Study On The Construction And Properties Of Layer By Layer Self-assembled Flame-retardant Coatings On The Surface Of Chinese Fir Based On PEI-APP System

As a kind of natural and renewable biomass polymer material,wood has many advantages such as light weight,high strength,beautiful texture,easy processing,adjustable humidity,good environmental characteristics and so on.However,it also has the deficiency of easy combustion,which limits its applications.Therefore,it is of great practical significance to carry out flame-retardant modification of wood to give wood good flame-retardant characteristics and effectively avoid the occurrence of fire.Aiming at the problems of large drug load and high cost of traditional wood flame-retardant impregnation process,this study prepared highly efficient flame-retardant coatings on the wood surface by using highly efficient flame-retardant ammonium polyphosphate（APP）and polyethyleneimine（PEI）as raw materials through layer by layer self-assembly（LBL）and ion cross-linking modification technology.The purpose was to give wood high efficient flame retardation under the condition of low drug loading and expand the application field of wood and wood products.Firstly,the layer by layer self-assembly process was discussed and optimized.Then the structure of metal ions cross-linked modified coatings,thermal stability of coated wood,flame-retardant property,flame-retardant mechanism and so on were discussed.Finally,the durability,wettability,wear resistance of the coatings and the effect of the coatings on the adhesive property of the wood were studied.The main research conclusions were as follows:（1）The Zeta potential tests,the dispersion state of PEI and APP mixture in water and Fourier transform infrared spectroscopy（FT-IR）characterization analysis confirmed the strong interaction between PEI and APP,which laid a theoretical foundation for the subsequent layer self-assembly of PEI and APP on wood surface.By optimizing the process conditions of PEI-APP layer self-assembly coated wood,the optimal process parameters were as follows:the number of assembly layers was 15,the concentration of PEI was 1%,the concentration of APP was 1%,the molecular weight of PEI was 10000,and the molecular weight of APP was greater than 1000.Under these conditions,the oxygen index（LOI）value of modified wood was 37.5%,which was 59.6%higher than that of uncoated wood.（2）Scanning electron microscopy equipped with energy-dispersive（SEM-EDX）and FT-IR characterization confirmed the successful construction of metal ion cross-linked flame-retardant PEI-APP coatings on wood surface.The results of combustion performance analysis showed that metal ion cross-linking modification and PEI-APP coatings modification had a strong synergistic effect on improving the flame-retardancy and smoke suppression properties of wood.The LOI values of wood coated by 15BL-Cu²⁺and 15BL-Co²⁺were 47%and 42.5%.which were 25.3%and 13.3%higher than those coated by PEI-APP alone.In addition,the peak heat release rate（p HRR）,total heat release（THR）and total smoke release（TSR）were also significantly reduced.The main reason was that Cu²⁺and Co²⁺had good catalytic carbonization,catalytic decomposition of APP in advance to produce NH₃,and catalytic conversion of CO to CO₂,which made the wood surface form a more stable carbon layer,blocking oxygen and heat,diluting the combustible gas on the wood surface,thereby improving the flame-retardant performance of wood and reducing the generation of smoke.（3）PEI-APP layer by layer self-assembled and metal ion cross-linking significantly affected the surface wettability and bonding strength of wood.With the increase of the number of PEI-APP assembled layers,the contact Angle of urea-formaldehyde resin（UF）adhesive on wood surface increased significantly,and the wettability decreased,and the bonding strength of the UF adhesive decreased significantly compared with the unmodified wood.When the number of PEI-APP assembled layers was15,the bonding strength decreased by 48.4%.However,after the cross-linking modification of metal ions(Cu²⁺and Co),the contact Angle of UF decreased and the surface free energy increased,and the bonding strength of UF was increased by 50.0%and 68.8%,respectively,compared with that of 15BL coated wood,indicating that ions could improve the adverse bonding effect caused by PEI-APP coatings.In addition,durability test showed that the flame-retardant coatings of PEI-APP had good physical wear resistance and good acid and water resistance,but poor alkali resistance,and Cu²⁺cross-linking modification could inhibit the dissolution of PEI-APP coatings in alkali solution and retain part of the flame-retardant property.The coating adhesion test showed that coatings adhesion reached grade 2 stipulated in GB/T17657-2013"Test Method for Physical and Chemical Properties of Wood-based Panels and veneer Panels"and the adhesion grade reached grade 1 after cross-linking modification of Cu²⁺and Co²⁺,indicating that the metal ion cross-linking modification effectively improved the adhesion of coatings.