Coating Far infrared drying method

1. Infrared

The light visible to the human eye is called visible light, which is light with a wavelength in the range of 350-750nm. According to the wavelength, it is subdivided into seven bands: red, orange, yellow, green, blue-green, blue, and purple. Among them, the longest wavelength is red light, and the shortest wavelength is violet light. Among the rays that cannot be seen by the human eye, the part with a wavelength longer than red light is called infrared. When absorbed by an object, it can often stimulate a chemical reaction of the substance. Ultraviolet rays and infrared rays are divided into several bands according to the wavelength range. For example, those with wavelengths below 4000nm are called near-infrared rays, and those with wavelengths above 4000nm are called mid- and far-infrared rays. Although all substances have the ability to absorb infrared rays, the wavelength range of infrared rays most easily absorbed by each material (called the photosensitive region) is different. Generally, the infrared absorption sensitive area of ​​the coating film is in the far-infrared band of 3~10μm, so the drying effect of the coating film is better by using the far-infrared radiation heating method.

2. Far-infrared radiation material

Far-red radiation drying uses monochromatic light with a high emissivity and a radiation peak in the far-infrared region. Inorganic materials with high emissivity include metal oxides such as chromium, cobalt, aluminum, aluminum, titanium, and aluminum, carbides of chromium, silicon, and titanium, and some borides. These materials are mixed and ground in proportion (sometimes sintered) to make infrared radiation paint, and then this kind of paint is coated on the radiator, put into the oven, and the radiator is heated by electric energy, gas and other heat sources, it It can emit infrared radiation and dry the coating film.

3. Advantages of far infrared radiation drying

After the substance absorbs infrared radiation, the radiant energy will be converted into heat energy, which will increase its own temperature. Since different substances have different sensitive absorption ranges for infrared radiation, it is necessary to control the infrared wavelength range emitted by the infrared radiator in the far infrared range. It is conducive to the absorption of the coating film, but the substrate is not easy to absorb, and the far infrared rays reflected by the substrate can be absorbed again when passing through the paint film. The difference between far-infrared radiation drying and general heating drying is that it only makes the coating absorb infrared energy to heat up and cure, without heating the coated workpiece, so that the energy use efficiency can be greatly improved and energy consumption can be reduced. . Moreover, when far-infrared radiation is cured, the coating film is cured in the order from the inside to the outside. The solvent in the inner coating film is evaporated first. When the inner coating film is cured, the outer layer is still wet. At this time, there is no solvent in the entire coating film, so it will not be like when heating and drying, because the outer coating film is first cured by heat, and as the curing progresses, the solvent in the inner coating film needs to pass through the partially cured part. Only when the outer coating film is evaporated, there will be disadvantages such as orange peel and pinholes.

Since the surface of the coating film formed by curing with infrared radiation is flat and bright, while the surface of the coating film dried by heat is often rough, this advantage is more obvious in industries such as furniture, automobile painting, and repairing that require high decorative properties. .

4. Types of Far Infrared Radiators

(1) Electrothermal infrared radiator The structure of various infrared radiators heated by electric energy is shown in Figure 1-12.

Electrothermal far-infrared radiators can be divided into three types: side heating type, direct heating type and semiconductor type. The side heating type can be divided into three types according to different shapes: tubular, lamp-shaped and plate-shaped. The tubular shape is made by coating far-infrared radiation material on the outside of nickel-chromium resistance wire. The electrothermal far-infrared radiator transmits the heat energy generated by electricity to the infrared radiation layer through direct or indirect means and emits infrared rays outward. It has the advantages of simple structure, convenient use, and clean use of electric energy, but it consumes more electricity. , suitable for drying small and medium-sized workpieces with simple shape and uniform film thickness.

(2) Gas-fired far-infrared radiators are far-infrared radiators heated by gas (or gas), which use the high temperature generated during combustion of gas (gas) to heat ceramic or metal substrates and the far-infrared radiation coatings on their surfaces and make them It radiates far infrared rays. There are two ways of heating: direct heating and heating by flowing high-heat flue gas in the radiator. The new gas-fired far-infrared radiator currently used is a catalytic thermal reaction radiator (also known as a low-temperature combustion radiator flameless burner) that uses natural gas or liquefied petroleum gas as energy, and is a more economical design. The structure of a new flameless far-infrared radiator heated by gas is shown in Figure 1-13.

In the reactor there is a special catalyst plate, which catalyzes the oxidation reaction between gas and oxygen and continuously converts the heat energy generated into infrared radiation. Since the temperature during combustion is around 500-600°C, the radiation is mainly far-infrared rays, and there is no open flame during the reaction. When the ratio of oxygen to gas is appropriate and the temperature is appropriate, the catalyst can catalyze the oxidation reaction. A resistance wire coil is arranged inside the catalyst plate, which can heat the catalyst plate to a desired temperature and control the temperature of the catalyst plate. It only needs to be powered on for a few minutes to heat the catalyst plate to the temperature required for the oxidation reaction, and the power will be automatically turned off to stop working. At this time, the solenoid valve controlling the gas is opened to make the gas reach the catalytic plate, and be catalyzed and oxidized, and the heat generated is converted into infrared radiation for curing the coating. This radiant unit features no spark plug igniter and no open flame. The whole device can be automatically controlled. When a fault occurs and the catalytic plate cannot reach the temperature required for the oxidation reaction, the internal control system will automatically shut down the reactor and stop the gas from entering. When the electrical system fails, the entire system will automatically shut down.

The gas flow is determined by the nozzle diameter of the air inlet and the gas supply pressure, and the oxygen required for the thermal oxidation reaction comes from the air. In order to ensure that the air entering the catalytic plate is clean and will not pollute and poison the catalyst, the air must be carefully filtered and purified before entering, and sent to the vicinity of the catalytic plate by a forced air supply device such as a fan. In order to ensure the cleanliness of the air filter, it is necessary to clean or replace the filter element frequently. Air is mixed with gas in the venturi tube and then enters the catalytic reactor. The biggest advantage of using this catalytic oxidation infrared radiator is safety. During the reaction process, even if the solvent vapor volatilized from the coating film enters the reactor, it will not cause combustion, because the catalytic oxidation reaction is extremely fast, and the volatile solvent has been catalyzed and oxidized before burning. The use of this new type of gas far-infrared radiator can save 90% of fuel compared with the traditional gas heating method. Since the far-infrared ray generated by this method has a narrow wavelength range, it only heats the coating film and the workpiece will not be too hot. It is convenient for workers to operate, and the curing temperature is easy to control, which will not cause excessive curing, nor will it cause discoloration of the paint film. When using it to dry the wet film of solvent-based coatings, the solvent can be effectively removed without causing orange peel, pinholes and other disadvantages. It can not only dry the coating film on the metal plate, but also suitable for drying the surface coating on textiles, leather products and other materials that are not resistant to high temperature.