Polyester technologies have proliferated from thermoset unsaturated polyester, used in fiberglass composites like SMC (left), to copolyester TP elastomers like DuPont’s Hytrel. (Photos: Dieffenbacher and DuPont) The versatility of polyester chemistry has been demonstrated over the nine decades since its original introduction. Much of the plastics industry today is focused on thermoplastic materials. However, one of the most important polyester chemistries is unsaturated polyesters and vinyl esters. These materials are created using chemical reactions similar to those already discussed for PET and PBT. However, in this case, a difunctional alcohol such as propylene glycol is reacted with a combination of saturated and unsaturated organic acids or anhydrides with reactive groups that extend in more than two directions. This chemistry allows for the process of crosslinking using a material like styrene monomer or vinyl acetate to create a thermosetting material. These chemistries were discovered by Wallace Carothers and his team at DuPont in the early 1930s at the same time that they were investigating the early polyester fiber chemistry. The original polymer developed by Carothers used fumaric acid, maleic anhydride, and ethylene glycol. The reaction required a relatively high temperature and resulted in a material of very high viscosity that was difficult to work with and cured very slowly. Like the aliphatic polyesters that DuPont developed at about this same time, these crosslinked versions were not commercialized. But in 1933, Carlton Ellis, an inventor and pioneer in organic chemistry, introduced the vinyl monomer styrene into the process as a crosslinking agent. This resulted in a lower-viscosity resin to which glass fibers could be added prior to curing. Cure rates also increased dramatically with this innovation. Resource: https://www.ptonline.com/articles/tracing-the-history-of-polymeric-materials-part-16