Unsaturated polyester resin, as a composite material, has been well applied in the fields of coatings, glass fiber reinforced plastics, artificial stone, handicrafts, etc. However, the yellowing of unsaturated resins has always been a problem that plagues manufacturers. Generally, the causes of yellowing of unsaturated resins include the following:
1. During the esterification synthesis of unsaturated resin, due to heat aging yellowing caused by high temperature, unsaturated resin generally esterification temperature is 180 ~ 220 ℃ or higher, at this temperature the resin is easy to yellow due to heat aging , Affecting the appearance of resin products;
2. The yellowing caused by the exposure of the resin to ultraviolet rays is mainly due to the presence of benzene rings in the resin (including aromatic dibasic acids/anhydrides and styrene rings introduced by styrene). Degradation, prone to electronic transitions in the π→π orbit, making the resin appear yellow;
3. In the production process of resin, the raw materials are exposed to oxygen due to poor sealing of the device, etc. The general unsaturated polyester molecular chain contains not only ester groups, hydroxyl groups, carboxyl groups, but also double bonds and aromatic rings. The most obvious manifestation of thermal oxidative degradation is the yellowing of the resin.
4. The influence of additives such as antioxidants, polymerization inhibitors, curing agents, etc., amine antioxidants are easily converted into nitrogen oxide radicals to color the products, commonly used polymerization inhibitors, such as hydroquinone, in trace oxygen When present, it oxidizes to quinones. The quinones themselves carry color, which affects the color of the resin. Some manufacturers still use acyl peroxide-tertiary amine systems and ketone peroxide-metal soap systems due to tertiary amines and metal soaps. All have colors, easy to color the resin.
Of course, there are other causes that cause the resin to turn yellow. In general, hot oxygen and ultraviolet light are the main causes of yellowing. Saturated dibasic acid (or anhydride) is used instead of aromatic dibasic acid (or anhydride). Although the resin color can be made lighter to a certain extent, this method is not enough considering various factors such as resin performance and cost. ideal.
According to experts, in addition to filling the inert gas during the production and storage process as far as possible from the contact with oxygen, a more effective method is to add antioxidants and ultraviolet absorbers, which can effectively prevent and delay the yellowing of polyester. The anti-yellowing solution of unsaturated resin recommended by experts is: use antioxidants that do not contain amines, and use the main and auxiliary antioxidants. The main antioxidants are usually hindered phenols, which can capture free radicals. ; The auxiliary antioxidants are phosphites, which can chelate metal ions while decomposing hydroperoxides, to prevent the resin from oxidizing and discoloring.
If you want to further improve the yellowing resistance and weather resistance, it is recommended to add ultraviolet absorbers to add in. Adding ultraviolet absorbers can effectively inhibit the yellowing phenomenon of polymer materials, under the action of ultraviolet light, and provide excellent protection for the product to effectively prevent The reduction of gloss, the generation of cracks, bubbles, and delamination significantly improve the weather resistance of the product, and it has a good synergistic effect when used together with antioxidants. Of course, the use of antioxidants and ultraviolet absorbers cannot fundamentally solve the problem of yellowing, but within a certain range, it can still effectively prevent the oxidation of unsaturated polyester products from yellowing, keep the product transparent and improve the product's color. Grade.