By adding adhesion promoters to coatings, hydrogen bonds and hydroxyl or carboxyl groups react between the substrate and the coating film, which is the most significant way to improve adhesion.
Common adhesion promoters are:
(1) Silane coupling agent
(2) Phosphate compound
(3) Titanate or zirconate
(4) High molecular organic resin
(5) Chlorinated polyolefin
1. Silane coupling agent
The coupling agent has two functional groups, one that is organic-friendly and the other that is inorganic-friendly, at both ends of the molecule. It can connect two materials of different types with very different chemical structures and affinities at the interface, and increase the bonding between the resin and the inorganic substrate, pigment and filler.
The general formula of the silane coupling agent is R-(CH2)n-Si-X(4-n), where R is a non-hydrolyzable organic functional group that can react with high molecular polymers. Such as OH group, vinyl group, amino group, epoxy group, methacrylate group, etc. X is a hydrolyzable functional group, which can cause hydrolysis when encountering water adsorbed on the surface of inorganic substances, and has good reactivity with the surface of inorganic substances. Typical X groups include methoxy (OCH3), ethoxy (OC2H5), etc.
The most common example of adding silane coupling agents to coatings is: in the coating of glass (wine bottles, cosmetics, etc.), after spraying, the silane coupling agent encounters the moisture on the surface of inorganic glass and can be decomposed into silanols, which then produce hydrogen bonds or condense into Si-O-M covalent bonds with the OH groups on the glass surface; the organic functional group at the other end reacts with the resin.
2. Phosphate compounds
The structure of phosphate compounds contains various organic functional groups and phosphate groups that can react with metal inorganic substrates. Epoxy phosphate is the most important type of phosphate ester. It has a very good effect of increasing adhesion on metal substrates under baking conditions.
3. Titanate or zirconate
The structure of the titanium or zirconate ester molecule is shown in the figure below. The organic chain segment can react with the water on the metal surface through hydrolysis to form a chemical bond; at the same time, the other end can react with the functional group of the coating resin to play the role of an adhesion promoter.
Our company has professional silane coupling agents and titanate crosslinkers, which are a good choice for our customers.