Preparation Of Fullerene (C₆₀)-Decorated Polyamide 6 And Expandable Graphite And Their Flame Retardant Applications In Polypropylene Composites

Intumescent flame retardants have become one of the most active fields in polypropylene?PP?flame retardant research because of its unique flame retardant mechanism and halogen-free,low smoke,low toxicity and other characteristics.However,common charring agents polyol in the mixed phosphorus-nitrogen intumescent flame retardants are incompatible with PP,and the polyols are easy to hydrolyze,so that the flame retardant PP has strong hygroscopicity;the chemical oxidation method used in the preparation of the physical intumescent flame retardant-expandable graphite?EG?has a large amount of acid consumption or heavy metal contamination.Therefore,in this paper,the preparation processes of EG and macromolecular nano charring agent C₆₀-d-PA6 were studied,the synergistic flame retardant effects of MPP and C₆₀-d-PA6 or EG on PP were studied by using melamine polyphosphate?MPP?as the main flame retardant.The details are as follows:First of all,a novel macromolecular nano charring agent C₆₀-d-PA6 was prepared by melt grafting reaction of fullerene(C₆₀)and polyamide 6?PA6?.The C₆₀-d-PA6 was characterized by means of Fourier transform infrared spectra?FTIR?,transmission electron microscopy?TEM?,scanning electron microscopy?SEM?and thermo-gravimetric analysis?TGA?.The results show that C₆₀ crystals are relatively uniform dispersed in the PA6 matrix with the size between 10²5 nm,which suggests that the approach by melt grafting reaction of C₆₀ and PA6 can overcome the shortcomings of easily aggregating of C₆₀ crystals.C₆₀ and PA6 exhibited a good synergistic effect in charring.Secondly,using 325 mesh,150 mesh and 50 mesh natural flake graphite?NFG?respectively as raw material,potassium dichromate as oxidant and concentrated sulfuric acid as intercalation agent,the technical parameters for preparing the EG with maximum exfoliated volume?EV?were optimized by orthogonal tests.It was found that the optimal technical parameters to prepare EG with maximum EV had relations with the graphite particle size.Under the optimal technical parameters,more quantities of oxidants and shorter reaction time were needed to prepare EG with small particle size NFG than with large one.Thirdly,in order to obtain the low-temperature expandable graphite?LTEG?by means of a low pollution method,50 mesh NFG was oxidized in the perchloric acid solution by means of the electrochemical method.The results show that the sulfur-free LTEG with large EV could be prepared in the perchloric acid solution with the mass fraction of more than 50% and in a wide range of current density of 30⁸0 mA/cm2 at room temperature.For example,the LTEG prepared in the perchloric acid solution with the mass fraction of 60%,in the current density of 50 m A/cm2 and the reaction duration for 5 hours has no sulfur,oxidizing agent,heavy metals or other impurities,and has large exfoliated volume and is expandable at low temperatures,the EV of the LTEG reaches up to 282 and 594 mL/g respectively at the exfoliating temperatures of 200 °C and 950 °C.Fourthly,a new way to prepare EG by means of ozone oxidation was proposed.The influences of reaction time,ozone flow and concentration of sulfuric acid on EV were discussed in detail.Too long or too short for the reaction time and too large or too small for the ozone flow will result in reduction of the EV;whereas EV will decrease with the decrease of sulfuric acid concentration,its suitable volume fraction should be more than 90 %.The optimum conditions with the maximum EV of 285 ml/g at 950? may be that reaction time,ozone flow,sulfuric acid concentration are 3.5 hours,80 ml/min and 98 wt.%,respectively.In addition,the intercalation solutions have a greater effect on the EV.Fifthly,the C₆₀-d-PA6 combined with MPP was introduced into PP to prepare PP/MPP/C₆₀-d-PA6 composites.The flame retardant and thermal properties of the composites were investigated by the limit oxygen index?LOI?,horizontal and vertical burning test?UL-94?,smoke density rank?SDR?,thermo-gravimetric analysis?TGA?and char morphology.The results show that the flame retardant mechanism of PP/MPP/C₆₀-d-PA6 composites is the flame retardant in condensed phase and gas phase.Adding MPP/C₆₀-d-PA6 to PP has played a role in promoting the early degradation,which can reduce the thermal loss rate during PP combustion and significantly increase the char residue and LOI of the composites and obviously reduced horizontal burning rate,there are good synergistic effects in enhancing charring and LOI between MPP and C₆₀-d-PA6.Compared with the PP/MPP/PA6 composites,the increase of the char residue and LOI of the PP/MPP/C₆₀-d-PA6 composites and the decrease in the combustion rate were not obvious,PP/MPP/25C₆₀-d-PA6 composite was only UL94-3.2 mm V-2 rating,but the SDR of PP/MPP/C₆₀-d-PA6 composites was much smaller than that of PP/MPP/PA6 composites.This suggests that C₆₀ has a good smoke suppression effect.Finally,the EG combined with MPP was introduced into PP to prepare PP/MPP/EG composites with MPP as the main flame retardant and EG as a synergistic agent.The flame-retardant and mechanical properties of the composites were investigated by LOI,UL-94,SDR,TGA,char morphology and mechanical performance test.The results show that the flame retardant mechanism of PP/MPP/EG composite is the flame retardant in condensed phase,the flame retardant effect of PP/MPP/EG composite depends mainly on the thickness of the char layer.Adding MPP/EG to PP can reduce the thermal weight loss rate during PP combustion and significantly increase the char residue of the flame retardant PP;but the SDR of the flame retardant PP increases with the increase of EG;EG and MPP has a significant synergistic flame-retardant effect in PP,when the total amount of the flame retardant is constant,the LOI of the composites increases with the increase of EG dosage,and the combustion rate of the composites decreases with the increase of EG dosage,PP/20MPP/15 EG composites can pass UL94-1.6 mm V-0 rating.In addition,replacing a part of the MPP with a small amount of 50 mesh EG can play a role in reinforcing and achieve both reinforcing and flame retardant effect.