JinPersistent
(Zhejiang Fulin Guorun Automotive Parts Co., Ltd., Zhejiang, Taizhou 318050)
Summary: This document primarily introduces the factors that affect the cleanliness of paints and coatings in automotive finishing, discusses methods for managing cleanliness, and provides solutions to common problems in the form of case studies.
Keywords: Paint Cleanability; Particle Size; Filtration; Metal Coatings
0 Introduction
Modern technological advancements have led to continuous innovation in automotive coating processes, with new materials and techniques emerging. However, the primary functions of automotive coating remain consistent regardless of these changes. Firstly, it provides protection by preventing corrosion and oxidation of metal surfaces through the exclusion of moisture, acids, alkalis, and ultraviolet radiation from the air. Secondly, it offers aesthetic appeal. Natural colors are diverse, and with the development of spraying techniques, various colors have been further developed on vehicles. This beautification and decoration enhance the richness and variety of automotive colors.
In the automotive coating industry, the pursuit of both the aforementioned functions is driving a gradual shift towards lightweight and environmentally friendly solutions. Specifically, the thickness of automotive coatings is decreasing, and in some B1B2 and 3C2B coating processes, the application of paint to the inner layer is being eliminated entirely. This contributes to reduced vehicle weight and manufacturing costs for automakers. As coating thicknesses continue to decrease and consumer demands become more stringent, the quality requirements for coating materials are also increasing. This necessitates the use of clean and high-quality coating materials.
1 Related concepts regarding paint cleanliness
Cleanliness, in its typical sense, refers to the size and quantity of all particles in a given volume of air, which reflects the air's cleanliness level.
1.1 Paint Cleanliness
The cleanliness of paint is an extension of the cleanliness of the air. In simple terms, it refers to the size, shape, and quantity of particles and fibers contained in a specific volume of paint.
1.2 Paint Viscosity
The fineness of paint reflects the particle size distribution of pigment and other powdered materials dispersed in the dispersion medium.
1. 3 Paint coating thickness
Typically, paints are applied in a way that results in a fixed and uniform coating thickness. With the advancement of modern detection methods, both wet and dry film thicknesses of paints can be measured. The term "coating thickness" usually refers to the dry film thickness. Table 2 lists common coating thicknesses for the 3C2B process.
1. 4 Methods of paint film formation
During the baking process after applying paint, the temperature of the paint film increases, and the solvents on the surface begin to evaporate first. After solvent evaporation, the surface tension increases. Within the liquid, the flow occurs from areas with lower tension to areas with higher tension. Therefore, the paint also flows from the bottom to the top. However, under the influence of gravity, some paint may also sink from the surface. As the solvent evaporates and the viscosity increases, the upward and downward flow gradually slows down and eventually stops, completing the curing of the paint film.
2 Maintaining cleanliness in the paint mixing room
2.1 The importance of cleanliness management
In general, after applying a coat of paint, particles with a diameter of 5 to 15 μm may appear on the vehicle surface. These can be repaired or polished using sanding and polishing methods. Particles with a diameter of 16 to 25 μm can be repaired or repainted. For particles larger than 25 μm, single-piece or full-vehicle repainting is required. To understand this more clearly, 15 μm is approximately the diameter of a human hair. With the increasing trend of lightweight vehicle construction, the thickness of the coating is also decreasing. As the paint dries, particles and fibers within the paint become more visible, leading to the formation of particles on the vehicle surface. Therefore, managing the cleanliness of the paint can effectively reduce the need for sanding, polishing, and repainting, thereby improving product quality.
2.2 Common Paint Defects
1) Impurities in paint: Paint manufacturers strictly control this aspect during production and conduct thorough inspections before shipment. However, due to the inherent properties of paint, noticeable impurities can form during storage, transportation, and use.
2) Metal Paint Particles: Metal paint, in its typical understanding, refers to the addition of metal particles, such as aluminum powder, to paint. Due to the high density and conductivity of metal particles, issues such as clumping and sedimentation often occur during construction, requiring stringent requirements. Furthermore, differences in materials used by various paint manufacturers result in variations in the particle size of the metal particles. If some metal particles have a particle size that is too large and are not effectively filtered during construction, they can easily form particulate problems after spraying.
The following two cases illustrate the issue of metal coatings causing overall surface imperfections that require polishing, which we encountered during production.
3 Examples related to cleanliness management
3.1 Large-area metallic particles on the surface
In June 2018, during the entire vehicle coating production process, after spraying the shimmering gold paint, significant surface granularity was observed on the entire horizontal surface (including the hood and roof), along with noticeable granularity issues on areas with thinner clear coat film on the sides (see Figure 1). Analysis of the clear coat layer using a 200x electron microscope revealed no other types of impurities, ruling out the possibility of granularity issues caused by environmental cleanliness. After using 2000# sandpaper for grinding and polishing, the granular areas showed no obvious abnormalities, and the electron microscope clearly showed the accumulation of aluminum powder in the granular areas.
based on the analysis of the results, the initial cause has been preliminarily identified as excessively coarse aluminum powder particle size or aluminum powder clumping in the color coating layer. Building on this, we conducted a series of comparative tests. When applying the clear coat using a robotic sprayer, we manually increased the coating thickness on the machine cover surface by approximately 15 μm, and the difference in particle count was more than 60% (see Table 3).
based on the above test results, and considering the location and method of robotic spraying, the final judgment is that the rapid increase in summer temperature caused the color coating thinner to evaporate too quickly, resulting in the rapid drying of the sprayed color coating surface. This caused some metal particles in the coating to settle on the surface of the color coating, and during the formation of the clear coat, they moved to the surface of the color coating, forming particles. The side of the vehicle was relatively less affected.
based on the analysis results, the following measures were taken: 1) Reduced the viscosity of the metal paint and increased the addition of the slow-drying agent. However, considering the potential for the solvent in the metal paint to evaporate slowly and cause blooming, the addition of the slow-drying agent was limited to around 1%, which also enhanced the effect of re-dissolving metal particles; 2) Used a 200-mesh filter for the metal paint.
The original paint should be filtered before mixing, as the use of filtration bags with a mesh size of 100 is common in coating mixing systems due to equipment considerations. Using a 100-mesh filtration bag can easily damage the bag.
- only particles with a size of 150 μm can be filtered. Using a 200-micrometer filter bag, the particle size of the metal particles can be reduced to 75 μm. After implementing these two temporary measures, the number of particles on the metal surface after applying the paint decreased by approximately 50% without adding a clear coat.
Following this incident, we have increased our requirements for the particle size of metal particles used in metal paints supplied by paint vendors. We have also improved the three-stage paint filtration system in the mixing room, which includes primary filtration of the original paint before mixing, secondary filtration of the paint circulating to the container, and tertiary filtration of the paint circulating system. This system effectively eliminates the recurrence of this quality issue.
3.2 Metal paint for complete vehicle
As we entered 2018 winter, the issue of metal flake particles, which had not appeared for a long time, resurfaced frequently. This time, the problem manifested as a widespread issue of particles, but there were also differences. Specifically, the particles were clearly visible on both horizontal and vertical surfaces. Furthermore, the particle issue was particularly pronounced in areas with thin film thicknesses.During the problem resolution process, we implemented corresponding measures based on the initial repair approach, but the results were not significant. However, the most severe instances of this issue were concentrated in the Coral Red and Amber Orange colors.
By comparing the conditions of these two colors of metal paint with other colors, we found that these two colors, due to their small color, experience air compression and exhaust, leading to frequent freezing of the exhaust port. As the exhaust port becomes blocked, the exhaust volume decreases, reducing the number of strokes of the plunger pump, and the speed at which the paint flows through the pipes also decreases significantly. Previously, we could only thaw the pipes by pouring hot water. As the ice in the exhaust port melts, the plunger pump also returns to normal operation. In normal operation, the number of particles on the Coral Red and Amber Orange painted vehicle bodies also decreases significantly.
based on this analysis, we also implemented a temporary measure: 0.5 hours before applying coral red and amber orange, we would heat the piston pump with hot water to thaw it, ensuring that the metal paint could circulate properly. This resulted in a significant reduction in the number of fine particles on the surface after applying the paint. Consequently, we concluded that the piston pump's paint circulation system slowed down due to freezing of the exhaust port at low winter temperatures, leading to the clumping and settling of metal particles. However, thawing with hot water cannot fundamentally solve this problem.
After continuous experimentation and exploration, we implemented three measures to ensure the normal functioning of the metal paint:
1) Heat the compressed air to ensure that the compressed air does not freeze after passing through the piston pump.
2) Connect the ventilation system of the spray booth to the mixing room, improving the temperature stability of the mixing room.
3) The color coat in the paint circulating system should be cleaned once every month, while the clear coat and primer should be cleaned every 3 months. Before cleaning, pre-filter the diluted solution using a 300-mesh filter bag to ensure the cleanliness of the cleaning solvent. By implementing these measures, the number of paint particles on the metal surface after coating with the color coat has decreased significantly (see Figure 2).
4. Conclusion
With the increasing popularity of automobiles in everyday use, car body colors are becoming more and more diverse. Metal paints are particularly popular due to their unique advantages. However, this also requires higher quality control and equipment standards for the painting process. While maintaining anti-corrosion properties, automotive coatings are moving towards lighter and thinner options. Therefore, maintaining the cleanliness of paint booths will be even more crucial in ensuring consistent quality and improving paint performance.
(Please refer to "Modern Coatings and Coatings" 2020-12 for details)