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Abstract:
In powder coatings and coating applications, particles are a common and relatively complex defect. This paper classifies and analyzes the causes of particle formation in powder coatings and coating processes, and then provides targeted control measures, with the aim of reducing particle issues during coating.

Introduction
Due to their environmental friendliness, energy efficiency, high production efficiency, wide color variety, and excellent physical and chemical properties of the coating film, powder coatings are increasingly being widely applied across various industries and represent a major trend in the future development of coatings.
During the electrostatic powder coating process, coating film particle defects are common and relatively complex in origin. This paper summarizes the causes of particle formation during electrostatic powder spraying and possible control measures, for joint discussion.

1. Classification of Particles

Coating film particles generally refer to dot-like raised particles on the surface of the film after complete curing. These disrupt the continuity and integrity of the coating, seriously affecting appearance quality and aesthetics, while also impacting the overall mechanical and chemical performance of the coating, thus lowering product quality.
According to their source, they can be divided into two categories: particles inherent in the powder coating itself, and those caused during coating application.
Particles inherent in the powder coating itself can be further divided into three types: mechanical impurities, gel particles, and oversized powder particles.
Particles caused during coating application are mainly divided into four types: substrate defects, pretreatment defects, spray system contamination, and environmental contamination.

2. Causes of Particles and Control Measures

2.1 Particles Inherent in Powder Coatings

2.1.1 Mechanical Impurities
Mechanical impurities in powder coatings mainly include:

• From pigments or fillers: Due to oil absorption requirements, pigments or fillers have inherent particle size ranges. Coarse pigment or filler particles can easily lead to coating defects.
  Control: Pigments or fillers should be fine enough to pass through a 325-mesh sieve to minimize particle generation.

• From insoluble matter in resins: Some mechanical impurities or large-molecular-weight gels exist in resins due to production processes. These do not dissolve in organic solvents and cannot melt under high temperatures.
  Control: Use to test raw materials to check insoluble content against supplier specifications.

• From mechanical abrasion during powder production: Wear and tear from extruders, grinders, or cyclone separators may lead to metallic debris.
  Control: Regular equipment maintenance; such particles feel sharp to the touch, making them relatively easy to identify.

• From textile fibers or dust during production: Packaging, workers’ gloves/clothing, and the environment may introduce fibers or dust.
  Control: Workers’ clothing, gloves, and tools must be non-contaminating; maintain proper humidity and good ventilation in production areas.

2.1.2 Gel Particles
Gel particles may form due to:

• Equipment reasons: Dead corners in extruder screws or gaps between screw and barrel may cause residual materials to remain, undergoing chemical reactions at extrusion temperatures (90–110°C), leading to gelation.
  Control: Adjust screw blade arrangement to minimize gel formation.

• Process reasons: Excessive extrusion temperature or prolonged material retention (e.g., due to feed interruptions) may cause premature chemical reactions.
  Control: Strictly follow process temperature requirements, monitor feeding and discharge speed.

2.1.3 Oversized Powder Particles
Oversized particles may arise due to:

• Excessive coarse fraction in powder, causing poor leveling and particle formation.
  Control: Real-time particle size testing or batch mixing before testing.

• Screen breakage during sieving, allowing flake-like materials to pass into finished product.
  Control: Periodically perform hand-sieving tests (100–140 mesh).

• Film thickness too thin: Ideally, film thickness ≈ 2× average particle size. Too thin films cannot properly level.
  Control: Ensure spraying thickness meets specifications.

2.2 Particles During Coating Application

2.2.1 Substrate Defects

• Surface protrusions or pits in aluminum/iron substrates can cause visible particles after coating.
  Control: Treat substrate surface to ensure smoothness (alkali etching for aluminum; grinding and degreasing for iron).

2.2.2 Pretreatment Defects

• Residual chemicals or metallic debris from pretreatment can cause particles.
  Control: Strengthen rinsing, add air-blowing steps before/after drying.

2.2.3 Spray System Contamination

• Powder fluidizing hopper contamination: Residual powders or agglomerates.
  Control: Clean hoppers regularly, empty if unused for >4h.

• Spray gun contamination: Uneven powder output or excessive discharge voltage can cause local melting.
  Control: Adjust spray gun settings for even atomization, avoid excessive voltage.

• Conveyor chain & jig contamination: Dust or detached coatings falling onto workpiece.
  Control: Regularly clean conveyor and jigs.

2.2.4 Environmental Contamination

• Powder booth contamination: Dirty clothing/tools, airborne dust, external intrusion.
  Control: Keep powder booth clean, maintain humidity, ensure enclosure.

• Curing oven contamination: Combustion residues or dust adhering to parts during curing.
  Control: Prefer indirect heating; maintain clean incoming air; clean ovens regularly.

3. Conclusion

In summary, particle problems must be controlled from powder materials, coating application, and environment. Strict adherence to process procedures in both powder preparation and coating application is essential. Since causes vary widely, specific analysis and targeted control measures are required to minimize particle occurrence and its impact on product quality.