Coffee waste-derived biochar as a flame retardant for epoxy nanocomposites

Starting from spent coffee grounds, the use of coffee-derived biochar (CB) as a flame retardant (FR) additive was explored following a waste-to-wealth approach. CB was employed alone and in combination with ammonium polyphosphate (APP) and a ternary (Si-Ti-Mg) mixed oxide to enhance the thermal, fire, and mechanical performances of a bisphenol A diglycidyl ether (DGEBA)-based epoxy resin modified with (3-aminopropyl)-triethoxysilane (APTES) and cured with a cycloaliphatic amine hardener. The presence of silicon-modified epoxy chains guaranteed the uniform distribution of CB throughout the resin. The combined FR action of fillers (CB, APP, and Si-Ti-Mg oxide) and the acidic characteristics of hybrid epoxy moieties enabled the achievement of a no dripping UL 94-V-0 classification for epoxy resin containing 20 wt% CB and 1 wt% of phosphorus loading, significantly increasing the flexural modulus (by ∼15%). Although it is not self-extinguishing, compared to pristine resin, the silicon-modified epoxy nanocomposite filled only with CB exhibited a remarkable decrease in the peak of heat release rate (pHRR) (by ∼65%) and a beneficial smoke suppressant effect with a notable decrease (∼11%) in the total smoke production. Cone calorimetry tests, pyrolysis combustion flow calorimetry analysis, and microscopy measurements helped to outline the combined mode of action of CB, APP, and Si-Ti-Mg oxide in the flame retardation of the hybrid epoxy resin, highlighting a strong FR action in the condensed phase, with the formation of a stable aromatic ceramic char, as well as the smoke suppressant character due to the basic nature of the ternary metal oxide and the ability of porous biochar to adsorb the generated gases.