空气中的氧气和阳光中的紫外光导致塑料材料发生热氧化和光氧化反应，使塑料制品的外观和物理力学性能变差，提前失去原有功能和使用价值。主要的原因是，光引起的自由基反应，化学键变化，链式反应，自催化等过程，从而使分子链断裂、交联等，特别是在不定型态区域，以断链等形式，老化降解，这种效果一旦启动，如不制止，将以几何倍数加速，表面最为明显，但也会逐渐涉及到内部。从而使制品，不能再使用。影响塑料光老化的因素主要有：氧气、污染物、热、紫外光、可见光、湿度和水等。因此，为了有效低抑制或降低塑料大分子的热氧化、光氧化反应速度，显著提高塑料材料的耐热、耐光性能，延缓塑料材料的降解、老化过程，延长塑料制品的使用寿命，要在其中加工抗氧剂、光稳定剂等助剂。
光稳定剂主要有受组胺光稳定剂和紫外线吸收剂两类产品。
受阻胺光稳定剂（HALS）
受阻胺类的作用机理包括三种，捕获自由基、分解过氧化物和猝灭单线态氧。它们本身不吸收任何大于260 nm的光线，也不能猝灭激发态分子，但在氧存在下，受阻胺能被氧化生成相应的氮氧自由基，这种化合物相当稳定，它们能非常有效的捕获聚合物光氧化降解产生的活性自由基，而且在稳定过程中具有再生功能，这种再生功能是其有别于其他稳定剂的特征。
作用机理
a）氧化所产生的自由基NO·捕获高分子材料所产生的具有破坏性的活性基团，例如：R·、RO·、ROO·等自由基；也使其变为相对稳定的化合物，从而中断反应过程。
b）HALS还具有猝灭单线态氧的功能，使其从激发态转变为基态，在光老化的链引发前干预光化反应的进行。
作用位置：全部范围、不依赖厚度起效，特别是表面保护效果很好。
保护作用效果：对塑料制品的光泽、粉化、颜色及力学性能有很好的保护作用，并且在＜100℃）条件下,提升长期热稳定性。
影响HALS效果的主要因素:分子量 / 迁移速率、树脂体系的相容性、捕获自由基的效率与树脂中被氧化区域的亲和力。
紫外线吸收剂
这类光稳定剂利用自身的分子结构，将光能转化为热能，避免塑料材料发生光氧化反应而起到光稳定作用。根据分子结构可以分为：二苯甲酮类和苯并三唑类有机物等。
保护作用效果
优点：一般来说，对颜色保护较好，可对颜料提供一定的保护。对于后制品的力学性能保护较好；
缺点：对树脂表面保护不足、对光泽度保护贡献有限、对薄制品的保护有限。
光稳定剂选择原则
1.相容性
塑料与光稳定剂的分子具有不同程度的极性，两者相容剂较差，通常在高温下使两者融合，因此为了使光稳定剂发挥作用，选用光稳定剂的熔点或熔程上限，不应低于塑料聚合物的加工温度。
2.迁移性
尤其是表面积和体积比数值较小的制品，氧化主要发生在制品表面，这就需要光稳定剂连续不断地从塑料制品内部迁移到制品表面而发挥作用。如果向制品表面迁移速度过快，迁移量过大，光稳定剂就会挥发，从而造成损失。因此，在选择的时候，可以选择分子量相对较大，熔点适当较高的品种。
3.稳定性
助剂在材料中应能保持稳定，在使用环境下及高温加工过程中挥发损失，不变色或不显色，不分解，不与其他助剂发生不利的化学反应等。
4.加工性
塑料制品加工中时，加入光稳定剂对树脂熔融黏度和螺杆转矩都有可能发生改变。如果其与树脂熔融范围相差较大，会使光稳定剂偏流或抱螺杆现象。因此，光稳定剂的熔点低于加工温度100℃以上时，应先将光稳定剂造成一定浓度的母粒，再与树脂混合加工制品，避免因偏流造成制品中光稳定剂分布不均及加工产量下降。
5.环境与卫生
光稳定剂应该无毒或低毒，无粉尘或低粉尘，在制品加工制造过程和使用中对人体无有害作用等。
Oxygen in the air and ultraviolet light in sunlight cause plastics to undergo thermo-oxidation and photo-oxidation reactions, which degrade the appearance and
physical-mechanical properties of plastic products, and cause them to lose their original function and value prematurely. The main reason is that light induces free
radical reactions, chemical bond changes, chain reactions, and autocatalytic processes, leading to molecular chain scission, crosslinking, and other effects,
especially in amorphous regions, resulting in aging and degradation such as chain breakage. Once this process starts, if not stopped, it accelerates exponentially.
The surface shows the effects first, but eventually the inside is affected as well. This makes the product unusable. Factors that affect plastic photoaging include:
oxygen, pollutants, heat, ultraviolet light, visible light, humidity, and water. Therefore, to effectively reduce or control the rate of thermo-oxidation and photo-oxidation
of plastics, significantly improve their heat and light resistance, delay degradation and aging, and extend the service life of plastic products, antioxidants, light stabilizers,
and other additives are added during processing.
Light stabilizers mainly include hindered amine light stabilizers (HALS) and ultraviolet absorbers.
**Hindered Amine Light Stabilizers (HALS)**
The mechanisms of HALS work through three ways: capturing free radicals, decomposing peroxides, and quenching singlet oxygen. They themselves do not absorb
any light above 260 nm and cannot directly quench excited molecules. However, in the presence of oxygen, hindered amines can be oxidized to form relatively stable
nitroxyl radicals. These compounds can effectively capture active radicals generated by polymer photo-oxidative degradation and have a regenerative function during
stabilization, which sets them apart from other stabilizers.
**Mechanism:**
1) Free radicals NO• formed from oxidation capture destructive active groups in polymers such as R•, RO•, ROO•, turning them into relatively stable compounds and
interrupting the reaction process.
2) HALS also quench singlet oxygen, converting it from the excited state to the ground state, thus intervening in photo-initiated reactions before chain-starting in
photoaging.
**Application Location:** Works throughout the material, not dependent on thickness, with particularly good surface protection.
**Protection Effect:** Provides excellent protection for the gloss, chalking, color, and mechanical properties of plastics, and improves long-term thermal stability under
conditions below 100°C.
**Factors Affecting HALS Performance:** Molecular weight/migration rate, compatibility with the resin system, efficiency in capturing free radicals, and affinity for
oxidized regions in the resin.
**Ultraviolet Absorbers**
These stabilizers use their molecular structure to convert light energy into heat, preventing photo-oxidation and thereby providing light stabilization. Based on
molecular structure, they can be categorized into types like benzophenones and benzotriazoles.
**Protection Effect:**
**Advantages:** Generally good for color protection, provides some protection for pigments, and good for mechanical property preservation of finished products.
**Disadvantages:** Provides insufficient surface protection for the resin, limited contribution to gloss protection, and limited protection for thin products.
