The Effect of UV Stabilizers Under UV and Daylight Curing: Characterization by RT FTIR and Confocal Raman Spectroscopy

The organic coatings used for outdoor applications, should be protected against the harmful solar radiation to overcome the photodegradation resulting in changes in optical and mechanical properties of the coating. The photostabilization can be improved by adding UV stabilizers to the photo-curable formulation: UV absorbers and Hindered Amin Light Stabilizers (HALS)1. UV absorbers protect the coating by absorbing the UV rays by itself to prevent the light penetration through the coating and have a strong resistance to sunlight to provide a long period protection. HALS does not absorb the light at 300-400 nm, they trap the radicals which are formed via photodegradation by generating nitroxyl radicals. Radical scavengers (HALS) don’t have any crucial effect on the photopolymerization process2. In the case of UV absorbers, an important issue that should be considered is the competition between photoinitiator and UV absorber for absorbing the photons needed for photoinitiator to produce photoinitiating radicals2. The filtering effect of the UV absorbers leads to a slower rate of production of initiating radicals, decrease on the conversion. However, the appropriate selection of UV absorbers, photoinitiator type and lamp output spectrum can enable the proper UV curing of weathering resistant UV coatings. Due to the wavelength of LED systems (> 380 nm) it is expected that UV absorbers that absorbs UVA range shouldn’t consequently affect the polymerization process.

Currently, the visible light sources as LED’s are preferred to the harmful radiations of conventional UV lamp systems due to the some advantages such as lower-heat generation, longer lifetime and narrower bandwidth, but therefore less emission intensity which can cause enhancement on the oxygen inhibition in the free radical photopolymerization3.