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Study on Processing Techniques and Mechanism of Straw/wood MDF with the Compound Fiber Refined Toget

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Tutor: ZhouDingGuo
School: Nanjing Forestry University
Course: Wood Science and Technology
Keywords: straw/wood MDF,refine together,refining disc,fiber quality,mechanical property
CLC: TS653.5
Type: PhD thesis
Year:  2013
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Abstract:
China has been the world¡¯s largest man-made panel producer, consumer and exporter. Thevolume of production has reached209million m3, in2011. The shortage of raw materialsrestricted the development of the industry, as the production increased year by year. As a bigagricultural country, the agricultural residues such as plant straw are extremely rich. Whilemore than30%of the straw has been burned in the farm, which caused serious environmentalpollution and traffic problems. It is urgent to find more ways out for the straw.Straw/wood MDF is an effective way to reduce the raw material problems of MDF factories; isa good method to rational utilize the agricultural residues; also is a good way to improve thefarmers¡¯ income and relieve the ¡°Three Rural Issues¡± problems. In study, the raw materialcontrol, production process adjustment, disc segment profile optimization, etc. of theStraw/wood MDF production are analyzied, by theoretical analysis, laboratory test, pilotproduction test. The conclusions are in the following:(1) Straw/wood raw material refine together is feasible in theory. We discovered the straw fiberis shorter and thinner, higher content of parenchyma, lower content of lignin, than wood fiber,by comparing the fiber characters of wood and straw. Therefore, the softening requirements ofcooking time and temperature for straw are lower than wood. During refining, because of thesmaller fibre diameter,the compressive stress and shear stress of straw fiber are smaller thanwood fiber, according to the research results of Kerekes, Martinez, Batchelor. This happen tomeet the two kinds of material¡¯s different separating power need. The straw¡¯s frictioncoefficient with the segment is lower than wood, because of the wax on the straw¡¯s surface andthe thiner and shorter fiber character, it is necessary to reduce the refining gap to improve thefiber quality, when the straw raw material content raise. From the panel¡¯s structure perspective,the soft wood with longer fiber length is more suitable than hard wood for refine together withstraw which has shorter and thinner fiber.(2) The panels made from the straw/wood fibers separated together performed better thanstraw/wood fibers isolated separated. Refine straw/wood raw material together has threeadvantages:1) improve the mixing uniformity of the two kinds of fiber and reduce the bondingdisadvantages of wax and silicone from the straw;2) effectively reduce the fiber powder byavoiding the segment excessively cutting straw fibers.(3)The segment of refining disc should be adjusted according to the content of straw.Considering straw fiber¡¯s shorter and thiner character, refining disc separating straw/wood fiberis necessary to reduce the segment width, depth, increase the segment angle, compared withcommon wood chips¡¯ separating. For fibers¡¯ good separation, the number of resist segmentsand its¡¯ location should be reasonable arranged to control the fibers¡¯ refining time.(4) Adjust the refining techniques, control fibers¡¯quality. In order to control reasonalbe contentsof stem fiber, qualified fiber and powder fiber in the compound fiber, it is necessary to narrow the disc gap, lower cooking preasure and cooking time when the straw raw material contentraising. The coordination of the three factors(disc gap, cooking preasure,cooking time) shouldbe based on the straw/wood raw material ratio, raw materials¡¯ moisture content and the targetfiber morphology, etc.(5) Control panel-making process, improve mechanical properties of panels. Themechanical properties of composites decreased when increasing straw content.The fiber lengthis positive related to the panel¡¯s MOE, MOR, nail holding power; The glue usage is positiverelated to the panel¡¯s IB and nail holding power. The panel density is positive related to everymechanical property of the panel. They all have a liner relationship between the factors.Therefore, it is crucial to set a reasonable cooking process, panel density and gule usageaccording to the straw/wood raw material ratio and panel¡¯s target mechanical properties.Considering the differences in cooking process and bonding performance between straw andwood, the straw raw material content should be around30%, in order to develop thestraw/wood MDF technique in existed wood MDF production line without additionalequipment cost and complicated process adjustment.(6) Hot-pressing process control of straw/wood MDF. The panel¡¯s mechanical properties can beimproved by adjust the hot-pressing process. The raw material should be dominated by wood orby straw, avoid equal content, that will be benefit to work out the reasonable hot pressingprocess. The panel¡¯s MOR and MOE can be raised by properly increasing surface moisturecontent and lowering core layers¡¯ moisture content of the slab. The panel¡¯s surface density canbe raised by increasing machine¡¯s closing speed to improve the panel¡¯s tensile and compressiveproperties, base on proper slab moisture content. To prevent panel¡¯s surface excessive hotdestruction, improve the heat conduction efficiency, ensure good bond between fibers, we canproperly lower the hot-pressing temperature increase the glue usage when the straw rawmaterial ratio increase. According to the experiment experience, hot-pressing temperatureshould not exceed180¡æ, the glue usage should control within12%. Increase panel¡¯s densitycan benefit mechanical properties, while the density increase may result in an increase ofpanel¡¯s internal steam pressure. So when the panel density increased, the slab¡¯s moisturecontent should be properly controlled to prevent the formation of defects such as delaminationand bubbling.In the laboratory and pilot plant, this study proved the feasibility in making straw/wood MDFwith the compound fiber refined together. In the further study the raw material cost control, thefactory production process, the panel¡¯s mechanical properties improvement, the productapplication can be deeply researched in order to realize straw/wood MDF industrialization innear future.
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