Coating Rheology - Outlinecalpoly.edu/~rhfernan/chem550/pdf/Chem550_Spring_04_4.pdf · Coating...

CHEM 550 – Formulation of Modern Coatings Spring 2006

Polymers and Coatings ProgramCalifornia Polytechnic State University 1

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Coating Rheology Coating Rheology -- OutlineOutline

Rheological Properties of CoatingsRheological Properties of Coatings–– Viscosity, ShearViscosity, Shear--thinning, thinning, ThixotropyThixotropy, , ViscoelasticityViscoelasticity, ,

Surface & Interfacial Tension, etc.Surface & Interfacial Tension, etc.

Impact of Rheology on Coating ProcessImpact of Rheology on Coating Process–– Paint Mixing, Application, Sag & Leveling, etc.Paint Mixing, Application, Sag & Leveling, etc.

Effect of Coating Formulation Aspects on Effect of Coating Formulation Aspects on RheologyRheology–– Solids Level, Binders, Pigments, Additives, Solids Level, Binders, Pigments, Additives,

InteractionsInteractions

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Rheological Properties of CoatingsRheological Properties of Coatings

Rheology & Viscosity Definitions

Effect of Shear Rate on Viscosity

Newtonian Vs Non-Newtonian Behavior

Shear-Thinning, Pseudoplasticity, Thixotropy, etc.

Shear Vs Extensional Viscosity

Viscoelasticity

Measurement of Rheological Properties



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Coating Process OverviewCoating Process Overview

Raw Materials

Mixing / Blending

Storage (Shelf / Pot)

Delivery to Applicator

Application

Flow & Leveling

Drying / Curing

What are the Requirementsin Each Step?

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RheologyRheology

Science concerned with the Science concerned with the flow and deformation of flow and deformation of

materialsmaterials



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Which is more viscous?Which is more viscous?

Water or cooking oil?

Cooking oil or honey?

Honey or mayonnaise?

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Viscosities of Common MaterialsViscosities of Common Materials

100 centipoise (cps) = 1 poise 10 poise = 1 Pa s

1 cps = 1 mPa s

5000Honey

1000Glycerine

100Olive Oil

1Water

Viscosity (cps)



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ViscosityViscosity

Viscosity is resistance of a material to flow

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ViscosityViscosity

Velocity = 0

Velocity = V

ViscosityViscosityMeasure of the resistance of a fluid to an Measure of the resistance of a fluid to an

imposed flowimposed flow



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Viscosity Viscosity –– Quantitative DefinitionQuantitative Definition

Shear Rate = (V1-V2)/d [ s-1]Shear Stress = F/A [N cm-2]

Velocity = V2

dVelocity = V1

A - Area

F - Force

Viscosity = Shear Stress / Shear Rate [Pa s]

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If Viscosity is Independent of Shear If Viscosity is Independent of Shear Rate Coating is NewtonianRate Coating is Newtonian

Shea

r Stre

ss, P

a

Visc

osity

, Pa s

Shear Rate, s-1 Shear Rate, s-1

Stress Rate

Viscosity

γητ &=



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Bubble ViscometersBubble ViscometersSimilar in Concept to Falling Ball. However, Bubble is Deformable -Theoretical Analysis is Difficult.– e.g. Gardner Bubble Viscometer

(ASTM D1545)

Falling Body Type has Very Limited Control Over Measurement Parameters (Shear Rates, Shear Stress)

Not Recommended for non-Newtonian Fluids

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Orifice Viscometers (Viscosity Cups)Orifice Viscometers (Viscosity Cups)η (poise)ν (stoke) = ρ (g/cm3)

For low viscosity (<10cps)Kinetic Energy Correctioncan be as high as 90%

Not Suitable for non-Newtonian Fluids



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Rotational Viscometers

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NonNon--Newtonian Viscosity BehaviorNewtonian Viscosity Behavior

RPM Viscosity (cps) Spindle #0.5 8000 41 5000 "

2.5 2560 "5 1520 "10 1000 "20 550 250 316 "100 227 "

Brookfield Viscosities @ Different RPMsWaterborne Coating



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ShearShear--Thinning BehaviorThinning Behavior

As viscometer RPM increases Shear Rate increasesCoating is Shear-Thinning

4""""2""

Viscosity Measured withBrookfield

100

1000

10000

0.1 1 10 100

RPM

Visc

osity

(cps

)

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Shear Thinning BehaviorShear Thinning Behavior

Viscosity Measured with ARES

10

100

1000

10000

100000

0.01 0.10 1.00 10.00 100.00 1000.00

Shear Rate (s-1)

Visc

osity

(cps

)



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Shear Thinning & Thickening BehaviorShear Thinning & Thickening Behavior

Shea

r Stre

ss, P

a

Shear Rate, s-1

γητ &=

)( 1<= nnKγτ &

)( 1>= nnKγτ &

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Continuous Phase Continuous Phase CompositionComposition–– Thickener, OtherThickener, Other

Hydrodynamic VolumeHydrodynamic VolumeChain EntanglementsChain Entanglements

Volume Fraction and Volume Fraction and Nature of Dispersed Nature of Dispersed ComponentsComponents–– Binder, Pigment, FillerBinder, Pigment, Filler

What makes a coating nonWhat makes a coating non--Newtonian?Newtonian?



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Polymer Conformation Hydrodynamic Polymer Conformation Hydrodynamic Volume Volume RheologyRheology

Intrinsic Viscosity, [η] = K Mva

Mv - Viscosity average molecular weightK - Huggins Constanta - Mark-Houwink Constant

a = 0.5 at theta(θ) conditions(e.g., polyisobutylene in benzeneat 24oC)a > 0.5 in a good solvent

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Polymer Concentration Polymer Concentration RheologyRheology

Viscosity dependence on shear-rate increases

Viscosity dependence on time increases

Dilute Critical(Overlapping)

Concentrated(Entanglements)



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Polymer Molecular Weight Effect on Polymer Molecular Weight Effect on ViscosityViscosity

Log

Vis

cosi

ty

Log Mc

1.0

3.4

24,000Polydimethyl siloxane

23,000Polyvinyl acetate

5,0001,4 polybutadiene

15,000Polyisobutylene

10,000Cis-polyisoprene

13,000Polycarbonate

28,000Polymethyl methacrylate

30,000Polystyrene

4,000Polyethylene

Critical MWPolymer

Chain entanglements above a critical polymer chain length

Ferry, Viscoelastic Properties of Polymers, 3rd ed, Wiley, New York, 1980

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High shear can break up High shear can break up entanglementsentanglements

Vis

cosi

ty

Shear Rate

Visc

osity

Molecular Weight

LSV

HSV



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Polymer MWD impacts shear Polymer MWD impacts shear --thinningthinning

Vis

cosi

ty

Shear Rate

Narrow

Broad

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Effect of Molecular Weight on Effect of Molecular Weight on ThickeningThickening

HMW Shear-thinning

LMW Newtonian



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Effect of HEC Concentration on Viscosity

Glass, J. Oil Colour Chem. Assoc., 58, 169 (1975)

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ThixotropyThixotropy

Vis

cosi

ty

Shear Rate

Vis

cosi

ty

Time

Shea

r Stre

ss

Shear Rate



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UltraUltra--Fine Clay ThickenersFine Clay ThickenersHighly Shear thinning

High Yield Stress

Low Thixotropy

++

_ _ _ _

Positive Edges & NegativeFaces – Weak Structure

e.g., Laponite

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Effect of Dispersed Phase on Viscosity

Visc

osity

% Volume Solids 64

Cubic PackingCritical Volume

Fraction - 0.5236

Hexagonal PackingCritical Volume

Fraction - 0.7405

For Random Packing, Critical For Random Packing, Critical Volume Fraction Volume Fraction -- 0.640.64



ppt00 29 29Krieger, Trans. Soc. Rheol., 7, 101 (1963)

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Effect of Particle Size Distribution Effect of Particle Size Distribution on Viscosityon Viscosity

Polydispersed

Monodispersed

“Vis

cosi

ty”

% Volume Solids 64

Monodispersed Polydispersed



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Effect of Particle Shape on Effect of Particle Shape on ViscosityViscosity

“Vis

cosi

ty”

% Volume Solids 64

Spheres

PlateletsNeedles

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Flocculation & Aggregation of Flocculation & Aggregation of Particles in CoatingsParticles in Coatings

DLVO Theory(Two Particles)

S

a - Van der Waals, Long-range (Attractive)b - Electrostatic, Long-range (Attractive orRepulsive)c - Steric, Short-range (Repulsive)d - Solvation, Short-range (Attractive orRepulsive)e - Born, Atomic-range (Repulsive)

Inter-Particle Forces



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Flocculation & Aggregation of Flocculation & Aggregation of Particles in CoatingsParticles in Coatings

Pot

entia

l Ene

rgy

S

Repulsive

Attractive

Pot

entia

l Ene

rgy

S

Repulsive

Attractive

a

bc

de

AggregationFlocculation/Agglomeration

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Effect of Particle Flocculation / Effect of Particle Flocculation / Aggregation on ViscosityAggregation on Viscosity

“Vis

cosi

ty”

% Volume Solids 64

Non-aggregated

Aggregated



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Effect of Solids Level on Viscosities Effect of Solids Level on Viscosities at Various Shear Ratesat Various Shear Rates

log

(Vis

cosi

ty)

log (Shear Rate)

SolidsSolidsIncreasesIncreases

LowLow--ShearShear MediumMedium--ShearShear HighHigh--ShearShear

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Effect of Solids Level on Viscosities Effect of Solids Level on Viscosities at Various Shear Ratesat Various Shear Rates

Vis

cosi

ty

%Volume Solids

LowLow--ShearShear MediumMedium--ShearShear HighHigh--ShearShear



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Types of Viscosity BehaviorTypes of Viscosity Behavior

Newtonian

Dilatant (Shear-thickening)Yield Stress

Rate

Stre

ss

Plastic

Shear-thinning (Pseudo-plastic)

2/12/12/10

2/1 γηττ &∞+= Casson

Approx.

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SummarySummaryViscosity– Hydrodynamic Volume

Shear Thinning– Chain Entanglements– Flocculation / Aggregation

Thixotropy– Chain Entanglements– Flocculation / Aggregation

Yield Stress– Weak Structure (gel)



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10-2 10-1 100 10+1 10+2 10+3 10+4 10+5 10+6

log (V

iscos

ity)

log (Shear Rate (s-1))

What is the Ideal Viscosity Profile?What is the Ideal Viscosity Profile?

Shear-Thinning

Newtonian

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Shear Rates for Coating SagShear Rates for Coating Sag

t - film thicknessρ - densityμ - viscosity

μρ2

gt=Shear Rate

For a 1.1g cm-3 density, 3 mil (0.0076cm) thick, 1Pa s (10 poise)

coating,sag shear rate is 0.4 s-1



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Shear Rates for ReverseShear Rates for Reverse--Roll ApplicationRoll Application

Reverse Roll Coater Nip Region

v

vd/2

V = 100 F/mind = 2 mil

Shear Rate = ___________200x12x103

60x2

Shear Rate = 2.0x104 s-1

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Shear Rates for Various SubShear Rates for Various Sub--ProcessesProcesses

10-2 10-1 100 10+1 10+2 10+3 10+4 10+5 10+6

log

(Vis

cosi

ty)

log (Shear Rate (s-1))

Settling

WickingMixing

(Slurries) RollCoating

SprayCoating

Sag & Leveling

Brush/RollPick Up



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V isco s ity M easu red w ith AR E S

10

10 0

10 0 0

10 0 0 0

10 0 0 0 0

0 .0 1 0 .10 1.0 0 10 .0 0 10 0 .0 0 10 0 0 .0 0

S h ear R ate (s -1 )

Visc

osity

(cps

)

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V isco s ity M easu red w ith AR E S

0

2 0 0 0

4 0 0 0

6 0 0 0

8 0 0 0

10 0 0 0

12 0 0 0

14 0 0 0

16 0 0 0

18 0 0 0

0 .0 0 50 .0 0 10 0 .0 0 150 .0 0 2 0 0 .0 0 2 50 .0 0

S h ear R ate (s -1 )

Visc

osity

(cps

)



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Other NonOther Non--Newtonian Phenomena Newtonian Phenomena ((ViscoelasticityViscoelasticity))

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Surface Tension? Other Forces?Surface Tension? Other Forces?

Externally applied force

Gravitational force

Inertial force

Surface (interfacial) tension forceOther forcesViscous resistance force

What are all the forces impacting flow?



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Surface TensionSurface Tension

Soap Film

Surface tension = Energy required to expand soap bubble (creating surface)

Soap Film

l

w

l

w

dl Force = σ w x 2Energy = σ w dl x 2

σ is surface tension (dynes/cm) or freesurface energy (ergs/cm2)

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Surface TensionSurface Tension

Liquid Surface Tension (dyne/cm)

Water 72 Ethyl Alcohol 22.8

Molecules at the surface aresubject to a net attractive forcetowards bulk liquid.

This is the origin ofSurface Tension



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Why does water haveWhy does water havesuch a high surface tension ?such a high surface tension ?

H

H OH O

H

CH HCH H

HH

O

H

H O

Small positive chargeon H atom

Small negative chargeon O atom

Hydrogen bonding attractions

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Surface (Interfacial) Tension Surface (Interfacial) Tension Causes FlowCauses Flow

Waxed Surface – 32 dynes/cmWater - 72 dynes/cm



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Surfactants are used to lower Surfactants are used to lower surface tension of watersurface tension of water

Surfactant concentrationCMC

Sur

face

Ten

sion

hydrophilic lipophilic

C12H25SO4-Na+

C2H5OH

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Solvent Surface Tension ValuesSolvent Surface Tension Values

24.6N-Butanol

24.0MEK

18.0Hexane

27.7Dowanol PM®

28.4Toluene

30.0O-Xylene

36.0Propylene Glycol

48.4Ethylene glycol

72.3Water

Surface Tension

(dynes/cm)Solvent



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Surface Tension of Common Surface Tension of Common SubstratesSubstrates

16PTFE

29-31PP, PE

41PMMA

35-50Most Organic Coatings

>55Steel

Surface Tension (dynes/cm)

Substrate

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Contact Angle Contact Angle –– Wetting Wetting

Interfacial tension between

Liquid / Air

Solid / Air

Solid / Liquid

Must be considered

θ

θ - Contact Angle

Zero Contact Angle

Spontaneous Wetting& Spreading



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Equilibrium Equilibrium vsvs Dynamic Dynamic Surface TensionSurface Tension

•What is the surface tension of just mixed surfactantl/water ?

•What is it sometime later ?•What is it in between ?

SurfaceTension

TimeThis is important when creating new surfaces/interfaces

e.g., creation of spray dropletscreation of coating films



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MarangoniMarangoni Effect Effect –– Flow Driven by Flow Driven by Surface Tension ForceSurface Tension Force

Evaporation of alcohol raises surface tension near the rim of glass

Alcohol-rich wine flows upwards due to surface tension force

Drops cascade down as gravity takes over

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Surface Tension flows

Crawling: When γimpurities < γfilm, the coating naturally begins to

minimize free surface energy by flowing from low γ to high γ, leaving a crater with the impurity in the center

Substrate

Low Surface Tension

Fluid Flow Fluid Flow

Low Surface TensionContaminant



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CRATERSCRATERS

Courtesy of Werner Blank, King Ind.

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DEWETTING

Courtesy of Werner Blank, King Ind.

Coating Rheology - Outlinecalpoly.edu/~rhfernan/chem550/pdf/Chem550_Spring_04_4.pdf · Coating...

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