What is Powder coating? Principle, coating, process, advantages and application of powder coating

What is Powder coating? Principle, coating, process, advantages and application of powder coating

Ⅰ Powder coating Introduction

Powder coating, as the name suggests, is a coating composed of solid fine particles that are uniformly coated on the surface of the coated material by electrostatic spraying, fluidized bed impregnation, or other methods, and fused at high temperatures to cure into a film. Unlike traditional liquid coatings, it does not require solvents, so it greatly reduces the pollution to the environment during the application process.

Ⅱ Type of coating

Powder coating is a completely different form from the general coating, it exists in the state of fine powder. Because it does not use solvents, it is called powder coating. The main characteristics of powder coating are: harmless, high efficiency, saving resources and environmental protection characteristics. It has two main categories: thermoplastic powder coatings and thermosetting powder coatings.

1.Thermosetting powder coating

Thermosetting powder coating refers to the thermosetting resin as a film forming material, adding the curing agent of crosslinking reaction after heating to form an insoluble and insoluble texture of hard coating. No matter how high the temperature is, the coating will not soften like the thermoplastic coating, but can only decompose. Because the resin used in the thermosetting powder coating is a prepolymer with low polymerization degree and low molecular weight, the coating has good leveling property and good decoration, and the prepolymer with low molecular weight can form a network of cross-linked macromolecules after curing, so the coating has good corrosion resistance and mechanical properties. Therefore, the development of thermosetting powder coatings is particularly rapid.

Epoxy powder coating

Due to its excellent adhesion with metal, corrosion resistance, hardness, flexibility and impact strength, it is the first variety used in thermosetting powder coatings. The preparation of epoxy powder coating is composed of epoxy resin, curing agent, pigment, filler and other auxiliaries. The contribution of these components to the performance of the powder coating is mutually restricted and affected, and a suitable formula is actually the result of the coordination of various components.

Polyester powder coating

Compared with other types of powder coatings, polyester powder coatings have unique properties. The properties of weathering resistance and UV rotation resistance are better than those of epoxy resin. In addition, because the polyester resin has polar groups, the powder rate is higher than the epoxy resin, the baking process is not easy to yellow, high gloss, good leveling, full paint film, light color and other characteristics, so it has a good decorative. Generally used in refrigerators, washing machines, vacuum cleaners, instrument shells, bicycles, furniture and other fields.

Acrylic powder coating

Acrylic resin powder coatings are thermoplastic and thermosetting. The biggest advantages of thermosetting acrylic resin powder coating are excellent weather resistance, color retention, pollution resistance, strong metal adhesion, excellent film appearance, suitable for decorative powder coating.

2.Thermoplastic powder coating

Thermoplastic powder coatings began to appear in 1950, it melts at spray temperature and solidifies into a film when cooled. Due to the simple processing and spraying methods, powder coatings only need to be heated and melted, levelled, cooled or extracted to form films, without the need for complex curing devices. Most of the raw materials used are common polymers on the market, which can meet the requirements of performance under most conditions. However, there are also some shortcomings, such as high melting temperature, low coloring level, and poor adhesion to the metal surface. However, the commonly used thermoplastic powder coatings still show some unique properties, among which polyolefin powder coatings have excellent solvent resistance; Polyvinylidene fluoride coatings have outstanding weatherability. Polyamide has excellent wear resistance; PVC has a good price/performance ratio; Thermoplastic polyester powder coating has the advantages of beautiful appearance and high artistic quality. These characteristics make thermoplastic powder coatings occupy a large proportion in the coatings market.

Polyvinyl chloride powder

PVC powder is one of the cheapest polymers produced on an industrial scale. It has excellent solvent resistance, good corrosion resistance to water and acid, impact resistance, salt spray resistance, can prevent food contamination and electrostatic spraying has high insulation strength. Mainly used for painting metal mesh board, steel furniture, chemical equipment and so on.

Polyethylene powder

Polyethylene powder coating has excellent corrosion resistance, chemical resistance and excellent electrical insulation and ultraviolet radiation resistance. The disadvantage is that the mechanical strength is not high and the adhesion to the matrix is poor. It can be used in impeller, pump, pipe wall, instrument shell, metal plate, refrigerator inner screen board, auto parts and so on.

Nylon powder coating

Nylon (Nylon) is also known as polyamide, because the N atom of the chlorine group on the molecular chain and the hydrogen atom on the adjacent chain segment are easy to form hydrogen bonds, so the melting point of polyamide resin is generally higher. Nylon has the advantages of high mechanical strength, impact resistance, hardness, wear resistance and small friction coefficient, low dust absorption, etc., can be used for special requirements of the parts. For example, it is used in water pump impeller, textile machinery parts, diesel engine starting piston parts, machine sailing propeller impeller, automobile wheels, motorcycle support, agricultural machinery, construction and sports equipment. In addition, due to the resistance of nylon to salt water and the inertia of mold and bacteria, it is suitable for manufacturing coating soaked in seawater or in contact with seawater, while nylon powder coating is non-toxic, tasteless, not corroded by mold, will not promote bacterial growth, it is suitable for spraying parts of the food industry, drinking water pipes and food packaging.

Fluorine resin powder coating

Polytetrafluoroethylene melting point is as high as 327 ° C, can be used in the range of -250 ~ 250 ° C for a long time, in addition to excellent corrosion resistance, even in king water is not corroded, excellent dielectric properties, very low friction coefficient and self-lubrication, so known as "plastic king" said. It is widely used in petroleum, chemical anti-corrosion coating, sealing, bearing lubrication materials, electronic and electrical materials, ship launching guide rail and non-stick coating.

Emulsion powder coating

Emulsion powder is made by spray drying emulsion, most of which are vinyl acetate copolymers. The coating produced by latex powder is called latex powder coating. It is the most environmentally friendly coating in the current wall coating.

Ⅲ Coating production

The production process of powder coating mainly includes raw material preparation, pre-mixing, melting extrusion, crushing and screening, packaging and other steps. Among them, melting extrusion is the key link, through high temperature to melt the resin, curing agent, pigment and other raw materials, and then through cooling, crushing, screening to obtain uniform fine powder particles.

Ⅳ Process flow

1.preconditioning

Pretreatment is a key step before powder spraying, which directly affects the quality and adhesion of the coating. It mainly includes:

• Oil removal: Use solvents or alkaline cleaning agents to remove oil from the surface of the workpiece.

• Rust removal: Removal of rust and oxide by means of sandblasting, shot blasting or chemical treatment.

• Phosphating/passivation: A phosphating film or passivation layer is formed on the metal surface to improve the adhesion and corrosion resistance of the coating.

• Cleaning and drying: Thoroughly clean and dry the surface of the treated workpiece in preparation for powder spraying.

2.Powder spraying

The powder coating was evenly sprayed on the pre-treated workpiece by electrostatic spray gun. The main steps are as follows:

• Powder feeding: The powder coating is delivered to the gun through the powder feeding system.

• Electrostatic spraying: The electrostatic field generated by the spray gun is used to charge the powder particles and adsorb them to the grounded workpiece.

• Leveling: Under the action of electrostatic force, the powder particles automatically adjust their position to form a uniform coating.

3.Solidification cooling

The sprayed workpiece is sent to the heating furnace for high temperature curing, so that the powder melts and levels and firmly adheres to the substrate, and then cooled to room temperature.

• Heating curing: According to the type of powder coating, set the appropriate temperature and time to melt the powder and level curing to form a continuous coating.

• Cooling: After curing, the workpiece needs to be gradually cooled to room temperature to ensure the uniform release of the internal stress of the coating and prevent cracking.

4.Inspection and repair

Check the quality of the cured coating and repair any defects.

• Appearance inspection: Check whether the coating is uniform, smooth, no leakage, flow hanging, shrinkage and other defects.

• Performance testing: Perform coating thickness, adhesion, hardness, corrosion resistance and other performance testing as required.

• Repair: The found defects are locally polished, resprayed and re-cured to meet the quality requirements.

Ⅴ Construction requirement

• In order to give full play to the characteristics of powder coating and extend the service life of the coating film, the surface of the coated material is first strictly treated

• When spraying, the coated material must be completely grounded to increase the spraying efficiency of powder coating.

• For the coated material with large surface defects, the conductive scraping putty should be applied to ensure the smooth and smooth feeling of the coating film

• After spraying, the object shall be heated and cured. The curing conditions shall be subject to the technical specifications of the powder product, but the curing temperature and time must be fully guaranteed. Avoid quality accidents caused by insufficient curing.

• Check immediately after spraying powder, if found defects should be treated in time, if the defects are found after curing, the range is small and only local and does not affect the surface decoration of the coated material, can be repaired after diluting with the same color powder and acetone, if the range is large and affects the surface quality, then use sandpaper, spray again or remove the coating with paint remover, and then spray powder again.

• The recovered powder must be screened to remove debris and mixed with the new powder in a certain proportion.

• Powder supply tank, powder room and recovery system should avoid contamination of other different colors of powder, so it must be blown clean every time the color change.

Ⅵ Advantage

1. Environmental protection: no solvent emission, VOC(volatile organic compounds) content is very low, in line with environmental requirements.

2. High efficiency and energy saving: One spray can achieve the required thickness, low energy consumption during curing.

3. Excellent coating performance: wear resistance, corrosion resistance, weather resistance, beautiful appearance, good adhesion.

4. Resource conservation: The use of powder coating saves non-renewable water resources, because compared with the wet spray painting method, it does not require circulating water as a raw material for process treatment, and also avoids secondary solid waste treatment.

5. Good economy: powder coating is 100% solid system, oversprayed powder can be recycled and reused, the utilization rate of paint up to more than 90%, and even in some cases up to more than 95%, reducing the waste of paint, reducing production costs and waste disposal costs.

6. Easy construction: The construction and operation of powder coating is convenient, there is no need to adjust the viscosity of the coating with seasonal changes, and the coating property is good for the corners of the workpiece, and it is not easy to produce defects such as flow hanging when thick coating. In addition, powder coating is easy to implement automated production line, improving production efficiency.

7. Good safety: powder coating odorless, non-toxic, does not contain heavy metals, no adverse effects on the human body, so the production and operation are relatively safe.

Ⅶ Powder coating application

Automobile: wheels, frames, brackets, etc

Home appliances: refrigerator, washing machine, dryer, etc

Building materials: pumps, pipes, metal sheets, steel furniture, etc

Ship: impeller, ship plate, ship equipment, etc

Chemical industry: chemical tank, petroleum equipment, etc

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