桉木在干燥过程中极易发生皱缩,使木材降等严重甚至报废,有效地解决桉木干燥皱缩问题是桉木资源高附加值实木化利用的重大难题之一.对桉木进行适度的预处理能够改变其内部细胞的微观结构,形成新的水分迁移通道.以尾巨桉(Eucalyptus urophylla × E.grandis)为试材,对其进行预冻、压缩及预冻-压缩预处理,然后进行常规干燥,研究预处理条件对桉木干燥速率、干燥应力应变及皱缩特性的影响.结果表明:3种预处理条件都能有效提高桉木的干燥速率,其中,预冻-压缩预处理后试件的干燥速率提高20%,幅度最大.预处理材与未处理材残余应力指标变化趋势一致,干燥后期预处理材指标值小于未处理材,残余应力小.3种预处理方式都不同程度地改变了细胞壁微观构造,破坏了皱缩发生的条件,抑制了木材的干燥皱缩.其中,预冻-压缩预处理材全干缩率最大减少15.8%,抑制木材皱缩效果最为显著.预冻-压缩预处理能够改变木材的微观结构,改善了木材的干燥特性,是一种有效抑制木材干燥皱缩的预处理技术.%Being widely planted in China, eucalyptus is one of the faster growing type with good mechanical and physi-cal properties, which are suitable for fabricating high value wood products, e.g.,furniture, door, and flooring. How-ever, as well known, the eucalyptus is difficult to be dried due to its native characteristics. Collapse is a very severe defect during drying, which would result in the degradation of wood products. The collapse is caused by the ununiform shrinkage during wood drying, especially occurred above the fiber saturated point (FSP). Resulted from the free water migration in the poor permeability lumens, the factors affecting the extent of collapse could be the permeability of wood species, drying rate, drying temperature, wood structures and extractive content in the wood. In order to im-prove the product quality and utilization ratio, it is important to control and reduce the collapses during drying. Cur-rently, several control methods including pre-treatment, low temperature kiln drying, continuous and intermittent dr-ying, etc. were carried out to change cell structures, reduce extractives, improve the permeability of wood for the pur-pose of reducing, or preventing collapse. In this study, Eucalyptus urophylla × E. grandis wood samples were pre-treated by freezing, compress or freezing-compress, and then the treated and un-treated samples were dried at a con-ventional kiln drying condition. The effects of pre-treatment on drying rate, residual stress and total shrinkage during the conventional kiln drying were examined. Results indicated that the drying rates of the samples pre-treated by freez-ing, compress and freezing-compress increased by 11.1%, 17.7%, 15.6% above FSP and 5.3%, 10.7%, 19.6% be-low FSP, respectively, compared with the drying rates of the un-treated samples. The results showed that the best pre-treatment method is the freezing-compress and the total drying rate was improved by 20%. The pre-treatment modified the micro-structure of cell walls, created new moisture migration channels and resulted in not only even moisture dis-tribution in wood cross sections, but also released of the drying stress during the drying process. Therefore, the residu-al deformation of the treated samples was reduced. It was concluded that the pre-treatment can reduce wood collapse in some degrees, and the total shrinkage of freezing-compress samples was reduced by 15.8%, which significantly re- duced the collapse during drying.
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