1 Introduction

In order to ensure the anti-rust quality of the whole vehicle, especially the chassis components in the maritime transport, at present, the major automobile factories have adopted the wax-proof protection for the exporter's whole vehicle, and at the same time, the coating of the chassis parts has also set strict standards to continuously Improve coating quality. Based on the anti-rust experience of export vehicles for many years and the needs of overseas markets, the company requires a total coating thickness of not less than 40μm for the export vehicle chassis parts, and the comprehensive performance of the coating meets the export vehicle quality standards. In order to meet the requirements of the export vehicle standard, the factory adopts the method of “electrophoretic primer + high-voltage electrostatic powder coating” on the chassis parts of the export vehicle. The process flow is as follows: pre-electrophoresis treatment (degreasing → water washing → surface adjustment → phosphating → water washing) → Moisture drying → electrophoresis and post-rinsing → electrophoretic paint drying (180 ° C, 30 min) → transfer → high-pressure electrostatic powder coating → powder curing (180 ° C, 25 min) using this production process, the total thickness of the paint film is 70 ~ 100μm, adhesion is 0, neutral salt spray test exceeds 800h, the coating has excellent comprehensive performance, and the coating quality has been stable. However, in mid-October 2009, after re-coating of the powder, the parts showed serious shrinkage of a large area, and mainly concentrated on the corners of the parts, and the adhesion of the paint film was extremely poor. Paint film defects are shown in Figure 1. According to the time and situation of the problem, by analyzing the working conditions of the production line one by one, combined with the related literature [1], there may be several reasons for the shrinkage of the coating during the recoating of the powder: 1 the compressed air is not clean. 2, oily, excessive water; 2 electrophoresis primer curing is not sufficient; 3 electrophoretic primer paint film contact with external dirt, contaminated; 4 a certain component of the electrophoretic paint released during the drying process attached to the surface of the electrophoretic paint film , changed the surface properties of the paint film.

2 accident test analysis and verification

In view of the above factors that may cause shrinkage of the powder coating, the analysis was carried out step by step through experiments.

2.1 compressed air is not clean, oily, too much water

The factory adopts WZG-6/8 type non-heat regeneration air dryer. After the decompressed air is treated by degreasing and drying gas, all parameters can meet the requirements of powder coating. The specific indexes are as follows: the pressure is 0.6-0.8MPa. The dew point temperature is not more than -40 ° C, oil, water and other debris meet the standard requirements. At the same time, the following tests were carried out: parts of the same type were used, some of them were sprayed after electrophoresis, and the remaining parts were directly sprayed with powder after pretreatment. The test results are as follows: after the electrophoresis, the parts of the powder are sprayed, and the surface of the paint film has a large area of shrinkage holes and the adhesion is extremely poor; and the parts directly subjected to the powder coating have good appearance and excellent adhesion. According to this, it is possible to eliminate the shrinkage of the compressed air, which is caused by excessive oil and water.

2.2 Electrophoresis primer curing is insufficient

In the electrophoretic coating process, if the drying process control is not strict, such as the drying temperature or time does not meet the minimum requirements, the curing agent containing the isocyanate groups after deblocking may not be completely reacted and remain on the surface of the electrophoretic paint film. The surface properties of the electrophoretic paint film changed, resulting in poor matching with the topcoat [1]. In order to verify this factor, the quality of the paint film after electrophoresis was checked. The hardness was 2H, the adhesion was 0, and the quality of the paint film completely satisfied the technical specifications of the original paint. At the same time, the absorbent cotton with acetone was applied to the electrophoretic paint film. About 9.8N) After repeated rubbing 10 times, the surface of the coating is in good condition and there is no paint film on the gauze; then the chassis part after electrophoretic paint is subjected to secondary curing at 180 ° C for 25 min in a powder coating line paint curing oven, and finally Further powder recoating revealed that the paint film still had large area shrinkage and poor adhesion. Accordingly, shrinkage cavities caused by insufficient curing of the electrophoretic primer can be eliminated.

2.3 Electrophoresis primer paint film is exposed to external dirt and is contaminated

In order to verify that the electrophoretic primer film is contaminated by external dirt, the following verification scheme is adopted: 1 the electrophoretic primer film is wiped with xylene, dried and then powder coated; 2 the electrophoresis tank is prepared in the laboratory. The electrophoresis template was then re-coated with powder. After the xylene wiping parts, the appearance quality of the repainted film has been improved, but there are still serious shrinkage holes, and the adhesion is basically not improved; the electrophoresis sample produced in the laboratory, the effect of the powder after recoating and the situation of the electrophoresis line production line The exact same: large area shrinkage, very poor adhesion. According to this, the shrinkage hole caused by the contamination of the electrophoretic primer paint film with the external dirt can be excluded.

2.4 A certain component of the electrophoretic paint is released during the drying process.

On the surface of the paint film, the surface properties of the paint film were changed. In the case that the cause of the paint was unsuccessful, the manufacturer of the electrophoretic paint was informed that the electrophoresis line was recently used in October 20 batches of original paint, and the manufacturer found the appearance quality. In the case of poor conditions, he joined the 435 leveling agent of Taiwan Deqian Company, so it is suspected that there may be shrinkage and adhesion caused by the leveling agent. In order to verify this factor, the following tests were carried out: October 20 batches of original paint and September 10 batches of original paint (excluding 435 leveling agent), re-made electrophoretic paint samples, and then powder recoating, October 20 The batch of original paint has a large number of shrinkage cavities, and the adhesion is very poor, which is consistent with the situation reflected on the spot; and the batch of the original paint is normal in September 10, according to which it can be judged that the 435 leveling agent causes poor recoatability after electrophoresis. The results are shown in Figure 2.

3 results analysis and discussion

The leveling agent is one of the coating additives, which can impart a smooth, smooth and uniform coating film during the dry film formation process. The main component of the 435 leveling agent is a modified polysiloxane, which belongs to the silicone resin additive, which has a low surface tension and is connected to the reactive functional group hydroxyl group at the side group of the main chain. Modification [2].

The leveling agent can be used to solve the problem of shrinkage of the surface of the coating caused by the contamination of the metal, the polymer and the wood substrate due to the release agent and the lubricating oil. When added to the coating, the surface tension of the coating can be significantly reduced, and the surface tension gradient of the coating film can be reduced. It promotes the leveling performance of the coating film, prevents the flowering and orange peel, improves the wettability to the substrate, increases the adhesion, and reduces the defects of the coating surface such as shrinkage holes and pinholes. However, since it is modified by a reactive functional group, it cross-links with a resin or a curing agent during film formation, and a large amount of methyl groups are enriched on the surface of the paint film, and the surface tension of the paint film becomes extremely low. According to the liquid spreading theory: when the surface tension of the recoating paint is greater than the surface tension of the primer paint film, the wet film is difficult to spread and spread, and the paint film has disadvantages, which seriously affects the appearance of the recoating layer and the interlayer adhesion. Focus on [3]. In order to systematically understand the effect of the addition amount of 435 leveling agent on the recoatability of powder coating, electrophoresis plates with different addition amounts (from no to 0.3 ‰) were made, and the recoatability was observed by powder coating.

4 Conclusion

There are many factors influencing the recoating compatibility, and the surface tension of the coating is an extremely important factor. If the surface tension of the primer is too low, it will cause great problems for subsequent recoating and even recoating. According to this quality accident and the recoating compatibility of silicone leveling agent, the author believes that: the electrophoretic primer should not add additives that have a great influence on the surface tension of the coating, such as silicone flow. Flat agent, especially unmodified polysiloxane, reactive functional group modified polysiloxane, because the leveling agent is only 0.025 ,, the recoating adhesion is significantly reduced, and the appearance of the paint film begins. Getting worse. In addition, when testing the electrophoretic primer, the user should also check the recoating suitability according to the usage. The paint manufacturer should communicate with the user in advance and change the relevant process test to ensure the process of the additive. Recoat the fit to avoid irreparable damage.