Case Study - Surface Technologies Course Notes 2010

Surface Treatment Technologies

Case Study for the IAC – January 2011

Coating and laminating

Coating and laminating processes are widely used to improve and

modify the physical properties and appearance of fabric, be it

knitted, woven or nonwoven. They have also facilitated the

development of entirely new products and have led to innovations

in the area of “smart” materials.

Coating and lamination cuts across virtually every product group in

the textile industry, including composites, where the potential is

especially broad.

CSIRO. Surface Technologies

Australian capability

Australias manufacturing capability for surface treatments on fabrics is estimated to be as follows:

• 12 coaters• 7 laminators• 1 plasma• 35 stenters with padding funtionality

• There are no dedicated training programs for personnel on this machinery

Applications for coated and laminated textiles

• Home furnishings • Protective apparel • Automotive interiors • Industrial textiles - filtration• Performance wear • Technical fabrics• Conveyor belts• Medical textiles• Agriculture textiles• Military textiles• Transport – train and aerospace• Marine textiles• Flooring

Industry Association consortium

In 2010 the TTNA commissioned the CSIRO to develop the workshop on “Surface technologies”.

Thirty industry personnel attended the workshop which was held at the Rio Tinto Innovation Centre in conjunction with the FSAA

workshop on “chemical finishes to enhance filtration properties.”

Following are the course notes.......

Course Outline• Adhesion theory

• Definitions• Adhesion theory• Surface energy and spreading• Failure modes

• Types of adhesives• Classification of adhesives• Properties of adhesives

• Surface preparation• Eroding techniques• Chemical modification

• Application and test methods• Preparation, application and curing• Test methods

• Examples

The Mechanics of Adhesion - Introduction

What is an Adhesive?

• Any substance that holds materials together in a functional manner.

• Terms used to describe adhesives include:• Cement, mucilage, glue, paste

• In this workshop we will only consider organic adhesives, but inorganic substances such as Portland Cement can be considered an adhesive

Contents

• Definitions (in notes)• History of Adhesives• Adhesion Theories• Surface Energy • Failure modes• Why use Adhesives

Definitions

• Absorption• The penetration of a liquid into a solid structure by capillary action

• Adherend or Substrate• Material to be bonded by an adhesive

• Adsorption• The interaction of a liquid and solid surface without penetration

• Catalyst• Chemical that accelerates a chemical reaction such as curing• Usually at low concentration and not consumed by the reaction

Definitions

• Cure• Change the physical properties of an adhesive by chemical

reaction

• Cohesive• Resistant to failure by rupture of the material (rather than the bond)

• Creep• Deformation of a material under constant load

• Laminate• Bond together layers of adherends/substrates

• Open time• Time in which dry adhesive layers may still be bonded (contact

adhesive)

Definitions

• Pot-life• The maximum time between preparing the adhesive and its

application

• Shelf-life• the maximum storage time before use

• Shrinkage• Reduction of volume on curing

• Tack• Resistance to detach from the material surface on immediate low

pressure contact

History of Adhesives

• Up to 19th century all glues were animal or plant based• Collagen based obtained from skin, bone, sinew, fish• Starches and dextrins obtained from plants

• 20th century synthetic adhesives developed• 30’s acrylics, 70’s second generation acrylics• 80’s aqueous based systems• 90’s curable hot melts and moisture cure urethanes

4000 BC

Tree sapresins

2000 1500 1000 500 0 500 1000 1500

Animal gluerecorded

Wood glues Veneeringmarquetry

glues refinedProtein, grains

furniture1750 patent

Post-it note

Pressure sensitive acrylic foamed hot melts

Hot melt

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

Synthetic polymers and resinsAcrylics, polyurethanes epoxy resins

BakelitePhenolic resins

Curable hot meltsMoisture cure urethanes

Adhesives

To form a good bond:• The adhesive must wet and spread on the surface of the

material being bonded• Generally the adhesive must harden to a cohesively strong

solid (Pressure sensitive adhesives remain liquid)• Many adhesives contain additives to improve the

performance of the adhesive• Stabilizers, plasticisers, fillers, tackifiers and coupling agents

Surface Tension, Surface Energy and Wetting

• Interactions between the molecules of a liquid and those of another insoluble liquid or gas results in the formation of an interface. Energy is required to change the form of this interface or surface.

• Surface or interfacial tension is the work required to change the shape of the interface.

• Surface tension is easily measured using a tensiometer

Surface Energy and Contact Angle

• Three interfacial forces balance at the edge of a liquid drop on a solid surface. Two are in opposite directions and one forms the “contact angle” to the surface.

• The surface energy of a solid surface (σS) can be indirectly determined from the drop shape of liquids of known surface tension (σL )when they are placed on the solid surface. As the interfacial tension between the liquid and solid (σLS) is unknown, a single liquid cannot be used. This method is not useful for fibrous surfaces.

fLS fVS

fLV = interfacial force of drop & vapour

fLS = interfacial force of drop & solid surface

fVS = interfacial force of solid & vapour

σσσθ −

Contact Angle

• Bond strength depends on contact angle• Low contact angle à stronger bond

• Non-wetting liquid – θ > 90o, wetting liquid θ < 90o

Surfaces wet when the solid surface energy is greater than the liquid surface tension

Wetting increases as the difference between the liquid surface tension and solid surface energy increases

Measurement of Surface Energy

Standard liquids

Contact angles

Apply a model

Surface energyσ = σP + σD

ZismanFowkesOwens, Wendt, Rabel, KaelbleWuSchultzEquations of state

• Drops of liquids of known surface energy• Observe spreading behaviour

• Wilhelmy plate method• Microbalance measurement of the force on the solid as it is

immersed and retracted from the liquid• F = M + lγLcosθ – buoyancy in liquidWhere M = mass of solid plate, l = length of liquid contact, γ L = liquid

surface tension• Goniometer

• Direct measurement

• Zisman plot is the simplest method for determining the surface energy of a solid surface. The surface energy is the surface tension where the two lines intersect.

Liquid surface tension mJm-20 20 40 60

N-decaneCyclohexane

n-tetradecanetoluene

Benzyl alcohol

Ethylene glycol

N-pentanen-hexane

Surface Free Energy – polar and disperse components

σσθ

PTFE e.g. surfacenonpolar

σσσσ

>≈==

or )7.5 ,3.32(PMA e.g.

surfacesPolar The contact angle depends on the polarity of the surface and probe liquids

Units in table mJm-2

Surface liquid Surface tension

Dispersive component

Polar component

Contactangle

PTFE n-decane 23.8 23.8 0 42.3

PTFE n-tetradecane 26.4 26.4 0 49.4

PTFE toluene 28.4 26.1 2.3 58.2

PMA nitromethane 36.5 22 14.5 16.5

PMA methyl benzoate 37.2 27 10.2 3.9

PMA benzyl alcohol 39 30.3 8.7 15.1

• Owens, Wendt, Rabel, Kaelble model gives both the surface energy and polar and disperse components of the surface energy.

y=mx+b

)(2 )(2 ))cos( (1 PL

DS γγγγγθ +=+ D

L σσ

σθσ

2)1(cos

σθσ

2)1(cos +

Measurement of Surface Energy of Fabrics

• Balance methodBalance

LIQUID

FABRIC

• Water Uptake Rate – comparative • Saturation level - comparative• δW = force due to surface tension – need to know perimeter precisely

Spreading Pressure

• Spreading Pressure• πe = γS - γSV

• γS = solid surface free energy, γSV = solid/vapour surface free energy

• S (spreading parameter) = γSG - γSL - γL• Must be negative for liquid to spread

Spreading Pressures

Liquid γL (mJm-2) Solid θ (o) πe (mJm-2)

Hexane 17.9 PTFE 12 3.28

Octane 21.1 PTFE 26 4.9

Water 72.8 PE 94 0

Methyleneiodide 50.8 PE 52 0

Hexadecane 27.2 PE 0 7.6

Hexane 17.9 PE 0 14.5

Surface energy PTFE 19.1 mJm-2, PE 33.2 mJm-2

Theories of Adhesion

• Mechanical Interlocking• Adhesive wets the surface, entering irregularities in the surface

before curing• Physical Adsorption

• Van der Waals forces across the interface between the adhesive and substrate

• Chemical Bonding• Chemical bonds (covalent, ionic, hydrogen) form across the

interface• Diffusion

• Interdiffusion of polymers in contact so the boundary is removed• Electrostatic

• Electrical double layer formed when two metals are placed in contact

Mechanical Interlocking

• Adhesive enters irregularities in the surface before hardening

• requires good wetting and flow properties in the adhesive• Surface roughness increases the apparent contact angle

• A very rough surface at the micron scale does not wet well• Keys into the surface to form a strong bond

• Similar action to hooks and loops in Velcro• Most common mechanism in textiles

• Interfacing using hot melt adhesive• Latex back on carpets

• Adhesive usually below Tg during use• Adhesive has glass like properties over the normal operating

temperature range

Poor wetting

Good wetting

Mechanical Interlocking

Glass surface viewed by AFM. Roughness height approximately 50nm.

Adhesive

Good wetting and flow into the surface roughness à good adhesion.

Physical Adsorption

• Van der Waals forces across the interface• Interaction of dipoles in the surfaces

• Atoms of different electronegativity in a molecule induce a non-uniform distribution of charge in the molecule called a dipole

• Adhesive must wet the surface as the forces only act over a short range (<1nm)

• Only top layer of the surface is involved• Surface energy of the adhesive and substrates are used to assess

adhesion• Three types of dipole interaction with decreasing strength

• Permanent dipole – permanent dipole• Water between glass

• Permanent dipole – induced dipole• Epoxy and polyethylene

• Instantaneous dipole (non-polar molecules)• Cling wrap – polyethylene film

Adhesion• Work of adhesion

• Where σ1 = surface free energy of substrate 1, σ2 = surface free energy of substrate 2 and σ12 = surface free energy of the interface between substrate 1 and 2. σ12 is minimised to give maximum bond strength.

• Polar/disperse mismatched

• Polar/disperse matched

INNNNNNNNNI

NIIIIIIIIIN

21212112

σσσσ

σσσσσσσ

σσσ

+−+=

nN/m40 nN/m,10 nN/m,5020mN.m 80mN/m,

nN/m10 nN/m,40 nN/m,50

Wσσσ

σσσσ

nN/m40 nN/m,10 nN/m,500mN.m 100mN/m,

nN/m40 nN/m,10 nN/m,50

Wσσσ

σσσσ

I I I IN NIII

Example - Gecko

• Gecko foot• Covered in hairs• Each hair splits into hundreds of spatula shaped ends• Van der Waals attraction between hairs and surface• Changing the hair to surface angle by curling the toe allows easy

removal and walking

Photo by Bjørn Christian Tørrissen (Wikipedia)

Chemical Bonding

• Formation of ionic, covalent or hydrogen bonds• Ionic – metal epoxy, some pressure sensitive adhesives

• water -dispersible sulfopolyester• Some easily disrupted by water

• Covalent – silicones, BAP on wool• Permanent strong bond

• Hydrogen – postage stamps (PVA to cellulose)• Easily debonded by water, humidity sensitive

• Stronger bonds than adsorption• Often requires a coupling agent or surface treatment

• Coupling agents are compounds that contain two reactive groups, one that bonds to the substrate and the other to the surface coating

Diffusion

• Interdiffusion of polymers in contact• Boundary between adherends is eventually removed• Requires mobile polymer chains (T>Tg)• Requires compatibility between polymers

• Same polymer• Polyethylene and polypropylene are not compatible

• Solvent welding of polymers• Used in wetsuit manufacture, contact adhesives

Electrostatic

• Electron transfer from one material to another at interface• Development of electrical double layer at interface (opposite

charges in materials)• Attraction between materials

• May be applicable to some metals• Most polymers are insulators

Weak Boundary Layer

• Presence of contaminants leads to a weak bond• Contaminants include processing oils, softeners, waxes• Oxides on metals can result in weak bonds

• Some adhesive designed to dissolve contaminants• Acrylics can dissolve some oilsà stronger bonds

Glass Transition Temperature• Temperature where polymer changes from glassy solid to a rubber• Mechanical properties radically change at Tg

• Below Tg limited translational and rotational movement of polymer backbone• Above Tg movement of backbone

• Polar groups increase Tg

• Non-polar groups decrease Tg

• Adding liquids to the adhesive lowers TgPolymer Abbreviation Tg (°C)Polymethacrylic acid PMAA 228

Poly(methyl methacrylate) PMMA 105

Poly (ethyl methacrylate) PEMA 65

Poly(n-propyl methacrylate) PPMA 35

Poly(n-butyl methacrylate) PBMA 20

Polychloroprene CR -50

Polyisoprene (natural rubber) rubber -75

Polydimethylsiloxane PDMS -127

Measurement of Glass Transition Temperature

• DSC

• Change in properties at Tg.• Rapid increase in temperature• Reduced stiffness

Sample Reference

Heaters

Tg Temperature à

RubberyGlassy

Temperature à

ume à

Glassy

Temperature à

à Rubbery

Glass Transition Temperature

It is unacceptable for an adhesive or surface coating to pass through the glass transition during service

Failure Modes

• Adhesive and adherend must be compatible• 5 elements to consider• Weakest element determines the joint strength

Adherend 1 Adherend 2Adhesive

Interface 1 Interface 2

Failure modes

• Failure of bond or material

Adhesive failure

Cohesive failure

Advantages of Adhesive Bonding

• Able to bond materials that would otherwise be difficult to join

• Thin sheet materials, laminates, fibres, paper products, carpets• Stress is distributed over a wider area• Dissimilar materials can be joined• Fabrication of complex shapes• Improved appearance• Reduced cost• Rapid assembly• Good sealing and insulating properties• Improved product performance

Disadvantages of Adhesive Bonding

• Need for surface preparation• Relatively long curing times

• Optimum strength develops over time• Joint design important

• Need to understand stresses applied to the bond• Temperature limitations

• Thermal and mechanical shock• Poor electrical and thermal conductivity• Degradation• Dismantling may be difficult

• New adhesives improve disassembly e.g. hot melt adhesives• Creep• Retooling costs

Why use Adhesives

• Adhesives may be the only solution to a bonding problem• Large area to be joined (e.g. bench tops), membrane fabrics

• Improved performance• Upholstery fabrics, plywood

• Join dissimilar materials• Wet suits (neoprene to nylon)

• Join heat sensitive materials• Thermoplastic components

• Laminated structures• Laminated fabrics

• Reinforced structures• Fibreglass, tyres

• Temporary fastening• Labels, ‘sticky tape’, ‘post-it notes’

Adhesive Materials – Properties and Selection

Contents

• Types of Adhesives• Structural adhesives• Pressure sensitive adhesives • Contact adhesives• Hot-melt adhesives• Reactive hot melt adhesives• Drying adhesives – solvent and water• UV cure adhesives

• Properties of Selected Adhesive• Selection of Bond Type and Adhesives

Classification of Adhesives

• Thermoplastic• Melt without degrading

• Thermoset• Heat curing• Chemical reaction• Catalysed

• Structural adhesives• Pressure sensitive adhesives • Contact adhesives• Hot-melt adhesives• Drying adhesives – solvent and water

Structural Adhesives

• Requirements• Good load carrying capacity• Long-term durability• Resistance to:

• Heat, Solvents, Fatigue

• Adhesive families• Epoxies, polyurethanes, acrylics, surface activated acrylics,

cyanoacrylates, silicones

Advantages DisadvantagesHigh strength Surfaces must be matched

Bond dissimilar materials Clean surfaces

Large surface area Max temperature 100oC

Distribute load Weather resistance

No weakening of bonded parts Design requirements

Where usedComposite materialsConstructionCarpet

Pressure sensitive adhesives

• Permanently tacky adhesives• Balance between adhesion and cohesion• Polar adhesives for high surface energy surfaces• Relatively low MW• Low Tg, often < ambient (rubber like)

• Requires pressure to achieve good bond• Low viscosity à better wetting at low pressure

• Adhesive often carried between a backing and release liner• PSA’s include natural and synthetic rubbers (SBR),

thermoplastic elastomers, polyacrylates, polyvinylalkyl ethers, and silicones.

PSA’s

Property Rubber Based Acrylic SiliconeCost Low Moderate HighTack High Low-high LowPeel Strength Mod-high Low-high Low-modService Temp 0 To 65oc -40 To 150oc -73 To 250ocEnvironment Indoor Indoor/Outdoor Indoor/OutdoorUV Resistance Poor Excellent ExcellentSolvent/Chem. Resistance

Poor Good Excellent

Plasticiser Resistance

Poor Poor-fair Good

Bond To High Energy Surface

Excellent Excellent Excellent

Bond To Low Energy Surface

Moderate Poor-high High

• Adhesive selection

PSA’s

Advantages DisadvantagesRemovable? Release liner

Invisible bonding Aging

Reduced weight PSA manufacture - solvents

Range of bond strengths and properties

Non-permanent bond

No open time Low sheer and peel strength

Can bond dissimilar materials Temperature sensitive

Contact adhesives• Similar to PSA• Semi-structural adhesives

• Sheer strength > 1000 kPa• Peel strength > 3kg/cm length

• Adhesive applied to both surfaces and solvent allowed to evaporate before joining pieces

• Diffusion mechanism involved as adhesive on each component diffuses across the interface. The rheology immediately before bonding is important for good bonding

• Usually based on solvent solutions of neoprene• -(CH2-C(Cl)=CH-CH2)- polychloroprene or poly-2-chlorobutadiene• Also polyurethanes, SBR, acrylic polymers

• Water dispersion versions replacing solvent systems• Reduced strength and durability

http://www.veganwares.com/factory.htm

Contact Adhesives

Advantages DisadvantagesNo mixing required Cannot be repositionedImmediate green strength Use of solventsStronger than PSA Open time

Water based systems have long drying times

Hot Melt Adhesives

• Thermoplastic resins• Ethylene vinyl acetate (EVA), polyamides, polyester, acrylics

• Applied as hot liquids• Application temperature 150-200oC• Must flow and wet surface• Solid at 80oC with amorphous and crystalline domains• Preheating of the surface to control cooling rate

• Limited by upper operating temperature – approximately 65oC

• Applications• Book binding, veneer coating, laminated textiles, labels, packaging,

construction

Hot Melt Adhesives

Property Ethylene Vinyl Acetate

Polyamide Polyester Polyethylene

Softening point 40°C 100°C 60 -200°C

Melting point 95°C 195-220°C 267°C 137°C

Crystallinity Low Low High Low or High

Melt flow index 6 2 5 5

Tensile strength MPa

18 13 31 13

Elongation, % 800 300 500 150

Cost Low to Mod Moderate High Low

Typical properties of hot melt adhesives

Melt flow index is the ease of flow of the molten adhesive

Hot Melt Adhesives

Advantages DisadvantagesNo solvents Poor temperature resistance

No mixing required Creep Immediate green strength Water and solvent permeation

Easy handling, several formats High viscosity

Reactive Hot Melt Adhesives

• Thermoplastic adhesives that react after application to become a thermosetting polymer

• Polyurethanes, silane modified urethanes, acrylates (UV), silicones• Moisture cure polyurethane most common

• Others include UV cure acrylates and silicones• Overcome many of the disadvantages of hot melts

• Higher operating temperatures - >100oC• Improved environment stability – humidity, chemical

• Able to apply at lower temperature – 65oC• Applications

• Automotive components, laminated textiles, carpet manufacture

Reactive Hot Melt Adhesives

Advantages Disadvantages

Lower application temperature Higher cost

Improved temperature resistance Full cure in several days

Improved adhesion Short open time

Improved creep resistance Moisture sensitive in applicator

Tough and flexible Special applicators needed

Applicator clean-up

Drying adhesives

Types of drying adhesives and common uses• Loss of organic solvent

• Contact adhesives• Loss of water

• Pastes of starch derivatives or PVA adhesives• Water moistenable

• Poly(vinyl alcohol) PVOH and poly(vinyl acetate)• Aqueous emulsions

• Latex e.g. PVA, acrylic

UV Cured Adhesives

• Cure on exposure to UV radiation• Very rapid cure possible

• Usually a low molecular weight prepolymer and initiator dissolved in monomer

• Photoinitiator produces radicals that begin polymerisation• Often large volume decrease on curing

• Reduced by use of particulate fillers• Must be visible to UV source

Adhesive Formulations

• Adhesives are usually not pure polymers• Formulations include

• Adhesive polymer• Fillers

• Improve the properties of the liquid and cured adhesive e.g. increase viscosity of liquid and shear strength of solid epoxy

• Examples: zinc oxide, titanium oxide, silica, clay, pigments• Tackifiers

• Added to increase the tack of the adhesive• Examples: rosin esters, polyterpene resins, hydrocarbons

• Plasticizers• Added to soften the cured adhesive – “make more plastic”• Examples: mineral oil, lanolin, lecithin, glycol

• Antioxidants• Inhibits oxidation of the adhesive, increases shelf and service life• Examples: metal chelating agents, common antioxidants

Selection of Adhesives

• Factors to consider when selecting adhesives• Surfaces to be bonded• Material properties

• Maximum operating temperature• Thermal and moisture expansion

• Joint design• Rate of stress load, total stress load and direction stress is applied to

the joint• Area of joint

• Cure time• Hot melt < drying < chemical reaction

• Open time• Creep• Flexibility• Peel strength

• Factors to consider when selecting adhesives• Application method• Process speed • Further processing – e.g. finishing operations• Service conditions

• Weathering • Washability, dry-clean• Temperature range• need for autoclaving

• Ability to disassemble goods for recycling

MaterialAdhesive

NR CR PU Si PU foam

PVC foam

PTFE PE Glass

Acrylic X X X X * X ** X **Cyanoacrylate *** *** ** X * * ** * X

Epoxy ** ** X X * * X X **Chloroprene *** *** ** X ** *** * X X

Urethane ** X *** X ** * X X **Silicone X X X *** X X * * *SBR X X X X * *** X X X

Nitrile rubber *** X *** X ** *** ** *** *

• Selection guide

NR – nitrile rubber, CR – polychloroprene rubber, PU – polyurethane, PVC – polyvinyl chloride, PTFE – poly tetrafluoroethylene, PE – polyethylene, SBR – styrene butadiene rubberX – not recommended, * - poor, ** - fair, *** - good

Surface Analysis

• Contact Angle (see above)• FT-IR Spectroscopy • X-ray Photoelectron Spectroscopy• Scanning Probe Microscopy

Fourier Transform –Infra Red Spectroscopy

• Attenuated Total Reflectance (ATR)• 2 micron penetration into surface by effervescent wave. Therefore

provides information about the surface chemistry.• Requires smooth surface and close contact with the crystal.

Multiple bounce crystals give better signals with fabrics.• Permanent dipole in the bond required to absorb IR radiation and

give a signal

• Chemical information about surface• Chemical bonding• Chemical groups• Oxidation states

Sample

ATR crystal

IR beam

• Other surface sensitive techniques• Grazing angle spectroscopy• Specular reflectance• Photoacoustic spectroscopy• Diffuse reflectance

microphone

PhotoacousticGrazing angle

Specular and diffusereflectance

X-ray Photoelectron Spectroscopy (XPS)

• Also called Electron Spectroscopy for Chemical Analysis (ESCA)

• High vacuum technique• Sample is irradiated with a monochromatic X-ray beam• Electrons with characteristic energy are ejected from atoms in the

surface

Bvcrist, Wikipedia

XPS Binding Energies

Element Binding energyEb (eV)

Relative sensitivity

Core level

C 285 0.25 IsO 530 0.66 1sF 690 1.0 1sNa 1072 2.3 1sSi 102 0.27 2p

• Energy of emitted electron (Ek) is unique to atom and oxidation state

Ek = hv – Eb – ψwhere hv = energy of x-ray, Eb = binding energy of electron,

ψ = work function of spectrometer (constant)

• Surface technique only• Electrons from <10nm below the surface• Reducing the angle can increase surface sensitivity

• Static Secondary Ion Mass Spectroscopy (SSIM)• Related technique

• High energy ion beam used to sputter material from surface• Mass spectrometer to analyse ions produced• Very surface specific – 1nm

Scanning Probe Microscopy

• Scanning Probe Microscopy (SPM or AFM)• Three modes of operation• Force-distance (non-contact) mode

• Tip of the cantilever maintains constant distance from sample surface• Distance determined by force on tip

• Contact mode• The tip is in contact with the surface at constant force

• Tapping mode• Tip is osculated near the surface and changes in deflection observed

• Able to determine some chemical information using functional tips

Surface Modification

Contents: Surface Modification Technologies

• Abrasion• Solvents• Coupling agents• Corona• Low Pressure Plasma• Atmospheric plasma• Flame

Abrasion

• Increase surface roughness and removes contaminants through mechanical means

• Increases surface area of contact• So increases adhesion strength

• Usually used on hard surfaces• Sand paper, emery paper, wet & dry• Shot blast, bead blast

Cleaning & Degreasing

• Solvents are often used to clean and degrease the surfaces before adhesion.

• They are very good at gross level cleaning • BUT: as little as 1g/m2 of contamination, e.g. a monolayer,

can affect adhesion unless the adhesive can absorb the contamination.

• 1g/m2 contamination is the residue of 0.1L/m2 of liquid containing 10ppm non-volatiles e.g. from a dirty container.

• Also many plastics and fibres contain additives designed to bloom at the surface

Cleaning & Degreasing

• Metals: • Trichloroethylene• N-propyl bromide

• Polycarbonates: • Methanol• isopropanol• detergent

• Fluorocarbons: • Trichloroethylene

• Polyesters: • Detergent, • Acetone, MEK

• Polyethylene: • Acetone,• MEK

• Polypropylene: • Acetone, • MEK

• Polystyrene: • Methanol, • Isopropanol, • detergent

• Polyurethane:• Acetone, • MEK

Commonly Used Degreasing Solvents

Wet Chemical Treatments

• Metals: • Various acidic etches

• Fluorocarbons e.g. PTFE: • 1% Sodium in ammonia; or epoxy primer & heat 10min at 370oC

plus 5min at 400oC• Polyesters:

• 20% Sodium Hydroxide at 95oC 10mins• Polyethylene, Polypropylene:

• Sodium dichromate + water + Sulphuric acid (93%) in proportions 5:8:100

Coupling Agents

• Adhesion promotors can be reactive (or nonreactive), if they contain a functional group that can react with a functional group on the substrates

• Coupling Agents• the term coupling agent is used if one of the components is an

inorganic component (filler, metal etc). • Reactive coupling agents will contain reactive groups.

• Reactive groups can be Carboxylic acid groups, Epoxy groups (e.g. glycidylmethacrylate, oxazoline), Maleic anhydride, or others.

• Non reactive coupling agents draw their functionality mainly from their polarity. They then represent an intermediate polarity between the adhering substrates and the adhesive. Adhesion is usually by Van der Waals forces.

Corona & Plasma Treatments

• Corona is the most common form of atmospheric pressure plasma

• It is a dielectric barrier discharge plasma (DBD) most commonly used on plastic films for printing and adhesion enhancement

• Higher treatment levels are obtained with other forms of plasmas, including low, medium and atmospheric pressure plasmas in a variety of process gases

ELECTRODES

DIELECTRIC BARRIER

CORONA or PLASMA

High Voltage AC

“STATIONARY” MICRODISCHARGES

What is a Plasma?

TAKE A GAS

ADD ENERGY

ELECTRICITY

ELECTRONS

IONISE THE GAS

Two Temperatures

Hot & Cold Plasmas

Electron temp

Ion & atom temp

Examples of Plasmas

Fluorescent light

Cool plasma

Hot, high pressure

Thermonuclear device

Welding arc

LighteningCool, low pressure

Xenon sputtering plasma

Hot, atmospheric pressure

Sun - Hot, high pressure (inside)

History

• Low Pressure Plasmas- vacuum, batch process • Effects & benefits well proven but it is industrially difficult to use

• Dielectric Barrier Discharge, “Corona” • less effective but industrially robust

• NEW Systems: Atmospheric Pressure, highly effective, industrially robust:

• Some commercial systems are available, many under development

Plasma cutter

Textile treatment with permission Textile World 2008,

Dielectric Barrier Discharge

Breakdown initiates Micro-filament forms

Electron concentrationElectron concentration

Corona Systems

• Discharge between ceramic coated HV electrodes and grounded steel rollers.

• Problem: micro-discharges recur in same spots:• Breakdown occurs at ions from last cycle• Leads to poor uniformity• Low concentration of reactive species• Relatively low surface energy modification

Dielectric Barrier Discharge

• For an effective surface treatment we need: • random distribution of micro-filaments

• To achieve:• Uniformity of treatment and • Maximise the concentration of reactive species

CORONA GLOW-LIKE

Glow-like Discharge

Advantages• Uniform plasma• Low temperature but very reactive• Generates free oxygen (at the fibre surface)• Penetrates permeable fabrics

Electrodes

Dielectric barrier

Plasma

High Voltage AC

Uniform plasma

Design considerationsGas composition & flowElectrode designVoltage & frequency

Plasma Surface Treatments & CoatingsFree radicals

UV photons

PlasmaElectric field

Produce reactive groups on fibre surface

e.g. Carbonyl, carboxylic acid, hydroxyl groups

Oxidise surface

UV & Ions cross-link surface molecules

Plasma Surface Treatment

Optimum Result:• UV Crosslinked layer covalently bonded to the adhesive

via plasma produced reactive groups. • Cross-linked layer:

• strong • stabilises the surface against reorientation and diffusion of low

molecular weight material from the bulk

Crosslinked layer

Bulk Polymer

Covalent Bonding

Adhesive

Enhance bonding

Enable printing

Increase or decrease wettability

Apply Functional Polymers

Monomer gas

Polymerisation& Grafting

Monomer gas

e.g. Fluoro-polymers

Addition of a monomer gas to the plasma produces a highly crosslinked polymer on the surface.

Surface Modification and Polymerisation

The surface can be tailored for the application. e.g.

• Surface energy can be raised for bonding, printing, hydrophilicity

• Surface energy can be lowered by grafting polymers either in the plasma or post plasma treatment for stain-blocking, cake release, increased hydrophobicity

Example: PE

Surface tension increase after plasma treatment (arbitrary units)

PE Fabric

untreated 1 sec 5 sec 10 sec 5 sec ( 4 days later)

1 0.03 0.25 0.37 0.34 0.29

2 0.07 0.25 0.3 0.30 0.33

3 0.11 0.29 0.29 0.3 0.31

Low and Medium Pressure Plasmas

• Consist of Vacuum vessels, pumping systems, RF drivers• Batch process, and expensive• Good control of the environment• Highly uniform plasma over large volume• Can use dangerous chemicals safely• Medium pressure plasmas can have quicker pump down

and cheaper pumping systems

www.europlasma.be

Low and Medium Pressure Plasmas

• High energy surfaces reorientate over time so that the reactive groups bury themselves into the bulk polymer

• Better control of process gases, less oxygen, may allow control of the competition between cross linking and oxidation.

• Cross-linked surface layer resists reorientation and is stronger

Industrial Plasmas

• Saturated treatment in 2 seconds• At 20m / min:

• Need a treatment length of ~ 0.7m

• Process can be in-line with other processes:• Printing• Laminating• Stenter• Coater

Optimum Treatments

• Each polymer & adhesive system requires optimization of the energy density and plasma chemistry.

• Under treatment leaves contamination, which may result in poor adhesion

• Over treatment can produce a layer of low molecular weight material – wettable and appears to have the right chemistry but is weakly bonded to bulk.

Flame Treatment

Courtesy, Eddie Grant, Aerogen

Flame Treatment

• Treatment level depends on the substrate, the time spent in the flame and the temperature and chemistry of the flame.

• Excess treatment results in flame-polishing, which can be useful but doesn’t enhance adhesion.

Flame Chemistry

• The most reactive species are not in the hottest part of the flame

• Small reactive species such as H , OH and O have higher concentrations near the tip of the flame

H2OCO2O2OH2OHO

COOH2OH

Temperature scaleLow high

Air Fuel

Flame Treatment• Flame Treatment can be more stable than Corona

treatment on polypropylene but Corona is more user friendly. Plasma treatments can give higher surface energies but lower stability but the plateau level is still usually higher than Corona or Flame.

Flame Treatment

• Gas Mixture: Fuel/Air Ratio

• The excess O2 level in the flame is critical to the surface energy enhancement attained.

Flame Treatment

Critical Parameters• Combustion conditions –

• Air / Gas ratio• The burner to substrate gap• The dwell time of the substrate

in the flame• The substrate• Mechanical handling• Flame energy

Courtesy Aerogen

Flame Treatment

• A water cooled backing roller is often used to dissipate the heat of the flame.

• The roller also controls the position of lightweight films and webs

Courtesy Aerogen

Aerogen Control System

Courtesy Aerogen

Measurement of Surface Modification

• Indirect Tests: • Surface energy, surface tension, contact angle, wicking test

• Chemical Composition • XPS, NMR, FTIR etc

• Direct tests: • adhesion, peel tests, bond strength

Summary

• Properties of plasma processes:• low energy (~10kW / m2 )• low effluent• high speed• cheap, reliable, efficient • inexpensive• replace solvents (environmental and/or OHS hazards)

• Properties of flame treatment:• high speed• more stable surface

• Properties imparted to fibres:• increased fibre surface energy• no reduction in strength• enhanced bonding• greater wettability• greater reactivity

Adhesive Materials – Application Methods for Textiles

Contents

• Spraying• Roll Coating• Knife Coating• Printing• Hot Melt• Foam• Powder• Release Coatings

Spraying

• Advantages:• Suitable for large areas and uneven surfaces• Good control of adhesive film thickness• Requires low viscosity solutions

• Disadvantages:• Overspray• Use of solvents• Aerosol generation

• Small areas can be applied using a hand spray• Very good for fabrics

• (Video of spray application)• http://www.youtube.com/watch?v=jw1DTpUqFeU

Roll Coating

• Direct roll and Gravure roll

Gap between the rollers determines add-on

• Kiss roll

Fabric and roller at different speeds and direction

• Doctor roll

Fabric and roller in same direction

• Reverse roll

Metering roller to control add-on

Knife Coating

• Knife over air Knife over roll

Thinner coating possible Gap determines thickness

• Knife over belt or table Blade shapes

Over air or surface options sharp, rounded, Jdetermines penetration and add-on.

• http://www.youtube.com/watch?v=LusEXygxUoo

Curtain Coating

• Falling curtain of adhesive coats material• Very even coating possible• Only adhesive touched the substrate

• Useful for delicate substrates

http://www.youtube.com/watch?v=UquzkUUqqwo

Printing

• Rotary screen printing• Non-continuous application possible• Control of add-on• Improved flexibility of textile• Surface application possible• Tensionless application

Hot Melt

• Large surface area Powder applicators• Roller coating• Doctor blade• Printing

• Small area• Glue gun

• Powder• Scatter• Electrostatic• Paste• Engraved roller

• Doctor blade• Slot • Reduces water usage

• Parabolic foam applicator head• Constant foam age• Eliminates head tracking effects

Release coatings

• Used on PSA tapes• Low surface energy coating

• Coating on liner has good adhesion to the liner but poor adhesion to adhesive on substrate

• Mould release agents• Needs good cohesion – must separate cleanly• Silicones most common release agent

• Bond to liner by mechanical interlocking• Release energy can be tailored to the application

AdhesiveTight releaseSubstrate

LinerAdhesive

Tight releaseEasy release

SummaryCoating method

Viscosity cP

Coating weightg/m2

Coating accuracy

Coating speedm/min

Adhesive types

Wire rod 100-1,000 15-100 10 100-150 Solution, emulsion

Knife over roll

4,000-50,000 25-750 10 100-400 Solution, emulsion, 100% solids

Reverse roll 300-50,000 25-250 5 100-700 Solution, emulsion

Gravure 15-1500 2-50 2 100-700 Solution, emulsion

Extrusion die

400-500,000 15-750 5 300-700 Emulsion, hot melt, 100% solids

Slot die 400-200,000 20-700 2 100-300 Emulsion, hot melt, 100% solids

curtain 50,000-125,000 20-500 2 100-500 Emulsion, hot melt

Viscosity of water at 20oC is 1cP, honey is 2,000-10,000cP

Adhesive Materials – Bonding in Textiles

Contents

• Coatings• Shoes• Laminates• Carpets• Non-wovens• Automotive

Examples of the use of adhesives

Fabric to foam flocks

non-wovens

Case Study – Coated Blind Fabric

• Typical blind fabric has several layers of resin. For a typical ‘blackout blind’ these are:

1. A stiff resin impregnated into the fabric to give the desired bending properties

2. A softer, usually white layer to protect the visual appearance of the fabric

3. A soft layer containing a black pigment – the blackout layer4. A soft, usually white layer, to improve the back appearance.

• Resins are usually acrylics with different Tg (glass transition temperature)

• A final layer may be applied to enhance the visual appearance of the product – e.g. a flock

http://blinds247.net/rollers_holland_blinds.html

Case Study - shoe

1.Fully supported Box-Toe for shape retention 2.Cotton Vamp Lining 3.Heavy Gauge Upper 4.Double eyestay Construction 5.Padded tongue6.Runner's Ortho Cup7.Foam padded collar8.Brushed suede Counter Insert9.Triad Heel10.Special Shock Retention Heel & Sole Design 11.Special Rubber Blended Sole12.High Density Foam Insole 13.Texon Insoles14.Comfort lining 15.Fore-part Pad & Flex Zone 16.Chevron Design Sole

• Adhesive requirements• Flexibility• Elongation• Moisture resistance• (chemical resistance)• Dissimilar materials – urethane/SBR to leather, cotton to leather• Fast tack• High green strength• Cure conditions – room temperature, elevated temperatures• Low fatigue

• Inner Sole to Upper• Polychloroprene (neoprene) contact cement

• Outer Sole to Upper• Traditional cement – contact adhesive (neoprene)• Urethanes and polyamides now also used• Some manufacturers cast the urethane sole on the inner sole

• Toe cap• Urethane contact cement• Trend to aqueous contact cement systems

• Sports shoes• Tend to hot melt polyamide

Laminated Films

• One substrate coated then nipped to a second substrate• Wet lamination

• Water based solutions or emulsions• Natural products e.g. starch, dextrin• Synthetic polymers e.g. polyvinyl acetate, acrylics• Reduced VOC

• 100% reactive liquids• Polyurethanes, polyesters

• Solvent based adhesives• Reduced drying time and energy• Potential environmental concerns – VOC emissions

• Conventional coating equipment• One substrate must be porous

Laminated Films

• Dry lamination• Hot melt adhesives e.g. ethylene vinyl acetate copolymers• Liquid adhesives partially dried before lamination e.g. acrylic

emulsions, silicones• 100% reactive solids e.g. polyurethanes, UV curable acrylates• Application methods include powder application

• Green strength important for handling of laminate• Full strength usually 24 hours• Coating method and adhesive depend on substrate

characteristics• Surface preparation• Sensitivity to moisture, solvents• Temperature stability

Laminated Films

• Adhesives• Other functional properties may be included – e.g. flame resistance• Consider gas permeability, optical clarity, thermoforming capability,

electrical properties, chemical and heat resistance• Resistance to tunnelling – local delamination caused by substrates

of different extensibilities• Adhesive properties – adhesion, cohesion, flow, flexibility• Water borne adhesives becoming more popular, improving in

properties - acrylics and polyurethanes• UV cure adhesives

Example – Gore-Tex

• The simplest rain wear is a two layer sandwich. The outer layer is typically nylon or polyester and provides strength. The inner one is polyurethane that provides water resistance at the cost of breathability.

• Early Gore-Tex fabric replaced the inner layer of PU with a thin, porous fluoropolymer membrane (Teflon) coating that is bonded to a fabric.

• However the exposed Teflon membrane layer was easily damaged. A third, PU layer, was added as the inner of the "protection" layers. Then either a loose fabric shell layer, or a bonded coating is added to the garment to protect the membrane sandwich.

http://www.youtube.com/watch?v=X5IgcmKXb8o

Non-woven

Thermal Mechanical ChemicalCalendering

PointOverall

EntanglementNeedle punchSpunlace

Emulsion adhesiveButadiene copolymersVinyl acetateVinyl chloride

Air oven Perforation Solvent bondingRadiant heat Pressure embossing Thermoplastic dry

bondingUltrasonic Stitching Powder resinFlame Hot melt bondingExtrusion

• Many ways to bond non-woven fibres

Latex Bonding of Non-wovens

• Non-woven strength function of fibre, binder and adhesion strength

• Good cohesion requires coalescence of latex droplets• Increase surface energy• Decrease particle size

• Adhesion to fibres• Latex and polymer must wet fibres• Surfactants added to reduce latex surface tension• Size added to fibre to improve wetting• Web density affects binder performance

Good bonding

Poor bonding

Latex Bonding of Non-wovens

Advantages Disadvantages

Low viscosity, easy to apply Entrainment of surfactant

Wide range of binders High temperature to dry

Easy to handle Polymer migration

Simple application machinery Environmental concerns -surfactants

No solvent, low VOC

Carpet production

• Move from conventional latex production

• Latex integrated into tufts• Difficult to separate• Generally non-recyclable

• to recyclable products• Thermoplastic adhesives• Easier separation for recycling by

heating

Images from http://www.shawcontractgroup.com/Html/PerformanceBackings

Case Study - AutomotiveComponent Adhesive use %Headliners 33Sound insulation 17Door and side panels 10Carpet bonding 14Dashboard assemblies 8Seat upholstery 8Other 10

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