Characterization and performance of surface-modified biochar-reinforced polyester composites with different glass/rockwool/sisal fiber stacking sequences

This study examines the influence of fiber stacking sequence and biochar filler content on the performance of hybrid polyester composites reinforced with sisal, glass, and rockwool fibers. Laminates with S/R/S and G/R/G stacking configurations were fabricated with biochar contents ranging from 0 to 5 vol.% and evaluated for mechanical, tribological, thermal, and flammability properties. The stacking sequence significantly affected composite behavior. The G/R/G configuration consistently outperformed S/R/S due to the higher stiffness, load-bearing capacity, and non-combustibility of glass fibers in the outer layers. Among all specimens, G3 (G/R/G with 3 vol.% biochar) exhibited the best mechanical performance, attributed to uniform biochar dispersion and improved fiber-matrix interfacial bonding, which enhanced crack resistance and load transfer. For tribological, thermal, and flammability performance, G5 (G/R/G with 5 vol. % biochar) was the optimal composition. It showed the lowest wear rate and coefficient of friction, improved thermal conductivity, and superior flame resistance, achieving a UL-94 V-0 rating without dripping or cotton ignition. Overall, the results demonstrate that both stacking sequence and biochar content play critical roles in tailoring the multifunctional performance of hybrid polyester composites.