Application of waterborne polyurethane anticorrosive coatings in ships

Shipbuilding industry is an important industry for the development of China's national economy. Since the 1990s, the completion of shipbuilding in China has continued to be high and long. In 2006, China's shipbuilding completion volume reached 14.52 million DWT, accounting for 19% of the world's shipbuilding volume. In 2007, the shipbuilding completion exceeded 18 million DWT, ranking the third in the world for 14 consecutive years; more than 60% of shipbuilding completions were exported ships. The varieties have also developed from ordinary bulk carriers and oil tankers to high-tech, high value-added ships with large-scale container ships, large-scale liquefied petroleum gas ships, giant oil tankers, dump ships and ro-ro ships with modern advanced level. Southeast Asia has expanded to include many countries and regions including developed countries in Europe and America.

In 2015, China's shipbuilding completion capacity will reach 30 million deadweight tons, which is expected to catch up with or surpass Japan and South Korea and become the world's largest shipbuilding country. Paint coating is one of the main sources of air pollution. In recent years, with the increasing shortage of petroleum resources and the increasing awareness of environmental protection, the coatings industry has gradually developed towards environmental protection and resource conservation. High-solids coatings, solvent-free coatings and water-based coatings have become important development directions for coatings. Waterborne coatings have become a hot spot in coating research due to their unique environmental friendliness and safety. According to statistics, in the western developed countries, US water-based coatings accounted for 52% of total paint production, Germany accounted for 46&, and Japan accounted for 17%, while China started late, accounting for only 8% of total paint production.

A large number of coatings are used in the shipbuilding process. Traditional industrial coatings contain a large amount of organic solvents under the construction viscosity. During the coating construction and drying process, the organic solvent volatilizes not only causes a lot of waste of resources and environmental pollution, but also causes fire hazards and harms construction. The health of the people. The use of water-based paint instead of solvent-based products for ship coating has a very important practical significance.

1. Classification and development of waterborne anticorrosive coatings and comparison with solventborne coatings

1.1 Classification and development of waterborne coatings

According to foreign classification, water-based coatings mainly include water-dilutable coatings, water-based latex coatings and emulsion coatings.

(1) Water dilutable coating

The water-dilutable coating resin itself is insoluble in water, but has a relatively low molecular weight in the molecule, a carboxylic acid group which can be partially neutralized with an amine or a low molecular weight, and can be partially neutralized with a carboxylic acid. The amine group, the resin liquid is a water-soluble high solid solution in which pigments and other auxiliaries are dispersed, and the solution is dissolved in water by the hydrophilic action of a water-soluble solvent and a salt group. Many resins such as acrylic resin, polyester resin, alkyd resin, urethane resin, epoxy resin, etc., can be obtained by introducing a specific hydrophilic group into a resin molecular chain. In this system, resins with higher acid numbers or more hydrophilic groups are mainly used as electrophoretic primers or dip-coating lacquers, and are rarely used in the preparation of air-drying coatings. American Willams and PPG, AKZO in the Netherlands, and Kansai in Japan all have such products.

A water-reducible resin having a lower acid value can be prepared by using a core I shell structure. Firstly, a resin (saturated polyester, alkyd, polyurethane, etc.) which meets the requirements of the grafting point is prepared, and then the acrylate monomer of a predetermined acid value is grafted onto the target molecular chain by a free radical polymerization method. After neutralization by the amine, the grafted resin can disperse itself in water. The resin synthesized by the core-shell structure self-emulsification technique is usually designed as a hydrophobic core layer and a hydrophilic shell layer, and the content of the hydrophilic group in the entire resin molecule is greatly reduced, and the core layer and the shell layer are classified. Different kinds of resins form a unified body with complementary advantages, which greatly improves the comprehensive performance of the resin. The water-reducible resin of this structure can be used for both self-drying paints and baking varnishes.

(2) Water emulsion type coating

The water latex type coating resin is prepared by emulsion polymerization, and the emulsion polymerization technology has been greatly developed, and its application has penetrated into many fields such as construction and industrial protection. The emulsion polymerization can be further divided into two types: homogeneous emulsion polymerization and heterogeneous emulsion polymerization. The former refers to one-step emulsion polymerization; the latter is a step-by-step emulsion polymerization method developed in recent years, also known as seed polymerization or core-shell emulsion polymerization. Such coatings mainly include acrylate latex coatings, styrene-acrylate coatings and modified products thereof.

Hybrid emulsion polymerization is a resin preparation and modification technology developed in the late 1990s. The emulsion polymerization and modification are completed in one step. The typical process is to dissolve the modified resin (epoxy resin, alkyd resin, polyurethane resin, etc.) in the acrylic monomer, pre-emulsified to obtain the microemulsion, and then use the emulsion polymerization technology to obtain the polymer with excellent film forming performance and stability. Latex, the resulting latex combines the advantages of acrylic resin and modified resin, and the overall performance is greatly improved.

(3) Emulsion coating

The emulsion type resin is obtained by forcibly dispersing an oil-soluble polymer plus an emulsifier by mechanical force. The emulsifier used in the early stage does not participate in film formation, but is freed in the paint film, thus adversely affecting the performance of the paint film in terms of water resistance and corrosion resistance. In recent years, emulsifier technology has also made great progress. The use of polymerizable (or reactive) emulsifiers has greatly improved the water resistance and corrosion resistance of the products. Such coatings mainly include water-based epoxy coatings and alkyd coatings. In addition, foreign countries have also reported the emulsification of nitrocellulose to prepare waterborne wood varnish.

2.2 Classification of waterborne coatings and comparison with solventborne coatings

Early waterborne coatings differed greatly from solventborne coatings in performance, as shown in Table 1.

3. Application of waterborne anticorrosive coating in ship coating in China

3.1 Development trend of marine coatings

The 17th National Congress of the Communist Party of China once again emphasized the need to strengthen resource conservation and ecological environmental protection and enhance the capacity for sustainable development. The soundness of national environmental regulations will promote the development of marine coatings in the direction of environmental protection and resource conservation. Although the traditional low solid content anticorrosive coatings still occupy more than 50% of the market in China, and it is difficult to withdraw from the market in a long period of time; in the high-end market, as well as in coastal economically developed areas, environmentally friendly marine coatings will gradually become mainstream. . In addition to the development of coating systems in the direction of high-solids coatings, solvent-free coatings and water-based coatings, pigments and fillers for coatings should also comply with the requirements of environmental protection. In the 90-year antifouling agent, organotin will be banned, and red rust in rust-preventing pigments will be banned. , chromate, etc. will also be gradually replaced by environmentally friendly products such as new phosphates.

3.2 Current status and main varieties of waterborne marine coatings in China

In the 1990s, the world's major marine coatings multinationals IP, JOTUN, HEMPLE, SIGMA, COSCO Kansai, KCC, ICC; etc. have landed in China. Due to the emphasis on environmental protection in developed countries such as Europe and the United States, these multinational companies attach great importance to the development of environmentally friendly marine coatings, most of which have waterborne marine coating products. When the Hudong Shipyard built the 3600T bulk carrier for Hong Kong Universal in the 1980s, it used the waterborne alkyd topcoat produced by foreign companies. In addition to the joint ventures of multinational companies in China, only a few manufacturers in China use water in batches. Marine anti-corrosion coating products are currently mainly used for the coating of ships' inner cabins with low corrosion resistance requirements, such as living compartments and engine rooms, and the ballast tanks with high corrosion resistance requirements have not been used.

Due to the small space inside the ship, inconvenient operation and poor ventilation, fire and poisoning accidents are likely to occur during construction. Especially in the soft covering construction of the insulation layer, the covering cloth and the insulating and insulating materials are porous materials, which are absorbed by the coating. The amount is large, and it is easy to enclose the organic solvent inside the porous substrate, resulting in long-term paint odor in the cabin, which seriously damages the health of the person working and living in the cabin. The use of water-based paint can cross-coat coating construction and other construction, improving shipbuilding efficiency. The environment inside the ship is not particularly harsh, so in addition to environmental indicators, the other requirements for interior tank coatings are not so harsh, so the application of waterborne anti-corrosion coatings in the interior of ships is very promising.

At present, the water-based coatings used for ship anti-corrosion can be matched with water-based coatings as well as with solvent-based systems. The main types of domestic waterborne marine coatings are shown in Table 2.

Although water-based paints have made great progress in environmental protection compared with solvent-based paints, due to the uneven quality of products on the market, not every product can meet the requirements of the Navy. For this reason, some domestic water-based paint manufacturers have conducted special research on this, but because the test standard is very demanding, for example, the formaldehyde concentration can not exceed 1.0×10—5, so only a few manufacturers have passed the corresponding toxicity test requirements. The toxicity test is carried out in accordance with GJB-3881-1999 "Procedures for Toxicity Evaluation of Non-metallic Materials for Ships". During the test, the samples are placed in a constant volume and constant humidity sealed box according to the actual volume ratio of the ship, and the sample release is periodically tested. Whether the concentration exceeds the standard, and the smoke after burning the paint film is also tested for the toxicity of the mouse. Through the 30d toxicity test, it can be used for the inner cabin painting of surface ships; it can be used for the inner cabin painting of conventional submarines through the 60d toxicity test; it can be used for the inner cabin painting of nuclear submarines after the 90d toxicity test. At present, the waterborne acrylic antirust primer and topcoat of Taicang Lanyan Coatings Factory can be used for the interior cabin coating of surface ships; the waterborne acrylic modified alkyd inner tank coating and waterborne epoxy inner tank developed by the Marine Chemical Research Institute The coatings have passed the 90d toxicity test of the Naval Medical Research Institute and can be used for the painting of various ship cabins including nuclear submarines.

3.3 The main problems in the development of waterborne ship anticorrosive coatings

(1) The product variety is small, the supporting facilities are not perfect, and the product performance needs to be further improved.

China's waterborne marine anticorrosive coatings have just started, with few varieties of products, mainly several antirust primers and topcoats. Compared with similar solvent-based products, the performance of primers is not small. In addition to water-based inorganic zinc silicate primers, other primers are mainly used for anti-corrosion of waterline parts. Topcoats are mainly single-component systems. There is not much difference in solvent-based products. The application areas of the product are mainly inner and above deck facilities.

(2) Construction performance, especially low temperature construction performance needs to be improved

Compared with the solvent, the latent heat of evaporation of water is much higher, and the evaporation rate is much slower, especially in low-temperature and humid conditions, only the hot air can be applied; otherwise, the moisture is not volatile, which tends to cause the paint to flow.

(3) Improve the functionality of waterborne marine coatings and improve their cost performance

At present, the main advantages of waterborne marine anticorrosive coatings are environmental protection and safety, and the incomplete use of its functions will affect its promotion and application; therefore, the functionality of the products should be further improved. For example, interior coatings should try to improve their flame retardancy and resistance. Stain, anti-condensation, anti-mold, etc., hull coatings should have heat reflection and heat insulation capabilities.