The application of thin, high-insulation powder coatings on new energy vehicle batteries. Powder coatings are energy-efficient, environmentally friendly, VOC-free, decorative, and safe to use. In recent years, powder coating technology has developed rapidly in China. With the improvement of people's living standards, in the rapidly developing automotive industry, new energy vehicles are developing rapidly in various places due to their low noise, high driving stability, and environmental friendliness.

Application of thin, high-insulation powder coatings on new energy vehicle batteries. Powder coatings are energy-efficient, environmentally friendly, VOC-free, decorative, and safe to use. Domestic powder coating technology has developed rapidly in recent years. With the improvement of people's living standards, in the rapidly changing automotive industry, new energy vehicles are rapidly developing in various places due to their low noise, high driving stability, and green environmental protection characteristics.

　　New energy vehicles have increasingly longer cruising ranges, multiple series battery packs, and small distances between adjacent batteries. At the same time, the vehicle battery pack is prone to heat generation and high temperatures during driving. Therefore, the quality of the insulation material between the batteries is a key factor determining the smooth operation of the automotive power battery. High-performance insulation materials are required to improve the applicability and extend the service life of new energy power batteries.

　　Traditional power batteries often use insulating paper for insulation protection and packaging between batteries. Insulating paper has good insulation and flexibility.

　　However, due to the complex power battery production process and the air gap between the insulating paper and the battery casing, partial discharge is easily generated, affecting the service life of the battery and increasing the uncertainty during the driving process of new energy electric vehicles.

　　Insulating powder coatings for new energy power vehicles are not only easy to apply but also easy to install and transport due to their one-time spray molding.

　　However, due to the space limitations of the automotive battery pack, the coating thickness of the aluminum battery casing is 100-150μm (while the traditional spray-coated insulating powder is more than 300μm), and impurities and metal particles have a greater impact on the electrical insulation performance of the coating. Therefore, special insulation powder production equipment must be used to ensure the excellent insulation and withstand voltage performance of the coating.

　　At the same time, compared with insulating films, insulating powder is simpler to produce, does not produce air gaps, has good adhesion, is not easily scratched, and has excellent insulation withstand voltage performance, corrosion resistance, and excellent wet heat stability. It also maintains excellent insulation withstand voltage performance after high and low temperature impact.

　　1. Experimental Section

　　① Raw materials for the formulation

　　The basic formula is shown in Table 1.

　　② Test equipment

　　Test equipment: extruder, electrostatic spray gun, oven, salt spray test chamber, thickness gauge, insulation withstand voltage tester, etc.

　　③ Sample preparation

　　Weigh the raw materials according to the formula, mix them evenly, extrude them with a twin-screw extruder, cool and crush them, and then crush them with a special high-speed crusher. Pass through a 180-mesh standard sieve to obtain the powder coating. The obtained powder coating is applied using high-pressure electrostatic spraying, with a curing time of 200℃/10min, and the film thickness is controlled to 0.1-0.15mm. Then, various tests are carried out.

　　④ Test methods

　　Test items, methods, and indicators are shown in Table 2.

　

 

　　2. Research content

　　Different raw materials, curing temperature and time, and workpiece pretreatment methods have a significant impact on the insulation performance of the powder coating of aluminum batteries. At the same time, the limited space of the automotive battery pack restricts the thickness of the coating used in new energy vehicle power batteries (0.1-0.15mm), and impurities introduced during the production of powder coatings also have a certain impact on the insulation performance.

　　This article studies the influence of different types of resins, curing agent amounts, catalysts, and fillers on the insulation withstand voltage performance, dielectric strength, and dielectric breakdown pass rate of the cured coating under a certain thickness.

　　① The influence of different epoxy resins on electrical insulation performance

　　Resin is the most important component of the film-forming material of powder coatings, and its properties inevitably have a significant impact on the insulation performance of the coating.

　　Bisphenol A epoxy resin itself has excellent electrical properties, heat resistance, mechanical strength, and chemical resistance, making it an excellent material for manufacturing insulating powder coatings.

　　This experiment selected one-step and two-step epoxy resins produced by different manufacturers, along with dicyandiamide curing agent, pigments, fillers, and catalysts, to compare the effects of different epoxy resins on the dielectric strength, mechanical strength, and heat resistance of the coating. The experimental results are shown in Table 3.

　　

 

　　Table 3 shows that the dielectric strength of the E-12 epoxy resin produced by the two-step method is significantly better than that of the one-step E-12 epoxy resin, both meeting the customer's requirement of a breakdown voltage ≥5kV.

　　■ In one-step synthesis, the molecular weight distribution of the obtained resin is relatively wide, and the organic chlorine content is high, which affects the curing speed of the coating, the crosslinking density of the coating is not high, and therefore the breakdown voltage of the coating is not high;

　　■ The two-step synthesis reaction proceeds homogeneously, the chain growth is relatively stable, the molecular weight distribution of the obtained resin is relatively narrow, the organic chlorine content is low, the crosslinking density of the cured coating is high, and the voltage resistance performance is good.

　　Therefore, in the preparation of thin-coated, high-insulation battery shell powder, it is recommended to use bisphenol A epoxy resin synthesized by the two-step method, with low chlorine content and a narrow molecular weight distribution.

　　In the combination of epoxy resins with different epoxy equivalents, due to the different viscosities of the two resins, the wrapping properties of the workpiece are better than those of a single epoxy resin, and the insulation performance is even better, easily forming a dense, stable, continuous, smooth, and flat coating.

　　In the production of insulating powder of a certain thickness, resins with different epoxy equivalents can be selected for matching, and suitable products can be matched according to the actual situation and customer requirements.

　　② Influence of curing agent amount on coating performance

　　

 

　　Table 4 shows that when the amount of latent dicyandiamide is relatively small, the voltage resistance and physical properties of the coating are relatively poor, and the electrolyte resistance is poor, which is caused by incomplete curing of the system.

　　The impact performance of the coating and other properties are poor. However, when the amount of curing agent exceeds a certain level, it will not only increase the cost of the formula but also affect the impact resistance of the coating due to the accelerated gelation speed.

　　The epoxy value range of E-12 epoxy resin produced by different manufacturers varies. Therefore, when designing the formula, the theoretical amount of curing agent should be calculated based on the specific epoxy value of the selected bisphenol A epoxy resin. Then, this theoretical basis should be verified through specific experiments to finally determine the actual amount of curing agent.

　　③ Influence of catalyst on coating performance

　　Imidazoles can significantly reduce the film-forming temperature or shorten the curing time of pure epoxy powder coatings. In powder coatings, they can greatly reduce the crosslinking reaction temperature of powder coatings, improve the crosslinking density of the coatings, save reaction time, and make the coating curing more complete.

　

 

　　As shown in Table 5, with the increase in the amount of catalyst, the reaction rate gradually increases, and the crosslinking density gradually increases, but the appearance of the coating gradually deteriorates. The corrosion resistance of the coating gradually improves, and the electrical insulation performance of the coating continuously improves. However, after a certain point, the electrical insulation impact performance of the coating deteriorates.

　　Considering performance, cost, and customer site requirements for appearance and breakdown strength, the recommended dosage of imidazole promoter is 0.3%~0.6%.

　　④ Influence of filler on coating performance

　　

 

　　Table 6 shows that after adding barium sulfate, the electrical insulation performance decreases. Although the insulation performance is excellent after adding aluminum oxide and light calcium carbonate, the coating has an orange peel effect because light calcium carbonate and aluminum oxide have high oil absorption, and light calcium carbonate has poor acid resistance.

　　Moreover, during the production of light calcium carbonate, a small amount of conductive impurities will remain. When using aluminum oxide as a filler, the coating has excellent thermal conductivity and good breakdown resistance. However, aluminum oxide has high Mohs hardness, resulting in high equipment wear during production and easy introduction of metal impurities, thus affecting the electrical insulation performance of the coating.

　　Therefore, silicon micropowder is the most commonly used insulation powder, and its high dielectric constant makes it widely used in insulation powder. Calcium carbonate has a high dielectric constant, but poor acid resistance. Therefore, only a small amount of calcium carbonate can be added during the preparation of insulation powder to improve the pulverization and powdering rate of the coating.

　　⑤ Influence of curing agents from different manufacturers

　　The reaction between epoxy resin and dicyandiamide is as shown in formula (1):

 

　

　　

　　Table 7 shows that dicyandiamide H has the best effect, J is second, and I is the worst. Dicyandiamides produced by different manufacturers have different processes, curing speeds, and purities, resulting in different crosslinking densities with E-12 epoxy resin. Dicyandiamide H has the best curing effect with epoxy resin.

　　The curing effect of dicyandiamide I and J with epoxy resin is not good, and the insulation withstand voltage is slightly worse than H. This may be due to the introduction of other impurities during the preparation of dicyandiamide I and J, resulting in poor electrical insulation of the coating.

　　For electrical insulation powder coatings with a certain thickness (0.1~0.15 mm), dicyandiamide with high purity, good compatibility with other materials, and excellent insulation performance after curing should be selected to ensure the insulation withstand voltage performance of the coating.

　　The mass-produced powder was spray-coated at the customer's site, and insulation withstand voltage tests were conducted simultaneously. The results are shown in Table 8, with a pass rate of 97%.

　　During spray coating, the spray coating environment should be strictly controlled. Since the spray coating line is newly installed, metal particles and other impurities may remain during installation. After the electrostatic spray powder is recycled, the metal particles and impurities in the pipeline and spray booth will be brought in, affecting the electrical insulation performance of the coating.

　　For the spray coating environment of insulation powder, attention should be paid to protecting the construction environment, and excellent insulation repair paint should be provided to repair the breakdown coating to improve the finished product rate of the customer's aluminum battery shell spray coating.

　　

 

　　In addition, during the production of powder coatings, some metal particles and impurities are inevitably mixed in, making it difficult to obtain completely pure powder coatings, thus affecting the insulation performance of the powder.

　　For the production of insulation powder, dedicated insulation powder production equipment should be used as much as possible, and simple and effective metal particle control and impurity treatment should be carried out to reduce the impact on the performance of insulation powder.

　　At the same time, the baking time should be controlled to avoid incomplete curing of the coating, resulting in poor insulation withstand voltage performance of the coating.

 

 

　　3. Conclusion

　　For new energy vehicle insulation powder coatings with a certain thickness (0.1~0.15 mm) requirement and high insulation withstand voltage requirement, various raw materials must be strictly screened.

　　Determine the appropriate formula according to customer standards, control the mixing of impurities during production, obtain powder coatings with excellent heat resistance, excellent corrosion resistance, and excellent electrical insulation performance, and match excellent insulation repair paint. The following suggestions are provided:

　　(1) Select epoxy resin produced by the two-step method, match resins with different epoxy equivalents, add an appropriate amount of imidazole catalyst to improve the crosslinking density of the coating, ensure that the coating appearance is smooth and flat, and ensure the edge coverage of the workpiece to form a uniform, complete, continuous, and dense coating;

　　(2) Select fillers with low dielectric constant and high resistivity, ensure good electrical insulation performance of the coating, and ensure the heat resistance and insulation performance of the coating at relatively high temperatures;

　　(3) Strictly screen various raw materials, isolate the mixing of impurities during production, use dedicated insulation powder production equipment, and avoid the introduction of impurities and metal particles to affect the electrical insulation performance of the coating.