Analysis of Poor Adhesion of Fan Blade Coating

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Author : JU LAI
Update time : 2023-06-15 09:52:28

The coating is the main production process for changing the color of fan blades. The use of powder coating spray baking has an important technical indicator, that is, the adhesion of the powder coating to the fan blade aluminum alloy substrate, which refers to the firmness of the bonding. If the adhesion is not qualified, the coating loses its protective effect on the substrate, and therefore adhesion is particularly important. Powder coatings rely on electrostatic adsorption to the conversion layer of the fan blade, and undergo four processes of sintering, gelling, leveling, and curing in the curing oven. Among them, in the processes of melting and gelling, and leveling, the powder appears liquid-like and will penetrate the pores of the conversion layer, thereby tightly bonding the powder and the conversion layer. If the powder layer cannot be tightly combined with the conversion layer during the melting, gelling, and leveling processes, the fan blade workpiece will exhibit the phenomenon of coating separation from the substrate under external force, which is commonly known as "plastic drop", "coating detachment", or "poor adhesion". In general, the fan blades of industrial fans are solved by special cleaning and surface conversion to solve the problem of adhesion with coatings. Even the treated aluminum alloy surface often does not have an ideal affinity with the coating. As the main material of the coating, the paint should be the first factor to consider. Usually, some polymeric compounds with affinity to the aluminum alloy surface are added to the system to improve the adhesion between the coating and the metal surface. However, improper addition of compounds can also result in poor adhesion.

  1. Insufficient effective substance (tree): If the content of the epoxy resin in low-temperature-cure epoxy powder coating or the content of polyamide in polyamide powder coating is low, the incomplete cross-linking during the curing process will cause the coating to become brittle. Under external force, the coating will fall off and the impact resistance test of the coating is also unqualified.

  2. The molecular weight distribution of the effective substance (resin) is too wide: Resin is an organic polymer compound with a certain molecular weight distribution range. High-quality resin usually has a normal distribution of molecular weight, while poor-quality resin has a wider molecular weight distribution range. This means that the proportion of low molecular weight area and high molecular weight area is higher, and the proportion of middle molecular weight area is lower or the distribution is not average. This result leads to a wider temperature range for resin melting, and the resin cannot be fully melted within a narrow temperature range, which seriously affects the cross-linking process of the resin and causes poor adhesion.

  3. Improper or improper amount of additives: Powder coatings generally include leveling agents, stabilizers, pigments, fillers, etc. If unqualified additives are used or the amount of additives is improper, the surface tension of the coating will change, and the wettability of the coating will become worse. The coating cannot completely penetrate the pores of the conversion layer, leaving gaps between the coating and the substrate, resulting in poor adhesion. At this time, the impact resistance test of the coating may be qualified or unqualified.

  4. The addition of recycled powder or excessive recycled powder: In high adhesion requirements coating, the recycled powder is generally not used or only a small amount of recycled powder is used, because the proportion of various materials in the recycled powder has deviated from the normal new powder, and the mixing of the two will cause the wetting effect of the coating to become worse. The coating cannot completely penetrate the pores of the conversion layer, leaving gaps between the coating and the substrate, resulting in poor adhesion. At this time, the impact resistance test of the coating may be qualified or unqualified.

  5. Use of different coatings: Mixing coatings from different manufacturers can easily lead to poor adhesion because coatings from different manufacturers have different proportions. Mixing them will make the wetting effect of the coating worse. The coating cannot completely penetrate the pores of the conversion layer, leaving gaps between the coating and the substrate, resulting in poor adhesion. At this time, the impact resistance test of the coating may be qualified or unqualified.

The coating is the most important factor affecting the adhesion of fan blades, but many industrial fan blade manufacturers do not have effective means to test the quality of coatings during the production of colored fan blades. Here we introduce two simple detection methods for coatings. Although these methods cannot truly test the quality of coatings, they are still feasible as general monitoring methods.

  1. Heap density test: that is, adding powder coating to a 100ml graduated cylinder, compacting it on a rubber or wooden pad until the powder height no longer drops, and reading the volume. Pour out the powder and weigh it. The weight divided by the volume is the heap density. Compare this value with the heap density values ​​of high-quality powders on the market. If it is higher, it means that there are too many additives in the coating. If it is lower, it means that the amount of additives or the molecular weight of the resin in the coating is too low.

  2. Particle size distribution test: Accurately weigh a certain weight of powder coating and add it to a set of sieves. Start from the top layer and cover the sieve with a lid. Manually shake the sieve. Take out the coating on each layer of the sieve, accurately weigh it, divide this weight by the total weight, and then multiply it by 100%. This is the composition of the particle size of the coating on this sieve. This composition should have relatively low amounts of large and small particles, accounting for about 10-30%; the middle range is high, accounting for about 60-80%. If the distribution is not within this range, it means that the quality of the coating is low. Although the adhesion of the coating after painting depends not only on the selected coating but also on the surface pretreatment of the substrate, construction methods, operating conditions, storage, and usage environment, as industrial fan manufacturers, they should balance various indicators and make the adhesion that measures the important index of colored fan blades as their core advantage.

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