1. Production Phenomenon
In August 2000. a powder coating company produced special powder coatings for pure polyester aluminum profiles for a certain group. Two batches were shipped simultaneously on the same day: 200 kg of STD 75 powder and 2000 kg of STD 76 powder. The customer reported that the STD 75 powder coatings were used normally without any abnormalities; however, some STD 76 products had quality problems, specifically manifested as follows:
(1) The control panel's electrical display, which previously had 5 bars, now only had 2 bars, indicating the powder was not charged.
(②) Occasionally, the spray gun would spit out powder during spraying.
(③) The mechanical properties (impact, cross-cut, etc.) of the production test samples passed the tests, but the surface leveling was worse than before.
2. Case Analysis
Based on the phenomena observed in the above coating process, theoretically, the cause is estimated to be moisture absorption of the powder coating. However, why do some products in the same batch have problems while others don't, even under identical operating conditions? With this question in mind, we analyzed the reasons from both the production and use aspects of powder coatings.
2.1. Powder Formulation System
Why does powder coating sometimes lack charge? The main factor affecting the charge acceptance and retention of powder particles is the dielectric constant of the powder. The lower the dielectric constant, the easier it is for the particles to become charged, but also the easier it is to lose charge. This is reflected in the weak adhesion of the powder to the workpiece, causing it to fall off with slight vibration. For electrostatic spraying powder coatings, a high dielectric constant should be used as much as possible, which will greatly improve the powder's adhesion. STD 76 is a mature and widely used product for aluminum profile spraying, with countless production batches. This batch followed the previous production formula, with no changes to the raw material selection or formula. Routine performance tests of the production samples, such as impact, cross-cut, gloss, film thickness, and color difference, all met the company's production standards. Therefore, the formula is not the cause of the above phenomenon.
2. 2Powder Particle Size
The particle size of powder coatings is a crucial indicator of their quality. For powder coatings suitable for electrostatic spraying, the particle size is ideally between 1 and 90 μm (i.e., >170 mesh). Powder with a particle size less than 10 μm is called ultrafine powder, and its content should not be excessive. It has been reported that the charge of powder coatings is directly proportional to the square of the powder particle size; the smaller the particle size, the lower the charge. In the aforementioned production process, we found that the charge was worse than usual. Initially, we suspected that too much ultrafine powder was causing the decrease in the charge of the powder coating. Therefore, we used an Omec STD 76 laser particle size analyzer to perform random checks on the fineness. The results are shown in Figure 1.
As shown in Figure 1:
The particle size of the STD 76 fully meets the requirements for electrostatic spraying of aluminum profiles. The particle size is not the cause of the aforementioned abnormal spraying. The production formula, raw material selection, and formulation remained unchanged. Routine performance tests on the production samples, such as impact resistance, cross-cut adhesion, gloss, film thickness, and color difference, all met the company's production standards. Therefore, the formula is not the cause of the aforementioned phenomena.
3. Fluidity of Powder Coatings
Powders with good fluidity flow easily during use, exhibiting a loose, flowing effect, like water. Powder transfer from the supply hopper to the spray gun is smooth, and the powder atomizes well from the nozzle, without clumping or spitting out. Powder coatings with poor fluidity will cause nozzle clogging and spitting out. The fluidity of powder coatings is usually tested using the Selier flowability test method, and its testing standard is shown in the figure below.
Figure 1 shows the relationship between the flowability coefficient and fluidization performance of powder coatings. We used a fluidization analyzer to test the STD 76. and its flowability coefficient was only around 85. indicating poor fluidization and thus spitting out and nozzle clogging. Previously, our company's powder coating products typically had a flowability coefficient of around 140. indicating good fluidization and easy powder application. The main reasons for poor fluidization are excessively fine powder or moisture absorption. Powder particle size analysis ruled out an excessive amount of ultrafine powder, suggesting the problem is likely caused by moisture in the powder coating.
4. Spraying Process Conditions
Considering the possibility of poor electrical conductivity due to moisture in the powder, we carefully examined the manufacturer's spraying process conditions, especially parameters related to powder electrical conductivity and powder discharge, such as operating voltage, compressed air pressure in the powder supply system, relative humidity in the spray booth, and the effectiveness of the oil-water separator using compressed air. No abnormalities were found. We tested the STD 76 using a fluidization analyzer, and its flow coefficient was only around 85. indicating poor fluidization and resulting in powder discharge and nozzle clogging. Previously, our company's powder coatings typically had a flow coefficient of around 140. resulting in good fluidization and easy powder spraying. The main reasons for poor fluidization are excessively fine powder or moisture absorption. Powder particle size analysis ruled out an excessive amount of ultrafine powder, suggesting the problem is likely caused by moisture in the powder coating.
5. Causes and Handling of the Accident
5.1. Causes
Based on the above analysis, it was determined that the phenomenon was caused by a quality issue with the powder coating, not a usage problem. Therefore, we carefully inspected and analyzed every step of the powder coating production process, finally identifying the root cause:
1. During the fine grinding process in the ACM mill, the collision of materials generates a large amount of heat. Simultaneously, it was summer with high temperatures, and the material could not dissipate heat in time due to ambient temperature limitations. The temperature measured was approximately 45°C. The skilled operator did not allow the material to cool down promptly, immediately packaging and storing it. Powder coating has poor thermal conductivity, preventing internal heat dissipation and accelerating the thermal motion of the powder coating components, leading to clumping. Simultaneously, the high-temperature air inside the plastic packaging condensed to room temperature, causing moisture to seep out. The combined effect of these two factors resulted in the powder coating becoming damp and clumping, leading to the aforementioned phenomenon during use.
5.2. Treatment Measures
In the powder coating manufacturing process, the air classifier (AC mill) is one of the key pieces of equipment for determining the quality of powder coating products. It determines the particle size and particle size distribution of the powder coating, thus affecting the coating performance. The ACM mill itself does not have a dedicated cooling system, and a large amount of air must be introduced to cool the materials. However, the ambient temperature in summer is generally high. Therefore, we have taken the following measures to address this:
(1) Sufficient material cooling. Before entering the ACM mill, the material needs to be sufficiently cooled during the tableting and crushing processes. According to production experience, the material temperature generally rises by 24°C after being crushed by the ACM mill. Only with sufficient cooling can the temperature of the crushed product not be too high;
(2) Controlling ambient temperature. The main cooling measures are as follows:
A. Manufacturers with good conditions can install central air conditioning in their production workshops to keep the ambient temperature below 32℃.
B. Local cooling can be achieved by using cold air to circulate through the air inlet of the ACM mill to keep the temperature of the powder coating product below 35℃. This involves installing a large air cooler or even an air conditioner with its vent connected to the air inlet of the ACM mill to cool the ACM mill system. In this way, even in the hot summer, the temperature of the material will not rise, and the powder coating will not clump or become damp.
(2) Adjust the feeding speed to reduce the heat generated during the crushing process; or increase the ventilation to remove the heat generated during the crushing process.
(3) Appropriately consider improving the stability of the powder coating in the formulation. Select resins with high glass transition temperatures to increase the temperature resistance to clumping; you can also add loosening agents or powder coating flow modifiers to the formulation. 5) Control of production process conditions is shown in Table
5.3.Conclusion
In summary, we learned from this case that insufficiently fine powder can lead to various problems.
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