Powder coating yellowing and countermeasures

The yellowing of powder coatings is a headache.
The color I made in the factory is very white and regular.
When the powder is sprayed in the user's factory, it turns yellow after passing through the drying room.
A few colors will come out.
It became a painted face, causing customer complaints, rework and claims
.

In order to understand the yellowing of the powder, we conducted a series of experimental tests.
The yellowing value of the coating can be measured by a spectrophotometer and expressed as a numerical value
.

The experiment uses a standard formula, and compares and tests the coating online coatingol.
com
.

The test recipe is as follows:

Extrusion: 100-110℃

Baking: 180℃*15min, 200℃*15min, 220℃*15min

1.
Yellowing test of different systems

Table 1: Yellowing test of different systems

b value: indicates yellowish phase.
The larger the value, the more yellowish it is.
When the b value is less than 3, the yellowing is not obvious by naked eyes.
When the b value is greater than 3, the yellowing is more obvious by naked eyes
.

Table 1 can be seen

(1) The yellowing is mainly caused by high-temperature baking.
The yellowing is not obvious below 200 ℃, and the yellowing becomes obvious when the temperature reaches 220 ℃
.

(2) The yellowing degree of polyester/epoxy hybrid type is similar to that of outdoor polyester/TGIC, and the yellowing of HAA is more serious
.

(3) Among Primid curing agents, the yellowing of 1260 curing agent is lighter than HAA (552), and the yellowing is similar to TGIC
.

(4) To control yellowing, the most important thing is to control the temperature not too high.
The mixed curing temperature is 180 degrees, and the yellowing is not obvious below 200 degrees; but the curing temperature of the outdoor TGIC system itself is 200 degrees, and the powder spraying manufacturer is to be safe , The temperature of the drying tunnel is usually adjusted to 220 degrees or more.
Due to the large space of the drying tunnel or drying room, the temperature distribution in different positions is uneven, and the temperature of some parts will be higher.
The higher the temperature, the more serious the yellowing is most likely to occur
.

2.
Dealing with yellowing

Since high-temperature baking is inevitable and yellowing is inevitable, how to solve this problem? We have carried out a series of tests and added an anti-yellowing agent, model L517, to observe the effect by adding 0.
5% and 1% respectively
.

Table 2: Effect of adding anti-yellowing agent

Note: L517 is an anti-yellowing agent of Wuhan Yincai Company

It can be seen from Table 2

(1) For indoor powder, adding anti-yellowing agent L517 has no effect; outdoor powder adding anti-yellowing agent L517 has obvious effect

(2) One disadvantage of HAA is yellowing.

Figure 1: For the same sample, the yellowing of baking at 220 ℃ doubles that of 200 ℃, indicating that the temperature over 200 ℃ has a great influence on the yellowing

Figure 2: The same formula, with 1% L517 anti-yellowing agent added on the right, the yellowing value has been reduced by half, and the effect is very obvious, indicating that the yellowing of HAA curing agent can be solved by adding anti-yellowing agent

Figure 3: At the same temperature of 200℃, the HAA sample plate on the right is added with 1% anti-yellowing agent, which is whiter than TGIC

Figure 4: At 220°C, HAA turns yellow more severely than TGIC, almost twice that of TGIC

Figure 5: Model 1 is HAA, Model 2 is HAA with 1% anti-yellowing agent, and Model 3 is TGIC

3.

 We tested multi-branched polyesters from many manufacturers and found that some polyesters are lightly yellowed, and some are heavy yellowing.

Table 3: Yellowness of different polyesters

Note: FL3301 is Yantai Fenglin polyester
.

in conclusion:

Yellowing is caused by high-temperature baking.
When the temperature reaches 220 degrees, the yellowing occurs faster.
Lowering the baking temperature is the most direct way to solve the yellowing
.

When yellowing is unavoidable, adding anti-yellowing agent L517 can effectively reduce yellowing
.

The yellowing of polyester from different manufacturers is quite different, and some manufacturers have better anti-yellowing effect of polyester at high temperature
.