Abstract: Polyester resin for outdoor weather resistant powder coatings was synthesized using a two-step method and characterized by ET-IR and UV Uis. The influence of IPA, TMP, and ADA dosage on the weather resistance and impact resistance of polyester resin was studied. The results showed that with the increase of IPA dosage, the weather resistance of polyester resin improved, while the impact resistance decreased; With the increase of TMP dosage, the weather resistance of polyester resin improves, while the impact resistance first increases and then decreases; With the increase of ADA dosage, the weather resistance of polyester resin decreases, while the impact resistance improves. The polyester resin synthesized under optimized formula has a light retention rate of 90% (240h B lamp aging) and excellent impact resistance.

1 Preface

In recent years, with the increasing awareness of environmental protection among people, the development of environmentally friendly coatings has received more and more attention from the public. Powder coating, as a solvent-free solid coating, has almost zero VOC emissions and greatly reduces environmental pollution, making it an ideal type of environmentally friendly coating.

Powder coating is exposed to ultraviolet radiation, water, acid rain, oxygen, and other factors in outdoor environments, causing the coating to continuously age and degrade, greatly reducing its service life. In addition, due to external factors such as collisions, the coating may also suffer mechanical damage such as cracks, affecting its service life. Therefore, improving the weather resistance and mechanical properties of powder coatings is of great significance for energy conservation and emission reduction, as well as increasing the service life of materials. Polyester resin, as an important component of powder coatings, is the key to optimizing weather resistance and mechanical properties.

Under the existing technological conditions, simply solving the weather resistance and mechanical properties of powder coating is not a problem. Gu Yuxin et al. conducted a systematic study on the factors affecting the weather resistance of powder coating, while Chen Longcai et al. studied the factors affecting the impact performance of polyester.

However, it is still difficult to simultaneously consider the weather resistance and mechanical properties of powder coatings, mainly due to the limitations of synthetic polyester resin monomers. So far, there is no monomer that has both good flexibility, good weather resistance, and suitable cost. Therefore, coordinating the weather resistance and mechanical properties of powder coatings has become one of the important directions for improving polyester resins. The author optimized the synthesis formula and performance parameters of polyester resin by studying the effects of different monomers on its weather resistance and impact resistance. As a result, a variety of polyester resin with excellent weather resistance and impact resistance was obtained.

2 Experimental section

2.1 Raw materials

PTA, IPA, ADA, TMP, NPG, TGIC, antioxidant 245, antioxidant 626, titanium dioxide, barium sulfate, leveling agent, benzoin, all of which are industrial grade.

2.2 Synthesis process of polyester resin

Add polyols, polyacids, and catalysts into a 100L reaction vessel according to the formula amount, and stir evenly. Under nitrogen protection, gradually raise the temperature to 240 ℃ for reaction. After reaching the target acid value and viscosity, add acid hydrolyzing agent for acid hydrolysis for 3 and 5 hours. After reaching the target acid value and viscosity, vacuum condensation is carried out. After 2 and 3 hours, products with acid value, hydroxyl value, viscosity, relative molecular weight and their distribution that meet the requirements are obtained.

2.3 Preparation process of powder coatings and coatings

Prepare powder coating according to the basic formula in Table 1, and the process flow is as follows:

Ingredients→ Pre mixed→ Extrusion→ Tablet pressing→ Crush→ Screening→ 3 products, the prepared powder coating is electrostatically sprayed and cured under certain curing conditions to obtain a coating, and the coating and coating properties are tested.

2.4 Sample Detection and Characterization

FT-IR: The structure of the sample was analyzed using the Nexus670 Fourier Transform Infrared Spectrometer from Nicolet Corporation in the United States;

UV Vis: The TU-1901 dual beam UV visible spectrophotometer from Beijing Puxi General Instrument Co., Ltd. was used to analyze the UV absorption performance of the sample;

Weather resistance test: The QUV/Spray model artificial aging box from Q-Lab Corporation in the United States was used for weather resistance testing. Experimental conditions: cyclic condition UVB: 60 ℃× 4-hour light exposure at 50 ℃× 4-hour condensation, irradiance 0.71W/m2/nm.

Impact performance test: Refer to GB/T 1732-1993 Coating Film Impact Resistance Test Method to determine the impact resistance of powder coatings.

3 Results and Discussion

3.1 Polyester resin formula and basic properties

Figure 1 shows the infrared spectrum of the synthesized weather resistant and impact resistant polyester resin sample. It can be seen from Figure 1 that the broad and scattered absorption peak near 3276 cm-1 is the - OH absorption peak of carboxyl groups. The C-H characteristic bond absorption peak of alkanes is located near 2963cm-1. The characteristic absorption peak of the C=O bond of the ester group is around 1713cm-1. The characteristic absorption peak of ester based C-O bond near 1266cm-1. The characteristic absorption peaks of the benzene ring are at 1600cm-1, 1585, 1500cm-1, and 1450cm-1, while the C=C absorption peak without olefins is observed between 1675cm-1 and 1640cm-1. As shown in the figure, the synthesized polyester is an aromatic carboxyl saturated polyester resin.

3.3 Effect of IPA dosage on the weather resistance and impact resistance of powder coatings

Figure 2 shows the relationship between different IPA contents and the weather resistance and impact resistance of polyester resin. From the graph, it can be seen that as the amount of IPA increases, the weather resistance of the polyester coating also increases, but the impact resistance shows a decreasing trend. The aging and degradation of polyester coatings during outdoor use are mainly related to ultraviolet radiation. Figure 3 shows the UV spectra of polyester resins obtained at different IPA ratios. From the graph, it can be seen that as the amount of IPA increases, the UV absorption intensity of polyester resin decreases. Thus reducing the degradation of polymers and improving weather resistance. However, due to the asymmetric structure of IPA, the mechanical properties of the resin synthesized from it are poor, and it is more likely to cause molecular chain breakage under external forces, resulting in a decrease in its impact resistance.

3.4 Effect of TMP dosage on the weather resistance and impact resistance of powder coatings

Figure 4 shows the effect of TMP dosage on the weather resistance and impact resistance of polyester coatings. From the graph, it can be seen that with the increase of TMP dosage, the weather resistance of the coating gradually improves, while the mechanical properties first increase and then decrease. TMP is a polyol with 3 functional groups, which does not contain beta in the monomer molecules; Hydrogen atoms are helpful in improving the weather resistance of polyester resins. The addition of this monomer can also increase the crosslinking density, transforming the molecule from a long-chain structure to a network structure, thereby improving the impact resistance of polyester. However, excessive addition can cause the coating to become too hard, thereby reducing its flexibility. In addition, it will also cause a rapid increase in viscosity, and even produce gel, so it is not easy to add too much.

The influence of ADA dosage on the weather resistance and impact resistance of powder coatings

Figure 5 shows the relationship between the amount of ADA added and the weather resistance and mechanical properties of polyester coatings. From the graph, it can be seen that with the increase of ADA dosage, the impact resistance of polyester resin improves. This is mainly because ADA is an aliphatic dicarboxylic acid, and its addition improves the flexibility of polyester polymer molecular chains, thereby enhancing their impact resistance. However, due to the high number of hydrogen atoms contained in ADA monomers, they are more likely to absorb energy and cause molecular chain breakage. Therefore, the addition of ADA will have a significant impact on the weather resistance of polyester resin.

4 Conclusion

Polyester resin for outdoor weather resistant powder coatings was synthesized using a two-step method. The infrared analysis results indicate that the synthesized polyester resin is an aromatic terminal carboxyl saturated polyester resin. With the increase of IPA dosage, the weather resistance of polyester resin improves, while the impact resistance decreases; With the increase of TMP dosage, the weather resistance of polyester resin improves, while the impact resistance first increases and then decreases; With the increase of ADA dosage, the weather resistance of polyester resin decreases and the impact resistance improves. The polyester resin synthesized under optimized formula has a light retention rate of 90% (240h B lamp aging) and excellent impact resistance. Source: Jingcheng, Anhui