We often say that coatings are semi-finished products. Only when they are applied to different substrate surfaces to form a complete coating 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 vital 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 rupture, 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, under normal circumstances, 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
The solid content of water-based wood coatings is relatively low, 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 vibrate rapidly, producing a significant thermal effect, so that the temperature of the inside and 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-concentrating effect of microwaves on water heating, it will inevitably heat the water in the wood materials at the same time. When the microwave power cannot be accurately controlled, it is possible that the water in the wood materials will overheat and be lost in the form of water vapor, thereby destroying the stability of the wood materials and causing defects such as deformation and cracking of the wood materials. 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. This is called the migration of the base color. Due to the poor wettability and dispersibility of water-based resins to a considerable part of 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. 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 and moisture gradients, far-infrared drying is not suitable for drying thicker coatings. The thicker the coating, the more obvious the gradient. Infrared drying can only heat the areas that infrared rays can irradiate and cannot be used to dry three-dimensional workpieces with complex shapes. Infrared drying also consumes a lot of energy. Hot air drying technology mainly uses the principle of thermal convection to wrap furniture workpieces and heat them evenly in all directions, significantly accelerating the drying speed of water-based wood coatings. It is highly adaptable and is currently a widely used drying technology, and a relatively stable technology for achieving film formation of water-based wood coatings. Because all the air in the drying room needs to be heated, and the walls, floor, carrier, and workpiece coatings will absorb or lose heat, the heat loss in this mode is relatively large. Therefore, the convection flow direction of the hot air needs to be reasonably designed to ensure that the energy carried by the hot air is mainly transferred to the coating on the surface of the workpiece. Currently, there are three main combinations of water-based wood coating application processes: water-based undercoat and topcoat, solvent-based undercoat and water-based topcoat, and UV-cured undercoat and water-based topcoat. Water-based undercoat and topcoat refers to the use of water-based wood coatings for both the undercoat and topcoat. 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, their practical application is limited. Solvent-based undercoat and water-based topcoat refers to the use of solvent-based coatings (commonly PU) for the undercoat and water-based wood coatings for the topcoat. For fixed furniture products, this is currently a relatively stable combination of water-based wood coatings, and the paint film is relatively stable. However, due to the use of solvent-based coatings for the undercoat, VOC emissions are still high. UV (LED) cured undercoat and water-based topcoat refers to the use of UV-cured or LED-cured water-based wood coatings for the undercoat and water-based wood coatings for the topcoat. This is a process combination with promising prospects. It is currently mainly used for the coating of flat workpieces within a certain thickness range, mainly using 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, and 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.