Aluminum profile is very popular in the market because of its many excellent properties. Aluminum profile refers to various aluminum materials obtained by processing aluminum rods. The production process of aluminum profile is mainly hot melting, extrusion, spraying and other processes, among which spraying process is one of the more difficult processes to control and operate, and it is generally difficult to spray successfully at one time, and most of them need to be reworked and adjusted. So what problems will occur in the spraying process of aluminum profile?

The reason why sandgrain powder coatings are more prone to delamination between layers must have its particularity.

Compared with plain powder coating, the formula of Shanghai aluminum profile sand grain powder coating has its particularity.

The main raw materials used in the formula, such as polyester resin and curing agent TGIC (or HAA), may be lower.

The grit agent used in the formula is a substance with low surface tension, and it is easier to form a coating surface with low surface energy after curing.

More wax is used in the formula to improve the surface hardness and scratch resistance, and floats on the coating surface after curing.

Samples to be inspected, stored powder and recycled powder may be used in the formula.

When cured, the sand grain powder also shows different curing characteristics from ordinary plain powder.

The essence of adhesion is the force between interfaces, and it is the degree of firmness between organic coatings and substrates through physical and chemical actions. It mainly includes two aspects: the ability of mutual bonding between organic coatings and metal substrates and the degree of intermolecular crosslinking of organic coatings. The greater the bonding force between coatings and substrates, the better. The more thorough the coating curing, the more stable the intermolecular crosslinking and the denser and firmer the formed coating.

When powder coating is sprayed, it is absorbed to the surface of the workpiece by static electricity, and then it goes through four processes: melting, leveling, gelatinization and curing in the curing furnace. Among them, in the three processes of melting, leveling and gelatinization, the powder is a Shanghai aluminum profile with liquid characteristics, which wets the substrate and penetrates into the surface pores of the substrate, so that the powder is closely combined with the coated substrate to form adhesion.

If the powder layer can't be closely combined with the substrate in the three processes of melting, leveling, gelatinization, then the coating will be separated from the substrate when the workpiece is acted by external force, which is often called "plastic shedding", "plastic stripping", "coating shedding" or "unqualified adhesion". Wettability is a necessary standard when examining adhesion, and it only works when the coating effectively wets the substrate. Wettability of the coating to the substrate is the key to forming adhesion. The wetting of the coated surface can be described from the thermodynamic point of view. The surface tension of the coating in liquid state and the surface energy of the substrate and the solid coating film are important parameters that affect the interfacial bonding strength and adhesion formation.

Therefore, it is very important for the wetting of the substrate to obtain good adhesion powder in the molten state, and the wetting process is related to the surface tension of the substrate, the viscosity of the melt and the contact time. Because of the high viscosity and short gelation time of sand grain powder during curing, it is just unfavorable to wet the bottom sand grain surface when it is sprayed back on the sand grain surface with low surface energy.

It is reported that adding a small amount of some nitrogen-containing groups can greatly improve the adhesion. Because of the ammonia-transesterification reaction between the two phases on the interface, amide bond is formed, but it has not been confirmed in practical operation.

Adhesion usually forms adhesion to the substrate in several forms as shown in Figure 1, and the adhesion depends on the surface of the substrate and the properties of the coating. In a broad sense, these binding forces can be divided into two categories: primary valence force and secondary valence force. The chemical bond is the principal force, which has much higher adhesion than the secondary force, and the secondary force is a much weaker force represented by hydrogen bond. When powder coating is sprayed on the substrate, adhesion is formed during the curing process. Covalent bonds may be formed between the interfaces of thermosetting polyester powder coatings, and the chemical groups that react with each other are firmly bound to the substrate and coatings. It is easier to combine with chemical bonds on the substrate with polar groups, such as conversion film (chromized film) containing crystal water (covalent bonds are formed between reactive carboxyl groups in polyester powder and the interface of conversion film on metal substrate, and this mutually reactive chemical group is firmly combined with the substrate and coating, which solves the problem of adhesion between polyester powder coating and aluminum profile surface), so this kind of connection belongs to chemical bonds and has the strongest durability. However, there are fewer chemical bonds on nonpolar surfaces, such as fully cured and cross-linked polyester powder coatings, so the back-sprayed coating will not react with the base coating chemically, and can only be physically attached or bonded with a small amount of hydrogen bonds. Therefore, it is more suitable for the theoretical explanation of mechanical connection to return sandblasting powder on the surface of sandblasting.

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