Analysis of fast or low temperature curing powder coatings suitable for different substrates

Powder coatings and coating systems are known for their ecology, economy and applicability, which can reduce VOC emissions and meet various regulatory requirements.

Most of these areas used poorly "ecologically compatible" technologies (such as liquid coatings) to paint online coatingol.

In recent years, the powder coating industry is vigorously promoting the research and development of new powder coating technologies such as rapid curing, low-temperature baking, peroxide curing, and UV (irradiation) curing, and has gradually formed a development trend of technological expansion in related fields.

1 New technologies, new products and new applications

For new application markets, there are a variety of powder coating coating technologies to choose from.

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Fast curing powder coatings can be cured by baking at high temperature in a short time, such as coil coating.

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The use of different polymerization kinetics can meet the needs of fast curing or low temperature curing, such as peroxide (thermally initiated) curing and UV curing (irradiation and photocatalytic initiation).

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3 Issues to be focused on

The film-forming products of powder coatings are limited by melt viscosity and storage stability, and various curing systems have their own inherent characteristics and deficiencies.

(1) Thermal curing ultra-low temperature baking system

①Extrusion process is not easy to control (resin system gelation); ②Poor flow and leveling performance; ③The variety of coatings suitable for medium density fiberboard (MDF) heat-sensitive substrates is limited, and most of them are textured products, which are difficult to achieve liquid coating.
The surface effect of the film; ④The curing cycle is longer; ⑤It is prone to edge cracking and poor outgassing.

(2) Near-infrared heating and curing (750 nm <near-infrared wavelength <1 500 nm) system

①High energy consumption (30-50 times that of ordinary infrared curing); ②Polar groups have selective energy absorption; ③Short curing period (1-15 s), the coating film is not easy to level; ④Filling is required ( CaCO3); ⑤ Water volatilizes; ⑥ Not suitable for wood composite materials.

(3) Peroxide curing system

①The degree of curing reaction is low, and there may be unreacted double bonds; ②The formation of free radicals is an exothermic reaction without heat; ③The adhesion is generally insufficient, which is more suitable for internal mold powder coating; ④The double bond reaction causes the coating film to shrink ; ⑤ It is difficult to obtain ideal mechanical properties.

(4) UV curing system

①It is difficult to cure thick coatings (>120μm); ②Dark yellow paint cannot be fully cured; ③Complicated workpieces cannot turn all the surfaces that need to be cured to the direction of UV radiation; ④It is difficult to design a multi-purpose coating line suitable for workpieces of different sizes and shapes ⑤The minimum curing temperature is currently at least 100 ℃; ⑥Special polymer materials with low melt viscosity, such as crystalline or semi-crystalline polymers are required; ⑦It is difficult to obtain high gloss and very low gloss coating films.

2 Thermal substrates and non-conductive materials

Thermal substrates are generally non-conductive materials, such as wood, MDF, plastics, ceramics, etc.
, so the following issues should be considered when electrostatic coating:

①Preheat the substrate or precoat conductive primer; ②Improve the conductivity of the plastic substrate; ③Increase the moisture content of the substrate when possible.

When coating heat-sensitive substrates, the curing temperature should be reduced as much as possible.
Therefore, the powder coatings used should be low-temperature curing or radiation curing.
The available curing techniques include:

①Low temperature curing, 125 ℃×25 min; ②Near-infrared curing, several seconds; ③UV curing; ④Peroxide curing, inner mold powder coating.

3 Ultra-low temperature baking curing system

Low-temperature thermal curing systems are still growing in the powder coating market.
A series of new polyesters are made by adjusting the accelerator mixture.
Epoxy or glycidyl compounds are used for baking at a low temperature of 125 ℃ for a long time to cure into a film, which is mainly used for the coating of heat-sensitive substrates.

The experimental polyesters and accelerators used in the ultra-low temperature baking system are listed in Table 1.

Low temperature thermal curing system is mainly used in MDF coating.
In recent years, the MDF industry has developed rapidly, and there is a strong demand for solvent-free coatings.
MDF itself has certain characteristics, and its powder coating is coated with the following special process conditions.

(1) The quality consistency between different batches of MDF: ①The density distribution is stable (about 780 kg/m3); ②The randomness of the moisture distribution.
From the perspective of electrical conductivity and degassing requirements, the moisture content of MDF should be controlled at 6 %～7%; ③The temperature stability of the size.

(2) Preheating requirements (irradiated by infrared light for 30 s) The temperature of the substrate should be between 70 and 80 ℃.

(3) Electrostatic spraying (corona charging/friction charging) control electrostatic spraying control, otherwise it is difficult to estimate the thickness of the coating film.

The polyester resin prepared in the laboratory was used to formulate powder coatings.
Tables 2 to 5 show the experimental formulas and the test results of coating on metal substrates and MDF substrates.

The powder coating is prepared using an experimental single-screw extruder with an extrusion temperature of 80-90 ℃ and a screw speed of 200 r/min; the corona electrostatic spray gun is sprayed to make the plate, and the substrate is steel plate, aluminum plate and MDF plate.

Table 2 and Table 4 are 60/40 polyester and 50/50 polyester/epoxy hybrid system.
And tested the performance of the coating film under different curing conditions (Table 3, Table 5).

4 Near-infrared curing technology

Near-infrared curing is a curing technology that can greatly shorten the process time.
The curing time is only 1-15 s, which means that the energy density is extremely high, which is 30-50 times that of mid-wave infrared light.

The comparison result of near infrared technology and conventional hot air/standard infrared curing technology is shown in Figure 1.
The curing process of this system takes advantage of the energy absorption characteristics of polar groups, and CaCO3 needs to be added to the coating formulation.

The characteristics of its curing mechanism give this curing system an advantage that is different from other systems, that is, the coated workpiece is not heated as a whole, and the heat-sensitive substrate suffers less thermal shock during the curing process.
The compact design of the near-infrared curing equipment can greatly save the space required by the production line.

5 Coil coating and metal pre-coating

The increasingly serious greenhouse effect forces people to further reduce VOC emissions and solvent combustion exhaust emissions.
Powder coatings are a good solution.

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(2) Compared with ultra-low temperature thermal curing powder coatings, UV curing powder coatings have the following advantages (see Figure 2):

①The reaction speed is fast; ②The leveling window is controllable, and the leveling process and the curing process are carried out independently; ③The apparent effect of the coating film is better; ④The baking conditions required for MDF coating are relatively mild, which is resistant to degassing and cracking.
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(3) Compared with UV curing liquid coatings, UV curing powder coatings have the following advantages:

①Because the material used has relatively high molecular weight and does not contain active monomers, it is less toxic to the human body and does not have skin irritation; ②It is easy to obtain a higher film thickness; ③The material utilization rate is very high, and it can be oversprayed.
Recycling; ④The coating film is uniform, and the corners and corners are well covered; ⑤Good adhesion to rigid substrates; ⑥Won't penetrate the wood texture.

UV curing system powder coatings will not undergo curing reaction in the melting stage, and can improve the leveling property to form a highly leveling coating film.
This is also one of its advantages over thermal curing systems.

The difference between the melt leveling and curing of powder coatings can be seen from the DSC chart (see Figure 3).
The reaction kinetics of the UV curing system has nothing to do with the heat flow applied to the system, which is different from the thermal curing system.

In order to make full use of this advantage, it is necessary to develop a new type of unsaturated resin system with the following characteristics:

①Resin reaction activity compatible with a sufficient amount of photoinitiator; ②The resin is transparent to UV, and the light absorption wavelength is different from that of the photoinitiator; ③Excellent adhesion performance; ④It has a clear molecular structure to ensure low viscosity at low temperatures, At the same time, it does not affect the storage stability of the powder coating.

8 Conclusion

Powder coating application technology has received active attention under the situation of increasing emphasis on environmental protection, and extensive and in-depth research has been carried out at home and abroad.

The low-temperature application of powder coating technology on heat-sensitive materials, the application in the field of coil coating in assembly line operation, and the rapid curing by UV curing, resulting in excellent leveling and decorative properties have been significantly improved.

As for the resin synthesis aspect of basic research, the powder coating formulated with polyester resin developed and developed has achieved ideal results in the application of MDF; the application of the developed accelerator in the pre-coating of metal plate and coil coating has also been achieved.
Satisfactory results.

Due to its technical characteristics of leveling and separate curing, UV-curing powders show the decorative ability close to traditional solvent-based coatings.
Its rapid low-temperature curing and continuous coating characteristics make it applicable

The scope is expected to be further expanded.