UV Absorbers

I. Mechanism of UV Absorbers

1. Benzophenone Series

    

   Benzophenone-based UV absorbers are the most widely used category among all UV absorbers. They exhibit slow absorption of UV-A, UV-B, and UV-C rays. The carbonyl and hydroxyl groups in their molecules form intramolecular hydrogen bonds, constructing a chelate ring. After absorbing UV light energy, the molecules undergo thermal vibration, which breaks the intramolecular hydrogen bonds and opens the chelate ring, converting UV energy into heat for dissipation. In addition, the carbonyl groups in the molecules are excited by the absorbed UV energy, resulting in tautomerism and forming an enol structure, which also consumes part of the energy.
    
   For this series of UV absorbers, the strength of intramolecular hydrogen bonds is correlated with their light stabilization effectiveness. Stronger hydrogen bonds require more energy to break, enabling the absorption and dissipation of more UV energy and thus delivering better performance, and vice versa. The stabilization effect is also related to the length of the alkoxy chain on the benzene ring. Longer chains offer better compatibility with polymers, leading to optimal stabilization results.
    
   Benzophenone-based UV absorbers must contain a hydroxyl group at the ortho-position of the carbonyl group; otherwise, intramolecular hydrogen bonds cannot form, and the compound cannot function as a UV absorber. UV absorbers with a single ortho-hydroxyl group can absorb UV rays in the wavelength range of 290–380 nm, while absorbing almost no visible light and causing no discoloration. They also have good compatibility with polymer materials.
    
   If there are two hydroxyl groups at the ortho-position of the carbonyl group, the absorber can absorb UV rays between 300–400 nm as well as part of visible light. The absorption of visible light disrupts the balance of complementary colors, causing yellowing of the products incorporated with such absorbers. Moreover, their compatibility with polymers is poor, which limits their application scope. Although benzophenones without ortho-hydroxyl groups also have UV absorption capacity, they decompose under light exposure and are therefore unsuitable for use as UV absorbers.
    
2. Salicylate Series

    

   Salicylate-based UV absorbers are the earliest applied category, and their molecules also contain intramolecular hydrogen bonds. Initially, these absorbers have low UV absorption capacity and a very narrow absorption range (below 340 nm). However, after exposure to UV light for a certain period, their absorption capacity gradually increases to the maximum level. This is because they undergo molecular rearrangement under UV irradiation, forming a benzophenone structure with strong UV absorption capacity, thereby enhancing their UV absorption effect. For this reason, they are called precursor-type UV absorbers.
    
   The dihydroxybenzophenone and its derivatives formed after molecular rearrangement can absorb part of visible light, resulting in yellowing of the materials added with these absorbers.
    
3. Benzotriazole Series

    

   The mechanism of action of benzotriazole-based UV absorbers is similar to that of benzophenone-based ones. They have a wide UV absorption range, capable of absorbing light with wavelengths of 300–400 nm, while absorbing almost no visible light above 400 nm. Therefore, the products treated with these absorbers do not experience discoloration.
    
4. Other Types of UV Absorbers

    

   In addition, substituted acrylonitrile and triazine-based UV absorbers are believed to convert light energy into harmless energy for dissipation through cis-trans isomerization. Substituted acrylonitrile UV absorbers can absorb UV light in the range of 290–320 nm without absorbing visible light, thus avoiding discoloration of the treated substrates. Triazine-based UV absorbers can absorb UV light with wavelengths of 300–400 nm.
    
   UV absorbers can absorb auxochromic groups (such as -NH, -OH, -SO₃H, -COOH, etc.) and chromophoric groups (such as C=N, N=N, N=O, C=O, etc.) that absorb light below 400 nm; these groups are all attached to aromatic nuclei.
    
   Organonickel compounds can also serve as UV absorbers, but they are generally classified as quenching agents (also known as deactivators, light stabilizers, excited-state quenchers, or energy quenchers). Their UV absorption capacity is lower than that of the aforementioned absorbers, but they can prevent the generation of free radicals in polymers caused by UV absorption. During UV irradiation, organonickel complexes interact with the excited-state molecules of polymers, returning the excited molecules to the ground state and converting UV energy into low-energy radiation for dissipation, thereby preventing damage to polymer materials.
    
   They are called quenching agents because their mechanism of action differs from that of conventional UV absorbers: UV absorbers dissipate energy through structural changes of their own molecules, while quenching agents achieve this through intermolecular energy transfer.
    
   Furthermore, light shielding agents such as carbon black, titanium dioxide, zinc oxide, and lithopone are also substances that can absorb or reflect UV rays. They act as a barrier between polymers and light sources, absorbing or reflecting UV rays before they reach the polymer surface, thus preventing UV rays from penetrating into the polymer interior. Among these, carbon black offers the highest efficiency.
    
   Free radical scavengers are another category of substances with light stabilization effects. They are piperidine derivatives with steric hindrance effects, known as hindered amine light stabilizers (HALS).
    

II. Classification of UV Absorbers

1. Benzophenone Series

    

   All benzophenone-based UV absorbers are derivatives of ortho-hydroxybenzophenone, including monohydroxy, dihydroxy, trihydroxy, and tetrahydroxy derivatives. This category is widely used in the post-finishing of polymers such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), acrylonitrile-butadiene-styrene (ABS), polystyrene (PS), polyamide (PA), and textile materials.
    
   They exhibit good compatibility with most polymers and excellent stability to light and heat, without decomposing at 200°C. However, they have high sublimation properties and are suitable for use in paints and plastics, with a recommended dosage of 0.1–0.5%.
    
2. Salicylate Series

    

   Salicylate-based UV absorbers have lower UV absorption rates and narrower absorption bands (absorbing light below 340 nm) compared to benzophenone-based ones. Additionally, they are not very stable to UV light themselves and can absorb visible light, causing yellowing of the treated substrates.
    
   Nevertheless, they are inexpensive and have good compatibility with polymers, making them suitable for use in cellulose, polyester (PET), PVC, PE, polyvinylidene chloride (PVDC), PS, and other polymer materials.
    
3. Benzotriazole Series

    

   Benzotriazole-based UV absorbers outperform benzophenone-based ones in performance. They can strongly absorb UV light in the range of 310–385 nm while absorbing almost no visible light. They also have good stability but are relatively expensive.
    
   • UV-P: Absorbs UV light with wavelengths of 270–380 nm and almost no visible light, resulting in minimal initial discoloration. It is mainly used in products such as PVC, PS, unsaturated polyester (UPR), polycarbonate (PC), polymethyl methacrylate (PMMA), PE, and ABS, and is particularly suitable for colorless, transparent, and light-colored products. However, it is not resistant to soap washing, as it is soluble in alkaline soap solutions, which can cause yellowing of fibers.
   • UV-326: Effectively absorbs UV light in the range of 270–380 nm. It is mainly used in polyolefins, PVC, UPR, PA, epoxy resin (EP), ABS, polyurethane (PU), and other products. It offers excel