The curing of UPR belongs to free radical copolymerization. The curing reaction has the characteristics of four radical reactions: chain initiation, chain extension, chain termination, and chain transfer.
Chain initiation-the process from the decomposition of peroxide initiators to form free radicals to the addition of such free radicals to unsaturated groups.
Chain growth-the process of continuous addition of monomers to newly generated free radicals. Compared with chain initiation, the activation energy required for chain growth is much lower.
Chain termination-the combination of two free radicals terminates the growing polymer chain.
Chain transfer-a growing large free radical can interact with other molecules, such as solvent molecules or inhibitors, so that the original active chain disappears and becomes a stable macromolecule, while the original inactive molecules become free radicals. The curing process of UPR is a process in which the unsaturated double bonds in the UPR molecular chain and the double bonds of the cross-linking monomer (usually styrene) undergo cross-linking polymerization to form a three-dimensional network structure from linear long-chain molecules. During this curing process, there are three possible chemical reactions, namely
1. The reaction between styrene and polyester molecules;
2. The reaction between styrene and styrene;
3. The reaction between polyester molecules and polyester molecules.
The occurrence of these three reactions has been confirmed by various experiments.
It is worth noting that when there are trans double bonds in the polyester molecular structure, the third reaction is prone to occur, that is, the reaction between the polyester molecule and the polyester molecule. This reaction can make the molecules combine More compact, which can improve the performance of the resin. The curing process of unsaturated polyester resin can be divided into three stages, namely:
1. Gelation stage (A stage): Count from the addition of curing agent and accelerator until the resin condenses into 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): Counting from the resin gel, until it becomes a stage with sufficient hardness to reach a basically non-sticky state. 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. The maturation stage (C stage): it is placed at room temperature, from the time of hardening, it reaches the required hardness of the product, and has stable physical and chemical properties 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. 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 initiators and accelerators, curing temperature, post-curing temperature, and curing time can all affect the curing degree of the polyester resin.