What Are Light Stabilizers and What Types Exist?

Any substance that is added to shield or inhibit the photo-redox or photo-ageing process is called a light stabilizer. Articles such as coatings, plastics, rubber and synthetic fibres, when exposed to sunlight or to strong fluorescent light (which contains more harmful ultraviolet than ordinary fluorescent lamps), absorb ultraviolet radiation and trigger self-oxidation, which causes polymer degradation and a deterioration of both the appearance and the physical and mechanical properties of the article; this process is called photo-redox or photo-ageing. Ultraviolet radiation can also pass through the surface layer of human skin, damage skin cells, gradually harden the dermis and make it lose its elasticity, and accelerate ageing and the appearance of wrinkles.

Definition of a light stabilizer. Light stabilizer, light stability agent, photo stabilizer.

When the electromagnetic waves of solar radiation 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 electromagnetic radiation between 290 nm and 3000 nm, of which the band from 400 nm to 800 nm (about 40%) is visible light, the band from about 800 nm to 3000 nm (about 55%) is infrared radiation, and the band from about 290 nm to 400 nm (only about 5%) is ultraviolet radiation.

Mechanism of action of light stabilizers. The mechanism of action of a light stabilizer differs according to its own structure and to its type.

Some stabilizers shield or reflect ultraviolet radiation, or absorb it and convert it into harmless heat energy; some quench the excited state of the molecules or groups that have been excited by ultraviolet radiation, returning them to the ground state and so removing or slowing down the possibility of a photo-redox reaction; and some capture the free radicals generated by photo-redox, thereby blocking the free-radical reactions that cause the article to age and protecting the article from ultraviolet damage.

In industry, effective prevention and retardation of photo-ageing is mostly achieved by compounding two or more anti-ageing agents that work by different mechanisms, because every anti-ageing agent — and UV absorbers in particular — has its own ultraviolet absorption band. A compounded formulation such as benzophenone + benzotriazole + hindered amine (HAL) can deliver an optimum effect that no single light stabilizer can reach.

The ultraviolet absorbers added to sunscreen cosmetics work on the same principle: they disperse or absorb the ultraviolet radiation incident on the surface of the skin, so that the skin is protected from, or is exposed to less, ultraviolet damage.

Classification of light stabilizers

First, classified by mechanism of action, they can be divided into:

1) light screening agents;

2) ultraviolet absorbers (UV absorber);

3) quenchers (quencher);

4) free-radical scavengers.

1) Light screening agents: they act like a barrier placed between the polymer and the light radiation, so that light cannot radiate directly into the interior of the polymer and the interior of the polymer is not exposed to ultraviolet damage, which effectively inhibits photo-oxidative degradation.

2) Ultraviolet absorbers: their mechanism of action lies in strongly absorbing the ultraviolet light to which the polymer is sensitive and releasing the energy in the form of harmless heat.

3) Quenchers: they can accept the energy absorbed by the chromophores in the plastic and dissipate that energy in the form of heat, fluorescence or phosphorescence, returning it to the base material and thereby protecting the polymer from ultraviolet damage.

Second, classified by chemical structure, they can be divided into:

1) salicylates;

2) benzophenones (benzophenone);

3) benzotriazoles (benzotriazol);

4) triazines;

5) substituted acrylonitriles;

6) oxanilides;

7) organic nickel compounds;

8) hindered amines (HAL), and so on.

Selection of light stabilizers. The following factors should be considered when selecting a light stabilizer:

1. It should effectively absorb ultraviolet radiation in the 290–400 nm wavelength range, or be able to quench the energy of excited-state molecules, or have the ability to capture free radicals;

2. it should have good light stability and thermal stability of its own;

3. it should have good compatibility and must not bleed out during use;

4. it should resist hydrolysis and resist extraction by water and other solvents;

5. it should have low volatility and cause little contamination;

6. it should be non-toxic or of low toxicity, inexpensive 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) ultraviolet absorbers: the benzophenones, salicylates and benzotriazoles are the most widely used in industry;

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

(4) free-radical scavengers: mainly hindered amine derivatives.

Among these, light screening agents reflect or absorb the ultraviolet radiation of sunlight — that is, they set a barrier between the polymer and the light source and prevent ultraviolet radiation from penetrating deep into the polymer, so that the polymer is protected. Ultraviolet absorbers are a class of organic compounds that selectively and strongly absorb the ultraviolet radiation of sunlight that is 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 the photosensitive chromophores in a polymer and dissipate it in a harmless form, so that the polymer is spared a photodegradation reaction. Free-radical scavengers, finally, work by capturing and clearing free radicals, thereby cutting off the autoxidation chain reaction.

Preparation of light stabilizers.

Benzophenone-type light stabilizer for information-recording materials: this light stabilizer can inhibit the ultraviolet photo-oxidation fading of dyes and of their laser materials and information-recording materials, and adding it improves the weatherability and the resistance to ultraviolet photo-oxidation fading of dyes, laser materials and information-recording materials.

The composition of this light stabilizer consists of a series of derivatives formed by introducing a benzophenone functional group with an ultraviolet-absorbing function together with nickel sulfonate with an ultraviolet-quenching function, as well as a series of compounds, and mixtures of them, formed by combining the benzophenone molecular functional group with phenols and their derivatives that act as anti-photo-oxidation stabilizers. Weathering test results show that this light stabilizer has a significant effect on improving the resistance of dyes, laser materials and information-recording materials to ultraviolet photo-oxidation fading.

P3395 — preparation method of a polyester for light stabilizers: the corresponding straight-chain hydroxypiperidinol and a straight-chain dibasic acid ester are used as monomer raw materials and undergo polycondensation in the presence of an organic titanate catalyst. The polycondensation is divided into two stages, atmospheric-pressure polycondensation and reduced-pressure polycondensation; the atmospheric-pressure stage takes 1–5 hours, the reduced-pressure stage takes 3–7 hours, the vacuum is 0.094–0.1 MPa, and the reaction temperature is 140–200℃ in both stages. An antioxidant, tris(2,4-di-tert-butylphenyl) phosphite, is added to the polycondensation system at 0.1–2.0 wt% of the total raw materials; in structural formula I, R is ethylene or propylene, R1 is a C3–C5 straight-chain carbon chain, R2 is methyl or ethyl, and n is a positive integer from 20 to 50.

Light stabilizer for dyes and their information-recording materials: this belongs to the technical field of preventing ultraviolet photo-oxidation fading of dyes and of their laser materials and information-recording materials. Its background technology comes from information technology and laser technology. Adding this light stabilizer improves the weatherability and the resistance to ultraviolet photo-oxidation fading of dyes, laser materials and information-recording materials.

The composition of this light stabilizer consists of benzotriazoles and their derivatives with the chemical formulae 2-(N'-N*'-hydroxyphenyl) N-N* hydroxybenzotriazole and 1-R,N-N*R* benzotriazole. Examples are 2-(2',4'-dihydroxyphenyl)-5,6-dihydroxybenzotriazole and 1,6-dihydroxybenzotriazole. Weathering test results show that this light stabilizer has an obvious effect on improving the resistance of dyes, laser materials and information-recording materials to ultraviolet photo-oxidation fading.

Preparation method of 2,2,6,6-tetramethyl-4-piperidamine compounds as light stabilizer intermediates: more precisely, a method for preparing intermediates for hindered amine light stabilizers 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, essentially completed through three reactions: pre-mixing, dehydration and hydrogenation. By-products are further reduced, the product yield and purity are high, post-treatment is further simplified, and the product has very good storage stability. After six months of storage the colour of the product still meets the standard.

Preparation method of a polymeric light stabilizer with a self-synergistic effect: a method for preparing a polymeric light stabilizer with a self-synergistic effect that carries both hindered amine (HA) and ortho-hydroxybenzophenone (HBP) groups on the side chain. It is prepared by free-radical polymerization of ethylenically unsaturated monomers that respectively contain the 2,2,6,6-tetramethylpiperidine (hindered amine — HA) and the 2-hydroxybenzophenone (HBP) light-stabilizing group. Light-stabilization tests on the polymer show that the polymeric light stabilizer prepared by this method has an obvious synergistic effect. The self-synergistic polymeric light stabilizer thus prepared is an entirely new type of light-stabilizing additive.

Definition of a light stabilizer. Light stabilizer, light stability agent, photo stabilizer. When the electromagnetic waves of solar radiation 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 electromagnetic radiation between 290 nm and 3000 nm, of which the band from 400 nm to 800 nm (about 40%) is visible light, the band from about 800 nm to 3000 nm (about 55%) is infrared radiation, and the band from about 290 nm to 400 nm (only about 5%) is ultraviolet radiation.

Mechanism of action of light stabilizers. The mechanism of action of a light stabilizer differs according to its own structure and to its type. Some stabilizers shield or reflect ultraviolet radiation, or absorb it and convert it into harmless heat energy; some quench the excited state of the molecules or groups that have been excited by ultraviolet radiation, returning them to the ground state and so removing or slowing down the possibility of a photo-redox reaction; and some capture the free radicals generated by photo-redox, thereby blocking the free-radical reactions that cause the article to age and protecting the article from ultraviolet damage. In industry, effective prevention and retardation of photo-ageing is mostly achieved by compounding two or more anti-ageing agents that work by different mechanisms, because every anti-ageing agent, and UV absorbers in particular, has its own ultraviolet absorption band. A compounded formulation such as benzophenone + benzotriazole + hindered amine (HAL) can deliver an optimum effect that no single light stabilizer can reach. The ultraviolet absorbers added to sunscreen cosmetics work on the same principle: they disperse or absorb the ultraviolet radiation incident on the surface of the skin, so that the skin is protected from, or is exposed to less, ultraviolet damage.

Classification of light stabilizers. First, classified by mechanism of action, they can be divided into: 1) light screening agents; 2) ultraviolet absorbers (UV absorber); 3) quenchers (quencher); 4) free-radical scavengers. 1) Light screening agents act like a barrier placed between the polymer and the light radiation, so that light cannot radiate directly into the interior of the polymer and the interior of the polymer is not exposed to ultraviolet damage, which effectively inhibits photo-oxidative degradation. 2) Ultraviolet absorbers strongly absorb the ultraviolet light to which the polymer is sensitive and release the energy in the form of harmless heat. 3) Quenchers can accept the energy absorbed by the chromophores in the plastic and dissipate that energy in the form of heat, fluorescence or phosphorescence, returning it to the base material and thereby protecting the polymer from ultraviolet damage. Second, classified by chemical structure, they can be divided into: 1) salicylates; 2) benzophenones (benzophenone); 3) benzotriazoles (benzotriazol); 4) triazines; 5) substituted acrylonitriles; 6) oxanilides; 7) organic nickel compounds; 8) hindered amines (HAL), and so on.

Selection of light stabilizers. The following factors should be considered when selecting a light stabilizer: 1. it should effectively absorb ultraviolet radiation in the 290–400 nm wavelength range, or be able to quench the energy of excited-state molecules, or have the ability to capture free radicals; 2. it should have good light stability and thermal stability of its own; 3. it should have good compatibility and must not bleed out during use; 4. it should resist hydrolysis and resist extraction by water and other solvents; 5. it should have low volatility and cause little contamination; 6. it should be non-toxic or of low toxicity, inexpensive 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) ultraviolet absorbers, of which the benzophenones, salicylates and benzotriazoles are the most widely used in industry; (3) quenchers, mainly metal complexes such as the organic complexes of nickel, cobalt and iron; (4) free-radical scavengers, mainly hindered amine derivatives. Among these, light screening agents reflect or absorb the ultraviolet radiation of sunlight — that is, they set a barrier between the polymer and the light source and prevent ultraviolet radiation from penetrating deep into the polymer, so that the polymer is protected. Ultraviolet absorbers are a class of organic compounds that selectively and strongly absorb the ultraviolet radiation of sunlight that is 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 the photosensitive chromophores in a polymer and dissipate it in a harmless form, so that the polymer is spared a photodegradation reaction. Free-radical scavengers, finally, work by capturing and clearing free radicals, thereby cutting off the autoxidation chain reaction.