What are the applications of antioxidants, light stabilizers and both in coatings?

In this paper, the types of light stabilizers and antioxidants are systematically introduced, and the application of antioxidants, light stabilizers and their combined use in powder coatings is studied respectively, and the effects of the two in inhibiting or delaying the photo/thermal oxidation of the coating film are expounded.

With the rapid development of the national economy, the application of powder coatings in outdoor is more and more common

There are many factors that affect the weather resistance of powder coatings, including internal factors such as the structure and performance of resins, curing agents, pigments and fillers, and other additives; as well as the role of sunlight (mainly ultraviolet), the composition of the atmosphere (oxygen, ozone, industrial Smoke, etc.

Ultraviolet radiation is the main cause of natural aging of powder coatings, and oxygen in the atmosphere is an important factor in promoting natural aging

Therefore, the influence of various factors cannot be ignored.

During the formation process of powder coating film, there will be weak chain bonds and macromolecular chains of diene structure.

In order to inhibit or delay the photo-oxidation speed of the coating film, people usually add antioxidants, ultraviolet absorbers or light stabilizers or a mixture of the three

1.

1.

Weigh the resin, curing agent, leveling agent, pigments and fillers and other additives according to the powder coating formula, put them into a mixing kettle for high-speed mixing, and then extrude them through a twin-screw extruder and cool them
.

Grind the extruded material through coffee grinding, then sieve it through a screen, and spray the obtained finished powder coating onto the sandpaper-polished steel plate with a 40 kV high-voltage electrostatic spray gun, and place it in an oven to bake and solidify.
Boilerplate
.

1.
2 Performance test method

Artificial accelerated aging test: use QUV for artificial accelerated aging test, QUV uses QUB313 light source, and runs for 200h, among which: a value test conditions: UV: 0.
72W/m2, 50℃, 4h; b value test conditions: condensation: 40℃, 4h
.

Baking test: In order to determine the heat resistance of the light stabilizer, the oven baking method is used, and the baking condition is 220 ° C, 30min to carry out the heat resistance test
.

2.
Results and discussion

2.
1 Application research of antioxidants

From the thermo-oxidative degradation mechanism of the polymer, it can be known that the thermo-oxidative degradation of the polymer is mainly caused by the chain free radical reaction caused by the generation of free radicals by the heating of hydroperoxide
.

Thus, polymer thermo-oxidative degradation can be inhibited by free radical trapping and hydroperoxide decomposition, as shown in FIG.
1
.
Among them, antioxidants are widely used for the above inhibition of oxidation
.

Antioxidants (or heat stabilizers) are additives used to inhibit or delay the degradation of polymers by the action of oxygen or ozone in the atmosphere, and are the most widely used additives in polymer materials
.

Powder coatings will undergo thermal oxidative degradation after being baked at high temperature or irradiated by sunlight, and aging, yellowing and other phenomena will seriously affect the appearance and performance of the product.
stabilizer to achieve
.

Antioxidants can be divided into three main categories according to their function (ie, intervention in auto-oxidative chemical processes):

The first category is called chain-terminating antioxidants, which mainly capture or scavenge free radicals generated by polymer auto-oxidation;

The second type is called hydroperoxide decomposer antioxidant, which mainly promotes the non-radical decomposition of hydroperoxide in the polymer;

The third type is called metal ion passivation type antioxidants, which can form stable chelates with harmful metal ions, thereby passivating the catalytic effect of metal ions on the auto-oxidation process of polymers
.

Among the three types of antioxidants, the first type is called the main antioxidant, mainly including hindered phenols and secondary aromatic amines; the second and third types are called auxiliary antioxidants, including phosphites, dithiols Carbamate metal salts,
etc.
In order to obtain a stable coating that meets the application requirements, a combination of various antioxidants is usually selected
.

In the following experiments, different antioxidants were compounded and added to the powder coating formula.
After spraying and curing, the samples were prepared, and the b value was measured with a colorimeter at the same film thickness.
The internationally accepted powder CIE Lab color system ( DIN 6174, ISO 10526 and ASTM 2244) evaluate the color of the coating films
.

Table 1 is the test results sorted by the color of the coating film from the second to the best.
It can be seen that:

(1) The basic formula 1 has serious loss of light.
Although the pigment has good heat resistance, it changes color after film formation.
The analysis believes that the pigment is oxidized at high temperature, and some groups in the pigment react under the action of oxygen
.

(2) The color changes of formula 2 and formula 3 are better than formula 1, but the improvement is not obvious, and formula 3 is better than formula 2
.

After analysis, the antioxidants prevented further oxidation and reduced the discoloration, and the effect of antioxidant 3 was better than that of antioxidant 2
.
Another reason may be that both of them are hindered amines, which prevent the production of dyeing groups after pigment oxidation, but the effect is not good, and can only prevent further reaction after partial oxidation, so the effect is not optimal
.

(3) Formula 4 is better than Formula 3, but not optimal
.
Because the phosphite antioxidant has good color protection ability, it has reducibility and can quickly reduce the oxidized pigment at high temperature, so it has a good antioxidant effect
.

(4) The effect of formula 5 is better than that of formula 4
.
In this formula, the main antioxidant and the auxiliary antioxidant are used together, which not only prevents the further oxidation of the pigment, but also rapidly reduces the oxidized groups, and the auxiliary antioxidant can make the dyeing produced by the main antioxidant.
The group becomes shallow and has a good synergistic effect
.

(5) The color retention effect of formula 6 using compound antioxidant is obviously better than that of formula 5
.
Antioxidant 4 is a mixture of high-efficiency phosphite and phenolic antioxidant, and the ratio of the two is appropriate, which has a good antioxidant effect
.

(6) Formula 7 is better than formula 6, and the color effect is basically the same as the original pigment
.
The recommended dosage of antioxidants is 0.
5% to 1.
0%, so the dosage of formula 6 is significantly less
.
It shows that the color effect remains better after the amount of composite antioxidant is increased
.

(7) The test of formula 8 shows that the use of antioxidants can effectively inhibit the oxidative degradation of the resin in the process of extrusion and film-forming curing of powder coatings and improve impact resistance
.

The formula with the addition of antioxidants can increase the pigment-to-base ratio to achieve the same performance as the smaller pigment-to-base ratio without the addition of antioxidants
.
This is because the addition of antioxidants reduces the tendency of the resin to decompose into low molecular weight products, allowing the macromolecular resin to better coat more fillers with the same performance
.

(8) It can be seen from the white coating film samples of formula 10 and formula 9 that the addition of antioxidants can effectively inhibit the yellowing of powder coatings during processing and post-curing, and improve the color properties of white powder coatings
.

The above test results show that although there are many factors that affect the oxidation of the coating film, such as the quality and type of resin, pigment and additives, the formulation design of the coating, the production process, temperature, atmosphere, humidity and other natural factors, the appropriate antioxidant Applications do reduce the occurrence of this trend
.

2.
2 Application research of light stabilizers

The degradation of polymers under the action of light and oxygen is called "photo-oxygen degradation"
.
Light stabilizer, also known as UV stabilizer, is a kind of stabilization assistant used to inhibit the photo-oxygen degradation of polymer resin and improve the weather resistance of powder coating film
.

According to the different stabilization mechanisms, light stabilizers can be divided into light shielding agents, ultraviolet absorbers, excited state quenchers and free radical scavengers
.

Due to the diversification and complexity of the formulation, curing process and curing form of powder coatings, the light curing and light protection of powder coatings are very important
.

Secondly, the light stabilizer has a very significant effect on the photoaging of the coating and prolonging the service life of the coating film, and the amount is very small, generally only 0.
5% to 1.
0% of the total formula
.

Therefore, the application of light stabilizers in powder coatings to improve their weather resistance is a very simple, low-cost and very effective method
.
The tests in Table 2 and Table 3 will help to illustrate the effect of light stabilizer on the properties of the coating film
.

According to the formula in Table 2, light stabilizer was added to the coating, and the coating film sample was obtained by spraying and curing.
The rapid weather resistance test evaluation method widely popularized in the world was adopted - artificial accelerated aging (QUV) test and baking test
.

Through the test results in Table 3, the application performance of the light stabilizer is evaluated as follows:

(1) Indoor powder has poor weather resistance, but adding light stabilizer will play an obvious role
.

(2) There is no light stabilizer in the formulas of A and D, and the test shows that they are obviously worse than the samples with light stabilizer added
.

(3) The formula samples of C and F show that the increase of the amount of light stabilizer can significantly improve the gloss retention and color retention of the coating film
.

(4) The results of the baking test show that the light stabilizer has no temperature resistance.
To solve the temperature resistance of the coating film, an anti-yellowing agent should be added
.

2.
3 Research on the synergistic application of antioxidants and light stabilizers

Through the above tests, we can know that the aging of the coating film is actually the result of the combined action of ultraviolet light and oxygen, and this process includes two different processes: photodegradation and photooxidation
.

However, the stabilization mechanisms of light stabilizers and antioxidants on the coating film are different.
Combining two stabilizers with different mechanisms of action is expected to obtain a better stabilization effect than a single stabilizer, that is, a synergistic effect
.

There are such stabilizers on the market at present, which is also a development trend of stabilizers
.
But at the same time of synergy, additive and antagonistic effects between two different stabilizers will also appear
.

Therefore, when antioxidants and light stabilizers are used together, it is crucial to have a good understanding of the different reactions between the two.
Only by mastering the potential chemical reactions of the combined effects of the two can an effective system of antioxidants and light stabilizers be designed
.

Among them, the most typical ones are the combination of HALS and antioxidants, the combination of UV absorbers and antioxidants, and the combination of UV shielding agents and antioxidants
.

Through the accelerated aging and baking test of the coating film, the effect of adding antioxidant and light stabilizer to the powder coating formulation was evaluated.
The light stabilizer used here is the HALS light stabilizer that people are more concerned about
.
The test formula and results are shown in Table 4 and Table 5
.

Through the test results, the light stabilizer is evaluated:

(1) The addition of light stabilizer will have an obvious effect on the weather resistance of the powder, but the yellowing resistance of the coating film will not change
.

(2) The combination of light stabilizer and antioxidant has a significant effect on the weather resistance and discoloration of the coating film, and the optimal amount of the two is 1:1
.

(3) It is better to use light stabilizer and antioxidant together in HAA system
.

The combination of light stabilizers and antioxidants is not as simple as described in the article
.
The effect of different light stabilizers and antioxidants in combination needs to be confirmed by further experiments based on theory
.

For example, the combination of HALS light stabilizers and sulfur-containing antioxidants will produce antagonistic effects and reduce the performance of the polymer; when HALS and phosphorus-containing polymers are used together, the synergistic effect should be maximized under the condition of 1:1 concentration.
The combination of low molecular weight HALS and HALS only has an additive effect, while the combination of high molecular weight HALS and low molecular weight HALS has a synergistic effect, and so on
.

3.
Conclusion

Antioxidants and light stabilizers are added to powder coatings, which can effectively inhibit and reduce the thermal oxidation and photooxidative reaction speed of polymer macromolecules in the production and application of powder coatings, significantly improve the heat resistance and light resistance of the coating film, delay the The degradation and aging process of the coating film prolongs the service life of the coating film
.

The combination of light stabilizer and antioxidant is applied to high-performance powder coatings.
If used properly, there will be a synergistic effect, which will significantly improve the weather resistance of powder coatings, especially Super-Duable powder coatings;

If used improperly, there will be additive effects or even antagonistic effects, which will reduce the stability of the coating film
.

The trend of combined use of stabilizers will develop towards the direction of multifunctionalization
.