Study On Drying Technology Of White Birch Disks

With the development of the national economy and the improvement of people's living standards,people's demand for wood products is increasing,and the contradiction between supply and demand of timber resources is increasing day by day.Curved trees and other lowquality timber resources along the direction of the vertical fiber cut into the appropriate thickness of the disks,which can be processed into a dial,solid wood furniture and other handicrafts,etc.In this way,it will be able to increase its added value and utilization,to alleviate the contradictions with significance.The key to realize the above product yield improvement is its good quality during its drying process.However,even if the moisture content distribution is even in the drying process,it will still produce stress caused by nonsynchronous of the tangential and radial shrinkage due to the dry shrinkage anisotropy of wood,and the stress lead to cracking,seriously affecting its application and value.In theory,the cracking will be able to restrain effectively during the drying process of wood disks with appropriate softening to produces creep and stress relaxation before cracking,but there is no application examples.Therefore,the study about the effect of moisture content on glass transition temperature,and the moisture content and temperature on the storage modulus of different water content can make contribution to restrain the cracking during the drying process for wood disks.Therefore,in this paper,the appropriate conventional drying schedule and high frequency vacuum drying schedule for 35 mm thick white birch disks was obtained.The relationship between the moisture content and its glass transition temperature were studied.The effect of the pretreatment on the shrinkage characteristics,drying rate,quality and dimensional stability were studied based on the thermodynamics characteristics such as the dynamic viscoelasticity under different temperature and moisture content of White Birch.The main results are as follows:(1)The average drying rates of the whole drying process were 0.376% / h,0.527% / h and 0.652% / h,respectively,in the conventional drying of the White Birch Disks under the drying schedule I,II and III.The drying schedule II(the constant temperature is 50 ?,and the relative humidity decreases according to the decrease of the moisture content of the disks)was the appropriate conventional drying schedule for its drying time consume was shorter and the drying quality was better,but it needed to combine with wet heat pretreatment to optimize it for it still exist cracks.(2)In the high-frequency vacuum drying for disks,the placement of the disks between the electromagnetic field of high frequency had a significant effect on the drying rate and drying quality of the disks,and the aluminum plate with the grooves between the upper and lower plates had a significant effect on the uniform drying rate of each White Birch disks.The drying rate was faster and drying quality was better for it produced only a little cracks around the sapwood under the drying schedule IV(the absolute pressure of the environment is 7.0KPa,the temperature of disks is 42 ?).Thus,the drying schedule IV was the appropriate high-frequency vacuum drying schedule.(3)The glass transition temperature of White Birch decreased linearly with the increase of moisture content.When the moisture content is 22%,the glass transition temperature was 86 ?.The storage modulus decreased with the increase of temperature and initial moisture content,and the effect of moisture content on it was more significant.When the initial moisture content is 15% and 22%,the storage modulus increased sharply at the temperature of about 100 ?.(4)The shrinkage ratio and shrinkage coefficients of the disks with pretreatment decreased significantly.Compared to the untreated disks,the total dry shrinkage ratio of the disks with pretreatment decreased by 5.08%,2.07% and 9.80% respectively.The dry shrinkage coefficient for disks with pretreatment increased to some extent.(5)In the conventional drying for disks,the moisture distribution along the radial direction the disks with pretreatment was more uniform.Compared with the untreated disks,the total number of edge cracks of the treatment group was significantly reduced.In the high frequency vacuum drying for disks,there were few cracks on the disks with 80 ? saturated wet air pretreatment,and the edge cracking on the sapwood reduced for the disks with 90 ? saturated wet air and 100 ? saturated steam pretreatment.The equilibrium moisture content and the change rate of the dimension for the pretreated disks decreased.The final moisture content distribution was more uniform for the disks with pretreatment after drying.