| The mechanical properties of wood plastic composites(WPCs)were directly affected by drying quality of wood fibers,however,there is no good drying method of wood fibers for wood plastic composites with account of high energy consumption,low efficiency and poor fibers dispersity of existing drying equipment.The impulse-cyclone airflow drying equipment was be designed and developed according to the demand of the uniformity of the moisture content,the fiber classification and the low energy consumption.The effect of drying conditions on the final moisture content of impulse-cyclone airflow was discussed,as well as fiber quality,chemical composition,crystal form and thermal stability were studied by using the(FTIR),X-ray diffraction(XRD)and thermogravimetric analyzer(TGA).The results of numerical simulation by Fluent software were used to verify the movement law of wood fibers in the drying process.The effects of different drying methods on the mechanical properties,hygroscopic desorption characteristics and wettability of wood plastic composites were analyzed compared.The main contents and results of this study are as follows:(1)Drawn from Drying method of the pharmaceutical and food industries,the impulse-cyclone airflow drying equipment,with medium temperature 160-240?,feed rate of 90?150kg/h,and the wind speed 9m/s-13m/s,was designed for wood fibers in WPCs industry.The evaluation of drying effect was carried out to explore the feasibility of impulse-cyclone airflow drying device and drying process for wood fibers.The results showed that good the dispersion and the final moisture content with the coefficient of variation was 0.0798,which reached the requirements of fiber preparation for WPCs.(2)The drying mechanism of wood fiber was studied by impulse-cyclone airflow drying system.Orthogonal test and single factor test method were used to reveal the mechanism of wood fiber impulse-cyclone airflow drying at high temperature.The results showed that the treatment temperature was in the range of 200?240?,the air velocity was 11m/s and the feed rate was 90kg/h,which can effectively control the final water content of wood fiber.When the water content was large,the process parameters can be regulated by reducing the airflow velocity and feed rate.The higher feed rate and air velocity were,the higher the final water content was and the lower precipitation rate was.The airflow temperature or fiber morphology was increasing,the final water content was a downward trend,the precipitation rate was on the rise.Wood fibers with high temperature drying process is high temperature modification of wood fiber.High temperature modification of wood fibers can improve the interfacial compatibility of wood fibers and plastic substrate,wood plastic composite mechanical properties and thermal stability.The results showed that hydroxyl and carbonyl absorption peak intensity of poplar fibers changed obviously with impulse-cyclone airflow drying of poplar wood fibers.The higher the temperature,the lower the wind speed and fiber geometry,the greater hydroxyl and carbonyl absorption peak intensity changed.High temperature drying conditions can also influence relative crystallinity of the wood fibers obviously.When the drying temperature was 160 ?,200 ? or 240 ?,the relative ciystallinity was 50.79%,66.18%or 60.92%and relative ciystallinity of wood fibers with 200? drying temperature increased by 25.42%,compared with 103? oven drying method.With the increase of wind speed and feed rate,relative ciystallinity of wood fibers first increased and then decreased.High temperature drying process can improve the initial thermal solution temperature and improved the wood fiber thermal stability.Under the high temperature and high humidity environment,wood fiber holocellulose reduced and acidity increased.(3)Through gas solid two phase flow analysis and numerical simulation of the impulse-cyclone airflow drying,the Euler-Lagrange model and computational fluid dynamics software Fluent were applied to simulate the gas-solid two-phase flow field,which revealed the mechanism of drying and heat and mass transfer properties.Results showed that airflow velocity changed obviously and 2m/s velocity difference between fibers and airflow showed in the simulation which was advantageous to the heat and mass transfer.The increase of feed speed led to a backflow area at the bottom of cyclone dryer,which easy to cause reflux of the material and long residence time of the wood fibers in the dryer.Through to calculate the curve of the temperature,speed,time compared with the measured values,the maximum relative error between simulation and test was less than 10%,which verified gas-solid two phase flow method of Fluent to establish the mathematical model of impulse-cyclone airflow drying was effective.(4)The effects of different drying methods and different coupling agents on the properties of wood-plastic composites were investigated.The effects of different drying methods on the properties of wood-modification mechanism of performance of Wood-Plastic Composites by high temperature drying.The results show that the impulse-cyclone high temperature heat treatment/A187 silane coupling agent co-modification of wood fibers showed the best mechanical properties of WPCs.The flexural strength,the elastic modulus,the tensile strength and impact strength were improved by 24.4%,29.4%,23.8%and 35.6%respectly.With the increase of high temperature heat treatment,the impact strength,flexural strength and elastic modulus increase first and then decrease.When the wind speed was more than 10m/s,mechanical properties were higher than the contrast plot.Dynamic thermal mechanical experimental analysis showed that the temperature of 220 ?drying condition of high temperature heat treatment/A187 silane coupling agent co-modification of WPCs showed the highest energy storage modulus and loss modulus,which verified wood fiber and plastic composite modification of interface bonding ability was increased.Hygroscopic property tests showed that with the increase of heat treatment temperature,moisture sorption isotherm showed a trend of "S" shape curve along the direction of the moisture absorption and desorption and equilibrium moisture content decreased.Static contact angle test shows that after 220 ? high temperature heat treatment of wood fiber/A187 composite co-modification of WPCs static contact Angle was 98 °,which increased 21.7 ° compared with using A187 alone of WPCs,enhancing the hydrophobicity of wood plastic composite material.Through the microstructure observation,dispersion of wood fiber after high temperature treatment in the plastic substrate was better.Therefore,the impulse cyclone airflow drying method which has a good application prospect is a kind of high temperature drying and thermal modification method in the field of the preparation of WPCs... |
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