[China Aluminum Industry Network] 1 Preface
The surface of the aluminum profile has good protection and decorative properties after powder coating. After decades of development, the aluminum profile coating process has become more and more perfect, and the coating yield has also been continuously improved. Under normal circumstances, if the pre-treatment and spraying process are strictly controlled, the production of defective products can be controlled to a large extent. However, due to the raw and auxiliary materials, extrusion billets, spraying equipment, production environment factors and employees' responsibilities, it is difficult to achieve a 100% yield of primary aluminum powder coating. Most of these defects require rework. It has been observed that when quality defects on the sand grain surface need to be returned to the spray, delamination between the spray coating layer and the first sand grain coating sometimes occurs, which is often referred to as “two skins†phenomenon. The delamination problem rarely occurs when spraying light powder coatings. Since the adhesion of the coating has a great influence on the weathering resistance, salt spray resistance, and acid and alkali resistance of the coating, once the adhesion is unqualified, the coating loses its protective effect on the substrate and thus the adhesion. It is particularly important.
2 back to the sand coating powder adhesion theory
The reason why the sand grain powder coating is more prone to interlayer delamination phenomenon, must have its particularity.
Adhesion is essentially the interaction force between the interfaces. It is the firmness of the organic coating and the matrix through the combination of physical and chemical effects. It mainly includes two aspects: the ability of the organic coating and the metal matrix to combine with each other and organic coating. The degree of intermolecular cross-linking between layers is such that the greater the binding force between the coating and the substrate, the more complete the coating is cured, and the more stable the cross-linking between the molecules, the denser and firmer the resulting coating will be.
When the powder coating is sprayed, it is electrostatically adsorbed on the surface of the workpiece and is then subjected to four processes of melting, leveling, gelling and solidification in the curing furnace. Among them, in the three processes of melting, leveling, and gelatinization, the powder is in a liquid state, wets the substrate and penetrates into the surface pores of the substrate, so that the powder is closely combined with the substrate to be coated to form an adhesion.
If the powder layer cannot be tightly bonded to the substrate in the three processes of melting, leveling, gelling, then the workpiece will be separated from the substrate when subjected to external forces, which is often referred to as "plastic elimination." , "plasticized", "coating off" or "adhesion failure." Wetting is an essential criterion when examining adhesion. It only works when the coating effectively wets the substrate. Wetting of the substrate by the coating is the key to adhesion. The wetting of the coated surface can be described from a thermodynamic point of view. The surface tension of the coating in the liquid state and the surface energy of the substrate and the solid coating film are important parameters affecting the formation of interfacial bonding strength and adhesion.
Therefore, in order to obtain a good adhesion powder in the molten state is essential to the wetting of the substrate, 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 gel time characteristics exhibited by the sand grains, the wetting of the underlying sand grain surface when spraying back onto a low surface energy sand grain surface is just unfavorable.
It has been reported that the addition of small amounts of certain nitrogen-containing groups can greatly increase the adhesion. Because the ammonia-transesterification reaction occurs between the two phases at the interface, an amide bond is formed, but it has not been confirmed in actual operation.
R1NH2+RCOOR2→RCONH-R1
Adhesion usually forms the adhesion to the substrate in several forms as shown in Figure 1. The amount of adhesion depends on the surface of the substrate and the nature of the coating. In a broad sense, these binding forces can be divided into two categories: primary price and secondary cost. The chemical bond is the main valence force, which has much higher adhesion than the sub-price force, and the sub-price force is a much weaker force represented by a hydrogen bond. When the powder coating is sprayed on the substrate, adhesion is formed during curing. Covalent bonds may form between the interfaces of thermoset polyester powder coatings, and reactive chemical groups are firmly bonded to substrates and coatings. It is easier to chemically bond on substrates with polar groups, such as conversion films (chromed films) containing water of crystallization (covalent bonds between the reactive carboxyl groups in the polyester powder and the conversion film of the metal substrate are formed. This mutual reactive chemical group is firmly bonded to the substrate and coating to solve the problem of adhesion of the polyester powder coating to the surface of the aluminum profile. Therefore, these types of bonds are chemically bonded and have excellent durability. On non-polar surfaces such as completely cured cross-linked polyester powder coatings, there are less chemical bonds, so the spray coating will not chemically cross-link with the base coat, but only physical attachment or a small amount of hydrogen Bonding. Therefore, it is more suitable for the theory of mechanical connection theory to return sand-grained powder on the sand grain surface.
Figure 1 common adhesion and bonding
Mechanical connection theory
When the molten powder comes in contact with a sand grain surface containing pores and holes, the melt can penetrate and act as a mechanical anchor. Although the surface roughness of sand grain can improve the adhesion, care must be taken to avoid deep and sharp coating shapes (sandwool gloss is too low, the surface is too dry sandpaper coating), this rough sand coating will The phenomenon of bottoming out occurs, and the deep and sharp bulges will form a non-uniform coating, creating stress points and reducing adhesion. When peeling off the sand-grained coating, it can be seen that the backside of the sand-grained coating is smooth, that is, the re-shot sanding powder does not actually penetrate into the first layer of rough sand coating when it is melted. Layer in the gap. If it cannot penetrate completely, the contact between the coating and the surface will be smaller than the corresponding geometric area, and there will be gaps between the coating and the substrate. Air bubbles trapped in the gap will cause the accumulation of water vapor, and zui will eventually lead to the loss of adhesion. . This is related to the properties of the sand grain powder: First, the first layer of sand grain coating has a lower surface tension after curing, and the surface tension becomes lower when the coating is over-cured; secondly, the viscosity of the sand grain powder when melted is relatively large, and The “heat-barrier†effect of the first layer of the coating during re-spraying results in a lower heating temperature of the actual workpiece than in the first spraying. The melt viscosity is more difficult to reduce at lower temperatures, and the sand pattern with high viscosity and low flowability can be reduced. The powder melt is difficult to penetrate into the surface of the first coating at a lower "workpiece temperature", and as the viscosity and the coating rigidity increase, it also gradually generates a large amount of stress and remains in the coating. The mechanical connection between the coatings is low and delamination occurs when subjected to external forces.
3 Analysis of interlayer delamination during re-spraying
Through the above theory combined with the characteristics of the sand grain powder, it is possible to conclude the possible delamination causes when the sand grain powder is returned:
3.1 Low content of polyester resin
If the content of polyester resin is low (the amount of filler is too large), the effective film-forming material is small, the cross-linking of the coating will be incomplete during the curing process, the coating will be brittle after curing, and the coating will fall off when the external force is encountered.
3.2 Low-grade polyester resin
Polyester resin is gradually polymerized by polycondensation reaction. Strict control of the polycondensation process (control of the temperature increase rate and the degree of vacuum at the end of the polycondensation reaction) gives a good quality product with a normal distribution of molecular weight, if the polycondensation process is not controlled properly (heating rate is too fast), especially in the late stages of polycondensation The wide molecular weight distribution of the obtained polyester resin means that there are a large number of thermolabile low-molecular-weight polycondensates in the resin. At a higher curing temperature, these low-molecular-weight polycondensates will decompose and volatilize from the coating and float. On the surface of the coating, a low molecular "oil film" is formed, which seriously affects the adhesion.
3.3 Improper or inappropriate dosage of additives
The sand grain powder coating formulation needs to add sand grain agent and add a certain amount of wax (usually polyethylene wax) to improve the scratch resistance. The main ingredient of the sand grain agent is PTFE, which is our common special The main components of fluorocarbon non-stick coating, PTFE and polyethylene wax will reduce the surface tension of the coating. The proper amount of additive will not have a great influence on the adhesion, but if it is used in excess, it will The surface tension of the sand grain coating is greatly reduced to form a similar non-stick surface with low surface energy, which deteriorates the wetting effect of the paint on the surface thereof, and the pores of the primer layer where the melted re-spray powder is difficult to penetrate completely. In the middle, and in the gap between the two layers of coating, so that the sand coating on the spray back to it is difficult to firmly bond with it, zui eventually lead to the adhesion of the coating is not qualified.
3.4 Incorporating too much recycled powder or expired powder when spraying sand grain powder
All aluminum profiles customers will have a large number of older inventory powders and recycled powders. In order to consume part of the powder inventory and reduce costs, aluminum profile sprayers usually store flat colored powders or sand powders with similar colors for a longer period of time. Mixing according to the proportion to the new powder spraying, since these mixed stock powders or recycled powders are likely to have produced pre-crosslinking reaction or even expired, the proportion of various materials in the sprayed sand grain powder has deviated from the normal new powder formula, When the two are used in combination, the wetting effect of the coating will be deteriorated, the coating cannot completely penetrate into the pores of the underlying sand grain coating, and there is a gap between the sprayed coating and the primer layer, and the zui final adhesion is not qualified. In the expired powder, since the polyester resin and the curing agent (TGIC or HAA) are partially pre-crosslinked, the effective reaction point for curing the coating is reduced (functionality is reduced), which will also seriously affect the adhesion of the coating.
3.5 The curing temperature is too low or time is too short
The low curing temperature or short time severely affects the cross-linking reaction between the polyester resin and the curing agent (TGIC or HAA), and the cured coating has poor flexibility and poor adhesion.
3.6 Curing temperature is too high or too long
Adhesion worsens when the coating is cured at a much higher temperature than the original temperature or the baking time is prolonged. The main reason is that in these two cases, some of the coatings are carbonized, which seriously affects the cross-linking between coating molecules. Polyester powder coatings for aluminum profiles are usually cured at temperatures of 180°C (HAA cure) or 200°C (TGIC cure) for 15 to 20 minutes. The over-crosslinked coating forms a surface with a low surface energy and the high-viscosity melt formed by the re-grit coating coating is difficult to wet on its low surface energy surface.
The reactive carboxyl groups in the moiré powder tend to adhere more firmly to substrates containing similar groups. If the first coating is properly cured, the remaining small amount of carboxyl groups of the first coating will be in the second coating layer. The curing agent TGIC or HAA reacts to facilitate the bonding of the sprayed coating with the primer layer. When the first coating is baked (long baking time and/or too high curing temperature), there is almost no carboxyl residue in the first coating, and the adhesion to the sprayed coating is significantly weakened, sometimes even without adhesion.
3.7 Using a Straight Gas Burner
The direct-discharge gas furnace directly discharges the combustion products into the solidification furnace. If impure gas is used, the small-molecule carbonized product formed when not fully burned will adhere to the surface of the solidified coating, forming an “oil film†and returning to the spray. If it is not removed, it will directly cause the failure of the re-spray coating. In addition, if some indirect exchange gas stoves are improperly maintained (eg, gas leaks occur), it may also cause combustion by-products to adhere to the surface of the coating and cause the loss of adhesion at the later time of re-injection.
3.8 Compatibility between coatings
Sand grain powder coatings produced by different manufacturers, due to the different reactivity and viscosity of the resins used, or different batches of sand grain coatings from the same manufacturer, due to differences in formulas (some batches may consume inventory powder, pending inspection Powder or even recycled powder) may have supporting problems. If re-spraying uses different manufacturer's sand grain powder or a batch with a large difference, it is very likely that delamination will occur during re-spraying. The precautionary measure is, do not easily change the paint supply manufacturer or the sand grain powder before entering the factory or before using the small batch of test spray to detect the adhesion of the back spray.
3.9 spray thickness thick
When the sprayed layer is thicker, the cohesive force is increased along with the increase of the viscosity and the coating rigidity and the adhesion to the substrate, and remains in the coating. The difference in stress between the coatings after curing is large, and when applied by external forces, the coating will naturally fall off.
4 The correct operation process of sandblasting powder
Polyester powder coating for aluminum profiles is a thermosetting powder coating. Due to the absence of chemical bonding between two spraying coatings during coating rework, only delamination occurs due to mechanical adhesion. According to the actual production experience of the spraying workshop, Use corrective measures and quality inspections to ensure adhesion to the spray coating.
Fig.2 Flowchart of production process of sandpaper powder with profile
Fig. 2 is a flow chart of the production process of re-sanded grain powder. After the profile is cured, the surface quality defects need to be returned to the spray. Before re-spraying, we must first grind the reworked material to smooth the surface coating defects, not only increasing the roughness of the coating surface. Degree (that is, increase the contact area with the re-sprayed layer), and the oil film or wax film on the surface of the first sand coating can be polished off to improve the surface tension of the coating, thereby improving the recoat layer and the first coating layer. Interlayer adhesion.
After polishing, clean it with a clean cloth, use compressed air to blow the surface of the base coat as much as possible, and do not wipe with a cloth that contains oil. Wipe the cleaned return material with alcohol and dry it. Grinding can increase the return injection success rate and ensure the interlayer adhesion of the coating.
The re-spraying spraying and curing process is basically the same as the normal spraying production process of the aluminum chromate material, but the spraying voltage should be appropriately reduced, generally set at 30KV-60KV, and the thickness of the re-sprayed layer is properly controlled (generally ≤ 40 μm).
After unloading, check the film thickness of the re-fed material and cross-cut test to check the inter-layer adhesion. You can also test the coating adhesion by sampling the sawing material head to do the boiling test.
5 Conclusion
Adhesion greatly affects the service life of the aluminum coating. Once the adhesion problem occurs, it cannot be repaired or reduced. It can only be scrapped or deplasticized and reprocessed. Therefore, in the electrostatic powder coating of aluminum profiles, the key factors of the process must be strictly controlled: the quality of the pre-treatment, the process parameters of the spray booth, the curing temperature and the time, and it is possible to reduce the number of unsatisfactory products for spraying in order to avoid any problems. If the defect must be returned to the spray, the first is to analyze the cause of the coating defects, and formulate appropriate measures to avoid the recurrence of similar problems. The quality of the sanding and the correct method of wiping should be ensured at the time of re-flooding to ensure that the interlayer adhesion of the coating after re- spraying is qualified.
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