Classification, role, and function of plastic antioxidants and light stabilizers

Air and sunlight are essential for human survival and plant growth on Earth, but they have detrimental effects during the storage, processing, and use of polymer plastic materials. Oxygen in the air and ultraviolet rays in sunlight lead to thermo-oxidative or photo-oxidative reactions in plastic materials, causing the appearance and physical mechanical properties of plastic products to deteriorate, resulting in the premature loss of their original functions and value.

Plastic materials, due to differences in molecular structure, or differences in polymerization processes, processing techniques, usage environments, and conditions even with the same molecular structure, exhibit significant variations in the rate of thermo-oxidative and photo-oxidative reactions and their ability to resist these reactions.

Antioxidants and light stabilizers are additives incorporated into plastic materials to effectively inhibit or reduce the rate of thermal oxidation and photo-oxidation reactions of plastic macromolecules. They significantly enhance the heat resistance and light resistance of plastic materials, delay the degradation and aging process of plastics, extend the service life of plastic products, and increase the value of plastic products as plastic additives.

We first introduce antioxidants, as they are the most widely used additives in plastics. Their extensive application refers to their use in various stages of plastics such as polymer synthesis, granulation, storage, processing, and usage. Antioxidants are applied to the widest variety of plastic materials, such as polyethylene, polypropylene, styrene polymers, engineering plastics, and modified plastics.

Commonly used plastic antioxidants are generally divided into five categories based on molecular structure and mechanism of action: hindered phenols, phosphite esters, sulfur-containing types, composite types, and hindered amines.

Hindered phenol antioxidants are the primary antioxidants in plastic materials.The main function is to react with oxidative free radicals R· and ROO· generated by oxidation in plastic materials, interrupting the growth of active chains. The primary structure of hindered phenolic antioxidants can be clearly seen from the diagram below. In the structural formula, X is a tertiary butyl group, and R can be hydrogen, methyl, tertiary butyl, or high carbon chain alkyl or aralkyl like nonyl, octadecyl, etc. The key to the antioxidant action of hindered phenols lies in the activity of the hydroxyl group it contains. The reactivity of the hydroxyl group with free radicals is influenced by the steric hindrance of the adjacent alkyl groups X and R. The smaller the molecular weight of the alkyl, the less the steric hindrance, the greater the reactivity, the faster the reaction speed, and the higher the thermal-oxidative stability efficiency. Therefore, the steric hindrance of substituents adjacent to the hydroxyl group is one of the important factors affecting the stability performance of the antioxidant.

Hindered phenolic antioxidants are classified into monophenols, biphenols, polyphenols, and nitrogen-containing heterocyclic polyphenols based on their molecular structure. Monophenol and biphenol antioxidants, such as BHT and 2246 products, have a relatively low molecular weight, resulting in higher volatility and migration. This can lead to discoloration of plastic products, and therefore, their consumption in plastics has significantly decreased in recent years.

Polyphenolic antioxidants 1010 and 1076 are the leading products among plastic antioxidants both domestically and internationally. Antioxidant 1010, in particular, is considered the best product among plastic antioxidants due to its high molecular weight, good compatibility with plastic materials, excellent antioxidant effects, and largest consumption.

The production and consumption of domestic 1010 and 1076 account for about 40% of the total domestic production and consumption of antioxidants.

Traditional hindered phenol antioxidants are derivatives based on 2,6-di-tert-butylphenol, with X and R both being tert-butyl groups: -C(CH3)3, known as fully hindered phenol or symmetrical hindered phenol antioxidants. Symmetrical hindered phenol antioxidants, such as 1010 and 1076, have two bulky tert-butyl groups at the ortho position of the hydroxyl group in the molecule. Due to the large steric hindrance, they have low reactivity and slow reaction rates, which can easily cause plastic products to become discolored.

The asymmetric hindered phenol structure, with a tert-butyl and a methyl group at the ortho position of the phenolic hydroxyl group, specifically with 2-methyl-6-tert-butylphenol as the backbone, not only possesses the characteristics of general hindered phenol antioxidants but also exhibits more prominent synergistic stabilization with auxiliary antioxidants and better color retention compared to traditional symmetric hindered phenols. It is suitable for polymer applications that are prone to thermal oxidative degradation and require high heat resistance and color stability. Examples include the antioxidant Irganox 245 developed by BASF (formerly Ciba SC), the antioxidant Cyanox 1790 developed by Cytec in the United States, and BASF's (formerly Ciba SC) brand Irganox 170.

Dialkyl phosphite antioxidants and sulfur-containing antioxidants

Phosphite antioxidants and sulfur-containing antioxidants are both auxiliary antioxidants.The primary mechanism of action of auxiliary antioxidants is to decompose highly active hydroperoxides in plastics into low-activity molecules through changes in the valence of phosphorus or sulfur atoms in their own molecules.

The domestic production and consumption of phosphite antioxidants account for approximately 40% of the total production and consumption of antioxidants in the country. The commonly used product grades of phosphite antioxidants are 168, 626, etc.

Domestic sulfur-containing antioxidants can be categorized into three types based on molecular structure: thioester antioxidants, thiodiphenol antioxidants, and thioether-type phenolic antioxidants.Common types of sulfur-containing antioxidants are thioester DLTP and DSTP.

Different types of primary and secondary antioxidants, or antioxidants with different molecular structures of the same type, exhibit differences in functionality and application effects, each having its strengths and weaknesses. Compound antioxidants are formulated by combining two or more different types of antioxidants or different varieties of the same type. In plastic materials, they can capitalize on each other's strengths and compensate for weaknesses, showing a synergistic effect to achieve optimal thermal-oxidative aging resistance with minimal addition and lowest cost. Synergistic effect refers to the phenomenon where the combined use of two or more additives results in an application effect greater than the sum of the effects of each additive used alone, i.e., 1+1>2.

The synergistic effect of combining antioxidants, combining light stabilizers, and combining antioxidants with light stabilizers can significantly enhance the anti-aging effects of antioxidants and light stabilizers.

The main functions of light stabilizers are: shielding light, absorbing and transferring light energy, quenching or capturing free radicals.Light stabilizers are generally classified into four types based on their mechanisms of action: light screeners, ultraviolet absorbers, quenchers, and hindered amine light stabilizers.

Hindered amine light stabilizers (HALS) are a class of organic amine compounds with steric hindrance effects. They are the most widely used light stabilizers domestically and internationally due to their ability to decompose hydroperoxides, quench singlet oxygen, capture free radicals, and regenerate active groups. The main product grades of hindered amine light stabilizers in China are 944, 770, and 622, accounting for about 65% of the total domestic consumption of light stabilizers.

Most types of Hindered Amine Light Stabilizers (HALS) are based on the 2,2,6,6-tetramethyl-4-piperidinyl group as the core structure, which is clearly illustrated in the diagram below.

R is a methyl group, R1 represents various groups, and R2 can be H, O·, CH3, OR, etc.

The stabilization mechanism of hindered amine light stabilizers on polymer materials is as follows:

The secondary amino group of tetramethylpiperidine is oxidized by peroxides such as hydroperoxides generated during the photo-oxidative and thermal oxidative aging of polymer materials, converting it into a nitroxide radical NO·. This oxidation reaction destroys some active substances that can initiate the degradation process of polymer materials, converting them into relatively stable hydroxyl compounds.

② The nitrogen-oxygen free radicals NO· generated by oxidation capture the destructive active groups produced by polymer materials, such as radicals R·, RO·, ROO·, etc.; they also convert them into relatively stable compounds, such as R-R, R-O-R, R-OO-R, etc.

In this process, the nitrogen-oxygen radical NO· is regenerated and continues to react with other radicals in the material, thus cycling repeatedly and significantly slowing down the photo-oxidation and thermal oxidation aging of the plastic material.

Hindered amine light stabilizers also have the function of quenching singlet oxygen, converting it from an excited state to a ground state, and intervening in the photo-oxidation reaction before the initiation of photo-aging chains. Therefore, hindered amine light stabilizers possess four self-synergistic abilities: decomposing hydroperoxides, quenching excited state oxygen, capturing free radicals, and self-regeneration. They are not only highly effective light stabilizers but also highly effective antioxidants.

We can clearly see from the table below the thermo-oxidative stability of 1mm thick PP injection molded sheets with aggregate hindered amine light stabilizers 622 and 944.

When hindered amine light stabilizers are used in conjunction with antioxidants, the thermo-oxidative stability effect is better. It is clear from the table below that when hindered amine light stabilizer 622, hindered amine light stabilizer 944 are used in conjunction with antioxidants 1010, 1076, and 168, the number of days to reach 50% elongation at break increases. Without any additives, it takes only 21 days, while adding only 0.03% of 1076 increases it to 225 days. Finally, by adding 0.01% of 1076, 0.04% of 168, and 0.05% of HALS 622, it can reach 635 days.

Ultraviolet absorbers, generally referred to as UV absorbers, are a type of light stabilizer that utilizes their molecular structure to convert light energy into thermal energy, thereby preventing photo-oxidative reactions in plastic materials and achieving light stabilization. UV absorbers can be categorized based on their molecular structure into benzophenone types and benzotriazole types, among others. In China, the main product grades of benzophenone-type light stabilizers and benzotriazole-type light stabilizers are 531, 326, and 329, with their consumption accounting for about 20% and 10% of the total domestic consumption of light stabilizers, respectively.

Quenchers and UV absorbers achieve photostability by transferring light energy. UV absorbers transfer energy when their own molecules directly absorb light energy, while quenchers interact with high-energy, highly chemically reactive excited state functional groups produced in plastic materials due to exposure to light, transferring the energy of the excited state functional groups. Because the mechanisms of energy transfer are completely different for quenchers and UV absorbers, quenchers are classified as one of the four major series of light stabilizers. The industrial products of quenchers are divalent nickel complexes, which contain the heavy metal nickel in their molecules. Considering environmental protection and human health, developed countries and regions such as Europe, North America, and Japan have stopped or restricted the use of quenchers.

The light shielding agents include carbon black, titanium dioxide, and zinc oxide. The industrial application of nanotechnology will significantly enhance the light resistance and weather resistance of light shielding agents in plastic materials.

Before and after the 1960s, three types of light stabilizers—domestic light shielding agents, ultraviolet absorbers, and quenchers—were industrially applied. Although hindered amine light stabilizers only began industrial production in the mid-1970s, the speed of increase in their product variety and output far exceeded that of the other three types of light stabilizers, making them a rising star in the family of plastic light stabilizers.

The product varieties, quality, and application effects of domestic plastic antioxidants and light stabilizers can basically meet the needs of the domestic petrochemical and plastics industries. Major varieties of antioxidants and light stabilizers are exported every year.

Today's Summary

Oxygen in the air and ultraviolet rays in sunlight cause thermal oxidation or photo-oxidation reactions in plastic materials, leading to deterioration in the appearance and physical-mechanical properties of plastic products, causing them to lose their original function and value prematurely.

Common plastic antioxidants are generally divided into five categories based on molecular structure and mechanism of action: hindered phenols, phosphites, sulfur-containing antioxidants, composite antioxidants, and hindered amines.

The main functions of light stabilizers are to screen light, absorb and transfer light energy, and quench or capture free radicals. Light stabilizers are generally classified into four categories based on their mechanisms of action: light screening agents, ultraviolet absorbers, quenchers, and hindered amine light stabilizers.

The synergistic effect of combining antioxidants, combining light stabilizers, and combining antioxidants with light stabilizers can appropriately enhance the anti-aging properties of antioxidants and light stabilizers.