Abstract
Maximum stability of any system is achieved when its free energy is minimum, in accordance with the second law of thermodynamics. Considering the adhesive bond as a thermodynamic system, it is proposed that the minimum interfacial energy coincides with (1) the maximum strength, and (2) the maximum durability, understood as bond resistance to degradation under environmental attack. The thermodynamic properties of bond components which play a key role in promoting conditions for maximum strength of adhesion have been identified. The general pattern of the relationship: strength = function (interfacial energy and related parameters), has been developed based on experimental data covering a variety of adhesives and substrates such as metals (steel and aluminium), plastics, ceramics and glass fibre composites. The influence of adhesion promoters (eg, silanes) has also been considered. It is shown that conditions for maximum strength coincide with the minimum interfacial energy of the system, acquired when the ratio of the surface energy of the substrate, γ1, to that of the cured adhesive, γ2 (ie, a = γ1/γ2), has a specific value denoted aMIN. Systems with energy ratios a ⩾aMIN were found to have engineering utility, because the strength deficiency for a >aMIN was found to be significantly less than for a <aMIN.
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