Performance And Gluing Mechanism Of High-density Bamboo-based Fiber Composites

Bamboo fiberreinforcedcomposite(BFRC)is ahigh-performance bamboo scrmber,which is made from bamboo fiber mat as basic manufacturing units and phenolic resin as adhesive by the process of cold pressing or hot pressing.The composite has many advantages such as adjustable size,controllable property,designable structure,and beautiful texture.Thus,its products are favored by domestic and foreign markets and they are widely used for indoor and outdoor floor,furniture,landscape,architecture,etc.Density is the material base of BFRC properties and thus definitively affects the physical-mechanical performance and application field of BFRC.At present,BFRC in the field of research and application is mainly the one with the density ranged from 0.85 g/cm³ to 1.20 g/cm³,but the one with the density over 1.20 g/cm³.In theory,bamboo cells can be completely densified and the density can reach the density level of cell walls.Therefore,there is a huge room for further researches of the physical-mechanical properties.Herein,the composites with high density(>1.20 g/cm³)was prepared using Ci bamboo(Sinocalamus affinis)as raw material.The ultimate density was explored as well as the corresponding physical-mechanical performance.Also,the effects of density and resin content on the mechanical performance,water resistance and surface properties were investigated in detail.After that the molding mechanism of BFRC were investigated using the modern instruments and analysis methods.The conclusions were as follows.(1)The ultimate density of BFRC was 1.37 g/cm³.The horizontal shear strength,longitudinal compressive strength,modulus of rupture(MOR)and Modulus of elasticity(MOE)were 28.83 MPa,215.53 MPa,354.00 MPa and 28.36 GPa,respectively.The water absorption and thickness swelling of the composites treated by 28h-intermittent boiling were 9.67%and7.52%,respectively.(2)The increase in density improved the shearing,compressive,bending and impact properties.The impact energy was 89.13 k J/m² at the ultimate density.Water resistance and dimensional stability were also improved as the density increased.The surface roughness,hydrophilicity and free energy were decreaed with the increasing density and the roughness Rawas as low as 1.71?m.(3)As resin content increased,the shearing and compressive strength firstly increased and then decreased.The maximum of them wererespectively 25.77 MPa and 169.24 MPa as the resin content was 13%.The bending and impact properties dropped with the increasing resin content.MOR and impact energy were respectively up to 328.27 MPa and 7.70 k J/m².The increase in resin content remarkably improved water resistance and dimensional stability,and water absorption and thickness swelling were as low as 9.14%and 6.71%,respectively.Also,the surface roughness,hydrophilicity and free energy were decreaed as the resin content increased.The minimum surface roughness Ra was 2.14?m.(4)During defibering,fiber cells survived but metaxylem vessels and parenchyma cells were hadly destroyed.Andduring the process of BFRC,fiber cells also survived,but parenchyma cells collapsed while metaxylem vessels were filled by other cells.Simultaneously,the porosity wasdecreased and its minimum was 5.52%.The diameters of the pores in BFRC were in the rangs of 58?0.5?m and 80?1.8 nm.The diameter gradually reduced while the diameter distribution moved to the low-value area during the pressing.The diameters of the pores in cell walls were between 2nm and 15nm and the pores in 3?10nm were over 85%of total pores.During the pressing,the diameters of the pores in cell walls gradually increased and the maximum was was 6.0 nm.Meanwhile,the crystal structure of cellulose stayed original but the relative crystallinity and microfibril angle were gradually increased,and their maximum were 62.23%and 9.86°,respectively.(5)During the hot pressing of BFRCbamboo fiber and its components promoted the condensation reaction of phenolic resin,andchanged the reaction termperture and heat enthalpy.Cellulose improved the reaction activity of the resin and eliminated the active groups such as phenolic hydroxyl(Ar-OH)and alcohol hydroxyl(-CH₂-OH)groups.Lignin showed the similar chemical structure with phenolic resin and it reacted with the resin through the active groups(e.g.Ar-OH and-CH₂-OH),thus they linked each other by the chemical bonds of-O-C-O-or-C-O-C-.Xylan participated in the cross-linking reaction of phenolic resin using the carbonyl group(-C=O)and they linked each other by the chemical bonds of-C-O-.