Powder Coating

Powder coating involves applying electrically charged particles (20–100 μm) of powder paint onto a conductive surface, such as metal. The deposited powder layer adheres to the surface due to electrostatic forces.

There are two primary techniques for applying powder paint in powder coating:

• Electrostatic spray deposition (ESD) – a high-voltage method (40–100 kV), commonly referred to as the “corona” method.
• Electrokinetic spray (triboelectric charging) – a friction-based method, known as the “tribo” method.

After application, the coated elements are heated to a temperature of 140–200°C, causing the powder to melt and undergo polymerization. The resulting coating is highly resistant to corrosion, chemicals, high temperatures, and mechanical damage.

Key Advantages of Powder Coating:

• No emission of solvents or thinners into the environment.
• Nearly 100% utilization of coating material, with the ability to recover unused powder particles.
• Significant energy savings due to the use of closed ventilation systems.
• Excellent coverage of surface imperfections thanks to coating thickness (typically 60–80 μm).
• Effective corrosion protection of coated materials.

Powder Coatings Typically Contain

• Binders (resins, curing agents)
• Additives
• Pigments
• Fillers

Powder Coating Production Process Consists of Three Stages:

Raw Material Mixing

All chemical substances included in the formulation are weighed and pre-mixed using a mixer for a specified time until a homogeneous mixture is obtained.

Extrusion

In the extrusion process, the mixture is melted and dispersed. After cooling the rollers and belt stages, the material is broken into small chips.

Grinding into Fine Powder

These small chips are ground for a specified time (particle size distribution) for various applications. Once the optimal particle size is achieved, the powder is packaged and ready for use.

• Qualicoat Quality Certificate
• RoHS Restriction of Hazardous Substances
• ISO 9001:2008 Quality Certificate
• OHSAS ISO 18001:2007 Certification
• ISO 14001:2004 (EMS)

The Qualicoat certificate is available for our PE coatings used in architectural applications on aluminum surfaces.

The RoHS (Restriction of Hazardous Substances) regulation can also be applied to our products.

Quality control and assurance in Boyasan's internal procedures are regulated by the ISO 9001:2008 Quality System.

We also hold the OHSAS ISO 18001:2007 certificate for the Occupational Health and Safety Management System (OHSMS), which is essential for reducing the risk of unexpected injuries, protecting both employees and their workers.

Boyasan is in the process of improving its Environmental Management System (EMS) by meeting the requirements of ISO 14001:2004, aiming to enhance environmental protection by reducing waste.

• Epoxy:
  These coatings are characterized by excellent chemical resistance (acidic, alkaline, and salt solvents), corrosion resistance, good mechanical properties, flexibility, high adhesion levels, and abrasion resistance. However, this group tends to chalk when exposed to sunlight, making it recommended for indoor applications.
• Epoxy-Polyester (Hybrid):
  These are epoxy powders blended with polyester resins for indoor applications. This series offers similar properties to epoxy coatings but with improved resistance to yellowing and weathering. However, they still tend to chalk under sunlight exposure. The series provides resistance to a wide range of solvents and chemicals, depending on the formulation. Chemical resistance requirements should be evaluated based on processing conditions and the final application of the finished product.
• Polyester-TGIC:
  This type of coating offers resistance to chalking, UV radiation, and weathering, making it suitable for both indoor and outdoor applications.
• Polyester - PRIMID (TGIC-Free):
  These coatings offer mechanical properties similar to TGIC-based polyester systems but have distinct advantages and disadvantages.
  Advantages include higher transfer efficiency, excellent storage stability, and improved surface flow.
  The only drawback of this system is coating thickness, but improved formulations have significantly addressed this issue.
  The most crucial feature of this group is that it is free of heavy metals and toxic substances harmful to human health.
• Polyurethane:
  These are polyester-based, caprolactam-free powder coatings used for both indoor and outdoor applications. They provide excellent chemical and physical resistance, exceptional thin-film appearance and durability, and outstanding weather resistance.
  
  Advantages include:
  • Higher transfer efficiency
  • Excellent storage stability
  • Improved surface flow
  • Superior damage resistance
  • High resistance to moisture and salt spray

The measurement method involves performing an appropriate indentation test. The hardness of the tested surface is determined based on the imprint left by a steel disc loaded with a 500 g weight for a duration of 30 seconds. The reading is conducted using an illuminated microscope with twentyfold magnification. The length of the imprint left is inversely proportional to the hardness of the tested coating.

The powder packaging is supplied in PP bags and cardboard boxes – up to 25 kg.

The powder should be stored under optimal conditions below 25°C and at a relative humidity of approximately 50-60%. Under these conditions, most powders should remain ready for use for at least 12 months from the production date. Higher temperatures and extended storage periods increase the risk of moisture absorption.

It is important to always keep the powders:

• Protected from high temperatures (>25ºC)
• Protected from moisture and water
• Protected from other dust and contaminants

Storage conditions may vary for each type of powder, so it is always necessary to refer to the product datasheet.

This process generally consists of three stages:

1 - Pre-treatment of the metal surface

Powder coating is mainly applied to surfaces made of steel, galvanized steel, aluminum, copper, and zinc alloys.

Metal surfaces are prone to rapid oxidation. To prevent oxidation, the surface is usually oily and greasy, which causes several issues during painting. For optimal performance, metal surfaces should be cleaned using various chemical methods before applying the coating. Oil, dirt, metal oxides, rubber, and plastics must be thoroughly removed.

Depending on the type of metal, there are two main cleaning methods:

To select the appropriate process and chemicals, the following points should be determined:

• The type of metal and its protective quality requirements
• The degree of contamination
• The application area

2 - Powder Coating Application Systems

The most common method for applying powder coating to metal objects is spraying the powder using an electrostatic gun. There are two generally known charging processes: corona charging and tribo charging.

- Corona Charging

In corona charging, powder particles are charged by voltage. An ion field is generated between the electrode and the grounded metal surface. Powder particles passing through this field become charged and are attracted to the grounded metal. Corona charging is suitable for all types of powder coatings.

Advantages:

• Strong electrostatic field enables fast charging
• The electrostatic field helps move powder particles toward the workpiece
• Suitable for various types of powder materials
• Differences in particle size distribution can be eliminated
• Coating thickness can be easily adjusted by changing the voltage
• Fast color changes and cleaning
• Lower equipment wear and reduced replacement costs

Disadvantages:

• A strong electric field induces ionization drop-off
• The strong electrostatic field leads to the Faraday effect (uneven coating on corners and edges)
• Voltage changes can cause an orange peel effect on the coating

- Tribo Charging

In tribo charging, powder particles are charged through friction inside the gun. Electrons are removed from the powder particles as they come into contact with the gun's interior, which is usually made of Teflon. The positively charged powder particles are then transported to the grounded product by an airflow exiting the gun.

Advantages:

• No Faraday effect; corners and edges can be better coated
• Uniform coating application
• Smooth flow without the orange peel effect
• Offers excellent automation potential
• Does not require a high-voltage generator

Disadvantages:

• The powder formulation must be adapted for the tribo charging process; it does not work efficiently with all powder types
• Performance is heavily affected by uncontrolled airflow
• Particles smaller than 10 microns are difficult to charge
• Color changes and cleaning are time-consuming and challenging
• Particle charging takes longer, reducing efficiency during extended operations
• Higher wear and shorter lifespan of spare parts
• Since particle charging takes longer, less powder is applied per unit time, requiring more tribo guns for coating

How to Choose the Application Method (Corona vs. Tribo)?

3 - Curing

When a thermosetting powder coating passes through the curing oven, it begins to melt, flow, and then undergo a chemical reaction to form cross-linking. Typically, powders cure at temperatures between 160-200°C for approximately 5-25 minutes. The curing time and temperature may vary depending on the type and specifications of the powder.

When powder coatings are applied to a substrate using an electrostatic spray gun, part of the sprayed powder adheres to the workpiece, while some do not. Transfer Efficiency (TE) is defined as the ratio of the amount of powder actually deposited on the coated part to the total amount of sprayed powder. It is expressed as a percentage, with 100% representing full efficiency.

Increasing transfer efficiency reduces the amount of oversprayed powder and the amount of recovered powder. High transfer efficiency results in lower costs, higher productivity, and improved quality.

Key Factors Affecting TE:

1. Gun Voltage/Current Settings:

• The optimal voltage range for the gun is 30 to 100 kV. Higher voltages generally lead to more powder recovery.
• The optimal current draw for good transfer efficiency is 10 to 20 microamperes (μA), which ensures proper deposition and penetration in Faraday cage areas.

2. Powder Flow Rate (Air Regulation):

• Excessive air velocity reduces transfer efficiency and makes application in corners more difficult.
• Small powder particles are directed toward the part by airflow and adhere due to electrostatic attraction.
• If the powder is propelled too quickly, it will hit the surface and bounce off, as the compressed air velocity can be stronger than electrostatic attraction.
• Lower airflow results in:
  • Higher transfer efficiency
  • More consistent coating thickness
  • Reduced orange peel effect
  • Less wear on consumable parts

3. Gun Positioning:

• The distance between the gun and the part is critical for transfer efficiency.
• If the gun is too far, gravity or airflow will pull powder away from the part.
• If the gun is too close, voltage drops and current increases.
• When the gun’s current exceeds the optimal level, more ions are generated, causing faster adhesion and leading to back ionization.
• Recommended gun distances:
  • Manual application: 15-20 cm
  • Automatic application: 20-30 cm

4. Conveyor Density:

• The positioning of hangers affects transfer efficiency.
• Placing hangers closer together increases TE by minimizing powder dispersion in the air.

5. Nozzle Type:

• Different nozzles influence transfer efficiency.
• Two common types:
  • Fan spray nozzles: Create a large, high-speed powder cloud.
  • Conical nozzles: Provide a softer forward speed with varied powder dispersion based on nozzle diameter.
• Testing different nozzles helps determine the best option for the application.

6. Humidity and Temperature:

• Both factors can impact powder coating system performance:
  • Temperature and humidity variations affect fluidization, filter efficiency, filter lifespan, and powder charging capabilities.
  • Excessive heat can alter the physical/chemical properties of the powder.
  • High humidity can cause powder clumping.
  • Extremely dry air can create charging issues.
  • Optimal conditions for maximum TE:
    • Room temperature below 25°C
    • Relative humidity: 50-60%

7. Grounding:

• Proper grounding is critical for transfer efficiency.
• Poor grounding causes powder dispersion in various directions, leading to:
  • Inconsistent coating thickness
  • Increased powder waste
• Grounding should always be maintained, ensuring all components have a ground resistance below 1 megaohm.

8. Powder Particle Size Distribution:

• Correct particle size distribution is crucial for transfer efficiency.
• Fine particles:
  • Harder to fluidize and pump.
  • Carry more charge per unit mass, leading to poor TE due to airflow disturbances.
  • Easily deposit on flat surfaces but struggle to penetrate Faraday cage areas.
• Larger particles:
  • Move in a straight line and are influenced by electrostatic forces or gravity.
  • If too large, they fall to the booth floor due to gravity.
  • Provide better penetration into internal corners.
• If penetration is difficult and coating thickness doesn’t increase, the powder mix may contain too many fine particles.

Optimizing these factors can significantly improve transfer efficiency, reducing powder waste and ensuring consistent, high-quality coatings.

The most advantageous feature of powder coating is its ability to be recycled. Considering the minimal losses in filtration and collection systems and on part hangers, approximately 95% of the powder coating can be recovered and reused.

In general, the recovery ratio ranges from 25% to 15% of the total powder volume used in the first application (25% reclaimed + 75% virgin powder). The ideal reclaim amount is 15%, which helps maintain the particle size distribution of the powder blend with very minimal changes.

When determining the amount of recycled powder, you should know your first-pass transfer efficiency (TE).

• First-pass transfer efficiency mainly depends on the entire application process and system maintenance, including:
  • Spray guns
  • Hoses
  • Grounding
  • Air regulation

Effects of Insufficient Maintenance on Reclaim:

• Poor maintenance will increase the amount of reclaim powder collected in the recovery system, leading to a higher reclaim ratio.
• Weak grounding will negatively impact transfer efficiency, causing an increase in reclaim powder usage.
• It is not possible to spray for long periods with fixed settings, as reclaim powder will alter the particle size distribution in the feed hopper. Gun settings must be adjusted for long-term use to maintain coating appearance.

NOTICE:

All the points mentioned above apply to smooth surfaces.
For fine, medium, or coarse texture coatings, as well as special effects including metallic pigments, the reclaim ratio may need to be lower to prevent changes in appearance.

Surface defect

Problems with mechanical and chemical properties