Study on the curing parameters of gas catalytic combustion infrared curing powder coatings

Abstract: Use gas catalytic combustion infrared curing polyester +TGIC powder coating to analyze the effect of curing time and curing temperature on the powder coating;

The performance of the coating under different curing parameters was studied, and the optimal curing process of the catalytic infrared curing powder coating was determined

introduction

As environmental problems have become more prominent, people have begun to increase environmental protection efforts.

1 Research background

With the gradual advancement of refined production in China's coatings industry and the trend of "paint to powder", powder coatings have been increasingly used due to their superior properties, including general industry, agricultural engineering machinery and automobiles, furniture, home appliances, and building materials.

The powder coating curing process has used hot air circulation heating for a long time [5], which has high energy consumption, and there is an urgent need for a new type of heating method that is efficient, energy-saving and environmentally friendly
.

In the production process, people gradually discovered that infrared technology can solve this problem, so infrared heating technology is gradually applied to the field of coating
.

Infrared heating technology has nearly 50 years of development history in China, and infrared curing coatings have been used for more than 20 years, including far infrared [6], high infrared [7-8], mid-wave infrared [9-10] and gas infrared [11- 12]
.

The first three types of infrared use electric energy as the input energy source, and use electric heating wire, resistance wire or special material heating wire as the heating element, which radiates infrared rays of different wavelengths, such as quartz lamps.
The temperature of the heating element is between 400 and 2000 ℃
.

High-infrared has high power and fast response time, so it needs to be strictly controlled when using.
In addition, its service life is short and it is not suitable for large-scale coating lines.
The other two types of electrical infrared are also due to cost, economy, and use.
Issues such as life span have not been widely used in the coating field
.

Gas infrared is divided into gas direct combustion infrared and catalytic combustion infrared.
Direct combustion infrared is the mixed combustion of gas and air in the honeycomb ceramic or metal mesh to directly heat the ceramic plate or metal mesh;

High-temperature ceramic plates and metal meshes can radiate energy, and the temperature of the heating element is around 900°C, which is an indirect infrared generator with low efficiency
.

Catalytic combustion infrared is to control the combustion reaction on the surface of the catalyst.
The reaction temperature is lower than 600°C, and a completely flameless combustion state can be achieved during the reaction
.

Catalytic infrared has only been gradually promoted and applied in the country in recent years.
At present, most of the catalytic infrared in coating equipment is used to preheat the workpiece and combine with the hot air drying tunnel to achieve complete curing of the coating
.

However, with the development of technology and the improvement of design, it is possible to completely use catalytic infrared to cure workpiece surface coatings.
However, the current research does not give the curing parameters of powder coatings using infrared heating.
This article will determine this parameter through experiments.
The application of catalytic infrared technology in actual production provides reference
.

2 Catalytic infrared fast curing coating principle

Energy is emitted and transmitted in the form of electromagnetic waves or photons as radiation, and electromagnetic waves with wavelengths from 0.
76 to 100 μm are generally called infrared rays
.

Infrared radiation is further divided into long-wave infrared radiation, medium-wave infrared radiation and short-wave infrared radiation [13].
The international standard industrial electric heating ICE60050-841 infrared band division is listed in Table 1, and its wavelength characteristics are listed in Table 2
.

The infrared absorption, reflection or transmittance of the coating depends on the spectral characteristics of the coating, including its emissivity and other surface characteristics
.

When the radiation spectrum of the radiation source matches the absorption spectrum of the surface coating of the workpiece, the thermal efficiency is the highest.
The coating composition mostly contains hydroxyl and carboxyl groups, and the wavelength of its natural oscillation frequency is mostly 2.
0~3.
5μm;

Therefore, when the emissivity of the infrared radiation source corresponds to the absorption band of the coating, the radiation energy directly acts on the chemical bond to form a shock-absorbing state and cause the bond to break, so as to achieve the purpose of rapid drying and curing [11]
.

Catalytic infrared is a 3~7μm medium and long-wave infrared [14] produced by the catalytic oxidation reaction of natural gas, liquefied petroleum gas, etc.
and oxygen under the action of the catalytic layer.
The wavelength spectrum of this infrared matches the absorption spectrum of the coating.
Higher
.

The function of the catalyst is to reduce the reaction activation energy of the fuel gas, accelerate the reaction rate, and not be consumed by itself.
The whole process is a flameless combustion reaction that is thousands of times more violent than ordinary combustion, which directly converts the hydrocarbons in the fuel gas into water And carbon dioxide, does not produce harmful gases
.

3 Features of catalytic infrared equipment

Catalytic infrared has all the advantages of electrical infrared curing coatings, including short curing time, high film quality, high safety factor, and simple operation [15]
.

In addition, catalytic infrared is directly converted into infrared radiation energy through gas, which is a direct infrared generator to avoid energy loss during the conversion process
.
Catalytic infrared equipment has a longer service life than electric infrared, and a single radiant panel can normally be used for more than 10 years
.

Although catalytic infrared technology has many advantages, because it heats the workpiece in the form of radiation, although it can quickly cure the coating surface directly exposed to infrared radiation, the incompletely exposed area may not be uniformly cured
.

In the design of infrared furnaces, there are several commonly used methods to solve this problem
.
One method is to use a variable controller to adjust the radiation intensity of a single infrared emitter
.

The second method is to apply heat to each area of ​​the part in the oven according to the requirements of high energy, low energy or no energy.
By using mirrors and baffles to focus the infrared, uniform intensity and intensity can be obtained in different areas.
Coverage, so the infrared drying tunnel needs to be designed according to the shape of the workpiece in the actual process
.

In addition, we generally need to increase the amount of radiation at the entrance of the equipment, so that the coating temperature quickly rises to the curing temperature, and then need to appropriately reduce the radiation amount according to the heat transfer of the workpiece to ensure that the temperature of the coating no longer rises, so catalytic infrared The "heating section" and "heat preservation section" need to be considered in the design of the drying tunnel
.

4 Experimental equipment and result analysis

The gas catalytic infrared oven is mainly assembled by a radiant panel system, a gas supply system, a circuit control system and an equipment frame
.

The oven is a semi-enclosed space, with stainless steel reflectors on both sides, two catalytic combustion infrared radiation panels on the top, and thermal insulation at the bottom.
The size of the radiation panels is 750mm×400mm×20mm
.

In the experiment, the catalytic layer is made of fibrous alumina as a carrier, platinum and other trace metal elements as additives, and calcined at high temperature
.
The specific surface area of ​​the fiber carrier can reach 100~200m2/g, and the packing density is 0.
2~0.
3g/m3
.

The maximum power of each radiator is 6kW, and the maximum power of the infrared oven is 12kW
.
The gas used in the test is canned liquefied petroleum gas, the outlet pressure is higher, and the pressure is adjusted to a suitable pressure through a pressure regulating valve
.

The instantaneous volume flow of gas can be controlled and read by a rotameter, and the inlet pressure of the radiant panel can be read by a pressure gauge on the gas pipeline
.

During the curing process of the infrared oven, the temperature of the sample is measured by a K-type thermocouple and transmitted to the computer for recording.
The temperature measurement range of the hot spot couple is -200~500℃
.
At the same time, the portable thermometer measures the surface temperature of the radiant panel in real time
.

The powder coating selected in this study is a widely used thermosetting powder coating—triglycidyl isocyanurate (TGIC) polyester system, composed of polyester resin, TGIC, acrylic additives, barium sulfate, and titanium dioxide.
It is composed of other pigments, and its characteristics are listed in Table 3
.

The substrate used in the study is galvanized steel, the size of the substrate is 150mm×70mm×1mm, the powder is sprayed with a high-voltage electrostatic spray gun, and chromium treatment is performed before spraying
.

Place the sample plate sprayed with powder coating under the radiant panel.
By controlling the gas flow into the radiator and combining the knowledge of heat transfer, the temperature will rise to 180℃, 190℃, 200℃, 210℃, 220 within 2 minutes.
℃, 230℃, 240℃, at the above 7 different temperatures, keep 1~5min in increments of 1min (total curing time is 3~7min), then take out the sample and cool it to room temperature
.

During the test, it was found that the three groups of test samples had obvious color difference, and no further performance test was done
.
The samples obtained from the above curing conditions were subjected to performance tests according to the requirements of Table 4
.

The test results of five representative parameters are given in the article, including adhesion, impact resistance, gloss, color, and pencil hardness, as listed in Table 5
.

It can be seen from Table 5 that as the curing temperature increases, the time required for the complete curing of the coating is reduced.
This is the same as the hot air curing, which may be due to the increase in the coating temperature and the faster reaction speed
.

When heating the coating with catalytic infrared, the maximum curing temperature (230°C) is higher than the hot air temperature (200°C), and the corresponding curing time is only 2min, which is 1/5 of the shortest curing time in the coating product manual
.

When the curing temperature is low, with the extension of the curing time, the coating performance changes less.
When the curing temperature is 180℃, the gloss of the coating is >90%, and the adhesion detection is greater than 0;

And other parameters are unqualified.
Therefore, the current test did not obtain the shortest time required for the coating to be completely cured at this temperature.
Next, it is necessary to continue the test and explore with the theory of heat transfer.
The curing temperature of 190℃ also needs to increase the test.

.

When the curing temperature is 200°C, only one set of coatings is qualified and the curing time is 5 minutes, which is half of the traditional hot air drying time
.

It can be concluded that when hot air and catalytic infrared are used to cure the same powder coating, under the premise that the powder coating is completely cured, the time for infrared curing is at least half that of hot air
.

When the curing temperature is 210°C and the curing time is 5 minutes, except for the gloss, several other parameters are qualified.
It can be considered that the curing time of 5 minutes is the longest curing time at this temperature
.

In order to obtain the best curing time at this temperature, we need to take a time between 4min and 5min for further experiments
.

When the curing parameters are 220℃×3min and 230℃×2min, all performance tests of the coating meet the requirements.
Considering the curing time and energy consumption, these two parameters can be considered as the best for catalytic infrared curing of this kind of powder coatings.
Curing parameters
.

When the curing temperature is 240°C, there is no test result that meets the requirements.
Therefore, it can be considered that the maximum curing temperature of this kind of powder coating is between 230°C and 240°C when catalytic infrared curing is used.
The specific temperature needs further testing
.

It can also be found from the test results that when the gloss of the coating is greater than 87%, the other properties of the coating are basically unqualified, indicating that the complete curing of this kind of powder coating with catalytic infrared will not give a higher gloss coating
.

Except for the curing parameter 240℃×3min, the appearance and color of the coating under other curing parameters are all normal, but this is the result obtained by our naked eyes
.

When we tested with a colorimeter, we found that as the curing time increases, the b value of the coating increases and the coating becomes yellowish, especially the higher the curing temperature, the greater the difference;

In view of this problem, we propose a solution to this problem is to make the powder coating bluer, which can ensure that the coating color meets the requirements, the coating curing temperature is increased, the curing time is shortened, and the energy consumption is saved
.

The pencil hardness corresponding to most of the curing parameters in Table 5 is ≥H, even for other unqualified coatings
.
Therefore, it can be considered that with catalytic infrared curing powder coating, the pencil hardness of the coating is better
.

Therefore, if there is no special requirement, the powder coating cured by catalytic infrared does not need to test the pencil hardness
.
The impact resistance of the coating is slightly stricter than the adhesion, which may be because the two test items are related to the bonding characteristics between the coating and the substrate
.

5 Conclusion

Gas catalytic infrared heating technology is an energy-saving, high-efficiency, and environmentally friendly green technology.
Compared with traditional hot-air heating technology, the productivity is greatly improved.
If this technology is used reasonably, it will have a significant impact on the coating market
.

The key to the application of this technology lies in the control of infrared radiation inside the drying tunnel, including the reasonable arrangement of radiant panels and reflectors, which need to be determined based on a large number of experiments, theoretical calculations and computer simulations
.
The application of this technology in the field of curing and drying in China is still in the development stage and has a good application prospect
.

Source: Mao Yadong, Pan Song, Liu Yiqiao, Wang Tongzhao, Qin Mingyuan, Wang Xinru, Si Junhong, Wu Jinshun (1.
Department of Urban Construction, Beijing University of Technology; 2.
Fire Rescue Detachment of Rizhao City, Shandong Province; 3.
Municipal and Environmental Engineering of Jilin Jianzhu University College; 4.
School of Emergency Technology and Management, North China University of Science and Technology)