Unsaturated polyester resins are widely used in coatings, artificial stone and other fields, but their color yellowing is often a problem for manufacturers. The main causes of yellowing of unsaturated resins usually include:
Yellowing by heat aging: In the process of esterification synthesis, unsaturated resins are prone to yellowing due to heat aging due to the reaction at a higher temperature (usually 180 220), which affects the appearance of the product.
UV-induced yellowing: The benzene ring structure in the resin (e.g., the benzene ring brought by aromatic anhydride and styrene) is prone to yellowing through electronic transitions under UV exposure.
Oxidative yellowing during production: Due to the poor sealing of production equipment, raw materials may come into contact with oxygen, causing thermal oxidative degradation promoted by trace amounts of oxygen, resulting in yellowing of the resin.
Additive effects: Some additives such as antioxidants, polymerization inhibitors, and curing agents themselves may cause discoloration of resins when they are oxidatively decomposed. For example, amine antioxidants are easily converted into nitrogen oxygen radicals, hydroquinone polymerization inhibitors may oxidize to colored quinones in the presence of oxygen, and tertiary amine or metal soap curing agents may also affect resins due to their own color.
In order to solve the yellowing problem of unsaturated resins, the following measures can be taken:
Improved raw material selection: Although the use of saturated diacids (or anhydrides) instead of aromatic diacids (or anhydrides) can slightly improve resin color, it is not necessarily an ideal solution as there is a trade-off between resin performance and cost.
Isolate oxygen contact: In the process of production and storage, try to isolate oxygen with inert gas filling to reduce the occurrence of thermal oxidative yellowing.
Add stabilizers: use amine-free hindered phenolic antioxidants and compound with phosphite auxiliary antioxidants. Hindered phenolic antioxidants trap peroxide radicals, while phosphite co-antioxidants break down hydroperoxides and chelate metal ions, thereby preventing oxidative discoloration of resins. For example, traditional triphenyl phosphite can be replaced by pentaerythritol diphosphite.
Use UV absorbers: The addition of UV absorbers can effectively delay the yellowing of resins under UV rays and provide additional protection against gloss loss, cracks, bubbles and delamination, thereby significantly improving product weatherability. The combination of UV absorbers and antioxidants also has a synergistic effect.
Through the above measures, the oxidative yellowing of unsaturated polyester resin products can be prevented to a certain extent, and the transparency of the product can be maintained and the product grade can be improved. It should be pointed out that although these measures can effectively delay yellowing, they may not be able to completely solve the problem, and the actual application conditions of the product still need to be comprehensively considered.