In coatings, coupling agents play an important role in changing the compatibility between resins and fillers and improving the adhesion between coatings and substrates. The mechanism of silane coupling agents in coatings can be attributed to two aspects: ① the role of coupling agents between coatings and substrates; ② The interaction between coupling agents, pigments, and fillers.
The chemical functional groups contained in the coupling agent react with the functional groups on the surface of the substrate to form chemical bonds. It is generally believed that silanol reacts with the oxide or hydrate layer on the metal surface to form chemical bonds. There are two macroscopic interfaces in the coating (more than eight layers of substrate, coating/air). Due to different surface tensions, the coupling agent migrates to the interface. For example, when organic coatings contain a small amount of silane coupling agent, after coating, silane will migrate to the interface between the coating and the substrate, react with the moisture on the inorganic surface, hydrolyze to form silanol groups, and then form hydrogen bonds or condense into a SIO-M covalent bond (M is the inorganic surface) with the substrate surface.
Another theory suggests that the wetting process of coupling agents on the surface of substrates is a complex physical and chemical process at the liquid-solid interface. Firstly, the coupling agent has low viscosity and surface tension, high wetting ability, and a small contact angle with the metal surface. It can quickly spread out on the surface, wetting the inorganic material surface with the coupling agent; Secondly, once the coupling agent spreads on its surface, the material surface is wetted, and the two functional groups on the coupling agent molecules diffuse towards surfaces with similar polarity. Due to the thin water layer adsorbed on the surface of the material in the atmosphere, the alkoxy group at one end hydrolyzes into silicon 9-group J oriented on the inorganic material surface, and undergoes hydrolysis condensation reaction with the 9-group on the material surface; Organic functional groups are oriented towards the surface of organic materials, and during crosslinking and solidification, they undergo chemical reactions to complete the coupling process between different materials.
In the process of coating production, it is necessary to disperse inorganic fillers and fillers into organic resins. As the former mostly belongs to hydrophilic polar substances (with water molecule layers adsorbed on the surfaces of both fillers and fillers), and the latter mostly belongs to hydrophobic non-polar substances, they lack affinity between each other, resulting in flocculation of fillers and fillers. In order to increase the affinity between inorganic substances and organic polymers, coupling agents or other surfactants are usually used to treat the surface of inorganic substances, making them change from hydrophilic to hydrophobic, thereby promoting the interfacial bonding between inorganic and organic substances.
Coupling agents can not only improve the adhesion of coatings, enhance the compatibility of pigments and fillers with resins, but also endow coatings with special functions, and have become an important component of coatings. However, the research on their coupling mechanism is still in the development stage, and there are many imperfect aspects in the existing theory. The study of their mechanism of action will still be the focus of future research. At present, most of the coupling agents used are silicon or Qin containing, with a relatively single type, and coupling agents suitable for water solvent systems are not yet common. With the improvement of environmental protection requirements and the development of water-based coatings, coupling agents that are soluble in water and suitable for water-based coating systems will become the focus of development.
