1. The diversified development pattern of coatings
In the field of coatings today, they can be mainly divided into three categories based on their differences in form: organic solvent-based coatings, water-based coatings, and powder coatings.
(1) Organic solvent-based coatings: the hidden worries behind the brilliance
Organic solvent-based coatings are prepared using organic solvents as diluents. After long years of development, they have evolved into a new type of building material that combines high decorative properties, long-lasting protection, versatility and diversity. Organic solvent-based coatings play an indispensable role in the modern industrial boom, the construction of a modern national defense, and cutting-edge fields such as information technology, biochemistry and new materials. For example, in the aerospace industry, the body coating of aircraft needs to have excellent weather resistance and protection. Organic solvent-based coatings can effectively resist the erosion of the aircraft body material by the extreme environment at high altitudes. In the development of marine resources, the protective coatings for offshore drilling platforms and ship hulls cannot do without the help of organic solvent-based coatings for their good adhesion and corrosion resistance.
However, this type of paint has serious drawbacks during the film formation stage. When the coating is applied, the solvent will evaporate into the atmosphere during the volatilization process. Most organic solvents contain toxic ingredients and pose a direct threat to human health. According to relevant environmental monitoring data, in some traditional industrial agglomeration areas, the impact of volatile solvents during the use of organic solvent-based paints on the surrounding air quality can reach a range of several kilometers, resulting in a significant increase in the content of volatile organic compounds (VOCs) in the air. Moreover, after the solvents have completely evaporated, not only is there an unwarranted waste of resources, but there is also a significant loss of energy. This has undoubtedly become a huge bottleneck in development at a time when environmental protection and sustainable development are advocated.
(2) Powder coatings: advantages and limitations coexist
Powder coatings are made from solid resins, mixed with pigments, fillers and additives to form a solid powder. The unique feature of powder coatings is that they use air as a diluent and exist in the form of tiny powders, which makes them easy to disperse in the air. Powder coatings show great flexibility in terms of application methods, and can be applied using a variety of processes such as electrostatic spraying, rolling and showering. For example, in the metal furniture manufacturing industry, electrostatic powder coating can evenly adhere the powder to the metal surface, forming a smooth and adherent coating. Moreover, 100% of the overspray powder can be recovered, greatly improving material utilization and reducing production costs. From the perspective of coating film properties, powder coatings have many advantages, such as good adhesion, excellent weather resistance, hardness and wear resistance, and excellent corrosion resistance. They can also achieve zero VOC emissions throughout the entire production and use cycle, and are a model of environmentally friendly coatings.
However, the shortcomings of powder coatings should not be ignored. During production and coating, a large amount of dust is generated. Despite the continuous development of existing dust removal technology, complete purification is still difficult to achieve. Some ultra-fine dust will escape into the atmosphere, adversely affecting air quality. Take a large powder coating manufacturer as an example. Despite being equipped with advanced dust removal equipment, a certain amount of ultra-fine dust is still emitted every year, which to some extent aggravates the particulate matter pollution in the surrounding area. In addition, the curing process of powder coatings requires high temperatures, which means higher energy consumption. At the same time, the application of powder coatings on non-metallic substrates, oversized metal parts, and metal parts with complex shapes faces many technical challenges. For example, in the surface coating of some large sculpture artworks, due to their complex shapes and mostly non-metallic materials, powder coatings have difficulty adhering evenly and forming an ideal coating effect.
(3) Water-based coatings: potential accompanied by challenges
Water-based coatings use water as a dispersing medium or solvent. Depending on the binder used, they can be subdivided into synthetic resin water-based coatings and natural water-based coatings based on minerals and natural substances. Their significant advantage is low cost. Compared to organic solvent-based coatings, they are much safer, do not pose a risk of flammability or explosion, and can be applied directly in humid environments. Whether spraying, brushing or electrophoretic coating, they can be easily controlled, and are suitable for objects of various materials and shapes. They are relatively less restricted by the coating method and curing conditions. For example, in the field of interior decoration, water-based paints are widely used for wall painting. Their good air permeability makes it difficult for the walls to blister or mold, and the loose requirements for the moisture content of the base layer also reduce the difficulty of construction.
However, the use of water as a raw material for water-based paints also causes some problems. Water evaporates slowly, which means that water-based paints require higher drying temperatures or longer drying times, which increases energy consumption. According to experimental data, under the same coating area, the drying time of water-based paint may be 30% – 50% longer than that of organic solvent-based paint. Moreover, water-based paint is sensitive to the humidity in the spray booth. When the relative humidity exceeds 85%, the speed of surface drying slows significantly. When the humidity is saturated (I>90%), the paint film may even run, which seriously affects the coating effect. Therefore, the use of water-based paints requires a good air circulation system and strict control of the temperature and humidity conditions in the spray booth. In addition, in highly decorative situations, such as the exterior painting of high-end cars or the coating of the shells of high-end electronic products, it is currently difficult for water-based paints to achieve the fine decorative effects exhibited by organic solvent-based paints.
2. The mystery of paint film formation: the synergy of physics and chemistry
The film-forming or curing process of paint is essentially the process of the paint changing from a liquid to a solid film after being applied to a solid surface, as the solvent evaporates and the cross-linking and curing reaction progresses.
The three main types of paint – solvent-based, water-based and powder – all have the ability to form a film physically. However, the physical film-forming method of powder paint is significantly different from the other two. Powder coatings adhere to the surface of the substrate mainly through electrostatic attraction or heat energy. Then, they are heated to above the melting temperature, at which point the molten polymer adhesive flows under the action of surface tension and levels out. After cooling, a solid paint film is formed. For example, in the powder coating process for metal doors and windows, the powder is first evenly adsorbed on the surface of the doors and windows through an electrostatic spray gun, and then sent into a high-temperature curing oven. After a series of steps such as heating and melting, leveling and cooling, a smooth, shiny coating with good protective properties is finally formed.
The physical film formation of water-based paints and paints with organic solvents, on the other hand, is mainly dependent on solvent evaporation. In this process, the particles of the emulsion come together, deform and gradually align densely. As the drying temperature increases, the particles diffuse and fuse with each other, eventually forming a continuous solid film. Take water-based emulsion paint as an example. After painting the wall, the water begins to evaporate slowly, and the particles of the emulsion gradually come together. When a certain degree of drying is reached, the particles fuse with each other to form a dense coating film.
It is worth mentioning that water-based paints and organic solvent paints can also chemically form a film through a chemical reaction between reactive groups, forming a film with a crosslinked network structure. This chemical film formation method can significantly improve the performance indicators of the coating film, such as hardness, wear resistance, and chemical resistance. For example, in some high-performance industrial coatings, a highly crosslinked network structure is formed through a special chemical crosslinking reaction, which maintains good protective properties in harsh industrial environments.
3. The future direction of the coatings industry: innovation-driven change and breakthroughs
To sum up, an in-depth analysis of the development status and film-forming mechanism of mainstream coatings can clearly reveal the current landscape and challenges of the coatings industry. Although water-based coatings and powder coatings have obvious advantages in terms of environmental protection, due to their respective limitations, it is difficult for them to completely replace the dominant position of organic solvent-based coatings in the short term.
Although organic solvent-based coatings have excellent performance, the pollution they cause to the environment needs to be urgently addressed. In the future, researchers should focus on improving the preparation process and film-forming methods, exploring the use of low-toxic or non-toxic organic solvent alternatives, and optimizing solvent recovery and recycling technology during the film-forming process, so as to minimize pollutant emissions and achieve green and sustainable development. For example, some scientific research institutions are trying to develop new organic solvent formulations, through molecular structure design, to reduce the volatility and toxicity of solvents while improving their utilization during the film-forming process.
Water-based coatings are an important representative of environmentally friendly coatings, but their inability to meet high decorative requirements limits their further expansion of application. Therefore, future research should focus on the modification of water-based coatings, by introducing new functional additives and optimizing the resin structure, to improve their decorative properties and enable them to excel in highly decorative applications. For example, by adding nano-scale pigments and fillers, the color vibrancy and gloss of water-based coatings can be improved, while also enhancing the hardness and wear resistance of the coating film.
In terms of powder coatings, the primary task is to overcome the technical difficulties of dust treatment, develop more efficient dust removal equipment and processes, and ensure that dust emissions during production and painting processes reach lower levels. At the same time, the preparation process of powder coatings is optimized to reduce its curing temperature, reduce energy consumption, and expand its application range on non-metallic substrates, oversized metal parts, and metal parts with complex shapes. For example, the use of new low-temperature curing agents can reduce the curing temperature of powder coatings and reduce energy consumption. Special pretreatment processes have also been developed to improve the adhesion of powder coatings to non-metallic substrates.
In short, the coatings industry is at a crossroads of innovative development. Only by continuously breaking through technical bottlenecks and solving existing problems can we achieve sustainable development in an era of increasingly stringent environmental protection requirements and increasingly diverse market demands, and provide better, more environmentally friendly, high-performance coating products for many fields such as construction, industry, and transportation.
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