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RHEOLOGY of Coatings

2

Overview

1 Simple Test Methods, and Rheometry

2 Flow behavior during the Application

3 Behavior after the Application

4 Long-term Storage Stability

5 Curing of Powder Coatings and UV – Coatings

3

1 Simple Test Methods

trowel test- high-viscosity fluids: “thick“- low-viscosity fluids: “thin“ e.g. for dispersions

finger test- tacky: “long“ - less tacky: “short“ e.g. for paints, offset-printing inks, pigment pastes

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Flow Cups

measurement of theflow timeof low-viscosity liquids

to determine the

kinematic viscosity(weight-dependent viscosity !)

Examples: oils, solvent-based coatings,gravure and flexo printing inks

1 Simple Test Methods

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Falling - rod Viscometers

determination of the timeof the rodto travel downwardsover a defined distance

e.g. for testing offset-printing inks (highly viscous) and pastes

weight

printing ink

falling rod

falling-rod viscometer, e.g. type Laray

1 Simple Test Methods

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Using the typical spindles

relative viscosity valuesare measured

- cylinders- disks- pins- T-bars

preset: rotational speedmeasurement: torque

Rotational Viscometersfor testing

„Low - shear Viscosity“ (LSV)(which is in fact not really low-shear)

1 Simple Test Methods

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Rotational Viscometers for testing

„Medium – shear Viscosity“ (MSV)

originallypreset: force (constant torque),using a freely falling weight (in grams),measurement: rotational speedof the rotational measuring system

nowadays: preset of the speed,measurement of the torque

Krebs spindles stirrer-like „paddles"

relative viscosity valuesare measured here; typically given in

Krebs Units, KU

1 Simple Test Methods

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Cone & Plate Viscometersfor testing

„High - shear Viscosity“ (HSV)

preset: rotational speedmeasurement: torque

Problem:Friction between cone and plate,since the tip of the cone is not truncated, sitting directly on the bottom plate.

Consequence:Friction influences the measuring results

1 Simple Test Methods

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all these kinds of stirrers are relative

measuring systems

helix 1 helix 2

stirrer forbuilding materials

starch stirrer

blade anchor ball measuring system

1 Simple Test Methods

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Concentric Cylinders, CCfor low-viscosity liquids,solvent-borne coatings

Cone & Plate, CPfor liquids;

for dispersions only witha limitted particle size

(usually < 10 µm)

Parallel - Plates, PPuseful for

dispersions containing coarse particles,

pastes, offset-printing inks,gel-like materials,

polymer melts

Measuring Geometries for rotational and oscillatory rheometeraccording to DIN 53019 and ISO 3219

1 Rheometry Measuring Systems for Absolute Values

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2 Application (flow behavior)

Flow behavior during the application

- Application behavior in the flowing state when stirring, painting, brushing, rolling, spraying when pumping, dosing, blading,

flatstream application, dip coating, pouring,using roboters or high-rotational disks or bells

® Test method: Flow curves, at medium and high shear rates

(rotation)

Requirements:- ability to brush- limited coating force - no spatters- roller resistance

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wet layerthickness (h = 200 µm)

brush velocity(v = 0.5 m/s)

calculation of the shear rate:

2 Application (flow behavior) Coating, Painting, Brushing

Brushing, Paintingat medium andhigh shear ratesbetween 100 and 10,000 s-1

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s 2500sm102

m0.5

Δh

Δv

Application Example

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Quelle: Fotos vom Daimler-Museum, Stuttgart

Application examples :

Automotive coatings- spray roboters- high-rotational atomizers,

electrostatically supported

Requirements:- ability to pump- ability to spray

2 Application (flow behavior) Industrial Spray Processes

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2 Application (flow behavior) Spraying of Automotive Coatings

car body degreasing & phosphatizing electro dip coating

seam sealing underbody spraying

filler base coat and clear coat cavity conservation

Spraying, Coating at high shear ratesof 1000 to 10,000 s-1

a) Plastisols: seam sealing and under-body sealingb) Coatings: dip coating, filler, base coat, clear coatc) Waxes: cavity conservation

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Process Shear Rates (s-1)

sedimentation < 0.001 to 0.01

surface levelling 0.01 to 0.1

sagging 0.01 to 1

dip coating 1 to 100

pipe flow, pumping, filling into containers 1 to 10,000

coating, painting, brushing 100 to 10,000

spraying 1000 to 10,000

(high - speed) coating, blade coating 100,000 to 1 mio.

2 Application (flow behavior) Shear Rate Range

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2 Application (flow behavior) Overview: Flow & Viscosity Curves

flow curves viscosity curves yield point

1 ideally viscous (Newtonian) 4 without a yield point2 shear-thinning (pseudoplastic) 5 having a yield point3 shear-thickening (dilatant)

- --

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18

0,1

1

10

mPas

lg h

0,01

0,1

1

10

1000

mPa

lg t

1 10 100s-1

DG 42(double - gap)T = +20°C

Water

lg

constant viscosity,ideally viscous flow behavior

Double-gap measuring systems are special systems designed for low - viscosity liquids.

2 Application (flow behavior) Flow Curves

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0.1

0.2

0.3

0.4

0.5

Pas

h

0

50

100

150

Pa

t

0 200 400 600 1000s-1

Wall Paper Paste

aqueous methylcellulosesolution

T = +23°C

shear rate

typical behavior

of polymer solutions: continuosly shear-thinning

2 Application (flow behavior) Flow Curves

Shear-thinning flow behavior

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material at rest: under shear: high viscosity decrease in viscosity

suspension with needle-shapedor platelet-shapedparticles(e.g. flakes inmetallic-effect automotive coatings)

The particles are The particles are suspended randomly orientated in(if there are no flow direction. interaction forces).

consequence: shear - thinning flow behavior, decreasing viscosity

2 Application (flow behavior) Shear-Thinning Behavior

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020406080

100120140160180200220240

300

Pa

0 100 200 300 400 500 600 700 800 1,0001/s

Shear Rate .

Gellant (Clay) Viscosifier (PUR)

0

5

10

15

20

25

30

35

40

45

50

Pa

0 5·100

101

1/s

0.1

1

10

100

1,000

Pa

0.1 1 10 100 1,0001/s

Shear Rate .

Gellant (Clay) Viscosifier (PUR)

2

2

1

1

- lg -

2

1

2 Application (flow behavior) Effect of rheological additives (1)

lg

Example: comparison of flow behavior of a water-based dispersion withadditive 1, a „gellant“ e.g. clayadditive 2, a „viscosifier“ e.g. an associative thickener

flow curves on a linear scale flow curves on a logarithmic scale

with yield point

Summary: The gellant shows is effective especially in the low-shear range(or at rest, resp.), and the viscosifier in the high-shear range.

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0.1

1

10

100

1,000

Pa·s

0.1 1 10 100 1,0001/s

Shear Rate .

Gellant (Clay) Viscosifier (PUR)

shear rate

lg -

- Brookfield -

- Krebs --Stormer

- flow cups -

low - shear range high - shear range stirring, painting, rolling, spatters (?)

spray coating

shear - thinning flow behavior

Summary:A single - point

viscosity measurement

is not sufficient.

- coating processes -

Coating 1 Coating 2

viscosity

lg

2 Application (flow behavior) Effect of rheological additives (2)

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Different rheological additives as thickeners

(example:water-based coatings)

(1) silica (clay, inorganic gellant

(2) cellulose derivative, polymer solution

(3a) unmodifiíed polymer dispersion

(3b) polymer dispersion with an associative thickener

(bar length: 100 nm = 0.1 µm)

left side:at rest

right side:when sheared

For polymer dispersions: lower viscosity even though the higher molar mass of the polymer

(1)

(2)

(3b)

(3a)

2 Application (flow Behavior) Effect of Rheobogical Additives (3)

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3a

3b

2

1

lg

0.01 0.1 1 10 100 1000 10,000 s-1

lg

low-shear high-shear

Viscosity functions of pigmented

water-based coatingscontaining different

rheological additives as thickeners, in principle:

(1) silica (clay), inorganic gellant

(2) cellulose derivative, polymer solution

(3a) unmodifiíed polymer dispersion (3b) polymer dispersion with an associative thickener

2 Application (flow behavior) Effect of Rheological Additives (4)

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3 Behavior after application

3 Behavior after the application

- levelling, gloss, de-aeration- sagging, wet layer thickness, edge cover- structure recovery, time-dependent „thixotropic behavior“

® Test method: step test, low – high – low shear (rotation or

oscillation)

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Application examples:

- brush coatings- spray coatings Requirements:

- Levelling without brush marks or other flow defects- controlled sagging- desired layer thickness

3 Behavior after application Levelling and Sagging

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at very low shear rates between 0.01 and 1 s-1

(or at rest, respectively)

Levelling, Brush Marks, Wet-layer Thickness, Sagging Example: Brush Paints

3 Behavior after application Levelling and Sagging

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Example for surface treatment of cars:1 car body mould metal sheet2 kathodic dipping process, anti-corrosion protection3 functional layer4 water-base coat5 clear coat

Quelle: Fotos vom Daimler-Museum, Stuttgart

Automotive

Coating:High-rotational

atomizer (bell),

electrostaticallysupported

spray process

spray coatingproblem:

sag control

3 Behavior after application Levelling and Sagging

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Application examples:

- printing inks

Requirements:- area printing: without levelling problems- halftone printing: dot sharpness

3 Behavior after application Printing Process

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Process Shear Rates (s-1)

sedimentation < 0.001 to 0.01

surface levelling 0.01 to 0.1

sagging 0.01 to 1

dip coating 1 to 100

pipe flow, pumping, filling into containers 1 to 10,000

coating, painting, brushing 100 to 10,000

spraying 1000 to 10,000

(high - speed) coating, blade coating 100,000 to 1 mio.

3 Behavior after application Shear Rate Range

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a) rotation (3 intervals)

Result:time - dependent viscosity

Preset:three steps low / high / low shear rate

3 After Coating Step Tests (Rotation): Structure Recovery

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0.1

1

10

100

Pas

0 100 200 300 400 500 600 700s

time t

- structure recovery

Comparison of two Formulations of Coatings :Step Test with 3 Intervals

= 0.1 s-1 = 0.1 s-1

= 100 s-1

Structure recoveryis fasterwith the „gellant“ - less sagging,- high wet-layer thickness,- but maybe poor leveling

Structure recoveryis slower with the „thickener“ - good leveling,- but maybe too much sagging

lg h

3 After Coating Step Tests (Rotation): Structure Recovery

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b) oscillation (3 intervals)

Preset: three steps low / high / low strain amplitude

Result: the two time-dependent functions of G'' (viscous) and G' (elastic behavior)

3 After Coating Step Tests (Oscillation): Structure Recovery

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preset:1 low-shear conditions

(strain in the LVE-range, oscillation)2 high-shear conditions (rotation)3 low-shear conditions

(strain in the LVE-range, oscillation)

measuring result:1 state of rest2 structure decomposition3 structure regeneration

Step test with 3 intervals, as oscillation / rotation / oscillation (measuring „thixotropic behavior“)

2nd test interval:liquid, at high shear rates

1st & 3rd test interval:G‘ > G‘‘ („gel-like structure“ at rest)

3 After Coating Step Tests (O-R-O): Structure Recovery

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3Spritzlack( )mit Additiv B

G'

G''

2Spritzlack( )mit Additiv A

G'

G''

1Spritzlack( )ohne Additiv

G'

G''

Comparison: 2 Spray Coatings, Step Tests in Oscillation / Rotation / Oscillation

time t

0.01

0.1

1

10

Pa

lg G'

lg G''

100 200 300 500 600 s

- crossover G‘ = G‘‘

g = 0.2%

g = 0.2%

= 15,000 s-1

Structure recovery

1) liquid, as long as G‘‘ > G‘ for leveling

2) „gel - like“, when G‘ > G‘‘ sagging is stopped

Analysis: Time point of

crossoverG‘ = G‘‘

can be optimizedby rheological

additives.

3 After Coating Step Tests (O-R-O): Structure Recovery

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a) rotation (3 intervals)

result: time-dependent viscosity (here, the viscous behavior is measured only !)

b) oscillation (3 intervals)

result: two time-dependent functions G'' (viscous) and G' (elastic) here, the whole viscoelastic behavior is measured.

3 After Coating Step Tests: Structure Recovery

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4 Storage Stability

4 Long-term storage stability

- settling (sedimentation), flotation- syneresis („blooding“), demixing- appearance after a time of rest („consistency“)- transport stability- gelation effects, fluidisation

- Test method:frequency sweep (oscillation), low frequencies

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Application examples: - emusion paints- coatings with metallic - effect

Requirements:- no demixing- no sedimentation- no syneresis

4 Storage Stability Sedimentation

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Process Shear Rates (s-1)

sedimentation < 0.001 to 0.01

surface levelling 0.01 to 0.1

sagging 0.01 to 1

dip coating 1 to 100

pipe flow, pumping, filling into containers 1 to 10,000

coating, painting, brushing 100 to 10,000

spraying 1000 to 10,000

(high - speed) coating, blade coating 100,000 to 1 mio.

4 Storage Stability Shear Rate Range

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Controlled stress rotational tests:

Flow Curves on a

linear scale Yield Point as a limiting value of the shear stress

1 without a yield point2 having a yield point -y

2

1

ty

Break of the structure - at - rest.Super - structure by a

chemical - physical network via interactive forces.

-

-

4 Storage Stability Simple Method: Yield Point

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Preset:constant amplitude, shear strain or shear stress (within the LVE - range)andvariable frequency

Precondition:First of all, the LVE - range has to be checkedby an amplitude sweep.

4 Storage Stability Frequency Sweep: Long-term Behavior

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g = 1 %T = +23°C

0.001

0.1

10

Pa

10-3

10-2

10-1

100

101

102

rad/s

Comparison of two Coatings: Dispersion Stability

Long - term storage stability:Evaluation at a low frequency

G' > G'' hence „gel - like“,

stable dispersion (Top Coat).

G'' > G' hence „liquid - like“,unstable dispersion (Primer).

G'' > G'

G' > G''

angular frequency lg -

lg G'

lg G''

1

0.01

4 Storage Stability Frequency Sweep: Long-term Behavior

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5 Curing (powder coatings, UV – coatings) - time - dependent and temperature - dependent melting and

curing

5 Curing Coatings

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Application examples:

- powder coatings- UV – curing coatings

Requirements:- melting- netting of the

subtrate- good levelling

Foto: BASF Coatings

Foto: AlzChem

Foto: DuPont Performance Coatings

5 Curing Coatings Examples

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gel formation and curing

preset: constant shear conditions (shear rate or shear stress)

result: viscosity / temperature curve showing a viscosity minimum

5 Curing Coatings Rotational Tests

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gel formation, hardening or curing process

preset: constant shear conditions (amplitude and frequency)

results: temperature-dependent G' and G'' curves

Tm ... melting temperature (when G' = G'')

TCR ... temperature at the onset of the hardening process,

gel formation, curing or chemical reactionTSG ... sol /gel transition (when again G' = G'')

5 Curing Coatings Oscillatory Tests

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102

103

104

105

106

Pa

G'

G''

0

20

40

60

80

100

120

140

160

180

200°C

T

0 100 200 300 400 500 600 700 800 1,000stime t

Powder Coat 1G'

G''

T

Powder Coat 2G'

G''

T

g = 0.1 %

ω = 10 rad/s preset: T = T(t)

Comparison of two Powder Coatings

5 Curing Coatings

Oscillatory Tests