Some recent progress in adhesion technology and science

Gluing is a complex process which associates the connection of an adhesive to a substrate, the chemistry of curing and the mechanical properties of the glassy polymer which is formed. We review some selected topics which illustrate progresses made in the bonding of plastics or metals and their understanding. Physical chemistry has explored the relation between the surface energy of the components and the energy of adhesion. However, real surfaces have a low superficial energy. Adhesives do not wet the surface and the spreading of adhesives has to be forced by pressure. Fracture mechanics has provided the frame which allows for measuring the fracture energy but the dissipation of energy by modem structural adhesives is such that it restricts the number of methods that may be used. Polymer/polymer bonding has been shown to occur through the interdiffusion of chains which also happens when plastics are glued. The process is obscured because it is due to unknown specific interactions. The effect of surface treatments is not always understood as for instance the exposure of plastics to a plasma. Successful surface treatment of metals comes from long trial and error experiments. The use of connectors so called 'adhesion promoters' has proven useful for glass and aluminium. In order to increase the dissipation of energy all modern adhesive are modified with a second phase that separates from the adhesive network in order to increase their toughness. Unexpectedly, a new class of hardeners, alkyl borannes, provide adhesives that are able to bond low energy surfaces without treatment. Finally, some recent results and explanations of the behaviour of adhesive in moist conditions are presented.