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    Home > Coatings News > Paints and Coatings Market > A common drying method for applying water-based coatings to furniture.

    A common drying method for applying water-based coatings to furniture.

    • Last Update: 2020-10-09
    • Source: Internet
    • Author: User
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    Coating drying of water-based coatings is the need to ensure the quality of coatings, is an indispensable process in the entire coating process, directly affecting production costs and efficiency.
    coating is dry, it has a great impact on the quality of the final product. If the coating drying is not good, the paint film will have poor gloss, orange peel, pinholes and other defects, paint film will produce serious internal stress, reduce the
    adhesion of the paint film
    ;
    , coating drying is a frequent and time-consuming process. Therefore, in modern production, how to speed up coating drying is not only related to shortening the production cycle, saving production area, but also to achieve construction continuity and automation needs to solve the key technical problems.
    refore, the study of water-based coating drying methods is of great significance to ensure the quality of coatings and accelerate the efficiency of coating construction, as well as an important way to improve the drying efficiency of coatings, or an important issue in the development of production, which has important application value.
    paint used in wood furniture is a water-based coating. The solid content of water-based coatings is very low, generally only 20%-30%, water content is about 70%. Therefore, the most important thing to pay attention to in the drying process of water-based coatings is the evaporation of water.
    the next step is to introduce the film-forming process of water-based coatings and the process of water evaporation.
    PART
    1
    The film-forming process of applying furniture to water-based coatings
    the film-forming process of water-based coatings is a molecular chain condensation phenomenon, which can generally be divided into three steps: water volatilization, particle deformation, particle merger.
    the construction of the water, the first is the evaporating of water. When the latex particles of water-based coatings account for 74% (volume) of the adhesive layer, the particles are close to each other and reach a dense filling state; The capillary pressure is higher than the anti-deformation force of the polymer particles. The more volatile the medium, the greater the pressure. The polymer particles then condense and fuse to form a continuous coating; When the pressure reaches to spread the molecular chain in each particle to another particle molecular chain, the polymer molecular chain gradually spreads to each other, so that the coating film is uniform and the curing and drying process is completed.
    part
    2
    to study the process of water evaporation
    In theory, this is a water-based coating evaporation process from surface to interior in line with the vertical drying theory. In the first stage, the resin particles have irregular Brown motion, the water evaporation rate on the water-air interface is the same as in pure water, after entering the second stage, the suspension medium of the resin particles is reduced, so that the resin particles close to each other, aggregation, condensation, resulting in the water-air interface shrinkage, reduce the total area of the water-air interface, and rapidly reduce the evaporation rate of water;
    the whole process develops from the surface of the resin particles to the inside, the evaporation rate of water decreases with the decrease of the moisture content, and the moisture content shifts from the gap between the particles to the polymer transfer.
    PART
    3
    Drying Mode
    The drying process of wood is the process of gradually removing moisture from the inside of the wood, while the drying process of the water-based coating used in furniture is largely the process of gradually removing moisture from the inside of the coating. In a sense, there are some similarities between the two. Therefore, in the further study of wood furniture water-based coating drying methods, we can learn from the more mature drying methods. In the field of wood drying, natural drying, hot air drying, microwave drying, ultraviolet drying, infrared drying, solar drying, high frequency drying, combined drying and other drying methods are theoretically suitable for the drying of water-based coatings, but various drying methods in water-based coating drying applications in the specific technical parameters still need to be studied.
    main factors affecting the drying speed of water-based coating wood furniture are ambient temperature, relative humidity, paint film thickness and airflow speed. Wu Zhongyue and others believe that water-based wood coatings can speed up the drying speed, improve the hardness of the coating, reduce VOC, to avoid the "slow drying" brought about by many disadvantages. Cai Jiabin, etc. The application of microwave drying in polyurethane water-based wood coating was studied. The results showed that 90% of the moisture in the coating was removed after microwave drying, while the dry water removal rate of the oven was only about 50%. After microwave drying, the coating can be dried, polished, stacked and packaged.
    different drying methods have a significant effect on the drying speed and film quality of water-based coatings. Below is an overview of the various drying methods for water-based coatings.
    naturally dry
    the evaporation potential heat of water is very high, reaching 2457.7kj/kg. Evaporation of water-based coatings requires the absorption of large amounts of heat. The volatility of water is affected by atmospheric temperature, humidity and wind speed. The rate of volatility increases with the increase of temperature, decreases with the increase of air humidity, and increases with the increase of wind speed. This is because the temperature rises, the molecules move more vigorously, and liquid water molecules are more likely to escape the attraction of other water molecules and become water vapor molecules. However, with the increase of air humidity, the proportion of water vapor in the air increases, the sub-pressure of water vapor increases, and more water vapor molecules become liquid water molecules per unit of time. As a result, the evaporation rate of water decreases with the increase of air humidity. The higher the wind speed, the faster the evaporation of the water molecules removed from the volatile surface in unit time.
    natural drying has the advantages of simple method and wide application, but it also has the disadvantage of slow drying speed. Under natural conditions, temperature, humidity and wind speed are constantly changing, and the drying speed and film quality are unstable. If the temperature is high, humidity is high or humidity is high, the coating tends to turn white and the drying speed is slow. At low temperatures, the drying rate is slow, especially at 5 degrees C, water-based coatings are difficult to film. These are important reasons that restrict the application of water-based coatings.
    there air drying
    hot air drying is based on 40-60 degrees C of hot air as a heat carrier, the heat energy is transmitted to the surface of the workware coating, the coating absorbs energy and cures into a membrane of a heating and drying method. Generally electric or steam as a heat source, first heat air, through the flow of hot air to the surface of the coating, so that the coating quickly dry.
    dry with hot air, the hot air around the coating is the heating medium. The coating always has a certain thickness. Heat transmission from the coating surface to the inner boundary takes some time. The speed of heat transmission depends on the thickness of the coating and its thermal conductivity. Therefore, when conflow heating, the coating surface is always heated first. In the early stages of drying, the surface moisture is most volatile, and the curing of the coating begins at the surface and then gradually expands to the bottom layer, causing the bottom layer to finally dry.
    application of hot air drying technology can obviously speed up the drying speed of coatings, with adaptable characteristics. It is a widely used form of drying. In large furniture factories, hot air drying line is the main form of forced drying of solvent-based coatings. It is used to dry water-based coatings. Because of the long drying time of water-based coatings, it is necessary to study the drying equipment on the one hand, and the drying process on the other. The temperature, humidity, operating speed (drying time) and ventilation conditions of the various parts of the drying line will affect the drying quality and the final coating filming is an important key.
    microwave
    microwave refers to the wavelength of 1mm-1m, the frequency of 300mhz-300ghz, with penetrating electromagnetic waves. The commonly used microwave frequency is 915-2450mhz.
    microwave heating using the principle of medium loss, the dielectric constant of water is much larger than the dielectric constant of dry matter. Most of the energy released by the electromagnetic field is absorbed by the water in the coating.
    microwave field periodically changes the direction of the applied electric field at a high speed of hundreds of millions of times per second, making the water molecules swing rapidly, producing a significant thermal effect, so that the temperature inside and outside the coating rises rapidly at the same time. The advantages of microwave heating are fast drying speed, different substances have selective absorption of microwaves, no shape requirements for drying materials, film heating is very uniform, no temperature gradient, thick film can be dried. For water molecules, the 2450MHz microwave has the best energy conversion and loss balance. Microwaves at this frequency penetrate 30mm thick layers of water and can be used to dry water-based coatings of various thicknesses.
    UV
    for water-based coatings, UV-cured drying can be used. The so-called UV curing refers to the water-based coating UV in the wavelength of 300-400nm of ultraviolet radiation curing drying method.
    In water-based coatings contains a small amount of photosensitive agent, in the ultraviolet radiation, photosensitive agent absorbs a specific wavelength of ultraviolet light, decomposition produces an active group, triggering a polymerization reaction of film-forming substances, forming a mesh structure and making the coating cured. Ultraviolet curing has the characteristics of fast coating curing speed and good film quality. But. This method can only be used against water-based coating coatings, and can only be dried into flat-panel furniture coating plates.
    infrared drying
    infrared curing can be used for the drying of water-based wood coatings. The so-called infrared curing, in fact, refers to the furniture panel and its coating in the infrared radiation exposure, absorb radiation energy and converted into heat energy, in order to achieve the curing of the coating.
    infrared is an invisible light between visible light and microwaves. The wavelength is 0.72-1000um. According to the wavelength range can be divided into "near", "medium" and "far" three kinds of infrared, often used for coating drying.
    infrared curing has the advantages of fast curing speed, fast heating and good curing quality. However, when using infrared heating drying coating, there is a clear temperature gradient, its drying extends from the surface to the inside, is not suitable for drying thick coating;
    .
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