Analysis of the reasons for poor adhesion of powder coating of metal packaging
Introduction
Metal packaging, especially various industrial packaging, such as steel drums, steel pails, metal buckets with lids, etc., all use powder coating. 200L steel drums often require an external paint layer, while 20L steel pails or commonly known as metal buckets with lids, and smaller tin cans or metal boxes or metal buckets used in the chemical or food industry, although they do not need powder coating on the outside--they are often used as paint buckets, popcorn oil bbuckets, ketchup cans, etc., so they need more delicate and beautiful surface patterns, so the surface is printed with beautiful pattern, but for steel pails or metal buckets containing water-based substances or acidic and saline substances, organic coatings are required inside to prevent corrosion and rust. However, the welding part is not coated, which requires that after the welding of these metal packaging is completed, the organic coating should be applied to the weld. Powder coating is currently the most hygienic and convenient way, and is widely used in the steel pail and tin can production industry.
Powder coating started late. In the 1940s, some countries began preliminary research, but there was no breakthrough until 1954 when James of Germany successfully coated polyethylene with a fluidized bed method and 1962 when the French company Semmes invented powder electrostatic spraying. Powder coating was officially adopted in production. With the continuous recognition and exploration of powder coating, it is found that powder coating has its special advantages in safety, environmental protection, economic benefits, process operation, product quality, etc., so powder coating has been developed and popularized on a large scale in the past ten years.
1 Powder coating
1.1 Principle of powder electrostatic spraying
The main equipment included in the powder electrostatic spraying process includes powder room, powder supply system, recovery system, control system, spray gun, reciprocating machine, etc. After the workpiece enters the powder room, the control system identifies the product demand, and the powder supply system uses compressed air to send the powder into the spray gun. The front end of the spray gun is added with high voltage generated by a high-voltage electrostatic generator. Due to corona discharge, dense charges are generated near it. When the powder is sprayed from the gun nozzle, charged paint particles are formed. It is attracted to the workpiece with the opposite polarity due to the electrostatic force. As the amount of powder sprayed increases, the charge accumulates more. When it reaches a certain thickness, electrostatic repulsion occurs, and it will not continue to adsorb, so that the entire workpiece obtains a certain thickness of powder coating, and then the powder is heated to melt, level, and solidify, that is, a hard coating is formed on the surface of the workpiece.
1.2 Powder coating quality problems
People often encounter many quality problems in the process of using powder coating. Common quality problems include shrinkage, particles, whitening, orange peel, color difference, pinholes, and pull holes on the surface of the workpiece. When these defects occur in the powder coating line, we can identify the problem workpiece through self-inspection and conduct targeted problem troubleshooting. Unqualified products generated during the troubleshooting process can be reworked by grinding and secondary back spraying.
There is a kind of unqualified problem in the powder coating process that is difficult to find through self-inspection and mutual inspection, and such unqualified products have a great impact on product performance and quality, and it is not easy to rework. This kind of unqualified problem is poor adhesion. When workpieces with poor adhesion are found through regular sampling, only batch workpiece performance testing can be carried out, and whether the workpieces should be scrapped or de-plasticized can be determined based on the test results. Rework and repair are relatively difficult. If no problems are found through regular sampling, when the problematic workpieces flow to the market, there will be great quality risks.
2 Analysis of poor adhesion of powder coating
For the influencing factors that may cause poor adhesion problems at the coating site, a summary analysis is conducted from five aspects: people, machines, materials, methods, and environment. At the same time, since there are many possibilities for people, machines, materials, methods, and environment, this article selects common causes for each aspect and proposes corresponding detection and verification methods, and provides solutions and prevention plans.
2.1 Poor adhesion caused by human factors
Generally, there is no problem of human contact during the pre-treatment powder spraying of the automated coating production line. However, due to equipment reasons or cost pressures, at many customer sites, the workpieces are hung or placed for a period of time after pre-treatment before powder spraying. The adhesion test of the hung or placed workpieces after powder spraying is qualified after 1 day. However, after 2 weeks, the adhesion of some powder sprayed workpieces that were touched by humans will be significantly reduced, and there will be obvious rust spots on the surface of the substrate.
For this problem, laboratory verification was carried out. A batch of test panels were treated with ceramics, and a layer of sweat was artificially applied on the test panels. After powder spraying, they were placed for 1 day to test the adhesion, and the adhesion was qualified. When the test panels were placed for 2 weeks and then tested for adhesion, it was found that the adhesion was significantly reduced. Under the same conditions, the adhesion of the ceramics pre-treated test panels that were not contaminated by sweat did not decrease after powder spraying.
Through experiments, it is proved that artificial contact has a great influence on the adhesion of products, and it is not easy to be discovered in a short time. Therefore, when the pre-treatment and powder spraying of the coating production line cannot be carried out continuously, it is recommended to ensure that the sweat of the operator will not contact the dried workpiece during the transfer process. This is mainly because human sweat contains about 0.3% NaCI, which will accelerate the rusting of the plate surface. As time goes by, the workpiece will gradually rust from the inside to the outside, resulting in poor adhesion.
2.2 Poor adhesion caused by equipment
Many production sites often encounter intermittent poor adhesion problems during batch production, and there are no abnormalities in the inspection of agents and powders. When encountering such problems, it is necessary to check the equipment. In the early stage of design, the equipment is strictly designed according to the requirements of workpiece cleaning, workpiece drying, powder spraying, powder curing, etc. for chain speed, temperature, and spraying. After the equipment has been used for a period of time, some equipment will be worn out or the equipment processing capacity will deteriorate, such as the curing temperature of the curing furnace.
A customer's workpieces have poor adhesion in batches, and the adhesion is only poor for a period of time during the process, and returns to normal after a period of time. At this time, it is necessary to check whether it is a problem with the oven baking temperature. You can use an oven temperature meter to test the oven temperature, or you can test the adhesion of unqualified workpieces after secondary curing. If the adhesion of the workpiece after secondary curing is qualified, and the furnace temperature curve is very different from the oven surface temperature, the equipment needs to be repaired.
For poor adhesion caused by equipment, it is recommended to conduct regular equipment inspection and maintenance to prevent large-scale quality problems caused by long-term use of equipment without maintenance.
2.3 Poor adhesion caused by materials
2.3.1 Effect of sheet on adhesion
Powder coating can be applied to multiple materials such as cold-rolled plates and galvanized plates. Adhesion problems will occur on different materials, but the reasons for different materials affecting adhesion are different.
The poor adhesion of the cold-rolled plate surface is mainly due to the rust on the plate surface or the rust on the plate during the pre-treatment process. The surface product of the rusted plate is mainly iron oxide, that is, rust. Rust is relatively loose and loses the bonding force between elements, so it is easy to peel off. When the rusted workpiece is powder-sprayed, the rust on the surface of the substrate is relatively loose, and the adhesion is greatly reduced, so it is easy to fall off. The judgment method for this problem is relatively simple. Visually check whether the detached substrate is rusty and whether the part where the powder and the plate are combined is yellow. If it is yellow, it is basically determined that the plate is rusted and causes poor adhesion. If further judgment is required, an XRF instrument can be used to perform full element analysis on the elements on the surface of the detached powder, and compare and analyze the difference between the Fe element content in the detached powder coating and the Fe element content in the normal powder coating. Figure 1 and Table 1 are photos and data of a problem analysis. The results show that the Fe element content in the powder coating on the surface of the workpiece with unqualified adhesion is much higher than that in the normal powder coating, and the other elements are not much different.
Picture
For galvanized sheets, there are many reasons for poor adhesion, but in most cases the main reason for poor adhesion is the presence of a passivation layer on the surface of the galvanized sheet. However, not all passivation layers have poor adhesion, and some Si-containing passivation layers have a greater impact on adhesion.
During the on-site production of a customer, the galvanized sheet suddenly had poor adhesion, while the cold-rolled sheet and other galvanized sheets produced along the line had qualified adhesion. After multiple inspections of on-site materials and equipment, no cause was found. Then, the surface element analysis of different galvanized sheets on site found that there were differences between the surface elements of galvanized sheets with unqualified adhesion and qualified galvanized sheets, and the number of Si elements on the surface of galvanized sheets with unqualified adhesion was relatively large. The relevant data is shown in Table 2. Among them, test plate 1 is an unpassivated galvanized sheet, test plate 2 is a passivated galvanized sheet with qualified adhesion, and test plate 3 is a passivated galvanized sheet with unqualified adhesion. Through element detection and understanding of the previous passivation process, it was determined that the adhesion was unqualified due to the passivation layer of the galvanized sheet. The problem was solved after replacing the passivated galvanized sheet.
Picture
2.3.2 Effect of pre-treatment agents and powders on adhesion
Poor adhesion caused by materials In addition to the plate itself, pre-treatment degreasing, ceramic agents and powders may also cause poor adhesion.
The effect of degreasing agent: The main function of degreasing agent is to remove oil stains on the surface of the plate. It will not directly cause poor adhesion, but it will indirectly cause poor adhesion. For example, the frequency of changing the washing water on site is too low, the degreasing agent is not cleaned before the workpiece is sprayed, or the degreasing liquid in the gap between the hanger and the chain falls in the oven or remains on the workpiece. After the workpiece is sprayed with powder, it is not obvious that the adhesion is poor, but after a period of time, it will start to rust from the inside of the plate.
The effect of ceramic agent: The main function of ceramic agent is to form a ceramic film on the surface of the plate, increase the surface area of the plate, increase the contact area between the powder coating and the plate after melting, and increase adhesion. We all know that even if there is no ceramic film on the bare board, there is no problem with adhesion after powder spraying. Therefore, if the ceramic agent parameters are not out of adjustment and the ceramic film layer is uniform, the ceramic agent will generally not cause poor adhesion.
Influence of powder: The effect of powder on adhesion is much greater than that of pre-treatment degreasing agent and ceramic agent. The more common powder influencing factors include: low content of effective substances (resins), too wide molecular weight distribution of effective substances (resins), unqualified additives or improper dosage, etc., which are all the reasons for unqualified adhesion? . Since many powder manufacturers do not have effective testing methods, it is impossible to predict whether the coating is qualified. When it is initially determined that there is a problem with the powder, an organic solvent can be used to remove the oil on the surface of the test plate and directly spray the powder to test the adhesion.
2.4 Poor adhesion caused by improper methods
2.4.1 Effect of recycled powder on adhesion
A large amount of recycled powder will be produced during the powder spraying process. In order to improve the utilization rate of powder, recycled powder is usually mixed with new powder. Since the amount of resin and additives in recycled powder has changed relative to new powder, when the mixed amount is large, the proportion of each component in the powder will be unbalanced, resulting in poor adhesion. Therefore, when using recycled powder, relevant tests need to be done in advance to determine the use ratio.
In many cases, the use of recycled powder will definitely affect adhesion, but it is difficult to detect the use of a small amount of recycled powder by testing. When the sprayed workpiece is seriously contaminated during the pre-treatment and baking process, the adhesion will be revealed. Therefore, it is generally not recommended to use recycled powder in situations with high quality requirements. Recycled powder can be considered for use in places such as the inside of products with low quality requirements.
2.4.2 Effect of washing water on adhesion
Before powder spraying, the workpiece surface needs to have a clean surface, and the quality of the pre-treatment washing water has a more obvious effect on the quality of powder spraying.
If the water used on site is groundwater, coastal tap water or tap water of poor quality, it is recommended to use pure water for ceramicization and washing water after ceramicization. Anions such as CI and SO, and cations such as Ca'* and Mg* in the washing water remain on the surface of the plate, affecting the long-term adhesion of the product. At the same time, if the washing water on site is not replaced in time, the washing water will be contaminated after long-term use, and the residue on the surface of the workpiece will cause poor adhesion of the workpiece, especially if the degreasing agent is not cleaned up. It will not affect the adhesion at the moment, but it will have a greater impact on the long-term adhesion.
2.5 Adhesion failure caused by the environment
The most common location of environmental influence is the moisture drying furnace. If there is a large amount of smoke in the moisture drying furnace, or if the local position is directly exposed to the smoke, the adhesion of the workpiece after powder spraying will be significantly reduced.
3 Daily management concerns of powder coating
Through the analysis and discussion of the common reasons affecting adhesion in terms of people, machines, materials, methods and environment, some common concerns of powder coating lines can be obtained:
1) Powder coating has very high requirements for the cleanliness of the workpiece before powder coating. The production line design and the later production process should minimize the secondary pollution of the workpiece by human beings. At the same time, special attention should be paid to the quality of local tap water;
2) Only good equipment can guarantee good product quality, so the equipment must be regularly inspected and maintained during the production and operation of the coating line to ensure that abnormal points of the equipment can be found in time and the risk of batch failure can be reduced;
3) In the early stage of powder coating acceptance, a relatively comprehensive test is required to ensure the matching of on-site materials and on-site materials The passivated galvanized sheet is the focus of the inspection. After the material and passivation method of the galvanized sheet are verified and determined, it cannot be changed at will;
4) Cold-rolled sheet workpieces are prone to rust during the pre-treatment process of coating, so the coating line needs to focus on the quality of the pre-treatment film layer of the cold-rolled sheet. Before spraying, it is necessary to ensure that the workpiece is not rusty, and the rusted workpiece needs to be removed in time for secondary treatment;
5) Pre-treatment degreasing and ceramic agents, powders need to be inspected for incoming materials during use, and important indicators of pre-treatment agents need to be regularly tested during production, and mechanical properties need to be tested regularly;
6) The use of all materials needs to be carried out in accordance with the process document standards and plans initially set, and cannot be changed at will. If changes are required, the process verification and evaluation must be re-performed.
The above are common precautions that need special attention in the management of the production line. In addition, there are many other items that need attention, such as the way the workpiece is hung, the design structure of the workpiece, the design of the oil pan, etc., which are all parts that cannot be ignored.
4 Conclusion
To sum up, powder coating is a relatively safe, environmentally friendly and economically efficient coating process. Although its process operation is simple and its performance advantages are obvious, serious quality problems often occur during the application process, especially poor adhesion. Therefore, in the process of powder coating, it is necessary to do a good job of process control to ensure quality. When encountering poor adhesion problems, the problem can be analyzed from multiple aspects. The above are some common factors affecting adhesion. There are many other possible factors that need to be considered comprehensively during the problem investigation and analysis process.
