Study on the Application of Medium High Solid Clear Coat for Automobiles

ZHANG Xing-wei (Sichuan Vocational and Technical College of Communications, Chengdu 611130, Sichuan, China)

Abstract: In this article, 1K medium high solid clear coat instead of traditional 1K clear coat is used to study the application of the 3C2B system for automobiles. Compared with traditional 1K clear coat with the same viscosity at application, VOC content, film appearance and performance are investigated using 1K medium high solid clear coat.

0 Introduction
Solvent-based varnishes, as the outermost coatings for automobiles, need to have good resistance to media, abrasion and weather. In order to reduce the VOC content of varnishes, one of the most effective means is to use two-component varnishes (VOC content ≤420 g / L), which will also bring the disadvantage of increased use costs. From an economic point of view, some OEMs prefer to use single-component varnishes. The VOC content of traditional one-component varnishes is usually ≥480 g / L, which is difficult to control below 420 g / L from a technical perspective. In view of this, local governments have also issued limits on the VOC content of solvent-based varnishes. For example: The regulations implemented by Beijing in 2015 stipulated that the solvent-based varnish VOC limit was 480 g / L. At the same time, the Beijing Environmental Protection Department also tested the solvent-based coatings used by the OEM in September 2016, and required the VOC content of the solvent-based coatings to be 480 g / L or less under construction. On the premise of ensuring the same construction viscosity, it is technically feasible to increase the construction solid content of the one-component varnish and reduce the VOC content. In this paper, a single-component medium-high solid varnish is used as a water-based three-coat two-bake system, and the practical application of the coating system is studied.

1 Experimental part
1.1 Construction process
The 3C2B process route is adopted. The traditional single-component varnish is replaced with a single-component medium-high solid varnish, and the entire production line is used for spraying. See Table 1 for construction conditions.

Table 1 Construction Environment Temperature and Relative Humidity in Different Working Procedures

Spraying method: The outer panel is robotic electrostatic spraying, and the inner panel is sprayed by manual air gun. The intermediate coating is flash-dried at room temperature for 7 minutes, pre-baked at 80-90 ℃ for 5-8 minutes, and then baked at 140 ℃ for 20-25 minutes. After the paint and varnish are flashed at room temperature for 5-6 minutes, Pre-bake at 80 ~ 90 ℃ for 5 ~ 8min; after spraying the varnish, level dry at room temperature for 7 ~ 8 minutes and bake at 150 ℃ for 20 ~ 25 minutes.

1.2 Main construction parameters of materials
The laboratory electrophoresis test boards used in this article and the commercial electrophoresis car body used for field spraying experiments are provided with a zinc-based phosphating pretreatment by a main engine factory in the north; all the middle coats, paints and varnishes used are Provided by a paint manufacturer.
Middle coating: 3C2B system water-based middle ash intermediate coating, construction solid content is 55% ± 2%, construction viscosity is (55 ± 5) s (4-4 cups, 25 ℃); color paint: 3C2B black color paint, construction The solid content is 25% ± 3%, the construction viscosity is (500 ± 100) mPa · s (3 # rotor, 60 r / min); the single component medium and high solid varnish (MH varnish): the construction solid content is 54%, Construction viscosity 27s (coated-4 # cup, 25 ℃); traditional one-component varnish (MS varnish): construction solid content is 49%, construction viscosity is 26 s (coated-4 # cup, 25 ℃).

2 Results and discussion
2.1 VOC content comparison

The VOC content of single component medium and high solid varnish (MH varnish) and traditional single component varnish (MS varnish) is shown in Table 2.

Table 2 Comparison of VOC Contents

It can be seen from Table 2 that the VOC contents of the two varnishes are 451 g / L and 503 g / L, respectively.

2.2 Appearance verification of two kinds of varnishes in the laboratory and after mixing
MH varnishes were firstly verified in the laboratory before being applied to the client, and compared with MS varnishes in use at the customer’s site. The results of the appearance comparison data are shown in Table 3.

Appearance Data of Two Clear Coats

Note: The film thickness of the middle coat is 38 μm, and the film thickness of the colored paint layer is 10-12 μm
It can be seen from the data in Table 3 that under the same application viscosity, the construction solid content of the varnish is increased from 49% to 54%, and the appearance of the final coating film has been significantly improved. Regardless of the horizontal plane or the vertical plane, the long and short waves of the MH varnish are lower than the long and short waves of the MS varnish; at the same time, the CF value of the former is significantly higher than the latter. Considering the actual production situation on the site, when applying MH varnish online, it can only be carried out by gradually adding and slowly replacing it. After the two varnishes are mixed in different proportions, the appearance data results are shown in Table 4.

Appearance Data of Two Clear Coats Mixed in Different Proportions

Note: The film thickness of the middle coat is 38 μm, and the film thickness of the colored paint layer is 13 μm
It can be seen from the appearance data in Table 4 that there is no abnormal appearance during the mixing process. Compared with MS varnish, with the increasing amount of MH varnish, the short wave change in the horizontal plane is not obvious, the long wave is reduced, and the CF value is significantly increased; while the long and short waves in the vertical plane are decreased, the CF value is significantly increased.

2.3 Body appearance comparison
After about one week of on-site production, the replacement rate of the two varnishes changed from 0 to 100%, indicating that the MS varnish in the circulation system had been completely consumed at this time. During the replacement process, the appearance was monitored in accordance with the customer’s required test points (a total of 27 front covers, doors, fenders and rear covers). The average value of all body appearance data is shown in Table 5.

Table 5 Influence of Different Displacement Rates on the Appearance of Car Body

From the data in Table 5, it can be seen that when the varnish is replaced, the average short-wavelength of the vehicle body is significantly lower than before, which shows that the use of medium and high solid content varnish can effectively improve the final coating under the same field construction conditions. The short wave of the film, and ultimately improve product quality. See Table 6 for exterior appearance data after MH varnish SOP.

Table 6 Appearance of Car Body after Use of MH Clear Coat

From the data in Table 6, it can be seen that the MH varnish has good construction stability. After continuously spraying multiple vehicles at different times, the appearance data of the vehicle body is basically consistent with the appearance data of the body after complete replacement, and the short-wave data is significantly better than the MS varnish SOP. Time of shortwave data.

2.4 Appearance Evaluation of Rework
According to the customer’s technical specifications, the varnish in the construction state is taken from the main tank on site to verify the initial, one and two rework appearances as shown in Table 7.

Appearance Data (Original, Recoating Once and Recoating Twice)

From the appearance data in Table 7, it can be seen that the appearance data of the MH varnish after SOP meets the customer’s technical specifications, and the appearance data after one rework and two rework are also consistent with the initial appearance data.

2.5 Coating film performance verification

The general performance, one-time rework performance, medium resistance, friction resistance, and workability test results of the MH varnish supporting composite coating film are shown in Tables 8 to 10, respectively.

General Performance Test Table 8 General Performance Test, Table 9 Performance Test after Recoating Once
Table 10 Medium and Scratch Resistance and Applicability Tests

Note:a Soaking method (0.05 mol / L H2SO4, 24 h);b immersion method (0.1 mol / L NaOH, 24 h);c. Measure 60˚ gloss retention after 15 friction cycles (Crockmeter dry friction meter and P2400 sandpaper);d drop method (with droplets closed at 85 ℃, 20% H2SO4, 1 h)
According to customer technical specifications, the adhesion, pencil hardness, cup protrusion, and impact resistance of the coating film of different baking conditions and varnishes with different baking conditions and the coating after recoating were tested (see Tables 8 and 9 for details). The test results show that all test indicators meet the technical specifications. In addition, the MH varnish has a pinhole limit and a sag limit of 60 μm and 50 μm, respectively. There is no problem in acid resistance, alkali resistance and gasoline resistance test. The gloss retention rate at 60 ° after dry friction is above 80%. The above performance test results also meet the technical specifications.

3 Conclusions
(1) The water-based 3C2B system is used to match the one-component medium-high solid varnish. Compared with the traditional one-component varnish, its product appearance has been improved, especially the short-wave improvement is particularly obvious.
(2) The use of a single-component medium-high solid varnish with a construction solid fraction of 54% and a VOC content of 451 g / L can effectively reduce VOC emissions from automobile manufacturers, reduce environmental pollution, and meet current local environmental protection regulations.
(3) The coating film prepared by using single-component medium-high solid varnish has main performances that meet customer technical specifications and good construction performance.

Leave a Reply

Your email address will not be published. Required fields are marked *