The end of 2020 will be a different beginning. The rampant COVID-19 pandemic forced our country to hit the pause button, with various industries shutting down and people staying home, leading to an unprecedented level of health protection. People's awareness of environmental protection and health has also become stronger. As an environmentally friendly water-based paint that can replace harmful oil-based paints, it will surely continue to attract huge development opportunities.

We often say that coatings are semi-finished products. Only when they are applied to different substrate surfaces to form a complete coating film can their use value be realized. Excellent formula development and good production processes are the guarantee for obtaining high-quality coatings, and the coating construction process also plays a crucial role. The pre- and post-treatment of water-based wood coatings due to the characteristics of water must be carried out under certain environmental temperature conditions. This environmental temperature includes the workpiece surface temperature, air temperature, and coating temperature. In summer, excessively high environmental temperatures will cause the water-based wood coating to evaporate quickly and dry quickly, and bubbles will not have time to gather, enlarge, and then burst, making leveling difficult to control. Therefore, in summer, the air in the leveling section needs to be cooled. In winter, when the temperature drops below 10℃, the surface temperature of the workpiece will also decrease accordingly. If water-based wood coatings are applied to the workpiece at this time, it will have a great negative impact on the subsequent leveling and drying of the coating. Slow water evaporation, easy biting and wrinkling, and the low-temperature coating encounters higher air heating in the drying area, which will cause a large temperature difference between the surface and the inside of the coating, the surface of the coating dries quickly, locking the moisture inside the coating, causing the coating to be unable to completely dry for a long time. Therefore, in winter, when the ambient temperature drops below 15℃, the workpiece should be preheated. Similarly, appropriate insulation measures should be taken for the storage and use of water-based wood coatings to ensure that the coatings are stored and applied at a temperature of 15℃~20℃. Insulation during the construction process can be achieved using a water bath heating device. In order to improve the utilization rate of coating equipment, generally, after the coating is dried for 2-4 hours and the film is basically dried, it can be removed from the drying room and left to stand for 24-48 hours or more to achieve the effect of basically complete curing of the paint film. However, due to the particularity of water-based wood coatings, during this standing period, the ambient temperature and humidity should also be kept suitable, and it should not be placed directly in an open space. Drying methods for water-based wood coatings Water-based wood coatings have a low solid content, generally only 20%~30%, and more than 70% is water. Therefore, the drying process of water-based wood coatings is mainly the evaporation of water. Common heating methods for the film formation process of water-based wood coatings include microwave, far-infrared, and hot air. Microwave heating Microwave heating uses the principle of dielectric loss, and the dielectric constant of water is much larger than that of dry matter, and most of the energy released by the electromagnetic field is absorbed by the water in the coating. The microwave field changes the direction of the applied electric field rapidly and periodically, causing the water molecules to oscillate rapidly, producing a significant thermal effect, so that the temperature inside and on the surface of the coating increases rapidly at the same time. ▲Microwave dryer The advantages of microwave drying are that the drying speed is particularly fast, different substances have selective absorption of microwaves, there are no shape requirements for the dried objects, the heating of the coating is very even, there is no temperature gradient, thick films can be dried, and the energy utilization rate is high. However, microwave heating drying also has some disadvantages. ①It is difficult to accurately control the paint film temperature. Microwaves heat all the water in the microwave field, but the amount of water-based coating on the surface of the coated workpiece in the microwave field is uncertain, so the water content is also very different. Under the same microwave power, the temperature difference between workpieces with large surface area and workpieces with small surface area will also be large. Therefore, when using microwaves to heat water-based coatings, it is difficult to accurately control the paint film temperature, and it is also difficult to automatically match the microwave power and time according to the coating area. ②The influence on the moisture content of wood materials Generally, when the moisture content of wood materials is 8~12%, it is most suitable for various machining and the most stable performance. When using microwaves to heat and dry water-based wood coatings, due to the energy-gathering effect of microwaves on water heating, it will inevitably heat the water in the wood material at the same time. When the microwave power cannot be accurately controlled, it is possible that the water in the wood material will overheat and be lost in the form of water vapor, thereby destroying the stability of the wood material and causing defects such as deformation and cracking. ③The influence on other chemical substances in wood materials Generally, wood materials contain a large number of aliphatic, terpene, and phenolic substances. These chemical substances mostly exist in the form of hydrates in wood materials. When microwave heating is used, these chemical substances will also be heated at the same time. The heated chemical substances may migrate to the surface of the wood material, thereby affecting the adhesion between the coating and the wood material, and in severe cases, it may even directly lead to coating failure. ④The influence of temperature on the migration of base color The liquid topcoat will re-dissolve the rubbed base color into the topcoat, thereby affecting the transparency of the coating, which is called the migration of the base color. Due to the poor wettability and dispersibility of water-based resins to a considerable part of the pigments and fillers, floating color and layering often occur; after microwave heating, local overheating will cause serious migration of the base color, making the entire or partial coating become a “big flower face”. Infrared drying technology, that is, a drying technology in which the coating absorbs infrared radiation energy and converts it into heat to finally achieve curing and film formation. At present, far-infrared drying is usually selected for infrared drying technology, which can penetrate into the coating, increasing the internal temperature of the coating, and the internal temperature of the coating is higher than the surface, so that the heat diffusion of the coating occurs from the inside to the outside to form a temperature gradient. At the same time, there is also a moisture gradient inside the coating, which causes water migration, and the inside with higher water content gradually diffuses to the outside with lower water content, which is consistent with the direction of heat diffusion, thereby accelerating the drying process. Due to the existence of temperature gradient and moisture gradient, far-infrared drying is not suitable for drying thicker coatings, and the thicker the coating, the more obvious the gradient. Infrared drying can only heat the areas that can be irradiated by infrared rays, and cannot be used to dry three-dimensional workpieces with complex shapes, and infrared drying also consumes a lot of energy. Hot air drying technology mainly uses the principle of thermal convection, which can wrap the furniture workpiece and heat it uniformly in all directions, significantly accelerating the drying speed of water-based wood coatings, with strong adaptability, and is a widely used drying technology at present, and it is also a relatively stable drying technology for film formation of water-based wood coatings under current conditions. Because it is necessary to heat all the air in the drying room, and at the same time, the walls, ground, carrier, workpiece coating, etc. will absorb or dissipate heat energy, so the heat energy loss is relatively large in this mode, and the convection flow direction of hot air needs to be reasonably designed to make the energy carried by the hot air mainly transferred to the coating on the surface of the workpiece. At present, there are three main combinations of water-based wood coating coating processes, namely water bottom and water surface, oil bottom and water surface, and UV bottom and water surface. Water bottom and water surface Water bottom and water surface means that the base coat and topcoat both use water-based wood coatings. Because the current water-based wood coating primers are inferior to commonly used solvent-based coatings in terms of adhesion, wear resistance, and surface hardness, and the use of water-based primers has a significant negative impact on the moisture content of wood materials, therefore, it is rarely used in practice. Oil bottom and water surface Oil bottom and water surface means that the primer uses solvent-based coatings (commonly PU), and the topcoat uses water-based wood coatings. For fixed furniture products, this is currently a relatively stable combination of water-based wood coatings, and the paint film is relatively stable. However, since the primer uses solvent-based coatings, the VOC emission is still very high. UV (LED) bottom and water surface UV bottom and water surface means that the primer is UV-cured or LED-cured water-based wood coating, and the topcoat is water-based wood coating. This is a coating process combination with relatively promising prospects. At present, it is mainly used for the coating of flat workpieces with a certain thickness range, and mainly uses automatic reciprocating spraying equipment for coating. Research on water-based wood coatings has never stopped, and there are still many problems to be solved, such as the influence of chemical substances in natural water, the influence of vacuum environment on film formation, what is the ideal film thickness, etc. The research work is arduous and long, but with the increasingly severe environmental situation, the promotion of water-based environmentally friendly coatings is imperative.

Relevant experts indicate that water-based paints are currently the VOCs source control measure that can be actively promoted. Oil-based paints have a VOCs content of 50%~70%, while water-based paints have a low VOCs content, capable of reducing VOCs from the raw material end, with higher effectiveness in raw material changes. VOCs control focuses on key regions and industries, involving key sectors such as petrochemicals, organic chemicals, surface coating, and packaging printing. The paint and ink industries have numerous scattered and polluting enterprises, and meeting environmental compliance has become a matter of survival for enterprises, necessitating the use of water-based products with low (or no) VOCs content as raw material substitutes. Waterborne polyurethane can provide downstream industries with water-based products with low (or no) VOCs content. Han Baole, deputy secretary-general of the China Polyurethane Industry Association, once revealed that waterborne polyurethane, using water as a solvent, has advantages such as being pollution-free, safe and reliable, having excellent mechanical properties, good compatibility, and being easy to modify. It can be widely used in coatings, adhesives, fabric coatings and finishing agents, leather finishing agents, paper surface treatment agents, fiber surface treatment agents, etc., and is a safe, environmentally friendly, green, high-performance polymer material. Producing 1 ton of waterborne polyurethane dispersion can replace solvent-based polyurethane, reducing 0.7 tons of organic solvent volatilization into the atmosphere. Its development is valued by governments worldwide. Is the water-based change smooth? Currently, the implementation of "oil-to-water" is not ideal. Liao, president of the Hunan Provincial Coatings Industry Association, introduced that the biggest bottleneck in the current implementation of "oil-to-water" lies in the limitations of water-based paint technology, especially in the fields of water-based industrial paints and anti-corrosion paints. Water-based paints applied indoors will not have a significant impact on the coating effect, but if applied outdoors and subjected to sun and rain, especially in areas with severe environmental pollution, the performance of water-based paints is not ideal, such as in film formation, hardness, adhesion, gloss, and stability. Currently, the global water-based rate is not very high. Although many large enterprises have introduced water-based paint projects, they still sell oil-based paints. Dr. Cui Yanjun, assistant director of the Shanghai Huafeng Materials Science and Technology Research Institute, told reporters that oil-based materials have been used for more than 30 years, while water-based materials have only been developed for more than ten years, and the supporting facilities are not yet mature. For synthetic leather enterprises, the cost of "oil-to-water" will increase by 3%, which is not an easy choice for an industry with already thin profit margins. In addition, switching to water-based materials also faces problems such as production process adjustments and reduced production efficiency. Lü Guohui, secretary-general of the China Polyurethane Industry Association, pointed out that China has become the largest producer and consumer of polyurethane and is expected to become the largest consumer base. However, the industry's innovation capacity is insufficient, a large amount of technology is in the follower stage, the industrial structure is not entirely reasonable, the concentration is relatively low, there is serious homogeneity, there is a surplus of low-end products, and there is a shortage of high-end products. He pointed out that the application of polyurethane products should shift from demand consumption to quality consumption, which is verified in the field of waterborne polyurethane. After 2015, the innovation and application of waterborne polyurethane have entered multiple fields and are currently in a period of rapid development. Zhang Haoming, chairman of Ausjia Materials Technology Co., Ltd. and director of the Waterborne Materials Professional Committee of the China Polyurethane Industry Association, expressed the hope that society will pay attention to the water-based conversion of polyurethane materials and guide waterborne polyurethane to become a safe, environmentally friendly, and competitive industry. Professor Fan Hong from the College of Chemical and Biological Engineering, Zhejiang University, pointed out that the market share of water-based coatings in countries such as Europe and the United States is above 60%, while in China it is less than 20%. The performance of water-based coating products internationally has been significantly improved, and some have approached or exceeded solvent-based products. In 2016, the global waterborne polyurethane market exceeded US$42 billion, but the domestic technological development level still needs further improvement.