Effects Of Expansion And Encapsulation Of Expandable Graphite On Flame Retardant Properties Of Polyethylene And Polyurethane Foams

Polyethylene?PE?and polyurethane foam?PU?have a large output and a wide range of application.However,the long chain of PE is almost completely cracked into a combustible gas during the combustion process,and the residue yield is near to zero.PU can release toxic and harmful substances for the human body and the environment.The fire risk caused by the flammability has aroused wide concern.To improve the flame retardancy of these materials,expandable graphite?EG?,expanded products of EG?EBG?at different temperatures,melamine-modified urea-formaldehyde?MUF?resin-coated EG microcapsule?EG@MUF?were prepared and applied to linear low density polyethylene?LLDPE?and PU.Scanning electron microscope-Energy dispersion spectrum?SEM-EDS?,X-ray power diffraction?XRD?,Fourier transform infrared spectroscopy?FTIR?and thermogravimetric analysis?TGA?were used to show the morphology,chemical structure and thermal stability of these flame retardants.The properities of the flame retardant composites were investigated by limit oxygen index?LOI?,vertical combustion level,cone calorimeter test?CCT?,thermogravimetric/differential thermogravimetric?TG/DTG?analysis,tensile tester,and thermal conductivity meter.The detailed contents and results were as follows.First,EG and EBG₃₀₀,EBG₆₀₀,EBG₉₀₀ were prepared by chemical oxidation intercalation and then expanded at 300°C,600°C and 900°C,respectively.Results show that the value of expansion volume?EV?increases with the expansion temperature increase,and the pore structures become fuler.The EV of EBG₃₀₀,EBG₆₀₀,and EBG₉₀₀ is 30mL/g,248mL/g,and405mL/g,respectively.In addition,the higher the expansion temperature,the higher the thermal stability of EBG.EG,EBG₃₀₀,EBG₆₀₀ and EBG₉₀₀ still retain the layered structure of graphite,but they show the increasing disorder.Second,EBG₃₀₀,EBG₆₀₀,EBG₉₀₀,mixture of each EBG and APP were incorporated,and the influence on the LLDPE properties was investigated.Results show that the addition of 30 wt%additive significantly improves the composites high temperature thermal stability and residue yield.The heat release rate?HRR?,total heat release?THR?,smoke generation rate?SPR?,and the peak values of the CO and CO₂ curves are also significantly reduced.The effective fire resistance index is increased by 4 to 9 times,and the fire spread rate index?FGI?is reduced by more than 84%.The LOI values of the tested composites are all above 29.8%.In the cooperative system,except for the 70LLDPE/20EBG₉₀₀/10APP,the UL-94 grade of the other synergistic components all reaches V-0.Results also show that the addition of the additives all reduces the mechanical properties and improves the thermal conductivity of the LLDPE composites.70LLDPE/30EBG₉₀₀ has the best thermal conductivity,which is nearly 7 times higher than the untreated LLDPE.Third,through pre-polymerization and in-situ polymerization,the encapsulated EG@MUF was prepared with EG?obtained by oxidized intercalation of 50 mesh graphite?as the capsule core and MUF as the capsule wall at different EG and MUF feed ratios of 3/1,5/1,7/1 and 9/1,respectively.The products were labeled as EG@MUF3/1,EG@MUF5/1,EG@MUF7/1 and EG@MUF9/1.The results show that the coating amount of MUF decreases as the initial feed ratio of EG to MUF.The larger the feed ratio,the smaller the thickness of the shell layer,and the worse the encapsulation for EG.At the ratio of 9/1,the incomplete shell layer is formed.Compared with the EV of EG of 405mL/g,the EV of EG@MUF5/1 and EG@MUF7/1increases by 21.7%and 18.5%,respectively.Fourth,EG and the series EG@MUF are added at a 13 wt%dose,the incorporation significantly increases the thermal stability of PU,which is determinated by the additives.Although the addition of EG and each EG@MUF also results in the decrease of thermal insulation performance and compression strength of PU composites,the effect of EG@MUF was significantly lower than that of uncoated EG.The thermal conductivity and compressive strength of PU/EG@MUF composites are closely related to the shell thickness of EG@MUF particles.The combustion tests confirm that each EG@MUF shows better flame retardancy than EG.Addition of EG@MUF7/1 increases the LOI value of 87PU/13EG@MUF7/1 from18.6%to 28.9%,and the composite passes the V-0 level.The CCT results indicate that the incorporation of EG and each EG@MUF can significantly reduce the HRR,SPR,THR,CO and CO₂ value.87PU/13EG@MUF7/1 and 87PU/13EG@MUF5/1 present the best flame retardancy,with PHRR decreased by 65.64%and 58.66%,and TSP decreased by 71.73%and75.95%,respectively.