As a composite material, unsaturated polyester resin has been well used in the fields of coatings, FRP, artificial stone, handicrafts and so on. However, the yellowing of unsaturated resin has always been a problem perplexing manufacturers. Generally, the yellowing causes of unsaturated resin include the following:

1. In the esterification synthesis process of unsaturated resin, the thermal aging yellowing caused by high temperature. Generally, the esterification temperature of unsaturated resin is 180 ~ 220 ℃ or even higher. At this temperature, the resin is easy to turn yellow due to thermal aging, affecting the appearance of resin products;

2. The main reason for the yellowing of the resin caused by UV exposure is the benzene ring in the resin (including the benzene ring introduced by aromatic dicarboxylic acid / anhydride and styrene), which may be due to the thermal oxygen degradation of aromatic compounds at high temperature, which is prone to electronic transition on π → π orbit, making the resin yellow;

3. In the process of resin production, raw materials are exposed to oxygen due to poor sealing of the device. The molecular chain of general unsaturated polyester contains not only ester group, hydroxyl group and carboxyl group, but also double bond and aromatic ring. A trace of oxygen can cause thermal oxidative degradation, and the most obvious manifestation is that the color of tree fat turns yellow;

4. The influence of additives, such as antioxidants, polymerization inhibitors, curing agents, etc. amine antioxidants are easy to change into nitrogen and oxygen free radicals to color the products. Common polymerization inhibitors, such as hydroquinone, are oxidized to quinones in the presence of trace oxygen. Quinones themselves have color, which affects the color of the resin, Some manufacturers of curing agent still use peroxide acyl tertiary amine system and peroxide ketone metal soap system. Because both tertiary amine and metal soap have color, it is easy to color the resin.

Of course, there are other reasons why the resin turns yellow. In general, hot oxygen and ultraviolet light are the main causes of yellowing. Using saturated dibasic acid (or anhydride) instead of aromatic dibasic acid (or anhydride) can lighten the color of the resin to a certain extent, but considering various factors such as resin performance and cost, this method is not ideal.

According to experts, in addition to filling inert gas in the process of production and storage to isolate the contact with oxygen as much as possible, the more effective method is to add antioxidant and ultraviolet absorber, which can effectively prevent and delay the yellowing of polyester. The anti yellowing solution of unsaturated resin recommended by experts is: choose the antioxidant without amine, and use the combination of main and auxiliary antioxidants. The main antioxidant is usually hindered phenols, which can capture peroxide free radicals; The auxiliary antioxidant is phosphite, which can chelate metal ions while decomposing hydroperoxide to prevent oxidation discoloration of the resin.

If you want to further improve the yellowing resistance and weather resistance, it is recommended to add ultraviolet absorbent. Adding ultraviolet absorbent can effectively inhibit the yellowing phenomenon of polymer materials under the action of ultraviolet light, provide excellent protection for products, effectively prevent the reduction of gloss, cracks, bubbles and delamination, and significantly improve the weather resistance of products, It has a good synergistic effect when used with antioxidants. Of course, the use of antioxidants and ultraviolet absorbers can not fundamentally solve the problem of yellowing, but within a certain range, it can effectively prevent the oxidation yellowing of unsaturated polyester products, keep the water color transparent and improve the grade of products