Polyester and Vinylester resins and gelcoats contain a significant amount of styrene; anywhere from 25% to 50% by weight. This chemical is a monomer, and serves several purposes in the resin system.

Styrene monomer is a reactive diluent for the resin system. The diluent part relates that it works to control viscosity of the resin (more styrene means lower viscosity). The reactive part relates that it is part of the crosslinking chemical reaction. Methyl Methacrylate (MMA) is also a commonly used monomer.

During the crosslinking reaction, most of the styrene monomer is captured and becomes part of the solid form of the resin. Open molding processes allow styrene to evaporate before final cure, which is the characteristic smell of polyester and vinylester resins. Worker exposure and environmental impact are negative side effects of the open molding processes. Federal and state laws require special air quality permits for industrial styrene emissions. Worker exposure to styrene must be monitored and minimized.