The weather resistance of the powder coating sprayed on the surface of aluminum profiles is a key factor affecting the lifespan of aluminum profiles. This article mainly analyzes it from technical perspectives such as the raw materials of powder coatings, formulation structure, manufacturing process, and curing conditions, and, combined with experimental results, summarizes several ideas for improving the weather resistance of powder coatings as a reference, thereby ultimately achieving the goal of extending the lifespan of aluminum profiles.
With the rapid development of the economy, the outdoor application of powder coatings has become increasingly common, and people are paying more attention to the weather resistance and durability of powder coatings, especially the weather resistance requirements for powder coatings on the surfaces of aluminum profiles, ceilings, curtain wall panels, and other outdoor products.
Powder coatings are mainly composed of resin, curing agents, additives, pigments, and fillers. The coating sprayed on the surface of aluminum profiles will gradually experience performance degradation over time due to natural environmental factors such as sunlight, rain, oxidation, temperature changes, and microorganisms, which is known as aging. The ability of powder coatings to inhibit or slow down natural aging is called natural aging resistance, or simply aging resistance, also referred to as weather resistance.
There are many factors that affect the weather resistance of powder coatings. These include internal factors such as the properties, quantities, and proportions of various raw materials in the powder coating, including resin, curing agents, pigments, and additives; the technological conditions during the powder coating manufacturing process and the degree of curing of the coating; as well as external factors related to the environment in which the coating is used, such as the effect of sunlight (mainly ultraviolet radiation), the composition of the atmosphere (oxygen, ozone, industrial smoke, etc.), humidity (including acid rain and salt spray), and temperature changes.
From the perspective of the powder coating itself, improving the weather resistance of powder coatings mainly involves aspects such as raw materials, formulation structure, and manufacturing process.
1. Raw Materials
Powder coatings are mainly composed of resin, curing agents, additives, pigments, and fillers. The weather resistance of these raw materials basically determines the weather resistance of the coating. Therefore, 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. These weather-resistant raw materials must also meet other performance requirements of the coating film. When there is a conflict between various performance characteristics of the coating film, specific properties can be prioritized according to customer requirements, but the results of accelerated aging tests must meet the specified accelerated weather resistance requirements.
1. Resin
Since resin is the main film-forming substance of powder coatings and the primary component that determines the properties of powder coatings and the performance of the coating film, the choice of resin is crucial. Polyester resins commonly used for aluminum profile powder coatings by major manufacturers on the market were selected. Using the same formulation structure and production process, powders were prepared separately for a 300-hour aging resistance test and for comparison of the coating film appearance.
(Accelerated aging conditions: 8 hours of light, 4 hours of condensation cycle; UVB-313EL lamp, irradiance 0.65 W/㎡, illumination temperature 60℃; condensation temperature 50℃)
From the test results, it can be seen that resins D, F, and H have relatively better weather resistance among the series of resins. However, the drawback of these resins is their high molecular weight and high melt viscosity. If applied in flat powders, they will ultimately lead to poor film leveling performance. Therefore, when selecting resins with good weather resistance to improve the weather resistance of flat powders in powder coating formulations, it is necessary to consider whether the film leveling will deteriorate and whether the customer can accept the deterioration.
2. Curing Agents
Although HAA-based curing agents are well-known for being environmentally friendly, their drawback is that the curing reaction produces by-products, which can lead to issues such as pinholes and craters during thick spraying. The over-baked film also has inferior yellowing resistance and durability compared to the TGIC system. [1] The aluminum profile industry currently finds it difficult to accept these shortcomings of the HAA system, or most powder manufacturers have not solved these issues, so powder coatings for aluminum profiles still mainly use the TGIC system. The curing agent TGIC also has a certain impact on the weather resistance of powder coatings.
3. Pigments
Pigments are the second most influential factor on the weather resistance of powder coatings after resins, because pigments can fade during use. Therefore, the choice of pigments for powder coatings for aluminum profiles is also very important. Even for a single color of pigment on the market, there are many varieties, and different varieties vary greatly in hue, coloring power, hiding power, weather resistance, heat resistance, and other properties, which brings considerable difficulty to powder coating manufacturers in making selections. Pigments are chemically classified into inorganic and organic pigments, and their respective advantages and disadvantages .
The selection of pigments needs to consider multiple factors, such as the pigment's hue, coloring strength, hiding power, heat resistance, weather and light resistance, boiling water resistance, toxicity, and so on. Additionally, since some pigments may contain active groups that can promote the action of resins and curing agents, it is also necessary to consider the impact of pigments on the coating's reaction speed and viscosity.
To improve the weather resistance of powder coatings, pigments should be chosen with a lightfastness rating of 7–8 (8 being the best) and a weather resistance rating of 4–5 (5 being the best). At the same time, heat resistance and boiling water resistance must meet the requirements of the aluminum profile industry.
Determined by the nature of the pigments themselves, organic pigments have limited lightfastness and weather resistance, while inorganic pigments are not bright enough. Therefore, the bright-colored powders mostly use organic bright pigments, which have limited lightfastness and weather resistance. This is the main reason why bright-colored coatings show greater aging-related color differences. Consequently, to ensure the weather resistance of bright-colored coatings, besides selecting other materials with excellent weather resistance, greater attention must be paid to pigment selection.
4. Fillers
The main function of fillers is that when added to powder coatings, they can improve the physical and mechanical properties of the coating film, such as hardness, rigidity, and scratch resistance, while also helping to improve the storage stability, looseness, and electrostatic properties of the powder coating. The most commonly used filler in aluminum profile powder coatings is barium sulfate. After conducting aging resistance tests on barium sulfate from multiple manufacturers, it was found that there is no significant difference in weather resistance.
Therefore, to ensure the weather resistance of the coating, it is necessary to use high-purity barium sulfate as a filler. Barium sulfate must be inspected before entering the warehouse. Using a 10% hydrochloric acid solution is the simplest and quickest method to quickly detect whether the barium sulfate contains calcium carbonate. Calcium carbonate can reduce the weather resistance of the coating and cannot be used as a filler for aluminum profile powder coatings.
5. Additives
The amount of additives in powder coating formulations is small, but they are indispensable components in general powder coating compositions and play a decisive role in the appearance and certain properties of the coating film. After conducting accelerated aging comparison tests on similar additives from large domestic manufacturers, it was found that leveling agents, gloss agents, and benzoin from different manufacturers have little effect on the aging resistance of the coating film. On the other hand, wax powders and matting agents from different manufacturers have a greater impact on the aging resistance of the coating. Therefore, to improve the weather resistance of powder coatings, it is advisable to use wax powders and matting agents with good aging resistance.
