Mechanism And Properties Of Flame Retardancy And Fire Warning Of Wood Based On MXene

As an environment-friendly material,wood is widely used in the fields of architecture and furniture decoration.The wood without flame retardant treatment has potential fire hazards,and the surface flame retardant coating technology can endow the wood with flame retardant characteristics.In addition,the further introduction of nanoparticles with fire identification ability can not only give wood fire early warning performance,but also improve the advanced functional application of wood.In this paper,a polydopamine（PDA）adhesive coating was constructed on the surface of wood as a"secondary reaction platform",the polyelectrolyte solution（flame retardant layer）of ammonium polyphosphate（APP）and titanium carbide（Ti₃C₂T_x MXene）nanocomposite solution（fire warning layer）modified by cellulose nanocrystal（CNC）were constructed on PDA surface by hydrogen bond of layer by layer self-assembly（LBL）technology to prepare PA/C-MXene with flame retardant and fire warning function.The microscopic morphology and chemical structure of the coating system were systematically analyzed,and the assembly mechanism was explored.The flame retardant,smoke suppression and fire warning properties and mechanism of wood after LBL treatment were further studied.The details are as follows:（1）The flatness of wood surface can be improved by constructing PA/C-MXene LBL coating system on wood surface.Moreover,the mechanical interlocking,hydrogen bond association and covalent crosslinking between PDA and wood jointly promoted its effective attachment on the wood surface,endowing the wood surface with better reactivity.At the same time,the wettability of wood surface was improved,so that O-P=O and NH₄⁺on APP were assembled on wood surface through hydrogen bond with catechol and imino groups on PDA surface.The assembled APP further formed hydrogen bond association with a large number of hydroxyl,C-O and other oxygen-containing functional groups on the surface of C-MXene,which promoteed the orderly assembly of PDA adhesion layer,APP flame retardant layer and C-MXene fire warning layer on the wood surface.（2）The wood surface with LBL of PA/C-MXene coating system can achieve better flame retardant performance,with a limit oxygen index（LOI）of 47.4%and UL 94 V-0 rating.The average heat release rate（HRR）and total heat release（THR）were 54.70k W/m² and 47.86 MJ/m²respectively,which were 50.6%and 19.0%lower than that of neat wood.Mechanism research findings,in the combustion process,the hydrogen bond crosslinking between the coatings in PA/C-MXene coating system and its adhesion with the wood matrix,combined with the synergistic effect of C-MXene nanoparticles on phosphorus containing flame retardants,jointly promoted the formation of continuous,dense and orderly carbon layer on the wood surface,effectively block the transfer of heat and oxygen in the combustion process,so as to improve the flame retardant performance of wood.In addition,during the combustion process,the adsorption of porous carbon layer generated by PA/C-MXene treated wood and the catalysis of Ti O₂ nanoparticles generated by high-temperature oxidation of MXene jointly reduce the generation of CO₂ and CO gas and effectively inhibit the release of smoke,resulting in only 119.9m²/m² and 0.251g/s of total smoke release（TSR）and CO₂ generation rate（CO₂P）,respectively.（3）After LBL of PA/C-MXene coating system on wood surface,the fire warning response performance of different sensitivity can be obtained,especially when the amount of C-MXene coating reaches 19.2g/m²（6 layers）,the fire warning performance was the best,the fire warning temperature was about 250℃,the response time was 2.1s,and the fire warning maintenance time was more than 60s.The mechanism study found that in case of fire attack,the CNC in the surface coating was completely pyrolyzed at about 250℃,and the gas compounds such as CO and CO₂promoted the surrounding MXene sheet to oxidize into anatase Ti O₂ at high temperature.These Ti O₂ nanoparticles combined with the C-C structure formed by carbonization on the surface of PA/C-MXene coating,forming a Ti O₂network containing a large amount of C element doping on the wood surface.Under the continuous flame attack,the thermal excitation of the C element-doped Ti O₂ network triggered the electronic transition inside the Ti O₂ network,and the electrons were excited from the valence band（VB）to the conduction band（CB）,which promoted the transition of this Ti O₂ network from an insulating state to a conducting state.The LBL of PA/C-MXene treated wood produced a sudden drop in surface resistance at around 250°C（the resistance droped from 0.93MΩat room temperature to 6.2kΩ）,so that the circuit can be turned on after being attacked by flame,resulting in fire warning phenomenon.