Ultraviolet (UV) radiation is the primary cause of aging in polymer materials. As a key polymer additive, light stabilizers can inhibit or delay UV-induced oxidative aging, extend the service life of products such as plastics and rubbers, and maintain their performance stability. Its scientific definition is: a substance that can block or slow down the chain reaction of photoaging, and its English translations are Light stabilizer and Photo stabilizer.

I. Core Functions

The core functions of light stabilizers focus on "resisting UV damage and protecting polymer materials", which can be summarized into three key points:

1. Blocking UV Damage: By absorbing, reflecting or shielding the harmful UV wavelength range of 290–400 nm, it reduces the penetration of high-energy photons into the material interior, thereby lowering the probability of photooxidation initiation from the source.

2. Terminating the Aging Chain Reaction: It traps active free radicals generated by UV excitation and quenches excited-state molecules, blocking the propagation of aging reactions and avoiding irreversible damage to the material molecular chains.

3. Maintaining Performance Stability: It prevents product discoloration, chalking, cracking, embrittlement and strength reduction, ensuring the appearance and service reliability under long-term sunlight exposure. Typical applications include extending the lifespan of agricultural films and automotive coatings.

II. Mechanisms of Action

Light stabilizers are classified into four categories based on their chemical structures and modes of action, each with distinct mechanisms. In practical applications, they are often used in combination to achieve synergistic enhancement effects.

(1) Light Screeners: Physical "Shielding" to Isolate UV Radiation

Light screeners form a protective barrier through physical reflection or absorption of UV rays, preventing UV radiation from penetrating into the material. They convert absorbed UV energy into thermal energy and dissipate it into the environment.

Common types include inorganic fillers such as carbon black, rutile titanium dioxide and zinc oxide. Among these, carbon black offers the best shielding performance, while nano-titanium dioxide balances transparency and shielding effectiveness. The protective effect depends on the uniformity of dispersion, which can be improved by using silane coupling agents.

(2) Ultraviolet Absorbers (UVA): Absorption, Conversion and Energy Dissipation

The core mechanism is "selective absorption + energy conversion". UVAs strongly absorb UV light in the 290–410 nm range while barely absorbing visible light. They convert high-energy UV radiation into thermal energy through intramolecular structural rearrangement and maintain their own structural stability.

The main categories include 2-hydroxybenzophenones, benzotriazoles and triazines. Benzotriazoles feature high temperature resistance; benzophenones offer excellent transparency; triazines boast high protective efficiency, making them the core component for surface protection of transparent products.

(3) Quenchers: Energy Transfer to Stabilize Excited States

Quenchers receive energy from excited-state molecules of the material through intermolecular energy transfer, dissipate the energy in a harmless form, and restore the molecules to a stable ground state, thereby preventing the occurrence of photooxidation reactions.

Traditional quenchers are divalent organonickel chelates, which are suitable for polyolefin films and fibers. However, nickel ions are toxic and tend to cause material discoloration. With the tightening of environmental regulations, organonickel chelates are being replaced by low-toxicity alternatives.

(4) Free Radical Scavengers: Targeted "Trapping" to Terminate Aging Chains

Photooxidative aging generates active free radicals such as alkyl and peroxy radicals, triggering chain degradation reactions. This type of stabilizer can selectively trap these free radicals and convert them into stable compounds, completely terminating the propagation of aging chains.

Hindered Amine Light Stabilizers (HALS) are the mainstream high-efficiency variety. HALS do not absorb light above 260 nm; instead, they are oxidized to form nitroxide free radicals in the presence of oxygen. These nitroxide free radicals can efficiently trap active free radicals and have a regenerative cycle capacity, providing long-term protection even at low addition levels.

HALS are suitable for weather-resistant applications such as polyolefins and automotive coatings. High-molecular-weight HALS reduce migration rate and enhance durability. Acidic components can inhibit their effectiveness; therefore, for halogen-containing polymers such as PVC, acid-resistant HALS should be selected or used in combination with acid scavengers.