The atoms and molecules that make up matter are in thermal motion, and their energy state is changed from time to time.When the energy state transitions from higher to lower levels, electromagnetic waves will be emitted, and the energy will be taken away in the form of photons.In our daily life, we encounter various types of radiation, such as solar rays, thermal radiation, radio waves and x-rays. Although they are different in form, they are the same in nature, collectively called electromagnetic radiation.All radiation obeys the same laws of reflection, refraction, diffraction and polarization, as well as the same propagation speed, which is called "speed of light". The difference between them is only the frequency.There are several ways to generate infrared ray, the most common is through substancesMolecular thermal motionThis is thermal radiation.[1]
Infrared is a kind of electromagnetic wave, with a wavelength range of 0.76-1000μm. Generally, the infrared ray is divided into two parts, and the wavelength is less than 5.6μm. Near red light is called near-infrared light;Wavelength greater than 5.6μm. The one far from the red light is called far infrared.The division of near-infrared and far-infrared is relative. Some people also divide infrared into near-infrared, mid infrared and far-infrared.Wavelength 1-3μM is called near infrared ray;Wavelength: 2-40μM is called mid infrared ray;Wavelength: 40~1000μM is called far infrared ray.
The generation of infrared ray is closely related to temperature.All objects in nature will radiate infrared when their temperature is higher than absolute zero (- 273.15 ℃).The size and distribution of its radiant energy by wavelength are determined by the surface temperature of the object.The radiant energy of the object surface is proportional to the fourth power of the object surface temperature.
Once the infrared ray is absorbed by the object, the infrared radiation energy will be converted into heat energy, heating the object to raise its temperature.When the electromagnetic wave (i.e. infrared) generated by the infrared radiator directly propagates to the surface of an object at the speed of light, and its emission frequency matches the natural frequency of the movement of the object's molecules, it will cause strong vibration of the object's molecules and generate heat due to intense friction inside the object.Therefore, infrared radiation is often called thermal radiation line, and infrared radiation is called thermal radiation or temperature radiation.[2]
Drying skills
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According to the above properties of infrared ray, when the infrared ray radiation coating is used, the coating can be heated to accelerate drying.When a beam of infrared ray irradiates the coating surface, part of it is reflected by the coating surface, part of it enters the coating interior and is absorbed by the coating, which is converted into heat energy to heat the coating from the interior of the coating, and part of it passes through the coating to the surface and interior of the substrate, and is converted from radiant energy to heat energy to heat the coating from below the coating.Because of this self heating effect, the coating can be heated quickly and effectively. The curing of the coating is carried out from inside to outside and from bottom to top. The drying process is similar to preheating drying, and the drying effect is good.See the Schematic Diagram of Infrared Radiation, Reflection, Absorption and Transmission for details.
Schematic diagram of infrared radiation, reflection, absorption and transmission
When near infrared radiation is applied to the coating surface, about 10% of its radiation energy is absorbed by the coating, about 30% is reflected by the coating surface, and about 60% is absorbed by the substrate through the coating, converted into heat energy, and then the coating is heated from below the coating.When far-infrared radiation is used to radiate the coating surface, about 50% of the radiation energy is absorbed by the coating. The reflectivity of the coating surface to the infrared ray is very low, less than 5%. The remaining 45% is absorbed by the substrate, converted into heat energy, and the coating is heated from below the coating.
The coating can well absorb infrared radiation energy because the oscillating wave potential of these organic macromolecule substances is 3~10μm. Yes, 3~50μThe far-infrared ray of m can be well absorbed. Because the infrared ray frequency of radiation matches the molecular oscillation frequency of the coating polymer material, causing the intense molecular resonance phenomenon of the coating polymer, the coating interior is heated rapidly and evenly, the heating speed is fast, and the effect is good. Therefore, the effect of drying the coating with far-infrared ray is higher and better than that with near-infrared ray.In far-infrared drying, due to the continuous evaporation and heat absorption of the solvent on the coating surface, the temperature of the coating surface decreases, resulting in the internal temperature being higher than the surface temperature, which is more conducive to the volatilization of the solvent, thus improving the quality of the paint film.Except photosensitive coatings and electron beam curing coatings, almost all coatings can be dried by far infrared heating.[2]
Drying characteristics
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Characteristics of infrared radiation drying
1. Fast drying speed and high production efficiency.Compared with hot air drying, the drying time can be shortened by 3~5 times.It is especially suitable for heating and drying large area of surface layer.
2. Good drying quality.In the process of infrared radiation, part of the infrared ray is absorbed by the coating, and the other part passes through the coating to the surface of the substrate, generating heat exchange between the surface of the substrate and the bottom of the coating, so that the direction of heat conduction is consistent with the direction of solvent evaporation.In this way, not only the heating speed is fast, but also the pinholes, bubbles, "orange peel" and other defects in the drying process are avoided.In addition, infrared drying does not require a lot of circulating air flow, so there is less flying dust, the coating surface is clean, and the drying quality is good.
3. Rapid temperature rise and high thermal efficiency.Radiation drying does not need intermediate media and can be directly transferred from heat source to coating, so the temperature rises rapidly.It has no heat consumption caused by the intermediate medium and reduces the heat carried away by some hot air, so it has high thermal efficiency.
4. Compact equipment and flexible use.Due to the short drying time of infrared radiation, the equipment is short in length and occupies a small area.The specific heat air drying equipment is simple and compact in structure, which is convenient for construction and installation.It is flexible to use, simple to operate, and convenient to adjust temperature with transformer.
5. There are certain requirements for workpiece shape.Due to the linear propagation of infrared ray, the coating is difficult to dry in some places that cannot be irradiated.The arrangement of radiators shall be considered, especially the design of reflector plate, and the irradiation efficiency must be improved as far as possible.For workpieces with complex geometric shapes, the exposure shadow is serious, and it is difficult to control that the exposure distance is roughly equal, which may cause discoloration of the paint film on the surface of workpieces with close radiation distance, and the phenomenon that the far or shaded parts are not completely dry.It is difficult to guarantee the drying quality of complex workpieces.
6. Due to the rapid temperature rise of the coating and the curing of the coating in a short time (20~30min), sometimes the solvent is too late to evaporate, which also affects the film quality, so it should be controlled.
7. If the temperature is too high, the paint film may change color and become brittle. The light red paint film is often more prone to change color.[2]
Desiccation Room
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Used in productionInfrared dryingWhen coating, a certain number of infrared radiators are usually assembled into a through drying chamber.The coated parts or products are carried by the conveyor and passed through the drying chamber to solidify the coating.
Far infrared radiation drying is an earlier radiation drying method, which is superior to hot air drying in many aspects, but also has shortcomings.Therefore, there have beenFar-infrared radiationHot air drying room.
At present, there is no finalized drying chamber in China, which should be designed according to the specific production conditions.When designing the drying chamber, appropriate radiators should be selected and reasonably arranged.Determine the optimal radiation temperature and distance, determine the size and structure of the drying chamber, and consider the factors such as heat preservation and ventilation of the drying chamber.
Structure diagram of infrared radiation drying chamber
Far infrared radiation drying room is mainly composed of room body, radiation heater, ventilation system, temperature control system, etc.
The room type and structural requirements of far infrared drying room can refer to the general hot air drying room.But its size is small, and there are few brick structures.
As the main body of the radiation drying chamber, its function is to maintain a certain temperature in the drying chamber, reduce heat loss, and improve the drying effect.The design of the section size and shape of the chamber body and the configuration of the radiator are determined according to the nature, shape and size of the heated workpiece, the type, temperature, irradiation distance and other factors of the selected radiator.The length and volume of the chamber are determined by the size of the workpiece, heating time, transportation speed and output.
Far infrared heating and drying is to use radiation heating, but in fact, it cannot be a simple radiation heating.When the far-infrared radiator works, it also heats the indoor air to a certain extent, so hot air heating also plays a role.Therefore, the drying room should also have appropriate thermal insulation measures to cover the inside of the roomThermal insulation materialTo reduce heat loss and improve operating conditions.
Radiation heater, also called radiation element, refers to the element that can emit far infrared ray.The radiant heater is composed ofFar-infrared coating, heating element, matrix and accessories.
Commonly used radiation coatings are oxides, carbides, nitrides, sulfides, borides, etc. of most elements located in cycles 2, 3, 4, and 5 of the periodic table of chemical elements. Under a certain temperature, they will radiate infrared rays of different wavelengths to varying degrees.One or more substances can be selected and mixed as required, and applied to the radiator surface by different processes.When selecting far-infrared components, far-infrared coatings with wavelength matching the coating shall be selected according to the requirements of different coatings.
The function of the heat source is to provide enough heat for the radiation coating to make it radiate far infrared ray.Theoretical research shows that the energy of far-infrared ray radiated by the radiation coating is proportional to the fourth power of the absolute temperature of the radiator surface.Therefore, increasing the temperature can increase the far infrared radiation.Generally, electricity, coal, steam, etc. are used as heat sources. The most practical application is resistance wire heating, that is, electrothermal far infrared ray.[3]
