Adhesion

Victor X. Lu, Ph.D.Cytec Industries, Inc.

Victor X. Lu, Ph.D.Cytec Industries, Inc.

Pressure Sensitive Adhesive Tapes for Sales and Marketing Professionals

Tape University Basics May 12, 2009

Disciplines of AdhesionDisciplines of Adhesion

Reference: Petrie, E.M., Handbook of Adhesives and Sealants, p. 9, McGraw-Hill, 2000.

Physics

Science of

Adhesion

SurfaceScience

JointDesign

Polymeric

Materials

Chemistry Mechanics

PSA FundamentalsPSA Fundamentals

• Tg

• -35 oC to +10 oC

• Viscoelastic Behavior• Solid component• Liquid component

• Surface Energy

• Interfacial Forces

• Tg

• -35 oC to +10 oC

• Viscoelastic Behavior• Solid component• Liquid component

• Surface Energy

• Interfacial Forces

Theories of AdhesionTheories of Adhesion

• Mechanical Interlocking

• Diffusion

• Electronic

• Adsorption

• Mechanical Interlocking

• Diffusion

• Electronic

• Adsorption

No one theory explains adhesion but Adsorption Theory is the most popular.

A List To Stick By ...A List To Stick By ...

SUBSTRATEThe material that the adhesive/tape

needs to stick to?

FUNCTIONALITYWhat is the tape supposed to do?

ENVIRONMENTAL CONDITIONS

What are the conditions the tape will function in?

APPLICATIONHow is the tape applied?

ADHESIONHow the adhesive is supposed to behave?

Things to know about PSA functionality

You cannot always assume that the customer knows

what he/she needs or what will or will

not work!

Knowing The Answers To These Parameters Will Help To Select ...

Knowing The Answers To These Parameters Will Help To Select ...

• Adhesive Type• Backing Type• Product Construction• Fitness For Use Requirements • Quality Control Tests• Cost Implications

SURFACE TEXTURESURFACE CONTOURSURFACE ENERGY

SUBSTRATE

FUNCTIONALITY

ENVIRONMENTAL CONDITIONS

APPLICATION

DURABILITYFLAMMABILITY RESISTANCEBARRIER PROPERTIES

WATER/MOISTURECHEMICAL EXPOSUREU.V. EXPOSUREHOT/COLDHAND APPLIEDMACHINE APPLIEDLINER REMOVALTEMPERATURE

ADHESIONPERMANENCEREMOVABLETACKSHEAR RESISTANCE

ADHESIVE THICKNESSADHESIVE TYPE

BACKING TYPEADHESIVE TYPECONSTRUCTION

BACKING TYPEADHESIVE TYPE

TACK VALUESADHESION VALUES

ADHESIVE PROPERTIES

PARAMETER CONDITION IMPLICATIONS

The focus of this presentation: To discuss ...

The focus of this presentation: To discuss ...

• What conditions must exist for maximum adhesion to occur

• How to maximize adhesion when these conditions are compromised

• What conditions must exist for maximum adhesion to occur

• How to maximize adhesion when these conditions are compromised

Critical Parameters For Bond Formation

Critical Parameters For Bond Formation

Bulk adhesive properties Viscous and elastic behavior

Surface properties Surface tension in high energy surfaces

Interfacial properties Ionic, Van der Waals and hydrogen bonds

Bulk adhesive properties Viscous and elastic behavior

Surface properties Surface tension in high energy surfaces

Interfacial properties Ionic, Van der Waals and hydrogen bonds

Bond FormationBond Formation• Adhesive must “wet” substrate* surface.

Surface energetics control wetting.

– Surface energy properties of both the adhesive and of the substrate

• Adhesive must “flow” over substrate. Rheology controls flow.

– Viscous flow properties of the adhesive (affects wet-out)

• Adhesive must “wet” substrate* surface. Surface energetics control wetting.

– Surface energy properties of both the adhesive and of the substrate

• Adhesive must “flow” over substrate. Rheology controls flow.

– Viscous flow properties of the adhesive (affects wet-out)

* Substrate refers to the material/surfacethat the adhesive is attempting to adhere to

(E.G. METAL, CONCRETE, PVC , etc.)

Bond EnergiesBond Energies

Covalent 60-700

Ionic 600-1000

Van der Waals 0.1-40

Polar 4-20

Hydrogen Up to 40

Bond Energy, Kj/mol

Adhesion is governed by Van der Waals, Polar and Hydrogen Bonds which require intimate contact

between the substrate and adhesive

Surface EnergySurface Energy

What Is Surface Energy?What Is Surface Energy?

LIQUID

LOW SURFACE ENERGYLOW SURFACE ENERGY

WAX

PAINTMETAL

Poor To AverageAdhesive Wet-out

LIQUID

HIGH SURFACE ENERGYHIGH SURFACE ENERGY

PAINTMETAL

GoodAdhesive Wet-out

Surface EnergyThe ability of an adhesive to wet out over the surface of a material is related to its surface energy. Low surface energy materials do not allow the surface to wet out - high surface energy surfaces do

Surface EnergyThe ability of an adhesive to wet out over the surface of a material is related to its surface energy. Low surface energy materials do not allow the surface to wet out - high surface energy surfaces do

The difference is similar to the behavior of water on the finish of a car. On a newly waxed car, water beads up (low surface energy) and on an older finish it wets out (high surface energy).

Surface EnergeticSurface Energetic

L s<s : Beading occursLIf >

For wet out to occurL s

Surface Energy is expressed as dyne/cm. It can be altered by corona and flame treatment or primer application of surface.

FOR WETTING TO OCCUR, THE SURFACE ENERGY OF THE ADHESIVE MUST BE LOWER THAN

THE SURFACE ENERGY OF THE SUBSTRATE.

Surface Energy

Surface Energy

HIGH SURFACE ENERGY MATERIALS DYNES/CM MATERIAL

1103 COPPER

840 ALUMINUM

753 ZINC

526 TIN

458 LEAD

700-1100 STAINLESS STEEL

250-500 GLASS

50 KAPTON (POLYIMIDE)

47 PHENOLLIC

46 NYLON

45 ALKYD ENAMEL

43 POLYESTER

43 EPOXY

43 POLYURETHANE

42 ABS

42 POLYCARBONATE

39 PVC

38 NORYL

38 ACRYLIC

LOW SURFACE ENERGY MATERIALS Dynes/cm Material

37 PVA

36 POLYSTYRENE

36 ACETAL

33 EVA

31 POLYETHYLENE

29 POLYPROPYLENE

28 TEDLAR

18 TEFLON

• Wets out easily on high surface energy substrates

• Wets out poorly on low surface energy substrates

• Wets out easily on high surface energy substrates

• Wets out poorly on low surface energy substrates

RheologyRheology

The study of the flow and deformation of matter.

ViscoelasticityViscoelasticity

A solid will not continuously change its shape when subjected to a given stress (elastic).

A liquid will continuously change its shape (ie, flow) when subjected to a given stress, irrespective of how small that stress is (viscous).

A viscoelastic material has mechanical properties that have both elastic and viscous components, properties that show a marked time and temperature dependence.

Log Modulus versus TemperatureLog Modulus versus TemperatureFrom: Polymer Structure, Properties, and Applications, Deanin, Cahners Books, © 1972,

p. 89.From: Polymer Structure, Properties, and Applications, Deanin, Cahners Books, © 1972,

p. 89.

Peel Adhesion Peel Adhesion Wetting

Surface Tension (Intermolecular forces and Polarity)

Rate of Wetting Surface roughness and viscosity of adhesive

Bonding Chemistry across interfaces to form interphase Displacement of weak boundary layer Viscoelasticity of Adhesive

Viscoelasticity Molecular Weight and MWD Glass Transition Temperature Degree of Cross-linking and Cross-link Density Polymer architecture

De-Bonding

Viscoelasticity of Adhesive

Wetting Surface Tension (Intermolecular forces and Polarity)

Rate of Wetting Surface roughness and viscosity of adhesive

Bonding Chemistry across interfaces to form interphase Displacement of weak boundary layer Viscoelasticity of Adhesive

Viscoelasticity Molecular Weight and MWD Glass Transition Temperature Degree of Cross-linking and Cross-link Density Polymer architecture

De-Bonding

Viscoelasticity of Adhesive

Other Issues To ConsiderFor Adhesive Bonding

Other Issues To ConsiderFor Adhesive Bonding

• Surface Contour The contour of the substrate will influence product

performance.

Substrate or Surface Contour

CORNERSCORNERSCONCAVE ANGLESCONCAVE ANGLES

CONVEX CURVESCONVEX CURVES

ADHESIVEADHESIVESTRESS ON ADHESIVESTRESS ON ADHESIVESUBSTRATESUBSTRATE

IDEAL SURFACE - FLATIDEAL SURFACE - FLAT1

2

3

4

• Substrate ContourRegardless of the strength of the adhesive formulation, it is

very difficult to overcome a continuous stress placed on it by a rigid backing trying to return to its original form.

Choose a conformable product

Consider adding stress relief to the converted part via scoring or perforation

Substrate Contour

IMPLICATIONS/RECOMMENDATIONS

The presence of surface contaminant can prevent contact of the adhesive to the substrate and results in weak bonding Plastisizers (PVC) Dirt/Dust Oils Chemicals Water Chemicals

Surface Contamination

EXAMPLE: DIRT PARTICLES

ADHESIVE

The areas highlighted in red are areas where there is no adhesive contact

SUBSTRATE

Most contamination is not visible to the eye but can be identified analytically

It may not be possible to obtain an acceptable bond without cleaning a contaminated surface

Surface contamination may be present if one can detect loose material on the surface of the substrate or if the material feels slippery, greasy etc.

Contamination may also be suspect if testing indicates poor bond strength and the adhesive feels “dead” after removal from the substrate

IMPLICATIONS/RECOMMENDATIONS

Surface Contamination

IMPLICATIONS/RECOMMENDATIONS - CONTINUED

Surface Contamination

The amount of surface preparation depends on the required bond strength, desired environmental aging resistance and economics. For maximum strength oxide films, oils, dust, release agents and all other surface contaminants must be completely removed. There are four principal ways for preparing surfaces:

Solvent degreasing; Abrasion, including emery paper, sand, shot or grit

blasting; Chemical etching and anodizing; Use of surface primers. Wiping.

Surface Contamination

Fortunately, most applications do not require preparation at this level, and usually an acceptable compromise can be found at some point in the sequence:

No treatment at all; Solvent wash; Solvent wash, abrade, solvent wash; Solvent wash, abrade, solvent wash,

chemically etch Any of the above plus an appropriate primer.

IMPLICATIONS/RECOMMENDATIONS - CONTINUED

• Surface Roughness It’s a common misconception that more adhesive means

better adhesion. On smooth surfaces, good surface contact can be achieved with relatively thin layers of adhesive, typically 2 mils. On rough or textured surfaces, more adhesive is required to fill in the hills and valleys, typically 5 mils.

Surface Roughness

ADHESIVEADHESIVE ADHESIVEADHESIVE ADHESIVEADHESIVE

Smooth Slight Texture Heavy Texture

Substrate texture has an impact on the strength of the adhesiveadhesive bond

Textured substrates do not allow for complete contact (wet-out) of the adhesive with the substrate

The less contact, the smaller the bonding area and lower the adhesion

IMPLICATIONS/RECOMMENDATIONS

Surface Roughness

Most tapes are good heat insulators and take several hours to reach room temperatures

A tape that is too warm can become soft & gummy

A tape that is too cold can become hard and lose tack and adhesion

Control the temperature of the tape and/or substrate

If temperature is difficult to regulate, consider the use of the following adhesives:

20 oF to 150 oF - RUBBER

0 oF to 400 oF - ACRYLIC

-20 oF to 500 oF - SILICONE

IMPLICATIONS / RECOMMENDATIONS

Substrate or Tape Temperature

PSA Performance Depends Upon

•Surface Energy•Rheology•Application Environment•Speed of Application•Surface Contour and Roughness•Surface Cleanliness•Chemicals and Moisture •Time for Bond Formation•Durability •Tape Chemistry and Construction

Understanding the functional performance attributes of the application is critical to the selection and design of PSA tape