Research on low temperature curing technology of medium density fiberboard (MDF) powder coating

Abstract: In order to prepare low temperature curing powder coatings for medium density fiberboard (MDF), a liquid tertiary amine accelerator was synthesized, and the polyester/epoxy powder coatings were successfully cured at 130℃ for 3min.

0 Preface

The electrostatic spraying technology of medium density fiberboard (MDF) powder coating has many advantages such as low pollution, high efficiency and low cost [1-2], but many factors limit its rapid development, such as the thermal sensitivity of the substrate, electrical conductivity, powder Coating low temperature curing technology, etc.

Polyester/epoxy powder coatings are the most important MDF powder coatings because of their good leveling properties, easy adjustment of texture and color, and a wide range of material choices [4]

In order to reduce the curing temperature of polyester/epoxy powder coatings and improve the curing reaction rate, this paper studies the effect of the dosage of a long-chain liquid tertiary amine accelerator on the curing rate and coating properties of powder coatings, and calculates by thermodynamic analysis The theoretical curing temperature of powder coatings was used to verify the feasibility of low temperature curing, in order to achieve 130 ℃, 3min fast curing

1 Experimental part

1.

Low temperature curing polyester resin: DSM; E12 epoxy resin: Guodu Chemical; n-dodecanol: Runfeng Petrochemical; Dimethylamine: Jufeng Chemical; Texture agent, leveling agent, charging agent, hardening wax powder: Jietongda new material; Titanium dioxide: Lanke Chemical; Carbon black pigment: Lily pigment

Differential scanning calorimeter: Q-200, American TA company; Infrared spectrometer: Nicolet is20, American Thermo Fisher Scientific

1.

Reference [9] Synthesize tertiary amine, put 4L n-dodecanol and 500mg catalyst into the reaction kettle, heat the temperature of the reaction kettle to 200 ° C and keep the temperature, and pass in a sufficient amount of nitrogen to remove the air in the reaction kettle.

1.

Mix 450g of polyester resin, 300g of epoxy resin, 200g of titanium dioxide, 2g of leveling agent, 5g of sanding agent, 15g of degassing agent, 3g of charging agent, 15g of hardening wax powder, 2g of pigment and an appropriate amount of tertiary amine accelerator , melted and extruded by an extruder (extrusion temperature of 80 ° C), then crushed and sieved to obtain polyester/epoxy powder coating

1.

Use differential scanning calorimeter to test powder coating thermodynamic parameters; use infrared spectrometer to test coating curing characteristics; test coating 60° gloss according to GB/T4893.

2 Results and discussion

2.
1 Effect of accelerator on curing temperature

Differential scanning calorimeter (DSC) was used to measure the melting and curing curve of powder coatings, analyze the influence of accelerator on the reactivity of powder coatings, and study the influence of accelerator dosage on curing reaction
.
The DSC test results are shown in Figure 1
.

It can be found from Figure 1 that the synthesized liquid tertiary amine can significantly change the reaction temperature and reaction rate of the curing system
.
When no accelerator is added, the initial curing temperature of the 1# sample is about 130 °C, the peak curing temperature is close to 200 °C, and the curing exothermic peak is wider, indicating that the reactivity is low and the curing rate is slow
.
When 1g of accelerator was added, the initial curing temperature of the 2# sample was significantly reduced, about 115 °C, the reactivity was improved, and the curing exothermic peak was significantly narrowed, and the curing rate was accelerated
.
When the additive amount of the accelerator reaches 2g, the initial curing temperature of the 3# sample is further reduced to about 110 ℃, the curing exothermic peak narrows obviously, and the reaction rate is further improved
.
However, when the additive amount of the accelerator increased to 3g, the curing temperature and reaction rate of the 4# sample no longer changed significantly
.
Therefore, the optimum dosage of accelerator is 2g
.

The principle of curing and crosslinking reaction between carboxyl polyester resin and E12 epoxy resin is shown in Figure 2.
The first step of the reaction is the addition esterification of carboxyl group and epoxy group, followed by the reaction product with carboxyl group and epoxy group.
Continue the esterification reaction
.
Alkaline accelerators can promote this series of esterification reactions, effectively improve the reactivity of polyester/epoxy powder coatings, reduce the curing temperature, and increase the curing rate [4]
.
The long-chain tertiary amine synthesized in this study has strong basicity and good catalytic effect due to the connection of three hydrocarbon groups on its nitrogen atom
.
On the other hand, low-temperature curing powder coatings have a low curing temperature, and conventional accelerators may be difficult to play due to their high melting point, while long-chain liquid tertiary amines have a low melting point and are liquid at room temperature, which can play a more significant role in low-temperature curing.
promotion
.
In addition, it is theoretically known that the application of liquid tertiary amines to polyester/epoxy powder coatings will lead to a decrease in the glass transition temperature of the powder, but from the DSC curve, it can be found that the glass transition temperature of the powder without accelerator is about 57 °C, The glass transition temperature of the powder was increased to about 65°C when 2g of accelerator was added
.
The reason may be that after the reactivity of the powder coating is improved, the raw material is prone to partial curing during extrusion, resulting in an increase in the glass transition temperature
.

2.
2 Effect of accelerator on coating properties

It can be seen from the above studies that the reactivity of polyester/epoxy powder coatings can be significantly improved by applying tertiary amine accelerators
.
In general, the higher the reactivity of the powder coating, the higher the cured crosslink density of the coating and the higher the coating density
.
Higher densities are beneficial for improving the corrosion and chemical resistance of the coating, but the flexibility of the coating is also reduced
.
Therefore, the relevant properties of the cured coatings of different formulations of powder coatings were tested, and the influence of the improvement of the reactivity of the powder coatings on the coating properties was studied.
The test results are shown in Table 1
.

It can be seen from Table 1 that the gloss of the 1# sample without the accelerator is relatively high, while the gloss is obviously reduced after the accelerator is added
.
This is because the catalysis of the accelerator accelerates the curing rate, and the powder melt leveling time is shortened and the gloss is reduced
.
Coating hardness and solvent rub resistance are improved after application of the accelerator due to increased coating cure
.
However, when the curing rate is too fast, the hardness will decrease because the hardened wax powder cannot migrate to the surface in time
.
In addition, tertiary amine accelerators can effectively improve the coating adhesion.
When no accelerators are used, the powder coating activity is low and cannot form a dense coating, so the adhesion is poor.
After the application of the accelerator, the coating density is improved on the one hand.
On the other hand, the liquid tertiary amine accelerator can improve the wettability of the molten coating film, so the coating adhesion is improved
.
When the amount of accelerator reaches 3g, the crosslinking density of the coating is too high, and the movement of molecular chains is hindered, resulting in a decrease in the impact resistance of the coating
.
Therefore, taking into account comprehensively, the optimal dosage of accelerator in this study is 2g
.

2.
3 Study on curing properties

It can be seen from the above studies that the application of tertiary amine accelerators in polyester/epoxy powder coatings can not only significantly improve the powder reactivity, reduce the curing temperature, but also significantly improve the coating performance
.
In order to verify the curing performance of the designed polyester/epoxy powder coating, 3# powder coating was selected, and the coating was prepared on the MDF surface under the curing conditions of 130 ℃ and 3 min, and the comprehensive properties of the powder and coating were tested
.
Figure 3 is the infrared spectrum of the prepared powder coating before and after curing
.

It can be seen from Figure 3 that after the powder is cured, the carboxyl peak (about 3430cm-1, weak peak) is weakened, and the hydroxyl peak (3300~2500cm-1) is enhanced, so the peak shape of hydroxyl absorption is from 3428cm-1 to about 3432cm-1 in the high frequency direction.
At the same time, the absorption peak of the epoxy group (914cm-1) disappeared, indicating that the accelerator played a significant catalytic role in the esterification reaction between the polyester carboxyl group and the epoxy group of the epoxy resin
.

The performance of the prepared coatings was tested with reference to the relevant performance testing methods in T/CSTM00456-2021 "Technical Specifications for Medium Density Fiberboard (MDF) Powder Spraying", and the results are shown in Table 2
.

It can be seen from Table 2 that after the designed polyester/epoxy powder coating is cured at 130 °C for 3 min, the comprehensive performance of the coating meets the relevant requirements
.
In order to further verify the feasibility of this curing condition, the curing behavior of coatings was studied by DSC thermal analysis at different heating rates.
The results are shown in Figure 4.
) as shown in Table 3
.

Combining with Figure 4 and Table 3, it can be seen that when the heating rate gradually increases, the initial curing temperature and peak curing temperature of the powder coating gradually increase, and the curing rate gradually increases
.
It shows that the heating rate has an important influence on the curing behavior of powder coatings
.
Usually, in actual construction, after the powder coating is sprayed on the surface of the MDF sheet, it will be cured in the drying tunnel that has reached the set temperature.
At this time, the drying tunnel temperature has reached the predetermined value, that is, the heating rate is zero
.
The different curing temperatures and heating rates measured by the above DSC analysis are linearly fitted, and the results are shown in Figure 5
.
According to the function model in Figure 5, when the heating rate is zero, that is, when x=0, T0=92°C, Tp=114°C, that is, the theoretical curing temperature of the powder coating is 114°C, and the theoretical gel temperature is 92°C
.
Considering that the actual situation is not ideal, but the curing temperature set in this study is 130 °C, it can be considered that the powder coating can be fully cured
.

3 Conclusion

In this paper, the effect of a long-chain tertiary amine accelerator on the reactivity, curing temperature and reaction rate of polyester/epoxy powder coatings for MDF was studied.
Performance and chemical resistance are improved and meet the requirements for use
.
In this study, the optimal dosage of tertiary amine accelerator is 2g, and further increasing the amount of accelerator does not improve the reactivity of powder coatings obviously, but reduces the coating performance
.
Through DSC thermodynamic behavior analysis, the theoretical curing temperature of the designed powder coating is 114 °C, so the curing condition of 130 °C in the experiment meets the powder curing requirements
.