Advantages and problems of water-based coatings
Time:2021-11-03

1. Introduction to Waterborne Coatings

   Water-based paints refer to paints that use water as a solvent or as a dispersion medium, including water-soluble paints, water-reducible paints, and water-dispersible paints (latex paints). Water-soluble coatings use water-soluble resin as the film-forming material, mainly including polyvinyl alcohol and various modified products, water-soluble alkyd resin, water-soluble epoxy resin and inorganic polymer water-based resin. Water-reducible paint mainly uses emulsified emulsion as a film-forming material to dissolve the resin, and then with the help of emulsifiers, the resin is dispersed in water to form an emulsion, called post-emulsified emulsion. Water can be used to dilute during construction. Water-dispersed coatings mainly refer to coatings prepared with synthetic resin emulsion as a film-forming material.

Solvent-based coatings are composed of four parts: pigments, fillers, resins, solvents and various additives. The composition of water-based coatings and solvent-based coatings is roughly the same, but water-based coatings require more additives and more complex formulations, mainly composed of water-based resins, pigment and filler additives, neutralizers, and water.

  2·Development advantages of water-based coatings

  2.1 Environmentally friendly

  The whole life cycle of water-based coatings from the source of raw materials to production and use is conducive to the healthy development of the environment. It not only replaces volatile organic solvents in the production process, but also the coating tools can be cleaned with water during use, which also reduces the consumption of cleaning solvents. The production of organic solvents requires the consumption of fossil raw materials, which is a major challenge to resources at this stage when resources are scarce. The emergence of water-based coatings has solved this problem. At the same time, the reduction of solvents has greatly reduced the emission of volatile organic compounds during production and use, and has made a positive contribution to improving the surrounding atmospheric environment. Ma Chunqing [3] and others compared the VOC emissions of different coating systems, and the results showed that the VOC emissions of the "solvent-based intermediate coating + solvent-based colored paint + single-component varnish" process system was 120g/m3, and " The emission of the process system of water-based intermediate coating + water-based color paint + water-based varnish is only 28g/m3, which is greatly reduced.

  On the other hand, water-based paint only uses a small amount of low-toxic alcohol ether organic solvents, and the volatilization of organic solvents is reduced, which greatly improves the environment of the workplace and is beneficial to the health of the staff. The organic solvent (accounting for the paint) of general water-based paint is between 10% and 15%, but the current cathodic electrophoretic paint has been reduced to less than 1.2%, which has a significant effect on reducing pollution and saving resources.

  2.2 Reduce risk

Take the most common toluene and xylene in the solvent as an example: the flash point of toluene is 4℃, vapor can form an explosive mixture with air, the explosion limit is 1.2%~7.0% (volume); the flash point of xylene is 29℃, and the vapor can be Air forms an explosive mixture, the explosion limit is about 1% to 7% (volume). Both are flammable substances. Therefore, the places where the flammable and explosive materials of solvent-based coatings are stored and used have the risk of fire and explosion accidents. In our country, there are frequent reports of fire and explosion accidents in the solvent storage tanks of paint factories.

The production and use of water-based coatings reduces the demand for flammable and explosive raw materials and greatly reduces risks.

  2.3 Reduce costs

  2.3.1 Raw material cost

  With the increasing demand for resources and energy, there is a shortage of resources and energy around the world, causing the price of petroleum and downstream products to continue to rise, and organic solvents as downstream products are no exception. Under the pressure of resource prices, the price advantage of using water as a diluent is gradually emerging [4].

  2.3.2 Environmental cost

   A large amount of organic waste gas is generated in the traditional solvent-based paint production process. In order to meet the emission requirements, enterprises need to configure exhaust gas treatment facilities and related supporting facilities, and bear the daily operation and maintenance costs. The organic waste gas emission of water-based paint is very small, which reduces the burden for enterprises.

   2.3.3 Security cost

   Water-based paint uses water as a diluent, making it a safer paint. The fire hazard from production to transportation to application is greatly reduced compared to traditional solvent-based paints. For managers, the input cost of security will also be reduced accordingly. Considering raw material costs, equipment costs, safety costs, and reduction of occupational diseases, water-based coatings are competitive in the market.

  2.4 Construction conditions and appearance performance

Water-based coatings have better air permeability, and the water vapor inside the base layer can diffuse outward; while solvent-based coatings form a dense film. For example, the water content of the base layer is high. In terms of requirements, the water content of the base layer of water-based paint construction should be less than 10%, and the water content of the base layer of solvent-based paint construction should be less than 8%. The water content requirement is stricter than that of water-based paint. The water-based paint has good adaptability to the surface of the material, and the coating has strong adhesion. It can be directly applied in a humid environment. The volatilization of water in water-based coatings mainly depends on temperature and humidity control, while solvent-based coatings can adjust the coating solids by adjusting the evaporation rate of the thinner. Generally, the coating solids of solvent-based coatings are as high as 60%~70%, while the coating solids of water-based coatings are only 20%~30%. It can be seen that the smoothness of water-based coatings is relatively good.

  3·Difficulties in the popularization of water-based coatings

  3.1 Technical aspects

   Although water-based coatings are better than solvent-based coatings in terms of environmental humidity and moisture content of the base layer, they are not as suitable as solvent-based coatings in other aspects.

  (1) Due to the high surface tension of water, it is difficult to infiltrate the substrate and is prone to shrinkage. Water-based coatings also require high cleanliness during the construction process and the surface of the material. Whether it is solvent-based paint or water-based paint, the quality of oil removal on the surface of the workpiece will affect the quality of the coating film. For the spraying of solvent-based coatings, the surface of the workpiece is not completely degreasing, and it is not easy to appear immediately; but for the spraying of water-based coatings, if the surface of the workpiece is not completely degreasing, it is difficult to form a complete paint film on the surface.

  (2) The melting points of the two types of coating dispersion media are very different. Water is 0°C, while the melting point of organic solvents is much lower. Toluene is -95°C, and xylene is about -25°C. Therefore, for the construction temperature, water-based coatings are generally required to be above 5°C, while solvent-based coatings can also be used at 0°C. This requirement severely limits the scope of application of water-based coatings.

(3) Spraying water-based paint also has certain requirements on the humidity of the spray booth. The water-based paint dries slowly when the relative humidity of the environment is above 85%, and the water solvent cannot volatilize normally in a saturated humidity environment. When the humidity is too high (up to 90%) Above), the coating film on the coated object will flow down [5]. Therefore, the spray booth needs to have good air circulation and maintain proper humidity.

  (4) Since the latent heat of evaporation of water is large, which is 5-7 times that of general solvents, it is often necessary to increase the drying temperature or extend the drying time.

  3.2 Performance

   At present, water-based coatings have the protective and decorative functions of ordinary solvent-based coatings, but in occasions with high decorative requirements, they cannot achieve high fullness, and solvent-based coatings are needed to achieve it.

  3.3 Thinking inertia

   At present, there are no mandatory requirements for the use of water-based coatings, and traditional solvent-based coatings are widely rooted in all aspects of social production and life. Professionals have long been familiar with its performance, whether it is production or use. To master the new thing of water-based coatings, you need to spend extra energy to understand and familiarize yourself with its characteristics. Therefore, affected by the inertia of use, most people are unwilling to take the initiative to abandon the stable use of solvent-based coatings and assume a certain investment risk to use water-based coatings. This is also a major obstacle to the popularization and application of water-based coatings.

  4 · Outlook

   Water-based paints that use water as a diluent have significantly lower VOC content than solvent-based paints. They are environmentally friendly and harmless to the health of constructors and users. There have been many successful cases of water-based paint companies in China.

In the future, to vigorously carry out basic research to reduce the difficulty of construction of water-based coatings and improve the performance of use is an urgent task for the expansion of application fields and large-scale popularization of water-based coatings. At the same time, strengthening the attention to policy guidelines, promoting technology, changing ideological concepts, and eliminating public prejudice and worries about water-based coatings will also promote the application and development of water-based coatings. In short, under the guidance of the three principles of developing the coating industry (no pollution, resource saving, and energy saving) and the "4E" principle (economy, efficiency, ecology, and energy), water-based coatings will have greater development.