Research and Development of Four Mainstream Water-based Anti-corrosion Coatings!

Traditional industrial coatings contain a large amount of organic solvents. During the application and drying process, most of these solvents volatilize, which not only seriously pollutes the environment and endangers human health, but also causes waste of resources. According to the data, the total amount of VOCs emitted during the coating process of steel products and their facilities accounts for about 20% to 25% of the global total VOCs, and has become one of the main sources of air pollution. Therefore, the application of water-based coatings instead of solvent-based coatings in industrial anti-corrosion coating has very important practical significance for China to build a harmonious society and achieve sustainable economic development.

Water-based coatings are coatings that use water as the solvent or dispersion medium. It features non-toxicity, odorlessness and abundant resources, which can significantly reduce the pollution to the atmosphere caused by the evaporation of organic solvents in coatings. Moreover, using water as the solvent can enhance the safety of coatings during storage, transportation and application. At present, water-based anti-corrosion coatings have been successfully applied in architectural decoration, automobile manufacturing, ship and container manufacturing, railway locomotives, aerospace, transportation Bridges and wood coating, etc. Water-based anti-corrosion coatings mainly include four categories: water-based inorganic zinc-rich coatings, water-based epoxy coatings, water-based acrylic coatings and water-based polyurethane coatings.

Water-based inorganic zinc-rich coating

Water-based inorganic zinc-rich coatings are generally classified into two types: self-curing inorganic zinc-rich and post-curing inorganic zinc-rich. At present, water-based self-curing inorganic zinc-rich coatings are widely used in the market. Water-based inorganic zinc-rich coatings mainly use inorganic silicate water solution as the base material and zinc powder as the anti-corrosion pigment. To improve the film-forming property and corrosion resistance of water-based inorganic zinc-rich coatings during construction, it is necessary to appropriately add corresponding additives, such as leveling agents and corrosion inhibitors.

Water-based inorganic zinc-rich anti-corrosion coatings mainly have the following characteristics:

(1) It dries quickly. At 20℃ and 75% relative humidity, it reaches touch dry in about 1 hour.

(2) The coating has good electrostatic conductivity and excellent welding performance.

(3) Excellent corrosion resistance, long-term resistance to chemical corrosion within the pH range of 5.5 to 10.5; Under Marine and atmospheric environmental conditions, the service life of water-based inorganic zinc-rich coatings can reach 25 years.

(4) It can withstand a temperature of 400℃ for a long time.

(5) Clean and environmentally friendly, it uses water as the diluent and cleaning agent, has low volatile organic compounds, and is friendly to the environment.

(6) Water-based inorganic zinc-rich coatings also have their shortcomings, mainly manifested in the fact that the dry film contains a large amount of zinc powder, resulting in the porosity of the coating. The processing requirements for the substrate are relatively high and the paint film is relatively brittle. Construction and curing are greatly affected by environmental temperature and humidity.

Water-based inorganic zinc-rich coatings are suitable for long-term anti-corrosion of steel structures in various environments such as Marine atmosphere and high temperature, including offshore platforms, ships, containers, large steel components, oil pipelines, and inner linings of various chemical storage tanks.

Waterborne epoxy coating

Waterborne epoxy coatings are mainly water-dispersible epoxy coatings, consisting of two components: hydrophobic epoxy resin and hydrophilic curing agent.

The film-forming process is divided into two stages: the first is the evaporation of water, reaching surface dryness; The second is the polymerization and cross-linking process of epoxy resin and curing agent. Waterborne epoxy coatings themselves contain moisture, which can, together with the moisture on the surface of damp substrates, form components of waterborne epoxy coatings.

The waterborne epoxy coating system has the following characteristics:

(1) It has excellent adhesion on the surface of substrates such as steel, concrete and aluminum. The coating film features high hardness, good wear resistance, corrosion resistance and outstanding chemical resistance.

(2) It has a low content of volatile organic compounds and will not cause air pollution. It uses water as the dispersion medium, which is low in price, odorless, non-flammable, and greatly improves safety during storage, transportation and use.

(3) Fast drying: At 20℃ and 75% relative humidity, it usually reaches touch dry in 2 hours, and the minimum curing temperature is 15℃.

(4) However, its chemical resistance is still slightly inferior to that of solvent-based epoxy coatings, and its service life is also shorter than that of general solvent-based epoxy coatings.

The commercialized waterborne epoxy anti-corrosion coatings mainly include: waterborne epoxy iron red primer, waterborne epoxy zinc phosphate primer, waterborne epoxy zinc-rich primer and waterborne epoxy mica intermediate paint.

Waterborne acrylic coating

Water-based acrylic coatings refer to those mainly formulated with acrylic copolymers as the base material, water as the dispersant, and the addition of pigments, fillers, and additives. Acrylic copolymers are synthesized from methyl methacrylate, butyl acrylate, and acrylic acid. They are classified into water-emulsion, water-sol, and water-soluble acrylic coatings based on the state of the resin in water. Because the relative molecular mass of the acrylic polymer water dispersion is larger than that of the solvent-based polymer, a harder paint film can be obtained after drying. This resin presents a long-chain three-dimensional network form. By choosing different monomers and monomer ratios, it can meet various application requirements.

The waterborne acrylic anti-corrosion coating system has the following characteristics:

Water-based acrylic topcoat has excellent weather resistance, UV resistance, water resistance and does not turn yellow. It can reach hard drying in 45 minutes and can be recoated in 2 hours. It can be used at a temperature as low as 5℃.

(2) Water-based acrylic topcoat can be used in combination with water-based epoxy or acrylic primer and intermediate coat, and can also be applied to solvent-based coatings to form a composite coating system.

(3) It has excellent anti-corrosion properties, good light and color retention, uses water as the diluent and cleaning agent, is non-flammable and safe.

(4) Waterborne acrylic resin coatings also have problems such as high hardness and poor solvent resistance.

Water-based acrylic anti-corrosion coatings can be made into glossy and semi-glossy types and can be used as primers, topcoats and sealing paints for concrete surfaces. It can be applied on the surface of steel, concrete and other substrates. The coating has good adhesion, weather resistance, low-temperature resistance, light and color retention and other characteristics. Waterborne acrylic resin coatings have a wide range of applications and can be used in various fields such as construction and automobiles, and have become a major system of anti-corrosion coatings.

Waterborne polyurethane coating

Waterborne polyurethane coating is formulated with waterborne polyurethane resin as the base material, water, pigments, additives, etc. Waterborne polyurethane resins can be classified into three types based on particle size and appearance: polyurethane aqueous solution (particle size <0.001μm, transparent appearance), polyurethane water dispersion (particle size: 0.001-0.1μm, semi-transparent appearance), and polyurethane emulsion (particle size >0.1μm, white and turbid appearance) water-soluble type.

Waterborne polyurethane can be classified into polyether type, polyester type and polyether and polyester mixed type according to the different synthetic monomers. Depending on the type of diisocyanate selected, waterborne polyurethane can be further classified into aromatic and aliphatic types, or specifically into TDI type, HDI type, MDI type, etc. The entire synthesis process of waterborne polyurethane can be divided into two stages. The first stage is pre-step-up polymerization, that is, the oligomer diol, chain extender, waterborne monomer and diisocyanate are step-up polymerized through solution to generate waterborne polyurethane prepolymer with a relative molecular mass of the order of 1000. The second stage is the dispersion of the prepolymer in water after neutralization.

Waterborne polyurethane anti-corrosion coatings have the following characteristics:

(1) The particle size is less than 0.1nm and it has good dispersion stability.

(2) Low production cost, safe and non-flammable, no environmental pollution, less likely to damage the coated surface, easy to operate and modify.

(3) It has good adhesion to wood, fiberboard, plastic film, metal, glass and leather, etc.

(4) Waterborne polyurethane coatings still have some disadvantages at present, such as long film-forming time, poor water resistance of the paint film caused by hydrophilic groups in the coating, easy formation of bubbles in the paint film, and relatively high raw material cost.

Waterborne polyurethane anti-corrosion coatings are mainly used as anti-corrosion topcoats and are widely applied in the coating of railway locomotives, Bridges, automobiles, wooden furniture, architectural decoration, etc. With the continuous improvement of environmental protection, human health and the requirements for coating quality, the comprehensive application of various modification methods and the development of new modification methods have become the main research directions of waterborne polyurethane coatings. In addition, how to improve the water resistance, antistatic and antibacterial properties of waterborne polyurethane coatings will also become a new research direction in the future.

The development of water-based coatings can not only alleviate the consumption of petroleum resources, but also significantly reduce the emission of VOCs. This has very important economic and social significance in today's increasingly deteriorating environment and increasingly tight petroleum resources.