When choosing UV photoinitiators for clear coatings, we should first know the properties about clear coatings.

Clear coatings is the most commonly used coatings in many applications, it can be applied on metal, wood, glass and plastic. Because it is a transparent paint, it will not cover the original color of the object completely when applied on the surface of the object, but also show the original texture of the object. So it belongs to the main paint in modern home decoration construction.

Coating thickness of clear coating varies according to applications, for example, for coating onto fiber optics, the coating thickness is less than 25 microns, for can coatings, the thickness is less than 50 microns, and for coatings onto floor tiles, it varies from 50 to 100 microns (2 to 4 mils).

Alpha hydroxyketone photoinitiators such as Irgacure 184 have found broad acceptance in clear coatings onto a wide range of substrates. As Irgacure 184 is with non-yellowing property. It is good for surface curing. To improve through curing and boost cure speed, Irgacure 819 is suggested. So Irgacure 184 and Irgacure 819 are often used as a package in many applications.

As clear coatings are more susceptible to environment or weather than pigmented coatings. To improve weather-ability of UV cured clear coatings, hindered amine light stabilizers (HALS) and Ultraviolet Absorbers (UVAs) have proven useful. They act synergistically and do not interfere with the UV cure process when using BAPO type photoinitiators such as Irgacure 819. Gloss retention can usually be improved and cracking and yellowing minimized by the use of an appropriate HALS. And UVAs help protect the physical integrity of the coating and to screen any potentially harmful UV radiation from the sun to reach the substrate once the coating is in service.

Below is the formulation for clear coating provided by Ciba:

Clear CoatingsFilm ThicknessPhotoinitiatorConc.
Clear coatings on
plastic, metal, wood
< 1 mil to > 4 mils
(< 25 microns to > 100
Irgacure 184
Irgacure 2959
Darocur 1173
Irgacure 500*
Irgacure 819** 0.5-1%

*Together with an amine synergist.
**Particularly recommended for thicker coatings as a cure speed booster when used in combination with 1-2% of the above photoinitiators.