| China has abundant resource of corn stalk, but the stalk utilization has low added-value and outstanding problems of combustion and idleness. Corn stalk has different components whose chemical constituents vary, among which corn stalk rind, as one of the most constituents, consists of rich lignin and cellulose similar to wood properties, has high mechanical strength and can be used as raw material of stalk artificial board and paper. Utilization of agro residues as a raw material for making composite board is beneficial to booming rural economy, protecting forest and constructing ecological agriculture. Corn stalk rind separated by the rindpith-leaf separation equipment, as the major component of corn stalk, can be used as raw material for producing composite board because of its high mechanical strength. Corn is a tall plant of gramineae that has multiple stem nodes and its stalk epidermis is a heterogeneous material with apparent anisotropy and variability. There are large differences considering the mechanical property, fiber morphology and chemical compositions in different stem nodes of corn stalk rind. Therefore, the initial objective of this study was to provide a theoretical basis and technological support for the technology of intact corn stalk rind board production and the improvement of physical and mechanical properties of the board, the main research contents and results were as following:I. Research of the biological and mechanical properties of corn stalk rind. The corn stalk rind consisted of epidermis and sarciniform tissue, the epidermis was a layer of the wax coat, and sarciniform tissue was made of fibrocytes and fiber bundles bonded by lignin, and the epidermis was around 0.04 mm in thickness and mainly consisted of Si O2. Corn stalk rind consisted primarily of lignin, cellulose and hemicellulose and had properties similar to those of wood. The corn stalk rind mainly contained C, O, Si, and K, as well as small cmounts of other elements including Mg, Cl, Ca, K, Fe, Cu, and Zn. Compared with wood, corn stalk rind got shorter fiber length with 1,870 ?m mean, fiber width was of 31.01 ?m mean, fiber wall thickness was of 4.59 ?m mean, fiber cavity diameter was of 6.55 ?m mean.II.Research of the mechanical properties of corn stalk rind.The effect of the experimental factors on the maximum compression force of corn stalk rind was obtained by the compression experiments. The ANOVA results showed that the sampling height, hot-pressing temperature, loading speed, and feeding quantity were all signficently effect on the maximum force of corn stalk rind. By a frequency analysis of the experimental results, the optimization parameters were as following: sampling height of the bottom(the third section of corn stalk), hot-pressing temperature at 165 to 185?, loading speed of 2.0 to 2.5 mm/min, and feeding quantity of 78 to 82 g. Based on the optimizition parameters, the maximum compression force was bellow 100.9 k N at the condition of the compression density was 0.610 g/cm3. The corn stalk rind had three stages of being loose, transitional and tight during the process of hot-pressing. At being loose stage, its compressibility was better, the compressibility began to decrease gradually at the transitional stage, and decreased sharply at the stage of being tight. The recent researches related to mechanical property mostly start with crop physiological characteristics, stalk mechanical model, stalk mechanical property index and equipment performance parameters, but make little analysis of internal factors of corn stalk rind like microstructure and fiber morphology. Taking full consideration of the impact of variety, growing environment and various conditions on corn stalk constituent, this paper had adopted the corn stalk rind of the same variety under the same planting condition to analyze fiber morphology characteristic distribution rule among different sampling heights and the changes of fiber morphology parameters, the tensile and shear strengths of different sampling heights were tested, the correlation between mechanical properties and fiber morphology parameters of corn stalk rind was analyzed, and the internal mechanism of crop stalk mechanical property was researched from the microscopic view to offer theoretical basis for physicochemical property research and deep processing and utilization of corn stalk rind. Nitric acid technique was adopted to obtain fiber and morphology parameters of corn stalk rind at different heights, meanwhile tensile and shear strength were tested, and grey correlation method was applied to analyze the correlation between fiber morphology parameters(like fiber length-width ratio and wall-cavity ratio) and mechanical properties of corn stalk rind. Through the experiments, it was confirmed that there was significant difference between fiber morphology parameters and mechanical properties of corn stalk rind at different sampling heights(p<0.01); with the increase of sampling heights, radial tensile and axial shear strength reduced gradually while axial tensile strength firstly increased and then reduced gradually. There was correlation between tensile and shear property and fiber morphology of corn stalk rind, between radial tensile strength and fiber wall-cavity ratio(r=0.9972), between axial tensile strength and length-width ratio(r=0.8885) and wall-cavity ratio(r=0.6648), between axial shear strength and fiber wall-cavity ratio(r=0.9643) of the fiber.III.Research of the surface wettability, thermal stability and gluing properties of corn stalk rind. The corn stalk rind had significant differences in fiber morphology, chemical composition, and mechanical properties at different heights. Nano-Si O2 composite material contained in the epidermis of corn stalk rind was not conducive to exert adhesive into a board. To study board-making technology by intact corn stalk rind, the wettability of corn stalk rind at different sampling heights was necessary to be analyzed by keeping or removing the epidermis. To analyze the surface wettability difference, the contact angle with water, element compositions and the chemical compositions of corn stalk rind at different sampling heights were studied before and after removing epidermis. A Fourier transform infrared(FTIR) analysis was performed. The results showed that the removal of the epidermis could significantly improved the hydrophilicity of corn stalk rind. Before and after removal of the epidermis, the surface wettability and chemical compositions of corn stalk rind differ greatly at different sampling heights. Before removing the epidermis, the contact angle of corn stalk rind with water and the mass fractions of lignin and cellulose reduced with the increase of the sampling height, while the mass fraction of hemicellulose increased. After removing the epidermis, as the sampling height increased, the contact angle of corn stalk rind with water increased, the mass fractions of lignin and cellulose decreased, and the mass fraction of hemicellulose increased. Removing the epidermis improved the surface wettability of corn stalk rind. Before removing the epidermis, the wettability was mainly influenced by the variation of mass fractions of C and Si, and the existence of hydrophobic materials including fat and silicide inhibit the hydrophilicity of corn stalk rind. After removing the epidermis, the mass fractions of Si and extracts reduced, and the mass fraction of hemicellulose was the dominant factor influencing the surface wettability of corn stalk rind. The gluing properties and the mechanism of gluing between intact corn stalk rind and adhesive were analysised by using the STA-IR-GC-MS technology, the scanning electron microscopy(SEM) and the Fourier transform infrared spectroscopy(FTIR). The analysis results of thermal stability of corn stalk rind before and after gluing showed that the hot-pressing temperature at below 200? was benefit to decrease the thermal decomposition of hemicellulose and adhesive; compared with corn stalk rind before gluing and the epidermis, the corn stalk rind after gluing had good thermal stability; the temperature of 30? to 200?, the weightlessness rate of corn stalk rind before gluing was 6.490%, and the weightlessness rate of corn stalk rind after gluing was 2.416%; the temperature of 200? to 400?, the weightlessness rate of corn stalk rind after gluing was increased greatly and the two large weightlessness points were at 299? and 344?; pyrolysis composition of corn stalk rind after gluing at 299? and 344? mainly CO2, CO, and H2 O, as well as some organic matter of Squalene, Acetic acid, Furfural, Bran ester, and Phenol. Seen from the microscopic observation, the adhesive was filling in the gap of corn stalk rind surface and formed plastic nails; the air and moisure in the layers of the composite board from corn stalk rind caused stratification of the board.IV. Research of preparation of corn stalk rind and removal of its epidermis. The experimental study of preparation of the intact corn stalk rind was carried out and the separation of rind, pith and leaf by using a rind-pith-leaf separation equipment was discribed. Under the conditions of rotational speed was 1080 r/min, clearance was 1.8 mm, and hellx angle was 40°, the leaf-stripping percent was more than 97%, the pith-stripping percent was more than 95%, and the integrity rate of rind was more than 95%. To remove the epidermis of the intact corn stalk rind, the experiments of mechanical grinding and bathing with Na OH solution were carried out. The result showed that the moisture content of corn stalk rind effect on the removel of the epidermis by mechanical grinding, reducing the moisture content could improve the removal of the epidermis; the epidermis dissoluted by Na OH quickly, and the mechanical properties could be improved by bathing with Na OH solution; the Na OH concentration, bathing temperature, and bathing time effect on the removal rate of Si; at the condition of Na OH solution was 9%, the bathing temperature at 80?, the bathing time was 90 min, the removal rate of Si was 95.16%, the mean value of axial tensile strength of corn stalk rind after bathing was 91.47±3.15 MPa.V. Manufacturing technology and performance analysis of laminated veneer lumber and there-level composite board from intact corn stalk rinds. The intact corn stalk rinds after removal of the epidermis in different heights and the polymeric diphenylmethane isocyanate(p MDI) were used as the material, and composite boards from the intact corn stalk rinds were produced by a new technology for the laminated veneer lumbers(LVL) in this study. The influences of the sampling height, the mass fraction of adhesive, the hot-pressing temperature, and the hot-pressing time on the physical and mechanical properties of the LVL were investigated. The following physical and mechanical properties were measured according to GB/T17657-2013: modulus of rupture(MOR), modulus of elasticity(MOE), internal bond strength(IB), thickness swelling in two hours(TS2h), and water absorption in two hours(WA2h). The results showed that the mechanical properties of the LVL at the root and the ear part were better than at top part except the internal bond strength; With an increase of the mass fraction of adhesive, the mechanical properties of the LVL such as the modulus of rupture, the modulus of elasticity, and the internal bond strength increased significantly(p<0.01), while the physical properties such as the thickness swelling in two hours and water absorption in two hours were decreased; The mechanical properties of the LVL were better when the hotpressing temperature at 150? and hot-pressing time of 6 min, but the physical properties were increased with the hot-pressing time increasing. Under the optimized conditions that the sampling height at root, the mass fraction of adhesive of 12%, the hot-pressing temperature at 150?, and the hot-pressing time of 6 min,the mechanical properties of the LVL could meet the national standard requirements for flooring fiberboard belonging to ordinary type and the LVL that the modulus of elasticity was 6000 MPa. In order to improve the adhesive properties of the corn stalk rind and the performance of the board, the epidermis removed from the intact corn stalk rind were used on the surface layer of the composite board, the intact corn stalk rinds after removal of the epidermis were used in the core layer, and the polymeric diphenylmethane isocyanate(p MDI) were used as adhesives for manufacturing the three-layer composite board. Effects of sampling height of corn stalk rind(the first, the third, the fifth, the seventh, and the ninth section along the growing direction of the corn stalk), surface layer content(0%, 4%, 8%, 12%, and 16%), hot-pressing temperature(110?, 130?, 150?, 170?, and 190?) and hot-pressing time(3 min, 6 min, 9 min, 12 min, and 15 min) on the mechanical and physical properties of the composite board from the corn stalk rind were investigated by single factor experiments and orthogonal experiments. The mechanical and physical properties of composite boards in different type including the modulus of rupture(MOR), the modulus of elasticity(MOE), the internal bond strength(IB), the thickness swelling(TS) in two hours, and the water absorption(WA) in two hours were tested according to GB/T4897.3-2003 and GB/T17657-2013 standard. The results showed that the mechanical and physical properties of the composite board at the ear and the following part were better than at top part except the internal bond strength; utilization of the epidermis as the surface layer could significantly improved the physical properties of the composite board. The optimal parameters were the following: the sampling height was the bottom part(the third section) of the corn stalk, the surface layer content was 12%, the hot-pressing temperature was 150?, and the hot-pressing time was 6 min. Under the conditions,the mechanical and physical properties of the composite board could meet the national standard requirements for particleboard.These findings provided theoretical basis and technological support for the board-making process by intact corn stalk rinds and the improvement of physical and mechanical properties of the board. |
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