Manufacturing And Characterizing Of The Surface Functional Layer On Poplar Using Combined Treatment Of Fire Retardancy And Thermal Modification

Heat treatment is an important wood modification approach,while it belongs to self-catalyzed reaction,therefore possesses shortcomings including high treatment temperature and long modification time.Meanwhile,the fire resistance of wood cannot be improved by this modification,so the thermal modified wood still has fire risk.These shortcomings limit the application of thermal modified wood,as well as the further development of the heat treatment.This study was aimed at addressing those problems,acidic intermediates created by the nitrogen phosphorus flame retardant(NP)under low temperature treatment was introduced to accelerate the thermal degradation process of wood,prompting in rapid thermal modification effect and improving the efficiency of the modification.The "Two-steps"method combined NP with heat treatment(NP-Ht)functionalized the wood surface layer,which improving surface properties,making it dimensional stability and fire retardant.Then the surface functional layer on the poplar wood was established using combined treatment of NP and thermo-compression,the color generation,surface strength,wetting performance,bonding properties,and fire resistance of surface functional layer on the combine treated wood were studied.Besides,NIR-FTIR,SEM-EDS,VDP,and CONE were applied to uncover the modification mechanism of the combined treatment used in this study.The main results of the study are summarized below:(1)Surface embrittlement phenomenon was found on the surface of conventional 160?-220?heat-treated poplar.The macroscopic performance of the surface embrittlement can be manifested by linearly decrease of the surface strength with treatment temperature or weight loss during the treatment(R2>0.878);surface application performance showed that coating and bonding strength were decreased.Studies carried on different layers from surface to inner part,revealing that the failure model of the bonding mainly occurred on wood itself and the bonding strength depended on the surface strength,after removing the 1 mm embrittlement surface layer.AFM showed the microcosmic performance of the surface embrittlement,including decline of intercellular binding,decrease of cell wall thickness,as well as stratification on S2 layer of the cell wall.The formation of the surface embrittlement layer could be caused by the excessive thermal degradation on the surface layer of wood.(2)TGA and NIR-FTIR analysis showed that the NP formed acid catalytic medium like phosphoric acid under 120?-160?,which increased the thermal degradation intensity of the hemicellulose under relatively low temperature,but the degradation intensity of cellulose and lignin was significantly lower than that of conventional 220? heat-treated wood(Ht-220).The combined treatment could alleviate the embrittlement of wood surface and bonding strength decline;the treatment temperature and holding time reduced about 30%and 75%,respectively.(3)Study showed that chemical reactions involved between the NP and wood during the combined treatment process.Meanwhile,the NP migrated into the deeper layer and finally formed functionalized layer with a thickness about 5.5 mm.The combined treatment transformed the NP from crystal particles state into a uniform coating layer.Hygroscopicity of the combine treated wood was about 60%lower than that of the only NP treated wood(NP-t),leaching resistance increased from 16%to about 27%.Besides,the volume anti-swelling efficacy(ASE)of the combine treated wood reached 51%because of the functionalized layer,and its bonding strength increased 53%when compared with that of Ht-220;heat release rate was about 22.7%lower than that of NP-t,total smoke release decline from 2.2 m2 to 0.4 m2.(4)During the combined treatment of NP and surface thermo-compression modification,the NP firstly decomposed to form phosphoric acid,water vapor and NH3,promoting the thermal degradation of cell wall components,as well as softening and plasticizing the wood surface.Then,the surface functional layer with a thickness about 2-3 mm was built via densification process under"acidic-humid-hot" condition.The results of SEM-EDS showed that NP made the functional layer denser and the cell wall less collapse because of the soften effect on wood.Besides,the fire retardant can fill in the pores of the cell wall and has a connection effect between the compressed cell walls.Surface hardness of the functional layer increased about 48%.The thickness resilient rate was only 6.07%,the cracking time of the burning surface increased 50%during the CONE test,fire resistance also enhanced.(5)In this study,synergistic effect existed in NP and heat treatment or surface thermo-compression on wood modification.Cluster analysis showed that the combine treated wood with the surface functional layers belong to the same group with conventional 200? and 220?heat-treated poplar,when taking the surface color into consideration.Similarly,flame retardant and smoke suppression properties of combine treated wood belong to the same group with only NP treated poplar,proving that the fire retardancy was remarkably enhanced.The combined treatment with NP and thermal modification could enhance color decoration,dimensional stability,and flame retardancy of treated wood at the same time.It was thought to be helpful for the develop and application of heat-treated wood.