The effects of allophane (ALL) as a synergistic agent on the flame retardancy and thermal stability of intumescent flame-retardant (IFR) polypropylene composites were studied by limiting oxygen index (LOI), UL-94 test, cone calorimeter test (CONE) and thermogravimetric analysis. The incorporation of ALL led to enhanced thermal stability, LOI value and UL-94 ratings. CONE tests indicated that heat release rate, peak rate of heat release, smoke production rate, total smoke production and mass loss values of PP/IFR/ALL sample were much lower than those of PP/IFR and pure PP samples. Fourier transform infrared spectrometry result proved the presence of silicoaluminophosphate which can act as a catalytic agent to enhance the oxidative dehydration crosslinking charring process. Scanning electron microscopy and Raman spectra observation demonstrated that ALL could promote forming homogenous and compact char layer. The mechanical properties of PP composites were also improved by the loading of ALL.

A novel oligomeric charring agent (PTCA) containing triazine and diphenyl group was synthesized by cyanuric trichloride, diphenylamine and ethylenediamine through nucleophilic reaction. The structure and thermal stability of PTCA were characterized by Fourier transform infrared spectrometry (FT-IR), element analysis testing and thermogravimetric analysis (TGA). The effectiveness of the novel intumescent flame retardant system (ammonium polyphosphate and PTCA, abbrev. IFRs) on fire retardancy and thermal stability of polypropylene (PP) was investigated through UL-94, limiting oxygen index (LOI) tests, cone calorimeter tests and thermogravimetric analysis (TGA). UL-94 vertical burning test revealed that the addition of 20 wt% IFRs into neat PP was enough to reach V-0 rating. LOI value of PP by addition of 20 wt% IFRs rose from 17.5 to 31 vol%. Moreover, a remarkable decrease in peak heat release rate (PHRR: −90%), total heat release (THR: −54%) and total smoke release (TSR: −51%) was revealed when the loading of IFRs was 20 wt%. TGA results showed that PTCA presented good char formation ability, and it would greatly increase the thermal stability of PP when combined with APP. The Fourier transformed infrared spectra (FTIR) revealed that the flame retardant mechanism might be ascribed to aromatic and phosphoric char formed during combustion. Additionally, the structure and morphology of char residues were further studied by SEM and Raman spectra.

The flammability and thermal degradation properties of polypropylene (PP) composites containing zinc hydroxystannate (ZHS) and intumescent flame retardant additives (IFR), i.e. ammonium polyphosphate (APP) and pentaerythritol (PER) were characterized respectively by limiting oxygen index (LOI), UL-94 measurements, Cone calorimeter test (CCT) and Thermogravimetry analysis (TGA) in this work. A synergistic effect in flame retardancy was observed when ZHS was used in combination with APP and PER. The experimental data indicated that ZHS enhanced the LOI value, UL-94 ratings and restricted the dripping of the composites. The PP/IFR composites passed the UL-94 V-0 rating test in the presence of 1 wt% ZHS. The CCT tests indicated that the heat release rate (HRR), peak rate of heat release (PHRR) and mass loss rate (MLR) values of the PP/IFR/ZHS samples were much lower than those of the PP/IFR and pure PP samples. The TGA results showed that ZHS could accelerate the char formation of IFR, therefore, greatly increase the thermal stability of PP composites. The Fourier transformed infrared spectra (FTIR) revealed that the flame retardant mechanism of ZHS could be ascribed to its catalysis degradation of the PP resin, which promoted the formation of charred layers with the P–O–P and P–O–C complexes in the condensed phase. SEM observation further indicated that ZHS could promote forming stable and compact intumescent char layer and effectively protect the underlying polymer from burning.