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Study on Micro Mechanical Properties and Interface Gluing Roperties of Silvergrass

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Tutor: DengYuHe
School: Nanjing Forestry University
Course: Wood Science and Technology
Keywords: silvergrass,micro mechanical properties,wettability,waterproof agent,interfacegl
CLC: TS653
Type: PhD thesis
Year:  2013
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Silvergrass (Miscanthus sacchariflorus, Family: Poaceae) is a tall, upright perennial grasswith high biomass. The purpose of this study is using silvergrass on wood-based panel industry,and realizing the effective development and utilization of silvergrass resource. The researchstudies micro mechanical properties of silvergrass firstly, including cell size, microstructure, andmechanical properties of cell wall, and building the relationship between micro and macroproperties. And then discussing the silvergrass stalk wettablity, material character and stalksilicon content, evaluating the feasibility of silica used as a waterproof agent in silvergrassparticleboard manufacture. On this basis, the model of thickness after absorbing water andtechnology and performance of silvergrass particleboard were studied. Finally, the stress andstain of bonding interface of silvergrass stalk and adhesive, and the bonding mechanism werediscussed. Research contents and the results were as following:1. Analysing the macroscopic morphology of silvegrass, and the microscopic morphology, tissuestructure and the cell types of cross section and out surface of stalk were discussed.The results show the lower silvergrass stalk was hollow, and the upper part was solid.Silvergrass stalk constituted epidermal tissue, parenchyma cells, vascular bundle and fiber texture.Upper stalk contained pith tissue, and it was made up of parenchyma cells. Epidermal tissueincluded long cells, suberin cells, silicon cells and pores. The waxy layer covered on epidermaltissue prevented adhesive from wetting and permeating.2. Silvergrass cell morphology parameter was determined, including diameter of fiber cells andparenchyma cells, cavity diameter of fiber cells and parenchyma cells, and the ratio of wall andcavity.The result displays silvergrass stalk mainly included fiber cells and parenchyma cells.Diameter of fiber cells was about8.62-23.60¦Ìm, diameter of parenchyma cells was33.05-66.65¦Ìm. The ratio of wall and cavity of fiber cells was2.94-3.44, the strength andstiffness of fiber cell was high; the ratio of wall and cavity of parenchyma cells was0.29-0.34,less than1.3. Microstructure and mechanical properties of silvergrass stalk were determined byNanoindentation, and the relationship between micro and macro properties was built.The fiber cell is an irregular polygonal with a diameter of5-20¦Ìm, the cell wall wasmultilayer feature. Nanoindentation hardness values of the fiber cell wall in lower and upperstalks were20.1and22.5GPa, and elastic modulus values were0.53and0.58GPa, respectively.The microfibril angle of lower and upper stalks were18.6¡ãand16.5¡ã. Macrohardness value of silvergrass stalk was0.330GPa evaluated from nanoindentation tests.Based on the properties determined, silvergrass is a good potential raw material for particleboardmanufacturing.4. Contact angle of adhesives on silvegrass stalk was measured to fit the model of dynamicwettablity, and the wettablity of the adhesives on silvergrass stalk was compared The surface freeenergy of silvergrass stalk was also measured.The result displays the model of dynamic wettablity could simulate the effect of contactangle on time exactly, the order of wettablity of adhesives on silvergrass stalk was MUF<UF<PF,mean free energy of silvergrass stalk was47.49mJ/m2, which was close to that of wood,andhigh than that of wheat straw.5. The pH and buffering capacity of silvergrass were determined, surface wettability wascompared with poplar and wheat straw. The surface and average silicon contents of silvergrassstalk were measured by XPS and EDAX respectively. Evaluating the feasibility of silica used as awaterproof agent in silvergrass particleboard manufacture.Results show silvergrass was weak acidic, and was favorable for UF curing. Acid bufferingcapacity of silvergrass was lower than poplar; alkali buffering capacity was close to that of poplar.The internal surface of the silvergrass stalk had better wettability than poplar. The main chemicalcompositions of silvergrass stalk internal surface were cellulose and hemi cellulose, andcompositions of external surface were hydrocarbon. Silicon content of internal surface wasalmost the same and that of external surface decreased from lower to upper stalk. Silicon occursin silvergrass stalk as compound silica, and silica content was3.49%. Silica could be used as awaterproof agent for silvergrass particleboard.6. Study the variation of silvergrass particleboard thickness after soaking in water for100h. Themodel of thickness variation was established, and the law of thickness varied as time wasconfirmed.The result shows the swell of silvergrass particleboard can be divided into elasticdeformation and viscosity deformation. Model of thickness swelling of silvergrass particleboardcan be expressed asTS(t) a be t. The max thickness swelling of silvergrass particleboardincreased as the density increasing in time0-100h.7. Density and adhesive content were key technological parameters, the effect of density andadhesive content on mechanical properties of silvergrass particleboard was studied. The physicaland mechanical properties of boards with wax and without it were also compared to analyzewaterproof function of silica.The result of the study indicates that the theory of silica used as waterproof agent ofsilvergrass particleboard was feasible. The silvergrass particleboard could be manufacturedwithout wax, at the14%of adhesive content, and density was higher than0.70g/cm3.8. The strain distribution of silvergrass stalk glue interface was measured by digital specklecorrelation method (DSCM) at loading, comparing the performances of stalk glued by different adhesives (UF, MUF and PF).The result shows the max shear strain occurs at the two ends of glue interface when loading,and two ends of glue interface were fractured firstly. When glue interface approach damage, themax shear strain throughout the glue interface. Shear strain appears a gradient decreasing fromglue interface to stalks at both sides. The law of max shear strain changes as loading was maxshear strain increased linearly, before damage, and increased suddenly approaching damage. Theorder of modulus of elasticity of three adhesives was UF<MUF<PF. The stalk glued by MUFneed the largest load to be damaged, and also had the max deformation.9. The reaction mechanism of adhesive and the surfaces of silvergrass stalk were studied byATR-FTIR.The result displays that after silvergrass stalk was glued by UF and PF, cellulose andhemicellulose of internal surface of stalk had chemical reaction with UF and PF, but the ligninnot. The external surface had wax coat and didn¡¯t react with UF and PF, only had a littledegradation at high temperature. After silvergrass stalk was glued by MUF, not only celluloseand hemicellulose of internal surface of stalk had chemical reaction with MUF, but also externalsurface of stalk reacted with MUF to a certain extent.
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