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The outdoor application of powder coating is becoming more and more common. People pay more attention to the weather-ability and durability of powder coating products. Especially for aluminium profiles, ceilings, curtain wall panels and other outdoor supplies surface. How to improve the weather-ability of powder coatings for aluminium is particularly important.
Powder coatings are mainly composed of resins, binders, additives, and color pigments. With the time going, the properties of the coating of aluminium profiles will decrease gradually due to the effects of natural factors such as sunlight, rain, oxidation, cold and heat changes, and microorganisms. This process is called"aging". The ability of powder coatings to inhibit or retard natural aging is called natural aging resistance, or aging resistance. Also known as weather resistance.
There are many factors that affect the weather-ability of powder coatings. It includes the internal factors such as the performance of resins, binders, additivers and color pigments. As well as the process conditions and curing degree of powder coatings. The use of film environment such as the role of sunlight(mainly ultraviolet), the composition of the atmosphere(oxygen,ozone, industrial smoke, etc), humidity(including acid rain, salt fog, etc.), temperature changes and other external factors.
To improve the weather ability of powder coatings, the first step is to select the raw materials which can meet the requirements of aluminum profile industry. And the qualified raw materials should also meet the other properties of the coating film.
Since resin is the main film-forming material of powder coatings. And it’s the most important component that determines the properties of films. The selection of Resin is very important.
Although the Curatives of HAA System is well known. Its disadvantage is that the by-product is formed in the curing reaction. And pinholes and porcine pores are easy to be produced in thick spraying. The yellowing resistance and durability of overbaking film are not as good as those of TG/C System At present, it is difficult for aluminum profile industry to accept these disadvantages of HAA system. Most powder manufacturers have not solved these problems. So the TGIC system is still the main powder coating for aluminum profile. The curing agent TGIC also has an effect on the weather ability of powder coatings.
The influence of pigment on the weather ability of powder coatings is the most important factor in raw materials except resin. Because pigment will fade during use. Even if one color is used, in the market, there are many kinds of pigments. And the properties of different pigments (such as hue, coloring power, covering power, weather ability and heat resistance) are very different. This gives us a great difficulty to choose.
Table: Comparison of Advantages and Disadvantages between Inorganic Pigment and Organic Pigment
|Inorganic Pigment||Good heat resistance, strong chemical inertia, strong hiding power, good weather resistance, color stability||Poor coloring, limited range of colors, not bright, some toxic|
|Organic Pigment||Strong coloring, good transparency, bright color, broad range of colors. Non-toxic.||High cost of raw materials, poor heat resistance, poor hiding power|
The important function of additiver is to improve the hardness, rigidity and scratch resistance of coating film after it is added to powder coatings. At the same time, it is beneficial to improve the storage stability, looseness and electrification of powder coatings. Barium Sulfate is the most commonly used additiver for aluminum profiles. Thus, to ensure the weather ability of coatings, it is necessary to use high purity barium sulfate as additiver.
In powder coatings formulation, the amount of crosslinker is very small. But in general powder coatings formulation composition, crosslinker is an indispensable component. And it plays a decisive role on film appearance and some properties . To improve the weather ability of powder coatings, consider using anti-aging wax powder and matting agent is available.
By calculating the acid value of polyester resin in powder coatings and the hydroxyl equivalent of curing agent, the suitable amount of curing agent is designed. The optimum amount of curing agent is used to make the coatings fully solidify when curing, which is verified by accelerated aging test. To achieve the best weather resistance.
On the premise of satisfying the properties of powder coatings, reduce the kinds of raw materials, especially the materials which have negative effect on the weather ability of coatings. On the premise of not affecting the covering power, hardness and wear resistance of the coating film. The amount of pigments and additivers in the formulation (especially the pigments with large oil absorption capacity) should be reduced appropriately. So that the pigments in the formulation can be fully covered by resin in the melting and mixing process. In the course of using the coating film, reduce the color change of the pigment. To improve the weather ability of the coating film.
The manufacturing process of powder coatings mainly includes four stages: pre-mixing, melting extrusion, crushing and grading crushing. The two processes of pre-mixing and Melt extrusion have great influence on the weather ability of powder coatings.
The purpose of premixing is to make the raw materials in the powder coating formula disperse evenly. The purpose of Melt extrusion is to make the various components in the composition of the powder coating mix evenly. To improve the weather resistance, in the melting extrusion stage, under the premise of no gelling particles, the temperature of extruder should be raised properly. (especially under the condition of lower temperature), so that the resin can melt into a kind of fluid. To ensure that the pigment can have a good wetting and obtain the maximum shear force. And it is conducive to high dispersion of pigments, so that the components into a uniform system.
While ensuring the production progress, the extrusion speed can be appropriately reduced to ensure that the materials have sufficient melting and mixing time. Thus improving the mixing effect. So that the pigments and additivers in the powder coatings are fully covered by resin. And the particles of various raw materials are dispersed evenly. Especially the curing agent and resin can be mixed according to the proportion of the formulation. The film can be solidified when cured, thus increasing the surface compactness of the film. Finally, the weather ability of the powder coating is improved.
Only when the powder coating is cured, the polymer chain of the resin will be fully cross-linked with the curing agent. And the Physical Chemistry Properties will reach the best state. Weather resistance is no exception. Thus, to improve the weather ability, the powder coating must be fully cured. And well controlling the temperature and time.
In summary, to improve the weather ability of powder coatings for aluminum profiles. We can refer to the following ideas. Choose the resin with good weather ability and leveling not too bad. The formulation structure is designed to improve the weather ability of the coating film. And the production process of the powder coating was well controlled.