Talking About How To Improve The Weatherability Of Powder Coatings For Aluminum Profiles

- Oct 30, 2019-

The weather resistance of the powder coating film sprayed on the surface of the aluminum profile is a key factor affecting the life of the aluminum profile. This paper mainly analyzes the raw materials, formula structure, manufacturing process and curing conditions of powder coatings, and combines the experimental results to summarize several ideas for improving the weather resistance of powder coatings. The purpose of aluminum profile life.


With the rapid development of the national economy, the outdoor application of powder coatings is becoming more and more popular, and people pay more attention to the weather resistance and durability of powder coatings, especially for the surface of outdoor products such as aluminum profiles, ceilings, curtain wall panels, etc. The weather resistance requirements of the coating film are getting higher and higher.


The powder coating is mainly composed of resin, curing agent, auxiliary agent, pigment and filler. The coating on the surface of the aluminum profile is prolonged with time, and is affected by surrounding natural factors such as sun, rain, oxidation, cold and heat, and microorganisms. The role of gradual degradation, that is, aging. The ability of powder coatings to inhibit or retard natural aging is called natural aging resistance, referred to as aging resistance, also known as weatherability.


There are many factors affecting the weather resistance of powder coatings, including the internal factors such as the properties, dosage and ratio of various raw materials such as resin, curing agent, pigment and filler, additives in powder coating composition, and the process conditions of the powder coating process. The degree of curing of the coating; there are also external factors such as the use of the coating film such as sunlight (mainly ultraviolet), composition of the atmosphere (oxygen, ozone, industrial smoke, etc.), humidity (including acid rain, salt spray, etc.), temperature changes, etc. .


From the point of view of the powder coating itself, improving the weather resistance of the powder coating mainly starts from the raw materials, the formulation structure and the production process.


First, raw materials


Powder coatings are mainly composed of resins, curing agents, additives, pigments and other materials. The weather resistance of these materials basically determines the weather resistance of the coating. Therefore, in order to improve the weather resistance of powder coatings, firstly, raw materials whose weather resistance is required to meet the requirements of the aluminum profile industry should be selected, and these weather-resistant raw materials should also satisfy other properties of the coating film. When the performance of the coating film conflicts with each other, it can be focused on a certain performance according to the customer's requirements, but the results of the artificial accelerated aging test should meet the requirements of accelerated weathering in GB 5237.4-2008.


Resin


Since resin is the main film-forming substance of powder coatings and is the main component determining the properties of powder coatings and film properties, the choice of resin is crucial. The powder coating polyester resin for aluminum profiles commonly used by large manufacturers in the market was selected, and the 300-hour aging resistance test and the appearance of the coating film were respectively prepared by the same formulation structure and the same manufacturing process, and the results are shown in Table 1.


Talking about How to Improve the Weatherability of Powder Coatings for Aluminum Profiles


(Accelerated aging conditions are 8 hours light, 4 hours condensation cycle; UVB-313EL lamp, irradiance 0.65W/m2, light temperature 60 °C; condensation temperature 50 °C)


It can be seen from the test results that the resins D, F and H have good weather resistance in a series of resins, but the disadvantage of this resin is that the molecular weight is large and the melt viscosity is high. If applied in the plane powder, the film flow will eventually be caused. Poor flat performance. Therefore, by using a resin with good weather resistance in the powder coating formulation to improve the weather resistance of the plane powder, it must be considered whether the leveling of the coating film will be deteriorated, and whether the customer can accept it after the deterioration.


2. Curing agent


Although the environmentally friendly type of curing agent of HAA system is well known, its disadvantage is that the curing reaction has the formation of by-products, and the disadvantages such as pinholes and pig pores are easily generated during thick spraying. The film is over-baked and resistant to yellowing and durability is inferior to TGIC system. . [1] The aluminum profile industry is currently difficult to accept these drawbacks of the HAA system, or most powder manufacturers have not solved these drawbacks, so the powder coating for aluminum profiles is still based on the TGIC system. The curing agent TGIC also has a certain influence on the weather resistance of powder coatings.


After aging resistance tests were conducted on TGICs of the top three manufacturers in China, it was found that their aging resistance was basically the same, and there was no major difference. Therefore, it does not make much sense to improve the weatherability of the powder coating by the selection of the curing agent TGIC.


3. Pigments


The effect of pigments on the weatherability of powder coatings is a major factor in the raw material in addition to the resin. Since the pigments will fade during use, the choice of pigments for powder coatings for aluminum profiles is also important. Even if it uses a pigment of a color, it has a wide variety of products. The color, tinting strength, hiding power, weather resistance and heat resistance of different varieties vary widely, which brings great choice to our powder coating manufacturers. Difficulty. Pigments are classified into inorganic pigments and organic pigments according to their chemical composition, and their advantages and disadvantages are shown in Table 2.


Talking about How to Improve the Weatherability of Powder Coatings for Aluminum Profiles


The choice of pigments should be considered in many ways, such as pigment hue, tinting strength, hiding power, heat resistance, weathering and light resistance, boiling water resistance, toxicity, etc.; and because some pigments may have a role in resin and curing agent Promote the active group, so also consider the effect of the pigment on the reaction speed and viscosity of the coating.


In order to improve the weather resistance of the powder coating, the pigment should be selected to have a light resistance of 7-8 (8 grade * good), weather resistance 4-5 grade (5 grade * good), and heat resistance and boiling water resistance should be satisfied. Requirements for use in the aluminum profile industry.


It can be seen from Table 2 that the organic pigment has limited light resistance and weather resistance due to the nature of the pigment itself, and the inorganic pigment is not bright enough. Therefore, some bright color powders mostly use organic bright pigments with limited light resistance and weather resistance. The main reason for the large difference in aging resistance of the bright color coating film. Therefore, in order to ensure the weather resistance of the bright color coating film, in addition to the selection of other materials with excellent weather resistance, it is necessary to pay attention to the selection of the pigment.


4. Filler


The important function of the filler is to improve the physical and mechanical properties such as hardness, rigidity and scratch resistance of the coating film after being added to the powder coating, and to improve the storage stability, looseness and charging properties of the powder coating. [3] The commonly used filler in aluminum powder coatings is barium sulfate. After aging resistance testing of barium sulfate from various manufacturers, it is found that there is no significant difference in weather resistance.


Therefore, in order to ensure the weather resistance of the coating, it is necessary to use barium sulfate having a high purity as a filler. Barium sulphate must be tested before entering the warehouse. The use of 10% hydrochloric acid solution is a simple and quick method. It can quickly detect whether sulphuric acid contains calcium carbonate. Calcium carbonate will reduce the weathering resistance of the coating and cannot be used as a filler for powder coatings for aluminum profiles. .


5. Additives


In the powder coating formulation, the amount of the auxiliary agent is small, but it is an indispensable component in the composition of the general powder coating formulation, and plays a decisive role in the appearance and certain properties of the coating film. After accelerating aging comparison tests on similar additives of large manufacturers in various countries, it was found that the leveling agent, brightener and benzoin of different manufacturers had little effect on the aging resistance of the coating film; while the wax powder and matting agent of different manufacturers applied The effect of film aging resistance is large. Therefore, in order to improve the weather resistance of the powder coating, it is possible to consider the use of a wax powder and a matting agent having good aging resistance.


Second, the formula structure


Through the calculation of the acid value of the polyester resin in the powder coating and the hydroxyl equivalent of the curing agent, the appropriate amount of curing agent is designed, and the accelerated curing test is used to verify that the coating can be fully cured when cured. Thereby achieving * good weather resistance.


Under the premise of meeting the performance of powder coatings, try to reduce the types of raw materials in the formulation, especially the materials that have a negative effect on the weather resistance of the coating film. Under the premise of not affecting the hiding power of the coating film and the hardness and wear resistance, the amount of the pigment in the formulation (especially the pigment with large oil absorption) and the filler are appropriately reduced, so that the pigment in the formulation can be fully utilized in the melt-kneading process. The resin coating reduces the color change of the pigment during the use of the coating film, thereby achieving the purpose of improving the weather resistance of the coating film.


Third, the production process


The powder coating production process mainly includes four stages of premixing, melt extrusion, tablet crushing and fractional crushing. The two processes of premixing and melt extrusion have great influence on the weather resistance of the powder coating. The role of premixing is to make the various raw material compositions in the powder coating formulation evenly dispersed, laying a good foundation for melt extrusion. In order to improve the weather resistance, in the pre-mixing stage, the raw materials must be fed in a certain order, and the amount of feed is controlled to be 20%-80% of the mixing tank capacity, and the mixing time is appropriately extended.


Melt extrusion is to homogenize the various components of the powder coating composition, that is, to achieve the composition of each particle in the finished powder coating. In order to improve the weather resistance, in the melt extrusion stage, under the premise of no gelatinized particles, the extruder temperature is appropriately increased (especially in the case of low temperature), and the resin is melted into a fluid to ensure that the pigment can have Good wetting and *large shearing force are beneficial to the high dispersion of pigments and the like, so that the components become a uniform system; while ensuring the production schedule, the extrusion speed can be appropriately reduced to ensure sufficient materials. Melt mixing time; thereby improving the mixing effect, so that the pigment filler in the powder coating is fully coated by the resin, and the particles of various raw material components are uniformly dispersed, especially the curing agent and the resin can be uniformly mixed according to the formula ratio, and when solidified The coating film can be sufficiently cured to increase the surface compactness of the coating film, and finally improve the weather resistance of the powder coating layer.


Fourth, curing conditions


When the powder coating is fully cured, the polymer chain of the resin will be completely cross-linked with the curing agent, and the physical and chemical properties will reach the best state, and the weather resistance is no exception. Therefore, to improve the weather resistance, the powder coating must be fully cured, and the temperature and time should be controlled during curing.


Conclusion


In summary, for improving the weather resistance of powder coatings for aluminum profiles, the following ideas can be used: selecting resins with good weather resistance and leveling is not too bad; pay attention to the selection of bright pigments, wax powders and matting agents; The formulation structure for improving the weather resistance of the coating film; controlling the production process of the powder coating.