Powder Coatings

Applied as a free-flowing, dry powder, powder coatings are typically applied thermostatically after which it is cured under heat. ChinaSavvy’s powder coating shop specializes in coating various metal substrates ranging from aluminum extrusions and automobile parts, to household appliances and drum hardware, to name but a few examples.

On this page you will find more information concerning:

1. The Production of Different Types of Powder Coatings
2. The Powder Coating Process
3. Advantages of Powder Coating

The Production of Different Types of Powder Coatings

There are two main categories of powder coatings. This includes thermoplastics and thermosets.

Thermosetting incorporates a cross-linker into the formulation and, when the powder is baked in the baking step of the process, it reacts with other chemical groups in the powder to polymerize.

In the baking step of the process, thermoplastics however flows out onto into the final coating and does not undergo additional actions.

Powder Production:

• 1. First, the powder granules are mixed with a number of powder ingredients including pigments and hardeners.
• 2. Next, this mixture is heated in an extruder.
• 3. The extruded mixture is now rolled flat, cooled and then broken into small chips.
• 4. These chips are now milled and sieved, creating a fine powder ready to be used in the powder coating process.

The most common polymers used in the creation of powders for the powder coating processes include polyester, polyurethane, acrylics, straight epoxy and polyester-epoxy.

The Powder Coating Process

Powder coatings are applied in three basic steps:

1. Pre-Treatment
2. Powder Application
3. Curing

Pre-Treatment

Before powder coatings can be applied the part has to be cleaned – this entails the removal lubrication greases, oil, welding scales, dirt and metal oxides, to name but a few examples. This cleaning can be done by using a variety of mechanical and chemical methods, the selection of the method depending on:

• The material of the part to be powder coated,
• The type of impurities that has to be removed,
• The size of the parts, and
• The performance requirements of the finished part.

In terms of chemical pre-treatments, phosphates or chromates either in spray application or submersion can is used. Note here that the pre-treatment process not only cleans, but also improves the bonding of the powder to the metal substrate.

Other methods used to clean parts also include shot blasting and sandblasting (or abrasive blasting). This method is used on glass, wood and plastic parts to clean and provide surface texturing.

Powder Application

An electrostatic gun (or corona gun) is most commonly used to apply powder coatings to metal parts. This electrostatic gun imparts a positive electric charge to the powder. This powder is then sprayed towards the grounded part by either mechanical or compressed air spraying.

The powder is now accelerated towards the work piece by the positive electric charge created earlier.

The Tribo gun is also commonly used and here the powder is charged by using friction. Here, the powder picks up a positive charge when rubbing against the Teflon tube, located inside the barrel of the Tribo gun. From here, the powder particles adhere to the grounded part.

Note that Tribo guns do require the use of a differently formulated powder.

The fluidized bed method is also used to apply coatings and entails heating the part (substrate) and then dipping it into an aerated, powder filled bed. This allows for the powder to stick to the part and then melt. This part then also undergoes further curing processes.

Note here that in a powder coating shop the fluidized bed coating process is more commonly used where a coating thickness of more than 300 micrometers is required.

Curing

When the thermoset powder used in previous steps of the coating process is exposed to elevated temperatures, it will begin to melt, flow out and then chemically react. This causes the formation of a higher molecular weight polymer in the form of a ‘network-like’ structure.

Also called crosslinking, the cure process needs a specific temperature for a specific length of time. This is required in order to reach full cure and establish the full film properties it was designed to achieve.

While powders usually require a temperature of 200°C (390°F) for 10 minutes to cure, this can vary according the requirements of the part.

Laser curing processes, infrared cure ovens and convection cure ovens is can be used the cure powder coatings.

Advantages of Powder Coating

Powder coatings offer various benefits in terms of:

1. Resistance
   • Coatings have a longer life than paint coatings and are also flexible, allowing for a part to ‘bend’ without cracking the coating.
   • Powder coated surfaces are more resistant to fading, wear, scratch and chipping when compared to other surface finishes.
2. Finish
   • Powder coated surfaces are highly durable and also more durable than paint.
   • Sags and runs, as seen in liquid coatings, are eliminated.
   • When compared to liquid coatings, powder is capable of producing a thicker coating and produces less hazardous waste.
   • Compared to paint, manufacturers can achieve a more uniform, higher film thickness.
   • Also note that, when compared to liquid coated parts, these types of coatings display fewer appearance differences between vertically coated and horizontally coated surfaces.
3. Texture and Color
   • There is a large variety of standard and custom color options available with powder coating. Metallic finishes, clear coatings and glosses can also be produced.
   • Powder coating also allows for a number of textures including smooth, matte, wrinkled and rough.
4. Durability
   • Powder coats offers resistance to corrosion, abrasion, heat and impact.
   • These types of coatings has a resistance to peeling, scratching, cracking and fading due to sunlight.
5. Environment
   • Because powder coatings do not contain any solvents, it releases negligible amounts of Volatile Organic Compounds (VOC).
   • This process has reduced fire, health and safety hazards.
6. Financial
   • Due to the negligible amounts of VOCs in this process, the need for pollution control equipment is minimized
   • In powder coating, the overspray or unused powder can be recycled, helping to reduce material waste. Material waste can be kept at less than 5%. Powder recovered can be blended with virgin powders and can result in up to 95% utilization.
   • Another financial benefit is the cost saving during the stoving (or curing process) as much less energy is needed. This is due to the fact that there is no need to evaporate solvents and evacuate it from the oven.
   • Time saving benefits in a powder coating shop is also great with this process as powders do not need to be mixed with solvents.

Note that powder coating also has its limitations.

Though it is relatively easy to achieve thick films, it is more difficult to achieve thin films when compared to wet paint processes. Color change is also slower as that seen with liquid paints and powders cannot be mixed in order to achieve a different color.

Also note that, in cases where multiple colors are sprayed, there is a limitation in the ability to recycle the overspray.

ChinaSavvy, based in Guangzhou, China, has access to world-class powder coating shops and technologies, offering high-quality metal coatings at affordable China prices.

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Further Suggested Reading:

• Corrosion Resistant Coatings
• Decorative Coatings