Renewable and non-hazardous methyl cinnamate (MC) and ethyl cinnamate (EC) are investigated as replacements of hazardous styrene in a commercial unsaturated polyester (UPE) resin that is a mixture of styrene and a UPE. The UPE is a polymerization product of propylene glycol (PG) and maleic anhydride (MA), thus being abbreviated as PG-MA. Both MC and the PG-MA are solid; however, a mixture of MC and PG-MA became a clear, homogenous, and easily pourable liquid. In this study, we demonstrate for the first time that both MC and EC serve as excellent crosslinking agents for PG-MA, enabling the formation of strong crosslinked polymer networks and the production of superior glass fiber-reinforced composites. The mechanical properties of the resulting glass fiber-reinforced MC-(PG-MA) or EC-(PG-MA) composites are comparable or even superior to those of glass fiber-reinforced styrene-(PG-MA) composites. The viscosity and pot life of MC-(PG-MA) and EC-(PG-MA) are investigated. Effects of the MC content on the mechanical properties of the resulting glass fiber-reinforced MC-UPE composites, glass transition temperatures of both the MC-UPE resins and the corresponding composites, and viscosities of the MC-UPE resins are studied. The curing mechanism of the MC-UPE resins is discussed.
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