Collapse phenomena in beechwood during and after NH3-impregnation

Summary When beechwood is brought into contact with anhydrous ammonia, it swells like in contact with water. Upon removal of the ammonia from the wood, shrinkage is twice as high as before the treatment. Subsequent watering and drying still increase the linear shrinkage of beechwood. In order to find out in which dimensional range there is a reduction of volume in the wood, swelling and shrinkage tests as well as microscopic observations, mercury porosimetry measurements, and density measurements were carried out. The experiments showed that the collapse of the beechwood samples after ammonia treatment is caused by a reduction of the volume of the cell lumina and the perforations of the cell wall. With increasing duration of treatment, first the cell lumina, then the small and smallest pores of the cell walls are partly closed or even eliminated. However, this collapse does not extend to the molecular range. Density measurements even indicate a loosening of the substance during the first hour of ammonia treatment. With increasing duration of treatment, however, this loosening of the substance disappears again.The changes in the wood caused by ammonia treatment are so substantial that-analogous to ammonia cellulose-the term ammonia wood may be applied.This investigation was sponsored by the Zentenarfonds of the Swiss Federal Institute of Technology. The support of this research work is gratefully acknowledged. This article represents a part of the habilitation thesis submitted by the author to the Swiss Fed. Inst. of Technology with the title Physikalische und physikalisch-chemische Änderungen im Holz während und nach NH₃-Behandlung.