscosity Loss

Quiescent polymer
coil in oil solution

Reversible Non‐reversible
Orientation of coil Rupture of coil
under shear forces under shear forces

Smaller cross‐ Lower molecular
section gives less weight gives less
thickening which thickening which 
equals lower viscosity equals lower viscosity

Temporary Viscosity Loss Permanent Viscosity Loss:
Shear Stability Index (SSI) measures loss of polymer added viscosity after 
30 cycle KO test.

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Shear stability index (SSI) (permanent viscosity loss)

• Shear Stability Index (SSI) measures loss of polymer added kinematic

viscosity after 30 cycles in Kurt Orbahn
• The higher the SSI the more viscosity loss upon oil shearing
• SSI is usually measured in a reference oil that represents polymer
behavior in SAE 15W-40 grade
• SSI depends on polymer chemistry, molecular geometry and molecular
weight
– Higher MW = less shear stable VM

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Newton’s law & shear thinning

(temporary viscosity loss)
Bingham plastic

Shearing stress, T
Shear thinning

Newtonian

Shear thickening

Rate of shearing strain, du

dy
• Newton’s Law of Viscosity: viscosity = shear stress / shear rate
• Newtonian Fluids = viscosity is a constant; does not change with shear stress or
shear rate
• Fluids that do not obey this are called Non-Newtonian
• The most common type is Shear-Thinning
– Viscosity decreases with increasing shear rate
– Viscosity modified fluids fall into this category
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Typical shear rates

25

20 Oil pump
Viscosity, mPa-s

15
Journal Bearing, piston rings,
Pouring valve train
10 Redline
oil
Idle Highway

0
Non-Reversible
102 103 104 105 106 107
Shear-Thinning
Hundred Shear Rate, sec-1 Million

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Shear rate & shear-thinning

15
Low shear Kinematic Viscosity
Shear Rate
viscosity of
Oil with VM engine oil Ring Zone,
Bearings, Valve Train
Shear Rate
VM
Viscosity, mPa-s

10 Thickening High Temperature

High Shear (HTHS)
Temporary Non-Newtonian Shear Rate
viscosity loss Viscosity decreases
with shear rate
Newtonian
Other additives Viscosity is constant
at all shear rates
5

Base Stocks

0
102 103 104 105 106 107
Shear Rate, sec -1

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Viscosity modifier types

• Common types
– Ethylene-Propylene Co-polymer (OCP)
• Semi-crystalline
• Amorphous
– Hydrogenated Styrene-diene Co-polymer (HSD)
– Polymethacrylate (PMA)

• Factors that need to be considered when selecting VM

– Cost to achieve required thickening (Cost vs. TE)
– Shear Thinning Properties
– Low Temperature Properties

• Other performance harms/credits

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Viscosity modifier chemistry

Can be semi-crystalline or
Ethylene-Propylene Co-polymer amorphous depending on
(OCP) structural details

Hydrogenated Styrene-Diene
Co-polymer :
Linear Polymer

Polymethacrylate:
(PMA)
O O O O O O
Loose physical
association due to
R R R polarity differences

Hydrogenated Chemically bonded

Styrene-Diene: together
Star Polymer

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Performance comparison
TE vs. SSI
Semi-Crystalline
Hydrogenated
OCP
Styrene-Diene
Hydrogenated Linear Polymers
Styrene-Diene Amorphous
Star Polymers OCP
Thickening Efficiency

PMA

0 10 20 30 40 50 60
SSI

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Pour point depressants

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Pour point depressants

• Pour Point Depressants

– Commonly referred to as PPDs
– Also know as Lube Oil Flow Improvers (LOFIs)
• Break up regularity of wax crystals
– Prevent large crystal sheets from forming
– Encourage small crystals - easier flow
– Minimize low-temperature viscosity and yield stress
• Types:
– Fumarate Vinyl Acetates (FVA)
– Polymethacrylates (PMA)

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Wax crystal modification by LOFI

• Wax crystals can cause the most

serious type of engine problem
• Engines can start but the oil does
not flow, leading to catastrophic
engine failure

50 micron + LOFI

50 micron

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SAE viscosity grades

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SAE J300 engine oil viscosity grades

(issued January 2015)
MRV Kinematic HTHS
mPa-s, Max Viscosity @ 106 Sec-1
w/No Yield at 100 C mm2/s at 150 C
SAE CCS
Grade mPa-s, Max Stress Min Max mPa-s, Min
0W 6200 at -35°C 60 000 at -40°C 3.8 – –
5W 6600 at -30°C 60 000 at -35°C 3.8 – –
10W 7000 at -25°C 60 000 at -30°C 4.1 – –
15W 7000 at -20°C 60 000 at -25°C 5.6 – –
20W 9500 at -15°C 60 000 at -20°C 5.6 – –
25W 13000 at -10°C 60 000 at -15°C 9.3 – –
8 – – 4.0 <6.1 1.7
12 – – 5.0 <7.1 2.0
16 – – 6.1 <8.2 2.3
20 – – 6.9 <9.3 2.6
30 – – 9.3 <12.5 2.9
40 – – 12.5 <16.3 3.5(1)
40 – – 12.5 <16.3 3.7(2)
50 – – 16.3 <21.9 3.7
60 – – 21.9 <26.1 3.7
(1) For 0W, 5W, 10W Multigrades – Changed from 2.9 in 11/2007
(2) For 15W, 20W, 25W Multigrades and SAE 40 Grade
Previously 5.6
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Viscosity measurement methods

ASTM
Temp Shear Performance Instrument Method

High Low Oil Consumption Kinematic Viscometer D 445

Quality Control

Low High Cold Starting Cold Cranking Simulator (CCS) D 5293

Low Low Cold Pumping Mini-Rotary Viscometer (MRV) D 4684

High High Wear/Fuel Economy Tapered Bearing Simulator (TBS) D 4683

Tapered Plug Viscometer (TPV) D 4741
Multi-Cell Capillary (MCC) D 5481

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SAE viscosity grades

60 0W‐60 5W‐60 10W‐60 15W‐60 20W‐60 25W‐60

50 0W‐50 5W‐50 10W‐50 15W‐50 20W‐50 25W‐50
‘Summer’ Grade

40 0W‐40 5W‐40 10W‐40 15W‐40 20W‐40 25W‐40

30 0W‐30 5W‐30 10W‐30 15W‐30 20W‐30 25W‐30
20 0W‐20 5W‐20 10W‐20 15W‐20 20W‐20
16 0W‐16 5W‐16
12 0W‐12
8 0W‐8
0W 5W 10W 15W 20W 25W
‘Winter’ Grade
Straight Grades

Some common viscosity grades for engine oils

•SAE 0W-X grades typically need synthetic base stocks

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SAE J300 engine oil viscosity grades

• Correct
– SAE 10W-30
• Incorrect
– 10W-30
– SAE 10W/30
– SAE 10W30
– SAE 10w-30
• Labeling
– Must label as the lowest ‘W’ grade
• An oil that meets 5W also meets 10W, 15W, etc.
– Oils with VM must be labeled as Multigrades
• Need to take care with CCS and KV100 labelling as there is overlap
between the SAE grades

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Summary

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Summary

• Viscosity is a measure of a fluid’s resistance to flow

– It depends strongly on temperature
– And it can depend on shear rate
• Viscosity modifiers in lubricants:
– Are used to reduce the influence of temperature on lubricant viscosity
– Chemical structure and molecular weight affect performance and efficiencies
– Exhibit temporary and permanent viscosity loss due to shear
– Three common types: OCPs, Hydrogenated Styrene-diene Co-polymer, PMAs
– Oil formulators must balance viscometric requirements, engine performance
and cost
• Viscosity grades are defined by SAE J300
– “Oils which are formulated with polymeric viscosity index improvers for the
purpose of making them multiviscosity-grade products are non-Newtonian
and must be labeled with the appropriate multiviscosity grade”.
Source: SAE J300

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Appendix: viscosity measurement

methods

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Kinematic viscosity

• Kinematic viscosity = viscosity/density

• Principle
– Measure time for known volume to flow
through capillary tube
• Driving force: gravity (mass of fluid)
Start • Low shear rate
Timing – Units
marks
• mm2/s
Stop • CentiStokes = cSt (discouraged)
• Saybolt Universal Seconds (SUS)
Capillary (obsolete)

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Cold cranking simulator

• Rotor/stator system
Oil

CCS cross-section
Stator

Rotor

• Viscosity range - 500 to 200,000 mPa-s

• Shear rates - 104 to 106 sec-1
• Shear stress - 50,000 Pa

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Mini-rotary viscometer

• Low shear rate/shear stress measurement

• Measurements
– Yield stress
(min. stress to cause flow)
– Viscosity @ 525 Pa stress)
• Relationship to pumpability
failure mechanism
– Yield stress/air-binding
– Viscosity/flow-limited

Rotor

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High temperature high shear

Increase Constant
pressure to Capillary speed
increase
shear rate
Gap controls
shear rate

Laminar
fluid
flow
profile

Rotational

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