Adhesion to Elastomers I: Viscoelasticity and Surfaces

1. Adhesion to Elastomers I: Viscoelasticity and Surfaces Larry R. Evans Presented at the 179th Meeting of the Rubber Division, American Chemical Society April, 19, 2011 Akron, Ohio

2. 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.

3. 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

4. Deformation of Adhesive Layer Tensile Loading Shear Loading Cleavage Loading

5. Potential Failure Sites Adherend 1 Adhesive Interphase Adherend 2 • 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

6. Viscoelastic Behavior • Viscoelasticbehavior 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.

7. 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

8. Surface Forces In this way, a drop of water will create an extremely strong bond between two plates of glass 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

9. These fundamental forces operate between all atoms The total potential energy is a function of force over a distance Fundamental Chemical Forces Where: A = Scalar of Attractive Forces B = Scalar of Repulsive Forces r = Intermolecular Distance Force ≈ - 6A + 12B r7 r13 • Electrostatic forces • van der Waal’s forces • Dipole-dipole • Dipole-Induced dipole • Dispersion forces • Electron pair sharing • Repulsive forces

10. Forces between positively / negatively charged particles the potential energy, Φ is Φ = q1q2 4πεr2 Electrostatic forces are on the order of 400 kJ/mole Electrostatic Forces Where: q1 and q2 are the charges on the particles ε is the dielectric constant of the medium r is the intermolecular distance

11. Dipole-Dipole Interactions 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 δ- δ+ Dipole-Dipole interactions can range from 5 to 100 kJ/mole • Many molecules do not share the electrons equally between the nuclei • Water is the most common example: H O H

12. Dipole – Induced Dipole Interactions Dipole – Induced Dipole forces are about 1 kJ/mole When a dipole comes into close contact with a symmetrical molecule the charge can distort the electron cloud producing a transient force

13. 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

14. Dipole – Dipole Φ = 2μ12μ22 3kTr6 Dipole – Induced dipole Φ = μ12α2 r6 Induced dipole – Induced dipole Φ = 3 α12α22 2I1I2 4r6 I1 + I2 Forces Where: μ = Dipole moment k = Boltzmann’s constant kT = Thermal energy α = polarizability r = Intermolecular distance I = Molecular constant .

15. Surface energy is a result of the unbalanced forces for molecules at the surface compared to molecules in the bulk Surface Energy γ = πn2A Where: 32r02 n = Molecular Density A = Attractive Force r0 = Intermolecular Distance

16. Surface Energy of Common Liquids

17. Wetting of Surfaces 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 θ

18. 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

19. 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

20. 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

21. The kinetics of pore penetration with respect to time are described by Poiseulle’s Law r2P 8η Scale of Surface Disparities 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