April 2014 Archives

Electrodeposition coatings can be can be broadly classified into cationic and anionic types. As shown in [Fig.1], the subject item (workpiece) to be painted for the cationic type is made into a cathode, and the counter electrode is made into the anode, and vise versa for the anionic type. When the paint subject is fully submerged and a DC current is applied from a rectifier, ionic paint particles precipitate by electrophoresis. The precipitated paint particles lose ionic nature and become insoluble forming nonconductive film resistance. Therefore, thin film coating is possible even on paint subjects with complex shapes. The Cathodic Electrodeposition Painting does not cause the base metals of the paint subjects to discolor due to oxidation since the subjects become cathodic. But with the Anionic Electrodeposition Painting, such reaction occurs, and therefore, it not used to paint copper, brass, or silver plated objects. For the electrodeposited film to grow thicker, the film needs to be conductive. But when the current flows, resin particles on the paint subject surface precipitate immediately, and form a nonconductive film and fuse due to joule heating. The insulating film should not grow thicker any further, but it actually does. It is supposed that gases from electrolysis keeping the surface porous, preventing formation of nonconductive surface film.

The porous coating film would melt while curing and becomes a continuous film, acquiring excellent anti-rusting effects. Comparisons between the anionic and the cathodic electrodeposition paintings in the [Table 1].

[Table 1] Comparisons between the anionic and the cathodic electrodeposition paintings

The explanations on the paint types have so far been rather extensive, but we will discuss the methods of the paint coating applications from this point on.

The paint coating application methods currently in use are summarized on [Table 1]. There are liquid-form flowable paints and powder paints in power form. The powder paints are applied by electrostatic powder coating or fluidized bed dip coating methods, but there are many coating methods for the liquid-form flowable paints.

Firstly, there is a difference in whether coating applied directly or indirectly by atomizing the paints. The former method employs paint brushes and the like and has been in use since the old times. Instead of the brushes, some methods employ directly dipping the workpieces in liquid paints or apply electrical power in water soluble paint liquids, as well as pouring paints over the workpieces. These methods will minimize the wasted paint.

In contrast with the above, atomized paint coating methods provide increased work efficiency, but cause more wasted paints and increased energy consumption. Some representative methods are explained below.

[Table 1] Classification of paint coating methods

(15) Powder coatings

Powder coatings are synthetic resin compositions in fine powder form with pigments, fillers, curing agents, and plasticizers added to thermosetting or thermoplastic resins.

For the thermoplastic coating resins, there are Vinyl chloride, Polyolefin (polyethylene), Polyamide (nylon), Cellulose ester (CAB), Fluorine resin. The coating formation of these paints is due to the thermal fusion of the powder particles.

For the thermoplastic coating resins, there are Vinyl chloride, Polyolefin (polyethylene), Polyamide (nylon), Cellulose ester (CAB), Fluorine resin. The coating formation of these paints is due to the thermal fusion of the powder particles.

Epoxy, acrylic, polyester resins are used for the thermosetting powder coatings. Of those, the epoxy resin powder coating has been produced in large quantities. These resins are low in molecular weights until baked, but when thermally fused, cross-linking reaction is initiated by the curing agent in the paint, and is polymerized. Characteristics of the powder coatings are as follow.

[1] Non-solvent nature alleviates the concerns for air pollutions. Powder form makes possible for recovery and reuse. [2] Thick coating is obtained by a single coating. [3] The application processes can be automated and the coating thickness can be managed to improve product lives.

(16) High solid paints

They are essentially the same as the conventional solvent type paints but with reduced amounts of solvents and increased solids without compromising the film performance and workability, and are collectively called high solid paints. Typically, they are of 75% or more solids during painting and 70% or more solids when painted.
The coating film main elements are the same type as the conventional paints, but the viscosity of the resin itself is lowered and use co-reactive resin or low viscosity curing agent.
Some of the paints are introduced.

(1) Acrylic or polyester high solid baked paints

Single liquid type paint with low viscosity resins used for automobiles, electrical equipment, and metal furniture.

(2) Epoxy high solid paints

Modified epoxy resin paints are paints with epoxy resins partially modified with non-reactive plasticizers such as coal tar pitch, phenolic resin and others. A representative type is the tar epoxy resin paint that is lower in organic solvent content, lower in cost, is excellent in anti-corrosion properties compared to the pure epoxy resin paints.

(3) Urethane high solid paints

This is a high solid paint with low viscosity polyol and polyisocyanate compounds and uses aliphatic isocyanate curing agents. Uses are the sames as the epoxy high solid paints such as for marine vessels and chemical plant facilities.

(4) Inorganic paints

Paints with alkyl silicate or alkali silicate are used as binders. The zinc-rich paint in combination with zinc powder is well known. It is used for chemical and corrosion resistance.