Preparation And Performance Of Unilaterally Surface-densification Wood And Its Flame Retardant Modification By In-situ Mineralized

Abstract:With the rapid development of the wood industry,the supply of tropical hardwood is not satisfying the growing market demand due to long growth cycles and stricter environmental regulations worldwide.Fast-growing wood is a promising alternative.However,there are some undesirable inherent disadvantages of fast-growing wood,including loose texture,inferior mechanical strength,dimensional instability and flammability,which significantly limit its applications.Unilaterally surface-densified treatment could improve the the mechanical properties,fix the compressed cell by cured water-based resin to form a high hardness surface with paint free,and retain the moisture absorption-desorption characteristics.The zinc borate in-situ mineralized modification can realize the deep impregnation in wood of flame-retardant,which has the advantages of non moisture absorption and anti loss.The realization of unilaterally surface-densification wood and flame-retardant modification can broaden the field of use of fast-growing wood,enhance the added value of fast-growing wood.In this paper,the fast-growing poplar wood was used as the research object.The study focuses on developing an energy-efficient and environmentally friendly densification process combined water-based acrylic resin impregnation to produce unilaterally surface-densified wood products（ISD-wood）with high dimensional stability,hardness,improved mechanical strength and high efficient flame retardancy.The effects of average molecular weight of water-based resin,resin solution concentration,impregnation time and hot pressing temperature on the properties of ISD-wood were revealed,and the effects of zinc borate flame retardant in situ formation of ISD-wood on the flame retardancy were explored.The main research contents and results of this paper are as follows:（1）The waterborne acrylic resin permeated into cell lumens and cured to form a rigid three-dimensional,water-resistant network.Set-recovery（SR）of the surface densified layers decreased dramatically when the deformation was fixed by the cured resin.A higher resin weight percent gain（WPG）resulted in a lower SR.Densified layers with WPG lower than4.7%yielded SR values lower than 2.0%.Surface properties of the surface densified wood were dependent on resin WPG and compression temperature.Surface hardness increased significantly with increasing resin WPG.Samples with WPG higher than 3.7%had a static hardness of 5.9 k N,which is more than double that of the un-densified control.The modulus of rupture（MOR）and modulus of elasticity（MOE）of the only surface densified（OSD）wood increased 28.2%and 37.6%,respectively,compared with the un-densified control.Samples with low WPG had comparable MOR and MOE to the OSD wood.The resin-impregnated and surface densified wood with excellent performance is suitable for flooring,desktop,tabletop,bathroom cabinet applications.（2）In same conditions,the WPG of ISD wood impregnated with Acrodur?DS3530 resin(80 000 g mol^-1)by vacuum impregnation is about 3.7%and that of high pressure impregnation is about 10%.The average molecular weight of Acrodur?DS3530 resin(12000 g mol^-1)is smaller,SR of ISD-wood impregnated Acrodur?DS3530 is smaller,roughness is smaller and contact angle is smaller.The XPS analysis showed that when impregnation time is 3h,the oxygen content of ISD-950L wood and ISD-DS3530 wood is reduced,the C1 content is increased by about 12%,the C2 content is decreased by 13%,and the C2/C1 value is decreased,which proves that the hydrophilicity of wood surface is reduced.（3）Zinc nitrate could form zinc ammonium complexation ion in ammonia water,andformed mixed aqueous solution with borax solution.When ammonia gas was removed,zinc nitrate and borax produced precipitation,and zinc borate flame retardant precipitation was in situ formed in wood.SEM showed that zinc borate particles were particularly abundant in the wood vessels as well as small amounts in ray cells,parenchyma cells and cell corners of vicinal cells.When impregnation-mineralization cycles were more than twice,the limiting oxygen index significantly reached upwards 40.0%,UL-94 test was evaluated as V1 rating.Zinc borate could reduce the thermal degradation temperature of wood components.This was conducive to thermal degradation towards tendency to form more carbon layer with stable structure.Carbon yield of mineralized-wood-2 was improved by 6.5%compared with the natural wood.Compared with natural wood,the heat release rate peak of mineralized wood-2 and M-ISD wood decreased by 24.6%and 30.7%,total heat release decreased by 50.4%and 38.6%,total production smoke decreased by 71.3%and 54.8%,respectively.The ignition time and heat release rate peak time of M-ISD wood were delayed 15 s and 55 s than that of mineralized wood-2,respectively.It was indicated M-ISD wood had high efficient flame retardancy and smoke suppression.In conclusion,unilaterally surface-densified treatment combined with unilaterally surface impregnation could significantly improve the mechanical strength,surface hardness,wear resistance and dimensional stability of fast-growing poplar,the in-situ mineralization of Zinc borate could enhance the flame retardancy of ISD-wood.This paper provides a new research idea for the high-value utilization of Plantation wood,which can expand the application fields.