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What are the characteristics of electrostatic spray thermosetting powder?

What are the characteristics of electrostatic spray thermosetting powder?

The successful application of powder coatings depends on the equipment parameters as well as the nature, composition and state of the powder.

There are many different types of powders to choose from, and the final choice depends on the end use of the coated substrate.

For thermoset systems, the molecular weight of the polymer is generally low, melting and flowing during melting, while forming a thermoset (cured or crosslinked) state by chemical conversion. Once this state is reached, the polymer does not melt again like a thermoplastic system to become plastic.

In the thermosetting powder, a crosslinking reaction occurs between the resin and the functional (reactive) group of the curing agent. Obviously, in order to obtain a smooth coating film, the polymer must first flow to form a continuous, uniform coating film, and then solidify.

Powder coatings must strike a balance between softening temperature, melt viscosity or melting characteristics, and crosslinking speed. As with solvent-based coatings, thermosetting powder coatings can also be formulated to achieve high-gloss and low-gloss decorative coatings, as well as aluminum, bronze and other metal, texture and hammer effects.

Powder coating composition
The basic components required for thermosetting powder production are:

Polymer (resin)
Hardener, catalyst, crosslinker or curing agent
Flow control additive
Factors affecting component selection are:

Film properties such as gloss, color, hardness, bending ability, adhesion and chemical resistance.
Painting process.
Curing time and temperature.
The powder coating uses a solid resin. The choice of resin type is extremely important because it determines the melting point, fluidity, leveling and film properties of the powder. The softening point of the resin is usually between 70 and 110 °C. Resins with lower melting points are prone to "caking" during storage and are extremely fluid when cured so that only poor "sharp coverage" is obtained. On the contrary, the resin having a higher softening point tends to have an orange peel effect due to insufficient fluidity at the time of curing, but the sharp edge coverage is good. In addition, during production, when the processing temperature is high, the problem that the hardener reacts with the polymer may occur.

Hardener, catalyst, crosslinker or curing agent
Production methods, coating processes, curing methods, and coating characteristics all affect the choice of curing agent for a particular polymer type. The hardener should not react at room temperature, remain late at 100 ° C, and react adequately between 100-180 ° C. The reaction rate does not hinder the sufficient flow of the molten resin, but for commercial reasons, it is necessary to avoid the curing time being too long.

Colorants and extenders
Conventional solvent-based surface coatings are currently used in powder coatings if they are chemically inert, light-resistant, and heat-resistant.

White, soft, and light-colored paints use titanium dioxide almost without exception. Black and grey paints are made from carbon black. Colorful paints use organic and inorganic pigments; in order to reduce the amount of heavy metals such as lead, organic pigments are currently used more. However, some organic red materials tend to react during processing, losing brightness and cleanliness, so care must be taken to ensure stability. Metal effect coatings use aluminum powder and bronze powder.

Inorganic extenders that do not affect gloss, flow and mechanical properties of the film can be added to the formulation. These extenders generally have a relatively high specific gravity, and although they reduce the cost of raw materials, they can reduce the coverage area of the powder coating. The economics of these extenders can be evaluated by calculating the unit coating area per unit weight of powder. Other types of extenders can be used if it is desired to specifically adjust the gloss and appearance of the coating.

Flow control additive
After selecting the appropriate resin/hardener/color system, it is often necessary to adjust the formulation to alter the fluidity and film properties of the powder to meet coating and curing conditions.

Under normal circumstances, the formulation needs to contain a flow control agent, otherwise the powder coating tends to have pinholes, fish eyes and craters during curing, and the orange peel effect. The flow control agent reduces the surface tension of the system, thereby reducing the tendency of the resin to shrink pores; in addition, the leveling of the coating is improved to obtain a smoother coating film. In this regard, acrylic polymers and other resin materials have been successfully applied.

In some cases, the leveling of the paint is too fast, resulting in poor "sharp edge coverage." To ensure that such workpieces are adequately covered and prevented from sagging, thixotropic agents such as "Bentones" particulate white carbon and other extenders may be used. However, the choice must be made cautiously, otherwise the film properties such as gloss and color may be affected.

Epoxy powder coating
The purpose of formulating this powder coating is to achieve a high gloss smooth coating with excellent adhesion, bending and chemical resistance. The main disadvantage is that it is easy to yellow at high temperatures or after exposure to sunlight. In addition, this coating will quickly pulverize when exposed to outdoor exposure. However, the integrity of the film under outdoor exposure is very good. The curing of epoxy powder coatings is additive curing and does not release volatiles during drying.

polyester powder coating
This powder coating has excellent outdoor weatherability and UV yellowing resistance, and has a longer drying time and higher temperature than epoxy powder. The chemical resistance of the polyester powder is slightly lower than that of the epoxy powder.

Epoxy-polyester mixed powder coating
This powder coating is a widely used thermosetting powder in which an epoxy resin acts as a polyester resin curing agent. The characteristics of the mixed powder coating vary between pure epoxy powder and pure polyester powder, depending on the resin selected and the relative proportions. The presence of an epoxy resin causes such a mixed resin to pulverize when exposed to the outside.

Polyurethane powder coating
Polyurethane powder coatings are based on hydroxyl polyesters and are capable of reacting with masked (or blocked) isocyanates. These powders have good, comprehensive physical and chemical properties and excellent weatherability.
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