Thermo-Mechanical Characterization of SMC/BMC Glass Fibers–Reinforced Polyester Composites: Toward a Novel Matrix With Zero Styrene Content

Glass fiber-reinforced polymer composites produced via sheet molding compound (SMC) and bulk molding compound (BMC) processes heavily rely on unsaturated polyester resins (UPRs) containing styrene, a volatile organic compound associated with environmental and health risks. In response to increasing regulatory pressure to reduce styrene emissions, this study investigates the formulation and performance of low-styrene (10%wt) and styrene-free UPR for SMC/BMC applications. Pure maleic-based and isophthalic/maleic-based polyester matrices were synthesized using 1,4-butanediol dimethacrylate as an alternative reactive diluent. These resins were reinforced with short glass fibers to produce composite laminates via a combined SMC or BMC and hot pressing production process, which were evaluated for thermo-mechanical properties and interfacial morphology. The isophthalic/maleic-based formulation with 10?wt% styrene demonstrated superior mechanical performance (i.e., flexural strength: 119.83?±?5.52?MPa) and comparable thermal stability (Tg of 155°C) to conventional 30?wt% styrene systems, owing to improved fiber-matrix adhesion. Conversely, pure maleic-based systems exhibited higher flexural modulus but inferior mechanical strength due to poor interfacial bonding. Morphological and dynamic mechanical analyses confirmed the role of matrix structure and styrene content in governing performance. These findings demonstrate the feasibility of reducing styrene content in UPRs when optimized backbone chemical structure are used while maintaining industrially relevant processing and mechanical performance, thus supporting the transition toward sustainable composite manufacturing.