SMC Glass Fiber-Reinforced Polyester Composites: Thermo-Mechanical Study Toward Styrene-Free Matrix Development

The development of Sheet Molding Compound (SMC) glass fiber-reinforced polyester composites is gaining increasing interest for applications demanding lightweight, mechanically robust, and thermally stable materials, by also representing a simple manufacturing method with freedom designing (Li et al., 2024; Görthofer et al., 2019). A critical challenge in advancing these composites lies in reducing or eliminating styrene content in unsaturated polyester resin (UPR) matrices to improve environmental safety and worker health without compromising performance (Sarigiannis et al., 2011). This study investigates the thermo-mechanical behavior of SMC composites manufactured via SMC (see Figure 1 for the fabrication process) with five different UPR matrices: Pure Maleic Styrene-free, Isophthalic Styrene-free, Pure Maleic with 10% Styrene, Isophthalic with 10% Styrene, and Orthophthalic UPR with 30% Styrene. The samples ID specifications are in Table 1. The reference composite showed a glass transition temperature (Tg) of 168.7 ± 0.5 °C, a flexural strength of 119.83 ± 5.52 MPa, a flexural modulus of 10.23 ± 0.70 GPa, and a strain at break of 1.80%. Using desirability function optimization, Sample 2A emerged as a promising alternative, offering comparable flexural strength, flexural modulus and strain at break with respect to the reference sample. Morphological analysis confirmed improved fiber/matrix interfacial bonding in Sample 2A, supporting effective stress transfer and overall mechanical performance. Thermally, Sample 2A exhibited a Tg close to that of the reference, reinforcing its viability. On the other hand, Samples 1 and 1A demonstrated inferior flexural properties, attributed to poor fiber/matrix interfacial adhesion, though both showed high Tg values. These findings suggest that even with reduced or zero styrene content, careful tuning of the polyester backbone chemistry can yield thermally superior composites. This study supports the feasibility of transitioning to low- or zero-styrene SMC formulations while maintaining or enhancing performance in targeted applications.