The Research On Characteristics And Mechanism Of Poplar Wood During High-intensity Microwave Pretreatment

High-intensity microwave pretreatment(HIMP)is a kind of wood modification technology with great potential.A series of research works have been conducted,which mainly focused on the study of microwave pretreatment process and physical and mechanical properties of wood pretreated by microwave.However,research on the mechanism of wood microwave pretreatment has seldom been systematic studied.In this study,the poplar wood(Populus deltoids cv.I-69/55)was used as the research object.The dielectric properties of wood over an ultra-wide frequency range were systematically studied.The response characteristics of HIMP on the the variation characteristics of temperature-humidity and macro-micro structure of wood were researched.The heat transfer model of wood with non-uniform moisture distribution and the prediction model during HIMP were developed.The cell expansion and critical conditions concerning pressure and temperature were simulated during microwave pretreatment.The mechanism of HIMP of poplar wood was revealed,which provides scientific basis and theoretical support for the follow-up research on the high-intensity microwave modification of poplar wood and manufacturing technology of high-valueadded wood-based composites.The main conclusions were showed as the following:(1)The dielectric properties of poplar wood have been studied over an ultra-wide range,the quantitative mathematical model of dielectric properties and influence factors was established and the thickness of wood in the microwave pretreatment has been confirmed.The dielectric constant and loss factor increased with increasing initial moisture content(MC)and temperature,and decreased slowly as the frequency increased.When the moisture content increased from 0% to 98.91%,the dielectric constant of wood increased by 820.22% and the loss factor increased by 8630.79% to the maximum.The dielectric properties of wood were anisotropic.The longitudinal dielectric properties were higher than those found at the transverse or radial directions,the latter two being similar to one another.The dielectric properties were mainly correlated with the MC,followed by temperature and grain direction.The squares of regression coefficients were above 0.90,which could appropriately describe the relations between the dielectric properties and the MC.If poplar wood with high MC(about 100%)was pretreated with microwave,the wood thickness should be controlled within 12 cm(915 MHz)and 4 cm(2450 MHz)respectively.(2)The effect of microwave pretreatment conditions on temperature change features and MC variation features of poplar wood has been revealed.The microwave field strength inside wood during HIMP was calculated.HIMP can rapidly increase the temperature of wood,and the maximum heating rate can reach 3.03 ? /s The temperature distribution inside wood is uneven and complicated during HIMP and there is not regular temperature gradient along the direction of wood thickness.The final temperature and the heating rate of wood increased with the increasing microwave power.The longer the microwave time,the longer the constant temperature period,the higher the final temperature.The temperature distribution inside wood with lower initial MC and greater thickness was more uniform.HIMP significantly reduced the MC of poplar wood,per kilowatt microwave energy could remove 0.4?1.1 kg water from wood.The highest rate of water loss was 1.22%/s.The water extraction rate increased with increasing microwave time and increased with decreasing initial MC.The microwave field strength inside wood increased with increasing temperature and decreased with increased MC.(3)The effect of HIMP conditions on macro and micro structures of poplar wood was studied,and the mechanism of crack generation during microwave pretreatment was revealed.Obvious cracks appeared on the cross section of wood after microwave pretreatment,and the cracks mainly distributed radially along the direction of wood rays with different crack lengths and widths.The cracks mainly occurred in the intercellular layer between wood fibers,vessel and ray parenchyma,wood fiber and ray parenchyma,and also occurred in the pits on cell wall of vessels and wood fibers.Increasing the microwave power,prolonging the microwave time and increasing the frequency of radiation are effective measures to improve the uniformity of crack distribution of microwave pretreatment wood.The cracks position was related to the chemical composition and the microfibril angle of different cell wall layers.Because the middle lamella has high lignin content and low fiber content,and the microfibril angle of the cell wall around the pits is large,both of them are the weak structure of cell wall,which is easy to be destroyed under the action of stress.During the microwave pretreatment,the shrinkage differential caused by different microfibril angle between different wall layers will result in cell wall microcracks.(4)The porosity distribution of poplar wood was characterized by digital slice scanner,and the quantitative characterization equations of the micro-density and microMC of poplar wood were obtained.The heat transfer model of wood during microwave pretreatment under non-uniform water distribution was established.The law of heat transfer in the process of microwave pretreatment was revealed.The results showed that the maximum difference in porosity between early wood and late wood was 47.5%.When porosity of wood was increased,the micro-density of poplar wood decreased,while the micro-MC increased.The difference of micro-MC of poplar wood was up to 59% under different water saturation conditions.The thermal transfer model based on non-uniform properties could accurately characterize the influence of the uneven water distribution on the temperature distribution during microwave pretreatment.The temperature difference in poplar wood during microwave pretreatment process was as high as 98?,and the maximum saturation steam pressure difference was up to 0.15 MPa.The heat migrated from regions with MC of 20%-60% to regions with higher MC(greater than 120%)and areas with lower MC(less than 20%).(5)A theoretical model to predict the critical pressure and temperature conditions of wood during HIMP has been developed,and the mathematical simulation has been conducted.The mechanism of HIMP of poplar wood was revealed.The simulation results show that the cell wall was in a state of three-direction stress(axial stress,radial stress and circumferential stress)during HIMP.The axial stress was equal everywhere in the direction of wall thickness,while the circumferential stress and radial stress were non-uniformly,and reached the maximum value in the inner wall.During microwave pretreatment of poplar wood,the minimum critical pressure and the temperature of vessel and ray parenchyma were 0.32 MPa and 133?,0.4 MPa and 140.2?,respectively.The yield and damage pressure of wood fiber was 1.19 MPa and 2.3 MPa,respectively,and the corresponding temperature in the wood were 180? and 206?.In the process of HIMP,the thermal strain and wet strain caused by the extremely uneven temperature distribution and rapid water loss are the main reasons for the extrusion,stretching and even destruction of pore membrane,intercellular layer and parenchyma cells of wood.The cracks first appeared in the pit membrane and the compound middle lamella,and then appeared successively in the cell wall of ray parenchyma,vessels and wood fiber,and the cracks expanded from the inner surface of cell wall to the outer surface.