Ammonia treated and mechanically densified beech wood (Fagus sylvatica L.): Fixation behaviour

The subject of this study was the fixation behaviour of mechanically densified beech wood specimens (Fagus sylvatica L.) prior plasticised with gaseous ammonia. The modification included both dry and conditioned specimens in combination with multiple pressure levels of the ammonia gas ranging from ambient pressure to almost saturated steam pressure. The mechanical densification was conducted in radial direction to a constant dimension regarding all different test series. Finally, the specimens were subjected to swelling in water to induce maximum recovery. The fixation behaviour was evaluated and investigated on the basis of three parameters: the direct elastic spring back after removing from the fixation device, the elastic spring back after drying and the recovery of set after the swelling cycle. The reference series of the dry and conditioned specimens showed as expected very high spring back and recovery of set values. The ammonia treatment led to a strong reduction of the recovery and caused at high pressure levels even negative spring back values. It could be suggested that there is a superposition of ammonia induced self-densification and relaxation of inner strain. Recovery of set is reduced to 10 % at high ammonia pressure levels and humid conditioning before ammonia treatment favours lower spring back and recovery of set values.