Why does car paint turn yellow?

Why does car paint turn yellow?

Text/Milan Resin Meite only

Yellowing of car paint is actually a relatively complex chemical and physical process that is largely due to characteristics of material itself.

Then what is this concept of car paint yellowing that we often talk about? Let me first talk about this traditional automotive paint coating process, which is divided into four layers: electrophoresis, intermediate coat, base paint, and varnish from inside to outside.

What we can directly observe with naked eye is varnish and colored paint. The layer of colored paint gives us most direct sense of color and determines color of car, white, black, red.

The outer layer of varnish is transparent and can be seen on our furniture. Generally, yellowing of car paint is yellowing of these two layers, base paint and varnish. Not only will white cars turn yellow, all cars will age, fade, and change color, but white cars are more obvious.

Why does car paint turn yellow?

Reason for car paint yellowing:

(1) Constant sunlight is main reason for shortening life of car paint. The ultraviolet rays of sun eventually cause car's coating to oxidize, which in turn leads to aging, discoloration and cracking of car's paint. . If there are still water drops on car body when it is exposed to sun, due to convex lens principle, oxidation rate is even more amazing.

Amino resin varnishes are more likely to oxidize and turn yellow, and their raw material prices are relatively low, so more will be needed for regular low-cost models. Amino resin varnishes are easily oxidized in air, and aromatic primary and secondary amines contained in them are especially sensitive to oxidizing agents, which is also one of causes of yellowing.

Acrylic resin varnishes also change color when exposed to sunlight. Acrylic resins also contain benzene rings. Conjugated double bonds in benzene rings are converted into Lu-type structures under action of light. The image's point of view is that it will produce yellow bands. The car's paint is actually yellow, speckled and flaky. But due to its relatively high density, whole piece appears yellowed from a distance, and type of yellowing is different. Wind yellowing is different from sun yellowing.

The varnish is based on acrylic resin, it also reacts with sulfuric and nitric acids, causing yellowing. Acid rain in cities contains not only carbon dioxide, but also small amounts of sulfuric, nitric and nitrous acids. As soon as acidic substance rises, it undergoes esterification and forms a light yellow acrylate.

The hardener will also yellow car paint, and hardener will also yellow car paint. It must be added to varnish. It is used to speed up curing speed of varnish. in spraying process and strengthen its hardness. Hardener L75, manufactured by Bayer in Germany, is extremely prone to yellowing. However, due to technical requirements of design, there is no other option than to use it, if it is not used, paint may not be possible.

Why does car paint turn yellow?

(2) In addition to problem of oxidation, second reason is that there are many gaps (paint pores) on surface of automotive paint, often waxing and sealing of glaze, a large number of paraffin wax substances are blocked into gaps, and these substances are easily absorb dust and other small particles, and oxidize easily, causing surface of white paint to turn yellow over time. It is difficult to handle original color and gloss with such paintwork. Some car paints that have been used for a long time cannot restore original color after repainting due to deep penetration. This is one of reasons why it is necessary to care for painted surface.

(3) The difference in paint color of white cars is more obvious. After a few years, white paint of a car should turn yellow, and white car body is especially easy to get dirty. The sun's ultraviolet rays are main cause of paint surface oxidation. In addition, air pollution such as exhaust fumes, dust, rain slurry, etc. come into contact with surface of vehicle, affecting color retention.