News

In order to improve the dispersion of nano-ZnO (VK-J30) in coatings, it was modified with a titanate coupling agent.

The anticorrosion performance of an acrylic polyurethane varnish, acrylic polyurethane coatings containing unmodified nano-zinc oxide (VK-J30), and acrylic polyurethane coatings containing nano-zinc oxide modified with a titanate coupling agent was studied. The results show that, after modification with the titanate coupling agent, the agglomeration of nano-ZnO (VK-J30s) largely disappears and the modified zinc oxide shows good compatibility with the coating. The impermeability of the resulting composite coating is clearly stronger than that of the varnish and of the coating containing unmodified nano-ZnO. The modified nano-ZnO markedly improves the anticorrosion performance of the acrylic polyurethane coating.

Keywords: acrylic polyurethane coating; nano-zinc oxide; VK-J30; modification; anticorrosion; Xuancheng Jingrui New Materials

The anticorrosion function of a coating is an important measure of its performance, and improving the anticorrosion performance of coatings has always been a major research goal. The emergence of nanomaterials has brought new opportunities and challenges to the coatings industry. Applying nanomaterials to coatings is expected to improve and enhance the anticorrosion and other properties of conventional coatings, producing new functional coatings.

Among the many nano-oxides, nano-zinc oxide has attracted wide attention for its unique UV resistance. Nano-zinc oxide (VK-J30) is a white powder and a new high-functionality fine inorganic material. Because of its excellent oxidation and corrosion resistance, high melting point and good electromechanical coupling, UV-shielding capability and antibacterial/deodorising properties, nano-zinc oxide is widely used in many fields, including optoelectronic devices, chemicals, coatings and pharmaceuticals. However, adding nano-zinc oxide, as an inorganic substance, directly to organic materials — especially coatings — is quite difficult. The reasons are that nano-zinc oxide has a small particle size, a large specific surface area and high surface energy, and exists in a thermodynamically non-equilibrium state; under the effects of intermolecular forces, hydrogen bonding and static electricity it therefore agglomerates very easily, losing the special functions of nanoparticles. At the same time, the zinc oxide surface is hydrophilic and oleophobic, and strongly polar, so it is difficult to disperse uniformly in organic media; its bonding to the binder is weak, easily causing interfacial defects and impairing coating performance. Therefore, to prevent the agglomeration of nano-zinc oxide and give full play to its nano effect, a titanate coupling agent was used to modify the surface of the nano-zinc oxide; the modified nano-zinc oxide was added to the acrylic polyurethane coating in a certain amount, and its effect on the protective performance of the coating was studied. At present, research at home and abroad on the protective effect of nano-oxide-modified coatings is still at an exploratory stage. In particular, studies on the effect of nano-zinc oxide modified with a coupling agent on the ability of a coating to resist permeation by media are still at a simple qualitative stage, and the anticorrosion mechanism is not yet fully understood.

During the experiment the coating maintained a high impedance value and remained in the early stage of immersion throughout. The coating containing 0.5% unmodified nano-zinc oxide showed good anticorrosion in the early stage of immersion, but its impedance fell rapidly with time, and after 16 days of immersion its impedance was lower than that of the varnish at the same stage. This is mainly because nano-ZnO has high surface activity and small size but, because of its high surface energy, agglomerates easily; when added directly to the coating it readily loses its nano effect through agglomeration, and the agglomerated particles tend to create pores in the coating, actually accelerating corrosion. Nano-ZnO modified with the titanate can disperse uniformly in the matrix as nano-scale particles and shows good compatibility with the matrix, thereby exerting its unique nano functions. Moreover, the modified nano-ZnO changes from hydrophilic to lipophilic, allowing more matrix resin to be adsorbed onto its surface; this increases the compactness of the coating so that the coating interface effectively hinders the permeation of media, improving the anticorrosion performance of the coating.

Three coating samples were immersed in a 3.5% NaCl solution for 40 days, then removed, with the corrosion products cleaned from the surface and dried. As the surface photographs show, after the 40-day immersion test the coating without nano-zinc oxide had completely lost its protective function — the base metal had been severely corroded, fully exposed and rusted. The coating containing 0.5% unmodified nano-ZnO showed relatively severe pitting corrosion. In contrast, the coating containing 0.5% modified nano-ZnO remained intact, with a bright and smooth surface. The modified nano-ZnO greatly improves the anticorrosion performance of acrylic polyurethane coatings.

Conclusions

(1) After modification with the titanate coupling agent, nano-ZnO changes from hydrophilic to lipophilic; agglomeration largely disappears, uniform dispersion of the nano-scale particles is achieved, and the modified zinc oxide shows good compatibility with the coating.

(2) The modified nano-ZnO gives full play to the nano effect; the impermeability of its composite coating is clearly stronger than that of the varnish and of the composite coating with unmodified nano-ZnO. It acts as an effective barrier and markedly improves the anticorrosion performance of the acrylic polyurethane coating.