Investigation of ammonium polyphosphate dilution with ground eggshells and lignin through the study of natural fibre composite flammability

The interactions of Calcium carbonate (e.g., eggshell powder) and Lignin with ammonium polyphosphate (APP) when used as fire retardants were investigated. Three mixing ratios - 1:3, 1:1, and 3:1, were used with natural fibre reinforced composites containing a hemp mat and an epoxy matrix manufactured using a light resin transfer moulding (L-RTM) process. The thermal decomposition of the retardant mixtures and composites was investigated using thermogravimetric analysis (TGA). The findings showed that even though the decomposition reactions of APP with eggshell powder and lignin mixtures interacted and overlapped, the same interactions could not be seen in the composites. In the composite form while the residue was affected by the retardant, the decomposition reactions were driven primarily by the hemp and epoxy. Flammability of the composites was studied by testing to 20, 35, 50, and 75 kW/m² with a cone calorimeter, and determining the critical heat flux. While the samples with eggshell powder had higher ignition times, the critical heat flux for ignition was 13 kW/m² for all sample groups except for a ratio of 1:3 APP to eggshell powder, which was 14 kW/m². The lowest burning rates (mass loss and heat release) occurred in composites containing only APP, however, the addition of eggshell powder or lignin at even a ratio of 3:1 APP to either provided a notable reduction.