Flame Retardant Design And Mechanism Of Cellulose Aerogel With Thermal Insulation

In order to achieve energy conservation,a large number of petrochemical based thermal insulation materials are used in industrial and civil cold chain transportation,new energy vehicles,industrial kilns and buildings,national defense aviation and other fields.However,because they are not easy to degrade,it will bring a series of environmental problems after consumption and abandonment,which is not conducive to sustainable development and the realization of the"Carbon peaking and Carbon neutralization".Cellulose aerogel（CA）has the advantages of ecological environmental protection and high efficiency in energy saving,which is one of the main directions for the future development of energy-saving materials.However,CA has the general flammability of polymer materials and causes fire hazards when used alone.In order to achieve active fire prevention,adding flame retardants is the most convenient and effective way.To realize the flame retardant of CA,the key lies in the following points:1.Inhibit the thermal decomposition of cellulose in the solid phase and promote its dehydration into carbon,so as to reduce the delivery of fuel to the flame zone;2.Inhibit the combustion and oxidation reaction of fuel in the gas phase to control the spread of flame;3.Inhibit the heat feedback generated by combustion and oxidation reaction,so as to realize the self extinguishing of materials after ignition.In addition,because heat radiation is not easy to dissipate in the pore structure,the suppression of open fire will lead to the formation of CA smoldering and dense smoke.Therefore,flame retardant can also solve the contradiction between flame retardant and easy smoke production,carbon formation and easy smoldering.Based on this purpose,this topic used waste cotton fiber from textile industry as cellulose raw material to prepare CA,and successively realized the high-efficiency,green and durable flame-retardant modification of CA with inorganic nano flame-retardant system and biomass flame-retardant system,and further studied the flame-retardant mechanism.The specific research contents and conclusions are as follows:（1）Design and synthesis of LDH ultrafine nano materials and their flame retardant and charring properties:Layered double hydroxides（LDH）inorganic flame retardant can interfere with the thermal decomposition process of cellulose through thermal absorption,so that cellulose can be transformed into carbon slag at a lower thermal decomposition temperature and slower decomposition rate.At the same time,its own thermal decomposition product layered double hydroxides（LDO）,which has high heat insulation and smoke suppression and can endow（2）cellulose with flame retardancy.However,due to the premature or too fast thermal decomposition of LDH,its interference effect on the thermal decomposition process of cellulose is not ideal,and the current synergistic method based on the synergistic modification of flame retardant elements is not significant to improve its efficiency.Therefore,in this chapter,through adjusting the types and composition of metal elements of LDH,six kinds of ultra-fine LDH nanoparticles containing transition metal elements Ni/Co,Mg and Al were designed and synthesized by hydrothermal titration（Ni@LDH,Ni₁@LDH and Ni₂@LDH and Co@LDH,Co₁@LDH and Co₂@LDH）,which were further used to prepare LDH/cellulose composite aerogel.The results show that the initial thermal decomposition temperature of LDH advances with the increase of transition metal content,and the residual amount decreases.The charring efficiency,charring quality and flame retardancy of LDH on CA were mainly improved with the increase of the content of transition metal elements.Grey correlation analysis results show that the decomposition of LDH dominated the flammability of LDH/CA,while its charring had a great impact on the flame propagation.Through comprehensive analysis,Ni₂@LDH was selected as the more suitable one for the thermal decomposition process of cellulose.It has the most lasting interference to the thermal decomposition process of CA and high charring property.In air atmosphere,when the addition amount was 6 wt%,the carbon residue of LDH/CA was9.99%,and the limiting oxygen index（LOI）was 27.6%.（3）Study on thermal insulation performance and flame retardant mechanism of Ni₂@LDH/cellulose composite aerogel:Flame retardants,while promoting the efficient carbonization and flame retardation of cellulose,are often prone to the formation of thick smoke,which is the biggest fatal threat factor in the fire scene.Therefore,it is very important to understand the effect of the addition of Ni₂@LDH on the exothermic smoke release and the flame retardant mechanism.In addition,the smoldering process of aerogel is caused by thermal irradiation and spreads in the carbon slag.The improvement of thermal insulation is contradictory to the improvement of charring,so it is also important to understand the thermal insulation of the substrate.The research in this chapter shows that the thermal insulation of CA was affected by both pore structure and Ni₂@LDH,and increased with the increasing of porosity and Ni₂@LDH.The study of exothermic smoke release shown that Ni₂@LDH can inhibit the combustion heat release and smoke suppression process of cellulose.When the addition amount of Ni₂@LDH was 9 wt%,the total heat release（THR）was reduced by 20.56%compared with CA,and the total smoke release（THR）was significantly reduced by 87.16%compared with CA.Ni₂@LDH had high-efficiency heat insulation,and could effectively promote the dehydration and carbonization of cellulose through long-range interference in the thermal decomposition process of cellulose on the solid phase.The LDO formed by its thermal decomposition could build a thermal insulation barrier together with carbon slag,so as to achieved high smoke suppression and heat radiation shielding at the same time of flame retardation,which solved the contradiction between flame retardation and dense smoke,high carbon slag and smoldering.（4）Construction of flame propagation suppression layer based on tea polyphenols&study on synergistic flame retardant mechanism of Ni₂@LDH with it:The gas-phase effect of Ni₂@LDH is mainly dilution effect rather than on the combustion free radical reaction,so the inhibitory effect on flame propagation is poor.Therefore,in this chapter,tea polyphenols（TP）with excellent adhesion were assembled on the surface of Ni₂@LDH/CA composite aerogel to prepare TP/Ni₂@LDH/CA composite aerogel by hydrothermal driving.Tea polyphenols formed an antioxidant layer at the interface between gas phase and solid phase through hydrogen bonding with Ni₂@LDH and cellulose,which can suppress the combustion free radical reaction by consuming oxygen,so as to effectively suppress the flame propagation.The vertical combustion reached V-0 level and realized self extinguishing.The results showed that tea polyphenols and Ni₂@LDH had synergistic flame retardant effect:Ni₂@LDH could inhibit the thermal decomposition process of tea polyphenols and cellulose at the same time,so that tea polyphenols remained in the solid phase,and built high-quality carbon slag with heat and mass shielding effect together with cellulose and LDO.When the deposition concentration of tea polyphenols was 0.6 g/L,and the dosage of Ni₂@LDH was 9 wt%,THR was 28.57%lower than CA,TSR was 90.54%lower than CA,and the fire safety was further improved.（5）Construction of tea polyphenols water-resistant flame retardant layer&study on synergistic flame retardant mechanism of Ni₂@LDH with it:The synergistic effect of tea polyphenols and Ni₂@LDH improves the flame retardancy of cellulose aerogel,but the flame retardancy of the material may be lost with the loss of water-soluble tea polyphenols,so it does not have flame retardancy durability.The traditional flame retardancy durability is achieved by chemical grafting,but the grafting process is easy to make the mechanical properties of cellulose brittle.Therefore,in this chapter,the complexation of metal ion flame retardant(Mg²⁺)was skillfully used to solidify the tea polyphenols（TP-Mg）,which could improve the mechanical strength of aerogel.After complexation modification,the LOI loss rate of modified aerogel deposited with 0.6 g/L tea polyphenols decreased from 26.75%to 2.63%when it was washed with water for 72 hours.TP-Mg and Ni₂@LDH had synergistic flame retardant effect:Ni₂@LDH could inhibit the thermal decomposition process of cellulose and TP-Mg at the same time,and the metal ion complexation modification not only improved the oxidation resistance of aerogel,but also significantly improved the charring capacity of tea polyphenols and cellulose.In addition,it converted into Mg O and built a high-efficiency anti-oxidation thermal shield layer with cellulose carbon residue,which further improves the flame retardancy and fire safety.When the deposition concentration of tea polyphenols is 0.6 g/L,the dosage of Ni₂@LDH was 9 wt%and the modified dosage of Mg²⁺was 0.20 mol/L,THR decreased by 30.31%and TSR decreased by 92.91%compared with CA.Focusing on the theme of"Carbon peaking and Carbon neutralization",this topic aims to achieve energy conservation,reuses the resources of waste cotton fibers in the textile industry,integrates inorganic nano flame retardant system and biomass flame retardant system to build a synergistic flame retardant system,realizes the active prevention of potential fire hazards of cellulose aerogel,and provides a new idea for the reuse of waste cotton fiber resources in the textile industry.