Application and repair of matt varnish on the car body

With the diversified needs of users for the appearance of lacquer, the lacquer with matt texture gradually enters the public's field of vision, and the matt varnish has also emerged because of its uniform and unique lacquer effect, which has won the favor of more consumers

1 Composition of matt varnish

In addition to the resins and additives used in traditional varnishes, matting agents are important components of matt varnishes

At present, the main components of the matt varnish used in the Polestar production base are acrylic resin and fumed silica

2 The mechanism of action of matt varnish

In the wet film state, the SiO2 matting agent is evenly distributed in the coating film.

At present, the main components of the matt varnish used in the Polestar factory are acrylic resin and fumed silica.

3 Construction technology of matt varnish

At present, the Polestar production line adopts a line-side system.

(1) Covering

Avoid defects such as particles

(2) Dust removal

(3) The first spraying

(4) Leveling after the first spraying

(5) The second spraying

(6) Leveling after the second spraying

(7) Baking
.
Low temperature baking, 80℃, 60min is used on site
.
After baking, the two-component mixing system should be cleaned within 1h to avoid the solidification of the curing agent in the pipeline and cause blockage
.

4 The main factors affecting the extinction effect

4.
1 Resin film-forming material

The resin film-forming substance directly affects the degree of difficulty of matting.
The paints that are easy to matte include nitro paint, amino alkyd paint, urea alkyd paint, and the paints that are difficult to matte include polyurethane paint, epoxy resin paint, and long oil alkyd paint.
, Urea alkyd paint
.
Experiments have proved that coatings with fast drying and lower solid content are easier to achieve the matting effect
.

4.
2 Particle size of matting agent

The particle size of the matting agent mainly refers to the particle size of the agglomerates of the matting agent and the average particle size after dispersion
.
The smaller the particle size, the worse the matting effect, the larger the particle size, the more obvious the matting effect, but too large a particle size will cause a rough hand feel.
The particle size of the matting agent in Polestar Factory is between 4-10μm
.

4.
3 Porosity of matting agent

Porosity is the volume occupied by the effective matting particles in the coating film.
The greater the porosity, the better the matting effect
.

4.
4 Degree of dispersion of matting agent

Dispersion is the process of depolymerization, wetting, uniform distribution and stabilization of pigment particles.
The matting agent can be dispersed at an online speed of about 6m/s and 10-20 minutes; for wax-treated matting agents, avoid a long time exceeding 60°C
.

4.
5 Coating film thickness

The film thickness has a great influence on the gloss of the coating film.
Figure 1 shows the gloss of the coating film at different film thicknesses
.

Figure 1 Extinction effect at different film thicknesses

It can be seen from Figure 1 that when the matte clear coating film thickness is 25μm, the matting effect is more obvious, but the hand feel is also rougher
.
Therefore, it is necessary to weigh the matte effect and the feel
.

5 Common problems in construction

(1) Uneven gloss
.
If the two leveling time is too short, it will cause uneven surface gloss of the coating film (as shown in Figure 2).
The specific leveling time should be determined according to the coating manufacturer and the site construction experience to find the best time
.

Figure 2 Uneven gloss

(2) Bite the bottom
.
If the coating film is too thick, the primer solvent is not completely volatilized, which will cause the bottom to bite
.
For manual spraying, the film thickness control is more difficult
.
In the early stage of production, process engineers test and cure parameters such as spray guns, forming air and spraying methods.
Operators conduct a large number of spraying exercises on body and test panels to minimize manual spraying errors and perform flow calibration every week to ensure that every time The error cannot exceed 5%
.

(3) Paint mist pollution
.
For a matte car body, if there is paint fog pollution, it will cause a rough feel
.
The Polestar site currently produces plastic parts and the car body on the same line.
For the car body, it is necessary to increase the shielding to prevent the paint mist from falling into the car; for the plastic spraying frame, a certain distance must be maintained between the parts, or By adding rotation and limit devices to the individual plastic parts, the individual parts will not cause paint mist pollution to the surrounding plastic parts during spraying
.

6 Defects and repair of matt varnish

6.
1 Defects of matt varnish

After spraying the same high-gloss varnish and matt varnish, and testing the total number of defects, it can be found that the visual defects of the matt varnish car body are reduced by at least 50% compared with the high-gloss varnish, and it has a good defect covering ability
.

6.
2 Repair of matt varnish

The matt varnish cannot be polished and repaired, otherwise it will increase the gloss, so the repair of the matt varnish is more difficult
.
The difficulty of rework determines that most of the matt varnishes have a separate production line
.

(1) Repair the whole vehicle
.
In order to avoid chromatic aberration and ensure gloss uniformity, the whole car is generally reworked, the whole car is polished and re-sprayed with varnish, but this method will increase the production operation time and increase the risk of other defects
.

(2) Partial sheet metal repair
.
This repair method is aimed at repairing body parts.
For example, if the door is defective, the door is removed, polished, and then installed on the spray frame for spraying.
This method can save time, but requires higher color paint, otherwise it will appear and the surrounding area.
The problem of mismatched component color
.
After testing, light-colored paint has a good repair effect in this way, but some dark-colored paints still have color differences in this way
.
Therefore, in the early stage of use, on-site testing and color difference evaluation are also required
.

(3) Point repair
.
After testing, there will be weaving marks and serious chromatic aberration after repairing with point repair.
The quality of this method of repair is basically unacceptable
.

(4) The future repair direction of the matt varnish is limited by the process.
At present, there are still many difficulties in the repair of the matt varnish.
For the factory, it can be repaired by needle picking or using the body structure line repair
.

7 Conclusion

Compared with high-gloss varnish, matt varnish has good defect covering ability and unique paint effect
.
The existing technology is more suitable for small-batch production, requires relatively high environmental cleanliness and anti-paint fog measures, is relatively affected by film thickness, and is difficult to repair
.