Powder coating is a type of coating that creates an extremely durable and hard finish on a product. It is more durable than conventional paint and protects even the roughest surface. Powder coating needs to be applied with an electrostatic gun and cannot be rolled on, unlike conventional liquid paint. When applying a powder coating the electrical charge in the powder is attracted to the surface of the metal. The final step is to cure the powder at a specific temperature using heat or ultraviolet light. Powder coating is generally used to cover metal products including household appliances, aluminium extrusions, drum hardware, automobiles, and bicycle frames. Recently, there have been advancements in the technology that has allowed other materials, such as plastics, composites, carbon fibre, and MDF (medium-density fibreboard), to be powder coated using slightly different methods that require a reduced amount of both heat and time.
Main Properties of Powder Coating
There are several advantages to using powder coating over traditional paint. Powder coating can provide a thicker and more attractive finish. Due to the lack of liquid in a powder coating, there is no risk of drips, runs or sags when a thick covering is needed. Whether or not the coating is applied horizontally or vertically the coating will appear the same.
A powder coating carries less toxic odours and less risk of harm to the environment or human health because the coating process emits limited volatile organic compounds (VOC). This is because there is no carrier fluid to evaporate into the air.
Finally, it is easy to create beautiful special effects with a powder coating. Numerous colours can be applied and cured together allowing the colours to blend in an aesthetically pleasing manner.
When using a powder coating process, thick coatings are much easier to apply than smooth thin coatings. When the coating becomes thinner the texture of the film becomes more orange peeled. This is because of two things; the particle size and glass transition temperature(Tg) of the powder.
There is a wide range of particle sizes when it comes to powder coatings; particle size can range from 2 to 50 μ (Microns). The average softening temperature Tg is about 80 °C and the average melting temperature is about 150 °C. Usually, a product will need to be cured for around 10 – 15 minutes at 200 °C but this could increase or decrease depending on thickness of the coat being applied. Thicker, smoother coatings are not always the desired outcome for every product and manufacturer. Sometimes it is best to have some orange peel texture because it helps to make any defects invisible to the consumer. Many consumers may prefer an orange peel effect because it effectively hides fingerprints.
It is more complicated to produce very thin coats with little film, but there is a specialized technique that can be used. A smooth, thin coating of less that 30 micrometres can be effectively produced by combining the fine powder particles with water, as well as further additives and filtration. This is known as the Powder Slurry process and it combines the advantages of powder coatings with the advantages of liquid coatings.
Advantages of powder coating over other coating processes
- Powder coatings contain no solvents and release very little or no volatile organic compounds (VOC) into the air. This saves money because it eliminates the need for finishers to purchase expensive pollution control equipment. It is easier for organizations to comply with the regulations of the U.S. Environmental Protection Agency.
- Powder coatings can produce much thicker coatings than conventional liquid coatings without worrying about runs, drips or sags. .
- Unlike painted items, powder coated items generally look the same whether it is a horizontally coated surface or a vertically coated surface.
- A wide range of beautiful colour effects are easily accomplished using powder coatings that would be impossible to achieve with traditional liquid paint.
- Curing time is significantly faster with powder coatings as compared to traditional liquid paint coatings, especially when using ultraviolet cured powder.
Main Types of Powder Coating
When investigating your choices of powder coatings, it is easiest to divide the types into the three main categories including: thermosets, thermoplastics and UV curable powder coatings.
Thermoset powder coatings are probably the most popular type of powder coatings because they incorporate a cross-linker into the formulation resulting in a durable and decorative finish. Usually the cross-linker in a thermoset powder coat is a solid epoxy resin in hybrid powders mixed in various ratios. These mixtures will vary, based on whether the product to be covered will be used indoors or outdoors. When the powder is baked, it reacts with other chemical groups in the powder to polymerize, making it smooth enough to be comparable to a liquid-based coating and durable enough for any product. This chemical cross-linking for hybrids and TGIC powders is the most common in the powder coating market. It is based on the knowledge of 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, thus allowing for very specific and customizable options.
In specific cases such as coil coatings or clear coats the most common product to use as a hardener is glycidyl ester. Similar to the epoxy resin, the crosslinking is once again based on carboxy-epoxy chemistry. With use of polyurethane powders, a different chemical reaction is used. In this case the binder resin carries hydroxyl functional groups that react with isocyanate groups of the hardener component. The isocyanate group is generally introduced in a blocked form and will require higher temperatures to become available for a cross-linking reaction with hydroxyl functionality.
Generally, all thermosetting powder formulations must contain binder resin and cross-linker additives for the necessary chemical reactions to occur. It is most common to use a flow promoter where the active ingredient (a polyacrylate) is absorbed on silica as carrier or as masterbatch dispersed in a polyester resin as matrix. The great majority of powders will contain benzoin as the degassing agent. This will help to avoid pinholes in the final powder coating film.
The thermoplastic type of powder coating is applied after heating. The coating melts and encapsulates the part being coated. The thermostat variety does not require additional processes after heating.
Powder coating cured with UV light has lower temperature requirements and a faster curing time than the other categories 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 photopolymerisable materials containing a chemical photoinitiator. The chemical photoinitiator immediately responds to UV light by starting the reaction that leads to the curing stage.
The polymers that are most often used are: 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 pre-treatment process involves cleaning the product to be powder coated. All oil, dirt, lubrication greases, metal oxides etc. must be completely and thoroughly removed. This can be achieved in a variety of different chemical and mechanical processes.
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
If the plastic or composite you are working with is heat sensitive and has a low surface tension plasma treating may be necessary to improve powder adhesion.
Chemical pre-treatments involve submersion or spraying of phosphates and chromates. This is not a simple process and often requires several stages such as degreasing, etching, de-smutting, various rinses and the final phosphating or chromating of the substrate and new nanotechnology chemical bonding. The pre-treatment process not only cleans the material, it also increases the ability of the powder to bond to the metal. As chromates are toxic to the environment, recent developments have included the use of titanium zirconium and silanes which offer similar benefits as the chromates with less toxicity.
Often, there is a middle step between pre-treatment and application of the powder and that is to electrocoat the part. This step helps with avoiding corrosion and increasing durability. It is especially useful in automotive painting and other high end applications.
Sometimes a powder coat application can require additional preparations before it can be completed. One such method of surface preparation is called abrasive blasting, also referred to as sandblasting and shot blasting. Abrasive blasting is done to add surface texture, such as etching or finishing or to smooth a rough surface. It may also be used to remove contaminants. This step is normally done for products made of wood, plastic or glass. When deciding what type of grit to use when sandblasting a part, it will be necessary to consider chemical composition, particle size and shape, as well as impact resistance.
There are several types of abrasives that can be used in the blasting process including: silicon carbide, ground up plastic stock, sand, glass and steel grit. Some are extremely abrasive, while others are milder. Silicon carbide is brittle and sharp, suitable for grinding metals while plastic media is suitable for de-coating and surface finishing. Sand is highly abrasive while glass bead blasting is a bit milder. Steel grit is also known as cast steel shot and is a highly abrasive material used to remove contaminants from the part before coating.
Shot blasting is an environmentally friendly process that is normally used on large steel parts such as I-beams, angles, pipes, tubes and fabricated pieces.
One newer development in the powder coating industry is the use of plasma pre-treatment for some materials. The materials that typically respond well to plasma pre-treatment include heat sensitive plastics and composites. The plasma pre-treatment will change the surface energy of the part being coated in order to increase adhesion. Some materials have low-energy surfaces, as well as a low degree of wettability which will make it difficult for the coating to properly anchor to the surface. Plasma treatment will clean the surface and allow a strong bond between the surface and the powder coating.
Generally, the addition of any pre-treatment services combined with a powder coating will be an additional charge for the consumer.
Usually the dry powder coating is sprayed onto metal products using an electrostatic gun, also known as a corona gun. The gun sprays the negatively charged powder onto the grounded object. The powder stays on the object until it is melted and then cured into a smooth, uniform coating. After the cooling process is complete, the product will be coated in a tough, durable finish. For further information on powder coating see the article “Fusion Bonded Epoxy Coatings”
Another type of gun used in the powder application process is known as a tribo gun. This type of spray application charges the powder by triboelectric friction. This variety of gun can help avoid some of the concerns associated with other electrostatic guns. A couple of the problems that can be avoided with a tribo gun include back ionization and the Faraday cage effect.
Unique electrostatic discs can also be used to apply a powder coating.
Another method of applying powder coating is the fluidized bed method. This process allows the uniform application of a powder coating by using heat to warm the object. Once the piece is extremely hot, it is submerged in a bed of the powder. The powder will melt onto the heated object. After the powder is melted, it will have to be cured to finish the process. This is an effective method if the goal is to get a thick, even coating. For example, dishwasher racks are typically coated in this manner.
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 a product is pre-treated and the powder coat is applied it must be cured. Curing ensures that the finish will be durable for several years. Curing an object means heating it to a specific temperature for a specific amount of time. The cure process is also called crosslinking and it is necessary to allow the coating materials to perform the way they were meant to. The architecture of the polyester resin and type of curing agent will have a significant effect on crosslinking.
It is most common to cure a coating powder at 200 °C (390 °F)/object temperature for ten minutes. This temperature has been slightly different in European and Asian markets where the industry standard is to cure objects at 180 °C (356 °F) for 10 minutes. Recently, this standard has shifted down to a temperature level of 160 °C (320 °F) for ten minutes. The more advanced systems cure at even a lower temperature level of 125-130 °C (257-266 °F). Some outdoor durable powders with triglycidyl isocyanurate (TGIC) used as a hardener can operate at this low a temperature level as well. TGIC-free systems with β-hydroxy alkylamides as curing agents need to be cured at 160 °C (320 °F).
The main advantages of curing the object at a lower temperature is that it will result in energy savings and increased productivity especially when the objects to be coated are very large. Significant savings can be seen if the temperature can be reduced by 25 degrees or more. There are fast-cure powders available that can allow an object to be cured at 180 °C (356 °F) for 2 minutes.
One of the major difficulties for any low bake system is maintaining the essential resin characteristics of the coating powder while maintaining such crucial aspects such as storage stability, chemical stability, flow out and simultaneous reactivity. When working with metal it is a challenge to provide the highest performance in all gloss levels and colours.
There are several different types of ovens that can be used to cure an object. The curing process can be done by using a convection cure oven, infrared cure oven, or for optimal curing time, by a laser curing process.
Another way to reduce curing time is to use an ultraviolet (UV) cured powder coating. This type of coating has been in commercial use since the 1990s. UV cured powder coatings have the significant advantage of using less heat energy, for this reason objects can be cured more quickly than with thermally cured powder coatings.
All TUV’s are independent organizations that strive to minimize danger and prevent hazards in all kinds of systems, facilities and objects. TUV’s do not allow curing at a temperature higher than 90 °C (194 °F) for alloy automotive wheel repair because it can be hazardous to the structure. The same can be true of any other highly stressed components such as diving cylinders.
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