Cellulose-Based Flame-Retardant Aerogels:Preparation And Study On Flame Retardant Properties

As report goes that there were 748,000 fire occurrences,2225 injuries,1987 fatalities and 6.75billion yuan in property loss due to fire just in 2021.Furthermore,most of the polymeric materials such as polyurethane（PU）,polyethylene（PE）,polypropylene（PP）,polybutylene terephthalate（PBT）,and other polymeric materials commonly used in civil equipment,industrial facilities,and medical applications are highly flammable.Materials with flame-retardant qualities have drawn a lot of research interest in increasingly urgent firefighting scenarios.Aerogel is a highly porous material with up to 99%porosity,low density,and high surface area.A variety of aerogels such as Polyimide（PI）,Si O₂,ceramics,carbon,organic and metal oxides have been extensively studied.However,these aerogels either lack flexibility and compressibility due to brittleness and have poor mechanical properties;are complicated to prepare and costly to produce;or are inherently toxic and harmful to humans,which is why today’s research is more inclined toward making aerogels using bioresources（cellulose,in particular）.Cellulose exhibits high tensile strength,stiffness,Young’s modulus,low density,and low coefficient of thermal expansion.Moreover,it possesses low toxicity,good biocompatibility,and biodegradability,making it an ideal raw material for the production of aerogels with green,good mechanical,and thermal insulation properties.The inherent hydroxyl group of cellulose facilitates cross-linking modification with flame retardants,thereby expanding the practical application of cellulose aerogels as flame-retardant materials.In this thesis,we aim to develop flame-retardant cellulose aerogels for practical applications.We prepared a series of flame-retardant cellulose aerogels were prepared by incorporating flame retardants into the cellulose matrix and carefully studied their physicochemical properties and flame-retardant performance.The main contents details of this work are as follows:（1）The cellulose（Cel）,polyvinyl alcohol（PVA）,and sodium alginate（SA）composite aerogels are prepared by adding PVA and SA to the Cel suspension,followed by freeze-drying.The structure and properties of Cel-PVA-SA composite aerogels are determined by FT-IR,scanning electron microscopy,thermogravimetric analysis,limiting oxygen index method,and cone calorimetry.The results showed that the best overall performance of the resulting cellulose composite aerogel was obtained when the mass ratio of sodium alginate to cellulose-polyvinyl alcohol was 2:8.Its compressive modulus reaches 69.6 k Pa,the limiting oxygen concentration value（LOI）increases to 37.6%,the total heat release（THR）decreases to 7.3 MJ·m^-2,the fire index（FIGRA）was 4.66 k W·m²·s,and the thermal conductivity was 48.2 m W·m^-1·K^-1.Moreover,the combustion experiments demonstrate that the Cel-PVA composite aerogels exhibited self-extinguishing ability after SA addition.Meanwhile,the addition of SA affects the compressive resilience of Cel aerogels and improves the mechanical properties of the Cel aerogels to a certain extent.The study on the flame retardant mechanism of Cel-PVA-SA aerogel showed that when the aerogel burns,it generated CO₂ and other non-flammable gases,which diluted the concentration of combustible gases,and in addition,calcium ions promoted the carbonization of cellulose and sodium alginate to form a carbon layer,that is,the gas phase flame retardant and condensed phase flame retardant act synergistically to realize the improvement of the flame retardant performance of Cel-PVA-SA composite aerogels.（2）A phosphorus-doped graphitized nitriding carbonic flame retardant（P-UCN）was prepared by phosphorus substitution,the P-UCN flame retardant was added to the cellulose solution,and the Cel-P-UCN aerogels were prepared by cross-linking with epichlorohydrin.The chemical structure,elemental distribution,and electronic state in each element of the flame-retardant aerogels were characterized by FT-IR,EDS,and XPS.The results showed that the P-UCN flame retardant was uniformly distributed in the cellulose aerogels.Among them,the LOI value of Cel-1P-UCN aerogel reached 42.1%and the TSP was 0.017m²/m²,which was 66.7%lower compared with the pure cellulose aerogel.The Cel-P-UCN aerogel’s flame retardancy was achieved through the synergistic effect of two factors.Firstly,the surface carbonization of the cellulose matrix by phosphorus-generated polyphosphate during combustion.Secondly,graphitized carbon nitride provided thermal insulation.