The addition protection effect of the combination of different light stabilizers
The main product categories of light stabilizers used for organic polymers are UV absorbers, hindered amine light stabilizers, excited state bursting agents, hydroperoxide decomposers, etc. These different types of light stabilizers have different degrees of effect on improving the light stabilization of polymer coatings. In the actual formulation of coating light stabilization, technicians often use a combination of two or more different mechanisms of light stabilizers to obtain a more effective and durable light stabilization effect on the coating, which has an additive effect of 1 + 1 > 2. A more classic combination is the use of UV Absorber with hindered amine light stabilizer (HALS). The former protects the coating from photo-oxidation at source by filtering out the harmful UV light from sunlight, but it is difficult to avoid consuming transformation during its own cyclic action, with irreversible changes such as fading and photolysis, and the effective concentration gradually decreases and slowly loses its photostabilizing effect. HALS interrupts the photo-oxidation process by removing free radicals and peroxides from the coating to maintain the gloss of the coating and inhibit yellowing and embrittlement. However, the nitrogen-oxygen radicals generated by HALS are basically colored and can absorb UV light, which has the chance of photolytic depletion. The combination of UV Absorber and HALS can play a complementary role to protect each other, as UV Absorber filters out harmful UV rays and protects nitrogen-oxygen radicals from photolytic depletion, while HALS scavenges free radicals and hydroperoxides to protect UVA from photolytic depletion. UV Absorber is protected from free radicals and peroxides. By complementing each other, UV Absorber and HALS can maintain a sufficiently high concentration during the photoaging of the coating for a long time to enhance the photostabilization effect.
The graph on the left shows the change in the concentration of nitrogen oxide radicals in the film of an acrylate copolymer coating containing styrene units during the photoaging process. In the system containing only HALS, the nitrogen-oxygen radicals rapidly decay to lower levels after experiencing higher concentrations for a short period of time; while in the combined system with benzotriazole UV Absorber, the intra-film nitrogen-oxygen radicals can be maintained at higher concentrations for a longer period of time.
UV Absorber alone, the effect of photostabilization of the coating is often not ideal, because it is based on the characteristics of light absorption to play the efficacy of the decision. When combined with HALS, which has a higher light stabilization effect, it is easier to obtain a significant improvement in the light stabilization effect.
As shown in the figure above, the effect of UV Absorber in combination with HALS on the light stability of aliphatic polyurethane coatings. In the system with only benzotriazole UVA ( Tinuvin234), the rate of urethane bond loss was only slightly lower than that of the blank sample as the photoaging time increased; when HALS alone was used, the light stabilization effect had been sharply improved; when HALS was combined with an equal amount of UV Absorber, the light stabilization effect was further improved.