0 1 Introduction

In recent years, with the emergence of new requirements for simplifying the processing and production technology of white goods, most white goods factories have also requested an upgrade in the technology of powder coating products. In order to solve the problem of difficult powder coating on corners and reduce processing costs, home appliance manufacturers have proposed that the workpiece can be cured at 180 ℃/10min after spraying powder coating, with a coating film thickness of 100-150 μ m; Under the condition of m, the process of processing and forming without cracking the coating at the bending point is called sheet metal process. It is a comprehensive cold working process for metal sheets (usually below 6mm), including cutting, punching/cutting/composite, folding, welding, riveting, splicing, and forming. This requires the coating to have excellent flexibility, be able to withstand the processing of the workpiece without damage, and at the same time, require the coating product to have excellent resistance to natural gas over baking yellowing performance. In response to these new technological requirements, we plan to develop a powder coating product with high toughness, and conduct T-bend (0T) testing and improvement on the coating after being refrigerated at room temperature and -10 ℃ for 1 hour; In a constant temperature oven at 200 ℃/1h, it is required to achieve coating color difference △ E≤ 0.8, achieving partial substitution for solvent based coatings.

1 Experimental section

1.1 Raw materials

Saturated polyester resin (A, B): Xinglong Resin Chemical Building Materials Company; Saturated polyester resin (C, D): Qingtian Material Technology Co., Ltd; Rutile titanium dioxide: DuPont; Barium sulfate: Xinmei Chemical Co., Ltd; Triglycidyl isocyanurate (TGIC), hydroxylamide substances, universal pigments for powder coatings, universal additives for powder coatings (such as benzoin, leveling agents, antioxidants, curing accelerators, etc.), toughening agents (A, B, C, D): commercially available. All of the above are industrial grade.

1.2 Main equipment and experimental instruments

Twin screw extruder (SLJ-30E type): Shandong Lingyu General Equipment Co., Ltd; Universal Traditional Chinese Medicine Grinder (DFY-500): Lin Da Machinery, Wenling City; Grinding System (ACM-02 Type): Yantai Dongyuan Chemical Equipment Co., Ltd; Small electrostatic spraying equipment (SURECOAT type); Noxin electrostatic spray gun; High temperature oven (GPH-30): Aspec Environmental Instruments (Shanghai) Co., Ltd; Impact Tester: Kunshan Jingjia Instrument Equipment Co., Ltd; Powder Coating Tester (FT-201): Ningbo Ruike Weiye Instrument Co., Ltd; Differential Scanning Calorimeter (DSC): Shanghai Hesheng Instrument Technology Co., Ltd; Bending testing machine (QZWT type): Jingke; Horizontal refrigerator (BDF-40H100): Boko.

1.3 Powder coating and coating preparation

Weigh each component according to the design formula, thoroughly pre mix it with a universal traditional Chinese medicine grinder, and then melt and extrude it through a twin-screw extruder at a set temperature. The extruded sheet material is cooled, crushed, sieved, electrostatically sprayed, and thermally cured into a film, and finally the performance of the coating is tested.

2 Results and Discussion

2.1 Selection of Resin

Polyester powder coating has been widely used in many outdoor painting fields such as road signs, automotive industry, transportation equipment, building materials, household appliances, agricultural machinery, etc. due to its excellent weather resistance, chemical resistance, and mechanical properties. This article studies high toughness powder coatings applied in the field of household appliances, such as air conditioning outdoor units, which are exposed to external environments for a long time, such as sunlight, rainwater erosion, etc. Therefore, the powder coatings applied to them require excellent weather resistance and chemical resistance. At the same time, as mentioned earlier, in order to simplify the processing process, solve the difficulty of powder coating in blind spots, and increase the cost caused by repeated processing, home appliance factories have adopted sheet metal technology, which is a process of spraying first and then processing and forming. This requires the coating to have excellent flexibility and not be damaged or cracked at bends during the processing, which will affect its subsequent protective function.

Based on the above requirements, this study selected polyester resin as the resin part of high toughness powder coating.

The commonly used polyester resins for powder coatings in the market are divided into two categories: saturated polyester resins and unsaturated polyester resins. Unsaturated polyester resin molecular chains also contain some unsaturated bonds, such as double bonds, which determine that this type of polyester resin will not be used in weather resistant powder coatings. Therefore, in this study, saturated polyester resin was chosen as the main resin. The commonly used saturated polyester resin in the powder coating industry is usually hydroxyl terminated polyester resin, while carboxyl terminated polyester resin is less commonly used due to its more complex synthesis technology. However, pure polyester powder coatings prepared from saturated carboxyl terminated polyester resins with low and medium acid values have advantages in weather resistance, decoration, workability, and storage stability. Taking into account the advantages of carboxyl terminated polyester resins and the performance requirements of the coating products in this study, four saturated carboxyl terminated polyester resins with relatively low acid values were used for experiments. Through multiple performance tests, suitable polyester resins were selected as the resin body of the powder coating.

This study used four two-step methods to synthesize saturated carboxyl terminated polyester resins (hereinafter referred to as resin A, resin B, resin C, resin D) for experiments. The acid values (mgKOH/g) of the four resins are 20~30, 30~45, 45~55, and 55~65, respectively. A powder coating was prepared using TGIC as a curing agent, and a universal leveling agent, benzoin, curing accelerator, antioxidant, and pigment were selected without adding fillers. The effects of different types of polyester resins on the coating properties were compared, and the results are shown in Table 1.

Table 1 data shows that to a certain extent, as the resin acid value increases, the flexibility performance of the coating becomes stronger. This is because the higher the resin acid value, the more active functional groups are contained in a certain quality of resin, the more complex the reaction with the curing agent, the higher the degree of crosslinking of the coating, and the better the various mechanical properties and chemical resistance of the coating.

The experimental results also showed that increasing the resin acid value resulted in stronger mechanical properties such as coating flexibility; But as the acid value of the selected resin reaches a certain level, due to the high acid value, the cross-linking degree of the coating is too high, the rigidity of the coating is too strong, and the coating cannot withstand the tension changes caused by bending deformation. The coating begins to break, and even detach from the substrate, unable to achieve the expected protective effect. On the other hand, it can be seen from the table that without adding toughening agents, the coating product prepared solely with resin B showed slight cracks at the bending point after being refrigerated at -10 ℃ for 1 hour and subjected to 0T. However, in terms of overall performance, resin B's performance is relatively outstanding compared to the other three resins. Therefore, resin B was chosen as the main resin for this study. Figure 1 shows the experimental results of the flexibility test.

2.2 Selection of curing agent

Curing agents are an important component of powder coatings, which play a crucial role in the production, transportation, storage, and application performance of coatings. Currently, there are several types of curing agents used for polyester resins, including melamine resin, blocked isocyanates, tri glycidyl isocyanurate (TGIC), acid anhydrides, and hydroxylamide substances. Among them, the most widely used are tri glycidyl isocyanurate (TGIC) and hydroxylamide substances.

During the experiment, resin B was used as the main resin, and universal titanium dioxide, leveling agent, benzoin, antioxidant, curing accelerator, and pigment were selected. The effects of two curing agents, namely TGIC and hydroxylamide, on the properties of the coating were compared. The coating thickness is between (150&plus 5)μ m。

Table 2 data shows that, purely in terms of flexibility, impact resistance, and adhesion, the polyester/TGIC and polyester/HAA systems are within the scope of this study, and both of these coating properties are within the expected performance requirements. However, in terms of other performance requirements such as over baking and coating appearance, the coating appearance and over baking yellowing resistance of polyester/TGIC are better than those of polyester/HAA system. Taking into account various aspects and performance requirements, this study chooses the polyester/TGIC system as the curing system for the coating.

2.3 Selection of Toughening Additives

Through preliminary experiments and research, the four selected toughening agents have varying degrees of impact on the flexibility and other properties of the coating. Therefore, it has been decided to conduct further experiments to comprehensively analyze the comprehensive performance of these four toughening agents and find the type of additive we want.

In order to further investigate the effect of toughening agents on the performance of coating materials, the experiment used polyester resin B as the resin system, TGIC as the curing agent, and selected universal titanium dioxide, leveling agent, benzoin, antioxidant, curing promoter, and pigment. The effects of four toughening agents (additive 1, additive 2, additive 3, and additive 4) on the coating properties were compared. Among them, toughening agent 1 is a thermoplastic elastomer that participates in the reaction; Toughening agents 2 and 3 are rubber toughening agents that participate in the reaction;, Toughening agent 4 is a non active toughening agent that does not participate in the reaction; The results are shown in Table 3. In the table, there are 200 parts of polyester resin, 11 parts of TGIC, 80 parts of titanium dioxide, 2.2 parts of universal leveling agent, 2 parts of benzoin, 1.5 parts of antioxidant, 0.8 parts of curing accelerator, and an appropriate amount of pigment. The curing conditions are 180 ℃/10min, and the coating thickness is (150± 5)μ m。

Table 3 shows that the four toughening agents have different effects on enhancing the flexibility of the coating. The effect of additive 1 is better compared to the other three additives. After adding additive 1, the flexibility of the coating was improved. Before the addition, the coating was refrigerated at -10 ℃ for 1 hour, and there were small cracks at the bend when the coating was bent at 0T. However, after the addition, the situation was greatly improved, and this performance also met the expected flexibility requirements of the coating. The reason for the significant improvement in coating toughness may be that thermoplastic elastomer additive 1 forms a semi interpenetrating network structure in the cross-linked network formed by the resin and curing agent during the curing reaction, greatly enhancing the flexibility of the coating; However, the other three toughening agents did not significantly improve the flexibility of the coating. Taking into account all factors, additive 1 was chosen as the toughening agent for the coating product in this study. Figure 2 shows a photo of the experimental results.

2.4 Determination of formula for high toughness powder coating for household appliances

In summary, the formula for high toughness powder coatings applied to household appliances is shown in Table 4. The coating thickness is between (150&plus 5)μ m， Curing process: 180 ℃/10min curing.

3 Conclusion

The high toughness powder coating products for household appliances prepared under the polyester/TGIC curing system as the film-forming material can meet the requirements of excellent adhesion and flexibility required by high toughness powder coatings in sheet metal processes. The coating exhibits excellent flexibility during 0T testing, whether at room temperature of 25 ℃ or after refrigeration at -10 ℃ for 1 hour. Meanwhile, the coating prepared in this study exhibited excellent resistance to over baking yellowing during over baking.

The high toughness powder coating prepared in this study for household appliances can be compared with solvent based coatings under the same working environment, and its various properties are basically the same as those of solvent based coatings, which can partially replace solvent based coatings.