Unsaturated polyester resin, which accounts for 75% of the resins we use daily. Unsaturated polyester resin is a kind of thermosetting resin. When under the action of heat or initiator, it can be cured into an insoluble and insoluble high Molecular network polymer.
Unsaturated polyester resin
The apparent characteristic changes of unsaturated polyester resin during curing:
The curing process of unsaturated polyester resin can be divided into three stages, namely:
1. Gel stage (A stage): from the addition of curing agent and accelerator, until the resin condenses into a jelly and loses fluidity. In this junction, the resin can be melted and can be dissolved in some solvents (such as ethanol, acetone, etc.). This stage takes about several minutes to tens of minutes.
2. Hardening stage (stage B): starting from the resin gel, until it becomes a stage with sufficient hardness and basically no stickiness. In this stage, the resin can swell but not dissolve when it comes into contact with some solvents (such as ethanol, acetone, etc.), and it can soften when heated but cannot completely melt. This stage takes about tens of minutes to several hours.
3, maturation stage (C stage): placed at room temperature, counting from the hardening, to reach the required hardness of the product, with stable physical and chemical properties available for use. In this stage, the resin neither dissolves nor melts. The post-curing we usually refer to refers to this stage. This knot is usually a very long process. It usually takes days or weeks or even longer.
Factors affecting the degree of curing of unsaturated polyester resin:
The curing of unsaturated polyester resin is a process of linear macromolecules forming a three-dimensional network through the action of crosslinking agents, but the curing process cannot consume all the active double bonds in the resin to achieve a 100% curing degree. In other words, the degree of curing of the resin is difficult to achieve complete. The reason is that in the late stage of the curing reaction, the viscosity of the system increases sharply and the diffusion of molecules is hindered. Generally, only when the material properties become stable, it is considered that the curing is complete. The degree of curing of the resin has a great influence on the performance of FRP. The higher the degree of curing, the mechanical properties, physical and chemical properties of FRP products will be fully utilized. (Someone has done experiments to test the physical properties of the UPR resin at different stages after curing. The results show that its bending strength does not increase with time, and it will not stabilize until one year later. In fact, for The FRP products that have been put into use, one year later, due to the effects of heat, light and other aging and medium corrosion, the mechanical properties have begun to gradually decline.)
There are many factors that affect the degree of curing. The components of the resin itself, the amount of initiator and accelerator, curing temperature, post-curing temperature and curing time can all affect the curing degree of polyester resin.