Photoinitiators of benzophenone and benzotriazole UV absorbers
Various types of light stabilizers have different light stabilization mechanisms. The protective mechanism of UV absorbers is based on the absorption of harmful UV radiation and the release of energy in the form of heat without causing photosensitization. uva should have a high photostability in addition to its own sufficient UV absorption capacity. Otherwise it will be quickly consumed in the non-stable secondary reaction.
As shown on the left, the obvious defect of the UV absorber is the requirement that the stabilized specimen has a certain thickness in order for the UV absorber to obtain a sufficiently high absorbance for the purpose of photostabilization. Therefore, UV absorbers such as used alone in thin-layer specimens, it is difficult to obtain the desired effect of light stabilization, and often used in combination with other types of light stabilizers. Here we discuss the common types of UV absorbers and their characteristics.
First, 2-hydroxybenzophenone type UV absorbers.
As shown on the left, 2-hydroxybenzophenone derivatives are a class of UV absorbers are quite widely used in traditional plastics, coatings and other polymer light stabilization fields have more mature applications. This class of UVA is generally derived from 2,4dihydroxybenzophenone. Sometimes, the parent compound also includes 2,2′,4-trihydroxybenzophenone or 2,2′,4,4′-tetrahydroxybenzophenone incompletely etherified derivatives. Regardless of the derivative structure, the hydroxyl group adjacent to the carbonyl group needs to be retained to ensure its photostabilization efficacy. 2-hydroxybenzophenone parent itself has a maximum absorption wavelength located at 260 nm and has no color. However, the higher the degree of alkoxy substitution of the 2-hydroxybenzophenone, the higher the absorption wavelength may be, and it even behaves yellow itself. 2-hydroxybenzophenone UA’s photostabilization mechanism mainly relies on the hydrogen bonding between the 2-hydroxy group and the carbonyl oxygen atom. The action process is shown in the figure below.
When the molecule absorbs UV light and reaches the excited state, the carbonyl oxygen atom becomes more basic and takes over the hydroxyl proton with which it was hydrogen bonded, forming an enol-quinone structure. The structure is unstable and the energy is released in the form of heat, and the enol-quinone structure rearranges back to its original structure, completing a cycle of protective action. Through such a no-damage cycle, the UV damage is dissolved and the UVA molecules can be recycled again.
2-Hydroxybenzophenone UVA is likely to cause blocking in photoinitiated radical polymerization due to the phenolic hydroxyl group in the structure, which affects the design curing of the coating. In addition, 2-hydroxybenzophenone UVA structure is not properly selected or with unreasonable, may itself play the role of photosensitizer, not only can not resolve the UV hazards, but also may aggravate the polymer system photoaging behavior, so this type of UVA in the light-curing coating system should be applied with caution.
Second, benzotriazole UV absorbers.
Benzotriazole (BTZ) class UV absorber is a common type of light stabilizer on the market, with a high market share and a wide range of applications. The parent compound is 2-hydroxyphenylbenzotriazole, and the common structure is shown in the figure on the left.
The 5-position chlorine substitution on the benzene ring of 2-hydroxyphenylbenzotriazole, as well as the alkyl substitution at the 3′ and 5′ positions will redshift the absorption peak at the maximum wavelength of the absorption spectrum. The electronic structure of the basal 2-hydroxyphenylbenzotriazole is more complex and can be seen as the result of mixing of several resonance structures as follows.
As shown above, after the absorption of photons by the 2-hydroxyphenylbenzotriazole molecule, the high electron cloud density center is transferred from the phenolic oxygen atom to the nitrogen atom, and the basicity of the nitrogen atom center is enhanced to take the proton from the phenolic hydroxyl group. The photoisomerization occurs roughly as shown on the left, and the principle of action is very similar to that of 2-hydroxybenzophenone.
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