Technical Study on Single-Coat Color Paint Formation in Waterborne Primer-Free Process

1. Research Background

With the deepening of energy conservation and environmental protection concepts, the water-based primer-less process has gradually become the preferred solution for automotive coating process design due to its shorter process flow, reduced pollutant emissions, and lower energy consumption. The main process flow is: body treatment process consisting of degreasing, surface conditioning, phosphating, electrodeposition, baking, electrodeposition sanding, sealing, seal baking, wiping, basecoat B1, basecoat B2, pre-evaporation drying, clearcoat, clearcoat baking, forced cooling, and inspection.

The basecoat B2 layer primarily supports the color effect and achieves the exterior color appearance of the vehicle body. Its application process mostly adopts a two-pass basecoat film formation method, which requires a longer spray line and more spray robots, resulting in relatively higher investment costs and greater operational energy consumption. If the B2 basecoat can achieve the same appearance effect with a single-pass film formation, it would reduce the number of robots and shorten the spray line length, significantly lowering both investment and operational energy consumption. Therefore, this paper mainly discusses the impact of the water-based primer-less process using single-pass basecoat film formation on coating appearance quality and the corresponding solutions.

2. Analysis of Control Challenges in Single-Pass Film Formation

When adopting the single-pass basecoat film formation process, the one-time film build is thicker, and the robot output volume is relatively larger compared to two-pass film formation, making it prone to sagging, bubbling, and challenges in aluminum flake orientation for color effect. Specific control challenges are as follows:

2.1 Prone to Sagging, Bubbling, Pinholing, and Other Film Defects

In the water-based primer-less new process, the B1, B2, and clearcoat layers are all applied wet-on-wet. During the application process, water and solvent evaporation is limited, primarily relying on the flash-off oven after B2 to achieve a dehydration rate of over 85 percent. When the B2 layer is applied in a single pass, the water and solvent in the B2 wet film are more difficult to evaporate. Compared to two-pass application, the one-time film build is thicker, making it more prone to sagging, bubbling, and pinholing at edges and corners. To achieve good leveling appearance and reduce film defects, the difficulty of optimizing the contour trajectory (especially for edge and corner areas) and parameters is increased compared to two-pass application.

2.2 Increased Difficulty in Controlling Film Thickness and Color Uniformity

The B2 layer is mainly used to achieve the color effect of the coating. When switching from two-pass to single-pass application, higher requirements are placed on robot contouring and the consistency of the spraying process to meet film thickness uniformity requirements. For colors such as Elegant White (color value L greater than 85 percent, b value between 1.8 and 2.2), the color hiding power is poor. If the film thickness uniformity at the B1 and B2 stations is not good, it will seriously affect the color effect and cause color differences on the vehicle body itself as well as color mismatch with plastic parts at edges and corners.

2.3 Difficulty in Achieving Color Effects with Metallic Aluminum Paint

For metallic aluminum colors such as Crystal Silver and Titanium Gray, the B2 station generally uses two-pass application to achieve the sparkling effect of the color. If the B2 station is changed to single-pass application, with the paint formulation unchanged, the arrangement of metallic aluminum flakes will be affected by the application method, easily causing color differences from the existing color. As shown in Figure 1, the arrangement on the left is not as good as that on the right.

Figure 1: Mottling

3. Experimental Conditions for Single-Pass Basecoat Film Formation

3.1 Basecoat Line Layout

The water-based primer-less process coating production line has 4 robots at the basecoat B1 station, 6 robots at the basecoat B2 station, and 5 robots at the CC (clearcoat) station. For the B2 station single-pass film formation, 4 robots are used for spraying (the other 2 robots are on standby), while the two-pass film formation process uses 3+3 robots. Therefore, under the same vehicle model and same overlap rate, the TCP speed of the spraying robots using the two-pass film formation process will be relatively higher.

3.2 Robot Trajectory Adjustment

To achieve the full-vehicle spray verification of single-pass B2 basecoat film formation, the robot contour trajectory was systematically optimized and adjusted, with special focus on edge and corner areas. The adjusted robot contour (using the Refine S5 as an example) is shown in Figure 2 below:

Figure 2: Example of Horizontal and Vertical Surfaces for Single-Pass Film Formation by Spraying Robot

3.3 Basic Paint Parameters

Table 1: Basic Parameters and Film Thickness of Water-Based Primer-Less Process Paints

Coating Layer	Color	Matching B1	Application Film Thickness / micrometers	Solid Content / percent
B1	Light Gray	---	16-20	41
	Medium Gray	---	16-20	32
	Dark Gray	---	15-18	29
B2	Crystal Silver, Titanium Gray	Medium Gray	10-16	18
	Elegant White	Light Gray	16-20	41
	Cool Black	Dark Gray	10-13	19
	Lafite Red	Medium Gray	10-16	18
CC	Transparent Clearcoat		45-55	58*

Note: 58* indicates the solid content after mixing at a main agent to curing agent volume ratio of 3:1.

4. Impact of Single-Pass Basecoat Film Formation on Coating Appearance Quality for Different Colors

To verify the potential impact of single-pass basecoat film formation on coating appearance quality, different types of basecoat paints (classified by effect pigments in the paint) were tested through full-vehicle spray verification. By adjusting process parameters to achieve the required film thickness for each coating layer, the differences in appearance data between single-pass and two-pass film formation were observed.

In this paper, three classic colors (white, black, silver) corresponding to three commonly used types of automotive paints were studied technically.

4.1 Solid Color Paint

Unlike metallic paints, solid color paints only contain traditional organic or inorganic pigments, such as the Elegant White commonly used on automotive bodies. This section uses Elegant White as an example to discuss the single-pass B2 basecoat film formation process for solid color paints:

4.1.1 B2 Two-Pass and B2 Single-Pass Film Formation Process Parameters

Table 2: Elegant White B2 Two-Pass and B2 Single-Pass Film Formation Process Parameters

The shaping air settings change between single-pass and two-pass film formation, adjusting from small spray width to large spray width, which can reduce contamination. Meanwhile, the output volume for single-pass film formation is higher than that for two-pass film formation.

4.1.2 Comparison of Elegant White B2 Two-Pass and B2 Single-Pass Vehicle Appearance

After robot contour optimization and debugging, the appearance and color difference of Elegant White with single-pass film formation are basically consistent with those of two-pass film formation. Specific values are shown in Table 3.

4.1.3 Recommended Measures for Elegant White B2 Single-Pass Film Formation Application

Based on experimental data and full-vehicle verification, solid color paints represented by Elegant White can fully adopt the B2 single-pass film formation process. However, the following recommended measures should be considered during practical application:

Considering the poor hiding power of Elegant White, special attention should be given to contour refinement and color effects at areas prone to color matching issues on production vehicles, such as A-pillars, rearview mirrors, front and rear fenders, and manual transition areas. Independent spraying parameter sets should be established, and film thickness should be verified multiple times to ensure whole-vehicle color consistency.

By increasing the B2 layer film thickness from 16-20 micrometers to 18-22 micrometers, the impact of Elegant White hiding power issues on whole-vehicle color can be reduced, raising the whole-vehicle color control range from the previous b value of -0.3 to 0, to match the positive b value trend of bumpers and other components.

4.2 Pearlescent Paint

Pearlescent paint refers to color paint that primarily contains mica effect pigments to achieve metallic effects, such as the Cool Black color commonly used on automotive bodies.

4.2.2 Comparison of Cool Black Spraying Method and Parameters

The parameter settings show minimal changes. Based on the production program, independent spraying parameter sets are set for positions such as door handles and rocker panels, with reduced output volume settings.

4.2.3 Comparison of Cool Black B2 Two-Pass and B2 Single-Pass Vehicle Appearance

After optimization and debugging, the color difference and appearance quality data of Cool Black with B2 single-pass film formation are basically consistent with those of two-pass film formation.

Note: For Cool Black, focus on controlling the 25 degree/45 degree L values.

4.2.3 Recommended Measures for Cool Black B2 Single-Pass Film Formation Application

Based on experimental data and full-vehicle verification, pearlescent paints represented by Cool Black can also adopt the B2 single-pass film formation process. However, the following recommended measures should be considered during practical application:

Due to the larger particle size of pearlescent pigments, the orange peel appearance data may show deviations with slightly higher values. Flowability can be improved and coating appearance enhanced by measures such as reducing viscosity.

4.3 Metallic Paint

Metallic paint mainly refers to color paint containing various effect pigments (aluminum flakes, mica) to achieve metallic effects, with a more pronounced metallic sparkle effect, such as the Crystal Silver commonly used on automotive bodies.

4.3.1 Comparison of Spraying Method and Parameters

After adopting single-pass film formation, the shaping air settings change from small spray width to large spray width, which can reduce bell contamination. The output volume is relatively higher than that of two-pass film formation. The bell rotational speed is reduced from 55 krpm to 50 krpm, combined with inner-ring shaping air control, which can reduce paint mist rebound and contamination.

4.3.2 Comparison of Production Vehicle and B2 Single-Pass Vehicle Appearance

After optimization and debugging, the appearance quality data of Crystal Silver with B2 single-pass staggered spraying are shown in Table 7.

The DOI and gloss values of Crystal Silver with B2 single-pass film formation are consistent with those of two-pass film formation. The long and short wave values of orange peel show changes, with the short wave values increasing significantly. The reason is that Crystal Silver is a metallic aluminum paint. When adjusted to single-pass application, the robot output volume increases, and the paint atomization effect and aluminum flake arrangement change, which not only affects the color effect but also increases the orange peel short wave values. However, the overall SW data does not exceed 30.

However, for metallic paints, single-pass film formation requires special attention to changes in coating color data (at 3 angles). After testing, it was found that the 45 degree and 75 degree lightness and a values of the coating changed significantly after adopting single-pass film formation.

4.3.3 Recommended Measures for Metallic Paint B2 Single-Pass Film Formation Application

Therefore, for metallic paints represented by silver paints, if single-pass film formation is to be implemented, the following recommended measures must be considered:

Fully evaluate whether the appearance quality (long and short wave) meets requirements. For trucks or products with relatively lower quality requirements, the appearance quality of single-pass film formation is acceptable.

Color effect implementation: Paint development can be conducted using the single-pass film formation method during the metallic paint color development process, and minor adjustments to production colors can be allowed based on on-site debugging results.

Pay close attention to the overlap rate during the spraying process. Ensure that the whole-vehicle spraying is conducted at a 66 percent to 75 percent overlap rate to prevent mottling issues with metallic paints.

5. Existing Issues and Improvement Measures

Overall, the water-based primer-less B2 single-pass film formation is no different from B2 two-pass film formation in terms of physical and mechanical properties. The main issues it brings are in the application process and color effect.

5.1 Existing Issues

(1) In terms of the application process: Due to single-pass film formation, the required flow rate for a single pass increases, which demands higher equipment capabilities. When high bell rotational speed is required, the flow rate should not be too high. Additionally, due to the thicker wet film from single-pass application, the probability of film defects such as sagging, pinholing, mottling, and bubbling increases.

(2) Based on the limitations of application, color development is also constrained in product development.

5.2 When adopting single-pass film formation, the following measures can be taken from the paint perspective to achieve appearance requirements or improve coating appearance quality:

From the paint development perspective, if high flow rate affects atomization effect, an appropriate increase in material solid content can be attempted. However, for a fixed paint system, the room for increasing solid content is limited.

From the paint development perspective, the thixotropy of the material can be effectively reduced by adjusting the resin ratio, or for existing fixed materials, flowability can be improved by reducing viscosity and other measures, but this carries a risk of sagging.

For metallic paints, the metallic effect and improved leveling for good appearance are mutually constraining.

6. Conclusion

The water-based primer-less coating process has been put into use in our company. Through more than a year of continuous process debugging and optimization, we have achieved production of single-pass B2 basecoat film formation for certain colors in the new process, with stable appearance data and good quality. Based on on-site process debugging and optimization, this paper analyzes the water-based primer-less B2 single-pass film formation from the perspectives of application performance, appearance quality, and color difference, providing a useful reference for the promotion and application of this process in the industry.

Originally published in "Modern Paint and Finishing" journal, Issue 7, 2014, authored by Xing Wenping, Zhang Zuoyang, and Ge Fei from the Process Engineering Institute of Anhui Jianghuai Automobile Co., Ltd. Shared on our WeChat public platform at readers' request.