The weather resistance of the powder coating film on the surface of aluminum profiles is a key factor affecting the service life of aluminum profiles. Powder coatings are mainly composed of resins, hardeners, additives, pigments and fillers, etc. The coatings sprayed on the surface of aluminum profiles are affected by the surrounding natural factors such as sun exposure, rain, oxidation, cold and heat changes, and microorganisms over time. There will be a phenomenon of gradually decreasing performance, that is, aging. The ability of powder coatings to resist or delay natural aging is called natural aging resistance, also known as aging resistance, also known as weather resistance.
There are many factors that influence the weather resistance of powder coatings, including internal factors such as performance, dosage and ratio.ng of various raw materials such as resins, hardeners, pigments and fillers, and additives in powder coatings; The process conditions of the paint production process and the degree of curing of the paint; and the use environment of the paint film such as the effect of sunlight (mainly ultraviolet rays), the composition of the atmosphere (oxygen, ozone, industrial smoke, etc.), humidity (including acid rain, salt spray, etc.), temperature changes and others external factors.
From the perspective of powder coating itself, improving the weather resistance of powder coating mainly starts with raw materials, formula structure and production process.
Powder coatings are mainly composed of resins, hardeners, additives, pigments and fillers and other materials. The weather resistance of these raw materials essentially determines the weather resistance of the materialn coatings. Therefore, in order to improve the weather resistance of powder coatings, it is first necessary to select raw materials whose weather resistance meets the requirements of the aluminum profile industry, and these raw materials with qualified weather resistance must also meet other properties of the coating film. When the properties of the coating film are in conflict, we can focus on a particular property according to customer requirements, but the artificial accelerated aging test results must meet the accelerated weather resistance requirements in GB5237.4-2008.
Since resin is the main film-forming substance of powder coatings and the main component that determines the properties of powder coatings and film performance, the choice of resin is very important. The powder coating polyester resin for aluminum profiles commonly used by large manufacturers in the market was selected, and the same formula structure and manufacturing process were used to make powders for 300 hours aging resistance test and coating film appearance comparison. The results are shown in Table 1.
(accelThe aging condition is 8 hours light, 4 hours condensing cycle; UVB-313EL lamp, irradiance 0.65W/㎡, light temperature 60℃; condensing temperature 50℃)
According to the test results, resins D, F and H have better weather resistance among a series of resins, but the disadvantages of this resin are large molecular weight and high melt viscosity. In aircraft flour, this will eventually lead to poor leveling performance of the coating film. Therefore, when improving the weather resistance of flat powder by choosing a resin with good weather resistance in the powder coating formulation, the coating film should be taken into account. Whether the equalization gets worse and whether the customer can accept it after it gets worseand.
Although the HAA system hardener is widely known to be environmentally friendly, the drawback is that by-products are formed in the curing reaction, and cavities and pig pores are susceptible to thick spraying arise other disadvantages, the yellowing resistance and durability of the coating film after overbaking are not as good as the TGIC system.
The influence of pigments on the weather resistance of powder coatings is the most influential factor in raw materials next to resin, because pigments fade during use, so for aluminum selection of pigments for powder coatings for profiles is also very important. Even if one color pigment is used in the market, there are many types and the performance of different varieties, such as tint, tinting strength, hiding power, weather resistance, heat resistance, etc. difficulty. Pigments are divided into inorganic pigments and organic pigmentsbased on their chemical composition, and their advantages and disadvantages are shown in Table 2.
The choice of pigments should be considered in many aspects, such as tint, tinting strength, opacity, heat resistance, weather resistance, light resistance, boiling water resistance , toxicity, etc.; and because some pigments may contain active groups that promote resins and hardeners, the influence of pigments on the reaction rate and viscosity of coatings must also be considered.
To improve the weather resistance of powder coatings, pigments should be selected with a light resistance of 7-8 degrees and a weather resistance of 4-5 degrees. At the same time, heat resistance and boiling water resistance must meet the requirements of the aluminum profile industry. Require.
Table 2 shows that due to the nature of the pigment itself, the lightfastness and weatherability of organic pigments are limited, whilel the inorganic pigments are not bright enough, so some brightly colored powders are usually used. Organic clear pigments with limited lightfastness and weather resistance are used, which is the main reason for the large color difference in the aging resistance of brightly colored coatings. Therefore, in order to ensure the weather resistance of brightly colored coatings, in addition to selecting other materials with excellent weather resistance, more attention should be paid to the selection of pigments.
The important function of filler is to improve the physical and mechanical properties of the coating film, such as hardness, stiffness and scratch resistance after addition to the powder coating, and at the same time it is it is beneficial to improve the storage stability, looseness and electrification of powder coatings.
The most commonly used filler in powder coatings for aluminum profiles is barium sulfate. After testingof the aging resistance of barium sulfate from several manufacturers, it appears that there is no significant difference in weather resistance.
Therefore, high purity barium sulfate should be used as a filler to ensure the weather resistance of the coating. Barium sulfate should be inspected before entering the department store. Using 10% hydrochloric acid solution is the easiest and fastest way to quickly detect whether sulfuric acid contains calcium carbonate. Calcium carbonate reduces the weather resistance of the coating and cannot be used as a filler for powder coatings for aluminum profiles. .
5. Auxiliary Agents
In the powder coating formula, the amount of additives is very small, but it is an indispensable part in the composition of the general powder coating formula, and it has a great influence on the coating film. Appearance and Security can play a decisive role. After conducting comparative tests withaccelerated aging on similar additives from large-scale domestic manufacturers, it was found that leveling agents, brighteners and benzoin from various manufacturers have little effect on the aging resistance of the coating film; Membrane resistance to aging is greater. To the weather resistance of powder coatings, washing powder and matting agent with good aging resistance can be considered.
2. Formula structure
By calculating the acid value of the polyester resin in the powder coating and the hydroxyl equivalent of the hardener, the correct amount of hardener is determined and verified by the accelerated aging test, use the best amount of hardener, so that the coating can fully be cured during curing, to achieve good weather resistance.
Based on meeting the different properties of powder coatings, try to reduce the number of raw material types in the formula, by name the materials that have a negative effect on the weather resistance of the coating film. Under the premise that the hiding power, hardness and wear resistance of the coating film are not affected, the amount of pigments (especially pigments with high oil absorption) and fillers in the formula should be appropriately reduced so that the pigments in the formula can be fully absorbed during the melting and kneading process. Resin coating reduces the color change of the pigment during the use of the coating film, so as to achieve the purpose of improving the weather resistance of the coating film.
3. Production Process
The production process of powder coating mainly includes four stages: pre-mixing, melt extrusion, tablet crushing and gradual crushing, of which pre-mixing and melt extrusion are two stages. The process has a major impact on the weather resistance of powder coatings. The role of pre-mixing is to control the various raw materialsin the powder coating formula to evenly distribute and provide a good foundation for melt extrusion. In order to improve the weather resistance, the raw materials in the pre-mixing stage should be fed in a certain order, and the feeding amount should be controlled at 20% -80% of the mixing tank capacity, and the mixing time should be appropriately extended.
Melt extrusion is making the different components in the powder coating composition uniformly mixed, that is to achieve the same composition of each particle in the finished powder coating. In order to improve weather resistance, at the stage of melt extrusion, under the premise of no gelled particles, the temperature of the extruder should be appropriately increased (especially in the case of low temperature), so that the resin can be melted into a liquid to ensure that the pigment can have good wetting and large shear are favorable toor the high dispersion of pigments, etc., making each component a uniform system; while ensuring the progress of production, the extrusion speed can be reduced appropriately to ensure that the material melts sufficiently Mixing time; thus improving the mixing effect, so that the pigment filler in the powder coating is completely covered by the resin, and the particles of different raw material components are evenly dispersed, especially the hardener and resin can be mixed evenly according to the ratio of the formula. The film can be fully cured, which increases the surface compactness of the coating film and improves the weather resistance of the powder coating.
Four, curing conditions
Only when the powder coating is fully cured will the polymer chain of the resin be fully cross-linked with the curing agent and the physical and chemical properties will be optimal. Condition, weather resistanceis no exception. Therefore, the powder needs to be improved to improve weather resistance. The paint is fully cured and temperature and time must be properly controlled during curing.
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