Factors influencing powder coating adhesion in automotive cabins

With the increasing attention and attention of the state and society to the environment, the national air control and prevention plan and the local air VOC emission limits have been introduced, which strictly restricts the emission of three wastes in the automobile coating industry.

The comprehensive performance of powder coating is higher than that of paint, and it has become the mainstream to replace liquid coating.

Adhesion is an important technical index related to the mechanical properties, corrosion resistance and weather resistance of coatings. Based on the characteristics of the cabin itself and its coating process, powder coatings differ from traditional paints in controlling the adhesion of coatings.

In this paper, the situation of powder coating technology promoted by automobile interior coating line is introduced, the factors affecting adhesion are discussed in detail, and the control methods and improvement measures are proposed.

1. Characteristics of the carriage coating process

The process of powder coating process for automobile compartment is as follows: the upper part is → wiped→ degreased→ washed with water→ surface adjustment→ phosphating → washed with water→ pure water washing→ electrophoresis→ UF (ultrafiltrate) cleaning→ pure water washing→ electrophoresis drying→ grinding and wiping→ gluing → wiping before powder spraying→ powder spraying→ drying → inspection→ fine → repair → the next parts.   

Table 1 compares the painting process of the carriage with other products, such as frames and appliances. It can be seen that compared with other workpieces, the process of powder coating of the carriage is the most complete. In the "composite coating" process, which is a "multi-process" process, it is necessary to ensure a balance of adhesion between each coating, taking into account the compatibility between each process.

2. Factors affecting the adhesion of powder coating 

The powder coating is solvent-free and bakes directly into a film in an "all-solids" state. Compared with liquid coatings, the liquid wetting time is shorter in the film-forming process, the baking time in the glue state is longer, the baking temperature is higher, the coating is thicker, and the dry film cohesion is greater.

According to GB/T9286–1998 "Scratch Test of Pigment and Varnish Paint Film", the adhesion of the coating after each process was checked in the laboratory to investigate the influence of each process on the adhesion. 

2.1 Pre-treatment process 

2.1.1 Wipe clean

The surface condition of the steel plate affects the adhesion. The oxide film on the surface of the cabin substrate has a weak bond with the metal, which makes the subsequent coating easy to stick off and peel off. Therefore, the workpiece needs to be polished by manual grinding to wipe the oxide scale or shot blasting before painting.

2.1.2 Degreasing 

The surface tension of oil stains is low, and its adhesion to metals and coatings is very weak, which has a great impact on adhesion. The degreasing process mainly includes pre-degreasing, degreasing, etc., and the low concentration of the liquid medicine, the pH change, the process temperature is not up to standard, and the treatment time is too short.

In addition, the anti-rust oil on some plates is particularly difficult to remove, so it is necessary to verify before choosing anti-rust oil, and qualified oil can be used. 

2.1.3 Tone 

Abnormal surface conditioning process parameters will lead to large crystalline particles and loose film layers, which will seriously affect the adhesion of powder coating. 

2.1.4 Phosphating 

The phosphating process is an important factor affecting adhesion, and its control parameters include total acidity, free acidity, accelerator concentration, bath temperature, phosphating time, etc. The workpiece of the carriage is generally phosphated by ternary zinc, and the temperature is usually controlled at 40~55 °C, and the time is 2~3min.

When the parameters change, such as the temperature of the bath liquid is too high, the phosphating reaction is violent, the effective ion content will be lost, resulting in irregular crystallization and even secondary crystallization, and the obtained phosphate film particles are seriously large, sparse and not dense, and finally the adhesion between the phosphate film and the substrate will decrease.

Spraying powder coating or electrophoretic coating (i.e., single coating process) directly on the phosphate film generally has a wide range of requirements for phosphating quality, the crystal particle size of the phosphate film is 5~25μm, and the adhesion of the coating film can be guaranteed when the film thickness is 1~10μm.

However, when examining the adhesion of different supporting systems (see Table 2), it was found that the adhesion of the "electrophoresis + powder coating" system was unqualified when the phosphate film thickness was about 9 μm. The test proves that the system has stricter requirements for the thickness and crystallization state of the phosphate film, and the adhesion of the final product can only be guaranteed when the crystalline particle size is 5~10μm and the film thickness is 1~3μm.

2.2 Electrophoresis 

2.2.1 Compatibility between electrophoretic paint and powder coating 

Electrophoretic paint can be divided into standard type and weather-resistant type according to the weather resistance level. Standard electrophoretic paints are usually based on epoxy resin with good corrosion resistance, and its free polar groups can react with the powder coating to form chemical bonds, so there is good adhesion between the two.

The resin of weather-resistant electrophoretic paint is mainly composed of epoxy, polyester and acrylic resin, because the surface tension is different, the resin with large surface tension sinks in the lower layer during film formation, and the acrylic resin with low surface tension floats on the surface, and there is no active group to cross-link with the powder coating of the upper layer, so the adhesion between the two is poor.

Before selecting a weather-resistant electrophoretic paint, it is necessary to verify the compatibility with the powder coating. 

2.2.2 High temperature resistance of electrophoretic paint 

The drying conditions of powder coating are (180~190)°C×20min, while the drying conditions of electrophoretic coating are usually (160~170)°C×20min, if it is baked at (180~190)°C×20min, it may be over-dried, and there is a risk of poor adhesion to powder coating.

Therefore, it is necessary to verify the high temperature baking resistance of the electrophoretic coating to ensure that it performs well at 190°C×60min, and the adhesion to the powder coating is still grade 0 after drying.

2.2.3 Drying conditions of electrophoretic paint 

The adhesion of the powder coating to the electrophoretic coating is stronger than that obtained by the pretreatment, and once the electrophoresis layer is not dried enough, it is easy to engage with the upper coating, but when the drying time is set too long or the temperature is set too high, the electrophoretic paint will be over-cured, and the surface tension will increase, resulting in the adhesion of the powder coating on the electrophoretic layer will decrease.

After online pretreatment and electrophoresis, the test plate is dried under different conditions, and the dryness is detected and then the on-line hanging plate is sprayed with powder, and the powder coating thickness is controlled to 100~120μm.

2.3 Powder coating 

2.3.1 Powder coating process parameters 

Using electrostatic spraying, the final process parameters used in the field are as follows: gun distance 20~40cm, voltage 50~80kV, current 10~40μA, powder output 100~300 g/min. After debugging multiple rounds of process parameters, it was found that the construction parameters only affected the utilization rate of the paint and the one-time powder rate, but did not affect the adhesion.

2.3.2 Selection of powder coatings 

Although powder coating has been maturely used in home appliances, auto parts and other fields for many years, due to the particularity of the cabin coating, the powder coating used is self-contained, which has higher requirements than other industries.

2.3.3 Powder coated film thickness 

When the coating is formed on a rigid substrate, it can counteract adhesion due to the internal stress caused by the inability to shrink, and only a small external force is required to break the bond. If the film layer is too thick, the internal shrinkage force will have a strong peeling effect on the lower coating, which will affect the adhesion between the coatings and the substrate.

When the powder coating model remains the same, the thicker the film layer, the greater the internal stress, and the worse the adhesion. As a rule of thumb, the powder coating thickness is controlled below 160 μm, and the adhesion can be stabilized at class 0.

And the car compartment mostly adopts corrugated plate, there are more concave and convex surfaces, and the thickness distribution of powder coating is uneven, so it is recommended to control the film thickness of the concave surface to meet the requirements of low engineering covering power, generally 70~80μm; The convex surface is closer to the powder spraying gun, and the powder amount will be higher, and the film thickness can generally reach 120~130μm.

2.3.4 Factors influencing the curing of powder coatings

Insufficient curing time and drying temperature will cause the coating to become more brittle, which will peel off when the adhesion is tested. Therefore, in order to ensure that the adhesion of the coating meets the standard, it is necessary to track and monitor the furnace temperature curve at the production site and check the adhesion at this temperature.

2.4 Key points of production control of powder coating line

2.4.1 Pretreatment line groove fluid 

The production site needs to monitor the bath parameters once per shift to monitor the debinding status, temperature, degreasing alkalinity, phosphating parameters, and ion content in the phosphating solution. At the same time, the pretreatment material manufacturers are required to provide on-site services and provide weekly reports on ion content testing.

2.4.2 Electrophoresis drying state 

Check whether the electrophoresis oven and conveyor equipment are in line with the process status every day, and regularly wipe with absorbent cotton dipped in acetone to test the dryness of the electrophoretic paint film. During the process debugging and trial production, the first article is used with a breadboard every day to check the adhesion of the electrophoresis layer and powder coating, and then produce after passing the test. After stabilization, it can be changed to random baffle detection. 

2.4.3 Furnace temperature

Once a week, the furnace temperature curve of the electrophoresis and powder drying furnace is checked by a furnace temperature tracker, and the adhesion of the coating at this temperature is tested. 

2.5 Solutions to powder coating adhesion problems at the production site

After the powder production line is put into use, under the condition that the coating is well matched and the equipment status meets the process requirements, once there is a problem of poor adhesion, the cause of the problem can be quickly determined and solved by the following methods on site.

2.5.1 Powder anomalies 

The powder is sprayed directly on the standard pretreatment plate and baked in a small laboratory oven to determine whether the powder is abnormal.

2.5.2 Pretreatment and electrophoresis abnormalities 

Pretreatment fluids and electrophoretic paints are provided by different suppliers, and it is difficult to immediately determine the source of the problem if there is an abnormality in production. The "standard phosphating plate" can be used in commissioning production, which can be directly electrophoresed and then sprayed with powder coating, and then the adhesion can be evaluated.

If it is qualified, it means that there is an abnormality in the pretreatment process, and if it is not, it is the electrophoresis process. The method is simple, fast, effective, and reproducible, which can effectively troubleshoot problems and ensure mass production schedule.

2.5.3 Selection of detection tools 

Replacing the scratching knife with a utility knife, stabilizing the utility knife with a steel ruler, and using a "test tape" such as 3M's Scotch4-1000 can effectively improve the accuracy of adhesion testing.

3. Conclusion 

In order to ensure the quality of the final coating, this paper investigates the influence of each process on adhesion according to the process flow, and gives the control points and solutions, which provides a certain reference for other factories to promote this process.