Solvent based products still account for a large proportion in China's industrial coatings industry. Analysis suggests that each year, the industry emits over 2 million tons of volatile organic compounds (VOCs) into the air, equivalent to the total amount of air inhaled by 1.3 billion Chinese people for 16 consecutive hours, with an average annual growth rate of 9%.

From the perspectives of protecting environmental resources, improving people's quality of life, and safeguarding human health and safety, the development of environmentally friendly coatings will become an inevitable trend.

Powder coatings, as solvent-free solid coatings, have almost zero VOC emissions and greatly reduce environmental pollution. Among them, the polyester/hydroxyalkylamide (HAA) system has the advantage of being more environmentally friendly due to the non-toxic nature of HAA, which does not pose a threat to human health.

The curing temperature of most systems in powder coatings is 180-200 ℃, which is higher than other coatings. High temperature means higher energy consumption than traditional systems, which is not conducive to energy conservation. Low temperature curing can reduce more energy consumption and greatly reduce the cost of powder coating. Compared with traditional high-temperature curing powder coatings, low-temperature curing powder coatings have at least two major advantages:

(1) The film-forming temperature is relatively low, which can save a lot of energy. For every 10 ℃ decrease in the curing film-forming temperature of the coating, about 10% of energy can be saved;

(2) Low curing temperature can greatly expand the application fields of powder coatings. Polyester/HAA has gone through a tortuous process in its development in recent years.

In 2010, due to the elimination of trichloroisocyanurate (TGIC) by the Ministry of Industry and Information Technology, the development of polyester/HAA system was accelerated. However, from the second half of 2012, some problems emerged in the application of polyester/HAA system, and the development of this system was greatly hindered.

But we believe that the polyester/HAA system still has development prospects through continuous improvement. It still has greater advantages than the polyester/TGIC system in terms of toxicity, reactivity, and storage stability.

This article synthesized carboxyl terminated polyester resin for low-temperature curing powder coatings in polyester/HAA system and studied the system.

2 Experimental section

2.1 Experimental materials: New pentanediol (NPG), ethylene glycol (EG), trimethylolpropane (TMP), 1,4-cyclohexanedimethanol (CHDM), terephthalic acid (PTA), isophthalic acid (IPA), adipic acid (ADA), esterification catalyst, acid hydrolyzer, antioxidant, HAA curing agent, titanium dioxide, leveling agent, benzoin, etc. are all industrial products.

2.2 Synthesis of Polyester Resin: Add polyols, polyacids, and catalysts according to the formula amount into a 2L reactor equipped with a distillation column, stirrer, and thermometer, and stir evenly. Under nitrogen protection, gradually raise the temperature to 180-250 ℃ for a certain period of time for condensation reaction;

Add acid hydrolyzing agent to adjust the acid value, then vacuum, control the endpoint of polyester, add antioxidant, cool down, and obtain samples with acid value and viscosity that meet the requirements, namely resin A, B, C, and D.

2.3 Powder Coatings and Coating Preparation The basic formula for powder coatings is shown in Table 1. According to the basic formula in Table 1, the process flow is as follows: batching, mixing, extrusion, crushing, and sieving. Electrospray the powder coating and cure it at 160 ℃ for 15 minutes to obtain the coating. Test the coating and its properties.

2.4 Performance characterization: The determination of acid value, softening point, gelation time of powder coating, and coating properties of polyester resin shall be carried out in accordance with the corresponding national standards; The melt viscosity was measured using an ICI vertebral plate viscometer at 175 ℃;

The glass transition temperature and curing curve were tested using NETZSCH DSC 200PC from Germany, with a heating rate of 10 K/min and a nitrogen atmosphere;

The aging resistance of the coating is tested according to ASTM G 154, using the QUV/Spray UV accelerated aging tester from Q-Lab in the United States. The UVB-313 fluorescent UV lamp has a wavelength of 310nm and an irradiation intensity of 0 71 W/m2, 4 hours of UV/60 ℃, 4 hours of condensation/50 ℃, 240 hours.

3 Results and Discussion

3.1 Performance of polyester resin P9250PR and its powder coating. The performance of polyester and its powder coating is shown in Table 2. 3.2.  The effect of EG dosage on artificial aging performance is shown in Table 3, and the effect of EG dosage on the weather resistance of coatings is shown in Figure 2. The polyols in Table 3 are a mixture of neopentyl glycol, ethylene glycol, and 1,4-cyclohexanedimethanol. When ethylene glycol is not included, the light retention rate after 240 hours of artificial accelerated aging with QUVB313 is 70%.

As the amount of ethylene glycol in polyols increases and the amount of neopentyl glycol decreases, the gloss retention rate sharply decreases after artificial accelerated aging. When 10% ethylene glycol is used, the gloss retention rate of the coating is only 43%, while when the ethylene glycol content increases to 30% to 50%, the gloss retention rate decreases to only 30%.

The weather resistance of the coating decreases significantly with the increase of ethylene glycol content. The weather resistance decreases, the coating is prone to aging, resulting in material brittleness, decreased mechanical properties, and shortened service life.

When synthesizing polyester, it is advisable to minimize the use of ethylene glycol that is not conducive to weather resistance. From Table 3, it can be seen that as the amount of ethylene glycol increases, the glass transition temperature of polyester decreases.

3.3 Reaction activity at different temperatures. The reaction activity at different temperatures is shown in Figure 3. The high or low reaction temperature directly affects the reaction activity, thereby affecting the curing rate. Figure 3 shows the gelation time of powder coatings measured at different temperatures.

From the relationship between gelation time and temperature, it can be seen that at low temperatures, the reaction activity is low, the reaction time is long, and the gelation time is long; As the temperature increases, the reaction activity increases, the reaction accelerates, and the gelation time becomes shorter. The time required for powder coating to cure gradually decreases with the increase of curing temperature.

β- The esterification reaction between the hydroxyl group in hydroxyalkylamide and the carboxyl group in polyester will produce water molecules. Therefore, if the curing rate is too fast, the melt viscosity of the coating will increase rapidly. The concentrated water molecules generated during the powder coating reaction are not easy to overflow from the coating, resulting in pinholes in the coating.

If the curing rate is slower and the melting viscosity of the coating increases more slowly, the water generated in the early stage of the reaction is easily removed, which can increase the thickness of the coating when pinholes occur and improve the density of the coating.

We chose to cure the coating at 160 ℃ for 15 minutes to achieve better surface leveling, and the highest film thickness that produces pinholes can reach 110 μ m; m。

3.4 Curing Reaction&beta- Hydroxyalkyl amides have four functional groups and exhibit high activity. Conventional esterification reactions of aliphatic alcohols require temperatures above 225 ℃ or the use of catalysts.

And&beta- Hydroxyalkylamide can react with carboxyl groups at relatively low temperatures starting from 150 ℃, and so far there is no effective curing accelerator to adjust the curing speed.

Activation energy is the minimum energy required for reactant molecules to reach activation in a chemical reaction. Effective collisions and reactions can only occur when the energy of the colliding molecules is equal to or exceeds this activation energy.

The rate of chemical reaction is closely related to the magnitude of its activation energy. A low activation energy can activate more reactant molecules, accelerating the reaction rate. Therefore, reducing the activation energy can effectively promote the progress of the reaction.

In addition, increasing the concentration of reactants can increase the total number of activated molecules and accelerate the reaction rate.

Low temperature curing can be achieved by adding catalysts or using raw materials with higher active functional groups to reduce the activation energy of the reaction.

However, there is no suitable catalyst to adjust the activation energy of the polyester/HAA system, so the only option is to increase the reaction activity of the polyester.

Shortening the curing time can be achieved by increasing the concentration of reactants. Acid value is the reactive functional group in resin; The indicator of high or low carboxyl content is that the acid value is high and the carboxyl content is high.

On average, more carboxyl groups can participate in the reaction, and as the total number of molecules participating in the reaction per unit volume increases, the number of activated molecules per unit volume will also increase, which can accelerate the chemical reaction rate. When the acid value is high, the carboxyl content increases, the amount of curing agent used increases, the cross-linking density is high, and the viscosity rapidly increases, which will affect the fluidity of the coating film.

In order to make the coating have good appearance and mechanical properties, on the one hand, suitable acid hydrolyzing agents with appropriate reactivity should be selected, and on the other hand, the viscosity of polyester needs to be reduced to achieve sufficient melt flow before polymer crosslinking.

But when reducing viscosity, it is necessary to ensure that the polyester resin has a sufficiently high glass transition temperature to ensure good storage stability of the powder coating.

Figure 4 shows the non isothermal curing curves of powder coatings with three different ratios of polyester/HAA systems, heated from 0 ℃ at 10 K/min to 250 ℃.

Table 4 shows the performance of three samples. There is an endothermic peak at 50-70 ℃ before the solidification of powder coatings, which is the glass transition zone of powder coatings. Due to the insolubility of HAA in polyester, its effect on the glass transition temperature (Tg) is not significant [2]. The glass transition temperature here varies with the glass transition temperature of polyester resin.

The temperature continues to rise and a step appears between 90-100 ℃, indicating that the powder begins to melt and causes a change in the material's heat capacity. Sample 1 has a melting temperature of only 91 At 9 ℃, it can fully melt and level before solidification.

In the powder curve of sample 1, a very obvious endothermic peak appeared at around 120 ℃, which is the melting endothermic peak of HAA. Due to the large amount of HAA in sample 1, the peak is more obvious, while in sample 3, there is almost no obvious peak. As the amount of HAA decreases, the absorption peak weakens.

As the temperature increases, the powder coating begins to undergo a curing reaction. There is a broad endothermic peak in the figure, which is due to the small molecule products generated during the reaction -; When water escapes, it absorbs a large amount of heat (usually an exothermic peak in TGIC and mixed powder coating systems).

The powder coatings of three different ratios of polyester/HAA systems did not add catalysts, and the activation energy of the systems was the same. Therefore, the initial curing temperature was basically the same, all at 124 ℃, and the reaction gradually progressed.

At the peak, the maximum reaction rate is reached, where the most water is produced and the most heat is dissipated. Then the reaction rate gradually slows down because there are fewer unreacted substances available for reaction.

4 Conclusion

The low-temperature cured carboxyl terminated polyester resin (P9250PR) of the synthesized HAA system was compared with HAA at a yield of 92 5: 7.5 Preparation of powder coatings. Although the acid value is high, the prepared powder coatings can produce pinholes with a maximum coating thickness of up to 110 μ m; m， The mechanical properties obtained by curing at 160 ℃/15 min are excellent.

Because no curing accelerator was added, the activation energy of the powder coating did not decrease, the storage stability was good, and there was no pre reaction caused by low activation energy during extrusion, resulting in a coating with good leveling properties.

At the same time, because ethylene glycol is not used as a polyol component, it has good weather resistance. This polyester resin has excellent comprehensive performance and high application value.

Energy conservation and emission reduction are prominent advantages of low-temperature curing powder coatings, and HAA system low-temperature curing powder coatings will have more market prospects due to their non toxicity.

With the increasing awareness of environmental protection and the strengthening of ecological civilization construction, the environmental and energy-saving advantages of low-temperature curing powder coatings have become more prominent, which will promote their development.