Introduction to Epoxy Powder Coatings

Epoxy powder coating is a thermosetting powder coating with corrosion resistance and toughness. It was one of the earliest and fastest-developing types of coating, composed of epoxy resin, pigments, fillers, additives, and a curing agent.

1. Introduction

Powder coating is a coating containing 100% solids, applied in powder form to form a film, using air as the dispersion medium. Epoxy powder coating is a thermosetting powder coating with corrosion resistance and toughness. It was one of the earliest and fastest-developing types of coating, composed of epoxy resin, pigments, fillers, additives, and a curing agent. The preparation of epoxy powder coating uses the only internationally accepted method for producing thermosetting powder coatings: melt-mix extrusion—mixing, melt-mix extrusion, and fine grinding.

Required conditions for the epoxy resin in epoxy powder coating: The epoxy equivalent should be a solid resin between 700 and 100; the molecular weight distribution should be narrow; at its curing temperature, the melt viscosity should be low, allowing for easy leveling and producing a smooth and thin film; good dispersibility for pigments and fillers.

Epoxy powder coating curing agents mainly include dicyandiamide, dicyandiamide derivatives, acid anhydrides, imidazoles, cyclic ethers, phenolic resins, polyester resins, and boron trifluoride amine complexes. Industrially, dicyandiamide, imidazoles, and cyclic ethers are generally used.

2. Characteristics

2.1 Strong Adhesion of Epoxy Powder Coatings

Adhesion is one of the important indicators for evaluating coating performance, and epoxy powder coatings excel in this aspect. They exhibit strong adhesion to various substrates, with particularly outstanding adhesion to metal substrates. Metal substrate surfaces are typically smooth, making it difficult for some coatings to adhere stably. However, epoxy powder coatings can form strong chemical bonds with metal surfaces, and even during long-term use, coating peeling and flaking are less likely to occur. This gives them an irreplaceable advantage in the coating and protection of metal products, such as metal pipes and metal components, for which epoxy powder coatings are widely used.

2.2 Excellent Mechanical Properties of the Paint Film

Epoxy powder coatings produce paint films with excellent mechanical properties. Firstly, they have high hardness, effectively resisting scratches and impacts from external objects, reducing the possibility of scratches, dents, and other damage to the paint film surface. Secondly, they have strong scratch resistance; even under slight friction and scratches during daily use or transportation, the paint film maintains its integrity. Most importantly, they have excellent corrosion resistance, effectively isolating the substrate from air, moisture, acids, alkalis, salts, and other corrosive substances, preventing corrosion and rust, and extending the substrate's service life. This characteristic makes them widely used in humid and highly corrosive environments, such as coating metal components and chemical equipment in marine environments.

2.3 Low Melt Viscosity and Excellent Leveling Properties

During the coating process, epoxy powder coatings have a low melt viscosity. When heated to a molten state, the powder coating can flow freely on the workpiece surface. This good fluidity allows the coating to fully fill the tiny depressions and gaps on the workpiece surface, ultimately forming a smooth and even paint film, with virtually no pinholes or craters. Pinholes and craters severely affect the appearance quality and protective performance of coatings. Epoxy powder coatings, with their excellent leveling properties, effectively avoid these problems, ensuring high-quality coatings.

2.4 Rich Variety of Colors and Finishes

To meet the diverse needs of different users for coating appearance, epoxy powder coatings offer a wide variety of colors and finishes. By adjusting the types and proportions of pigments and fillers, and adding different special additives, epoxy powder coatings with various appearance effects, such as matte, glossy, textured, and hammered finishes, can be formulated. Matte epoxy powder coatings are suitable for applications with lower gloss requirements, such as internal components of some mechanical equipment; glossy epoxy powder coatings have high gloss, enhancing the aesthetics of workpieces and are suitable for products such as household appliances and furniture; textured and hammered epoxy powder coatings have unique decorative effects, adding a personalized appearance to workpieces and are widely used in fields with high decorative requirements.

2.5 Large Coating Thickness in a Single Application, Powder Recyclable

Unlike traditional solvent-based coatings that typically require multiple coats to achieve the desired thickness, epoxy powder coatings can achieve a film thickness of 50-300μm in a single application. This not only significantly improves coating efficiency and reduces coating processes and time costs, but also ensures uniform film thickness, avoiding problems such as uneven film thickness and poor interlayer bonding that can occur with multiple coats. Furthermore, the powder generated during the coating process that is not coated on the workpiece can be collected and processed using appropriate recycling equipment. After screening and impurity removal, it can be reused. This feature not only reduces paint waste and production costs but also reduces environmental pollution, aligning with the concept of green and environmentally friendly development.

2.6 High Requirements for Coating Equipment and Large Investment

Despite the many advantages of epoxy powder coatings, there are high requirements for coating equipment. Because epoxy powder coatings are applied in powder form, specialized coating equipment is required to achieve processes such as uniform powder spraying and electrostatic adsorption. This equipment is typically complex in structure and requires high precision. For example, electrostatic spraying equipment needs precise control of parameters such as spraying voltage and powder quantity to ensure coating results. Furthermore, a complete epoxy powder coating production line also includes pretreatment equipment, curing equipment, and recycling equipment, resulting in a significant overall investment. This presents a barrier to entry for small businesses or companies with limited funds.

3. Main Types of Epoxy Powder Coatings

The main types of epoxy powder coatings include glossy epoxy powder coatings, matte epoxy powder coatings, non-glossy epoxy powder coatings, semi-gloss or matte epoxy powder coatings, epoxy electrophoretic powder coatings, and functional epoxy powder coatings such as antibacterial, conductive, flame-retardant, and artistic types.

4. Epoxy Powder Coating Application Considerations

4.1 Diverse Coating Methods, Electrostatic Spraying is the Main Method

Epoxy powder coatings have strong adaptability and can be applied using various methods, including electrostatic spraying, fluidized bed dip coating, electrostatic current bed dip coating, and flame spraying. Among these, electrostatic spraying is currently the most widely used method. This method involves electrostatically charging the epoxy powder coating and using electrostatic adsorption to evenly adhere the powder coating to the grounded workpiece surface. It offers advantages such as high coating efficiency, uniform coating, high automation, and suitability for mass production. Fluidized bed dip coating involves immersing the workpiece in a fluidized powder coating, allowing the powder coating to adhere to the workpiece surface. It is suitable for coating small, simple-shaped workpieces. Electrostatic current bed dip coating combines the characteristics of electrostatic spraying and fluidized bed dip coating, further improving coating effect and efficiency. Flame spraying utilizes the high temperature of a flame to melt the powder coating and spray it onto the workpiece surface to form a coating film. It is suitable for coating some special materials or large workpieces, but its application is relatively limited.

4.2 Precise Control of Electrostatic Coating Parameters

When using electrostatic spraying, precise control of relevant parameters is necessary to ensure coating effectiveness. Firstly, the coating voltage should generally be controlled between 40-80kV. Excessive voltage may cause the powder coating to break down into air, generating sparks and posing a safety hazard. It may also cause defects such as pinholes in the coating film. Insufficient voltage will result in insufficient electrostatic adsorption of the powder coating, leading to uneven film thickness and reduced adhesion. Secondly, the spraying distance should typically be maintained between 150-300mm. Too close a distance will cause excessively high local charge density on the workpiece surface, resulting in an overly thick coating or orange peel-like appearance. Too far a distance will cause charge loss during powder coating flight, reducing adsorption efficiency and wasting powder. Finally, the powder output should be kept balanced and stable, generally controlled between 70-200g/min. Excessive powder output can lead to an overly thick coating and sagging; insufficient powder output will result in an overly thin coating, failing to achieve the expected protective effect and reducing coating efficiency.

4.3 Controlling the Number of Spray Coats

The number of electrostatic spray coatings for epoxy powder coatings should not be excessive. Generally, one spray coat is sufficient to achieve the required film thickness. Multiple spray coats not only increase construction time and cost but may also lead to poor interlayer adhesion, resulting in coating peeling, cracking, and other problems. Only in special circumstances, such as when the workpiece surface has significant defects requiring repair, or when extremely high film thickness is required, should a small number of touch-up sprays be considered. Before touch-up spraying, the workpiece surface must be properly treated to ensure interlayer adhesion.

4.4 Strictly Controlling Baking and Curing Conditions

Baking and curing is a crucial step in the formation of the final epoxy powder coating film. It is necessary to strictly adhere to the specified temperature and time, while ensuring uniform baking temperature. Different types of epoxy powder coatings have different curing temperatures and times, which generally need to be set according to the product instructions. Excessively high baking temperatures or excessively long baking times may lead to problems such as discoloration, aging, and increased brittleness of the coating film; excessively low temperatures or excessively short times will result in incomplete curing of the coating, failing to meet requirements for film hardness, adhesion, corrosion resistance, and other properties. During the baking process, it is also necessary to ensure uniform temperature throughout the oven to avoid localized overheating or underheating. This can be achieved by installing multiple temperature sensors inside the oven for monitoring and adjusting the heating device in a timely manner to ensure that all parts of the workpiece are heated evenly and fully cured.

4.5 Emphasizing Safety and Environmental Protection

Dust pollution and dust explosions are key safety and environmental concerns during epoxy powder coating application. Epoxy powder coatings exist in powder form, and dust is easily generated during handling, mixing, and spraying. If this dust is inhaled by operators, it can damage the respiratory system and pollute the surrounding environment. Therefore, the construction site needs to be equipped with good ventilation and dust removal equipment to control the dust concentration in the air within a safe range, and operators must wear appropriate protective equipment, such as dust masks and safety glasses. Furthermore, epoxy powder is a flammable dust. When the dust reaches a certain concentration in the air, it can easily explode upon contact with a source of ignition. Therefore, open flames must be strictly prohibited at construction sites to avoid generating static sparks, electrical sparks, or other ignition sources. Appropriate fire-fighting equipment, such as dry powder fire extinguishers and fire sand, must be provided, and a comprehensive safety emergency plan must be developed to ensure construction safety.

5. Application Scope of Epoxy Powder Coatings

Epoxy powder coatings are not suitable for substrates with poor heat resistance because their baking and curing temperature is high, reaching 140℃. They can be used for decorative protection of electrical switch cabinets, electronic instruments, and metal silicon boxes; electrical insulation protection for motor rotors or copper busbars; and corrosion and rust prevention for kitchen utensils, automotive parts, ships, building materials, and underground facilities.