By improving the flexibility of the resin, the impact strength and crack resistance after curing of the resin can be improved, so that flexibility can also be referred to, and the flexural resistance of the resin directly corresponds to it.
The softness of the resin depends mainly on the following factors.
1 The molecular chain of the polyester has a long chain length, a small amount of unsaturated bonds, less cross-linking points, few branches, and high flexibility. For this reason, the ratio of the amount of the saturated acid to the unsaturated acid is increased and the branching is prevented to increase the flexibility.
2 In the saturated dibasic acid, the use of an aliphatic acid is more effective than the aromatic acid for improving the flexibility, and the flexibility and flexibility increase with the increase of the saturated acid chain length.
3 Increasing the saturated diol chain length can also achieve the effect of improving softness. However, it is easier to control the degree of softening than the growth of the dibasic acid chain, and the final properties of the resin are good.
4 When the number of polar groups (such as ester bond or aromatic ring group) in the polyester molecular chain is large, it is not conducive to flexibility, and the number of polar groups increases to increase the embrittlement point. A structure having a long methylene chain can reduce the embrittlement point. If the polyester does not contain an aromatic group and the number of ester bonds per 5 chain lengths in the molecular chain is less than one, the embrittlement point after curing can be lowered to -40 °C.