Powder coating is a type of coating used as a free-flowing dry powder. The main difference between conventional liquid paint and powder coatings is that the powder coating does not require a solvent to keep the binder and filler parts in the form of a liquid suspension. This layer is usually applied electrostatically and then healed under heat to allow it to flow and form "skin". The powder may be a thermoplastic or thermoset polymer. Usually used to make hard finishing harder than conventional paint. Powder coating is mainly used for metal coating, such as household appliances, aluminum extrusions, drum hardware and automobile and bicycle parts. The newer technology allows other materials, such as MDF (medium density fiberboard), to be powder coated using different methods. The powder coating process was discovered around 1945 by Daniel Gustin US Patent 2538562.
Video Powder coating
Powder coating properties
Since the powder layer lacks a liquid carrier, it can produce a thicker layer than conventional liquid coatings without running or sagging, and the powder layer results in minimal difference in appearance between horizontally coated surfaces and vertically lined surfaces. Since no carrier fluid evaporates, the coating process emits several volatile organic compounds (VOCs). Finally, some powder colors can be applied before healing them all together, allowing the blending of color and special effects bleeding in one layer.
While it's relatively easy to apply a thick layer that heals to smooth, texture-free layers, it's not as easy as applying a fine thin film. As the film thickness decreases, the film becomes more and more orange peel in texture due to particle size and glass transition temperature (Tg) of the powder.
Most powder coatings have particle sizes in the range of 2 to 50? (Microns), the softening temperature of Tg is about 80 ° C, the melting temperature is about 150 ° C, and recovers approximately 200 ° C. for a minimum of 10 minutes to 15 minutes (the exact temperature and time can depend on the thickness of the item being coated). For such a powder coating, the film accumulation is more than 50? (Mikron) may be required to get a very fine film. The surface texture that is considered desirable or acceptable depends on the final product. Many manufacturers actually prefer to have a certain level of orange peel because it helps hide metal defects that occur during manufacture, and the resulting layer is less prone to show fingerprints.
There is a very special operation where powder coating less than 30 micrometers or with Tg below 40 Â ° C is used to produce a fine thin film. One variation of the dry powder coating process, the Powdered Porridge process, combines the advantages of powder coating and aqueous coating by dispersing fine particle-sized powder 1-5 micrometers into water, which then allows very fine. , a low layer of film thickness to be produced.
For garage scale work, "small cans" spray paint is cheaper and more complicated than powder coating. On a professional scale, the cost of capital and time required for powder guns, booths and ovens is similar to a spray gun system. Powder coatings have a great advantage because overload can be recycled. However, if multiple colors are sprayed in a single spray booth, this may limit the ability to recycle overpayments.
Maps Powder coating
Advantages of powder coating than other coatings
- Powder coatings do not contain solvents and release little or no Volatile Organic Compounds (VOCs) into the atmosphere. Thus, there is no longer a need for people who buy expensive pollution control equipment. Companies can more easily and economically comply with U.S. Environment Protection Agency regulations.
- Powder coatings can produce a thicker layer than conventional liquid coatings without running or sagging.
- Powder coated articles generally have less difference in appearance between horizontally coated surfaces and vertically lined surfaces than liquid-coated items.
- A wide range of special effects is easily achieved by using powder coatings that are not possible with other coating processes.
- The curing time is significantly faster with the powder layer compared to the liquid layer.
Type of powder layer
There are two main categories of powder coatings: thermosets and thermoplastics. Various thermosetting incorporates cross-linker into the formulation. When the powder is roasted, react with other chemical groups in the powder for polymerization, improving the performance properties. Thermoplastic varieties do not take additional measures during the roasting process as they flow to form the final layer.
The most commonly used polymers are: polyester, polyurethane, polyester-epoxy (known as hybrid), fusion bonded epoxy and acrylic.
Production
- The polymer granules are mixed with hardener, pigment and other powder materials in industrial mixers, such as turbomixer
- The mixture is heated in the extruder
- The extruded mixture is flat-rolled, cooled and broken into small chips
- Chips are ground and sifted to make fine powder
Powder coating process
The powder coating process involves three basic steps:
- Preparation section or pre-treatment
- Powder application
- Heal
Parts of the preparation process and equipment
Removal of oil, dirt, grease lubricant, metal oxide, welding scale etc is very important before powder coating process. This can be done by various chemical and mechanical methods. The choice of method depends on the size and material of the part to be powder coated, the type of impurities to be removed and the performance requirements of the finished product.
Chemical pre-treatments involve the use of phosphate or chromate in immersion or spray applications. This often occurs in several stages and consists of degreasing, etching, de-smutting, various rinsing and phosphate end or chromate substrate & amp; new chemical bonds of nanotechnology. The pre-treatment process both cleans and enhances the bonding of the powder with the metal. An additional process has recently been developed that avoids the use of chromates, as this can be toxic to the environment. Titanium zirconium and silane offer similar performance against powder corrosion and adhesion.
In many high end applications, this section is electrocoated after pretreatment process, and after application of powder coating. This is particularly useful in automotive and other applications that require high performance end characteristics.
Another method of preparing the surface before coating is known as abrasive blasting or sandblasting and shot blasting. Blast and blasting abrasive media are used to provide surface texture and preparation, etching, finishing, and degreasing for products made of wood, plastic, or glass. The most important properties to consider are chemical composition and density; particle shape and size; and impact resistance.
Silicon carbide grit blast is fragile, sharp, and suitable for grinding metal and low tensile strength, non-metallic materials. Plastic blast media equipment uses abrasive plastics that are sensitive to substrates such as aluminum, but are still suitable for de-coating and surface finishing. The blast medium sand uses high purity crystals that have low metal content. Medium bead blast glass contains glass beads of various sizes.
Cast steel shot or steel grit is used to clean and prepare the surface before coating. Shot blasting recycles media and environmentally friendly. This preparation method is highly efficient in steel parts such as I-beams, corners, pipes, tubes and large pieces made.
Different powder coating applications may require alternative preparation methods such as abrasive blasting before coating. The online consumer market usually offers media blasting services coupled with their coating services at an additional cost.
Powder application process
The most common way to apply powder coating to a metal object is to spray the powder using an electrostatic gun, or a corona gun. The gun provides a positive electrical charge to the powder, which is then sprayed towards the grounded object by mechanical or compressed air spraying and then accelerated to the workpiece with a strong electrostatic charge. There are various kinds of spray nozzles available for use in electrostatic coatings. The type of nozzle used will depend on the shape of the workpiece to be painted and the consistency of the paint. The object is then heated, and the powder melts into a uniform film, and then cooled to form a hard layer. It is also common to heat the metal first and then spray the powder onto the heat substrate. Initial heating can help to achieve a more uniform end result but can also create other problems, such as processes caused by excess powders. See article "Bonded Fusion Epoxy Coatings"
Another type of weapon is called a tribo pistol, which fills the powder with friction (triboelectric). In this case, the powder takes a positive charge while rubbing along the Teflon tube wall inside the gun barrel. The charged particles are then attached to the grounded substrates. Using tribo weapons requires different powder formulations than the more common corona rifles. Tribal weapons are not subject to some of the problems associated with corona weapons, however, such as re-ionization and the effects of Faraday's cage.
Powders can also be applied with customized electrostatic discs.
Another method of applying powder coating, termed as fluidized bed method, is to heat the substrate and then dip it into a filled, powdered bed. The powder sticks and melts into a hot object. Further heating is usually required to complete the coating process. This method is generally used when the desired layer thickness exceeds 300 micrometers. This is how most dishwashing shelves are coated.
Electrostatic fluidized bed layers
The application of an electrostatic fluidised bed uses the same fluidization technique as conventional fluidized bed dips but with much less powder depth in the bed. An electrostatic filling medium is placed inside the bed so that the powder material becomes filled when the fluidizing air lifts it. Powdered particles move upward and form a charged powder cloud above the liquid bed. When the ground is passed through a charged cloud, the particle will be attracted to its surface. The parts are not heated as they are for conventional fluidized bed processes.
Electrostatic magnetic brush (EMB) coating
Coating method for flat material using powder with roller, enables relatively high speed and accurate layer thickness between 5 and 100 micrometers. The basis for this process is the conventional photocopier technology. Currently used in some coating applications and looks promising for commercial powder coating on flat substrates (steel, aluminum, MDF, paper, board) as well as sheets for sheets and/or roll processes for rolling. This process is potentially integrated in existing coating lines.
Curing
When the thermoset powder is exposed to high temperatures, it begins to melt, flows out, and then reacts chemically to form polymers of higher molecular weight in tissue-like structures. This healing process, called crosslinking, requires a certain temperature for a certain period of time to achieve full recovery and builds the full film properties of which the material is designed. Usually the medicine powder at 200 Â ° C (390 Â ° F) for 10 minutes. The curing schedule may vary according to the manufacturer's specifications. Energy applications for products to be cured can be achieved by convection healing ovens, infrared healing ovens, or by laser preservation processes. The latter shows significant reduction in preservation time.
Eliminating powder layer
Methylene chloride and acetone are generally effective in removing powder coating. Most other organic solvents (diluents, etc.) are completely ineffective. Recently, the alleged chlorine methylene chlorine as a human carcinogen was replaced by benzyl alcohol with great success. Powder coating can also be removed by abrasive blasting. 98% commercial grade sulfuric acid also removes the film powder coating. Certain low-grade powder coats can be removed with steel wool, although this may be a more labor-intensive process than desired.
The powder layer can also be removed by the combustion process, where the parts are inserted into a large temperature oven with temperatures typically reaching a temperature of 300-450 ° C. This process takes about four hours and requires parts to be cleaned completely and re-coated powder. Parts made with thinner gauges should be burned at a lower temperature to prevent the material from curling.
Market
According to market reports prepared in August 2016 by Grand View Research, Inc., the global powder coating market is expected to reach USD 16.55 billion by 2024. Increase the use of powder coatings for aluminum extrusions used in windows, doorframes, building facades, kitchens Bathroom and electrical appliances will drive industrial expansion. Increased construction spending in various countries including China, US, Mexico, Qatar, UAE, India, Vietnam and Singapore will boost growth over the forecast period. Increasing government support for eco-friendly and economical products will stimulate demand during the forecast period. The general industry is a leading application segment and accounts for 20.7% of global volume by 2015. Increased demand for tractors in the US, Brazil, Japan, India and China is expected to increase the use of powder coatings due to corrosion protection, extremely outdoor resistance good, and high temperature performance. In addition, increased use of agricultural equipment, sports equipment, file drawers, computer cabinets, laptop computers, cell phones, and electronic components will drive industry expansion.
See also
- The laser printer
- Fusion bonded epoxy layer
- Powder layer on glass
References
Source of the article : Wikipedia
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