| Because of its porous core structure,the wood-based composite grid sandwich structure has the potential for light weight,high strength,heat preservation,heat insulation,sound insulation,and moisture resistance.Carbon fiber and glass fiber were used as the reinforcement of the traditional composite sandwich structure.With the increasing problems of resources and environmental issues,the development of sustainable structural and functional materials has become one of the research hotspots in recent years.Natural plant fibers have high initial modulus,good mechanical properties,extensive resources,biodegradable,sustainable and low price.Laminated composite materials made from continuous natural fiber fabric reinforced resin matrix have good mechanical properties,which meets the requirements of composite grid sandwich structure for tensile dominant lightweight materials.Therefore,combining the natural fiber fabric reinforced composite with the wood-based sandwich structure can expand the application field of the composite sandwich structure,and can promote the comprehensive development and utilization of natural fibers.Jute fabric/epoxy laminated composite was used as the core,spruce was used as the panel to prepare wood-based composite grid sandwich structure.In order to solve the interface compatibility between jute fabric and resin matrix,the surface treatments were carried out by using hot-alkali treatment that caused changes in the chemical composition of jute fibers(cellulose,hemicellulose and lignin)and KH-560 treatment that does not cause chemical composition changes of jute fibers.The effects of yarn density(30/1 Ocm,50/10cm,80/10cm),different concentrations of hot-alkali treatment(0%,2%,4%,6%,8%,10%)and different KH-560 silane coupling agent treatment time(0min,10min,30min,60min,90min,120min)on chemical composition,elemental composition,crystallinity of jute fabric,thermal stability,surface morphology and mechanical properties of laminated composites,were analyzed with the aids of Fourier transform infrared spectroscopy(FTIR),X-ray diffraction(XRD),optical microscopy,scanning electron microscopy(SEM),X-photoelectron spectroscopy(XPS),plant fiber composition analysis method and thermogravimetric analysis method(TG/DTG).Therefore,the best processing method was obtained and applied to the preparation of wood-based composite grid sandwich structure.The effects of two yarn density(50/10cm and 80/10cm)on compressive performance were studied.The failure mode,load capacity,energy absorption and bending properties of three kinds of grid sandwich beams(SSS,DCSS and CSS)were studied.The research results showed that jute fiber bounds were similar as "bamboo joint" in the growth direction,and the fibers cross section were polygonals.The contents of cellulose,hemicellulose and lignin in jute fibers were 53.69%,25.56%and 11.1%,respectively.The cell wall of jute fiber was composed of macromolecular cellulose filled in hemicellulose and lignin,which was the backbone of jute fiber and mainly supported by force.Jute fiber had the largest degradation in the temperature range of 240℃-395 ℃,which was 71.77%.When the temperature was 365.18℃,the degradation rate of jute fiber was the fastest.The tensile strength and flexural strength of the jute fabric reinforced laminated composite with a yam density of 80/10cm were increased by 29.2%and 29.4%than those of the laminated composite with a yarn density of 30/10cm.The porosity,good thermal stability and mechanical properties of the jute fabric indicated that it was suitable for the preparation of core material of wood-based grid sandwich structure.Hot-alkali treatment can effectively remove most of the lignin and hemicellulose on jute fibers surface,and improve the crystal structure of jute fibers.When the NaOH concentration was 6%,the relative content of cellulose was the maximum,and the mechanical properties of laminated composites was the most excellent,its tensile strength,tensile modulus,flexural strength and flexural modulus were respectively increased by 37.5%,23.2%,72.3%and 72.2%compared with untreated jute fabric reinforced laminated composites.When the NaOH solution concentrations were 8%and 10%,the cellulose in the jute fiber began to be degraded.KH-560 silane coupling agent was successfully grafted onto the surface of jute fabric.FTIR detected that C-O-Si(1105cm-1)and Si-OH(986cm-1)functional groups were present in the modified jute fiber.It was observed by SEM that the surface of the modified jute fiber was wrapped with a thick layer of resin.The interface compatibility,crystallinity,and thermal stability were improved.When the modification time was 60 minutes,the crystallinity and grain size of the jute fiber were the largest,the mechanical properties of laminated composite materials were the best,and their tensile strength,tensile modulus,flexural strength,and flexural modulus were respectively 42.32%,39.74%,49.43%,and 49.45%higher than those of the untreated ones.Thermogravimetric analysis shown that the residual amount of the laminated composite was large and had good thermal stability,when the KH-560 modified times were 30min and 60min.Compared with the hot-alkali treatment,KH-560 had the best tensile properties of the laminated composite treated for 60 minutes,which meets the requirements of composite grid sandwich structure for tensile dominant lightweight materials.The silane coupling agent treatment does not produce waste liquid,which was more environmentally friendly,had no damage to jute fiber,and the treatment process was simpler and more effective.Therefore,KH-560 modification was applied to the preparation of the grid sandwich structures.KH-560 treated jute fabric/epoxy laminate composites wtth yarn densities of 50/10cm and 80/10cm were used as cores,and spruce wood was used as panel.The square sandwich structures with yarn densities of 50/10cm and 80/10cm had almost the same maximum load capacity,load-to-mass ratio and specific strength.The carrying capacity of the square core with jute fabric yarn density of 80/10cm was stronger than the core with yarn density of 50/10cm.The total energy absorption,specific energy absorption,average load and load efficiency of the core structure with a yarn density of 80/10cm were 20.3%,16.5%,23.4%,and 23%higher than those of the core structure with a yarn density of 50/10cm,respectively.The compression failure modes of the square grid sandwich structure were panel cracking,core buckling,core breaking,and panel breaking.The bending properties of three grid sandwich structures(SSS,DCSS and CSS)shown that KH-560 modification improved the interfacial compatibility and mechanical properties of laminated composite.The silicon content in the modified laminated composite was about 4 times than that of unmodifited laminated composite.The load-carrying capacity and energy absorption of the square sandwich long beam were the most excellent.The bending stiffness,shear stiffness,shear modulus and shear strength of the square sandwich beam(SSS)were 2.10×108N·mm2,1.97×105N,143.94MPa,0.948MPa respectively,which were significantly higher than that of double cross(DCSS)and cross sandwich beams(CSS).The load-to-mass ratio of DCSS short beams was similar to that of SSS long beams.The three failure modes of grid sandwich beams included core shear failure,overall shear failure,panel crushing,panel breakage,and core and panel separation. |
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