adhesion to elastomers i: viscoelasticity and surfaces larry r. evans presented at the 179 th...
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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