Unified theory and guidelines on adhesion

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Abstract
A new approach was proposed to consolidate the many adhesion theories into one coherent concept. The maximum attractive force between two sets of molecules (adhesion strength) is derived from the Lennard–Jones potential function and calculated with measured bond length and bond energy. It leads to two criteria for strong adhesion: intimate molecular contact of closer than 9 Å (necessary condition), and maximum attractive force with minimum potential energy (sufficient condition). The criteria conform to the key elements of most prior adhesion theories. Seven prior adhesion theories and their relevance to these two criteria were briefly reviewed/discussed. In order to draw up a set of guidelines on adhesion and supplement the missing pieces of information, 21 model polymers of varied functionality were synthesized to study (1) the effect of polar groups on adhesion and (2) the effect of polymer conformations on adhesion. The results indicate that polar groups are more effective in polymer backbone than in side chains for promoting adhesion. The presence of both hydrogen donors and acceptors in the same backbone maximizes adhesion. True (active) solvents enhance adhesion, cosolvents (latent solvents) boost adhesion by inducing favorable conformation of polymers in solution, but thinners (diluents) reduce adhesion. The set of guidelines covered the effects of functional groups, solvent blends, pigment loadings, adhesion promotion, and adhesion loss.

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