Preparation And Performance Of Nacre-inspired Flame Retardant Nanocoating With Fire-warning Capability

Frequent fire accidents in recent years have revealed two safety problems:（1）the wide application of combustible lightweight materials brings huge fire-safety risk;（2）the low sensitivity of the existing fire-warning system leads to the delayed fire-warning response.How to effectively solve these problems has become the key to reduce the fire incidence and fire loss.Multifunctional flame-retardant coatings have attracted much attention because of their high efficiency,intelligent responsiveness and sensitivity,which provides new ideas for improving the flame retardancy of lightweight materials and researching sensitive fire-warning systems.In this thesis,firstly,a nacre-inspired flame retardant nanocoating was prepared via one-step co-assembly of epoxy silane-modified montmorillonite（MMT）and carboxymethyl chitosan（CCS）.The effect of MMT/CCS nanocoating on the flame retardancy of combustible lightweight materials was studied.Secondly,a functionalized cellulose（FC）containing 2-ureido-4[1H]-pyrimidinone（UPy）group was combined with graphene oxide（GO）to construct a flame-retardant nanocoating with self-healing and thermal resistance response fire-warning capability.Subsequently,a sandwich-like flame retardant nanocoating was prepared by fabricating silver nanowire/fluorinated polyvinyl butyral（Ag NW/FPVB）conductive layer on the both sides of the GO/FC nanocoating,which further improved the fire-warning sensitivity and environmental adaptability of the GO/FC nanocoating.On this basis,a flame-retardant nanocoating with repeatable temperature detection and fire-warning capabilities was constructed by the combination of silver selenide（Ag₂Se）/Ag NW/polyvinyl butyral（PVB）thermoelectric layer and MMT/CCS flame-retardant layer.Finally,thermoelectric graphene（TEG）nanosheet was synthesized by in-situ polymerization of3,4-ethylenedioxythiophene on graphene,and a thermoelectric response fire-warning flame retardant nanocoating with good flexibility was constructed via one-step co-assembly of TEG and CCS.The flame retardant and fire-warning mechanisms of the nanocoatings mentioned above were thoroughly studied.The main research contents and results are listed as following:（1）MMT was firstly modified by[3-（2,3-epoxypropoxy）propyl]trimethoxysilane,and mixed with CCS to prepare waterborne dispersion.MMT/CCS nanocoating with nacre-inspired layered structure was fabricated on polyester（PET）film,cotton fabric（CF）and polyurethane（PU）foam through dip-coating or spray-coating.The effect of MMT/CCS nanocoating on the flame retardancy of the combustible substrates was investigated,and the flame-retardant mechanism of the MMT/CCS nanocoating was explored by thermogravimetry analysis（TGA）,X-ray diffraction（XRD）,scanning electron microscopy（SEM）and energy dispersive X-ray spectrometry（EDX）.The results showed that the MMT/CCS nanocoating endowed PET film,cotton fabric and PU foam with excellent flame retardancy,made them self-extinguish in flame retardant test,and effectively suppressed their heat release and smoke production.Compared with PU,the peak heat release rate（PHRR）,total heat release（THR）,peak smoke production rate（PSPR）and total smoke production（TSP）of PU-MMT/CCS-50/d1 were decreased by 84.1%,89.4%,84.4%and 95.2%,respectively.The flame-retardant mechanism of the MMT/CCS nanocoating was as follows:when being exposed to high temperature or burnt,CCS/MMT underwent synergistic catalytic carbonization reactions,and a compact porous char layer was formed on the substrate surface,which effectively blocked the transmission of oxygen,combustible gases and heat.（2）Functionalized cellulose（FC）containing UPy group was synthesized with1,6-diisocyanatohexane,2-amino-4-hydroxy-6-methylpyrimidine and hydroxypropyl methyl cellulose,and then it was mixed with GO to prepare waterborne dispersion.Nacre-inspired GO/FC nanocoating with self-healing and thermal resistance response fire-warning capabilities was fabricated via dip-coating.Its fire-warning and flame-retardant mechanism was explored by X-ray photoelectron spectroscopy（XPS）,SEM,EDX and thermogravimetry-Fourier transform infrared（TG-FTIR）.The results showed that at high temperature,GO was rapidly reduced to graphene with excellent conductivity,and was bonded together by the carbonization product of FC to form stable multiple conductive paths,thereby triggered the fire-warning system.The fire-warning response time of GO/FC-50nanocoating was only 2.7 s.Simultaneously,the GO/FC char layer was expanded by pyrolysis gases to form porous structure,which exhibited high flame-retardant efficiency on PP,wood and PU foam.The limiting oxygen index（LOI）of PP-GO/FC-40 was as high as 31.0%.It could self-extinguish in the horizontal and vertical burning tests,and reached FH-1 and UL-94 V-0 ratings,respectively.More importantly,the UPy group endowed GO/FC nanocoating with excellent self-healing capability.The lost fire-warning capability and flame retardancy of the damaged GO/FC nanocoating could be restored as the nanocoating was self-healed.（3）Fluorinated polyvinyl butyral（FPVB）was firstly synthesized with PVB and triethoxy-1H,1H,2H,2H-tridecafluoro-n-octylsilane,and then mixed with Ag NW and GO/FC to fabricated sandwich-like flame retardant（SFR）nanocoating via spray-coating.The hydrophobicity,fire-warning capability and flame retardancy of the SFR nanocoating were investigated,and its fire-warning and flame-retardant mechanism was explored by XPS,SEM,TGA and TG-FTIR.It was found that the sandwich structure changed the conductive path of the coating from the centimeter level in the horizontal direction to the micron level in the vertical direction,thus significantly reducing its fire-warning response time and expanding its application area.The fire-warning response time of SFR-100 nanocoating was only 0.83 s.Besides,SFR nanocoating exhibited high flame-retardant efficiency on PP and wood.The LOI of PP-SFR-100 was as high as 31.5%.It could self-extinguish in the horizontal and vertical burning tests and reached FH-1 and UL-94 V-0 ratings,respectively.Compared with PP,the PHRR,THR,PSPR and TSP of PP-SFR-100 were decreased by 57.6%,18.3%,65.5%and 40.8%,respectively.In addition,SFR nanocoating also exhibited good water resistance and self-cleaning performance.（4）Ag₂Se nanorod was firstly synthesized by selenium dioxide,β-cyclodextrin and ascorbic acid,and combined with Ag NW,PVB and MMT/CCS to fabricate thermoelectric（TE）layer and flame retardant（FR）layer successively,and the thermoelectric flame retardant（TE-FR）nanocoating was constructed.The temperature detection capability,fire-warning capability and flame retardancy of TE-FR nanocoating were investigated,and its fire-warning and flame-retardant mechanism was explored by the infrared thermal imaging test,TGA,SEM,XRD and XPS.The results showed that when being burnt,FR layer could improve the thermal insulation by forming porous structure,and exhibited high flame-retardant efficiency on PP and wood.The LOI of PP-TE4-FR100 was as high as 32.0%.It could self-extinguish in the horizontal and vertical burning tests and reached FH-1 and UL-94 V-0 ratings,respectively.Compared with PP,the PHRR,average heat release（AV-HRR）,THR,PSPR and TSP of PP-TE4-FR100 were decreased by 33.5%,47.4%,23.8%,36.3%and 30.3%,respectively.Meanwhile,under the protection of the FR layer,Ag₂Se could maintain the alpha crystal form with high thermoelectric efficiency,and underwent carrier migration along the direction of the temperature gradient,thus generated voltage signals and triggered the fire-warning system.The fire-warning response time of TE4-FR100 nanocoating was only 2.0s,and it could also trigger the fire-warning system within 2.8 s when it was reburned.Additionally,the maximum output voltage of the TE-FR nanocoating exhibited a stable linear relationship with the temperature between 100℃and 300℃,which could be used for temperature detection.（5）Thermoelectric graphene（TEG）nanosheet was synthesized by in-situ polymerization of 3,4-ethylenedioxythiophene on graphene,and mixed with CCS to prepare waterborne dispersion.TEG/CCS nanocoating with nacre-inspired structure was fabricated via spray-coating or dip-coating,and its temperature detection capability,fire-warning capability and flame retardancy were investigated.The fire-warning and flame-retardant mechanism of the TEG/CCS nanocoating was explored by TGA,SEM and XPS.The results showed that TEG/CCS possessed good flexibility,repeatable temperature detection and fire-warning capabilities,and exhibited high flame-retardant efficiency on PP,wood and PU foam.The fire-warning response time of TEG/CCS-4 nanocoating was as short as 1.5 s,and its maximum output voltage exhibited a stable linear relationship with the temperature between50℃to 350℃.The LOI of PP-TEG/CCS-4 was as high as 33.5%.It could self-extinguish in the horizontal and vertical burning tests and reached FH-1 and UL-94 V-0 ratings,respectively.Compared with PP,the PHRR,THR,PSPR,TSP,CO and CO₂ yields of PP-TE4-FR100 were decreased by 31.6%,16.1%,53.7%,30.7%,36.2%and 42.4%,respectively.When encountering fire,TEG and CCS in the high temperature region of the coating underwent synergistic carbonization reaction to form a porous char layer with compact surface,which played an effective role in thermal insulation,reducing the lateral thermal conductivity of the coating and providing multiple paths for the carrier migration,thus generated voltage signals and triggered the fire-warning system.