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Study on Heat-conduction of Fiberboard and Plywood in the Hot-pressing Process

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Tutor: ZhangYang
School: Nanjing Forestry University
Course: Wood Science and Technology
Keywords: fiberboard,plywood,hot-pressing,heat-conduction,thermal thermal physicalproperit
CLC: TS653
Type: PhD thesis
Year:  2013
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Abstract:
Hot pressing is an important process in wood-based panel production that affects both thequality and the output of the products. Many factors influence heat and mass transfer during hotpressing, including temperature, resin content, target density, mat thickness and raw materialproperties like thermal physical properties, moisture content and mat structure. In this paper,heat transfer during the hot pressing of Poplar plywood bonded with UF and soybean proteinresin and fiberboard prepared by Poplar and pennisetum was investigated by means ofadvanced online temperature test technology. The research was mainly focused on the effect oftarget density, initial moisture content, resin content and mat thickness on the heat transferprocess, and results as follows were obtained:The hot pressing of poplar fiberboard and pennisetum fiberboard can be divided into fourdistinct phases: short constant temperature phase at the beginning of hot pressing, fast heatingphase, constant temperature phase due to core layer water evaporation and slow heating phase.This process was influenced by hot pressing temperature, mat density, panel moisture contentand resin content, among which, the influence of hot pressing temperature was pronounced. Atthe same temperature, the thermal conductivity of poplar fiberboard was a little bit better thanthat of pennisetum fiberboard. The core-layer heating rate increased significantly with the riseof temperature. Panel density had an influence on the heat conduction during the hot pressing.Both the time needed to evaporate the core layer moisture and the time of constant temperaturephase due to core layer water evaporation increased with the increase of panel density. At theslow heating phase, the core layer heating rate was lowered with the increase of the density.The resin content had an influence on fast heating phase and constant temperature phase due tocore layer water evaporation. When the resin content increased, the core layer heating rate atthe fast heating phase was lowered, while the water evaporation time was prolonged. At theslow heating phase, the resin content exerted a limited influence on the heat conduction. Themoisture content had an impact on the time of fast heating phase and constant temperaturephase due to water evaporation. The time increased with the rise of the moisture content. At theslow heating phase, the moisture content had little impact on the core layer heating rate.The hot pressing of poplar plywood bonded with UF resin can be divided into three phases:short constant temperature phase, fast heating phase and slow heating phase. The thickness ofthe slab has a very significant impact on heat transfer process of the hot pressing. The core layerheating rate was lowered with the increase of the thickness during the hot pressing, while theheating time became longer. The hot pressing temperature has a significant impact on heattransfer process. The core layer heating rate increased with the rise of the temperature. Duringthe fast heating phase the heating rate of the core layer increased with the increase of moisturecontent while the slow heating phase did not show any significant impact of moisture contenton the corresponding heating rate. The impact of resin content on the hot-pressing heat transferprocess was not significant.There exists difference between hot-pressing process of the poplar plywood bonded withsoybean protein adhesive and the conventional plywood. For the former plywood, prepressing is needed before hot-pressing. The heat transfer process of prepressing can be divided into fastheating phase and constant temperature phase. The time of prepressing was effectedsignificantly by the moisture content of the mat, which was significantly prolonged with theincrease of the moisture content. The time of prepressing was also affected by the resin content.The heat conduction during the process of hot pressing after prepressing could be divided tofast heating phase and slow heating phase. The heat transfer during hot-pressing of the poplarplywood bonded with soybean protein adhesive was greatly influence by hot-pressingtemperature. The core layer heating rate was lowered with the increase of the resin contentduring the hot pressing, while the heating time increased with the rise of resin content. Themoisture content had no effect on the heat conduction during the process of hot pressing.A theoretical model for heat transfer in poplar plywood bonded with UF resin wasdeveloped.It can be used to simulate and predict the temperature change during hot-pressingprocess in different conditions. The simulation results were consistent with experimental data.The model showed a guiding value for the manufacturing practice of plywood and LVL.Chemical and instrument analyses were performed to research the thermal chemicalcharacteristics of the material. The results showed that poplar had similar thermal chemicalcharacteristics with pennisetum. The chemical structure was intact if the treatment temperaturewas lower than200¡æ.No mechanical property change of the material was found in thiscondition. Pyrolysis process of pennisetum mainly occurred at220-500¡æ.Among the reactionproducts were carbon monoxide, carbon dioxide, paraffin, acid and aldehyde. No more qualitychanged of the samples were found after the temperature climbed to500¡æ. Pyrolysis processof poplar mainly occurred at225-500¡æ,and the reaction products were similar with those ofpennisetum.In order to investigate the thermal physical properties of plywood and fiberboard, a quasisteady method was applied. The results showed that the panel density had a significant impacton the thermal physical properties of the fiberboard. The thermal conductivity of poplarfiberboard was higher than that of pennisetum fiberboard with the same density. The thermalconductivity of the fiberboard mat increased linearly with the increase of density. Specific heatof fiberboard was not significantly affected by density. The thermal diffusivity of the fiberboardmat decreased with the increase of density. The initial moisture content of panel had asignificant impact on the thermal physical properties of fiberboard and plywood. With the riseof moisture content, the thermal conductivity and specific heat of the fiberboard and plywoodmat increased significantly. Obvious linear relationship was found between the thermalconductivity and the moisture content of the mat. The impact of moisture content on the thermaldiffusivity of the fiberboard and plywood was not significant.
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