Adhesion to Elastomers I: Viscoelasticity and Surfaces Larry R. Evans Presented at the 179 th Meeting of the Rubber Division, American Chemical Society April, 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 … 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. 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. 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: 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 adherend(s) The material properties of an adhesive material The material properties of an adhesive material The properties of the actual interfacial bond 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 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 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: Failure may occur cohesively: In either adherend (which may be different materials) In either adherend (which may be different materials) In the adhesive In the adhesive Failure may occur adhesively between materials Failure may occur adhesively between materials Many polymeric joints develop an interphase during adhesive joining and cure Many polymeric joints develop an interphase during adhesive joining and cure May be result of blending of material components May be result of blending of material components May have completely different properties from adherend/adhesive May have completely different properties from adherend/adhesive Adherend 1 Adherend 2 Adhesive Interphase

Viscoelastic Behavior Viscoelastic behavior is a result of molecular rearrangements during the loading and unloading cycle 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 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). 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. 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. 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 a Tg = 17.4 (T – T g ) (T – T g ) (T – T g ) Where log a Tg is the function of the ratio of test rates at temperature T and at T g in Kelvins. This also represents the relative rates of Brownian motion of individual molecular segments at temperatures T and T g. 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 Where log a Tg is the function of the ratio of test rates at temperature T and at T g in Kelvins. This also represents the relative rates of Brownian motion of individual molecular segments at temperatures T and T g. 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 surfaces In this way, a drop of water will create an extremely strong bond between two plates of glass

Fundamental Chemical Forces Electrostatic forces Electrostatic forces van der Waal’s forces van der Waal’s forces Dipole-dipole Dipole-dipole Dipole-Induced dipole Dipole-Induced dipole Dispersion forces Dispersion forces Electron pair sharing Electron pair sharing Repulsive forces Repulsive forces These fundamental forces operate between all atoms The total potential energy is a function of force over a distance Force ≈ - 6A + 12B r 7 r 13 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 = q 1 q 2 4πεr 2 Electrostatic forces are on the order of 400 kJ/mole Where: q 1 and q 2 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 Many molecules do not share the electrons equally between the nuclei Water is the most common example: Water is the most common example:HO H δ+δ+ δ-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. 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 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 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 Also called London dispersion forces or induced dipole – induced dipole interactions

Forces Dipole – Dipole Φ = 2μ 1 2 μ 2 2 3kTr 6 Dipole – Induced dipole Φ = μ 1 2 α 2 r 6 Induced dipole – Induced dipole Φ = 3 α 1 2 α 2 2 2I 1 I 2 4r 6 I 1 + I 2. 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 = πn 2 A Where: 32r 0 2 n = Molecular Density A = Attractive Force r0 = Intermolecular Distance

Surface Energy of Common Liquids Liquidγ, mN/m 2 Acetone25.2 Dichloroethane33.3 Benzene28.9 Bromobenzene36.5 Chlorobenzene33.6 Iodobenzene39.7 Ethylbenzene29.2 Toluene28.4 Nitrotoluene41.4 Liquidγ, mN/m 2 Methanol22.7 Ethanol22.1 isoPropanol23.0 Hexane18.4 Perfluorohexane11.9 Epoxy Resin43.0 Glycerol63.0 Water72.8 Mercury425.4

Wetting of Surfaces SurfacemN/m 2 Tetrafluoroethylene18 Dimethylsiloxane21 Polyethylene31 Polystyrene33 Polyvinyl chloride39 Cured Epoxy Resin43 PET43 Nylon-6,646 Diene Rubbers 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 The kinetics of pore penetration with respect to time are described by Poiseulle’s Law r2P 8η Where: r = pore radius P = capillary pressure η = viscosity * Source: Packham, D.E. Adhesion Aspects of Polymeric Coatings, K. L. Mittal, (Ed), 1983, Plenum Press, NY