Chemical adhesion occurs via molecular contact at the surface of the substrate. It is the strongest mechanism of adhesion, with bonds occurring between functional groups in the adhesive and atoms/molecules on the substrate surface. This mechanism contributes to the structural strength of many adhesives such as epoxies bonding aluminum.
Mechanical interlocking occurs as the adhesive flows into the pores on the substrate’s surface. The interlocking provides a physical impediment to crack propagation at the interface. It also serves to increase surface area, increasing total contact between adhesive and substrate. Liquid adhesives readily flow upon application to the substrate prior to drying or curing. Bonding tapes continue to flow slowly over time, building strength as they do.
Diffusion occurs when the adhesive polymer is capable of penetrating and entangling with a polymeric substrate. An interface is created with entangled polymer chains bonding adhesive and substrate. This mechanism is common in assemblies with difficult-to-bond low surface energy materials such as polypropylene.
Electrostatic adhesion occurs as the charged surface of the adhesive is drawn toward the oppositely charged substrate, and it is most common in tape applications. It’s the phenomenon often observed when trying to tape a box or wrap a present.