Preparation and Research of Low Temperature Powder

Paint Industry

Coating Industry:

abstract

Abstract: The effect of different types of curing accelerators and different amounts of low-temperature powder curing was studied. After selecting 1.5 ‰ 4 # accelerator, the effects of ADA and BEPD on the low-temperature curing performance of polyester were investigated. A low-temperature curing polyester resin was synthesized and a TGIC curing low-temperature powder coating was prepared.

Keywords: powder coating, polyester resin, low-temperature curing accelerator

0 Introduction

Powder coating is a new type of coating that completely does not contain organic solvents but is coated in powder form and forms a film through high-temperature melting, leveling, and curing. It is a 4E type coating product with high production efficiency, excellent film performance, ecological and environmental protection, and economy. Compared to conventional coatings, powder coatings have the disadvantages of higher curing temperature (usually 180-200 ℃) and longer baking time. The curing temperature of low-temperature cured powder coatings is generally 140-160 ℃ or even lower than 120-130 ℃. In terms of energy conservation, cost reduction, efficiency improvement, and expanding the scope of powder application, the development of low-temperature cured powder coatings is of great significance. However, the technical difficulty of low-temperature curing lies in balancing the storage stability of the powder and the leveling appearance of the coating film.

Based on the existing ester formula, this article selects different curing accelerators and tests and selects the appropriate curing accelerators and their dosage by adding different amounts; Further study the effects of different polyols and acids on the leveling and impact properties of powder coatings, and select the optimal formula for outdoor low-temperature curing polyester resin to produce low-temperature powder cured at 140 ℃/20 min.

1 Experimental part

1.1 Experimental materials

Neopentanediol, ethylene glycol, diethylene glycol, 2-methyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, hexanediol, cyclohexanedimethanol, trimethylolpropane, pentaerythritol; Terephthalic acid, isophthalic acid, hexanediol, cyclohexanedicarboxylic acid, trimellitic anhydride; Monobutyl tin oxide; Triphenyl phosphite; TGIC, titanium dioxide, barium sulfate, benzoin, leveling agent, brightener, etc. All are industrial grade.

1.2 Main instruments

Electric heating sleeve, one set of 3L glass reactor, viscosity analyzer, differential thermal analyzer, twin screw extruder, electrostatic spraying equipment, film thickness gauge, impact tester, gelatinizer, etc.

1.3 Experimental process

1.3.1 Polyester synthesis

Step 1: Add the prescribed amount of alcohol into the reaction kettle, heat until the alcohol melts, and then start stirring. Add the prescribed amount of alcohol and monobutyl tin oxide, slowly raise the temperature to 245 ℃, and keep it warm for 3 hours. Sampling and testing, with an acid value controlled at 15-20mgKOH/L.

Step 2: Lower the temperature to 220 ℃, add the formula amount of acid, raise the temperature to 240 ℃, and keep it warm for 3 hours. Sampling and testing, with an acid value controlled at 45-50mgKOH/g.

Step 3: Conduct vacuum polycondensation reaction at a vacuum degree of -0.095MPa or above for 2-3 hours, and take samples to detect acid value and viscosity.

Step 4: Lower the temperature to 200 ℃, add the formula amount of additives, stir for 0.5 hours, and pour out the resin for cooling.

1.3.2 Powder preparation

Mix the synthesized polyester, TGIC, titanium dioxide, barium sulfate, leveling agent, benzoin, etc. according to the ratio shown in Table (1), mix them evenly, extrude them through a twin screw extruder, press them into pieces, cool them, crush them, and sieve them to produce powder coatings. Spray the prepared powder coating onto the sample using electrostatic spraying. Place the sample in an oven and bake it to solidify into a film. Testing of coatings and their performance.

Table 1 Distribution ratio of powder coating groups

1.4 Performance testing

Polyester acid value: Test according to the indicator method in Method A of GB/T 6743-2008; Polyester viscosity: tested according to the provisions of GB/T 9751.1-2008; Glass transition temperature: tested according to the provisions of GB/T 19466.2-2004, with a heating rate of 10 ℃/min; Film thickness: measured using an ultrasonic thickness gauge in accordance with GB/T 37361-2019; Gloss: Measure using a 60 ° glossmeter according to GB/T 9754-2007; Impact performance: According to the provisions of GB/T 1732-2993, use a paint film impact tester to perform positive and negative impact on the sample coating, and observe the cracking of the coating; Gelation time: Test according to the provisions of GB/T 16995-1997.

2 Results and Discussion

2.1 Selection of curing accelerator

Curing accelerators can promote the reaction between the carboxyl group of polyester resin and the epoxy group of epoxy resin or TGIC. Commonly used are imidazoles, imidazolines, tertiary amines, and phase salts, which can be added externally during powder making. However, due to the poor dispersion effect of external mixing, a large amount of addition can cause serious surface problems such as orange peel. Therefore, in addition to imidazoles, imidazolines, and other curing accelerators that can cause changes in polyester color, Generally, it is added as an additive to the resin during the production process for melt dispersion. Four types of curing accelerators were selected for the experiment by adding different amounts of them to the same polyester formula. Make polyester into powder, 10min@200 Test and compare the effectiveness of the sample after baking and curing at ℃, and use a differential thermal analyzer to analyze and compare the promotion effect on the powder. The comparison results of the sample are shown in Table 2, and the analysis results of the differential thermal analyzer with the same amount of accelerator added are shown in Figure 1.

Table 2 Effect of Different Types and Dosages of Curing Accelerators on 200 ℃/10min Curing Samples

Figure 1 DSC Effect of Adding 1 ‰ Different Curing Accelerators

From Table 2, it can be seen that polyester itself can cure completely without adding a curing accelerator, with good surface effect, normal impact performance, and longer gelation time; After adding different curing accelerators, the promoting effects of different accelerators are different. With the increase of the amount of accelerator added, the gelation time decreases, the surface effect gradually deteriorates, and the impact can pass through; From Figure 1, it can be seen that different curing accelerators with the same amount of addition have different promotion effects, with 1 # accelerator having the worst effect and 4 # accelerator having the best effect, which is consistent with the data in Table 2.

To achieve low-temperature curing at 140 ℃/15min, the above sample powder was baked at 140 ℃/15min and compared with the sample. The specific situation is shown in Table 3.

Table 3 Curing samples of different types and dosages of curing accelerators at 140 ℃/15min

From Table 3, it can be seen that the sample with 1.5 ‰ of 4 # accelerator achieved low temperature curing impact performance at 140 ℃/15min, and the surface was slightly better.

2.2 Optimization of polyester performance

Based on the above experimental results, the addition of 1.5 ‰ 4 # curing accelerator to polyester was selected to produce low-temperature powder, which can achieve the impact performance of the coating film after curing at 140 ℃/15 minutes. The sample surface has slight orange peel. To adjust surface leveling, a portion of flexible monomers such as hexanediol or adipic acid (ADA) were introduced into the polyester resin synthesis formula to reduce system viscosity and further improve leveling and mechanical properties; However, excessive flexible monomers significantly reduce the glass transition temperature of the resin and affect the storage performance of the resin and powder. Therefore, it is considered to introduce 2-ethyl-2-butyl-1,3-propanediol (BEPD) to improve it. To investigate the effects of different monomers and ratios on the properties of polyester resin through experiments.

2.2.1 Effect of ADA dosage on low-temperature curing performance of polyester

During the polyester synthesis process, isophthalic acid and adipic acid were used as acidolysis agents, while 1.5 ‰ 4 # curing accelerator was added to investigate the effect of adipic acid on the low-temperature curing performance of the resin. The results are shown in Table 4.

Table 4 Effect of ADA dosage on low-temperature curing performance of polyester

From Table 4, it can be seen that different contents of adipic acid have an impact on the low-temperature curing properties of polyester. Due to the fact that adipic acid is a long straight chain flexible monomer, molecules are prone to rotate around a single bond, and chain segment rotation activity is easy. At the same time, a decrease in benzene dosage leads to a decrease in phenyl rigid groups. Therefore, as the content of adipic acid increases, the viscosity and Tg of the resin significantly decrease, and the leveling and impact properties of low-temperature powders gradually improve. But ADA is a type of polyester monomer that is not weather resistant, which has a negative impact on the weather resistance of polyester. Moreover, in the case of poor process control, it can cause the resin color to turn yellow, so its dosage needs to be controlled.

2.2.2 Effect of BEPD dosage on low-temperature curing performance of polyester

On the basis of the formula in Experiment 4 above, BEPD was used to replace part of neopentyl glycol to investigate the effect of 2-ethyl-2-butyl-1,3-propanediol on the low-temperature curing performance of the resin. The results are shown in Table 5.

Table 5 Effect of BEPD dosage on low-temperature curing performance of resin

From Table 5, it can be seen that different levels of BEPD have an impact on the low-temperature curing performance of polyester based on the presence of 6% ADA. Due to the presence of one ethyl and one butyl on the carbon atom in the middle of BEPD, it affects the activity of the two hydroxyl groups at the end and also has a shielding effect on the ester bonds formed at both ends, limiting the activity of resin molecular chains and reducing intermolecular forces. Therefore, as the amount of BEPD increases, the Tg of polyester resin gradually increases, the mechanical properties gradually decrease, and the viscosity of the system does not change much. Meanwhile, due to the protective effect of butyl, the weather resistance of the resin has been improved.

Based on the above experimental results, the polyester resin prepared in Experiment 6 can achieve a low-temperature cured powder with good surface and excellent impact performance.

3. Conclusion

Through comparative experiments, different curing accelerators and their dosages were studied on the coating conditions of conventional curing and low-temperature curing. Finally, 1.5 ‰ 4 # accelerator was selected to be added; Then, the effect of different amounts of ADA and BEPD on the low-temperature curing performance of polyester was investigated. Finally, an experimental formula with 6% ADA and 6% BEPD was selected to synthesize polyester resin. TGIC curing agent was used to prepare low-temperature powder coating, and the sample surface was good with good impact performance and good storage performance. The advantage of low-temperature curing powder coatings lies in energy conservation and emission reduction, while expanding the application scenarios of powder coatings, which is of great significance.