Synergism effect of CeO2 on the flame retardant performance of intumescent flame retardant polypropylene composites and its mechanism

Abstract

The synergism of CeO2 on the flame retardancy and char-forming mechanism of efficient intumescent flame retardant polypropylene composites (PP/IFR) are investigated. It is found that CeO2 enhances the limited oxygen index (LOI) value of the PP/IFR composites dramatically and the materials pass the vertical burning test (UL-94) V-0 rating test. The catalytic effectivity (CAT-EFF) results show that when 1wt% CeO2 is introduced into PP/IFR composite, it has the highest CAT-EFF, and the LOI value of the composites increases from 27.1 to 32.6%. The presence of CeO2 enhances the fire retardant performance, resulting consequently in a great reduction in peak heat release rate (p-HRR), total heat release (THR), heat release capacity (HRC) of the PP/IFR composite obtained from microscale cone calorimeter (MCC) tests. CeO2 reduces the initial decomposition temperature of the IFR and PP/IFR, and it improves the thermal stability and char formation ability. When CeO2 is incorporated into IFR, it forms numerous closed foams without holes, and the char contains more P. The morphological structures observed by digital photos and scanning electron microscopy (SEM) demonstrated that CeO2 promotes to form the more continuous and more compact intumescent char layer. The Energy dispersive X-ray spectrometry (EDX) results indicate that it remains more P and O elements for PP/IFR/1%CeO2 composite. The Laser Raman spectroscopy (LRS) measurements reveal that the strength of the char residue with CeO2 is enhanced with lower R value. The X-ray photoelectron spectroscopy (XPS) analysis demonstrates that CeO2 helps to form crosslinking network containing more O and P in the char layer. Thus, a suitable amount of CeO2 plays a critical role in the flame retardancy and char-forming behavior of PP/IFR, and promotes the formation of more compact char structures in the PP/IFR composites.