Effects of ultrasound pretreatment on microstructure and drying characteristics of Eucalyptus urophylla × E. grandis

Abstract

Wood collapse-shrinkage is a severe defect that occurs in Eucalyptus timber during drying. To solve this problem and provide the technical support for the Eucalyptus application as high value-added wood products, Eucalyptus urophylla × E. grandis samples were pre-treated by ultrasound at 400 W power for 1, 3, or 6 h and then dried in the kiln. The effects of ultrasound pre-treatment on drying rates, residual stress, and total shrinkage after kiln drying were investigated. The results indicated that ultrasound pre-treatment increased drying rates, which was increased by 5%, 13%, and 11% when moisture content (MC) was above 24%MC according to the treating duration, and below 24%MC, which were increased by 25%, 28%, and 23%. Drying rate increased below 24% MC. The residual stress decreased during the later drying stage, especially for the condition of 3 h of ultrasound pre-treatment compared with the untreated samples, while it increased in the early stage of the drying process. The total shrinkage decreased after ultrasound pre-treatment, and it decreased 14.9% after 3 h pre-treatment. Ultrasound pre-treatment broke the pit membranes and modified the microstructures of the wood. The microstructure changes not only affect the drying rate, but also affect the developing of stress and strain, collapse, and its recovery.