July 2014 Archives

The mechanisms of paints applied in various methods becoming coating layers differ based on the types of paints used. For the lacquer paints, the coatings are formed just by the solvents drying, but for the crosslinked paint types, polymerizing will advance by paint components' molecular weights increasing during drying. Usually, the paint baking is the typical drying method for the crosslinked type paints.
Currently, paint drying equipment are roughly classified into hot air convection type, radiation type, and electromagnetic wave curing type, and each method is used individually or used in conjunction.

（1）Hot air convection type

Most of the paint coat baking equipment used in Japan are the hot air furnaces in either direct flame type or indirect types that use heat exchangers. The hot air convection type has long furnace length and requires large occupied area, but has large heat capacity and ability to uniformly cure complex shaped objects and is in use for many years of trusted use and track record.

（2）Radiation type

This type cures the paint coatings by infrared radiation heat. Based on the wavelength of the radiated heat, it can be classified into Near infrared (Heater surface temperature 450℃), Medium wave infrared (700℃), and Short wave infrared (2500℃) types. [Table 1] shows the effectiveness on shapes and types of cured/dried subjects. It is considered that the near infrared is effective in preheating and shortening of temperature rise time of thick objects, medium wave infrared is effective in heat penetration into the coating and shortening of baking time, and far infrared is suitable for heating objects in varying shapes and materials of painted subjects.

[Table 1] Effectiveness of various infrared types by shapes and types

（3）Electromagnetic wave type

For the electromagnetic wave type, there is ultraviolet cure (UV) type, and electron beam (EB) type.
The UV curing is capable of "seconds-order" short time curing, and is excellent in work efficiency, energy saving, and environmental concerns.
Especially for the environmental concerns that are becoming of the importance, the advancements in solvent-free paints and UV type changeover of water based paints are desired.

The EB method tends to be costly, preventing the popularization.

At any rate, the baking as the final process of the painting is an important process that largely affects the paint coatings consuming the most energy amongst the painting processes, and environmentally impacting due to the VOC (Volatile Organic Compounds). In the future, more productive, high quality, and lower cost advancements are desired.

This is a painting method that electrically charged powder paints are air sprayed on paint subjects, used in many fields.
High DC voltage of 30~90kv is applied between the electrically grounded paint subject and the electrode of a spray gun.
When the powder paint is sprayed from the gun by air, the airborne paint particles are charged by colliding with the ionized air due to the corona discharge at the gun's electrode.
The paint particles fly toward the paint subject by the effects of the electrostatic field and the force of air, forming the film by the electrostatic force attraction.

[Fig.1] shows an example of Electrostatic Powder Spray system.

The characteristics of this method are as the following.

	1.	There aren't very many size and shape restrictions on the paint subjects.
	2.	Uniform film thickness is easily obtained.
	3.	Wide range of film thicknesses can be obtained. (50~150μm)

[Fig.2] shows an example of a equipment of this powder painting method.