Preparation And Manufacturing Mechanism Of Reconsolidated Crop-Stalks

Crop straw is a reproducible resource,whose effective utilization has drawn more andmore attention worldwide.In order to produce high strength and environmental safereconsolidated timber which would be used as an alternative to wood,the five raw materialsused in this experiment were cotton-stalks,soybean-stalks,capsicum-stalks,tobacco-stalksand corn-stalks.The main research of the dissertation was conducted in the areas of raw materialcharacteristics,processing technologies,forming mechanisms and key technologies ofreconsolidated crop-stalks.First of all,the characteristics of five crop stalks were analyzed inthe paper through observation via optical microscopy,fiber isolation,analysis of chemicalcomponents and so on.Secondly,through single factor and orthogonal experiments,theprocessing technologies of reconsolidated crop-stalks and the methods of improving waterresistance of reconsolidated cotton-stalks were studied in the article after determining theoptimum softening process and scrimming technology.In the third place,the temperature ofthe center layer of mat was measured continuously and automatically by advanced instrumentin the hot-pressing process.Thermal conductivity,specific heat and thermal diffusivity ofreconsolidated crop-stalks were measured with non-steady state conditions.Under fivesuppositions,mathematic model of thermal transfer in the process of hot-pressing wasdeduced.Variation of cotton-stalks cells before and after the hot-pressing were observed withoptical microscopy.By means of fourier transform infrared spectroscopy(FTIR)and scanningelectron microscope(SEM),mechanisms of adhesive joints of reconsolidated cotton-stalksbonded with urea-formaldehyde(UF)resin were analyzed.Lastly,the process for industrialproduction of reconsolidated crop-stalks was given and scrimming equipment and layingequipment were studied in the dissertation.The main results of the dissertation are as follows:(1)Cotton-stalks,soybean-stalks,capsicum-stalks and tobacco-stalks are all consist ofxylem,phloem and pith.Corn-stalks is consist of epidermis,vascular bundles andthinner-walled cells.The chemical components of fiber of five crop stalks are similar to woodfiber.Five crop stalks can substitute for wood as raw materials of reconsolidated timber.Among five crop-stalks,cotton-stalks is an excellent raw material of reconsolidated timber. (2)The optimum scrimming technology under the current conditions is that cotton-stalks,soybean-stalks,capsicum-stalks and the solid part of tobacco-stalks,whose moisture contentare all 11%,are cooked 3 hours(90℃)and then crushed.Corn stalks and the hollow part oftobacco-stalks soaked 1～2 hours(20℃)and then crushed can also obtain the betterscrimming effect.(3)Manufacturing UF reconsolidated cotton-stalks with the size 350mm×350mm×10mm,the optimum technological parameters obtained under the lab conditions are:0.7g/cm~3density,150℃press temperature,12% UF and pressing time of 12min.The mechanicalproperties of the UF reconsolidated cotton-stalks can meet the requirements of industrialstandard(GB/T4897.2-2003)except for 2h thickness swell(2hTS).Under the sametechnological conditions,the mechanical properties of the UF reconsolidated soybean-stalksand capsicum-stalks are similar to reconsolidated cotton-stalks and the internal bond strengthand 2hTS of UF reconsolidated tobacco-stalks can not meet the requirements of industrialstandard.The bunch of corn stalks can not be bonded with UF.Under the same technologicalconditions,the mechanical properties of reconsolidated cotton-stalks,soybean-stalks,capsicum-stalks and tobacco-stalks bonded with phenol-formaldehyde(PF)resin can alsomeet or exceed the requirements of industrial standard except for PF reconsolidatedcorn-stalks.(4)As the increasing of paraffin latex consumption,2hTS of UF reconsolidatedcotton-stalks can reduce gradually on the condition of optimum technological parameters.Even if the value of paraffin latex consumption is up to 2.5%,the 2hTS of the panel can notreach to the standard value.When the temperature of hot-pressing reaches to 180℃or 200℃and add 1.5% paraffin latex,or the weight ratio of UF to PF is up to 4:2 in the mixture ofadhesive,2hTS of UF reconsolidated cotton-stalks can all meet the requirement of industrialstandard.(5)The curves of temperature changing of mat' center layer can be divided into threestages:the fast temperature rising stage in which the moisture in the mat is not vaporizing,theconstant temperature stage in which the moisture is vaporizing,and the slow temperaturerising stage in which the moisture vaporizing process is near to the end.During thehot-pressing process of reconsolidated cotton-stalks,glue spread rate,moisture content,hot-pressing temperature,density and thickness have an effect on velocity of elevatedtemperature of mat' center layer,the times of moisture vaporizing process and the times thattemperature of mat' center layer is up to glass transition temperature.(6)Moisture content and density are all significantly correlated with thermalconductivity and specific heat of reconsolidated cotton-stalks,and are not closely related with its thermal diffusivity.The thickness change no long has an impact on thermal properties ofreconsolidated cotton-stalks.(7)Mathematic model is established in the paper,which can basically reflect the rule forthe changes of mat' center layer temperature varying with time,and whose improvingthoughts are also presented.(8)Raw materials are compressed in radial and tangential section during the hot-pressingprocess.Hot-pressing technology makes the wood rays bent,makes the vessels crushed,andmakes the wood fibers compressed into irregular form.Compress is the main factorinfluencing deformation of tissue structure.There are some other factors,such as temperatureand moisture content.(9)The results of FTIR show that free hydroxyls form hydrogen-bonding duringhot-pressing of UF reconsolidated cotton-stalks,resulting in strengthened fiber self-bonding,that the position and shape of carbonyl' absorption peak of hemicellulose have changed afterhot-pressing which indicates that hemicellulose may take part in chemical reaction,and thatthe position of carbonyl' absorption peak of lignin has changed and the relative absorbingintensity of peak of benzene ring has reduced after hot-pressing which indicates that ligninmay take part in chemical reaction.(10)The result of SEM shows that there are adhesive joints of glue nail in the UFreconsolidated cotton-stalks.(11)The mechanism of adhesive joints of UF reconsolidated cotton-stalks are analyzedin the aspects of mechanical bonding,absorption theory,chemical bond,weak bonding layerand rheology.(12)Industrial production of reconsolidated crop-stalks is divided into six sections thatare pretreatment,scrimming,drying,glue blending and lay-up,hot pressing andafter-treatment.Technological process of industrial production is made.The technologicalplane layout chart of reconsolidated crop-stalks is also drawn.(13)The design principles and limited conditions of main parameters of scrimmingmachine and scrimming quality expressed by tearing coefficient are proposed.Assemblingdesign of scrimming machine has been finished.A new laying equipment is developed,whichcan realize mechanized mat forming.