Brief Discussion on Technical Difficulties in the Development of Waterborne Container Coatings
Ye Fengying, Chu Guangcheng, Chen Jinghua
According to statistics, in 2007, the international container production and sales reached 3.92 million TEU, of which about 3.746 million TEUs were produced in China, accounting for 95.6% of the total. The container-based coatings used were about 300,000 tons, which produced about 15 10,000 tons of xylene organic solvent emissions. With the establishment of various environmental laws and regulations, the environmental protection of container coatings has become inevitable. Water-based paint uses water as a solvent, and can adjust the construction viscosity according to needs. It is characterized by convenient construction, easy control of thickness and low loss, and has become the development trend and research focus of container coating. The International Container Leasing Association (IICL) also held a special meeting to put forward proposals for the development, production and application of container waterborne coatings. In recent years, many companies at home and abroad have been conducting research on the water-based coating of container coatings. At present, the main problem is that the production speed of the container is continuously increasing, and the fastest production cycle has reached 3.5 minutes per standard box, which undoubtedly increases the difficulty of water-based container coating. The development of water-based container coatings needs to solve three technical problems of anti-corrosion performance, production line construction performance and high coating cost, as well as a series of problems such as capital and management. At present, there are only a few manufacturers of three-coat full-water container coatings that can fully meet the requirements of the production line. Hong Kong Weixin Group is one of them. In the past few years, the author has been conducting research on waterborne container coatings. Here, I mainly discuss technical problems and hope to provide some help for researchers who are conducting research in this area.
1. Technical requirements for container coatings
The operation of the container needs to go through the entire process from land to sea, and will play an increasingly important role in the international transportation and logistics system. Container coatings are required to withstand any harsh weather conditions on the planet and to accommodate any type of transportation. The performance requirements of container coatings are: anti-corrosion performance should be tested by KTA; coating film should have good wear resistance and flexibility; have resistance to various environmental changes, should have strong anti-pulverization, color retention, Light-proof; has good construction performance, meets high-volume and high-efficiency production requirements; the box should be wear-resistant, corrosion-resistant, solvent-resistant, and should also obtain FDA certificate for transportation of food; according to international practice, container manufacturing plant And the supplier must provide guarantees to the owner. Ensure that the area of ​​the container coating rust and peeling within 5a is not more than 10% of the entire coating area. In addition, some container manufacturers also require salt spray testing of the matching coatings to reach 1000h. The following are the specific requirements for container coatings in the production line with the highest production efficiency and the fastest pace in China:
(1) The production rhythm is 3 to 6 minutes per process, the construction is not affected by the environment, and the coating adaptability is required;
(2) Adopt high-pressure airless spraying, requiring the coating to carry out the wet-on-wet construction process;
(3) The three coatings are baked together, the baking temperature is 60-80 ° C, about 15-40 min; the coating film is required to be dry and can not be foamed;
(4) Labeling is carried out about 0.5 to 1 hour after baking and cooling, and it is required not to tear the coating film when labeling;
(5) After the baking is cooled, the water sealing test is carried out for about 1 to 1.5 hours, and the coating film is required to be blister-free;
(6) The surface of the coating film is required to be flat, no sag, cracking, and no obvious defects such as pinholes and shrinkage holes.
It can be seen that container coatings have high requirements on construction performance and coating performance, and coating manufacturers are required to adjust according to different usage environments. General paint manufacturers will send paint representatives to the production line for 24h service to quickly solve the coating problems appearing on site to ensure the production efficiency of container manufacturers.
2. Technical difficulties and countermeasures for waterborne container coatings
2.1 anti-corrosion performance
The corrosion resistance of container coatings is primarily determined by the resin in the coating. The anti-corrosion performance of water-based container coatings must reach the same anti-corrosion performance as solvent-based ones. There are certain technical difficulties, because the difference in film-forming mechanism between water-based and solvent-based coatings leads to differences in coating properties. Film-forming mechanism of solvent-based epoxy resin coating: epoxy resin and curing agent are dissolved in organic solvent in molecular form, the system formed is homogeneous, and the curing reaction proceeds between molecules, so the curing reaction proceeds relatively completely. Forming a homogeneous, dense coating film with outstanding anti-corrosion properties. Film-forming mechanism of waterborne epoxy resin coating: After most of the water is evaporated, the epoxy resin latex particles contact each other to form a closely packed structure, and the residual moisture and curing agent molecules are in the gap of the dispersed phase particles; the water is further evaporated and dispersed. The phase particles begin to coagulate, forming a compact hexagonal structure; at the same time, the curing agent molecules diffuse to the interface of the dispersed phase particles and the internal curing reaction occurs, the glass transition temperature of the resin gradually increases, the diffusion speed becomes slow, and the particles are difficult to coagulate. Film formation. The curing reaction is incomplete, and a uniform and dense coating film cannot be formed, resulting in a difference in corrosion resistance from the solvent type. Whether the curing of waterborne epoxy coatings is sufficient depends mainly on the following two factors:
(1) Particle size of epoxy resin dispersed phase particles.
Under the same circumstances, when the particle size of the dispersed phase of the epoxy resin is small, the concentration of the curing agent on the surface of the particle is moderate, the surface curing speed is slow, and the curing agent molecule has sufficient time to diffuse to the entire dispersed phase of the epoxy resin. The curing is complete, so that a uniform, fully cured coating film can be formed. Conversely, when the particle size of the dispersed phase is large, the concentration of the curing agent on the surface of the particle is relatively high, resulting in rapid surface solidification. As the curing reaction proceeds, the apparent viscosity of the dispersed phase of the epoxy resin increases continuously, and the glass transition temperature also increases. Gradually, the diffusion rate of the curing agent molecules into the epoxy resin particles gradually becomes slower, and the inside of the epoxy resin particles cannot be cured, resulting in incomplete curing of the coating film.
(2) Compatibility of curing agent with epoxy resin.
The compatibility between the curing agent and the epoxy resin is improved, and the more easily the curing agent diffuses into the interior of the epoxy resin particles, the more favorable the curing reaction is, and the stability of the system is improved.
In recent years, water-based epoxy resin products have become increasingly mature. Many companies at home and abroad such as Dow, Cytec, Hansen, and Anbang have developed water-based epoxy resins with a particle size of nanometers; A compatible aqueous epoxy curing agent product is also maturing. These provide the basis for the study of water-based coatings for containers. By rationally using these mature products, the performance of water-based epoxy anti-corrosion coatings and solvent-based epoxy anti-corrosion coatings are very close, which can basically meet the anti-corrosion requirements of containers.
2.2 Construction performance
In recent years, due to the rapid development of the world's shipping industry, the demand for containers has grown rapidly, container production has increased substantially, production efficiency has also increased rapidly, and the production line with the fastest pace of production has reached a rhythm of 3 minutes, which has increased the construction of waterborne container coatings. The difficulty. It is well known that water-based coatings dry and cure at very low temperatures and high humidity. How to make water-based coatings meet such fast production rhythm all the time becomes the second problem that restricts the development of water-based container coatings. The research unit of waterborne container coatings hopes to continue the construction performance test on the production line. However, due to the consideration of production efficiency, the container manufacturer hopes that the number of times the research unit will go to the site for construction testing as little as possible, so as not to affect normal production. Therefore, it is necessary to properly resolve this contradiction, and it is required that the water-based paints studied by the research unit have a wide construction window, and fully simulate the environmental conditions, construction conditions and drying conditions of the site in the laboratory as much as possible to reduce the pressure on the tank factory. . The working position, production process status and requirements for water-based container coatings of a production line with the fastest pace of production in China are shown in Table 1.
The water-based container coating should be subjected to high-pressure airless spraying without changing the efficiency of the existing solvent-based production line. The three coatings are baked together, requiring fast drying speed, good coating adhesion, good water resistance, no sag, Cracking, pinhole and other coating defects. These requirements pose unprecedented challenges for the construction of waterborne container coatings. Based on years of research experience in waterborne container coatings, the author proposes technical difficulties and solutions for coating construction, as shown in Table 2.
2.3 cost problem
Although water-based coatings have the advantages of environmental protection, energy saving, and convenient construction, they are also the direction of coating development. However, since the cost of water-based container coatings is always higher than that of corresponding solvent-based coatings, today, the competition in the container manufacturing industry is fierce, persuading the box factory and the box. It is very difficult for the Lord to take out more funds to pay for environmental protection, so reducing costs is the only way to apply the scale of waterborne container coatings. The cost reduction of waterborne container coatings is mainly the independent research and development of water-based resins, anti-rust pigments and water-based additives. Domestic research in this area started late, and there is still a certain gap between product performance and foreign companies. Mainly due to the lack of domestic professional research institutions, weak foundations, fast flow of technical personnel, and low technical exchanges, these materials are heavily dependent on foreign imports, and domestic lack of independent innovation. The cost reduction of waterborne container coatings can be started from the following aspects: (1) Strengthening the cooperation with professional research institutions and enterprises, independently developing low-cost water-based resins and additives; (2) Stabilizing a scientific research team, in accordance with scientific research procedures Conduct rigorous, scientific and systematic experiments; (3) Strengthen technical exchanges, strengthen exchanges with the industry, and keep abreast of domestic and international information.
3. Conclusion
China is a big container manufacturing country. Research on water-based container coatings can save energy, reduce environmental pollution, and reduce damage to production line workers. It is an inevitable trend in coating development. The development of water-based container coatings needs to solve three technical problems of corrosion resistance, construction performance and cost. It is a long-term and systematic work. It is necessary to strengthen close cooperation and exchanges with research institutions, raw material manufacturers and enterprises and enterprises. With the addition of more paint manufacturers, it will inevitably promote the continuous improvement of waterborne container coating technology and promote the transformation of the container industry. The development and application of water-based container coatings can play an exemplary role in the field of industrial anti-corrosion coatings, drive the development of new water-based resins, additives, anti-rust pigments and other raw materials, and promote the innovation of water-based anti-corrosion coating technology in other industries, with extensive social benefits and economy. benefit.
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