Operating Mechanism of Coupling Agent in Coatings

The Difference between Coupling Agent and Surfactant

In the process of coating manufacturing, it is necessary to disperse hydrophilic polar substances such as pigments and fillers into hydrophobic non-polar organic substrates. In order to increase the affinity between inorganic substances and organic polymers, coupling agents or other surfactants are generally used to treat the surface of inorganic substances, making them hydrophobic instead of hydrophilic, thus promoting the interface binding between inorganic and organic substances.

Coupling agents and surfactants have some similarities in molecular structure and application performance, but there are also differences. Both of them are composed of hydrophilic and hydrophobic groups. Surfactants adsorb on the surface of inorganic pigments and fillers by orienting their hydrophilic groups to form a monolayer, which is a physical adsorption phenomenon, thus improving the dispersion and wetting of pigments and fillers in the substrate. Therefore, it is only physical adsorption, so the surfactant has a migration phenomenon that affects the gloss, appearance, and adhesion. Coupling agent form a chemically bonded coupling with the surface of inorganic pigments and fillers and silane crosslinking with the polymer substrate, bridging the two different materials and improving the binding strength, dispersion degree of pigments, fillers in the substrate, and reducing the interfacial free energy significantly better than surfactants.

Coupling Mechanism of Coupling Agent

Regarding the mechanism of coupling agent, it is generally believed that only one isopropoxy group in the hydrolyzed group of single alkoxyl titanium coupling agent can couple with inorganic materials. Therefore, a monolayer can form on the surface of the inorganic pigment and filler compared with other coupling agents, titanium ester coupling agents can more tightly connect inorganic pigments and fillers with organic polymer materials, and fully exert the effect of each molecule of titanium ester. Therefore, the amount needed is small, and the effect is significant. Because the titanium ester coupling agent replaces the adsorbed trace water and gas on the surface of inorganic pigments and fillers with a monolayer state, and increases the compatibility with the organic polymer substrate through a long-chain hydrophobic group in the molecule, it reduces the interfacial free energy, which is conducive to the wetting and dispersion of the powder aggregates by the organic polymer substrate.

Application Functions of Coupling Agent

Due to the characteristics of the six different functional areas in the molecular structure of titanium ester coupling agents, titanium ester coupling agents with different groups can be designed according to the needs of the coating industry to make them specific or multi-functional coupling agents, and give coatings and inks the following functions.

• Good dispersing and wetting function, which can significantly improve the dispersion of most inorganic pigments and fillers in organic materials, especially effective for dispersing carbon black, phthalocyanine blue, iron red, middle chrome yellow, etc.

• Good anti-settling performance to improve storage stability.

• Aid grinding action, which can shorten the number and time of grinding paths. The same grinding time can make particles ground finer.

• Silane coupling agents can increase the adhesion of the paint film to the substrate, improve the bond strength of the paint film to various metals, glass, and inorganic materials, improve abrasion resistance, increase impact strength, and improve flexibility.