School of Mechanical EngineeringFACULTY OF ENGINEERING

School of Mechanical EngineeringFACULTY OF ENGINEERING

Thin Film Coating & Spreading Flows

Harvey Thompson, Nikil Kapur, Jon Summers,

Mark Wilson & Phil Gaskell School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK

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Overview:

1. Introduction

2. Brief Review of Industrial Coating & Drying Flows1. Self-metered processes

2. Pre-metered processes

3. Drying

3. Droplet formation and spreading

4. Modern simulation techniques

5. Conclusions

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School of Mechanical Engineering:

• 5** rated under the Government Research Exercise:

• 3 multidisciplinary research institutes:

1. Institute of Medical and Biomedical Engineering

2. Institute of Engineering Thermo-fluids, Surfaces and Interfaces

3. Institute of Engineering Systems and Design

Broad groupings designed to allow cross-fertilisation of research areas

• Excellent research staff and facilities

School of somethingFACULTY OF OTHER

School of Mechanical EngineeringFACULTY OF ENGINEERING

•Combustion •Thin Films and Fluids •Tribology •Corrosion

•Engineering Optics/Metrology •Biomimetics •Microfluidics•Computational Simulations

Growth AreasCore Strengths

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Optics and Flow Diagnostics

Interferometric sensing:

Full field shape / colour / texture

Ultrahigh dynamic range distance metrology (to 1: 1011)

High bandwidth 10MHz

Flow Metrology:Multiphase / multi-constituent flows

Mixing

GDI sprays

Micro-reactors

4D flow metrology:

Time varying turbulent flow

In-cylinder automotive applications

Biophotonics:Signal processing

4D sensing fluorophor labelled cells

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Thin

Films

and

Fluid

Flow

Coating and PrintingNumerical and experimental investigations of industrial processes

Free Surface Flows and WettingDroplet motion on surfaces

Spreading films on heterogeneous surfaces

MicrofluidicsMicrofluidic device characterisation

Mixing

MicroPIV

Flow Modelling and SimulationMultiscale modelling: molecular dynamics / lattice Boltzmann

Computational Fluid Dynamics for Engineering Applications

Engineering Simulation Tools

Rheology and Fluid Characterisation

Review of Industrial Coating Flows

Industrial coating flows have several key stages:

Fluids preparation, coating, drying and winding.

Key distinction in coating stage: self-metered, pre-meteredLiquid supply

Substrate

Coater

Dryer/CurerWinder

Self-Metered Processes

Self-metered Coating Processes: where the wet film thickness is controlled by the process itself as opposed to controlled flow rate to the coater – hence self metering.

Simple example: roll coating Substrate

Bath

Self-metered processes

Many different forms

Just a few basic principles

Fighting the same sets of problems

Substrate

Bath

Self-metered processes

Pick up some liquid – Dip Coating

Split it between rolls

Get the split ratio right

Hope there’s no ribbing, barring or runback

Hope we have a wide coating window

Self-metered processes

Liquid Pick-up:

Key parameter is Capillary number:

For vertical pick-out (Wilson 1982):

Can be modified to take account of angle of pick-up and effect of plate.

Simple tools aid coating design

Speed ViscosityCa

SurfaceTension

6/1

2/1

944.0 Cag

UH lift

lift

s r o r

Coated Substrate

Wiper

Reservoir

Reservoir-fed Reverse Roll Coating:

Industrially-important variation of the robust reverse roll configuration used for manufacturing a variety of films and foils

Self-metered processes

Reservoir-fed Reverse Roll Coating: Comparison with Experiment

S=0.1

S=0.5

Self-metered processes

Controlling position of wetting line position is key.

Wetting line position vs speed ratio:

Hydrostatic head is important, as is the variation of dynamic wetting angle with metering speed.

Self-metered processes

Self-metered processes

Negative Gaps: Deformable roll coating

One roller rubber

Great way to make really thin coatings …

… if you’re not too bothered about coat quality

Depends critically on rubber, which can change with time or between batches

Set load or (-ve) gap

Back-up

Applicator

Metering

Elastomer Web

Three-roll pan reverse

Applicator Metering

Back-upWeb

Three-roll reservoir reverse

Self-metered processes

A PRE-SET GAP is specified and the separation of the roll centres is set by the adjustment of mechanical stops.

A LOAD is specified and the separation of the roll centres is set by applying a force across the roll pair.

Deformable roll coating can be very complex – requires sophisticated Finite element analysis

BUT simple models can be developed which do a good enough job for practical design – take account of roll speeds, viscosity, load,…

Self-metered processes

Gravure Coating is increasingly popular – small film thicknesses (a few microns) and good stability

Fluid properties

• viscosity, surface tension

Web & roll speeds

Doctor blade position

Gravure cell

Note - web tension and wrap don’t affect transfer!

Direct Gravure nip

Gravure rollDoctor blade

Reservoir

Web

Self-metered processes

Gravure Cell Shape is Key

• Quadrangular

• Pyramidal

• Laser engraved ceramics

• various shapes

• also QCH

• trihelical

Volume (microns)

Density (lines per inch /lines per cm)

Screen angle

Photographs of gravure roll surfaces

Self-metered processes

The film thickness & pickout is also sensitive to fluid properties, doctor blade pressure and roll speed.

Need for accurate predictive models!

Pre-metered processes

You deliver just the right amount of liquid to the web through, for example, a slot.

Pre-metering is used to smooth the liquid surface without the need to throw away or recirculate any liquid.

You know your flow rate Q m3/s and the line speed U m/s – wet film thickness is then Q/U.

Pre-metered processes are often used in high precision applications.

Pre-metered processes

Slide-bead Slot

Curtain

and others

Slot coating

Slot coating is a versatile method for applying single layers to a web.

Examples include photosensitive materials, such as photo-resist, magnetic suspensions, waxes, inks, silicon, rubber and foams and hot melt adhesives, in addition to low viscosity melts of alloys, metals and organic materials

Slot Die

Coated Substrate

Roll

Controlling slot coating

Slot coating is affected by:

Lip shape

Lip land length

Angle of slot

Type of backing (roll/web)

Radius of backing roller

Angle

RollerRadius

Web speed

Gap WetThickness

Land length

web (wrapped around roll of radius, R)

dynamic contact line

downstreamm eniscus

Q = UH

G

H

L d

static contact line

upstream meniscus

D

S

SLOTDIE

U

Simple models useful – may need more detailed analyses of the velocity or pressure field – can use Finite Elements or other CFD methods.

Typical streamlines in a

slot coating system.

BUT simpler models are often effective…

shows eddies etc…

Slot coating

A happy slot

A nice downstream meniscus

A healthy balance of pressure

Upstream meniscus under control …

… able to absorb fluctuations

Too much pressure and your pump has problems, too little and you’re out of control

Downstream meniscus

Upstream meniscus

Pressure curve

Unhappy slots

Upstream overspill Unstable inflow

Ribbing

Slide Coatingpopular in the photographic industry for producing multi-layer coatings

Substrate

Cascade

Slide coating

Typical Finite Element Grid

Slide coating

Streak-Line Formation

w eb

upper free surface

lower free surface

rec ircu la tion reg ion

(a ) = 25 S

o

(b ) = 45 S

o

in te rna lin te rface

Slide coating

Curtain Coating

High impingement speed of

falling curtain enables high

coating speeds - up to ~600m/m

Very versatile due to large gap

Not so mechanically demanding

Highly robust against lines

Predictable performance

Can coat several layers at once

Curtain Flow Zone

Simple and highly predictable

impingement velocity

But can be inherently unstable – minimum

flow rates to avoid break-up of curtain

X

X

V

V

0

0 0

20

2 2 XXgVV

Curtain Impingement Zone

An unwanted heel can form

This can trap particles

and bubbles - causing lines

Can also entrain air

Slot exits often used to supply pre-metered coatings such as slide and curtain

Defects Caused by Feed Flow

Back-wetting of uppermost slot may occur during start-up – can lead to defect-causing solids deposits due to degradation in recirculation regions.

Back wetting at the upper slot: (a) experimental, (b) CFD prediction

Defects Caused by Feed Flow

Geometrical modifications to slots – effect of a curved diffusor (Schweizer (1988))

Diffusor can also remove downstream eddy

Defects Caused by Feed Flow

CFD used to identify a more practical solution:Merging flow out of slot exits – effect of chamfering lower corner

Chamfer can remove eddies in both liquid layers simultaneously!

CFD Slot Optimisation

Film drying is often the limiting factor in industrial coating systems:

In drying, two processes must occur simultaneously:

(a) The transfer of energy (heat) from the surrounding environment to the product in order to evaporate the surface moisture

(b) The transportation of solvent held within the

product to its surface where it can be removed

by process (a)

Film Drying

Liquid supply

Substrate

Coater

Dryer/CurerWinder

For aqueous coatings, most of drying is in the Constant Rate Period.

For solvent-based coatings,

most drying is in the Falling Rate Period.

Film Drying

For aqueous coatings, most of drying is in the Constant Rate Period.

For solvent-based coatings,

most drying is in the Falling Rate Period.

The Drying Curve

Air Floatation Dryers

Double-sided floatation dryers usually have nozzles in a staggered arrangement on opposite sides with a stable, sinusoidal web profile.

Practical Dryer Design

Understanding web stability is crucial

A typical nozzle design show below –

two angled jets separated by a Coanda

plate.

Practical Dryer Design

Practical Dryer Design

Accurate predictions require sophisticated numerical models (Computational Fluid Dynamics):

But simpler models can also be extremely useful.

Aqueous coating, 30% solids By weight

Single sided drying, Temp = 125oC, h = 100 W/m2K

Single zone – 4m in length

No recirculation

Water not completely evaporated by end of zone

Practical Dryer Design

Effect of more volatile solvent

Methanol, 30% solids by weight

Single sided drying, Temp = 125oC, h=100W/m2K

Single zone – 4m in length

No recirculation

All methanol evaporated after 1.1m.

Practical Dryer Design

Droplet Formation and Spreading

Droplet flows are increasingly important – coating of electronic circuits, spray deposition onto leaves, heat mitigation in circuits, ink-jet printing.

At Leeds, have developed range of experimental and numerical methods for analysing droplet flows: formation, coalescence and migration.

Courtesy of S.L. Turner & T.P. Comyn, University of Leeds

Droplet Formation – Ink Jet Printing on Textiles

Droplet Formation – Ink Jet Printing on Textiles

CFD predictions agree well with experiment – very useful design tool.

Droplet coalescence is of fundamental importance for a variety of applications

Experiments Lubrication Theory

(a)

(c)

(e)

(b)

(d)

(f)

(a)

(c)

(e)

(g)

(b)

(d)

(f)

(h)

Droplet Coalescence

Droplet motion on chemically- and topographically patterned surface

Through a small trench

Droplet Migration on Complex Surfaces

Droplet motion on chemically- and topographically patterned surface

Through a larger trench

Droplet Migration on Complex Surfaces

Droplet evaporation – formation of classical ‘coffee-ring’

Droplet Evaporation

Lattice Boltzmann Method

The Lattice Boltzmann Method (Jon Summers, Mark Wilson)

•Capabilities to deal with complex physics are increasing rapidly.

• Can deal with flow in very complex geometry

• Can couple up to continuum and couple down to molecular

Rayleigh-Taylor Instability

Fluid-fluid interface as heavier fluid on top plunges into lighter fluid below

Droplet dynamics

Animations by Alan Davies

Passive mixing with blocks.

Electric field

Species concentration

Electric field and fluid flow both solved using LBM. Very useful for microfluidic flows in micro-channels.

Slug in a pore-throat

Lattice Boltzmann very powerful for porous media flows

Heterogeneous surface causes snap-off

Conclusions

Coating and Drying phenomena can be very complex

Often best optimised by combination of targeted experiments, simple models and/or more complex numerical models

Simple models can often be produced which capture the essence of the process.