Powder coating is a process that provides a durable and tough finish to products, protecting even the roughest surfaces better than conventional paint. Unlike liquid paint, powder coating must be applied using an electrostatic gun, and it can’t be rolled on. When the powder coating is applied, the electrical charge in the powder is attracted to the metal surface. The final step involves curing the powder at a specific temperature through the use of heat or UV light.
Powder coating is used to cover a wide variety of metal products, including household appliances, aluminium extrusions, drum hardware, automobiles, and bicycle frames. Recent advancements in technology have enabled other materials to be powder-coated, including plastics, composites, carbon fibre, and MDF (medium-density fibreboard). The methods used to powder coat these materials require reduced amounts of heat and time
What are the Benefits of Powder Coating?
Powder coating provides numerous benefits over traditional paint. Firstly, it gives a thicker and more attractive finish, and there are no worries about dripping, running or sagging when a thick covering is required. The coating will look the same whether you’re applying it horizontally or vertically.
Powder coating also releases limited volatile organic compounds (VOC) and poses less of a risk to the environment and human health. There is no carrier fluid to evaporate into the air, and it has fewer toxic odours.
In addition, powder coating enables the creation of amazing special effects, such as the ability to apply and cure numerous colours together, allowing the colours to blend in a visually appealing manner.
What are the Properties of Powder Coating?
Powder coating is ideal for applying thick coatings, as it becomes thinner, the texture of the film can start to feel like orange peel. This is because of the particle size and glass transition temperature (Tg) of the powder.
There is a diverse range of particle sizes when it comes to powder coatings, and particle sizes can range from 2 to 50 μ (Microns). The average softening temperature Tg is approximately 80 °C, and the average melting temperature is about 150 °C. In normal circumstances, a product will need to be cured for around 10 – 15 minutes at 200 °C. However, this could increase or decrease depending on the thickness of the coat being applied.
Powder slurrying is a special technique that can be used when you need to produce very thin coats with little film. A smooth, thin coating of fewer than 30 micrometres can be produced when fine powder particles are combined with water and extra additives and filtration. The powder slurry process blends the benefits of powder coatings with the advantages of liquid coatings.
What are the Advantages of Powder Coating over Other Coating Processes?
Powder coatings have no solvents and are known for releasing little or no volatile organic compounds or VOCs into the air. This can be cost-effective as it removes the need for finishers to purchase expensive pollution control equipment. It also makes it easier for organizations to comply with environmental regulations.
Powder coatings are also able to produce much thicker coatings than conventional liquid coatings and eliminate worries about runs, sags, and drips. Powder-coated products generally look the same whether the surface has been coated horizontally or vertically.
Additionally, powder coating is known for its swift curing time, especially when ultraviolet-cured powder is used.
Main Types of Powder Coating
The three most popular types of powder coatings are thermosets, thermoplastics and UV-curable powder coatings.
Thermoset powder coatings incorporate a cross-linker into the formulation to deliver a durable and decorative finish. A solid epoxy resin in hybrid
powder mixed in various ratios is typically the cross-linker in a thermoset powder coat. The mixtures can vary depending on factors such as whether the product is designed for indoor or outdoor use. When the powder is baked, it reacts with other chemical groups in the powder to polymerize. This makes it smooth enough to be comparable to a liquid-based coating and tough enough for any product.
Chemical cross-linking for hybrids and TGIC (Triglycidyl Isocyanurate) powders is the most common in the powder coating industry. It is based on the reaction of organic acid groups with an epoxy resin. The different reactions of various formulations of powder coating have been thoroughly researched and are well understood. This means that very specific and customizable options are available to consumers.
In specific cases such as coil coatings or clear coats, glycidyl ester is the most common product to use as a hardener. Similar to epoxy resin, the cross-linking is once again based on carboxy-epoxy chemistry. With the use of polyurethane powders, a different chemical reaction is generated. In this case, the binder resin carries hydroxyl functional groups that react with isocyanate groups of the hardener component. The isocyanate group is normally introduced in a blocked form and needs higher temperatures to deliver a cross-linking reaction with hydroxyl functionality.
Generally, all thermosetting powder formulations must contain binder resin and cross-linker additives so that the necessary chemical reactions can occur. A flow promoter is commonly used, and the active ingredient (a polyacrylate) is absorbed on silica as a carrier or as a masterbatch dispersed in a polyester resin as a matrix. Most powders will contain benzoin as the degassing agent. This will help prevent pinholes in the final powder coating film.
The thermoplastic type of powder coating is applied following the heating process. The coating melts and encapsulates the part undergoing coating. The thermostat variety does not require additional processes once the heating process is complete.
Powder coating that’s cured with UV light has reduced temperature requirements and offers a faster curing time than the other types of powder coating. The coating powder will melt in one to two minutes (when it reaches a temperature between 110°C and 130°C). This type of powder coating requires photopolymerizable materials that contain a chemical photoinitiator. The chemical photoinitiator immediately responds to UV light by beginning the reaction that leads to curing.
The process of powder coating involves applying a layer of powdered polymer to a metal object, which is then melted and cured to form a smooth, durable finish. There are several different polymers that can be used in powder coating, including polyester, polyurethane, polyester-epoxy (known as hybrid), straight epoxy (fusion bonded epoxy), and acrylics.
Process of Powder Coating
There are three steps to the powder coating process. These include:
- Powder Application
Pre-treatment or Part preparation
The method used to complete the pre-treatment process is dependent on three things:
- The size and the material of the part to be powder coated
- The type of impurities to be removed
- The performance requirement of the finished product
The pre-treatment stage is essential for ensuring that the product to be powder-coated is free of any impurities that could affect the adhesion of the polymer. During this stage, oil, dirt, lubrication greases, and metal oxides are removed using a combination of chemical and mechanical processes.
The specific method used for pre-treatment depends on the size of the part, the material it is made from, and the performance requirements of the finished product. Chemical pre-treatments, such as submersion or spraying of phosphates and chromates, are commonly used. However, due to environmental concerns, there has been a shift towards using less toxic alternatives like titanium zirconium and silanes.
Plasma treatment may also be used to boost adhesion, particularly for heat-sensitive plastics or composites that have a low surface tension. Additionally, abrasive blasting may be required to add surface texture, remove contaminants, or smoothen out rough surfaces. The type of abrasive used depends on the chemical composition, particle size and shape, and impact resistance required.
The powder coating is applied to the product using an electrostatic gun or a tribo gun, which sprays the negatively charged powder onto the grounded object. Alternatively, the powder can be applied via the fluidized bed method, which involves submerging the heated object into a bed of powder, or the electrostatic fluidized bed application, which involves creating a cloud of charged powder that adheres to the grounded object.
Electrostatic fluidized bed coating
Similar to the fluidized bed method discussed above is the electrostatic fluidized bed application. This method uses the same fluidizing technique as described, but the bed is much deeper with more powder. The object does not need to be preheated, instead, the powder in the bed is electrostatically charged, which causes the powder to move upward and form a cloud. When the grounded object passes through the cloud of charged powder the particles are attracted to its surface. The particles stick to the surface, effectively coating the object.
Electrostatic magnetic brush (EMB) coating
Another effective coating method is using an electrostatic magnetic brush. This method of coating is suitable for flat objects such as steel sheets, aluminium, MDF, or even paper. The electrostatic magnetic brush coating method was created based on conventional copier technology. This method allows one to apply a powder coating quickly and precisely using a roller. This process is so accurate that it allows the finisher to choose a specific thickness of between 5 and 100 micrometres.
After pre-treatment and powder coat application, curing is necessary to achieve durability and performance in the finished product. Curing, also known as cross-linking, involves heating the object to a specific temperature for a fixed period of time to ensure that coating materials are performing as expected.
Influence of Polyester Resin and Curing Agent The type of curing agent used and the structure of the polyester resin can significantly affect cross-linking. TGIC-free systems with β-hydroxy alkylamides as curing agents must be cured at 160 °C (320 °F), while outdoor durable powders with triglycidyl isocyanurate (TGIC) as a hardener can operate at lower temperatures, even down to 125-130 °C (257-266 °F).
Curing Temperature and Time While coating powders are typically cured at 200 °C (390 °F)/object temperature for ten minutes, some European and Asian companies have opted for 180 °C (356 °F) for 10 minutes. However, the standard has recently changed to a lower temperature level of 160 °C (320 °F) for ten minutes, with some advanced systems able to cure at even lower temperatures. Lower temperatures offer energy savings and enhanced productivity, especially for larger items, with fast-cure powders enabling objects to be cured at 180 °C (356 °F) for just two minutes.
Curing Challenges Maintaining the essential resin characteristics of coating powders while ensuring storage and chemical stability, flow out, and simultaneous reactivity can be a challenge for low bake systems. This is particularly true for those working with metal, who must strive to achieve optimal performance across all gloss levels and colors.
Curing Methods Different types of ovens can be used to cure objects, including convection, infrared, and laser cutting ovens. UV-cured powder coatings, which have been used commercially since the 1990s, require less heat energy and can cure more quickly than thermally cured powder coatings.
TUV Standards for Safety To ensure safety, TUVs, which are independent organizations that aim to prevent hazards across all types of objects, facilities, and systems, do not allow curing at any temperature higher than 90 °C (194 °F) for alloy automotive wheel repair and other highly stressed components such as diving cylinders.
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