Introduction

Antioxidants (or heat stabilizers) are additives used to inhibit or delay the degradation of polymers due to oxygen or ozone in the atmosphere. They are the most widely used additives in polymer materials. Coatings will undergo thermal oxidation degradation after being baked at high temperatures or exposed to sunlight. Phenomena such as aging and yellowing will seriously affect the appearance and performance of the product. In order to prevent or reduce the occurrence of this trend, antioxidants are usually added.

Thermal oxidation degradation of polymers is mainly caused by the chain-type free radical reaction initiated by the free radicals generated by hydroperoxides when heated. Thermal oxidation degradation of polymers can be inhibited by free radical capture and hydroperoxide decomposition, as shown in the figure below. Among them, antioxidants can inhibit the above oxidation and are therefore widely used.

Types of antioxidants

Antioxidants can be divided into three categories according to their functions (i.e., their intervention in the auto-oxidation chemical process):

chain terminating antioxidants: they mainly capture or remove free radicals generated by polymer auto-oxidation;

hydroperoxide decomposing antioxidants: they mainly promote the non-radical decomposition of hydroperoxides in polymers;

metal ion passivating antioxidants: they can form stable chelates with harmful metal ions, thereby passivating the catalytic effect of metal ions on the auto-oxidation process of polymers.

Among the three types of antioxidants, chain-terminating antioxidants are called primary antioxidants, mainly hindered phenols and secondary aromatic amines; the other two types are called auxiliary antioxidants, including phosphites and dithiocarbamate metal salts. In order to obtain a stable coating that meets application requirements, a combination of multiple antioxidants is usually selected.

Application of antioxidants in coatings

1. Used in alkyd, polyester, unsaturated polyester
In the oil-containing components of alkyd, there are double bonds to varying degrees. Single double bonds, multiple double bonds, and conjugated double bonds are easily oxidized to form peroxides at high temperatures, making the color darker, while antioxidants can decompose hydroperoxides to lighten the color.

2. Used in the synthesis of PU curing agent
PU curing agent generally refers to the prepolymer of trimethylolpropane (TMP) and toluene diisocyanate (TDI). When the resin is exposed to heat and light during synthesis, the urethane decomposes into amines and olefins and breaks the chain. If the amine is aromatic, it is oxidized to become a quinone chromophore.

3. Application in thermosetting powder coatings
A mixed antioxidant of high-efficiency phosphite and phenolic antioxidants, suitable for protecting powder coatings from thermal oxidative degradation during processing, curing, overheating and other processes. Applications include polyester epoxy, blocked isocyanate TGIC, TGIC substitutes, linear epoxy compounds and thermosetting acrylic resins.

Nanjing Reborn New Materials provides different types of antioxidants for plastic, coating, rubber industries.

With the innovation and progress of the coatings industry, the importance of antioxidants for coatings will become more obvious, and the space for development will be broader. In the future, antioxidants will develop in the direction of high relative molecular mass, multifunctionality, high efficiency, newness, compositeness, responsiveness and green environmental protection. This requires practitioners to conduct in-depth research from both the mechanism and application aspects to continuously improve them, conduct in-depth research on the structural characteristics of antioxidants, and further develop new and efficient antioxidants based on this, which will have a profound impact on the processing and application of the coatings industry. Antioxidants for coatings will increasingly exert their huge potential and bring excellent economic and technological benefits.