How to improve the weatherability of powder coatings through antioxidants and light stabilizers?
With the rapid development of national economy, the application of powder coating in outdoor is becoming more and more common. Thereby, the weatherability and durability of powder coating coating as protection and decoration are also receiving more and more attention, especially the coating film of indoor and outdoor items such as ceilings, curtain wall panels, drinking fountains, air conditioners, washing machines, aluminum profiles, etc.
There are many factors affecting the weatherability of powder coatings, including internal factors such as the structure and performance of resins, curing agents, color fillers, and other additives; and natural factors (external factors) such as the effect of sunlight (mainly UV), the composition of the atmosphere (oxygen, ozone, industrial smoke, etc.), humidity (including acid rain, salt spray, etc.), temperature changes, etc.
Ultraviolet radiation is the main cause of natural aging of powder coatings, and oxygen in the atmosphere is an important factor to promote natural aging. Under the action of UV and oxygen, it triggers automatic oxidation reaction of powder coating, i.e. oxidation chain reaction, which degrades powder coating. Water and heat accelerate this reaction and play a role in promoting photo-oxidation.
The powder coating film will have weak chain bonds and macromolecular chains of diene structure during the formation process, which are prone to photo-induced oxidative degradation reaction (aging) after UV radiation, leading to fading and chalking of the coating film.
In order to inhibit or slow down the photo-oxidation of the coating film, people usually use adding antioxidants, UV absorbers or light stabilizers or a mixture of the three.
Research on the application of antioxidants
From the mechanism of thermal oxygen degradation of polymers, it is known that the thermal oxygen degradation of polymers is mainly caused by the occurrence of chain-linked radical reactions triggered by the generation of free radicals from hydroperoxides by heat.
Therefore, the thermal oxygen degradation of polymers can be inhibited by radical trapping and hydroperoxide decomposition, as shown in Figure 1. Among them, antioxidants are widely used for the above inhibition of oxidation.
Antioxidants (or heat stabilizers) are additives that are 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 are subject to thermal oxygen degradation after baking at high temperatures or sunlight, aging, yellowing and other phenomena seriously affect the appearance and performance of the product, in order to prevent or reduce the occurrence of this trend, usually using the addition of antioxidants or heat stabilizers to achieve.
Antioxidants can be divided into three main categories according to their function (i.e., the intervention behavior of the automatic oxidation chemical process).
The first category is called chain-terminating antioxidants, which mainly capture or scavenge free radicals generated by the auto-oxidation of polymers.
The second category is called hydroperoxide decomposer-type antioxidants, mainly to induce non-radical-type decomposition of hydroperoxides in polymers.
The third category is called metal ion passivator-type antioxidants, which can form a stable chelate with harmful metal ions, thereby blunting the catalytic effect of metal ions on the polymer auto-oxidation process.
The first of the three types of antioxidants is called the main antioxidant, mainly phenol blockers, seco-aromatic amines; the second and third categories are called auxiliary antioxidants, phosphite, dithiocarbamate metal salts, etc.. In order to obtain a stable coating to meet the application requirements, usually to choose a variety of antioxidant compounding.
The following test uses different antioxidant compounding added to the powder coating formulation, after spraying and curing, the sample is made, and the b-value is measured in the same film thickness with a colorimeter, and the color of the coating film is evaluated using the international common powder CIE Lab color system (DIN 6174, ISO 10526 and ASTM 2244).
The test results after sorting the color of the coating film in the order from sub-increasing to excellent, it can be seen that.
1, the basic formulation 1 shows serious loss of light, although the pigment heat resistance is good, but after the film is formed, the analysis is that the pigment is oxidized under high temperature, and some groups within the pigment react under the action of oxygen.
2, the color change of formula 2 and formula 3 is better than formula 1, but the improvement is not obvious, and formula 3 has better effect than formula 2.
After analysis, the antioxidant prevented further oxidation and made the color change less, and the effect of antioxidant 3 was better than that of antioxidant 2. Another reason may be due to both are hindered amine, prevent the production of dyeing group after the oxidation of pigment, but the effect is not good, only prevent further reaction after partial oxidation, so the effect can not reach the best.
3, formulation 4 is better than formulation 3, but not the best. Because phosphite antioxidant has good color protection ability, it has reducing property and can make the pigment oxidized at high temperature to restore quickly, so it has better antioxidant effect.
4, the effect achieved by formula 5 is better than formula 4. This formula uses the main antioxidant and auxiliary antioxidant together, so it not only stops the further oxidation of pigment, but also restores the oxidized group quickly, and the auxiliary antioxidant can make the dye group produced by the main antioxidant become lighter, so it has good synergistic effect.
5, the use of composite antioxidant formulation 6 color preservation effect is significantly better than formulation 5. antioxidant 4 is a mixture of high efficiency phosphite and phenolic antioxidant, and the appropriate ratio of the two, has a good antioxidant effect.
6, formulation 7 is better than formulation 6, and the color effect is basically the same as the original pigment. The recommended dosage of antioxidant is 0.5% to 1.0%, so the dosage of formulation 6 is significantly less. It shows that the color effect is maintained better after the dosage of compound antioxidant is increased.
7, formulation 8 test shows that in the powder coating powder making extrusion and film curing process, the use of antioxidants can effectively inhibit the resin in the process of oxidative degradation, improve the impact resistance.
The formulation when adding antioxidants can increase the face to base ratio, to achieve the same performance without the addition of antioxidants when the smaller face to base ratio. This is because the addition of antioxidants reduce the tendency of resin decomposition into low-molecular-weight products, so that large molecules of resin better cover more fillers, while the performance remains unchanged.
8, Formulation 10 and Formulation 9 white coating film samples can be seen, plus antioxidants can effectively inhibit the processing of powder coatings and post-curing process yellowing, improve the color performance of white powder coatings.
The above test results show that although there are many factors affecting the appearance of oxidation in the coating film, such as the quality and type of resin, pigment, additives, formulation design of the coating, production process, temperature, atmosphere, humidity and other natural factors, the application of suitable antioxidants does reduce the occurrence of this trend.
Research on the application of light stabilizers
The degradation of polymers in the presence of light and oxygen is called “photo-oxidative degradation”. Light stabilizers, also known as UV stabilizers, are a class of stabilization additives used to inhibit the photo-oxidative degradation of polymer resins and improve the weatherability of powder coating films.
According to the different stabilization mechanisms, light stabilizers can be divided into light shielding agents, UV absorbers, excited state bursting agents and free radical capture agents.
Due to the diversity and complexity of powder coating formulation, curing process and curing form, it makes the light conservation and light protection of powder coating very important.
Secondly, light stabilizer is very effective for light aging of coating and prolonging the service life of coating film, and the amount is very small, generally only 0.5%~1.0% of the total formulation.
Therefore, the application of light stabilizers in powder coatings to improve their weatherability is a very simple, low-cost and very effective method.
According to the formula in Table 2, light stabilizer is added to the coating, and the coating film sample is cured by spraying.
The application performance of light stabilizer is evaluated as follows.
1, indoor powder weatherability is very poor, but the addition of light stabilizers will play a significant role.
2, A and D formulations are not added to the light stabilizer, the test shows that are significantly worse than the sample added to the light stabilizer.
3, C and F formulations show that the amount of light stabilizer increases, the light and color retention of the coating film has a significant improvement.
4, baking test results show that the light stabilizer does not have the ability to resist temperature, to solve the temperature resistance of the coating film should add anti-yellowing additives.
Research on the synergistic application of antioxidant and light stabilizer
Through the above test, we can understand that the aging of the coating film is actually the result of the joint action of ultraviolet light and oxygen, and this process includes two different processes of photodegradation and photo-oxidation.
However, light stabilizers and antioxidants have different stabilization mechanisms on the coating film, and the combination of two stabilizers with different mechanisms of action is expected to achieve better stabilization effect than a single stabilizer, i.e. synergistic effect.
Currently there are such stabilizers on the market, which is also a development trend of stabilizers. But the synergistic effect at the same time, two different stabilizers between the additive and antagonistic effect will also appear.
Therefore, in the antioxidant and light stabilizer with a good understanding of the different reactions between the two is critical, only to master the effect of the two with the potential chemical reaction, in order to design an effective antioxidant and light stabilizer with the system.
By accelerated aging and baking tests on the coating film, the effect of adding antioxidants and light stabilizers to the powder coating formulation is evaluated. The test formulations 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 play a significant role in the weatherability of the powder, but the yellowing resistance of the coating film did not change.
2, light stabilizer and antioxidant with the coating film weatherability and discoloration has a significant effect, and the amount of both 1:1 when the best.
3、Light stabilizer and antioxidant have better effect in HAA system.
The use of light stabilizers and antioxidants is not so simple as introduced in the article. The effect of different light stabilizers and antioxidants need to be confirmed by further experiments according to the theory.
The addition of antioxidants and light stabilizers to powder coatings can effectively inhibit and reduce the rate of thermal and photo-oxidation of polymeric macromolecules in the production and application of powder coatings, significantly improve the heat and light resistance of the coating film, delay the degradation and aging process of the coating film, and extend the service life of the coating film.
Light stabilizers and antioxidants used in high-performance powder coatings, if used properly, there will be a synergistic effect, significantly improve the weathering properties of the powder coating film, especially Super-Duable powder coating film.
If used improperly, there will be an additive effect, or even an antagonistic effect, which makes the stability of the coating film decrease. The trend of stabilizers and will be developed in the direction of multi-functionality
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