Study On Synergistic Intumescent Flame Retardant Polypropylene

Due to the increasing requirement of high-efficient and eco-friendly flame retardantpolymers according to new legislations, polypropylene (PP) as the third consumed generalplastic after polyvinylchloride and polystyrene has a wide range of applications in the fieldsof electric-electronic, architecture, transport and packaging, especially with its flameretardant type. Intumescent flame retardant system exhibits advantages of eco-friendly, lowsmoke release and high efficiency, which makes it to be the developing trend ofhalogen-free flame retardacy. Some layered nano-silicates, metal oxides and new types ofnano-carbon materials show synergy with intumescent flame retardants on improving flameretardant efficiency of polymers, thus it is essential to carry on the study of synergisticmechanism between the components.In this research, PP was melt-blended with combinations of intumescent flameretardant system (ammonium polyphosphate (APP) as the acid resource anddipentaerythritol (DPER) as the carbon resource) with layered nano-silicates, metalhydroxides or novel graphite nanosheet (GN) as synergists. By using thermogravimetric,infrared, X-ray photoelectron spectrum, X-ray diffraction and scanning electron microscopeanalyses, the morphology, synergistic and condensed flame retardant mechanisms werestudied. As well, the effects of synergists on combustion behavior, thermal decomposition,crystallinity and mechanic property were discussed.(1) Study on flame retardant mechanism based on migration of layered nano-silicatesas barriers and acidic catalysis. Various types of montmorillonite (MMT) and sepiolite (SEP)were applied to form PP/layered nano-silicates composites. The chemical state and amountof Si on the surface of PP/MMT and PP/SEP layered nano-silicate composites at severaltemperatures was analyzed by pseudo-in-situ XPS, to illustrate their correspondingcombustion behaviors. By comparing the amount and distribution of acidic sites on MMTand SEP, the relationship between acidic sites of layered silicate and combustion orcatalytic charring of PP/layered nano-silicates composites was established. Results showedthat the fast migration and transformation of silicate to SiO2, and the high ratio of Br nsted and Lewis acid would lead to a better barrier char residue and higher char yield, eventuallya better flame retardancy.(2) Study on synergistic flame retardant mechanism of layered silicate and intumescentsystem. Incorporating pristine MMT (MMT-Na) or SEP (S9) with APP/DPER into PP, theresultant composites were investigated in terms of combustion and thermal decomposition.Based on the investigation, the synergistic mechanism was proposed. Results showed thatsynergists could decrease the heat release rate, increase the oxygen index and the charresidue of flame retardant PP. MMT-Na and S9could accelerate the decomposition of APP,the release of NH3and H2O and the crosslinking between polyphosphate with DPER, aswell as suppress the release of P2O5. Polyphosphate reacted with Si-OH to give out bondsof Si-O-P, bridging MMT or SEP with APP. The existence of these bonds improved both theviscosity of molten char precursor and thermal stability of char residue, adjusting theviscosity to match with the intumescent process.(3) Study on synergistic flame retardant mechanism of metal oxides and intumescentsystem. Incorporating nick oxide (Ni2O3) or bismuth oxide (Bi2O3) with APP/DPER into PP,the resultant composites were investigated in terms of combustion and thermaldecomposition. Results showed NiO (from thermal decomposition of Ni2O3) and Bi2O3would react with polyphosphate (from thermal decomposition of APP), and the resultantnick phosphate or bismuth phosphate could suppress the decomposition of polyphosphateinto P2O5. The P-O-Ni and P-O-Bi bonds in the char residue were more stable than theP-O-Si bond in layered silicate containing samples.(4) Study on synergistic flame retardant mechanism of modified MMT andintumescent system. Based on results from (2) and (3), Ni2+(MMT-Ni) and Bi3+(MMT-Bi)containing MMT were obtained by ion-exchange method. Incorporating MMT-Ni orMMT-Bi with APP/DPER into PP, the synergistic effect of modified MMT was investigated.Results showed that MMT-Ni and MMT-Bi exhibited better synergistic effect thanMMT-Na in terms of flame retardancy. The mechanism of synergistic effect of modifiedMMT lay on the effects of migration of silicate layers as barriers, acidic catalysis and charresidue promotion originated from MMT itself, as well as the char residue promotion frombridging effect of metal oxide. (5) Study on synergistic flame retardant mechanism of graphite nanosheet andintumescent system. graphite nanosheet(GN) was prepared and incorporated withAPP/DPER into PP, the resultant composites were investigated in terms of combustion andthermal decomposition. Results showed diffusion of oligomer phosphate andpyrophosphate from APP decomposition was hindered by the GN layers, thus it couldpromote the reactions of phosphates with DPER to result in formation of more stable charresidue. Besides, GN could improve the thermo-oxidative stability, integrity andgraphitization, as well as increase the crystallinity.The above results have enriched the theory of synergistic flame retardant mechanism,and it provides a theoretical reference for manufacturing intumescent flame retardantsystem and intumescent flame retardant PP according to the ultimate applications.