The difference between Epoxy and Polyester
Epoxies and polyesters are both families of thermosetting resins. The fundamental
difference between these two resins is the type of reactive chemical group that is involved in the crosslinking reaction. For epoxies, the reactive group is a small 3-membered circle of two carbon atoms and an oxygen atom – called the epoxy circle.
For polyesters, the reactive group is a double bond between two carbon atoms. We can appreciate the differences in crosslinking reactions and conditions without getting into it on a scientific level. Epoxies do not use organic peroxides. Curing epoxy is accomplished by adding a curing agent (hardener), but the conditions for curing can vary widely depending on the natures of resin and the curing agent, as well as the requirements of the manufacturing operation and the properties of the final product
 
Polyesters are crosslinked by adding a small amount of organic peroxide (catalyst) to a solvent system of the resin. Then, either with applying heat or at room temperature the resin will cure. When compared with the polyester crosslinking process and the properties of polyester products, the epoxy system and product properties are far more versatile. This greater flexibility is both good and bad. Good because the manufacturer can select curing conditions and properties that are just right for the product and manufacturing application. Bad because the system can become so complex that the level of skill needs to be very high in order to choose the proper components of the resin mixture, the type of curing agent, and the environment conditions.  The production of epoxy resins requires the optimization of cost, performance, and processing aspects. Typically ingredients are selected for their chemical and physical attributes and then various formulations are tried in small test mixes until the desired properties are achieved.  

But it is not just what is put together that matters, how the ingredients are assembled and used affects the consistency of properties, the processing characteristics, safety in scale up, and full-scale production practice.

To summarise polyester resins are tried and tested to be suitable for mass production but the flexibility and superior technical qualities of epoxy does attract the more demanding projects. At the end of the day it comes down to cost and how extensively a product needs to be engineered.

We hope you have found this case study helpful in determining which products best suite your design needs.