Upcycling of magnesium from saline lakes into functionalized Si–MgO for synergistic flame-retardant PVC

Abstract

Polyvinyl chloride (PVC) is inherently flammable and produces copious amounts of smoke upon combustion, while conventional flame retardants often compromise its mechanical properties. To address this challenge, a halogen-free synergistic flame-retardant system featuring low loading, high efficiency, and balanced flame retardancy and mechanical performance was developed. Specifically, γ-aminopropyltriethoxysilane was employed to surface-modify MgO, yielding silane-modified MgO (Si–MgO). Meanwhile, expandable graphite (EG) was acidified with a mixed acid to obtain acidified EG (A-EG). PVC composites were then fabricated by blending Si–MgO and A-EG in various ratios. At a total additive content of 5 wt% and an Si–MgO-to-A-EG mass ratio of 2 : 3, the composite exhibited an optimal overall performance: a limiting oxygen index (LOI) of 28.67% and a UL-94 V-1 rating. Cone calorimeter tests (CCTs) revealed that the peak heat release rate (HRR), total heat release (THR), and smoke production rate (SPR) were reduced by 30.99%, 31.68%, and 71.96%, respectively, compared to neat PVC. Smoke density measurements showed a 91.8% reduction in the specific optical density (Ds), while mechanical properties were largely preserved. The flame-retardant mechanism involves the expansion of A-EG upon heating to form a porous carbon skeleton, followed by Si–MgO catalyzing the crosslinking of PVC into amorphous carbon. Their synergy constructs a dense, barrier-type composite carbon layer that effectively blocks mass and heat transfer and traps smoke particles, thereby achieving simultaneous flame retardancy and smoke suppression. Through this strategy of interfacial modification and component compounding, a high-performance Si–MgO/A-EG synergistic flame-retardant system for PVC is successfully established.