Improve the corrosion resistance formula of epoxy heavy anticorrosion coatings

1, Introduction
can be used in more severe corrosion environment, can protect the substrate (mainly steel) from all kinds of corrosion (electrochemical corrosion, chemical media corrosion, biological deface, atmospheric aging, mechanical wear, etc.), in order to obtain relatively long-term use of coatings called heavy corrosion coatings. In other words, heavy anti-corrosion coatings are protective coatings in harsh environments of use, and they can achieve relatively long-lasting protection purposes. In the chemical atmosphere and marine environment, heavy anti-corrosion coatings can generally be used for more than 10 or 15 years, and under the corrosive conditions of acid, alkali, salt and solvent media at a certain temperature, they can be used for more than 5 years
China
. At present, heavy anti-corrosion coatings mainly have the following varieties: epoxy anti-corrosion coatings, polyurethane coatings, chlorine-containing anti-corrosion coatings, organic fluorine coatings. Among them, epoxy coatings, due to their unique properties, are used in almost all areas of corrosion protection. At present, how to design the formula to improve the corrosion resistance of epoxy heavy anti-corrosion coatings is the focus of research.2, the corrosion path of epoxy heavy anti-corrosion coatings
The most common means to prevent corrosion of steel structures is to apply heavy anti-corrosion coatings to their surfaces, resulting in corrosion-resistant coatings that cut off their electrical connection to corrosive media. The anti-corrosion function of the coating can be divided into two kinds: one is to prevent the corrosive substance outside the coating from invading the substrate, and the other is to rely on anti-corrosion pigments and fillers to inhibit corrosion. These two functions complement each other and act together to prevent corrosion.
But from a molecular structure, even the densest corrosion-resistant coatings have a large number of micro-defects, which result in permeability that causes the corrosion environment of the steel structure surface at various points of the region with differences in the lead, resulting in electrochemical corrosion. The iron in the anode region with a positive inductive position loses electrons to become iron ions, and the negative ions in the medium combine into active rust, and loose rust not only does not prevent corrosion, but also depolarizes the electrode reaction of the corrosion cell under certain inductive conditions and accelerates the corrosion rate.
It can be seen that the corrosion of metal under organic coating is mainly caused by electrochemical corrosion of phase interface, due to the permeation and diffusion of electrolytes on the coating, resulting in the deterioration of coating performance, coating loss of protective effect, so that water molecules, oxygen and other corrosive ions reach the coating-metal interface, causing corrosion of the substitial metal.
three elements of corrosion are water, oxygen and ions. The coating first absorbs water, only then the ions diffuse, and the cation is penetrated mainly through the channel of water in the paint film.
ions through the paint film is much slower than water and oxygen, epoxy heavy anti-corrosion coating film contains hydroxyl dissosis so that it is negatively charged, and therefore will selectively absorb cation into the paint film, the study confirmed that the general paint film will absorb a large number of cations (such as Na) into the paint film, and anions (such as Cl-) is not easy to penetrate. After the ions penetrate the paint film, the paint film foams and falls off, causing corrosion.3, improve epoxy heavy anti-corrosion coatings, corrosion-resistant formulation design
3.1 improve the cross-link density of epoxy resin
3.1.1
add reactive resin
add phenolic epoxy resin to epoxy resin, which increases the number of epoxy groups involved in the reaction in the unit mass resin, thereby improving the cross-link density of the coating system, showing better chemical resistance. You can also add styrene, methyl acrylates, methyl acrylates, acrylics and other monomers in epoxy coatings, but also to add free-based polymerization agents, so that in the anti-corrosion coating coating, there will be a reaction between the carbide and epoxy base, forming a mutually permeable polymer network (IPN) structure, causing epoxy resin cross-linking into a membrane. After the crosslinking density of the coating system is increased, the transmission rate of water, oxygen and other ions is greatly reduced, which can effectively reduce the corrosion of the substrate.
3.1.2
Avoid the use of esters or ketone solvents
solvent-based coatings solvent role: (1) the formation of a certain degree of viscosity solution to facilitate the construction of the coating; In solvent-based epoxy heavy antiseptic coating, the general use of curing agent is polyamide, if the solvent is selected as an ester or ketone solvent, due to ester or ketone solvent and amine curing agent reaction, reduce the curing effect of amine, affecting the cross-link density of epoxy resin.
3.1.3
the epoxy resin of multi-official group has been tested
to show that the epoxy resin of multi-official group can produce higher cross-linking. For example, when the three-function epoxy resin of the 5mol molecule reacts with the polyamine of the 5mol molecule, it can produce an effective high cross-link density network, and when the two-function epoxy resin of the same number of molecules reacts with the multi-amine of the same number of molecules, a low-effective cross-link density network is produced. See Figure 1.
3.2 Improves the wet adhesion of epoxy heavy anticorrosion coatings and substrates
Whether due to water absorption expansion, hydrogen analysis, osmosis pressure, electrical penetration, phase separation or other reasons, when the internal stress of the paint film exceeds adhesion and foaming away from the metal surface, it loses its protective effect and begins to corrode. In addition to shielding water, oxygen and ions, a good corrosion-resistant paint film must have and maintain a high degree of adhesion. Epoxy resin has many hydroxyl and ether bonds, can attract with the substrate, so the substrate has a good bonding force, and some polar substrate has a good adhesion. But when the paint film is immersed in water, the water molecules reach the metal interface through the paint film. Adsorption between the original metal surfactant point and the polar group in the paint film is reduced due to the intervention and replacement of water molecules. In fact, the anti-corrosion effect of the paint film is in the wet state to play a role, so high dry adhesion can not guarantee corrosion resistance of the paint film, paint film must have a high wet adhesion. In order to obtain high wet adhesion, the metal surface is generally sprayed with hair or phosphate treatment, or improve the film temperature of epoxy anti-corrosion coating, at a higher temperature, the temperature of the lacquer film environment exceeds the glass temperature of the high polymer, can make the active hydrogen fully react cross-link curing, and make the solvent fully escape the paint film before the reaction. In addition, high temperature can make the polymer chain easy to rotate, eliminate stress in the paint film, and make the lively polar group of the paint film adsorbed to the active point of the metal and improve adhesion. Silicone conjoiners, aluminum zirconium additives, and titanium and niobium conjoined agents can also be added to epoxy-heavy antiseptic coatings. The use of silicone alkane coupled agent is the role of the chemical bonds on the surface of the silane coupled agent and substrate to improve the adhesion of the coating, but also improve the corrosion resistance of the coating. Or add corrosion inhibitors to epoxy heavy antiseptic coatings, wherein corrosion-resistant substances can effectively prevent the occurrence and development of metal corrosion under the coating, so that the intermediate process of decreased adhesion for a considerable period of time, but the dry adhesion of the coating added corrosion inhibitors may not be as good as the dry adhesion of the coating without corrosion inhibitors.4. Conclusion
Epoxy heavy anti-corrosion coating in the formulation design to improve corrosion resistance, can be added to the reactive resin, avoid the use of esters or ketone solvents, the use of multi-official group epoxy resin to improve the cross-linking of epoxy resin, can also be increased by increasing the film temperature of epoxy corrosion-resistant coatings, add silicone conjoined agents or corrosion inhibitors to improve the wet adhesion of epoxy corrosion-resistant coatings.