Study On The Flame Retardant Properties And Mechanism Of Poplar And Radiata Pine Enhanced By Sodium Silicate And Boric Acid Treatment

Wood is a natural and renewable material,which is inseparable from people’s lives;however,its flammability may poses a safety hazard.Therefore,fire-retardant treatment of wood is very important to reduce the fire risk.Sodium silicate(SS)has the effect of physical coverage and thermal insulation and thus may be used as wood flame retardant.However,the disadvantage of SS-treated wood is the lack of effective fixation in wood,which is easy to leach from wood in a humid environment,resulting in a decrease in the flame retardant performance of the wood.Boric acid(BA)exhibits a high flame retardant efficacy during combustion of wood and mainly performs to promote dehydration of wood into charcoal and suppress the produciton of smoke.BA is also easily lost from wood at humid environment.In order to solve the leachability of individual SS and BA,the combinative SS/BA flame retardant system was established to treat the fast-growing planation poplar and radiata pine wood via vacuum pressure..SS and BA can react to form gel in wood and resultantly reduce the leachability.In addition,Physical coverage of SS has a thermal insulation effect BA promotes the dehydration of wood into charcoal during combustion of wood,thereby providing a synergistic effect on imparting efficient flame retardancy to wood.Scanning electron microscope and cone calorimeter were used to analyze the micro-morphology,comustion behaviour and smoke release properties,and residual charcoal of wood.The individual chemical components of wood were treated with SS/BA and the flame retardant mechanism was explored using Fourier transform infrared spectrometer,X-ray diffractometer and thermogravimetric analysis.The physical and mechanical properties of the treated wood were characterized by spectrophotometer and universal mechanical testing machine.The main results of this study are given as follows:(1)According to the reaction rate and particle size of the gel at different ratios of SS and BA,the treating solutions of 5% SS,10% SS,and 20% SS plus 5% BA,respectively,were used to treat poplar and radiata pine woods in two-steps under vacuum,followed with pressure.The weight percent gain and microscopic morphology analysis of the treated wood showed that the fire retardant system can react in wood cell lumens forming gel;SEM-EDX analysis indicates that Si and B elements are also uniformly distributed in the wood cell wall,indicating that both SS and BA can distribute in the cell lumens and cell wall;Compared to poplar wood,radiata pine wood exhibits better permeability and resistance to alkali deterioration.(2)Cone calorimeter was used to analyze the combustion performance of the wood treated with solutions at different SS/BA ratios.Compared to the untreated woods,poplar and pine wood exhibit a decrease in total heat release by 56.9% and 51.5% and in total smoke production by 90.9% and 84.2%,and an increase in ignition time by 20 s and 17 s,respectively.The microscopic morphology of the residual charcoal was anaylzed using SEM.The result showed that BA melted to form a boron oxide film attached to the inner wall of the cell cavity;SS existed in the form of fine particles on the inner wall of the cell cavity;and SS/BA formed a thicker layer adhering to the inner wall of the cell cavity to protect the wood.Among the treated woods,wood treated with 10% SS/5% BA shows the optimum flame retardant effect.(3)In order to further analyze the flame retardant mechanism of the systems,10% SS,5%BA and 10% SS/5% BA flame retardant systems were used to treat MCC(model compound of cellulose),xylan(model compound of hemicellulose),enzymatically hydrolyzed lignin(model compound of lignin),poplar wood flour,and radiata pine wood flour.The performance changes before and after treatment were analyzed using infrared spectroscope,X-ray diffraction and thermogravimeter.The results show that treatments with 10% SS/5% BA considerably improved the carbon residue of the model compounds,especially the MCC and xylan.Neither BA nor SS can chemically react with the model compounds;however,the gels formed by the reaction of BA and SS can react with C=O of wood to form Si-O-C bonds.In the combustion process,BA mainly plays the role of catalytic char formation at relatively low temperature,while SS works synergistically to physically cover the surface.(4)The physical and mechanical properties of the treated wood were tested.Compared with wood treated with 10% SS or 5% BA alone,wood treated with 10% SS/5% BA exhibits higher weight percent gain due to formation of a gel in wood.The improved fixation of SS/BA facilitates to enhance the durability of wood in the humid environment.Incorporation of BA into SS can weaken the darkening of poplar and pine woods by treatment with SS alone.Compared with the untreated poplar and pine woods,woods treated with 10% SS/5% BA shows considerable increase in flexural strength by 37.5% and 35.4%,respectively;however,the impact strength decreased by 44.2% and 42.5%,respectively.