Effect Of Ultrasonic Pretreatment On Drying Characteristics Of Poplar

In order to solve the drying problem of poplar wood,enhance utilization efficiency of fast.growing wood and alleviate the contradiction between supply and demand of timber resources in our country,this study pretreated poplar wood by ultrasound in water medium.The effects of ultrasonic pretreatment on drying characteristics of poplar were explored and the meehanism was explained.The main results were as follows.(1)The ultrasonic pretreatment can improve the moisture content,drying rate and effective moisture diffusion coefficient of poplar.The drying rate and effective moisture diffusion coefficient of poplar increased with the prolonging of treatnent time and increase of ultrasonic power below 350W.When the power reached 400W,however,the drying rate and effective moisture diffusion coefficient of poplar decreased.In addition,the moisture content deviation and residual stress index of ultrasonic treated wood were lower than that of untreated wood,and the drying quality was better.(2)All of the samples showed similar temperature change trend with different thickness during heating process,but the ultrasonic pretreated samples reached stable temperature earlier.Moreover,the final and average temperature along with the difference between out layer and center layer temperature of treated samples were lower than that of untreated samples.However,the treatment did not change the difficulty of poplar wood temperature rising.Compared with the control group,the drying kinetics curves of treated and untreated samples were similar,and the drying rate of treated wood was faster than that of untreated wood.In addition,it was found that the drying rate of free water and bound water of poplar wood was increased by ultrasonic pretreatment.(3)The dry density of ultrasound pretreated samples were slightly lower than that of untreated samples.The water absorption and swelling coefficient of poplar wood decreased after pretreatment and the dimensional stability of treated samples were better consequently.Additionally,the ratio of tangential and radial swelling of treated samples dreased,hence the shape stability increased after ultrasonic treatment.The compressive and bending strength of treated samples decreased slightly after conditioned at 20? and 65%relative humidity,compared to untreated samples,while the modulus of elasticity increased slightly.The bending strain at failure of treated samples increased,and the elastic strain decreased specifically while the inelastic strain increased which was computed by subtracting the elastic strain from the failure strain.The effect of ultrasonic on the mechanical properties of poplar wood is the result of the change of the moisture absorption,density and the composition of wood eell wall.(4)FTIR analysis showed that the ultrasonic pretreatment interrupted weak chemical bonds between the wood cell wall components,thus enabling more functional groups exposed and weakened the connection between the tissue of poplar wood.The results of X ray diffraction showed that the ultrasonic treatment had little effect on the aggregation structure of wood,and the crystallinity of the treated wood was improved.In the process of ultrasonic treatment,"crystallization" and"decrystallization" phenomenon exists simultaneously.The changes of the crystallinity and the chemical functional groups of the ultrasonic treated wood greatly explained changes of dimensional stability and mechanical properties of poplar.The results of scanning electron microscopy showed that the ultrasonic wave destroyed the weak structure such as the pit membrane of wood tissue,and formed new pores and channels in the wood,as a result,the drying rate of poplar wood was increased.Ultrasonic pretreatment can not only improve the drying characteristics of poplar wood,but also improve its dimensional stability,and the negative influence on its mechanical strength is a very small.Therefore,ultrasonic pretreatment is an ideal modification method.