Analysis of delamination of powder layer in back sandblasting of aluminum profile

The surface of aluminum profile has good protective and decorative properties after powder spraying. After decades of development, the spraying process of aluminum profile is becoming more and more perfect, and the spraying yield is also increasing. In general, if the pretreatment and spraying process are strictly controlled, the production of unqualified products can be controlled to the maximum extent. However, due to raw and auxiliary materials, extrusion bad materials, spraying equipment, production environment factors and staff's sense of responsibility, it is difficult to achieve 100% yield of aluminum profile powder spraying once, and most of the defects need to be reworked and repaired. It has been observed that when there are quality defects on the sanding surface and need to be re sprayed, sometimes there will be delamination between the back spraying coating and the first sanding coating, which is commonly known as "two skins", but rarely occurs when the smooth powder coating is sprayed back. Because the adhesion of the coating has a great impact on the weather resistance, salt spray resistance, acid and alkali resistance of the coating, once the adhesion is not qualified, the coating will lose the protective effect on the substrate, so the adhesion is particularly important.

Powder adhesion theory of 2-back sandblasting pattern

The reason why sanding powder coating is more prone to interlayer phenomenon must have its particularity.

Compared with smooth powder coating, the formula of sand grain powder coating has its own particularity

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

It is easy to form the coating surface with low surface energy after curing when using sanding agent as surface tension material;

More wax was used in the formulation to improve the surface hardness and scratch resistance, and floated on the coating surface after curing;

The samples to be inspected, powder in stock and recycled powder may be used in the formula;

When cured, the sand textured powder also shows different curing characteristics from ordinary smooth powder

The melting viscosity is high;

The gelation time is short.

The essence of adhesion is the force between the interface, which is the firmness of the organic coating and the substrate through physical and chemical interaction. It mainly includes two aspects: the mutual bonding ability between the organic coating and the metal substrate and the degree of the bonding between the organic coating and the metal substrate. The greater the bonding force between the coating and the substrate, the more complete the coating curing, the more stable the cross-linking between the molecules, The more dense and firm the coating is.

The powder coating is absorbed to the surface of the workpiece by static electricity during spraying, and it goes through four processes of melting, leveling, gelation and curing in the curing furnace. In the three processes of melting, leveling and gelation, the powder has liquid characteristics, wetting the substrate and infiltrating into the surface space of the substrate, so that the powder and the coated substrate are closely combined to form adhesion.

If the powder layer can't be tightly combined with the substrate in the three processes of melting, leveling and gelatinization, the coating and substrate will be separated when the workpiece is subjected to external force, that is, "plastic peeling", "plastic peeling", "coating falling off" or "unqualified adhesion". The wettability is a necessary standard when testing the adhesion. Only when the coating effectively moistens the substrate can it work. The wetting of the coating on the substrate is the key to the adhesion. The wettability of the coated surface can be described by thermodynamics. The surface tension of the coating in liquid and the surface energy of the substrate and the solid film are the important parameters affecting the interfacial bonding strength and adhesion.

Therefore, in order to obtain good adhesion, it is very important for the powder to wet the substrate 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. Due to the high viscosity and short gelatinization time of the sand grain powder during solidification, the wetting of the bottom sand grain surface is just disadvantageous when it is sprayed back on the surface with low surface energy.

It has been reported that the adhesion can be greatly improved by adding a small amount of some nitrogenous groups. However, it has not been confirmed in practice because of the formation of amide bond due to the ammonia ester exchange reaction between the two phases on the interface.

Adhesion usually forms an adhesion to the substrate in several forms shown in Figure 1. 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 types: the main valence force and the secondary valence force. Chemical bond is the main valence force, which has much higher adhesion than the secondary valence force. The secondary valence force is much weaker than the hydrogen bond. When the 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 reactive chemical groups are firmly bonded to the substrate and coating. On the substrate with polar groups, such as the conversion film containing crystal water (chromized film), it is easier to combine with chemical bond (covalent bond is formed between the reactive carboxyl group in polyester powder and the metal substrate conversion film, and this reactive chemical group is firmly bonded to the substrate and coating to solve the problem of adhesion between polyester powder coating and aluminum profile surface) It belongs to chemical bonding and has the strongest durability. However, on the non-polar surface, such as fully cured and cross-linked polyester powder coating, there is less chemical bond, so the back spray coating will not react with the base coating, and can only carry out physical adhesion or a small amount of hydrogen bonding. Therefore, it is more suitable to explain the mechanical connection theory that the powder with sand grain surface is back sandblasted.

Mechanical connection theory

When the melted powder contacts with the sand grain surface containing holes and holes, the melt can penetrate into it and act as a mechanical anchor. Although the surface roughness of sand grain can improve the adhesion, attention must be paid to avoid the deep and sharp coating shape (sandpaper like coating with too low luster and too dry surface). The rough sand grain coating will penetrate the bottom, and the deep and sharp bulge will form uneven coating, thus forming stress concentration points and reducing adhesion. When the back sandblasting coating is peeled off, it can be seen that the back surface of the back sandblasting coating is smooth, that is to say, when the back sprayed sand grain powder is not melted, it does not really penetrate into the gap of the first coarse sand surface coating. If it can not be completely penetrated, the contact between the coating and the surface will be smaller than the corresponding geometric area, and there will be a gap between the coating and the substrate. The bubbles in the gap will lead to the accumulation of water vapor and eventually lead to the loss of adhesion. This is related to the characteristics of sanding powder: firstly, the surface tension of the first layer of sanding coating is lower after curing, and the surface tension becomes lower when the coating is over solidified; secondly, due to the higher viscosity of sand grain powder during melting and the "thermal barrier" effect of the first coating during back spraying, the heating temperature of the real workpiece is lower than that of the first spraying, and the melt viscosity is more difficult to reduce at lower temperature The sand grain powder melt with low viscosity and low fluidity is difficult to penetrate into the surface of the first coating at a low "workpiece temperature". With the increase of viscosity and coating rigidity, it will gradually form and generate a large amount of stress and remain in the coating. Therefore, the mechanical connection force between coatings is low, and delamination will occur when receiving external force.

Cause analysis of interlayer delamination during 3 backflow injection

Based on the above theory and the characteristics of sand grain powder, the possible causes of delamination can be summarized

3.1 polyester resin content is low

If the content of polyester resin is low (the amount of filler is too large) and the effective film-forming material is small, the crosslinking of the coating will be incomplete during the curing process, and the coating will be brittle after curing, and the coating will fall off when encountering external force.

3.2 low grade polyester resin

Polyester resin is polymerized by polycondensation step by step. Strictly controlling the polycondensation process (controlling the heating rate and the vacuum degree at the later stage of polycondensation reaction) can obtain high quality products with normal molecular weight distribution. If the polycondensation process is not properly controlled (the heating rate is too fast), especially the vacuum degree at the late stage of polycondensation, the molecular weight distribution of polyester resin is wide, which means that there are a lot of low molecular weight heat-resistant polymers in the resin, At higher curing temperature, these low molecular weight polymers will decompose and volatilize from the coating and float on the surface of the coating, forming a low molecular "oil film", which seriously affects the adhesion.

3.3 improper additives or dosage