Wide application of polyurethane coatings in metal corrosion protection

Polyurethane coatings are widely used in petrochemical equipment, marine, marine, electromechanical, leather, wood, construction, etc. due to their high wear resistance and toughness, excellent adhesion, chemical resistance, corrosion resistance and insulation properties. field. However, traditional solvent-based polyurethane coatings require the addition of many organic solvents during preparation and construction, posing a hazard to human health and the environment. With the restrictions on VOCs in various countries' environmental regulations and people's attention to environmental protection, waterborne polyurethane coatings have been rapidly developed and are constantly replacing solvent-based polyurethane coatings.

Waterborne polyurethane coating not only has the advantages of non-toxic, odorless, non-polluting, non-flammable, low cost, easy to damage the surface to be painted, easy to clean, etc. It also has the excellent properties of high hardness and wear resistance inherent in solvent-based polyurethane coatings. Therefore, it is widely used in various fields. The damage caused by corrosion to humans is staggering. The annual economic loss caused by corrosion is about $1 billion, accounting for about 10% of the total metal output in the year. In 2008, the economic losses caused by corrosion in China amounted to 1.2 trillion to 2 trillion yuan. Therefore, vigorous development of anti-corrosion technology has become an urgent need to reduce corrosion losses and promote resource conservation. Among the many anti-corrosion methods, coating protection has been chosen. In the past, anti-corrosion coatings were mainly solvent-based coatings. With the development of water-based coatings, many successful metal anti-corrosion coating systems have been applied. The waterborne polyurethane coating is a coating based on waterborne polyurethane and water as a dispersion medium. There are also one-component waterborne polyurethane coatings and two-component waterborne polyurethane coatings, as well as modified waterborne polyurethane coatings.

1. One-component waterborne polyurethane coating

One-component waterborne polyurethane coatings are the earliest waterborne polyurethane coatings. The first-component waterborne polyurethane coatings used were some of the thermoplastics with a certain number of hydrophilic groups as the film-forming material, but the coating generally does not undergo chemical reaction during drying and curing, and the solvent-based double Compared with the component polyurethane coating, its mechanical properties, chemical resistance, water resistance and solvent resistance are poor. In the continuous research and exploration, it has been found that the introduction of cross-linking agent or internal cross-linking method to improve the cross-linking degree of the coating film can improve the comprehensive performance of the one-component water-based polyurethane coating. For example, linseed oil and diethanolamine are subjected to amine hydrolysis to form a linoleyl diethanolamine intermediate, and the intermediate and multifunctional monomer dimethylolpropionic acid (DMPA) are used as a composite diol component and TDI/HDI. The diisocyanate composite component is subjected to a stepwise addition polymerization reaction to obtain a room temperature self-crosslinking aqueous polyurethane resin. The coating plays a decisive role in the film formation process, which is the oxidative crosslinking of the double bond, so that the coating film forms a crosslinked network structure at normal temperature, thereby greatly improving the mechanical properties and chemical resistance of the coating film. The presence of a carboxyl group in the resin provides a basis for the water solubility of the resin. By introducing a hydrophilic group into the polymer chain, a polyurethane prepolymer is first prepared, and the prepolymer is neutralized, and simultaneously extended and dispersed in water to obtain a one-component self-crosslinking aqueous polyurethane dispersion. . The one-component self-crosslinking aqueous polyurethane dispersion belongs to an oxidatively crosslinked polyurethane aqueous dispersion, and introduces an unsaturated bond-containing vegetable oil or a fatty acid thereof into a molecular chain of an active hydrogen component, and is a metal drier (such as cobalt, Manganese, zirconium salt) to catalyze self-crosslinking. The coatings produced therefrom have excellent durability, impact resistance, chemical resistance and corrosion resistance. With the continuous development of single-component waterborne polyurethane coatings and the gradual improvement of properties, it has been applied in the field of anti-corrosion. Water-based polyurethane is the base material, and pigmented anti-corrosion coatings are added by adding pigments and additives. Change the mass ratio of pigment to filler and binder in the coating formulation, make coating samples on stainless steel plate, discuss the influence of P/B on the corrosion performance of the coating through polarization curve and electrochemical impedance test. The test results show that the coating shows The overall performance of the coating is better when P/B = 0.8 and 1.5 in the formulation. However, due to its own defects, one-component waterborne polyurethane coatings have certain limitations in anti-corrosion applications, and their anti-corrosion properties are improved by modification methods. Bayhdrol VP LS2917, a dispersion of water-based fatty acid-modified polyester-polyurethane developed by Bayer, has good corrosion resistance and can be used for drying and drying various types of topcoats, single-layer paints and primers. . The salt spray test shows that the one-component self-drying anti-corrosion waterborne polyurethane primer has better corrosion resistance than the two-component waterborne epoxy resin and the water-soluble polyurethane modified alkyd resin coating.

In these cross-linking one-component aqueous polyurethane coatings, since two or more functional groups are in the same place, in order to achieve a crosslinking reaction in a film formation state, a reaction switch is often required. According to different initiation conditions, it can be divided into acid catalysis, thermal catalysis and photocuring crosslinking reaction. The acid-catalyzed crosslinking reaction is mainly applied to the use of an internal crosslinking agent such as siloxane, aziridine, carbodiimide and methylimine. These internal crosslinking agents are stable in the emulsion, and During the drying process of the coating film, the pH of the coating film decreases due to the volatilization of the water and the neutralizing agent, and the crosslinking reaction occurs. When the polyisocyanate is blocked by a phenol or other compound containing a monofunctional active hydrogen atom, the blocked isocyanate group does not react with the hydroxyl group at room temperature, and when the temperature rises to a certain extent, it is automatically deblocked. And cross-linking curing reaction with the crosslinking agent. The photocuring cross-linking reaction is a high-strength radiation using electron beam radiation or ultraviolet radiation to initiate a cross-linking curing of a low-activity polymer system.

2. Two-component waterborne polyurethane coating

The two component aqueous polyurethane consists essentially of an aqueous polyol system and a polyisocyanate system. In terms of polyols, it is classified into a dispersion type polyol and an emulsion type polyol. Among them, dispersion type polyols are often produced by a multi-step synthesis reaction, including polymerization in an organic solvent or a bulk, followed by neutralization and transfer to water, and sometimes removal of an organic solvent. The resulting dispersion is water soluble and has a small particle size (<0.08 μm). The emulsion type polyol is obtained by emulsion polymerization and is directly dispersed in water. Unlike the dispersion type, the emulsion type polyol has a typical clear boundary between the organic phase and the aqueous phase, and has a large particle size. The dispersion type polyol can be used in combination with a conventional isocyanate, and the emulsion type polyol is suitable for a water-dispersible isocyanate.

Commonly used aqueous polyol systems mainly include acrylic polyols, polyester polyols, polyether polyols, polyurethanes or alkyd polyols, and may also be hybrid mixtures of two or more of the above. In general, polyester polyols provide good weatherability, abrasion resistance, chemical resistance, and toughness, but are poor in water resistance. Polyether polyols have good hydrolytic stability and flexibility, but are less resistant to oxidation and UV degradation. The two-component waterborne polyurethane coating prepared by the acrylic polyol has good solvent resistance, chemical resistance, hydrolysis resistance and weather resistance. In the process of use, different polyol systems can be used according to the application field to achieve the desired effect.

In the isocyanate system, a conventional type of isocyanate curing agent and a water-dispersed isocyanate curing agent are mainly included. Among them, when a suitable polyol is selected, the conventional isocyanate curing agent can be emulsified in the polyol component. At the same time, in order to obtain a better dispersion effect, the viscosity of the curing agent is required to be as low as possible, and mechanical stirring is required. Bayer has developed a trimeric curing agent for ultra-low viscosity, high-NCO functionality polyisocyanate VLV AST, which can be used to provide corrosion-resistant coatings with a high degree of cross-linking.

The water-dispersed isocyanate curing agent has a non-ionic modification and an ion modification. Since isocyanate reacts with water to form CO2 and replace urea, which affects the quality of the coating film, an aliphatic or alicyclic isocyanate which is slow to react with water or an adduct thereof should be selected. In the non-ionic modification, a commonly used method is to graft a hydrophilic polyether, and a polyoxyethylene fatty alcohol can also be used. In the ionic modification, the carboxyl group-containing diol and isocyanate addition are mainly used, and the most commonly used one is DMPA, which is added with an isocyanate to introduce a carboxyl group onto a molecular pendant group, and the carboxyl group reacts with an isocyanate at a high rate. It is far lower than the reaction rate of hydroxyl group and isocyanate, and basically does not participate in the reaction, which can be neglected. At the same time, the introduction of carboxyl group greatly improves the hydrophilicity of the adduct, and then neutralizes with organic or inorganic base to further enhance its dispersion in water. Sex.

Two-component waterborne polyurethane coatings not only have excellent physical properties, but also have excellent aging resistance, acid resistance, alkali resistance, salt spray resistance, salt water resistance, oil resistance and other chemical properties. Therefore, two-component waterborne polyurethane coatings are becoming waterborne polyurethanes. The leader in anti-corrosion coatings.

A three-coat anticorrosive system was prepared by using waterborne epoxy zinc-rich primer as the primer, preparing the intermediate paint with the same resin, and preparing the two-component waterborne polyurethane based on the emulsion type acrylic polyol as the top coat. This aqueous three-coat system provides excellent corrosion protection. After 3 700 h of salt spray aging, the coating applied to the blasted steel plate was only slightly corroded and had similar properties to the solvent type. Its chemical resistance test shows that this system is suitable for use under the liquid. Using BASF hydroxy acrylic emulsion (Luhydran S937T) and water-based HDI curing agent (Basonat HW160PC) as base materials, combined with non-toxic and high-efficiency anti-rust pigments, it can be used to formulate water-based heavy-duty anti-corrosion coatings with excellent performance. It has excellent anti-corrosion properties and can completely replace most solvent-based anti-corrosion coatings.

A two-component waterborne polyurethane coating for metal was developed using waterborne polyurethane resin produced by Bayer. Through the test of the film properties, it was found that the film not only has excellent physical properties, but also has good performance in terms of alcohol resistance, salt water resistance, and resistance to cold and heat cycles. Through the exploration of its online construction process, it is found that the performance and construction performance of the water-based paint coating is good, and it can be used on a large scale on metal parts. In the 1996 protection of the Daytona USA play center floor, the intermediate paint and topcoat were made of Bayhydur two-component waterborne polyurethane paint from Bayer, providing excellent gloss retention, high gloss and high wear resistance. Sex. In a two-component aqueous polyurethane coating, water is excessive, and the reaction is mainly based on the reaction of an isocyanate group with a hydroxyl group because the reaction rate of the isocyanate group with a hydroxyl group, water, or the like is lower than that of water. The chemical reactions involved in the film formation of the two-component waterborne polyurethane coating are shown in Figure 1.

In these reactions, if the degree of side reaction between water and isocyanate groups is high, a large amount of CO2 and amines are formed, wherein the reaction rate of the isocyanate groups with the amine is greater than the reaction rate with the hydroxyl groups, which will result in a coating film performance and appearance. Great impact. Therefore, how to control the side reaction between isocyanate groups and water is the key to the development of two-component waterborne polyurethane coatings.

3. Modified waterborne polyurethane coating

Although waterborne polyurethane coatings meet the trend of environmentally friendly coatings in the coatings industry, due to the introduction of hydrophilic groups in their molecules, the chemical resistance and solvent resistance of the coatings are poor, the hardness is low, and the surface gloss is not high. Use in the field of anti-corrosion is limited. Therefore, people often use some excellent materials to modify it. 3.1 Epoxy modified waterborne polyurethane coating epoxy resin has many excellent properties, such as many types, easy to cure, high mechanical strength, strong adhesion, molding shrinkage Low rate, good chemical stability, good electrical insulation, low cost, high modulus, high strength and good thermal stability. It has become a widely used thermosetting plastic, which plays a role in machinery, electronics, paints, etc. An increasingly important role. Therefore, the waterborne polyurethane coating modified by epoxy resin has many excellent properties. For example, the coating film not only has strong adhesion, but also has excellent oil resistance, acid resistance, salt and alkali resistance, water resistance and solvent resistance. Epoxy modified waterborne polyurethane coatings have been widely used in the field of corrosion protection. The synthesis of epoxy modified waterborne polyurethane resin for anticorrosive coatings was studied. The research found that: through the modification of epoxy resin, the water resistance, chemical resistance, hardness and tensile strength of the coating film are significantly improved, and its comprehensive performance has reached or exceeded the comprehensive performance of the waterborne polyurethane resin of foreign anticorrosive coatings. Then, an epoxy resin is introduced into the synthesis process of the waterborne polyurethane to prepare an aqueous polyurethane epoxy resin emulsion, and the emulsion is used as a base material, and the non-toxic and high-efficiency composite iron-titanium anti-rust pigment is selected through formula design, and is combined with other pigments and fillers. Auxiliary, research and preparation of high performance waterborne anti-rust coatings. Introduced epoxy resin modified waterborne polyurethane to water

Epoxy polyurethane was used as the base material to prepare zinc-rich coating. Corrosion electrochemistry of zinc-rich coating with different content of zinc powder in 3% NaCl solution was studied by analyzing its corrosion potential and electrochemical impedance spectroscopy (EIS). Behavior and comparison with zinc-rich coatings and traditional zinc-rich coatings with a small amount of aluminum powder. The results show that the anti-corrosion ability of waterborne epoxy polyurethane zinc-rich coating is stronger than that of traditional epoxy zinc-rich primer; the addition of zinc powder has a certain influence on the anti-corrosion effect of the coating. Adding a small amount of aluminum powder can improve the coating. Corrosion resistance.

3.2 Acrylic modified waterborne polyurethane coating

Although the waterborne polyurethane coating has good physical and mechanical properties and excellent cold resistance, the self-thickening property of the single polyurethane emulsion is poor, the water resistance of the latex film is poor, the gloss is poor, the mechanical strength is not as good as that of the acrylic resin, and its application is affected. Certain restrictions. Polyacrylate emulsion has good water resistance, physical and mechanical properties and weather resistance. Therefore, acrylic modified waterborne polyurethane coating is a new type of coating which combines the advantages of waterborne acrylic coating and waterborne polyurethane coating. It has excellent weather resistance and self-heating. Good thickening and high solid content.

The modification of aqueous polyurethane by acrylate compounds can be divided into two methods: physical blending modification and synthetic copolymer emulsion modification. The physical blending method is a simple physical mixing of the polyurethane emulsion and the polyacrylic acid emulsion by mechanical stirring, and the coating performance of the obtained coating is poor. Therefore, a modification method of a synthetic copolymer emulsion is commonly used. The method can be further divided into: an external crosslinking agent method, a seed emulsion polymerization method, an interpenetrating network method, and an emulsion copolymerization method.

Air products' Hybridur 870 emulsion, an acrylic-polyurethane hybrid emulsion, has excellent wettability, adhesion and shielding properties by adding suitable anti-corrosive pigments such as Heucophos ZBZ, ZPO, ZPA, ZMP and ZPZ. The prepared film was not bubbled at 1 000 h at 37 ° C and 100% humidity.

In addition, water-based polyurethane was prepared by in-situ polymerization using toluene diisocyanate (TDI-80), polyether diol (N220), epoxy resin (E20) and methyl methacrylate (MMA) as raw materials. Epoxy resin-acrylic acid (WPUEA) composite dispersion. The storage period of the dispersion is more than 10 months, the freeze-thaw cycle is greater than 5, the film hardness is greater than 0.70, the tensile strength is greater than 10 MPa, and the water resistance, acid and alkali resistance, solvent resistance, etc.