In the production and application of powder coatings, "particle size detection" is an easily overlooked yet crucial step. Many people wonder: if the powder can be sprayed onto the workpiece and cured into a film, why spend time measuring particle size? In fact, the particle size and distribution of powder directly affect coating quality, spraying efficiency, and even production costs. Today, we'll break down the necessity of particle size detection from two aspects: the core indicators of particle size detection and the correlation between particle size and powder quality, helping you understand its key significance.
1. First, understand: What can a laser particle size analyzer measure?
When it comes to particle size detection, the most commonly used equipment is the laser particle size analyzer. It can provide three core data points, which are like the powder's "identity card," clearly reflecting the size characteristics of the powder particles:
1.1 Average Particle Size (Median Diameter)
The average particle size (usually expressed as D50) is the most intuitive indicator. It represents that "half of the particles in the sample are smaller than it, and half are larger than it"—like the average height of students in a class, it can quickly determine the overall size level of the powder particles. For example, if a powder has a D50 of 30 micrometers, it means that 50% of the particles are smaller than 30 micrometers and 50% are larger than 30 micrometers. This value is a basic reference for powder selection and directly affects the subsequent coating effect.
1.2 Boundary Particle Size
Many people ask, "What is the largest and smallest particle size of the powder?" However, describing it using "maximum/minimum particle size" is not scientific—because powder particle size is continuously distributed, it is difficult to find an absolute "maximum" or "minimum." Therefore, the industry commonly uses "boundary particle size" to define the range, most commonly (D10. D90). For example, if the test results show (D10. D90) = (8. 60), it means that particles with a diameter less than 8 micrometers account for 10% of the total, and particles with a diameter less than 60 micrometers account for 90% of the total. These two values clearly indicate the "lower limit" and "upper limit" of the powder particles, avoiding an excessive number of overly fine or coarse particles.
1.3 Particle Size Distribution Dispersion
The dispersion is calculated simply as: (D90 - D10) / D50. The smaller this value, the more concentrated the powder particle size—like soldiers in a formation with small height differences, resulting in a more orderly formation; conversely, the greater the dispersion, the greater the difference in particle size, including both ultrafine and coarse particles. Dispersion directly affects the spraying stability of the powder. For example, excessive dispersion can easily lead to "separation of coarse and fine particles" during spraying, resulting in an uneven coating.
2. Key Significance: Particle Size Directly Determines the "Quality" of Powder
Many people think that "particle size is just a number," but in reality, it is closely related to core quality indicators such as powder appearance, powder application rate, and fluidization effect, and can even affect production costs. Specifically, it is reflected in these 5 aspects:
2.1 Appearance, Leveling, and Particle Size
Everyone wants a smooth, even coating surface free of small particles (particles), and this largely depends on the powder particle size. Generally, the smaller the powder particle size, the better the leveling properties during curing—small particles can spread more evenly on the workpiece surface, filling tiny gaps like "fine sand"; while large particles, like "coarse pebbles," easily leave bumps on the coating surface, forming particles. For example, in the spraying of appliance casings, using powder with a small and evenly distributed particle size results in a mirror-like gloss after curing; if a powder with a large particle size and high dispersion is used, the coating surface may have a noticeable grainy texture, affecting the product's appearance. At the same coating thickness, small-particle-size powder is less likely to produce particles, reducing rework waste.
2.2 Charging Effect and Powder Application Rate
The core of electrostatic spraying is "powder charging and adsorption," and the powder application rate is directly related to the charge of the powder particles. In principle, the charge of the powder is proportional to the square of the particle size—the larger the particle size, the more charge it carries, and theoretically, the stronger the force adsorbed onto the workpiece, resulting in a higher powder application rate. However, this doesn't mean "the larger the particle size, the better." If the particles are too coarse (e.g., exceeding 100 micrometers), their own gravity will exceed aerodynamic and electrostatic forces, like a large stone that won't travel far before falling to the workpiece surface, thus reducing powder application rate. Conversely, if the powder is too fine (especially ultrafine powder less than 10 micrometers), its charge will be significantly reduced, almost unable to carry a charge. During spraying, it will float in the air like "dust," resulting in low powder application rate, environmental pollution, and impact on worker health. Furthermore, ultrafine powder is more difficult to produce, increasing costs. Therefore, the ideal powder particle size strikes a balance between "charge" and "gravity," typically in the 20-60 micrometer range. This ensures sufficient charge without causing the powder to fall due to gravity, achieving optimal powder application rate.
2.3 Powder Fluidization Effect
During spraying, the powder needs to be "fluidized" in the powder supply tank—similar to boiling water—so that the powder particles can be evenly dispersed and smoothly transported to the spray gun through the pipeline. Particle size directly affects the fluidization effect: If the powder is too fine, the ultrafine powder will act like "flour," easily adsorbing and agglomerating, unable to disperse even after fluidization, clogging the pipeline during transport, and forming "small bumps" on the workpiece, damaging the coating appearance; if the powder is too coarse, the friction between particles is high, making it difficult to flow in the powder supply tank, like "sand piled up," unable to fluidize evenly, leading to uneven powder supply and inconsistent coating thickness. Only powder with a uniform particle size distribution, without excessive ultrafine or coarse particles, can achieve a good fluidization effect and ensure a stable spraying process.
2.4 Storage Stability
The storage stability of powder coatings is related to "how long it can be stored before it can be used." Generally speaking, the finer the powder, especially the higher the content of ultrafine powder (below 10 micrometers), the stronger its hygroscopicity. Ultrafine powder has a large surface area, making it easy to absorb moisture from the air, leading to powder clumping.
Clumped powder cannot flow and spray properly, and it can clog spray guns, causing "poor powder delivery," resulting in waste. For example, in humid southern regions, if the stored powder has a high fine powder content, it may clump within half a month if the packaging is not tight or the storage environment has high humidity; while powder with moderate particle size and low dispersion has weak hygroscopicity and better storage stability, generally lasting 6-12 months.
2.5 Recovery Rate
Both powder manufacturers and coating plants generate unabsorbed "recycled powder," and the recovery rate directly affects production costs. Recovery rate is closely related to particle size: Ultrafine powders with a particle size less than 10 micrometers are lightweight and easily float, making them difficult to capture by collection equipment during recovery, resulting in extremely low recovery rates. When the particle size is greater than 10 micrometers, the recovery rate increases rapidly, and it also increases with increasing particle size (as long as it does not exceed the upper limit affecting powder coating yield).
3. Summary
After reviewing this, it's clear that particle size testing is not a "redundant step," but rather a crucial "quality gatekeeper" in powder coating production and application. By detecting average particle size, boundary particle size, and dispersion, it helps us determine the particle characteristics of the powder; furthermore, by examining the correlation between particle size and appearance, powder coating yield, fluidization effect, etc., it ensures that the powder meets production and coating requirements—guaranteeing coating quality while reducing waste and lowering costs. For manufacturers, particle size testing can optimize formulations and production processes, producing higher-quality powders; for coating companies, selecting suitable powders through particle size testing can improve spraying efficiency and reduce rework. In conclusion, paying attention to particle size detection is essential to fully leveraging the advantages of powder coatings, enabling the production of high-quality products while simultaneously reducing costs and increasing efficiency.
