What Are Light Stabilizers and What Types Exist?

Substances added to shield or inhibit the photo-redox or photoaging process are called light stabilizers. Products such as coatings, plastics, rubber, and synthetic fibers, under sunlight or strong fluorescent light (which contains more harmful ultraviolet rays than ordinary fluorescent lamps), absorb UV and trigger self-oxidation, causing polymer degradation and deterioration of the product's appearance and physical-mechanical properties—a process called photo-redox or photoaging. Ultraviolet rays can also penetrate the superficial layer of human skin, damage skin cells, gradually harden the dermis and lose elasticity, and accelerate aging and the appearance of wrinkles.
Definition of light stabilizer   Light stabilizer, light stability agent, photo stabilizer.
   When solar electromagnetic waves pass through space and the ozone layer, radiation below 290 nm and above 3000 nm is almost entirely filtered out; what actually reaches the ground is 290 nm — 3000 nm electromagnetic waves, of which the 400 nm — 800 nm band (about 40%) is visible light, the 800 nm — 3000 nm band (about 55%) is infrared, and the 290 nm — 400 nm band (only about 5%) is ultraviolet.
Mechanism of light stabilizers   The mechanism of light stabilizers varies with their own structure and type. Some shield, reflect, or absorb UV and convert it into harmless heat; some quench the excited state of molecules or groups excited by UV, returning them to the ground state and eliminating or slowing the possibility of photo-redox reactions; some capture free radicals generated by photo-redox, thereby preventing the free-radical reactions that cause product aging and protecting products from UV damage.
   In industry, effective prevention and retardation of photoaging is mostly achieved by combining two or more anti-aging agents with different mechanisms, because each anti-aging agent, especially UV absorbers, has its own UV absorption band. A combined formulation such as benzophenone + benzotriazole + hindered amine (HAL) can achieve an optimal effect that a single light stabilizer cannot.   
The UV absorbers added to sunscreens also work by dispersing or absorbing UV incident on the skin surface, thereby protecting the skin from or reducing UV damage. Classification of light stabilizers
I. By mechanism:
1) Light screening agents,
2) UV absorbers (UV absorber),
3) Quenchers (quencher),
4) Free-radical scavengers.   
1) Light screening agents: they act like a barrier set between the polymer and radiation, so light cannot directly reach the interior of the polymer, protecting it from internal UV damage and effectively inhibiting photo-oxidative degradation.   
2) UV absorbers: their mechanism is to strongly absorb the UV sensitive to polymers and release the energy as harmless heat.   
3) Quenchers: they can accept the energy absorbed by chromophores in plastics and dissipate it as heat, fluorescence, or phosphorescence, returning it to the base material and protecting the polymer from UV damage.   
II. By chemical structure:
1) Salicylates,
2) Benzophenones (benzophenone),
3) Benzotriazoles (benzotriazol),
4) Triazines,
5) Substituted acrylonitriles,
6) Oxanilides,
7) Organic nickel compounds,
8) Hindered amines (HAL), etc.
Selection of light stabilizers
The following factors should be considered when selecting a light stabilizer:
1. Effectively absorb UV of 290 — 400 nm, or quench excited-state molecular energy, or have free-radical capturing ability;

2. Good own light stability and thermal stability;

3. Good compatibility, no bleed during use;

4. Hydrolysis resistance, water and other solvent extraction resistance;

5. Low volatility, low pollution;

6. Non-toxic or low-toxic, cheap and readily available.   

Light stabilizers can be divided into four categories:

(1) Light screening agents, mainly carbon black, zinc oxide, titanium dioxide, and lithopone;

(2) UV absorbers: benzophenones, salicylates, and benzotriazoles are most used industrially;

(3) Quenchers: mainly metal complexes, such as organic complexes of nickel, cobalt, and iron;

(4) Free-radical scavengers: mainly hindered-amine derivatives.    Among them, light screening agents reflect or absorb solar UV, setting a barrier between the polymer and the light source to prevent UV from penetrating into the polymer interior, thereby protecting the polymer. UV absorbers are a class of organic compounds that selectively and strongly absorb solar UV harmful to polymers while themselves having high light resistance. Quenchers are a class of light stabilizers that can effectively transfer the excited-state energy of photosensitive chromophores in polymers and dissipate it in a harmless form, thereby protecting the polymer from photodegradation. Free-radical scavengers achieve protection by capturing and clearing free radicals to cut off the autoxidation chain reaction. Preparation of light stabilizers   Benzophenone-type information-recording-material light stabilizer: this light stabilizer can inhibit the UV photo-oxidation fading of dyes and their laser materials and information-recording materials, and by adding it, improve the weatherability and anti-photo-oxidation fading ability of dyes, laser materials, and information-recording materials.

This light stabilizer's composition consists of a series of derivatives formed by introducing a benzophenone functional group with UV-absorption function and nickel sulfonate with UV-quenching function, as well as compounds and mixtures formed by the reaction of benzophenone molecular functional groups with phenols and their derivatives that exert anti-photo-oxidation stabilization. Weathering test results show that this light stabilizer has a significant effect on improving the anti-UV-photo-oxidation fading of dyes, laser materials, and information-recording materials.    P3395 — Preparation method of polyester for light stabilizers: straight-chain hydroxypiperidinol and straight-chain dibasic acid ester are used as monomer raw materials for polycondensation in the presence of an organic titanate catalyst. The polycondensation is divided into two stages: atmospheric-pressure polycondensation and reduced-pressure polycondensation; atmospheric-pressure polycondensation takes 1–5 hours, reduced-pressure polycondensation takes 3–7 hours, vacuum degree 0.094–0.1 MPa, reaction temperature 140–200°C. An anti-oxidant, tris(2,4-di-tert-butylphenyl) phosphite, is added to the polycondensation system at 0.1–2.0 wt% of total raw materials; structural formula I: R is ethylene or propylene, R1 is a C3–C5 straight-chain carbon chain, R2 is methyl or ethyl, n is a positive integer 20–50.    Dye and information-recording-material light stabilizer: belongs to the technical field of preventing UV-photo-oxidation fading of dyes and their laser materials and information-recording materials. Its background comes from information technology and laser technology. By adding this light stabilizer, the weatherability and anti-photo-oxidation fading ability of dyes, laser materials, and information-recording materials are improved. The composition of this light stabilizer consists of benzotriazoles with the chemical formula 2-(N’-N*’-hydroxyphenyl) N-N* hydroxyphenylbenzotriazole and 1-R, N-N*R* benzotriazole and their derivatives. Examples include 2-(2’,4’-dihydroxyphenyl)-5,6-dihydroxybenzotriazole and 1,6-dihydroxybenzotriazole. Weathering tests show that this light stabilizer has an obvious effect on improving anti-UV-photo-oxidation fading of dyes, laser materials, and information-recording materials.    Preparation method of 2,2,6,6-tetramethyl-4-piperidamine intermediates for light stabilizers: specifically a method for preparing hindered-amine light stabilizer intermediates by reacting 2,2,6,6-tetramethyl-4-piperidone with amine compounds. A three-step method is used to prepare the 2,2,6,6-tetramethyl-4-piperidamine intermediate, mainly through pre-mixing, dehydration, and hydrogenation. This further reduces by-products, gives high yield and purity, simplifies post-treatment, and the product has good storage stability; after six months of storage, the color still meets the standard.    Preparation method of polymeric light stabilizer with self-synergistic effect: a preparation method of a polymeric light stabilizer with self-synergistic effect that has both hindered-amine (HA) and ortho-hydroxybenzophenone (HBP) groups on the side chain. It is prepared by free-radical polymerization of ethylenically unsaturated monomers respectively containing the 2,2,6,6-tetramethylpiperidine (hindered amine — HA) and 2-hydroxybenzophenone (HBP) light-stabilizing groups. Light-stabilization tests of the polymer show that the high-molecular-weight light stabilizer prepared by the invention's method has an obvious synergistic effect. The self-synergistic polymeric light stabilizer prepared is a brand-new type of light-stabilization additive.