Effect of Multiwalled Carbon Nanotubes and Ammonium Polyphosphate on the Thermal Degradation and Flame-Retardant Behavior of Carbon/Hemp Hybrid Composites

Hybrid composites are increasingly attractive due to their lightweight nature and enhanced flame-retardant properties. This study investigated flame-retardant hybrid laminates by using carbon fibers and multiwalled carbon nanotubes (MWCNT) coated hemp fibers embedded in a novolac phenolic resin. Ammonium polyphosphate (APP) was incorporated at 0 wt% (Group 1), 5 wt% (Group 2), and 10 wt% (Group 3). Hemp fibers were evenly coated with 1 wt% MWCNT. Laminates were formed with varying carbon-to-hemp fiber weight ratios (8C:0 H, 6C:1 H, 5C:2 H, 3C:3 H, 2C:4 H, and 0C:5 H). Thermal stability and flame-retardant performance were assessed using thermogravimetric analysis (TGA), limiting oxygen index (LOI), and UL-94 horizontal and vertical burning tests. TGA results revealed that Group 3 had better thermal stability, with the 8C:0 H and 6C:1 H layout showing the best performance. Notably, the 6C:1 H hybrid laminate demonstrated a T10 of around 895°C and a char residue of 89.9%, showcasing high thermal stability within the investigated carbon/hemp hybrid materials. LOI and UL-94 results confirmed that Group 3 samples significantly improved flame retardancy. Overall, 6C:1 H hybrid composites with 10 wt% APP exhibit enhanced flame-retardant performance and thermal stability.