Dec 23 2015

Adhesion

Published by at 9:51 pm under Quantum Mechanics

Go to an adhesion seminar, like I did years ago with a fellow engineer, and the speaker may or may not tell you that the main component of adhesion is due to the most fundamental of van der Waals forces, that being dipole-dipole electrostatic attraction, which force falls off at a rate proportional to one over distance to the fourth power. The seminar leader is likely to tell you nevertheless that the surfaces should be clean and dry.

Obviously adhesion can also, and usually does, have a mechanical component. This is especially true in the shear direction when a surface is course, or purposely roughed up first. Pressure is often applied when adhering surfaces to force the adhesive into crevices, for the mechanical component to grab better, and to bring molecules in closer contact for the electrostatic component. If the instructions for the adhesive say to hold the pressure for a certain amount of time at room temperature, that is to let the molecules creep into crevices and to allow the dipoles to move themselves into positions that increase the number of potential energy wells that relate to movement and positioning of the dipoles.

The electrostatic component is strongest in the first 3 or 4 molecular layers of relatively complex adhesive molecules, so this makes it easier to see why pressure helps. The ‘dry’ rule is mostly because water does not make a good adhesive. The ‘clean’ rule is a little more complex. Chemists have made adhesives good at inducing dipoles into relatively non-polar material, but it is best if the material being bonded to has consistent isotropic structure. When the dipoles are setting up the energy wells, it is more efficient when dipoles of a locale ‘see’ a uniform structure across a hemispherical view. Nature prefers mathematical order. Also, an adhesive may stick to a piece of debris, but the piece of debris will probably not stick to the substrate.

Some of the strongest adhesives are solidified by heat curing, when cross linking occurs. If the dipole positions end up more rigid, they can maintain strength under high strain.

For a good description of dipole-dipole bonding, see:

Tipler, Paul A. and Llewellyn, Ralph A., Modern Physics, Sixth Edition, W. H. Freeman and Company, New York, c. 2012, pgs. 387-388

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