The formation mechanism of the adhesive is very complicated. For porous materials, hot melt adhesive can penetrate into the pores, and after curing, it will be like a rivet to fix the bonded materials together. However, this situation is only part of the bonding state, and in some cases, the hot melt adhesive polymer will diffuse into the material being bonded. For example, under normal circumstances, there will be a drop of solvent between two plastic plates. After hours, the two plastic plates will be glued together. Another situation is the chemical reaction between the hot melt adhesive and the adherend, such as the hot melt adhesive with coupling agent. Of course, this situation is rare.
The physical force at the interface between the hot melt adhesive and the adherend is the more common source of bonding strength. Under normal circumstances, the attraction of positrons and electrons is one of the reasons for the formation of viscosity. The two materials on the interface do not necessarily have free charges. Many molecules have polarity, and the same attraction exists between dipoles. If two objects are very close, such as at a nanometer distance, a huge attraction will be generated, and this force is the dispersion force. Since the development of the bonding theory, the dispersion force has been generally accepted, and it is considered to be an important part of the bonding ability of the hot melt adhesive and the adherend.