Sustainable Hybrid Flame-Retardant Architectures for LDPE: Effects of Waste-Derived Cellulose, SiO₂, and APP/ADA

This study evaluates the flame retardant efficiency of hybrid systems based on natural cellulose extracted from agricultural wastes (pecan nutshell and guayule), combined with azodicarbonamide (ADA), porous silicon dioxide (SiO₂) scaffolds, and ammonium polyphosphate (APP), incorporated into low-density polyethylene (LDPE). The waste-derived cellulose fractions were isolated via alkaline extraction and steam explosion, yielding microfibrils that preserved native lignin fractions. Porous SiO₂ particles were modified through the adsorption of ADA and APP, which in turn decorated the surface of the cellulose particles. Hybrid LDPE composites containing 20 wt% of total additives were melt-compounded and characterized. Thermogravimetric analysis (TGA) revealed an early intumescent activation that promoted robust charring, achieving final char residue yields up to 9.5 wt% for the pecan nutshell composite. Horizontal UL-94 and cone calorimeter tests demonstrated the complete elimination of hazardous flaming melt-dripping across all formulated hybrid systems, accompanied by a significant reduction in the burning rate to 18.3 mm/min and a 45% reduction in PHRR for the pecan nutshell-derived compound, consistent with a protective contribution of the multifunctional particles in the condensed phase, significantly outperforming its commercial nanocellulose counterparts. Cone calorimetry highlighted a drastic flattening of the peak heat release rate (PHRR) kinetics of composites. FTIR, TEM and TGA analyses confirmed the formation of a APP-ADA-loaded SiO₂ particle scaffold ("grenade-like" morphology), with combustion-residue FTIR and SEM/EDS supporting a protective contribution in the condensed phase.