Effect of expandable graphite content on the physical, thermal and mechanical properties of novolac matrix composites: Halogen-free flame-retardant polymer composites

Flame-retardant properties are particularly important for materials used in high-temperature applications. This study focuses on novolac matrix composites reinforced with expandable graphite (EG) particles, produced through a hot pressing process using powders prepared by mechanical milling. The research examines the particle size of both the matrix and the reinforcing particles used in composite production. Additionally, the morphology of the powders, the microstructural properties of the composites, and the fracture surfaces after tensile testing were analyzed using scanning electron microscopy (SEM). Phase analysis of the samples was performed using X-ray diffraction (XRD). Hardness and tensile tests were conducted to evaluate the mechanical properties. The effect of EG particles on the thermal stability of the composites was assessed using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and thermal conductivity tests. Furthermore, flammability was evaluated by determining the Limit Oxygen Index (LOI) values. The experimental results identified the optimum reinforcement ratio as 20 wt% EG. TGA results showed residue values of approximately 37.39 % for pure novolac and 57.87 % for novolac matrix composites reinforced with 20 wt% EG. The highest thermal conductivity (0.72 W/mK) and LOI values (40.64 %) were achieved with 20 wt% EG reinforcement, resulting in an LOI value approximately 1.25 times greater than that of the pure novolac sample (32.45 %). Additionally, tensile strength increased by approximately 2.7 times compared to the pure novolac sample. This research highlights the potential of the novolac/EG composites for advanced high-temperature applications where enhanced flame retardancy and structural integrity are essential.