Factors that affect the adhesion of powder coating in car compartments

Zhang Fujia 1, Pi Wei 1, Wei Wenping 1, Luochuan 2, Cheng Jin 2
1. Anhui Jianghuai Automobile Co., Ltd. Technology Center
2. AkzoNobel (China) Investment Co., Ltd.
0 Abstract
introduced the application of powder spraying process in the car car coating line. According to the process, the effect of each process on the adhesion of the final coating system is explained, and the control points and solutions are given.With the increasing attention and attention of the state and society to the environment, the national air control prevention and control plan and the local limits on atmospheric VOC emissions have been introduced, strictly restricting the three waste discharges of the automobile coating industry, no waste water, no waste slag, no exhaust emissions of powder coating new technology has a very obvious energy-saving advantages
the comprehensive performance of powder coatings is higher than that of paints, which replace liquid coatings into the mainstream today. The plain color coating of the car compartment became the first coating line of Jianghuai Automobile using powder coating technology.
adhesion is an important technical indicator involving the mechanical properties, corrosion resistance and weather resistance of coatings. Based on the characteristics of the carrier itself and its coating process, powder coating is different from traditional paint in controlling coating adhesion.
this paper introduces jianghuai automobile in the car car coating line to promote powder spraying technology, discusses in detail the factors affecting adhesion, and puts forward control methods and improvement measures.
01 Car coating process features
Automotive car car powder spray process is: the upper part →washing→ de-oiling→ washing→ table tone →phosphorification→washing→ pure water washing→ electrophoresis→UF (Ultrafiltration liquid) cleaning →semploy water washing→ electrophoresis drying→ grinding cleaning→ applying glue→ cleaning before spraying powder→ powder spraying→ drying→ checking →repairing→ repair→ parts.1 compares the coating process of the carrier with other products, such as frames and appliances. It can be seen that the process of coating the carrier powder is the most complete compared to other work parts. As a "multi-process" "composite coating" process, the matching between processes must be considered to ensure the balance of adhesion between the coatings.
02 factors that affect the adhesion of the powder coatingPowder coatings are solvent-free and are baked directly into film in a "full solid" state. Compared with liquid coating, the liquid wetting time is short in the film-forming process, the baking time is longer under the glue solid state, and the baking temperature is higher, the coating is thick, and the cohesion of the dry film is greater.
according to GB/T9286-1998 "Color paint and varnish paint film" in the laboratory to check the adhesion of the coating after each process, to examine the impact of each process on adhesion.
2.1 pre-treatment process
2.1.1 wipe
the surface status of the steel plate will affect adhesion. The oxide film on the surface of the carrier substrate is less binding to the metal, resulting in the subsequent coating easily adhesive peeling off. Therefore, the work piece needs to be wiped by artificial grinding or blasting before coating.
2.1.2 oil
the surface pressure of oil pollution is low, its binding force with metals and coatings is very weak, the effect on adhesion is very large. The oil removal process mainly includes pre-skimming, degreasing, etc., low concentration of liquid, pH change, process temperature is not up to standard, treatment time is too short, etc. will cause oil removal is not clean.
oil on other plates is particularly difficult to remove, so before selecting rust-proof oil needs to be verified, qualified oil can be used.
2.1.3 table tuning
table adjustment process parameters abnormal will lead to phosphate film crystalline particles large, loose film layer, and thus seriously affect the adhesion of the powder coating.
2.1.4 phosphate
phosphate process is an important factor affecting adhesion, and its control parameters include total acidity, free acidity, promoter concentration, tank temperature, phosphate time, etc. Carrier work parts generally use triple zinc phosphation, the temperature is usually controlled at 40 to 55 degrees C, time 2 to 3min.
When parameters change, such as tank temperature is too high, phosphate reaction is intense, will lose the effective ion content, resulting in irregular crystallization or even secondary crystallization, the resulting phosphate membrane particles are seriously large, and sparse, not dense, eventually make phosphate film and substrate adhesion between the decline.directly sprayed powder coating or electrophoresis coating (i.e., single coating process) on the phosphate film, the general quality requirements for phosphate are relatively broad, phosphate film crystalline particle size of 5 to 25 m, film thickness of 1 to 10 m can ensure the adhesion of the coating film.
the adhesion of different supporting systems (see Table 2), it was found that the "electrophoresis and powder coating" system was not adhesive when the phosphate film was about 9 m thick. After test and demonstration, the system on the thickness of phosphate film and crystallization status requirements are more stringent, crystalline particle size of 5 to 10 m, film thickness of 1 to 3 m to ensure the end product adhesion.
1 shows gold-phase photographs of different crystalline particle sizes and film-thick phosphate layers. It can be seen that the phosphate film, which is 9 m thick, is loose and not dense enough.
2.2 electrophoresis
2.2.1 matching of electrophoresis paint with powder paint
electrophoresis paint can be divided into standard and weather-resistant types according to weather resistance level. Standard electrophoretic paint is usually good corrosion-resistant epoxy resin as the main body, its free polar group can react with the powder coating to form a chemical bond, so there is a good adhesion between the two.
weather-resistant electrophoretic paint resin is mainly composed of epoxy, polyester and acrylic resin, because the surface tension is different, when the surface tension of the film when the resin sinks in the lower layer, the surface tension is small acrylic resin floating on the surface, and there is no active group can be cross-linked with the powder coating of the upper layer, so the adhesion between the two is poor.
to verify the match with the powder coating before selecting weather-resistant electrophoresis paint.
2.2.2 High temperature resistance of electrophoresis paint
the drying conditions of powder coatings are (180 to 190) ×20min, while the drying conditions of electrophoresis coatings are (1 60 to 170 degrees C×20min, if it passes through (180 to 190) degrees C×20min baking, may be over-drying, there is a risk of poor attachment with the powder coating.
it is therefore important to verify the high temperature baking resistance of the electrophoresis coating to ensure that it performs well at 190 degrees C×60min and that the adhesion to the powder coating after drying is still level 0.
2.2.3 electrophoresis paint drying conditions
powder coating than the pretreated membrane layer and electrophoresis coating binding force, once the electrophoresis layer drying is insufficient, easy to bite with the upper coating, but when the drying time is set too long or the temperature setting is too high, electrophoresis paint will be over-cured, surface pressure increases, resulting in the powder coating on the electrophoresis layer adhesion decreased.
drying the test plate under different conditions after on-line pre-treatment and electrophoresis, detecting dryness and then on-line hanging plate spray powder, control powder coating thickness of 100 to 120 m. Each condition selects 3 test boards to test adhesion, the results are listed in Table 3.
as can be seen from Table 3, electrophoresis drying conditions have a greater impact on coating adhesion, drying time is too long or the temperature is too high (more than 200 degrees C) easy to cause the "electrophoresis and powder" system adhesion is worse. If conditions change or the work piece appears multiple times in the line rework situation, the adhesion of the coating needs to be controlled with emphasis. 2.3 Powder Spraying
2.3.1 Powder Spraying Process Parameters
Using electrostectration spraying, it was determined that the process parameters used in the field were: gun distance 20 to 40 cm, voltage 50 to 80kV, current 10 to 40 sA, powder output 100 to 300 g/min. After debugging the multi-wheel process parameters, it is found that the construction parameters only affect the utilization rate of the coating and the powder rate at once, and do not affect the adhesion.
2.3.2 Powder coating selection
Although powder coating has matured for many years in home appliances, automotive parts and other fields, but due to the special nature of the car coating, the powder coating used in its own system, higher than other industries.
the powder coating makes it different in its matching with electrophoresis coatings. A variety of powder coatings are selected to match the common 30 kinds of electrophoresis paints, some of which are listed in Table 4.
can be seen that only AkzoNobel Coatings' Interpon A5000 and the selected electrophoresis paint all match qualified, the rest of the powder paint is more or less with some kind of electrophoresis paint match is not good, can not meet the adhesion requirements.
The Interpon A5000 has a wide match with electrophoresis paint, excellent adhesion, high product masquebability, many years of experience in many countries, performance is guaranteed.
The main raw material of the coating is polyester resin, and its coating film can meet the quality requirements of the carriers in terms of appearance, salt spray resistance, stone resistance, weather resistance, aging resistance, etc., and its key performance indicators are shown in Table 5.
2.3.3 The film thickness of the powder coating
when the coating is filmed on a rigid substrate, it produces internal stress due to its inability to contract, which can counteract adhesion and destroy the bonding bond with only a small external force. The film layer is too thick, and the internal contraction force has a strong peeling effect on the lower coating, which will affect the adhesion between the coating and its substrate.
powder coating model remains unchanged, the thicker the film layer, the greater the internal stress, the worse the adhesion. As a rule of experience, the powder coating thickness is controlled below 160 m and the adhesion is stable at level 0.
and car compartments are mostly ripple plate, there are more concave surfaces, powder coating thickness distribution is uneven, so it is recommended to control the concave surface of the film thickness to meet the requirements of low engineering cover force can be, generally 70 to 80 m;
2.3.4 Factors that affect the curing of powder coatings
The lack of curing time and drying temperature will result in increased brittleness of the coating, which will peel off when testing adhesion. Therefore, in order to ensure that the adhesion of the coating meets the standard, it is necessary to track the furnace temperature curve at the production site and check the adhesion at that temperature. Figure 2 shows the furnace temperature curve of the powder drying furnace after multi-wheel commissioning, and the standard drying condition is 180 degrees C × 20 min.
2.4 Powder coating line production control points
2.4.1 pre-treatment line tank fluid
the production site each shift to monitor the tank fluid parameters once, monitoring the work parts degreasing status, temperature, degreasing alkalinity, phosphate parameters, ion content in phosphate liquid, etc. At the same time, the pre-processing materials manufacturers are required to station services, and provide ion content detection weekly report.
2.4.2 electrophoresis drying state
daily detection of electrophoresis oven and conveying chain equipment in line with the process state, and regular use of skim cotton to take acetone wipe to detect the dryness of electrophoresis paint film. During process commissioning and trial production, the test board is used as the first piece every day to check the adhesion of electrophoresis layer and powder coating, and then produce after passing. After stabilization, it can be changed to random board detection.
2.4.3 Furnace Temperature
the furnace temperature curve of the electrophoresis and powder dryer is detected once a week through the furnace temperature tracker, and the adhesion of the coating at that temperature is tested.
2.5 Solutions to the problem of adhesion of powder coatings at the production site Powder production line put into use, in the paint supporting good, equipment conditions are in line with the requirements of the process, once the problem of poor adhesion, the site can quickly determine the cause of the problem and solve it by the following methods.
2.5.1 powder abnormal
sprayed directly on the standard pre-treatment plate and baked in a small laboratory oven to determine if the powder was abnormal.
2.5.2 pre-treatment and electrophoresis anomalies
pre-treatment fluids and electrophoresis paints are provided by different suppliers, and it is difficult to determine the source of the problem immediately if there is an anomaly in production. Can be used in commissioning production "standard phosphate plate", its direct electrophoresis after spraying powder coating, and then assess adhesion.
if qualified, the pre-processing process appears abnormal, and the reverse is the electrophoresis process. The method is simple, fast and effective, reproducible and can effectively troubleshoot problems and ensure the progress of mass production.
2.5.3 Selection of detection tools
replace the rowing knife with an art knife, use a steel ruler to assist in stabilizing the art knife, and use "test-specific tape" (e.g. 3M's Switch4-1000) can effectively improve the accuracy of adhesion testing.
03 Conclusion
3. Conclusion
Carrier powder coating project is the first car car production line using powder coating, the resulting "phosphate and electrophoresis and powder composite coating" appearance, mechanical performance, corrosion resistance, weather resistance, etc. are better than the traditional "electrophoresis and solvent-based single coating."
Based on the powder coating is different from other coatings, phosphate film, electrophoretic layer and coating adhesion between the adhesion, pull a hair and move the whole body, in order to ensure the quality of the final coating, this paper according to the process to examine the impact of each process on adhesion, and give control points and solutions, for other factories to promote this process to provide a certain reference.