Classification, Roles, and Functions of Plastic Antioxidants and Light Stabilizers (Part II)

Classification, Roles, and Functions of Plastic Antioxidants and Light Stabilizers

Air and sunlight are essential for human survival and plant growth on Earth, but they play a detrimental role during the storage, processing, and use of polymer plastic materials. Oxygen in the air and ultraviolet rays in sunlight cause thermal oxidation or photo-oxidation reactions in plastic materials, degrading the appearance and physical mechanical properties of plastic products, leading to premature loss of their original functions and value.

Due to differences in molecular structure, or even the same molecular structure having variations in polymerization process, processing technology, and usage environment and conditions, plastic materials exhibit significant differences in their rates of thermo-oxidative and photo-oxidative reactions, as well as in their resistance to thermo-oxidation and photo-oxidation.

Antioxidants and light stabilizers are additives incorporated into plastic materials to effectively inhibit or reduce the rate of thermo-oxidative and photo-oxidative reactions of polymer chains. They significantly enhance the heat and light resistance of plastics, delay the degradation and aging processes, extend the service life of plastic products, and increase their value in use.

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

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.

Hindered phenol antioxidantsHindered phenolic antioxidants are the main antioxidants for plastic materials. Their primary function is to react with the oxidation free radicals R· and ROO· generated in plastic materials due to oxidation, interrupting the growth of active chains. The main structure of hindered phenolic antioxidants can be seen clearly from the diagram below. In the structural formula, X is a tert-butyl group, and R can be hydrogen, methyl, tert-butyl, or higher carbon chain alkyls or aralkyls such as nonyl and octadecyl. The key to the antioxidant action of hindered phenols lies in the activity of the hydroxyl group they contain. The reactivity of the hydroxyl group with free radicals is influenced by the steric hindrance of its adjacent alkyl groups X and R. The smaller the molecular weight of the alkyl group, the smaller the steric hindrance, the greater the reactivity, the faster the reaction rate, 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 antioxidants.

Hindered phenolic antioxidants are classified into monophenols, bisphenols, polyphenols, nitrogen-containing heterocyclic polyphenols, and other types based on their molecular structures. Monophenol and bisphenol antioxidants, such as BHT and 2246, have relatively low molecular weights, which results in higher volatility and mobility, leading to discoloration of plastic products. Consequently, their consumption in plastics has significantly decreased in recent years.

Phenolic antioxidants 1010 and 1076 are the leading antioxidant products for plastics both domestically and internationally. Among them, antioxidant 1010 stands out as the best plastic antioxidant due to its high molecular weight, excellent compatibility with plastic materials, outstanding antioxidant performance, and the largest consumption.

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

Traditional hindered phenolic antioxidants are derivatives based on 2,6-di-tert-butylphenol, with both X and R being tert-butyl groups: -C(CH3)3, known as fully hindered phenols or symmetrical hindered phenolic antioxidants. Symmetrical hindered phenolic antioxidants such as 1010 and 1076 have two large 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 discolor.

The asymmetric hindered phenol structure with a tert-butyl group and a methyl group at the ortho position of the phenolic hydroxyl group, that is, the asymmetric hindered phenol antioxidant with 2-methyl-6-tert-butylphenol as the backbone, not only possesses the characteristics of general hindered phenol antioxidants, but also exhibits more pronounced synergistic stabilization effects with auxiliary antioxidants and color stability 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 high 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) commercial brand Irganox 170.

2. Diphosphite Antioxidants and Sulfur-Containing Antioxidants

Phosphite antioxidants and sulfur-containing antioxidants are both secondary antioxidants. The main mechanism of secondary antioxidants is to decompose highly reactive hydroperoxides in plastics into less reactive molecules through changes in the valence state of phosphorus or sulfur atoms within their own molecules.

Phosphite antioxidants account for approximately 40% of the total production and consumption of antioxidants in China. Common product grades of phosphite antioxidants include 168, 626, etc.

Domestically produced sulfur-containing antioxidants can be classified according to their molecular structure into thioester antioxidants, thiodiphenol antioxidants, and thioether-type phenols. Commonly used sulfur-containing antioxidants include thioester types such as DLTP and DSTP.

Different types of primary and secondary antioxidants, or antioxidants with different molecular structures within the same type, have varying functions and effects, each with its own strengths and weaknesses. Compound antioxidants are formulated by combining two or more different types or varieties of antioxidants. In plastic materials, they complement each other’s strengths, demonstrating a synergistic effect, achieving optimal thermal oxidative aging resistance with minimal addition and the lowest cost. The synergistic effect refers to the phenomenon where the combined use of two or more additives results in a greater effect than the sum of the effects of each additive used individually, i.e., 1+1>2.

The synergistic effects 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, and quenching or capturing free radicals. Light stabilizers are generally classified into four categories based on their mechanisms of action: light blockers, ultraviolet absorbers, quenchers, and hindered amine light stabilizers.

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

Most hindered amine light stabilizers (HALS) are based on the 2,2,6,6-tetramethyl-4-piperidyl structure, and their representative structural formula is clearly shown in the figure below.

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

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

The secondary amine group of tetramethylpiperidine is oxidized by peroxides such as hydrogen peroxide generated from the photo-thermal oxidative aging of polymer materials, converting into the nitroxide radical NO·. This oxidation reaction destroys certain active substances that can initiate the degradation process of polymer materials, transforming them into relatively stable hydroxylated compounds.

② The nitrogen oxide free radical NO· generated by oxidation captures the destructive active groups produced by polymer materials, such as R·, RO·, ROO· and other free radicals; it also converts 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-oxidative and thermal oxidative aging of plastic materials.

In addition, hindered amine light stabilizers (HALS) also have the ability to quench singlet oxygen, enabling its transition from an excited state to the ground state, thereby intervening in the photo-oxidation reaction before the initiation of photoaging chains. Therefore, HALS possess four self-synergistic abilities: decomposing hydroperoxides, quenching excited-state oxygen, capturing free radicals, and self-regeneration. They are not only highly efficient light stabilizers, but also highly effective antioxidants.

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

The thermal-oxidative stability effect is better when hindered amine light stabilizers are used in conjunction with antioxidants. It is clear from the table below that the synergistic use of hindered amine light stabilizer 622, hindered amine light stabilizer 944, and antioxidants 1010, 1076, 168, achieves a number of days to 50% elongation at break from only 21 days without any additives, to 225 days with 0.03% 1076 alone, and finally to 635 days with the addition of 0.01% 1076, 0.04% 168, and 0.05% HALS 622.

Ultraviolet light absorber type light stabilizers are commonly referred to as UV absorbers. These light stabilizers utilize their molecular structure to convert light energy into heat energy, thereby preventing photoxidative reactions in plastic materials and achieving light stabilization. UV absorbers are divided into benzophenone types and benzotriazole types based on their molecular structure. The main product grades for benzophenone-type and benzotriazole-type light stabilizers in China are 531, 326, 329, etc. Their consumption accounts for approximately 20% and 10% of the total domestic consumption of light stabilizers, respectively.

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

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

The three types of light stabilizers in China—light screen agents, ultraviolet absorbers, and quenchers—were industrially applied around the 1960s. Although hindered amine light stabilizers (HALS) only began industrial production in the mid-1970s, the variety and output of these products have increased at a much faster rate than the other three types of light stabilizers, making them a rising star in the family of plastic light stabilizers.

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

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

(2) Common plastic antioxidants are generally classified into five categories based on their molecular structure and mechanism of action: hindered phenols, phosphite esters, sulfur-containing compounds, composites, and hindered amines.

The main functions of light stabilizers are to block 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 screeners, ultraviolet absorbers, quenchers, and hindered amine light stabilizers.

The synergistic effect of antioxidant compounding, light stabilizer compounding, and the combination of antioxidants and light stabilizers can appropriately enhance the anti-aging function of antioxidants and light stabilizers.