Synthesis and Study of Low Temperature and Ultra Weathering Resistant Flexible Polyester Resin for TGIC System Powder Coatings

preface

Energy conservation and environmental protection are the main theme of the world today. Due to the outstanding environmental performance of powder coatings with almost zero VOC emissions, powder coatings will gradually replace solvent based coatings.

In recent years, with the increasing demand for energy conservation and emission reduction, powder coatings have become increasingly common in outdoor buildings, automobiles, road facilities, and other occasions, leading to an increasing demand for low-temperature and ultra weather resistant powder coatings.

Furthermore, the key factor for powder coatings to achieve ultra-high weather resistance and low temperature curing performance, as well as relatively low prices, is still the development and research of low-temperature ultra-high weather resistant polyester resins.

Nowadays, TGIC system low-temperature ultra weather resistant polyester resin has appeared on the market. Due to the need to ensure weather resistance, a large amount of IPA (isophthalic acid) is used in the formula for dicarboxylic acid.

However, due to the rigid structure of IPA (isophthalic acid), the flexibility of powder coatings is poor. Therefore, obtaining ultra weather resistant and low-temperature curing polyester resin with certain flexibility is currently a research direction needed in the industry.

                                                                 

                                                                 

This article adjusts the various properties of polyester by examining the amount of TMP (trimethylolpropane), the ratio of CHDA (1,4-cyclohexanedicarboxylic acid), PTA (terephthalic acid) and IPA (isophthalic acid), and the amount of curing promoter;

A super weather resistant saturated polyester resin suitable for curing at TGIC 170 ℃ for 15 minutes has been synthesized, solving the flexibility problem of the super weather resistant polyester resin.

The effect of TMP dosage on the reaction activity and leveling of polyester resin

                                                            

There are currently two solutions to obtain low-temperature cured polyester resin for TGIC system. One is to increase the amount of TMP to improve the functionality of polyester.

Another approach is to increase the content of curing accelerators, but too much TMP content can cause high polyester viscosity, which seriously affects the flowability of powder coatings;

If the amount of curing promoter is too high, the activity of the powder will be too high, which can easily cause gelation during melt extrusion or seriously affect the storage stability of the powder.

So it is necessary to find a suitable TMP dosage. Chen Longcai et al. studied the relationship between TMP dosage in polyester and powder gelation time, and found that the TMP dosage of conventional short gelation polyester resin is generally 1.5% -1.6% of the total mass of the formula.

Research has found that for β- In the synthesis of polyester resin for low-temperature curing of hydroxyalkylamide system, the addition of TMP at 2.38% of the total mass of the formula can achieve relatively low viscosity and impact performance.

Based on relevant information, we selected several TMP dosages from the table below for the experiment, without considering the influence of curing accelerators for the time being.

                                                                      

After the above experiments, we found that the addition of TMP in the range of 1.0% -1.5% is reasonable and can obtain a suitable viscosity, resulting in relatively satisfactory surface leveling under low-temperature curing conditions.

The effect of CHDA dosage on coating performance

The impact of CHDA dosage as a percentage of the mass in the formula on the toughness of the coating is shown in the table below

                                                                        

Note: In the above polyester formulas, alcohols are NPG, TMP, and acids are IPA and CHDA

The above experiments indicate that:

Firstly, when the CHDA dosage is below 5%, the glass transition temperature of polyester does not decrease significantly, but the decrease is relatively large after exceeding 7.5%;

Secondly, with the increase of CHDA dosage, the gelation time does not change much and remains basically unchanged;

Thirdly, with the continuous increase of CHDA dosage, the flexibility of polyester continues to improve.

Due to consideration of the cost of polyester and the impact of glass transition temperature, the experiment did not further increase the amount of CHDA used. Research by Huang Jiyue and others has shown that polyester resin synthesized with CHDA has good hardness and elasticity.

From the viscosity in the table, it can be seen that as the amount of CHDA added increases, the viscosity of polyester resin remains basically unchanged.

The effect of CHDA dosage on aging performance is shown in Figure 1

                                                                      

Note: The CHDA dosages for polyester 1, 2, 3, 4, and 5 are 0%, 2.5%, 5%, 7.5%, and 12.5%, respectively. Polyester 6 is a regular weather resistant outdoor polyester, and the aging standard is GSB AL631

From Figure 1, we can see that as the amount of CHDA added increases, the aging of polyester deteriorates, but it is much better than ordinary outdoor polyester with weather resistance. This also indicates from another perspective that the weather resistance level of CHDA is better than that of PTA, but worse than that of IPA.

Research by Nan Renzhi and others has shown that the poor weather resistance of polyester resins synthesized with terephthalic acid is due to its absorption of short 297nm ultraviolet light.

I chose to use CHDA as a monomer because it can be seen as a product of PTA benzene ring hydrogenation, and the maximum UV absorption wavelength should not be at 297nm, so its weather resistance will be better than TPA.

Analysis of curing reaction

Figure 2 shows the DSC non isothermal curing curve of a powder coating made of ultra weather resistant and low-temperature cured flexible polyester resin synthesized experimentally, with a heating rate of 10 ℃/min starting from 10 ℃ and rising to 250 ℃;

Powder 1 is a powder made from synthetic super weather resistant low-temperature flexible polyester resin, while powder 2 is a powder coating made from ordinary super weather resistant polyester resin.

From it, we can see that the glass transition temperature of powder 1 is 53.17 ℃, slightly lower than the glass transition temperature of powder 2, which is 55.31 ℃, but higher than the safe value of 50 ℃ for powder storage stability.

From the perspective of curing temperature, the curing PEAK value of powder 1 is 161.92 ℃, and the curing PEAK value of powder 2 is 186.87 ℃. Therefore, powder 1 can completely cure under the condition of 170 ℃ for 15 minutes.

                                                                    

                                                                   

Figure 3 shows the DSC curve obtained by heating the synthesized resin from 10 ℃ to 250 ℃ at a rate of 10 ℃/min. From the figure, we can see that the glass transition temperature of the synthesized polyester resin is 61.96 ℃, which also meets the glass transition temperature requirements of outdoor polyester resin and meets the storage stability requirements of polyester.

Comparison of Main Performance Indicators of 4 Powder Coatings

The following table compares the powder properties of synthetic polyester resin and ordinary super weather resistant polyester resin

                                                                    

It can be seen that the powder coating prepared by synthetic resin has better flexibility, higher hardness, and lower curing temperature compared to the powder coating prepared by ordinary super weather resistant polyester.

Due to the addition of unconventional monomers such as CHDA, there is a slight sacrifice in terms of weather resistance. However, from Figure 1, we can see that the weather resistance of the powder prepared by synthetic resin is far ahead of that of ordinary outdoor weather resistant polyester resin, and super weather resistant powder coatings can still be prepared.

5 Conclusion

Through experimental research, it has been determined that TMP accounts for 1.25% of the feed amount, and an appropriate amount of unconventional monomers such as CHDA are added, combined with a certain amount of curing promoter, to obtain a super weather resistant polyester resin suitable for low-temperature curing at 170 ℃ for 15 minutes.

This polyester has better bending performance than ordinary super weather resistant polyester resin, so it has a wider range of applications and energy-saving and environmental protection performance than ordinary super weather resistant polyester.