In the field of furniture manufacturing and decoration, the drying process of water-based paint coatings is a key link. It is not only a prerequisite for ensuring that the quality of the finish meets standards, but also an important process that is indispensable throughout the entire finishing process. Its impact on production costs and work efficiency should not be underestimated.

Historically, the neglect of coating drying conditions in early furniture finishing processes has led to a large number of product quality problems. For example, in some wooden furniture custom-made by European aristocrats in the Middle Ages, due to the unknown drying principle of water-based paints at that time, many of the furniture developed defects such as poor gloss, a rough surface similar to orange peel, and pinholes shortly after use. In severe cases, internal stress developed within the paint film, greatly reducing adhesion. In subsequent use, these pieces of furniture have cracks, losing their protective and decorative functions, and their appearance and durability have been greatly reduced. This profoundly reflects the enormous impact of the drying conditions of the coating on the quality of the final product.

Coating drying is a process that is not only frequently repeated in production, but also extremely time-consuming. In the context of modern industrial mass production, how to speed up this process has become the key to shortening production cycles and saving production space. It is also a technical bottleneck that must be broken through to achieve continuous construction and automation. This involves complex scientific principles and the optimization of production processes. For example, the “process optimization theory” in modern production management emphasizes that improving the efficiency of key processes such as coating drying has a multiplier effect on improving the efficiency of the entire production system.

This shows that in-depth research into the drying of water-based paint coatings is of great significance for ensuring the quality of the finish and improving the efficiency of the finish. This is an important way to improve drying efficiency and an important topic for promoting the development of the furniture manufacturing industry, with extremely high application value. In particular, water-based paints for wooden furniture use water as a medium, with a very low solid content, usually only 20% – 30%, and a high moisture content of about 70%. This means that the drying process is mainly a water evaporation process. Below we will take a closer look at the film formation process of water-based paints and the water evaporation process.

Film formation process of water-based paints for furniture

The film formation process of water-based paints is a subtle phenomenon of molecular chain condensation, which can be roughly divided into three steps: water evaporation, particle deformation and particle coalescence.

Once the application is complete, the first stage is the evaporation of the water. According to research in the field of paint chemistry, when the volume of the latex particles in the aqueous coating layer reaches 74%, the particles move closer together and enter a dense, packed state. This is like a group of small particles arranged closely together, with the distance between them gradually decreasing. Then, as the water continues to evaporate, the polymer particles begin to deform. At this point, the capillary pressure exceeds the deformation resistance of the polymer particles, and pressure builds up between the particles. The more the medium evaporates, the greater the pressure becomes, and the particles then coalesce and merge to form a continuous coating film. This process is like tiny building blocks gradually being joined together under pressure. Finally, as the water continues to evaporate, the polymer chains gradually diffuse into each other, level by level, when the pressure is sufficient to cause the molecular chains in each particle to spread into the molecular chains of another particle. This process is like a fine mesh gradually being woven together, with every link closely connected and indispensable.

Research on the water evaporation process

In-depth theoretical analysis shows that the moisture evaporation process of water-based paint follows the vertical drying theory from the surface to the inside. This theory holds that the evaporation of moisture during the formation of the film of water-based paint can be divided into three stages.

In the first stage, the resin particles undergo random Brownian motion, and the water evaporates at the same rate as in pure water, along the water-air interface. This is like a lively dance floor, where people (water molecules) dance freely (Brownian motion) and gradually move away (evaporation). In the second stage, the medium in which the resin particles are suspended decreases, and the resin particles move closer together, accumulate and agglomerate, just like a crowd begins to gather together. This causes the water-air interface to shrink, the total area to decrease, and the rate of water evaporation to rapidly decrease. In the final stage, the rate of water evaporation continues to decrease, and the remaining water needs to diffuse through the capillaries in the membrane to the film-air surface to evaporate. The whole process develops from the surface layer of the resin particles to the inside, and the rate of moisture evaporation decreases due to the continuous reduction of moisture and the change of transfer mode from the gap between particles to the transfer through the polymer. This series of changes is like a well-choreographed dance, with each movement having its own specific rhythm and logic.

Drying method

The drying process of wood is a process of gradually removing the internal moisture of the wood, and the drying of furniture coated with water-based paint is also, to a large extent, a process of gradually removing the moisture from the interior of the coating. In a sense, there are similarities between the two. Analogy is a common research method in modern materials science. Therefore, when exploring the drying of water-based paint coatings on wooden furniture, we can draw on the relatively mature wood drying methods. In the field of wood drying, methods such as natural drying, hot air drying, microwave drying, ultraviolet drying, infrared drying, solar drying, high-frequency drying, and combined drying are all theoretically applicable to the drying of water-based paint coatings. However, each drying method needs to be studied in depth for its specific technical parameters when applied to the drying of water-based coatings.

In fact, the main factors affecting the drying speed of water-based wood coatings include ambient temperature, relative humidity, film thickness, and air flow velocity. Just as environmental factors affect the growth of living things, these factors interact with each other to affect the drying process. Wu Zhongyue and other experts pointed out in their research that the use of water-based wood coatings for line painting not only speeds up drying, but also improves film hardness, reduces VOCs, and effectively avoids a series of problems caused by “slow drying”. Cai Jiabin and other scholars conducted research on the application of microwave drying in polyurethane water-based wood coatings. The results showed that after microwave drying, the moisture removal rate in the water-based coating film is as high as 90%, while in the same time, oven drying can only remove about 50% of the total moisture. Moreover, the coating film after microwave drying can immediately reach a dry state, and can be sanded, stacked and packaged. These research results provide a strong basis for us to choose the appropriate drying method.

The drying speed and film-forming quality of water-based coatings vary significantly depending on the drying method. The following section provides a detailed description of the various drying methods for water-based coatings.

Natural drying

The latent heat of evaporation of water is as high as 2457.7KJ/Kg, and the evaporation of water in a water-based coating requires the absorption of a large amount of heat. This characteristic determines that the evaporation of water is affected by the atmospheric temperature, humidity and wind speed. From a physical point of view, when the temperature rises, the thermal motion of molecules intensifies, and liquid water molecules are more likely to escape the attraction of other water molecules and become steam molecules. Therefore, the evaporation rate increases with the increase of temperature. On the contrary, as the air humidity increases, the proportion of water vapor in the air increases, and the partial pressure of water vapor increases, which will make more water vapor molecules turn back into liquid water molecules, thus reducing the evaporation rate of water. The effect of wind speed is that the greater the wind speed, the more water molecules are removed from the surface of the evaporator per unit time, and the faster the evaporation rate.

Although natural drying is a simple and widely used method, its obvious drawback is its slow drying speed. In the natural environment, the temperature, humidity and wind speed are constantly changing, which leads to unstable drying speed and film formation quality. For example, in high temperature and humidity or high humidity conditions, the coating is prone to whitening and the drying speed is significantly reduced. In low-temperature environments, the drying speed is even slower, and at 5°C in particular, water-based paints can hardly form a film at all. These problems have largely limited the application of water-based paints. It is like growing crops in changing weather conditions, where unstable environmental factors can affect the harvest. The instability of natural drying hinders the application of water-based paints.

Hot air drying

Hot air drying is a heating drying method that relies on the principle of convection. Hot air at a temperature of 40–60°C is used as a heat carrier to transfer heat energy to the coating on the surface of the workpiece. The coating absorbs the energy and then cures and forms a film. Usually electricity or steam is used as the heat source to heat the air first, and then the heat is transferred to the surface of the coating by convection, so that the coating dries quickly.

In this process, the hot air surrounding the coating acts as the heating medium. Since the coating has a certain thickness, it takes time for the heat to transfer from the surface of the coating to the inner boundary. The heat transfer rate depends on the thickness of the coating and its thermal conductivity. Therefore, when convection heating, the surface of the coating is always heated first. During the initial drying phase, the surface layer evaporates the most moisture, and the coating cures from the surface layer, gradually spreading to the underlying layers, which dry last. This drying method can significantly speed up the drying of the coating and is highly adaptable. It is currently one of the most widely used drying methods. In large furniture factories, hot air drying lines were originally the main form of forced drying for solvent-based coatings. When applied to the drying of water-based coatings, due to the long drying time of water-based coatings, on the one hand, the drying equipment needs to be studied in a targeted manner, and on the other hand, the drying process needs to be optimized. Factors such as the temperature, humidity, speed of the workpiece (drying time) and ventilation conditions at various points along the drying line are crucial to ensuring the drying quality and the final quality of the paint film. It is like a complex cooking process, where the adjustment of each parameter can affect the taste (quality) of the final dish (paint film quality).

Microwave drying

Microwaves are penetrating electromagnetic waves with wavelengths of 1 mm to 1 m and frequencies of 300 MHz to 300 GHz. The most commonly used microwave frequencies are 915 MHz to 2450 MHz. Microwave heating uses the dielectric loss principle. The dielectric constant of water is much higher than that of dry matter, so most of the energy released by the electromagnetic field is absorbed by the water in the paint.

The microwave field periodically changes the direction of the applied electric field at a high speed of several hundred million times per second, causing the water molecules to swing rapidly and producing a significant thermal effect, thereby rapidly increasing the temperature inside and on the surface of the coating at the same time. The advantages of microwave heating are outstanding: extremely fast drying speed, selective absorption of microwaves by different substances, no shape requirements for the object being dried, very even heating of the coating film, no temperature gradient, and the ability to dry thick films. Microwaves with a frequency of 2450 MHz have the best energy conversion and loss balance for water molecules. Microwaves at this frequency can penetrate a 30 mm thick layer of water and can be used to dry water-based paint coatings of various thicknesses. It is as if countless miniature heaters have been installed inside the paint, evenly and efficiently completing the drying process.

UV drying

For water-based UV wood coatings, UV curing can be used, i.e. the water-based UV wood coating is cured by ultraviolet light with a wavelength of 300 – 400 nm. Water-based UV coatings contain a small amount of photosensitizer. When exposed to ultraviolet light, the photosensitizer decomposes after absorbing ultraviolet light of a specific wavelength, producing active radicals that initiate the polymerization reaction of the film-forming material to form a network structure, thereby curing the coating. UV curing has the advantages of fast curing and good coating quality. However, this method has its limitations: it can only be used to dry water-based UV coatings and only flat furniture panels can be dried. It is like a special key that can only open a specific lock (drying a specific type of coating and shape of object).

Infrared drying

Infrared curing can be used to dry water-based wood coatings. Specifically, the coated furniture panels and their coatings are exposed to infrared light, which absorbs radiant energy and converts it into heat, thereby curing the coating. Infrared is an invisible ray between visible light and microwaves with a wavelength of 0.72 – 1000um. It can be divided into “near”, “medium” and “far” infrared according to the wavelength range. Far infrared is commonly used to dry coatings.

Infrared curing has the advantages of fast curing speed, rapid temperature rise and good curing quality. However, when using infrared heating to dry coatings, there is a significant temperature gradient in the coating, and the drying extends from the surface to the inside, which makes it unsuitable for drying thicker coated films. Moreover, infrared drying can only heat the area that can be irradiated by infrared rays, and cannot be used to dry three-dimensional objects. This is similar to sunlight shining on an object, which only illuminates the surface, and has no effect on the inside and shadowed parts. Infrared drying also has similar limitations.