The curing process of unsaturated polyester resin can be divided into three stages, namely:
1. Gel stage (stage A): the stage from the addition of the curing agent and accelerator until the resin coagulates into a jelly and loses its fluidity. In this junction, the resin can melt and be soluble in certain solvents (such as ethanol, acetone, etc.). This stage takes about a few minutes to tens of minutes.
2. Hardening stage (stage B): It starts from the resin gel until it has enough hardness to reach the state of not sticking hands. In this stage, the resin can swell but not dissolve when contacted with certain solvents (such as ethanol, acetone, etc.), and can soften but not completely melt when heated. This stage takes about tens of minutes to several hours.
3. Aging stage (stage C): It is placed at room temperature, after hardening, it reaches the required hardness of the product, and has stable physical and chemical properties available for use. At this stage, the resin neither dissolves nor melts. What we usually refer to as post-curing refers to this stage. This knot is usually a very long process. It usually takes days, weeks or even longer