A Brief Discussion on How to Improve the Weather Resistance of Powder Coatings for Aluminum Profiles

The weather resistance of the powder coating applied to the surface of aluminum profiles is a key factor affecting the lifespan of aluminum profiles. This article mainly analyzes it from a technical perspective, including the raw materials of powder coatings, formulation structure, production process, and curing conditions, and combines the experimental results to summarize several ideas that can be referenced to improve the weather resistance of powder coatings, thereby ultimately achieving the goal of extending the lifespan of aluminum profiles.

Powder coatings are mainly composed of resins, curing agents, additives, pigments, and fillers. The coating sprayed on the surface of aluminum profiles gradually experiences a decline in performance over time due to natural environmental factors such as sunlight, rain, oxidation, temperature changes, and microorganisms, a phenomenon known as aging. The ability of powder coatings to suppress or delay natural aging is called resistance to natural aging, also referred to as aging resistance, or weather resistance. 

There are many factors that affect the weather resistance of powder coatings, including internal factors such as the performance, dosage, and ratio of various raw materials in the powder coating composition, such as resins, curing agents, pigments and fillers, and additives; as well as process conditions during powder coating production and the degree of curing of the coating; in addition to external factors related to the use environment of the coating film, such as sunlight (mainly ultraviolet light), atmospheric composition (oxygen, ozone, industrial smoke, etc.), humidity (including acid rain, salt spray, etc.), and temperature changes.

From the perspective of the powder coating itself, improving the weather resistance of powder coatings mainly focuses on raw materials, formula structure, and production processes.

一. Raw Materials

Powder coatings are mainly composed of resin, curing agent, additives, pigments, and fillers. The weather resistance of these raw materials essentially 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. In addition, these raw materials with qualified weather resistance must also meet other performance requirements of the coating film. When there is a conflict between different performance aspects of the coating film, emphasis can be placed on a specific performance according to customer requirements.

1. Resin

Since resin is the main film-forming substance of powder coatings and is the primary component that determines the properties of powder coatings and the performance of the coating film, the choice of resin is crucial. Large manufacturers on the market commonly use polyester resin for aluminum profile powder coatings. Using the same formula structure and the same production process, powders were made separately for a 300-hour aging resistance test and a comparison of the coating film appearance.

From the test results, it can be seen that resins D, F, and H have better weather resistance among a series of resins, but the drawback of these resins is their high molecular weight and high melt viscosity. If they are used in flat powder, they will ultimately lead to poor leveling performance of the coating film. Therefore, when using resins with good weather resistance in powder coating formulations to improve the weather resistance of flat powder, it is necessary to consider whether the leveling of the coating film will deteriorate and whether the customer can accept it if it does.

2. Curing Agent

Although the curing agents in the HAA system are well known for being environmentally friendly, their drawback is that the curing reaction produces by-products, which can lead to defects such as pinholes and black spots during thick spraying. In addition, the baked coating's resistance to yellowing and durability are not as good as the TGIC system. Currently, the aluminum extrusion industry finds it difficult to accept these disadvantages of the HAA system, or rather, most powder coating manufacturers have not solved these issues. Therefore, 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 most influential factor on the weather resistance of powder coatings after resins in raw materials. Since pigments can fade during use, selecting pigments for powder coatings for aluminum profiles is also very important. Even for a single color pigment on the market, there are many different types, and their properties such as hue, coloring power, covering power, weather resistance, and heat resistance vary greatly, which brings significant challenges to powder coating manufacturers in making choices. Pigments can be divided into inorganic pigments and organic pigments according to their chemical composition.

The selection of pigments needs to be considered from multiple aspects, such as the pigment's hue, tinting strength, hiding power, heat resistance, weather resistance and light fastness, boiling water resistance, toxicity, and so on; moreover, since some pigments may contain active groups that can promote the reaction of resins and curing agents, the effect of pigments on the coating's reaction rate and viscosity also needs to be considered.

Determined by the inherent properties of the pigments, organic pigments have limited lightfastness and weather resistance, while inorganic pigments are not bright enough. Therefore, most powders of bright colors use organic bright pigments with limited lightfastness and weather resistance. This is the main reason why coatings of bright colors have relatively large aging-related color differences. Therefore, to ensure the weather resistance of bright-colored coatings, in addition to selecting other materials with excellent weather resistance, more attention should be paid to the choice of pigments.

4. Fillers

The important function of fillers is that after being added to powder coatings, they can improve the physical and mechanical properties of the coating, such as hardness, rigidity, and scratch resistance, while also helping to improve the storage stability, flowability, and charging properties of the powder coatings. Common fillers used in aluminum profile powder coatings are barium sulfate. After conducting aging resistance tests on barium sulfate from multiple manufacturers, it was found that there was no significant difference in weather resistance.

Therefore, in order to ensure the weather resistance of the coating, high-purity barium sulfate must be used as the filler. Barium sulfate must be inspected before entering the warehouse. Using a 10% hydrochloric acid solution is a simple and quick method that can rapidly detect whether the sulfate 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. Additives

In powder coating formulations, the amount of additives is very small, but they are indispensable components in general powder coating formulations 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 various large manufacturers, it was found that leveling agents, brighteners, benzoic acid, etc., from different manufacturers had little effect on the coating film's aging resistance; however, wax powders and matting agents from different manufacturers had a significant impact on the coating film's aging resistance. Therefore, to improve the weather resistance of powder coatings, the use of wax powders and matting agents with good aging resistance can be considered.

二. Formulation Structure

By calculating the acid value of the polyester resin and the hydroxyl equivalent of the curing agent in the powder coating, an appropriate amount of curing agent is designed. Through verification by accelerated aging tests, the optimal amount of curing agent is used so that the coating can fully cure during curing, thereby achieving the best weather resistance.

Under the premise of meeting the various performance requirements of the powder coating, the variety of raw materials in the formulation should be minimized as much as possible, especially materials that negatively affect the weather resistance of the coating. Without affecting the coating's coverage, hardness, or wear resistance, the amounts of pigments (especially high oil absorption pigments) and fillers in the formulation can be appropriately reduced, allowing the pigments in the formulation to be fully coated by the resin during the melt mixing process. This reduces color changes of the pigments during the use of the coating, thereby improving the weather resistance of the coating.

三. Manufacturing Process

The production process of powder coatings mainly includes four stages: premixing, melt extrusion, flaking and crushing, and classification and grinding. Among these, the premixing and melt extrusion processes have a significant impact on the weather resistance of powder coatings. The purpose of premixing is to ensure that the various raw material components in the powder coating formulation are evenly dispersed, laying a good foundation for melt extrusion. To improve weather resistance, during the premixing stage, raw materials must be added in a specific sequence, with the feeding amount controlled at 20%-80% of the mixing tank capacity, and the mixing time should be appropriately extended.

Melt extrusion aims to uniformly mix the various components in the powder coating composition, ensuring that each particle in the finished powder coating has the same composition. To improve weather resistance, during the melt extrusion stage, the extruder temperature should be appropriately increased under the premise of avoiding gelled particles, allowing the resin to melt into a fluid state. This ensures good wetting of the pigments and generates high shear forces, promoting the high dispersion of pigments and other components, forming a uniform system. While ensuring production progress, the extrusion speed can be appropriately reduced to provide sufficient melt kneading time. This improves the mixing effect, ensuring that pigments and fillers in the powder coating are fully coated by the resin, and that the particles of various raw material components are evenly dispersed. In particular, curing agents and resins can be evenly mixed according to the formulation, allowing the coating to cure fully during crosslinking, thereby increasing the surface density of the coating and ultimately improving the weather resistance of the powder coating.

四. Curing Conditions

Powder coatings can only achieve full cross-linking between the resin's polymer chains and the curing agent when they are fully cured, allowing all physical and chemical properties to reach their optimal state, and weather resistance is no exception. Therefore, to improve weather resistance, powder coatings must be fully cured, and the temperature and time during curing must be properly controlled.