Industries are searching for new coating systems that contain less organic solvents. A variety of approaches are used to achieve this. The switch from solvent-based to water-based systems has been made in several applications. Also, liquid systems that contain less solvent (high-solids coatings) or no solvent at all (solvent-free coatings) are being used. Powder coatings is a winning technology for specific industrial applications.
A powder paint consists of solid particles. Each particle contains the key ingredients of the system.
First, a binder system that can consist of more than 1 component is present. Both resin and crosslinker are present in a thermosetting system. Secondly, solid particles, pigments and/or fillers are present in most powder paints. Finally, the system contains a low percentage of additives. These can be additives that assure the liberation of air, see Film Formation, or additives that improve flow and levelling.
Powder paint is storage stable when the glass transition temperature (Tg) of the binder system is high enough. Preferably, the Tg of the paint is above 50 °C.
Powder paint is produced in a few steps. The raw materials that are solid at room temperature are mixed on a macroscopic level in a mixer. The solid mixture of raw materials is fed into a kneader, called an extruder.
The core of an extruder is a screw that rotates in a heated shaft. The material is transported through the extruder, because the screw rotates. The shaft of the extruder is heated to a temperature of 70 – 120 °C. The binder system and the additives melt in the Melting zone of the extruder. Mixing of the molten raw materials up to molecular level takes place in the last part of the extruder. Also, wetting and dispersion of the pigments and fillers takes place in the Mixing & Dispersion zone. The homogeneous melt that comes out of the extruder is cooled down, flaked and ground into solid particles of the desired size.
Most powder paints are applied on metal objects. The solid particles are electrically charged in the spray gun. The particles adhere onto the metal because of the electric potential difference between paint and substrate.
During film formation, the solid particles transform into a strong and smooth coating. Several processes play a crucial role during film formation.
The object, covered with the solid particles, goes into an oven. The particles melt and they merge together to form a uniform and smooth layer. A critical aspect is that the air present between the solid particles must be able to escape. In a thermosetting system, a chemical reaction of resin and crosslinker, called crosslinking, takes place. A thermoplastic powder does not contain crosslinker. The binder only melts, flows and levels. The film becomes hard upon cooling.
A key challenge in powder coating technology is to develop and improve systems that can be applied on heat sensitive substrates like wood, MDF (Medium Density Fibreboard) and plastics. DSM Coating Resins, the global leading company in powder coating resin technology, succeeded in developing an LTC (Low Temperature Cure) powder coating system that can be applied on MDF.
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