Li Liangjian and Xie Pinxiong/Foshan Nanhai Jiaduocai Powder Coating Co., Ltd.

1 Preface

Under the general trend of global green and environmentally friendly production, General Secretary Xi Jinping solemnly declared at the 75th United Nations General Assembly: "China will reach carbon peak before 2030 and achieve carbon neutrality by 2060." From then on, my country has entered the "30/60 dual carbon" era of carbon peak and carbon neutrality.

Although powder coatings have the "5E" advantages of Efficient, Energy Saving, Environmental Friendly, Excellent

finishing (excellent performance), and Enabling (wide adaptability). However, during the spraying construction, the heating and curing process of powder consumes a lot of manpower and material resources, and there is still a lot of economic space for improvement. If the idle employees during the heating process are fully utilized, and the heating rate of baking is increased, the powder curing time is shortened, it can also save energy and improve production efficiency. It can alleviate the difficult problems of enterprise recruitment and peak electricity consumption, and also respond to the call of "30/60 double carbon".

2 The connection and difference between low-temperature curing and fast-curing powders

The standard curing temperature of powder coatings is between 180℃~200℃. If the curing temperature is significantly lower than the standard curing temperature, the powder is called low-temperature curing powder. Some heat-sensitive substrates and electronic components are sensitive to temperature, and high temperature will damage them. Therefore, the coatings selected for decoration and protection should be suitable for low-temperature curing. For example:

MDF special powder and electronic encapsulation special powder can be cured at 120℃~140℃/10min~30min, which are common low-temperature curing powders.

The standard curing time of powder coatings is between 10min~15min. If the curing time is significantly shorter than the standard curing time, the powder is called fast-curing powder.

Epoxy steel bar special powder and coil steel special powder are common fast-curing powders. Epoxy steel bar special powder has a particularly high reaction activity and can be fully cured at 230℃/9s.

Low temperature curing is not equal to fast curing. Low temperature curing is about temperature; fast curing is about time. However, low temperature curing and fast curing are related. As the curing temperature increases, the reaction activity increases and the required curing time becomes shorter. On the contrary, as the curing temperature decreases, the reaction activity decreases and the required curing time becomes longer. For example, a special powder for a coil of steel cured at 130℃/5min in the laboratory can meet the curing conditions of 240℃~260℃/30s in the actual production line.

3 Obstacles to the development of low temperature curing powder

Low temperature curing powder can usually be divided into two categories: thermal curing and UV (ultraviolet light) curing. Thermal curing is cross-linking curing when heated to a certain temperature, which is well known to everyone; UV curing is curing after the powder is heated and melted, and then cured by UV radiation. If there is no UV radiation, the powder will not cure even if it is heated to a very high temperature.

The melting and curing process of thermal curing powder is continuous, which is a curing mechanism of polymerization. The softening point of the powder base resin is between 87℃ and 120℃, and the extrusion processing temperature must be slightly higher than the softening point. However, the extrusion temperature is not far from the curing temperature, and gel particles are prone to appear during the processing. Moreover, low-temperature heat-curing powders require low temperatures for storage and transportation, otherwise, powder agglomeration or slow reaction will occur, which will bring inconvenience to the application.

The separation of the melting and curing process of UV-curing powders is the curing mechanism of chain polymerization. The curing process is composed of chain initiation, chain growth, chain termination and chain transfer in series/parallel. The schematic diagram of melting and curing is shown in Figure 1. UV-curing powders avoid the influence of temperature on heat curing extrusion processing, storage and transportation. However, some photoinitiators have problems such as migration and reproductive toxicity. For example, photoinitiators containing aromatic amine groups are carcinogenic, and photoinitiator 907 is reproductively toxic and developmentally toxic, both of which are listed as toxic and hazardous substances by EU REACH and North American TSCA. This is contrary to the direction of developing "green and environmentally friendly coatings". To a certain extent, it has affected the development of UV-curing powders. In addition, very few domestic companies produce resins and equipment for UV curing powders, which also hinders the development of UV curing powders.

Figure 1 Schematic diagram of the melting and curing mechanism of UV curing powder

Therefore, the current domestic low-temperature curing powder is still mainly thermally cured.

4 Several heat transfer methods

Rapid curing mainly emphasizes time efficiency at the same temperature. There are three heat transfer methods to reach a certain temperature: conduction, convection, and radiation. For example: boiling water includes these three heat transfer methods, as shown in Figure 2 below.

Figure 2 Schematic diagram of conduction, convection and radiation

When the parts of an object do not move relative to each other, the heat energy transfer generated by the thermal motion of microscopic particles such as molecules, atoms and free electrons is called conduction, or heat conduction for short. Metal conductors have good conduction effects, while air has very poor conduction effects.

Convection refers to the heat transfer process caused by the relative displacement between the parts of the fluid caused by the macroscopic movement of the fluid, and the mixing of cold and hot fluids. Convection must be accompanied by conduction. Water transfers heat much more strongly than air.

The way an object transfers energy through electromagnetic waves is called radiation. The way to transfer heat through electromagnetic waves is called radiation heat transfer. Both conduction and convection can only be achieved in the presence of matter, while thermal radiation can be transferred in a vacuum, and in fact, the transfer of radiation energy in a vacuum is more effective.

Above the scale of the thermodynamic temperature scale "absolute zero" OK (Kelvin temperature K = Celsius temperature ℃ + 273.15), all objects can emit infrared rays. Infrared emission is achieved through the change of dipole moment. According to Wilhelm Wien's Wien displacement law. The corresponding relationship between the operating temperature range and the radiation peak λmax of various commercially available infrared radiators is shown in Table 1.

Table 1 Correspondence between radiation source temperature and radiation peak wavelength

5 Infrared radiation heating

5.1 Understanding infrared

Infrared is also an electromagnetic wave that can radiate heat. Radiative heat transfer not only produces energy transfer, but also is accompanied by the conversion of energy forms, that is, from thermal energy to radiation energy during emission, and from radiation energy to thermal energy during absorption. The wavelength of electromagnetic waves ranges from zero to infinity, and the naming of electromagnetic waves in the entire spectrum is shown in Figure 3. However, the actual meaningful thermal radiation wavelength is between 0.1~100μm, and most of the energy is in the infrared range of 0.78~20μm. my country's GB/T18497-2019 Characteristics of electric infrared emitters for industrial heating divides infrared heaters into three categories: shortwave infrared Shortwave IR (0.78~2μm), mediumwave infrared Mediumwave IR (2~4μm), and longwave infrared Longwave IR (>4μm) as shown in Figure 4. Different industries may have different ways of dividing infrared. Infrared is invisible light and cannot be observed by the human eye.

　图3电磁波的波谱

Figure 4 Classification of infrared spectral range

5.2 Infrared heating

Infrared heating technology is already very mature. Representative manufacturers include Heraeus of Germany, SUNKISS of France

MATHERM, INFRAGAS of Italy, Adphos of Germany, and Guangzhou Xuluxing Electronics Co., Ltd. of China. The characteristics of infrared heating are safety and environmental protection; heat transfer does not need to rely on space medium, and energy is transferred in a contactless manner; it has strong directionality, high heating rate, and can control local or all heating materials; it can flexibly control heat output and has low thermal inertia.

The principle of infrared heating is: when an object that matches infrared is irradiated by infrared rays with continuously changing wavelengths, its molecules and atoms absorb radiation energy of certain wavelengths, and the vibration and rotation are accelerated, which not only produces energy level transitions from the ground state to the excited state, but also enhances the amplitude of various movements centered on the equilibrium position, the dipole moment (the product of the center distance of the positive and negative charges and the charge carried by the charge center) increases, and the internal energy of the particle increases. The macroscopic reflection of the intensified movement of microstructure particles is the increase in the temperature of the object. Therefore, the object heats up quickly after absorbing infrared radiation energy.

The energy of infrared rays is expressed in wave numbers, not wavelengths or frequencies. When cm is used as the wavelength unit, the wave number is defined as the reciprocal of the wavelength. Short-wave infrared wavelength

0.78~2μm (wave number is 12820~5000cm-1); medium-wave infrared wavelength is 2~4μm (wave number is 5000~2500cm-1); long-wave infrared wavelength is 4~1000μm (wave number is 2500~10cm-1). It can be seen that the shorter the wavelength, the greater its energy. Among the three types of infrared rays, the energy of short-wave is greater.

Short-wave infrared radiation in the over-band region changes the dipole moment heating by absorbing energy level transitions of the double frequency and combined frequency of the stretching vibration of the chemical bonds of O-H, N-H and C-H. Near Infrared (NIR) is a new heating technology with a radiation peak of 0.8~1.1μm. It has a strong ability to penetrate coatings, and the shorter the radiation wavelength, the greater the penetration. High-energy radiation source and high-focus reflector system can activate the polar groups of the heated object to stimulate the thermal motion of the object's molecules, so that the entire coating is heated three-dimensionally and evenly at the same time without heating the entire substrate, and can complete rapid curing in a very short time (1min). NIR heating tubes and reflectors, and German NIR infrared curing laboratory equipment are shown in Figure 5.

NIR infrared radiation lamp and reflector German NIR infrared curing laboratory device

Figure 5 NIR heating tube and reflector, German NIR infrared curing laboratory device

The medium-wave infrared radiation in the fundamental frequency vibration zone changes the dipole moment heating by atomic vibration in molecules and molecular rotation energy level transition; the long-wave infrared (or far-infrared Far-IR) radiation in the rotation zone changes the dipole moment heating by molecular rotation and vibration energy level transition of skeleton, lattice, etc.

The medium-wave infrared thermal radiation with a radiation peak of 2~4μm only heats the shallow layer of the heated body, and will not penetrate the heated object and penetrate into the substrate; the long-wave infrared thermal radiation with a radiation peak of 4~10μm has a lower coating penetration ability than the medium wave, and is particularly suitable for substrates that need to slightly penetrate the heated surface or have high thermal sensitivity (such as plastics, wood and electronic components, etc.).

The infrared radiation with a radiation peak of 2~10 μm matches the absorption of organic products and can be used for heating powder coatings or drying water. The medium-wave infrared heater and the domestic medium-wave infrared curing laboratory device are shown in Figure 6.

Figure 6 Mid-infrared heater and mid-wave infrared curing laboratory device

6 Infrared fast curing powder

When designing the formula of infrared radiation curing powder, it is necessary to pay attention to the ability of the coating component to absorb infrared rays, that is, to select raw materials that match infrared rays. Carbon, graphite, oxides, carbides, nitrides and other materials can absorb infrared rays well. Carboxyl, hydroxyl and epoxy resins used in powders all have the ability to absorb infrared rays, and the absorption capacity of various pigments and fillers will vary. The darker the color of the coating, the stronger the infrared absorption, and the order is black> gray> color> white. Powder formula test is shown in Table 2 below.

Figure 6 Mid-infrared heater and mid-wave infrared curing laboratory device

6 Infrared fast curing powder

When designing the formula of infrared radiation curing powder, it is necessary to pay attention to the ability of the coating component to absorb infrared rays, that is, to select raw materials that match infrared rays. Carbon, graphite, oxides, carbides, nitrides and other materials can absorb infrared rays well. Carboxyl, hydroxyl and epoxy resins used in powders all have the ability to absorb infrared rays, and the absorption capacity of various pigments and fillers will vary. The darker the color of the coating, the stronger the infrared absorption, and the order is black> gray> color> white. Powder formula test is shown in Table 2 below.

Table 2 Comparison of different curing methods for several powders

The above curing test substrates are all made of tinplate. The convection heat adopts a 2400W ordinary electric oven; NIR curing adopts a high-power halogen-filled tungsten emitter imported from Germany, a high-efficiency gold reflector, a power of 5800W, a radiation peak λmax0.9~1μm, and a distance of 50mm between the workpiece and the radiation source; mid-infrared curing adopts a domestic infrared radiation board, a power of 2400W, a radiation peak input max of 3~5 μm, and a distance of 50mm between the workpiece and the radiation source. From the experimental results, it can be seen that various infrared radiation heating can quickly cure the coating to varying degrees.

7 Application of infrared radiation heating

Infrared radiation heating is fast, energy-saving, environmentally friendly, and will not produce secondary pollution. It is widely used in 4D wood grain, MDF, coiled steel, automobiles, electronic appliances and other fields with special requirements. The application of infrared heating must be aware of its emission and absorption laws. Infrared radiation is still the linear propagation characteristic of visible light. In addition to reflection and transmission losses, the remaining radiation energy is absorbed by the heated object. After the heated object absorbs the radiation energy, the internal molecules and atoms transition from the ground state to the high energy state, and the temperature rises rapidly.

First, during radiation heating, the emission source should have a wavelength, radiation angle and power output suitable for the coating or substrate to be treated. Precise matching determines the effectiveness and speed of the heating process. Note that the radiation intensity of the emission source at different temperatures is also different.

Secondly, the distance between the radiation source and the heated object can be shortened, and appropriate reflectors can be used to converge the rays to improve efficiency.

In addition, when infrared rays radiate energy outward, they follow three basic laws of radiation. They reveal from different perspectives the amount of blackbody radiation energy per unit surface at a certain temperature, and its distribution with wavelength and in spatial direction. These theories provide a scientific basis for the application of infrared heating.

7.1 Application of Stefan-Boltzmann Law

The Stefan-Boltzmann law stipulates the relationship between the blackbody radiation force and the thermodynamic temperature (K). It shows that the black body radiation force is proportional to the fourth power of the surface temperature. The Stefan-Boltzmann law shows that the radiation force increases sharply as the surface temperature rises.

Radiation force is the sum of the energy of all wavelengths radiated in all directions of the hemispherical space above it per unit surface area per unit time. Unit W/m².

Eb=σT4,E=εAσT4

Eb--- Black body radiation force, E object radiation force, unit W/m2; σ---Black body radiation constant, its value is 5.67×10-8W/(m2 ·K4);

T---Black body thermodynamic temperature, unit K;

ε---Emissivity of the object, less than 100% (the emission of the black body is 1);

A---Radiation surface area, unit m².

The radiation force of the actual radiation source does not strictly follow the fourth power law, and the deviation is included in the emissivity value determined by the experiment. To ensure sufficient radiation power, high-temperature solidified powders should not be heated by long-wave infrared heating at low temperatures.

7.2 Application of Planck's law

Planck's law explains the law of blackbody radiation energy distribution by wavelength. It shows that: the higher the temperature, the greater the spectral radiation power at the same wavelength; at a certain temperature, the spectral radiation power of a blackbody at a certain wavelength has a maximum value; the spectral radiation power of a blackbody first increases with the increase of wavelength, and after reaching the peak value, it decreases with the increase of wavelength.

The spectral radiation power is the energy per unit wavelength, including the wavelength λ, radiated from each unit surface area to all directions of the hemispherical space above it per unit time. The unit is W/(m²·m), and the denominator m represents the width of the unit wavelength. Since m is too large for the wavelength width, μm is often used instead, that is, W/(m²·μm).

Ebλ--- Blackbody spectral radiation power, unit W/m³; λ--- Wavelength, unit m;

T--- Blackbody thermodynamic temperature, K; e--- Base of natural logarithm;

C1--- First radiation constant, 3.7419×10-16(W·m2); C2--- Second radiation constant, 1.4388×10-2(m ·K).

Although Planck's law and the law of the distribution of spectral radiation power of actual radiation sources according to wavelength are quantitatively different, they are qualitatively consistent. This law provides a reference for choosing which peak wavelength radiation source to purchase.

Planck's law gives the dependence of blackbody monochromatic radiation power on wavelength and temperature. As the temperature rises, the wavelength maximum value of the spectral radiation power of a certain wavelength of the blackbody, max, becomes smaller and smaller, that is, it moves from the λ coordinate to the shorter wavelength direction. There is the following relationship between the wavelength λmax corresponding to the maximum spectral radiation power and the temperature T:

In maxT=2.8976×10-³m.K≈2.9×10-3m·K

This is the Wien displacement law. The Wien displacement law determines the maximum wavelength corresponding to the peak of the blackbody spectral radiation power. Its wavelength λmax is inversely proportional to the temperature T. The wavelength-integrated thermal energy function with λT as the independent variable can calculate the heat distribution ratio of various infrared heaters at different wavelengths at typical working temperatures, as shown in Table 3.

Table 3 Energy distribution of different heaters at each wavelength at typical temperatures

The operating temperature adjustment range of different heaters must be based on the temperature range corresponding to the radiation peak wavelength in Table 1, otherwise the output power will be greatly reduced, and the economy will be lost. Wien's displacement law has a good guiding significance for determining the operating temperature of the radiation source.

7.3 Application of Lambert's law (cosine law)

Lambert's law gives the distribution law of blackbody radiation energy according to the spatial direction. The directional radiation intensity is the energy emitted from the blackbody unit visible area and falls into the unit solid angle (Figure 7) in any direction of space. The unit is W/(m² ·Sr), Sr is the steradian.

Figure 8 Schematic diagram of visible area

Lambert's law states that the directional radiation intensity l of a black body is a constant, which has nothing to do with the spatial direction, and the radiation intensity in all directions of the hemispherical space is equal. However, the energy radiated by a black body per unit area is unevenly distributed in different directions in space, and its directional radiation force changes with the latitude angle θ in a cosine law. This is another way to express Lambert's law, called the cosine law. As shown in the following formula:

(Missing formula)

dφ(θ,φ)---Energy radiated within the micro-element solid angle dΩ; θ---Space latitude angle;

dA--- Black body micro-element area; dQ--- Micro-element solid angle;

dAcosθ---The area seen from the θ direction is called the visible area, as shown in Figure 8.

The cosine law shows that the distribution of black body radiation energy in different directions in space is uneven: the normal direction (perpendicular to the surface) is the largest, and the tangential direction (parallel to the surface) is the smallest (zero).

LANBERT cosine law is only applicable to black bodies and gray bodies. For radiation that obeys this law, the black body radiation force Eb is numerically equal to π times its directional radiation intensity lb. Actual objects only approximately obey LANBERT cosine law. Non-conductive materials are basically correct when the normal θ angle of the radiation surface is 0~60°. When the θ angle is >60°, there is a deviation. In order to obtain the maximum radiation intensity, when installing and arranging the radiation source, the radiation surface will be slightly larger than the heated surface to maintain normal radiation.

8 Discussion on the application of infrared heating

8.1 Reasonable selection of infrared radiators and powder raw materials

The radiation wavelength of the heater theoretically covers the entire wavelength range, but there is only one peak value in its radiation range, so the heater is often named after the radiation peak. For example: short-wave infrared heater, medium-wave infrared heater, etc. The naming of products varies from company to company. When selecting a radiator, to obtain accurate information on the radiation peak wavelength, it is best to ask for the radiation peak spectrum instead of relying solely on the name of the radiator. The radiation peak spectra of several infrared heaters at a certain operating temperature are shown in Figure 9 below. Standard medium wave

When the working temperature is 850℃, the radiation peak λmax of the heater is 2.6μm, and the radiation intensity is relatively large in the range of 2~4 μm (khaki area), covering most non-

The absorption peak of metal materials can be matched to heat the powder base material. The so-called positive matching is that the radiation peak band corresponds to the absorption peak in a positive phase, so that the incident radiation enters the shallow layer of the heated body and causes strong resonance and heating. The service life of the standard medium wave is about 15000-20000 h, which is about 3 times longer than that of NIR, and the maintenance cost is low. Considering the heating matching and economy, it is more appropriate to choose medium wave infrared heating for powder curing.

图 9 几种红外辐射器的辐射峰

　　图10一些物质的吸收峰

Polyester, water, and other non-metals absorb medium waves better, while metals absorb short waves more strongly. The absorption peaks λmax of some substances are as follows: polyester has multiple absorption peaks λmax of 2.8μm,

3.3μm, 5.8μm, 7.8μm, etc., rutile titanium dioxide has three absorption peaks λmax of 15.6 μm, 18.9μm, 25~29μm,

quartz powder has multiple absorption peaks λmax of 8~9 μm, 12.5~13.3μm, etc., barium sulfate has multiple absorption peaks λmax of 8.5~9.3 μm, 10.2μm, 16.7μm, etc.

λmax, water has three absorption peaks λmax of 2.9μm, 6μm, 15.4μm, and steel absorption peak λmax of 0.8~1μm. The absorption characteristics of some substances are shown in Figure 10 above. Generally, the radiation peak λmax of the curing furnace is fixed. When designing the formula, the absorption peak characteristics of the powder raw materials and the spray substrate should be considered. Although the powder base materials can absorb infrared rays and produce resonant heating, it does not affect the curing. However, the absorption peak differences of the resins, pigments, and additives of each system, as well as the coating substrates with different characteristics are worth paying attention to. The different absorption rates will affect the performance of convection and equalization. Special formulas are more suitable for infrared curing.

8.2 NIR low temperature and fast curing applications

NIR technology originated from ink drying. At the European Powder Annual Conference in Amsterdam, the Netherlands in 1998, it was reported that it was used for powder curing, and commercial applications began in 1999. The emergence of the new NIR near-infrared technology has further subdivided short-wave infrared into NIR (wavelength 0.78~1.1μm) and short-wave infrared with a wavelength of 1.1~2 μm. In a broad sense, part of short-wave infrared also belongs to the category of NIR.

NIR has the highest energy in the infrared region, with a lamp power of 250~4000 watts, a filament temperature of up to 3000℃, and an energy density of up to 1500KW/m2. It also has a high energy conversion rate (60%), which is four times the energy conversion rate of hot air (15%); NIR has a fast response time, and can output or shut down the heat source instantly within 1s, providing rapid heating and cooling. When the powder is melted, even if the temperature is higher than 100℃, or even 120℃, the temperature of the heat-sensitive substrate does not exceed 80℃. This solves the problem that short-wave infrared cannot protect heat-sensitive substrates at low temperatures in the past; when the powder is cured, the essential difference between NIR and medium-wave is that medium-wave heating only has positive matching shallow layer heating, while NIR heating also has partial matching deep three-dimensional heating to compensate for the reflected energy of the substrate. The so-called partial matching refers to the incident wavelength deviating from the wavelength blocked by the absorber to varying degrees, thereby heating the inside and outside at the same time. NIR enables the entire coating to obtain the required curing temperature at the same time, and the total time from melting to curing does not exceed 20s. Extremely fast heating rate, heating the coating without heating the entire substrate.

Powder coating of heat-sensitive substrates often considers UV curing. The emergence of new NIR rapid heating technology, melting and leveling are completed in a few seconds, perfectly matching the rate of UV curing, expanding the imagination of dealing with heat sensitivity, and the melting process → curing process can be connected into an assembly line with the same line speed. NIR has absorption peaks for hydrogen-containing groups and metals, which is not only suitable for UV curing with the same speed melting, but also suitable for the rapid curing of heat-curing powders.

8.3 Application of infrared rapid curing in coiled steel

The coiled steel powder curing assembly line has a short furnace body (about 35~45m), a fast line speed (25~100m/min), and high leveling requirements. The total time from melting to complete curing is only about one minute. The infrared radiation heating design scheme of "melting, leveling, solidification, and repairing" is often implemented. The so-called "melting" means melting. First, NIR heating can be used to heat the coating three-dimensionally and melt quickly; "flat" means leveling. Then standard medium wave heating is used to heat only the melted coating, which is easy to fully level. This not only takes advantage of the different characteristics of NIR and standard medium wave in penetrating the coating, but also takes advantage of the long service life of standard medium wave; "solid" means curing. Next, standard medium wave is used to set a higher temperature than the melting and leveling section to allow the powder to cure quickly; "supplement" means supplementary curing. Next, standard medium wave radiation and hot air convection are used to heat and supplement curing to compensate for the shadow area and the influence that linear radiation cannot fully heat. At the same time, convection also makes full use of the residual heat of "melting, leveling, and solidification" to ensure the adequacy of coating curing. As for which section should be set to what temperature, it should be controlled by adjusting the radiation power of the radiation source according to the specific requirements of the actual line speed and powder characteristics.

8.4 Renovation of the original curing furnace

The traditional heating is mainly based on hot air convection, which has slow heat transfer efficiency and poor curing effect. It is easy to cause the surface to melt quickly and the inside to melt slowly, and it is easy to produce pinholes, bubbles, orange peel and other defects. The application of infrared heating is a qualitative reform in the powder coating industry. It is necessary to upgrade and improve the original curing line. For example, if curing is required within one minute, the NIR radiation source can be used in conjunction with the existing hot air circulation; if curing is required within 1 to 5 minutes, the standard medium-wave radiation source can be used in conjunction with the existing hot air circulation. Although the one-time cost of upgrading and transformation is large, the length of the production line can be greatly shortened after the transformation, and less space is occupied. Infrared radiation can directly heat the coating, with fast heating, low energy consumption, high efficiency, and theoretical calculations. It can save more than 50% of energy costs, and the economic benefits are significant in the long run. The more enthusiastic the transformation is, the more companies use infrared heating, and the more conducive it is to further accumulate experience.

Conclusion

Combining the radiation peak in Figure 9 and the absorption characteristics in Figure 10, medium-wave heating is the first choice for matching the positive absorption of powder; from the radiation peak in Figure 9 and the Stefan-

Boltzmann/Planck law, NIR radiation has a short wavelength, high working temperature, and the strongest radiation in the infrared region, making it the first choice for matching the biased peak of powder absorption; NIR heaters and standard medium-wave heaters are matched with each other, and infrared radiation and hot air convection heating can adapt to different requirements of curing; the logical relationship between temperature, wavelength, irradiation angle and thermal energy, and the law of radiation make a deep interpretation, providing application guidance for infrared heating.

When producing powder coatings, the formula is designed according to the radiation conditions of the customer's heater, which is the same as the principle of designing the formula according to the customer's furnace temperature curve when we do thermal convection curing (infrared directly heats the coating, and the furnace temperature tracker that measures the temperature of the air and substrate is not suitable for measurement). Infrared curing is a radiation-based thermal curing. There is no need to deliberately select powder raw materials or design low-temperature curing. Rapid curing infrared heating can also improve efficiency and save energy. my country's infrared heating started late. In the application of low-temperature rapid curing, we need to conduct practical verification and accumulate experience together. We will work together to promote environmental protection and energy conservation, and achieve the "30/60 dual carbon" goal as soon as possible. We look forward to blue sky and white clouds. Source: 2021 China Powder Coatings and Coatings Bulletin