The application of antioxidants and light stabilizers in powder coatings is discussed.

Abstract: The
system introduces the types of light stabilizers and antioxidants, studies the antioxidants, light stabilizers and both and applies them in powder coatings, and describes the mechanism of the two in suppressing or delaying the light/thermal oxidation speed of the coating film.
foreword . With the rapid development of the national economy, powder coatings are more and more commonly used outdoors. Thus, as a protective and decorative powder coating, its weather resistance and durability has also received more and more attention, especially such as ceilings, curtain wall panels, water dispensers, air conditioners, washing machines, aluminum profiles and other indoor and outdoor supplies coating .
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 other additives, as well as natural factors such as the effects of sunlight (mainly ultraviolet light), the composition of the atmosphere (oxygen, ozone, industrial fumes, etc.), humidity (including acid rain, salt spray, etc.), temperature changes, etc.
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 effect of ultraviolet rays and oxygen, it causes the powder coating to produce an automatic oxidation reaction, i.e. an oxidation chain reaction, which degrades the powder coating. Water and heat accelerate this reaction and promote photooxidation.
refore, the influence of various factors can not be ignored, only to understand how various factors on the powder coating works, grasp the main contradictions, in order to find out how to improve its weather resistance.
powder coating film in the formation process will exist weak chain bonds and biene structure of the large molecular chain, after ultraviolet radiation, prone to light-induced oxidation and degradation reaction (aging), resulting in the film fade, powdering.
in order to inhibit or delay the photooxidation rate of the coating film, people usually use a mixture of antioxidants, UV absorbers or light stabilizers or 3 kinds. In this paper, the application of antioxidants and light stabilizers in powder coatings and their effects on performance are discussed in combination with the test.
1, test part
1.1 test model preparation
according to the powder coating formula called resin, curing agent, leveling agent, pigment and other additives, put into the mixture kettle high-speed mixing, and then through the double screw extruder extruded after cooling.
will be squeezed through the coffee polished pieces, and then through the screen through the screen, the resulting finished powder paint with a golden horse spray gun 40 kV high-pressure electrostic spray to the sandpaper polished steel plate, and placed in the oven baking curing to obtain a model.
1.2 Performance Test Method
Artificial Accelerated Aging Test: Using QUV for artificial accelerated aging testing, QUV using QUB313 light source, running 200h, where: a value test conditions: UV:0.72W/m2, 50 degrees C, 4h;
test: In order to determine the heat resistance of light stabilizer, the oven baking method is used, the baking condition is 220 degrees C, 30min heat resistance test.
2, Results and Discussion
2.1 Applied Research on Antioxidants
The thermooxygenation of polymers is known to be mainly caused by chain-link free-based reactions caused by the heat-producing free-forming free-agents of hydrogen peroxides.
, polymer thermooxygenation can be inhibited by free-form capture and hydrogen peroxide decomposition, as shown in Figure 1. Among them, antioxidants are widely used to inhibit oxidation.
antioxidants (or thermal stabilizers) are additives used to inhibit or delay the degradation of polymers by oxygen or ozone in the atmosphere and are the most widely used additives in polymeric materials.
Powder coating after exposure to high temperature baking or sunlight will occur thermal oxygen degradation, 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 by adding antioxidants or thermal stabilizers to achieve.
antioxidants can be divided into three broad categories according to their function (i.e., intervention in automatic oxidation chemical processes):
first class is called chain termination antioxidants, mainly to capture or remove free fundamentals produced by automatic oxidation of polymers;
The second class is called hydrogen peroxide decomposition agent type antioxidant, mainly to promote the hydrogen peroxide in the polymer non-free-form decomposition,
the third class is called metal ion passivation agent type antioxidant, which can form a stable chelate with harmful metal ions, thus passivating metal ions on the polymer autooxidation process catalytic effect.
3 types of antioxidants, the first category is called the main antioxidants, mainly phenolic, amphetamines, and the second and third categories are called auxiliary antioxidants, there are paraphosphates, disulfide methamphetamine metal salts and so on. In order to obtain a stable coating that meets the requirements of the application, a variety of antioxidants are often used for compounding.
the following tests using different antioxidant compounds added to the powder coating formula, after spraying and curing, the model, in the same film thick color difference meter measured b value, using the international common powder CIE Lab color system (DIN 6174, ISO 10526 and ASTM 2244) to evaluate the color of the coating film.
Table 1 is based on the color of the coating film from the second increment to the excellent order after sorting the test results, it can be seen:
(1) the basic formula 1 appears serious loss of light, although the pigment heat resistance is better, but after the film, the analysis is that the pigment is oxidized at high temperature, under the oxygen action of the pigment in some of the group reaction.
(2) Formula 2 and Formula 3 color changes better than Formula 1, but the improvement is not obvious, and Formula 3 works better than Formula 2.
analysis, antioxidants prevent further oxidation and reduce color change, antioxidant 3 effect is better than antioxidants 2. Another reason may be because both are blocked amines, prevent pigment oxidation after the production of dyeing groups, but the effect is not good, can only prevent some oxidation after further reaction, so the effect is not optimal.
(3) Formula 4 is better than Formula 3, but not optimal. Because phosphate antioxidants have good color protection ability, it has reductive properties, can make high temperature oxidized pigments quickly reduced, so has a better antioxidant effect.
(4) Formula 5 achieves better results than Formula 4. This formula uses the main antioxidant and auxiliary antioxidant, which not only prevents the further oxidation of the pigment, but also the oxidized base group is quickly reduced, and the auxiliary antioxidant can make the dyeing group produced by the main antioxidant lighter, with good synergy.
(5) using a compound antioxidant formula 6 color preservation effect is significantly better than formula 5. Antioxidant 4 is a mixture of high-efficiency phosphates and phenolic antioxidants, and the ratio of the two is appropriate, with good antioxidant effect.
(6) formula 7 is better than formula 6, the color effect is basically the same as the original pigment. The recommended dosage of antioxidants is 0.5% to 1.0%, so the amount of formula 6 is less. It is shown that the color effect remains better after the amount of compound antioxidant is increased.
(7) Formula 8 test shows that the use of antioxidants can effectively inhibit the oxidation degradation of resins in the process and improve impact resistance during extrusion and film curing in powder preparation.
when adding antioxidants can increase the yanki ratio, to achieve the same performance without antioxidants, smaller yanji ratio. This is because the addition of antioxidants reduces the tendency of resins to break down into low molecular weight products, making large molecular resins better covered with more fillers, while the performance remains the same.
(8) Formula 10 and Formula 9 white coating film model can be seen that the addition of antioxidants can effectively inhibit the processing of powder coating and later curing process yellow denaturation, improve the color performance of white powder coating.
The above test results show that although there are many factors affecting the oxidation of coating film, such as resin, pigment, the quality and type of additives, coating formulation design, production process, temperature, atmosphere, humidity and other natural factors, but the application of suitable antioxidants does reduce the occurrence of this trend.
of 2.2 photostabilizers
the degradation of polymers under the effects of light and oxygen is called photooxygenation. Light stabilizer, also known as UV stabilizer, is a kind of stabilizer used to inhibit photooxygenation of polymer resin and improve the weather resistance of powder coating film.
according to the stability process, light stabilizers can be divided into light shield, ultraviolet absorbent, excitation annihilation agent and free-based capture agent.
the diversity and complexity of powder coating formulation, curing process and curing forms, light conservation and light protection of powder coatings are very important.
second light stabilizer for the light aging of the coating and extend the service life of the coating film is very significant, and the dosage is very small, generally only 0.5% to 1.0% of the total formulation.
it is a very simple, low cost and very effective method to apply photostabilizers to powder coatings to improve their weather resistance. Table 2 and Table 3 tests will help explain the effect of light stabilizers on film properties.
According to the formula in Table 2, light stabilizers are added to the coating, the film model is obtained by spray curing, and the rapid weather resistance test evaluation method popularized internationally - artificial accelerated aging (QUV) test and baking test.
table 3 test results, the application performance of light stabilizers is evaluated as follows:
(1) in-house powder weather resistance is very poor, but the addition of light stabilizers will play a significant role.
(2) A and D formulations were not added to the photostabilizer, and tests showed that they were significantly worse than the models of the light stabilizers added.
(3) C and F formula models showed an increase in the dosage of light stabilizers, which significantly improved the preservation and color preservation of the coating film.
(4) baking test results show that the light stabilizer has no temperature resistance, to solve the temperature resistance of the coating film should be added anti-yellowing additives.
2.3 Collaborative Application Of Antioxidants and Light Stabilizers
Through the above tests, it can be learned that the aging of the coating film is actually the result of a joint action of ultraviolet light and oxygen, a process that includes two different processes of photodegradation and photooxidation.
However, the stability of the film by light stabilizer and antioxidant is different, and the stabilizer of the two different action agents is combined in the hope of obtaining a better stable effect than a single stabilizer, i.e. synergistic effect.
is currently on the market for such stabilizers, which is also a trend of stabilizers. But at the same time, synergy between the two different stabilizers and the effect of confrontation will also appear.
In this way, it is critical to understand the different reactions between the two when using antioxidants and light stabilizers, and only by mastering both and using the potential chemical reactions of the effect can an effective antioxidant and photostabilizer system be designed.
most typical of these are HALS and antioxidants, UV absorbers and antioxidants, as well as UV shields and antioxidants, and so on.
Through the accelerated aging and baking test of the coating film, the effect of adding antioxidants and light stabilizers to the powder coating formulation and using them is evaluated, and the light stabilizers used here are the HALS light stabilizers that people pay more attention to. Test recipes and results can be found in Tables 4 and 5.
the results of the test, the light stabilizer was evaluated:
(1) the addition of photostabilizer will play a significant role in the weather resistance of the powder, but the anti-yellowing of the coating film has not changed.
(2) light stabilizers and antioxidants have a significant effect on film weather resistance and color change, and the two dosage 1:1 is the best.
(3) light stabilizers and antioxidants are more effective in HAA systems.
light stabilizers and antioxidants are not as simple as described in the article. The effects of different light stabilizers and antioxidants need to be confirmed by further experiments based on theory.
e.g. HALS-type photostabilizers and sulfur-containing antioxidants will produce adverbial effects to reduce the performance of polymers, HALS and phosphorus-containing polymers should be used in the same conditions, the concentration should be guaranteed to be the best synergy under 1:1 conditions, low molecular weight HALS and only add-on effect, and high molecular weight HALS and low molecular weight HALS use synergies and so on.
3, Conclusion . Antioxidants and photostabilizers added to powder coatings can effectively inhibit and reduce the speed of thermal oxidation and photooxidation reactions of polymer polymer molecules 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.
Applying light stabilizers and antioxidants to high-performance powder coatings, if used properly, there will be synergies that significantly improve the weather resistance of powder coating films, especially Super-Duable powder coatings, and

trend of using stability agents will develop in the direction of multi-functionalization.
source: DuPont Huajia Chemical Co., Ltd
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