Various application methods for wet paints (Topic: 14058)

Spray painting

This was earlier the most important method for organic coating of profiles, but due to pollution problems and health reasons caused by solvent emission, powder coating has taken over as the main coating method.

In spray painting the spray gun consists essentially of a cup from which the paint is atomised and projected by means of a constant stream of cleansed, compressed air. Spray painting is carried out in spray booths. Both in order to prevent dirt or dust settling on the extrusions and to avoid the escape of volatile solvents into the shop, the booth is fitted with extraction fans. Absolute cleanliness is essential. The profiles must not be handled without gloves as a finger mark might prevent paint from adhering to the metal. Dust which might stick to the surface of the paint must also be excluded.

Electrostatic spraying

Electrostatic spraying is a special type of spray painting. One of the most serious drawbacks to spray painting is the waste of paint in over-spray. In an automatic plant this may be 40ñ50%, depending upon the shape and spacing of the profiles. A method to avoid over-spray is to pass the atomised paint particles through a high-potential electrostatic field. In the zone of this field, the particles become positively charged and are attracted to the profiles which are cathodic and are earthen on the conveyor. In this process the spraying pressure is lower and the aim is to produce a "fog" around the profiles which are then coated uniformly.

To obtain the best coverage with an electrostatic material, it is important to have the electrical resistance of the paint correct, and in order to maintain good durability, this is usually adjusted by modifying the solvent balance.

Electrophoresis

Electrophoresis or electrophoretic painting is a coating process which is carried out with the use of anodes and cathodes in the painting tank. There are two types of electrophoretic painting: Anaphoretic and cataphoretic painting. In anaphoretic painting the profiles will be the anode, and in cataphoretic painting the profiles will be the cathode. In the electrophoretic painting process evolution of oxygen gas will take place at the anode, and hydrogen gas will be evolved at the cathode. During anaphoretic painting formation of an anodic oxide coating occurs on the profile surface, and a conversion coating (chromate layer) pre-treatment is destroyed. Anodising is suitable as a pre-treatment for this type. The thickness of the anodic oxide coating is usually between 3ñ8 µm.

The profiles may be pre-treated by conventional alkaline degreasing, deoxidising, and yellow chromating (or zinc phosphating) as the main steps prior to cataphoretic painting. Anodising is not suitable as a pre-treatment prior to cataphoretic painting due to evolution of hydrogen gas on the profile surface, which may give spalling of the oxide layer.

Prior to electrophoretic painting the profiles should be thoroughly rinsed in deionised water. The anaphoretic paint may contain a water-soluble acrylic based paint, the resin being rendered soluble by the addition of suitable amines such as melamine. When current is passed through such a solution the resin and pigments will migrate to the anode whilst the amine will be discharged at the cathode. During this paint deposition the level of amine will increase, and this causes a decrease in paint deposition rate if allowed to continue. The most usual processes used for removal of excess amine are ultrafiltration often in combination with reverse osmosis.

The electropaint is continuously circulated to avoid settling of the paint solids, and heat resulting from the pumping process as well as from the passage of electric current, needs to be extracted in order to maintain the desired temperature. Dry coating thicknesses of the order of 20 µm are normally applied, using a voltage of 150ñ200 volts for a time of 1ñ2 minutes. After painting, the profiles are rinsed before stowing conventionally at temperatures of 180ñ200ƒC for 20ñ30 minutes.

Electrophoresis is very suitable for large volumes of products coated with the same colour, for instance electrodeposition of a white paint on window and door frames. The main disadvantage with electrophoretic painting is the problem regarding change of colour. There is no possibility (in practical life) for colour change. The colour is more - or less - a colour for life. The colour shade chosen has to be a frequently used colour in the market, and white shades are mostly used.