Increasing the flexibility of the resin can improve the impact strength and crack resistance of the resin after curing, so it can also be called flexibility, and the flexural and flexible properties of the resin directly correspond to it.
The flexibility of the resin mainly depends on the following factors.
① The length of the saturated part of the polyester molecular chain is small, the amount of unsaturated bonds is small, the number of cross-linking points is few, the number of branches is very few, and the flexibility is high. For this reason, increasing the amount of saturated acid to unsaturated acid and preventing the occurrence of branched chains can improve the flexibility.
②In the saturated dibasic acid, the use of aliphatic acid is more effective than aromatic acid in improving the flexibility, and the flexibility and flexibility increase with the increase of saturated acid chain length.
③ Increase the chain length of saturated glycol can also achieve the effect of improving softness. But compared to the growth of the dibasic acid chain, it is not as easy as the latter to control the degree of softening, and the final performance of the resin is good.
④When the number of polar groups (such as ester bonds and aromatic ring groups) in the polyester molecular chain is large, it is not conducive to flexibility. The increase of polar groups increases the embrittlement point. The structure with a long methylene chain can reduce the embrittlement point. If the polyester does not contain aromatic groups, and the number of ester bonds per 5 atomic chain lengths in the molecular chain is less than 1, the embrittlement point after curing can be reduced to -40°C.