Thread Content
Substances such as metals and glass have a high surface tension and are considered high-energy surfaces. The cured products of isocyanate adhesives and PU adhesives contain urethane bonds and urea bonds with high cohesive energy, which can aggregate at the bonding interface under certain conditions to form an adhesive layer with high surface tension. Generally speaking, the higher the percentage of isocyanate or its derivatives in the adhesive, the greater the surface tension of the adhesive layer; the adhesive becomes more resilient, and it can bond well with substrates such as metal, resulting in generally high adhesion strength. The adhesion mechanism of isocyanate adhesives containing one NCO group to metals is as follows: Hydrates of adsorbed water are generally present on metal surfaces (even on polished metal surfaces, there are trace amounts of adsorbed water or metal oxide hydrates). The urea bonds formed by the reaction between the –NCO group and water chelate with metal oxides via hydrogen bonds, thereby forming a ureido–metal oxide complex. Additionally, the –NCO group can also form covalent bonds with metal hydrates. In the absence of any NCO groups, hydrates on the metal surface as well as van der Waals forces and hydrogen bonds arise between metal atoms and urethane and urea bonds. Polyurethane adhesives based on TDI and MDI contain benzene rings and possess an electron-rich system, enabling them to form coordinate bonds with metals. The composition of metal surfaces is relatively complex, and the types of various chemical bonds or secondary bonds (such as hydrogen bonds) formed between them and PU adhesive are also quite complex. Inorganic materials such as glass, slate, and ceramics are generally composed of components like Al2O3, SiO2, CaO, and Na2O; their surfaces also contain adsorbed water and hydroxyl groups. The adhesion mechanism is roughly the same as that for metals.