Overview of UV absorbers
What are UV absorbers? Definition of UV absorbers
These additives preferentially absorb incident UV radiation, thereby protecting the polymer from radiation. UV absorbers themselves do not degrade rapidly, but they convert UV energy into harmless heat that is dissipated throughout the polymer matrix. The effectiveness of UV absorbers is limited due to physical limitations of the absorption process, and their absorptive capacity depends on the need for high concentrations of additives and polymer thickness before sufficient absorption occurs to effectively delay photodegradation.
However, high concentrations of additives would be uneconomical and technically limiting, and many applications (such as polyolefins) are in very thin parts such as films and fibers. Benzophenone is a good general UV absorber for transparent polyolefin systems and can also be used in colored compounds. Benzotriazole is mainly used in polystyrene. Both are also available in polyester. The concentration is usually about 0.25-1.0%
Synonyms for UV absorber
UV absorber
UV stabilizer
Classification of UV absorbers
The main function of UV absorbers is to absorb UV rays in the presence of chromophores (Ch) in the polymer Radiation, the purpose is to filter out UV rays that are harmful to the polymer before Ch* is formed. Most importantly, UV absorbers must work in the 290 and 350nm range. The purpose of UV absorbers is to absorb harmful UV rays and quickly convert them into harmless heat. In this process, the absorbed energy is converted into vibrational and rotational energy of the molecular components. For a UV absorber to be effective, this process must occur faster than the corresponding reaction within the substrate, and neither the UV absorber nor the polymer it is intended to stabilize is damaged during the energy conversion process. The most important UV absorbers are:
a) 2-(2-Hydroxyphenyl)-benzotriazole
b) 2-Hydroxyacetophenone
c) Hydroxyphenyl- s-triazine
d) Oxalimide
These UV absorbers Each of the groups can be characterized by typical absorption and transmission spectra.
How do UV absorbers work? The mechanism of UV absorbers
The main function of UV absorbers is to absorb UV radiation in the presence of chromophores (Ch) in the polymer, with the purpose of filtering the chromophore free radicals Ch* before they are formed Remove UV rays that are harmful to polymers. Most importantly, UV absorbers must work in the 290 and 350nm range. The effectiveness of UV absorbers depends not only on their absorption properties but, above all, on the Lambert-Beer law.
Extinction is wavelength dependent and can be considered a measure of the stabilizing or shielding effectiveness of a UV absorber. In other words, the higher the extinction, the higher the UV shielding and the greater the stabilizing effect – always assuming that the UV absorber itself is not destroyed by the absorption of light. Therefore, extinction depends on the extinction coefficient, the concentration “c” of the UV absorber in the polymer, and the film thickness “d” of the unpigmented polymer.
For a UV absorber to be effective, it must absorb UV rays better and faster than the polymer, which means stabilizing and dissipating the absorbed energy before triggering undesirable side reactions. This means that the conversion of energy absorbed into the form of UV light must occur in the singlet state. Intersystem crossover (transition S1 to T1), so phosphorescence must be excluded.
The synergistic effect of light stabilizers and UV absorbers
UV absorbers cannot absorb coatings Exposure to all UV radiation. Some UV radiation will penetrate the coating surface. Therefore, HALS is incorporated into the polymer. These molecules work by scavenging any free radicals that form – unlike UV absorbers, which first prevent their formation by scavenging free radicals from the system and then regenerating themselves. For this reason, most formulators will use a combination of absorbents and HALS.
The synergistic combination of UV absorbers and HALS is optimal for polymer stabilization. UV absorbers are governed by the Beer-Lambert law, so the absorbance is linearly related to the concentration of UVA (320 to 400 nm (for photocuring)), its molar absorptivity (extinction coefficient) and path length (coating thickness). HALS It is a free radical scavenger that is not bound by Beer’s Law and is effective anywhere in the coating system. HALS is particularly effective on coatings
