Adhesion to Elastomers I: Viscoelasticity and

Surfaces

Larry R. Evans

Presented at the 179th Meeting of the Rubber Division, American Chemical SocietyApril, 19, 2011

Akron, Ohio

Testing for Adhesion

Testing for Adhesion should be simple – stick things together and see how hard it is to pull back apart – But …

There are 40 ASTM test methods for determining adhesion with an equal number of tests in ISO, as well as performance tests such as SAE tests for automotive components, etc. And there may be 3 or 4 variations in

each method.

Why so many tests? Adhesion is usually thought of as the

strength of an adhesive joint. This may involve: The material properties of adherend(s) The material properties of an adhesive

material The properties of the actual interfacial

bond The adhesive and possibly the

adherends are viscoelastic materials. Part of the energy is retained as kinetic energy, and part is converted into heat energy

The type of deformation experienced in service varies

Deformation of Adhesive Layer

Tensile Loading

Shear Loading

Cleavage Loading

Potential Failure Sites

Failure may occur cohesively: In either adherend (which may be different

materials) In the adhesive

Failure may occur adhesively between materials

Many polymeric joints develop an interphase during adhesive joining and cure May be result of blending of material

components May have completely different properties from

adherend/adhesive

Adherend 1Adherend 2

Adhesive

Interphase

Viscoelastic Behavior Viscoelastic behavior is a result of

molecular rearrangements during the loading and unloading cycle

Therefore it changes with temperature and with the rate of the loading strain

As the temperature is reduced, the molecules are not able to rearrange – eventually it reaches the glass transition temperature (Tg). The Williams, Landel, Ferry equation

describes the relationship between rate of strain and temperature.

WLF Equation For non-crystallizing systems above their glass

transition temperature, Tg, the measured peel force is also increased as the speed of testing is increased or as the testing temperature is decreased, often with a change in the locus of failure. These changes follow the Williams, Landel and Ferry (WLF) equation.

log aTg = 17.4 (T – Tg)

51.6 + (T – Tg)

Where log aTg is the function of the ratio of test rates at temperature T and at Tg in Kelvins. This also represents the relative rates of Brownian motion of individual molecular segments at temperatures T and Tg. Using this equation, we can correlate a series of test temperatures and test rates onto a single continuous master curve. For compounds which have a high degree of strain-induced crystallization, the effects of temperature and testing rate may have significant deviation from the WLF equation

Surface Forces

In the simplest model: an adhesive bond is created when there is sufficient energy to keep the joined surfaces in contact

Once the bond is created, separating the surfaces creates two new surfacesIn this way, a drop of

water will create an extremely strong bond between two plates of

glass

Fundamental Chemical Forces

Electrostatic forces

van der Waal’s forces Dipole-dipole Dipole-Induced

dipole Dispersion forces

Electron pair sharing

Repulsive forces

These fundamental forces operate between all atoms The total potential

energy is a function of force over a distance

Force ≈ - 6A + 12B r7

r13

Where:

A = Scalar of Attractive Forces

B = Scalar of Repulsive Forces

r = Intermolecular Distance

Electrostatic Forces

Forces between positively / negatively charged particles the potential energy, Φ is

Φ = q1q2

4πεr2

Electrostatic forces are on the order of 400 kJ/mole

Where:

q1 and q2 are the charges on the particles

ε is the dielectric constant of the medium

r is the intermolecular distance

Dipole-Dipole Interactions

Many molecules do not share the electrons equally between the nuclei Water is the most common example:

HO

Hδ+ δ-

The electronegative Oxygen tends to pull electrons closer to its nucleus leaving a partial positive charge on the Hydrogen end of the molecule

The partial charges result in significant molecular interaction

Dipole-Dipole interactions can range from 5 to 100 kJ/mole

Dipole – Induced Dipole Interactions

When a dipole comes into close contact with a symmetrical molecule the charge can distort the electron cloud producing a transient force

Dipole – Induced

Dipole forces are about 1

kJ/mole

Dispersion Forces The electrons of all molecules are in

constant motion. Symmetrical molecules will have more electrons on one side of the nucleus at times. Molecules in close contact will

influence neighboring molecules to create a weak interaction

Forces are only 0.01 to 0.1 kJ/mole, however they exist between all molecules Also called London dispersion forces or

induced dipole – induced dipole interactions

ForcesDipole – Dipole

Φ = 2μ12μ2

2

3kTr6

Dipole – Induced dipoleΦ = μ1

2α2

r6

Induced dipole – Induced dipole

Φ = 3 α12α2

2 2I1I2

4r6 I1 + I2

.

Where:

μ = Dipole moment

k = Boltzmann’s constant

kT = Thermal energy

α = polarizability

r = Intermolecular distance

I = Molecular constant

Surface Energy Surface energy is a result of the

unbalanced forces for molecules at the surface compared to molecules in the bulk

γ = πn2A Where:

32r02 n = Molecular Density

A = Attractive Forcer0 = Intermolecular Distance

Surface Energy of Common Liquids

Liquid γ, mN/m2

Acetone 25.2

Dichloroethane

33.3

Benzene 28.9

Bromobenzene 36.5

Chlorobenzene 33.6

Iodobenzene 39.7

Ethylbenzene 29.2

Toluene 28.4

Nitrotoluene 41.4

Liquid γ, mN/m2

Methanol 22.7

Ethanol 22.1

isoPropanol 23.0

Hexane 18.4

Perfluorohexane

11.9

Epoxy Resin 43.0

Glycerol 63.0

Water 72.8

Mercury 425.4

Wetting of Surfaces

Surface mN/m2

Tetrafluoroethylene

18

Dimethylsiloxane

21

Polyethylene 31

Polystyrene 33

Polyvinyl chloride

39

Cured Epoxy Resin

43

PET 43

Nylon-6,6 46

Diene Rubbers 27 - 33

θ

When a drop is brought into contact with a smooth horizontal surface the wetting (tendency of the drop to spread) is measured at the solid/liquid/gas interface

Surface Considerations

Breaking an adhesive bond requires energy to create a new surface

If the energy of an adhesive interface is greater than the energy of cohesion, the new surface is created in the adhesive (or adherend) Force depends

on configuration All this theory

assumes perfectly flat and perfectly clean surfaces

Surface Contamination

Removal of surface contamination is a major part of preparation of materials for adhesive bonding High energy

methods such as flame, corona discharge, …

Chemical cleaning with solvent, acid

Surface activation

Mechanical cleaning

Mechanical Interlocking

Real surfaces are not flat on a molecular scale Actual surface

area is increased

Instead of a plane of cleavage, a shear force will encounter an array of vectored forces

Alternatively each surface disparity is a flaw, inducing a stress concentration

Scale of Surface Disparities

Pore radius, m-6

* Distance penetrated by molten polyethylene, m-6

1000 220

10 22

1 7

0.1 2.2

0.01 0.7

The kinetics of pore penetration with respect to time are described by Poiseulle’s Law

r2P8η

Where:r = pore radiusP = capillary pressureη = viscosity

* Source: Packham, D .E. Adhesion Aspects of Polymeric Coatings, K. L. Mittal, (Ed), 1983, Plenum Press, NY